II-III-N semiconductor nanoparticles and method of making same

Abstract

The present application provides nitride semiconductor nanoparticles, for example nanocrystals, made from a new composition of matter in the form of a novel compound semiconductor family of the type group II-III-N, for example ZnGaN, ZnInN, ZnInGaN, ZnAlN, ZnAlGaN, ZnAlInN and ZnAlGaInN. This type of compound semiconductor nanocrystal is not previously known in the prior art. The invention also discloses II-N semiconductor nanocrystals, for example ZnN nanocrystals, which are a subgroup of the group II-III-N semiconductor nanocrystals. The composition and size of the new and novel II-III-N compound semiconductor nanocrystals can be controlled in order to tailor their band-gap and light emission properties. Efficient light emission in the ultraviolet-visible-infrared wavelength range is demonstrated. The products of this invention are useful as constituents of optoelectronic devices such as solar cells, light emitting diodes, laser diodes and as a light emitting phosphor material for LEDs and emissive EL displays.

Claims

1. A semiconductor nanoparticle having nanoscale dimensions and composed of a first compound having a formula II-III-N, where II denotes one or more elements in Group II of the periodic table and III denotes one or more elements in Group III of the periodic table, the semiconductor nanoparticle being light emissive and having a photoluminescence quantum yield of at least 20%, the semiconductor nanoparticle having a peak emission wavelength of 450 nm-850 nm.

2. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle is composed of ZnGaN.

3. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle is composed of ZnInN.

4. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle is composed of ZnAlN.

5. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle is composed of ZnGaInN.

6. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle is composed of MgInN.

7. A semiconductor nanoparticle as claimed in claim 1 wherein the first II-III-N compound is single crystal in structure.

8. A semiconductor nanoparticle as claimed in claim 1 wherein the first II-III-N compound is polycrystalline in structure.

9. A semiconductor nanoparticle as claimed in claim 1 wherein the first II-III-N compound is amorphous in structure.

10. A semiconductor nanoparticle as claimed in claim 1, wherein the first compound having the formula II-III-N forms a core of the nanoparticle and wherein the nanoparticle further comprises a layer disposed around the core and composed of a semiconductor material having a different composition to the first II-III-N compound.

11. A semiconductor nanoparticle as claimed in claim 10, wherein the layer is composed of a second II-III-N compound, the second II-III-N compound having a different composition to the first II-III-N compound.

12. A semiconductor nanoparticle as claimed in claim 1 and having a photoluminescence quantum yield of at least 50%.

13. A semiconductor nanoparticle as claimed in claim 1, wherein the formula II-III-N contains at least 1% by volume of each of the group II, III, and V element atoms.

14. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle has dimensions of the order of 1 to 100 nm.

15. A semiconductor nanoparticle as claimed in claim 1, wherein the first compound has the formula Zn-III-N.

16. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle has a crystal structure, the one or more constituent elements of the formula in Group II are incorporated into the crystal structure, and the one or more constituent elements of the formula in Group III are incorporated into the crystal structure.

17. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle has a peak emission wavelength of 450 nm-600 nm.

18. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle has a peak emission wavelength of 500 nm-850 nm.

19. A semiconductor nanoparticle as claimed in claim 1, wherein the first compound has the formula II-Ga-N.

20. A semiconductor nanoparticle as claimed in claim 1, wherein the nanoparticle has a crystal structure, and the one or more constituent elements of the formula in Group II, the one or more constituent elements of the formula in Group III, and the N are arranged over regular sites in the crystal structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the present invention will be described by way of example with reference to the accompanying figures, in which:

(2) FIG. 1: shows PL emission spectra of a set of zinc gallium nitride nanocrystal solutions obtained from a single reaction at different times.

(3) FIG. 2: shows the room temperature PL emission spectra of ZnGaN nanocrystal solutions containing having gallium: zinc molar ratios of 1:3, 1:1 and 3:1.

(4) FIG. 3 shows the variation in the peak PL emission wavelengths of ZnGaN nanocrystal solutions obtained for different reaction times and using different zinc to gallium ratios.

(5) FIG. 4: shows the room temperature PL emission spectra of ZnInN nanocrystal solutions obtained from a single reaction at different times.

(6) FIG. 5 shows the variation in the peak PL emission wavelengths of ZnInN nanocrystal solutions obtained for different reaction times and using different zinc to indium ratios.

(7) FIG. 6: shows the room temperature PL emission spectra of ZnAlN nanocrystal solutions obtained from a single reaction at different times.

(8) FIG. 7: shows the room temperature PL emission spectra of zinc nitride nanocrystal solutions obtained from a single reaction at different times.

(9) FIGS. 8(a) and 8(b) are Transmission Electron Micrographs of ZnAlN nanoparticles obtained by a method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) This invention relates to nanoparticles, for example nanocrystals, of semiconducting compounds. More specifically it relates to nanoparticles, for example nanocrystals, of group compound semiconductors of the general formulae or II-N where II is an element, or elements, from group II of the periodic table, III is an element, or elements from group III of the periodic table and N is nitrogen.

(11) The present invention makes possible the fabrication of nanoparticles, for example nanocrystals. The nanocrystals may be fabricated such that their diameters range from about 1 nm to about 100 nm and more specifically from about 1 nm to about 30 nm. The invention may be used to fabricate nanocrystals of a range of shapes such as roughly spherical or teardrop-shapes. In addition the nanocrystals provided by this invention may have a core-shell structure where a shell of a second material is grown directly onto the surface of the nanocrystal (which forms the core of the core-shell structure). More than one such shell may be grown. This shell may be made the same material or from a different material to that used for the core or an alternative III-V or II-VI semiconductor or any other suitable material. Ideally the band gap of the shell material will be larger that that of the material which forms the core to help confine the excited state within the core of the nanocrystals; this is known to improve the intensity of the emission from such materials.

(12) In a preferred embodiment, the present II-III-N semiconductor nanoparticles may exist in the form of crystalline nanoparticles.

(13) In another preferred embodiment, the present II-III-N semiconductor nanoparticles may exist in the form of polycrystalline nanoparticles.

(14) In another preferred embodiment, the present II-III-N semiconductor nanoparticles may exist in the form of amorphous nanoparticles.

(15) In another preferred embodiment, the II-III-N nanoparticles may be light emissive and have a photoluminescence quantum yield of at least 5%, or of at least 20%, or of at least 50%.

(16) In another preferred embodiment, the present II-III-N semiconductor nanocrystals consist of zinc gallium nitride. This material alloy has an energy gap of between 1.0 eV and 3.4 eV, depending on the Zn:Ga ratio, which traverses the visible spectral region.

(17) In another preferred embodiment, the present II-III-N semiconductor nanocrystals consist of zinc aluminium gallium indium nitride. This material has an energy gap of between 0.6 eV and 4.0 eV, again depending on the exact composition, that traverses the solar spectral region.

(18) In another preferred embodiment, the present II-III-N semiconductor nanocrystals consist of zinc aluminium nitride. This material alloy can yield a wide energy gap up to 6.2 eV for emission of ultraviolet light

(19) In another preferred embodiment, the present II-III-V semiconductor nanocrystals consist of zinc indium nitride. This material alloy can yield a small energy gap of 0.6 eV for emission of infrared light.

(20) In another preferred embodiment the II-III-N semiconductor nanocrystals can be doped with one or more impurity elements. Examples of impurity elements are silicon, magnesium, carbon, beryllium, calcium, germanium, tin and lead.

(21) An application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals to provide a phosphor which is excited by a light source such as a light emitting diode or laser diode.

(22) An application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals to provide large area illumination panels which are excited by a light source such as a light emitting diode or laser diode.

(23) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a solar cell.

(24) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a photovoltaic device.

(25) A further application of the novel material of the current invention is the use of compound semiconductor nanocrystals in a light emitting diode.

(26) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a light emitting device.

(27) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a laser diode device.

(28) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a laser

(29) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in an electronic device.

(30) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a transistor device.

(31) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a microprocessor device.

(32) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in an amplifier device.

(33) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a power switching device.

(34) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a power regulator device.

(35) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a light detecting device.

(36) A further application of the novel material of the current invention is the use of an II-III-N compound semiconductor nanocrystals to provide fluorescent fibres, rods, wires and other shapes.

(37) A further application of the novel material of the current invention is the use of an electrical current to generate the excited state which decays with the emission of light to make a light emitting diode with direct electrical injection into the II-III-N semiconductor nanocrystals.

(38) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as part of the back light used in a liquid crystal display.

(39) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as the emissive species in a display such as a plasma display panel, a field emission display or a cathode ray tube.

(40) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as the emissive species in an organic light emitting diode.

(41) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as the emissive species in a solar concentrator, where the light emitted by the solar concentrator is matched to a solar cell used to convert the collected light to an electrical current. More than one such concentrator may be stacked on one another to provide light at a series of wavelengths each matched to a separate solar cell.

(42) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as the light harvesting species in an organic solar cell or photo detector.

(43) A further application of the novel material of the current invention is the use of compound semiconductor nanocrystals as the light harvesting species in a dye sensitised solar cell or photo detector.

(44) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals to generate multiple excitons from the absorption of a single photon though the process of multiple exciton generation in a solar cell or photo detector.

(45) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals to assist identification in combat.

(46) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals to assist in asset tracking and marking.

(47) A further application of nanocrystals of this invention is the use of II-III-N compound semiconductor nanocrystals as counterfeit inks.

(48) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as bio markers both in-vivo and in-vitro.

(49) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in photodynamic therapy.

(50) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals as bio markers in for example cancer diagnosis, flow cytometry and immunoassays.

(51) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in flash memory.

(52) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in quantum computing.

(53) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in dynamic holography.

(54) A further application of the novel material of the current invention is the use of II-III-N compound semiconductor nanocrystals in a thermoelectric device.

(55) A further application of the novel material of this invention is the use of II-III-N compound semiconductor nanocrystals in a device used in telecommunications.

(56) A further application of the novel material of this invention is the use of II-III-N compound semiconductor nanocrystals for any application.

EXAMPLES

(57) In the following examples, several methods of fabricating a form of the present invention are described. Other methods of forming II-III-N semiconductor nanoparticles are, but not exclusively: metal organic vapour phase epitaxy (MOVPE), molecular beam epitaxy (MBE), chemical vapour deposition (CVD), sputtering, plasma assisted vacuum deposition, pulsed laser deposition (PLD), Hydride vapour phase epitaxy (HVPE), sublimation, thermal decomposition and condensation, annealing, powder or metal nitridation.

(58) Photoluminescence quantum yield (PLQY) measurements are carried out using the procedure described in Analytical Chemistry, Vol. 81, No. 15, 2009, 6285-6294. Dilute samples of the nitride nanocrystals in cyclohexane with absorbance between 0.04 and 0.1 are used. Nile red PLQY 70% (Analytical Biochemistry, Vol. 167, 1987, 228-234) in 1,4-dioxane was used as a standard.

(59) It should understood that the examples are given by way of illustration only, and that the invention is not limited to the examples. For example, although Examples 1 to 8 use a carboxylate, in particular a stearate, as the source of the group II element the invention is not limited to this and other precursors of the group II element may be used, such as, for example, amines, acetoacetonates, sulfonates, phosphonates, thiocarbamates or thiolates. Moreover, although Examples 1 to 8 use 1-octadecene or diphenyl ether as a solvent the invention is not limited to these particular solvents.

(60) The methods described below have been found effective to obtain nanoparticles having three dimensions of the order of 1 to 100 nm, or having three dimensions of the order of 1 to 30 nm. The size of the obtained nanoparticles may be determined in any suitable way such as, for example, taking a Transmission Electron Micrograph (TEM) image of the nanoparticles and estimating the size of the nanoparticles from the TEM image.

Example 1: Colloidal ZnGaN Semiconductor Nanocrystals

(61) Gallium iodide (270 mg, 0.6 mmol), sodium amide (500 mg, 12.8 mmol), hexadecane thiol (308 ?l, 1.0 mmol), zinc stearate (379 mg, 0.6 mmol) and 1-octadecene (20 ml) were heated rapidly to 250? C. and maintained at that temperature. Of the reaction constituents, Gallium iodide provided a Group III metal (Gallium), sodium amide provided the Nitrogen atoms, hexadecane thiol is a capping agent with an electron-donating group, zinc stearate provided a Group II metal (Zinc) and 1-octadecene acts as a solvent. Over the course of 60 minutes a number of 0.25 ml portions of the reaction mixture were removed and diluted with toluene (3 ml) and any insoluble materials were removed using a centrifuge. The resulting clear solutions were analysed by emission spectroscopy and showed a change in the peak emission wavelength from 450-600 nm over the course of the reaction, as shown in FIG. 1. The peak in the emission spectrum has a full width at half the maximum intensity of the order of 100 nm.

(62) The resultant ZnGaN nanoparticles were found to have a Ga:Zn ratio of approximately 1:1.3.

(63) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high quantum yield of ZnGaN nanostructures obtainable by the present invention.

(64) The corresponding emission spectra of these samples are shown in FIG. 1. The lefthand-most emission spectrum (shown as a dashed line) was obtained for a sample of the reaction mixture removed a few minutes after the start of the reaction, in this example 10 minutes after the start of the reaction. The righthand-most emission spectrum (shown as a dotted line) was obtained for a sample of the reaction mixture removed approximately one hour after the start of the reaction. The emission spectra between the lefthand-most emission spectrum and the righthand-most emission spectrum were obtained for samples of the reaction mixture removed at intermediate times.

(65) It should be noted that the peak wavelength of the emission spectrum does not change uniformly with time. Initially the peak emission wavelength increases rapidly with time, but as the reaction proceeds the rate of increase, with time, of the peak emission wavelength falls.

(66) As can be seen from FIG. 1, the emission spectra of samples removed at times up to about one hour after the start of the reaction span much of the visible region from blue to orange-red. Thus, nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution.

(67) The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of greater than 30%.

(68) Using the same synthesis procedure, several other ZnGaN nanocrystal compounds were formed. For example:

(69) The ratio of gallium iodide to zinc stearate was varied in order to produce compounds of zinc gallium nitride containing different amounts of gallium and zinc. FIG. 2 shows the PL spectra from samples made with different zinc to gallium ratios. The emission spectra for nanoparticles with a Ga:Zn ratio of 3:1 was obtained for a sample of the reaction mixture removed approximately 90 minutes after the start of the reaction, and the emission spectrum for nanoparticles with a Ga:Zn ratio of 1:1 was also obtained for a sample of the reaction mixture removed approximately 90 minutes after the start of the reaction. The emission spectra for nanoparticles with a Ga:Zn ratio of 1:3 was obtained for a sample of the reaction mixture removed approximately 20 minutes after the start of the reaction. Thus, the emission spectra of samples removed at times up to about 90 minutes were found to span the ultraviolet-visible-infrared regions.

(70) FIG. 3 shows the variation in the peak PL emission wavelengths of ZnGaN nanocrystals obtained for different reaction times and using three different zinc to gallium ratios. This result demonstrates that nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution, and from the appropriate choice of quantities of zinc and gallium in the synthesis reaction. Thus, as an example, a person wishing to fabricate nanoparticles having a peak emission wavelength of approximately 450 nm (in the blue region of the spectrum) may see from FIG. 3 that this made be done by fabricating ZnGaN nanoparticles as described in Example 1, by choosing the quantities of the constituents such that the nanoparticles have a Ga:Zn ratio of 3:1, and removing the sample from the reaction about 35 minutes after the start of the reaction.

(71) For a nanocrystal sample made with a Ga:Zn ratio of 4:1 in the reaction constituents a photoluminescence quantum yield value of 45% was obtained using a reaction time of 40 minutes.

(72) It can therefore be seen that the present invention makes possible the formation of zinc gallium nitride nanocrystals, or more generally, nanocrystals of the Group II-III-N compound semiconductor family, which have extremely good light-emissive properties.

Example 2: Colloidal ZnInN Semiconductor Nanocrystals

(73) Indium iodide (300 mg, 0.6 mmol), sodium amide (500 mg, 12.8 mmol), hexadecane thiol (308 ?l, 1.0 mmol), zinc stearate (379 mg, 0.6 mmol) and diphenyl ether (20 ml) were heated rapidly to 250? C. and maintained at that temperature. Of the reaction constituents, Indium iodide provided a Group III metal (indium), sodium amide provided the Nitrogen, hexadecane thiol is a capping agent with an electron-donating group, zinc stearate provided a Group II metal (Zinc) and diphenyl ether acts as a solvent. Over the course of 60 minutes a number of 0.25 ml portions of the reaction mixture were removed and diluted with cyclohexane (3 ml) and any insoluble materials were removed using a centrifuge. The resulting clear solutions were analysed by PL emission spectroscopy and showed a change in the maximum emission wavelength from 500-850 nm over the course of the reaction, as shown in FIG. 4. (The lefthand-most emission spectrum in FIG. 4 was obtained for a sample of the reaction mixture removed approximately 5 minutes after the reaction started, and the other emission spectra were obtained for samples of the reaction mixture removed approximately 10 minutes, 15 minutes, 20 minutes, 25 minutes, 35 minutes and 60 minutes after the reaction started.) The peak in the emission spectrum has a full width at half the maximum intensity of the order of 100 nm.

(74) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high photoluminescence quantum yield of ZnInN nanostructures obtainable by the present invention.

(75) The corresponding PL emission spectra of these samples are shown in FIG. 4. The emission spectra of samples removed at times up to about one hour span substantially the whole visible region and extend into the infra-red. Thus, nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution.

(76) The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of 10%.

(77) Using the same synthesis procedure, several other ZnInN nanocrystal compounds were formed. For example:

(78) The ratio of indium iodide to zinc stearate was varied in order to produce compounds of zinc indium nitride containing different amounts of indium and zinc. FIG. 5 shows the variation in the peak PL emission wavelengths of ZnInN nanocrystals obtained for different reaction times and using different zinc to indium ratios. This result demonstrates that nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution, and from the appropriate choice of quantities of zinc and indium in the synthesis reaction.

(79) For a nanocrystal sample made with a In:Zn ratio of 1:4 a photoluminescence quantum yield value of 30% was obtained using a reaction time of 20 minutes.

(80) It can therefore be seen that the present invention makes possible the formation of zinc indium nitride nanocrystals, or more generally, nanocrystals of the Group II-III-N compound semiconductor family, which have extremely good light-emissive properties.

Example 3: Colloidal ZnAlN Semiconductor Nanocrystals

(81) Aluminium iodide (102 mg, 0.25 mmol), sodium amide (468 mg, 12 mmol), hexadecane thiol (259 ?l, 1.0 mmol), zinc stearate (474 mg, 0.75 mmol) and 1-octadecene (25 ml) were heated rapidly to 250? C. and maintained at that temperature. Of the reaction constituents, Aluminium iodide provided a Group III metal (Aluminium), sodium amide provided the Nitrogen atoms, hexadecane thiol is a capping agent with an electron-donating group, zinc stearate provided a Group II metal (Zinc) and 1-octadecene acts as a solvent. Over the course of 60 minutes a number of 0.25 ml portions of the reaction mixture were removed and diluted with toluene (3 ml) and any insoluble materials were removed using a centrifuge. The resulting clear solutions were analysed by absorption and emission spectroscopy and showed a change in the maximum emission wavelength from 420-950 nm over the course of the reaction, as shown in FIG. 5. The peak in the emission spectrum has a full width at half the maximum intensity of the order of 100 nm.

(82) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high quantum yield of ZnAlN nanostructures obtainable by the present invention.

(83) The corresponding emission spectra of these samples are shown in FIG. 6. The lefthand-most emission spectrum in FIG. 6 was obtained for a sample of the reaction mixture removed a few minutes after the start of the reaction, and the righthand-most emission spectrum was obtained for a sample of the reaction mixture removed approximately 60 minutes after the start of the reaction. The emission spectra between the lefthand-most emission spectrum and the righthand-most emission spectrum were obtained for samples of the reaction mixture removed at intermediate times.) The emission spectra of samples removed at times up to about one hour span the ultraviolet to visible region and extend into the infra-red. Thus, nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution.

(84) The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of greater than 55%.

(85) FIG. 8(a) is a Transmission Electron Micrograph of ZnAlN nanoparticles obtained by a method as described in this example. The nanoparticles have a dimension of approximately 3 nm. The image of FIG. 8(a) was obtained for a sample of the reaction mixture removed approximately 12 minutes after the start of the reaction.

(86) FIG. 8(b) is a second Transmission Electron Micrograph of ZnAlN nanoparticles obtained by a method as described in this example. The image of FIG. 8(b) was obtained for a sample of the reaction mixture removed approximately 60 minutes after the start of the reaction. It can be seen that the nanoparticles of FIG. 8(b) have a dimension of approximately 5 nm, compared to the dimension of approximately 3 nm for the nanoparticles of FIG. 8(a).

(87) Methods as described herein may be used to fabricate nanoparticles having dimensions of more than 5 nm, by using longer reaction times. It should however be noted that many of the applications envisaged for nanoparticles of the invention require nanoparticles that emit light in the visible region of the spectrum and, in general, this requires that the nanoparticles have dimensions of 5 nm or belownanoparticles having dimensions of more than 5 nm will, in most cases, have a peak emission wavelength of 750 nm or greater. Also, fabricating nanoparticles having dimensions of more than 5 nm would require the use of larger quantities of source chemicals as well as requiring longer reaction times.

(88) It can therefore be seen that the present invention makes possible the formation of zinc aluminium nitride nanocrystals, or more generally, nanocrystals of the Group II-III-V compound semiconductor family, which have extremely good light-emissive properties.

Example 4: Colloidal MgInN Semiconductor Nanocrystals

(89) MgInN nanocrystals were fabricated by a method similar to that described in example 2, except that magnesium stearate was used as a starting material instead of zinc stearate.

Example 5: Colloidal ZnN Semiconductor Nanocrystals

(90) Sodium amide (500 mg, 12.8 mmol), zinc stearate (379 mg, 0.6 mmol) and 1-octadecene (20 ml) were heated rapidly to 250? C. and maintained at that temperature. Of the reaction constituents, sodium amide provided the Nitrogen atoms, zinc stearate provided a Group II metal (Zinc) and 1-octadecene acts as a solvent. Over the course of 60 minutes a number of 0.25 ml portions of the reaction mixture were removed and diluted with toluene (3 ml) and any insoluble materials were removed using a centrifuge. The resulting clear solutions were analysed by PL emission spectroscopy and showed a change in the maximum emission wavelength from 450-850 nm over the course of the reaction, as shown in FIG. 7. (The lefthand-most emission spectrum in FIG. 7 was obtained for a sample of the reaction mixture removed a few minutes after the start of the reaction, and the righthand-most emission spectrum was obtained for a sample of the reaction mixture removed approximately 60 minutes after the start of the reaction. The emission spectra between the lefthand-most emission spectrum and the righthand-most emission spectrum were obtained for samples of the reaction mixture removed at intermediate times.) The peak in the emission spectrum has a full width at half the maximum intensity of the order of 100 nm.

(91) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for early stage samples emitting in the visible region. This illustrates the high quantum yield of ZnN nanostructures obtainable by the present invention.

(92) The corresponding emission spectra of these samples are shown in FIG. 7. The emission spectra of samples removed at times up to about one hour span from ultraviolet through the whole visible region and extend into the infra-red. Thus, nanocrystals having particular optical properties (such as a desired peak emission wavelength) can be obtained by appropriate choice of the reaction period before the nanocrystals are recovered from the solution.

(93) The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of 25%.

(94) It can therefore be seen that the present invention makes possible the formation of zinc nitride nanocrystals, in particular nanocrystals of the Group II-N compound semiconductor family, which have extremely good light-emissive and crystalline properties.

Example 6: Colloidal ZnInGaN Core with ZnGaN Shell Semiconductor Nanocrystals

(95) Gallium iodide (113 mg, 0.25 mmol), indium iodide (124 mg, 0.25 mmol), sodium amide (390 mg, 10 mmol), hexadecane thiol (153 ?l, 0.5 mmol), zinc stearate (316 mg, 0.5 mmol) and 1-octadecene (40 ml) were heated rapidly to 225? C. After 20 minutes the mixture was cooled to room temperature and centrifuged to remove any insoluble material. This mixture consisted of the core only ZnInGaN nanoparticles. To form the ZnGaN shell around the nanoparticle core, 20 ml of the core solution was further treated with gallium iodide (113 mg, 0.25 mmol), zinc stearate (316 mg) and sodium amide (185 mg, 5 mmol) and heated to 225? C. for 20 minutes.

(96) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high quantum yield of ZnInGaN-ZnGaN nanostructures (that is, nanostructures with a ZnInGaN core and a ZnGaN shell) obtainable by the present invention. The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of 30%.

Example 7: Colloidal ZnInGaN Core with ZnS Shell Semiconductor Nanocrystals

(97) Gallium iodide (113 mg, 0.25 mmol), indium iodide (124 mg, 0.25 mmol), sodium amide (390 mg, 10 mmol), hexadecane thiol (153 ?l, 0.5 mmol), zinc stearate (316 mg, 0.5 mmol) and 1-octadecene (40 ml) were heated rapidly to 225? C. After 20 minutes the mixture was cooled to room temperature and centrifuged to remove any insoluble material. This mixture consisted of the core only ZnInGaN nanoparticles. To form the ZnS shell around the nanoparticle core the highly coloured solution was decanted from the solids and a 4 ml sample was treated with zinc diethyldithiocarbamate (100 mg, 0.27 mmol) for 40 minutes at 175? C.

(98) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high quantum yield of ZnInGaN-ZnS nanostructures (that is, nanostructures with a ZnInGaN core and a ZnS shell) obtainable by the present invention. The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of 23%.

Example 8: Colloidal ZnInGaN Core with Double Shell of ZnGaN and ZnS Semiconductor Nanocrystals

(99) Gallium iodide (113 mg, 0.25 mmol), indium iodide (124 mg, 0.25 mmol), sodium amide (390 mg, 10 mmol), hexadecane thiol (153 ?l, 0.5 mmol), zinc stearate (316 mg, 0.5 mmol) and 1-octadecene (40 ml) were heated rapidly to 225? C. After 20 minutes the mixture was cooled to room temperature and centrifuged to remove any insoluble material. This mixture consisted of the core only ZnInGaN nanoparticles. To form the ZnGaN inner shell around the nanoparticle core, 20 ml of the resulting solution was further treated with gallium iodide (113 mg, 0.25 mmol) and sodium amide (185 mg, 5 mmol) and heated to 225? C. for 20 minutes. The resulting solution was centrifuged to remove any insoluble material and then treated with zinc diethyldithiocarbamate (500 mg, 1.35 mmol) and heated to 175? C. for a period of 60 minutes to form the ZnS outer shell.

(100) When samples from such a reaction are illuminated with a UV light sources, the resultant emission is easily visible with the naked eye for samples emitting in the visible region. This illustrates the high quantum yield of ZnInGaN-ZnGaN-ZnS nanostructures (that is, nanostructures with a ZnInGaN core and a double shell of ZnGaN and ZnS) obtainable by the present invention. The photoluminescence quantum yield of a sample removed from this reaction was measured and gave a value of 22%.

(101) In Examples 6 to 8 the core, and optionally the shell, of the core-shell nanoparticles of these examples are made of a II-III-N or II-N material. In a further application of the invention, the invention may be used to provide a shell of a II-III-N or II-N material in a core-shell nanoparticle in which the core is not composed of a II-III-N or II-N material.