WO2012168192A2

WO2012168192A2 - Synthesis of highly fluorescing semiconducting core-shell nanoparticles based on ib, iib, iiia, via elements of the periodic classification.

Info

Publication number: WO2012168192A2
Application number: PCT/EP2012/060515
Authority: WO
Inventors: Werner Hoheisel; Susan TIAN; Shizhe TIAN
Original assignee: Bayer Intellectual Property Gmbh; Bayer (South East Asia) Pte Ltd
Priority date: 2011-06-07
Filing date: 2012-06-04
Publication date: 2012-12-13
Also published as: TW201313877A; WO2012168192A3

Abstract

A method for the preparation of semiconducting core-shell nanoparticles comprising elements of the groups IB, IIB, IIIA, IVA, VA or VIA of the periodic classification wherein the composition of the core nanoparticle is selected from the group consisting of IB-VIA, IIB- VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA or IIB-IVA-VA or mixtures thereof, and at least one shell comprises elements of the groups IIB and VIA,, wherein in case the core composition is IIB-VIA the shell comprises at least a further element selected from the groups IB, IIIA, IVA or VA, said method comprising the following steps: i.)Core reaction mixture comprising at least one cation metal precursors and an anion precursor is dissolved in a ligand and solvent mixture at a temperature that is still beneath the threshold at which nucleation takes place, j.)Core reaction mixture is heated above nucleation threshold temperature and then kept at the reaction temperature for core nucleation, k.)Optionally the heated reaction mixture is kept at a temperature well suited for core growth but lower than the nucleation temperature, l.)Shell reaction mixture prepared separately comprising at least one cation metal precursors and an anion precursor and heated at a temperature that is still beneath the threshold at which nucleation takes place is added to the reaction mixture of step b) or c) for shell coating of the core nanoparticles, m.)reaction mixture of step d) is heated and hold to a shell growth temperature, n.)optionally for ZnS shell preparation shell reaction mixture comprising ZnS shell material is prepared separately and heated at a temperature that is still beneath the threshold at which nucleation takes place is added to the reaction mixture of step d) or e) for shell coating of the nanoparticles, o.)reaction mixture of step f) is heated and hold to a shell growth temperature, p.)reaction mixture is then cooled down to prevent further particle growth.

Description

Synthesis of highly fluorescing semiconducting core-shell nanoparticles based on

IB, MB, IMA, VIA elements of the periodic classification.

The present invention relates to the preparation of highly fluorescing semiconducting core- shell nanoparticles based on IB, MB, IIIA, VIA elements of the periodic classification.

Fluorescing semiconducting nanoparticles (also called quantum dots) are gaining increasing commercial interest. They can be utilized as efficient fluorescent agents which are considered as more stable than organic dyes.

Varying chemical composition and particle size allows flexible tuning of fluorescing semiconducting nanoparticles depending on commercial use. For example depending on the chemical composition, the whole visible range can be covered including parts of the UV and Near I R which are hard to reach with organic materials. Furthermore, due to quantum confinement effects the variation of the particles size has a strong effect on the absorption and emission wavelengths which can be utilized to considerably tune the emission colour (hundreds of nanometers) to wavelengths below the ones corresponding to the bulk band gap. However, up to now the best performing materials still contain toxic elements like cadmium, lead or mercury which strongly hinders a broad commercialization. Alternative materials are being developed worldwide but there are no materials yet which show sufficient performance and can be produced economically. Therefore there is a need for an alternative material with very good performance and low toxicity.

US 7833506 B2 describes a general process technology to synthesize semiconductor nanoparticles in a continuous way with micro reactor technology. With this technology scale up of nanoparticle production can be done economically to enable quantum dot based applications like 0(Q)LEDs, white or specifically colored LEDs for display and lighting, solar cells, anti counterfeiting, light converters, labels for biomolecules etc. To realize quantum dots with high fluorescent quantum yield it is essential to manufacture them with an advanced design which is composed of a core-shell structure with certain requirements on composition, number of shells, their thickness and ligand structure to ensure a proper surface passivation. As previously described quantum dots with a composition that does not contain cadmium, lead or mercury are strongly needed for their commercial use in a broad field of material and life science applications. Ternary or quaternary systems like IB-IIIA-VIA (I B = Cu, Ag; IIIA = In, Ga, Al; VIA = S, Se, Te) were found to be promising candidates whereby CulnS2 gained the largest interest recently due to its tunability of its efficient fluorescence [US 201 1 -0039104 A1 ]. Since a high fluorescence quantum yield of quantum dots requires a very good crystallinity with a minimal number of lattice, point and surface defects the manufacture of ternary systems pose a greater challenge than binary systems. This is since donor-acceptor defects like vacancies or anti-site defects of the cations are generally more likely due to deviation from the ideal stoichiometry which may cause quenching of photoluminescence. This is well known and especially true for CulnS2. Addition of Zinc as an additional cation is known to stabilize the CulnS2-lattice and cause an increase of the fluorescence quantum yield (H. Nakamura et al., Chem. Mater. 18 (2006) pp. 3330). At the same time incorporation of Zinc into the CulnS2 lattice, thereby forming a quaternary lattice provides the opportunity to shift the emission wavelength further to the blue due to the widening of the band gap by adding the wide band gap semiconductor material ZnS. The formation of homogenously alloyed nanocrystals is simplified if the crystalline-structure of the constituents matches with each other. ZnS generally exists in a cubic or hexagonal phase which can be obtained by statistically replacing the Zn²⁺ sites of ZnS with Cu⁺ and ln³⁺ while maintaining the original ZnS crystalline symmetry. In addition, the difference in the lattice constants between ZnS and CulnS2 is very small (approx. 2.2%), which makes it possible and likely to synthesize homogenous alloyed nanoparticles Cu_xln_yZn2-x-_yS2 or (CulnS2)x(ZnS)i_-x, respectively (D. Pan et al.; Chem. Commun. (2009), pp. 4221 ).

Furthermore, ZnS has a suited wide band gap material for an inorganic shell which is necessary for almost all types of quantum dots to get high fluorescence quantum yields. This is also true for CulnS2 nanoparticles and the position of valence and conduction band of ZnS provides an effective confinement of the charge carriers' wave functions which is typical for a type I semiconductor heterojunction. Thus, ZnS efficiently passivates the surface traps, preventing largely a leakage of the created charge carriers and therefore their non-radiative recombination. As a consequence the quantum yield is increased (e.g. R. Xie et al., J. Am. Chem. Soc. 131 (2009) pp. 5691 ; K. Kuo et al., Thin Solid Films 517 (2008) pp. 1257).

However all nanoparticles disclosed in these prior art are only moderately luminescent. It is known that the synthesis of a ternary or even quaternary nanoparticle relies on precursor materials that have, in a certain process parameter window, similar reactivity to ensure that the nanoparticle's stoichiometry is formed as required (US 201 1 -0039104 A1 ). Further coordinating ligands are included to control the particle formation, limit the growth process and secure a stable dispersion of the particles. For ternary nanoparticles like CulnS2 or even quaternary su lphides nanoparticles long chai n alkanethiols like dodecanthiol are advantageous which also act as sulphur source for the particle at the same time (US 201 1 -0039104 A1 ). Cationic long-chain (unsaturated) surfactants like oleylamine are potential ligands as well which are often coexisting in the reaction solution.

There is a need for a convenient method for the preparation of highly fluorescing semiconducting nanoparticles based on I B, M B, I MA, VIA elements of the periodic classification.

The method for the preparation of the nanoparticles of the present invention comprises the following steps:

1 .) precursors for the core and shell are separately dissolved in a ligand or surfactant, resp. and solvent mixture at a temperature that is still beneath the threshold at which nucleation takes place being typically from room temperature to 190 °C, preferably from 90 °C to 180 °C so a core reaction mixture and a shell reaction mixture are prepared. The terms ligand and surfactant are used synonymously in the following.

In case a ternary core is used the molecular ratio of metal precursors in the core reaction mixture usually is close to the stoichiometric ratio of the according bulk material and varies preferred by a factor of <2, most preferred by a factor of <1 .1 . This means for example in the case of CulnS2 that the molecular ratio of the Cu- and In-precursor ranges preferably from (1 - 2):(1 - 2) most preferably (1 - 1 .1 ):(1 - 1 .1 ). The molar content of the anion precursor is preferable in excess of the molar content of the metal salt. In case the anion precursor is also ligand a molar ratio from (100-1 .5):1 for anion:cation are usually used, preferred from (50-2):1 , most preferred from (12-3):1. It is preferred that core solution mixture comprises a total amount of cation salts (i.e. Copper, Indium, Silver and Gallium) in a concentration from 0.0005 M to 1 M, preferably from 0.001 M to 0.5 M, and more preferably from 0.005 to 0.1 M in at least one solvent.

Preferably core reaction mixture and a shell reaction mixture are clear and flowable solutions with a viscosity below 50 mPas preferably below 10 mPas homogeneity.

3.) core reaction mixture is heated above nucleation threshold temperature typically from 100°C to 350°C, preferably from 190°C to 300°C, most preferably from 210°C to 270°C and then kept at the reaction temperature for core nucleation . The heating procedure should be preferably done with a heating rate of > 1 K/s, preferably with a heating rate of > 10 K s and most preferably with a heating rate of > 100 K/s, wherein preferred procedure is that the reaction mixture is pumped through a micro-heat exchanger to heat up the mixture with the highest possible heating rate. The temperature is kept for a time period of preferably between 10 minutes and 10 hours, more preferably between 10 minutes and 1 hour. Preferably the heated reaction mixture flows into a residence microreactor with micro heat exchanger to adjust and hold the above given temperatures.

4.) optionally and for larger particles or for better particle quality the heated reaction mixture is then kept at a temperature being well suited for core growth but lower than the nucleation temperature wherein a second injection of the core reaction mixture can be done. The temperature range is from 150°C to 270°C, preferred from 180°C to 250°C and most preferred from 200°C to 240°C and can thereby be adjusted by heating oil. The temperature is kept for a time period of preferably between 10 minutes and 10 hours, more preferably between 10 minutes and 1 hour. Preferably the heated reaction mixture flows into a residence microreactor with micro heat exchanger to adjust and hold the above given temperatures.

5. ) shell reaction mixture containing precursors of shell material is added to the reaction mixture for shell coating of the core nanoparticles prepared in step 3.) or 4.) respectively.

Alternatively the reaction solution containing the core material is cooled down and the additional reaction mixture containing precursors of shell material is added and mixed at temperatures below 150°C, preferred below 100°C. Then the reaction mixture containing the core and precursors of shell material is heated up again . Step 5 is preferably conducted in a micro-mixer.

6. ) the reaction mixture is heated and hold to a shell growth temperature from 150°C to 260°C, preferred from 180°C to 250°C and most preferred from 200°C to 230°C usually for a time period of preferably between 5 minutes and 10 hours, more preferably between 10 minutes and 3 hours and most preferred between 15 min to 90 min. The reaction temperature can thereby adjusted by adjusting the temperature of heating oil . I n a preferred embodiment a residence reactor with heat exchanger is used to grow the shell material under the conditions given above. Most preferably the reaction mixture flows through a residence microreactor connected to a micro heat exchanger to grow the shell material under the conditions given above. Optionally the residence microreactor system can be segmented into a microreactor and an attached conventional tube system which diameter is not restricted to be less than about 2 mm. It is preferred that the outer shell of step 6) shows a thickness for from 0.3 to 4 nm of almost pure shell material that is a percentage of from 80% to 100 % of shell material in the outer shell is preferred. Preferably steps 5.) and 6.) are repeated to obtain the outer shell in the preferred mentioned thickness and composition.

7.) the reaction mixture is then cooled down to prevent further particle growth preferably by flowing into a second micro heat-exchanger or capillary tube to cool the reaction solution.

8) usually the nanoparticles are then separated by adding anti-solvents and dried. The core of the nanoparticles of the present inventions is nanoparticles comprising elements of the group:

IB such as Cu, Ag or combination thereof,

MB in particular Zn,

IIIA such as Al, Ga, In, or combinations thereof

further referred to as metal precursors and

VIA such as O, S, Se, Te or combinations thereof, further referred to as anion precursor,

wherein the composition of the nanoparticles is selected from the group consisting of l-lll- VI or l-ll-IV-VI, ll-lll-VI or ll-IV-V or mixtures thereof.

I- lll-VI group semiconductors in the sense of the present invention are CulnS2, AglnS2, CulnSe₂, AglnSe₂, CuGaS₂, CuGaSe₂, AgGaS₂, AgGaSe₂, CulnGaS, CuAglnS, CulnGaSe, CuAglnSe, AglnGaS, AglnGaSe, CuAgGaS, CuAgGaSe. Preferably, l-lll-VI group semiconductors in present invention are CulnS₂, AglnS₂, CulnSe₂, AglnSe₂, CuGaS₂, CulnGaS, CuAglnS, AglnGaS. Most preferably, l-lll-VI group semiconductors in present invention are CulnS₂, AglnS₂ because of lower toxicity.

II- IV-V group semiconductors in the sense of the present invention are e.g. ZnSiP₂, ZnGeP₂, ZnSnP₂, ZnSiN₂, ZnGeN₂, ZnSnN₂. ll-lll-VI group semiconductors in the sense of the present invention are e.g. ZnX₂Y₄ (X = Al, Ga, In; Y = S, Se, Te).

I-II-IV-VI group semiconductors in the sense of the present invention are e.g. Cu₂ZnSiS4, Cu?ZnGeS.i , Cu?ZnSnS.i .

Precursor for the element of the groups IB, MB or IIIA may be any inorganic or organometallic compound or elemental metal. For example Cu-precursors can be Cu-salts such as copper (I) acetate, copper (II) acetate, copper (II) chloride, copper (I) chloride, copper (I) iodide, copper (II) sulfate, or any mixture thereof. In-precursors can be indium salts such as indium acetate, indium chloride, indium sulfate, indium nitrate, or any mixture thereof. Silver source can be selected from silver nitrate, silver sulfate, silver acetate, or any mixture of them. Gallium source can be selected from gallium chloride, gallium acetate, gallium sulfate, gallium stearate or any mixture of them.

Se source can be selected from Selenium and bis(trimethylsilyl) selenide, or any mixture of them. Potential S-source may be S or S-containing compounds such as Thiourea, Bis(trimethylsilyl) sulphide or alkanethiol, etc. The alkanethiols can be mercaptans having one or more sulfhydryl functional groups, or a mixture of the mercaptans having one or more sulfhydryl functional groups. The mercaptan having one sulfhydryl functional group is preferably octyl mercaptan, iso-octyl-mercaptan, dodecyl mercaptan, hexadecanethiol or octadecanethiol, etc. The mercaptans having more sulfhydryl functional groups are preferably 1 ,8-dioctyl mercaptans or 1 ,6-dioctyl mercaptans, etc.

The source of metal chalcogenides (S, Se and Te) can also comprise so called "single source precursors", i. e. precursor molecules that consist of all necessary components needed for the synthesis of the intended product. It decomposes accordingly into constituents suited for the synthesis. In this case it can be but not restricted to e.g. metal mono- or dithiocarbamate/selenocarbamate, metal thiolates/selenolates, metal thiocaboxylates/selenocarboxylates, metal xanthate, metal mono- or dithiophosphate, metal trithiocarbonate, metal mono- or dithiol-complex, metal polysulfides, or metal-thiol-ether- complex (see also Narayan Pradhan et al.; J. Phys. Chem. B 107 (2003) 13843-13854) but also according compounds of this class which is know to an expert in the field.

Preferred shell material is Zn precursor showing good solubility at room temperature in the surfactant / solvent mixture. ZnS is preferred when octadecene (ODE) and dodecanethiol (DDT) are used.

It was found that during the ZnS coating process a diffusion of Zinc-ions into the CulnS2- core occurs resulting in a smaller becoming core of pure Cu l nS2 as a resu lt of an annealing/alloying process taking place at high temperatures used. In other words alloying process causes that the protecting shell thins out. This results in a blue shift of the fluorescence due to the then smaller core which results in a particle shell composed of Cu_xln_yZn2-x-yS2 with a gradual increase of x and y from the particle su rface to the remaining core of CulnS2. Thereby, x and y is 0 < x, y <1 with no preference on the value for |x - y|, but preferred is |x - y| < 0.3. Thereby the inner part of the particle with a CulnS2-rich composition becomes smaller (resulting in a stronger quantum confinement of the excited exciton) and at the same time the band gap widens approaching that of ZnS. The gradient layer Cu_xln_yZn2-x-_yS2 works further as buffer layer in-between CulnS2 and ZnS minimizing the lattice mismatch and the surface/interface defect density even more. The fluorescence quantum yield was found to be strongly increased. In some cases it cannot be excluded that the annealing/alloying process affects the whole particle and does not leave a pure CulnS2 core. Especially in the latter but also in the cases described above the tasks of the shell, effective saturation of the particle core and prevention of exciton leakage to the particle surface, cannot be provided efficiently any more. In the method of the present invention the fluorescence quantum yield is further increased when the ZnS coating process with subsequent annealing procedure of step 6) is repeated one or several times. Each ZnS treatment causes a fresh ZnS coating which may then initiate a new diffusion process of Zn-ions into the interior of the particle. Repetition of the ZnS coating process and annealing procedure allows the Zn- concentration within the particle develop a gradient which may affect the entire particle if suited process parameters (e.g. reaction time and temperature) are given. As a result an increase of up to 50% relative to the former quantum yield (i.e. increase from e.g. 40% to 60% or from 50% to 75%), was achieved for the first overcoating process whereby the fluorescence emission maximum stays almost constant, e.g. for a typical preparation of yellow-orange fluorescing particles at wavelengths between 590 and 600 nm (only a slight blue shift was observed), with an almost constant band width (range of maximum emission wavelength to be defined). The process and structure of the nanoparticles of the present invention is exemplified in Fig. 1.

It was found that a reasonable thick shell from 0.3 to 4 nm, preferably from 0.5 to 3 nm, most preferably from 0.5 to 1 .5 nm, pure ZnS outer shell is essential for a high and stable quantum yield.

The nanoparticles of the present inventions can have various shapes, morphologies (spherical particles, rods, plates, tetrapods, etc) and sizes. The nanoparticles of the present invention show a characteristic average particle size of up to 40 nm, in a preferred embodiment of from 0.5 to 20 nm , and in a very particularly preferred embodiment particles with characteristic dimensions of from 1 to 10 nm, characteristic dimension meaning the property-determining dimension, for example the diameter of rods or the diameter of tetrapod arms. The particle size distribution which can be achieved has usually a standard deviation of ± 10 nm, preferably of ± 5 nm and particularly preferably of ± 2 nm. A particle size distribution may be established and evaluated, for example, by a statistical analysis of transmission electron microscopy images.

The size and morphology of particles in particular core size and shell thickness can be varied by adjusting concentration and concentration ratios of precursor materials, ligand types and solvents as well as process parameters like temperature, temperature ramps, order and dynamics of addition of precursors and so on.

Solvent can be any non-coordinating long chain molecules (1 0 or more than 1 0 C) preferably non polar or low polar. The polarity index can vary from 4 to 0, preferably 1.5 to 0, most preferably 0.8 to 0, based on the polarity index of water being 9, according to the polarity scale (V.J. Barwick, Trends in Analytical Chemistry, vol. 16, no. 6, 1997, p.293ff, Table 5). The solvent is a high boiling point solvent, preferably without any double bond in the chain to prevent any side reaction. The organic solvents, during the reaction temperature, should be stable and degrade as little as possible. Preferably the boiling point of organic compound is above 200 °C, more preferably above 240 °C. Among others suitable solvents are octadecene, 1 -hexadecene, 1 -eicosene, paraffin wax, diphenyl ether, benzyl ether, dioctyl ether, squalane, trioctylamine, heat transfer fluids or any solvent mixture thereof.

Ligands can be phosphine (oxide), phosphonic acid, phosphinic acid, alkyl, amine, thiol and carboxylic acid. Suitable ligands are oleylamine, oleic acid, trioctylphosphine, trioctylphosphine oxide, myristic acid, low molecular weight thiols (from 8 to 18 C ) such as dodecanethiol, tetradecanethiol, hexadecanethiol, as well as mixture thereof. For better luminescence quantum yield in the production of CulnS2 thiols are preferred and dodecanethiol, tetradecanethiol and hexadecanethiol are most preferred.

The properties of nanoparticles such as particle size and particle morphology are characterized using transmission electron microscopy (TEM, Philips CM 20). The photoluminescence of nanoparticles are tested by UV / VIS absorption (Jena Analytics, Specord) and photoluminescence spectroscopy (Fluorolog 3, Jobin Yvon).

Characterization further includes Thermogravimetric Analysis-Mass Spectroscopy (TGA- MS), X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Photoluminescence (PL). The method of the present invention can be extended for the preparation of nanoparticles with different morphology. For example, it can include ternary copper indium sulphide or quaternary copper indium zinc sulphide or doped/alloyed copper indium sulphide so composition is CUxln_yZn_zSn. Reverse type I particles of morphology ZnS@ Cu_xln_yS_n.@ZnS can also be synthesized to widen the accessible color range.

A further advantage of the process of the present invention is that color can be tuned from 500 to 920 nm. Surprisingly, it turned out that when starting with an alloy of Cu_xln_yZn2-x-_yS2 the blue shift of the fluorescence emission maximum was much less pronounced than when starting with pure CulnS2 as a core. Thus, for the preparation of particles with longer (more reddish) emission wavelengths it is advantageous to start with alloys. Applying the procedure above on an alloy core (but modified to achieve larger core diameters) with an coating of ZnS results in deep red fluorescing particles with quantum yields of approx. 40%. Additional ZnS treatments and subsequent annealing again raised the quantum yield for up to 50% relative to the original value.

To synthesize green fluorescing particles the synthesis conditions (higher amount of coordinating ligands, shorter reaction times and temperatures) were changed to start with smaller particles sizes and pure CulnS2 was used as core to utilize the stronger blue shift after addition of ZnS for the shell growth . The final particle structure was therefore CulnS2@Cu_xln_yZn2-x-_yS2@ZnS . Agai n , add itional ZnS treatments and su bseq uent annealing for the outer shell raised the quantum yield for up to 50% relative to the original value.

The method of the invention also allows starting with a pure ZnS core followed by the growth of a first CulnS2 shell before the above proposed ZnS treatments and subsequent annealing. This procedure ensures a larger band gap inner core which after the above described, annealing induced diffusion processes, provides blue and green fluorescing nanoparticles with high quantum yield.

I n a further step of the method of the present invention the surface ligand can be exchanged to modify solubility of the obtained nanoparticle in a given environment. For example solubility in matrices like siloxanes, polyepoxides can be obtained for embedding the nanoparticles into special polymers or water soluble nanoparticles can be obtained for bio-imaging applications. Important for a commercial application is the adaptability of the method of the invention to a contin u ous prod uction process. U ntil now, as far as we know, syntheses of nanoparticles on CulnS2 and its related structures are all based on the batch route, which inevitably meet the difficulties of heat and mass transfer. The recently introduced microfluidic methods can realize operation in closed systems, and the precise control of synthetic conditions offered by the strengthened heat and mass transfer makes the reproducible reaction achievable. Applicability of the synthesis of ternary or quaternary nanoparticles with the composition IB-IIIA-VIA (IB = Cu, Ag; IIIA = In, Ga, Al; VIA = S, Se, Te) according to the present invention by a continuous route based on microreaction technology is shown.

Comparing to the batch synthesis, the continuous synthesis route

1 ) allows the synthesis of the mentioned nanoparticles with narrow size distribution due to separation of nucleation and crystal growth

2) has the flexibility to adjust the particle size by parameters' adjustments

3) has the flexibility to grow in principle an unlimited number of shells onto the nanoparticles

4) shows an excellent reproducibil ity due to the precise reaction cond itions' controlling

5) is highly suitable for large scale production

For the synthesis of Cu_xln_yZn2-x-_yS2@ZnS semiconductor nanoparticles it was found that they can be synthesized when one starts from a reaction mixture that contains cation and anion precursors, surfactants and an organic reaction medium. The reaction mixture is pumped into a suitable micro-heat exchanger to heat up the reaction solution, then runs through a residence microreactor before a reaction mixture that contains cation and/or anion precursors for an additional shell coating is added. The latter process can be repeated as often as necessary to achieve a high fluorescence quantum yield and high stability. Then the particle containing solution is cooled quickly.

First object of the present invention is therefore a method for the preparation of semiconducting core-shell nanoparticles comprising elements of the groups I B, M B, I I IA, IVA, VA or VIA of the periodic classification wherein the composition of the core nanoparticle is selected from the group consisting of IB-VIA, IIB-VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA or IIB-IVA-VA or mixtures thereof, and at least one shell comprises elements of the groups MB and VIA, wherein in case the core composition is IIB- VIA the shell comprises at least a further element selected from the groups IB, II IA, IVA or VA, said method comprising the following steps:

a. ) Core reaction mixture comprising at least one cation metal precursors and an anion precursor is dissolved in a ligand and solvent mixture at a temperature that is still beneath the threshold at which nucleation takes place, b. ) Core reaction mixture is heated above nucleation threshold temperature and then kept at the reaction temperature for core nucleation,

c. ) Optionally the heated reaction mixture is kept at a temperature well suited for core growth but lower than the nucleation temperature,

d. ) Shell reaction mixture prepared separately comprising at least one cation metal precursors and an anion precursor and heated at a temperature that is still beneath the threshold at which nucleation takes place is added to the reaction mixture of step b) or c) for shell coating of the core nanoparticles, e. ) reaction mixture of step d) is heated and hold to a shell growth temperature, f. ) optionally for ZnS shell preparation shell reaction mixture comprising ZnS shell material is prepared separately and heated at a temperature that is still beneath the threshold at which nucleation takes place is added to the reaction mixture of step d) or e) for shell coating of the nanoparticles, g. ) reaction mixture of step f) is heated and hold to a shell growth temperature, h. ) reaction mixture is then cooled down to prevent further particle growth.

Optional steps f) and g) were shown to lead to a higher quantum yields.

In particular reverse type I particles of morphology ZnS @ABC2@A_xB_yZn2-x-_yC2@ZnS, ZnS@AxByZn2-x-yC2@ABC2@AxByZn₂-x-yC2@ZnC o r Z n S @A_xByZn₂-x-yC2@ZnS are prepared with this method.

In a particular embodiment core size and shell thickness are varied by way of adjusting concentration and concentration ratios of precursor materials, ligand types and solvents as well as process parameters like temperature, temperature ramps, order and dynamics of addition of precursors. Also the repetition of one or several of the individual steps belongs to the particular embodiment.

In a particular embodiment the method of the present invention is conducted in a microreactor system comprising elements selected from the group comprising for step a) and d) micromixers, for step b), c) and e) residence reactors preferred residence microreactors, and for step f) a micro heat exchanger, wherein micromixers and residence reactors also comprise micro heat exchangers and elements are connected with each other so the reaction mixture flows continuously from one to the next element.

Further objects of the present invention are therefore:

- Semiconducting core-shell nanoparticle comprising elements of the groups I B, M B, I IIA, IVA, VA or VIA of the periodic classification wherein:

• the composition of the core nanoparticle is selected from the group consisting of IB-VIA, IIB-VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA, IIB-IVA- VA or mixtures thereof, and

· at least one shell comprises elements of the groups M B and VIA, , wherein in case the core composition is IIB-VIA the shell comprises at least a further element selected from the groups IB, IIIA, IVA or VA,

• having an emission maximum at wavelengths around 580-800 n m and a quantum yield≥ 60 %.

In particular a semiconducting core-shell nanoparticle based on IB, MB, IIIA, VIA elements of the periodic classification as mentioned above obtainable by the method of the invention,

- semiconducting core-shell nanoparticles with the general formula:

- ABC2@AxByZn2-x.yC2 or

- A_xByZn2-x-yC2@ZnS or

- ABC2@A_xByZn₂-x-yC2@ZnS or

- ZnS @ABC2@A_xByZn₂-x-yC2@ZnS,

- ZnS@ A_xByZn₂-x-yC2@ZnS or

- ZnS@AxByZn2-x-yC2@ABC2@AxByZn₂-x-yC2@ZnS

wherein A = one or more element of the group IB, B = one or more element of the group IIIA or C = one or more element of the group VIA, and

wherein x and y is 0 < x and y < 1 with no preference on the value for |x - y| with a gradual increase of x and y from the particle surface to the remaining core

In particular wherein A = Cu or Ag, B = Al, Ga or In, C = O, S, Se or Te and wherein x and y is 0 < x,y <1 with no preference on the value for |x - y| with a gradual increase of x and y from the particle surface to the remaining core.

In particular, semiconducting core-shell nanoparticles with the general formula:

- CulnS2@Cu_xln_yZn2-x-yS2 or

- Cu_xln_yZn2-x-yS2@ZnS - CulnS2@CUxln_yZn2-x-yS2@ZnS or

- ZnS @CulnS2@CUxln_yZn₂-x-yS2@ZnS

- ZnS@ CUxln_yZn2-x-yS2@ZnS or

- ZnS@CUxln_yZn2-x-yS2@CulnS2@ Cu_xln_yZn2-x-yS2@ZnS wherein x and y is 0 < x,y <1 with no preference on the value for |x - y| with a gradual increase of x and y from the particle surface to the remaining core.

The nanoparticles prepared with the process provided of the present invention can be applied in the fields of bio-labeling, light-emitting diode, thin-film solar cell, polymer solar cell, etc.

Further objects of the present invention are therefore a formulation in particular an ink or device comprising the semiconducting core-shell nanoparticle of the present invention.

Example 1 : Preparation of red luminating CulnS2 core particles

74 mg Copper(l)acetate (97%), 175 mg lndium(lll)acetate (99.99%), 15 ml 1 -Octadecene (90%) and 1 .5 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2. The solution was rapidly heated up to 240 °C under N2 for 60 min and cooled down to room temperature. The fluorescence emission peak of the dispersion was located at 685 nm with a full width half maximum (FWHM) of 140 nm. The quantum yield is 5 % based on sulforhodamine B as reference.

Example 2: Preparation of red luminating CulnS2 core particles

26.6 mg Copper(l) Iodide (98%), 40 mg lndium(lll)acetate (99.99%), 19.7 ml 1 - Octadecene (90%) and 0.3 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 5 min and purged with N2. The solution was rapidly heated up to 220 °C under N2 for 40 min get CulnS2 core particles. The quantum yield is 15.2 % based on sulforhodamine B as reference.

Example 3: Preparation of yellow luminating CulnS2@Cu_xln_yZn2-x-_yS2 core - gradient shell particles

74 mg Copper(l)acetate (97%), 175 mg lndium(lll)acetate (99.99%), 15 ml 1 -Octadecene (90%) and 1 .5 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2 for several times (Precursor 1 ). Then 220 mg Zinc acetate (99.99%), 6 ml 1 -Octadecene (90%) and 3 ml Dodecanethiole (98%) were added into a sample vial and heated to 100°C in an oil bath to dissolve chemicals until a transparent or almost transparent solution was formed (Precursor 2). Precursor 1 was rapidly heated up to 240^°C under N2 (for 60 min). Precursor 2 was then rapidly injected into precursor 1 so that the reaction mixture rapidly cooled down to 220^°C. The reaction mixture was hold at this temperature for 240 min before cooled slowly to room temperature. The fluorescence emission peak of the dispersion was located at 594 nm with a full width half maximum (FWHM) of 1 15 nm. The quantum yield was 55 % based on sulforhodamine B as reference.

Example 4: Preparation of yellow luminating CulnS2@CuxlnyZn2-x-yS2@ZnS core - gradient shell - shell particles

In the synthesis, 24ml of obtained CulnS2@Cu_xln_yZn2-x-_yS2 described in example 3 was given into a 3-neck RBF, degassed for 30 min and purged with N2. 220 mg Zinc acetate (99.99%), 6 ml 1 -Octadecene (90%), 3 ml Dodecanthiole and 2 ml oleic acid was prepared and added in 3 consecutive batches. The reaction was carried out at 220°C. The first batch was injected by the syringe pump (1 1 ml at 1 ml/min), the second batch was injected after 30 min and the third batch was injected after another 30 min. Compared to the preparation of example 2 the quantum yield has increased to 78 % based on sulforhodamine B as reference. The fluorescence emission peak of the dispersion was located at 595 nm with a "full width of half maximum" (FWHM) at about 1 15 nm.

Example 5: Preparation of green luminating CulnS2@CuxlnyZn2-x-yS2@ZnS core - gradient shell - shell particles

76 mg Copper(l)acetate (97%), 175 mg lndium(lll)acetate (99.99%), 15 ml 1 -Octadecene (90%) and 3 ml Dodecanethiole (98%) (70%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2 (Precursor 1 ). The solution was heated to 220°C (10 mins). Then 1 .7554 g Zinc acetate (99.99%), 24 ml 1 -Octadecene (90%), 12 ml Dodecanethiole (98%), 8 ml Oleylamine (70%) and 8 ml TOP were added into a sample vial and heated to 50 °C in an oil bath to dissolve chemicals until a transparent solution was formed (Precursor 2). Precursor 1 temperature was rapidly cooled to 50oC from 220oC (in 2mins) using the water bath. At 50 °C Precursor 2 was injected into reaction. Reaction was left to mix at 50 °C for 5 mins and the temperature was raised to 240 °C. Reaction was carried out 240 °C for 30min . I n the further overcoating synthesis, 28ml of obtained CulnS2@Cu_xln_yZn2-x-_yS2 described above was added in to a 3-neck RBF, degassed for 30 min and purged with N2. Then separately 2 batches were prepared with each containing 220 mg Zinc acetate (99.99%), 6 ml 1 - Octadecene (90%), 3 ml Dodecanethiole (98%) and 2ml Oleylamine (70%) which were added into a sample vial and heated to 100°C in an oil bath to dissolve chemicals until a transparent or almost transparent solution was formed (Precursor 3). First batch of precursor 3 was injected into 28ml of obtained CulnS2@Cu_xln_yZn2-x-_yS2 reaction solution at 220°C using a syringe pump (1 1 ml at 1 ml/min). Then after 1 ^st injection, let it react for 30 min. Then the second batch was injected at 220°C using a syringe pump (1 1 ml at 1 m l/mi n ). Then the reaction solution was held at 220° C for another 30 mi n . The fluorescence emission peak of the dispersion was located at 545 nm with a "full width of half maximum" (FWHM) at about 1 15 nm. The quantum yield was 33 %.

Example 6: Preparation of red luminating CulnS2@CuxlnyZn2-x-yS2@ZnS core - gradient shell - shell particles

76 mg Copper(l)acetate (97%), 175 mg lndium(lll)acetate (99.99%), 1 10 mg Zinc acetate (99.99%), 15 ml 1 -Octadecene (90%) and 1 .5 ml Dodecanethiole (98%) (70%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2 (Precursor 1 ). The solution was heated to 190°C. 6 ml Oleylamine was injected at 190°C and the solution was heated to 280°C until the solution turns dark red and held for 30 min. Then the solution was rapidly cooled to 220°C. 877 mg Zinc acetate (99.99%), 24 ml 1 - Octadecene (90%), 12 ml Dodecanethiole (98%) and 8 ml Oleylamine (70%) were added into a sample vial and heated to 50 °C in an oil bath to dissolve chemicals until a transparent solution was formed (Precursor 2). Precursor 2 was injected into precursor 1 reaction solution at 220oC using the syringe pump (30ml+30ml at 3ml/min) for 10min injection. Reaction was carried out 220oC for 90min. In the further overcoating synthesis, 26 ml of obtained CulnS2@Cu_xln_yZn2-x-_yS2 described above was added in to a 3-neck RBF, degassed for 30 min and purged with Is .Then separately 2 batches were prepared with each containing 220 mg Zinc acetate (99.99%), 6 ml 1 -Octadecene (90%), 3 ml Dodecanethiole (98%) and 3 ml Oleylamine (70%) which were added into a sample vial and heated to 100°C in an oil bath to dissolve chemicals until a transparent solution was formed (Precursor 2). The first batch of the preheated Precursor 2 was injected into precursor 1 reaction solution at 220 °C using the syringe pump (12 ml at 1 ml/min) for 12 min. After further 30 min. the second batch was injected at 220°C with the same rate (12 ml at 1 ml/min) and thereafter held at that temperature for another 30 min. Thereafter the solution was cooled down to room temperature. The fluorescence emission peak of the dispersion is located at approx. 660 nm with a "full width of half maximum" (FWH M) at about 130 nm. The quantum yield was 54 %.

Example 7: Preparation of IR luminating CulnS2@CuxlnyZn2-x-yS2@ZnS core - gradient shell - shell particles

76 mg Copper(l)acetate (97%), 175 mg lndium(lll)acetate (99.99%), 15 ml 1 -Octadecene (90%) and 0.9 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2 (Precursor 1 ). The solution was heated to 260°C and reaction was carried out at 260°C for 15mins. Then 438.9 mg Zinc acetate (99.99%), 12 ml 1 -Octadecene (90%), 6 ml Dodecanethiole (98%), 4 ml Oleylamine (70%) and 4 ml TOP were added into a sample vial and heated to 50 °C in an oil bath to dissolve chemicals until a transparent solution was formed (Precursor 2). Precursor 1 temperature was rapidly cooled to 50°C from 260°C (in 2mins) using the water bath. At 50°C Precursor 2 was injected into reaction . Reaction was left to mix at 50oC for 5 mins and the temperature was raised to 200°C. Reaction was carried out 200°C for 30min. In the further overcoating synthesis, 16.5 ml of obtained CulnS2@Cu_xln_yZn2-x-_yS2 described above was added in to a 3-neck RBF, degassed for 30 min and purged with Is Then separately 2 batches were prepared with each containing 220 mg Zinc acetate (99.99%), 6 ml 1 - Octadecene (90%), 3 ml Dodecanethiole (98%) and 3 ml Oleylamine (70%) which were added into a sample vial and heated to 100 °C in an oil bath to dissolve chemicals until a transparent solution was formed (Precursor 3). The obtained CulnS2@Cu_xln_yZn2-x-_yS2 was heated to 220°C before Precursor 2 was injected using the syringe pump (12 ml at 1 ml/min) for 12 min. After further 15 min. the second batch was injected at 220°C with the same rate (12 ml at 1 ml/min) and thereafter held at that temperature for another 15 min before it was cooled down to room temperature. The fluorescence emission peak of the dispersion is located at approx. 740 nm with a "full width of half maximum" (FWH M) at about 150 nm. The quantum yield was 42 %.

Example 8: Preparation of orange highly luminescing CulnS2@CuxlnyZn2-x- yS2@ZnS core - gradient shell - shell particles with shortened reaction times

Core precursor solution preparation: 1 03.9 mg Copper(l )acetate (97%), 240.1 mg lndium(lll)acetate (99.99%), 50 ml 1 -Octadecene (90%) and 3 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2. The solution was rapidly heated up to 190 °C under N2 for 45 min and cooled down to room temperature, then 67 ml of ODE was added and continue stirring for another 10 min to get core precursor solution.

Shell precursor solution preparation: 120.7 mg Zinc acetate (99.99%), 83 ml 1 - Octadecene (90%), 16 ml Dodecanethiole (98%), 1 1 ml Oleylamine (70%) and 1 1 ml TOP were added into a 3-neck round bottomed flask (RBF) and degassed for 30 min and purged with N2. The solution was heated up to 100 oC under N2 for 30 min and cooled down to room temperature to dissolve chemicals until a transparent shell precursor solution was formed.

Extract 10ml of core precursor solution into RBF, degassed for 10min and switched to N2. The solution was heated to 240 °C and maintained heating for 30 min and cooled down to 60 °C in 3 min with cooling water bath. Then 10ml_ of previously prepared shell precursor solution was added and stirred for 5min at 60 °C. The mixture was heated to 230 °C and maintained heating for 15 min. The fluorescence emission peak of the dispersion is located at approx. 604 nm with a "full width of half maximum" (FWHM) at about 120 nm. The quantum yield was 62%.

Example 9: Preparation of red highly luminating CulnS2/ZnS core / shell particles with with shortened reaction times Core preparation: 26.6 mg Copper(l) Iodide (98%), 40 mg lndium(ll l)acetate (99.99%), 19.7 ml 1 -Octadecene (90%) and 0.3 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 5 min and purged with N2. The solution was rapidly heated up to 220 °C under N2 for 45 min to get the CulnS2 core particles with a quantum yield of 1 1 %.

Shell precursor solution preparation: 1207 mg Zinc acetate (99.99%), 83 ml 1 -Octadecene (90%), 16 ml Dodecanethiole (98%), 1 1 ml Oleylamine (70%) and 1 1 ml TOP were added into a 3-neck round bottomed flask (RBF), degassed for 30 min and purged with N2. The solution was heated up to 100 °C under N2 for 30 min and cooled down to room temperature to dissolve chemicals until a transparent shell precursor solution was formed.

Sample preparation of core-shell particles: 30 mL of above shell precursor solution was added to the core solution and thereby stirred, degassed, purged with N2. The solution was continued to heat to 230 °C and at this temperature held for 75 min to achieve the CulnS2/ZnS core/shell particles. The fluorescence emission peak of the dispersion is located at approx. 663 nm with a "full width of half maximum" (FWHM) at about 160 nm. Thereafter the solution was cooled down to room temperature. The fluorescence emission peak of the dispersion is located at approx. 670 nm with a "full width of half maximum" (FWHM) at about 160 nm. The quantum yield was 81 %.

Example 10: Preparation of CulnS2 core particles by continuous route with single injection

The modular microreactor system was set up consisting of Microreactor 1 (Sandwich reactor, article no. 0213, Ehrfeld Mikrotechnik BTS GmbH) and heat exchanger (Coaxial heat exchanger, article no. 0309, Ehrfeld Mikrotechnik BTS GmbH) was set up according to the figure 3.

The whole modular microreactor was cleaned with ODE until the outlet solution becomes transparent and no bubbles remained in the tube; the sealing effect of all modules was checked and the temperature increased to 250°C for the precursor injection.

P reparation of precu rsor sol ution 1 : 1 60 mg Copper(l ) I od ide (98% ), 240 mg lndium(lll)acetate (99.99%), 1 16 ml 1 -Octadecene (90%) and 4 ml Dodecanethiole (98%) were added in a 3-neck round bottomed flask (RBF), degassed for 5 min and purged with N₂.

The precursor 1 solution was injected at 2 ml/min into the microreactor being adjusted to 250°C. The so formed CulnS2 solution exited the microreactor and was cooled via a heat exchanger with cooling water and collected after 5 minutes. The fluorescence emission peak of the dispersion was located at approx. 690 nm with a "full width of half maximum" (FWHM) at about 135 nm. The quantum yield was 22%.

Example 11 : Preparation of CulnS2 core particles by continuous route with double injection

Core precursor solution preparation is done according to example 7. The modular microreactor system was set up consisting of Microreactor 1 and 2 (Sandwich reactor, article no. 0213, Ehrfeld Mikrotechnik BTS GmbH), Micromixer (Valve-assisted mixer, article no. 01 1 1 , Ehrfeld Mikrotechnik BTS GmbH) and heat exchanger (Coaxial heat exchanger, article no. 0309, Ehrfeld Mikrotechnik BTS GmbH) was set up according to the figure 4.

The whole modular microreactor was cleaned with ODE until the outlet solution becomes transparent and no bubbles remained in the tube; the sealing effect of all modules was checked and the temperature increased to 250°C and 220°C for the first precursor injection and the second precursor injection, respectively.

The precursor 1 solution was injected at 2 ml/min into the first microreactor being adjusted to 250°C. The so formed CulnS2 solution exited the first microreactor and was flown into a micromixer. Further precursor solution was injected into the same micromixer at 2 ml/min. The two solutions were then mixed in the micromixer before entering the second microreactor being adjusted to a temperature of 220°C. The color of the solution became much lighter when it exited the second microreactor. Then the solution was cooled via a heat exchanger with cooling water and collected after 5 minutes. The fluorescence emission peak of the dispersion was located at approx. 674 nm with a "full width of half maximum" (FWHM) at about 120 nm. The quantum yield was 6.1 %.

Example 12: Preparation of CulnS2@CuxlnyZn2-x-yS2@ZnS core - gradient shell - shell particles CulnS2 core particles by continuous route

Core and shell precursor solution preparation is done according to example 7.

Core precursor solution preparation is done according to example 7. The modular microreactor system was set up consisting of Microreactor 1 and 2 (Sandwich reactor, article no. 0213, Ehrfeld Mikrotechnik BTS GmbH), Micromixer (Valve-assisted mixer, article no. 01 1 1 , Ehrfeld Mikrotechnik BTS GmbH) and heat exchanger (Coaxial heat exchanger, article no. 0309, Ehrfeld Mikrotechnik BTS GmbH) was set up according to the figure 4. The whole microreactor system was cleaned with ODE until the outlet solution becomes transparent and no bubbles remained in the tube; the sealing effect of all modules was checked and the temperature increased to 250°C and 240°C for the first precursor injection and the second precursor injection, respectively.

The core precursor solution was injected at 1 ml/min into the first microreactor being adjusted to a temperature of 250°C. The so formed CulnS2 solution exited the first microreactor and was flown into a micromixer. Further the shell precursor solution was injected into the same micromixer at 1 ml/min. Both solutions were mixed before entering the second microreactor being adjusted to 240°C. The color of the solution became much lighter when it exited the second microreactor. Then the solution was cooled via heat exchanger with cooling water and collected after 5 minutes. The fluorescence emission peak of the dispersion was located at approx. 612nm with a "full width of half maximum" (FWHM) at about 120 nm. The quantum yield was 31 %.

Figures:

Fig. 1 : Structure of the nanoparticles of the present invention.

Fig. 2: Absorption and Photoluminescence spectra of CulnS2 core shell nanoparticles as synthesized in the examples: a (example 5), b (example 4), c (example 6), d

(example 7).

Fig 3: Diagram of the microreactor system as used in example 10.

Fig 3: Diagram of the microreactor system as used in example 1 1 und 12.

Claims

Method for the preparation of semiconducting core-shell nanoparticles comprising elements of the groups IB, MB, IIIA, IVA, VA or VIA of the periodic classification wherein the composition of the core nanoparticle is selected from the group consisting of IB-VIA, IIB-VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA or IIB-IVA-VA or mixtures thereof, and at least one shell comprises elements of the groups MB and VIA, , wherein in case the core composition is IIB-VIA the shell comprises at least a further element selected from the groups IB, IIIA, IVA or VA, said method comprising the following steps:

Method according to claim 1 wherein a ternary core or a ternary first shell is respectively used and a molecular ratio of metal precursors in the core reaction mixture of step a) is from (1 - 2):(1 - 2), the anion precursor being in excess.

Method according to one of the claims 1 or 2 wherein reaction mixture of step c) cooled down before step d).

Method according to one of the claims 1 to 3 wherein core size and shell thickness respectively are varied by way of adjusting concentration and concentration ratios of precursor materials, ligand types and solvents as well as process parameters like temperature, temperature ramps, order and dynamics of addition of precursors.

Method according to one of the claims 1 to 4 wherein shell growth is allowed to proceed until the outer shell shows a thickness for from 0.3 to 4 nm of pure shell material.

Method according to claim 5 wherein a ZnS shell is prepared with a thickness from 0.3 to 4 nm of pure shell material is obtained by way of repeating steps for ZnS shell preparation.

Method according to one of the claims 1 to 6 conducted in a microreactor system comprising elements selected from the group comprising micromixers for mixing steps, residence microreactors for reaction steps, and a micro heat exchanger for cooling step, wherein micromixers and residence reactors also comprise micro heat exchangers and elements are connected with each other so the reaction mixture flows continuously from one to the next element.

Semiconducting core-shell nanoparticle comprising elements of the groups IB, MB, IIIA, IVA, VA or VIA of the periodic classification wherein:

the composition of the core nanoparticle is selected from the group consisting of IB- VIA, IIB-VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA, IIB-IVA-VA or mixtures thereof, and

at least one shell comprises elements of the groups MB and VIA, , wherein in case the core composition is IIB-VIA the shell comprises at least a further element selected from the groups IB, IIIA, IVA or VA,

having an emission maximum at wavelengths around 580-800 nm and a quantum yield ≥ 60 %.

Semiconducting core-shell nanoparticle according to claim 8-obtainable by the method according to one of the claims 1 to 7.

10. Semiconducting core-shell nanoparticle according to one of the claims & 8 to 9 comprising a ZnS shell wherein ZnS shell have a thickness from 0.3 to 4 nm of pure shell material.

1 1 . Semiconducting core-shell nanoparticle with the general formula:

- ABC2@AxByZn2-x.yC2 or

- A_xByZn2-x-yC2@ZnS or

- ABC2@A_xByZn₂-x-yC2@ZnS or

- ZnS @ABC2@A_xByZn₂-x-yC2@ZnS or

- ZnS@ A_xByZn₂-x-yC2@ZnS

- ZnS@AxByZn2-x-yC2@ABC2@AxByZn₂-x-yC2@ZnS

wherein A = one or more element of the group IB, B = one or more element of the group IMA or C = one or more element of the group VIA, and

wherein x and y is 0 < x and y < 1 with no preference on the value for |x - y| with a gradual increase of x and y from the particle surface to the remaining core.

12. Semiconducting core-shell nanoparticles according to claim 1 1 wherein A = Cu or Ag, B = Al, Ga or In, C = O, S, Se or Te and wherein x and y is 0 < x, y <1 with no preference on the value for |x - y| with a gradual increase of x and y from the particle surface to the remaining core.

13. Semiconducting core-shell nanoparticle according to claim 12 wherein A=Cu,

B=ln and C=S.

14. Formulation comprising the semiconducting core-shell nanoparticle according to one of the claims 8 to13.

15. Device comprising the semiconducting core-shell nanoparticle according to one of the claims 8 to13.

16. Device according to claim 15 wherein the device is an electronic device.