Claims:We Claim:
1. Semiconductor nanocrystals with extended protected defects to induce sub-band gap emission.
2. The semiconductor nanocrystals as claimed in claim 1, wherein more than 50% of the extended protected defect volume is located away from the nanocrystal surface.
3. The semiconductor nanocrystals as claimed in claim 1, wherein the semiconductor nanocrystal is comprised of Group II-VI, Group III-V, Group IV compounds and mixtures thereof.
4. The semiconductor nanocrystal as claimed in claim1, wherein the extended protected defects are selected from group comprising twin planes and intergrowth boundaries.
5. The semiconductor nanocrystals as claimed in claim 1,whereinsize of the nanocrystals are altered to systematically tune wavelengths of sub band gap emissions.
6. The semiconductor nanocrystals as claimed in claim 1, wherein the quantum efficiency of the emission is relatively insensitive to the optical density of the sample indicating a suppressed effect of self-absorption.
7. A light emitting device comprising the semiconductor nanocrystal of claim 1.
8. The light emitting device as claimed in claim 6, wherein the light emitted is selected from wavelengths comprising of visible spectrum.
9. The light emitting device as claimed in claim 6, wherein the light emitted is white light.
, Description:Field of invention:
The present invention relates to semiconductor nanocrystals having luminescent properties, high quantum yield and photostable emissions. In particular, the invention relates to nanocrystals of Groups II-VI, III-V, IV specifically to Group II-VI; with extended protected defects residing within the nanocrystals, for reduced self-absorption and high efficiency emission effects. The semiconductor nanocrystals with extended protected defects also find application in generation of white light.
Background of invention:
Semi-conductor nanocrystals also referred to as quantum dots are light emitting nanocrystals exhibiting high efficiency emissions. An important feature of these semiconductor nanocrystals is the quantum confinement effect due to which the electronic charge carriers are confined within the nanocrystals. By modifying the size and shape of the nanocrystals researchers have made use of this effect to tune the electronic energy states and optical transitions in the nanocrystals. Subsequently, the light emissions are varied throughout ultra-violet, visible and infra-red spectral ranges.
Nanocrystals absorb light from a source and in this process electrons get excited from the valence band to the conduction band. These electrons emit light in the subsequent de-excitation process, transferring the electron from the bottom of the conduction band to the top of the valence band. Thus, the colour of the light depends on the energy difference between the bottom of the conduction band and the top of the valence band. Smaller the size of the nanocrystal more will be the energy difference resulting in a deep blue colour. When the size of the nanocrystal is more, the energy difference is less, shifting the light towards red. In other words, the electronic transitions in the gap between the conduction band and the valence band of the nanocrystal gives the photoluminescence effect. Smaller the size of the nanocrystal the photoluminescence intensity increases by several folds and there is an energy shift towards red.
In comparison with other phosphors, the semiconductor nanocrystals have exceptional properties of size-tunable light emissions, broad absorption spectra. Because of their tunability property, the nanocrystals are of interest in various research applications such as light emitting diodes, solar cells and liquid crystal displays.
However, the functionality of the nanocrystals have impediments, profound being self-absorption of the light emission along with surface degradation of the nanocrystal. Self-absorption in semiconductor nanocrystals limits their emission efficiencies and thereby their technological applications. There have been efforts to curb self-absorption and intensify the photoluminescence effect of the nanocrystals. Presence of defects has been shown to affect the self-absorption and photoluminescence effect in the nanocrystals. There are many types of defects that are present in semiconductor nanocrystals. For example,(a)Surface defects are present in all nanocrystals due to the confined nature of the nanocrystals;(b) Interface defects present in all core-shell nanocrystals and coupled nanocrystals. These generally form local defects and lead to the lowering of photoluminescence quantum efficiencies. Point defects are present in nanocrystals due to substitution of one element with another. These defects are confined and have a non-extended nature. Prior art search reveal usage of defects in the nanocrystals for reducing self- absorption in the nanocrystals
US7850943 discloses the use of cadmium sulphide nanocrystals emitting light at multiple wavelengths and most of the atoms of the nanocrystals are present on the surface as defects for emitting light at multiple wavelengths. However, photoluminescence from these kind of defects are not tunable with size and hence adversely affects the ability to control the emission wavelength and to improve the quantum efficiency.
WO2015002995 discloses luminescent solar concentrators that include photo-luminescent nanoparticles. This document discloses the semiconductor nanocrystals wherein the defects are incorporated into the semiconductor nanocrystal or adsorbed onto, or otherwise associated with the surface of the semiconductor nanocrystal. The defects that are disclosed in this particular invention are in the form of an atom, or a cluster of atoms, or a lattice vacancy. These defects are confined and hence affect the quantum yield of the nanocrystals.
US20110025224 describes a light emitting device with semiconductor nanocrystals. The invention discloses semiconductor nanocrystal wherein the nanocrystal is doped with transition metals such as Fe, Ni, Mn, and Cu, or with a lanthanide, such as Eu, Er, Tm, or Tb to get white light emissions. The method is tedious and expensive to be practiced on a large scale.
The present invention provides semiconductor nanocrystals with extended protected defects residing within the nanocrystal for enhanced optical properties and high efficiency emissions.
Summary of invention:
Accordingly, the present invention relates to semiconductor nanocrystals with extended protected defects that are residing within the nanocrystals for high efficiency emissions; wherein the semiconductor nanocrystal is comprised of Group II-VI, Group III-V, Group IV compounds and mixtures thereof; and to a light emitting device comprising the semiconductor nanocrystal with extended protected defects residing within the nanocrystal for generating light in the visible spectrum and white light.
Brief description of figures:
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1 shows total and partial density of states of a nanocrystal of size 4.2 nm with (a) defect free spherical cluster, (b) intergrowth of zinc blend and wurzite structure and (c)with an anionic (111) twin boundaries.
Figure 2 explains empirically observed correlation between the extended defect states and the sample emission structure. Optical absorption and emission spectra of (a) with predominant defect state emission and (b) with predominant band edge emission. (c)-(h) HRTEM images corresponding to sample (a) (with extended protected defects) and (i)-(n) corresponding to sample (b) (without the defects) . The appearance of the broadband white light emission is seen when the sample contains extended protected defects.
Figure 3 illustrates a demonstration of reduced self-absorption of the defect emission, even though the band edge emission is strongly attenuated by the presence of high concentration of QDs in the medium.
Figure 4. shows (a) Prototype device which is emitting white light after coating an UV LED with as prepared CdS semiconductor nanocrystals embedded in PMMA. (b). Coordinate in CIE diagram shows emission in white region.
Detailed description of invention:
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the drawings, description and claims. It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Similarly the term quantum dot, nanocrystals are synonymous unless the context mean otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention relates to semiconductor nanocrystals with extended protected defects to induce sub-band gap emission.
In an embodiment of the present invention, wherein more than 50% of the extended protected defect volume is located away from the nanocrystal surface.
In an embodiment of the present invention, the semiconductor nanocrystal comprises Group II-VI, Group III-V, Group IV compounds, preferably to the group II-VI and mixtures thereof;
In another embodiment of the present invention, the extended protected defects comprising of, but not limited to, twin planes and intergrowth boundaries.
In another embodiment of the invention, size of the semiconductor nanocrystals with extended protected defects are altered to systematically tune wavelengths of sub band gap emissions.
In one embodiment of the invention, the quantum efficiency of the emission is relatively insensitive to the optical density of the sample indicating a suppressed effect of self-absorption
The present invention relates to a light emitting device comprising the semiconductor nanocrystals with extended protected defects.
In one embodiment of the present invention, the light emitted from the light emitting device comprising the semiconductor nanocrystals with extended protected defects is selected from the wavelengths comprising of visible spectrum.
In another embodiment of the invention, the light emitted from the light emitting device comprising the semiconductor nanocrystals with extended protected defects is white light.
Various embodiments of the invention provide for nanocrystals with extended protected defects to minimize self- absorption and blinking effects and improve photo-stability with high emission efficiency. The invention also discloses the synthesis of the nano-crystals by solution process and also the manner in which the extended protected defects are introduced in the nano-crystals. The embodiments also provide for the application of the nanocrystals with the defects in the generation of white light.
Self-absorption in semiconductor nanocrystals has been one of the major obstacle towards the development of high efficiency nanocrystal based devices. The present invention shows that extended crystal defects can be constructively utilized to enhance optical properties if they can be introduced in a controlled and “protected” manner. Specifically, the invention discloses the introduction of defects including but not limited to intergrowth and twin planes as examples of well-defined defects exhibiting similar properties, known to be present in such semiconductor nanocrystals. The experimental and theoretical results show that though these defects do not affect the absorption spectrum of the nanocrystal, they provide a unique method to red shift the emission. The energy positions of these defect states are found to be dependent on the nanocrystal size. Being extended in nature, these retain the functionality of the emission wavelength, that is, colour tunability via the quantum size effect. Thus, such defect engineering appears to be an attractive route to achieve a Stokes' shifted emission, while retaining all the other interesting properties of the material, thereby eliminating the persistent problem of self-absorption.
Defect scan be introduced in the semiconductor nanocrystals of Group II-VI, Group IV, Group III-V compounds for example CdS, ZnS, CdSe, ZnSe, CdTe and ZnTe and mixtures thereof; wherein the compounds of the said Groups exhibit more than one thermodynamically favoured phases like the presence of wurtzite and zinc-blende phases. The semiconductor nanocrystals display same type mid gap states which can have similar photoluminescence properties. Typically for the present invention, Cadmium Sulphide (CdS) of Group II-VI is chosen for experimentation.
CdS nanocrystals (NCs) of different sizes are generated by shell-wise growth of the cation followed by the anions in a sequential manner. Two distinct types of extended protected defects in semiconductor nanocrystals are grown by the solution process, namely intergrowth of wurtzite and zinc blende structures and twin boundaries. Wurtzite has a hexagonal structure while the zinc blende a cubic one. The essential difference between them is in the stacking of alternate layers of the cation (Cd) and anion (S) layers along the (0001) direction in the hexagonal form or the equivalent (111) direction in the cubic structure. The stacking being ABAB.. type and ABCABC... type, respectively, the intergrowth of one structure within the other is achieved by a simple alteration of the stacking sequence, for example by incorporating a layer of ABC stack within a ABAB one as follows: ...ABABABCABABAB..., where the inserted intergrowth is shown by italics. Alternately, we may have a sequence like ...ABCABCABABCABC... or multiple layers of intergrowth as well as multiple intergrowths within the same nanocrystal. The twin boundaries at the anion and cation layer along different crystal planes are introduced by slicing the spherical NCs along the various {111} crystal planes and reflecting the atoms along the twin planes to obtain the other half of the NC.
Defects in the nanocrystals are obtained by carefully controlling the reaction conditions through a kinetically controlled state. This is achieved by nucleation of precursors at low temperature to ensure mixed phase followed by quick quenching to room temperature to maximize these defects while existent defects are removed by long time annealing at high temperature. In this specific case of CdS, nucleation of semiconductor nanocrystalsis initiated at temperature of 220° ensuring CdS nanocrystals grow in mixed zinc blende and wurzite phases followed by quick quenching to room temperature. However, if the existing defects have to be removed which is also prepared as a reference, the samples nucleated are annealed at low temperature of 220° C to higher temperature (250° C) for a long time (5 hr). Such an annealing at a high temperature effectively removes all the defects leading to an essentially defect free quantum dot.
Using semi-empirical tight binding (TB) approximation, with the parameters being obtained by fitting of the ab-initio bulk band structure obtained from linearized augmented plane wave (LAPW) method, the electronic structure calculations for the various atoms of the NCs in real space is calculated for both intergrowth and twin defect cases. The total density of states (DOS) is obtained by summing the number of available states contributed by all the atoms present in a nanocluster and is shown in the black curve in Fig. 1(a), Fig. 1(b) and Fig. 1(c) for a typical 4.2 nm spherical defect free cluster, an intergrowth structure and a twinned cluster, respectively. The zero of the energy scale is defined as the valence band edge of the bulk CdS. The presence of substantial number of states above the Fermi energy in both defective clusters, namely the intergrowth and the twinned NCs is noticed and this is not observed in spherical, defect free NCs. Having performed the calculations in real space, the contributions have been plotted as partial density of states (PDOS) for sulfur (red) and cadmium (blue) atoms and shown in Fig.1(a), 1(b) and 1(c) for a typical 4.2 nm cluster. From the contributions as seen from the PDOS, it is evident that the major contribution for the states above the Fermi level arises from anionic states which concentrate at top of the valence band.
These differences in the PDOS of the interfacial and bulk atoms would have remarkable consequences on the electromagnetic interaction of light with the material. The presence of these precise well-defined states in the mid-gap region would be enough to have photo-luminescence transition from the conduction band to these states arising from the interfacial atoms. Hence these data suggest that defect states arising from intergrowth and twin boundaries can be used to obtain the much needed Stokes' shift to avoid self-absorption without the inclusion of other external agents like transition metal dopants. The extensive calculations on various sizes of nanocrystals show that not only the optical absorption energy of the sample varieswith changes in their size, as already known to be arising from quantum confinement effect, but also the energy of the states related to the extended protected defects, discussed above, move with the size of the crystal, giving rise to the very useful and attractive property of tunability of the emission wavelength by varying the size of nanocrystals.
In one embodiment of the invention, samples of CdS nanoparticles are synthesised with and without one class of extended protected defects, namely the intergrowth. The experimental data corroborate the theoretical expectations in that there is a clear correlation between the presence of extended protected defects in samples and the intensity of Stokes’ shifted light emission from the ensemble. Figure 2(a) shows highly Stokes shifted emission which arises from CdS nanoparticles with extended protected defects as seen from the high resolution transmission electron microscope (HRTEM) images Figure 2(c)-(h). This Stokes’ shifted emission however diminishes completely with a consequent increase of band edge emission upon prolong annealing (Figure 2(b)). Annealing removes extended protected defects present inside the particle completely as seen from HRTEM images Figure 2(i)-(h). The band edge emission resulting from these defect free particle suffers enhanced self-absorption problem due to strong overlap with absorption spectra (Figure 2(b)). The experiments also establish that although the band edge emission is strongly diminished at high sample concentrations, the emission of these defects is almost entirely invariant of the number density of nanocrystals in a medium. This is shown in Figure 3 where increasing concentration of CdS nanocrystals in solution strongly suppress band edge emission but defect state emission sustain and remains invariant with concentration. The changes in respective quantum yields is shown in Table 1. This is a direct verification of the reduced self-absorption that is possible in these materials.
Table 1: Quantitative change in quantum field
Total quantum yield(%) Band Edge Emission quantum yield(%) Defect Emission quantum yield(%)
[A] 10.18 1.64 8.54
[B] 13.79 1.78 12.01
[C] 15.49 0.19 15.30
[D] 14.76 ___ 14.76

An additional benefit of this method is that the broad band emission from the specific intergrowth structures created here provide an essentially white light, such that it is possible to be used directly for white light generation applications. The white light emitting material dispersed in PMMA and over-coated a UV diode is used, which then acts as the colourless excitation source. When the UV diode is excited, the resulting UV radiation from the diode is absorbed by the material to emit white light, thereby effectively converting a simple and inexpensive UV diode into a white light emitting device, as shown in Figure 4(a). The coordinate in the CIE diagram (Figure 4(b)) calculated from the emission spectra of this nanocrystal coated UV diode further confirms white light emission.
Experimental:
Cadmium acetate (26.7 mg, 1mmol), Myristic acid (76 mg, 1mmol) and 4 ml ODE are taken in a reaction flask and heated to 220°C for 10 minutes in argon medium. An optically clear solution is obtained. On cooling to room temperature, a turbid white suspension of cadmium myristate is obtained. Sulfur (3.2 mg, 1 mmol) and 2 ml of ODE are added. The mixture is degassed thoroughly for 10 minutes and subsequently heated to 220°C under argon. This colourless solution turns yellow at this point. At 220oC, 1 ml of oleyl amine is added dropwise. The reaction mixture is maintained at this temperature for 10 minutes to complete the growth. For defect free cadmium sulfide the same procedure is followed, and the resulting sample is annealed at 250°C for 5 hours. After the synthesis all semiconductor nanocrystals are cleaned thoroughly in methanol and re-dispersed in toluene to record optical absorption and emission spectra and transmission electron microscope image. Absolute quantum yield of the samples is measured using an integrating sphere placed inside an Edinburg Instruments FLS920 Series Fluorescence Spectrometer with a 450W continuous Xe arc lamp as an excitation source.
The invention provides for nanocrystals with extended and protected defects that can absorb light strongly, using the host, and then emit the light through the intentionally created defect levels. This enables the conversion of the UV/blue emission from a typical UV/blue LED source into white light. These engineered defective semiconductor nanocrystals have improved photostability and reduced self-absorption as compared to normal semiconductor quantum dots. At the same time, these materials still exhibit size tunability and are solution processable.
The present invention is also directed to light emitting devices comprising the semiconductor nanocrystals with the extended protected defects to give photoluminescence effect in the visible spectrum.
The process of fabricating the semiconductor nanocrystal with extended protected defects as the light emitting device can be carried out in several ways such as printing method, coating method or ink-jet method. As an illustrative example, the process of fabricating the semiconductor nanocrystal with extended protected defects as a light emitting device is carried out by first mixing as prepared nanocrystals thoroughly in a PMMA matrix and then coating this PMMA-nanocrystal composite as a thin layer on top of a UV/blue LED.
The aforesaid description is enabled to capture the nature of the invention. It is to be noted however that the aforesaid description and the appended figures illustrate only a typical embodiment of the invention and therefore not to be considered limiting of its scope for the invention may admit other equally effective embodiments.
It is an object of the appended claims to cover all such variations and modifications as can come within the true spirit and scope of the invention.