NANOCRYSTALS

Abstract

The present invention provides the use of a lead (IV) containing compound to prepare a lead chalcogenide nanocrystal and a method for producing broadband lead chalcogenide nanocrystals in a low cost, size-controllable and scalable method, the method comprising contacting a lead (IV) containing compound with an organic acid and a chalcogen-containing reagent.

Claims

1. The use of a lead (IV) containing compound to prepare a lead chalcogenide nanocrystal.

2. The use according to claim 1, wherein the lead (IV) containing compound comprises lead (II, IV) oxide.

3. The use according to claim 1, wherein the lead chalcogenide nanocrystal exhibits absorption in the range of 500 to 4500 nm, preferably 500 to 2400 nm.

4. A method for producing a lead chalcogenide nanocrystal, the method comprising contacting a lead (IV) containing compound with an organic acid and a chalcogen-containing reagent.

5. A method according to claim 4, wherein the lead (IV) containing compound comprises lead (II, IV) oxide.

6. A method according to claim 4, wherein the lead (IV) containing compound is contacted with the organic acid to produce a lead salt and the lead salt is contacted with the chalcogen-containing reagent.

7. A method according to claim 4, which is conducted in the presence of a solvent, preferably wherein the solvent comprises a non-polar solvent, such as octadecene, or a polar solvent, such as DMF, NMP, DMAc, THF, acetone.

8. A method according to claim 4, which comprises: forming a first solution of the lead (IV) containing compound and organic acid in a first solvent; forming a second solution of the chalcogen-containing reagent in a second solvent; heating the first solution to a first temperature in the range of from 120 to 250° C. and maintaining the first solution at the first temperature for a predetermined length of time; reducing the temperature of the first solution to a reduced temperature in the range of from 20 to 100° C. adding the second solution to the first solution at the reduced temperature to produce a reaction mixture; maintaining the reaction mixture at a temperature of from 20 to 300° C. for a predetermined length of time.

9. A method according to claim 4, which comprises: forming a first solution of the lead (IV) containing compound and organic acid in a first solvent; heating the first solution to a first temperature in the range of from 120 to 250° C. and maintaining the first solution at the first temperature for a predetermined length of time; providing the first solution at a second temperature in the range of from 50 to 150° C.: adding the chalcogen-containing reagent to the first solution at the second temperature to produce a reaction mixture; maintaining the reaction mixture at a temperature of from 50 to 300° C. for a predetermined length of time.

10. A method according to claim 8, further comprising quenching the reaction mixture, for example by adding a quenching solvent to the reaction mixture.

11. A method according to claim 8, further comprising purifying the lead chalcogenide nanoparticle.

12. A method according to claim 4, wherein the organic acid is a fatty acid, preferably oleic acid.

13. A method according to claim 4, wherein the chalcogen-containing reagent is selected from an oxygen-, sulphur-, selenium- and tellurium-containing reagent, and mixtures thereof.

14. A method according to claim 8, wherein the chalcogen-containing reagent comprises bis(trimethylsilyl)sulphide.

15. A method according to claim 9, wherein the chalcogen-containing reagent comprises thioacetamide.

16. A method according to claim 9, wherein the chalcogen-containing reagent comprises tri-n-octylphosphine selenide (TOPSe) and diphenylphosphine (DPP).

17. A method according to claim 6, wherein the lead salt is contacted with the chalcogen-containing reagent at a temperature of from 20 to 100° C., preferably of from 30 to 60° C.

18. A method according to claim 6, wherein the lead salt is contacted with the chalcogen-containing reagent at a temperature of from 50 to 300° C., preferably from 50 to 180° C.

19. A method according to claim 4, comprising the step of modifying a reaction condition so as to control the size of the nanocrystal prepared.

20. A method according to claim 19, wherein the reaction condition to be modified comprises one or more of the following: (i) solvent type; (ii) amount of solvent; (iii) organic acid type; (iv) amount of organic acid; (v) mode of addition of the reactants (particularly of chalcogen-containing reagent); (vi) reaction temperature; (vii) reaction time (viii) ratio of Pb to chalcogen-containing reagent; and (ix) addition of a secondary solvent.

21. A method according to claim 4, comprising monitoring an optical property so as to monitor the progress of the production of the nanocrystals.

22. A method according to claim 21, wherein the optical property is a UV-visible-near infrared absorbance spectrum.

23. A use or method according to claim 1, wherein the nanocrystals comprise quantum dots.

24. One or more (preferably a plurality of) lead chalcogenide nanocrystals obtained by the method according to claim 4.

25. A lead chalcogenide nanocrystals composition obtained by the method according to claim 4.

26. A lead chalcogenide nanocrystal composition comprising nanocrystals having a mean particle size in the range of 2 to 20 nm, and a relative size dispersion of less than 25%.

27. The lead chalcogenide nanocrystal composition according to claim 26, which exhibits absorption in a range of from about 500 to about 4500 nm, preferably from about 500 to about 2400 nm, such as from about 530 to about 2400 nm, such as from about 530 to 1600 nm.

28. The lead chalcogenide nanocrystal composition according to claim 26, which exhibits emission in the range of 600 to 4500 nm, preferably 600 to 2500 nm.

29. The lead chalcogenide nanocrystal composition according to claim 26, which exhibits absorption full width at half maximum (FWHM) values of less than 250 nm, preferably less than 230 nm, preferably less than 130 nm, preferably less than 110 nm and emission full width at half maximum (FWHM) values of less than 250 nm, preferably less than 230 nm, preferably less than 150 nm, preferably less than 110 nm.

30. The lead chalcogenide nanocrystal composition according to claim 26, which exhibits Quantum Yield (QY) greater than 10%, preferably greater than 20%, preferably greater than 40%, preferably greater than 50%.

31. The lead chalcogenide nanocrystal composition according to claim 26, having a mean particle size in the range of about 2 to about 20 nm, preferably about 2 to about 17 nm, preferably about 2 to about 10 nm.

32. The lead chalcogenide nanocrystal composition according to claim 26, comprising greater than 0.001% by weight of lead chalcogenide nanocrystals, preferably greater than 0.01% by weight, preferably greater than 0.1% by weight, preferably greater than 1% by weight, preferably greater than 5% by weight.

33. The lead chalcogenide nanocrystal composition according to claim 26, having a maximum absorption wavelength of 500 to 1000 nm and having an absorption FWHM of less than 115 nm.

34. The lead chalcogenide nanocrystal composition according to claim 26, wherein the nanocrystals have a molar ratio of lead atoms to chalcogen atoms in the range of from 1.2:1 to 4:1, preferably 1.6:1 to 3:1.

35. The lead chalcogenide nanocrystal composition according to claim 26, wherein the lead chalcogenide nanocrystal comprises PbS, PbSe, PbTe or mixtures thereof, preferably PbS.

36. Lead chalcogenide nanocrystal compositions according to claim 26, obtainable by the method comprising contacting a lead (IV) containing compound with an organic acid and a chalcogen-containing reagent.

37. A device selected from the group consisting of IR sensor, photodetector, sensor, solar cell, a bio-imaging or bio-sensing composition, photovoltaic system, display, battery, laser, photocatalyst, spectrometer, injectable composition, field-effect transistor, light-emitting diode, photonic or optical switching device or metamaterial, fiber amplifier, optical gain media, optical fiber, infrared LEDs, lasers, and electroluminescent device, comprising a lead chalcogenide nanocrystal composition according to claim 25.

38. A device according to claim 37, wherein the IR sensor or photodetector is modified for applications such as 3D cameras and 3D Time of flight cameras in mobile and consumer, automotive, medical, industrial, defense or aerospace applications.

39. A device according to claim 37, wherein the bio-imaging or bio-sensing compositions are modified for use as bio-labels or bio-tags in in vitro or ex vivo applications.

40. A device according to claim 37, wherein the infrared LEDs and electroluminescent devices are modified for use in telecommunication devices, night vision devices, solar energy conversion, thermoelectric or energy generation applications.

41. A film comprising the lead chalcogenide nanocrystal composition according to claim 25.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0241] For a better understanding of the invention, and to show how exemplary embodiments of the same may be carried into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0242] FIG. 1 shows the powder XRD pattern for the PbS nanocrystals prepared according to Example 1;

[0243] FIG. 2 shows the absorption spectrum of the PbS nanocrystals prepared in Example 1 in toluene;

[0244] FIG. 3 shows the absorption spectra for the PbS nanocrystals obtained with different amounts of oleic acid in Example 2;

[0245] FIG. 4 shows the absorption spectra for the PbS nanocrystals obtained at 40° C. or 60° C. in Example 3; FIG. 5 shows the absorption spectra for the PbS nanocrystals obtained with a single or multiple addition of (TMS).sub.2S in Example 4;

[0246] FIG. 6 shows the absorption spectra for the PbS nanocrystals obtained with different amounts of ODE in Example 5;

[0247] FIG. 7 shows the absorption spectra for the PbS nanocrystals obtained using TAA and (TMS).sub.2S in Example 6;

[0248] FIGS. 8a and 8b shows the TEM image of PbS nanocrystals (λ.sub.max=1314 nm) and the histogram of particle measurements by TEM in Example 6;

[0249] FIG. 9 shows the absorption spectra for the PbS nanocrystals obtained with and without the addition of acetone in Example 7;

[0250] FIG. 10 shows the absorption spectra for the PbS nanocrystals obtained with and without the addition of hexane in Example 8;

[0251] FIG. 11 shows the absorption spectra for the PbS nanocrystals obtained using different amounts of oleic acid and different reaction temperatures in Example 9;

[0252] FIG. 12 shows the absorption spectra for the solution PbS nanocrystals and thin film sample as prepared in Example 10;

[0253] FIG. 13 shows an example of a particular method of the present invention;

[0254] FIG. 14 shows an example of a particular method of the present invention;

[0255] FIG. 15 shows the synthesis scheme of PbSe nanocrystals using Pb.sub.3O.sub.4 and TOPSe;

[0256] FIG. 16 shows the powder XRD pattern for the PbSe nanocrystals as prepared in Example 11.

[0257] FIG. 17 shows the absorption spectra for the PbSe nanocrystals as prepared in Example 11.

[0258] FIG. 18 shows the absorption spectra of PbSe nanocrystals demonstrating the wide spectral tunability for the PbSe nanocrystals obtained from different reaction conditions as prepared in Example 11.

[0259] FIG. 19 shows the TEM images for the PbSe nanocrystals ((λ.sub.max=1926 nm) prepared from different conditions as prepared in Example 11.

[0260] FIGS. 20a and 20b show the high resolution transmission electron microscopy (HRTEM) images of colloidal PbSe nanocrystals ((λ.sub.max=2046 nm) and the histogram of the particle measurement by TEM as prepared in Example 11. The PbSe dots are highly crystalline and free from stacking faults and lattice defects.

[0261] FIGS. 21a and 21b show the absorption and emission spectra of PbS nanocrystals (λ.sub.max=950 nm, PL=1060±5 nm) and PbSe (λ.sub.max=1000 nm, PL=1100±5 nm) as prepared in Example 6 and 11 respectively.

EXAMPLES

[0262] Several examples and comparative examples are described hereunder illustrating the methods according to the present disclosure.

[0263] Whereas particular examples of this invention have been described below for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

[0264] Unless otherwise indicated, all parts and all percentages in the following examples, as well as throughout the specification, are parts by weight or percentages by weight respectively.

[0265] Absorption spectra were obtained on a JASCO V-770 UV-visible/NIR spectrometer (which can provide measurements in the 400 to 3200 nm wavelength range).

Example 1: Synthesis of Lead Sulphide (PbS) Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S

[0266] 0.19 g Pb.sub.3O.sub.4, 0.6 ml oleic acid and 7 ml octadecene (ODE) were loaded in a 3-necked flask, vacuumed and held under a N.sub.2 atmosphere for 30 minutes at 180° C. to produce lead oleate solution. After clear lead oleate solution formed, the temperature was reduced to 40° C. 45 μl bis(trimethylsilyl)sulphide ((TMS).sub.2S) in 4 ml of ODE was swiftly injected. The solution changed from light yellow to light brown then dark brown in the next 5 minutes, suggestive of controllable nucleation. The reaction was maintained at 40° C. for 60 minutes (monitored by UV-Vis-NIR spectroscopy), then slowly cooled down to room temperature. Then 20 ml of distilled acetone was added into the reaction mixture. PbS nanocrystals were precipitated through centrifugation, re-dispersed in toluene, and precipitated again through a combination of acetone and centrifugation. Finally, the nanocrystals were dispersed in toluene. The purification process was carried out in air.

[0267] The PbS nanocrystals prepared in Example 1 were characterised by powder XRD and EDX measurements.

[0268] XRD samples were prepared by dropcasting the particles from a toluene solution onto the end of a microscope slide until a relatively thick opaque film was formed. This film was then analysed using a Panalytical X'Pert PRO MPD instrument.

[0269] EDX samples were prepared by dropcasting the particles from a toluene solution onto SEM specimen stubs covered with a carbon-based adhesive tab. The toluene was allowed to evaporate prior to analysis. Samples were analysed on a Philips/FEI XL30 ESEM equipped with an Oxford Instruments Energy 250 energy dispersive spectrometer system.

[0270] FIG. 1 shows the powder XRD pattern for the PbS nanocrystals prepared according to Example 1. The XRD pattern shown in FIG. 1 confirms that Example 1 produced PbS nanocrystals with a rock-salt structure.

[0271] The EDX measurements were as shown in Table 1 and confirmed that the nanocrystals prepared contain lead and sulphur.

TABLE-US-00001 TABLE 1 Element Weight % Atomic % S K 8.48 37.46 Pb M 91.52 62.54 Totals 100.00

[0272] The absorption spectra of the PbS nanocrystals prepared in Example 1 in toluene was obtained and the results are shown in FIG. 2.

Example 2: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S with Different Amounts of Oleic Acid

[0273] The synthesis outlined above in Example 1 was repeated using an amount of oleic acid of 0.6 ml and using an amount of 14.4 ml.

[0274] The absorption spectra for the nanocrystals obtained are shown in FIG. 3 (in which the solid line corresponds to oleic acid 0.6 ml; and the dashed line corresponds to oleic acid 14.4 ml), which shows that different amounts of oleic acid provided different sized crystals.

[0275] FIG. 3 shows that larger PbS nanocrystals were obtained when the amount of oleic acid used was 14.4 ml compared to 0.6 ml.

Example 3: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S at 60° C.

[0276] The synthesis outlined above in Example 1 was repeated except that the temperature for the reaction of the lead salt with the bis(trimethylsilyl)sulphide ((TMS).sub.2S) was changed from 40° C. to 60° C.

[0277] The absorption spectra for the nanocrystals obtained at 40° C. and 60° C. are shown in FIG. 4 (in which the solid line corresponds to the reaction at 40° C.; and the dashed line corresponds to the reaction at 60° C.), which shows that different temperatures provided different sized crystals.

[0278] FIG. 4 shows that larger PbS nanocrystals were obtained when the temperature for the reaction of the lead salt with the chalcogen-containing reagent was increased.

Example 4: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S with Multi-Step (TMS).SUB.2.S Additions

[0279] 0.19 g Pb.sub.3O.sub.4, 9.6 ml oleic acid and 7 ml octadecene (ODE) were loaded in a 3-necked flask, vacuumed and held under a N.sub.2 atmosphere for 30 minutes at 180° C. to produce lead oleate solution. After clear lead oleate solution formed, the temperature was reduced to 40° C. 90 μl bis(trimethylsilyl)sulphide ((TMS).sub.2S) in 4 ml of ODE was swiftly injected. The reaction was maintained at 40° C. for 60 minutes, then slowly cooled down to room temperature. Then 20 ml of distilled acetone was added into the reaction mixture. PbS nanocrystals were precipitated through centrifugation, re-dispersed in toluene, and precipitated again through a combination of acetone and centrifugation. Finally, the nanocrystals were dispersed in toluene. The purification process was carried out in air.

[0280] The above method was repeated except that the bis(trimethylsilyl)sulphide (TMS) solution in ODE was divided into two equal portions and each portion added to the reaction mixture separately with a 15 minutes interval between each injection. 30 minutes after the last injection the reaction was completed.

[0281] The absorption spectra for the nanocrystals obtained with a single or multiple addition of (TMS).sub.2S are shown in FIG. 5 (in which the solid line corresponds to the reaction with a single addition of (TMS).sub.2S; and the dashed line corresponds to the reaction with two additions of (TMS).sub.2S), which shows that different modes of addition of the (TMS).sub.2S provided different sized crystals.

[0282] FIG. 5 shows that larger PbS nanocrystals were obtained when the (TMS).sub.2S was added by two additions compared to by a single addition.

Example 5: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S with Different Amounts of ODE

[0283] 0.19 g Pb.sub.3O.sub.4, 9.6 ml oleic acid and 7 ml octadecene (ODE) were loaded in a 3-necked flask, vacuumed and held under a N.sub.2 atmosphere for 30 minutes at 180° C. to produce lead oleate solution. After clear lead oleate solution formed, the temperature was reduced to 40° C. 45 μl bis(trimethylsilyl)sulphide ((TMS).sub.2S) in 4 ml of ODE was swiftly injected. The reaction was maintained at 40° C. for 60 minutes, then slowly cooled down to room temperature. Then 20 ml of distilled acetone was added into the reaction mixture. PbS nanocrystals were precipitated through centrifugation, re-dispersed in toluene, and precipitated again through a combination of acetone and centrifugation. Finally, the nanocrystals were dispersed in toluene. The purification process was carried out in air.

[0284] The above method was repeated except that the amount of ODE used to dilute the Pb.sub.3O.sub.4 and oleic acid before the addition of (TMS).sub.2S into the lead oleate solution was changed from 7 ml to 21 ml.

[0285] The absorption spectra for the nanocrystals obtained with different amounts of ODE are shown in FIG. 6 (in which the solid line corresponds to the reaction using 21 ml of ODE; and the dashed line corresponds to the reaction using 7 ml of ODE), which shows that different amounts of ODE provided different sized crystals.

[0286] FIG. 6 shows that smaller PbS nanocrystals were obtained when the amount of ODE used was increased.

Example 6: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and Thioacetamide (TAA)

[0287] The synthesis outlined above in Example 1 was repeated except that the (TMS).sub.2S was replaced by the same molar amount of thioacetamide (TAA). The latter can be added to the lead oleate as a powder or it can be dissolved in a solvent, such as Acetone, DMF or THF before injection. The reaction mixture is then heated to 50° C. and maintained at that temperature for 1 hour or to 90° C. and maintained at that temperature for 20 minutes

[0288] The absorption spectra for the nanocrystals obtained using TAA and (TMS).sub.2S are shown in FIG. 7 (in which the solid line corresponds to the reaction using (TMS).sub.2S; and the dashed line corresponds to the reaction using TAA), which shows that the different reagents provided different sized crystals.

[0289] FIG. 7 shows that larger PbS nanocrystals were obtained when TAA was used instead of (TMS).sub.2S.

[0290] FIGS. 8a and 8b shows TEM image of PbS nanocrystals and the histogram of particle measurements by TEM. The PbS nanocrystals with maximum absorption of 1314 nm were obtained when TAA was used instead of (TMS).sub.2S.

[0291] Table 2 shows maximum absorption wavelength, size and composition of PbS nanocrystals obtained when TAA was used instead of (TMS).sub.2S shown as below. Increasing Pb to S ratio leads to increase in the maximum absorption wavelength of PbS dots;

TABLE-US-00002 TABLE 2 Maximum absorption Pb/S molar ratio wavelength λ.sub.max (nm) Size by TEM (nm) by ICP-OES 950 3.10 ± 0.52 1.69:1 1314 6.28 ± 0.82 1.93:1 1540 8.32 ± 1.41 1.97:1

[0292] The absorption FWHM for these nanocrystals is about 100 nm. For example, the FWHM for the nanocrystals exhibiting a maximum absorption of 950 nm is 105 nm.

Example 7: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S Promoted by Addition of Acetone During Reaction

[0293] The synthesis outlined above in Example 1 was repeated except that 5 ml of acetone was quickly injected into the solution 10 seconds after the (TMS).sub.2S solution in ODE was added to the lead oleate solution.

[0294] The absorption spectra for the nanocrystals obtained with and without the addition of acetone are shown in FIG. 9 (in which the solid line corresponds to the reaction with the addition of acetone; and the dashed line corresponds to the reaction without addition of acetone, i.e. according to Example 1), which shows that the different reagents provided different sized crystals.

[0295] FIG. 9 shows that smaller PbS nanocrystals were obtained when acetone was added compared to Example 1.

Example 8: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and (TMS).SUB.2.S, Quenched by Cold Hexane During Reaction

[0296] The synthesis outlined above in Example 1 was repeated, except that cold hexane was quickly injected into the solution 1.5 minutes after the (TMS).sub.2S in ODE solution was added to the lead oleate solution.

[0297] The absorption spectra for the nanocrystals obtained with and without the addition of hexane are shown in FIG. 10 (in which the solid line corresponds to the reaction with the addition of hexane; and the dashed line corresponds to the reaction without addition of hexane, i.e. according to Example 1), which shows that the different reagents provided different sized crystals.

[0298] FIG. 10 shows that smaller PbS nanocrystals were obtained when hexane was added compared to Example 1.

Example 9: Synthesis of PbS Nanocrystals Using Pb.SUB.3.O.SUB.4 .and Thioacetamide (TAA)

[0299] 0.35 g Pb.sub.3O.sub.4, 1 ml oleic acid and 5 ml octadecene (ODE) were loaded in a 3-necked flask, vacuumed and held under a N.sub.2 atmosphere for 30 minutes at 180° C. to produce lead oleate solution. After clear lead oleate solution formed, the temperature was reduced to room temperature. 40 mg TAA was directly loaded into the flask and the reaction mixture was heated up to 50° C. and maintained at this temperature for 60 minutes. The reaction mixture was then slowly cooled down to room temperature. Then 20 ml of distilled acetone was added into the reaction mixture. PbS nanocrystals were precipitated through centrifugation, re-dispersed in toluene, and precipitated again through a combination of acetone and centrifugation. Finally, the nanocrystals were dispersed in toluene. The purification process was carried out in air.

[0300] The above method was then repeated, except that the amounts of oleic acid and the temperatures of the reaction of the lead oleate with the TAA were as follows:

TABLE-US-00003 Amount of oleic acid Temperature (lead oleate + TAA) 1.4 ml 50° C. 1.4 ml 70° C. 1.4 ml 85° C.

[0301] The wide absorption spectra for the PbS nanocrystals obtained in Example 9 are shown in FIG. 11, in which the spectra from left to right corresponds to the reaction with 1 ml oleic acid at 50° C.; 1.4 ml oleic acid at 50° C.; 1.4 ml oleic acid at 70° C. and 1.4 ml oleic acid at 85° C. The spectra show that the different reaction conditions and different amounts of oleic acid provided different sized PbS nanocrystals.

Example 10: Comparison of PbS Nanocrystals Solution and Drop Casting Thin Film

[0302] The synthesis outlined above in Example 1 was repeated. Thin film samples were prepared by drop casting the particles from a toluene solution onto a glass slide. The toluene was allowed to evaporate prior to analysis.

[0303] The solution and thin film absorption spectra for the PbS nanocrystals are shown in FIG. 12 (in which the solid line corresponds to the solution nanocrystals; and the dashed line corresponds to the thin film sample). The absorption peaks of the both solution and thin film are very similar and well overlapped indicating there was no change on their electronic states and no self-assembly taken place after drop casting.

Example 11: Synthesis of Lead Selenide (PbSe) Nanocrystals Using Pb.SUB.3.O.SUB.4 .and TOPSe

[0304] 1.5 g Pb oleate (with Pb and OA molar ratio of 1:4) and 3.9 ml octadecene (ODE) were loaded into a three-neck round bottom flask, then degassed via three cycles of vacuum/nitrogen and held under a N.sub.2 atmosphere for 30 minutes at 80° C. 3.6 g TOPSe (with TOP and Se molar ratio of 2:1) was rapidly injected. After temperature of the reaction mixture reached back to 80° C., 0.06 ml DPP was injected quickly. Colour of the reaction mixture changed immediately from light brown to dark brown after DPP addition, suggesting that nucleation of PbSe nanocrystals occurred. The occurrence of nucleation is a vital initial step in the formation of PbSe nanocrystals and allows their further growth to desired size. The reaction solution was then quickly heated from 80° C. to 150° C., aliquots were taken at different temperature, quickly cooled down to room temperature (20° C.) and kept under N.sub.2 prior to analysis. To purify the PbSe nanocrystals, the aliquot samples and the final reaction mixture were diluted with hexane (volume ratio of 1:1) then acetone was added (volume ratio of hexane/acetone was 1:3). The PbSe nanocrystals were precipitated through centrifugation, re-dispersed in hexane, and precipitated again through a combination of acetone and centrifugation. The purification process was carried out in air.

[0305] The PbSe nanocrystals prepared in the Example 11 were characterised using UV-Vis-NIR spectroscopy, XRD, TEM and ICP-OES.

[0306] For absorbance, the PbSe nanocrystals were dispersed either in tetrachloroethylene or hexane and their absorbance was monitored using a Jasco V-770 UV-VIS/NIR spectrometer.

[0307] XRD samples were prepared by drop casting the PbSe nanoparticle dispersions in hexane onto a microscope slide until a relatively thick opaque film was formed. This film was then analysed using a Bruker D2 Phaser instrument.

[0308] TEM samples were prepared by drop casting the PbSe nanoparticle dispersions in hexane onto a copper grid. The solvent was allowed to evaporate prior to analysis. Samples were analysed on a FEI Titan G2 80-200 kV (S-)TEM ChemiSTEM instrument.

[0309] FIG. 15 shows the synthesis scheme of PbSe nanocrystals using Pb.sub.3O.sub.4 and TOPSe

[0310] FIG. 16 shows the powder XRD pattern for the PbSe nanocrystals prepared according to Example 11. The XRD pattern shown in FIG. 16 confirms that Example 11 produced cubic phase PbSe nanocrystals.

[0311] FIG. 17 shows the absorption spectra for the PbSe nanocrystals prepared according to Example 11. The spectra from left to right correspond to the reaction temperature at 90° C., 100° C. and 110° C.

[0312] Table 3 shows the maximum absorption wavelength, size and composition of PbSe nanocrystals. Lower Pb to Se ratios (or increase in Se molar ratio) result in larger particles and longer maximum wavelength.

TABLE-US-00004 TABLE 3 Maximum absorption Pb/Se molar ratio wavelength λ.sub.max (nm) Size by TEM (nm) by ICP-OES 934 3.15 ± 0.65 2.91:1 1336 3.48 ± 0.42 1.84:1 2046 5.82 ± 0.65 1.62:1

[0313] The absorption FWHM of these nanocrystals is about 98, 87 and 100 respectively.

[0314] The molar ratio of the lead atoms (in the lead (IV) containing compound) to organic acid can be varied to achieve desired nanocrystal size and maximum absorption wavelength as shown in the PbS nanocrystal synthesis. This parameter can be applied to PbSe synthesis, but for the sake of simplicity, fixed lead to oleic acid molar ratio was used. Different Pb:Se molar ratio were also used in the PbSe synthesis. The amounts of TOPSe ranging from 1.8 g to 12.15 g correspond to Pb:Se molar ratio from 1:4 to 1:27.

[0315] FIG. 18 shows the wide spectral tunability for the PbSe nanocrystals obtained from different reaction conditions. The spectra show that maximum absorbance wavelength of PbSe nanocrystals can be controlled by using different Pb:Se ratio and different reaction temperature.

[0316] FIG. 19 shows the TEM image for the PbSe nanocrystals (λ.sub.max=1926 nm).

[0317] FIGS. 20a and 20b show the high resolution transmission electron microscopy (HRTEM) images of PbSe nanocrystals (λ.sub.max=2046 nm) and the histogram of the particle measurement by TEM. The PbSe dots are highly crystalline and free from stacking faults and lattice defects.

[0318] FIGS. 21a and 21b show the absorption and emission spectra of PbS nanocrystals (λ.sub.max=950 nm, PL=1060±5 nm) and PbSe (λ.sub.max=1000 nm, PL=1100±5 nm) as prepared in Example 6 and 11 respectively. The dots were excited with 808 nm laser.

[0319] Table 4 shows maximum absorption wavelength, photoluminescence peak, emission FWHM and quantum yield (QY) of some PbS and PbSe nanocrystals.

[0320] Photoluminescence quantum yield (PLQY) and Full Width at Half Maximum (FWHM) of some PbS and PbSe nanocrystals are shown in the Table 4.

TABLE-US-00005 TABLE 4 Max absorption Excitation Emission wavelength wavelength FWHM Sample (nm) (nm) PL (nm) (nm) QY(%) PbSe 1000 808 1100 ± 5 139 27 ± 2 PbS 1300 1064 1340 ± 5 140 19 ± 2 PbS 950 808 1060 ± 5 123 20 ± 2

[0321] PLQY data was recorded and analysed according to the methods of de Mello, J. C., Wittmann, H. F., & Friend, R. H. An improved experimental determination of external photoluminescence quantum efficiency. Adv. Mater. 1997, 9(3),230-232; doi: 10.1002/adma.19970090308.

[0322] The spectra were recorded outside of an integrating sphere using aliquots of each sample prepared to determine the initial absorbance before preparing aliquots for PLQY. 1 ml of each sample in a glass cuvette was mounted inside a Prolite Labsphere Spectraflect integrating sphere. The samples were excited with either a THOR labs M9-A64-0200 laser diode at 1064 nm, or a THOR labs L808P200 laser diode at 808 nm. The optical power from the 808 and 1064 nm beams were 125 and 165 mW respectively. Each beam was directed through an optical chopper operating at 173 Hz before entering the integrating sphere. Emission from the sphere was focussed through a Bentham TMc300 monochromator, after which it was detected by a Newport 818-IG photodiode. Signal from the photodiode was collected using a lock-in amplifier using the chopper frequency as a reference. PLQY was determined by recording both scattered laser radiation and emitted photoluminescence for the sample, with the sample positioned both in and out of the excitation beam by means of a rotating mount. A vial of sample solvent was also recorded and its contribution to the signal removed. All data was corrected for the spectral response of the system by measurement of a Bentham IL1 halogen lamp of known spectral intensity, illumination from which was directed into the empty integrating sphere.