WO2014147555A2

WO2014147555A2 - Cd-based-chalcogenide/cds core-shell nanomaterial, defective/defect-free core nanocrystal, methods and applications thereof

Info

Publication number: WO2014147555A2
Application number: PCT/IB2014/059938
Authority: WO
Inventors: Ranjani VISWANATHA; Avijit Saha; Kavassery Sureswaran NARAYAN; Kishore Velichappattu CHELLAPPAN
Original assignee: Jawaharlal Nehru Centre For Advanced Scientific Research
Priority date: 2013-03-18
Filing date: 2014-03-18
Publication date: 2014-09-25
Also published as: WO2014147555A3

Abstract

The present disclosure relates to semiconductor nanomaterials, preferably quantum dots (QDs), methods to obtain said nanomaterials and applications thereof. More particularly, the present disclosure relates to Cd-based-chalcogenide/CdS core-shell nanomaterial starting from a defective core nanocrystal and thick shell structure (greater than 3 nm thickness) with a high quantum yield (greater than 80%), and methods of obtaining the said nanomaterial. The present disclosure further relates to defective/defect-free Cd-based-chalcogenide core nanocrystal and method for synthesizing the defective/defect-free core. The present disclosure also relates to devices and applications of said nanomaterial and the defective/defect-free Cd-based-chalcogenide core.

Description

Cd-BASED-CHALCOGENIDE/CdS CORE-SHELL NANOMATERIAL, DEFECTIVE/DEFECT-FREE CORE NANOCRYSTAL, METHODS

AND APPLICATIONS THEREOF"

TECHNICAL FIELD

The present disclosure relates to semiconductor nanomaterials, preferably quantum dots (QDs) of Cd-based-chalcogenide/CdS core-shell nanomaterial, methods to obtain said nanomaterials and applications thereof. More particularly, the present disclosure relates to Cd-based-chalcogenide/CdS core-shell nanomaterial starting from a defective core nanocrystal and thick shell structure (greater than 3 nm thickness) with a high quantum yield (greater than 80%), and methods of obtaining the said nanomaterial. The present disclosure further relates to defective/defect-free Cd-based-chalcogenide core nanocrystal and method for synthesizing the defective/defect-free core. The present disclosure also relates to devices and applications of said nanomaterial and the defective/defect-free Cd-based-chalcogenide core.

BACKGROUND OF THE DISCLOSURE

A semiconductor is a material which has electrical conductivity between that of a conductor such as copper and an insulator such as glass. The conductivity of a semiconductor increases with increasing temperature, behaviour opposite to that of a metal. Semiconductors are very useful in devices for amplification of signals, switching, and energy conversion as the conductive properties of a semiconductor can be modified by controlled addition of impurities or by the application of electrical fields or light. Understanding the properties of semiconductors relies on quantum physics to explain the motions of electrons through a lattice of atoms. Quantum dots (QDs) or nanocrystal Quantum dots (NQDs) are tiny semiconducting nanocrystals having size less than excitonic Bohr radius and excitons are confined in all three dimensions. The most interesting property of these QDs is that their band gap can be changed by varying size and that results in change in optical emission over a wide range. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. Generally, the smaller the size of the crystal, the larger the band gap, which results in greater difference in energy between the highest valence band and the lowest conduction band. Therefore, more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. Such quantum dots have been the subject of scientific and technological research with an emphasis to develop novel colloidal nanomaterials for energy harvesting and conversion applications. The chemical and physical properties of QDs are dependent on its unique size, shape and composition and QDs have shown great potential as promising materials for a diverse set of applications including active materials in photovoltaics and as size tunable phosphors in lighting and displays. This has propelled efforts towards controlled synthesis and in-depth characterization with programmable composition and geometric features of the QDs. However, until recently, though it was known that internal microstructure of the QDs affect the properties of the material, it was not systematically understood. Recently, with the study of the microstructure using ultra high resolution TEM or X-ray photoemission, it has been shown that several photo-physical properties like quantum yield, blinking etc are dramatically affected by the microstructure. Thus, QDs with optimized properties obtained by fine tuning the size, shape and microstructure, have shown a lot of promise as active layers in photovoltaic devices like solar cells and photoemitting devices like light emitting diodes (LEDs).

Core/Shell QDs are a class of materials in which core semiconducting QDs are overcoated with a different semiconducting material. This shell material passivates the surface traps over the QDs that increase the quantum efficiency as well as the stability of the QDs.

Among the multitude of methods available to obtain such core/shell structures, the properties of the final materials obtained depend on various factors like the actual total coverage of the shell, whether all nanocrystals are equally coated and defects on the surface and/or at the shell interface. Though the lattice mismatch between the core and shell materials is known to play an important role in the design of robust core/shell quantum dots, in most cases the optical properties are mainly dominated by surface defects of the shell material as well. Recently, this dependence of optical properties on surface chemistry and chemical environment was overcome by the growth of ultra thick inorganic shell (> 5nm thick shells) of CdS over CdSe core particles. This increased shell thickness has not only shown direct correlation to the spectroscopic properties of the particles like lifetime and emission intensity, but also to the stability. Surprisingly, these particles were found to not only rid the dots of the detrimental effects of labile nature of organic ligands but have also shown to suppress a more intrinsic phenomenon, known as Auger recombination, believed to give rise to blinking in quantum dots.

The interesting optical properties of ultra thick shell Auger engineered CdSe/CdS quantum dots (QDs) have justifiably drawn a lot of attention since their original discovery as non- blinking dots a few years ago as prospective lasing materials, efficient electroluminescent materials or stable down conversion phosphors. Extensive investigations into the recombination dynamics and PL intermittency have shown that QDs under intense pulsed excitation is generally dominated by multi exciton Auger recombination that is not only the cause of blinking behavior but also detrimental for lasing applications and the efficiencies of the light emitting diodes and other applications. Recently, it has been shown by a couple of independent groups that increasing the shell thickness in these QDs successfully suppresses blinking as well as decreases the Auger recombination. This, in turn, has led to a flurry of activity in the last couple of years to extensively study the properties of these materials starting from blinking statistics to photostability to optical gain performance as well as practical demonstration of light emitting diodes. However, despite several improved qualities of the devices obtained from these materials, as well as extensive studies performed to optimize the synthesis of the QDs, the so- far best quantum yields are typically below 50%, and most often in the order of 30-40%. Clearly, it is evident that improving the quantum yield beyond this point would lead to even better efficiencies of the devices based on these QDs. A recent publication has given some insight into the complexities of shell growth by varying the precursor injection times, solvent ligand interactions as well as the core size. However, fundamental theoretical studies of these materials makes it evident that the Auger recombination can be suppressed not only by efficient shell growth but more importantly by obtaining a smooth interface between the core and the shell, thereby reducing the momentum change that impedes the Auger recombination. Nonetheless, in spite of major theoretical advances claiming the need to present smooth core/shell interface, practically it has not yet been possible to create such a perfect interface. Photovoltaics ultimately aim to absorb light, and convert the photogenerated excitons into spatially separate electrons and holes. On the other hand, the quality of the photoemitter is regulated by the efficiency of the recombination of the electron-hole pair leading to the emission of the absorbed energy. This naturally implies that photo-absorptive materials are in general not efficient as active materials for a photo-emitting device and vice versa. However, here in this instant disclosure, it is shown that tweaking of the microstructure also provides a playground to regulate the electron hole overlap particularly in type II semiconductor interfaces leading to efficient photo-absorber as well as a photo-emitter obtained from the same material.

CdTe, with high optical absorption coefficient and an optimal direct bandgap for solar photovoltaics, has received much attention as absorber materials for efficient, low-cost solar cells. The perfect match of electron affinity of CdS and the type II bandgap between the hetero-j unction of the CdTe and CdS have led to the widespread use of CdTe/CdS as active materials for photovoltaic devices with cell efficiencies as high as 16%. However, it is so far, not extensively used as a photo-emissive material for the well known reason of low quantum yield. Nevertheless, in the instant disclosure depending on the synthesis procedure of the CdTe or CdSe core QDs, both high quantum yield materials of CdTe/CdS (CdTe-A/CdS) or CdSe/CdS suitable for photo-emitters and low quantum yield, long lifetime CdTe/CdS (CdTe-C/CdS) suitable for photovoltaic applications is obtained. The major limitation of the existing products is the relatively low quantum yield. Typically, the quantum yield of these thick shell materials have been around 30-40% with some rare reports of about 50% quantum yield. In spite of this low quantum yield, these materials have shown some interesting photophysical properties that has been correlated to the smooth interface of the core and the shell material using theoretical studies. However, in spite of major theoretical advances claiming the need to present smooth core/shell interface, practically it has not yet been possible to create such a perfect interface. Therefore, the present disclosure addresses the challenge of transforming this theoretical knowledge to practical materials thus leading to greater than 90% quantum yield of fluorescence in case of CdSe/CdS Quantum dots and obtaining CdTe/CdS for efficient photovoltaic applications or greater than 80% quantum yield photoemitters based on the requirement of the application from the same material. STATEMENT OF THE DISCLOSURE

A Cd-based-chalcogenide/CdS core-shell nanomaterial comprising a defective/defect-free Cd based chalcogenide core nanocrystal and thick CdS shell; a method for obtaining a Cd-based- chalcogenide/CdS core-shell nanomaterial comprising a defective/defect-free Cd-based- chalcogenide core nanocrystal and thick CdS shell, said method comprising acts of a) synthesizing the defective/defect-free Cd-based-chalcogenide core nanocrystal, b) adding Cd and S precursors consecutively in a reaction mixture comprising the nanocrystal and c) raising the temperature of the reaction mixture for allowing annealing of the core and the shell to obtain said Cd-based-chalcogenide/CdS core-shell nanomaterial; a defective/defect- free Cd-based-chalcogenide core nanocrystal; a method for synthesizing a CdSe defective core nanocrystal, said method comprising acts of a) reacting trioctyl phosphine oxide (TOPO), octadecene (ODE) and Cadmium oleate (Cd012) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1 hour to about 2 hour to obtain a reaction mixture, b) raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere and c) adding a solution comprising trioctyl phosphine selenium TOP/Se, oleylamine (OAm) and octadecene (ODE) to the reaction mixture of step (b), followed by lowering the temperature of the reaction to obtain said CdSe defective core nanocrystal; a method for synthesizing a CdTe-C defect-free core nanocrystal, said method comprising acts of a) reacting cadmium oxide (CdO), octadecene (ODE) and oleic acid (OA) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1 hour to about 1.30 hour to obtain a reaction mixture, b) raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere and maintaining the temperature for obtaining a precipitate and c) lowering the temperature of the precipitate and adding a solution comprising TOP/Te, trioctyl phosphine (TOP) and octadecene (ODE) to the precipitate, followed by lowering the temperature of the reaction to room temperature to obtain said CdTe-C defect-free core nanocrystal; a method for synthesizing a CdTe-A defective core nanocrystal, said method comprising acts of a) reacting cadmium acetate dihydrate, oleic acid(OA), trioctyl phosphine oxide (TOPO), octadecene (ODE) and trioctyl phosphine (TOP) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1.30 hour to about 2 hour under evacuation to obtain a reaction mixture, b) raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere and c) adding a solution comprising TOP/Te and ODE to the reaction mixture of step (b), followed by lowering the temperature of the reaction to room temperature to obtain said Cd/Te-A defective core nanocrystal; a device comprising Cd-based- chalcogenide/CdS core-shell nanomaterial as above or defective/defect-free Cd-based- chalcogenide core nanocrystal as above; a Cd-based-chalcogenide/CdS core-shell nanomaterial as above or the defective/defect-free Cd-based-chalcogenide core nanocrystal as above, for use in a device selected from a group comprising a semiconductor, electroluminescent device, photolumine scent device, light emitting diode and laser or any combination thereof.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure wherein:

Figure 1: Figurel(a)shows schematically, the synthesis technique of the high quantum yield CdSe/CdS nanocrystals. Figurel(b)shows typical TEM image of the thick shell CdSe/CdS nanocrystals showing the formation of a thick CdS overcoating and inset of Figurel(b)shows typical absorption and emission spectra of the CdSe core and thick shell overcoating of CdS nanocrystals. Figurel(c) x-ray diffraction pattern of different CdSe cores and their core/shell structure along with their standard bulk counterparts.

Figure 2: Figure2(a) shows the absolute scatter and emission of the solvent, QD and Rhodamine 101 dye and the inset shows the digital image of the same, excited at a wavelength of 520 nm. The equal intensity of the two fluorophores suggests that both of them have similar quantum yield of greater than 90%. Figure2(b)shows the variation of quantum yield for the defective core (CdSe-4) and a smooth core (CdSe-8 and CdSe- 16) as a function of shell thickness. Figure 3: shows the lifetime decay dynamics of the QD emission for the core and core/shell structures are shown for defective core (CdSe-4) in Figure 3(a) and for smooth core (CdSe- 8) in Figure 3(b). The insets show the corresponding steady state emission obtained for the 405 nm excitation.

Figure 4:shows high resolution TEM image of the Figure 4(a) thin shell CdS over defective core showing the presence of defects in different directions (red arrows) as compared to Figure 4(b) a similar shell over smooth core showing defects along one direction. A similar high resolution TEM image of thick overcoating of CdS Figure 4(c) on defective core and Figure 4(d) smooth core showing the absence of defects and presence of sharp interfacial defects (red arrows) respectively.

Figure 5: shows nanostructure characterization using high resolution TEM: High resolution TEM images showing the formation of defective and non-defective thick shell core/shell nanostructures for (a) CdSe-14/CdS (b) CdSel6/CdS (c) CdSe-8/CdS and (d) CdSe-4/CdS. Red arrows in Fig. 5(a) points to the directions of the defects. The insets show the size distribution histogram obtained by measuring the sizes of about 300-400 nanostructures.

Figure 6: shows the increase in Quantum yield (QY) of the sample with increasing number of non-defective particles. Dots show the experimental points and the curve indicates guide to the eye.

Figure 7: Figure 7(a) shows the schematic of the device architecture and the photograph of the LED under operation (Inset) and Figure 7(b) LED emission spectrum with various driving voltages Figure 7(c) shows spectral profile of the LED device and comparison with the PL of the QD film on glass and Figure7(d)shows typical I-V characteristics as a function of the bias for the ITO|PEDOT|P3HT|QD|Al device using different efficient QDs.

Figure 8: I-V characteristics of devices obtained from 40% (CdSe-16) and 60% (CdSe-8) QY materials in comparison with that of near unity QY (CdSe-4) materials. Figure 9: TEM images of (a) 4.5 nm CdTe-C and (b) 6.8 nm CdTe-C/CdS (c) 4.1 nm CdTe- A (d) 6.4 nm CdTe-A/CdS QDs and their size distributions are shown in the corresponding insets showing the formation of spherical QDs of specified sizes.

Figure 10: (a) Absorption (dotted line) and PL (solid lines) of CdTe-C core and CdTe-A/CdS core/shell nanocrystals with increasing CdS shell, (b) Lifetime decay plots for core CdTe-C and CdTe-C/CdS core/shell nanocrystals. (c) The variation of QY (black) and average lifetime (red) as a function of size starting from core CdTe-C to CdTe-C/CdS core/shell nanocrystals. (Dots show experimental points and lines show fit to the eye).

Figure 11: (a) Absorption (dotted lines) and PL (solid lines) of CdTe-A core and CdTe- A/CdS alloy interface structure nanocrystals with increasing CdS. (b) Lifetime decay plots for core CdTe-A and CdTe-A/CdS alloy interface structure nanocrystals. (c) The variation of QY (black) and average lifetime (red) as a function of size starting from core CdTe-A to CdTe-A/CdS nanocrystals. (Dots show experimental points and lines show fit to the eye.)

Figure 12:shows schematics of CdTe/CdS (a) core/shell structure with spatially non overlapping electron (blue) and hole (red) wavefunction and (b) alloy interface structure with spatially overlapping electron (blue) and hole (red) wavefunction and their relative bandgap alignment for the largest size of CdTe/CdS (c) Typical X-ray diffraction patterns obtained for the different CdTe cores, CdTe-A/CdS and CdTe-C/CdS nanocrystals along with the bulk cubic CdTe and CdS. (d) Experimental Cd-edge and (e) Te-edge EXAFS spectra (circles) and their fits (solid lines) for CdTe-C/CdS and CdTe-A/CdS. Dotted lines show different paths used for the fitting.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to a Cd-based-chalcogenide/CdS core-shell nanomaterial comprising a defective/defect-free Cd based chalcogenide core nanocrystal and thick CdS shell.

In an embodiment of the present disclosure, the nanomaterial is a semiconductor nanomaterial or a quantum dot. In another embodiment of the present disclosure, the Cd-based-chalcogenide/CdS core-shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the defective/defect-free Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe core and CdTe core; and wherein the defective core is single crystalline core having crystal structure defects in multiple directions along with surface defects.

In yet another embodiment of the present disclosure, the CdTe/CdS core-shell nanomaterial is selected from a group comprising CdTe-A CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; and wherein the CdTe core nanocrystal is selected from a group comprising CdTe-A defective core and CdTe-C defect-free core.

In still another embodiment of the present disclosure, the thickness of the CdS shell ranges from about 3nm to about 8nm.

The present disclosure, also relates to a method for obtaining a Cd-based-chalcogenide/CdS core-shell nanomaterial comprising a defective/defect-free Cd-based-chalcogenide core nanocrystal and thick CdS shell, said method comprising acts of:

a. synthesizing the defective/defect-free Cd-based-chalcogenide core nanocrystal;

b. adding Cd and S precursors consecutively in a reaction mixture comprising the nanocrystal; and

c. raising the temperature of the reaction mixture for allowing annealing of the core and the shell to obtain said Cd-based-chalcogenide/CdS core-shell nanomaterial.

In an embodiment of the present disclosure, quantum yield of the nanomaterial comprising defective core is at least 80%.

In another embodiment of the present disclosure, the Cd-based-chalcogenide/CdS core-shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the CdTe/CdS core-shell nanomaterial is selected from a group comprising CdTe-A/CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; wherein the defective/defect-free Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

In yet another embodiment of the present disclosure, the CdSe defective core nanocrystal is synthesized by a process comprising acts of:

a. reacting trioctyl phosphine oxide (TOPO), octadecene (ODE) and Cadmium oleate (Cd012) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1 hour to about 2 hour to obtain a reaction mixture; b. raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere; and

c. adding a solution comprising trioctyl phosphine selenium TOP/Se, oleylamine

(OAm) and octadecene (ODE)to the reaction mixture of step (b), followed by lowering the temperature of the reaction to obtain said CdSe defective core nanocrystal. In still another embodiment of the present disclosure, the CdTe-C defect-free core nanocrystal is synthesized by a process comprising acts of:

a. reacting cadmium oxide (CdO), octadecene (ODE) and oleic acid (OA) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1 hour to about 1.30 hour to obtain a reaction mixture;

b. raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere and maintaining the temperature for obtaining a precipitate; and

c. lowering the temperature of the precipitate and adding a solution comprising trioctyl phosphine tellurium (TOP/Te), trioctyl phosphine (TOP) and octadecene (ODE) to the precipitate, followed by lowering the temperature of the reaction to room temperature to obtain said CdTe-C defect-free core nanocrystal. In still another embodiment of the present disclosure, the CdTe-A defective core nanocrystal is synthesized by a process comprising acts of:

a. reacting cadmium acetate dihydrate, oleic acid(OA), trioctyl phosphine oxide (TOPO), octadecene (ODE) and trioctyl phosphine (TOP) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1.30 hour to about 2 hour under evacuation to obtain a reaction mixture;

b. raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere; and

c. adding a solution comprising trioctyl phosphine tellurium (TOP/Te) and ODE to the reaction mixture of step (b), followed by lowering the temperature of the reaction to room temperature to obtain said Cd/Te-A defective core nanocrystal.

In still another embodiment of the present disclosure, the trioctyl phosphine selenium (TOP/Se) is prepared by dissolving selenium (Se) in trioctyl phosphine; and wherein the trioctyl phosphine tellurium (TOP/Te) is prepared by dissolving tellurium (Te) in trioctyl phosphine.

In still another embodiment of the present disclosure, the Cd-based-chalcogenide/CdS core- shell nanomaterial is obtained by the method comprising acts of:

a. synthesizing the defective/defect-free Cd-based-chalcogenide core nanocrystal according to the process as above;

b. reacting the defective/defect-free Cd-based-chalcogenide core nanocrystal with octadecene (ODE) and oleylamine (OAm) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1 hour to obtain a reaction mixture;

c. raising the temperature of the reaction mixture to a temperature ranging from about 200°C to about 300°C under inert gas atmosphere; and

d. adding Cadmium oleate (Cd012), followed by sulphur dissolved in octadecene (ODE) to the reaction mixture of step (b) followed by raising the temperature of the reaction mixture for allowing annealing of the core with a shell of CdS to obtain said Cd-based-chalcogenide/CdS core-shell nanomaterial.

In still another embodiment of the present disclosure, the Cd-based-chalcogenide/CdS core- shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the CdTe/CdS core-shell nanomaterial is selected from a group comprising CdTe-A/CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; wherein the defective/defect-free Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

The present disclosure also relates to a defective/defect-free Cd-based-chalcogenide core nanocrystal.

In an embodiment of the present disclosure, the defective/defect-free Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

The present disclosure also relates to a method for synthesizing a CdSe defective core nanocrystal, said method comprising acts of:

c. adding a solution comprising trioctyl phosphine selenium TOP/Se, oleylamine (OAm) and octadecene (ODE) to the reaction mixture of step (b), followed by lowering the temperature of the reaction to obtain said CdSe defective core nanocrystal. The present disclosure also relates to a method for synthesizing a CdTe-C defect-free core nanocrystal, said method comprising acts of:

c. lowering the temperature of the precipitate and adding a solution comprising TOP/Te, trioctyl phosphine (TOP) and octadecene (ODE) to the precipitate, followed by lowering the temperature of the reaction to room temperature to obtain said CdTe-C defect-free core nanocrystal.

The present disclosure also relates to a method for synthesizing a CdTe-A defective core nanocrystal, said method comprising acts of:

a. reacting cadmium acetate dihydrate, oleic acid(OA), trioctyl phosphine oxide

(TOPO), octadecene (ODE) and trioctyl phosphine (TOP) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1.30 hour to about 2 hour under evacuation to obtain a reaction mixture;

c. adding a solution comprising TOP/Te and ODE to the reaction mixture of step (b), followed by lowering the temperature of the reaction to room temperature to obtain said Cd/Te-A defective core nanocrystal. In an embodiment of the present disclosure, the trioctyl phosphine selenium (TOP/Se) is prepared by dissolving selenium (Se) in trioctyl phosphine; and wherein the trioctyl phosphine tellurium (TOP/Te) is prepared by dissolving tellurium (Te) in trioctyl phosphine.

In another embodiment of the present disclosure, the Cd-based-chalcogenide/CdS core-shell nanomaterial, the defective/defect-free Cd-based-chalcogenide core nanocrystal and the methods as above, wherein the defective core comprises multidirectional surface defects and defect-free core comprises unidirectional surface defects. The present disclosure also relates to a device comprising Cd-based-chalcogenide/CdS core- shell nanomaterial as above or defective/defect-free Cd-based-chalcogenide core nanocrystal as above.

In an embodiment of the present disclosure, the device is selected from a group comprising a semiconductor, electroluminescent device, photoluminescent device, light emitting diode and laser or any combination thereof. The present disclosure also relates to a Cd-based-chalcogenide/CdS core-shell nanomaterial as above or the defective/defect-free Cd-based-chalcogenide core nanocrystal as above, for use in a device selected from a group comprising a semiconductor, electroluminescent device, photoluminescent device, light emitting diode and laser or any combination thereof. The present disclosure relates to study of microstructure of CdS shell over Cd based chalogenide core nanocrystals or nanocrystal material and the formation of semiconductor nanomaterial comprising greater than 3 nm CdS shell over Cd based chalcogenide core nanocrystals. The Cd based chalcogenide is selected from CdSe or CdTe. These nanocrystals or nanomaterials are characterized by unique photo-physical properties aiding in devices such as stable down conversion phosphors and efficient electroluminescent materials such as LEDs and lasers.

In an embodiment of the present disclosure, the microstructure of the interface is modulated by using different density of surface defects of the core leading to materials with different percentages of the defective structures. Cores that are single crystalline but have crystal structure defects in multiple directions along with surface defects are called as defective cores in this disclosure. By studying the microstructure of these materials, the photophysical properties are correlated to the interfacial defects. Interestingly, a defective core (CdSe-4 having quantum yield (QY) -4%) is found to be more efficient in reducing interfacial defects in these materials. Additionally, by refining the microstructure of the material, it is shown that the quantum yield of the material is increased to almost 100%, that is till now not observed in any of the earlier inventions. This failure in the art can be attributed to largely empirical investigations by varying several synthesis parameters rather than a logical understanding and correlation of the microstructure with the photo physical properties of these core/shell structures as is carried out in the present disclosure. In another embodiment of the present disclosure, the result of the studies herein suggest that almost 100%, quantum yield is obtained by following the processes described in the instant disclosure for nanomaterials such as CdSe/CdS QDs by starting from a highly defective low quantum yield Cd based chalcogenide core nanocrystal and overcoating by a thick CdS shell. This is further applied to CdTe/CdS to obtain greater than 80% quantum yield for CdTe- A/CdS QDs by starting with a defective core which is suitable for photoemission applications. Low quantum yield but efficient for photovoltaic applications is CdTe-C/CdS QDs which is obtained by starting with defect-free high quantum yield core. The Cd based chalcogenide core nanocrystal is selected from CdSe or CdTe. The present disclosure thus relates to a correlation between microstructure and their photo physical properties leading up-to an increase in the quantum yield of greater than 90%, preferably greater than 80%.

In an embodiment of the present disclosure, thick shell Cd-based-chalcogenide/CdS nanocrystals are synthesized using colloidal techniques by first synthesizing the core nanocrystal. This step is followed by subsequent slow addition of the Cd and S precursors consecutively followed by annealing at high temperature leading to the formation of smooth defect free thick shell nanocrystals. Thus, the method for obtaining a Cd-based- chalcogenide/CdS core-shell nanomaterial comprising a defective Cd-based-chalcogenide core and thick CdS shell, comprises acts of:

a. synthesizing the defective Cd-based-chalcogenide core nanocrystal; b. adding Cd and S precursors consecutively in a reaction mixture comprising the nanocrystal; and

c. raising the temperature of the reaction mixture in the range of 200-300°C for allowing annealing of the core and the shell to obtain said Cd-based- chalcogenide/CdS core-shell nanomaterial. In another embodiment of the present disclosure, in order to correlate the microstructure of the nanocrystals with their photo-physical properties, CdSe core is prepared using different amounts of ligands as discussed in the Example 2 for the synthesis of CdSe core to obtain defective surfaces and smooth surfaces. The formation of the defective and less defective core is not limited to the ligands exemplified in the present disclosure and is possible using any other ligands of similar nature as employed in the instant disclosure. The surface of the nanocrystal is characterized by studying the PL lifetime decay plots at the band edge emission energy. The percentage of non-radiative decay as well as the quantum yield is used to determine the surface defect density on the nanocrystal core.

In still another embodiment of the present disclosure, based on the characterization studies, both the cores (defective and smooth cores) are overcoated with a thick shell of CdS using a similar overcoating procedure in both cases with identical annealing conditions. Surprisingly, the photoluminescence lifetime studies show that highly surface defective CdSe core(CdSe-4) leads to thick shell high quantum yield sample while a similar size defect-free CdSe core (CdSe-8 or CdSe- 16) yields a very low quantum yield sample under similar conditions for the shell formation.

In yet another embodiment of the present disclosure, in order to obtain more insight into the nature of the core/shell, quantification of the quantum yield measurements of these particles using an integrating sphere is carried out. The integrating sphere measurements are also further verified by comparing the quantum yield of typical QDs with the quantum yield of known dyes, like rhodamine 101, exciting both of them at around 520 nm. In yet another embodiment of the present disclosure, the scatter of the solvent as well as that of the dye/QD is studied, which suggests that the absorption of both these materials (CdSe/CdS Quantum dots and rhodamine 101 dye - standard) are identical. Similarly, the emission as obtained for both the dye as well as the QD are also found to be similar and found to be greater than 90%. From the studies, it is evident that the brightness of these materials are very similar, thus verifying that the quantum yield of certain shell thicknesses is indeed greater than 90%. Further, the variation of quantum yield as a function of shell thickness for both a defective core as well as a smooth surface core is also analysed. From the analysis, it is not only evident that the quantum yield can reach greater than 90% for certain shell thicknesses but also that the quantum yield is found to be substantially higher for core/shell originating from a defective surface core rather than a smooth surface core. In yet another embodiment of the present disclosure, microscopic study of a thin and thick shell of CdSe/CdS and CdTe/CdS QDs, obtained from cores with different extent of surface defects, using very high resolution transmission electron microscopy is performed. Such a study reveals that the thin shell CdSe/CdS and CdTe/CdS QD obtained from surface defective core indeed presents microstructural defects in different directions, while the defects in sample obtained from a core with smooth surface is highly prevalent along one fixed direction. However, in the thick shell CdSe/CdS and CdTe/CdS QD obtained from highly defective CdSe and CdTe core respectively, yielded a defect free core/shell QDs, while that obtained from low defect CdSe and CdTe core respectively retains the unidirectional defects observed in thinner shells. This study clearly shows that while the presence of surface defects leads to large number of defects during the shell formation in the initial stages, given sufficient time, the defects anneal out forming a smooth alloy interface at the core-shell junction. However, the presence of smooth core surface presents a large energy barrier for the introduction of slightly lattice mismatched CdS surface. As a consequence, this sharp interface is carried over to thick shells. This model is verified for the first time and hence QDs samples close to 100% quantum yield is obtained.

The present disclosure further relates to study of microstructure of CdS shell over CdTe nanocrystals or nanocrystal material and the formation of type-II semiconductor nanomaterial. These nanocrystals or nanomaterials are characterized by unique photo- physical properties aiding in devices efficient electroluminescent materials such as LEDs and lasers and photovoltaic devices like solar cells.

In an embodiment of the present disclosure, the microstructure of the interface is modulated by using different density of surface defects of the core leading to materials with different percentages of the defective structures. By studying the microstructure of these materials, the photophysical properties are correlated to the interfacial defects. Interestingly, it is found that starting from two different core leads to two different kind of core/shell particles-one having good photo absorbing property promising for photovoltaics applications (CdTe-C/CdS) and the other one having good emitting property promising for efficient electroluminescent applications (CdTe-A/CdS). In another embodiment of the present disclosure, two different CdTe core (labeled CdTe-A and CdTe-C) is prepared using different synthesis technique as discussed in the Example 6 and Example 7. Based on the characterization studies, both the cores are overcoated with a thick shell of CdS using almost similar overcoating procedure as detailed in Example 8. In still another embodiment of the present disclosure, the result of TEM analysis shows that in spite of the similarity of the particle size, the absorption and fluorescence characteristics of the two materials [CdTe-A/CdS (having alloyed interface) and CdTe-C/CdS (having sharp interface)] are unexpectedly found to be different. The optical characterization obtained from steady state PL spectroscopy and Time resolved PL (TrPL) measurement reveals that CdTe- C/CdS quantum dots shows signature of type-II semiconductor heterostructures as expected from the alignment of the energy states giving rise to spatially indirect recombination. In addition, the QY first increases due to the protection of CdTe from surface oxidation and then decreases due to the "spatially indirect" nature of the electron-hole pair leading to lower efficiency of radiative recombination. On the other hand, CdTe-A/CdS quantum dots show unexpectedly contrasting observations. Steady state PL study shows small Stokes' shift, suggestive of a direct transition instead of the expected indirect transition for type-II recombination. More interestingly, the fluorescence of these materials shows drastic changes in emission energy with very high quantum efficiency. In fact, detailed comparison of TrPL as well as QY data suggests that CdTe-C (average lifetime of CdTe-C- 22.6 ns and CdTe-A- 18.5 ns) has a lower percentage of non-radiative decay pathways and higher QY compared to CdTe-A (CdTe-C- 30.5% and CdTe-A- 18%). This suggests that the CdTe-A core have more surface defects compared to CdTe-C core which is also supported by the synthesis method. These contrasting results in CdTe-A/CdS can be attributed due to the formation of alloyed interface as the starting cores are defective. In CdTe-C/CdS formation of a sharp interface combined with a type-II alignment leads to the lower overlap of e-h wavefunction resulting in a "spatially indirect" recombination. However, in CdTe-A/CdS formation of an alloy at the interface leads to a more "spatially direct" recombination due to the greater e-h wavefunction overlap.

In yet another embodiment of the present disclosure, x-ray diffraction (XRD) illustrate that while the XRD pattern of CdTe-A matches with that of cubic CdTe (bulk), the peaks of CdTe-A/CdS is only slightly shifted compared to the core and does not show characteristic Cd-S peaks. In contrast, in case of CdTe-C/CdS it is observed that it is characteristic of both CdS and CdTe crystal lattices. In yet another embodiment of the present disclosure, local structure around the Cd and Te atoms is studied using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and the analysis of Fourier transformed (FT) Cd K-edge EXAFS data show that the CdTe-C/CdS sample undoubtedly shows evidence for the presence of both Cd-Te bond and Cd-S bond as observed by the peaks (in Fig 12(c) ) at about 2.5 A (green) and 1.9 A (magenta) respectively. On the other hand, the Cd K-edge FT data for CdTe-A/CdS sample shows a single major peak that is very similar to the Cd-S structure but is slightly larger bond length of 2.50 A compared to 2.47 A with pure CdS. This suggests that while CdTe-C/CdS is indeed a core/shell structure of CdTe and CdS, CdTe-A/CdS is most likely an alloy of CdTe and CdS with its bond length close to CdS structure. This is further substantiated by studying the Te K-edge in these samples. It is further observed that while the local environment around the Te in CdTe-C/CdS can be simply explained with Cd-Te bond and Te-Te bond, the same is not true for CdTe-A/CdS. In fact, it is not surprising to note that Te edge in the case of CdTe- A/CdS shows clear evidence of oxidation of the Te. This suggests that Te is exposed to the atmosphere in these materials and is consistent with the formation of CdTe/CdS alloy structure consistent with the Cd edge measurements.

A more complete understanding can be obtained by reference to the following specific examples, which are provided for purposes of illustration only and are not intended to limit the scope of the disclosure. Nonetheless, the examples provided herein, form part of the detailed description of the instant disclosure.

EXAMPLES Experimental details

Materials- Cadmium oxide (CdO), oleic Acid (OA, 90%), 1-octadecene (ODE, 90%), trioctylphosphine oxide (TOPO, 90%), oleylamine (OAm, 70%), trioctylphosphine (TOP, 90%) Te-shots (99.99%) Se pellets and sulphur powder (99.5%) are purchased from Sigma Aldrich. Cadmium acetate dehydrate (99%) is purchased from S D fine chem limited. All these chemicals are used without further purification.

Example 1:

Cd01?(cadmium oleate) Synthesis: For preparing 0.2M Cd01₂, 2.5 mmol of CdO, 6.18gm of Oleic acid and 8-10 ml of ODE are taken into a three necked round bottom flask having temperature controller probe attached with it. This reaction mixture is degassed in vacuum at 80°-100°C for l-2hr under vigorous stirring followed by increasing the temperature under Ar flow. At around 200°-230°C the reaction mixture is changed from brown to clear Cd01₂ solution. As soon as the Cd01₂ solution is formed, temperature is quenched to room temperature.

Example 2:

Synthesis of CdSe cores- In the present disclosure different kinds of CdSe cores are prepared by varying the chemical amounts. Firstly, Cadmium oleate (Cd01₂) is synthesized as mentioned above and 2M trioctyl phosphine selenium (TOP/Se) solution is prepared by dissolving Se in TOP inside a glove box.

In a typical synthetic method, 500mg of trioctyl phosphine oxide (TOPO), 5ml of 1- octadecene (ODE) and 1ml of 0.2M Cd01₂are taken in a 50ml round-bottom flask equipped with a temperature controller probe and degassed in vacuum under constant stirring. After lhr of evacuation at 80°C, the temperature of the reaction mixture is raised to 290°C under Ar atmosphere. 1ml of 2M TOP/Se, mixed with 1.5 ml of oleylamine (OAm) and 0.5 ml of ODE, is quickly injected into the reaction system at high temperature. For the growth of nanocrystals, the temperature of the system is then lowered to 260°C. After 1 minute, the solution is cooled down to room temperature to get quantum dots of 3.3nm size. Samples are washed by centrifugation once with acetone and afterwards using hexane and methanol twice. Sample's precipitations are re-dispersed in a small amount of hexane for further experiments. Following the above procedure, CdSe QDs which are almost defect-free and have single exponential lifetime are synthesized. Additionally, by reducing the amount of TOPO (~300mg) and OAm(-lml), CdSe QDs which are defective and have multi -exponential lifetime are synthesized.

Example 2(a)

CdSe defective core nano crystal is synthesized by the following process:

a. degassing about 200-300 mg of trioctyl phosphine oxide (TOPO), about 5-8ml of Octadecene (ODE) and about 1-1.5 ml of 0.2M Cd01₂ at a temperature ranging from about 80°C to about 100°C for about 1-2 hr under constant stirring.

b. raising the temperature of the reaction mixture to a temperature ranging from about 270°C to about 300°C under Ar atmosphere.

c. adding a mixture of about 1-1.5 ml 2M TOP/Se, about 1-1.2 ml of OAm and about 0.5-1 ml of ODE at high temperature.

d. annealing at lower temperature for few mins (about 3 to about 5mins)and then quenching the temperature to room temperature.

Example 2(b)

CdSe smooth core nano crystal is synthesized by the following process:

a. degassing about 500-800mg of trioctyl phosphine oxide (TOPO), about 5-8 ml of

Octadecene (ODE) and about 1-1.5 ml of 0.2M Cd01₂ at a temperature ranging from about 80°C to about 100°C for about 1-2 hr under constant stirring,

c. adding a mixture of about 1-1.5ml 2M TOP/Se, about 1.5-2ml of OAm and about

0.5-lml of ODE at high temperature.

d. annealing at lower temperature for few mins (about 3 to about 5mins) and then quenching the temperature to room temperature. Example 3:

Synthesis of CdSe-CdS core-shell QDs - The core-shell CdSe-CdS QDs are fabricated by the familiar successive ionic layer adsorption and reaction (SILAR) technique. The amount of Cd and S precursors required for each individual layer are calculated considering CdSe and CdS being present in wurtzite structure and the average thickness of each monolayer is about 0.35nm. In a typical experiment, ~2>< 10^"7 mol CdSe cores (size ~ 3.3nm), 5ml of ODE and 5ml of OAm are taken into a round-bottom flask and degassed at 80°C. After 1 hr of evacuation the temperature is raised to 240°C under Ar flow. 0.2M of sulphur dissolved in ODE and 0.2M Cd01₂ are used as S and Cd precursors respectively. At 240°C the required amounts of Cd01₂ as calculated for the growth of each monolayer to increase the thickness of the nanocrystal by 0.35 nm followed by S precursor are injected into the reaction mixture for each cycle of monolayer (ML) formation and the sample aliquots are taken out after the completion of each cycle. The annealing time for Cd amounted up to approximately 2.30 hrs and lhr for the S precursor, thus resulting in an alloy with core-shell interface. For the 5th to 8th shell monolayer cycles, Cd:OA ratio in 0.2M Cd01₂is kept at 1 : 10, while that for the other shell formation is 1 :4. The reaction is stopped after a shell formation of about 5-8 nm thickness. All the samples are washed using a hexane methanol mixture and centrifuged to obtain precipitation and then dissolved in distilled hexane. Following the above synthesis method, two reactions are performed at the same time taking two different cores CdSe- one having almost defect free QDs or QDs possessing smooth core and the other one having very defective core QDs. The scheme for the synthesis of these nanocrystals is shown in Fig. 1(a)

Example3(a):

CdSe/CdS core shell nano crystal having smooth interface is synthesized by the following process:

a. degassing ~2>< 10^"7mols CdSe defective cores (size ~ 3.3nm), about 4-6ml of OAm and about 5-8ml of Octadecene (ODE) and at a temperature ranging from about 60°C to about 80°C for about 1 hr under constant stirring.

b. raising the temperature of the reaction mixture to a temperature ranging from about

220°C to about 250°C under Ar atmosphere.

c. calculating the amount of Cd and S required for 1st shell. d. adding Cd precursor and annealing for about 2:30 hrs and then adding the S precursor and annealing for about lhr.

e. Similarly calculating the amounts of Cd and S for next shells and following the same annealing procedure and obtaining required amount of thick shells.

f. Here for the 5th to 8th shell monolayer cycles, Cd:OA ratio in 0.2M Cd01₂is kept at about 1 : 10, while that for the other shell formation is about 1 :4.

Example3(b):

CdSe/CdS core shell nano crystal having sharp interface is synthesized by the following process:

a. degassing ~2>< 10^"7mols CdSe smooth cores (size ~ 3.3nm), about 4-6ml of OAm and about 5-8 ml of Octadecene (ODE) and at a temperature ranging from about 60°C to about 80°C for 1 hr under constant stirring.

b. raising the temperature of the reaction mixture to a temperature ranging from about 220°C to about 250°C under Ar atmosphere.

c. calculating the amount of Cd and S required for 1st shell.

d. Adding Cd precursor and annealing for about 2:30 hrs and then adding the S precursor and annealing for about lhr.

e. Similarly calculating the amounts of Cd and S for next shells and following the same annealing procedure.

f. Here for the 5th to 8th shell monolayer cycles, Cd:OA ratio in 0.2M Cd01₂ is kept at 1 : 10, while that for the other shell formation is 1 :4.

The calculation followed for the amount of Cd and S for the 1st and subsequent shell is provided below:

The average thickness of one monolayer of CdS is taken as 0.35 nm, based from XRD crystal structure and assumed that one additional layer growth would increase the diameter of a nanocrystal by 0.7 nm. In a typical experiment 2>< 10^"7mols of CdSe nanocrystals (size 3.3 nm) is taken and calculated as to how many moles of CdS will be required to coat the CdSe cores/nanocrystals so that it forms one monolayer CdS shell. Similarly, the amount of CdS required for the next layer is calculated considering the diameter will increase 0.7 typical calculation table has been attached below.

No. of particles Diameter of the core Diameter of core/sell Cd and S precursor (mol) particles (nm) particles (nm) required (mol)

2x l0^"7 3.3 4 2.82x l0^"5

2x l0^"7 4 4.7 4x l0^"5

2x l0^"7 4.7 5.4 5.04x l0^"5

2x l0^"7 5.4 6.1 7x l0^"5

Example 4:

Characterization - Transmission electron microscopy (TEM) is carried out using Technai F30 UHR version electron microscope, using a field emission gun (FEG) operating at an acceleration voltage of 200kV. High resolution transmission microscope (HRTEM) images are obtained from the FEI TITAN (cube) 80-300kV double aberration corrected transmission electron microscope with a negative spherical aberration coefficient of Cs ~-30μπι to see the microstructure of core-shell QDs.

UV visible spectra of samples are recorded using Agilent 8453 UV-visible spectrometer. Steady state PL spectra are obtained using a 450W xenon lamp as the source on the FLSP920 spectrometer, Edinburgh instrument, while the lifetime measurements are carried out on an EPL-405 ps. A pulsed diode laser is used as an excitation source (λ= 405nm). Absolute quantum yield is determined using integrating sphere for both samples and dye.

Samples of the synthesized nanocrystals are initially characterized using transmission electron microscopy (TEM) as well as absorption and fluorescence measurement. Typical TEM image shown in Fig. 1(b) shows the formation of a thick shell of CdS leading to spherical core/shell particles in the size range of about 10 to about 15 nm. A typical absorption and PL measurement for the core and a thick shell CdSe/CdS sample is shown inset of Fig. 1(b). From this figure, it is evident that the PL emission peak is highly red-shifted compared to the core nanocrystal as expected from earlier studies. The x-ray diffraction patterns, shown in Fig. 1(c), on comparison with the bulk wurtzite and zinc blende CdSe and CdS patterns reveal the formation of wurtzite core and core/shell nanostructures obtained both from cores with low as well as with high surface passivation with no traces of zinc-blende structure. The percentage of non-radiative decay as well as the quantum yield is used to determine the surface defect density on the nanocrystal core. Fig. 3a and Fig. 3b show the lifetime decay dynamics of the QD emission for two different CdSe core particles while the insets show the relative intensities of the corresponding steady state emission. From the figure, it is evident that cores giving rise to the spectra in Fig. 3a is more defective than that of Fig. 3b, both due to presence of higher percentage of the non-radiative component as well as due to the lower quantum yield.

The photoluminescence lifetime studies are carried out for characterizing the cores (both smooth and defective) overcoated with a thick shell of CdS using a similar overcoating procedure in both cases with identical annealing conditions. The studies show that highly surface defective CdSe core leads to thick shell high quantum yield sample (shown in the inset to Fig. 3a) while a similar size defect free CdSe core yields a very low quantum yield sample under similar conditions for the shell formation (inset to Fig. 3b). Further, quantification of the quantum yield measurements of these particles using an integrating sphere is carried out. The integrating sphere measurements are also further verified by comparing the quantum yield of typical QDs with the quantum yield of known dyes, like rhodamine 101, exciting both of them at around 520 nm. Figure 2(a) shows the scatter of the solvent as well as that of the dye/QD suggesting that the absorption of both these materials are identical. Similarly, the emission as obtained for both the dye as well as the QD are also found to be similar and found to be greater than 90%. i.e: Figure 2(a) shows the measurement of quantum yield of the sample of the instant disclosure along with Rhodamine 101 under similar excitation conditions. The PL emission of the material under question as well as dye are similar and the dye is known to be having greater than 95% quantum yield. Additionally, this is an absolute QY measurement. Difference of the integration of the area under the curve for the scattered solvent light and the scattered sample light gives the amount of light absorbed. The area under the curve for the amount of light emitted is equal to that absorbed and hence quantum yield is close to 100% within experimental error. Nonetheless, in the prior art comparison with dyes as the instant method have reported 40% or lower quantum yield. Typical image of the emission of the dye as well as the QD under 520 nm excitation is shown in the inset to Fig. 2(a). From the figure, it is evident that the brightness of these materials are very similar, thus verifying that the quantum yield of certain shell thicknesses is indeed greater than 90%. Fig. 2(b) shows the variation of quantum yield as a function of shell thickness for both a defective core as well as a smooth surface core. From the figure, it is not only evident that the quantum yield can reach greater than 90% for certain shell thicknesses but also that the quantum yield is found to be substantially higher for core/shell originating from a defective surface core rather than a smooth surface core or defect-free surface core.

Furthermore, microscopic study of a thin and thick shell of CdSe/CdS QDs, obtained from cores with different extent of surface defects, using very high resolution transmission electron microscopy is performed. Such a study reveals that the thin shell CdSe/CdS QD obtained from surface defective core indeed presents microstructural defects in different directions as shown by the red arrows in Fig. 4a while the defects in sample obtained from a core with smooth surface is highly prevalent along one fixed direction (Fig. 4b). However, in the thick shell CdSe/CdS QD obtained from highly defective CdSe core yielded a defect free core/shell QDs (Fig. 4c) while that obtained from low defect CdSe core (Fig. 4d) retains the unidirectional defects observed in thinner shells. This study clearly shows that while the presence of surface defects leads to large number of defects during the shell formation in the initial stages, given sufficient time, the defects anneal out forming a smooth alloy interface at the core-shell junction. However, the presence of smooth core surface presents a large energy barrier for the introduction of slightly lattice mismatched CdS surface. As a consequence, this sharp interface is carried over to thick shells. The panels of Fig. 5(a)-(d) shows the typical HRTEM images of samples obtained from thick shell samples starting with differing defective particles (CdSe-14, CdSe-16, CdSe-8, CdSe-4 respectively). The size and size distribution obtained from the analysis of about 300-350 particles in every case is shown in the corresponding insets. The percentage of size distribution in all cases is found to be below 10% and mostly spherical particles. The percentage size distribution as well as the number of defective particles and the QY of the sample is shown in Fig.6. It appears that though the particles have almost same size of core and shell, with a small decrease in the percentage of defective particles, we observe a rather large change in the QY of the samples. Thus from the analysis of the present disclosure, it is clear that the defects of different types form a major contribution to the quenching of QY. The defect-free core/shell QDs obtained from a surface defective core are shown for the first time to have highly stable, near unity QY, in spite of their wurtzite crystal structure.

Example 5:

Application of Quantum Dots and LED Fabrication - A proof of concept or verification of device efficiency is carried out by fabricating a simple light emitting diode (LED) using near unity quantum yield nanocrystals as the active layer. The LED device schematic is shown in Figure 7(a). Poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) (Baytron P) (-50 nm) is spin coated on to pre-cleaned indium tin oxide anode (ITO) substrate and annealed at 150°C for 1 hour under low vacuum (0.1 Torr). A layer of poly(3- hexylthiophene) (P3HT) (-40 nm) is spin coated on top of the PEDOT:PSS layer and annealed at 140°C for 1 hour under N2 atmosphere. The emissive layer of CdSe:CdS quantum dots (-120 nm) is then spin coated on the P3HT layer followed by 12 h drying under N2, the device is completed by depositing aluminium top electrode (-100 nm) by physical vapour deposition.

Current-voltage characteristics are acquired using a Keithley 2400 Source Measure Unit as the voltage source and a Keithley 6512 electrometer. Spectral measurements are done using a fibre coupled Hamamatsu mini spectrophotometer (Model: TM-VIS/NIR). Luminance measurements are done using a calibrated silicon photodiode, which collected the emission from the ITO side of the device. The light lost through wave guiding and absorption in the various layers within the device is not accounted for in the measurements of luminescence. The performance of these un-optimized simple devices is comparable to the best quantum dot based LED devices (1-3%) in spite of the absence of electron injecting buffer layer. The devices retained all the advantages of the earlier devices like remarkably low turn on voltage as well as possibility of driving the device to yield luminance to a range of 7000 Cd/m². The sizable luminance at low input power leading to efficient devices (~ 1.5 Lm/W) is comparable to efficiency magnitudes obtained for solution processed LEDs; 2.4 Lm/W for cross-linked colloidal dots and 4.2 lm/W for a transfer printed LED.

I(V) of the quantum dot based active layer device exhibited typical diode characteristics (Fig. 7d). In absence of the P3HT layer in the device, the threshold voltage magnitude is substantial (> 8 V) and weakly emits in the injection regime. Upon introducing the P3HT layer, the light emission from the device is observed beyond the threshold voltage (3- 5 V range) for all the devices measured with substantial improvement in the magnitude and stability. The light emission band is centred at 628 nm (for device structure with QDs where PL is observed and centred at 628 nm) with a full width at half maximum (FWHM) < 45 nm (Fig. 7b). All the devices tested in this device configuration exhibit light emission. The emission flux shows linear dependence with current density. It is possible to drive the device to 9V with injection current density of 0.3 A/cm2 to result in emission exceeding 6500 cd/m². As is seen in Figure 7(c), the equivalence of PL and EL emission is clearly observed and strongly indicates common excitonic origin in the quantum dot layer as the source of emission. The trends in the present disclosure, by observation, points out that the improvement in PL yields are translatable to the light emitting device attributes. It also shows that the thin P3HT layer, despite having absorption in the visible region does not affect the emission characteristics of the quantum dots and merely facilitates the hole injection into the active quantum dot layer. It is to be noted that LED's made without the P3HT layer exhibit weak emission but display similar spectral characteristics. It is to be pointed out that further optimization of the device with a choice of different thickness of the active layers, hole injecting layers and cathodes should yield improved device characteristics. Similar devices made out of lower QY materials of identical composition are highly unstable and inefficient as shown in the main panel of Fig. 8. Observation of these trends points out that the improvement in PL yields are translatable to the LED attributes. Example 6:

Synthesis of CdTe core QDs for CdTe-C- CdTe core for CdTe-C nanostructure are synthesized by the following method. Briefly, 0.4 M TOP/Te solution is prepared by dissolving Te shots in TOP inside a glove box. In a typical synthesis method, 0.2 mmol (25.6 mg) of CdO, 0.2 ml of oleic acid, 8 ml of ODE were taken in a three necked round bottom flask equipped with temperature controller probe. The temperature is maintained at 80°C for degassing under vigorous stirring. After 1 :30 hr of evacuation, temperature of the reaction mixture is raised to 310°C under constant Ar flow. After some 20-30 min, as soon as a white coloured precipitate appeared, the temperature is brought down to 290°C and a solution containing 0.13 ml of TOP/Te diluted with 1ml of TOP and 1.12 ml of ODE is quickly injected into the hot mixture. After a few seconds, the temperature is quickly quenched down to room temperature using ice bath. Samples are washed twice by centrifugation using hexane and methanol mixture.

Example 6a:

CdTe core QDs for CdTe-C is synthesized by the following process:

a. dissolving Te shots in TOP and making 0.4MTOP/Te solution .

b. taking 0.15-0.25 mmol of CdO, 0.2-0.3 ml of oleic acid, 6-10 ml of ODE and degassing it for 1-1 :30hr under evacuation at temperature 80°-100°C.

c. raising the temperature to 300°-310° C under constant Ar flow and keeping it for 30- 50 mins until a white colored precipitate appears.

d. bringing down temperature to 285°-295°C immediately and adding a solution of 0.10- 0.15 ml of TOP/Te, 1-1.5 ml TOP and 1-1.2 ml ODE.

e. cooling down the temperature of the reaction mixture at room temperature.

Example 7:

Synthesis of CdTe core QDs for CdTe-A- In a typical synthesis, 20 mg (0.075 mmol) Cadmium acetate dihydrate, 0.3 ml of oleic acid, 0.4 gm of TOPO and 5ml of ODE are taken in a three necked flask and degassed at 80°C under constant stirring. 3 ml TOP is added into this reaction mixture during this procedure. Meanwhile, tellurium precursor is prepared using 0.1 ml of 0.4 M TOP/Te solution mixed with 0.9 ml of TOP and 1ml of ODE. After 1 :30 hr of degassing, the temperature is raised to 320°C under steady Ar flow. At 320°C the tellurium precursor is injected to the hot reaction mixture. After few seconds, the temperature is quenched down to room temperature using ice bath. Similar to the previous CdTe core, samples are washed and preserved for further use.

Example 7a:

CdTe core QDs for CdTe-A is synthesized by the following process:

a. dissolving Te shots in TOP and making 0.4 M TOP/Te solution .

b. taking 0.050 mmol (15 mg) to about O. lmmol (25mg) of Cadmium acetate dihydrate, 0.2 ml to about 0.5 ml of oleic acid, 0.3gm to about 0.6gm of TOPO, 5 ml to about 10ml of ODE, 2ml to about 5ml of TOP and degassing it for l :30hr to about 2hr under evacuation at temperature 80°C to about 100°C .

c. raising the temperature to 320°C to about 330°C under constant Ar flow and quickly adding a solution of 0.1 ml to about 0.2 ml of 0.4M TOP/Te and 1-2 ml of ODE.

d. cooling down the temperature of the reaction mixture at room temperature.

Example 8:

Synthesis of CdTe/CdS (CdTe-C/shell and CdTe- A/shell) heterostructures- SILAR technique is followed to synthesize these core/shell QDs. Considering CdTe and CdS are present in cubic structure the amount of Cd and S precursor required for each individual layer is calculated. In a typical experiment for the CdTe-C/shell, CdTe-C cores Q lO^"7 mol), 3 ml of OAm and 4 ml of ODE are taken into a round bottom flask and degassed under vacuum with the temperature gradually increased to 70°C. After 1 :30 hr of degassing the reaction flask is backfilled with Ar and the temperature is raised to 170°C and required amount of Cd(OA)₂ for the first shell is injected into the reaction mixture. After 10 min, the temperature is further raised to 220°C and same amount of S precursor is injected into the reaction. After 5 min, cycles of Cd followed by S precursors are injected and annealed at this temperature for all subsequent shell formation and aliquots are taken out after each cycle. After the addition of Cd precursor, the reaction mixture is annealed for 10 min and 5 min after the S precursor addition. Similarly, CdTe-A/CdS QDs are also synthesized just by taking CdTe-A cores (l x lO^"7 mol) and changing the annealing time to 15 min and 10 min after the addition of Cd and S precursor respectively, keeping all other conditions similar for both synthesis. In both the cases, aliquots are taken out after completion of each monolayer addition. All the samples are washed by centrifugation using hexane methanol mixture and re-dissolved in hexane. Example 8a:

CdTe-C/CdS heterostructures is synthesized by the following process.

a. degassing l x lO^"7 mol CdTe-C cores, 2-4 ml of OAm and 4-6 ml of ODE at a temperature ranging from about 60°C to about 80°C for 1 hr under constant stirring.

b. raising the temperature of the reaction mixture to a temperature ranging from about 160°C to about 180°C under Ar atmosphere.

c. calculating the amount of Cd and S required for the first shell.

d. adding Cd precursor and annealing for about 10-15 min and then increasing the temperature ranging from about 220°C to about 240°C and adding the S precursor and annealed it for 10-15 min.

e. similarly calculating the amounts of Cd and S for next shells and following the same annealing procedure (at temperature 220°C) and obtaining required amount of thick shells.

Example 8b:

CdTe-A/CdS heterostructures is synthesized by the following process.

a. degassing 1 * 10^"7 mol CdTe-A cores, 2-4 ml of OAm and 4-6 ml of ODE at a temperature ranging from about 60°C to about 80°C for 1 hr under constant stirring.

c. calculating the amount of Cd and S required for the first shell.

d. adding Cd precursor and annealing for about 15-20 min and then increasing the temperature ranging from about 220°C to about 240°C and adding the S precursor and annealed it for 10-15 min.

e. similarly calculating the amounts of Cd and S for next shells and following the same annealing procedure (at temperature 220°C) and obtaining required amount of thick shells. Example 9

Characterization TEM is performed on a Technai F30 UHR version electron microscope, using a field emission gun (FEG) operating at an accelerating voltage of 200 kV.

Absorption spectra of samples are recorded using Agilent 8453 UV-visible spectrometer. Steady state PL spectra are obtained using a 450W xenon lamp as the source on the FLSP920 spectrometer, Edinburgh instrument, while the lifetime measurements are carried out using the EPL-405 ps pulsed diode laser.

X-ray diffraction patterns for the QDs were recorded on Bruker D8 Advance diffractometer using Cu-Κα radiation.

The Cd K-edge (26711 eV) and Te K-edge (31814 eV) EXAFS measurements are carried out at the MRCAT 10-ID beam line at the Advanced Photon Source, Argonne National Laboratory The data is collected in fluorescence Stern-Heald geometry with samples loaded in cylindrical cuvettes. Cd foil and Te powder tapes are measured in transmission geometry with help of the reference ion chamber for every scan taken at Cd edge and Te edge respectively. The spot size of the incident x-ray beam on the sample is 500 micron by 500 micron. Platinum mirror is used for harmonic rejection. Incident ion chamber had full nitrogen gas; transmission and reference ion chambers are filled with 80% Argon mixed with 20% nitrogen. Fluorescence ion chamber had Krypton gas. Data collected is processed using Athena software by extracting the EXAFS oscillations x(k) as a function of photoelectron wave number k. The theoretical paths are generated using FEFF6 and the models are done in the conventional way using the fitting program called Artemis. Fitting parameters are obtained by modeling the EXAFS data of each sample in R-space until a satisfactory fit describing the system is obtained. Data sets are simultaneously fitted in R-space with k- weights of 1, 2 and 3.

Typical transmission electron microscope (TEM) images of the cores and the CdS overcoated materials along with their size distribution analysis are shown in Fig. 9. From the figure it is evident that both the sizes of the two cores (4.1 nm and 4.5 nm) and the overcoated (6.4 nm and 6.8 nm) materials are very similar. However, in spite of similar particle size, the absorption and photoluminescence characteristics of the two materials are unexpectedly found to be dramatically different. Fig 10 shows the evolution of absorption (dotted line) and emission spectra (solid line) of the QDs during the growth of the CdS shell on CdTe-C cores emitting at 2.09 eV with a sharp absorption peak at 2.15 eV.CdS shell leads to broadening eventually smearing out of the absorption feature completely along with a shift to lower energies accompanied by a significant increase in the Stokes shift. This is characteristic of a weak spatially indirect transitions consistent with the formation of a type-II structure. Further signatures of type II semiconductors is observed in the TrPL data shown in Fig. 10(b) as well as the evolution of QY as a function of shell formation as shown in Fig. 10(c). The electron-hole recombination lifetime increases with increasing shell thickness as shown in Fig. 1(b) and the average lifetime (red) plotted in Fig. 10(c).

In contrast, the optical characterization obtained from CdTe-A samples show unexpectedly different behaviour as shown in Fig. 11(a). Nevertheless, it is interesting to note that the Stokes shift in the case of CdTe-A/CdS is not as high as expected for type II systems due to the small shift of both the absorption edge to lower energies and emission peak to higher energies as observed in Fig. 11(a). In addition the emission energy of 1.8 eV in 6.4 nm CdTe-A/CdS (Fig. 11(a)) compared to 1.4 eV in 6.8 nm CdTe-C/CdS (Fig. 10(a)) cannot be explained either by experimental error or as a consequence of small changes in the size of the nanocrystals (-0.4 nm). More interestingly, the lifetime and quantum yield of these materials show drastic changes as seen in Figs. 10b, 10c, 1 lb and 11c. From the Fig. 1 lb and 1 lc, it is apparent that the lifetime of the excitonic recombination of CdTe-A/CdS does not increase with increasing shell thickness and retains a high QY of ~ 80% in the thickest shell sample consistent with the absorption and the emission data. This provides clear signatures of spatially direct recombination of the charge carriers within the heterostructure in an established type-II band structure alignment.

A schematic of sharp interface combined with a type-II alignment in case of CdTe-C/CdS and an alloy at the interface in case of CdTe-A/CdS are shown in Fig. l2(a)and 12(b) respectively. Crystal structure analysis using XRD analysis is shown in Fig.12(c). Local stmcture analysis around the Cd and Te atoms are done using EXAFS spectroscopy. The Fourier transform of the Cd and Te K-edge EXAFS with their corresponding fitting is shown in Fig. 12(d) and 12(e) respectively. This study suggests that CdTe-A/CdS is most likely an alloy of CdTe and CdS with its bond length close to CdS structure whereas CdTe- C/CdS is more likely to be a core shell structure.

Claims

We claim:

1. A Cd-based-chalcogenide/CdS core-shell nanomaterial comprising a defective/defect- free Cd based chalcogenide core nanocrystal and thick CdS shell.

2. The nanomaterial as claimed in claim 1, wherein the nanomaterial is a semiconductor nanomaterial or a quantum dot.

3. The nanomaterial as claimed in claim 1, wherein the Cd-based-chalcogenide/CdS core-shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the defective/defect- firee Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe core and CdTe core; and wherein the defective core is single crystalline core having crystal structure defects in multiple directions along with surface defects.

4. The nanomaterial as claimed in claim 3, wherein the CdTe/CdS core-shell nanomaterial is selected from a group comprising CdTe-A/CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; and wherein the CdTe core nanocrystal is selected from a group comprising CdTe-A defective core and CdTe-C defect-free core.

5. The nanomaterial as claimed in claim 1, wherein the thickness of the CdS shell ranges from about 3nm to about 8nm.

6. A method for obtaining a Cd-based-chalcogenide/CdS core-shell nanomaterial comprising a defective/defect-free Cd-based-chalcogenide core nanocrystal and thick CdS shell, said method comprising acts of:

7. The method as claimed in claim 6, wherein quantum yield of the nanomaterial comprising defective core is at least 80%.

8. The method as claimed in claim 6, wherein the Cd-based-chalcogenide/CdS core-shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the CdTe/CdS core-shell nanomaterial is selected from a group comprising CdTe-A/CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; wherein the defective/defect- firee Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

9. The method as claimed in claim 8, wherein the CdSe defective core nanocrystal is synthesized by a process comprising acts of:

c. adding a solution comprising trioctyl phosphine selenium TOP/Se, oleylamine (OAm) and octadecene (ODE)to the reaction mixture of step (b), followed by lowering the temperature of the reaction to obtain said CdSe defective core nanocrystal.

10. The method as claimed in claim 8, wherein the CdTe-C defect-free core nanocrystal is synthesized by a process comprising acts of:

c. lowering the temperature of the precipitate and adding a solution comprising trioctyl phosphine tellurium (TOP/Te), trioctyl phosphine (TOP) and octadecene (ODE) to the precipitate, followed by lowering the temperature of the reaction to room temperature to obtain said CdTe-C defect-free core nanocrystal.

11. The method as claimed in claim 8, wherein the CdTe-A defective core nanocrystal is synthesized by a process comprising acts of: a. reacting cadmium acetate dihydrate, oleic acid(OA), trioctyl phosphine oxide (TOPO), octadecene (ODE) and trioctyl phosphine (TOP) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1.30 hour to about 2 hour under evacuation to obtain a reaction mixture; b. raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere; and

12. The method as claimed in claims 9, 10 and 11 wherein the trioctyl phosphine selenium (TOP/Se)is prepared by dissolving selenium (Se) in trioctyl phosphine; and wherein the trioctyl phosphine tellurium (TOP/Te) is prepared by dissolving tellurium (Te) in trioctyl phosphine.

13. The method as claimed in claim 6, wherein the Cd-based-chalcogenide/CdS core-shell nanomaterial is obtained by the method comprising acts of:

a. synthesizing the defective/defect-free Cd-based-chalcogenide core nanocrystal according to the process of claims 9, 10 or 11;

14. The method as claimed in claim 13, wherein the Cd-based-chalcogenide/CdS core- shell nanomaterial is selected from a group comprising CdSe/CdS core-shell nanomaterial and CdTe/CdS core-shell nanomaterial; wherein the CdTe/CdS core- shell nanomaterial is selected from a group comprising CdTe-A/CdS core-shell nanomaterial and CdTe-C/CdS core-shell nanomaterial; wherein the defective/defect- free Cd-based-chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

15. A defective/defect-free Cd-based-chalcogenide core nanocrystal.

16. The nanocrystal as claimed in claim 15, wherein the defective/defect-free Cd-based- chalcogenide core nanocrystal is selected from a group comprising CdSe defective core and CdTe core; and wherein the CdTe core is selected from a group comprising CdTe-C defect-free core and CdTe-A defective core.

17. A method for synthesizing a CdSe defective core nanocrystal, said method comprising acts of:

c. adding a solution comprising trioctyl phosphine selenium TOP/Se, oleylamine (OAm) and octadecene (ODE) to the reaction mixture of step (b), followed by lowering the temperature of the reaction to obtain said CdSe defective core nanocrystal.

18. A method for synthesizing a CdTe-C defect-free core nanocrystal, said method comprising acts of:

19. A method for synthesizing a CdTe-A defective core nanocrystal, said method comprising acts of:

a. reacting cadmium acetate dihydrate, oleic acid(OA), trioctyl phosphine oxide (TOPO), octadecene (ODE) and trioctyl phosphine (TOP) at a temperature ranging from about 60°C to about 100°C, for a time duration of about 1.30 hour to about 2 hour under evacuation to obtain a reaction mixture; b. raising the temperature of the reaction mixture to a temperature ranging from about 250°C to about 350°C under inert gas atmosphere; and

c. adding a solution comprising TOP/Te and ODE to the reaction mixture of step (b), followed by lowering the temperature of the reaction to room temperature to obtain said Cd/Te-A defective core nanocrystal.

20. The method as claimed in claims 17, 18 and 19 wherein the trioctyl phosphine selenium (TOP/Se) is prepared by dissolving selenium (Se) in trioctyl phosphine; and wherein the trioctyl phosphine tellurium (TOP/Te) is prepared by dissolving tellurium (Te) in trioctyl phosphine.

21. The Cd-based-chalcogenide/CdS core-shell nanomaterial, the defective/defect-free Cd-based-chalcogenide core nanocrystal and the methods as claimed in any of the claims 1 to 20, wherein the defective core comprises multidirectional surface defects and defect-free core comprises unidirectional surface defects.

22. A device comprising Cd-based-chalcogenide/CdS core-shell nanomaterial as claimed in claim 1 or defective/defect-free Cd-based-chalcogenide core nanocrystal as claimed in claim 15.

23. The device as claimed in claim 22, wherein the device is selected from a group comprising a semiconductor, electroluminescent device, photoluminescent device, light emitting diode and laser or any combination thereof.

24. A Cd-based-chalcogenide/CdS core-shell nanomaterial as claimed in claim 1 or the defective/defect-free Cd-based-chalcogenide core nanocrystal as claimed in claim 15, for use in a device selected from a group comprising a semiconductor, electroluminescent device, photoluminescent device, light emitting diode and laser or any combination thereof.