TECHNICAL FIELD

The present subject matter relates, in general, to a light source made by using zinc oxide (ZnO) nanostructures and to a method for making a light source by using the ZnO nanostructures.

BACKGROUND

White light sources, such as white light emitting incandescent lamps, white light emitting fluorescent lamps, and solid state white light sources, for example, white light emitting diodes (LEDs), are commonly known. White light constitutes a spectrum of electromagnetic (EM) radiations or light of variable amplitudes in a wavelength range, typically, from about 400 nm to about 700 nm.

The solid-state white light sources are known to have high energy conversion efficiency, long durability, and low power consumption, when compared to conventional incandescent and fluorescent lamps. These efficient solid state white light sources, particularly in the form of light emitting diodes (LEDs), are widely researched. LEDs constitute a thin film structure that typically emits light of single colour in a narrow band of wavelength, monochromatic wavelength. White light is usually generated by mixing three primary colours, that is, blue, green, red, by using a multilayer structure in LEDs.

SUMMARY

White-light sources and methods of making white-light sources from ZnO nanostructures, prepared using microwave irradiation-assisted method, are described herein. In an implementation, a white-light source includes a surface coated with ZnO nanostructures, and a source of light of at least one predefined monochromatic wavelength. The coated surface is excited by the light of the at least one predefined monochromatic wavelength to obtain a white light emission from the coated surface. In an implementation, the method of making a white-light source comprises

irradiating, by microwaves, a solution having a compound of zinc to prepare the ZnO nanostructures of a predefined structure in the solution, and coating a surface of a light source with the ZnO nanostructures by immersing the surface in the solution.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the subject matter are set forth in the appended claims hereto. The subject matter itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein:

Figure 1 shows a schematic drawing of the apparatus for the microwave irradiation-assisted method for synthesis and coating of ZnO nanostructures, according to an implementation of the present subject matter.

Figure 2 illustrates a method flowchart showing schematically the steps involved in synthesis of ZnO nanostructures and in the coating process, according to an implementation of the present subject matter.

Figure 3(a) shows a scanning electron micrograph of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of poly(vinylpyrrolidone) (PVP) of molecular weight = 360000; microwave power = 800 W; irradiation time = 30 sec).

Figure 3(b) shows a scanning electron micrograph of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of PVP of molecular weight = 360000; microwave power = 800 W; irradiation time = 1 min).
Figure 3(c) shows a scanning electron micrograph of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of PVP of molecular weight = 360000; microwave power = 800 W; irradiation time = 5 min).

Figure 4 shows a scanning electron micrograph of hexagonal ZnO nanorods deposited on Si substrate (precursor = Zn(acac)2; solvent - ethanol; surfactant = aqueous solution of PVP of molecular weight = 360,000; substrate = Si(100); microwave power = 800 W; irradiation time = 5 min).

Figure 5(a) shows a photoluminescence spectrum of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of PVP of molecular weight = 360000; microwave power = 800 W; irradiation time = 30 sec).
Figure 5(b) shows a photoluminescence spectrum of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of PVP of molecular weight = 360000; microwave power = 800 W; irradiation time = 1 min).
Figure 5(c) shows a photoluminescence spectrum of ZnO nanostructures (precursor = Zn(acac)2; solvent = ethanol; surfactant = aqueous solution of PVP of molecular weight = 360000; microwave power = 800 W; irradiation time = 5 min).

Figure 6(a) shows a photoluminescence spectrum of ZnO nanostructures deposited on Si(100) substrate (surfactant = aqueous solution of PVP of molecular weight =10000).
Figure 6(b) shows a photoluminescence spectrum of ZnO nanostructures deposited on Si(100) substrate (surfactant = aqueous solution of PVP of molecular weight - 55000).

Figure 6(c) shows a photoluminescence spectrum of ZnO nanostructures deposited on Si(100) substrate (surfactant = aqueous solution of PVP of molecular weight = 360000).

Figure 7(a) shows photoluminescence spectra of ZnO nanostructures excited by laser radiation of wavelength of 325 nm.
Figure 7(b) shows photoluminescence spectra of ZnO nanostructures excited by laser radiation of wavelength of 488 nm.
Figure 7(c) shows photoluminescence spectra of ZnO nanostructures excited by laser radiation of wavelength of 532 nm.
Figure 7(d) shows photoluminescence spectra of ZnO nanostructures excited by laser radiation of wavelength of 632 nm.

Figure 8 shows white light emission from ZnO nanostructures upon excitation by a laser of a monochromatic wavelength.

Figure 9 illustrates a method flowchart showing schematically the steps for making a white-light source by using the ZnO nanostructures, according to an implementation of the present subject matter.

DETAILED DESCRIPTION

The present subject matter relates to a white-light source made by using zinc oxide (ZnO) nanostructures and to a method of making a white-light source by using the ZnO nanostructures. The present subject matter also relates to a method for producing white light from the ZnO nanostructures. The ZnO nanostructures used for production of white light are made using microwave irradiated-assisted chemical method, in which, microwaves of a predefined power are made incident for a predefined time period on a solution having a suitable compound of zinc.

Nanocrystals or nanostructures of certain materials are known to be highly fluorescent with substantially high fluorescence quantum efficiencies. These

nanostructures are capable of converting electromagnetic (EM) waves of short wavelengths into long wavelengths. This phenomenon may be understood as 'down-conversion.' Further, materials such as rare earth ions and phosphors are capable of converting EM waves of long wavelengths into short wavelengths. This phenomenon may be understood as 'up-conversion'. Such materials and/or nanostructures can convert one or more colours into other colour(s), thus making them ideal candidates for making white-light emitting sources, such as white LEDs (light emitting diodes).

Conventional white-light emitting sources based on the concept of down-conversion, can be made using GaN, a blue light emitting diode, coated with a yellow phosphor. Further, white light emissions based on the concept of down-conversion from different inorganic solids, such as Mn-doped ZnS, ZnS incorporated in porous Silicon, ZnSe, "magic sized" CdSe nanocrystals, core shell structures such as CdSe/ZnS/CdSe/ZnS/, doped ZnO with dopants Al, Ga, In and Li, Na, and Ga203, are typically known. These solid state white-light sources have various compositional and processing issues. For example, the "magic sized" nanocrystals are difficult to make; doping of host materials with fluorescent dyes involves a tedious process; and more than one rare-earth ion and phosphors are required for making conventional white-light sources. With these issues the conventional white-light sources are not economical to manufacture.

Some other conventional white light sources may involve complex multi-layered structure, wherein each layer emits light of substantially one colour. Mixture of colours from individual layers leads to white light emission. The fabrication of such multi-layered structures is often tedious, expensive, may require prolonged treatment at elevated temperatures, or may involve toxic and hazardous constituents like cadmium.
Therefore, simplified and economical methods of making white-light sources from ZnO nanostructures are described herein. Also, white-light sources made from ZnO nanostructures by simple and economical methods are described herein. The

emission of white light involves the down- and up-conversion from ZnO nanostructures, which leads to conversion of all primary colors to white light. The white-light sources, of the present subject matter, emit white light in ambient conditions, that is, at atmospheric temperature and atmospheric pressure.

In an implementation, the white-light source includes a surface coated with ZnO nanostructures, and a source of light of at least one predefined monochromatic wavelength. The light from the source of light of the at least one predefined monochromatic wavelength excites the surface coated with ZnO nanostructures in ambient conditions resulting in the emanation of white light from the aforesaid coated surface. The predefined wavelength of light may include one or more monochromatic wavelengths suitable for exciting the ZnO nanostructures for producing the white light emission. This suitable monochromatic wavelength(s) hereinafter may be referred to as 'excitation wavelength(s)'. In an implementation, the surface, which is coated with the ZnO nanostructures, may be planar or non-planar.

In an implementation, the method of making a white-light source includes irradiating, by microwaves, a solution having a suitable compound of zinc, and coating a surface of a light source with the ZnO nanostructures by immersing the surface of the light source in the solution having the ZnO nanostructures. In another implementation, a surface of a light source may be immersed in a solution have a suitable compound of zinc and the solution is subsequently irradiated by microwaves for preparing ZnO nanostructures and coating the surface of the light source with the ZnO nanostructures. The method of producing ZnO nanostructures using microwave irradiation hereinafter may be referred to as the microwave irradiation-assisted chemical method. The light source, coated with the ZnO nanostructures, may be capable of emitting light of one or more excitation wavelengths. Such light may excite the surface of the light source coated with the ZnO nanostructures to produce a white-light emission in ambient conditions. In an implementation, the excitation wavelength may be in a range from about 325 nm to about 700 nm. With the method

of the present subject matter, a light source of any primary colour, that is, blue, green, red, may be converted economically to a white-light source.

In an implementation, for the production of ZnO nanostructures, the solution having a ZnO compound is irradiated, for a predefined time period, by the microwaves of a predefined power. In an implementation, the predefined power may be in the range from about 25 W to about 5000 W, depending on the thickness of the ZnO nanostructures layer to be coated on a surface and the number of surfaces or light sources to be coated in a single process. Further, in an implementation, the predefined time period for microwave irradiation may be in a range from about 5 sec to 5 min, depending on the thickness of the ZnO nanostructures layer to be coated and the microwave power employed. The predefined time period for which the solution is irradiated with the microwaves hereinafter may be referred to as 'irradiation time'.

In an implementation, the white-light source may include a light source, such as a Light Emitting Diode (LED) or a Laser Diode (LD) capable of producing light of one or more excitation wavelengths. An inner and/or an outer surface of the LED or the LD may be coated with a thin film of ZnO nanostructures prepared using the microwave irradiation-assisted chemical method. The thin film may be of a thickness in a range from about 50 nm to about 25 um.

In an implementation, the light source may include a Blue LED/LD, a green LED/LD, a red LED/LD, and/or an LED/LD capable of emitting light of at least one excitation wavelength. The surface of such an LED or LD may be coated with a layer of ZnO nanostructures. When the LED or the LD is switched on, the internal light from the LED or the LD may impinge the ZnO nanostructures, coated on the surface, to produce a white light emission in ambient conditions.

Further, in an implementation, the white-light source may include a light bulb capable of producing light of one or more excitation wavelengths. In an implementation, the light bulb may enclose one or more LEDs or LDs for producing

light of one or more excitation wavelengths. An inner surface and/or an outer surface of the light bulb may be coated with a thin film of ZnO nanostructures prepared using the microwave irradiation-assisted chemical method. The thin film may be of a thickness in a range from about 50 nm to about 25 urn.

In an implementation, the surface of the light source, such as the LED or the LD or the light bulb, may be coated with the ZnO nanostructures through the microwave irradiation prior to the sealing of the light source in the manufacturing process. In another implementation, only the outer surface of the light source is coated with the ZnO nanostructures after the sealing of the light source in the manufacturing process.

Further, a method of producing white light from ZnO nanostructures, in a simple manner, is described herein. The ZnO nanostructures, prepared using the microwave irradiation-assisted method, are excited by light of at least one excitation wavelength to obtain white light emission from the ZnO nanostructures in ambient conditions. For this, the light of the at least one excitation wavelength is made incident on the ZnO nanostructures.

In an implementation, the ZnO nanostructures may be in a powder form, or in the form of a thin film of thickness in a range from about 50 nm to about 25 um coated on a substrate.

The substrate may include a surface of a light source capable of producing light of one or more excitation wavelengths. The light source may include one or more LEDs, or a light bulb that encloses one or more LEDs of suitable excitation wavelength(s). The ZnO nanostructures possess crystalline defect structures, thereby making these ZnO nanostructures emit white light in ambient conditions. The emission of white light from ZnO nanostructures, upon suitable excitation, enables making of solid state sources of white light. The concept of white light emission from ZnO nanostructures may have applications in the fields of solid state lighting and green technology-based light sources.

The description herein describes a method and related apparatus for the synthesis of ZnO nanostructures, at a rapid rate, through the microwave irradiation of a solution containing appropriate chemical reactants in a suitable liquid solvent.

Figure 1 illustrates an apparatus 1 for producing ZnO nanostructures using the microwave irradiation-assisted method, according to an implementation of the present subject matter.

The apparatus 1 includes a reaction chamber 2 in which microwave radiations are guided using devices such as waveguides. The reaction chamber 2 has a microwave source 3, such as a magnetron, which emits microwave radiations in the reaction chamber 2. In an implementation, the reaction chamber 2 may be a domestic-type microwave oven.

In an implementation, a reaction vessel 4, in which the ZnO nanostructures are synthesized, is placed in the reaction chamber 2. The reaction vessel 4 contains a solution having zinc compounds and is irradiated with suitably generated microwave radiations. Microwave radiations may span the frequency range, typically, from about 900 MHz to about 10 GHz. In an implementation, the microwave radiations of a frequency of about 2.45 GHz are used for irradiation purposes. The reaction vessel 4 may be made of a material that is transparent to microwaves, such as glass or plastic. The reaction vessel 4 may not be made of a metal or an alloy, as they are not transparent to microwaves. The apparatus 1 further includes a reflux system 5 outside the reaction chamber 2. The reflux system 5 includes a condenser 6, which is water-cooled during the operation of reaction chamber 2. For this, water is re-circulated to-and-fro from a reservoir 7 and is flown over the condenser 6 via a channel 8, as shown in figure 1. Further, a pump 9 is provided in the reservoir 7 for the purpose of the circulation of water. The ZnO nanostructures are produced in the reaction vessel 4 upon irradiation of the solution in the reaction vessel 4 by microwave radiations.

An adherent coating of ZnO nanostructures on suitable surfaces or substrates takes place in the reaction vessel 4. For this, a suitable surface or a substrate (not shown in figure 1) may be immersed in the reaction vessel 4 and adherently coated

with the ZnO nanostructures therein. In an implementation, a surface may be coated with the ZnO nanostructures for making a white-light source.

In an implementation, the solution in the reaction vessel 4 contains a solvent, which particularly is a microwaves-absorbing liquid. Choice of this solvent, or a mixture of solvents, depends on the solute used to make the solution. The solute is a zinc-containing chemical compound (or compounds) suitable for synthesizing ZnO nanostructures. Such a chemical compound may be understood as a chemical precursor. The solution may also contain a chemical called surfactant, which is employed to control the structure and morphology of the resulting ZnO nanostructures. Further, the quantity of the solution taken depends on the size of the reaction vessel 4.

In an implementation, the solvent includes, but not limited to, water, alkanes, alcohols, or a combination thereof. The solvent may, particularly, include ethanol, n-hexane, or a combination thereof.

In an implementation, the solute includes, but not limited to, metalorganic complexes of zinc, zinc acetates, zinc alkoxides, beta-diketonate complexes of zinc, or adducts of such complexes, or any other zinc compound which dissolves in a solvent of the kind described above. In an implementation, the solute is a metalorganic complex of zinc, such as zinc acetylacetonate.

In an implementation, the surfactant is selected from a polymeric, an ionic, or a non-ionic surfactant, a mixture of surfactants or a mixture of surfactants and solvents in appropriate ratio. The solution may only contain a solvent which efficiently works both as solvent and surfactant. In an implementation, the surfactant includes, but not limited to, poly(vinylpyrrolidone) (PVP), hexadecyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), and Triton X 100, in an appropriate concentration. In an implementation, the surfactant is

poly(vinylpyrrolidone) of a specific molecular weight. The molecular weight includes 10000, 55000, 360000,1300000, etc.

Figure 2 illustrates a method 20, which leads to production of ZnO nanostructures using microwave irradiation-assisted synthesis, according to an implementation of the present subject matter. At step 22, a suitable compound of zinc is added in a suitable solvent and stirred well to dissolve the compound. At step 24, a suitable surfactant is added in a suitable solvent and stirred well to dissolve the surfactant. At step 26, the solutions prepared at steps 22 and 24 are mixed and stirred well. The solution prepared at step 26 is taken in the reaction vessel 4 and placed in the reaction chamber 2. At step 28, the solution in the reaction vessel 4 is subjected to microwave radiations. The solution is irradiated by microwave radiations of a predefined frequency, a predefined power and for a predefined time period. The microwave irradiation of the solution results in a cloudy and white colloidal suspensions in the solution. The white colloidal suspensions include ZnO nanostructures.

In an implementation, at step 30, the white colloidal suspension obtained at step 28 is collected in the form of a precipitate (powder form) by centrifugation, followed by washing with acetone and distilled water. In case a polymeric surfactant is used, the precipitate is heated at a predefined temperature for a predefined time duration, at step 32, for removal of surfactant. In case, a surfactant other than a polymeric surfactant is used, this post-heating of the precipitate is not required.

Further, in an implementation, the ZnO nanostructures produced in the reaction vessel 4 after microwave irradiation may be adherently coated on a suitable surface for making a white-light source. In this implementation, the suitable surface may be immersed in the reaction vessel 4 before putting the solution of step 26 in the reaction vessel 4, or before the microwave irradiation of the solution of step 26. In this implementation, the suitable surface may be immersed in the reaction vessel 4 after the microwave irradiation of the solution of step 26.

The description below describes examples for preparation of ZnO nanostructures and coating of the ZnO nanostructures on a substrate. The examples described herein are not intended to be restrictive or to imply any limitations on the scope of the present subject matter.

Example 1: Preparation of ZnO nanostructures

To prepare ZnO nanostructures, about 1 gram of a metalorganic complex of zinc, such as zinc acetylacetonate (precursor) is added in about 40 ml of ethanol and stirred well to dissolve zinc acetylacetonate. To this solution, a solution of about 0.3 gram of poly(vinylpyrrolidone) (surfactant) in about 40 ml of water, is added and stirred well. Poly(vinylpyrrolidone) is of molecular weight of 360000. This final solution is taken in a reaction vessel 4 and the vessel 4 is placed in a reaction chamber 2 (microwave oven).

Microwave radiations of wavelength of about 2.45 GHz are turned on at a power of about 800 W. The microwave radiations at the said power are maintained for about 30 sec to 5 min, including 1 min, and then turned off. As mentioned earlier, the time for which the solution is irradiated with the microwave radiations is referred to as the irradiation time.

The microwave irradiation results in a white colloidal suspension in the solution, which is collected through centrifugation, subsequently washed, and heated at about 500 °C for about 5 min to get the ZnO nanostructures in a powder form.

Figures 3(a), 3(b) and 3(c) show scanning electron micrographs of ZnO nanostructures prepared with the irradiation time of about 30 sec, about 1 min and about 5 min, respectively, as described in example 1 above. It can be observed: from figure 3(a) that the ZnO nanostructures obtained are like nanoparticles of ZnO measuring 15 nm - 20 nm when the irradiation time is about 30 sec; from figure 3(b) that the ZnO nanostructures obtained are like nanorods of ZnO when the irradiation time is about 1 min; and from figure 3(c) that the ZnO nanostructures obtained are like nanotubes of ZnO when the irradiation time is about 5 min. This elucidates that by varying the irradiation time the structure of ZnO nanostructures can be altered.

Example 2: Coating of ZnO nanostructures on a Si(100) substrate
For coating ZnO nanostructures of a single crystalline substrate of silicon, Si(100), the steps described in example 1, above, are carried out with same parameters, except that the Si(100) substrate is placed in the reaction vessel 4 before putting the final solution in it. The Si(100) substrate is of dimension of about 20 mm x 20 mm. After irradiating the reaction vessel 4 with the microwave radiations, the substrate is taken out of the reaction vessel 4 and heated at about 500 °C for about 5 min.

Figure 4 shows scanning electron micrographs of ZnO nanostructures prepared, as described in the above example, with the irradiation time of about 5 min. It can be observed from figure 4 that the ZnO nanostructures obtained on the Si(100) substrate are like hexagonal nanorods of ZnO. This explains that the structure of ZnO nanostructures varies when coated on a substrate.

In other implementations, the ZnO nanostructures in a variety of forms, such as nanoparticles, nanorods, nanotubes, nanobipod and nanoflower, may be obtained through the variation of at least one from the precursor, the solvent, and the surfactant, in terms of their type and amount. Further, in other implementations, the ZnO nanostructures a variety of forms may be obtained through the variation in at least one from the type of substrate, power of microwave radiations and irradiation time. Variation in any of the mention parameters results in a variation in the structure of the synthesized ZnO nanostructures. The structure of the ZnO nanostructures may be understood to include shape and size of the ZnO nanostructures. For example, the structure of ZnO nanostructures can be controlled by: alteration of the ionic character and the polymeric nature of the surfactant; variation of the molecular weight of the polymeric surfactant; using a mixture of surfactants (such as, ionic+polymeric or ionic+non-ionic, cationic+anionic) in solution in appropriate ratio.

The ZnO nanostructures of the present subject matter are synthesized at a rapid rate and are of uniform morphology and composition. The synthesis method desribed herein consumes less time and is energy efficient, and the ZnO nanostructures are reproducible.

In an implementation, the ZnO nanostructures may be conformally coated on suitable surfaces. For example, LEDs, LDs or light bulbs may be immersed in the reaction chamber 4 to adherently coat a layer (thin film) of ZnO nanostructures on the outer surface of the LEDs, LDs or light bulbs. In another example, the outer surface of LEDs, LDs or light bulbs is coated substantially uniformly with the ZnO nanostructures obtained in powder form.

In an implementation, the ZnO nanostructures, prepared as described in example 1 earlier, are excited by He-Cd laser of wavelength of about 325 nm. Figures 5(a), 5(b) and 5(c) show photoluminescence (PL) spectra of ZnO nanostructures, prepared as described in example 1 earlier, with the irradiation time of about 30 sec, about 1 min and about 5 min, respectively. PL spectra of figures 5(a), 5(b) and 5(c) elucidate that ZnO nanostructures emit broadband white light upon suitable excitation. Further, figure 5(a) shows intensity peaks at 380 nm, 515 nm and 560 nm. Figure 5(b) shows intensity peaks at 380 nm, 520 nm, 585 nm, 635 nm and 660 nm. Figure 5(c) shows intensity peaks at 380 nm, 525 nm, 585 nm and 638 nm. PL spectra of figures 5(b) and 5(c) differ in the magnitude of intensity.

In an implementation, the ZnO nanostructures, prepared with the surfactant as Poly(vinylpyrrolidone) of different molecular weights, such as 10000, 55000 and 360000, and deposited on Si(100) substrate as described in example 2, are excited by He-Cd laser of wavelength of about 325 nm. Figures 6(a), 6(b) and 6(c) show PL spectra of ZnO nanostructures deposited on Si(100) substrate, with surfactant used as Poly(vinylpyrrolidone) of molecular weight of 10000, 55000 and 360000, respectively.

Figures 6(a), 6(b) and 6(c) show peaks at similar wavelength, however the intensity of wavelength peaks differs.

PL spectra in figures 5(a) to 5(c) and 6(a) to 6(c) show that a monochromatic light can be converted to a spectrum of white light by the ZnO nanostructures, whether in powder form or in thin film form. The spectrum of white light may be understood as an intensity distribution of electromagnetic waves as a function of wavelength.

Reason for differences in white light emission spectra (PL spectra) in figures 5(a) to 5(c) and figures 6(a) to 6(c) is difference in the structure of ZnO nanostructures. Another reason for this behaviour may be difference in crystalline defect structure in ZnO nanostructures. This shows that the spectrum of white light emitted from the ZnO nanostructures can be controlled or tuned based on the structure and crystalline defect structure of ZnO nanostructures. Difference in structure and crystalline defect structure of ZnO nanostructures depend on various synthesis parameters during the preparation of ZnO nanostructures. The synthesis parameters include type and amount of precursor, solvent, and surfactant; type of substrate; power of microwave radiations; and irradiation time. Thus, by varying the synthesis parameters, the spectrum of white light emitted from ZnO nanostructures can be tuned.

Further, in an implementation, ZnO nanostructures, prepared by the method 20 as described in figure 2, are exciting by light of at least one monochromatic wavelength. In an implementation, a Xenon lamp as the source of light of a plurality of monochromatic wavelengths, for excitation of ZnO nanostructures, may be used. Figures 7(a), 7(b), 7(c) and 7(d) show PL spectra of ZnO nanostructures excited by laser radiations of excitation wavelengths of 325 nm, 488 nm, 532 nm and 632 nm, respectively. These PL spectra show that light of a monochromatic wavelength, when made incident on the ZnO nanostructures, can be converted efficiently to a white light. This elucidates that the ZnO nanostructures can convert light of all primary colours to white light. Figures 7(a), 7(b), 7(c) and 7(d) show that the spectrum of

white light emitted from the ZnO nanostructures can be controlled by varying the excitation wavelength.

Figure 8 shows white light emission from ZnO nanostructures upon excitation by a laser of a monochromatic wavelength. The white light emission is of a substantially high intensity or is substantially bright. The white light emission has a hue or a tint around it.

This hue or tint depends on the excitation wavelength, and can be controlled by varying the excitation wavelength. Further, the tint can also be varied by varying the structure of ZnO nanostructures or coatings the ZnO nanostructures on a suitable surface.

The production of white light from the ZnO nanostructures, as described herein, make the ZnO nanostructures suitable for making white-light sources. The white-light source, according to an implementation of the present subject matter, may include a surface coated with the ZnO nanostructures prepared using the microwave irradiation-assisted chemical method. The surface may be coated with a thin film of ZnO nanostructures, where the thin film is of thickness in a range from about 50 nm to about 25 \im. In an implementation, the surface, which is coated with the ZnO nanostructures, may be planar, whereas, in another implementation, the surface may be non-planar. Further, in an implementation, the surface may be coated during the synthesis of the ZnO nanostructures. The white-light source also includes a source of light, of at least one predefined monochromatic wavelength, which excites the surface coated with ZnO nanostructures to obtain white light emission in ambient conditions. The predefined monochromatic wavelength of light includes a wavelength suitable to excite the ZnO nanostructures for the production of white light emission. In an implementation, the predefined monochromatic wavelength may include a wavelength in a range from about 325 nm to about 700 nm.

Figure 9 illustrates a method 90 of making a white-light source by using the ZnO nanostructures, according to an implementation of the present subject matter. The ZnO nanostructures are prepared using microwave irradiation-assisted method.

At step 92, a solution having a suitable compound of zinc is prepared following the steps 22, 24 and 26 of the method 20 as described in figure 2. The suitable compound of zinc may include any of the earlier mentioned zinc compounds. The solution may also include a suitable solvent and a surfactant, as mentioned earlier in the specification. The solution prepared at step 92 is taken in a reaction vessel 4, and placed in a reaction chamber 2. At step 94, the solution in the reaction vessel 4 is irradiated with microwave radiation of a predefined frequency, of a predefined power and for a predefine time period, to prepare ZnO nanostructures in the solution. At step 96, a surface of a light source is immersed in the solution and thereby adherently coated with the ZnO nanostructures. The light source may be understood to emit light of at least one predefined monochromatic wavelength and excite the surface coated with the ZnO nanostructures by that light to produce a white light emission.

In an implementation, a surface of a typical light bulb may be immersed in the solution prepared at step 92, and conformally coated with the ZnO nanostructures to make the light bulb a white-light source. The light bulb may be understood to have a suitable excitation source of light therein, which excites the ZnO nanostructures coated on the bulb to yield white light emission in ambience. In an implementation, a bulb capable of emitting light of any one colour, for example blue, green, yellow, red, etc., may be coated with the ZnO nanostructures to make the bulb a white-light source.

Further, in an implementation, a surface of a typical LED capable of emitting light of a suitable monochromatic wavelength may be immersed in the solution prepared at step 92, and conformally coated with the ZnO nanostructures to make the LED a white-light source. As the internal light, of a monochromatic wavelength, of the LED falls on the coating of ZnO nanostructures, the LED emits white light in ambience. In an implementation, an LED capable of emitting light of any one colour, for example blue, green, yellow, red, etc., may be coated with the ZnO nanostructures to make the LED a white-light source.

In an implementation, any other light source, for example an LD, capable of emitting light of a substantially monochromatic wavelength can be made a white-light source by coating it with ZnO nanostructures prepared using microwave irradiation-assisted method.

In an implementation, the ZnO nanostructures are coated on the surface of a light bulb, an LED, an LD or any monochromatic light source, during the synthesis of the ZnO nanostructures. For this, the light bulb or the LED or the LD is immersed in the reaction vessel 4, which has the solution to synthesize ZnO nanostructures, before the microwave irradiation at step 94 of the method 90 as described in figure 9. Suitable arrangements are made for immersing the light bulb or the LED or the LD in the reaction vessel 4 for the coating purpose.

In an implementation, the surface of light bulb, LED, LD or any monochromatic light source, is coated with a thin film of ZnO nanostructures. The thin film may be of thickness in a range from about 50 nm to about 25 (im. The thickness of the film depends on the time duration for which the surface is exposed to the ZnO nanostructures for coating, the power of microwaves and the irradiation time.

In an implementation, the spectrum of white light emission from the white-light source is controlled based on the wavelength (excitation wavelength) of light suitable for exciting the coated ZnO nanostructures. The spectrum of white light is characteristic of the excitation wavelength. In an implementation, the resulting white light emission from the white-light source may include a coloured hue depending upon the excitation wavelength.

In an implementation, the spectrum of white light emission from the white-light source is controlled based on geometric structure and the crystalline defect structure of the coated ZnO nanostructures. The geometric structure and the crystalline defect structure of ZnO nanostructures depends on the synthesis process

and various parameters, such as type of zinc-based precursor, type of solvent, type of surfactant used, type of substrate or surface on which ZnO nanostructures are coated, power of microwave irradiation, and duration of microwave irradiation. In an implementation, by varying or tailoring the geometric structure of ZnO nanostructures, the "whiteness" of the light emitted can be manipulated. Further, in an implementation, the resulting white light emission from the white-light source may include a coloured tint depending upon the structure of the ZnO nanostructures.

Although the subject matter has been described in detail with reference to certain embodiments thereof, other embodiments are also possible. As such, the spirit and scope of the present invention disclosed should not be limited to the description of the preferred embodiments contained therein.

I We claim:

1. A white-light source comprising:
a surface coated with ZnO nanostructures; and
a source of light of at least one predefined monochromatic wavelength, wherein the coated surface is excited by the light to obtain a white light emission from the coated surface.

2. The white-light source as claimed in claim 1, wherein the ZnO nanostructures are prepared using a microwave irradiation-assisted chemical method.

3. The white-light source as claimed in claim 1, wherein the surface is planar.

4. The white-light source as claimed in claim 1, wherein the surface is non-planar.

5. The white-light source as claimed in claim 1, wherein coating on the surface is a thin film of ZnO nanostructures, and wherein the thin film has a thickness in a range from about 50 nm to about 25 um.

6. The white-light source as claimed in claim 1, wherein the source of light is a light emitting diode (LED) capable of emitting the light of the at least one predefined monochromatic wavelength, and wherein the surface is at least one of an inner surface and an outer surface of the LED.

7. The white-light source as claimed in claim 1, wherein the source of light is a light bulb capable of emitting the light of the at least one predefined monochromatic wavelength, and wherein the surface is at least one of an inner surface and an outer surface of the light bulb.

8. The white-light source as claimed in claim 1, wherein the at least one predefined monochromatic wavelength is in a range from about 325 nm to about 700 nm.

9. The white-light source as claimed in claim 1, wherein the coated surface is excited in ambient conditions by the light.

10. A method of making a white-light source, the method comprising:

irradiating, by microwaves, a solution comprising a compound of zinc to prepare ZnO nanostructures of a predefined structure in the solution; and
coating a surface of a light source with the ZnO nanostructures by immersing the surface in the solution.

11. The method as claimed in claim 10, wherein the compound of zinc comprises one of metalorganic complexes of zinc, zinc acetates, zinc alkoxides and beta-diketonate complexes of zinc.

12. The method as claimed in claim 10, wherein the solution comprises a solvent and a surfactant, wherein the solvent comprises one of water, an alkane and an alcohol, and wherein the surfactant comprises one of a polymeric surfactant, an ionic surfactant and a non-ionic surfactant.

13. The method as claimed in claim 10, wherein the microwaves are of a predefined frequency in a range from about 900 MHz to about 10 GHz, and wherein the microwaves are of a predefined power in a range from about 25 W to about 5000 W.

14. The method as claimed in claim 10, wherein the solution is irradiated by the microwaves for a predefined time period in a range from about 5 seconds to about 5 minutes.

15. The method as claimed in claim 10, wherein the light source is capable of:
emitting light of at least one predefined monochromatic wavelength; and exciting the surface by the light to produce a white light emission.

16. The method as claimed in claim 15, wherein the method comprises:
controlling a spectrum of the white light emission based on the at least one predefined monochromatic wavelength of the light.

17. The method as claimed in claim 15, wherein the method comprises:
controlling a spectrum of the white light emission based on the predefined structure of the ZnO nanostructures.

18. A method for producing white light, the method comprising:
preparing ZnO nanostructures using a microwave irradiation-assisted chemical method; and
exciting the ZnO nanostructures by light of at least one predefined monochromatic wavelength to obtain white light emission from the ZnO nanostructures in ambient conditions..

19. The method as claimed in claim 18, wherein the ZnO nanostructures are in a
powder form.