DESC:FIELD OF DISCLOSURE

[0001] The present disclosure pertains to methods and techniques of doping semiconductor materials with colloidal nanocrystals (may be abbreviated as “NCs” hereinafter) for varying electrical properties thereof. More particularly, the present disclosure pertains to systems, mechanisms, techniques, and methods for using redox active colloidal nanocrystals to dope semiconducting materials, wherein the redox active colloidal nanocrystals are produced by incorporating redox active inorganic ions into nanocrystals.

BACKGROUND OF THE DISCLOSURE

[0002] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosure, or that any publication specifically or implicitly referenced is prior art.
[0003] Doping of semiconductors is a well-known concept where impurities are introduced into an extremely pure (also referred to as intrinsic) semiconductor for the purpose of modulating its electrical properties. These impurities are also commonly referred to as dopants.
[0004] Nanocrystals (NCs) are small pieces of material, less than one micrometer in their largest dimension. These materials still show evidence of a crystal lattice. They may be prepared in solution or by powdering bulk material. Colloidal semiconductor nanocrystals that include quantum dots (may be abbreviated as “QDs” hereinafter) have attracted much attention recently with their unique properties of size-tunable emission, continuous absorption profile, and stability against photo bleaching. In construction, colloidal semiconductor nanocrystals or quantum dots are small pieces of semiconductor that are large enough to still have a crystal lattice but small enough to exhibit quantum con?nement e?ects. Semiconductor quantum dots are usually one of two types, Epitaxial quantum dots and colloidal quantum dots. Epitaxial QDs are grown or patterned on a surface, whereas colloidal quantum dots are grown in solution from precursors.
[0005] Several doping strategies are discussed in prior art for introduction of dopants in semiconductors. The most widely used doping strategy is doping a particularly charged or valence mismatched dopants into active layer of semiconductors. However there are two main drawbacks for this method. Firstly, there exists the problem of compensation of dopants due to defects formed automatically at higher doping levels. Secondly, the dopants act as scattering centers, which lead to poor device performance.
[0006] Alternative methods for doping include delta doping and modulation doping, wherein, in modulation doping, a semiconductor nanostructure with a high band offset dumps charge into the active layer, and, in delta doping, dopant, from an inert secondary layer, is interfaced into the active layer. However, these methods are very expensive to implement for most commercial devices. Furthermore, with the reduction in sizes of active device elements, doping small circuit components is a much more challenging task.
[0007] There is therefore a need for developing a doping process that exhibits elimination of carrier impurity scattering, involves simple steps for doping, doping on large substrates and at the same time is cost-effective.
[0008] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0009] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the disclosure are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the disclosure may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[00010] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[00011] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the disclosure and does not pose a limitation on the scope of the disclosure otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the disclosure.

OBJECTS OF THE DISCLOSURE

[00012] An object of the present disclosure is to obviate short comings of the conventional methods of doping semiconductors.
[00013] An object of the present disclosure is to provide a doping process that eliminates of carrier impurity scattering.
[00014] An object of the present disclosure is to provide a doping process that involves simple steps for doping,
[00015] An object of the present disclosure is to provide a doping process capable of doping on large substrates.
[00016] An object of the present disclosure is to provide a doping process that is cost-effective.
[00017] An object of the present disclosure is to provide a doping process that maintains the dopants in a layer different from the active layer.
[00018] An object of the present disclosure is to provide a doping process in which system is kinetically frozen.
[00019] An object of the present disclosure is to provide a doping process in which dopant is completely un-ionized at time of synthesis so that there is very little tendency of development of compensating defects.
[00020] An object of the present disclosure is to provide a doping process using Nano crystals that are very small, allowing for the possibility of creating very small doped architectures on a large substrate.

SUMMARY OF THE DISCLOSURE

[00021] The present disclosure relates to techniques and methods for doping colloidal nanocrystals including quantum dots and, more particularly, to a method for doping semiconductor materials with redox active colloidal nanocrystals/quantum dots.
[00022] In an exemplary aspect of the present disclosure, redox active colloidal nanocrystals can be prepared by doping redox active inorganic ions into nanocrystals to form dopants (redox active colloidal nanocrystals), and bringing the dopants (doped nanocrystals) in proximity of active semiconductor to enable charge transfer to take place as a redox reaction.In another exemplary aspect of the present disclosure, redox active nanocrystals can be prepared from compounds that contain a redox active ion.
[00023] The present disclosure therefore enables development of a kinetic method of doping, whereby adopant that is isovalent with a semiconductor can be introduced into one quantum dot made wholly or partly from that semiconductor and then brought into close proximity of another semiconductor that is to be doped. The dopant undergoes a redox process, transferring charge to the semiconductor. The resulting electrostatic attraction attracts the quantum dot to the semiconductor, leading to a coulomb bound aggregate.
[00024] In an exemplary embodiment, the dopants can be maintained in a layer different from the active layer, ensuring that dopant carrier scattering is eliminated. In another embodiment, the dopant can be configured such that it is completely un-ionized at the time of synthesis, making the system of the present disclosure kinetically frozen. In another embodiment, nanocrystals are very small in size (<10nm), allowing the possibility of creating very small doped architectures on a large substrate.
[00025] In another exemplary aspect of the present disclosure, the semiconducting material to be doped is selected from a group comprising micro structured, nano structured, bulk structured semiconductors.
[00026] In an exemplary embodiment of the present disclosure, redox active inorganic ions can be selected from a group comprising Cu+2, Fe+2, V+2, Cr+2, or a combination thereof.
[00027] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components

BRIEF DESCRIPTION OF THE DRAWING

[00028] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[00029] Fig. 1 illustrates Absorption and PL of PbSe MSNCs, in accordance with the present disclosure.
[00030] Fig. 2 illustrates Absorbance of PbSe/CdSe QDs, in accordance with the present disclosure.
[00031] Fig. 3 illustrates Absorption and PL of typical copper doped CdS QDs, in accordance with the present disclosure.
[00032] Fig. 4illustrates Absorption and emission spectra of ZnTe/CdS QDs, in accordance with the present disclosure.
[00033] Fig.5(A), 5(B), F(C), 5(D) illustrate TEM (Transmission Electron Microscopy) images of individual QDs ZnTe/CdS, PbSe, Cu:CdS and Cu:ZnSe respectively, in accordance with the present disclosure.
[00034] Fig.6 illustrates IR spectra for QDs after purification, in accordance with the present disclosure.
[00035] Fig. 7 illustrates Spectroscopic profile of charge transfer observed in combinations of ZnTe/CdS and Cu:CdS QDs, in accordance with the present disclosure.
[00036] Fig.8illustrates schematic diagram of Ionic interactions between QDs, in accordance with the present disclosure.
[00037] Fig.9 (A), 9(B) and 9(C) illustrate PL quench, Absorption bleach and intraband spectrum, in accordance with the present disclosure.
[00038] Fig.10 (A), 10(B), 10(C), 10(D) & 10(E) illustrates glassiness, structures formed in the QD solid.

DETAILED DESCRIPTION OF THE DISCLOSURE

[00039] For those skilled in the art, numerous embodiments described below are for illustration purpose only and do not limit the scope of the present disclosure in any manner.
[00040] Before describing the present disclosure in detail, it is to be understood that this disclosure is not limited to specific compositions or process steps, as such may vary. It must be noted that, as used in this specification and the appended claims, the singular form “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
[00041] As used herein, the terms "comprise" "comprises" "comprising" "include", "includes", and "including" are meant to be non-limiting, i.e., other steps and other ingredients which do not affect the end of result can be added. The above terms encompass the terms "consisting of and "consisting essentially of".
[00042] The present disclosure relates to techniques and methods for doping colloidal nanocrystals and, more particularly, to a method for doping redox active colloidal nanocrystals in semiconducting materials. It is to be understood that for the purpose of this disclosure unless specifically stated or implied, the terms Nanocrystals (NCs) and Quantum Dots (QDs) have been used interchangeably and any reference to one automatically covers other also.
[00043] In an exemplary aspect of the present disclosure, redox active colloidal nanocrystals can be prepared by doping redox active inorganic ions into nanocrystals to form dopants (redox active colloidal nanocrystals), and bringing the dopants (doped nanocrystals) in proximity of active semiconductor to enable charge transfer to take place as a redox reaction.
[00044] In an exemplary embodiment, the dopants can be maintained in a layer different from the active layer, ensuring that dopant carrier scattering is eliminated. In another embodiment, the dopant can be configured such that it is completely un-ionized at the time of synthesis, making the system of the present disclosure kinetically frozen. In another embodiment, nanocrystals are very small in size (<10nm), allowing the possibility of creating very small doped architectures on a large substrate.
[00045] In another exemplary aspect of the present disclosure, the semiconducting material is selected from a group comprising micro structured, nano structured, bulk structured semiconductors.
[00046] In an exemplary embodiment of the present disclosure, redox active inorganic ions can be selected from a group comprising Cu+2, Fe+2 or a combination thereof.
[00047] As used herein, Nanocrystals (NCs) are small pieces of material, less than one micrometer in their largest dimension. These materials still show evidence of a crystal lattice. They may be prepared in solution or by powdering bulk material and the term “quantum dot” refers to a single spherical nanocrystal of a semiconductor material where the radius of the nanocrystal is less than or equal to the size of the exciton Bohr radius for that semiconductor material (the value for the exciton Bohr radius can be calculated from data found in handbooks containing information on semiconductor properties, such as the CRC Handbook of Chemistry and Physics, 83rd ed., Lide, David R. (Editor), CRC Press, Boca Raton, Fla. (2002)). Quantum dots are known in the art, as they are described in references, such as Weller, Angew. Chem. Int. Ed. Engl. 32: 41-53 (1993), Alivisatos, J. Phys. Chem. 100: 13226-13239 (1996), and Alivisatos, Science 271: 933-937 (1996).
[00048] As used herein, the term “interaction” refers to the properties of binding or attraction between two entities. It refers to any kind of chemical bonding such as covalent bond, hydrogen or ionic bond, and any kind of physical bonding such as Coulomb forces, dipole-dipole interaction, hydrophobic interaction or Van der Waals forces.
[00049] As used herein, the terms “Coulomb force” or “Coulomb interaction” or “Electrostatic force” refer to attraction or repulsion of particles or objects because of their electric charge.
[00050] As used herein, the term “redox active inorganic ions” refers to inorganic ions which can participate in redox reactions that involve a transfer of electrons between two species.
[00051] As used herein, the term “Fermi level” refers to an energy level between the forbidden band gap where probability of electron occupancy is 50%. In a semiconductor the Fermi level is in the middle of the band gap between the valence band and the conduction band.
[00052] As used herein, the term “dopant” refers to ions, atoms, compounds, quantum dots, nanocrystals or any aggregates or combinations of these that are introduced into or near a material, usually in small quantities, to affect the material's chemical, electrical or physical properties. As used herein, dopants include, atoms, compounds, or any aggregates or combinations of these that are introduced in a semiconductor to affect the semiconductor's electrical characteristics, such as the semiconductor's electrical conductivity and resistance.
[00053] As used herein, the term “semiconductor material” refers to any material that is an insulator in its pure form at a very low temperature. Semiconducting materials can be nano structured, micro structured, or bulk structured. Semiconductors useful in the present disclosure may comprise elemental semiconductors, such as silicon, germanium and diamond, and compound semiconductors, such as group IV compound semiconductors such as SiC and SiGe, group III-V semiconductors such as AlSb, AlAs, Aln, AlP, BN, GaSb, GaAs, GaN, GaP, InSb, InAs, InN, and InP, group III-V ternary semiconductors alloys such as AlxGa1-xAs, group II-VI semiconductors such as CdSe, CdS, CdTe, ZnO, ZnSe, ZnS, and ZnTe, group I-VII semiconductors CuCl, group IV-VI semiconductors such as PbS, PbSe, PbTe and SnS, layer semiconductors such as PbI2, MoS2 and GaSe, oxide semiconductors such as Cu2O. The term semiconductor includes intrinsic semiconductors and extrinsic semiconductors that are doped with one or more selected materials.
[00054] The present disclosure pertains to techniques and methods for doping colloidal quantum dots and, more particularly, to a method for employing redox active colloidal nanocrystals to dope semiconducting materials. A redox active nanocrystal may be a nanocrystal doped with a redox active ion such as copper doped CdS, or else a compound like CuInS2, CuO, Fe3O4, etc.
[00055] In one exemplary embodiment of the present disclosure, redox active colloidal nanocrystals can be prepared by doping a redox active inorganic ion into colloidal nanocrystals.
[00056] In an exemplary implementation, Copper doped CdS QDs can be mixed with PbSe based QDs to enable holes to be injected into PbSe based material and enable QDs to interact with each other via coulomb interactions and precipitate from the solution.
[00057] In an aspect, semiconductor nanocrystals can initially be doped with redox active inorganic ions to form doped nanocrystals, and then the doped nanocrystals can be brought in proximity of active semiconductor to enable charge transfer to take place as a redox reaction.
[00058] In another aspect/configuration, redox active dopant can be implanted into a semiconductor nanocrystal (acting as a doped QD), which nanocrystal can then be brought in proximity to a second semiconductor (such as active semiconductor) that can participate in a redox reaction with the dopant/QD combination. The two semiconductors can be held together by coulomb interactions. In an aspect of the above embodiment, the second semiconductor can include a different species of redox active dopants, wherein a redox reaction takes place between the two dopants, causing bending of Fermi levels in both materials.
[00059] In another embodiment, one or more nanocrystals can be doped with redox active dopant and then brought into contact with a bulk semiconductor so as to enable charge to be injected into the semiconductor. Such a process of doping does not cause development of compensatory defects into the system since the systems are frozen at temperatures well below individual synthetic temperatures. Also, in another aspect, no impurity levels are created within the bulk semiconductor. In yet another embodiment, two different redox active dopants can be doped into two species of quantum dots, which doped quantum dots can then be brought into proximity of two different areas of a bulk semiconductor, enabling charge to be transferred across the two different QDs, leading to establishment of an electrostatic field, causing the bending of bands of the bulk semiconductor without formation of any impurity levels within the same. In another embodiment, a single doped nanocrystal and another nanocrystalline donor or acceptor can be used to achieve the same effect. In another embodiment, two different redox active Nanocrystals may be used to achieve the same effect.
[00060] The present disclosure therefore uses redox doping using nanocrystals and enables creation of small doped architectures on a large substrate. The present disclosure and method/technique therein also enables synthesis of materials using nanocrystal building blocks held together by doping induced coulomb forces.

EXPERIMENTS

[00061] The present disclosure is further explained in the form of following examples. However it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the disclosure. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the disclosure.

Example 1: Lead-Selenium magic sized nanocrystals (PbSe MSNs) Synthesis

[00062] Lead Selenide magic sized nanocrystals are undoped QDs. As prepared, these do not contain electrons in the conduction band or holes in their valence band, and also do not contain any inorganic dopants. In subsequent steps, charges are introduced into these QDs by reacting them with other QDs. MSNs are found to be stable and do not degrade when charged in this manner. No protective shell growth was therefore needed in this case.
[00063] Lead oxide (0.892g, 4mmol), oleic acid (5ml, 9.5mmol) and Octadecene (12ml) were added into a round bottom flask and the flask was heated to 150°C under argon for 1.5hr. The flask was cooled to room temperature and 4ml of 2M trioctylphosphine-selenium was injected into the flask at room temperature (trioctylphosphine selenide was prepared by dissolving 1.248g of selenium in 8ml of trioctylphosphine). The mixture was left in air for 4hr. After 4hrs, the solution turned to brown color from transparent color which indicated the synthesis Lead-Selenium magic sized nanocrystals. Cleaned MSNs were obtained by centrifugation with Methanol, Ethanol and isopropanol. Absorption and PL graph of PbSe MSNs is shown in Fig.1.

Example 2: Synthesis of Lead selenide/Cadmium selenide core shell Quantum Dots/Nanocrystals(PbSe/CdSe)

[00064] While PbSe MSNs exhibit high stability, larger PbSe QDs can degrade when charged. One way to stabilize these systems is to build a protective shell of a more stable semiconductor on top. We employed a coating of CdSe on PbSe cores. These PbSe core/ CdSe shell QDs are undoped at this step of the reaction, and do not contain electrons in their conduction band levels or holes in their valence band levels at the time of preparation. Also, no dopants are present within the quantum dot.
Preparation of Lead selenide core:
[00065] 2.9mmol (0.69g) of Lead oxide was transferred into a three neck round bottom flask. To this flask, 8.5mmol (2.7ml) of oleic acid and 4ml of Octadecene were added. The whole solution was then degassed and was placed in Argon atmosphere. Reaction mixture was heated to 180°C for 1.5hr with constant stirring. 4ml of 2M trioctylphosphine selenide was added to reaction mixture at 180°C. Reaction was immediately quenched by adding 10ml of toluene once desired size achieved. Lead selenide quantum dots/Nanocrystals (PbSe QDs) were then collected from the flask by multiple step centrifugations with methanol, ethanol and isopropanol.
Preparation of CdSe shell on PbSe core:
[00066] PbSe QDs obtained from the above step was dispersed in 10ml of toluene in a round bottom flask. About 10ml of 0.1mmole Cadmium oleate was then added to the flask and then the flask was kept under Argon atmosphere at 100°C for 1hr with constant stirring. Reaction was quenched by cooling to room temperature and PbSe/CdSe QD precipitate was collected by centrifuging with methanol, ethanol and isopropanol. In this synthesis, cadmium myristate was used as the precursor for Cd. The precursor was obtained by annealing 0.1mmol (28mg) of Cadmium acetate dihydrate with 70mg of myristic acid in presence of 1ml ODE at 230°C for 5min, after 5min the temperature of flask was brought to room temperature (RT). Absorption and PL graph is shown in Fig.2.

Example 3: Synthesis of Cu-doped CdS(Cu-CdS) Quantum Dots/Nanocrystals
[00067] 1mg of copper chloride dihydrate and 3.2mg of elemental sulphur were added to cadmium myristate with 3ml of Octadecene, then reaction mixture was heated up to 230°C under argon. Once temperature was reached to 230°C, the color of the reaction turned brown, immediately 1ml of oleylamine was injected drop wise to the reaction. In all our experiments a 1% doping level was employed. The QD-QD Solid formation was not observed when dots with higher doping levels were employed. Immediately reaction was quenched by removing heat. Cu-CdS QDs was collected by centrifuging with methanol, ethanol and isopropanol. Absorption and PL graph for Cu-CDS QDs is shown in Fig.3.

Example 4: Synthesis of ZnTe/CdS QDs/NCS:

[00068] ZnTe QDs can degrade when charged or exposed to the atmosphere. One way to stabilize these systems is to build a protective shell of a more stable semiconductor on top. We employed a coating of CdS on ZnTe cores. These ZnTe core/CdS shell QDs are undoped at this step of the reaction, and do not contain electrons in their conduction band levels or holes in their valence band levels at the time of preparation. Also, no dopants are present within the quantum dot.
Preparation of ZnTe core:
[00069] The synthesis of ZnTe core involves two steps: The first step is making reactive telluride ion precursor by treating elemental tellurium with a reducing agent i.e. Sodium borohydride, with 1, 4-Butanediol as solvent at 60°C under argon atmosphere for 5 min. Then 0.1mmole of dextrose is dissolved in 1ml 1, 4-Butanediol added to consume excess hydrogen gas evolved during heating.
[00070] In the second step 0.1mmole (13.6mg) of zinc chloride anhydrous was heated in another flask at 100°C with 4ml ODE and 1ml oleyamine. Once temperature of flasks reached 100°C, contents from the above step is rapidly injected, Te2- from the first step reacted with Zn2+ forming ZnTe QDs.
Preparation of CdS shell:
[00071] To grow CdS on ZnTe core we followed methods available in literature. Cd-oleate and S in oleyamine at 230°C under argon atmosphere were added drop wise into the reaction vessel containing ZnTe core until desired shell thickness on core was obtained. The temperature of the Reaction mixture was then brought to RT. The obtained ZnTe/CdS core/shell dots were cleaned by using multiple step centrifugations with Methanol, Ethanol and isopropanol. Absorption and PL graph is shown in Fig.4.

Example 5: Synthesis of Cu doped ZnSe (Cu-ZnSe) Quantum Dots/Nanocrystals:

[00072] 0.1mmole of zinc chloride, 4ml Octadecene and 1ml oleyeamine and 0.1 mmol (7mg) of Selenium were added to a three neck round bottom flask. The mixture was then heated to 230°C under argon for 2min and the solution was cool down to RT. To this cooled solution, 1ml of 0.1mmol Zn-DMB , 1mg of Copper chloride and 7mg of selenium were added to the flask and heated to 230°C under argon for 2min, allowed the flask to cool down to room temperature, and Cu-ZnSe QDs were collected by centrifuge with methanol, ethanol and isopropanol.
[00073] Fig.5 illustrates low magnification images of individual QDs through TEM (Transmission Electron Microscopy) and inset, wherein Fig.5(a) represents ZnTe/CdS QDs, Fig.5(b) represents PbSe QDs, Fig.5(c) represents Cu:CdS QDs, and Fig.5(d) represents Cu:ZnSe QDs

Example 6: Purification of Nanocrystals

[00074] Before proceeding towards other experiments, NCs were cleaned thoroughly by using Methanol, Ethanol and isopropanol to remove excess ligand and unreacted precursor. The cleaned NCs were then re-dispersed in hexane or tetrachloroethylene. The solution was then centrifuged to eliminate any NC aggregates as well as solid by products. The resultant NC solutions had no detectable optical scattering, as well as extremely clean infrared profiles, suggesting the absence of any free ligands. Optical absorption was characterized in hexane or tetrachloroehtylene. Infrared measurements were done in a tetrachloroethylene solution. The spectrum was taken for 6 nm Cu:CdS NCs dispersed in tetrachloroethylene. The NC cross section was taken to be 5.06x10-15 cm2 at the S exciton. The two dips at 0.24 and 0.31 eV are solvent bands. Infrared Spectra for Cu-CDS NCs is shown in Fig.6, which illustrates IR cross sections obtained for QDs after the cleaning procedure, wherein the spectrum has been taken for 6nm Cu:CdS NCs dispersed in tetrachloroethylene. The NC cross section was taken to be 5.06*10-15 cm2 at the S exciton.

Evaluation of NCs interactions:
Example 7: Photoluminescence Test (PL test)

[00075] Photoluminescence is phenomenon which a substance absorbs incident light, and then subsequently emits light usually of a lower photon energy. Quantum Dots also exhibit strong photoluminescence because of excitation of electrons from the valence band into the conduction band. Absorption of light of the appropriate wavelength causes a hole to appear in the valence band and an electron to be generated in the conduction band. This resulting electron-hole pair (sometimes called exciton) can annihilate, emitting light. In the case of QDs that carry spectator charges in their quantum confined levels, exciton annihilation can also occur non-radiatively, that is, without the emission of a photon. Photoluminescence (PL) quenching can thus be a sensitive indicator of the presence of charges in the quantum confined levels of quantum dots. Introduction of charges into quantum confined levels of quantum dots automatically causes a fall in the emission efficiency. This is described here.
[00076] In this experiment, absorption and PL spectra of ZnTe/CdS (Optical density was 0.1 at S exciton) and Cu-CdS QDs (OD was 0.3 at S exciton) was taken separately in 2ml of hexane. The solutions were then mixed and dried by evaporation at room temperature. The QDs were then re-dispersed in 2ml of hexane and absorption as well as PL spectra of the mixture were taken. It was observed that there was bleaching in the absorption spectrum as well as a decrease in the intensity of PL spectra when compared to original spectra taken before mixing. All parameters such as volume of solvents used, concentrations of each QD were kept constant for each measurement. PL quenching in this experiment occurred because of enhanced non radiative recombination in a trion or a highly charged exciton. That was typically ascribed to auger decay whereby an interband exciton recombination event is coupled to an intraband excitation event for the spectator carrier. Furthermore, it was observed that a quenching of PL from the copper center is suggestive of its conversion from Cu+2 to Cu+ state.

Example 8: Bleach Test

[00077] Absorption spectra of cleaned MSNs and Cu-ZnSe samples were taken in 2ml of hexane separately. Those were dried and then mixed them together by dispersing them in minimum amount of hexane. The volume of the mixture is made up to 2ml and the absorption spectrum of the mixture was taken. The bleach spectrum which is plotted in Fig.7 is obtained by subtracting pure Cu-ZnSe spectra by spectra which was obtained from the mixture of MSNs and Cu-ZnSe.
[00078] In another experiment, PbSe/CdSe QDs was dispersed in 1:9 mixtures of octane & hexane and dropcast PbSe/CdSeQD solution on glass plate to form film of QDs on glass plate. The film was crossed linked by using mixture of butylamine and isopropanol (2ml butylaminemixed with 8ml of isopropanol) and then immediately the film was washed with ethanol. The film was allowed to dry. After the film had completely dried, the absorption spectra were taken.
[00079] Similarly Cu-CdS QDs film was also drop casted and crossed linked on glass plate to get the absorption spectra of Cu-CdS sample. Finally absorption spectra of PbSe/CdSe and Cu-CdS mixture were taken by dropcasting QD mixture on glass plate. Band edge bleach was observed in the spectrum. Bleach in absorption spectra taken place in solution as well as on film. All experimental parameters, optically density and number of layer of QDs on film were constant in all the cases. Cu-CdS introduced no absorption in the vicinity of PbSe QD band edge.
[00080] A similar procedure was followed for ZnTe/CdS QDs. The results of the experiment are shown in Fig.7. In this graph, the spectra of the single component and binary films have been normalized at 500 nm. The graph below shows the difference of the binary and the single component film corresponding to negative bleach.

Example 9: Intraband Spectra

[00081] In PbSe/CdSe the electron from the conduction band is injected to Cu: level of Cu-CdS. After the electron transfer the PbSe/CdS QDs are left with one or more holes in the valence band edge. These holes absorb light energy typically in the infrared, and undergo transitions to nearby levels in the valence band. The absorption feature due to such transitions is quite distinctive, and appears typically as a Gaussian curve in the infrared region. This is one of the most distinctive evidences of the presence of charges in the quantum confined level of a quantum dot, particularly because it is absent for unexcited, uncharged quantum dots.

[00082] For Intraband spectra, FTIR (Fourier Transform Infrared) spectra of PbSe/CdSe QD film was taken on a silicon wafer and then drop casted Cu-CdS film on top of PbSe/CdSe QD film to take FTIR spectra of resultant film. The intraband spectrum was inferred as the difference of the IR absorptions of the binary film from the IR spectra of the two separate QD films.

Example 10: Photoluminescence evolution

[00083] It is already demonstrated the existence of a photoluminescence quench due to the charge transfer process. In this section, we use the quantitative magnitude of this quench to study order in quantum dot solids. The magnitude of the luminescence quench is related to the charge distribution in the solid and thence to the structure. We therefore mechanically perturb the solid, and track the evolution of its order using the associated quench.
[00084] This experiment was done in solution phase as follows: 30µl of ZnTe/CdS (OD is 6.5 at S exition) was added to a vial containing 1.97ml and 2ml of hexane, the resulting solution optical density was 0.1 at exition and then PL of these two solutions were taken. Similarly, 30µl of Cu-CdS (OD is 6.56 at S exition) was added to a vial containing 1.97ml and 2ml of hexane, optical density of resulting solution was 0.15 at exition and then PL of these two solutions were taken.
[00085] 300µl of ZnTe/CdS(OD is 6.5) was mixed with 300µl Cu-CdS (OD is 6.56) and then solvent was removed by applying vacuum carefully. To the obtained solid 300µl of hexane was added. From this obtained solution, 30µl of the solution was taken out and then it was diluted up to 2ml by adding 1.97ml of hexane and then PL was taken. Again we evaporated 270µl solvent from the remaining solution mixture by applying vacuum, then wetted the solid with little amount of hexane. The solution was then dried up to get solid. It was then re-dispersed in 270µl of hexane by sonication. 30µl of the solution was taken out and was diluted to 1.97ml by adding hexane and then PL of it was taken. This experiment was performed by having the copper doped QDs as a limiting reagent. This ensures that the precipitate formed upon mixing was not particularly dense and can be re-dispersed readily. Further, it prevented a complete PL quench in the first step itself, and allowed the PL evolution to be tracked as a function of drying and wetting cycles. This procedure (drying and wetting) was repeated for 15 times. It corresponds to number of cycles (X-axis) in Fig.10a.

Example 11: Preparation of a NC-NC Solid

[00086] The solid which is obtained by mixing of redox metal ion doped dot with the undoped dots is referred to as a NC-NC solid. The solid formation occurred because of columbic attraction between redox metal ion doped NCs and with undoped NCs after the charge transfer. This solid is one manifestation of the charge transfer, and is composed of electrically doped quantum dots (the semiconductor to be electrically doped) in close association with dots containing metal ions.
[00087] To get solid precipitate out of NCs, we take cleaned NCs of PbSe /CdSe or ZnTe/CdS mixed with Cu-CdS or Cu-ZnSe and the solvent were evaporated from the solution. In order to perform this step in a reasonable amount of time, a vacuum pump was employed to speed up evaporation. After that the residual solid was wetted with minimum amount of solvent (0.1-1 ml) and again the solution was dried. This process was repeated 2-3 times. The solid was separated from supernatant by centrifugation in hexane. The quantity of solid obtained by mixing of MSNs and Cu:CdS was less comparable to other dots having same OD. This is most likely a result of the small size of MSNs relative to Cu:CdS. This size difference can for example prevent the achievement of a high coordination number in a NC solid formed from the two species. Fig.8 illustrates an exemplary schematic representation of emergence of ionic interactions between NCs. Step “a” shows a charge neutral quantum dot pair undergoing a redox process whereby a valency band electron from one NC is transferred into a dopant ion embedded in another NC. At step “b”, the resultant hole causes bleaching of interband transitions and also causes the occurrence of an interband transition in the valance band. At step “c”, the two NCs are no longer neutral and interact through coulomb forces.
[00088] Fig.9a illustrates charging of a mixture of Nanocrystalsby means of a band edge Photoluminescence (PL) Quench. Fig.9b, on the other hand, illustrates charging by means of a bleach. Fig.9c illustrates charging by means of occurrence on an intraband spectrum.Fig.10a illustrates Photoluminescence (PL) counts with respect to number of cycles, whereas Figs.10b through 10e illustrate size of mixture of Nanocrystals/nanocrystals.Fig.10 further illustrates that the solids formed from mixture are kinetically inert and therefore exhibit great structural diversity.
[00089] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE DISCLOSURE

[00090] The present disclosure obviates short comings of the conventional methods of doping semiconductors.
[00091] The present disclosure provides a doping process that eliminates of carrier impurity scattering.
[00092] The present disclosure provides a doping process that involves simple steps for doping,
[00093] The present disclosure provides a doping process capable of doping on large substrates.
[00094] The present disclosure provides a doping process that is cost-effective.
[00095] The present disclosure provides a doping process that maintains the dopants in a layer different from the active layer.
[00096] The present disclosure provides a doping process in which system is kinetically frozen.
[00097] The present disclosure provides a doping process in which dopant is completely un-ionized at time of synthesis so that there is very little tendency of development of compensating defects.
[00098] The present disclosure provides a doping process using Nano crystals that are very small, allowing for the possibility of creating very small doped architectures on a large substrate.
,CLAIMS:1. A method for electrically doping a semiconductor material comprising steps:
Preparing one or more species of redox active Nanocrystals out of materials that contain one or more redox active ions;
and
bringing the one or more species of the redox active Nanocrystals in proximity of the semiconductor material to enable charge transfer to take place as a redox reaction resulting in bonding between the semiconductor material and the one or more species of redox active Nanocrystals due to coulomb interactions.
2. The method of claim 1, wherein the one or more species of redox active Nanocrystals are colloidal Nanocrystals.
3. The method of claim 1, wherein the one or more species of the redox active Nanocrystals have size smaller than 10nm.
4. The method of claim 1, wherein the one or more redox active ions are inorganic ions.
5. The method of claim 4, wherein the one or more redox active ions are selected from a group comprising Cu+, Cu+2, Fe+2, Fe+3, V+2, V+3, Cr+2, or a combination thereof.
6. The method of claim 1, wherein two different species of the one or more species of redox active Nanocrystals are prepared with two different of one or more redox active ions and the two different species of the one or more species of redox active Nanocrystals are brought in proximity of two different areas of a the semiconductor material to enable transfer of charge across the two different species of the one or more redox active species of Nanocrystals wherein the transfer of charge results in bending of bands of the semiconductor material.
7. The method of claim 1, wherein the method results in creation of a small doped architecture on a large substrate.
8. The method of claim 1, wherein the method is a kinetic method of doping and wherein the one or more redox active ions is isovalent with the semiconductor material and is doped into one of the one or more species of a semiconductor Nanocrystalsmade wholly or partly from the semiconductor material.
9. The method of claim 1, wherein the one or more species of redox active Nanocrystals comprise wholly or partly of materials from group comprising FeO, Fe3O4, Fe2O3, CuO, CuO.Al2O3, CuS, In2O3 or SnO2.
10. The method of claim 1,wherein preparing one or more species of redox active Nanocrystals involves doping of one or more species of nanocrystals with one or more redox active ions.
11. The method of claim 10, wherein the one or more species of nanocrystals is a semiconductor material comprising wholly or partly of materials from group comprising PbS, PbSe, PbSe/CdSe, ZnTe, CdS, CdSe, CdSe/CdS.