LEAD SULFIDE NANOCRYSTALS, PREPARATION METHOD AND USES THEROF

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 as a starting material to prepare a lead chalcogenide nanocrystal, wherein the lead (IV) constitutes at least 50 molar % of all the lead present in the lead compound starting material.

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

3. The use according to claim 1 or 2, wherein the lead chalcogenide nanocrystal exhibits absorption in the range of 500 to 4500 nm, preferably in the range of 500 to 2400 nm, preferably in the range of 950 to 1600 nm, preferably in the range of 1350 to 1600 nm.

4. A method for producing a lead chalcogenide nanocrystal, the method comprising contacting a lead (IV) containing compound starting material with an organic acid and a chalcogen-containing reagent, wherein the molar ratio of lead (IV) oxide to any lead (II) oxide present is greater than 1:1, preferably greater than 2:1, preferably greater than 3:1, preferably greater than 5:1, preferably greater than 10:1, preferably greater than 20:1.

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

6. A method according to claim 4 or 5, wherein substantially no lead (II) containing compounds are present in the starting material.

7. A method according to any of claims 4 to 6, 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.

8. A method according to any of claims 4 to 7, 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.

9. A method according to any of claims 4 to 8, 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.

10. A method according to any of claims 4 to 8, 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.

11. A method according to claim 9 or 10, further comprising quenching the reaction mixture, for example by adding a quenching solvent to the reaction mixture.

12. A method according to any of claims 9 to 11, further comprising purifying the lead chalcogenide nanoparticle.

13. A method according to any of claims 4 to 12, wherein the organic acid is a fatty acid, preferably oleic acid.

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

15. A method according to claim 9, wherein the chalcogen-containing reagent comprises bis(trimethylsilyl)sulphide.

16. A method according to claim 10, wherein the chalcogen-containing reagent comprises thioacetamide.

17. A method according to claim 7 or 8, 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 7 or 8, wherein the lead salt is contacted with the chalcogen-containing reagent at a temperature of from 50 to 300° C., preferably from 50 to 150° C.

19. A method according to any of claims 4 to 18, 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) ratio of Pb to chalcogen-containing reagent; and (viii) addition of a secondary solvent.

21. A method according to any of claims 4 to 20, 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 any preceding claim, wherein the nanocrystals comprise quantum dots.

24. One or more (preferably a plurality of) lead chalcogenide nanocrystals obtained by the method according to any of claims 4 to 22.

25. A lead chalcogenide nanocrystals composition obtained by the method according to any of claims 4 to 22.

26. A lead chalcogenide nanocrystal composition comprising nanocrystals having a mean particle size of greater than 5 nm, preferably in the range of 6 to 25 nm, preferably 7 to 20 nm, preferably 8 to 15 nm, and a relative size dispersion of less than 25%, preferably less than 15%, preferably less than 10%.

27. The lead chalcogenide nanocrystal composition according to claim 26, which exhibits absorption in a range of from about 500 to 4500 nm, preferably suitably in the range of 500 to 2400 nm, preferably suitably in the range of 950 to 1600 nm, preferably in the range of 1350 to 1600 nm, preferably a maximum absorption wavelength (λ.sub.max) of greater than 1300 nm, preferably in the range of 1350 to 2500 nm, preferably 1400 to 1750 nm, preferably 1450 to 1600 nm.

28. The lead chalcogenide nanocrystal composition according to claim 26 or 27, which exhibits emission in the range of 600 to 4500 nm, preferably 600 to 2500 nm, preferably in the range of 950 to 1600 nm, preferably in the range of 1350 to 1600 nm.

29. The lead chalcogenide nanocrystal composition according to any of claims 26 to 28, which exhibits emission full width at half maximum (FWHM) values of less than 150 nm, preferably less than 130 nm, preferably less than 115 nm, preferably less than 105 nm. Preferably, the FWHM range is in the range of 75-150 nm, preferably 80-130 nm, preferably 85-110 nm, preferably 90-105 nm.

30. The lead chalcogenide nanocrystal composition according to any of claims 26 to 29, 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 any of claims 26 to 30, 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.

32. The lead chalcogenide nanocrystal composition according to any of claims 26 to 31, having a maximum absorption wavelength of 500 to 1000 nm and having an absorption FWHM of less than 115 nm.

33. The lead chalcogenide nanocrystal composition according to any of claims 26 to 32, 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.

34. The lead chalcogenide nanocrystal composition according to any of claims 26 to 33, wherein the lead chalcogenide nanocrystal comprises PbS, PbSe, PbTe or mixtures thereof, preferably PbS.

35. The PbS nanocrystal composition according to claim 34, wherein the nanocrystals adopt a substantially cubic structure.

36. Lead chalcogenide nanocrystal compositions according to any of claims 26-35, obtainable by the method according to any of claims 4 to 22.

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 any of claims 25-36.

38. A device according to claim 37, wherein the IR sensor or photodetector are modified for application as 3D cameras and 3D Time of flight cameras in mobile and consumer, automotive, medical, industrial, defence 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 any of claims 25 to 36.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0201] 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:

[0202] FIG. 1 shows absorption spectra of PbS nanocrystals using PbO.sub.2 as lead source and (TMS).sub.2S multiple additions.

[0203] FIG. 2 shows TEM images of the PS nanocrystals prepared from PbO.sub.2 lead source with FWHM=89 nm at different magnification. Cubic structure appears dominant for the lead (IV)-based nanocrystals and the nanoparticles show high crystallinity.

[0204] FIG. 3 shows absorption spectra of PbS nanocrystals using Pb.sub.3O.sub.4 as lead source and the (TMS).sub.2S multiple additions.

[0205] FIG. 4 shows TEM images of the PS nanocrystals prepared from Pb.sub.3O.sub.4 lead source with FWHM=94 nm at different magnification. Spherical structure appears dominant for the lead (II, IV)-based PbS nanocrystals and the nanoparticles show high crystallinity.

[0206] FIG. 5 shows absorption spectra of PbS nanocrystals using PbO as lead source and the (TMS).sub.2S multiple additions.

[0207] FIG. 6 shows TEM images of the PS nanocrystals prepared from PbO as the lead source with FWHM=91 nm at different magnification. Spherical or rounded edge structure appears dominant for the lead (II)-based PbS nanocrystals and the nanoparticles show high crystallinity.

[0208] FIG. 7 shows Time dependent absorption spectra of PbS nanocrystals dispersion in hexane stored in absence of light and in air and at room temperature. The nanocrystals showed significant blue shift after 42 days storage indicating nanocrystals were involved in oxidation reaction.

[0209] FIG. 8 shows absorption spectra of ammonium chloride treated-PbS nanocrystals dispersion in hexane in the dark and in air and at room temperature appear unchanged along with the storage time. This suggests that surface lead atoms of nanocrystals are covalently bound with halide protecting the nanocrystals from (photo)oxidation.

[0210] FIG. 9 shows the maximum absorption wavelength (λ) of PbS nanocrystal films upon heating at different temperatures. The nanocrystals were prepared from Pb(II), Pb(IV), Pb(II, IV) lead source and (TMS).sub.2S multiple addition. No blue shift was observed when films were heated to 180° C. in air indicating Pb(IV) and Pb(II,IV) based-PbS nanocrystals show comparable thermal stability as Pb(II) based-PbS nanocrystals.

[0211] FIG. 10 shows the FWHM of PbS nanocrystal films upon heating at different temperature. The nanocrystals were prepared from Pb(II), Pb(IV), Pb(II, IV) lead source and (TMS).sub.2S multiple addition. No significant FWHM broadening was observed upon being heated to 120° C. in air for all films indicating Pb(IV) and Pb(II,IV) based-PbS nanocrystals show comparable thermal stability as Pb(II) based-PbS nanocrystals.

[0212] FIG. 11 shows a HRTEM image of PbS quantum dots made from lead (II) oxide precursors. The quantum dots appear in truncated octahedral crystals. (002), (111) and (−111) facets are visible.

[0213] FIG. 12 shows a HRTEM image of PbS quantum dots made from lead (IV) oxide precursors. The quantum dots appear in truncated octahedral crystals (major) and in cuboctahedral crystals (minor). The (002), (111) and (022) facets are visible in truncated octahedral crystals while the cuboctahedral crystals appear with the (002) facet.

EXAMPLES

[0214] Several examples and comparative examples are described hereunder illustrating the methods according to the present disclosure.

[0215] 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.

[0216] Unless other 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.

[0217] Absorption spectra of colloidal quantum dots or quantum dots films were obtained on a JASCO V-770 UV-visible/NIR spectrometer which can provide measurements in the 400 to 3200 nm wavelength.

[0218] XRD data were collected on a Panalytical X'Pert PRO MPD diffractometer using Cu K.sub.a1 X-radiation (I=1.5406 Å) at room temperature over a range of 10<2q<90°. In each case a few drops of the dispersed sample were placed on a glass microscope slide and allowed to evaporate. Data were analysed using Rigaku SmartLab Studio II software and the search and match carried out using the Crystallographic Open Database.

[0219] TEM images and high-resolution transmission electron microscope (HRTEM) images were obtained with an FEI Talos F200X microscope equipped with an X-FEG electron source. The experiment was performed using an acceleration voltage of 200 kV and a beam current of approximately 5 nA. Images were recorded with an FEI CETA 4k×4k CMOS camera. In each case a few drops of the dispersed quantum dots in solvent were placed on a carbon coated copper grid and allow to evaporate. Samples were used as such or treated with acetone then methanol to clean unwanted organic materials before imaging.

[0220] ICP-OES data were obtained on an Agilent 720 ICP-OES. Each dispersion of the nanocrystals in toluene was added to water and heated to evaporate off the solvent then the solid was digested and remained in aqua regia (2HCl:1HNO.sub.3). This was then made up to volume in a volumetric flask, and then diluted as necessary to run within the calibration range on our ICP. The samples were run on separate calibrations for Pb and S calibration standard. The certified calibration CRM solution that contained Pb is a 28 element multi standard from SPEX CertiPrep sourced from Fisher Scientific, and the certified calibration CRM solution that contained S is a multi-element standard labelled CCS-5 supplied by Inorganic Ventures. Both the Pb & S calibrations were run using 0.5 and 10 ppm concentrations.

Materials

[0221] PbO (99.999% trace metal basis, Sigma-Aldrich), Pb.sub.3O.sub.4 (99%, Sigma-Aldrich), PbO.sub.2 (99.998% trace metal basis, Sigma-Aldrich), Hexamethyldisilathiane ((TMS).sub.2S, synthesis grade, Sigma-Aldrich) Oleic acid (OA, 90%, Fisher Scientific),

[0222] Thioacetamide (TAA, ≥99%, Sigma-Aldrich), Trioctylphosphine (TOP, 97%, Sigma-Aldrich), Se, Octadecene (ODE, 90%, Fisher Scientific), Diphenyl Phosphine (DPP, 98%, Sigma-Aldrich). NaCl (99.5%, Fisher Scientific), NaI (≥99%, Sigma-Aldrich), NH.sub.4Cl (99.99% trace metal basis, Sigma-Aldrich). All solvent (Hexane, Acetone, Methanol) were purchased from Fisher Scientific.

Example 1: Synthesis of Lead Sulfide (PbS) Nanocrystals Using Pb(IV) Oxide (PbO.SUB.2.) and Multiple Addition of (TMS).SUB.2.S

[0223] 1.25 g (5.23 mmol Pb) PbO.sub.2 and 10 mL oleic acid (28.40 mmol) were added to a 50 mL three neck-round bottom flask. The mixture was degassed under vacuum then held under a nitrogen atmosphere for 60 min at 250° C. to produce lead (IV) oleate solution. After the clear brown oleate solution formed, the temperature was reduced to about 40° C. and 1.08 g (0.56 mmol Pb) of the lead(IV)oleate solution was used to add to a 100 mL three neck round bottom flask containing 13.50 mL previously degassed octadecene (ODE). The mixture was further degassed under vacuum at 90° C. for 30 min and kept under nitrogen at 100° C. 0.8 mL of the 1.sup.st (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/8 v/v) was injected. After 7 min reaction at 100° C., 0.8 mL of the 2 nd (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/12 v/v) was added and the reaction mixture changed from light to dark brown within next few minutes indicating nanocrystals formation and growth. 0.8 mL of the 2 nd (TMS).sub.2S stock solution was then added every 5 min until target absorption wavelength was obtained. The reaction was then cooled down to room temperature (20° C.-30° C.) and the PbS nanocrystals were purified through precipitation and re-dispersion in in access (four times volume) acetone and hexane respectively. The nanocrystals were then re-dispersed in required solvents such as n-hexane, n-octane or toluene.

[0224] FIG. 1 shows absorption spectrum of PbS nanocrystals using PbO.sub.2 as lead source and (TMS).sub.2S multiple additions. Table 1 summarizes their maximum absorption, FWHM and peak to valley ratio.

TABLE-US-00001 λ(nm) FWHM(nm) P/V 1541 89 5.3

[0225] FIG. 2 shows TEM images of the PS nanocrystals prepared using PbO.sub.2 lead source with λ=1541 nm, FWHM=89 nm at different magnification. Cubic structure appears dominant for the lead (IV)-based nanocrystals which also show high crystallinity.

Reference Example 2: Synthesis of PbS Nanocrystals Using Pb(II,IV) Oxide (Pb.SUB.3.O.SUB.4.) and Multiple Addition of (TMS).SUB.2.S

[0226] 2.4 g (10.50 mmol Pb) Pb.sub.3O.sub.4 and 20 mL (56.70 mmol) oleic acid were added to a 50 mL three neck-round bottom flask. The mixture was degassed under vacuum then held under a nitrogen atmosphere for 60 min at 230° C. to produce lead (II, IV) oleate solution. After the clear light brown oleate solution was formed, the temperature was reduced to about 40° C. and 1.07 g (0.556 mml) of the lead(IV)oleate solution was used to add to a 100 mL three neck round bottom flask containing 13.50 mL previously degassed octadecene (ODE). The mixture was further degassed under vacuum at 90° C. for 30 min and kept under nitrogen at 100° C. 0.8 mL of the 1.sup.st (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/8 v/v) was injected. After 7 min reaction at 100° C., 0.8 mL of the 2.sup.nd (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/12 v/v) was added and the reaction mixture changed from light to dark brown within next few minutes indicating nanocrystals formation and growth. 0.8 mL of the 2 nd (TMS).sub.2S stock solution was then added every 5 min until target absorption wavelength was obtained. The reaction was then cooled down to room temperature (20° C.-30° C.) and the PbS nanocrystals were purified through precipitation and re-dispersion in in access (four times volume) acetone/methanol and hexane respectively. The nanocrystals were then re-dispersed in required solvents such as n-hexane, n-octane or toluene.

[0227] FIG. 3 shows absorption spectrum of PbS nanocrystals using Pb.sub.3O.sub.4 as lead source and (TMS).sub.2S multiple additions. Table 2 summarizes their maximum absorption, FWHM and peak to valley ratio.

TABLE-US-00002 λ(nm) FWHM(nm) P/V 1549 94 4.76 1556 92 4.88

[0228] It can be seen that, compared to the production of PbS nanocrystals using Pb.sub.3O.sub.4, the production of PbS nanocrystals using PbO.sub.2 produces higher P/V ratios at similar absorption wavelengths. Similarly, the production of PbS nanocrystals using PbO.sub.2 produces lower FWHM values than the corresponding production of PbS nanocrystals using Pb.sub.3O.sub.4.

[0229] FIG. 4 shows TEM images of the PS nanocrystals prepared using Pb.sub.3O.sub.4 as lead source with λ=1549 nm, FWHM=94 nm at different magnifications. Near spherical or rounded edge structure appears dominant for the lead (II, IV)-based nanocrystals which also show high crystallinity.

Reference Example 3: Synthesis of PbS Nanocrystals Using Pb(II) Oxide (PbO) and Multiple Addition of (TMS).SUB.2.S

[0230] 1.17 g (5.24 mmol Pb) Pb.sub.3O.sub.4 and 20 mL oleic acid (28.40 mmol) were added to a 50 mL three neck-round bottom flask. The mixture was degassed under vacuum then held under a nitrogen atmosphere for 60 min at 150° C. to produce lead oleate solution. After the clear light brown oleate solution was formed, the temperature was reduced to about 40° C. and 1.07 g (0.556 mmol Pb) of the lead (II) oleate solution was used to add to a 100 mL three neck round bottom flask containing 13.50 mL previously degassed octadecene (ODE). The mixture was further degassed under vacuum at for 30 min and kept under nitrogen at 100° C. 0.8 mL of the 1.sup.st (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/8 v/v) was injected. After 7 min reaction at 100° C., 0.8 mL of the 2 nd (TMS).sub.2S stock solution in degassed ODE ((TMS).sub.2S to ODE equal to 1/12 v/v) was added and the reaction mixture changed from light to dark brown within next few minutes indicating nanocrystals formation and growth. 0.8 mL of the 2 nd (TMS).sub.2S stock solution was then added every 5 min until target absorption wavelength obtained. The reaction was then cooled down to room temperature (20° C.-30° C.) and the PbS nanocrystals were purified through precipitation and re-dispersion in in access (four times volume) acetone/methanol and hexane respectively. The nanocrystals were then re-dispersed in required solvents such as n-hexane, n-octane or toluene.

[0231] FIG. 5 shows absorption spectrum of PbS nanocrystals using PbO as lead source and (TMS).sub.2S multiple additions.

TABLE-US-00003 Table 3 summarizes their maximum absorption, FWHM and peak to valley ratio. λ(nm) FWHM(nm) P/V 1514 92 5.00

[0232] As with PbS nanocrystals produced using Pb.sub.3O.sub.4, the production of PbS nanocrystals using PbO produces lower P/V ratios at similar absorption wavelengths compared to PbS nanocrystals produced using PbO.sub.2. Similarly, the production of PbS nanocrystals using PbO.sub.2 produces lower FWHM values than the corresponding production of PbS nanocrystals using PbO.

[0233] FIG. 6 shows TEM images of the PS nanocrystals using PbO as lead source at different magnifications. Near spherical or rounded edge structure appears dominant for the lead (II)-based nanocrystals which also show high crystallinity.

Example 4: Surface Passivation of PbS Nanocrystals with Halide Salt and Storage Stability of the Resultant Colloidal PbS Quantum Dots

[0234] The procedure is summarized as in Scheme 1, which illustrates the preparation of PbS nanocrystals using Pb(IV) oxide as lead source and surface passivation reaction.

[0235] Surface of PbS nanocrystals were treated with different halide salts to improve their storage stability and thermal stability.

[0236] PbS nanocrystals were synthesized as outlined above in Examples 1. The typical procedure for surface passivation reaction is as follows. After PbS nanocrystals reached the required absorption wavelength, the reaction mixture was rapidly cooled to 60° C. and 1 mL of 0.19M halide salts such NaCl, NaI, NH.sub.4Cl in degassed methanol was added dropwise to the reaction mixture of 1.07 g lead oleate (0.556 mmol Pb) while stirring under nitrogen. The passivation reactions could proceed for 30 min to and the resultant nanocrystals were purified with acetone and methanol as the non-solvents. The obtained solids were dispersed in required solvent such as n-octane. The obtained solids were dispersed in required solvent such as n-octane. The obtained dispersions might need to further centrifuge to remove unwanted solid (excess salt) precipitation. The halide treated nanocrystals typically show approximate redshift compared to untreated PbS nanocrystals (see Table 4).

TABLE-US-00004 TABLE 4 Stability of untreated and halide treated PbS nanocrystals dispersion in air and room temperature. Storage time FWHM Batch (day) λ(nm) (nm) P/V ratio Untreated PbS 0 1375 91 5.4 nanocrystals 1 1359 92 5.1 9 1302 84 6.4 23 1285 83 6.0 42 1279 83 6.7 NaCl treated PbS 0 1448 91 6.3 nanocrystals 1 1450 90 6.0 14 1448 90 6.3 28 1445 89 5.3 47 1442 89 5.8

[0237] FIGS. 7 and 8 show the absorption spectra of untreated and NH.sub.4Cl treated PbS nanocrystals dispersed in hexane and stored in air at room temperature (20° C.).

[0238] Table 4 compares stability of halide salt treated and untreated PbS nanocrystals. Without halide salt passivation, the PbS nanocrystals show 96 nm blue shift after 42 days stored in air and at room temperature suggesting the nanocrystals were subject to the oxidation reaction. In contrast, halide passivated PbS nanocrystals show only 6 nm blue shift after the same time under the same storage conditions.

Example 5: Film Formation of PbS Nanocrystals and their Thermal Stability

[0239] The synthesis outlined above in Examples 1 was repeated. The PbS nanocrystal surface was passivated with halide as in Example 4. Thin films of PbS and were prepared using spin coating of dispersions of PbS nanocrystals in n-hexane, n-octane or toluene on a glass slide.

[0240] For thermal stability study, spin coating films on glass slides with thickness in the range of 200 nm were heated on hotplate in air at different temperature and their film absorption wavelength and FWHM were monitored. FIGS. 9 and 10 show the change of films absorption wavelength and FWHM of PbS prepared using lead (II), lead (IV) and lead (II,IV) as the lead source and (TMS)S.

Example 6—Synthesis of PbS Quantum Dots

[0241] 6.1—PbS Quantum Dots from Lead (II) Oxide

[0242] PbO (0.1723 g, 0.772 mmol) was charged into a 3-necked RBF equipped with a magnetic stirring bar and a condenser. The system was evacuated on a Schlenk line and placed under N2, triplicating vacuum cycles. Oleic acid (1.465 mL, 4.15 mmol) was then injected into the flask and degassed thrice at room temperature, holding the vacuum for 10-minute intervals. The temperature was then increased to form lead oleate, which began to occur at 115° C. The temperature was further increased to 150° C. where it was held for 15 minutes to complete the reaction. 20 mL of dry, degassed octa-1-decene (ODE) was then injected into the lead oleate solution and the temperature allowed to plateau at 100° C. for 30 minutes. 1.18 mL of a 0.093 M solution of (TMS).sub.2S in ODE was then injected at once into the lead oleate solution. The solution was seen to blacken at 40 seconds after the injection. After 7 minutes, 1.28 mL of a 0.033 M solution of (TMS).sub.2S in ODE was injected at once into the lead oleate solution. After an additional 5 minutes, the reaction was quenched in an ice-water bath before reaction flask was sealed and purged into the glovebox. 12.5 mL aliquots of the reaction solution were combined with anhydrous IPA (30 mL) and centrifuged (4.5k, 3 mins) to precipitate the product. The precipitates were combined in anhydrous hexanes (˜5 mL) and anhydrous IPA (10 mL) was added before centrifuging (4.5k, 3 minutes). The IPA wash was repeated before the precipitates were dissolved in anhydrous octane (5 mL). A final centrifuge was performed to remove insoluble precipitates and the supernatant containing the purified product was stored in the glovebox under N.sub.2.

6.2—PbS Quantum Dots from Lead (IV) Oxide

[0243] PbO.sub.2 (0.1847 g, 0.772 mmol) was charged into a 3-necked RBF equipped with a magnetic stirring bar and a condenser. The system was evacuated on a Schlenk line and placed under N.sub.2, triplicating vacuum cycles. Oleic acid (1.465 mL, 4.15 mmol), was injected into the flask and degassed thrice at room temperature, holding the vacuum for 10-minute intervals. The temperature was then increased to form lead oleate, which began to occur at 200° C. The temperature was further increased to 220° C. where it was held for 15 minutes to complete the reaction. 20 mL of dry, degassed octa-1-decene (ODE) was then injected into the lead oleate solution and the temperature allowed to plateau at 100° C. for 30 minutes. 1.18 mL of a 0.093 M solution of (TMS).sub.2S in ODE was then injected at once into the lead oleate solution.

[0244] The solution was seen to blacken at 40 seconds after the injection. After 7 minutes, 1.28 mL of a 0.033 M solution of (TMS).sub.2S in ODE was injected at once into the lead oleate solution. After an additional 5 minutes, 0.15 mL of a 0.033 M solution of (TMS).sub.2S in ODE was injected at once into the lead oleate solution. After 3 minutes, the reaction was quenched in an ice-water bath before reaction flask was sealed and purged into the glovebox. 12.5 mL aliquots of the reaction solution were combined with anhydrous IPA (30 mL) and centrifuged (4.5k, 3 mins) to precipitate the product. The precipitates were combined in anhydrous hexanes (˜5 mL) and anhydrous IPA (10 mL) was added before centrifuging (4.5k, 3 minutes). The IPA wash was repeated before the precipitates were dissolved in anhydrous octane (5 mL). A final centrifuge was performed to remove insoluble precipitates and the supernatant containing the purified product was stored in the glovebox under N.sub.2.

6.3—Characterisation of Examples 6.1 and 6.2

[0245] Absorption spectra of PbS quantum dots were obtained on a JASCO V-770 UV-visible/NIR spectrometer which can provide measurements in the 400 to 3200 nm wavelength range.

[0246] The High-Resolution Transmission Electron Microscope (HRTEM) characterisations were conducted on a FEI (Thermo Fisher) Talos FX200A transmission electron microscope equipped with high brightness electron source (200 kV super-X field emission gun—FEG). The images from TEM characterisation were recorded with a CETATM 16M (4096×4096 pixel) CMOS camera. Atomic resolution images of nanoparticles were obtained in a high-resolution transmission electron microscopy (HRTEM) mode of the microscope from which lattice fringes of nanocrystals are visible. TEM images were analysed with digital micrograph (Gatan Digital Micrograph 2.3) and the analysis of crystals orientation was done with CrysTbox.

[0247] PbS CQDs with similar maximum absorption wavelength (1330-1340 nm) and band gap (0.92-0.93 eV) were synthesised (according to Example 6) using different lead oxide precursors, as summarized in Table 5.

TABLE-US-00005 TABLE 5 Peak absorption wavelength and band gap of quantum dots prepared from lead (II) and lead (IV) oxide. Samples λ.sub.max (nm) Eg (eV) Lead (II) oxide-based PbS quantum dots 1330 0.93 Lead (IV) oxide-based PbS quantum dots 1340 0.92

[0248] The shape of PbS colloidal quantum dots (CQDs) changes from octahedral toward cubic as their size (or absorption wavelength) increases. In particular, smaller PbS CQDs (<3 nm; Eg>1.3 eV) show octahedral shapes dominated by (111) facets. As the CQDs size increases, the (100) facet is expected to form gradually, altering the (111) shape facet-only octahedron to the (111) and (100) truncated octahedron and cuboctahedron. The (111) facet is lead-rich and polar while the (100) facet is of lower surface energy and non-polar. HRTEM images of PbS CQDs prepared from lead (II) and lead (IV) are shown in FIGS. 11 and 12 respectively.

[0249] It should be noted that the (200) and (002) facets are in the (100) group with interplanar spacing of ca 0.29 nm, the (022) facet is in (110) group. The (111) and (−111) facets have interplanar spacing of ca. 0.35 nm.

[0250] As can be seen in FIG. 11, PbS CQDs made from lead (II) oxide precursors according to the present invention are in truncated octahedral crystals, with visible (002), (111) and (−111) facets. PbS CQDs made from lead (IV) show a significantly higher proportion of cuboctahedrons as the major shape (FIG. 12A-D). The (002), (111) facets are major whilst the (022) facet is sometimes visible in cuboctahedral crystals of lead (IV) PbS CQDs.

[0251] Lead (IV) PbS CQDs having larger proportion of cuboctahedrons should have a higher area of the non-polar, lower surface energy (100) facets than the only truncated octahedral crystals based on lead (II) dots. The increase in (100) facet areas of lead (IV) CQDs at similar maximum absorption wavelength and bandgap to lead (II) CQDs can result in higher packing density of CQDs via (100)-(100) coupling, thereby improving charge transport in films comprising said CDQs. In fact, Sargent and co-workers reported that both hole mobility and time response in PbS photodetectors could be improved by surface modification making (100) facet dominant to increase coupling.sup.[1]. By directly measuring facet-dependent electrical properties of an n-type large PbS nanocrystal, Tan and co-workers reported that both (110) and (100) facets are highly conductive while the (111) facets can remain nonconductive even at 5 V.sup.[2]. These demonstrate that lead (IV) PbS CQDs provide better charge transport compared to lead (II)-based PbS, resulting in higher performance, especially in optoelectronic devices.

[0252] In conclusion, the nanocrystals and nanocrystal compositions of the present invention have some improved electronic properties compared to equivalent nanocrystals and nanocrystal compositions made from Pb(II) and Pb(II, IV) reagents. Said nanocrystals adopt a different morphology compared to prior art materials made from made from Pb(II) and Pb(II, IV) reagents. Other properties such as stability were at least as good as the equivalent nanocrystals and nanocrystal compositions made from Pb(II) and Pb(II, IV) reagents.

REFERENCES

[0253] 1. Biondi et al. Facet-Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodectectors Adv. Mater 2021, 33, 2101056; https://doi.10.1002/adma.202101056 [0254] 2. Tan et al, Facet-dependent electrical conductivity properties of PbS nanocrystals, 2016; http.//doi.org/10.1021/acs.chemmater.6b00274