Process for manufacturing colloidal nanosheets by lateral growth of nanocrystals

Abstract

A process for manufacturing colloidal nanosheet, by lateral growth, on an initial colloidal nanocrystal, of a crystalline semiconductor material represented by the formula M.sub.nX.sub.y, where M is a transition metal and X a chalcogen. The process includes the following steps: The preparation of a first organic solution, non or barely coordinating used as a synthesis solvent and including at least one initial colloidal nanocrystal; The preparation of a second organic solution including precursors of M and X, and including an acetate salt. And the slow introduction over a predetermined time scale of a predetermined amount of the second solution in a predetermined amount of the first solution, at a predetermined temperature for the growth of nanosheets. The use of the obtained material is also presented.

Claims

1. A colloidal nanosheet comprising: one first portion comprising an initial colloidal crystalline nanosheet completely surrounded laterally, and not in a thickness, by a second adjacent extended portion having a semiconductor material represented by the formula M.sub.nX.sub.y, wherein M is selected from the group consisting of Cd, Zn, Pb, Hg, Cu, In, Ag, Fe, Al, Ti, or a mixture thereof, and X is selected from the group consisting of O, S, Se, Te, P, or a mixture thereof, wherein the initial colloidal crystalline nanosheet has a different composition compared to the M.sub.nX.sub.y material of the second adjacent extended portion, thereby forming different semiconductor regions, and the initial colloidal crystalline nanosheet is made of CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, PbO, PbS, PbSe, PbTe, HgS, HgSe, HgTe, CuInS.sub.2, CuInSe.sub.2, AgInS.sub.2, AgInSe.sub.2, CuS, Cu.sub.2S, Ag.sub.2S, Ag.sub.2Se, Ag.sub.2Te, FeS, FeS.sub.2, InP, Cd.sub.3P.sub.2, Zn.sub.3P.sub.2, FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, Al.sub.2O.sub.3, TiO.sub.2, or an alloy thereof.

2. The colloidal nanosheet according to claim 1, wherein said crystalline semiconductor material MnXy is a compound selected from the group consisting of CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, PbO, PbS, PbSe, PbTe, HgS, HgSe, HgTe, CuInS.sub.2, CuInSe.sub.2, AgInS.sub.2, AgInSe.sub.2, CuS, Cu.sub.2S, Ag.sub.2S, Ag.sub.2Se, Ag.sub.2Te, FeS, FeS.sub.2, InP, Cd.sub.3P.sub.2, Zn.sub.3P.sub.2, FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, Al.sub.2O.sub.3, TiO.sub.2 and alloy thereof.

3. The colloidal nanosheet according to claim 1, wherein said semiconductor crystalline M.sub.nX.sub.y material is doped by a transition metal.

4. The colloidal nanosheet according to claim 1, wherein said colloidal nanosheet has a lateral size larger than 10 nm.

5. A large specific area material for catalysis comprising the colloidal nanosheet according to claim 1.

6. A light emitting diode comprising the colloidal nanosheet according to claim 1 as an active element.

7. A solar cell comprising the colloidal nanosheet according to claim 1 as an active element in an absorber and or in a collector of photogenerated charges.

8. A method of preparing a semiconductor ultrathin film comprising building said semiconductor ultrathin film on a substrate at low temperature from the colloidal nanosheet according to claim 1.

9. A colloidal nanosheet having a lateral extension larger than 10 nm comprising: one first portion comprising an initial colloidal nanocrystal nanosheet surrounded laterally on all sides, and not in a thickness, by a continuous second adjacent extended portion having a semiconductor material represented by the formula M.sub.nX.sub.y, wherein M is selected from the group consisting of Cd, Zn, Pb, Hg, Cu, In, Ag, Fe, Al, Ti, or a mixture thereof, and X is selected from the group consisting of O, S, Se, Te, P, or a mixture thereof, wherein the initial colloidal nanocrystal nanosheet has a different composition compared to the M.sub.xX.sub.y material of the continuous second adjacent extended portion, thereby forming different semiconductor regions, and wherein the one first portion comprising an initial colloidal nanocrystal nanosheet surrounded laterally on all sides is a semiconductor material.

Description

(1) Other characteristic and advantages of the process depending of the invention will appear while reading the detailed description of the realization example. The latter are given as non limiting illustrations and refer to the figure given in the annexes.

(2) FIG. 1 shows rolled sheets of CdSe emitting at 462 nm and synthetized according to example 1 as a possible realization of the current invention

(3) FIG. 2 shows aggregated sheets of CdSe emitting at 393 nm

(4) FIG. 3 shows rolled sheets of CdSe emitting at 462 nm synthetized according to a modified version of example 1 where the cadmium acetate is only in the flask (solution 1).

(5) FIG. 4 shows a top view of a scheme of a heterogeneously composed sheet.

DETAILED DESCRIPTION OF THE INVENTION

(6) Some embodiments of the invention, which will be described latter, show some bidimensional growth method of the semiconductor nanocrystals. These processes allow reaching lateral size above the micrometer while keeping the thickness constant and controlled within an atomic monolayer. It is also possible to control the thickness of the obtained nanosheets by controlling some synthesis parameters such as the precursor nature, the reaction temperature and/or the presence of nanocrystals in the reaction mixture.

(7) These growth processes according to the current invention allow obtaining, without post synthesis purification, pure nanosheets, free of any parasitic isotropic nanocrystals.

(8) The new process of crystalline growth confronts the paradigm nucleation/growth by quick injection of precursors at high temperature which is currently used in all organic syntheses of colloidal semiconductor nanocrystals. Indeed in one embodiment of the current invention the precursors are slowly introduced in the flask during the synthesis. The nucleation rate is controlled by the temperature of the flask and the injection speed, the system steers to a growth state at equilibrium in which all the injected precursors are consumed by the nanosheets growth, the initial nucleation leads to enough seeds in the reaction medium to consume at any time the introduced precursors. The final size of the nanosheets (its lateral dimensions) is consequently controlled by the introduced precursors.

(9) Moreover, the growth process proposed also confronts the descriptions of syntheses of nanosheets by the method of <<soft templating>> in which a solution lamellar complex (it can be done with the acetate salt) leads the growth to the formation of nanosheets.

(10) In one embodiment of the current invention, a solution of precursors including finely ground and well dispersed acetate salt is slowly added in the reaction mixture which can simply be a warm organic solvent, without acetate salt.

(11) By controlling the temperature and the precursor nature, it is thus possible to control the thickness of the sheets, while their concentration is controlled by the injection speed and their lateral size is controlled by the amount of introduced precursors. The sheet geometry is in particular determined by the nature of the used acetate salt.

(12) In the following, we will designate the nanocrystalline material binary compound by the general formula MX, M is a transition metal and X a chalchogen. The sheets which can be synthetized by the described process are CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, PbO, PbS, PbSe, PbTe and alloy of thereof. It is also possible to grow the same previously mentioned material doped with Fe, Cu, Mn, Mg, Co . . . .

(13) The synthesis consist in the slow introduction of M and X precursors, as well as an acetate salt in a flask including a non or barely coordinating organic solvent, as well as nanocrystals seeds.

(14) In order to get a bidimensional growth an acetate salt is used. It can be of any kind, and the use of different acetate salts leads to different geometries of nanosheets. It should be noticed that in the case of CdSe, the use of cadmium acetate leads to square shapes sheets.

(15) In one embodiment of the invention, the acetate salt is finely ground with a mortar and dispersed in the solution to introduce. In that case the synthesis is controlled and does not lead to the apparition of undesired compound such as isotropic nanocrystals or oxides induced by the thermal decomposition of the acetate salt. It is then possible to get nanosheets with large lateral dimensions superior to the micrometer, which is impossible if the acetate salt is directly present in the reaction medium. Indeed, in that case, it gradually damages with the synthesis and it does not lead to large pure nanosheets.

(16) In general the M precursor used is a M(carboxylate).sub.2 made out of a fatty acid. The M precursor can also come as a M(acetate) complex. More precisely, the M precursor can be an M(oleate) complex, an M(stearate) complex or an M(myristate) complex.

(17) The X precursor can be a liquid containing X or an homogeneous dispersion of a X powder. More precisely, the X precursor can be X dissolve in a phosphine (trioctylphosphine, tributylphosphine, triphenylphosphine, . . . ) with a concentration from 0.1M to the stoichiometry or it can be X dissolved in an alkene such as 1-octadecene with concentration from 0.01M to 0.2MThe X precursor can be also of II oxidation degree such as H.sub.2X or Na.sub.2X.

(18) The solvent can be any kind of organic solvent non or barely coordinating. More precisely, the solvent can be 1-octadecene, trioctylamine, toluene or benzylbenzoate.

(19) The flask temperature during the precursors introduction can be in the range from 20 C. to 250 C. It depends on the precursors and on the thickness of the nanosheets we want to synthesize. In particular, the temperature can be in the range from 150 C. to 200 C.

(20) The synthesis is preferably run under inert atmosphere (argon or nitrogen), to avoid the formation of unwanted oxides, but it can also be done in air.

EXAMPLES

(21) The invention will be described in references to the following examples, as illustrations but non limiting.

Example 1: Synthesis of Nanosheets Emitting at 462 nm

(22) In a 100 ml three necks flask, 10 ml of 1-octadecene are introduced with 40 mg of Cd(Acetate).sub.2,2H.sub.2O previously ground in a mortar. The mixture is magnetically stirred and degassed under vacuum for 30 minutes. The reaction medium is then passed under inert atmosphere (argon) and heated at 180 C.

(23) Simultaneously, a mixture of 40 mg of Cd(Acetate).sub.2, 2H.sub.2O previously ground in a mortar, 240 mg of Cd(myristate).sub.2 and 4 ml of trioctylamine are heated under stirring until complete dissolution of Cd(myristate).sub.2. 4 ml of a solution of selenium in ODE at 0.1M is then added. The mixture forms a gel by cooling down.

(24) This gel is injected over 2 hours at 180 C. in the reactive medium, leading to the nucleation and the growth of nanosheets with lateral dimensions larger than 200 nm.

Example 2: Growth of Nanosheets Emitting at 510 nm

(25) In a 100 ml three necks flask, 10 ml of 1-octadecene are introduced with 40 mg of Cd(Acetate).sub.2,2H.sub.2O previously crushed in a mortar and 10 nmol of CdSe nanocrystals synthesized through the method describe in reference.sup.12. The mixture is magnetically stirred and degassed under vacuum for 30 minutes. The reaction medium is then brought under inert atmosphere (argon) and heated at 180 C.

(26) Simultaneously, a mixture of 40 mg of Cd(Acetate).sub.2, 2H.sub.2O previously ground in a mortar, 240 mg of Cd(myristate).sub.2 and 4 ml of trioctylamine are heated under stirring until complete dissolution of Cd(Myristate).sub.2. 4 ml of a solution of selenium in ODE at 0.1M is then added. The mixture forms a gel by cooling down.

(27) This gel is injected over 4 hours at 200 C. in the reactive medium, leading to the nucleation and the growth of nanosheets with lateral dimensions larger than 100 nm.

(28) In an other embodiment of the invention:

(29) In a 100 ml three necks flask, 10 ml of 1-octadecene are introduced with 10 nmol of CdSe nanocrystals synthesized through the method describe in reference.sup.12. The mixture is magnetically stirred and degassed under vacuum for 30 minutes. The reaction medium is then brought under inert atmosphere (argon) and heated at 240 C.

(30) Simultaneously, a mixture of 96 mg of Cd(Acetate).sub.2, 2H.sub.2O previously ground in a mortar dissolved in 1 ml of ethanol, 46 l of oleic acid, 1 ml of butanol and 4 ml of a solution of selenium in ODE at 0.1M is prepared.

(31) This solution is injected in 10 minutes at 240 C. in the reactive medium, leading to the nucleation and the growth of nanosheets with lateral dimensions larger than 100 nm.

Example 3: Synthesis of Nanosheets Emitting at 393 nm

(32) 10 ml of toluene is introduced in a 100 ml three necks flask. The flask is heated at 100 C., while a syringe containing 5 ml of toluene, 133 mg of Cd(Acetate).sub.2,2H.sub.2O, 30 mg of benzoic acid and 100 l of stoichiometric TOPSe is prepared.

(33) The syringe is then injected over 1 hour, with a speed of 5 ml/h in the hot flask of toluene.

(34) The reactive medium slowly gets cloud, indicating the formation of large nanosheets emitting at 393 nm.

(35) These are separated from the reactive medium via centrifugation and resuspended in toluene.

(36) This new path of fabrication of nanocrystals following the examples presented allows the control of the thickness and the tuning of the lateral dimensions of the obtained nanosheets. It opens the way to new applications for these materials in domains as diversify as photovoltaic, electronic and optic.

(37) The lateral dimensions are controlled through the quantity of precursors introduced while the thickness is control by the synthesis parameters: temperature, precursors and the initial presence of nanocrystals in the reaction medium.

(38) It has been noticed that the syntheses of nanosheets following the exposed procedures minimize the formation of spherical nanocrystals and the reaction of acetate salt decomposition. Especially, a slow injection of the precursors and a thin powder of the acetate salt totally avoid from the undesired reaction such as the formation of spherical nanocrystals and thus allow to obtain pure nanosheets.

Exemple 4

Growth of Nanosheets CdSe/CdS

(39) In a 100 mL three necks flask, 10 ml of 1-octadecene (ODE) are introduced with 10 nmol of CdSe nanocrystals synthetized according to the method described in ref 12. The mixture is magnetically stirred and degassed for 30 minutes. The reaction atmosphere is switched to Argon and the solution warmed at 240 C.

(40) In the meanwhile a mixture composed of 96 mg of Cd(Acetate).sub.2,2H.sub.2O dissolved in 1 ml of ethanol, 46 L of oleic acid, 1 m of butanol and 4 mL of sulfur in ODE at 0.1M is prepared.

(41) This solution is injected over 30 minutes at 240 C. in the reaction mixture, leading to the growth of core/crown nanosheets of CdSe/CdS with lateral size larger than 50 nm.

(42) The structure of the core/crown sheets is schematized on FIG. 4, where A1 is CdSe and A2 is the CdS.