Novel synthesis of colloidal quantum dot ink and applications in semiconductor devices

Abstract

The present invention provides a method for producing a nanocrystal composition comprising an inorganic ligand via a one-pot synthesis. The present invention also provides nanocrystals comprising novel inorganic ligands and the use of these nanocrystals. The present invention further provides the use of the novel inorganic ligands in the preparation of nanocrystals.

Description

[0001] NOVEL SYNTHESIS OF COLLOIDAL QUANTUM DOT INK AND APPLICATIONS IN SEMICONDUCTOR DEVICES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates in general to nanocrystals. In particular, the present invention relates to a method for producing a nanocrystal composition comprising an inorganic ligand via a one-pot synthesis. The present invention also extends to nanocrystals comprising novel inorganic ligands and to the uses of these nanocrystals, and the use of the novel inorganic ligands in the preparation of nanocrystals.

[0004] BACKGROUND

[0005] Nanocrystals are useful in a wide range of applications, for example because their optical properties can be finely tuned to provide the desired properties. The optical properties (for example light absorption and emission characteristics) of nanocrystals can be finely tuned by controlling their size. The largest nanocrystals produce the longest wavelengths (and lowest frequencies), while the smallest nanocrystals produce shorter wavelengths (and higher frequencies). The size of the nanocrystals may be controlled by means of the method by which they are produced. This ability to finely tune the optical properties of the nanocrystals, by controlling their size, makes nanocrystals suitable for use in a wide range of applications, including, for example, photodetectors, sensors, solar cells, bioimaging and bio-sensing, photovoltaics, displays, lighting, security and counterfeiting, batteries, wired high-speed communications, quantum dot (QD) lasers, photocatalysts, spectrometers, injectable compositions, field-effect transistors, light-emitting diodes, lasers, photonic or optical switching devices, hydrogen production and metamaterials.

[0006] Pristine lead- and lead-free colloidal quantum dots (CQDs) are commonly synthesized via wet chemistry using long-chain organic ligands to control the growth of the nanocrystals and stabilize the colloidal system in the solvent. In order to construct high performance CQD devices, the long chain insulating organic ligands need to be replaced with shorter organic chain ligands or conducting inorganic ligands in order to improve the charge transfer of the system.

[0007] In the methods of the prior art, before the ligand replacement reaction (ligand exchange) takes place, there are multiple purification steps that are performed on the crude mixture comprising the nanocrystals in order to remove the by-products of the initial nanocrystal synthesis from the mixture. A flow diagram of the prior art method for nanocrystal synthesis is shown in Figure 1 , illustrating the formation of pristine nanocrystals, the purification steps, and then the ligand exchange reaction. Finally, the ink composition comprising the nanocrystals is formed. The conducting semiconductor nanocrystals can then be used in photodevices.

[0008] The purification steps have previously been reported to be an essential step to achieve quantum dots with optimal performance, for example in King et al (Importance of QD Purification Procedure on Surface Adsorbance of QDs and Performance of QD Sensitized Photoanode, J. Phys. Chem. C 2012, 116, 3349- 3355) and Chen et al (IEEE Access, vol. 8, pp. 159415-159423, 2020). In King et al, for example, it is explained that, prior to sensitization, QDs are subject to a common purification; cycles of alternate precipitation / redispersion in a nonsolvent / solvent. King et al concluded that the purification steps were critical and that, with enhanced purification, the surface bound QD concentration may increase 5-fold.

[0009] However, these purification steps are extremely time-consuming and use a significant amount of solvent. Furthermore, it is believed that the semiconductor nanocrystals of the prior art comprise partially oxidised surfaces which result in poor reproducibility and poor optoelectronic properties of the semiconductor nanocrystals.

[0010] There is therefore an unmet need for a method of producing nanocrystals with reduced surface oxidation. Furthermore, there is an unmet need for a method of producing nanocrystals that is quicker and produces less waste. Additionally, the use of lead and other heavy metals in nanocrystals can be problematic in some industries due to their toxicity. However, current alternative materials and ligand systems are outperformed by lead-based systems. There is therefore a further need in the industry to provide alternative ligand systems that avoid the use of toxic heavy metals such as lead, whilst also giving excellent optoelectronic properties.

[0011] SUMMARY OF INVENTION

[0012] The inventors have found a method of producing nanocrystals with improved optical properties. In particular, the inventors have found that a method of producing nanocrystals that does not comprise purification steps after the initial synthesis steps and before the ligand exchange reaction leads to improved optical properties of the resultant nanocrystals.

[0013] As such, according to a first aspect, the present invention provides a method for producing a nanocrystal composition, the method comprising: a) a contacting step taking place in a non-polar solvent, the contacting step comprising contacting: a plurality of first organic ligand compounds; a metal-containing compound comprising a metal element; and at least one reagent comprising a pnictogen element or a chalcogen element, to form a mixture comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of the first organic ligand compounds coordinated to a surface of the crystalline core, wherein the crystalline core comprises (i) the metal element, and (ii) the pnictogen element or the chalcogen element, and b) adding to the mixture an exchange composition comprising an exchange ligand precursor and a polar solvent, wherein there are no purification steps between step a) and step b).

[0014] According to a second aspect, the present invention provides a nanocrystal composition obtainable by the method of the first aspect. According to a third aspect, the present invention provides an ink composition comprising the nanocrystal composition of the second aspect. The ink composition will generally comprise the nanocrystals of the nanocrystal composition and the polar solvent used to dissolve the nanocrystals.

[0015] BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 shows a simplified flow diagram of a prior art method for quantum dot synthesis and contrasts this with the method of the present invention.

[0017] Figure 2 shows a simplified drawing of the in-situ ligand exchange process of PbS quantum dots using a novel inorganic ligand.

[0018] Figure 3 shows the absorption spectra of quantum dots at different stages of the synthesis, and contrasts a prior art method for quantum dot synthesis with a method of the present invention.

[0019] Figure 4 shows a photograph of phase separated quantum dots in a polar inorganic phase with a final ink.

[0020] Figure 5 shows the absorption spectra of nanocrystals produced according to a method of the present invention (in-situ) and the change over time.

[0021] Figure 6 shows the absorption spectra of nanocrystals produced according to a method of the prior art (ex-situ) and the change over time.

[0022] Figure 7 shows the absorption spectra of in-situ and ex-situ PbS quantum dots, with CsPbl3used as the inorganic ligand.

[0023] Figure 8 shows the absorption spectra of in-situ PbS quantum dots, with CsZnl3used as the inorganic ligand.

[0024] Figure 9 shows the absorption spectra of PbS quantum dots, with Asl3used as the inorganic ligand. Figure 10 shows a photograph of a ligand exchanged QD ink using Csl (left) and Znl2(right) as the inorganic ligand.

[0025] Figure 11 shows (a) Current density-voltage characteristic of a photodiode device fabricated using in-situ and ex-situ PbS QDs, and (b) EQE spectrum of as fabricated photodiode devices using in-situ and ex-situ QDs using the same device architecture in the same batch.

[0026] Figure 12 shows (a) Absorption spectrum of ligand exchanged InAs QDs in 2,6- difluoropyridine, and (b) a photograph of as fabricated films along with an AFM image to show the film quality.

[0027] Figure 13 shows (a) Current density-voltage characteristic of a photodiode device fabricated using in-situ and ex-situ InAs QDs, and (b) EQE spectrum of as fabricated photodiode devices using in-situ and ex-situ QDs using the same device architecture in the same batch.

[0028] Figure 14 shows the absorption spectrum of the InAs nanocrystal ink using the Csl-lnBr3ligand system.

[0029] Figure 15 shows a thermogravimetric analysis (TGA) curve for InAs nanocrystals, demonstrating how the extent of inorganic ligand exchange may be measured.

[0030] Figure 16 shows the X-ray diffraction (XRD) data for PbS nanocrystals with CsPbl3ligands.

[0031] Figure 17 shows the high resolution TEM micrographs for PbS nanocrystals with CsPbl3ligands, showing the superlattice structure formation on the surface of the nanocrystals.

[0032] Figure 18 shows the inductively coupled plasma time of flight mass spectrometry (ICP-TOF MS) of PbS nanocrystals with CsPbl3ligands, confirming the presence of CsPbl3. Figure 19 shows the absorption spectra of nanocrystals produced according to a method of the present invention (in-situ).

[0033] DETAILED DESCRIPTION

[0034] When describing the aspects of the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

[0035] As used in the specification and the appended claims, the singular forms "a", "an," and "the" include both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a nanocrystal" means one nanocrystal or more than one nanocrystal. By way of example, “an indium-containing compound” means one indium-containing compound or more than one indium- containing compound. References to a number when used in conjunction with comprising language include compositions comprising said number or more than said number.

[0036] The terms "comprising", "comprises" and "comprised of’ as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms "comprising", "comprises" and "comprised of” also include the term "consisting of’.

[0037] As used herein, the term "and / or," when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a list is described as comprising group A, B, and / or C, the list can comprise A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

[0038] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts of percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, indicates that a value includes the standard deviation of error for the device or method being employed to determine the value. The term "about" is meant to encompass variations of + / - 10% or less, + / -5% or less, or + / -0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosure. It is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0039] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0040] Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. By means of further guidance, definitions for the terms used in the description are included to better appreciate the teaching of the present disclosure. All publications referenced herein are incorporated by reference thereto.

[0041] As used herein, unless otherwise defined, the term "composition" may be open ended or closed. For example, “composition” comprises the specified material, i.e., the nanocrystals, and further unspecified material, or may consist of the specified material, i.e., to the substantial exclusion of non-specified materials.

[0042] In situ synthesis

[0043] According to a first aspect, the present invention provides a method for producing a nanocrystal composition, the method comprising: a) a contacting step taking place in a non-polar solvent, the contacting step comprising contacting: a plurality of first organic ligand compounds; a metal-containing compound comprising a metal element; and at least one reagent comprising a pnictogen element or a chalcogen element, to form a mixture comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of the first organic ligand compounds coordinated to a surface of the crystalline core, wherein the crystalline core comprises (i) the metal element, and (ii) the pnictogen element or the chalcogen element, and b) adding to the mixture an exchange composition comprising an exchange ligand precursor and a polar solvent, wherein there are no purification steps between step a) and step b).

[0044] The method of the present invention does not involve the purification steps of the prior art, and in turn surprisingly leads to several advantages. In other words, the present invention provides a one-pot method for producing nanocrystals.

[0045] The ink formation process of the prior art is time consuming and expensive due to repeatable processing / purification of the nanocrystals. Moreover, the purification process may cause surface oxidation which results in poor reproducibility of nanocrystal ink formation and poor optoelectronic properties. The inventors of the present invention have discovered a simple and cost-effective ligand exchange process via an in-situ technique in which the ligand exchange processes are carried out directly on as-synthesized pristine nanocrystals without the purification of the nanocrystals beforehand. The inventors estimate that this method allows for improved optical properties of the resultant nanocrystals, as well as providing a 30% cost reduction for the nanocrystal ink production thanks to the lower use of solvents, energy and labour and simpler process for scale up. Moreover, the inks prepared by this technique offer good colloidal stability in polar solvents as well as excellent optoelectronic properties with a high degree of reproducibility. These results are surprising due to the accepted belief in the art that purification is an essential step after the initial formation of the nanocrystals in order to achieve optimal performance of the nanocrystals. Traditionally, pristine nanocrystals, in particular quantum dots (QDs), are purified by a solvent / antisolvent method and dissolved in a non-polar solvent, before going through a ligand exchange process with more electrically-conductive short chain organic ligands or inorganic ligands to arrive at the nanocrystal ink for use in devices.

[0046] This solvent / antisolvent traditional purifying process is not only time consuming but also less efficient and costly. Without wishing to be bound by theory, it is believed that the post synthetic solvent / antisolvent purification of pristine nanocrystals also causes surface oxidation of the nanocrystals, which results in poor device performance and an inconsistent shift in the absorption spectrum towards lower wavelength. Here, we introduce the in-situ ligand exchange in which the exchange ligand is introduced into the crude solution comprising pristine nanocrystals without any purification steps in between. As stated above, this route offers a more cost effective and efficient solution. The present invention provides a stable colloidal ink composition comprising nanocrystals with improved performance by a method which is not only much cheaper, but is also more scalable. The final ink compositions are generally then used to fabricate devices via a single step deposition technique.

[0047] The advantages of the present method include: prevention of surface oxidation of nanocrystals, provision of higher quality nanocrystal compositions and devices, significantly cheaper production, provision of a less time consuming process, and provision of a process that is easy to scale up.

[0048] As used herein, the term “nanocrystal” is used to refer to a crystalline particle with at least one dimension measuring less than 100 nanometres (nm).

[0049] As used herein, the term “semiconductor nanocrystal” is used interchangeably with the term “quantum dot”, and is used to refer to a semiconductor crystalline material exhibiting quantum confinement effects that allow it to mimic the properties of an atom. Quantum dots may also be known as zero-dimensional nanocrystals. As used herein, the term “semiconductor nanocrystal composition” is used to refer to a composition comprising at least one semiconductor nanocrystal. The nanocrystals of the present invention are generally semiconductor nanocrystals and quantum dots.

[0050] As used herein, the term “Group I metal” is used to refer to a metal in Group IA and Group I B of the periodic table. For example, a Group I A metal may be lithium, sodium, potassium, rubidium or caesium, and a Group IB metal may be copper, silver or gold.

[0051] As used herein, the term “Group II metal” is used to refer to a metal in Group IIA or Group IIB of the periodic table. For example, a Group IIA metal may be beryllium, magnesium, calcium, strontium or barium, and a Group IIB metal may be zinc, cadmium or mercury.

[0052] As used herein, the term “Group III metal” is used to refer to a metal in group 13 of the periodic table. For example, a Group III metal may be aluminium, gallium, indium or thallium.

[0053] As used herein, the term “Group IV metal” is used to refer to a metal in group 14 of the periodic table. For example, a Group IV metal may be silicon, germanium, tin or lead.

[0054] As used herein, the terms "Group V element” and “pnictogen” are used to refer to an element in group 15 of the periodic table. For example, a pnictogen may be nitrogen, phosphorus, arsenic, antimony or bismuth.

[0055] As used herein, the term “indium pnictogenide semiconductor nanocrystal” is used to refer to a semiconductor nanocrystal comprising indium and a pnictogen.

[0056] As used herein, the terms "Group VI element” and “chalcogen” are used to refer to an element in group 16 of the periodic table. For example, a chalcogen may be oxygen, sulfur, selenium, tellurium or polonium. As used herein, the term “ligand” is used to refer to a compound capable of forming a complex with the nanocrystal by coordinating to a surface of the nanocrystal. A nanocrystal generally comprises a crystalline core with dimensions in the order of tens of nanometres. They are typically stabilised as a colloidal solution by surface capping ligands, which may coordinate to the crystalline core as Lewis acidic (Z- type), Lewis basic (L-type) or anionic (X-type) species.

[0057] As used herein, the term “organic compound” is used to refer to a compound comprising carbon atoms covalently bound to other atoms. As used herein, the term “Cx-Cy” organic compound, wherein x and y are integers, is used to refer to an organic compound containing at least x and no more than y carbon atoms.

[0058] As used herein, the term “inorganic compound” is used to refer to a compound other than an organic compound.

[0059] As used herein, the term “purification steps” is used to refer to any method resulting in the isolation of the nanocrystals in a solution, or any method by which the ratio of impurities to nanocrystals in the mixture is decreased. As used herein, the term “impurities” is used to refer to components in the solvent other than the nanocrystals (not including the solvent). For example, impurities may be free organic ligands or free inorganic ligands in solution, or salts thereof. In particular, the term “purification steps” includes the purification steps disclosed in the prior art such as cycles of alternate precipitation / redispersion in a nonsolvent / solvent. These purification steps generally involve precipitating the nanocrystals via centrifugation with a solvent / nonsolvent followed by redispersion in solvent.

[0060] As used herein, the letter “X” is used to refer to a halide. Where X is a halide, X is preferably Cl, Br or I. X may or may not be a mixed halide, for example X4may be l4or Br3l .

[0061] Preferably, the nanocrystals produced by the method of the first aspect are semiconductor nanocrystals, for example colloidal quantum dots. The present invention more specifically produces colloidal quantum dot inks. Preferably, step a) and step b) take place in a reaction vessel, and the mixture remains in the reaction vessel between step a) and step b). Preferably, step b) is carried out immediately after step a). In other words, there are preferably no further steps between step a) and step b). Preferably, the method is a one-pot method. Preferably, there are no components removed from the mixture between step a) and step b). Preferably, there are no components added to the mixture between step a) and step b).

[0062] As used herein, the term “reaction vessel” is used to refer to a container that isolates one reaction (e.g. the contacting step) from another, or that provides a space in which a reaction can take place. As used herein, the term “step b) is carried out immediately after step a)” is used to refer to a sequence of steps in which there are no further reaction steps taking place between step a) and step b). As used herein, the terms “one-pot method” and “in situ method” are used to refer a reaction in which all steps are carried out in the same reaction vessel. As used herein, the term “components” is used to refer to the compounds that form the mixture.

[0063] Preferably, steps a) and b) take place under inert conditions. For example, the reaction may take place under argon or nitrogen gas. Typically, inert conditions are used to avoid the surface oxidation of the nanocrystals. After the ligand exchange reaction with the exchange ligands, the nanocrystals may be exposed to air as they become less sensitive to oxygen.

[0064] The metal element of the present invention may be any suitable metal element for preparing nanocrystals, as would be understood by the skilled person. For example, the metal element may be any suitable post-transition metal element or transition metal element.

[0065] Preferably, the metal element is selected from the consisting of a Group I metal, a Group II metal, a Group III metal, a Group IV metal, and mixtures thereof, and the metal-containing compound is selected from the group consisting of an elemental metal, a metal halide, a metal acetate, a metal nitrate, a metal carbonate, a metal oleate, a metal oxide, a metal peroxide, a metal alkoxide, a metal hydroxide, a metal sulfate, a metal acetylacetonate, a metal perchlorate, a metal carboxylate, a metal cyanide or mixtures thereof. Preferably, the metal element is selected from the group consisting of indium, gallium, lead and silver.

[0066] In a preferred embodiment, step a) comprises contacting the plurality of first organic ligand compounds, a metal-containing compound comprising a metal element, wherein the metal element is indium or gallium, and at least one reagent comprising a pnictogen element. In other words, in a preferred embodiment, the metal element is indium or gallium, and the at least one reagent comprises a pnictogen element. As shown in Example 10, InAs quantum dots produced by a method of the present invention allow for photodiode devices to be produced with excellent EQE properties.

[0067] In a preferred embodiment, the metal element is indium. Where the metal element is indium, preferably the metal-containing compound is selected from the group consisting of an indium halide, indium acetate, indium nitrate, indium carbonate, indium oleate and mixtures thereof. In a particularly preferred embodiment, the metal-containing compound is indium acetate.

[0068] In an alternative embodiment, the metal element is gallium. Where the metal element is gallium, preferably the metal-containing compound is selected from the group consisting of a gallium halide, gallium acetate, gallium nitrate, gallium carbonate, gallium oleate and mixtures thereof.

[0069] The inventors have found that a range of suitable pnictogen elements may be used to prepare nanocrystals according to the method of the present invention. Preferably, the pnictogen element is selected from the group consisting of phosphorus, arsenic and antimony.

[0070] In an embodiment, the pnictogen element is phosphorus. Where the pnictogen element is phosphorus, the at least one reagent may be any suitable phosphorus- containing precursor, but preferably the at least one reagent is selected from the group consisting of triphenylphosphine, tris(trimethylsilyl)phosphine, tris(dimethylamino)phosphine, triethylphosphine, tributylphosphine, tricyclohexylphosphine, trioctylphosphine, and mixtures thereof.

[0071] In a preferred embodiment, the pnictogen element is arsenic or antimony. In a particularly preferred embodiment, the pnictogen element is arsenic. Where the pnictogen element is arsenic, preferably the at least one reagent is selected from the group consisting of an arsenic halide, tris(trimethylsi lyl)arsine, arsenic oxide, arsenic sulfate, and mixtures thereof. In a preferred embodiment, the pnictogen element is antimony. Where the pnictogen element is antimony, preferably the at least one reagent is an antimony halide or tris(trimethylsilyl)antimony.

[0072] Where the metal element is indium or gallium, the method of the present invention is particularly useful for preparing secondary, tertiary and quaternary nanocrystals. For example, the method is suitable for preparing indium pnictogenide and gallium pnictogenide nanocrystals. For example, the present invention may be used to prepare InAs, InSb, InAsSb, GaAs, GaSb, InGaAs, InGaSb, and InGaAsSb nanocrystals.

[0073] In an embodiment wherein the at least one reagent comprises a pnictogen element, the contacting step may also include contacting the other components with a compound selected from the group consisting of secondary amines, zinc carboxylates, and mixtures thereof, preferably wherein the compound is dioctyl amine. This compound is used to further enhance the shape control of the nanocrystals by complexing to the pnictogen-containing compound. In this way, it is possible to reduce the reactivity of the pnictogen-containing compound, which leads to better control over the size and the size distribution of the nanocrystals. Preferably, this compound is used when the pnictogen-containing compound is tris(trimethylsilyl)arsine or tris(trimethylsilyl)antimony, as these compounds are especially reactive.

[0074] In an embodiment, step a) comprises contacting the plurality of first organic ligand compounds, a metal-containing compound comprising a metal element, wherein the metal element is lead or silver, and at least one reagent comprising a chalcogen element. In otherwords, in a preferred embodiment, the metal element is lead or silver, and the at least one reagent comprises a chalcogen element.

[0075] In a preferred embodiment, the metal element is lead. Preferably, the metalcontaining compound is a lead oxide or lead acetate. Preferably, the metalcontaining compound is a lead oxide. Preferably, the metal-containing compound is selected from the group consisting of PbO, Pb3O4, PbO2, and mixtures thereof. In an alternative embodiment, the metal-containing compound is lead acetate.

[0076] In an alternative embodiment, the metal element is silver. Preferably, the metalcontaining compound is selected from the group consisting of silver acetate, silver nitrate, silver oxide, silver halide and mixtures thereof.

[0077] The inventors have found that a range of suitable chalcogen elements may be used to prepare nanocrystals according to the method of the present invention. Any suitable chalcogen-containing precursor may be used. Preferably, the chalcogen element is selected from the group consisting of sulfur, selenium and tellurium.

[0078] In an embodiment, the chalcogen element is sulfur. Examples of suitable sulphur- containing reagents include bis(trialkylsilyl)sulphide compounds such as bis(trimethylsilyl)sulphide, bis(triethylsilyl)sulphide and bis(tripropylsilyl)sulphide, thioacetamide, tri-n-octylphosphine sulphide, tributylphosphine sulphide, alkyl substituted and / or phenyl thiourea compounds such as N,N’-disubstituted and N,N,N’-trisubstituted thioureas, alkyl substituted thioamide compounds, elemental sulphur, mercaptopropylsilane, sulfur-triphenylphosphine ("S-TPP"), sulfurtrioctylamine ("S-TOA"), ammonium sulfide, sodium sulfide, hexanethiol, octanethiol, decanethiol, dodecanethiol, hexadecanethiol, and mixtures thereof.

[0079] In an alternative embodiment, the chalcogen element is selenium. Examples of suitable selenium-containing compounds include bis(trimethylsilyl)selenide, trioctylphosphine selenide, 1 -octadecene selenium, tributylphosphine selenide, selenium-triphenylphosphine, and mixtures thereof. In an alternative embodiment, the chalcogen element is tellurium. Examples of suitable tellurium-containing compounds include tellurium powder, triphenylphosphine telluride, tri-n-octylphosphine telluride, tributylphosphine telluride, and mixtures thereof.

[0080] Where the metal element is lead or silver, the method of the present invention is suitable for preparing secondary and tertiary nanocrystals. For example, the method is suitable for preparing lead chalcogenide and silver chalcogenide nanocrystals. For example, the present invention may be used to prepare PbS, PbSe, PbTe, Ag2S and Ag2Se nanocrystals.

[0081] Suitably, the lead chalcogenide nanocrystals or lead chalcogenide nanocrystal composition exhibits absorption in the visible and near infra-red ranges, suitably in the range of 500 to 4500nm, preferably suitably in the range of 500 to 2400 nm, preferably suitably in the range of 950 to 1600 nm, preferably in the range of 1350 to 1600 nm. In a preferred embodiment, lead chalcogenide nanocrystals or lead chalcogenide nanocrystal composition exhibit absorption of greater than 1300 nm.

[0082] The first organic ligand compound is capable of forming a complex with the nanocrystals by coordinating to a surface of the crystalline core of the nanocrystals. Generally, the first organic ligand compound coordinates to the surface of the nanocrystal via the functional group of the organic ligand as a Lewis acidic (Z-type), Lewis basic (L-type) or anionic (X-type) species. Generally, the organic ligand coordinates to the surface of the nanocrystal via the functional group of the organic ligand.

[0083] Typically, the metal-containing compound may be contacted with a molar excess of the organic ligand. For example, the molar ratio of the metal atoms (from the metal-containing compound) to the organic ligand may be in the range of from 1 :1.5 to 1 :200, such as from 1 :1.5 to 1 :60.

[0084] The first organic ligand compound any be any suitable organic compound as known in the art. In a preferred embodiment, the first organic ligand compound has a formula selected from the group consisting of RH2PO, R2HPO, R3PO, RPO(OH)2, or R2POOH, RH2P, R2HP, R3P, ROH, RCOOH, RCOOR’, RSH, RNH2, R2NH, and R3N, wherein R and R’ are each independently selected from the group consisting of Ci-C24 alkyl, C2-C24 alkenyl, C6-C24 aryl, and mixtures thereof. Preferably, the first organic ligand compound is a C2-C24 organic compound comprising a functional group selected from the group consisting of amino, thiol, hydroxyl and carboxylic acid. Examples of suitable first organic ligand compounds include, but are not limited to, aminobenzoic acids, dicarboxylic acids, aminoalkylcarboxylic acids, mercaptopropionic acid, mercaptobenzoic acid, thioalkanes, dithioalkanes, thiocarboxylic acids, thioglycolic acid, polyethylene glycol), polyethylene glycol) bis(3-aminopropyl) terminated, didodecyldimethylammonium bromide, n-dodecylammonium bromide, dodecyltrimethylammonium bromide, dimercaptosuccinic acid, oleic acid, oleylamine, bis iphenylphosphino)methane and alkylamines.

[0085] Preferably, the first organic ligand compound is an organic acid or organic amine. Preferably, the first organic ligand compound is a compound selected from the group consisting of mercaptopropionic acid, mercaptobenzoic acid, thioglycolic acid, dimercaptosuccinic acid, oleic acid and oleylamine. Preferably, the first organic ligand compound is oleic acid or oleylamine. In a preferred embodiment, the first organic ligand compound is oleic acid. In a preferred embodiment, the first organic ligand compound is oleylamine.

[0086] The exchange ligand precursor forms a ligand capable of forming a complex with the nanocrystals by coordinating to a surface of the crystalline core of the nanocrystals as known in the art. The exchange ligand precursor may comprise any suitable compounds, for example organic or inorganic compounds.

[0087] In some embodiments, the exchange ligand precursor comprises a second organic ligand compound. The preferred second organic ligand compounds are generally the same as for the first organic ligand compound as described above. Preferably, however, the first organic ligand compound and the second organic ligand compound used in any individual reaction are different compounds. In a particularly preferred embodiment, the second organic ligand compound is a C2-C24 organic compound comprising a functional group selected from the group consisting of amino and thiol. Preferably, the second organic ligand compound is selected from the group consisting of 3-mercaptopropionic acid, thioglycerol, 1 ,2- benzenedithiol, 1 ,3-benzenedithiol, 1 ,4-benzenedithiol, ethane-1 ,2-dithiol, meso- 2,3-dimercaptosuccinic acid, dimercaptopropanol, 5-(trifluoromethyl)pyridine-2- thiol, 3-(trifluoromethyl)pyridine-2-thiol, butylamine, phenethylamine, benzylamine, amylamine and mixtures thereof. In a preferred embodiment, the thiol comprises an aromatic group. Preferably, the thiol is selected from the group consisting of 5-(trifluoromethyl)pyridine-2-thiol, 3-(trifluoromethyl)pyridine-2-thiol and mixtures thereof.

[0088] In some embodiments, the exchange ligand precursor comprises an inorganic ligand precursor comprising one or more inorganic compounds. For example, the inorganic ligand precursor may comprise one or more metal halide compounds. Furthermore, the present inventors have discovered that several inorganic compounds, which previously had not been used in the preparation of nanocrystals, may be used as inorganic ligands. The use of these novel inorganic ligands results in nanocrystals comprising excellent optoelectronic properties.

[0089] Without wishing to be bound by theory, it is believed that the inorganic ligands of the present invention form a shell surrounding the nanocrystal during step b). In this way, the nanocrystals formed in the nanocrystal composition of the first aspect comprise a core-shell structure, wherein the nanocrystals comprise a crystalline core comprising (i) the metal element, and (ii) the pnictogen element or the chalcogen element, and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor during step b) of the reaction. The structure of the shell may be amorphous or crystalline. Preferably, the shell is crystalline and / or forms a lattice layer surrounding the crystalline core. As used herein, the term “lattice” is used to refer to an ordered array of points describing the arrangement of particles that form a crystal.

[0090] The crystalline core for all aspects disclosed herein typically comprises (i) a metal element and (ii) a pnictogen element or a chalcogen element. The crystalline core has a different chemical composition to the shell, such that the shell surrounding the crystalline core is structurally distinguished from the crystalline core. In other words, the crystalline core has a different chemical composition to the shell or lattice layer surrounding the crystalline core. Preferably, the crystalline core does not contain a halide. Preferably, the crystalline core comprises substantially (i) a metal element and (ii) a pnictogen element or a chalcogen element. More preferably, the crystalline core consists of (i) a metal element and (ii) a pnictogen element or a chalcogen element.

[0091] It may be that the composition and structure of the inorganic ligand precursor and the resultant inorganic ligand in the shell are the same, or they may be different. In other words, the inorganic compounds in the inorganic ligand precursor may react together and / or with the nanocrystal to form the shell surrounding the crystalline core, thereby changing from their original composition and structure which they had when initially added to form the inorganic ligand precursor. For example, the inorganic compounds added to the inorganic ligand precursor may be Csl and Znl2. These compounds may be present in the inorganic ligand precursor as ions in solution, or as Csl and Znl2, or as CsZnl3. The inorganic compounds may then react together and / or with the nanocrystal to form the shell surrounding the crystalline core, wherein the shell comprises amorphous or crystalline CsZnl3. In a particular embodiment, the shell may comprise crystalline CsZnl3, optionally having a perovskite crystal structure, formed from an inorganic ligand precursor initially comprising Csl and Znl2.

[0092] Examples of suitable inorganic compounds in the inorganic ligand precursor include PbX2, CsX, I nBr3, MgBr3, AgBr, FeX3, AsX3, TIX, CsZnl3, Csl nBr3l , CsPbl3, ZnX2or Csl, wherein X is a halide. In a preferred embodiment, the inorganic ligand precursor comprises Csl and Znl2. In a preferred embodiment, the inorganic ligand precursor comprises Csl and lnBr3. Of particular note is that the present invention discloses the advantageous use of novel pure inorganic ligand (a) Csl and Znl2, and (b) Asl3for colloidal lead based and lead-free quantum dots. Both of these ligand systems, as well as other novel inorganic ligands systems disclosed herein such as TIX and CslnBr3l, provide complete surface passivation and offer high charge carrier mobilities.

[0093] In an embodiment, the inorganic ligand precursor comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3. Examples of suitable organic ammonium salts include methylammonium and formamidinium.

[0094] In a preferred embodiment, the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is a Group IA metal, thallium or an organic ammonium salt, M is a metal having +2 oxidation state, and X is a halide. In a preferred embodiment, the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, potassium, lithium, thallium and sodium, M is Pb, Sn, Cu or Zn, and X is a halide. Preferably, the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, potassium, lithium, and sodium, M is Pb or Zn, and X is a halide.

[0095] In a preferred embodiment, M is Pb. Where M is Pb, the metal halide is preferably CsPbl3. In a preferred embodiment, M is Zn. Where M is Zn, the metal halide is preferably CsZnl3.

[0096] In an embodiment, the inorganic ligand precursor comprises an inorganic compound of the formula CsZnxAsyl3, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1 . In an embodiment, y is less than 0.01 or in the range of 0.01 to 0.1.

[0097] In an embodiment, the inorganic ligand precursor comprises an inorganic compound of the formula Cs3ZnxASyl5, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1 . In an embodiment, y is in the range of 0.01 to 0.1. In an alternative embodiment, the inorganic ligand precursor comprises an inorganic compound of the formula Cs3BX6, wherein B is indium or antimony and X is a halide.

[0098] In an embodiment, the metal halide has a perovskite crystal structure. As used herein, the term “perovskite” takes on its standard meaning in the art. As such, the term “perovskite” is used to refer to an inorganic compound comprising a three-dimensional crystal structure related to that of CaTiO3, such as CsZnl3and CsPbl3.

[0099] Further examples of suitable metal halide inorganic ligand precursors include inorganic compounds having a perovskite-like crystal structure such as Cs3lnX6, CslnX4, and Cs2lnX5, wherein X is a halide. In an embodiment, the inorganic ligand precursor comprises a metal halide of the formula Cs3lnX6, CslnX4, or Cs2lnX5, wherein X is a halide. Preferably, the shell comprises CslnBr3l.

[0100] Step b) involves adding to the mixture an exchange composition. Once the exchange composition is added, a reaction generally occurs between the exchange ligand precursor and the nanocrystals capped with the first organic ligand compounds.

[0101] Where the exchange composition comprises second inorganic ligand compounds, a reaction generally occurs between the second organic ligand compounds and the nanocrystals capped with the first organic ligand compounds. Specifically, a reaction will take place to remove the first organic ligand compounds from the surface of the nanocrystal. The first organic ligand compounds are replaced by the second organic ligand compounds.

[0102] As such, the method preferably further comprises: c) displacing at least a portion of the first organic ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a plurality of second organic ligand compounds coordinated to the surface of the crystalline core. Where the exchange composition comprises an inorganic ligand precursor, a reaction generally occurs between the inorganic compounds in the inorganic ligand precursor and the nanocrystals capped with the first organic ligand compounds. Specifically, a reaction will take place to remove the first organic ligand compounds from the surface of the nanocrystal. The first organic ligand compounds are replaced by the ligand formed from the inorganic compounds of the inorganic ligand precursor.

[0103] As such, the method preferably further comprises: c) displacing at least a portion of the first organic ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor.

[0104] Preferably, step c) takes place under inert conditions. Preferably, steps a), b) and c) all take place under inert conditions.

[0105] Preferably, step c) comprises stirring the mixture for at least two hours, preferably between 2 hours and 18 hours, preferably between 4 hours and 18 hours.

[0106] Preferably, where the exchange composition comprises an inorganic ligand precursor, step c) comprises displacing at least 10%, preferably at least 20%, more preferably at least 30%, more preferably at least 40%, more preferably at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90%, of the first organic ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor, as determined by thermogravimetric analysis (TGA) in air at a ramp rate of 10 °C / min.

[0107] The percentage by weight of different ligands on the surface of the nanocrystal is derived from the characteristic trace of the TGA curve for these nanocrystals, as exemplified in Figure 15. It is assumed that the first mass decrease (generally between 0 °C and 200 °C) corresponds to the evaporation of solvent. It is assumed that the second mass decrease (generally between 200 °C and 450 °C) corresponds to the decomposition of the organic ligands. It is assumed that the third mass decrease (generally between 450 °C and 750 °C) corresponds to the decomposition of the inorganic ligands. It is assumed that any further mass decrease (generally above 750 °C) corresponds to the decomposition of the nanocrystal crystalline core, for example PbS.

[0108] As mentioned above, the inorganic compounds in the inorganic ligand precursor are preferably metal halides. The inorganic compounds in the inorganic ligand precursor generally react to form a shell layer at partially surrounding the crystalline core of the nanocrystal. This shell may be amorphous or crystalline. Preferably, the shell is crystalline. Where the shell is crystalline, the shell may have a perovskite crystal structure. The core-shell structure of the resultant nanocrystals in the nanocrystal composition is described above.

[0109] As mentioned above, it may be that the composition and structure of the inorganic ligand precursor and the resultant inorganic ligand in the shell are the same, or they may be different. As such, the inorganic ligand precursor and the shell may have the same composition. As such, the inorganic ligand precursor and the shell may have the same structure.

[0110] Preferably, the shell comprises a metal halide. The shell may comprise any suitable metal halide as known in the art. Examples of suitable metal halides include PbX2, CsX, I nBr3, MgBr3, AgBr, FeX3, AsX3, TIX, CsZnl3, Csl nBr3l , CsPbl3, ZnX2or Csl, wherein X is a halide. In a preferred embodiment, the shell comprises Csl and Znl2. In a preferred embodiment, the shell comprises Csl and lnBr3.

[0111] In an embodiment, the shell comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3. Preferably, the organic ammonium salt is methylammonium or formamidinium.

[0112] In an embodiment, the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, thallium and sodium, M is Pb, Sn, Cu or Zn, and X is a halide. Preferably, the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium and sodium, M is Pb or Zn, and X is a halide. In a preferred embodiment, M is Pb. Where M is Pb, the metal halide is preferably CsPbl3. In a preferred embodiment, M is Zn. Where M is Zn, the metal halide is preferably CsZnl3.

[0113] In an embodiment, the shell comprises a metal halide of the formula CsZnxAsyl3, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1. Preferably, y is in the range of 0.01 to 0.1 .

[0114] In an embodiment, the shell comprises a metal halide of the formula Cs3ZnxAsyl5, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1. Preferably, y is in the range of 0.01 to 0.1 .

[0115] In an embodiment, the shell comprises a metal halide of the formula Cs3BX6, wherein B is indium or antimony and X is a halide.

[0116] In an embodiment, the shell comprises a metal halide of the formula CslnX4, wherein X is a halide. Preferably, the shell comprises CslnBr3l.

[0117] In some embodiments, the shell has a perovskite crystal structure, particularly wherein the shell is of formula AMX3as defined above. In particular, where the shell comprises CsPbl3and / or CsZnl3, the shell may comprise a perovskite crystal structure.

[0118] Whilst lead-based nanocrystals are shown in this application to exhibit good performance, their use can be problematic in some industries due to their toxicity. There is a need in the industry to provide alternative ligand systems that avoid the use of toxic heavy metals such as lead. The metal halides above which do not contain lead solve this problem, providing excellent performance whilst also having reduced toxicity.

[0119] Where the inorganic ligand precursor is lead-free, preferably the inorganic ligand precursor comprises Csl and Znl2. The mixing of these two components is believed to form CsZnl3on the nanocrystal surface during step b), and the mixture is preferably prepared by dissolving Znl2, Csl, and a stabilising agent such as ammonium acetate in a solvent such as dimethyl formamide (DMF). In an alternative embodiment where the inorganic ligand precursor is lead-free, the inorganic ligand precursor may comprise AsX3or TIX, wherein X is a halide.

[0120] A potential method of determining whether any purification steps have taken place is to verify whether the solution comprising ligand-exchanged nanocrystals comprises impurities resulting from the contacting step or any other previous step in the reaction. These impurities may be a salt of the metal element and the organic ligand, ora plurality of free organic ligands remaining in the solution, which are not coordinated to the surface of the crystalline core of a nanocrystal. Therefore, preferably, the mixture formed in step c) (comprising the ligand- exchanged nanocrystals) further comprises at least one of: a) a salt comprising the metal element and the first organic ligand compound; and b) a second plurality of first organic ligand compounds, wherein the second plurality of first organic ligand compounds are not coordinated to the surface of the crystalline core.

[0121] Preferably, the second plurality of first organic ligand compounds are free in solution. Preferably, the second plurality of first organic ligand compounds are present in an amount of at least 2 wt%, preferably at least 3 wt%, preferably at least 4wt%, preferably at least 5 wt%, preferably at least 10 wt%, preferably at least 20 wt%, with respect to the total weight of the first organic ligand compounds in the mixture. The total weight of the organic ligands in the mixture includes the weight of the organic ligands coordinated to the surface of the nanocrystal and the weight of the salts of the organic ligands in solution. Once again, this may be determined by TGA in air at a ramp rate of 10 °C / min.

[0122] In a preferred embodiment, forming the second composition further comprises adding a stabilising agent to the polar solvent, wherein the stabilising agent is selected from the group consisting of sodium acetate, lithium acetate, rubidium acetate, caesium acetate ammonium acetate, butylamine, and mixtures thereof. As such, the exchange composition preferably further comprises a stabilising agent, wherein the stabilising agent is selected from the group consisting of sodium acetate, lithium acetate, rubidium acetate, caesium acetate ammonium acetate, butylamine, trimethylsilyl halide and mixtures thereof. In a particularly preferred embodiment, the stabilising agent is selected from the group consisting of sodium acetate, ammonium acetate, trimethylsilyl halide and mixtures thereof. The stabilising agent, particularly sodium acetate and ammonium acetate, helps to remove the long chain organic ligands from the nanocrystals of the first nanocrystal composition during the contacting step and also helps to provide colloidal stability during this ligand exchange process. Trimethylsilyl halide works in a very similar manner to ammonium acetate and sodium acetate and is a harsher chemical than ammonium acetate, which makes it a particularly preferred stabilising agent for concentrated nanocrystal crude solutions.

[0123] In a preferred embodiment, the non-polar solvent has a relative permittivity at 20 °C of less than 3, preferably less than 2.5. As used herein, the term “relative permittivity” is used to refer to the ratio of the permittivity of a substance to the permittivity of a vacuum; it is a dimensionless number. As defined in Permittivity (Dielectric Constant) of Liquids, by Christian Wohlfarth, the permittivity of a substance (often called the dielectric constant) is the ratio of the electric displacement D to the electric field strength E when an external field is applied to the substance. The relative permittivity is measured with a Bl -870 Dielectric Constant Meter available from Brookhaven Instruments, using the sensitivity range of 1 to 200. The instrument may be calibrated with a liquid of known relative permittivity.

[0124] The non-polar solvent may be any C6-C3o aliphatic or aromatic organic compound. Preferably, the non-polar solvent is selected from the group consisting of a Ce-C3o amine, a Ce-Cso acid, a Ce-C3o phosphine, a Ce-C3o ether, and mixtures thereof. Examples of suitable non-polar solvents include octadecene, oleylamine, oleic acid, octylamine, butylamine, dioctylamine, diphenylphosphine, trioctylphosphine, heptadecane, hexadecane, and mixtures thereof. Preferably, the polar solvent has a relative permittivity at 20 °C of more than 3, more preferably more than 5, more preferably more than 10. Examples of suitable polar solvents include octylamine, ethylene glycol, N,N-dimethylformamide, dimethylsulfoxide, triphenylphosphite, 1 ,2-dichlorobenzene, hexamethylphosphoramide, trioctylphosphine, trioctylphosphine oxide, diphenylether, glycerol, propylene carbonate, dipropylene glycol, tetraethylene glycol, dihydrolevoglucosenone, dimethyl isosorbide, glycofurol, sulfolane, gamma butyrolactone, diformylxylose and mixtures thereof.

[0125] The contacting step may be carried out in a number of different ways, as described below. The methods are all used to produce nanocrystals and are largely known in the prior art. In one embodiment, the nanocrystals are produced via a simple addition method. In an alternative embodiment, the nanocrystals are produced via a heat-up method, optionally in combination with a continuous addition. In an alternative embodiment, the nanocrystals are produced via a hot addition method, optionally in combination with a continuous addition. In an alternative embodiment, the nanocrystals are produced via a continuous addition. In an alternative embodiment, the nanocrystals are produced via a combination of the available methods. The methods generally involve heating the components of the mixture to a crystal growth temperature in the range of 200 °C to 350 °C and maintaining this growth temperature for a predetermined length of time. However, the methods vary by way of the order of addition of the components, the temperatures at which each component is added, the timings and so on.

[0126] Typically, the metal-containing compound and the plurality of organic ligands are mixed in a suitable solvent until the reaction (i.e. the formation of a salt) is substantially complete and a solution of the salt in the solvent is produced. The at least one reagent may then be added to the solution of the salt and allowed to react to form the nanocrystals and / or compositions thereof. The at least one reagent may be added with or without solvent.

[0127] Various aspects of the methods of the invention, such as the particular reagents and / or reaction conditions, may be varied so as to provide nanocrystals of a desired size so as to achieve desired optical properties, such as desired absorption and emission (for example for a particular use of the nanocrystals).

[0128] For example, the reagents used (particularly the metal-containing compound) in the methods may be varied to provide nanocrystals of a desired size so as to achieve desired optical properties, such as desired absorption and emission (for example for a particular use of the nanocrystals).

[0129] For example, the reaction conditions of the methods may be varied to provide nanocrystals of a desired size so as to achieve desired optical properties, such as desired absorption and emission (for example for a particular use of the nanocrystals).

[0130] In other words, the uses and methods of the invention may be used to prepare nanocrystals having size-tuneable optical properties. Examples of the reagents and / or reaction conditions that may be varied are discussed herein.

[0131] The method of the present invention specifies at least one reagent. More than one reagent may be used comprising the same pnictogen element or chalcogen element, or a different pnictogen or chalcogen element.

[0132] Simple addition method

[0133] The simple addition method of preparing the nanocrystals is a preferred embodiment. This method generally comprising contacting the components of the mixture in step a) at a temperature above 50 °C. Preferably, the contacting step comprises contacting the plurality of organic ligands, the metal-containing compound and the at least one reagent at a temperature of at least 50 °C, preferably at least 60 °C, preferably at least 70 °C, preferably at least 80 °C, preferably at least 90 °C.

[0134] In a first step of the simple addition method, the metal-containing compound and the plurality of organic ligands are preferably dissolved in a non-polar solvent. Generally, this is done under an inert atmosphere. This solution is preferably then degassed. Typically, degassing is carried out for a period of time in the range of 0.5 to 1.5 hours at between 50 °C and 150 °C, for example approximately 100 °C.

[0135] In a further step, the temperature of the mixture may then be increased to above 200 °C, which usually forms a salt of the metal element and the organic ligand. The temperature may be reduced to between 50 °C and 150 °C, preferably approximately 100 °C, for the addition of the at least one reagent, which is generally added by injection. The reagent may preferably be added via a series of injections. The reaction is cooled down after the final injection, preferably about 3 to 15 minutes after the final injection.

[0136] In the simple addition method, preferably the contacting step comprises:

[0137] A) admixing the metal-containing compound and the plurality of first organic ligand compounds in the non-polar solvent to form a precursor mixture;

[0138] B) degassing the precursor mixture;

[0139] C) heating the precursor mixture to above 150 °C; and

[0140] D) cooling the mixture to below 150 °C and adding the at least one reagent to the precursor mixture.

[0141] Preferably, the contacting step comprises contacting the plurality of first organic ligand compounds, the metal-containing compound and the at least one reagent at a temperature of at least 50 °C, preferably at least 60 °C, preferably at least 70 °C, preferably at least 80 °C, preferably at least 90 °C.

[0142] The exchange ligand (e.g. an inorganic ligand dissolved in a polar solvent) is then added to the reaction mixture. The reaction mixture is then generally left for a number of hours (for example, 6 to 18 hours) under stirring in an inert atmosphere. In other words, in the simple addition method, the method preferably further comprises stirring the mixture for at least two hours, preferably between 2 hours and 18 hours, preferably between 4 hours and 18 hours. This is usually part of step c) as explained below. The temperature of the mixture is preferably in the range of 0 °C to 150 °C, preferably in the range of 20 °C to 50 °C. The resultant nanocrystals comprise exchange ligands (e.g. an inorganic ligand shell) coordinated to the surface of the crystalline core of the nanocrystals. These nanocrystals are typically then washed and dissolved in a further polar solvent for use an ink.

[0143] Hot addition method

[0144] As described in the review by Tamang et al. (Chem. Rev. 2016, 116, 10731-10819), in the case of the hot addition method, the separation of nucleation and growth can be achieved by the rapid injection of the reagents into the hot solvent, which raises the concentration in the reaction flask above the nucleation threshold. The hot injection leads to a nucleation burst, which is quickly quenched by two factors: (i) the fast cooling of the reaction mixture, enhanced by the fact that the solution to be injected is at room temperature; (ii) the decreased supersaturation due to precursor / monomer consumption during nucleation.

[0145] Where the hot addition method is used, the contacting step comprises:

[0146] A) heating a precursor mixture to a first temperature, wherein the precursor mixture comprises the metal-containing compound and the plurality of first organic ligand compounds,

[0147] B) adding the at least one reagent to the precursor mixture,

[0148] C) maintaining the precursor mixture at the first temperature for a first predetermined length of time to form a plurality of seed particles.

[0149] The precursor mixture may optionally be degassed prior to heating to a first temperature. Typically, degassing is carried out for a period of time in the range of 0.5 to 1 .5 hours at between 50 °C and 150 °C, for example approximately 100 °C.

[0150] Preferably, the first temperature is in the range of 275 °C to 350 °C, preferably in the range of 275 °C to 325 °C, preferably in the range of 290 °C to 310 °C.

[0151] Preferably, prior to addition to the precursor mixture in step B), the at least one reagent has a temperature in the range of 5 °C to 50 °C, preferably in the range of 10 °C to 35 °C, preferably in the range of 15 °C to 25 °C, preferably about room temperature.

[0152] Preferably, the first predetermined length of time is in the range of 10 minutes to 60 minutes, preferably in the range of 15 minutes to 50 minutes, preferably in the range of 20 minutes to 40 minutes, preferably in the range of 25 minutes to 35 minutes.

[0153] Preferably, the contacting step further comprises:

[0154] D) reducing the temperature of the precursor mixture to a second temperature,

[0155] E) continuously adding the at least one reagent to the precursor mixture for a second predetermined length of time to produce the mixture of step a).

[0156] Preferably, the second temperature is lower than the first temperature and is in the range of 200 °C to 275 °C, preferably in the range of 225 °C to 275 °C, preferably in the range of 240 °C to 260 °C.

[0157] Preferably, the second predetermined length of time is in the range of 60 minutes to 180 minutes, preferably in the range of 90 minutes to 150 minutes, preferably in the range of 100 minutes to 140 minutes, preferably in the range of 110 minutes to 130 minutes.

[0158] Continuous addition method

[0159] Where the contacting step uses a continuous addition method, the contacting step comprises:

[0160] A) heating the precursor mixture to a third temperature, wherein the precursor mixture comprises the metal-containing compound and the plurality of first organic ligand compounds,

[0161] B) continuously adding the at least one reagent to the precursor mixture for a third predetermined length of time to produce the mixture of step a). Preferably, the third temperature is in the range of 200 °C to 275 °C, preferably in the range of 225 °C to 275 °C, preferably in the range of 240 °C to 260 °C.

[0162] Preferably, the third predetermined length of time is in the range of 60 minutes to 180 minutes, preferably in the range of 90 minutes to 150 minutes, preferably in the range of 100 minutes to 140 minutes, preferably in the range of 110 minutes to 130 minutes.

[0163] Heat-up method

[0164] As outlined in Tamang et al., the heat up method relies on attaining the degree of supersaturation necessary for homogeneous nucleation via the in situ formation of reactive species upon supply of thermal energy.

[0165] In an embodiment, the contacting step comprises:

[0166] A) adding the at least one reagent to a precursor mixture, wherein the precursor mixture comprises the metal-containing compound and the plurality of first organic ligand compounds, and

[0167] B) heating the precursor mixture to a fourth temperature.

[0168] The precursor mixture may optionally be degassed. Typically, degassing is carried out for a period of time in the range of 0.5 to 1.5 hours at between 50 °C and 150 °C, for example approximately 100 °C.

[0169] Preferably, the fourth temperature is in the range of 200 °C to 275 °C, preferably in the range of 225 °C to 275 °C, preferably in the range of 240 °C to 260 °C.

[0170] Preferably, the contacting step further comprises:

[0171] C) maintaining the precursor mixture at the fourth temperature for a fourth predetermined length of time.

[0172] Preferably, the fourth predetermined length of time is in the range of 60 minutes to 180 minutes, preferably in the range of 90 minutes to 150 minutes, preferably in the range of 100 minutes to 140 minutes, preferably in the range of 110 minutes to 130 minutes. Heat-up + continuous addition method

[0173] Where the heat-up method is used in conjunction with the continuous addition method, the contacting step further comprises:

[0174] D) continuously adding the at least one reagent to the precursor mixture for a fifth predetermined length of time to produce the mixture of step a).

[0175] Preferably, the fifth predetermined length of time is in the range of 60 minutes to 180 minutes, preferably in the range of 90 minutes to 150 minutes, preferably in the range of 100 minutes to 140 minutes, preferably in the range of 110 minutes to 130 minutes.

[0176] According to a second aspect, the present invention provides a nanocrystal composition obtainable by the method of the first aspect.

[0177] The method of the present invention is especially useful for the preparation of nanocrystal ink compositions, in particular quantum dot ink compositions. These ink compositions have a wide range of uses in optoelectronic devices. The ink composition is generally formed by the dissolution of the nanocrystals comprising the inorganic ligands in a polar solvent. Before this dissolution, the nanocrystals are generally washed to remove the impurities from the solution comprising the ligand-exchanged nanocrystal compositions.

[0178] When the method of the present invention is used to make nanocrystal inks, the method further comprises: d) washing the nanocrystal composition with a solution comprising at least one of acetone, methyl acetate, ethyl acetate, and acetonitrile, and e) dissolving the nanocrystal composition in a second polar solvent to form a nanocrystal ink composition. Preferably, the second polar solvent is selected from the group consisting of 2,6-difluoropyridine, gamma-butyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof.

[0179] By using the in-situ ligand exchange process, the quality of ink can be improved. The final QD inks via in situ method using conducting inorganic ligands show excellent performance in devices. According to a third aspect, the present invention provides an ink composition comprising the nanocrystal composition of the second aspect. The ink composition will generally comprise the nanocrystals of the nanocrystal composition and the polar solvent used to dissolve the nanocrystals.

[0180] Novel metal halide ligands

[0181] According to a fourth aspect, the present invention provides a nanocrystal comprising a crystalline core and a lead-free inorganic shell at least partially surrounding the crystalline core. The present invention provides a nanocrystal composition comprising a plurality of nanocrystals according to the fourth aspect. Without wishing to be bound by theory, it is believed that the nanocrystals of the fourth aspect comprise a core-shell structure, wherein the nanocrystals comprise a crystalline core comprising (i) the metal element, and (ii) the pnictogen element or the chalcogen element, and a lead-free inorganic shell at least partially surrounding the crystalline core. Preferably, the shell surrounds the crystalline core.

[0182] As used herein, the term “lead-free” is used to refer to a component that is free of lead. For example, a lead-free inorganic ligand precursor refers to an inorganic ligand precursor that does not comprise any lead-containing compounds.

[0183] As described above, in the fourth aspect, the crystalline core typically comprises (i) a metal element and (ii) a pnictogen element or a chalcogen element. The preferences for the metal element, the pnictogen element and the chalcogen element are the same for each aspect of the present invention.

[0184] As discussed above, there is a need in the industry to provide nanocrystals, and in particular nanocrystal ligand systems, that avoid the use of toxic heavy metals such as lead. The inventors of the present invention have found that lead-free inorganic compounds may be used as ligands on the surface of all types of nanocrystals to produce nanocrystals with enhanced performance. For example, the nanocrystals of the present invention are stable under air for several months and have excellent P / V ratios. Caesium-containing metal halide ligands have shown particularly impressive performance thus far.

[0185] The nanocrystals comprising a lead-free inorganic shell may be prepared by the method of the present invention or by any other suitable method. For example, the method of preparing the nanocrystals of the fourth aspect may include a step for purifying the pristine nanocrystals. The abovementioned simple addition method, heat up method, hot addition method, and continuous addition methods are all suitable for preparing the nanocrystals comprising lead-free perovskite ligands, as would be understood in the art.

[0186] Preferably, the shell comprises a metal halide ligand. In a preferred embodiment, the shell comprises a metal halide selected from the group consisting of CsX, ASX3, TIX, CsZnl3, Csl nBr3l , ZnX2, Csl and mixtures thereof, wherein X is a halide.

[0187] In particular, the inventors have found that arsenic halides (AsX3) and thallium halides (TIX) may be used as inorganic ligands for nanocrystals. These inorganic ligands have also been found to form a shell layer surrounding the crystalline core. In a preferred embodiment, the shell comprises AsX3or TIX. In an embodiment, the shell comprises AsX3. In an embodiment, the shell comprises TIX.

[0188] Without wishing to be bound by theory, due to the high mobility of Asl3, nanocrystals comprising AsX3ligands advantageously offer very high charge carrier extraction and therefore high external quantum efficiency (EQE) in devices. Preferably, X is Cl, Br or I, more preferably X is I.

[0189] Without wishing to be bound by theory, it is believed that the use of thallium halide ligands leads to lower dark current in photodevices. Preferably, X is Cl, Br or I, more preferably I .

[0190] In a preferred embodiment, the shell comprises Csl and Znl2. In a preferred embodiment, the shell comprises Csl and lnBr3.

[0191] In a preferred embodiment, the shell comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3. Preferably, the organic ammonium salt is methylammonium or formamidinium.

[0192] In a preferred embodiment, the shell comprises a metal halide of the formula AMX3, wherein A is a Group IA metal, thallium or an organic ammonium salt, M is a metal having +2 oxidation state, and X is a halide. In a preferred embodiment, the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, thallium and sodium, M is Sn, Cu or Zn, and X is a halide. Preferably, the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium and sodium, M is Zn, and X is a halide. Preferably, the metal halide is CsZnl3.

[0193] In a preferred embodiment, the shell comprises a metal halide of the formula CsZnxAsyl3, wherein the sum of x and y is equal to 1 , wherein y is in the range of O to 0.1. Preferably, y is in the range of 0.01 to 0.1.

[0194] In a preferred embodiment, the shell comprises a metal halide of the formula Cs3ZnxAsyl5, wherein the sum of x and y is equal to 1 , wherein y is in the range of O to 0.1. Preferably, y is in the range of 0.01 to 0.1.

[0195] In a preferred embodiment, the shell comprises a metal halide of the formula CS3BX6, wherein B is indium or antimony and X is a halide. In a preferred embodiment, the shell comprises a metal halide of the formula Cs3lnX6, CslnX4, or Cs2lnX5, wherein X is a halide.

[0196] In a preferred embodiment, the shell comprises CslnBr3l.

[0197] As described above, the structure of the shell may be amorphous or crystalline. Preferably, the shell is crystalline and / or forms a lattice layer surrounding the crystalline core. As used herein, the term “lattice” is used to refer to an ordered array of points describing the arrangement of particles that form a crystal.

[0198] Preferably, the metal halide has a perovskite crystal structure. The shell is typically disposed on the surface of the crystalline core and coordinated to the crystalline core of the nanocrystal. Preferably, the shell surrounds the crystalline core. In a preferred embodiment, the crystalline core has a radius and the shell has a thickness, and the thickness of the shell is smaller than the radius of the crystalline core.

[0199] In a preferred embodiment, the nanocrystal has at least one dimension measuring less than 50 nm, or less than 40 nm, or less than 30 nm, or less than 20 nm, or less than 10 nm.

[0200] In a preferred embodiment, the nanocrystal is a semiconductor nanocrystal. In a preferred embodiment, the nanocrystal is a quantum dot.

[0201] According to a fifth aspect, the present invention provides a nanocrystal composition comprising a plurality of nanocrystals according to the fourth aspect.

[0202] In a preferred embodiment, the nanocrystals of the nanocrystal composition have a mean particle diameter in the range of 2 nm to 20 nm, or in the range of 2 nm to 17 nm, or in the range of 2 nm to 15 nm, or in the range of 2 nm to 12 nm, or in the range of 2 nm to 10 nm, or in the range of 5 nm to 20 nm, or in the range of 5 nm to 17 nm, or in the range of 5 nm to 15 nm, or in the range of 5 nm to 12 nm, or in the range of 5 nm to 10 nm.

[0203] In a preferred embodiment, the nanocrystals have a relative size dispersion of less than 25%, preferably less than 22%, preferably less than 20%, preferably less than 17%, preferably less than 15%, preferably less than 12%, preferably less than 10%.

[0204] In a preferred embodiment, the nanocrystal composition exhibits absorption full width at half maximum (FWHM) values of less than 250 nm, preferably less than 225 nm, preferably less than 200 nm, preferably less than 175 nm. As used herein, the term “full width at half maximum” (or “FWHM”) refers to the width of an absorption peak at half of its maximum amplitude. FWHM values are preferably measured by UV-vis spectroscopy. In particular, the FWHM value for an absorption peak is determined by measuring the distance between two points on the y-axis (for absorbance) where the intensity of the curve is half of its peak value.

[0205] In a preferred embodiment, the nanocrystal composition exhibits emission full width at half maximum (FWHM) values of less than 250 nm, preferably less than 225 nm, preferably less than 200 nm, preferably less than 175 nm.

[0206] According to a sixth aspect, the present invention provides an ink composition comprising the nanocrystal composition of the fifth aspect and a polar solvent.

[0207] Preferably, the polar solvent is selected from the group consisting of 2,6- difluoropyridine, gamma-butyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof.

[0208] The present invention also provides a device selected from the group consisting of IR sensor, photodetector, sensor, solar cell, a bio-imaging or bio-sensing composition, photovoltaic system, display, battery, laser, photocatalyst, spectrometer, injectable composition, field-effect transistor, light-emitting diode, photonic or optical switching device or metamaterial, fiber amplifier, optical gain media, optical fiber, infrared LEDs, lasers, and electroluminescent device, comprising the nanocrystal composition of the fourth aspect.

[0209] In a preferred embodiment, the IR sensor or photodetector are modified for application as 3D cameras and 3D Time of flight cameras in mobile and consumer, automotive, medical, industrial, defence or aerospace applications. In a preferred embodiment, the bio-imaging or bio-sensing compositions are modified for use as bio-labels or bio-tags in in vitro or ex vivo applications. In a preferred embodiment, the infrared LEDs and electroluminescent devices are modified for use in telecommunication devices, night vision devices, solar energy conversion, thermoelectric or energy generation applications.

[0210] According to a seventh aspect, the present invention provides the use of lead-free metal halide ligands in the preparation of nanocrystals. In other words, the seventh aspect provides a method for preparing nanocrystals comprising contacting nanocrystals with lead-free metal halide ligands. The nanocrystals may comprise any kind of native ligands, for example organic ligands or inorganic ligands. These native ligands are coordinated to a surface of the crystalline core of the nanocrystals.

[0211] According to an eight aspect, the present invention provides a method for nanocrystal ligand exchange, the method comprising a contacting step comprising contacting:

[0212] (i) a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligand compounds coordinated to a surface of the crystalline core, and

[0213] (ii) a second composition comprising a lead-free inorganic ligand precursor, wherein the inorganic ligand precursor comprises one or more metal halide compounds.

[0214] Like the method of the first aspect, this method generally involves a reaction between the metal halide compounds in the inorganic ligand precursor and the nanocrystal, resulting in the formation of a shell layer disposed on the surface of the crystalline core. The shell layer is formed from the metal halide compounds in the inorganic ligand precursor. More information about this reaction is given above in relation to the first aspect.

[0215] The method of the eighth aspect is typically carried out under inert conditions, such as those described above in relation to the first aspect.

[0216] In a preferred embodiment, the native ligand compounds do not comprise a metal halide compound. In other words, the native ligand compounds and the inorganic ligand precursor are different. The native ligand compounds and the inorganic ligand precursor preferably have a different chemical composition and / or a different structure.

[0217] In a preferred embodiment, the native ligand compounds are organic ligand compounds. In this embodiment, the first nanocrystal composition preferably further comprises a non-polar solvent. The organic ligand compounds are as described above in relation to the first organic ligand compounds of the first aspect. The preferred relative permittivities of the non-polar solvents are the same for each aspect of the present invention. The preferred non-polar solvents are the same for each aspect of the present invention.

[0218] In an alternative embodiment, the native ligand compounds are inorganic ligand compounds. In this embodiment, the first nanocrystal composition preferably further comprises a polar solvent. Preferably, the polar solvent has a relative permittivity at 20 °C of more than 3, more preferably more than 5, more preferably more than 10. Preferably, the polar solvent is selected from the group consisting of octylamine, ethylene glycol, N,N-dimethylformamide, dimethylsulfoxide, triphenylphosphite, 1 ,2-dichlorobenzene, hexamethylphosphoramide, trioctylphosphine, trioctylphosphine oxide, diphenylether, glycerol, propylene carbonate, dipropylene glycol, tetraethylene glycol, dihydrolevoglucosenone, dimethyl isosorbide, glycofurol, sulfolane, gamma butyrolactone, diformylxylose and mixtures thereof.

[0219] As mentioned above, the inorganic compounds in the inorganic ligand precursor are preferably metal halides. The inorganic compounds in the inorganic ligand precursor generally react to form a shell layer at partially surrounding the crystalline core of the nanocrystal. This shell may be amorphous or crystalline. Preferably, the shell is crystalline. Where the shell is crystalline, the shell may have a perovskite crystal structure. The core-shell structure of the resultant nanocrystals in the nanocrystal composition is described above.

[0220] In a preferred embodiment, the inorganic ligand precursor comprises CsX, AsX3, TIX, CsZnl3, CslnBr3l , ZnX2or Csl, wherein X is a halide. The halide may all be the same halide, for example X3may represent l3, or may be a mixed halide, for example Br2l .

[0221] In a preferred embodiment, the inorganic ligand precursor comprises Csl and Znl2.

[0222] In a preferred embodiment, the inorganic ligand precursor comprises Csl and lnBr3. In a preferred embodiment, the inorganic ligand precursor comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3. Examples of suitable organic ammonium salts include methylammonium and formamidinium.

[0223] In a preferred embodiment, the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, thallium and sodium, M is Sn, Cu or Zn, and X is a halide. In a preferred embodiment, the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium and sodium, M is Zn, and X is a halide. In a preferred embodiment, the metal halide is CsZnl3. The CsZnl3precursor is generally formed by the addition of Csl and Znl2into a nonsolvent (e.g. DMF) to form the second composition, wherein the second composition preferably additionally comprises a stabilising agent such as ammonium acetate or other suitable compounds.

[0224] In a preferred embodiment, the inorganic ligand precursor comprises CslnBr3l.

[0225] In a preferred embodiment, the metal halide has a perovskite crystal structure.

[0226] As explained above in relation to the first aspect, once the second composition is added, a reaction generally occurs between the inorganic compounds in the inorganic ligand precursor and the nanocrystals capped with the organic ligand compounds. Specifically, a reaction will take place to remove the organic ligand compounds from the surface of the nanocrystal. The organic ligand compounds are replaced by the ligand formed from the inorganic compounds of the inorganic ligand precursor.

[0227] As such, the contacting step further comprises displacing at least a portion of the native ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor. Preferably, the contacting step comprises stirring the compositions for at least two hours, preferably between 2 hours and 18 hours, preferably between 4 hours and 18 hours. Preferably, the contacting step comprises displacing at least 10%, preferably at least 20%, more preferably at least 30%, more preferably at least 40%, more preferably at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90%, of the native ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor, as determined by thermogravimetric analysis (TGA) in air at a ramp rate of 10 °C / min.

[0228] As in all aspects of the present invention, the shell may be amorphous or crystalline. Preferably, the shell is crystalline.

[0229] In a preferred embodiment, the second composition further comprises a stabilising agent, wherein the stabilising agent is selected from the group consisting of sodium acetate, lithium acetate, rubidium acetate, caesium acetate ammonium acetate, butylamine, trimethylsilyl halide and mixtures thereof. Preferably, the stabilising agent is selected from the group consisting of sodium acetate, ammonium acetate, trimethylsilyl halide and mixtures thereof. The stabilising agent is as described above in relation to the first aspect.

[0230] EXAMPLES

[0231] Examples are described hereunder illustrating the methods according to the present disclosure.

[0232] Whereas particular examples of this invention have been described below for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

[0233] Unless other indicated, all parts and all percentages in the following examples, as well as throughout the specification, are parts by weight or percentages by weight respectively. Absorption spectra of colloidal quantum dots or quantum dots films were obtained on a JASCO V-770 UV-visible / NIR spectrometer which can provide measurements in the 400 to 3200nm wavelength.

[0234] XRD data were collected on a Panalytical X’Pert PRO MPD diffractometer using Cu Ka1 X-radiation (I = 1 .5406 A) at room temperature over a range of 10 < 2q < 90 °. In each case a few drops of the dispersed sample were placed on a glass microscope slide and allowed to evaporate. Data were analysed using Rigaku SmartLab Studio II software and the search and match carried out using the Crystallographic Open Database.

[0235] TEM images and high-resolution transmission electron microscope (HRTEM) images were obtained with an FEI Talos F200X microscope equipped with an X- FEG electron source. The experiment was performed using an acceleration voltage of 200kV and a beam current of approximately 5 nA. Images were recorded with an FEI CETA 4k x 4k CMOS camera. In each case a few drops of the dispersed quantum dots in solvent were placed on a carbon coated copper grid and allow to evaporate. Samples were used as such or treated with acetone then methanol to clean unwanted organic materials before imaging.

[0236] Thermogravimetric analysis (TGA) measurements were conducted using a Hitachi STA200-RV TGA. Samples were prepared by first being dried under vacuum and then placed in an alumina crucible. Measurements were conducted under nitrogen gas flow with 10 °C / min ramp rate from 25 °C to 900 °C.

[0237] For ICP-TOF MS measurements, all imaging data was acquired using ImageBIO 266nm laser ablation system fitted with the TwoVolume 3 sample chamber and dual concentric injector (ESL, Bozeman), coupled to ICP-ToF-MS 2R (TOFWERK, Thun). A summary of the instrumental parameters may be found in the table below. The aerosol particles were transferred to the ICP via 0.9 L min-1 He gas flow and the data acquisition carried out in standard mode with no added reaction gases.

[0238] For the sample the 10nm quantum dot layer was chosen for imaging due to the high Pb content of the samples. Three regions across the samples were measured ( x= 2.1 mm, y = 0.2 mm, spacing between mapped regions = 1 mm) with each region having a corresponding mapped area. For all images the data baseline subtraction and mass calibration were performed using TofWare (version 3.2.0).

[0239] Example 1 - In-situ preparation of PbS nanocrystals using CsZnl3

[0240] To underpin the advantage of in-situ ligand exchange process, PbS nanocrystals were synthesised in a large flask. The resultant mixture was split into two halves to perform in-situ ligand exchange (Example 1) and ex-situ purification and ligand exchange process (Comparative Example 2). Figure 2 shows a simplified diagram of the ligand exchange reaction.

[0241] All processing and ligand exchange procedures were conducted in a N2filled glovebox with anhydrous solvents.

[0242] Part 1 : 1 .765 g of Pb3O4 with 14.6 mL of oleic acid, and 200 mL of 1 -octadecene were taken in a 3 neck round bottom flask under inert atmosphere. The reaction mixture was degassed at 100 °C for 1 h before increasing the temperature to 220 °C to form lead-oleate. Once a clear solution is formed, the reaction temperature is reduced to 100 °C to for the injection of Bis(trimethylsilyl)sulfide solution. In order to prepare Bis(trimethylsilyl)sulfide solution, firstly 1 mL of Bis(trimethylsilyl)sulfide is dissolved in 50 mL ODE, which is named solution A. Then, take 17.8 mL of solution A and dissolve in 32.2 mL of ODE to make solution B. When reaction temperature stabilises at 100 °C, 11 .8 mL of solution A is injected to the reaction flask followed by 12.8 ml_ of solution B after 8 minutes. 5 minutes after the injection of solution B, another 12.8 mL of solution B is injected into the reaction media and finally 5 minutes later another 6.4 mL of solution B is injected to achieve the desired wavelength. The reaction is cooled down 6 minutes after the final injection.

[0243] Part 2: Once the reaction is completed half of the colloidal solution is taken out to process ex-situ experiment (Comparative Example 2). The remaining half (this Example) was treated with CsZnl3inorganic ligand precursor which was prepared by dissolving 3.1 g of Znl2, 2.5 g of Csl, 305 mg of ammonium acetate in 100 mL of dimethyl formamide (DMF). The solution was added to the crude reaction mixture to conduct the phase transfer from long chain unstable organic ligand to inorganic stable ligand in the same flask. Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and further using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink.

[0244] It should be noted that the same reaction can be conducted using heat up method as well as continuous injection, in which Bis(trimethylsilyl)sulfide-ODE solution is added before the heating up or continuously added during the synthesis when the temperature is reduced to 100 °C after the formation of lead oleate. The rest of the ligand exchange process (i.e. washing and ink formation) remains the same once the PbS QDs are synthesised by any of the above methods. This applies to Comparative Example 2 as well. As can be seen by the results of these examples, no specific method of the contacting step is required in order to form the nanocrystals, and contacting the components is generally sufficient to produce the desired nanocrystals.

[0245] Figure 3 depicts the absorption properties of the resulting QDs, also shown in Table 1 below. Pristine nanocrystals show an absorption wavelength of 1450 nm with FWHM of 92 nm and PA / of 6.4, and after in-situ ligand exchange the resulting QDs show extremely good solubility in 2,6 difluoro pyridine in comparison to ex- situ ligand exchange QDs. Moreover, the absorption wavelength and FWHM of absorption peak stays the same i.e. 1448 nm absorption wavelength and 91 nm FWHM.

[0246] Comparative Example 2 - Ex-situ preparation of PbS nanocrystals using CsZnl3

[0247] Part 1 : Same as Example 1.

[0248] Part 2: Once the reaction is completed half of the colloidal solution is taken out to process ex-situ experiment. The crude solution was purified using crude solution:acetone:isopropyl alcohol (IPA) 1 :1 :1 twice by centrifuging at 6,000 RPM. The precipitate was dissolved in octane to form the purified pristine nanocrystal solution. This process was conducted inside a nitrogen filled glove-box. For ex- situ ligand exchange, to the 100 ml solution of 7mg / mL QDs-octane solution was added CsZnl3inorganic ligand precursor which was prepared by dissolving 3.1 g of Znl2, 2.5 g of Csl, 305 mg of ammonium acetate in 100 mL of dimethyl formamide (DMF). Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and further using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink.

[0249] On viewing Figure 3, it can be seen that the ex-situ ligand exchange QDs show blue shift of 30 nm with 10 nm of increment in FWHM indicating the poorer quality of QDs in comparison to in-situ QDs. In comparison, there is no blue shift or broadening of the FWHM for the nanocrystals formed by the in situ method, and these nanocrystals also have a higher PA / ratio than the ex situ nanocrystals of the prior art. This proves that the in-situ ligand exchange process offers better quality of QDs. Table 1 : Absorption spectra data for the nanocrystals prepared according to Example 1 and Comparative Example 2.

[0250] Example 3 - In-situ preparation of PbS nanocrystals using CsPbl3

[0251] Part 1 : 0.88 g of Pb3O4 with 7.3mL of oleic acid, and 100 mL of 1 -octadecene were taken in a 3 neck round bottom flask under inert atmosphere. The reaction mixture was degassed at 100 °C for 1 h before increasing the temperature to 220 °C to form lead-oleate. Once a clear solution is formed, the reaction temperature is reduced to 100 °C to for the injection of Bis(trimethylsilyl)sulfide solution. In order to prepare Bis(trimethylsilyl)sulfide solution, firstly 0.5 mL of Bis(trimethylsilyl)sulfide is dissolved in 25 mL ODE, which is named solution A. Then, take 8.9 mL of solution A and dissolve in 16.1 mL of ODE to make solution B. When reaction temperature stabilises at 100 °C, 5.9 mL of solution A is injected to the reaction flask followed by 6.4 mL of solution B after 8 minutes. 5 minutes after the injection of solution B, another 6.4 mL of solution B is injected into the reaction media and finally 5 minutes later another 3.2 mL of solution B is injected to achieve the desired wavelength. The reaction is cooled down 6 minutes after the final injection.

[0252] Part 2: Once the reaction is completed, the reaction mixture was treated with CsPbl3inorganic ligand precursor which was prepared by dissolving 4.57 g of Pbl2, 2.5 g of Csl, 305 mg of ammonium acetate in 100 mL of dimethyl formamide (DMF). The solution was added in crude reaction mixture to conduct the phase transfer from long chain unstable organic ligand to inorganic stable ligand in the same flask. Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and purified using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink.

[0253] It should be noted that the same reaction can be conducted using heat up method as well as continuous injection, in which Bis(trimethylsilyl)sulfide-ODE solution is added before the heating up or continuously added during the synthesis when the temperature is reduced to 100 °C after the formation of lead oleate. The rest of the ligand exchange process (i.e. washing and ink formation) remains the same once the PbS QDs are synthesised by any of the above methods.

[0254] Once the clear phase separation occurs, these QDs are then washed using acetone and finally dissolved in polar solvent such as 2,6, difluoropyridine, propylene carbonate, and / or DMF. Figure 4 shows a photograph of phase separated QDs in a centrifuge tube and a photograph of the final colloidal nanocrystal ink. As evident from the photograph of the colloidal ink, the QDs shows excellent colloidal stability in polar solvent after ligand exchange process.

[0255] Nanocrystals were produced by the method of Example 3 in order to produce an ink composition with an absorption wavelength of approximately 1400 nm (Sample 3A) and an ink composition with an absorption wavelength of approximately 1200 nm (Sample 3B). This shows the versatility of the method of the present invention. Further nanocrystal ink compositions were produced according to Example 3 which exhibited absorption peaks at 1550 nm, 1650 nm, 2000 nm and 2300 nm as shown in Figure 19. The different absorption peaks were obtained by varying the timings in Part 1 . Again, this further demonstrates the versatility of the method of the present invention.

[0256] The optical properties of resulting ink are impressive. The ink composition maintains its excitonic properties with high peak to valley (P / V) which indicates the high quality of the ligand exchanged colloidal QDs ink. Table 2 below shows the optical aerobic stability of Sample 3B of these quantum dots.

[0257] As used herein, the term “peak to valley ratio” (or “P / V ratio”) refers to the ratio between the maximum absorption value of an absorption peak to the lowest absorption value in the range of 150 nm before the maximum value of the absorption peak. The P / V ratio is preferably measured by UV-vis spectroscopy.

[0258] Comparative Example 4 - Ex-situ preparation of PbS nanocrystals using CsPbl3

[0259] Part 1 : Same as Example 3.

[0260] Part 2: The crude solution was purified using crude solution:acetone:IPA: 1 :1 :1 twice by centrifuging at 6,000 RPM. The precipitate was dissolved in octane to form purified QDs solution. This process was conducted inside a nitrogen filled glove-box. For ex-situ ligand exchange, 100 ml solution of 7mg / mL QDs-octane solution was added in CsPbl3inorganic ligand precursor which was prepared by dissolving 4.5 g of Pbl2, 2.5 g of Csl, 305 mg of ammonium acetate in 100 mL of dimethyl formamide (DMF). Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and purified using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink.

[0261] Nanocrystals were produced by the method of Comparative Example 4 in order to produce an ink composition with an absorption wavelength of approximately 1400 nm (Comp. Sample 4A) and an ink composition with an absorption wavelength of approximately 1520 nm (Comp. Sample 4B).

[0262] As evident in Table 2 and Figure 7, in-situ ligand exchange QDs offer higher PA / in comparison to post-synthetic (ex-situ) ligand exchange having a similar absorption wavelength, which indicates the improved surface passivation and solubility of these in situ QDs in solvent.

[0263] Table 2: Optical properties of in-situ ligand exchanged (CsPbl3) QDs (Sample 3A) vs post-synthetic ligand exchanged QDs (ex-situ Comp. Sample 4A).

[0264] The aerobic stability of the nanocrystal ink compositions Sample 3B and Comp. Sample 4B was also analysed. As discussed earlier, in-situ ligand exchange works more efficiently than the ex-situ method, and prevents the surface oxidation of pristine QDs and offers higher quality QDs than the ex-situ method. Figures 5 (in situ) and 6 (ex situ) show the absorption spectra of the QDs and how this changes over time. Figures 5 and 6 show that the QDs produced according to the above method are stable. As evident from Table 3 and Figures 5 and 6, the resulting ink formed by the method of the present invention shows aerobic stability up to 97 days, which is similar to the ink formed by the ex situ method (post-ligand exchanged QDs ink) as the CsPbl3ligand system is very stable. Therefore, it can be concluded that the in-situ ligand exchange process produces nanocrystal ink compositions that are at least as aerobically stable as the nanocrystal inks of the prior art, while offering some additional advantages which are discussed in further sections.

[0265] Table 3: Absorption spectroscopy parameters of in-situ vs post-synthetic ligand exchanged PbS QDs

[0266] Generally, the longer the absorption wavelength of the nanocrystals, the higher the PA / ratio. Therefore, as there is a large difference in the absorption wavelengths between Sample 3B and Comp. Sample 4B (>200 nm), the P / V ratios of these samples are not directly comparable. Example 5 - In-situ preparation of PbS nanocrystals using CsZnl3

[0267] Part 1 : Same as Example 1

[0268] Part 2: Same as Example 1

[0269] Asimilartype of in-situ ligand exchange was performed using a Csl and Znl2ligand system, which showed a successful phase transfer with resulting high quality quantum dots ink. Csl and Znl2results in successful phase transfer to polar inorganic phase. The resulting QDs showed good colloidal stability in 2,6- difluoropyridine.

[0270] Figure 8 shows the absorption spectrum of Csl-Znl2based in-situ ligand exchanged PbS QDs, which shows promising optical absorption properties. The similar ligand exchange approach can also be implemented on AsX3(X= Cl, Br, I), TIX (X= Cl, Br, I). Both of these materials offer their own advantages, for instance, due to high mobility of Asl3, its use should offer nanocrystals with very high charge carrier extraction and therefore high external quantum efficiency in devices, whereas the use of thallium halide-based ligands should lead to nanocrystals suitable for lower dark current.

[0271] Example 6 - Ligand exchange of synthesised PbS nanocrystals with Asl3

[0272] Part 1 : Same as Example 1

[0273] Part 2: Same as Comparative Example 2, with the exception that the ligand exchange DMF solution contained 0.070 M Asl3with 21 mg of ammonium acetate in 7 mL DMF and the amount of nanocrystals were 7 mL of 7 mg / mL.

[0274] Whilst this experiment involved ex-situ ligand exchange with the Asl3ligand, this example demonstrates that Asl3is an acceptable inorganic ligand. Asl3may simply be applied using in-situ ligand exchange system as well. Figure 9 shows the absorption spectrum of Asl3ligand based PbS QDs in 2,6, difluoro pyridine. Example 7 - Monitoring the Csl and Znl2ligand system

[0275] In order to verify the role of Csl and Znl2, a controlled ligand exchange experiment using Csl and Znl2was performed to see how the ligands perform individually and when combined.

[0276] Part 1 : Same as Example 1

[0277] Part 2: Same as Comparative Example 2, except that Csl, Znl2and CsZnl3were each used in separate ligand exchange reactions. For Csl, the same molar amount of Csl solution was made as in Comparative Example 2, and for Znl2the same amount of Znl2was used in exactly same amount of DMF.

[0278] The inventors found that Csl as an inorganic ligand was able to cause the phase separation from nonpolar phase of pristine QDs to polar phase of pristine QDs, however, the resulting QDs ink did precipitate and could not form a stable colloidal solution (Figure 10). When Znl2was used the stable quantum dot ink was formed, but the optical properties were not as good as the one ligand exchanged using Csl and Znl2, which is likely to be due to the better surface passivation. This result suggests that Csl in combination with Znl2forms a crystalline ligand structure around the nanocrystals, with superior optical properties.

[0279] Figure 10 is a photograph of ligand exchange QDs ink using Csl (left) and Znl2(right). As evident, the QDs ink is found to be unstable using Csl ligand but in case of Znl2the final ink was stable but for a diluted solution in comparison to Csl and Znl2together.

[0280] Table 4 shows the optical properties of final ink formed using three different types of ligands. As evident, Csl-Znl2(CsZnl3) ligand based QDs system results in the best optical properties.

[0281] Table 4: Absorption properties of QDs ligand exchanged ink using different inorganic ligands.

[0282] Example 8 - Fabrication of photodiode devices incorporating PbS-CsPbh prepared via in-situ method

[0283] In-situ ligand exchanged based QDs were also implemented in photodiode devices comprised of ITO / ZnO / PbS QDs / Organic ligand exchanged PbS QDs / MoO / Au. In this example, the PbS ink was prepared using the method of Example 3 - in-situ ligand exchange - with CsPbl3as the ligand system.

[0284] Device fabrication method: The fabrication of the photodiode devices involves multiple steps. Firstly, ZnO sol-gel solution is prepared, for which 1.09 g of zinc acetate dihydrate is dissolved in 10 mL of methoxyethanol and 0.3mL of ethanol amine. The ZnO solution is spin-coated on a cleaned ITO substrate at 1 ,500 RPM for 60s followed by heating at 210 °C for 15 min. A second layer of ZnO is coated on top followed by heating at 210 °C for 30 min. The ZnO coated films are then transferred into the nitrogen filled glovebox for PbS ink addition, wherein PbS ink was spin-coated at 1 ,500 RPM for 60s followed by spin coating a further layer at the same spin-speed, where this time the coating comprises a quantum dot ink with organic ligands coordinated to the surface. The devices are then transferred to a thermal evaporator for MoO and Au deposition under shadow masking.

[0285] Firstly, CsPbl3was used as the inorganic ligand for the PbS QDs implemented in the photodiode devices. The nanocrystals in the photodiode devices were prepared using either in-situ (Example 8) or ex-situ (Comparative Example 9) methods as described in the examples above, and the performance of each device was analysed. Figure 11 shows a comparison between the in-situ and ex-situ photodiode devices. In particular, Figure 11 shows (a) the current density-voltage characteristic of the photodiodes fabricated using in-situ and post-synthetic ligand exchanged PbS QDs and (b) the EQE spectrum of as fabricated photodiodes using in-situ and post-synthetic ligand exchanged based QDs using the same device architecture in the same batch.

[0286] A dark current density of 1.6E-5 A / cm2with EQE of 16.6% at -1V was achieved while in-situ ligand exchanged QDs were used. The EQE spectrum shows the excitonic feature of PbS QDs at 1300 nm indicating that the QDs maintained quantum confinement during the entire in-situ ligand exchange and device fabrication process.

[0287] Comparative Example 9 - Fabrication of photodiode devices incorporating PbS-CsPbh prepared via ex-situ method

[0288] For comparison, ex-situ ligand exchanged QDs were implemented in photodiode devices comprised of ITO / ZnO / PbS QDs / Organic ligand exchanged PbS QDs / MoO / Au. In this example, the PbS ink was prepared using the method of Comparative Example 4 - ex-situ ligand exchange - with CsPbl3as the ligand system.

[0289] Device fabrication method: Same as Example 8.

[0290] With the same type of device structure, this time with optimised post-synthetic ligand exchanged QDs (ex-situ), the photodiode device shows relatively lower device performance with an EQE of 13.8%.

[0291] Table 5 summarises the device data of in-situ vs post-synthetic ligand exchanged QD. As evident, in-situ ligand exchanged QDs device offers noticeably high external quantum efficiency and similar dark current, which proves that in-situ ligand exchange process is not only cost effective but also results in high quality product including QDs ink and devices. Table 5: Device metrices comparison of in-situ vs post synthetic ligand exchanged (ex-situ) QDs

[0292] Example 10 - Fabrication of photodiode devices incorporating InAs-CsZnh To further extend this work, Csl-Znl2ligand based InAs QDs were also implemented in a photodiode device and showed promising performance.

[0293] The dark and light current ratio (Table 6) clearly confirms the photodiode behaviour of the device.

[0294] Table 6: Device performance of photodiode using Csl-Znl2ligand based QDs

[0295] In this example, it is demonstrated that CsZnl3is an acceptable inorganic ligand for InAs nanocrystals. Here, InAs nanocrystals and devices including these nanocrystals are prepared using an ex-situ method as set out below.

[0296] Step 1: Preparation of amorphous InAs clusters In a 3-neck 250 m L_ flask, 3.0 mmol of ln(OAc)3, 9.2 mmol of oleic acid and 15 mL of heptadecane were taken and evacuated under vacuum (~0.1 mbar) at 110 °C for 90 minutes. During this step, indium oleate was formed and 9 mmol of acetic acid was collected in the cold trap. ln(OAc)3+ R-COOH ( = CI7H33) -> In-COOR + CH3COOH

[0297] The flask was switched to nitrogen atmosphere and slowly cooled down to room temperature. The flask was transferred into a glovebox. In the glovebox, 0.96 mmol of tris(trimethylsilyl)arsine ((TMSi)3As), 1.98 mmol dioctyl amine and 2.5 mL degassed heptadecane were taken in a 10 mL vial and vortexed thoroughly. Under constant stirring, indium oleate solution was mixed with TMSi-As solution to provide a composition comprising InAs clusters.

[0298] The composition comprising InAs clusters produced according to Step 1 was used as the composition comprising group lll-V clusters in Step 2.

[0299] Step 2: Preparation of InAs nanocrystals with an absorption peak of approximately 1400 nm

[0300] In Step 2, the preparation of InAs nanocrystals is split into two periods of crystal growth, followed by purification, ligand exchange with CsZnl3, ink formation and then device fabrication. First, InAs nanocrystals showing an absorption peak at around 1100 nm are formed (first growth time of 4.5 hours). The UV-vis spectra of these nanocrystals are shown in Figure 2. Second, these nanocrystals are used to prepare nanocrystals showing an absorption peak at around 1400 nm (second growth time of 8 hours). The UV-vis spectra of these nanocrystals are shown in Figure 1. The total growth time is 12.5 hours.

[0301] 1100 nm QDs

[0302] In a 3-neck 250 mL flask, 0.4 mmol of ln(OAc)3, 1.2 mmol of oleic acid and 6 mL heptadecane were taken and degassed under vacuum (~0.1 mbar) at 110 °C for 90 minutes. During this step, Indium oleate was formed and 1.2 mmol of acetic acid was collected in the cold trap. ln(OAc)3+ R-COOH (R= C17H33) -> In-COOR + CH3COOH

[0303] The flask was switched to nitrogen atmosphere and temperature was maintained at 100 °C. In the glovebox, 0.32 mmol of (TMSi)sAs, 0.56 mmol of dioctyl amine and 1 ml_ of degassed heptadecane were loaded into a syringe. This solution is added to the indium oleate solution at 100 °C. There was an immediate color change after the injection of arsenic precursor solution. The solution was simply heated up to 287 °C, once the temperature reached 287 °C, the growth was continued for 15 minutes. 10 mL of amorphous clusters solution of Example 1 was loaded into a syringe (diameter 20mm) and added for nearly 4h30 minutes at an injection rate of 2.230 mL / hour. After injecting all of the amorphous clusters solution, the solution was maintained at 287 °C for another 10 minutes. While the cluster addition was taking place, 270 pL of dilute HBr solution in acetone (dilute HBr solution was prepared by diluting 40 pL of HBr in 2 mL acetone solution) was added over 4h30 minutes time. The heating was turned off and the solution was cooled down naturally. CQDs were transferred to the glovebox for further use. CQDs growth was monitored by taking aliquots and measuring the absorption spectra.

[0304] 1400 nm QDs

[0305] In a 3-neck 250 mL flask connected under vacuum, 6 mL of InAs CQDs (50 mg / mL) (with an absorption max around 1100 nm) and 4 mL of degassed heptadecane was added. The flask contents were evacuated under vacuum (~0.1 mbar) at 100 °C for 30 minutes. The flask was switched to nitrogen. 200 pL of dilute HBr solution was added to the flask and maintained at 100 °C for 10 minutes. Then the temperature was increased to 285 °C. When temperature reaches around 285 °C, amorphous InAs clusters were loaded into a syringe and added using syringe pump at a rate of 1.16 mL / h for about 8 hours. While the cluster addition was taking place, 120 pL of dilute HBr was added over 8 hours. The heating was turned off and the solution was cooled down naturally. CQDs were transferred to the glovebox for further use. CQDs growth was monitored by taking aliquots and measuring the absorption spectra. To prepare InAs nanocrystals with an absorption peak at around 1550 nm to 1600 nm, Step 2 is the same as above, except that it requires a second growth time of 14 hours and addition of the amorphous clusters solution at a rate of 1 .1 ,6mL / hour.

[0306] Purification

[0307] The flask contents were transferred to the glovebox. 5 mL of hexane was added to the QDs followed by 40 mL acetone. The solution was divided into centrifuge tubes and centrifuged at 6000 RPM for 10 minutes. The clear supernatant was discarded and the CQDs pellet was redispersed in hexanes. Acetone, I PA were added in 2:1 volume ratio and centrifuged the dots once again at 6000 RPM for 10 minutes. The supernatant was discarded and the CQDs pellet was redispersed in n-octane. The CQDs in octane were centrifuged at 6000 RPM for 10 minutes to remove any solid impurities. The sediment was discarded and clear QD solution is filtered through a 0.1 pm PTFE to yield the product.

[0308] Ligand exchange and ink formation

[0309] After purification, the InAs nanocrystals comprise oleic acid capping the surface and are dissolved in octane. This organic ligand is exchanged for CsZnl3as set out below.

[0310] To a 100 ml solution of 7mg / mL QDs-octane solution was added CsZnl3inorganic ligand precursor which was prepared by dissolving 3.1 g of Znl2, 2.5 g of Csl, 305 mg of ammonium acetate in 100 mL of dimethyl formamide (DMF). Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and further using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink. Device fabrication

[0311] The device stack structure may be described as ITO / ETL / active layer (InAs- CsZnl3ink) / HTL / Au. ETL is used to refer to the electron transport layer. HTL is used to refer to the hole transport layer.

[0312] The advantage of the caesium zinc halide-based perovskite like system is that it is heavy metal free and therefore it can be used for ligand exchange for environmentally friendly QDs system. For instance, caesium zinc iodide can be used for InAs QDs ligand exchange, in which the precursor solution of caesium iodide and zinc iodide is dissolved in DMF and added in hexane QDs solution followed by stirring for 15 min. This results in a clear phase transfer of InAs QDs from non-polar phase to polar inorganic ligand system. Figure 12 shows the optical properties and atomic force microscopic image of ligand exchanged QDs. As evident, the resulting QDs show the excitonic feature along with high quality ink (Figure 12a). Figure 12b shows a photograph of the as fabricated film using spin-coating technique with AFM image which shows less than 10 nm RMS roughness.

[0313] In addition, dark current and EQE were measured for lnAs-CsZnl3nanocrystals. These results are shown in Table 7 below and Figure 13. As evident, when CsZnl3ligand was used the dark current density was reduced from 170 pA / cm2to 9.2 pA / cm2with a noticeable increase in EQE from 15.3% to 16.5%. These results are encouraging and show the superior surface passivation using our novel CsZnl3system for lead-free InAs QDs.

[0314] Comparative Example 11 - Fabrication of photodiode devices incorporating lnAs-lnBr3

[0315] For comparison, photodiode devices were prepared using the traditionally-used lnBr3inorganic ligand system. To ensure consistency, these devices were fabricated from the same batch of InAs nanocrystals as Example 10, but with a different ligand (lnBr3) used instead of CsZnl3. The device stack structure may be described as ITO / ETL / active layer (lnAs-lnBr3ink) / HTL / Au. The performance of these devices was compared in Table 7 and Figure 13.

[0316] Table 7: Device performance of photodiodes using prior art lnBr3ligands and the CsZnl3ligands of the present invention.

[0317] These results are discussed above under Example 10.

[0318] Example 12 - In-situ preparation of InAs nanocrystals using Csl-lnBr3

[0319] In this example, it is demonstrated that CsZnl3is an acceptable inorganic ligand for nanocrystals, in particular InAs nanocrystals. Here, these InAs nanocrystals and devices including these nanocrystals are prepared using an ex-situ method as set out below.

[0320] Photodiode devices were also prepared using the novel Csl-lnBr3inorganic ligand system. For this example, the method was the same as Example 10, but with the new ligand (Csl-lnBr3) used instead of CsZnl3and a different ligand exchange and ink fabrication step as set out below. The device stack structure may be described as ITO / ETL / active layer (lnAs-(Csl-lnBr3) ink) / HTL / Au.

[0321] Ligand exchange and ink formation

[0322] After purification, the InAs nanocrystals comprise oleic acid capping the surface and are dissolved in octane. This organic ligand is exchanged for Csl-lnBr3as set out below.

[0323] QDs-octane mixture was treated with CslnBr3l inorganic ligand precursor which was prepared by dissolving 90 mmol of lnBr3, 90 mmol g of Csl, 77 mmol of ammonium acetate in 10 mL of dimethyl formamide (DMF) in a reaction vessel. The solution was added to the QDs-octane mixture to conduct the phase transfer from long chain unstable organic ligand to inorganic stable ligand. Reaction mixture was left for 12 h under stirring under inert atmosphere. After 12 h, there is a clear phase transfer of QDs from ODE to DMF phase, they are then washed with 100 mL of hexane and purified using 100 mL of acetone by centrifuging at 6,000 RPM for 10 min. The resulting precipitate is dissolved in 2,6, difluoropyridine to use as an ink.

[0324] The ligand exchange showed a successful phase transfer with resulting high quality quantum dots ink. The use of Csl-lnBr3as a ligand therefore results in a successful phase transfer to the polar inorganic phase. The resulting QDs showed good colloidal stability in 2,6-difluoropyridine. The resulting solution forms a very stable ink with higher than 80 mg / mL concentration and a high PA / of 1.30, as shown by the absorption spectra in Figure 14.

[0325] Example 13 - Further analysis of PbS-CsPbl3nanocrystals

[0326] PbS-CsPbl3nanocrystals were prepared according to the method in Example 3.

[0327] TGA:

[0328] The nanocrystals were further analysed to determine the extent of the ligand exchange, i.e. how much of the organic ligand remains on the nanocrystals surface after the inorganic ligand exchange reaction. This may be determined via TGA via the method described above (illustrated in Figure 15).

[0329] The sample is prepared by drying a small amount of solution comprising the nanocrystals under high vacuum for a few hours until it stops losing further weight and has become a powder. The sample is then transferred to the TGA furnace for measurements.

[0330] For this ligand exchange, Figure 15 suggests that the ratio of inorganic ligands to organic ligands on the surface of the nanocrystals is 19.3:0.9 (21.4:1) by weight. This indicates that the ligand exchange reaction comprised exchanging 95.5% by weight of the organic ligands coordinated to the surface of the crystalline core with the inorganic ligand system shell.

[0331] XRD:

[0332] Figure 16 shows the XRD pattern of PbS-CsPbl3nanocrystals. The diffraction pattern demonstrates that the nanocrystals have a cubic crystal structure. However, it is difficult to identify the presence of CsPbl3due to the overlap between the crystal structure of cubic perovskite with PbS.

[0333] HRTEM:

[0334] Figure 17 shows the HRTEM micrographs of the PbS-CsPbl3nanocrystals. As can be seen from the micrographs, the CsPbl3forms a superlattice surrounding the nanocrystal surface.

[0335] ICP-TOF MS:

[0336] The MS graphs in Figure 18 confirm the presence of CsPbl3in this elemental ratio. The estimated ratio of Cs:l is 1 :3, which corresponds to the CsPbl3formula of the perovskite. This confirms the formation of a core-shell structure of the nanocrystals. Further, the uniform distribution of caesium and iodine demonstrates that the ligand uniformly covers the surface of the nanocrystals for this system, and indicates that corresponding ligand systems in the other examples also form core-shell structures.

[0337] REFERENCES

[0338] King et al, Importance of QD Purification Procedure on Surface Adsorbance of QDs and Performance of QD Sensitized Photoanode, J. Phys. Chem. C 2012, 116, 3349-3355 Chen et al, IEEE Access, vol. 8, pp. 159415-159423, 2020

[0339] Tamang et al., Chem. Rev. 2016, 116, 10731-10819

Claims

CLAIMS1. A method for producing a nanocrystal composition, the method comprising: a) a contacting step taking place in a non-polar solvent, the contacting step comprising contacting: a plurality of first organic ligand compounds; a metal-containing compound comprising a metal element; and at least one reagent comprising a pnictogen element or a chalcogen element, to form a mixture comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of the first organic ligand compounds coordinated to a surface of the crystalline core, wherein the crystalline core comprises (i) the metal element, and (ii) the pnictogen element or the chalcogen element, and b) adding to the mixture an exchange composition comprising an exchange ligand precursor and a polar solvent, wherein there are no purification steps between step a) and step b).

2. The method of claim 1 , wherein the metal element is selected from the group consisting of a Group I metal, a Group II metal, a Group III metal, a Group IV metal, and mixtures thereof, and wherein the metal-containing compound is selected from the group consisting of an elemental metal, a metal halide, a metal acetate, a metal nitrate, a metal carbonate, a metal oleate, a metal oxide, a metal peroxide, a metal alkoxide, a metal hydroxide, a metal sulfate, a metal acetylacetonate, a metal perchlorate, a metal carboxylate, a metal cyanide or mixtures thereof.

3. The method of claim 1 or claim 2, wherein the metal element is selected from the group consisting of indium, gallium, lead and silver.

4. The method of any one of the preceding claims, wherein the metal element is indium or gallium, and wherein the at least one reagent comprises a pnictogen element, andwherein the pnictogen element is selected from the group consisting of phosphorus, arsenic and antimony.

5. The method of any one of the preceding claims, wherein the metal element is lead or silver, and wherein the at least one reagent comprises a chalcogen element, and wherein the chalcogen element is selected from the group consisting of sulfur, selenium and tellurium.

6. The method of any one of the preceding claims, wherein the first organic ligand compound has a formula selected from the group consisting of RH2PO, R2HPO, R3PO, RPO(OH)2, or R2POOH, RH2P, R2HP, R3P, ROH, RCOOH, RCOOR’, RSH, RNH2, R2NH, and R3N, wherein R and R’ are each independently selected from the group consisting of Ci-C24 alkyl, C2-C24 alkenyl, Ce-C24 aryl, and mixtures thereof.

7. The method of any one of the preceding claims, wherein the exchange ligand precursor comprises a second organic ligand compound.

8. The method of any one of the preceding claims, wherein the exchange ligand precursor comprises an inorganic ligand precursor comprising one or more inorganic compounds.

9. The method of claim 8, wherein the inorganic ligand precursor comprises one or more metal halide compounds.

10. The method of claim 9, the inorganic ligand precursor comprises PbX2, CsX, lnBr3, MgBr3, AgBr, FeX3, AsX3, TIX, CsZnl3, CslnBr3l, CsPbl3, ZnX2or Csl, wherein X is a halide, or wherein the inorganic ligand precursor comprises Csl and Znl2, or wherein the inorganic ligand precursor comprises Csl and lnBr3.11 . The method of claim 9, wherein the inorganic ligand precursor comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3.

12. The method of claim 9, wherein the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is a Group IA metal, thallium or an organic ammonium salt, M is a metal having +2 oxidation state, and X is a halide, or wherein the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, potassium, lithium, thallium and sodium, M is Pb, Sn, Cu or Zn, and X is a halide, or wherein the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, potassium, lithium, and sodium, M is Pb or Zn, and X is a halide.

13. The method of any one of the preceding claims, wherein the method further comprises: c) displacing at least a portion of the first organic ligand compounds coordinated to the surface of the crystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor, preferably wherein step c) comprises stirring the mixture for at least two hours, preferably between 2 hours and 18 hours, preferably between 4 hours and 18 hours, preferably wherein the shell comprises a metal halide.

14. The method of claim 13, wherein the mixture formed in step c) further comprises at least one of: a) a salt comprising the metal element and the first organic ligand compound; and b) a second plurality of first organic ligand compounds, wherein the second plurality of first organic ligand compounds are not coordinated to the surface of the crystalline core, preferably wherein the second plurality of first organic ligand compounds are free in solution.

15. The method of any one of the preceding claims, wherein the exchange composition further comprises a stabilising agent, wherein the stabilising agent is selected from the group consisting of sodium acetate, lithium acetate, rubidium acetate, caesium acetate ammonium acetate, butylamine, trimethylsilyl halide and mixtures thereof, or wherein the stabilising agent is selected from the group consisting of sodium acetate, ammonium acetate, trimethylsilyl halide and mixtures thereof.

16. The method of any one of the preceding claims, wherein the non-polar solvent is a C6-C3o aliphatic or aromatic organic compound, optionally wherein the non-polar solvent is selected from the group consisting of a Ce-Cso amine, a Ce-C3o acid, a Ce-C3o phosphine, a Ce-C3o ether, and mixtures thereof.

17. A nanocrystal composition obtainable by the method of any one of the preceding claims.

18. The method of any one of claims 1 to 16, further comprising: d) washing the nanocrystal composition with a solution comprising at least one of acetone, methyl acetate, ethyl acetate, and acetonitrile, and e) dissolving the nanocrystal composition in a second polar solvent to form a nanocrystal ink composition, optionally wherein the second polar solvent is selected from the group consisting of 2,6-difluoropyridine, gamma-butyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof.

19. An ink composition comprising the nanocrystal composition of claim 18.

20. A nanocrystal comprising a crystalline core and a lead-free inorganic shell at least partially surrounding the crystalline core.

21. The nanocrystal of claim 20, wherein the shell comprises a metal halide ligand.

22. The nanocrystal of claim 20 or claim 21 , wherein the shell comprises a metal halide selected from the group consisting of CsX, AsX3, TIX, CsZnl3, Csl nBr3l , ZnX2, Csl and mixtures thereof, wherein X is a halide, or wherein the shell comprises AsX3or TIX, or wherein the shell comprises Csl and Znl2, or wherein the shell comprises Csl and lnBr3.

23. The nanocrystal of claim 20 or claim 21 , wherein the shell comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3, optionally wherein the organic ammonium salt is methylammonium or formamidinium.

24. The nanocrystal of claim 20 or claim 21 , wherein the shell comprises a metal halide of the formula AMX3, wherein A is a Group IA metal, thallium or an organic ammonium salt, M is a metal having +2 oxidation state, and X is a halide, or wherein the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, thallium and sodium, M is Sn, Cu or Zn, and X is a halide, or wherein the shell comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium and sodium, M is Zn, and X is a halide, or wherein the metal halide is CsZnl3.

25. The nanocrystal of claim 20 or claim 21 , wherein the shell comprises a metal halide of the formula CsZnxAsyl3, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1 , optionally wherein y is in the range of 0.01 to 0.1 , or wherein the shell comprises a metal halide of the formula Cs3ZnxAsyl5, wherein the sum of x and y is equal to 1 , wherein y is in the range of 0 to 0.1 , optionally wherein y is in the range of 0.01 to 0.1 .

26. The nanocrystal of claim 20 or claim 21 , wherein the shell comprises a metal halide of the formula Cs3BX6, wherein B is indium or antimony and X is a halide, or wherein the shell comprises a metal halide of the formula Cs3lnX6, CslnX4, or Cs2lnX5, wherein X is a halide, or wherein the shell comprises CslnBr3l.

27. A nanocrystal composition comprising a plurality of nanocrystals according to any one of claims 20 to 26.

28. An ink composition comprising the nanocrystal composition of claim 27 and a polar solvent.

29. A device selected from the group consisting of IR sensor, photodetector, sensor, solar cell, a bio-imaging or bio-sensing composition, photovoltaic system, display, battery, laser, photocatalyst, spectrometer, injectable composition, fieldeffect transistor, light-emitting diode, photonic or optical switching device or metamaterial, fiber amplifier, optical gain media, optical fiber, infrared LEDs, lasers, and electroluminescent device, comprising the nanocrystal composition of claim 27, optionally wherein the I sensor or photodetector are modified for application as 3D cameras and 3D Time of flight cameras in mobile and consumer, automotive, medical, industrial, defence or aerospace applications, optionally wherein the bio-imaging or bio-sensing compositions are modified for use as bio-labels or bio-tags in in vitro or ex vivo applications, optionally wherein the infrared LEDs and electroluminescent devices are modified for use in telecommunication devices, night vision devices, solar energy conversion, thermoelectric or energy generation applications.

30. A method for nanocrystal ligand exchange, the method comprising a contacting step comprising contacting:(i) a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligand compounds coordinated to a surface of the crystalline core, and(ii) a second composition comprising a lead-free inorganic ligand precursor, wherein the inorganic ligand precursor comprises one or more metal halide compounds.31 . The method of claim 30, wherein the native ligand compounds are organic ligand compounds, and wherein the first nanocrystal composition further comprises a non-polar solvent.

32. The method of claim 30 or claim 31 , wherein the inorganic ligand precursor comprises CsX, AsX3, TIX, CsZnl3, Csl nBr3l , ZnX2or Csl, wherein X is a halide, or wherein the inorganic ligand precursor comprises AsX3or TIX, or wherein the inorganic ligand precursor comprises Csl and Znl2, or wherein the inorganic ligand precursor comprises Csl and lnBr3.

33. The method of claim 30 or claim 31 , wherein the inorganic ligand precursor comprises a metal halide of the formula ApZnqXr, wherein A is a Group IA metal or an organic ammonium salt, X is a halide, and p, q and r are each independently 1 , 2 or 3, optionally wherein the organic ammonium salt is methylammonium or formamidinium.

34. The method of claim 30 or claim 31 , wherein the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium, thallium and sodium, M is Sn, Cu or Zn, and X is a halide, or wherein the inorganic ligand precursor comprises a metal halide of the formula AMX3, wherein A is selected from the group consisting of caesium, methylammonium, rubidium and sodium, M is Zn, and X is a halide, or wherein the metal halide is CsZnl3.

35. The method of claim 30 or claim 31 , wherein the inorganic ligand precursor comprises CslnBr3l.

36. The method of any one of claims 30 to 35, further comprising displacing at least a portion of the native ligand compounds coordinated to the surface of thecrystalline core to form nanocrystals comprising the crystalline core and a shell surrounding the crystalline core, wherein the shell is formed from the inorganic ligand precursor, optionally wherein the contacting step comprises stirring the compositions for at least two hours, preferably between 2 hours and 18 hours, preferably between 4 hours and 18 hours.

37. The method of any one of claims 30 to 36, wherein the second composition further comprises a stabilising agent, wherein the stabilising agent is selected from the group consisting of sodium acetate, lithium acetate, rubidium acetate, caesium acetate ammonium acetate, butylamine, trimethylsilyl halide and mixtures thereof, or wherein the stabilising agent is selected from the group consisting of sodium acetate, ammonium acetate, trimethylsilyl halide and mixtures thereof.

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