Novel ligand system for colloidal quantum dot ink compositions

Abstract

The present invention provides nanocrystals comprising chalcogenide halide ligands, and further provides inks and devices comprising these nanocrystals. The present invention also provides uses of the nanocrystals, inks and devices. The present invention additionally provides a method for nanocrystal ligand exchange to form these nanocrystals, the method comprising a contacting step comprising contacting a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligands coordinated to a surface of the crystalline core, and a second composition comprising a plurality of chalcogenide halide ligands.

Description

[0001] NOVEL LIGAND SYSTEM FOR COLLOIDAL QUANTUM DOT INK COMPOSITIONS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates in general to nanocrystals. In particular, the present invention relates to nanocrystals comprising chalcogenide halide ligands and to inks and devices comprising these nanocrystals, and uses thereof. The present invention also relates to a method for nanocrystal ligand exchange used to form these nanocrystals, the method comprising a contacting step comprising contacting a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligands coordinated to a surface of the crystalline core, and a second composition comprising a chalcogenide halide.

[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. Nanocrystals such as colloidal quantum dots (CQDs) are commonly synthesized using long-chain organic ligands to control the growth of the nanocrystals and stabilize the colloidal system in the solvent, which is generally non-polar. Although useful in controlling and stabilising the synthesis, the long chain organic ligands are electrically insulating and result in the nanocrystals having poor charge transfer. Therefore, these long-chain organic ligands are generally exchanged with more electrically-conductive ligands, such as shorter organic chain ligands or conducting inorganic ligands, in order to improve the charge transfer of the system.

[0006] Traditionally, the inorganic ligands used to prepare nanocrystals with improved charge transfer comprise lead. Whilst ligand systems comprising lead and other heavy metals have been shown to exhibit good performance, their use can be problematic in some industries due to their toxicity. For example, lead-based ligand systems such as PbX2, where X is a halide, have been used to prepare nanocrystals in the literature.

[0007] Chen et al (J. Phys. Chem. Lett. 2021 , 12, 1567-1572) discloses ligand exchanged PbS-Pbl2quantum dots, wherein the Pbl2is claimed to form a shell surrounding the PbS nanocrystal. However, the use of toxic lead in this system is problematic. Further, Pbl2has a relatively wide bandgap of approximately 380 nm. Having a wider band gap creates energy barrier for electron / hole transport for infrared photodetector.

[0008] Sargent et al (Nat. Commun. 2017, 8, 14757) discloses quantum-dot-in-perovskite materials. However, the use of toxic lead in the perovskite material is problematic.

[0009] Moreover, current inorganic ligands offer more n-type characteristics and exhibit defects in their structure.

[0010] There is a need in the industry to provide inorganic ligand systems for nanocrystals that avoid the use of toxic heavy metals such as lead, whilst maintaining good performance and few defects. SUMMARY OF INVENTION

[0011] According to a first aspect, the present invention provides a nanocrystal comprising a crystalline core and a plurality of ligands coordinated to a surface of the crystalline core, wherein the ligands have a structure according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0012] According to a second aspect, the present invention also provides a nanocrystal composition comprising a plurality of nanocrystals according to the first aspect.

[0013] According to a third aspect, the present invention also provides a nanocrystal ink composition comprising the nanocrystal composition of the second aspect and a polar solvent.

[0014] According to a fourth aspect, the present invention also provides the use of compounds of Formula (I) as ligands for the preparation of nanocrystals, wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0015] According to a fifth aspect, the present invention provides a method for nanocrystal ligand exchange, the method comprising a contacting step comprising contacting:

[0016] (i) a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligands coordinated to a surface of the crystalline core, and

[0017] (ii) a second composition comprising a chalcogenide halide according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0018] BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram showing the process for forming the nanocrystals of the present invention. Figure 2 shows the absorption spectrum of PbS nanocrystals according to the present invention comprising the BiSI ligand system.

[0020] Figure 3 shows the absorption spectrum of InAs nanocrystals according to the present invention comprising the BiSI ligand system.

[0021] Figure 4 shows the absorption spectra of PbS nanocrystals with different ligand systems to analyse the role of different precursors and ligand systems in the process.

[0022] Figure 5 shows the aerobic stability of PbS nanocrystals with varying ligand systems to analyse the role of the different precursors in the process.

[0023] Figure 6 shows the absorption spectrum of InAs nanocrystals comprising the BiSI ligand system, wherein the process for forming the nanocrystals comprised surface treating the nanocrystals with an amine such as butylamine.

[0024] Figure 7 shows the absorption spectrum of InAs nanocrystals comprising the BiSI ligand system, wherein the process for forming the nanocrystals comprised using N,N-dibutylthiourea as the Chalcogen-containing compound and gammabutyrolactone as the second polar solvent for the final ink composition.

[0025] Figure 8 shows the current density-voltage characteristics of the InAs photodiode device.

[0026] Figure 9 shows the EQE spectrum of the InAs photodiode device.

[0027] DETAILED DESCRIPTION

[0028] 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.

[0029] 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.

[0030] 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’.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] Nanocrystals

[0038] According to a first aspect, the present invention provides a nanocrystal comprising a crystalline core and a shell at least partially surrounding the crystalline core, wherein the shell comprises a chalcogenide halide according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0039] The nanocrystals of the present invention use a novel ligand system, given by Formula (I). The novel ligand system has a high defect tolerance, preventing the trapping of charge carriers in photodiode devices and leading to high external quantum efficiency (EQE). Importantly, the ligand system also avoids the use of toxic heavy metals such as lead. Additionally, the ligand system of the present invention offers more p-type characteristics, helping to improve the structural- function and optoelectronic properties of the nanocrystals. Furthermore, the ligand system forming the shell, given by Formula (I), allows for tuning of the band gap of the ligand system by varying the metal, chalcogen and halide component, for example by changing bismuth to antimony, sulphur to selenium, or chlorine to bromine.

[0040] 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).

[0041] The nanocrystals of the present invention are typically semiconductor nanocrystals and / or quantum dots. The quantum dots are preferably colloidal quantum dots.

[0042] 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.

[0043] As used herein, the term “pnictogen” is 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.

[0044] As used herein, the term “chalcogen” is 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.

[0045] As used herein, the term “halide” is used to refer to an element in group 17 of the periodic table. For example, a halide may be fluorine, chlorine, bromine or iodine.

[0046] 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.

[0047] Without wishing to be bound by theory, it is believed that the chalcogenide halide of the present invention forms a shell surrounding the nanocrystal during synthesis. In this way, the nanocrystal of the first aspect comprises a core-shell structure, wherein the nanocrystal comprises 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 chalcogenide halide during the synthesis. 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. In other words, without wishing to be bound by theory, it is believed that the ligands of Formula (I) of the present invention form a lattice layer surrounding the crystalline core of the nanocrystal. The lattice layer comprises the ligands of Formula (I). Preferably, the shell (i.e. the lattice layer) consists of the ligands of Formula (I). The shell (i.e. the lattice layer) is coordinated to the surface of the crystalline core.

[0048] The shell is typically disposed on the surface of the crystalline core and coordinated to the crystalline core of the nanocrystal. 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.

[0049] The chalcogenide halides of the present invention provide complete surface passivation and offer high charge carrier mobilities.

[0050] The inventors have found that the nanocrystals produced by the method of the present invention comprise a shell (i.e. a lattice layer) surrounding the crystalline core, wherein the lattice layer comprises the chalcogenide halide of Formula (I).

[0051] Preferably, A is a metal selected from the group consisting of bismuth, indium and antimony. Preferably, Ch is sulfur or selenium. More preferably, A is a metal selected from the group consisting of bismuth, indium and antimony and Ch is sulfur or selenium.

[0052] In a preferred embodiment, A is bismuth. Where A is bismuth, Ch is preferably sulfur or selenium.

[0053] In a preferred embodiment, Ch is sulfur. Where A is bismuth and Ch is sulfur, the chalcogenide halide is preferably selected from the group consisting of BiSCI, BiSBr, BiSI, and mixtures thereof. In a preferred embodiment, X is iodine or bromine. In this embodiment, preferably the chalcogenide halide is BiSI or BiSBr. BiSI exhibits excellent charge transport due to its small effective mass of carrier along the BiSI chain direction.

[0054] In an alternative preferred embodiment, Ch is selenium. Where A is bismuth and Ch is selenium, the chalcogenide halide is preferably selected from the group consisting of BiSeCI, BiSeBr, BiSei, and mixtures thereof. In a preferred embodiment, X is iodine or bromine. In this embodiment, the chalcogenide halide is preferably BiSei or BiSeBr.

[0055] When compared to the prior art, for example to Chen et al, this ligand system advantageously excludes the use of toxic lead. The BiSX ligand absorbs at 700 nm, whereas Pbl2absorbs at -380 nm, which makes Pbl2a wider band gap semiconductor. As described above, having a wider band gap creates an energy barrier for electron / hole transport for infrared photodetectors. BiSX absorbs in the visible-NIR region, which is ideal for light absorption on its own but also offers a smaller energy barrier for infrared photodetector devices. Wthout wishing to be bound by theory, it is also believed that BiSX is a more p type material which is desirable for extreme n-type InAs QDs.

[0056] This ligand system possesses properties such as sulfur (S) interaction with arsenic and enhanced passivation of iodine, which make it unique and desirable. Additionally, this ligand is well-suited for PbS quantum dots, and could potentially enhance their p-type characteristics. It is also hypothesized that this ligand system may provide benefits such as superior band alignment, and improved stability of the ligand, the colloidal solution, and the quantum dots.

[0057] Whilst lead-based nanocrystals 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 chalcogenide halides above which do not contain lead solve this problem, providing excellent performance whilst also having reduced toxicity.

[0058] To fully understand whether Bi, S, and I are being utilised in the ligand exchange, the inventors designed some controlled experiments, eliminating each salt during an exchange process on PbS QDs. The inventors found that when BiCh, Nal and thiourea were all used together, the nanocrystals had the highest peak to valley ratio (P / V ratio) of 3.25. These experiments are discussed in more detail in the examples. As used herein, the term “peak to valley ratio” (or “PA / 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.

[0059] In an alternative preferred embodiment, A is indium or antimony. Where A is indium or antimony, the chalcogenide halide is preferably selected from the group consisting of InSCI, InSBr, InSI, InSeCI, InSeBr, InSei, AsSCI, AsSBr, AsSI, AsSeCI, AsSeBr, AsSel, and mixtures thereof. In a preferred embodiment, X is chlorine, iodine or bromine.

[0060] 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.

[0061] The crystalline core 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.

[0062] For example, the metal element may be any suitable post-transition metal element or transition metal element. The crystalline core may therefore comprise a metal chalcogenide or a metal pnictogenide. Suitable metal elements include, but are not limited to, indium, gallium, lead and silver. Preferably, the metal element is selected from the group consisting of indium, gallium, lead, silver and mixtures thereof.

[0063] The nanocrystals may be secondary, tertiary or quaternary nanocrystals. In other words, the crystalline core may comprise or consist of two, three or four component elements. For example, the crystalline core may comprise or consist of one metal element and one chalcogen element, forming a secondary nanocrystal (e.g. PbS). Alternatively, the crystalline core may comprise or consist of two metal elements and one pnictogen element (e.g. InGaAs). Alternatively, the crystalline core may comprise or consist of two metal elements and two pnictogen elements (e.g. InGaAsSb).

[0064] In a preferred embodiment, the crystalline core comprises a pnictogen element. Typically, where the crystalline core comprises a pnictogen element, the crystalline core comprises or consists of (i) a metal element and (ii) a pnictogen element. Preferably, the crystalline core consists of (i) a metal element and (ii) a pnictogen element.

[0065] Where the crystalline core comprises a pnictogen element, preferably the metal element is selected from the group consisting of indium, gallium and mixtures thereof. Preferably, the pnictogen element is selected from the group consisting of phosphorus, arsenic, antimony and mixtures thereof. In an embodiment, the pnictogen element is arsenic. In an embodiment, the pnictogen element is phosphorus. In an embodiment, the pnictogen element is antimony.

[0066] Suitably, the crystalline core may comprise or consist of InAs, InP, InSb, InAsP, InAsSb, GaAs, GaP, GaSb, GaAsP, GaAsSb, InGaAs, InGaP, InGaSb, InGaAsP or InGaAsSb. In a particularly preferred embodiment, the crystalline core comprises InAs. More preferably, the crystalline core consists of InAs. Typically, where the crystalline core comprises indium, the nanocrystals will exhibit an absorption maximum in the range of 400 nm to 2500 nm.

[0067] In addition to all the advantages mentioned above for the novel ligand system, the inventors have found that the BiSI ligand system is particularly beneficial when used with InAs nanocrystals, i.e. wherein the crystalline core comprises InAs. This is because the InAs crystalline core is very n-type, making it difficult to find suitable transport layers for device fabrication. However, the BiSI ligand system offers more p-type characteristics, making charge extraction easier with the InAs crystalline core, leading to improved EQE.

[0068] In an alternative preferred embodiment, the crystalline core comprises a pnictogen element. Typically, where the crystalline core comprises a pnictogen element, the crystalline core comprises or consists of (i) a metal element and (ii) a chalcogen element. Preferably, the crystalline core consists of (i) a metal element and (ii) a chalcogen element.

[0069] Preferably, the metal element is selected from the group consisting of lead, silver and mixtures thereof. More preferably, the metal element is lead or silver. Preferably, the chalcogen element is selected from the group consisting of sulfur, selenium, tellurium and mixtures thereof. More preferably, the chalcogen element is sulfur or selenium. More preferably, the metal element is lead or silver and the chalcogen element is sulfur or selenium. As such, the crystalline core preferably comprises PbS, PbSe, Ag2S orAg2Se. Preferably, the crystalline core consists of PbS, PbSe, Ag2S orAg2Se. Typically, where the crystalline core comprises lead or silver, the nanocrystals will exhibit an absorption maximum in the range of 400 nm to 2600 nm.

[0070] Preferably, the nanocrystal of the first aspect 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.

[0071] Suitably, the nanocrystal of the first aspect may be a semiconductor nanocrystal and / or a quantum dot. In an embodiment, the nanocrystal is a semiconductor nanocrystal dot. In an embodiment, the nanocrystal is a quantum dot.

[0072] According to a second aspect, the present invention also provides a nanocrystal composition comprising a plurality of nanocrystals according to the first aspect.

[0073] Preferably, the nanocrystals of the nanocrystal composition of the second aspect 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.

[0074] Preferably, the nanocrystals of the nanocrystal composition of the second aspect have a relative size dispersion of less than 25%, or less than 22%, or less than 20%, or less than 17%, or less than 15%, or less than 12%, or less than 10%.

[0075] Preferably, the nanocrystals of the nanocrystal composition of the second aspect have an absorbance maximum in the range of 400 nm to 3000 nm, preferably 600 nm to 2400 nm. According to a third aspect, the present invention also provides a nanocrystal ink composition comprising the nanocrystal composition of the second aspect and a polar solvent.

[0076] Preferably, the polar solvent of the nanocrystal ink composition of the third aspect is selected from the group consisting of 2,6-difluoropyridine, gammabutyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof. For InAs nanocrystals comprising ligands according to Formula (I), especially BiSI or BiSBr, gamma-butyrolactone is particularly preferred polar solvent. The present invention therefore provides a nanocrystal ink composition comprising a plurality of nanocrystals dissolved in gammabutyrolactone, wherein the nanocrystals comprise a crystalline core and a plurality of ligands of Formula (I) coordinated to a surface of the crystalline core, wherein the crystalline core comprises InAs.

[0077] 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 a nanocrystal composition according to the first aspect.

[0078] Preferably, 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.

[0079] Preferably, the bio-imaging or bio-sensing compositions are modified for use as bio-labels or bio-tags in in vitro or ex vivo applications.

[0080] Preferably, the infrared LEDs and electroluminescent devices are modified for use in telecommunication devices, night vision devices, solar energy conversion, thermoelectric or energy generation applications. The present invention also provides a film comprising a nanocrystal composition according to the second aspect.

[0081] According to a fourth aspect, the present invention also provides the use of compounds of Formula (I) as ligands for the preparation of nanocrystals, wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0082] Method for ligand exchange

[0083] According to a fifth aspect, the present invention provides a method for nanocrystal ligand exchange, the method comprising a contacting step comprising contacting:

[0084] (i) a first nanocrystal composition comprising a plurality of nanocrystals, wherein the nanocrystals comprise a crystalline core and a plurality of native ligands coordinated to a surface of the crystalline core, and

[0085] (ii) a second composition comprising a chalcogenide halide according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

[0086] As used herein, the term “native ligand” is used to refer to a ligand coordinated to a surface of the crystalline core of the nanocrystals in the first nanocrystal composition.

[0087] The first nanocrystal composition may be prepared by any suitable method. For example, the first nanocrystal composition may suitably be prepared by the simple addition method, heat up method, hot addition method, and continuous addition methods as well-known in the art.

[0088] The preferred features and embodiments relating to the crystalline core of the nanocrystals are the same for the fifth aspect as they are for the first aspect of the invention.

[0089] The crystalline core 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 ligands of Formula (I), such that the lattice layer surrounding the crystalline core is structurally distinguished from the crystalline core. In other words, the crystalline core has a different chemical composition to the 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.

[0090] For example, the metal element may be any suitable post-transition metal element or transition metal element. The crystalline core may therefore comprise a metal chalcogenide or a metal pnictogenide. Suitable metal elements include, but are not limited to, indium, gallium, lead and silver. Preferably, the metal element is selected from the group consisting of indium, gallium, lead, silver and mixtures thereof.

[0091] The preferred features and embodiments relating to Formula (I) are the same for the first aspect as they are for the fifth aspect of the invention.

[0092] The metal A may be any suitable metal element, as would be understood by the skilled person. For example, A may be any suitable post-transition metal or transition metal.

[0093] In a preferred embodiment, A is a metal selected from the group consisting of bismuth, indium and antimony. More preferably, A is bismuth. In a preferred embodiment, Ch is sulfur or selenium. Preferably, A is a metal selected from the group consisting of bismuth, indium and antimony and Ch is sulfur or selenium, preferably sulfur.

[0094] In a preferred embodiment, A is bismuth. Where A is bismuth, Ch is preferably sulfur or selenium.

[0095] In a preferred embodiment, Ch is sulfur. Where A is bismuth and Ch is sulfur, the chalcogenide halide is preferably selected from the group consisting of BiSCI, BiSBr, BiSI, and mixtures thereof. In a preferred embodiment, X is iodine or bromine. In this embodiment, preferably the chalcogenide halide is BiSI or BiSBr.

[0096] In an alternative preferred embodiment, Ch is selenium. Where A is bismuth and Ch is selenium, the chalcogenide halide is preferably selected from the group consisting of BiSeCI, BiSeBr, BiSei, and mixtures thereof. In a preferred embodiment, X is iodine or bromine. In this embodiment, the chalcogenide halide is preferably BiSei or BiSeBr.

[0097] In an alternative preferred embodiment, A is indium or antimony. Where A is indium or antimony, the chalcogenide halide is preferably selected from the group consisting of InSCI, InSBr, InSI, InSeCI, InSeBr, InSei, AsSCI, AsSBr, AsSI, AsSeCI, AsSeBr, AsSel, and mixtures thereof. In a preferred embodiment, X is chlorine, iodine or bromine.

[0098] In a preferred embodiment, the first nanocrystal composition further comprises a non-polar solvent. Preferably, 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.

[0099] Preferably, the non-polar solvent is selected from the group consisting of oleic acid, oleyl amine, butylamine, dioctylamine, octylamine, dichloromethane, pentane, cyclohexane, hexane, heptane, octane, toluene, tetrachoroethylene, trioctylphosphineheptadecane, chloroform, carbon tetrachloride, and mixtures thereof. More preferably, the non-polar solvent comprises at least one of hexane, octane and toluene. In a preferred embodiment, the first nanocrystal composition has a concentration of nanocrystals in the range of 0.01 mg / mL to 100 mg / mL, or in the range of 0.02 mg / mL to 50 mg / mL, or in the range of 0.05 mg / mL to 20 mg / mL, or in the range of 0.1 mg / mL to 10 mg / mL, or in the range of 0.2 mg / mL to 5 mg / mL, or in the range of 0.5 mg / mL to 2 mg / mL.

[0100] The first nanocrystal composition may be commercially available, or the method of the present invention may comprise forming the first nanocrystal composition. The first nanocrystal composition may be formed by any suitable nanocrystal synthesis known in the art, particularly by any of the methods disclosed in Tamang et al Chem. Rev. 2016, 116, 10731-10819). For example, the first nanocrystal composition may be produced by via a simple addition method, or via a heat-up method, optionally in combination with a continuous addition, or via a hot addition method, optionally in combination with a continuous addition, or via a continuous addition method, or via a combination of the abovementioned methods.

[0101] In a preferred embodiment, the second composition further comprises a polar solvent. Preferably, the polar solvent has a relative permittivity at 20 °C of more than 3, preferably more than 5, preferably more than 10. Preferably, the polar solvent is selected from the group consisting of acetonitrile, gammabutyrolactone, ethyl acetate, 2,6-difluoropyridine, butanol, ethylene glycol, sulfolane, dimethylformamide and mixtures thereof.

[0102] Preferably, the second composition has a concentration of the chalcogenide halide in the range of 1 mmol dm-3to 10000 mmol dm-3, or in the range of 5 mmol dm-3to 5000 mmol dm-3, or in the range of 10 mmol dm-3to 1000 mmol dm-3, or in the range of 50 mmol dm-3to 500 mmol dm-3, or in the range of 100 mmol dm-3to 300 mmol dm-3.

[0103] Preferably, the first nanocrystal composition comprises in the range of 0.01 mg to 10 mg, or in the range of 0.05 mg to 5 mg, or in the range of 0.1 mg to 1 mg, or in the range of 0.2 mg to 0.8 mg, or in the range of 0.3 mg to 0.7 mg, or in the range of 0.4 mg to 0.6 mg, of nanocrystals per mmol of the chalcogenide halide in the second composition. Preferably, the method further comprises forming the second composition, wherein forming the second composition comprises adding an A-containing compound and a Ch-containing compound to a polar solvent.

[0104] Preferably, the A-containing compound is a metal halide comprising X.

[0105] As mentioned above, A may be bismuth. Where A is bismuth, in a preferred embodiment, the A-containing compound is a bismuth halide. Preferably, the A- containing compound is selected from the group consisting of Bil3or BiCI3, BiBr3, Bil3and mixtures thereof. More preferably, the A-containing compound is Bi l3.

[0106] As mentioned above, A may be indium. Where A is indium, in an alternative preferred embodiment, the A-containing compound is an indium halide. Preferably, the A-containing compound is selected from the group consisting of InCi, lnCI2, lnCI3, InBr, lnBr2, lnBr3, Ini, lnl2, lnl3and mixtures thereof. More preferably, the A-containing compound is selected from the group consisting of lnCI3, lnBr3, lnl3and mixtures thereof.

[0107] As mentioned above, A may be antimony. Where A is antimony, in a preferred embodiment, the A-containing compound is an antimony halide. Preferably, the A-containing compound is selected from the group consisting of SbCI3, SbBr3, Sbl3and mixtures thereof.

[0108] In a preferred embodiment, the Ch-containing compound comprises sulfur, selenium, tellurium, or mixtures thereof.

[0109] Where Ch is sulfur, the Ch-containing compound comprises sulfur. Preferably, the Ch-containing compound is selected from the group consisting of thiourea, butylthiourea, bis(trialkylsilyl)sulphide compounds, wherein the alkyl is methyl, ethyl or propyl, thioacetamide, tri-n-octylphosphine sulphide, tributylphosphine sulphide, alkyl substituted thioamide compounds, elemental sulphur and mixtures thereof. Examples of suitable thiourea compounds include mono- and disubstituted thioureas, preferably of the formula (RI NH)CS(NHR2), wherein Ri and R2are each independently selected from the group consisting of phenyl, alkyl (e.g. C1-C12; straight chained, branched, or cyclo), and substituted aryl (e.g. F3C-Ph, MeO-Ph, CN-Ph).

[0110] Where Ch is selenium, the Ch-containing compound comprises selenium. Preferably, the Ch-containing compound is selected from the group consisting of bis(trimethylsilyl)selenide, tri-n-octylphosphine selenide (TOPSe) and tributylphosphine selenide.

[0111] Where Ch is tellurium, the Ch-containing compound comprises tellurium. Preferably, the Ch-containing compound is selected from the group consisting of tri n-octylphosphine telluride, tellurium powder, and mixtures thereof.

[0112] 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, trimethylsilyl halide, and mixtures thereof. As such, 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. In a particularly preferred embodiment, the stabilising agent comprises ammonium acetate. The stabilising agent, particularly 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.

[0113] The inventors noted that ammonium acetate was a particularly useful stabilizing agent for use in the second composition, especially where the crystalline core of the nanocrystals comprises InAs. However, it was also noted that ammonium acetate was liable to undergo an undesired side reaction with BiCI3when BiCI3was used as the A-containing compound, resulting in a lower amount of bismuth available to form the ligand system on the surface of the crystalline core of the nanocrystals. Consequently, whilst the InAs nanocrystals exhibited good solubility and optoelectronic properties (and were therefore suitable for use in optoelectronic devices), the inventors believed that the solubility of the InAs nanocrystals could be improved further.

[0114] The inventors replaced BiCI3with Bil3, which has a less ionic nature. Surprisingly, the inventors observed that the undesired side reaction with ammonium acetate was avoided when Bil3was used as the A-containing compound in the second composition. Furthermore, the inventors discovered that treatment of the nanocrystals after the contacting step with an amine such as butylamine resulted in efficient surface passivation of the InAs nanocrystals. The combination of these two improvements allowed InAs quantum dots to be produced with excellent solubility, aerobic stability, and optoelectronic properties. It is believed that the same improvement may be seen with Bi Br3.

[0115] In a particularly preferred embodiment, the second composition comprises Bil3, butylthiourea and ammonium acetate. Preferably, the second composition further comprises sodium iodide. Preferably, the method also includes surface treating the nanocrystals of the second nanocrystal composition with an amine selected from the group consisting of butylamine, amylamine, phenylethylamine, hexylamine, cysteine, and mixtures thereof, preferably wherein the amine is butylamine.

[0116] In a preferred embodiment, the second composition has a concentration of the stabilising agent in the range of 0.1 mmol dm-3to 500 mmol dm-3, or in the range of 1 mmol dm-3to 200 mmol dm-3, or in the range of 5 mmol dm-3to 100 mmol dm-3, or in the range of 10 mmol dm-3to 70 mmol dm-3, or in the range of 20 mmol dm-3to 50 mmol dm-3.

[0117] The nanocrystals may be secondary, tertiary or quaternary nanocrystals. In other words, the crystalline core may comprise or consist of two, three or four component elements. For example, the crystalline core may comprise or consist of one metal element and one chalcogen element, forming a secondary nanocrystal (e.g. PbS). Alternatively, the crystalline core may comprise or consist of two metal elements and one pnictogen element (e.g. InGaAs). Alternatively, the crystalline core may comprise or consist of two metal elements and two pnictogen elements (e.g. InGaAsSb).

[0118] In a preferred embodiment, the crystalline core comprises a pnictogen element. Typically, where the crystalline core comprises a pnictogen element, the crystalline core comprises or consists of (i) a metal element and (ii) a pnictogen element. Preferably, the crystalline core consists of (i) a metal element and (ii) a pnictogen element.

[0119] Where the crystalline core comprises a pnictogen element, preferably the metal element is selected from the group consisting of indium, gallium and mixtures thereof. Preferably, the pnictogen element is selected from the group consisting of phosphorus, arsenic, antimony and mixtures thereof. In an embodiment, the pnictogen element is arsenic. In an embodiment, the pnictogen element is phosphorus. In an embodiment, the pnictogen element is antimony.

[0120] Suitably, the crystalline core may comprise or consist of InAs, InP, InSb, InAsP, InAsSb, GaAs, GaP, GaSb, GaAsP, GaAsSb, InGaAs, InGaP, InGaSb, InGaAsP or InGaAsSb. In a particularly preferred embodiment, the crystalline core comprises InAs. More preferably, the crystalline core consists of InAs.

[0121] In an alternative preferred embodiment, the crystalline core comprises a pnictogen element. Typically, where the crystalline core comprises a pnictogen element, the crystalline core comprises or consists of (i) a metal element and (ii) a chalcogen element. Preferably, the crystalline core consists of (i) a metal element and (ii) a chalcogen element.

[0122] Preferably, the metal element is selected from the group consisting of lead, silver and mixtures thereof. More preferably, the metal element is lead or silver.

[0123] Preferably, the chalcogen element is selected from the group consisting of sulfur, selenium, tellurium and mixtures thereof. More preferably, the chalcogen element is sulfur or selenium. More preferably, the metal element is lead or silver and the chalcogen element is sulfur or selenium. As such, the crystalline core preferably comprises PbS, PbSe, Ag2S or Ag2Se. Preferably, the crystalline core consists of PbS, PbSe, Ag2S orAg2Se.

[0124] Preferably, the native ligand is an organic ligand. Preferably, the native ligand is a C2-C24 organic compound comprising a functional group selected from the group consisting of amino, thiol, hydroxyl and carboxylic acid. Preferably, the native ligand is a Ce-C24 organic compound comprising a functional group selected from the group consisting of amino, thiol, hydroxyl and carboxylic acid.

[0125] 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. As used herein, the term “inorganic compound” is used to refer to a compound other than an organic compound.

[0126] Preferably, the native ligand is a compound selected from the group consisting of 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, alkylamines, dioctylamine, phosphonic acid, thiols and mixtures thereof.

[0127] Preferably, the native ligand is selected from the group consisting of oleic acid, oleylamine, 3-mercaptopropionic acid and mixtures thereof.

[0128] Preferably, the contacting step takes 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 inorganic ligands, the nanocrystals may be exposed to air as they become less sensitive to oxygen. Preferably, prior to the contacting step the first nanocrystal composition and the second composition have a temperature in the range of 0 °C to 150 °C, preferably in the range of 10 °C to 100 °C, more preferably in the range of 15 °C to 70 °C, more preferably in the range of 20 °C to 50 °C.

[0129] The method involves mixing a first nanocrystal composition and a second composition. Once the second composition is added, a reaction generally occurs between the chalcogenide halide and the nanocrystals capped with the native ligands. Specifically, a reaction will take place to remove the native ligands from the surface of the nanocrystal. The native ligands are replaced by the chalcogenide halide.

[0130] As such, the contacting step preferably comprises exchanging at least a portion of the native ligands with the chalcogenide halide to form a second nanocrystal composition, wherein the nanocrystals comprise a crystalline core and a shell at least partially surrounding the crystalline core, wherein the shell comprises a chalcogenide halide according to Formula (I). Preferably, the shell surrounds the crystalline core.

[0131] It may be that the composition and structure of the chalcogenide halide in the second composition and the resultant chalcogenide halide in the shell are the same, or they may be different. In other words, the chalcogenide and halide compounds added to form the precursor mixture 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 shell. For example, the chalcogenide halide may be present as ions in solution in the second composition, but the chalcogenide halide may be crystalline in the shell of the nanocrystal. The chalcogenide halide may then react together and / or with the nanocrystal to form the shell surrounding the crystalline core, wherein the shell comprises amorphous or crystalline chalcogenide halide. In a particular embodiment, the shell may comprise crystalline chalcogenide halide. Preferably, the contacting step further comprises surface treating the nanocrystals of the second nanocrystal composition. Preferably, surface treating the nanocrystals of the second nanocrystal composition comprises contacting the nanocrystals with an amine selected from the group consisting of butylamine, amylamine, phenylethylamine, hexylamine, cysteine, and mixtures thereof, preferably wherein the amine is butylamine.

[0132] Preferably, the contacting step comprises exchanging at least 10 wt%, preferably at least 20 wt%, more preferably at least 30 wt%, more preferably at least 40 wt%, more preferably at least 50 wt%, more preferably at least 60 wt%, more preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 90 wt%, of the native ligands coordinated to the surface of the crystalline core with the chalcogenide halide as determined by XPS.

[0133] Preferably, after the contacting step, at least 40%, preferably at least 50%, preferably at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90%, of the surface of the crystalline core is covered by the shell. This value may be determined by surface-bulk characterisation techniques such as 2D NMR.

[0134] The first nanocrystal composition and second composition may be contacted (or reacted) in any suitable manner, typically by mixing in a suitable reaction vessel. Preferably, the contacting step comprises mixing the first nanocrystal composition with the second composition to form a mixture. Preferably, the contacting step comprises mixing the first nanocrystal composition and the second composition for a first length of time of more than 1 second. Preferably, the first length of time is more than 5 seconds, preferably more than 10 seconds, preferably more than 20 seconds, preferably more than 40 seconds, preferably more than 1 minute, preferably more than 2 minutes, preferably more than 5 minutes, preferably more than 10 minutes, preferably more than 30 minutes, preferably more than 1 hour, preferably more than 2 hours, preferably more than 4 hours, preferably more than 6 hours, preferably more than 8 hours, preferably more than 10 hours. 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.

[0135] Preferably, the contacting step comprises agitating the mixture. Agitating the mixture may comprise stirring the mixture, for example with a magnetic stirrer, or may comprise placing the mixture under vortex.

[0136] Preferably, the mixture has a temperature in the range of 0 °C to 150 °C, or in the range of 10 °C to 100 °C, or in the range of 15 °C to 70 °C, or in the range of 20 °C to 50 °C.

[0137] The method of the present invention may further be used for the preparation of inks comprising the nanocrystals prepared by the method of the fifth aspect. Preferably, the method further comprises (a) washing the resultant nanocrystal composition, and (b) dissolving the resultant nanocrystal composition in a second polar solvent to form a nanocrystal ink composition.

[0138] Preferably, step (a) comprises washing the resultant nanocrystal composition with acetone, ethyl acetate, methyl acetate or mixtures thereof. Preferably, the second polar solvent is selected from the group consisting of 2,6-difluoropyridine, gammabutyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof. Where the crystalline core of the nanocrystals comprises InAs, gamma-butyrolactone is a particularly preferred solvent, particularly when the chalcogenide halide is BiSI or BiSBr.

[0139] The method of the present invention may further be used for the preparation of devices comprising the nanocrystals and inks prepared by the method of the fifth aspect. Preferably, the method further comprises (c) applying the nanocrystal ink composition onto a substrate; and (d) heating the substrate. Preferably, step (c) comprises spin-coating the nanocrystal ink composition onto the substrate. EXAMPLES

[0140] Examples are described hereunder illustrating the methods according to the present disclosure.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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. Example 1 - Ligand exchange of PbS nanocrystals with BiSI ligand

[0146] The ligand exchange of the present invention is demonstrated in this example using lead sulphide nanocrystals comprising an organic native ligand, oleic acid, formed via the simple addition method. The chalcogenide halide in this example is the chalcogen halide BiSI.

[0147] A ligand exchange for PbS nanocrystals (the first nanocrystal composition) was performed using a BiSI precursor solution (the second composition) in DMF; in which 182 mmol BiCI3, Nal , and thiourea with 39 mmol of ammonium acetate were dissolved in 9 mL of dimethyl formamide (DMF). The solution was filtered through a 450 nm PES filter. Simultaneously, ~2 mg / mL solution of PbS was prepared in hexane and added to BiSI precursor solution followed by vortex for 2 min. A clear phase transfer was observed, which demonstrates the QDs phase transfer from non-polar hexane phase to polar DMF phase (Figure 1). A clear phase transfer also indicates the replacement of long chain ligands with inorganic BiSI ligands in ionic form. After the phase transfer, the DMF phase is separated and washed with acetone, which then is dissolved in 2,6-difluoropyridine. The resultant ink can be spin-coated on glass substrate or devices followed by heating to form BiSI crystal structure on the surface of CQDs. It should be noted that, even without heating the ligand should offer more p type characteristics with high mobility.

[0148] Figure 1 shows photos of the PbS nanocrystals from Example 1 and the InAs nanocrystals of Example 2 at different stages of the process. In particular, the photos under “Step 1” are taken after the phase transfer of the nanocrystals from the non-polar solvent (hexane) to the polar solvent (DMF). The “Step 2” photos are taken after the nanocrystals have been washed and purified. Finally, the “Step 3” photos show the nanocrystal inks dissolved in the second polar solvent (2,6- difluoropyridine) on which the absorption spectra analysis was done.

[0149] Figure 2 shows the absorption spectrum for the PbS nanocrystal ink composition produced by the above method. As evident, the composition shows excitonic features after ligand exchange, indicating the high quality of the colloidal quantum dot ink. The PbS nanocrystal ink shows an absorption peak at 1350 nm with a FWHM of 100 nm and a P / V of 3.7, which is similar to state-of-the-art PbS QDs.

[0150] 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.

[0151] Example 2 - Ligand exchange of InAs nanocrystals with BiSI ligand

[0152] The ligand exchange of the present invention is demonstrated in this example using indium arsenide nanocrystals comprising an organic native ligand, oleic acid, formed via the simple addition method. The chalcogenide halide in this example is the chalcogen halide BiSI. The method was the same as Example 1 , replacing the PbS nanocrystals with the InAs in the first composition.

[0153] Figure 3 shows the absorption spectrum of the resulting inks for InAs CQDs. As evident, the InAs nanocrystal ink composition also shows excitonic features after ligand exchange, again indicating the high quality of the nanocrystal ink. The InAs nanocrystal ink shows an absorption peak of 1352 with PA / of 1.22, which is similar to state-of-the-art InAs QDs. This invention will pave the way for next generation CQDs inks for high performance optoelectronic devices.

[0154] Example 3 - Determining the role of BiSI in the ligand exchange

[0155] As far as the inventors are aware, the use of BiSI as a ligand in the synthesis of nanocrystals has not been reported until now. As such, the inventors undertook to verify the role of the compounds in the second composition to determine whether BiSI was formed and that BiSI also acted as a ligand on the surface of the nanocrystals. In order to assess this, the inventors repeated the method of Example 1 to prepare each of Samples 3Ato 3E, varying the compounds included in the second composition for each of these samples as shown in Table 1 below. Table 1 also includes a summary of the optoelectronic properties of each sample. Table 1 : Summary of nanocrystals

[0156] Sample 3E did not result in a stable solution in DMF, and therefore the experiment could not be continued. On the other hand, Bil3solubilised well in DMF and a clear phase transfer was obtained. However, the resulting ink was found to be very dilute as the nanocrystals were not very soluble in 2,6-difluoropyridine. When the experiment was conducted with Bil3and thiourea (Sample 3C), the resulting QDs looked soluble and dark in colour, but it is believed that a thiourea bismuth iodide complex was formed. As a result, the complex was unable to passivate QDs efficiently, which resulted in a very dilute ink and QDs with poor solubility, which was also evident in the absorption spectroscopy data (FWHM of 138 nm, PA / of 1.65). When no thiourea was used (Sample 3B), the ligand exchange process was successful and the resulting QDs offered similar solubility to the control experiment (Sample 3A), however, the higher FWHM of 127 and lower P / V clearly indicated the compromise in QD quality. In the case of the control experiment (Sample 3A), when BiCI3, Nal, and thiourea were utilised, the resulting QDs showed the best colloidal and optical properties as evident in Figure 4 and Table 1. The BiSI ligand showed the lowest FWHM of 115 nm, and highest P / V of 3.25. More importantly, the control sample showed the longest aerobic stability of 10 days so far, in comparison to other candidates (Figure 5 and Table 2). The nanocrystals of Sample 3A have excellent aerobic stability as shown in Table 2 and Figure 5. The absorption peak position stayed the same for the ligand exchanged QDs when BiCI3, thiourea and Nal were used as a ligand, indicating the combination of all of them is needed to passivate the surface. As can be seen from Table 1 , Sample 3A has the highest PA / ratio.

[0157] Table 2: Aerobic stability data of Samples 3A to 3C

[0158] Example 4 - Optimisation of InAs nanocrystals with BiSI ligand - treatment with butylamine

[0159] As discussed above, the process for preparing InAs nanocrystals may be optimized by selecting preferred components for the second composition and the final solvent to produce the highest quality of ink possible. In particular, the inventors noted that ammonium acetate was a particularly useful stabilizing agent for use in the second composition, and the inventors sought to optimise its use for preparing InAs nanocrystals. However, it was also noted by the inventors that ammonium acetate was liable to undergo an undesired side reaction with BiCI3when BiCI3was used as the A-containing compound, resulting in a lower amount of bismuth available to form the ligand system on the surface of the crystalline core of the nanocrystals. Consequently, whilst the InAs nanocrystals exhibited good solubility and optoelectronic properties (and were therefore suitable for use in optoelectronic devices), the inventors believed that the InAs nanocrystals could be improved further.

[0160] The nanocrystals of this example were prepared by the method of Example 3, with the exception that Bil3was used instead of BiCI3(182 mmol) as the A-containing compound and no Nal was used. Furthermore, the method for preparing these nanocrystals included the additional step of adding 20 mmol of butylamine before the DMF phase is separated and washed.

[0161] The inventors found that, when the nanocrystals are prepared using Bil3as the A- containing compound, the side reaction with ammonium acetate is avoided. Furthermore, the combination of the use of Bil3and treatment with butylamine ensured that the surface of the nanocrystals were sufficiently passivated and the optoelectronic properties were improved when compared to the InAs nanocrystals of Example 2. The inventors have discovered that this method optimises the surface passivation of the surface of the crystalline core.

[0162] Figure 6 shows the absorption spectrum of InAs nanocrystals comprising the BiSI ligand system, wherein the nanocrystals have been prepared according to the method of this example. These nanocrystals exhibit improved colloidal stability, potentially due to the lack of the NaCI impurity and / or poorer surface passivation.

[0163] Example 5 - Optimisation of InAs nanocrystals with BiSI ligand - n,n-dibutylthiourea

[0164] The inventors have found that the InAs nanocrystals may be prepared by the use of n,n-dibutylthiourea as the Ch-containing compound in the second composition and that a different solvent may be used in which the solubility of the nanocrystals is significantly higher. In particular, the nanocrystals of this example were prepared by the method of Example 3, with the exception that n,n-dibutylthiourea was used as the Ch-containing compound instead of thiourea, Bil3was used as the A-containing compound instead of BiCI3, and gamma-butyrolactone was used as the final polar solvent for the ink instead of 2,6-difluoropyridine.

[0165] The solvent of this example (gamma-butyrolactone) was able to solubilise an amount of nanocrystals of this example which was 10 times higher than the amount of nanocrystals of Example 4 that the solvent of Example 4 (2,6- difluoropyridine) was able to solubilise. This increased solubility of the nanocrystals of this example is a major advantage for preparing nanocrystal inks of the present invention. This demonstrates the versatility of the method of the present invention. Furthermore, the nanocrystals of this example exhibit impressive optoelectronic properties as shown in Figure 7.

[0166] Example 6 - Fabrication of InAs photodiode device using BiSI ligand system

[0167] In order to demonstrate the suitability of the chalcogenide halide ligand system of the invention for use in a photodiode device, InAs nanocrystals comprising the BiSI ligand system were prepared according to the method of the invention.

[0168] Ink Fabrication

[0169] Ink was prepared using a BiSI precursor solution (the second composition) in DMF; in which 182 mmol BiCI3, Nal, and thiourea with 39 mmol of ammonium acetate were dissolved in 9 mL of dimethyl formamide (DMF). The solution was filtered through a 450 nm PES filter. Simultaneously, ~1 mg / mL solution of InAs QDs was prepared in hexane and added to BiSI precursor solution followed by vortex for 2 min. A clear phase transfer was observed, which demonstrates the QDs phase transfer from non-polar hexane phase to polar DMF phase (Figure 1). A clear phase transfer also indicates the replacement of long chain ligands with inorganic BiSI ligands in ionic form. After the phase transfer, the DMF phase is separated and washed with acetone, which then is dissolved in 2,6- difluoropyridine. The resultant ink can be spin-coated on glass substrate or devices followed by heating to form BiSI crystal structure on the surface of CQDs. It should be noted that, even without heating the ligand should offer more p type characteristics with high mobility.

[0170] Device fabrication

[0171] AZnO solution is spin-coated on a cleaned ITO substrate. A second layer of ZnO is then coated on top. The ZnO coated films are then transferred to the nitrogen filled glovebox for InAs ink fabrication (which was prepared as described above) InAs ink was spin-coated. The hole transport layer (HTL) is then coated on top of active layer. The devices are then transferred to a thermal evaporator for Au deposition under shadow masking. The device stack structure may be described as ITO / ETL / active layer (InAs-BiSI ink) / HTL / Au.

[0172] The inventors prepared a photodiode using InAs nanocrystals capped with the BiSI ligand system. These nanocrystals were prepared by the same method as Example 3, with the exception that Bil3was used as the A-containing compound instead of BiCI3.

[0173] It is evident from Figures 8 and 9 that photodiode devices comprising the InAs QDs of the present invention show a diode characteristic with an EQE spectrum showing the absorption feature of the QDs (-1360 nm) and EQE of 4.3% at -4 V of applied bias. The EQE of this device was similar to those from control devices fabricated from this batch. This novel class of ligand system (AChX) therefore may be used to prepare nanocrystals exhibiting desirable photodiode behaviour.

Claims

CLAIMS1. A nanocrystal comprising a crystalline core and a shell at least partially surrounding the crystalline core, wherein the shell comprises a chalcogenide halide according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

2. The nanocrystal of claim 1 , wherein the shell surrounds the crystalline core.

3. The nanocrystal of claim 1 or claim 2, wherein A is a metal selected from the group consisting of bismuth, indium and antimony and / or wherein Ch is sulfur or selenium.

4. The nanocrystal of any one of the preceding claims, wherein the chalcogenide halide is selected from the group consisting of BiSCI, BiSBr, BiSI, and mixtures thereof, or wherein the chalcogenide halide is BiSI or BiSBr, or wherein the chalcogenide halide is selected from the group consisting ofBiSeCI, BiSeBr, BiSei, and mixtures thereof, or wherein the chalcogenide halide is BiSei or BiSeBr, or wherein the chalcogenide halide is selected from the group consisting of InSCI, InSBr, InSI, InSeCI, InSeBr, InSei, AsSCI, AsSBr, AsSI, AsSeCI, AsSeBr, AsSel, and mixtures thereof.

5. The nanocrystal of any one of the preceding claims, wherein the crystalline core comprises or consists of (i) a metal element and (ii) a pnictogen element or a chalcogen element, optionally wherein the metal element is selected from the group consisting of indium, gallium, lead, silver and mixtures thereof.

6. The nanocrystal of any one of the preceding claims, wherein the crystalline core comprises or consists of (i) a metal element and (ii) a pnictogen element,optionally wherein the metal element is selected from the group consisting of indium, gallium and mixtures thereof, preferably wherein the crystalline core comprises or consists of InAs, InP, InSb, InAsP, InAsSb, GaAs, GaP, GaSb, GaAsP, GaAsSb, InGaAs, InGaP, InGaSb, InGaAsP or InGaAsSb, more preferably wherein the crystalline core comprises or consists of InAs.

7. The nanocrystal of any one of claims 1 to 5, wherein the crystalline core comprises or consists of (i) a metal element and (ii) a chalcogen element, preferably wherein the crystalline core comprises or consists of PbS, PbSe, Ag2S orAg2Se.

8. The nanocrystal of any one of the preceding claims, wherein the nanocrystal is a semiconductor nanocrystal, or wherein the nanocrystal is a quantum dot, or a colloidal quantum dot.

9. A nanocrystal composition comprising a plurality of nanocrystals according to any one of the preceding claims.

10. The nanocrystal composition of claim 9, wherein the nanocrystals 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, and / or wherein the nanocrystals have a relative size dispersion of less than 25%, or less than 22%, or less than 20%, or less than 17%, or less than 15%, or less than 12%, or less than 10%.

11. A nanocrystal ink composition comprising the nanocrystal composition of any one of claims 9 to 10 and a polar solvent, wherein the polar solvent is selected from the group consisting of 2,6-difluoropyridine, gamma-butyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof.

12. 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 a nanocrystal composition according to any one of claims 9 to 11 , 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, or 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, or 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.

13. A film comprising a nanocrystal composition according to any one of claims 9 to 11.

14. Use of compounds of Formula (I) as ligands for the preparation of nanocrystals, wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

15. 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 ligands coordinated to a surface of the crystalline core, and(ii) a second composition comprising a chalcogenide halide according to Formula (I), wherein Formula (I) is AChX, wherein A is a metal, wherein Ch is a chalcogen, and wherein X is a halide.

16. The method of claim 15, wherein A is a metal selected from the group consisting of bismuth, indium and antimony, and wherein Ch is sulfur or selenium, optionally wherein X is bromine or iodine.

17. The method of any one of claims 15 to 16, wherein the chalcogenide halide is selected from the group consisting of BiSCI, BiSBr, BiSI, and mixtures thereof, or wherein the chalcogenide halide is BiSI or BiSBr, or wherein the chalcogenide halide is selected from the group consisting of BiSeCI, BiSeBr, BiSei, and mixtures thereof, or wherein the chalcogenide halide is BiSei or BiSeBr.

18. The method of any one of claims 15 to 17, wherein the chalcogenide halide is selected from the group consisting of InSCI, InSBr, I nSI , InSeCI, InSeBr, InSei, AsSCI, AsSBr, AsSI, AsSeCI, AsSeBr, AsSel, and mixtures thereof.

19. The method of any one of claims 15 to 18, wherein the first nanocrystal composition further comprises a non-polar solvent, optionally wherein the non-polar solvent has a relative permittivity at 20 °C of less than 3, preferably less than 2.5, optionally wherein the non-polar solvent is selected from the group consisting of oleic acid, oleyl amine, butylamine, dioctylamine, octylamine, dichloromethane, pentane, cyclohexane, hexane, heptane, octane, toluene, tetrachoroethylene, trioctylphosphineheptadecane, chloroform, carbon tetrachloride, and mixtures thereof, or wherein the non-polar solvent comprises at least one of hexane, octane and toluene.

20. The method of any one of claims 15 to 19, wherein the second composition further comprises a polar solvent, optionally wherein the polar solvent has a relative permittivity at 20 °C of more than 3, preferably more than 5, preferably more than 10, and / or wherein the polar solvent is selected from the group consisting of acetonitrile, gamma-butyrolactone, ethyl acetate, difluoropyridine, butanol, ethylene glycol, sulfolane, dimethylformamide and mixtures thereof.21 . The method of any one of claims 15 to 20, further comprising forming the second composition, wherein forming the second composition comprises adding an A-containing compound and a Ch-containing compound to a polar solvent.

22. The method of claim 21 , wherein the A-containing compound is selected from the group consisting of Bi l3or BiCI3, BiBr3, Bil3and mixtures thereof, or wherein the A-containing compound is selected from the group consisting of InCi, I nCl2, lnCI3, InBr, lnBr2, lnBr3, Ini, lnl2, lnl3and mixtures thereof, or wherein the A-containing compound is selected from the group consisting of SbCI3, SbBr3, Sbl3and mixtures thereof.

23. The method of claim 21 or claim 22, wherein the Ch-containing compound is selected from the group consisting of thiourea, butylthiourea, bis(trialkylsilyl)sulphide compounds, wherein the alkyl is methyl, ethyl or propyl, thioacetamide, tri-n-octylphosphine sulphide, tributylphosphine sulphide, alkyl substituted thioamide compounds, elemental sulphur and mixtures thereof, or wherein the Ch-containing compound is selected from the group consisting of bis(trimethylsilyl)selenide, tri-n-octylphosphine selenide (TOPSe) and tributylphosphine selenide, or wherein the Ch-containing compound is selected from the group consisting of tri n-octylphosphine telluride, tellurium powder, and mixtures thereof.

24. The method of any one of claims 21 to 23, wherein 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, trimethylsilyl halide, and mixtures thereof, or 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, preferably wherein the stabilising agent comprises ammonium acetate.

25. The method of any one of claims 15 to 24, wherein the second composition comprises Bil3, butylthiourea and ammonium acetate.

26. The method of any one of claims 15 to 25, wherein the native ligand is an organic ligand, preferably wherein the native ligand is a C2-C24 organic compound comprising a functional group selected from the group consisting of amino, thiol, hydroxyl and carboxylic acid, preferably wherein the native ligand is a C6-C24 organic compound comprising a functional group selected from the group consisting of amino, thiol, hydroxyl and carboxylic acid, preferably wherein the native ligand is a compound selected from the group consisting of 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, alkylamines, dioctylamine, phosphonic acid, thiols and mixtures thereof, preferably wherein the native ligand is selected from the group consisting of oleic acid, oleylamine, 3-mercaptopropionic acid and mixtures thereof.

27. The method of any one of claims 15 to 26, wherein the contacting step comprises exchanging at least a portion of the native ligands with the chalcogenide halide to form a second nanocrystal composition, wherein the nanocrystals comprise a crystalline core and a shell at least partially surrounding the crystalline core, wherein the shell comprises a chalcogenide halide according to Formula (I).

28. The method of any one of claims 15 to 27, wherein the shell surrounds the crystalline core.

29. The method of any one of claims 15 to 28, wherein the contacting step further comprises surface treating the nanocrystals of the second nanocrystalcomposition, preferably wherein surface treating the nanocrystals of the second nanocrystal composition comprises contacting the nanocrystals with an amine selected from the group consisting of butylamine, amylamine, phenylethylamine, hexylamine, cysteine, and mixtures thereof, preferably wherein the amine is butylamine.

30. The method of any one of claims 15 to 29, wherein the contacting step comprises exchanging at least 10 wt%, preferably at least 20 wt%, more preferably at least 30 wt%, more preferably at least 40 wt%, more preferably at least 50 wt%, more preferably at least 60 wt%, more preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 90 wt%, of the native ligands coordinated to the surface of the crystalline core with the chalcogenide halide as determined by thermogravimetric analysis.

31. The method of any one of claims 15 to 30, wherein after the contacting step, at least 40%, preferably at least 50%, preferably at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90%, of the surface of the crystalline core is covered by the shell.

32. The method of any one of claims 15 to 31 , further comprising:(a) washing the resultant nanocrystal composition, and(b) dissolving the resultant nanocrystal composition in a second polar solvent to form a nanocrystal ink composition, preferably wherein step (a) comprises washing the resultant nanocrystal composition with acetone, ethyl acetate, methyl acetate or mixtures thereof and / or wherein the second polar solvent is selected from the group consisting of 2,6-difluoropyridine, gamma-butyrolactone, propylene carbonate, dimethylformamide, sulfolane and combinations thereof.

33. The method of claim 32, further comprising:(c) applying the nanocrystal ink composition onto a substrate; and(d) heating the substrate, preferably wherein step (c) comprises spin-coating the nanocrystal ink composition onto the substrate.

Images