CLIAMS:We Claim:
1. A tunable quantum dot emitter, the emitter comprising:
a first semiconductor nanocrystal forming a core; and
a second semiconductor deposited over the core to form a plurality of layers;
wherein the emitter obtained contains dopants and has at least one layer of the second semiconductor grown for inducing tunability of the dopants.
2. The emitter according to claim 1, wherein the first semiconductor is at least one selected from the group comprising a metal chalcogenide, a metal pnictide, a ternary I-III-VI2 compound, a quaternary I-II-IV-VI compound, or an alloy.
3. The emitter according to claim 1, wherein the second semiconductor is at least one selected from the group comprising a metal chalcogenide, a metal pnictide, a ternary I-III-VI2 compound, a quaternary I-II-IV-VI compound, or an alloy.
4. The emitter according to claim 1, wherein at least one semiconductor is doped with a divalent metal ion.
5. The emitter according to claim 1, wherein the metal ion is selected from the group comprising of manganese, chromium, cobalt, iron, nickel, neodymium, erbium, terbium and ytterbium.
6. The emitter according to claim 1, wherein the emitter is capable of light emission in a wavelength range of about 450nm to about 1500nm.
7. The emitter according to claim 1, wherein the wavelength of light emitted is determined by the concentration of the doped divalent metal ion, the differences in lattice constants of various semiconductor layers of the semiconductor nanocrystal or a combination thereof.
8. A light emitting device comprising of
a. a plurality of layers of a semiconductor nanocrystals doped with at least one divalent ion; and
b. a medium,
wherein the arrangement of the dopant layers of semiconductor nanocrystals within the medium result in emission of light in the near infrared region of the electromagnetic spectrum and/or the visible region of the electromagnetic spectrum upon excitation by a source.
9. The light emitting device according to claim 8, wherein the medium is selected from a group comprising of a fluid host, a non-fluid host or a compact close packed film.
10. The light emitting device according to claim 8, wherein the source for excitation of the device is selected from a group comprising of a monochromatic source of light, a collimated light beam, a laser, a direct electric excitation or an indirect electric excitation. ,TagSPECI:A TUNABLE QUANTUM DOT EMITTER
FIELD OF INVENTION
The invention generally relates to the field of semiconductor materials and particularly to synthesis of a semiconductor nanocrystal material with an emission that is capable of being across a wide region of the electromagnetic spectrum.

BACKGROUND
Semiconductor nanocrystalline materials commonly referred to as quantum dots have been synthesized from group IV, III-V, II-VI semiconductor materials. The quantum dots thus formed exhibit size tunability. All these phosphors suffer from the problems of reabsorption of light near the band edge that limit their utility. A second problem is that carriers are free to sample the surface or interact with quenchers, in the case of dyes, that reduce stability and quantum yield. Doped quantum dots are materials where impurities such as but not limited to copper (Cu), Indium (In), Gallium (Ga), Manganese (Mn) are included into the semiconductor host. While Cu doped quantum dots allow size tunability and also have a sizeable stokes shift, a high degree of stability is still lacking, since one of the carriers is still free to sample the nanocrystal surface. Manganese also has been incorporated, as a dopant, into many quantum dot compositions available in the prior art. However, the tunability of the quantum dot thus obtained is also limited to a narrow range in the visible region, specifically to the yellow and the orange region. The tunability of the quantum dots is not enhanced with replacement of type of semiconductor material incorporated for core development.

BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG.1 illustrates tunability of Mn ions doped quantum dots across the visible spectrum, according to an embodiment of the invention.
FIG. 2 illustrates typical PL spectrum of a NIR emitting Mn doped quantum dots, according to an embodiment of the invention.
FIG. 3 illustrates emission and absorption of a typical sample of Mn doped quantum dots, according to an embodiment of the invention.
FIG. 4 illustrates invariance of quantum yield of Mn doped quantum dots, according to an embodiment of the invention.

SUMMARY OF THE INVENTION
One aspect of the invention provides a tunable quantum dot emitter. The emitter includes a first semiconductor nanocrystal forming a core; a second semiconductor nanocrystal deposited over the core to form a plurality of layers; the emitter obtained has at least one layer of the second semiconductor nanocrystal grown epitaxially for inducing tunability of the emitter.

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide for a tunable quantum dot emitter. The nanocrystals prepared consist of one or more component semiconductors. The semiconductor component is a semiconducting compound or alloy such as but not limited to ZnSe. Different semiconductors typically have varied lattice constants. By the epitaxial overgrowth of one material on the other, it is possible to generate strain in the semiconductor material. The strain that was generated in these hybrid core/shell particles was a function of the radius of the inner material, the thickness of various overcoatings as well as the lattice mismatch of each layer with the others. Emission from dopants such as manganese that are incorporated into these layers can also be tuned by strain. The effects of strain on the internal d-d transitions of a transition metal ion may be understood in terms of perturbative techniques such as crystal field theory or else through more rigorous approaches.
The semiconductor quantum dots were synthesized through colloidal techniques. A solution or a suspension of a metal precursor that may be a metal salt such as but not limited to Zinc acetate or an organometal such as but not limited to dimethylcadmium was mixed with a solution or suspension of a chalcogenide precursor that may be the element itself (e.g. Sulfur, Selenium, etc.) or else a compound such as thiourea. In one example, the solution or suspension of a single molecular species such as but not limited to zinc diethyldithiocarbamate was used as a precursor. Other molecules such as but not limited to oleylamine were employed to serve as ligands for the nanocrystals or were used to regulate the reaction. The reaction was accomplished by mixing the individual components at a certain temperature followed by regulated annealing of the reaction mixture at a temperature that was different from the original temperature. In one embodiment of the invention, this step was followed by separation of the nanocrystals from the reaction medium, and redispersion into a fresh medium. Layer growth was accomplished by addition of controlled amounts of cation and anion growth precursors or a single source precursor. Compounds of Manganese such as but not limited to Manganese(II) chloride or Dimethyl Manganese were used as reactant precursors. Each precursor was introduced into the reaction medium either in their pure form or dispersed into a buffer medium.
Example 1:
Synthesis of a typical blue emitting sample: ZnSe nanocrystals were synthesized by literature methods. For the as-prepared quantum dots, the temperature was raised to 250 oC while keeping the mixture in argon atmosphere. It was kept at this temperature for 2-3 minutes, and subsequently requisite amount (varied from 0.1 to 0.2 ml depending on the thickness of CdSe aimed at) of cadmium dimethylbutyrate (Cd-DMB)from a stock solution prepared by mixing 0.1 ml 0.1M Cd-DMB and 0.9 ml 1-ODE was injected to the mixture. It was allowed to react for 2-3 minutes. 0.2 ml of manganese stock solution prepared by dissolving 0.0025 g of MnCl2 in 1 ml propylene carbonate was then injected to the solution mixture. After 2-3 minutes the temperature was lowered down to room temperature. The overcoating of the ZnSe layer was performed by adding 0.4 ml of 0.25M zinc dimethylbutyrate (Zn-DMB) and 0.01g Se at 210 oC for 5 minutes. Three such ZnSe shells were grown. The final (optional) layer of ZnS was achieved by treating the mixture with 1-dodecanethiol (1-DDT) at 250 oC.
Example 2:
Synthesis of a typical red-emitting sample: CdS core was first synthesized as follows: 0.1 mmol (0.0260g) of cadmium acetate and 0.0850g of myristic acid were mixed in 2 ml 1-ODE, and purged with argon after putting under vacuum at low temperature. The mixture was heated to 230 oC, and the temperature was lowered down immediately. The mixture was kept under vacuum while cooling. At room temperature, 0.1 mmol (3.2 mg) sulphur and 2 ml 1-ODE were added to it and the temperature was again raised to 240 oC. At around 230 oC, 1 ml of oleic acid was quickly injected. The mixture was kept at 240 oC for 2-3 minutes. The CdS nanocrystals thus formed were separated and cleaned thoroughly by methanol-ethanol mixture. The cleaned CdS was taken in a mixture of 4 ml 1-ODE and 1 ml oleyl amine. 2 mg of S and 0.1 ml of Mn stock solution prepared by dissolving 0.0045 g of MnCl2 in 2 ml propylene carbonate was also added to the mixture, and the temperature was raised to 250 oC after pumping to vacuum and purging with argon. 0.1 ml of Zn-DMB stock solution (prepared by mixing 0.2 ml 0.25M Zn-DMB, 0.8 ml oleyl amine and 4 ml 1-ODE) and 0.1 ml 0.1M S-ODE solution was injected drop by drop to the above mixture. Further ZnS shells were grown by adding, drop by drop, 0.2 ml 0.25M Zn-DMB and 0.4 ml 0.1M S-ODE solution in two steps. The final CdS shell was grown by adding 2.3 ml of 0.1M Cd-DMB and 2.3 ml of 0.1M S-ODE solutions at 210 oC. The CdS shell growth was done very slowly to ensure no separate CdS nucleation.
Characterisation of tunable quantum dot emitters:
Figure 1 illustrates tunability of Mn ions across the visible spectrum. The PL spectra of Mn varied from 450 nm to 650 nm. From left to right the emission maxima lie at 480, 505, 525, 540,560 and 580 nm. The emission spectra at 580 nm is exhibited by most Mn doped nanocrystals.
Figure 2 illustrates typical PL spectrum of a NIR emitting Mn sample. The spectrum shown is centered about 850 nm and extends into the near-IR spectral region.
FIG. 3 illustrates emission and absorption of a typical sample of Mn doped quantum dots. The comparison of absorbance and emission spectra shows negligible self reabsorption of light by the Mn doped quantum dots.

INDUSTRIAL APPLICABILITY:
The tunable quantum dot emitters of the invention have negligible self–absorption, a property that make them useful in applications such as lighting and displays. Also at concentrations relevant to devices (typically OD> 1 at the S exciton), traditional quantum dots e.g. based on CdSe and other such semiconductors exhibit significantly lower quantum yields in dilute solution. This occurs due to significant re-absorption of emitted light at concentrations relevant to devices. A second deleterious effect that arises in these traditional quantum dots systems is the possibility of catastrophic device failures. As the device is used, the quantum yields fall, however the performance of the device degrades non-linearly with the decreasing quantum yields(QYs) of the quantum dots. The tunable quantum dot emitters do not exhibit these problems. Since Mn doped systems lack re-absorption, their quantum yields do not vary significantly as a function of concentration. Further, at concentrations relevant to devices, Mn doped nanocrystals can easily outperform traditional quantum dots. For example, FIG. 4 shows the concentration dependent QYs of three different quantum dots. Mn doped quantum dots with QYs as low as 1% outperform CdSe quantum dots with quantum yields that are 10 times higher at practical device concentrations. A further advantage of Mn doped quantum dots is their insensitivity to the quantum dot surface. This effect arises due to the local nature of the emitting exciton. In conventional quantum dots, the exciton is free to sample the quantum dot surface making it potentially more susceptible to the crystal environment. Mn doped quantum dots will thus find use in displays and solid state lighting.
The invention described therefore allows for the preparation of a tunable quantum dot emitter with a large, broadband absorption coefficient, negligible self absorption losses and a wide tunability in the visible and near infrared. In addition, this material may in principle be free of heavy metals such as cadmium, that are constituents of other phosphors in the visible e.g. CdSe.
The foregoing description of the invention has been set for merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.