Quantum dot surface preparation

Abstract

The invention relates to the synthesis of a surface-stabilized quantum dot by contacting a quantum dot bound to an initial ligand layer with an exchange composition in a solvent composition during a one-pot process, wherein the exchange composition comprises a metal halogenide, a further, protic ligand, and a sterically hindered base, wherein the solvent composition comprises an aprotic polar solvent.

Description

[0001] Quantum dot surface preparation

[0002] Field

[0003] The invention relates to the synthesis of a surface-stabilized quantum dot by contacting a quantum dot bound to an initial ligand layer with an exchange composition in a solvent composition during a one-pot process, wherein the exchange composition comprises a metal halogenide, a further, protic ligand, and a sterically hindered base, wherein the solvent composition comprises an aprotic polar solvent.

[0004] Background

[0005] Quantum dots have the property of absorbing blue (and UV) light and emitting that light at a longer wavelength, for example as green or red light. Because they can be made as efficient luminescent materials, where the color of the emission can be tuned by changing the crystal size, they can be applied as a down-convertor material in lighting and LED displays. A particularly interesting application subfield is found in microLED displays, where every pixel contains a blue, green and red emission source (so called self-emissive screens) and where the pixel size shrinks to below 10 pm. At this length scale, these quantum dots have a distinct advantage over conventional down-converting materials or native green and red emitting materials because they are more efficient.

[0006] There are however significant barriers to the integration of quantum dots in microLED displays. For these applications, quantum dots must be highly compatible with a polymer resin to form inks that can be deposited and patterned with the necessary excellent uniformity and resolution. It is also essential that the quantum dot-monomer ink retains the high luminescence efficiency and refined optical properties.

[0007] These properties can be achieved by binding an organic ligand layer to the quantum dot. However, common synthesis methods result in a layer of apolar organic ligands, incompatible with common inks.

[0008] There is an ongoing need for efficient methods to exchange the apolar organic ligand layer of pristine quantum dots by a ligand layer that is compatible with an application of choice, while preserving photoelectronic properties.

[0009] Description of the invention

[0010] In an aspect, the invention provides a method of preparing of a surface-stabilized quantum dot comprising the steps of:

[0011] (a) obtaining a quantum dot bound to an initial ligand layer; and

[0012] (b) contacting the quantum dot bound to the initial ligand layer with an exchange composition in a solvent composition during a one-pot process, wherein the exchange composition comprises a zinc halogenide, a further ligand, and a sterically hindered base, wherein the further ligand is a protic ligand, and wherein the solvent composition comprises an aprotic polar solvent. Such a method may be called a method according to or of the invention herein.

[0013] Quantum dots

[0014] A quantum dot (QD) is a (semi)spherical nanoparticle comprising a core, and optionally one or more layers or shells on the core. It is understood that a quantum dot is different from a quantum rod, which is an elongated semiconductor nanoparticle.

[0015] A core comprised in a quantum dot is a (semi)spherical, semiconductor nanocrystal, having optical and electronic properties that are distinct from larger particles of the same materials due to quantum mechanical effects. A core can be considered a quantum dot in its own right.

[0016] In the context of this application, a quantum dot is able to absorb and emit electromagnetic radiation, wherein the wavelength of the emitted radiation is higher than the wavelength of the absorbed radiation. Preferably, the emitted radiation is in the visible spectrum (“visible light”).

[0017] It is an advantage of a method according to the invention that a quantum dot can be obtained with good optical properties, i.e. with a high photoluminescence, high photostability and high absorbance of UV or blue light, via a robust reaction, meaning that the methods are cost-effective.

[0018] A suitable measure for photoluminescence is the "photoluminescent 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 may also be called the internal PLQY, in contrast with the external PLQY which is 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 embodiments, a method according to the invention results in a quantum dot 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] In embodiments, a method according to the invention results in a quantum dot exhibiting enhanced photostability for extended operational lifetimes, suitable for use in high-demand applications such as display technologies, and optoelectronic devices. Specifically, the disclosed quantum dots maintain a photostability defined by a T90 at 1 W / cm2greater than 300, 400, 500, 600, 700, 800, 900, 1000, 1 100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, or 2000, hours, where T90 represents the time required for the quantum dots to retain 90% of their initial photoluminescence intensity under continuous illumination. This high photostability is achieved through surface stabilization, which results in improved resistance to photobleaching and oxidative damage, thus significantly extending photostable emission properties.

[0021] In embodiments, a method according to the invention results in a quantum dot designed to exhibit high absorbance of ultraviolet (UV) or blue light, enhancing their functionality in applications requiring efficient light harvesting and energy transfer, such as display technologies. Specifically, the quantum dots are engineered to achieve an absorbance coefficient greater than 0.1 pm-1in the blue light wavelength range (440-460 nm) and / or greater than 0.5 pm-1in the UV wavelength range (365-405 nm).

[0022] It is understood that the application of a method according to the invention generally results in a plurality of quantum dots. Wherever a reference is made to a property of a single quantum dot, reference is preferably made to the average value of the property over the plurality of quantum dots. The average may be a number-weighted average or a mass-weighted average.

[0023] In embodiments, the quantum dot has a diameter from 5 up to 30 nm, up to 29 nm, up to 28 nm, up to 27 nm, up to 26 nm, up to 25 nm, up to 24 nm, up to 23 nm, up to 22 nm, up to

[0024] 21 nm, up to 20 nm, up to 19 nm, up to 18 nm, up to 17 nm, up to 16 nm, up to 15 nm, up to

[0025] 14 nm, up to 13 nm, up to 12 nm, up to 11 nm, up to 10 nm, up to 9.5 nm, up to 9 nm, up to

[0026] 8.5 nm, up to 8 nm. In embodiments, the quantum dot has a diameter from 6 up to 30 nm, up to 29 nm, up to 28 nm, up to 27 nm, up to 26 nm, up to 25 nm, up to 24 nm, up to 23 nm, up to 22 nm, up to 21 nm, up to 20 nm, up to 19 nm, up to 18 nm, up to 17 nm, up to 16 nm, up to 15 nm, up to 14 nm, up to 13 nm, up to 12 nm, up to 11 nm, up to 10 nm, up to 9.5 nm, up to 9 nm, up to 8.5 nm, up to 8 nm. Quantum dots having an average diameter in this range can provide good optical properties for down-conversion because the absorption coefficient at wavelengths corresponding to the pump light strongly exceeds the absorption coefficient at wavelengths corresponding to the quantum dot emission.

[0027] In embodiments, the quantum dot comprises one or more shells on the core, preferably one or more shells of a binary or ternary ll-VI material.

[0028] In embodiments, the quantum dot comprises a core and a first shell or layer on the core.

[0029] In embodiments, the quantum dot comprises a core, a first shell or layer on the core, and a second shell or layer on the first layer. Such a quantum dot may also be called a core / shell / shell quantum dot.

[0030] A core / shell / shell quantum dot may be represented by core / first layer / second layer. For example, InP / ZnSe / ZnS refers to a quantum dot comprising an InP core (i.e. a core comprising or (essentially) consisting of InP), a ZnSe first layer (i.e. a first layer comprising or (essentially) consisting of ZnSe), and a ZnS second layer (i.e. a second layer comprising or (essentially) consisting of a ZnS, wherein the molar ratio between the elements are as indicated). In this context, the terminology AB or ABC core, layer or shell, refers to a core, layer or shell comprising or (essentially) consisting of AB or ABC, respectively.

[0031] The composition of the quantum dot, the core, the first layer and the second layer (i.e. the elements comprised therein and their molar ratios) may be determined by EDX (Energy- dispersive X-ray spectroscopy) on an ensemble of quantum dots.

[0032] In embodiments, the first layer and the second layer are (semi)spherical layers arranged concentrically around the core.

[0033] In embodiments, the first layer surrounds the core and the second layer surrounds the second layer. In embodiments, the first layer and the second layer are solid layers.

[0034] In embodiments, step (a) comprised in a method according to the invention comprises the synthesis of the quantum dot via a hot-injection quantum dot synthesis.

[0035] In embodiments, the method according to the invention comprises a step of (c) obtaining a surface-stabilized quantum dot via purification of the product of step (b); preferably wherein the purification comprises washing with an antisolvent. An antisolvent is a solvent in which the surface-stabilized quantum dot is not soluble.

[0036] Surface stabilization

[0037] Surface stabilization, also known as surface passivation, plays a critical role in the performance and stability of quantum dots. Because of the high surface-to-volume ratio of the core, quantum dots have a significant number of surface atoms with unsaturated bonds, leading to a high density of surface defects. These defects can act as trap sites for charge carriers (electrons and holes), which causes non-radiative recombination, quenching the quantum dots' fluorescence and reducing its efficiency and stability.

[0038] A surface-stabilized quantum dot relates to a quantum dot wherein this challenge has been addressed by using materials to stabilize the surface atoms and eliminate defects (i.e. “surface stabilization” or “surface passivation”). Two commonly used strategies include the use of shells on the core, and / or the binding of the quantum dot to a ligand layer. Preferably, the shells consist of an inorganic material, and the ligand layer consists of organic compounds called ligands in this context. These approaches effectively 'cap' the surface, reducing the likelihood of charge trapping and non-radiative recombination, thereby enhancing the quantum dots’ photoluminescence efficiency and extending its operational lifetime. Moreover, these processes can improve the chemical and photostability of quantum dots, making them more robust for applications in display technology, bioimaging, and solar energy.

[0039] In the context of this application, the shells are considered to be comprised in the quantum dot, whereas the ligand layer is considered to be bound to the quantum dot. A surface- stabilized quantum dot is defined as the quantum dot, comprising any shells on the core if present, and the ligand layer bound to the quantum dot if present. The ligand layer may consist of one or more types of organic compounds. To be bound means to be covalently or non- covalently bound (i.e. complexed with).

[0040] Surface stabilization also tailors quantum dots for compatibility with diverse environments, expanding their applications across fields. By carefully selecting the ligand layer, the interaction between the quantum dots and various hydrophobic or hydrophilic media can be controlled, making the quantum dots more suitable for specific applications. For instance, applications in optoelectronics, like display technologies and light-emitting devices, often benefit from organic ligands that ensure long-term stability in slightly polar environments. These ligands help maintain quantum dot brightness and prevent oxidation or degradation in response to environmental factors like UV light exposure and thermal fluctuations, extending device longevity. Other applications such as bioimaging even take place in highly polar environments.

[0041] Ligand layer

[0042] The embodiments and preferences in this section apply to both the initial ligand layer and the ligand layer obtained at the end of step (b).

[0043] In embodiments, the ligand layer is a (semi)spherical layer arranged concentrically around the core (in case of a core quantum dot), around the first shell (in case of a core / shell quantum dot) or around the second shell (in case of a core / shell / shell quantum dot).

[0044] In embodiments, the ligand layer surrounds the core (in case of a core quantum dot), the first shell (in case of a core / shell quantum dot) or the second shell (in case of a core / shell / shell quantum dot).

[0045] The ligand layer may also be called the ligand shell, or the external shell or layer.

[0046] It is understood that the ligands comprised in the ligand layer may be bound to the core, first shell or second shell, covalently or otherwise.

[0047] In embodiments, the ligand layer comprises from 10 up to 2000, from 10 up to 1900, from 10 up to 1800, from 10 up to 1700, from 10 up to 1600, from 10 up to 1500, from 10 up to 1400, from 10 up to 1300, from 10 up to 1200, from 10 up to 1100, from 10 up to 1000, from 10 up to 900, from 10 up to 800, from 10 up to 700, from 10 up to 600, from 10 up to 500, from 10 up to 400, from 10 up to 300 organic, from 50 up to 800, from 100 up to 700, from 150 up to 600 or from 200 up to 500 ligands per quantum dot.

[0048] Ligand exchange

[0049] Ligand exchange is the complete or partial substitution of one or more types of ligands comprised in the ligand layer by another type of ligands. In the context of this invention, ligand exchange preferably refers to the substitution of the ligands comprised in the initial ligand layer by the further ligands.

[0050] The synthesis of quantum dots often results in “pristine” quantum dots bound to an apolar ligand layer, i.e. a ligand layer completely or mainly consisting of apolar ligands. In the context of the invention, the initial ligand layer is apolar. Yet, most applications require quantum dots to be bound to (slightly) polar ligands, preferably corresponding to the further ligands herein. The methods of the invention allow for a ligand exchange resulting in the complete or partial substitution of ligands comprised in the initial ligand layer for the further ligand in an efficient manner, while preserving the photoelectronic properties of the quantum dots.

[0051] In embodiments, step (b) of contacting the quantum dot bound to the initial ligand layer with the exchange composition comprising the further ligand, comprised in a method according to the invention, results in the ligand exchange of the initial ligand layer for the further ligand. Herein, the ligand exchange corresponds to the substitution of at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91 , 92, 93, 94, 95, 96, 97, 98, 99, 99.1 , 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, or 99.9 percent of the ligands comprised in the initial ligand layer for the further ligand (on a molar basis).

[0052] The method of the invention makes it possible to perform the ligand exchange as a one- pot process. Surprisingly, the use of weakly polar aprotic solvents such as cyclohexanone or methyl acetate in step (b) allows the solubilization of the metal halogenide, which is necessary for initial surface treatment of the quantum dots, while not degrading the quantum dots. Without being bound to this theory, the metal halogenide first replace the ligand comprised in the initial ligand layer, after which the further ligand is bound to the surface of the quantum dots. Yet, common solvents used for the metal halogenides degrade the quantum dots, complicating the process. According to the invention, this process can be simplified during a one-step process. Pristine quantum dots, which typically comprise a ligand layer consisting of apolar organic ligand and can thus only be formulated (i.e. dispersed) in aprotic solvents such as toluene, may be contacted with the exchange composition and the aprotic polar solvent during the one-step process to perform a robust ligand exchange. In this context, the solvent composition during step (b) of the method of the invention may consist of the apolar solvent in which the pristine quantum dots are formulated, and the aprotic polar solvent added in step (b). As such, the photoelectronic properties of the obtained surface-stabilized quantum dots are enhanced due to a lower number of manipulations and the use of a benign solvent.

[0053] A one-pot process means that any ligand exchange taking place in step (b) can be achieved without intermediate purification, isolation or solvent change.

[0054] In embodiments, the one-pot process of step (b) is performed in a batch operation and / or is performed in a single container and / or does not comprise isolating or purifying the quantum dot and / or is performed without changing the solvent composition.

[0055] In embodiments, step (b) results in the ligand exchange of the initial ligand layer for the further ligand, wherein a substitution of at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91 , 92, 93, 94, 95, 96, 97, 98, 99, 99.1 , 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, or 99.9 percent of the ligands comprised in the initial ligand layer for the further ligand (on a molar basis) is achieved during the one-pot process, i.e. before the quantum dot is isolated or purified.

[0056] In embodiments, step (b) is performed at a temperature from 0°C to 150°C, from 0°C to 140°C, from 0°C to 130°C, from 0°C to 120°C, from 0°C to 1 10°C, from 0°C to 100°C, from 0°C to 90°C, from 0°C to 80°C, from 0°C to 70°C, from 0°C to 60°C, from 0°C to 50°C.

[0057] In embodiments, step (b) is performed at a temperature from 20°C to 150°C, from 20°C to 140°C, from 20°C to 130°C, from 20°C to 120°C, from 20°C to 110°C, from 20°C to 100°C, from 20°C to 90°C, from 20°C to 80°C, from 20°C to 70°C, from 20°C to 60°C, from 20°C to 50°C.

[0058] In embodiments, step (b) is performed from 1 to 72 hours, from 1 to 66 hours, from 1 to 60 hours, from 1 to 54 hours, from 1 to 48 hours, from 1 to 42 hours, from 1 to 36 hours, from 1 to 30 hours, from 1 to 24 hours, from 1 to 18 hours, from 1 to 12 hours. In embodiments, step (b) is performed from 1 to 72 hours, from 1 to 66 hours, from 1 to 60 hours, from 1 to 54 hours, from 1 to 48 hours, from 1 to 42 hours, from 1 to 36 hours, from 1 to 30 hours, from 1 to 24 hours, from 1 to 18 hours, from 1 to 12 hours at a temperature from 0°C to 100°C.

[0059] In embodiments, step (b) is performed from 1 to 72 hours, from 1 to 66 hours, from 1 to 60 hours, from 1 to 54 hours, from 1 to 48 hours, from 1 to 42 hours, from 1 to 36 hours, from 1 to 30 hours, from 1 to 24 hours, from 1 to 18 hours, from 1 to 12 hours at a temperature from 0°C to 80°C.

[0060] In embodiments, step (b) is performed from 1 to 72 hours, from 1 to 66 hours, from 1 to 60 hours, from 1 to 54 hours, from 1 to 48 hours, from 1 to 42 hours, from 1 to 36 hours, from 1 to 30 hours, from 1 to 24 hours, from 1 to 18 hours, from 1 to 12 hours at a temperature from 0°C to 60°C.

[0061] Initial ligand layer

[0062] In embodiments, the initial ligand layer consists of organic compounds. These organic compounds may be called initial (organic) ligands.

[0063] In embodiments, the initial ligand layer consists of organic compounds having at least 8, 9,

[0064] 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms.

[0065] In embodiments, the initial ligand layer consists of organic compounds having no more than 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , or 20 carbon atoms.

[0066] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10,

[0067] 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 36 carbon atoms.

[0068] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 24 carbon atoms.

[0069] In embodiments, the initial ligand layer consists of organic compounds having 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, or 36 carbon atoms.

[0070] In embodiments, the initial ligand layer consists of organic compounds having at least 8, 9,

[0071] 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, wherein the organic compounds are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides. Preferably, a trialkylphoshines is trioctylphosphine and / or a trialkylphosphine oxide is trioctylphosphine oxide.

[0072] In embodiments, the initial ligand layer consists of organic compounds having no more than 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , or 20 carbon atoms, wherein the organic compounds are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides. Preferably, a trialkylphoshines is trioctylphosphine and / or a trialkylphosphine oxide is trioctylphosphine oxide.

[0073] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10,

[0074] 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 36 carbon atoms, wherein the organic compounds are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides. Preferably, a trialkylphoshines is trioctylphosphine and / or a trialkylphosphine oxide is trioctylphosphine oxide.

[0075] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 24 carbon atoms, wherein the organic compounds are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides. Preferably, a trialkylphoshines is trioctylphosphine and / or a trialkylphosphine oxide is trioctylphosphine oxide.

[0076] In embodiments, the initial ligand layer consists of organic compounds having 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, or 36 carbon atoms, wherein the organic compounds are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides. Preferably, a trialkylphoshines is trioctylphosphine and / or a trialkylphosphine oxide is trioctylphosphine oxide.

[0077] In embodiments, the initial ligand layer consists of organic compounds having at least 8, 9,

[0078] 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, wherein the organic compounds are carboxylic acids, or alkyl amines.

[0079] In embodiments, the initial ligand layer consists of organic compounds having no more than 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , or 20 carbon atoms, wherein the organic compounds are carboxylic acids, or alkyl amines.

[0080] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10,

[0081] 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 36 carbon atoms, wherein the organic compounds are carboxylic acids, or alkyl amines.

[0082] In embodiments, the initial ligand layer consist of organic compounds having from 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or 20 to 24 carbon atoms, wherein the organic compounds are carboxylic acids, or alkyl amines.

[0083] In embodiments, the initial ligand layer consists of organic compounds having 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, or 36 carbon atoms, wherein the organic compounds are carboxylic acids, or alkyl amines.

[0084] In embodiments, the initial ligand layer consists of one or more of oleic acid, oleylamine, trioctylphosphine, trioctylphosphine oxide, octadecylphosphonic acid, dodecylamine, stearic acid, or hexadecylamine.

[0085] In embodiments, the initial ligand layer consists of apolar organic compounds.

[0086] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL measured at 25°C and 1 atm.

[0087] Further ligand

[0088] In embodiments, the further ligand is an organic compound.

[0089] In embodiments, the further ligand is represented by formula (I): x Xs) (I) wherein X is a group capable of binding to the quantum dot; wherein L is a linear chain of at least four units independently selected from methylene (-CH2-), ethenylene (-CH=CH-), and the ketone (-C(=O)-), ester (■C(=O)O‘), thioester (-C(=S)O-), amide (-C(=O)NH-), primary ether (-0-CH2-), and primary thioether (-S-CH2-); wherein R is 5- or 6-membered ring, preferably comprising a N, O or S ring atom, wherein the ring is optionally substituted; and wherein the further ligand comprises 30 non-hydrogen atoms or less.

[0090] In preferred embodiments, X in formulae (I), (II), (III), (IV), (V), (VI) or (VII) is SH, SOOH, N(RN)2, N(RN)3+, SO3H, or PO3H, wherein each instance of RNis independently H or a C1-4 alkyl.

[0091] In preferred embodiments, L in formulae (I), (IV), (V), (VI) or (VI I) comprises 1 to 3 -Z-CH2- CH2- or -Z-C(=O)-CH2-CH2- units, wherein Z is O or S.

[0092] In preferred embodiments, the further ligand is represented by formula (II) or (III), wherein L’ is a linear chain of at least two units independently selected from methylene (-CH2-), ethenylene (-CH=CH-), and the ketone (-C(=O)-), ester (-C(=O)O-), thioester (-C(=S)O-), amide (-C(=O)NH-), primary ether (-O-CH2-), primary thioether (-S-CH2-) functional groups:

[0093] In preferred embodiments, R in formulae (I), (II), (III), (IV), (V), (VI) or (VII) is an unsubstituted ring.

[0094] In preferred embodiments, R in formulae (I), (II), (III), (IV), (V), (VI) or (VII) is a saturated heterocycle, preferably comprising a N, O or S atom.

[0095] In preferred embodiments, R in formulae (I), (II), (III), (IV), (V), (VI) or (VII) is a phenyl.

[0096] In preferred embodiments, the further ligand is represented by formula (IV), (V), (VI) or (VII), wherein Rais NH, O or S; wherein one of Rband Rcis NH, O or S, and the other is CH2; and wherein one of Rd, Reand Rfis NH, O or S, and the others are CH2:

[0097] (IV) (V) (VI) (VII).

[0098] In preferred embodiments, the further ligand comprises 25 non-hydrogen atoms or less, preferably 20 non-hydrogen atoms or less.

[0099] In preferred embodiments, the further ligand is represented by formula (VIII), (IX), (X), (XI) or (XII):

[0100]

[0101] (XII), wherein n is 1 , 2, 3, 4 or 5.

[0102] In embodiments, the further ligand is a polar further organic ligand, preferably a slightly polar further organic ligand.

[0103] In embodiments, the further ligand has a solubility in hexane of less than 1 g per 100 mL measured at 25°C and 1 atm. In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL measured at 25°C and 1 atm, and the further ligand has a solubility in hexane of less than 1 g per 100 mL measured at 25°C and 1 atm.

[0104] Exchange composition The exchange compositions comprises several compounds involved in the ligand exchange, besides the further ligand. Without being bound to this theory, the metal halogenides first replace the ligand comprised in the initial ligand layer, after which the further ligand is bound to the surface of the quantum dots.

[0105] For example, a quantum dot QD bound to an initial ligand L' via a zinc atom first reacts with a zinc chloride (a metal halogenide):

[0106] In embodiments, the metal halogenide is a Lewis acid.

[0107] In embodiments, the metal halogenide is a zinc halogenide, an aluminum halogenide or a titanium halogenide. In embodiments, the metal halogenide is a metal chloride, a metal bromide, or a metal iodide; preferably a metal chloride or bromide; more preferably a metal chloride.

[0108] In embodiments, the metal halogenide is zinc chloride, zinc bromide, zinc iodide, aluminum chloride, aluminum bromide, aluminum iodide, titanium chloride, titanium bromide, or titanium iodide; preferably zinc chloride.

[0109] The aprotic polar solvent is able to dissolve the metal halogenide, while not degrading the quantum dot.

[0110] In embodiments, the aprotic polar solvent is cyclohexanone, methyl acetate, dimethyl formamide, dimethyl sulfoxide, propylene carbonate, ethyl acetate, tetra hydrofuran, dichloromethane, diethyl ether, acetone, 1 ,4-dioxane, chloroform, or mixtures thereof. Preferably, the aprotic polar solvent is cyclohexanone, methyl acetate, dimethyl formamide, dimethyl sulfoxide, propylene carbonate, ethyl acetate, tetra hydrofuran, acetone, 1 ,4-dioxane, chloroform, or mixtures thereof. More preferably, the aprotic polar solvent is cyclohexanone.

[0111] In embodiments, the aprotic polar solvent is a solvent having a similar polarity, boiling point, density, viscosity and / or solubility in water as cyclohexanone.

[0112] In embodiments, the concentration of the aprotic solvent during the one-pot process is at least 5%, at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, or at least 45%.

[0113] In embodiments, the concentration of the aprotic solvent during the one-pot process is equal to or lower than 75%, equal to or lower than 70%, equal to or lower than 65%, equal to or lower than 60%, or equal to or lower than 55%.

[0114] In embodiments, the concentration of the aprotic solvent during the one-pot process is from 25% to 75%, from 25% to 70%, from 25% to 65%, from 25% to 60%, or from 25% to 55%.

[0115] In embodiments, the concentration of the aprotic solvent during the one-pot process is from 5% to 75%, from 10% to 75%, from 15% to 75%, from 20% to 75%, from 25% to 75%, from 30% to 75%, from 35% to 75%, from 40% to 75%, or from 45% to 75%.

[0116] In embodiments, the solvent composition comprises an apolar solvent, alongside the aprotic polar solvent. Preferably, the solvent composition consists of the apolar solvent and the aprotic polar solvent.

[0117] In embodiments, the quantum dots in step (a) are obtained in an apolar solvent, and the solvent composition in step (b) comprises, preferably consists of, said apolar solvent and the aprotic polar solvent.

[0118] In embodiments, the apolar solvent is a hydrocarbon. Preferably, the hydrocarbon comprises at least 6 carbon atoms. More preferably, the hydrocarbon comprises from 6 to 16, from 6 to 14, from 6 to 12 carbon atoms.

[0119] In embodiments, the apolar solvent is a hydrocarbon having a boiling point at 1 atm of at least 50°C, at least 55°C, at least 60°C, at least 65°C, at least 70°C, at least 75°C, at least 80°C, at least 85°C, at least 90°C, at least 95°C, at least 100°C, at least 105°C, at least 1 10°C, at least 115°C, at least 120°C, at least 125°C, at least 130°C, at least 135°C, at least 140°C, at least 145°C, or at least 150°C. Preferably, the hydrocarbon has a boiling point at 1 atm of 50°C to 250°C, of 55°C to 250°C, of 60°C to 250°C, of 65°C to 250°C, of 70°C to 250°C, of 75°C to 250°C, of 80°C to 250°C, of 85°C to 250°C, of 90°C to 250°C, of 95°C to 250°C, of 100°C to 250°C, of 105°C to 250°C, of 110°C to 250°C, of 115°C to 250°C, of 120°C to 250°C, of 125°C to 250°C, of 130°C to 250°C, of 135°C to 250°C, of 140°C to 250°C, of 145°C to 250°C, or of 150°C to 250°C.

[0120] In embodiments, the apolar solvent is benzene, toluene, xylene, pentane, hexane, heptane, octane, nonane, decane, cyclohexane, or any mixture thereof. Preferably, the apolar solvent is benzene, toluene, xylene, hexane, heptane, octane, nonane, decane, cyclohexane, or any mixture thereof.

[0121] After the first reaction, in the same solvent composition, the further ligand Lfis bound to the quantum dot QD in the presence of a sterically hindered base (R3N):

[0122] QD-ZnCh + 2LfH + 2R3N QD-ZnLf2+ 2[R3NH][CI]

[0123] In embodiments, the sterically hindered base is a Bransted base wherein the atom binding a proton in the corresponding Bransted reaction is bound to at least one, preferably at least two, preferably at least three, bulky substituents.

[0124] Preferably, a Bransted base has a conjugate acid pKa in water higher than 5, more preferably higher than 7.5, most preferably higher than 10.

[0125] The parameter “pKa” denotes the negative decimal logarithm of the acid dissociation constant (Ka) of the conjugate acid of the compound in question. Unless otherwise indicated, pKa values are determined at 25 °C by potentiometric titration in aqueous solution using a calibrated glass electrode, in accordance with internationally recognised measurement standards (e.g. IUPAC recommendations). For compounds of insufficient aqueous solubility, the pKa may be determined in a mixed aqueous-organic solvent system (for example water / methanol or water / acetonitrile) using the same titration principles, with the pKa value obtained from the inflection point or half-neutralisation point of the titration curve. Established extrapolation or correction methods for solvent effects may be applied. Spectrophotometric or NMR titration methods may alternatively be used, provided they yield a value that is consistent with the Henderson-Hasselbalch relationship.

[0126] Preferably, a bulky substituent has a Taft steric parameter Esequal to or higher than 2.

[0127] “Taft steric parameter (Es)” refers to the quantitative measure of steric hindrance originally defined by Taft (J. Am. Chem. Soc. 1956, 78, 335-341). The Esvalue of a substituent X is determined relative to the reference substituent methyl (Es= 0) using the Taft equation: where kx is the first-order rate constant for the acid-catalyzed hydrolysis of the ester CH3COOX under standardized conditions, and kMe is the corresponding rate constant for the methyl ester (X = Me). Rate constants shall be measured under identical reaction conditions, typically in 80% aqueous ethanol at 25 °C, using established kinetic methods such as continuous conductivity monitoring or titrimetric determination of liberated acetic acid. Positive Esvalues indicate larger steric bulk relative to methyl. Where direct kinetic measurement is impractical, Esmay be assigned by reference to published Taft steric parameter tables, which are incorporated herein by reference for substituents commonly characterized in the literature.

[0128] In embodiments, the sterically hindered base is a Bransted base wherein the atom binding a proton in the corresponding Bransted reaction is bound to at least one, preferably at least two, preferably at least three, bulky substituents, wherein the Bransted base has a conjugate acid pKa in water higher than 7.5, and wherein the bulky substituent has a Taft steric parameter Esequal to or higher than 2.

[0129] In embodiments, the sterically hindered base is a Bransted base wherein the atom binding a proton in the corresponding Bransted reaction is bound to at least one, preferably at least two, preferably at least three, bulky substituents, wherein the Bransted base has a conjugate acid pKa in water higher than 10, and wherein the bulky substituent has a Taft steric parameter Esequal to or higher than 2.

[0130] In embodiments, the sterically hindered base is a non-nucleophilic base.

[0131] In embodiments, the sterically hindered base is an amine, preferably a secondary amine or a tertiary amine, more preferably a tertiary amine.

[0132] In embodiments, the sterically hindered base is a dialkyl amine or a trialkyl amine, preferably a trialkyl amine, more preferably trioctylamine (TOA), diisopropylethylamine (DIPEA), or triethylamine (TEA).

[0133] In embodiments, the sterically hindered base is an amine; preferably trioctylamine (TOA), diisopropylethylamine (DIPEA), triethylamine (TEA), lithium diisopropylamide (LDA), 2,6-di- tert-butylpyridine (DTBP), 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,5- diazabicyclo[4.3.0]non-5-ene (DBN), or 2,2,6, 6-tetramethylpiperidine (TMP).

[0134] In embodiments, the concentration of each of the zinc halogenide, the second ligand, and the sterically hindered base during the one-step process is from 0.1 mol / L to 2 mol / L.

[0135] A possible complication during both reactions described above is the dissociation of the metal halogenide from the quantum dot surface. In order to prevent this, a metal salt of a conjugate base ofthe further ligand can be added. In the example above, the addition of Zn(Lf)2 prevents the following equilibrium of shifting towards the side of the products:

[0136] QD-Zn(Lf)2+ RSH QD° + RHS ^Zn(Lf)2

[0137] In embodiments, the exchange solution comprises a metal salt of a conjugate base of the further ligand. Preferably, the corresponding metal halogenide is a Lewis acid. More preferably, the metal is zinc, aluminum or titanium. Most preferably, the metal is zinc.

[0138] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm.

[0139] In embodiments, the metal halogenide is a zinc chloride; the sterically hindered base is a tertiary amine. In embodiments, the metal halogenide is a zinc chloride; the aprotic polar solvent is cyclohexanone.

[0140] In embodiments, the metal halogenide is a zinc chloride; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0141] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine.

[0142] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the aprotic polar solvent is cyclohexanone.

[0143] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0144] In embodiments, the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone.

[0145] In embodiments, the sterically hindered base is a tertiary amine; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0146] In embodiments, the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0147] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine.

[0148] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the aprotic polar solvent is cyclohexanone.

[0149] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0150] In embodiments, the metal halogenide is a zinc chloride; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone.

[0151] In embodiments, the metal halogenide is a zinc chloride; the sterically hindered base is a tertiary amine; the exchange composition comprises a zinc salt of a conjugate base of the further ligand. In embodiments, the metal halogenide is a zinc chloride; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0152] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone.

[0153] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0154] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0155] In embodiments, the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0156] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone.

[0157] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0158] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0159] In embodiments, the metal halogenide is a zinc chloride; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0160] In embodiments, the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0161] In embodiments, the metal halogenide is a zinc chloride; the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, and the further ligand has a solubility in hexane of less than 1 g per 100 mL, wherein the solubility is measured at 25°C and 1 atm; the sterically hindered base is a tertiary amine; the aprotic polar solvent is cyclohexanone; the exchange composition comprises a zinc salt of a conjugate base of the further ligand.

[0162] Core

[0163] The core is (made) of a binary, ternary or quaternary material (or compound). A binary, ternary or quaternary material is a material consisting of 2, 3 or 4 different elements, respectively. It is understood that the order of the elements in the formula of a tertiary or quaternary material is a matter of convention and has no bearing on the composition of the material.

[0164] In embodiments, a binary, ternary or quaternary material is a binary, ternary or quaternary lll-V material or a binary or ternary ll-VI material.

[0165] In embodiments, a binary, ternary or quaternary material is a binary or ternary material.

[0166] In embodiments, a binary, ternary or quaternary material is InP, InGaP, InAs, InSb or InSbAs.

[0167] In embodiments, a binary material is InP, InAs, InSb, GaP, GaAs, GaSb, AIP, AlAs or AlSb.

[0168] In embodiments, a tertiary material is InPAs, InPSb, InAsSb, GaPAs, GaPSb, GaAsSb, AlPAs, AlPSb, AlAsSb, InGaP, InGaAs, InGaSb, InAlP, InAIAs, InAISb, GaAlP, GaAIAs or GaAISb.

[0169] In embodiments, a tertiary material is InGaP or InSbAs.

[0170] In embodiments, a quaternary material is InPAsSb, GaPAsSb, AlPAsSb, InGaPAs, InGaPSb, InGaAsSb, InAIPAs, InAIPSb, InAIAsSb, GaAIPAs, GaAIPSb, GaAIAsSb, InGaAlP, InGaAIAs or InGaAISb.

[0171] In embodiments, the binary, ternary or quaternary material is InP. Such cores are highly attractive for downconverter purposes as they emit light in the visible spectrum upon illumination with blue (and UV) light when they are 2 nm to 4 nm in diameter.

[0172] Obtaining a quantum dot bound to an initial layer in step (a) in a method according to the invention may comprise preparing the core with any commonly known technique. For example, and without being limiting, a core may be synthesized by mixing a halogenide of each of the first core elements with a metal halogenide, preferably a zinc halogenide, and injecting the resulting mixture with a precursor of the second core element, preferably wherein the injection is performed at a temperature from 150°C up to 250°C, more preferably from 150°C up to 200°C. As an example, an InP core may be synthesized by mixing InCh and ZnCh in oleylamine and injecting a phosphor precursor (tris(diethylamino)phosphine for example) at an elevated temperature (180°C).

[0173] In embodiments, the preparation of the core in step (a) comprised in a method according to the invention is a colloidal synthesis.

[0174] In embodiments, the preparation of the core in step (a) comprised in a method according to the invention comprises a hot-injection quantum dot synthesis. Preferably, the hot-injection quantum dot synthesis results in the quantum dot in an apolar solvent.

[0175] In embodiments, the core has a diameter from 1 nm up to 5 nm, preferably from 1 .5 nm up to 4.5 nm, more preferably from 2 nm up to 4 nm. A suitable core diameter ensures that the quantum dots emit light in the visible spectrum upon illumination with blue (and UV) light.

[0176] In embodiments, the yield of a preparation of the core in step (a) 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%.

[0177] First shell

[0178] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is a (semi)spherical layer arranged concentrically around the core.

[0179] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer surrounds the core and, optionally, the second layer surrounds the first layer.

[0180] In embodiments, the first layer is a solid layer.

[0181] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of a binary or ternary ll-VI material.

[0182] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe or HgTe, preferably ZnS, ZnSe or ZnTe, more preferably ZnS or ZnSe, most preferably ZnSe.

[0183] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of ZnCdS, ZnCdSe, ZnSSe or CdSSe.

[0184] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of ZnSe or ZnCdSe.

[0185] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of ZnCdSe, wherein the molar ratio Cd / (Cd+Zn) in the first layer (i.e. the molar fraction of Cd) is between 0.001 and 1 .0, more preferably from 0.02 up to 0.2, most preferably from 0.025 up to 0.133.

[0186] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of ZnCdSe, wherein the molar ratio between the number of Se atoms and the total number of Zn and Cd atoms comprised in the first layer is from 0.50 up to 1 .50, from 0.55 up to 1 .45, from 0.60 up to 1 .40, from 0.65 up to 1 .35, from 0.70 up to 1 .30, from 0.75 up to 1 .25, from 0.80 up to 1 .20, from 0.85 up to 1.15, from 0.90 up to 1 .10, from 0.95 up to 1 .05, from 0.96 up to 1 .04, from 0.97 up to 1 .03, from 0.98 up to 1 .02, from 0.99 up to 1 .01 . This ratio can be determined by EDX (Energy-dispersive X-ray spectroscopy) on an ensemble of quantum dots.

[0187] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer is of ZnSe, wherein the first layer does not comprise Cd.

[0188] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the first layer has a thickness up to 1.0 nm, preferably between 0.1 nm and 0.9 nm, more preferably between 0.2 nm and 0.8 nm.

[0189] In embodiments, the preparation of the first shell in step (a) comprised in a method according to the invention is a colloidal synthesis.

[0190] In embodiments, the preparation of the first shell in step (a) comprised in a method according to the invention comprises a hot-injection quantum dot synthesis.

[0191] Second shell

[0192] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the second layer is a (semi)spherical layer arranged concentrically around the first layer or the core, respectively.

[0193] In embodiments, the quantum dot is a core / shell or core / shell / shell quantum dot, wherein the second layer surrounds the first layer or the core, respectively.

[0194] In embodiments, the second layer is a solid layer.

[0195] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of a binary or ternary ll-VI material.

[0196] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe or HgTe, preferably ZnS, ZnSe or ZnTe, more preferably ZnS or ZnSe, most preferably ZnS.

[0197] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of ZnCdS, ZnCdSe, ZnSSe or CdSSe.

[0198] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of ZnS or ZnCdS.

[0199] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of ZnCdS, wherein the molar ratio Cd / (Cd+Zn) in the second layer (i.e. the molar fraction of Cd) is between 0.001 and 1 .0, more preferably from 0.02 up to 0.2, most preferably from 0.025 up to 0.133.

[0200] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of ZnCdS, wherein the molar ratio between the number of S atoms and the total number of Zn and Cd atoms comprised in the second layer is from 0.50 up to 1 .50, from 0.55 up to 1 .45, from 0.60 up to 1 .40, from 0.65 up to 1 .35, from 0.70 up to 1 .30, from 0.75 up to 1 .25, from 0.80 up to 1 .20, from 0.85 up to 1.15, from 0.90 up to 1 .10, from 0.95 up to 1 .05, from 0.96 up to 1 .04, from 0.97 up to 1 .03, from 0.98 up to 1 .02, from 0.99 up to 1 .01 . This ratio can be determined by EDX (Energy-dispersive X-ray spectroscopy) on an ensemble of quantum dots. In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is of ZnS, wherein the second layer does not comprise Cd.

[0201] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer has a thickness up to 10 nm, preferably between 1 nm and 10 nm.

[0202] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the ratio between the volume of the second layer and the volume of the core is from 10 up to 50, from 15 up to 45, from 15 up to 40, from 15 up to 35, from 15 up to 30, or from 15 up to 25. If the volume of the second layer is increased, and thus the volume of the quantum dots, the absorption per quantum dot increases too.

[0203] In embodiments, the preparation of the second shell in step (a) comprised in a method according to the invention is a colloidal synthesis.

[0204] In embodiments, the preparation of the second shell in step (a) comprised in a method according to the invention comprises a hot-injection quantum dot synthesis.

[0205] Preferred quantum dots

[0206] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer is a (semi)spherical layer arranged concentrically around the first layer, and the first layer is a (semi)spherical layer arranged concentrically around the core.

[0207] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the second layer surrounds the first layer, and the first layer surrounds the core.

[0208] In embodiments, both the first layer and the second layer are solid layers.

[0209] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first and the second layer are independently of a binary or ternary ll-VI material, preferably wherein the core is of a binary, ternary or quaternary material lll-V material or of a binary or ternary ll-VI material.

[0210] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first and the second layer are independently of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe or HgTe, preferably ZnS, ZnSe or ZnTe, more preferably ZnS or ZnSe. Preferably, the core is of InP.

[0211] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first layer is of ZnSe and the second layer is of ZnS.

[0212] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first and the second layer are independently of ZnCdS, ZnCdSe, ZnSSe or CdSSe. Preferably, the core is of InP.

[0213] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first and the second layer are independently of ZnS, ZnSe, ZnCdS or ZnCdSe. Preferably, the core is of InP.

[0214] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of InP, InGaP, InAs, InSb or InSbAs. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0215] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of InP, InAs, InSb, GaP, GaAs, GaSb, AIP, AlAs or AlSb. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0216] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of InPAs, InPSb, InAsSb, GaPAs, GaPSb, GaAsSb, AlPAs, AlPSb, AlAsSb, InGaP, InGaAs, InGaSb, InAlP, InAIAs, InAISb, GaAlP, GaAIAs or GaAISb. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0217] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of InGaP or InSbAs. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0218] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of InPAsSb, GaPAsSb, AlPAsSb, InGaPAs, InGaPSb, InGaAsSb, InAIPAs, InAIPSb, InAIAsSb, GaAIPAs, GaAIPSb, GaAIAsSb, InGaAlP, InGaAIAs or InGaAISb. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0219] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe or HgTe, preferably ZnS, ZnSe or ZnTe, more preferably of ZnS or ZnSe. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0220] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the core is of ZnCdS, ZnCdSe, ZnSSe or CdSSe. Preferably, the first and the second layer are independently of ZnS or ZnSe. More preferably, the first layer is of ZnS or ZnSe and the second layer is of ZnS. Most preferably, the first layer is of ZnSe and the second layer is of ZnS.

[0221] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first layer is of ZnCdSe and the second layer is of ZnCdS, wherein the molar ratio Cd / (Cd+Zn) in the first and the second layer (i.e. the molar fraction of Cd) is between 0.001 and 1 .0, more preferably from 0.02 up to 0.2, most preferably from 0.025 up to 0.133.

[0222] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first layer if of ZnCdSe and the second layer is of ZnCdS, wherein the molar ratio between the number of Se or S atoms, respectively, and the total number of Zn and Cd atoms comprised in the first or the second layer is from 0.50 up to 1 .50, from 0.55 up to 1 .45, from 0.60 up to 1 .40, from 0.65 up to 1 .35, from 0.70 up to 1 .30, from 0.75 up to 1 .25, from 0.80 up to 1 .20, from 0.85 up to 1.15, from 0.90 up to 1.10, from 0.95 up to 1.05, from 0.96 up to 1.04, from 0.97 up to 1.03, from 0.98 up to 1 .02, from 0.99 up to 1 .01 . This ratio can be determined by EDX (Energy- dispersive X-ray spectroscopy) on an ensemble of quantum dots.

[0223] In embodiments, the quantum dot is a core / shell / shell quantum dot, wherein the first layer is of ZnSe and the second layer is of ZnS, wherein neither the first layer nor the second layer comprises Cd.

[0224] Further aspects

[0225] In a further aspect, the invention provides a quantum dot obtained by, obtainable by or prepared by any of the methods according to the invention as described herein. Such a quantum dot may be called a quantum dot according to the invention.

[0226] In a further aspect, the invention provides an ink comprising a quantum dot according to the invention. Methods according to the invention had the advantage that they can product surface-stabilized quantum dots that can be put in inks with a long shelf-life at a high solidloading (more than 20% by weight or more than 7% by volume of quantum dots). These inks can be deposited and cured to obtain quantum dot films or patterns with high absorbance of UV or blue light, high quantum efficiency and high photostability, as described above. Due to the ligand exchange resulting in a ligand layer compatible with the inks, the quantum dots maintain their properties when processed under ambient conditions.

[0227] In a further aspect, the invention provides a polymer film comprising a quantum dot according to the invention. A quantum dot according to the invention keeps its advantageous properties when embedded in a polymer film, which is a solid layer. Such a polymer film may be called a polymer film according to the invention.

[0228] In a further aspect, the invention provides a luminescent downconverter for converting down light frequency, comprising a quantum dot according to the invention or a polymer film according to the invention. In the frame of the present document, a luminescent downconverter is a device able to convert light with a higher frequency to light with a lower frequency (i.e. downconversion). The properties of a quantum dot according to the invention are especially advantageous for downconversion.

[0229] In a further aspect, the invention provides a method for preparing a luminescent downconverter, the method comprising a method for preparing a quantum dot according to the invention.

[0230] Definitions

[0231] In this document and in its claims, the verb "to comprise" and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition the verb “to consist” may be replaced by “to consist essentially of’ meaning that a product, an assay device respectively a method or a use as defined herein may comprise additional component(s) respectively additional step(s) than the ones specifically identified, said additional component(s), respectively step(s) not altering the unique characteristic of the invention.

[0232] In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

[0233] The concentration of a compound during the one-pot process means the concentration of the compound in the solvent composition, including all other compounds dissolved or suspended in the solvent composition, during the one-pot process, i.e. during step (b) of the method of the invention. Such concentrations may be expressed in any units, such as mol / L, m.% (mass percentage) or V.% (volume percentage).

[0234] All patent and literature references cited in the present specification are hereby incorporated by reference in their entirety.

[0235] Legend to the figures

[0236] Figure 1 . A bar graph showing the relative decrease in PLQY after ligand exchange with a 2, 3, or 4 component ligand mixture. Both R3N and Zn(RS)2 reduce the decrease of PLQY.

[0237] Examples

[0238] The invention is explained in more detail below with a number of examples, which are not to be construed as limiting the scope of the invention. The invention is not limited to the forms of implementation described in the cases given as examples. The invention also extends to each combination of measures as described above, independently from each other.

[0239] Example 1 - InP / ZnSe / ZnS synthesis

[0240] An example dataset is shown below, for the system where ZnX2 = ZnCh, protic ligand = RSH, metal salt of a protic ligand = Zn(RS)2, and sterically hindered base = R3N.

[0241] In an inert atmosphere (nitrogen-filled glovebox), an equimolar mixture of RSH, trioctylamine (TOA) and Zn(RS)2 was added to a solution of ZnCL in cyclohexanone (75 mg / mL). The molar ratio of RSH / TOA / Zn(RS)2:ZnCl2 was 2:1. This mixture was added to a dispersion of InP / ZnSe / ZnS quantum dots in heptane (400 pM). This final mixture was heated to 90 °C with stirring for 16 hours.

[0242] Afterwards, the mixture was taken out of the glovebox into the ambient environment for purification. Purification involved 8 precipitation steps by centrifugation with an antisolvent, alternating between an apolar (hexane) and polar (methanol) antisolvent. The quantum dot precipitate was redispersed in toluene after each step.

Claims

Claims1 . A method of preparing a surface-stabilized quantum dot comprising the steps of:(a) obtaining a quantum dot bound to an initial ligand layer; and(b) contacting the quantum dot bound to the initial ligand layer with an exchange composition in a solvent composition during a one-pot process, wherein the exchange composition comprises a metal halogenide, a further ligand, and a sterically hindered base, wherein the further ligand is a protic ligand, and wherein the solvent composition comprises an aprotic polar solvent.

2. The method of claim 1 , wherein the concentration of the aprotic polar solvent during the one-pot process is at least 10% by volume, preferably between 25% and 75% by volume.

3. The method of claim 1 or 2, wherein the initial ligand layer consists of organic compounds having a solubility in hexane of more than 10 g per 100 mL, wherein the solubility of the further ligand in hexane is less than 1 g per 100 mL, and wherein the solubility is measured at 25°C and 1 atm.

4. The method of any one of the preceding claims, wherein the initial ligand layer consists of initial organic ligands having from 8 to 36 carbon atoms, wherein the initial organic ligands are carboxylic acids, alkyl amines, trialkylphoshines, or trialkylphosphine oxides.

5. The method of any one of the preceding claims, wherein the further ligand is a thiol, preferably a thiol represented by formula (VI 11) or (IX):

6. The method of any one of the preceding claims, wherein the exchange solution comprises a metal salt of a conjugate base of the further ligand; preferably wherein the metal is zinc.

7. The method of any one of the preceding claims, wherein the aprotic polar solvent is cyclohexanone.

8. The method of any one of the preceding claims, wherein the sterically hindered base is a tertiary amine; preferably trioctylamine (TOA), diisopropylethylamine (DIPEA) or triethylamine (TEA).

9. The method of any one of the preceding claims, wherein the metal halogenide is a zinc halogenide.

10. The method of any one of the preceding claims, wherein the metal halogenide is a metal chloride, a metal bromide, or a metal iodide; preferably a metal chloride; more preferably zinc chloride.11 . The method of any one of the preceding claims, wherein the concentration of each of the metal halogenide, the further ligand, and the sterically hindered base during the one- pot process is from 0.1 mol / L to 2 mol / L.

12. The method of any one of the preceding claims, wherein step (a) comprises a hot- injection quantum dot synthesis.

13. The method of any one of the preceding claims, comprising a step of (c) obtaining a surface-stabilized quantum dot via purification of the product of step (b); preferably wherein the purification comprises washing with an antisolvent.

14. The method of any one of the preceding claims, wherein the quantum dot comprises a core of a binary, ternary or quaternary lll-V material or a binary or ternary ll-VImaterial, preferably a core of indium phosphide, preferably wherein the quantum dot comprises one or more shells of a binary or ternary ll-VI material on the core, more preferably an first shell of zinc selenide on the core and an second shell of zinc sulfide on the zinc selenide shell.

15. A surface-stabilized quantum dot obtainable by a method as defined in any one of claims 1 to 14.

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