Quantum dot shell synthesis

Abstract

The invention relates to synthesis of core / shell / shell quantum dots, in particular to a novel shell synthesis method.

Description

Technical Field

[0001] The present invention relates to the synthesis of core / shell / shell quantum dots, and more particularly to novel shell synthesis methods. Background Art

[0002] Core / shell / shell quantum dots have the property of absorbing blue light (and ultraviolet light) and emitting light at longer wavelengths (such as green or red light). Because they can be made into highly efficient light-emitting materials, where the emitted color can be adjusted by changing the crystal size, they can be used as down-convertor materials in lighting and LED displays. A particularly interesting sub-field of applications is in micro-LED displays, where each pixel contains blue, green, and red emission sources (so-called self-emitting screens), and where the pixel size is reduced to below 10 µm. At this length scale, these quantum dots have significant advantages over conventional down-convertor materials or native green and red emission materials because they are more efficient.

[0003] The first QDs incorporated into commercial displays were QDs containing cadmium selenide (CdSe), a direct bandgap semiconductor with emission that can be tuned across the visible spectrum by changing the size of the CdSe microcrystals. However, due to strict restrictions on the use of cadmium in consumer products, Cd-based quantum dots are generally considered unsuitable, and there has been a shift away from Cd-based quantum dots towards Cd-free alternatives such as indium phosphide (InP) QDs. The photo-thermal instability of InP-based QDs is addressed by the core / shell / shell structure.

[0004] In the case of metal-S / Se shells, the synthesis of the shell relies on the reaction between metal carboxylates and tertiary phosphine sulfides / selenides. However, this reaction is slow and often not robust due to the presence of impurities in commercially available secondary phosphine sulfides / selenides. This results in undesirable optical properties (low PLQY) and makes it difficult to scale up the process.

[0005] Therefore, there is a need in the art for a rapid and robust shell synthesis for producing core / shell / shell quantum dots with good optical properties (high PLQY). Description of the Invention

[0006] In one aspect, the present invention provides a method for preparing quantum dots, the method comprising the steps of:

[0007] (a) preparing a core of a binary, ternary or quaternary material, the binary, ternary or quaternary material comprising:

[0008] - one or more first core elements selected from the group consisting of In, Ga, and Al, and

[0009] - one or more second core elements selected from the group consisting of P, As, and Sb;

[0010] (b) forming a first layer on the core by contacting the core with a mixture comprising a metal precursor, a secondary phosphine selenide, and a tertiary phosphine selenide;

[0011] (c) forming a second layer on the first layer by contacting the product of step (b) with a mixture comprising a metal precursor, a secondary phosphine sulfide, and a tertiary phosphine sulfide.

[0012] This method can be referred to as the method according to the present invention or the method of the present invention.

[0013] Quantum dots prepared by the method according to the present invention are (semi)spherical nanoparticles comprising a core, a first layer on the core, and a second layer on the first layer. Such quantum dots can also be referred to as core / shell / shell quantum dots. It should be understood that quantum dots are different from quantum rods, which are elongated semiconductor nanoparticles.

[0014] Core / shell / shell quantum dots can be represented as core / first layer / second layer. For example, InP / ZnSe / Zn 1-x Cd x S refers to quantum dots comprising an InP core (i.e., a core containing InP or (mainly) consisting of InP), a ZnSe first layer (i.e., a first layer containing ZnSe or (mainly) consisting of ZnSe), and a Zn 1-x Cd x S second layer (i.e., an alloy containing Zn, Cd, and S or (mainly) consisting of an alloy of Zn, Cd, and S, wherein the molar ratios between the elements are as specified). In another example, InP / Zn(S,Se) / ZnS refers to quantum dots comprising an InP core, a Zn(S,Se) first layer (i.e., an alloy containing Zn, S, and Se or (mainly) consisting of an alloy of Zn, S, and Se, wherein the molar ratio of Zn to S + Se is substantially 1), and a ZnS second layer. In this document, the terms AB or ABC core, layer, or shell respectively refer to a core, layer, or shell containing AB or ABC or (mainly) consisting of AB or ABC.

[0015] The composition of the quantum dots, core, first layer, and second layer (i.e., the elements contained therein and their molar ratios) can be determined by performing EDX (energy-dispersive X-ray spectroscopy) on an ensemble of the quantum dots.

[0016] In the context of the present application, the quantum dots are capable of absorbing and emitting electromagnetic radiation, wherein the wavelength of the emitted radiation is greater than the wavelength of the absorbed radiation. Preferably, the absorbed radiation is within the visible spectrum ("visible light").

[0017] One advantage of the method according to the present invention is that quantum dots having good optical properties (i.e., having high photoluminescence) can be obtained in high yield at a relatively low temperature (e.g., 240 °C) through a robust and rapid reaction, which means that the method is cost-effective. In addition, the method enables good control of the shell thickness.

[0018] A measure suitable for photoluminescence is the "photoluminescence quantum yield" (PLQY), which is the ratio of the number of emitted photons that can be collected to the number of photons absorbed by the quantum dots. This PLQY can also be referred to as the internal PLQY, which is different from the external PLQY defined as the ratio of the total number of emitted photons to the number of photons provided to the quantum dots. Unless explicitly mentioned, PLQY refers to the internal PLQY herein.

[0019] In an embodiment, the method according to the present invention produces quantum dots having a PLQY of at least 85%, at least 85.5%, at least 86%, at least 86.5%, at least 87%, at least 87.5%, at least 88%, at least 88.5%, at least 89%, at least 89.5%, at least 90%, at least 90.5%, at least 91%, at least 91.5%, at least 92%, at least 92.5%, at least 93%, at least 93.5%, at least 94%, at least 94.5%, at least 95%, at least 95.5%, at least 96%, at least 96.5%, at least 97%, at least 97.5%, at least 98%, at least 98.5%, at least 99%, or at least 99.5%.

[0020] The method according to the present invention is also robust, rapid, and provides a high yield. Methods in the art for preparing core / shell / shell quantum dots typically rely only on tertiary phosphine selenides and sulfides during the formation of the first and second layers, respectively. Without being bound by this theory, since these substances are relatively inert, these methods are vulnerable to the presence of impurities in the phosphine (which are slightly more reactive), resulting in significant between-batch and within-batch differences in the shell composition and, consequently, significant between-batch and within-batch differences in properties such as photoluminescence. Such impurities are inevitable in commercial sources. In addition, the relative inertness of the tertiary phosphine results in slow reactions and low yields.

[0021] The present invention overcomes this problem by including secondary phosphine selenides and sulfides, which are more reactive, in addition to the less reactive tertiary phosphine selenides and sulfides, resulting in a faster (at a relatively low temperature, such as 240 °C), more robust (i.e., more uniform) synthesis with a higher overall yield, which is more suitable for scale-up. The higher yield at a relatively low temperature has better cost-effective economic benefits. Robustness means that the yield of the method is not vulnerable or not significantly affected by the presence of impurities or small changes in the reaction conditions.

[0022] The presence of the reactive secondary phosphine outweighs any reactive impurities in the tertiary phosphine. Without being bound by this theory, the secondary phosphine is used as a reactive substance during the formation of the first and second shells, respectively, while the tertiary phosphine is used as a reservoir for the reactive selenide and sulfide, respectively. It should be understood that this combination of the reactive secondary species and the inert tertiary species used as a sulfide / selenide reservoir can depend on a set of equilibrium reactions not further discussed herein.

[0023] The presence of more reactive phosphine during the reaction also makes it feasible to use relatively inert metal precursors (such as long-chain metal carboxylates containing more than 5 carbon atoms) and to perform a controlled synthesis of the second layer, which typically has a low rate and requires high temperature or long reaction times in the absence of secondary phosphine.

[0024] It should be understood that the application of the method according to the present invention generally produces multiple quantum dots. Wherever the properties of a single quantum dot are mentioned, preferably the average value of the properties of multiple quantum dots is referred to. The average value can be a quantity-weighted average or a mass-weighted average.

[0025] In an embodiment, the first layer and the second layer are (semi)spherical layers concentrically arranged around the core.

[0026] In an embodiment, the first layer surrounds the core, and the second layer surrounds the first layer.

[0027] In an embodiment, the first layer and the second layer are solid layers.

[0028] In an embodiment, the diameter of the quantum dots ranges from 5 up to a maximum of 30 nm, 29 nm, 28 nm, 27 nm, 26 nm, 25 nm, 24 nm, 23 nm, 22 nm, 21 nm, 20 nm, 19 nm, 18 nm, 17 nm, 16 nm, 15 nm, 14 nm, 13 nm, 12 nm, 11 nm, 10 nm, 9.5 nm, 9 nm, 8.5 nm, 8 nm. In an embodiment, the diameter of the quantum dots ranges from 6 up to a maximum of 30 nm, 29 nm, 28 nm, 27 nm, 26 nm, 25 nm, 24 nm, 23 nm, 22 nm, 21 nm, 20 nm, 19 nm, 18 nm, 17 nm, 16 nm, 15 nm, 14 nm, 13 nm, 12 nm, 11 nm, 10 nm, 9.5 nm, 9 nm, 8.5 nm, 8 nm. Quantum dots with an average diameter within this range can provide good optical properties for downconversion because the absorption coefficient at the wavelength corresponding to the pump light far exceeds the absorption coefficient at the wavelength corresponding to the emission of the quantum dots.

[0029] The core is a (semi)spherical semiconductor nanocrystal, whose optical and electronic properties are different from those of larger particles of the same material due to quantum mechanical effects. Although the core is not a core / shell / shell quantum dot, the core itself can be considered a quantum dot.

[0030] The core is a binary, ternary or quaternary material (or compound) (made of a binary, ternary or quaternary material (or compound)). Binary, ternary or quaternary materials are materials composed of 2, 3 or 4 different elements respectively. It should be understood that the order of the elements in the chemical formula of a ternary or quaternary material relates to convention and is independent of the composition of the material.

[0031] In an embodiment, the binary, ternary or quaternary material is a binary or ternary material.

[0032] In an embodiment, the binary, ternary or quaternary material is InP, InGaP, InAs, InSb or InSbAs.

[0033] In an embodiment, the binary material is InP, InAs, InSb, GaP, GaAs, GaSb, AlP, AlAs or AlSb.

[0034] In an embodiment, the ternary material is InPAs, InPSb, InAsSb, GaPAs, GaPSb, GaAsSb, AlPAs, AlPSb, AlAsSb, InGaP, InGaAs, InGaSb, InAlP, InAlAs, InAlSb, GaAlP, GaAlAs, or GaAlSb.

[0035] In an embodiment, the ternary material is InGaP or InSbAs.

[0036] In an embodiment, the quaternary material is InPAsSb, GaPAsSb, AlPAsSb, InGaPAs, InGaPSb, InGaAsSb, InAlPAs, InAlPSb, InAlAsSb, GaAlPAs, GaAlPSb, GaAlAsSb, InGaAlP, InGaAlAs, or InGaAlSb.

[0037] In an embodiment, the binary, ternary, or quaternary material is InP. Such nuclei are very attractive for downconverter purposes because when their diameter is from 2 nm to 4 nm, they emit light in the visible spectrum after irradiation with blue light (and ultraviolet light).

[0038] The preparation of the nuclei in step (a) included in the method according to the present invention can be carried out by any known technique. By way of example but not limitation, the nuclei can be synthesized by mixing a halide of each first nuclear element with a metal halide (preferably zinc halide) and injecting a precursor of the second nuclear element into the resulting mixture, preferably wherein the injection is carried out at a temperature of 150 °C to 250 °C, more preferably 150 °C to 200 °C. As an example, InP nuclei can be synthesized by mixing InCl3 and ZnCl2 in oleylamine and injecting a phosphor precursor (such as tris(diethylamino)phosphine) at a high temperature (180 °C).

[0039] In an embodiment, the preparation of the nuclei in step (a) included in the method according to the present invention is colloidal synthesis.

[0040] In an embodiment, the diameter of the nuclei is from 1 nm to 5 nm, preferably from 1.5 nm to 4.5 nm, more preferably from 2 nm to 4 nm. A suitable nuclear diameter ensures that the quantum dots emit light in the visible spectrum after irradiation with blue light (and ultraviolet light).

[0041] In an embodiment, the yield of step (a) included in the method according to the present invention is at least 90%, at least 90.5%, at least 91%, at least 91.5%, at least 92%, at least 92.5%, at least 93%, at least 93.5%, at least 94%, at least 94.5%, at least 95%, at least 95.5%, at least 96%, at least 96.5%, at least 97%, at least 97.5%, at least 98%, at least 98.5%, at least 99%, or at least 99.5%.

[0042] The first layer (which may also be referred to as the first shell, inner shell, or inner layer) is formed by contacting the core with a mixture comprising a metal precursor, a secondary phosphine selenide, and a tertiary phosphine selenide during step (b) of the method according to the present invention. The first layer is beneficial during the preparation of the quantum dots, but is not expected to have a significant impact on the optical properties of the quantum dots, as it creates a barrier thin enough to allow for rapid movement through tunneling of charge carriers between the second layer and the core.

[0043] The secondary phosphine selenide has the general formula structure R 1 R 2 P-Se, where R 1 and R 2 are organic groups, preferably where the bonds between P and R 1 and R 2 are formally phosphorous-carbon single bonds, respectively.

[0044] In an embodiment, R 1 and R 2 are hydrocarbon groups. Preferably, R 1 and R 2 are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl, or cycloalkynyl. More preferably, R 1 and R 2 are independently alkyl, cycloalkyl, or aryl.

[0045] In an embodiment, R 1 and R 2 are C 2-10 hydrocarbon groups. Preferably, R 1 and R 2 are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl, or C 3-10 cycloalkynyl. More preferably, R 1 and R 2 are independently C 2-10 alkyl, C 3-10 cycloalkyl, or C3-10 Aryl.

[0046] In an embodiment, R 1 and R 2 are C 2-6 hydrocarbon groups. Preferably, R 1 and R 2 are independently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 1 and R 2 are independently C 2-6 alkyl, C 3-6 cycloalkyl or C 3-6 aryl.

[0047] In an embodiment, R 1 and R 2 are the same. R 1 =R 2 The disubstitution of P can be with any group as defined above.

[0048] In an embodiment, R 1 and R 2 are different. R 1 and R 2 The asymmetric substitution of P can be with any two different groups as defined above.

[0049] In an embodiment, the secondary phosphine selenide is diphenylphosphine selenide, bis(2-norbornyl)phosphine selenide, diisobutylphosphine selenide, di-tert-butylphosphine selenide, dicyclopentylphosphine selenide, dicyclohexylphosphine selenide or 9-phosphabicyclononane selenide.

[0050] In an embodiment, the secondary phosphine selenide is diphenylphosphine selenide.

[0051] Aryl is defined herein as a monovalent aromatic hydrocarbon moiety. The C x-y moiety is defined herein as a moiety having a total number of carbon atoms ranging from x to y (including y).

[0052] The tertiary phosphine selenide has the general formula structure R 3 R 4 R 5 P=Se, where R 3 、R 4 and R 5 are organic moieties, preferably where P is bonded to R 3 、R4 and R 5 The bonds between them are formally phosphorus-carbon single bonds respectively.

[0053] In an embodiment, R 3 , R 4 and R 5 are hydrocarbon moieties. Preferably, R 3 , R 4 and R 5 are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl or cycloalkynyl. More preferably, R 3 , R 4 and R 5 are independently alkyl, cycloalkyl or aryl.

[0054] In an embodiment, R 3 , R 4 and R 5 are C 2-10 hydrocarbon moieties. Preferably, R 3 , R 4 and R 5 are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl or C 3-10 cycloalkynyl. More preferably, R 3 , R 4 and R 5 are independently C 2-10 alkyl, C 3-10 cycloalkyl or C 3-10 aryl.

[0055] In an embodiment, R 3 , R 4 and R 5 are C 2-6 hydrocarbon moieties. Preferably, R 3 , R 4 and R 5 are independently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 3 , R 4 and R 5 are independently C 2-6 alkyl, C 3-6 cycloalkyl or C3-6 Aryl.

[0056] In an embodiment, R 3 , R 4 and R 5 are the same. 3 =R 4 =R 5 The tri-substitution on P may be with any of the groups defined above.

[0057] In an embodiment, R 3 , R 4 and R 5 is different. 3 , R 4 and R 5 Asymmetric substitution of P may be with any three different groups as defined above.

[0058] In an embodiment, the tertiary phosphine selenide is tri-n-octyl phosphine selenide, triethyl phosphine selenide, tri-n-propyl phosphine selenide, tri-n-butyl phosphine selenide, tri-isobutyl phosphine selenide, tri-n-hexyl phosphine selenide, tri-tert-butyl (n-butyl) phosphine selenide, or triphenyl phosphine selenide.

[0059] In embodiments, the tertiary phosphine selenide is tri-n-octylphosphine selenide.

[0060] In an embodiment, in addition to secondary phosphine selenide and tertiary phosphine selenide, the mixture used in step (b) of the method according to the present invention also contains secondary phosphine sulfide and tertiary phosphine sulfide. Preferably, in the case of the second layer, the secondary phosphine sulfide and the tertiary phosphine sulfide can be as described in any of the following embodiments.

[0061] In an embodiment, the first shell comprises Zn(Se,S) or ZnSe 1-x S xAn alloy of Zn, S, and Se represented thereby or consisting thereof, where x is the molar ratio of the number of sulfur atoms to the total number of sulfur and selenium atoms. Such a first shell can be obtained by adding secondary phosphine sulfides and tertiary phosphine sulfides, in addition to secondary phosphine selenides and tertiary phosphine selenides, to the mixture used in step (b) of the method according to the invention. Preferably, x is equal to or greater than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99.Alternatively, x is equal to or less than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99.

[0062] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a metal carboxylate or a metal thiolate.

[0063] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a metal C 10-22 carboxylate, preferably C 10-22 zinc carboxylate or C 10-22 cadmium carboxylate, more preferably C 10-22 zinc carboxylate.

[0064] In the context of the present invention, a metal thiolate is any organic compound in which a metal ion is bonded or coordinated to a formally negatively charged sulfur atom and can thus be represented as M-S-R or M +- S-R, where M is a metal, S is a sulfur atom, and R is an organic moiety. For example, a metal thiolate can be, but is not limited to, a metal thiocarboxylate, a metal dithiocarboxylate, a metal thiocarbamate, or a metal dithiocarbamate.

[0065] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a metal dithiocarbamate or a metal thiocarbamate, preferably zinc dithiocarbamate, zinc thiocarbamate, cadmium dithiocarbamate or zinc thiocarbamate, more preferably zinc dithiocarbamate or cadmium dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, and / or the thiocarbamate is an alkylthiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl. In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a metal dithiocarbamate, preferably zinc dithiocarbamate or cadmium dithiocarbamate, more preferably zinc dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl.

[0066] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is zinc diethyldithiocarbamate.

[0067] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a metal oleate, stearate or myristate, preferably zinc oleate, cadmium oleate, zinc stearate, cadmium stearate, zinc myristate or cadmium myristate, more preferably zinc oleate, zinc stearate or zinc myristate.

[0068] In an alternative embodiment, the metal precursor used in step (b) of the method according to the present invention is a C 2-6 carboxylate, preferably a C 2-6 zinc carboxylate or a C 2-6 cadmium carboxylate. In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a C 2-4 carboxylate, preferably a C 2-4 zinc carboxylate or a C 2-4 cadmium carboxylate. In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a C 2-3 carboxylate, preferably a C 2-3 zinc carboxylate or a C 2-3 cadmium carboxylate. In an embodiment, the metal precursor used in step (b) of the method according to the present invention is an acetate, preferably zinc acetate or cadmium acetate.

[0069] In an embodiment, the metal precursor used in step (b) of the method according to the invention is a zinc or cadmium precursor, preferably a carboxylate or thiolate of zinc or cadmium. In a preferred embodiment, the metal precursor is a zinc precursor, preferably a carboxylate or thiolate of zinc. The properties of the metal determine the composition of the first layer. For example, if a zinc precursor is used, the first layer will contain or (primarily) consist of ZnSe, where Zn is derived from the zinc precursor and Se comes from secondary phosphine selenide and tertiary phosphine selenide. As another example, by using a mixture of a zinc precursor and a cadmium precursor, a first shell containing an alloy of Zn, Cd, and Se or (primarily) consisting of an alloy of Zn, Cd, and Se is obtained. In this case, the symbols Zn 1-x Cd x Se refer to the composition of the first shell, where the molar ratio Cd / (Cd + Zn) is x. This molar ratio is determined by the molar ratio Cd / (Cd + Zn) in the mixture of the zinc precursor and the cadmium precursor used in step (b) of the method according to the invention and preferably (substantially) equals the said molar ratio Cd / (Cd + Zn).

[0070] In an embodiment, the metal precursor used in step (b) of the method according to the invention is zinc oleate.

[0071] In an embodiment, the metal precursor used in step (b) of the method according to the invention is a mixture of a zinc precursor and a cadmium precursor, preferably a mixture of zinc carboxylate and cadmium carboxylate; more preferably C 10-22 zinc carboxylate and C 10-22 cadmium carboxylate; even more preferably a mixture of zinc oleate and cadmium oleate, or a mixture of zinc stearate and cadmium stearate, or a mixture of zinc myristate or cadmium myristate; most preferably a mixture of zinc oleate and cadmium oleate.

[0072] In an embodiment, the metal precursor used in step (b) of the method according to the invention is a mixture of a zinc precursor and a cadmium precursor as defined above, preferably a mixture of zinc carboxylate and cadmium carboxylate, where the molar ratio Cd / (Cd + Zn) (i.e., the mole fraction of Cd) in the mixture is from 0.001 to 1.0, more preferably from 0.02 to 0.2, and most preferably from 0.025 to 0.133.

[0073] In an alternative embodiment, the metal precursor used in step (b) of the method according to the invention is not a cadmium precursor or does not contain a cadmium precursor, preferably the metal precursor is a (pure) zinc precursor, preferably a (pure) zinc carboxylate or zinc thiolate.

[0074] In an embodiment, in the mixture used in step (b) of the method according to the present invention, the molar ratio of the number of molecules of the secondary phosphine selenide on the one hand to the total number of molecules of the secondary phosphine selenide and the tertiary phosphine selenide on the other hand is 1% to 90%, 1% to 85%, 1% to 80%, 1% to 75%, 1% to 70%, 1% to 65%, 1% to 60%, 1% to 55%, 1% to 50%, 1% to 45%, 1% to 40%, 1% to 35%, 1% to 30%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 1% to 5%, 5% to 90%, 10% to 90%, 15% to 90%, 20% to 90%, 25% to 90%, 30% to 90%, 35% to 90%, 40% to 90%, 45% to 90%, 50% to 90%, 55% to 90%, 60% to 90%, 65% to 90%, 70% to 90%, 75% to 90%, 80% to 90%, 85% to 90%, 5% to 15%, 10% to 20%, 15% to 25%, 20% to 30%, 25% to 35%, 30% to 40%, 35% to 45%, 40% to 50%, 45% to 55%, 50% to 60%, 55% to 65%, 60% to 70%, 65% to 75%, 70% to 80%, 75% to 85%, 80% to 90%, 85% to 95%, 5% to 25%, 10% to 30%, 15% to 35%, 20% to 40%, 25% to 45%, 30% to 50%, 35% to 55%, 40% to 60%, 45% to 65%, 50% to 70%, 55% to 75%, 60% to 80%, 65% to 85%, 70% to 90%, 75% to 95%, 5% to 35%, 10% to 40%, 15% to 45%, 20% to 50%, 25% to 55%, 30% to 60%, 35% to 65%, 40% to 70%, 45% to 75%, 50% to 80%, 55% to 85%, 60% to 90%, 65% to 95%.

[0075] In an embodiment, in addition to secondary phosphine selenides and tertiary phosphine selenides, the mixture used in step (b) of the method according to the invention further comprises secondary phosphine sulfides and tertiary phosphine sulfides. Preferably, in the mixture used in step (b) of the method according to the invention, the molar ratio of the number of molecules of secondary phosphine sulfides on the one hand to the total number of molecules of secondary phosphine sulfides and tertiary phosphine sulfides on the other hand is 1% to 90%, 1% to 85%, 1% to 80%, 1% to 75%, 1% to 70%, 1% to 65%, 1% to 60%, 1% to 55%, 1% to 50%, 1% to 45%, 1% to 40%, 1% to 35%, 1% to 30%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 1% to 5%, 5% to 90%, 10% to 90%, 15% to 90%, 20% to 90%, 25% to 90%, 30% to 90%, 35% to 90%, 40% to 90%, 45% to 90%, 50% to 90%, 55% to 90%, 60% to 90%, 65% to 90%, 70% to 90%, 75% to 90%, 80% to 90%, 85% to 90%, 5% to 15%, 10% to 20%, 15% to 25%, 20% to 30%, 25% to 35%, 30% to 40%, 35% to 45%, 40% to 50%, 45% to 55%, 50% to 60%, 55% to 65%, 60% to 70%, 65% to 75%, 70% to 80%, 75% to 85%, 80% to 90%, 85% to 95%, 5% to 25%, 10% to 30%, 15% to 35%, 20% to 40%, 25% to 45%, 30% to 50%, 35% to 55%, 40% to 60%, 45% to 65%, 50% to 70%, 55% to 75%, 60% to 80%, 65% to 85%, 70% to 90%, 75% to 95%, 5% to 35%, 10% to 40%, 15% to 45%, 20% to 50%, 25% to 55%, 30% to 60%, 35% to 65%, 40% to 70%, 45% to 75%, 50% to 80%, 55% to 85%, 60% to 90%, 65% to 95%.

[0076] In an embodiment, in addition to secondary phosphine selenides and tertiary phosphine selenides, the mixture used in step (b) of the method according to the present invention further comprises secondary phosphine sulfides and tertiary phosphine sulfides. Preferably, in the mixture used in step (b) of the method according to the present invention, the molar ratio of the total number of molecules of secondary phosphine sulfides and secondary phosphine selenides on the one hand to the total number of molecules of secondary phosphine sulfides, secondary phosphine selenides, tertiary phosphine sulfides and tertiary phosphine selenides on the other hand is 1% to 90%, 1% to 85%, 1% to 80%, 1% to 75%, 1% to 70%, 1% to 65%, 1% to 60%, 1% to 55%, 1% to 50%, 1% to 45%, 1% to 40%, 1% to 35%, 1% to 30%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 1% to 5%, 5% to 90%, 10% to 90%, 15% to 90%, 20% to 90%, 25% to 90%, 30% to 90%, 35% to 90%, 40% to 90%, 45% to 90%, 50% to 90%, 55% to 90%, 60% to 90%, 65% to 90%, 70% to 90%, 75% to 90%, 80% to 90%, 85% to 90%, 5% to 15%, 10% to 20%, 15% to 25%, 20% to 30%, 25% to 35%, 30% to 40%, 35% to 45%, 40% to 50%, 45% to 55%, 50% to 60%, 55% to 65%, 60% to 70%, 65% to 75%, 70% to 80%, 75% to 85%, 80% to 90%, 85% to 95%, 5% to 25%, 10% to 30%, 15% to 35%, 20% to 40%, 25% to 45%, 30% to 50%, 35% to 55%, 40% to 60%, 45% to 65%, 50% to 70%, 55% to 75%, 60% to 80%, 65% to 85%, 70% to 90%, 75% to 95%, 5% to 35%, 10% to 40%, 15% to 45%, 20% to 50%, 25% to 55%, 30% to 60%, 35% to 65%, 40% to 70%, 45% to 75%, 50% to 80%, 55% to 85%, 60% to 90%, 65% to 95%.

[0077] In an embodiment, the molar ratio between the number of Se atoms contained in the first layer and the total number of Zn atoms and Cd atoms is from 0.50 to 1.50, from 0.55 to 1.45, from 0.60 to 1.40, from 0.65 to 1.35, from 0.70 to 1.30, from 0.75 to 1.25, from 0.80 to 1.20, from 0.85 to 1.15, from 0.90 to 1.10, from 0.95 to 1.05, from 0.96 to 1.04, from 0.97 to 1.03, from 0.98 to 1.02, from 0.99 to 1.01. This ratio can be determined by performing EDX (Energy Dispersive X-ray Spectroscopy) on an ensemble of quantum dots.

[0078] In a further embodiment, the molar ratio between the total number of Se atoms and S atoms on the one hand and the total number of Zn atoms and Cd atoms on the other hand, contained in the first layer, is from 0.50 to 1.50, from 0.55 to 1.45, from 0.60 to 1.40, from 0.65 to 1.35, from 0.70 to 1.30, from 0.75 to 1.25, from 0.80 to 1.20, from 0.85 to 1.15, from 0.90 to 1.10, from 0.95 to 1.05, from 0.96 to 1.04, from 0.97 to 1.03, from 0.98 to 1.02, from 0.99 to 1.01.

[0079] In an embodiment, the thickness of the first layer is at most 1.0 nm, preferably from 0.1 nm to 0.9 nm, more preferably from 0.2 nm to 0.8 nm. The first layer prevents the growth of the second layer (e.g., CdSe) on the core (e.g., InP). In order to obtain this effect and have good optical properties for downconversion, the optimal thickness of the first layer is within this range.

[0080] Another advantage of the method according to the invention is that it provides good control over the shell thickness, in particular over the thickness of the first layer. In an embodiment, the standard deviation of the thickness of the first layer of a plurality of quantum dots prepared by the method according to the invention is equal to or less than 0.10, preferably equal to or less than 0.05, more preferably equal to or less than 0.025.

[0081] In an embodiment, after a reaction time of 30 minutes, the yield of step (b) included in the method according to the present invention is at least 60%, at least 60.5%, at least 61%, at least 61.5%, at least 62%, at least 62.5%, at least 63%, at least 63.5%, at least 64%, at least 64.5%, at least 65%, at least 65.5%, at least 66%, at least 66.5%, at least 67%, at least 67.5%, at least 68%, at least 68.5%, at least 69%, at least 69.5%, at least 70%, at least 70.5%, at least 71%, at least 71.5%, at least 72%, at least 72.5%, at least 73%, at least 73.5%, at least 74%, at least 74.5%, at least 75%, at least 75.5%, at least 76%, at least 76.5%, at least 77%, at least 77.5%, at least 78%, at least 78.5%, at least 79%, at least 79.5%, at least 80%, at least 80.5%, at least 81%, at least 81.5%, at least 82%, at least 82.5%, at least 83%, at least 83.5%, at least 84%, at least 84.5%, or at least 85%. As explained above, one advantage of the method according to the present invention is that a high yield can be obtained during the synthesis of the first layer. In an embodiment, the secondary phosphine selenide in the mixture used in step (b) of the method according to the present invention is generated in situ after adding secondary phosphine and selenium to the mixture used in step (b).

[0082] In an embodiment, the secondary phosphine selenide in the mixture used in step (b) of the method according to the present invention is added to the mixture used in step (b) as such and in crystalline form.

[0083] In an embodiment, in addition to secondary phosphine selenide and tertiary phosphine selenide, the mixture used in step (b) of the method according to the present invention further contains secondary phosphine sulfide and tertiary phosphine sulfide. Preferably, the secondary phosphine sulfide is generated in situ after adding secondary phosphine and sulfur to the mixture used in step (b). Alternatively, the secondary phosphine is added to the mixture used in step (b) as such and in crystalline form.

[0084] The second layer (which may also be referred to as the second shell, outer shell or outer layer) is formed during step (c) of the method according to the present invention by contacting the core with a mixture comprising a metal precursor, secondary phosphine sulfide and tertiary phosphine sulfide. Forming a high-quality second layer is crucial for obtaining quantum dots with a high photoluminescence quantum yield (PLQY).

[0085] The secondary phosphine sulfide has a general formula structure R 6 R 7 P-S, where R 6 and R 7 are organic moieties, preferably where P is bonded to R6 and R 7 The bonds between them are formally phosphorus-carbon single bonds respectively.

[0086] In an embodiment, R 6 and R 7 are hydrocarbon moieties. Preferably, R 6 and R 7 are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl or cycloalkynyl. More preferably, R 6 and R 7 are independently alkyl, cycloalkyl or aryl.

[0087] In an embodiment, R 6 and R 7 are C 2-10 hydrocarbon moieties. Preferably, R 6 and R 7 are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl or C 3-10 cycloalkynyl. More preferably, R 6 and R 7 are independently C 2-10 alkyl, C 3-10 cycloalkyl or C 3-10 aryl.

[0088] In an embodiment, R 6 and R 7 are C 2-6 hydrocarbon moieties. Preferably, R 6 and R 7 are independently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 6 and R 7 are independently C 2-6 alkyl, C 3-6 cycloalkyl or C 3-6 aryl.

[0089] In an embodiment, R 6 and R 7 are the same. R 1 =R 2 The disubstitution of P can be with any group as defined above.

[0090] In an embodiment, R 6 and R 7 are different. The asymmetric substitution of P by R 6 and R 7 can be carried out with any two different groups as defined above.

[0091] In an embodiment, the secondary phosphine sulfide is diphenylphosphine sulfide, bis(2-norbornyl)phosphine sulfide, diisobutylphosphine sulfide, di-tert-butylphosphine sulfide, dicyclopentylphosphine sulfide, dicyclohexylphosphine sulfide, or 9-phosphabicyclononane sulfide.

[0092] In an embodiment, the secondary phosphine sulfide is diphenylphosphine sulfide.

[0093] The tertiary phosphine sulfide has the general formula structure R 8 R 9 R 10 P=S, where R 8 、R 9 and R 10 are organic moieties, preferably where the bonds between P and R 8 、R 9 and R 10 are formally phosphorous-carbon single bonds, respectively.

[0094] In an embodiment, R 8 、R 9 and R 10 are hydrocarbon moieties. Preferably, R 8 、R 9 and R 10 are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl, or cycloalkynyl. More preferably, R 8 、R 9 and R 10 are independently alkyl, cycloalkyl, or aryl.

[0095] In an embodiment, R 8 、R 9 and R 10 are C 2-10 hydrocarbon moieties. Preferably, R 8 、R 9 and R 10 are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl, or C 3-10 cycloalkynyl. More preferably, R 8 、R9 and R 10 are independently C 2-10 alkyl, C 3-10 cycloalkyl or C 3-10 aryl.

[0096] In an embodiment, R 8 , R 9 and R 10 are C 2-6 hydrocarbon groups. Preferably, R 8 , R 9 and R 10 are independently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 8 , R 9 and R 10 are independently C 2-6 alkyl, C 3-6 cycloalkyl or C 3-6 aryl.

[0097] In an embodiment, R 8 , R 9 and R 10 are the same. R 8 = R 9 = R 10 The trisubstitution of P can be with any of the groups defined above.

[0098] In an embodiment, R 8 , R 9 and R 10 are different. R 8 , R 9 and R 10 The asymmetric substitution of P can be with any three different groups as defined above.

[0099] In an embodiment, the tertiary phosphine sulfide is tri-n-octylphosphine sulfide (TOP-S), triethylphosphine sulfide, tri-n-propylphosphine sulfide, tri-n-butylphosphine sulfide, triisobutylphosphine sulfide, tri-n-hexylphosphine sulfide, tri-tert-butyl(butyl)phosphine sulfide.

[0100] In an embodiment, the tertiary phosphine sulfide is tri-n-octylphosphine sulfide (TOP-S).

[0101] In an embodiment, the metal precursor used in step (c) of the method according to the present invention is a metal carboxylate or a metal thiolate.

[0102] In an embodiment, the metal precursor used in step (c) of the method according to the invention is metal C 10-22 carboxylate, preferably C 10-22 zinc carboxylate or C 10-22 cadmium carboxylate, more preferably C 10-22 zinc carboxylate.

[0103] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a metal dithiocarbamate or a metal thiocarbamate, preferably zinc dithiocarbamate, zinc thiocarbamate, cadmium dithiocarbamate or zinc thiocarbamate, more preferably zinc dithiocarbamate or cadmium dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, and / or the thiocarbamate is an alkylthiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl.

[0104] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a metal dithiocarbamate, preferably zinc dithiocarbamate or cadmium dithiocarbamate, more preferably zinc dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl.

[0105] In an embodiment, the metal precursor used in step (c) of the method according to the invention is zinc diethyldithiocarbamate.

[0106] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a metal oleate, stearate or myristate, preferably zinc oleate, cadmium oleate, zinc stearate, cadmium stearate, zinc myristate or cadmium myristate, more preferably zinc oleate, zinc stearate or zinc myristate.

[0107] In an alternative embodiment, the metal precursor used in step (c) of the method according to the invention is C 2-6 carboxylate, preferably C 2-6 zinc carboxylate or C 2-6 cadmium carboxylate. In an embodiment, the metal precursor used in step (c) of the method according to the invention is C 2-4 carboxylate, preferably C 2-4 zinc carboxylate or C 2-4 cadmium carboxylate. In an embodiment, the metal precursor used in step (c) of the method according to the invention is C 2-3 carboxylate, preferably C2-3 Zinc carboxylate or C 2-3 cadmium carboxylate. In an embodiment, the metal precursor used in step (c) of the method according to the invention is an acetate, preferably zinc acetate or cadmium acetate.

[0108] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a zinc or cadmium precursor, preferably a carboxylate or thiolate of zinc or cadmium. In a preferred embodiment, the metal precursor is a zinc precursor, preferably zinc carboxylate or zinc thiolate. The properties of the metal determine the composition of the second layer. For example, if a zinc precursor is used, the second layer will contain ZnS or (mainly) consist of ZnS, where Zn is derived from the zinc precursor and S comes from secondary phosphine sulfide and tertiary phosphine sulfide. As another example, by using a mixture of a zinc precursor and a cadmium precursor, a second shell containing an alloy of Zn, Cd, and S or (mainly) consisting of an alloy of Zn, Cd, and S is obtained. In this case, the symbols Zn 1-x Cd x S refer to the composition of the second shell, where the molar ratio Cd / (Cd + Zn) is x. This molar ratio is determined by the molar ratio Cd / (Cd + Zn) in the mixture of the zinc precursor and the cadmium precursor used in step (c) of the method according to the invention and is preferably (substantially) equal to said molar ratio Cd / (Cd + Zn).

[0109] In an embodiment, the metal precursor used in step (c) of the method according to the invention is zinc oleate.

[0110] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a mixture of a zinc precursor and a cadmium precursor, preferably a mixture of zinc carboxylate and cadmium carboxylate; preferably C 10-22 zinc carboxylate and C 10-22 a mixture of cadmium carboxylate; more preferably a mixture of zinc oleate and cadmium oleate, or a mixture of zinc stearate and cadmium stearate, or a mixture of zinc myristate or cadmium myristate; most preferably a mixture of zinc oleate and cadmium oleate.

[0111] In an embodiment, the metal precursor used in step (c) of the method according to the invention is a mixture of a zinc precursor and a cadmium precursor as defined above, preferably a mixture of zinc carboxylate and cadmium carboxylate, wherein the molar ratio Cd / (Cd + Zn) (i.e., the mole fraction of Cd) in the mixture is from 0.001 to 1.0, preferably from 0.02 to 0.2, more preferably from 0.025 to 0.133.

[0112] In an alternative embodiment, the metal precursor used in step (c) of the method according to the invention is not a cadmium precursor or does not contain a cadmium precursor, preferably the metal precursor is a (pure) zinc precursor, preferably (pure) zinc carboxylate or zinc thiolate.

[0113] In an embodiment, in the mixture used in step (c) of the method according to the present invention, the molar ratio of the number of molecules of secondary phosphine sulfide on the one hand to the total number of molecules of secondary phosphine sulfide and tertiary phosphine sulfide on the other hand is from 1% to 90%, 1% to 85%, 1% to 80%, 1% to 75%, 1% to 70%, 1% to 65%, 1% to 60%, 1% to 55%, 1% to 50%, 1% to 45%, 1% to 40%, 1% to 35%, 1% to 30%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 1% to 5%, 5% to 90%, 10% to 90%, 15% to 90%, 20% to 90%, 25% to 90%, 30% to 90%, 35% to 90%, 40% to 90%, 45% to 90%, 50% to 90%, 55% to 90%, 60% to 90%, 65% to 90%, 70% to 90%, 75% to 90%, 80% to 90%, 85% to 90%, 5% to 15%, 10% to 20%, 15% to 25%, 20% to 30%, 25% to 35%, 30% to 40%, 35% to 45%, 40% to 50%, 45% to 55%, 50% to 60%, 55% to 65%, 60% to 70%, 65% to 75%, 70% to 80%, 75% to 85%, 80% to 90%, 85% to 95%, 5% to 25%, 10% to 30%, 15% to 35%, 20% to 40%, 25% to 45%, 30% to 50%, 35% to 55%, 40% to 60%, 45% to 65%, 50% to 70%, 55% to 75%, 60% to 80%, 65% to 85%, 70% to 90%, 75% to 95%, 5% to 35%, 10% to 40%, 15% to 45%, 20% to 50%, 25% to 55%, 30% to 60%, 35% to 65%, 40% to 70%, 45% to 75%, 50% to 80%, 55% to 85%, 60% to 90%, 65% to 95%.

[0114] In an embodiment, the molar ratio between the number of S atoms comprised in the second layer and the total number of Zn atoms and Cd atoms is from 0.50 to 1.50, 0.55 to 1.45, 0.60 to 1.40, 0.65 to 1.35, 0.70 to 1.30, 0.75 to 1.25, 0.80 to 1.20, 0.85 to 1.15, 0.90 to 1.10, 0.95 to 1.05, 0.96 to 1.04, 0.97 to 1.03, 0.98 to 1.02, 0.99 to 1.01. This ratio can be determined by performing EDX (Energy Dispersive X-ray Spectroscopy) on an ensemble of quantum dots.

[0115] In an embodiment, the thickness of the second layer is at most 10 nm, preferably from 1 nm to 10 nm.

[0116] In an embodiment, the ratio between the volume of the second layer and the volume of the core is from 10 to 50, from 15 to 45, from 15 to 40, from 15 to 35, from 15 to 30, or from 15 to 25. If the volume of the second layer increases and thus the volume of the quantum dots increases, the absorption per quantum dot also increases.

[0117] Another advantage of the method according to the invention is that it provides good control over the shell thickness, in particular over the thickness of the second layer. In an embodiment, the standard deviation of the thickness of the second layer of a plurality of quantum dots prepared by the method according to the invention is equal to or less than 0.10, preferably equal to or less than 0.05, more preferably equal to or less than 0.025.

[0118] In an embodiment, after a reaction time of 10 minutes, the yield of step (c) included in the method according to the invention is at least 60%, at least 60.5%, at least 61%, at least 61.5%, at least 62%, at least 62.5%, at least 63%, at least 63.5%, at least 64%, at least 64.5%, at least 65%, at least 65.5%, at least 66%, at least 66.5%, at least 67%, at least 67.5%, at least 68%, at least 68.5%, at least 69%, at least 69.5%, at least 70%, at least 70.5%, at least 71%, at least 71.5%, at least 72%, at least 72.5%, at least 73%, at least 73.5%, at least 74%, at least 74.5%, at least 75%, at least 75.5%, at least 76%, at least 76.5%, at least 77%, at least 77.5%, at least 78%, at least 78.5%, at least 79%, at least 79.5%, at least 80%, at least 80.5%, at least 81%, at least 81.5%, at least 82%, at least 82.5%, at least 83%, at least 83.5%, at least 84%, at least 84.5%, or at least 85%. As explained above, one advantage of the method according to the invention is that a high yield can be obtained during the synthesis of the second layer.

[0119] In an embodiment, the secondary phosphine sulfide in the mixture used in step (c) of the method according to the invention is generated in situ after adding the secondary phosphine and sulfur to the mixture used in step (c).

[0120] In an embodiment, the secondary phosphine sulfide in the mixture used in step (c) of the method according to the invention is added to the mixture used in step (c) as such and in crystalline form. In an embodiment, the secondary phosphine selenide and the secondary phosphine sulfide share the same general formula structure R 1 sec R 2 sec P-X, where X is S or Se. In these embodiments, R 1=R 6 =R 1 sec and R 2 =R 7 =R 2 sec , where R 1 、R 2 、R 6 and R 7 have been defined above.

[0121] In an embodiment, R 1 sec and R 2 sec are hydrocarbon moieties. Preferably, R 1 sec and R 2 sec are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl or cycloalkynyl. More preferably, R 1 sec and R 2 sec are independently alkyl, cycloalkyl or aryl.

[0122] In an embodiment, R 1 sec and R 2 sec are C 2-10 hydrocarbon moieties. Preferably, R 1 sec and R 2 sec are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl or C 3-10 cycloalkynyl. More preferably, R 1 sec and R 2 sec are independently C 2-10 alkyl, C 3-10 cycloalkyl or C 3-10 aryl.

[0123] In an embodiment, R 1 sec and R 2 sec are C 2-6 hydrocarbon moieties. Preferably, R 1 sec and R 2 secIndependently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 1 sec and R 2 sec are independently C 2-6 alkyl, C 3-6 cycloalkyl or C 3-6 aryl.

[0124] In an embodiment, R 1 sec and R 2 sec are the same. R 1 sec =R 2 sec The disubstitution of P can be with any group as defined above.

[0125] In an embodiment, R 1 and R 2 are different. R 1 sec =R 2 sec The asymmetric substitution of P can be with any two different groups as defined above.

[0126] In an embodiment, the secondary phosphine contained in the secondary phosphine selenide and sulfide is diphenylphosphine (DPP), bis(2-norbornyl)phosphine, diisobutylphosphine, di-tert-butylphosphine, dicyclopentylphosphine, dicyclohexylphosphine, or 9-phosphabicyclononane.

[0127] In an embodiment, the secondary phosphine contained in the secondary phosphine selenide and sulfide is diphenylphosphine (DPP).

[0128] In an embodiment, the tertiary phosphine selenide and tertiary phosphine sulfide share the same general formula structure R 1 tert R 2 tert R 2 tert P=X, where X is S or Se. In these embodiments, R 3 =R 8 =R 1 tert ,R 4 =R 9 =R 2 tert ,and R5 =R 10 =R 3 tert , where R 3 、R 4 、R 5 、R 8 、R 9 and R 10 have been defined above.

[0129] In an embodiment, R 1 tert 、R 2 tert and R 3 tert is a hydrocarbyl moiety. Preferably, R 1 tert 、R 2 tert and R 3 tert are independently alkyl, cycloalkyl, aryl, alkenyl, cycloalkenyl, alkynyl or cycloalkynyl. More preferably, R 1 tert 、R 2 tert and R 3 tert are independently alkyl, cycloalkyl or aryl.

[0130] In an embodiment, R 1 tert 、R 2 tert and R 3 tert is a C 2-10 hydrocarbyl moiety. Preferably, R 1 tert 、R 2 tert and R 3 tert are independently C 2-10 alkyl, C 3-10 cycloalkyl, C 3-10 aryl, C 2-10 alkenyl, C 3-10 cycloalkenyl, C 2-10 alkynyl or C 3-10 cycloalkynyl. More preferably, R 1 tert 、R 2 tert and R 3 tert are independently C 2-10 alkyl, C 3-10 cycloalkyl or C 3-10 aryl.

[0131] In an embodiment, R 1 tert 、R 2 tert and R 3 tert are C 2-6 hydrocarbon groups. Preferably, R 1 tert 、R 2 tert and R 3 tert are independently C 2-6 alkyl, C 3-6 cycloalkyl, C 3-6 aryl, C 2-6 alkenyl, C 3-6 cycloalkenyl, C 2-6 alkynyl or C 3-6 cycloalkynyl. More preferably, R 1 tert 、R 2 tert and R 3 tert are independently C 2-6 alkyl, C 3-6 cycloalkyl or C 3-6 aryl.

[0132] In an embodiment, R 1 tert 、R 2 tert and R 3 tert are the same. R 1 tert =R 2 tert =R 3 tert The trisubstitution of P can be with any of the groups defined above.

[0133] In an embodiment, R 1 tert 、R 2 tert and R 3 tert are different. R 1 tert 、R 2 tert and R 3 tert The asymmetric substitution of P can be with any three different groups defined above.

[0134] In an embodiment, the tertiary phosphines included in the tertiary phosphine selenides and sulfides are tri-n-octylphosphine (TOP), triethylphosphine, tri-n-propylphosphine, tri-n-butylphosphine, triisobutylphosphine, tri-n-hexylphosphine, di-tert-butyl(n-butyl)phosphine.

[0135] In an embodiment, the tertiary phosphine included in the tertiary phosphine selenides and sulfides is tri-n-octylphosphine (TOP).

[0136] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both metal carboxylates and metal thiolates.

[0137] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both metal C 10-22 carboxylates, preferably C 10-22 zinc carboxylate or C 10-22 cadmium carboxylate, more preferably C 10-22 zinc carboxylate.

[0138] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both metal dithiocarbamates or metal thiocarbamates, preferably zinc dithiocarbamate, zinc thiocarbamate, cadmium dithiocarbamate or zinc thiocarbamate, more preferably zinc dithiocarbamate or cadmium dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, and / or the thiocarbamate is an alkylthiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl.

[0139] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both metal dithiocarbamates, preferably zinc dithiocarbamate or cadmium dithiocarbamate, more preferably zinc dithiocarbamate. Preferably, the dithiocarbamate is a dialkyldithiocarbamate, wherein each of the alkyl groups is independently C 1-10 alkyl, preferably C 2-5 alkyl, more preferably ethyl.

[0140] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both zinc diethyldithiocarbamate.

[0141] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both metal oleates, stearates or myristates, preferably zinc oleate, cadmium oleate, zinc stearate, cadmium stearate, zinc myristate or cadmium myristate, more preferably zinc oleate, zinc stearate or zinc myristate.

[0142] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both zinc or cadmium precursors, preferably zinc precursors. In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both carboxylates of zinc or cadmium, preferably zinc carboxylate. In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both thiolates of zinc or cadmium, preferably zinc thiolate.

[0143] In an embodiment, the metal precursor used in steps (b) and (c) of the method according to the present invention is zinc oleate.

[0144] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both a mixture of zinc precursor and cadmium precursor, preferably a mixture of zinc carboxylate and cadmium carboxylate; more preferably a mixture of C 10-22 zinc carboxylate and C 10-22 cadmium carboxylate; even more preferably a mixture of zinc oleate and cadmium oleate, or a mixture of zinc stearate and cadmium stearate, or a mixture of zinc myristate or cadmium myristate; most preferably a mixture of zinc oleate and cadmium oleate.

[0145] In an embodiment, the metal precursors used in steps (b) and (c) of the method according to the present invention are both a mixture of zinc precursor and cadmium precursor as defined above, preferably a mixture of zinc carboxylate and cadmium carboxylate, wherein the molar ratio of Cd / (Cd + Zn) in each of these mixtures is from 0.001 to 1.0, more preferably from 0.02 to 0.2, and most preferably from 0.025 to 0.133.

[0146] In an embodiment, the metal precursor used in step (b) of the method according to the present invention is a zinc precursor, preferably zinc carboxylate, and the metal carboxylate used in step (c) of the method according to the present invention is a mixture of zinc precursor and cadmium precursor, preferably a mixture of zinc carboxylate and cadmium carboxylate. In these embodiments, the precursors are preferably C 10-22 carboxylates; more preferably oleates, stearates or myristates; most preferably oleates. The molar ratio of Cd / (Cd + Zn) in the mixture is preferably from 0.001 to 1.0, preferably from 0.02 to 0.2, and more preferably from 0.025 to 0.133.

[0147] In an embodiment, the core is InP, the metal precursor used in step (b) is zinc carboxylate, preferably zinc oleate, and the metal precursor used in step (c) is a mixture of zinc carboxylate and cadmium carboxylate, preferably zinc oleate and cadmium oleate, preferably wherein the molar ratio of Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InP / ZnSe / Zn 1-x Cdx S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0148] Such InP / ZnSe / ZnS quantum dots are very efficient and narrow emitters in the visible spectrum, making them useful as, for example, luminescent down-converters in LED displays.

[0149] In an embodiment, the core is InGaP, the metal precursor used in step (b) is zinc carboxylate, preferably zinc oleate, and the metal precursors used in step (c) are a mixture of zinc carboxylate and cadmium carboxylate, preferably zinc oleate and cadmium oleate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InGaP / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0150] In an embodiment, the core is InAs, the metal precursor used in step (b) is zinc carboxylate, preferably zinc oleate, and the metal precursors used in step (c) are a mixture of zinc carboxylate and cadmium carboxylate, preferably zinc oleate and cadmium oleate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InAs / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0151] In an embodiment, the core is InSb, the metal precursor used in step (b) is zinc carboxylate, preferably zinc oleate, and the metal precursors used in step (c) are a mixture of zinc carboxylate and cadmium carboxylate, preferably zinc oleate and cadmium oleate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InSb / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0152] In an embodiment, the core is InSbAs, the metal precursor used in step (b) is zinc carboxylate, preferably zinc oleate, and the metal precursors used in step (c) are a mixture of zinc carboxylate and cadmium carboxylate, preferably zinc oleate and cadmium oleate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InSbAs / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0153] In an embodiment, the core is InP, the metal precursor used in step (b) is zinc thiolate, and the metal precursors used in step (c) are a mixture of zinc thiolate and cadmium thiolate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InP / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0154] In an embodiment, the core is InGaP, the metal precursor used in step (b) is thiolate, and the metal precursors used in step (c) are a mixture of zinc thiolate and cadmium thiolate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InGaP / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0155] In an embodiment, the core is InAs, the metal precursor used in step (b) is zinc thiolate, and the metal precursors used in step (c) are a mixture of zinc thiolate and cadmium thiolate, preferably where the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133. The resulting quantum dots can be represented as InAs / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, 0.02 to 0.2, or 0.025 to 0.133.

[0156] In an embodiment, the core is InSb, the metal precursor used in step (b) is zinc thiolate, and the metal precursors used in step (c) are a mixture of zinc thiolate and cadmium thiolate, preferably wherein the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, from 0.02 to 0.2, or from 0.025 to 0.133. The resulting quantum dots can be represented as InSb / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, from 0.02 to 0.2, or from 0.025 to 0.133.

[0157] In an embodiment, the core is InSbAs, the metal precursor used in step (b) is zinc thiolate, and the metal precursors used in step (c) are a mixture of zinc thiolate and cadmium thiolate, preferably wherein the molar ratio Cd / (Cd + Zn) in the mixture is from 0.001 to 1.0, from 0.02 to 0.2, or from 0.025 to 0.133. The resulting quantum dots can be represented as InSbAs / ZnSe / Zn 1-x Cd x S, where x is preferably from 0.001 to 1.0, from 0.02 to 0.2, or from 0.025 to 0.133.

[0158] In an embodiment, the method according to the invention comprises a step (d) of forming a ligand layer on the second layer. The quantum dots prepared by this method result in quantum dots comprising a core, a first layer on the core, a second layer on the first layer, and a ligand layer on the second layer. The ligand layer can also be referred to as a ligand shell, a third shell or layer, or an outer shell or layer.

[0159] In an embodiment, the first layer, the second layer, and the second layer are (semi)spherical layers arranged concentrically around the core.

[0160] In an embodiment, the first layer surrounds the core, the second layer surrounds the second layer, and the ligand layer surrounds the second layer.

[0161] In an embodiment, the ligand layer comprises an organic compound. Preferably, the ligand layer consists (mainly) of one or more organic compounds. In this case, the organic compound can be referred to as an organic ligand.

[0162] It should be understood that the organic compound can be an organic moiety that binds to another layer (preferably the second layer) contained in the quantum dots. For example, a ligand layer comprising thiol can represent the -SH moiety of the thiol binding to ZnS contained in the second layer. Thus, the number of organic compounds or ligands contained in the ligand layer includes the number of organic groups that bind to the said another layer.

[0163] In an embodiment, the ligand layer contains 10 to 2000, 10 to 1900, 10 to 1800, 10 to 1700, 10 to 1600, 10 to 1500, 10 to 1400, 10 to 1300, 10 to 1200, 10 to 1100, 10 to 1000, 10 to 900, 10 to 800, 10 to 700, 10 to 600, 10 to 500, 10 to 400, 10 to 300 organic, 50 to 800, 100 to 700, 150 to 600, or 200 to 500 organic ligands (per quantum dot).

[0164] In an embodiment, the ligand layer contains oleylamine.

[0165] In an embodiment, steps (b) and / or (c) included in the method according to the present invention are carried out at the following temperatures: 180 °C to 350 °C, 180 °C to 340 °C, 180 °C to 330 °C, 180 °C to 320 °C, 180 °C to 310 °C, 180 °C to 300 °C, 180 °C to 290 °C, 180 °C to 280 °C, 180 °C to 270 °C, 180 °C to 260 °C, 180 °C to 250 °C, 190 °C to 350 °C, 190 °C to 340 °C, 190 °C to 330 °C, 190 °C to 320 °C, 190 °C to 310 °C, 190 °C to 300 °C, 190 °C to 290 °C, 190 °C to 280 °C, 190 °C to 270 °C, 190 °C to 260 °C, 190 °C to 250 °C, 200 °C to 350 °C, 200 °C to 340 °C, 200 °C to 330 °C, 200 °C to 320 °C, 200 °C to 310 °C, 200 °C to 300 °C, 200 °C to 290 °C, 200 °C to 280 °C, 200 °C to 270 °C, 200 °C to 260 °C, 200 °C to 250 °C, 210 °C to 350 °C, 210 °C to 340 °C, 210 °C to 330 °C, 210 °C to 320 °C, 210 °C to 310 °C, 210 °C to 300 °C, 210 °C to 290 °C, 210 °C to 280 °C, 210 °C to 270 °C, 210 °C to 260 °C, 210 °C to 250 °C, 220 °C to 350 °C, 220 °C to 340 °C, 220 °C to 330 °C, 220 °C to 320 °C, 220 °C to 310 °C, 220 °C to 300 °C, 220 °C to 290 °C, 220 °C to 280 °C, 220 °C to 270 °C, 220 °C to 260 °C, 220 °C to 250 °C, 230 °C to 350 °C, 230 °C to 340 °C, 230 °C to 330 °C, 230 °C to 320 °C, 230 °C to 310 °C, 230 °C to 300 °C, 230 °C to 290 °C, 230 °C to 280 °C, 230 °C to 270 °C, 230 °C to 260 °C, or 230 °C to 250 °C.

[0166] In an embodiment, the reaction time of step (b) included in the method according to the present invention is 5 to 60 minutes, 5 to 55 minutes, 5 to 50 minutes, 5 to 45 minutes, 5 to 40 minutes, 10 to 60 minutes, 10 to 55 minutes, 10 to 50 minutes, 10 to 45 minutes, 10 to 40 minutes, 20 to 60 minutes, 20 to 55 minutes, 20 to 50 minutes, 20 to 45 minutes, or 20 to 40 minutes.

[0167] In an embodiment, the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, 1 to 25 minutes, 1 to 20 minutes, 5 to 30 minutes, 5 to 25 minutes, or 5 to 20 minutes.

[0168] In an embodiment:

[0169] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes; or

[0170] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes; or

[0171] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes; or

[0172] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes.

[0173] In an embodiment:

[0174] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 350°C; or

[0175] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 350°C; or

[0176] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 350°C; or

[0177] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 350°C; or

[0178] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 300°C; or

[0179] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 300°C; or

[0180] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 300°C; or

[0181] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 300°C

[0182] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 250°C; or

[0183] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 1 to 30 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 250°C; or

[0184] - The reaction time of step (b) included in the method according to the present invention is 15 to 45 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 200°C to 250°C; or

[0185] - The reaction time of step (b) included in the method according to the present invention is 20 to 40 minutes, and the reaction time of step (c) included in the method according to the present invention is 5 to 20 minutes, and step (b) and / or step (c) is carried out at a temperature of 300°C to 250°C.

[0186] In another aspect, the present invention provides a quantum dot obtainable by, capable of being obtained by, or prepared by any one of the methods according to the present invention as described herein. Such a quantum dot may be referred to as a quantum dot according to the present invention.

[0187] In another aspect, the present invention provides a polymer film comprising the quantum dots according to the present invention. The quantum dots according to the present invention retain their advantageous properties when embedded in the polymer film, and the polymer film is a solid layer. Such a polymer film may be referred to as a polymer film according to the present invention.

[0188] In another aspect, the present invention provides a luminescent downconverter for downconverting the optical frequency, the luminescent downconverter comprising the quantum dots according to the present invention or the polymer film according to the present invention. Within the framework of this text, a luminescent downconverter is a device capable of converting light with a higher frequency into light with a lower frequency (i.e., downconversion). The properties of the quantum dots according to the present invention are particularly advantageous for downconversion.

[0189] In another aspect, the present invention provides a method for preparing a luminescent downconverter, the method comprising the method for preparing quantum dots according to the present invention.

[0190] In this text and its claims, the verb "to comprise" and its inflected forms are used in their non-limiting sense to mean including the items following the word, but not excluding items not specifically mentioned. Additionally, the verb "to consist of" may be replaced by "to consist essentially of", indicating that a product, a measuring device, a method, or a use as defined herein may respectively include one or more additional components / parts, one or more additional steps in addition to the specifically specified components / parts, steps, and these one or more additional components / parts, one or more additional steps do not change the unique characteristics of the present invention.

[0191] In addition, unless the context clearly requires an element that is one and only one, an element modified by the indefinite article "a" or "an" does not exclude the possibility of there being more than one element. Thus, the indefinite article "a" or "an" generally means "at least one".

[0192] All patents and literature references cited in this specification are incorporated herein by reference in their entirety. BRIEF DESCRIPTION OF THE DRAWINGS

[0193] Figure 1 . Mass-corrected absorbance (a.u.) of InP / ZnSe quantum dots synthesized using only tertiary phosphine (dash), 85% tertiary phosphine + 15% secondary phosphine (long dash), and 50% tertiary phosphine + 50% secondary phosphine (solid line) at various wavelengths.

[0194] Figure 2 . Mass-corrected absorbance (a.u.) of InP / ZnSe / ZnS quantum dots synthesized using only tertiary phosphine (dash), 85% tertiary phosphine + 15% secondary phosphine (long dash), and 50% tertiary phosphine + 50% secondary phosphine (solid line) at various wavelengths.

[0195] Figure 3 . Absolute PLQY values (circles) of InP / ZnSe / ZnS quantum dots obtained by synthesis using only tertiary phosphine, 85% tertiary phosphine + 15% secondary phosphine, and 50% tertiary phosphine + 50% secondary phosphine, and their chemical yields (triangles).

[0196] Figure 4 . Absolute PLQY values (circles) and full width at half maximum (FWHM, triangles)) of InP / ZnSe / ZnS quantum dots obtained by synthesis using only tertiary phosphine, 85% tertiary phosphine + 15% secondary phosphine, and 50% tertiary phosphine + 50% secondary phosphine. EXAMPLES

[0197] The present invention will be described in more detail below by way of a plurality of examples, which should not be construed as limiting the scope of the present invention. The present invention is not limited to the forms of the embodiments described in the context of the examples given. The present invention also extends to every combination of the measures that are independent of each other as described above.

[0198] Example 1 - InP / ZnSe / ZnS Synthesis

[0199] InP Synthesis

[0200] InP was synthesized by mixing InCl3 and ZnCl2 in oleylamine and injecting a phosphor precursor (such as tris(diethylamino)phosphine) at a high temperature (180 °C).

[0201] InP / ZnSe Synthesis

[0202] 5 mol% of DPP was added to the Se-saturated (2.24 M) TOP-Se precursor before injection, resulting in quantum dots with a higher photoluminescence quantum yield (PLQY) compared to those synthesized without adding DPP to TOP-Se. In both cases, the same InP nanocrystals were used as the core, and the same synthesis conditions and amounts were maintained.

[0203] Figure 1 An example of the evolution of the emission properties of the quantum dots during synthesis is shown. The QDs with DPP added to the TOP-Se precursor showed generally higher relative PLQY compared to the QDs synthesized without using DPP. When using DPP, at the last point taken from the ZnSe shell growth, the PLQY increased by a factor of 6. Absolute PLQY measurements were obtained from another synthesis using the same conditions (and DPP-ZnSe addition) for reproducibility purposes, and the same trend was observed: a high and stable PL increase during the reaction. Figure 1 (Right) shows the absolute PLQY values of these points. For InP / ZnSe, a PLQY between 40% and 60% was observed at this point, representing a large increase relative to InP / ZnSe synthesized without using DPP (whose estimated QY is in the range of 5% to 15%). The shell growth was completed within 30 minutes.

[0204] InP / ZnSe / ZnS Synthesis

[0205] Compared to ZnSe, ZnS growth on the outer shell is more challenging. No evidence of ZnS shell growth was observed using TOP-S and zinc oleate as precursors at 300˚C. Figure 2 Absorbance and emission spectra, as well as peak parameters, of aliquots taken during the hypothesized ZnS formation on InP / ZnSe quantum dots are shown. No growth was seen in the absorbance spectrum, and no significant emission peak changes were observed.

[0206] ZnS growth can be achieved by increasing the reactivity of one of the precursors, i.e., using a more reactive carboxylate of Zn (such as acetate) or a more reactive phosphine (such as DPP, diphenylphosphine). DPP reacts with sulfur to form diphenylphosphine sulfide DPP-S, which is a white precipitate. DPP-S can be dissolved in TOP-S: a 15 mol% solution of DPP-S / TOP-S was used for synthesis in these experiments. The results are provided in Figure 2 and a significant PL increase (photoluminescence) was observed.

[0207] Total synthesis

[0208] Finally, we were able to combine the three previous results to synthesize InP / ZnSe / ZnS quantum dots with high QY: DPP in TOP-Se for a brighter ZnSe shell and DPP-S in TOP-S for forming a ZnS shell with zinc oleate. Figure 5 compiles the absolute quantum yield measurements for the final synthesis of ZnSe, ZnS, and the purification steps. The resulting QDs exhibited a PLQY of 90% to 95% at the final synthesis stage. The final chemical yields of S and Se in the ZnS and ZnSe shells in the presence of DPP were 85% and 75%, respectively.

[0209] During the purification step using different anti-solvents (ethanol and acetone), the PLQY remained above 90%, showing a slight increase to 100%.

Claims

1. A method for preparing quantum dots, the method comprising the following steps: (a) Prepare a core of a binary, ternary or quaternary material, the binary, ternary or quaternary material comprising: - one or more first core elements selected from the group consisting of In, Ga and Al, and - one or more second core elements selected from the group consisting of P, As and Sb; (b) Form a first layer on the core by contacting the core with a mixture comprising a metal precursor, a secondary phosphine selenide and a tertiary phosphine selenide; (c) Form a second layer on the first layer by contacting the product of step (b) with a mixture comprising a metal precursor, a secondary phosphine sulfide and a tertiary phosphine sulfide.

2. The method according to claim 1, wherein one of the first core elements is In, preferably wherein the binary, ternary or quaternary material is InP, InGaP, InAs, InSb or InSbAs.

3. The method according to claim 1 or 2, wherein the secondary phosphine selenide is diphenylphosphine selenide, bis(2-norbornanyl)phosphine selenide, diisobutylphosphine selenide, di-tert-butylphosphine selenide, dicyclopentylphosphine selenide, dicyclohexylphosphine selenide or 9-phosphabicyclononane selenide.

4. The method according to any one of claims 1 to 3, wherein the tertiary phosphine selenide is a trialkylphosphine selenide.

5. The method according to claim 4, wherein the trialkylphosphine selenide is tri-n-octylphosphine selenide (TOP-Se).

6. The method according to any one of claims 1 to 5, wherein the second phosphine selenide is a trialkylphosphine sulfide.

7. The method according to claim 6, wherein the trialkylphosphine selenide is tri-n-octylphosphine sulfide (TOP-S).

8. The method according to any one of claims 1 to 7, wherein the metal used in step (b) and / or step (c) is zinc or cadmium, preferably zinc.

9. The method according to any one of claims 1 to 8, wherein the metal carboxylate used in step (b) and / or step (c) is a metal carboxylate or a metal thiolate, preferably a metal oleate, a metal stearate or a metal myristate.

10. The method according to any one of claims 1 to 9, wherein step (b) and / or step (c) is carried out at a temperature of 200 °C to 350 °C, preferably 200 °C to 300 °C.

11. The method according to any one of claims 1 to 10, wherein the reaction time of step (b) is 15 to 45 minutes, preferably 20 to 40 minutes.

12. The method according to any one of claims 1 to 11, wherein the reaction time of step (c) is 1 to 30 minutes, preferably 5 to 20 minutes.

13. The method according to any one of claims 1 to 12, wherein the molar ratio between the diphenylphosphine selenide and the second phosphine selenide is 40% to 60%.

14. The method according to any one of claims 1 to 13, wherein the molar ratio between the diphenylphosphine sulfide and the second phosphine sulfide is 40% to 60%.

15. A quantum dot obtainable by the method according to any one of claims 1 to 14.

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