Inorganic nanocrystals with an organic coating and preparation process thereof

TECHNICAL AREA

The invention relates to inorganic nanocrystals which are provided with an organic coating layer, and to the materials formed by these nanocrystals.

More specifically, the invention relates to materials constituted by nanocrystals constituted by at least one metal and / or at least one semiconductor compound comprising at least one metal, and possibly a shell consisting of one or more layers of a metal and / or at least one semiconductor compound comprising at least one metal, in which the external surface of said nanocrystals is provided and coated with an organic coating layer, in particular a layer of ligands such as chelate ligands intended in particular to ensure the stabilization of nanocrystals.

In the case where they comprise one (or more) shell, the nanocrystals are also called "core / shell nanocrystals" (or "core-shell" in English). The invention relates in particular to luminescent materials consisting of nanocrystals comprising a semiconductor core A (II) B (VI), preferably surrounded by a shell, and an organic coating layer. More specifically, the invention relates to nanocrystals with a CdSe core covered by a ZnSe shell and by an organic coating layer.

The invention further relates to a process for the preparation of these nanocrystals and materials.

The technical field of the invention can be generally defined as that of inorganic nanocrystals, more precisely nanocrystals consisting of at least one metal and / or at least one semiconductor compound, said semiconductor compound comprising at least one metal .

Nanocrystals can be defined as "nanometric" crystalline objects, that is to say that their size is generally less than 150 Å and preferably in the range from 12 to 150 Å.

Inorganic nanocrystals find applications in many sectors.

A large number of applications of inorganic nanocrystals and particularly semiconductor nanocrystals is based on their ability to emit light. Such fluorescent semiconductor nanocrystals can be used, for example, as fluorescent markers for chemical or biological molecules or in electroluminescent devices.

In order to obtain an emission in the visible spectrum, one works essentially with the nanocrystals of semiconductors with small prohibited bandwidth ("gap") by taking advantage of the fact of being able to choose, for a given material, the color emitted according to of the size of the nanocrystal.

The luminescent semiconductor nanocrystals generally consist of semiconductors of formula A (II) B (VI), in which A represents a metal or a metalloid in the oxidation state + II, and B represents an element chemical, such as a metal or a metalloid, in the oxidation state -II.

Semiconductor crystals A (II) B (VI) such as ZnS, ZnSe, CdS, CdSe or CdTe are materials known for their photoluminescence properties for several decades.

In the years 1980-90, it was shown that their absorption and emission spectra depend on the size of the crystal when it becomes sufficiently small. For crystals whose size is approximately in the range from 1 to 10 nm, which are then called "nanocrystals" or "quantum dots" ("quantum dots" in English), this dependence is extremely pronounced (Angew. Chem. Int. Ed. Engl., 32, pages 41-53 (1993)).

Therefore, the entire palette of visible and near infrared and ultraviolet colors can be obtained with semiconductor nanocrystals A (II) B (VI) by the appropriate choice of their size and composition.

In general, the optical quality of a luminescent material made of nanocrystals depends on several parameters, the most important of which are:<ul id="ul0001" list-style="dash" compact="compact"><li>the size of the nanocrystals, which regulates the wavelength of the emission and therefore the color emitted, as already indicated;</li><li>the size distribution of the nanocrystals, which controls the width of the emission line;</li><li>the passivation of the nanocrystals surface which is responsible for the quantum fluorescence yield.</li></ul>

There are several methods for preparing nanocrystals A (II) B (VI).

The first processes for the preparation of these crystals were developed in an aqueous medium (J. Am. Chem. Soc., 109, pages 5 649-5 655 (1987); J. Phys. Chem., 98, pages 7 665-7 673 (1994)) or in mixtures of water and organic medium: this is, for example, the process known as "reverse micelles" (Langmuir, 13, pages 3,266-3,276 (1997)).

These aqueous synthesis routes of the nanocrystals of semiconductors A (II) B (VI) nevertheless lead to samples whose reduced luminescence efficiency and the large spectral width are major technological drawbacks.

It is for these reasons that other preparation methods have been developed.

The article by J. Am. Chem. Soc., 115, pages 8 706-8 715 (1993) thus describes an organometallic synthesis pathway for CdB nanocrystals (where B = S, Se, Te). The main advantage of this organometallic process compared to the processes in aqueous medium mentioned above lies in the fact that the nanocrystals thus prepared have a much better size distribution, which is generally less than 10%.

This size distribution leads to a much narrower emission spectrum, which is of course advantageous for technological applications. The main routes of organometallic synthesis for the preparation of CdSe nanocrystals are as follows:<ul id="ul0002" list-style="dash" compact="compact"><li>the conventional organometallic synthesis route consists in reacting dialkylcadmium, preferably dimethyl or diethylcadmium with selenium, both dispersed in trioctylphosphine (TOP); this dispersion is injected into trioctylphosphine oxide (TOPO) at high temperature, which serves as a reactive medium. This synthesis is described in the document J. Am. Chem. Soc., 115, pages 8 706-8 715 (1993);</li><li>a modification of the preceding synthesis process has been described in the document Nanoletters, 1 (4), pages 207-211 (2001), in this process, the reactive medium, instead of TOPO, is a mixture of TOPO and hexadecylamine (HDA). The authors report an improvement in the width of the size distribution which is around 5%;</li><li>another modification was proposed by ZA PENG and X. PENG, in the document J. Am. Chem. Soc., 123, pages 183-184 (2001); these authors describe an inorganic / organometallic synthesis pathway for CdB nanocrystals (where B is Se, S or Te) in which they replace the pyrophoric and toxic dialkylcadmium compounds with cadmium oxide (CdO) complexed by an alkylphosphonic acid. The main advantage of this process is that CdO constitutes a much less reactive and easier to handle cadmium source than dialkylcadmium compounds. Another advantage is that the nanocrystals thus prepared have a very small size distribution, generally less than 10%, which leads to a narrow emission spectrum with a line width at half height of the order of 30 nm.</li></ul>

The preparation methods described above do not solve the problem of the quantum fluorescence yield which remains low, namely typically between 5 and 10% for CdSe crystals.

In the article J. Phys. Chem., 100, pages 468-471 (1996), a method is proposed for increasing this yield. It consists in passivating nanocrystals which are in CdSe, in this case, by growing around this “core” a shell of a second semiconductor with a bandwidth gap greater than that of the core, this semiconductor being constituted by ZnS. This system is called "heart / shell" in the scientific literature. The procedures used for the deposition of the shell are essentially the same as those used for the preparation of the heart. The deposition of a ZnS shell is done, for example, with highly reactive compounds, which are diethyl zinc and bis (trimethylsilyl) sulfide. After growth of the shell, the authors observe an increase in the quantum fluorescence yield towards values ​​of between 10 and 50%, or even greater than 50%, at ambient temperature.

MG BAWENDI et al. describe the preparation of this same type of CdSe / ZnS core / shell nanocrystals in the article J. Phys. Chem. B, 101, pages 9,463-9,475 (1997) and US Patents 6,207,229 and WO-99/26299.

The article J. Am. Chem. Soc., 119, pages 7 019-7 029 (1997) presents the preparation of CdSe / CdS core / shell nanocrystals. The authors report an increase in the quantum yield of fluorescence at room temperature up to a value of at least 50%.

The document by P. REISS et al. Nano Lett. 2 (7), p. 781-783 (2002) demonstrates that with the CdSe / ZnSe core / shell system, the surface of which is provided with an organic passivation layer consisting of at least one primary amine such as hexadecylamine (HDA) associated with at least a phosphine oxide compound such as TOPO, and / or a phosphine selenide, yield values ​​of 60 to 85% are reached. The same nanocrystals and their synthesis in a solvent associating for example HDA and TOPO are described in French patent application FR-A-2 838 241.

It follows from the above that among the methods known to date the so-called inorganic / organometallic synthesis method makes it possible to prepare semiconductor nanocrystals A (II) B (VI) having the best optical properties. Indeed, the nanocrystals thus prepared, thanks to their small size dispersion have narrow emission lines and the core / shell systems have high fluorescence quantum yield values ​​(FR-A-2 838 241).

In the field of biological labeling, semiconductor nanocrystals could replace organic dyes, currently used as fluorescent markers, with the main advantage of better resistance to aging under light beams (M. Bruchez et al., <i>Science</i><b>281,</b> p. 2013-2015 (1998)).

However, this application requires a priori an exchange of organic molecules resulting from inorganic / organometallic synthesis which are found on the surface of nanocrystals for the following reasons:<ul id="ul0003" list-style="none" compact="compact"><li>-1) the molecules present on the surface of the nanocrystal give it a hydrophobic character, which prevents its use in a physiological medium, that is to say an essentially aqueous medium,</li><li>-2) these molecules do not allow a chemical bond with the biological molecules that we want to mark.</li></ul>

The molecules in question are surfactants, for example of the trialkylphosphine type such as TOP, trialkylphosphine oxide such as TOPO, alkylphosphonic acid, carboxylic acid or alkylamine which are linked by their polar head to the surface of the crystal, while their alkyl chains form a hydrophobic layer towards the outside. Thus, in document FR-A-2 838 241 already cited above, a layer formed by the association of a primary amine and a phosphine oxide or phosphine selenide compound is formed on the surface of the nanocrystals.

One possibility of making the nanocrystals water-soluble and of allowing their binding with biological molecules is to exchange the molecules on their surface by bifunctional ligands. The latter have both a function with an affinity for the surface of nanocrystals and a function which allows the solubilization of nanocrystals in water and / or the binding with a biological molecule (WCW Chan et al.,<i>Science</i><b>281</b>, P. 2016-2018 (1998)).

Another application of semiconductor nanocrystals which does not rely on their photoluminescence properties is their incorporation into films of conjugated polymers or oligomers in order to obtain new materials for light-electricity conversion for photovoltaic effect in solar cells. . Nanocrystals act as absorbers of sunlight and transport of electrons to the electrodes (WU Huynh et al.,<i>Science</i><b>295</b>, p. 2425 (2002)).

The effectiveness of these materials is currently limited by the difficulty of controlling their morphology: they are produced by mixing two constituents, namely on the one hand nanocrystals and on the other hand, a conjugated polymer, which leads to a phenomenon of phase segregation into areas rich in polymers and areas rich in nanocrystals. In addition, the interface between the two constituents is not controlled, which prevents optimizing the transfer of charge carriers and reduces the efficiency of the material. One possibility for improvement consists in grafting, by forming a chemical bond, the polymer onto the nanocrystal. Thus, phase segregation can be avoided and the interface between the nanocrystals and the polymer is better defined. This approach has been put into practice recently in the document by DJ Milliron et al.,<i>Adv. Mater.</i><b>15(1),</b> P. 58 (2003) in which a conjugated oligomer is chemically modified in order to introduce a function which allows grafting onto the nanocrystal. However, the proposed method requires a multi-stage organic synthesis, which is difficult to adapt to a large number of different conjugated polymers / oligomers.

On the contrary, the design of the bifunctional ligands which serve as binding molecules between the nanocrystal on the one hand and the polymer / oligomer on the other hand, gives much more flexibility in terms of the choice of materials because it is not necessary to change the well-established ways of preparing polymers or oligomers.

These ligands of the XYZ type carry an X function with a strong affinity for the surface of the nanocrystals, a so-called "spacer" group Y which can be conjugated or non-conjugated and a Z function which allows the bond with the polymer / oligomer.

It appears from the above, that the potential applications of nanocrystals require, first of all, an exchange of organic molecules on their surface by bifunctional ligands XYZ which allow dispersion in water or grafting to other molecules.

The type of bifunctional ligand currently used in most of the examples in the literature contains one or more thiol groups (-SH) as an anchoring function X for the bond with the surface of the nanocrystal, reference may be made in this connection to the articles of WCW Chan et al., <i>Science</i><b>281,</b> p. 2016-2018 (1998), and of SF Wuister et al.,<i>Nano Lett.</i><b>3(4),</b> p. 503-507 (2003).

Nanocrystals functionalized with thiols in order to make them water-soluble are, for example, the subject of application WO 00/17656 of March 30, 2000.

However, the publication <i>J. Am. Chem. Soc.</i><b>123(36),</b> p. 8844-8850 (2001) shows that the CdSe nanocrystals with thiols on the surface are sensitive to the photo-oxidation phenomenon. Under UV irradiation, the thiols detach from the surface, forming disulfides. Thus, the organic layer which protects the colloidal nanocrystal disappears, which leads to its precipitation. This process is observed for all types of thiol ligands and a comparison of the stability shows that thiols carrying long alkyl chains are more stable than short chain thiols and aromatic thiols. This study also shows that the stability of ligands with several thiol groups on the surface of nanocrystals is not greater than that of monothiols. The photo-oxidation phenomenon which results in a desorption of the thiol ligands is not limited to nanocrystals of semiconductors; Another important example is gold nanocrystals because thiols are the most used ligands for their functionalization.

In conclusion, it can be seen that the thiol ligands currently used for the functionalization of nanocrystals are not photostable, which will greatly limit the applications of the nanocrystals thus modified. The problem lies in particular in the anchoring function (-SH) which does not provide a sufficiently stable bond with the surface of the different types of nanocrystals.

Other documents relating to functionalized nanocrystals are documents US-A-5,990,479 (of 11/23/99), document US-B2-6,444,143 (of September 3, 2002) and document WO-A -02/073155 (from 19/09/2002).

Document US-A-5,990,479 relates to luminescent semiconductor nanocrystals which are linked to an affinity molecule via a linking agent.

The affinity molecule is capable of binding to a detectable substance, and the nanocrystal can then play the role of probe, in particular in biological applications to detect the presence of said substance in a material.

The affinity molecule is chosen, for example, from ligands and the binding agent is, for example, thiol N- (3-aminopropyl-3 mercapto-benzamide).

Document US-B2-6 444 143 describes water-soluble fluorescent nanocrystals, preferably with a core-shell structure, which comprise an outer layer comprising a compound having at least one linking group for hanging the compound on the surface of the nanocrystal and at least one hydroxyl group separated from the linking group by a hydrophobic region sufficient to prevent charge transfers through it.

To prepare the nanocrystals, the conventional ligands such as TOPO found on the surface of the nanocrystals are replaced by other ligand compounds which may be bidentate or tridentate ligands carrying several linkage groups such as thiols, the compound may thus be a dithiol such as dihydrolipoic acid.

The dithiolate compounds are not mentioned in this document.

Document WO-A-02/073155 likewise describes semiconductor nanocrystals with a core-shell structure which are provided with a water-solubilizing agent chosen in particular from hydroxamates, or derivatives of hydroxamic acid; multidentate complexing agents; a bilayer consisting of a layer of TOPO attached to the surface of the nanocrystal and a layer of surfactant; a molecule containing several anchoring groups corresponding to the very general formula (R¹) _at-R²- {(R³)_b (R⁴)_vs}_d in which R₁ may, among 18 possibilities represent -C (S) SH, no particular example of a molecule of this type is mentioned.

No information concerning the stability of the bond of these molecules with the surface of the nanocrystals is given.

In addition, the nanocrystals claimed in document WO-A-02/073155 are exclusively water-soluble nanocrystals; whereas, in the present invention, the XYZ ligands (see below) confer on the nanocrystals specific properties which are not limited to their water solubility.

Furthermore, the nanocrystals claimed in document WO-A-02/073155 are exclusively semiconductor nanocrystals with a semiconductor shell, while the present invention also relates to metallic nanocrystals, semiconductor nanocrystals without shell and nanocrystals with multiple shells.

It should be noted that in the last two documents cited, the ligands which cover the nanocrystals necessarily have hydrophilic functions in order to make them soluble in water.

It is important to mention that there are a large number of applications of nanocrystals, especially outside the field of biological labeling, which do not require that they be water-soluble.

With regard to the study carried out above, it appears that there is a need for functionalized nanocrystals which have a stability, in particular a colloidal stability, improved compared to nanocrystals functionalized by the ligands described in the documents of the art earlier and especially compared to nanocrystals functionalized with thiol ligands which are the most commonly used.

In addition, there is generally a need for nanocrystals which have a wide variety of properties, which are luminescent or non-luminescent, soluble in water or in other solvents and whose structure, in particular the functionalization, ensures a wide range of properties.

There is in particular a need for nanocrystals functionalized with ligands which exhibit improved photostability, in particular to ultraviolet light. In the case of photoluminescent nanocrystals, there is a need for nanocrystals whose photoluminescence properties are preserved for an extended period.

In addition, such crystals with improved stability must be able to be prepared by a simple, reliable, safe process and comprising a limited number of steps. Likewise, the ligands must be able to be prepared by a simple, reliable and flexible process ensuring access to a wide variety of structures.

The object of the invention is to provide nanocrystals meeting inter alia the needs mentioned above and satisfying the requirements and criteria mentioned above.

The object of the invention is also to provide nanocrystals which do not have the drawbacks, defects, limitations and disadvantages of nanocrystals of the prior art and which solve the problems posed by nanocrystals of the prior art.

This object, and still others, are achieved in accordance with the invention by a nanocrystal comprising an inorganic core constituted by at least one metal and / or at least one semiconductor compound comprising at least one metal, the external surface of said nanocrystal being provided with an organic coating layer constituted by at least one ligand compound of formula (I):         XYZ (I) in which :<ul id="ul0004" list-style="dash" compact="compact"><li>X represents a 1,1-dithiolate or 1,1-diselenoate group which is linked by the two sulfur or selenium atoms to a metal atom of the external surface of said nanocrystal;</li><li>Y represents a spacer group, such as a group capable of allowing charge transfer or an insulating group;</li><li>Z is a group chosen from groups capable of communicating properties specific to the nanocrystal.</li></ul>

The nanocrystals according to the invention differ fundamentally from nanocrystals of the prior art in that the organic coating layer which is provided with the external surface of the nanocrystal, which can be defined as a layer of ligands, comprises compounds or specific ligands of formula (I), in which the group X, which ensures the binding of the ligand with the surface of the nanocrystal is, specifically, according to the invention, a 1,1-dithiolate or 1,1-diselenoate group.

These groups provide a strong bond with the surface of the nanocrystals, whether they are semiconductors or metallic.

This bond is much stronger than with the ligands of the prior art and in particular the thiol ligands, and the multidentate ligands such as the dithiols. The stability of the nanocrystals according to the invention is consequently greatly improved compared to the functionalized nanocrystals stabilized by different ligands.

It appears that each ligand molecule (I) preferably binds by the two sulfur or selenium atoms of group X on the same metal atom on the surface of the nanocrystal thus forming a 4-atom ring and acting as a bidentate chelate ligand .

That a connection with such a force, and such an improvement in stability could be obtained with the specific ligands dithiolate or diselenoate according to the invention could in no way be expected in view of the prior art.

The nanocrystals of the invention meet all of the needs listed above and provide a solution to the problems of nanocrystals of the prior art, and in particular nanocrystals whose ligands are linked to the external surface of the nanocrystal by thiol groups or dithiols.

In other words, the main advantages of the nanocrystals of the invention are, among others:<ul id="ul0005" list-style="dash" compact="compact"><li>great ease of preparation, for example thanks to a simple substitution of the ligands already present on the surface of the nanocrystals by the ligands (I) characteristic of the invention, due to the great affinity of the latter for the metal atoms;</li><li>high colloidal stability of the covered nanocrystals, coated with the chelate ligands (I) thanks to their strong bond with the metal atoms on the surface of the nanocrystals;</li><li>a much improved resistance to photodegradation compared to nanocrystals of the prior art;</li><li>photoluminescence properties also retained longer than with the nanocrystals of the prior art and in particular than with the nanocrystals covered with thiol ligands, in particular in the case of core / shell nanocrystals.</li></ul>

The properties of nanocrystals can be easily adapted, modified thanks to the great flexibility which exists in the choices of the spacer Y (which can be conjugated or non-conjugated, containing or not heteroatoms, etc.) and of the group Z This flexibility stems in particular from the fact that the chelating ligand compounds (I) can be prepared by simple synthetic routes.

The Z function can thus make it possible to make the nanocrystals soluble in solvents of various polarities such as hydrocarbons, alcohols, water, and mixtures of these. In addition, the Z function can allow binding with other molecules such as biological molecules, conjugated polymers, etc.

Finally, it should be noted that the effects and advantages of the nanocrystals of the invention are not limited to the nanocrystals of semiconductors A (II) / B (VI) but are also presented by other semiconductors or other metals, whether these crystals are simple nanocrystals or core-shell nanocrystals.

In fact, the advantages and effects of the nanocrystals of the invention are inherently linked to the specific nature of the ligand compounds (I) and not to the nature of the nanocrystal or to its structure.

The nanocrystal provided with an organic coating layer constituted by the ligands (I) according to the invention can be a fluorescent, photoluminescent nanocrystal.

The photoluminescent fluorescent nanocrystal may have been subjected to irradiation with light, preferably ultraviolet light.

Surprisingly, the luminescent, photoluminescent crystals which have been subjected to such light irradiation, preferably by ultraviolet light have an improved, improved, increased, fluorescence intensity, photoluminescence, which is due to a photochemical process.

The invention further relates to a process for the preparation of nanocrystals as described above.

In a first embodiment, the process for preparing the nanocrystals as described above comprises the following successive steps:<ul id="ul0006" list-style="none" compact="compact"><li>a) a solution is prepared in a solvent, of nanocrystals having an inorganic core constituted by at least one metal and / or at least one semiconductor compound comprising at least one metal and optionally one or more shell (s) surrounding said core, said shell (s) being (each) made up of a layer of a metal and / or of at least one semiconductor compound comprising at least one metal, the external surface of said nanocrystal being provided with an organic coating layer constituted by at least a first ligand compound different from the ligand compound of formula (I);</li><li>b) adding to the solution of step a) ligand compounds of formula (I) soluble in the solvent of said solution of step a) so that the ligand compounds of formula (I) are in excess compared to the first ligand compounds different from the ligand compound of formula (I) with which the nanocrystals are provided;</li><li>c) the ligand compounds of formula (I) and the nanocrystals are left in contact, preferably with stirring, for a sufficient time to carry out an essentially total exchange of the first ligand compounds on the surface of the nanocrystals with the ligand compounds of formula (I) ).</li></ul>

By excess, in step b), it is generally understood that the molar ratio of the ligand compounds of formula (I) added to the first ligand compounds is for example from 5: 1 to 15: 1.

Generally, in step b), the compounds of formula (I) are added to the solution of step a) in the form of HS (S) CYZ or HSe (Se) CYZ acid, or in the form of salt M⁽⁺⁾ S (S) CYZ ^(-) or M⁽⁺⁾ Se (Se) CYZ ^(-) with M⁽⁺⁾ = Li⁽⁺⁾, N / A⁽⁺⁾, K ⁽⁺⁾, Rb ⁽⁺⁾, Cs ⁽⁺⁾, NH₄⁽⁺⁾, PR₄⁽⁺⁾.

Generally, the duration of the contacting of step c) is from 1 to 5 hours.

Generally, the solvent is chosen from chloroform, dichloromethane, toluene, heptane, another non-polar or low polarity solvent or a mixture of these.

In a second embodiment, the process for preparing the nanocrystals as described above comprises the following successive steps:<ul id="ul0007" list-style="none" compact="compact"><li>a) a solution is prepared in a first solvent, of nanocrystals having an inorganic core constituted by at least one metal and / or at least one semiconductor compound comprising at least one metal and optionally one or more shell (s) surrounding said core , said shell (s) being (each) made up of a layer of a metal and / or of at least one semiconductor compound comprising at least one metal, the external surface of said nanocrystal being provided with an organic coating layer constituted by at least a first ligand compound different from the ligand compound of formula (I);</li><li>b) a solution of the ligand compounds of formula (I) in a second solvent is added to the solution of step a), said ligand compounds of formula (I) not being soluble in said first solvent, whereby we obtain a two-phase mixture comprising a first phase consisting of the solution of the nanocrystals in the first solvent and a second phase consisting of the solution of the ligand compounds of formula (I) in the second solvent;</li><li>c) the said first and second phases are left in contact, preferably with stirring, for a sufficient time so that the nanocrystals are essentially completely transferred in said second phase and an essentially total exchange of the first ligand compounds is carried out on the surface of the nanocrystals by the ligand compounds of formula (I).</li></ul>

Generally, in step b), the compounds of formula (I) are added to the solution of step a) in the form of HS (S) CYZ or HSe (Se) CYZ acid, or in the form of salt M⁽⁺⁾S (S) CYZ^(-) or M⁽⁺⁾ Se (Se) CYZ ^(-) with M⁽⁺⁾ = Li ⁽⁺⁾, N / A ⁽⁺⁾, K ⁽⁺⁾, Rb ⁽⁺⁾, Cs⁽⁺⁾, NH₄⁽⁺⁾, PR₄⁽⁺⁾.

Generally, the duration of contacting in step c) is from 1 to 5 hours.

Generally, the first solvent is chosen from apolar solvents and solvents of low polarity such as chloroform, dichloromethane, toluene, heptane, hexane and their mixtures.

Generally, the second solvent is chosen from polar solvents such as alcohols, such as methanol and ethanol, water and their mixtures.

The process of the invention both in its first embodiment and in its second embodiment has a limited number of simple steps, easy to carry out, and allows great flexibility in the choice of the ligand (s) of which wishes to be able to cover nanocrystals.

The reagents used are also readily available and can be easily prepared by known reactions, thus a Grignard synthesis allows the preparation of dithiocarboxylic acid type HS (S) CRZ ligands in a simple manner and with good yields (see examples 3 , 4 below).

Most other types of ligands, such as xanthates, dithiocarbamates, etc., can be prepared from alcohols, amines, phosphines, etc. that react with carbon disulfide (CS₂) in the presence of a base.

Both in its first embodiment and in its second embodiment, the method according to the invention can comprise at the end of step c) and in the case where the nanocrystals are fluorescent, photoluminescent, an additional step to during which the nanocrystals provided with an organic coating layer constituted by the ligands (I) (said nanocrystals having optionally been precipitated, separated, washed, then dried) are subjected to irradiation with light, preferably ultraviolet (UV) light.

In other words, the nanocrystals are subjected to exposure to light, preferably ultraviolet light for a given period, generally from one or a few minutes to one or more hours, for example from one minute to 10 hours.

This irradiation, exposure, involves a photochemical process which improves the efficiency of fluorescence, photoluminescence.

The invention will be better understood on reading the detailed description which follows, given by way of illustration and not limitation, this description being made with reference to the accompanying drawings, in which:<ul id="ul0008" list-style="dash" compact="compact"><li>Figures 1A to 1F represent the spectra ¹H-NMR (200 MHz, CDCl₃) 4-methoxythiophenol (Fig. 1A), 4-dithiotoluic acid (Fig. 1B) and core / shell nanocrystals CdSe / ZnSe (with a diameter of approximately 6 nm) after the exchange reactions of following ligands:<ul id="ul0009" list-style="bullet" compact="compact"><li>a) 1.5 ml of a nanocrystals solution (3 mg / ml) + 20 mg of 4-methoxythiophenol (reaction time: 72 h) (FIG. 1C),</li><li>b) 1.5 ml of a solution of nanocrystals (3 mg / ml) + 20 mg of 4-dithiotoluic acid (reaction time: 1.5 h) (FIG. 1D),</li><li>solution a) + 20 mg of 4-dithiotoluic acid (reaction time: 1 h) (Fig. 1E),</li><li>solution b) + 20 mg of 4-methoxythiophenol (reaction time: 72 h) (Fig. 1F).</li></ul> In all cases, the exchange is carried out at a temperature of 30 ° C and the nanocrystals are purified / dried as described in Example 5 below and then redispersed in 1.5 ml of CDCl₃. </li><li>FIGS. 2A to 2C are graphs representing the evolution of the absorption spectra of colloidal solutions of CdSe nanocrystals with a diameter of approximately 4.5 nm, during continuous irradiation at 365 nm. On the ordinate, the absorbance is plotted (in arbitrary units ua) and on the abscissa is the wavelength (in nm).</li></ul>

The surface of the nanocrystals is covered by different ligands:<ul id="ul0010" list-style="none" compact="compact"><li>- a) trioctylphosphine oxide (TOPO) / trioctylphosphine (TOP) (resulting from the synthesis) (Fig. 2A);</li><li>- b) after exchange with 1-dodecanethiol (Fig. 2B);</li><li>- c) after exchange with 1-dithiotridecanoic acid (Fig. 2C).</li><li>FIG. 3 is a graph which represents the evolution of the intensity of the photoluminescence of a sample of CdSe / ZnSe core / shell nanocrystals (in accordance with the invention and prepared in accordance with Example 5), dispersed in chloroform, as a function of the time of exposure to UV irradiation coming from a mercury vapor lamp (365 nm, 100 W) (example 9).</li></ul>

On the ordinate is the intensity of photoluminescence I_ph (in arbitrary units ua) and on the abscissa is the time (t) (in seconds).

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The materials according to the invention are defined as being inorganic nanocrystals with an organic coating layer.

Inorganic nanocrystals generally consist of at least one metal and / or at least one semiconductor compound.

The nanocrystals are for example constituted by at least one metal.

The metal can be any, but it is generally chosen from transition metals, rare earth metals, metals from groups IIIA, IVA and VA of the periodic table of the elements, and their alloys, and mixtures of these metals and alloys. .

Preferably, the metal is chosen from aluminum, copper, silver, gold, indium, iron, platinum, nickel, molybdenum, titanium, tungsten, antimony, palladium, zinc, tin, their alloys, and mixtures of these metals and alloys.

Preferably, the metal is gold.

The nanocrystals can consist of at least one semiconductor compound. The semiconductor compound can be a semiconductor of formula AB in which A represents a metal or a metalloid in the oxidation state + II and B represents an element in the oxidation state -II.

A is generally selected from Mg, Ca, Sr, Ba, Zn, Cd, Hg, Sn, Pb and mixtures thereof, and B is generally selected from O, S, Se, Te and mixtures thereof .

Examples of these compounds A (II) B (VI) are MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, Base, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe and mixtures thereof.

The semiconductor compound can also be a semiconductor of formula CD (C (III) D (V)) in which C represents a metal or a metalloid in the oxidation state + III and D represents an element with l 'oxidation state -III.

C is generally chosen from Ga, In, and their mixtures, and D is generally chosen from Sb, As, P, N and their mixtures.

Examples of these C (III) D (V) semiconductor compounds are GaAs, GaSb, GaN, InGaAs, InN, InGaN, InP, InAs, InSb and mixtures thereof.

It is also possible to use group IV semiconductor compounds such as silicon or germanium.

It is even possible to use a mixture of the compounds AB, CD and of semiconductors of group IV.

In one embodiment of the invention, the inorganic nanocrystal has a core / shell structure, said core being constituted by a nanocrystal as described above consisting of at least a metal and / or at least one semiconductor compound while the shell (s) is (are) each constituted (s) of a layer of a metal and / or at least one semi-compound conductor comprising at least one metal.

The core has for example a diameter of 15 to 150 Å while the shell (s) has (have) a thickness of 3 to 30 Å.

When the nanocrystal does not have a shell (s), it generally has a diameter of 15 to 150 Å.

The organic coating layer generally has a thickness of 5 to 100 Å.

All the combinations are possible for the materials forming the heart on the one hand and the shell (s) on the other hand, but, preferably, the heart is in a first semiconductor compound, while the surrounding shell said heart (case of a single shell) or the first shell containing the heart in the case where the heart is surrounded by several shells, is in a second semiconductor compound different from the first semiconductor compound (forming the heart).

The first and second semiconductor compounds are chosen from the semiconductor compounds already described above.

Preferably, the heart is in a first semiconductor compound of type A (II) B (VI) described above, such as CdSe, while the shell surrounding the heart or the first shell surrounding the heart, is in a second semiconductor compound of type A (II) B (VI) different from the first semiconductor compound chosen for example from znSe, ZnS and CdS.

In the case of multiple shells, two successive shells are generally made of different semiconductor compounds.

Thus, in the case of multiple shells, the materials forming the shells can be chosen from all the possible combinations of the compounds mentioned above, for example these compounds can be chosen from ZnSe, CdS and ZnS. For example, we could have a first shell in ZnSe or in CdS and a second shell in ZnS.

According to the invention, the outer layer of inorganic nanocrystal is coated with an organic coating layer constituted by specific ligands corresponding to the general formula (I):         (I) XYZ

Basically, according to the invention, as already indicated above, the group X, which may also be called the anchoring group, is a 1,1-dithiolate group (-C (S) S^-) or a 1,1-diselenoate group (-C (Se) Se^-) .

The main advantageous properties of the nanocrystals of the invention are due to this particular group X.

The group Y or spacer group can be chosen from a wide variety of groups, which can allow charge transfer or else be insulating.

Y generally responds to the following formula:         -R₁-R- (II)    where R₁ represented :<ul id="ul0011" list-style="bullet"><li>a single bond;</li><li>a group<chemistry id="chem0001" num="0001"><img file="EP1548431A1_D0001.tif" /></chemistry> where R₂ and R₃ independently represent hydrogen, an alkyl radical, an aryl radical, an alkoxy radical, a halogen;</li><li>a group<chemistry id="chem0002" num="0002"><img file="EP1548431A1_D0002.tif" /></chemistry> where R 'represents a hydrogen, an alkyl radical, an aryl radical, an alkoxy radical, a halogen;</li><li>a group<chemistry id="chem0003" num="0003"><img file="EP1548431A1_D0003.tif" /></chemistry> where R 'has the meaning already given above;</li><li>a group<chemistry id="chem0004" num="0004"><img file="EP1548431A1_D0004.tif" /></chemistry> where R 'has the meaning already given above;</li><li>-O-;</li><li>-S-;</li><li>-Se-;</li></ul>    and R represents a linear or branched alkylene group having from 1 to 30 carbon atoms, preferably from 1 to 8; one or more carbon atoms of said alkylene group being optionally replaced by one or more heteroatoms chosen from O, N, S, P and Si; said alkylene group further optionally comprising one or more double and / or triple bonds such as carbon-carbon double and / or triple bonds; and said alkylene group also being optionally substituted by one or more groups chosen from halogens, such as chlorine, bromine, iodine and fluorine, heterocycles, aryl, hydroxyl, alkoxy, amino, acyl, carboxamido radicals ; = O, -CHO, -CO₂H, -SO₃H, -PO₃H₂, -PO₄H₂, -NHSO₃H, sulfonamide; monoalkylamino, trialkylammonium, or alternatively by a dialkylamino radical, in which the two alkyl groups can form, together with the nitrogen atom of said dialkyl group (C₁-VS₄) amino, to which they are linked, a cycle which can be interrupted by one or more nitrogen, oxygen or sulfur atoms, and the Z groups; or R represents a heterocycle, an aryl radical, an aryl radical condensed on one or more other aryl rings and / or alkyl or heterocycles rings, a cycloalkyl radical, a cycloalkyl-alkyl radical, an alkyl-cycloalkyl radical, an arylalkyl radical, a alkylaryl radical, a heterocycloalkyl radical, an alkyl-heterocycloalkyl radical.

According to the invention, the term alkyl for alkyl radicals, as well as for groups comprising an alkyl part, means, unless otherwise indicated, a carbon chain, linear or branched, comprising from 1 to 30 carbon atoms, preferably from 1 to 8, which can be carried and / or interrupted by one or more oxygen, sulfur, nitrogen, phosphorus or silicon atoms and which can also be substituted by one or more groups chosen from halogen atoms, such as chlorine, bromine, iodine and fluorine; heterocycles; aryl radicals; hydroxyl; alkoxy; amino; acyl; carboxamido; = O; -CHO; -CO₂H; -SO₃H; -PO₃H₂ ; -PO₄H₂ ; -NHSO₃H; sulfonamide; monoalkylamino; trialkylammonium; or alternatively by a dialkylamino radical, in which the two alkyl groups can form, together with the nitrogen atom of said dialkyl group (C₁-VS₄) amino, to which they are linked, a cycle which can be interrupted by one or more nitrogen, oxygen or sulfur atoms.

According to the invention, the term alkoxy used for the alkoxy radicals as well as for groups comprising an alkoxy part, means, unless otherwise indicated, an O-alkyl chain, the term alkyl having the meaning indicated above. The alkoxy radicals of the alkoxycarbonyl groups preferably have from 1 to 4 carbon atoms. The acyl groups preferably have from 2 to 4 carbon atoms.

According to the invention, the term “heterocycle” is intended to mean an aromatic or non-aromatic cycle containing 5, 6 or 7 vertices, and from 1 to 3 heteroatoms chosen from nitrogen, sulfur and oxygen atoms. These heterocycles can be condensed on other heterocycles, or on other notably aromatic rings such as a phenyl group. These heterocycles can, in addition, be quaternized by an alkyl radical. The terms alkyl and alkoxy have the meanings indicated above.

Among the heterocycles, mention may in particular be made, by way of example, of the cycles: thiophene, benzothiophene, furan, benzofuran, indole, indoline, carbazole, pyridine, dehydroquinoline, chromone, julodinine, thiadiazole, triazole, isoxazole, oxazole, thiazole, isothiazole, imidazole, pyrazole, triazine, thiazine, pyrazine, pyrazine, pyrazine diazepine, oxazepine, benzotriazole, benzoxazole, benzimidazole, benzothiazole, morpholine, piperidine, piperazine, azetidine, pyrrolidine, aziridine.

According to the invention, the term “cycloalkyl” is intended to mean a radical having from 3 to 10, preferably from 4 to 8 C, optionally substituted by one or more groups as defined above. Examples of cycloalkyl radicals are cyclobutyl, cyclopentyl and cyclohexyl.

According to the invention, the term “aryl”, unless otherwise specified, means a C aryl radical₆ at C₃₀ may be substituted by one or more alkyl radicals from 1 to 18 C; alkoxy; acyl; cyano; carboxamido; = O; -CHO; -CO₂H; -SO₃H; -PO₃H₂ ; -PO₄H₂ ; hydroxyl; amino; monoalkyl (C₁-VS₄) amino; or dialkyl (C₁-VS₄) amino; in which the two alkyl groups can form, together with the nitrogen atom of said dialkyl group (C₁-VS₄) amino, to which they are linked, a cycle which can be interrupted by one or more nitrogen, oxygen or sulfur atoms. Preferably, the aryl group is a phenyl group or a naphthyl group which may be substituted as indicated above.

R₁, R₂ and R₃ can also carry one or more other Z groups.

Preferred Y groups are alkylene chains - (CH₂)_not- with n = 1 to 18, preferably n = 12, the optionally substituted aromatic rings such as:<chemistry id="chem0005" num="0005"><img file="EP1548431A1_D0005.tif" /></chemistry>    and alkoxy chains - [(CH₂)_mO (CH₂)_m]_not-, with m = 1 to 4, and n = 1 to 6.

Depending on its chemical structure, the Y group or spacer group can influence the electronic properties of the XYZ ligand as well as the stability and solubility of the nanocrystal covered by these ligands.

For example, a group Y consisting of an aromatic ring -C₆H₄- can allow an electronic transfer through the ligand XYZ, while a group Y constituted by a long alkylene chain such as -C₁₂H₂₄- can prevent this electronic transfer. A polyether type Y chain such as - (CH₂) ₂-O- (CH₂) ₂-O- (CH₂) ₂- can increase the solubility of nanocrystals in a polar medium compared to ligands where Y does not contain heteroatoms such as oxygen. Furthermore, depending on its chemical structure, the spacer Y can carry one or more identical or different Z functions.

The group Z can be any, but it is generally chosen in order to obtain specific properties, such as for example a solubility of the nanocrystals in a specific solvent defined by a given polarity.

The group Z can therefore be chosen from the groups which allow the solubilization of the nanocrystals in apolar solvents and / or solvents with low polarity, such as hydrocarbons; or group Z can be chosen from groups which allow the solubilization of nanocrystals in polar solvents such as water, alcohols and their mixtures.

Group Z, in addition to allowing the solubility of nanocrystals in solvents of given polarity, can also allow the bonding of nanocrystals with other molecules such as biological molecules, polymers and conjugated oligomers and others, and / or group Z can already contain conjugated polymers or oligomers or biological molecules.

In addition, the group Z can allow the bond between nanocrystals (that is to say the bond of the nanocrystal with one or more other nanocrystals).

The group Z can therefore be chosen from the groups which allow the nanocrystals to bond with other molecules.

Advantageously, group Z is chosen from hydrophilic groups, in particular polar groups, such as hydroxyl groups, formyl groups, alkoxides, carboxylic acids, amines, polyethers, such as polyethylene glycol, and charged groups. such as carboxylate groups, sulfonates, phosphates, nitrates, ammonium salts and the like.

Z can also be identical to X, thus allowing the bond between nanocrystals.

Z can also simply be a hydrogen atom.

Most preferred Z groups are: hydroxyl (-OH) to allow dispersion of the nanocrystals in polar solvents; formyl (-CHO) and carboxylic acid (-COOH) to allow the bonding of nanocrystals with other molecules.

Several identical or different Z groups can be present on the same ligand which then has a formula of the type:<chemistry id="chem0006" num="0006"><img file="EP1548431A1_D0006.tif" /></chemistry>    where Z₁ and Z₂ identical or different have the meaning already given for Z.

In addition, several X groups can be present on the same ligand which then has for example a formula of the type:<chemistry id="chem0007" num="0007"><img file="EP1548431A1_D0007.tif" /></chemistry>    where Y₁, Y₂, Y_i identical or different and Z₁, Z₂, Z₃, Z₄, Z_2i-1, Z_2i identical or different have the meaning already given previously for Z and Y.

In addition, several ligands which differ by the group (s) Y and / or Z which they contain can be found on the same nanocrystal, which makes it possible to communicate to this nanocrystal all the desired properties and to adapt "tailor-made" these properties depending on the dissolution medium and the desired application.

Thus, it will be possible to introduce onto the nanocrystal both ligands which ensure the solubility of the nanocrystals in a given solvent and ligands which have functions allowing the bond with other molecules, or even the same ligands can carry functions ensuring solubility in a given solvent and functions allowing the bond with other molecules.

Thanks to the nanocrystals preparation process, it is very easy to control, to adjust, the number of functions of a category compared to another by changing the ratio of the two ligands during the functionalization of the nanocrystals.

The nanocrystals according to the invention, provided with an organic coating layer constituted by the ligands (I), can be fluorescent, photoluminescent nanocrystals.

Those skilled in the art can easily choose the nature of the organic core, and / or of the shell and / or of the ligands (I) in order to ensure that the nanocrystal according to the invention is fluorescent, photoluminescent.

When the nanocrystals are fluorescent, luminescent, in other words when the desired application of the nanocrystals is based on their fluorescence, photoluminescence, they can be subjected to irradiation with light, preferably an ultraviolet light which improves, increases, the efficiency, intensity, fluorescence, photoluminescence, as a result of a photochemical process.

The process for preparing the nanocrystals according to the invention will now be described.

In a first step, an inorganic nanocrystal is prepared, with a simple "core" structure or else with a "core-shell" structure on the surface of which the coating layer, of organic ligands according to the invention will then be fixed.

Any known method can be used to synthesize nanocrystals.

However, a preferred method is that described in document FR-A-2 838 241 already mentioned above and which allows the preparation of nanocrystals in semiconductor of formula AB where A is a metal or a metalloid in the state of oxidation + II and B represents a chemical element in the oxidation state -II.

In this process, in order to synthesize the semiconductor nanocrystals AB, we start by reacting an oxide of A with a powder of B.

An oxide of A, such as CdO, or HgO, is chosen as the specific reagent, because such an oxidizing reagent, for example CdO, proves to be less reactive and easier to use than the dialkyl metal compounds, for example , such as dialkylcadmium compounds commonly used in other processes.

It is generally necessary to complex the oxide of A, such as CdO, with an alkylphosphonic acid.

Various high-boiling alkylphosphonic acids can, in principle, be used as complexing agents, such acids correspond, for example, to the formula:<chemistry id="chem0008" num="0008"><img file="EP1548431A1_D0008.tif" /></chemistry> where n is an integer from 5 to 15.

The preferred complexing agent is dodecylphosphonic acid (DDPA).

The powder of B is the second specific reagent of this process. It may preferably be a powder of Se, Te, or S, Se being preferred. This reagent is generally dissolved in trialkylphosphine, the alkyl group of which has from 4 to 12 carbon atoms, the preferred trialkylphosphine being trioctylphosphine. The concentration of B powder in trialkylphosphine is generally 0.1 M to 0.5 M.

The reaction takes place according to this process in a specific solvent consisting of a mixture of at least one trialkylphosphine oxide and at least one primary alkylamine.

The primary alkylamine is generally chosen from long chain primary amines, that is to say comprising from 6 to 24, preferably 14 to 24 carbon atoms, and with a high boiling point, the amine, again preferred is hexadecylamine (HDA).

The trialkylphosphine oxide is generally chosen from trialkylphosphine oxides in which the alkyl group comprises from 4 to 12 carbon atoms, trioctylphosphine oxide (TOPO) being preferred.

It has been found that the relative molar concentration of each of the compounds entering the solvent mixture influences the growth kinetics and the size distribution of the nanocrystals, the mixture therefore generally comprises from 50 to 90 mol% of alkylamine, for example of HDA.

A preferred molar concentration is 60 to 80 mol% of alkylamine, for example HDA.

During a typical implementation of this process, the solvent mixture, for example of TOPO / HDA and the oxide, for example, CdO, are heated in a balloon above the melting temperature of the solvents, this temperature is therefore generally above 60 ° C, preferably from 150 to 250 ° C.

The complexing agent, such as DDPA, is then added, then the temperature increased to approximately 260-300 ° C. During this heating, the oxide powder, such as CdO, dissolves and a colorless and transparent solution, in the case of CdO, is formed.

The temperature of the complexing reaction strongly depends on the relative amounts of the two compounds forming the solvent, for example HDA and TOPO, and decreases when the relative concentration of trialkylphosphine oxide, for example in TOPO, increases.

The solution of B, for example Se, in trialkylphosphine, for example TOP, is prepared separately at room temperature, then very quickly injected into the flask. The temperature of the reaction mixture in the flask during injection is of crucial importance. It should preferably be between 240 and 300 ° C.

It should be noted here that the temperature of the reactive mixture strongly influences the nucleation and growth of nanocrystals. In general, for similar reaction times, smaller nanocrystals are formed at low temperatures. For a given temperature, the final size of the nanocrystals is determined by the reaction time. The reaction time is, for example, 2 to 90 minutes.

The reaction can be stopped at any time by cooling the reaction mixture to obtain a sample of nanocrystals of the desired size. The nanocrystals are then precipitated by the addition of an adequate mixture, for example of methanol and of n-butanol, then separated, for example, by centrifugation and purified.

It should be added that this method described above, ensures better quality of the core nanocrystals than other methods.

This is due to a very narrow size distribution, which is an important advantage of this process. In the processes of the prior art, a step - costly in material - of sorting the nanocrystals into fractions of given size, must imperatively be used to obtain samples whose width of the size distribution is as narrow as that which directly provides the process of the invention, namely less than 5%.

The method described in document FR-A-2 838 241 has the advantage of not comprising a step of sorting the nanocrystals into fractions of given sizes (fractionation) and is considerably simpler than other methods.

At the end of the preparation of the semiconductor nanocrystals, the synthesis, the growth of a shell which is for example made of ZnSe, can optionally be carried out.

The core / shell pair (CdSe / ZnSe) constitutes, according to this document, one of the best choices, even the best possible choice for the compounds A (II) B (VI).

Two types of zinc precursors can be chosen for the preparation of a ZnSe shell: zinc oxide (ZnO) and a zinc carboxylate of general formula:         Zn (R_atCOO)₂where R_at represents an aliphatic alkyl group of 1 to 24 carbon atoms, the preferred zinc carboxylate is zinc stearate of formula:         [CH₃ (CH₂)₁₆CO₂]₂Zn.

The advantage linked to their use comes in particular from the fact that these are industrially produced compounds, inexpensive, and much easier to handle than dialkyl zinc compounds.

In the case of ZnO, complexation with an alkylphosphonic acid, already described above, is required, as for the preparation of the heart, starting, for example, from CdO.

ZnO is complexed, for example, with dodecylphosphonic acid (DDPA) in trialkylphosphine oxide, for example TOPO between 350 and 360 ° C.

The colorless solution obtained is diluted to a concentration of 0.1 M to 0.5 M, for example at about 60 ° C in a small amount of toluene or any other solvent, for example, allowing the improvement of its rheological properties .

The use of a zinc carboxylate, such as zinc stearate, is even simpler, since the product is simply diluted in a suitable solvent, such as toluene, without a complexing step.

The source of selenium is the same as for the preparation of the heart: selenium powder dissolved in a trialkylphosphine, preferably in the TOP. This solution is mixed at room temperature with the solution containing the source of zinc (oxide or carboxylate). The resulting mixture can be easily injected using a syringe, in addition, into the flask where the synthesis takes place.

For the growth of the shell, the injection takes place in a solvent consisting of a mixture of at least one trialkylphosphine oxide and at least one primary alkylamine, the preferred mixture consisting of a mixture of TOPO and HDA . The mixture of solvents, for example TOPO / HDA, contains a dispersion of core nanocrystals, for example of CdSe.

The temperature of this mixture is of primary importance. Too low, no crystal growth reaction of ZnSe takes place. Conversely, if this temperature is too high, crystal seeds of ZnSe are formed and nanocrystals of ZnSe appear in place of the epitaxial growth of ZnSe on hearts, for example in CdSe. In addition, too high a temperature can lead to a widening of the size distribution of the nanocrystals, for example of CdSe, by the fact of an exchange of matter between the nanocrystals by dissolution, then redeposition. In this context, the use of a trialkylphosphine oxide / primary alkylamine solvent mixture, for example TOPO / HDA, with a high level of HDA, namely from 50 to 80 mol%, plays a crucial role. HDA is less strongly bound to Zn than TOPO, thus allowing the shell to form at lower temperatures. In addition, thanks to a less steric hindrance, HDA ensures better chemical passivation of the surface than TOPO (or TOPSe), in particular for the larger sizes of the core / shell systems, having the surface of less curvature. .

However, an alkylamine, such as pure HDA, is not a suitable solvent for the growth of the shell.

The best results are therefore obtained with a solvent mixture consisting, preferably of 60 to 80% of alkylamine, preferably of HDA, ie 40 to 20% of trialkylphosphine oxide TOPO, in molar fractions.

The presence of HDA on the surface of the shell can be quantitatively determined by measuring the spectra ¹H-NMR (200 MHz, CDCl₃) nanocrystals, for example, CdSe / ZnSe prepared by the process of the invention, thanks to the peak of the α-CH protons₂ of the HDA alkyl chain.

Finally, the choice of temperature depends on the size of the core nanocrystals, for example in CdSe, used for the synthesis. In the vast majority of cases, the temperature is in the range 170-210 ° C, the highest values ​​being used for the largest core nanocrystals. As the formation of ZnSe germs is facilitated by a high concentration of zinc and selenium precursors, the injection of their solution should preferably be extremely slow, with a constant speed, for example from 3 to 10 ml per hour for a total volume of 5 ml to be injected.

Preferably, a commercial automated microinjection system of the syringe pump type will be used for this purpose.

When the addition of the ZnSe precursors is finished, the reaction mixture is kept at the synthesis temperature for a period, for example of one to two hours, to allow the "annealing" of the shells and to improve their crystalline quality. This mixture is then cooled, for example, to about 60 ° C.

The steps of precipitation with a mixture, for example methanol / n-butanol for separation and purification are then carried out in exactly the same way as for the preparation of the core nanocrystals.

The width at mid-height of the emission line for core nanocrystals, for example, in CdSe, as for the core / shell, for example, CdSe / ZnSe is extremely narrow, typically 25 to 30 nm, as synthesized, without any size sorting procedure.

The quantum fluorescence yield at room temperature of the core / shell nanocrystals A (II) B (VI) thus prepared is very high. It exceeds 60%, even in some cases 80%.

At the end of this process, nanocrystals are obtained comprising a core optionally surrounded by a shell, said core being constituted by a semiconductor nanocrystal of formula AB in which A represents a metal or a metalloid in the state d oxidation + II (Cd, Hg) and B represents a chemical element, such as a metal or a metalloid, in the oxidation state -II (Se, S, Te) and the shell consists for example of a layer of ZnSe, the surface of the nanocrystals being provided with an organic passivation layer consisting of at least one primary amine such as hexadecylamine (HDA) associated with at least one phosphine oxide compound such as trioctylphosphine oxide (TOPO) and / or phosphine selenide such as triocthylphosphine selenide (TOPSe).

The inorganic nanocrystal obtained at the end of the first step, generally comprises, due to the very process of preparation used for preparing it, an organic coating layer made up of ligands which are different from the specific ligands of formula (I) according to the invention .

They are generally ligands chosen from phosphine oxides such as trialkylphosphine oxide, the alkyl group of which comprises from 4 to 12 carbon atoms such as trioctylphosphine oxide (TOPO); phosphine selenides such as trialkylphosphine selenides in which the alkyl group comprises from 4 to 12 carbon atoms, such as trioctylphosphine selenide (TOPSe); trialkylphosphines such as trialkylphosphines in which the alkyl group comprises from 4 to 12 carbon atoms such as trioctylphosphine (TOP); primary amines such as alkylamines, the alkyl group of which comprises from 6 to 24 carbon atoms such as hexadecylamine (HDA); and carboxylic acids such as stearic acid; and mixtures thereof.

In a second step, in order to prepare the nanocrystal according to the invention, nanocrystals prepared above and which comprise on their external surface a coating layer constituted by one or more ligands different from the ligands of formula (I) specific are dissolved nanocrystals according to the invention, in a solvent.

In a first embodiment of the second step, the ligands (I) in the form of HS (S) CYZ or HSe (Se) CYZ acid, according to the invention, are soluble in the same solvent as that used to prepare the nanocrystals solution described above (so-called "before exchange" nanocrystals solution), for example chloroform, dichloromethane, toluene, heptane, another nonpolar, or low polarity solvent, or a mixture of those -this.

It should be noted that throughout the description, the term "ligand" is used to designate either the organic molecule fixed on the external surface of the nanocrystal which is a dithiolate or a diselenoate and the corresponding organic molecule in solution which is found then in its form of dithiocarboxylic acid or diselenocarboxylic acid.

The exchange of molecules on the surface of nanocrystals by these new dithiolate or diselenoate ligands according to the invention of formula (I) is done simply by adding the new ligands of formula (I) to the solution of nanocrystals from the first step of so as to obtain in the solution an excess (namely a molar ratio of 5: 1 to 15: 1 for example) of these new ligands (I) compared to the ligands present on the nanocrystals.

A complete complete exchange of the ligands initially present is obtained by the ligands (I) in a period generally of 1 to 5 hours with stirring at room temperature (25-30 ° C).

The nanocrystals provided with an organic coating layer constituted by the ligands (I) are then generally precipitated by adding to the nanocrystal solution a solvent or a mixture of solvents having a different polarity. These solvents are generally chosen from methanol, ethanol, propanol, butanol or other polar solvents.

Optionally, the nanocrystals are then generally separated, for example by filtration, then they are generally washed, rinsed from 1 to 4 times with the same solvent which served for the precipitation. The nanocrystals are then generally dried, for example for a period of 1 to 5 hours, preferably under vacuum, at a temperature of 20 to 60 ° C.

A second embodiment of the second step is implemented in particular in the case where the new ligands are only soluble in solvents which are too polar to disperse nanocrystals such as alcohols such as methanol, ethanol, etc., or water and their mixtures.

This is particularly the case using the ligands (I) in the form of salt of formulas M⁽⁺⁾ S (S) CYZ^(-) or M⁽⁺⁾ Se (Se) CYZ ^(-) in which M⁽⁺⁾ is usually chosen from Li⁽⁺⁾, N / A⁽⁺⁾ , K⁽⁺⁾, Rb⁽⁺⁾, Cs ⁽⁺⁾, NH₄⁽⁺⁾, and PH₄⁽⁺⁾.

A heterogeneous biphasic exchange is then carried out in which the phase of nanocrystals (with the "old" ligands) in a preferably apolar solvent such as chloroform and the phase of the new ligand (I) (in a polar solvent such as methanol or water) are brought into intimate contact, preferably with vigorous stirring, until the nanocrystals are completely transferred into the polar phase.

Generally, the nanocrystals covered with ligands (I) can be precipitated, can be separated, can be washed, and can be dried, in the same manner as in the first embodiment. However, the solvent used for the precipitation and rinsing, washing, is this time an apolar solvent chosen, for example, from the solvents heptane, hexane or toluene.

At the end of step c) both in the first and in the second embodiment, the nanocrystals provided with an organic coating layer constituted by the ligands (I), optionally precipitated, separated, washed, then dried , are subjected to irradiation with light, preferably ultraviolet light, for example UV light with a wavelength of 365 nm.

This irradiation is carried out in the case where the nanocrystals provided with an organic coating constituted by the ligands (I) are fluorescent, photoluminescent nanocrystals.

In other words, in the case where fluorescent, photoluminescent nanocrystals are used, the latter may be, following step c), and optionally the steps of precipitation, separation, washing (rinsing) and drying, subjected to a photochemical treatment, to increase, improve, efficiency, intensity of their fluorescence, photoluminescence. This irradiation is preferably carried out by dispersing the nanocrystals provided with an organic coating layer constituted by the ligands (I) in an appropriate solvent, and by exposing the nanocrystals to light irradiation, preferably to ultraviolet light.

Said solvent for dispersing can be chosen from non-polar solvents and solvents of low polarity such as chloroform, dichloromethane, toluene, heptane, hexane, and mixtures thereof; or among polar solvents such as alcohols, water, and mixtures thereof.

The irradiation time required to obtain maximum fluorescence of the sample can range from 1 or a few minutes to 1 or more hours, for example 10 hours, it is in particular 1 hour.

This irradiation time can vary according to the experimental conditions such as the specifications of the light source, the distance between the source and the nanocrystals sample, the concentration of the sample (when a dispersion of nanocrystals is irradiated in a solvent ), this time can be determined by photoluminescence spectroscopy.

More precisely, the time, the duration of the irradiation, in particular with UV light is generally from 1 minute to 10 hours, preferably from 10 minutes to 200 minutes, more preferably from 50 to 100 minutes, for example from 60 minutes.

By way of example, the sample of nanocrystals, preferably suspended in a solvent such as chloroform, is placed in a quartz bowl at a distance of 3 to 5 cm from a 100 W mercury lamp which emits at 365 nm; and the sample is exposed to irradiation for one hour (see Fig. 3).

The invention will now be described with reference to the following examples, given by way of illustration and not limitation.

Examples

All handling of air-sensitive materials is done using a standard technical vacuum ramp (or Schlenk type) or a glove box under argon.

All products come from ALDRICH. Cadmium oxide (purity 0.999), zinc oxide (purity 0.999), selenium powder (purity 0.99999), zinc stearate (purity 0.95), magnesium (purity 0.9998) , 1-bromododecane (purity 0.97), 4-bromotoluene (purity 0.98), carbon disulfide (purity 0.999), 1-dodecanethiol (purity 0.985), 4-methoxythiophenol (purity 0.97) ) as well as toluene, THF, methanol and anhydrous n-butanol are used as such; trioctylphosphine (TOP, purity 0.9), trioctylphosphine oxide (TOPO, purity 0.9) and hexadecylamine (HDA, purity 0.9) are purified by distillation.

As dodecylphosphonic acid (DDPA) is not commercially available, it is synthesized using the method published in Method. org. Chem. 12/1, pages 352-353, 435 (1963). The product is identified by NMR spectroscopy of the nuclei¹H, ¹³C and ³¹P.

Optical characterization: the UV-visible absorption spectra are measured on a HEWLETT-PACKARD® 8 452A spectrophotometer, the photoluminescence spectra are acquired by a CCD camera coupled to a JOBIN-YVON® HR 460 monochromator whose resolution is 0.1 nm, the excitation wavelength (365 nm) being rejected by a high-pass filter (cut at 400 nm).

For these spectroscopic measurements, the colloidal solutions diluted of nanocrystals in toluene are placed in quartz cuvettes with 1 mm optical path.

The NMR spectra are carried out under the following conditions: BRUKER® AC 200, 200 MHz.

Example 1

In this example, the preparation of CdSe core nanocrystals covered by TOPO / TOP is described.

These crystals will later be used for the preparation of CdSe / ZnSe core / shell nanocrystals.

51.4 mg of CdO (0.4 mmol) are placed in a two-necked flask under constant flow of purified argon. Then 1.15 ml of TOPO and 2.85 ml of HDA are added, forming the solvent mixture with a molar fraction of 80% HDA. The flask is then heated to 200 ° C, with magnetic stirring. At this point, 230 µL of DDPA (0.8 mmol) is added and the temperature is increased to around 270 ° C for a period of 1 h. The complexation reaction - namely the reaction of cadmium oxide with DDPA to give calcium phosphate - takes place during this time and is manifested by the formation of a colorless solution. Thereafter, the temperature dropped to 250 ° C and stabilized.

A 0.2 mol / L selenium solution is prepared separately, at room temperature, by dissolving 157.9 mg (2 mmol) of Se powder in 10 mL of TOP. 2.5 mL of this solution is rapidly injected into the flask containing the Cd precursor solution, always with vigorous stirring. Amounts of 100 µL are periodically taken from the reaction medium, so as to follow the growth process of the nanocrystals. The narrowest size distribution is obtained when the reaction is stopped in the interval between 3 and 30 min after the injection, the average diameter of the nanocrystals ranging from 3.5 nm (for 3 min of reaction) to 4.6 nm (30 min). The reaction medium is cooled to approximately 60 ° C when the desired size is reached. The core nanocrystals are then precipitated by adding a mixture of 10 ml of methanol and 1 ml of n-butanol. They are then separated from the excess liquid by centrifugation, washed with methanol and dried with a stream of argon. The nanocrystals can be redissolved in various organic solvents such as toluene, chloroform, alkanes, ethers, etc., to carry out measurements of their optical properties.

The absorption and photoluminescence spectra of the series of samples taken during the reaction described above make it possible to draw a certain number of conclusions.

The absorption spectrum presents, in addition to the exciton peak, accidents characteristic of excited states of higher energy. The photoluminescence peak is of low width at half height, namely between 25 and 29 nm, which indicates, without question, that the size distribution of the prepared nanocrystals is very narrow. This is confirmed by the determination of the size distribution made by MET: the maximum deviation from the average is less than 5%, without using any size sorting process.

The TEM image, obtained with an acceleration voltage of 300 kV on a JEOL 3010 electron microscope, of CdSe nanocrystals taken after 10 min. during the above reaction shows that their size distribution is sufficiently narrow, thanks to the method according to the invention, to allow their "crystallization" on a super three-dimensional network.

Example 2

In this example, the preparation of core / shell nanocrystals in CdSe / ZnSe, covered with TOPO / TOP / HDA with zinc stearate as a source of zinc, is described.

In a flask, a solution of zinc stearate (ZnSt₂) of concentration 0.2 mol / L is prepared by mixing 632.3 mg of zinc stearate (ZnSt₂) (1 mmol) with 5 mL of toluene and heating moderately.

2.5 mL of this dispersion, which remains cloudy, are mixed at room temperature with the same volume of a Se solution in the TOP with a concentration of 0.2 mol / L, prepared as in Example 1. All of this mixture is placed in a syringe.

In the flask intended for synthesis, approximately 20 mg of core nanocrystals with an average diameter of 3.6 nm and with a dispersion of size less than 5% prepared as described in Example 1, are dispersed in a solvent mixture consisting of 2, 0 mL TOPO and 2.0 mL HDA (i.e. a molar ratio of approximately 2 to 3). This mixture is heated to 190 ° C. As soon as the temperature has stabilized, the solution containing the ZnSt mixture₂/ Was slowly injected into the balloon by a syringe pump, at the rate of 5 mL / h, the contents of the balloon being vigorously agitated. The growth of the shell is followed by periodic sampling, as described in Example 1. When the injection of the Zn St mixture₂/ Ended, the flask is left at 190 ° C for 90 min, in order to "anneal" the nanocrystals, to eliminate the stacking faults in the shell and at the core / shell interface. The synthesis ends with the cooling of the flask to 60 ° C. The nanocrystals obtained are precipitated and purified by the method described in Example 1. They are redissolved in an organic solvent for the determination of their optical properties (cf. Example 1).

Example 3

In this example, the synthesis of 1-dithiotridecanoic acid is described.

In a three-necked flask under an inert atmosphere, 5 equivalents of magnesium (0.1 mol / 2.44 g) are placed covered with 10 ml of anhydrous THF. 1 equivalent of bromododecane (0.02 mol / 5.01 g), dissolved in 20 ml of anhydrous THF, is added dropwise. When the addition is complete, the mixture is brought to 60 ° C. for 2 hours. A gray suspension is obtained.

In a glove box, a flask (bicol) is prepared with 3 equivalents of carbon disulfide (0.06 mol / 4.56 g) in 10 mL of THF and, outside the box, this mixture is cooled at -5 ° C. Under an inert atmosphere, the gray suspension previously prepared is added dropwise. The resulting mixture turns yellow, then orange. The temperature is then brought back up to room temperature and left to stir for 12 hours.

The reaction mixture is hydrolyzed with 40 ml of a diethyl ether / water mixture (1: 1). The organic phase becomes yellow-orange, the aqueous phase is yellow. Acidify with 15 mL of HCl (0.2 M): the aqueous phase becomes colorless because the product in the form of acid goes into the organic phase. In order to optimize the yield, the aqueous phase is extracted several times with ether (pH ≤ 7). The organic phases are combined and extracted with water (100 mL), dried over MgSO₄ and concentrated with a rotary evaporator. 3.46 g (70%) of a bright yellow oil are recovered, which partially crystallizes. The product is identified by NMR spectroscopy of the nuclei¹H and ¹³C as well as by elementary analysis.

Example 4

In this example, the synthesis of 4-dithiotoluic acid is described.

5 equivalents of magnesium (0.05 mol / 1.21 g) covered with 10 mL of anhydrous THF are placed in a three-necked flask under an inert atmosphere. 1 equivalent of 4-bromotoluene (0.01 mol / 1.71 g), dissolved in 15 ml of anhydrous THF, is added dropwise. After a few drops, the reflux begins and the solution becomes cloudy. The mixture is then brought to 60 ° C. for 2 hours. The solution becomes dark brown.

In a glove box, a flask (bicol) is prepared with 3 equivalents of carbon disulfide (0.03 mol / 2.34 g) in 10 mL of THF and, outside the box, this mixture is cooled at -5 ° C. Under an inert atmosphere, the brown suspension previously prepared is added dropwise. After a few drops, the solution becomes yellow, then dark red. The temperature is allowed to rise to room temperature and the mixture is left to stir for 12 hours. The reaction mixture is hydrolyzed with 40 ml of a diethyl ether / water mixture (1: 1). The organic phase becomes yellow-orange. In order to purify it, the aqueous phase is extracted with ether (150 mL) and the combined organic phases containing secondary products are rejected. Then the aqueous phase is acidified with 25 mL of HCl (0.2 M) and extracted with ether: the organic phase is dark purple. Acidification and extraction are continued until the aqueous phase is colorless (25 mL 0.2 M HCl, 200 mL ether). The organic phases are combined and extracted with water (100 mL). After drying on MgSO₄, the organic phases are concentrated with a rotary evaporator. 720 mg (43%) of a purple oil are recovered, which partially solidifies. The product is identified by NMR spectroscopy of the nuclei¹H (see Fig. 1B) and ¹³C as well as by elementary analysis.

Example 5

In this example, the preparation of nanocrystals according to the invention coated with ligands of the dithiolate type is described.

The nanocrystals of CdSe or CdSe / ZnSe, prepared as described in Examples 1 and 2, are redispersed in anhydrous chloroform with a concentration of approximately 3 mg / ml. For the exchange of ligands, 20 mg of dithiocarboxylic acid, which is either 1-dithiotridecanoic acid or 4-dithiotoluic acid, are added to a microtube which contains 1.5 ml of the solution of the nanocrystals with the "old" ligands. The mixture is left to stir for 2 hours at 30 ° C. to ensure complete exchange of the old ligands with the new ligands according to the invention. The nanocrystals covered by the dithiolate are then precipitated with 3 ml of methanol and washed several times with the same solvent in order to remove the excess of 1-dithiotridecanoic or 4-dithiotoluic acid, and of ligands desorbed from the surface. The nanocrystals are dried on a vacuum ramp and redispersed in chloroform or CDCl₃. The exchange can be characterized by standard analysis techniques such as NMR, UV-Vis or IR.

Example 6

In this example, the preparation of nanocrystals coated with thiol-type ligands is described.

The same method as that used for the preparation of nanocrystals coated with dithiolate type ligands (see example 5) can be applied to effect the exchange with thiol type ligands which are 4-methoxythiophenol (ZY-SH with Y = VS₆H₄ and Z = OCH₃) and dodecanethiol (ZY-SH with Y = (CH₂)₁₂ and Z = H).

However, the reaction time must be increased (up to at least 72 h) in order to obtain an almost complete exchange.

Example 7

In this example, the affinity for the surface of the nanocrystals of the dodecanethiol and tridecanodithiolate ligands is compared.

In order to study the stability of the nanocrystals with a thiol or dithiolate type ligand with respect to the ligand carrying the other anchoring function, the nanocrystals prepared as in Examples 5 and 6 are redissolved in CHCl₃ and the complementary ligand is added in excess.

In this example, 4-dithiotoluic acid, prepared as in Example 4, is used because the aromatic protons allow simple detection of the exchange rate in spectra ¹H-NMR (see NMR spectrum of 4-dithiotoluic acid in Fig. 1B), thanks to the strong shift in their chemical displacement compared to the protons of the alkyl chains in TOPO, TOP and HDA.

The thiol chosen for the comparison is 4-methoxythiophenol because it allows a good distinction of 4-dithiotoluic acid in the NMR spectra because of the methoxy function (see NMR spectrum of 4-methoxythiophenol in FIG. 1A). The purification was carried out in the same manner as before.

Specters ¹H-NMR show that the thiol-type ligand (namely 4-methoxythiophenol) is completely replaced after 1 h by the dithiolate-type ligand (namely dithiotoluic acid) (see FIGS. 1A to 1F). Otherwise, even after 72 h, approximately 5-10% of the dithiolate-type ligand remains on the surface of the nanocrystals. This clearly shows that the bond formed during the exchange reaction is more stable in the case of the ligand type dithiolate compared to thiol.

In detail, in FIGS. 1A to 1F, the greater affinity of the new dithiolate ligands for the surface of the nanocrystals compared to thiol ligands of the prior art is demonstrated by NMR spectroscopy.

As described above, initially, the ligands on the surface of the core / shell nanocrystals CdSe / ZnSe are replaced by 4-methoxythiophenol (ligand of thiol type ZY-SH with Y = C₆H₄ and Z = OCH₃ ; spectrum of Fig. 1C) (Example 6).

Another sample of the same batch of nanocrystals is treated with 4-dithiotoluic acid (ligand type dithiolate ZYC (S) SH with Y = C₆H₄ and Z = CH₃ ; spectrum of Fig. 1D) (example 5). Despite the much longer reaction time for exchange with 4-methoxythiophenol compared to 4-dithiotoluic acid (70 h instead of 1.5 h), the replacement of the starting ligands (trioctylphosphine oxide, trioctylphosphine, hexadecylamine) is not total. The significant peaks in the NMR spectrum between 0.8 ppm and 1.8 ppm, due to the protons in the alkyl chains of these ligands, are proof of this. We also observe a disappearance of the peak of the function - SH (δ = 6.62 ppm) in the spectrum of FIG. 1D, which shows the deprotonation of 4-dithiotoluic acid during the bond with the surface of the nanocrystal.

For the rest, as already described, an excess of complementary ligand is added to each of the two samples (see example 7). The thiols are quickly replaced (after one hour) by the dithiolate type ligands, as indicated in particular by the absence of the peak of the protons of the methoxy group (δ = 3.7 ppm) in the NMR spectrum of FIG. 1E. In the opposite case, the exchange of ligands cannot be detected after 1 h and, even after a reaction time of 72 h, there remain ligands of the dithiolate type on the surface of the nanocrystals, visible in particular thanks to the protons of the methyl group. (δ = 2.3 ppm) in the NMR spectrum of Fig. 1F.

Example 8

In this example, photo-degradation experiments are carried out to show the best stability under light beams of the nanocrystals covered with ligands of the dithiolate type (according to the invention) compared to the nanocrystals covered with thiols.

For these experiments, the samples are in solution in chloroform and have the same optical densities (0.1-0.5 at the exciton peak) and therefore identical concentrations. They are placed in quartz cuvettes and irradiated continuously by a mercury vapor lamp (irradiation wavelengths 365 or 254 nm). The desorption of the ligands on the surface of the nanocrystals decreases their colloidal stability and leads to their precipitation. This is visible in the UV-visible absorption spectra, which are recorded regularly. The faster the signal decreases, the less stable the nanocrystals are. The photo-degradation is also visible by a shift of the exciton peak towards blue which signifies a reduction in the average size of the crystals. A thiol with a long alkyl chain, 1-dodecanethiol, is chosen because it is this type of ligand which resists best in the photo-degradation experiments carried out by Aldana et al. ((<i>J. Am. Chem. Soc.</i><b>123(36),</b> p. 8844-8850 (2001)). This ligand is compared to 1-dithiotridecanoic acid, prepared according to example 3. Thus, the only difference between the two ligands XYZ is the anchoring function X (thiol or dithiolate) while the chemical groups YZ are identical ( VS₁₂H₂₅).

FIG. 2A shows the spectra obtained for the samples of CdSe nanocrystals before the exchange of ligands on their surface, hereinafter called base crystals. After 33 h of irradiation at 365 nm practically all the nanocrystals are precipitated and the exciton peak at approximately 570 nm is no longer visible in the spectrum. The ligands on the surface of the CdSe nanocrystals are exchanged with 1-dodecanethiol, as described in Example 6; we make sure by NMR spectroscopy that the exchange is complete. Under irradiation at 365 nm, the photo-degradation time remains approximately the same: after 50 h all the crystals have precipitated (Fig. 2B).

A second sample of the same batch of nanocrystals is treated with 1-dithiotridecanoic acid, as described in Example 5 in order to completely cover the surface of the crystals with the dithiolate ligand. In this case, even after 67 h of irradiation, no precipitation of the nanocrystals takes place, as shown by the presence of the exciton peak in the spectra (Fig. 2C) and the absence of deposit on the walls of the quartz cuvette. which contains the colloidal solution.

The use of higher energy light (254 nm) for irradiation gives a shorter time scale for degradation (1-2 h), however the nanocrystals covered by dithiolate ligands are again more stable than the base crystals and the crystals covered by the thiols. In the case of CdSe / ZnSe core / shell nanocrystals, the same order of stability is observed as for CdSe crystals. Furthermore, the fluorescence of CdSe / ZnSe crystals before exchange of ligands and after exchange with thiol decreases rapidly in less than 2 h of irradiation at 365 nm, while CdSe / ZnSe crystals covered by ligands of the dithiolate type fluoresce still after 90 h without changing the emission wavelength. This is a great advantage for applications that are based on the photoluminescence properties of nanocrystals.

Example 9

In this example, a photochemical treatment is carried out of a sample of fluorescent nanocrystals covered with ligands of the 1,1-dithiolate type (according to the invention), in order to increase their fluorescence efficiency.

For this experiment, a quartz cuvette containing a dispersion in chloroform of the CdSe / ZnSe nanocrystals core / shell, treated with 1-dithiotridecanoic acid prepared analogously to Example 5, is placed at a distance of 4 cm a 100 W mercury vapor lamp.

The sample is irradiated continuously at a wavelength of 365 nm.

FIG. 3 shows the evolution of the intensity of the fluorescence as a function of the irradiation time. It reaches its maximum which corresponds to approximately 8 times the initial value, after approximately 50 min. Continued exposure to UV light for a period exceeding approximately 100 min again results in a decrease in fluorescence.

It should be noted that the wavelength and the width of the fluorescence line do not change during irradiation.