Trap State Emission Research Articles

Excitation-Power-Dependent Color Tuning in a Single Sn-Doped CdS Nanowire.

Multicolor emission and dynamic color tuning with large spectral range are challenging to realize but critically important in many areas of technology and daily life, such as general lighting, display, multicolor detection and multi-band communication. Herein, we report an excitation-power-dependent color-tuning emission from an individual Sn-doped CdS nanowire with a large spectral range and continuous color tuning. Its photoluminescence (PL) spectrum shows a broad trap-state emission band out of Sn dopants, which is superposed by whispering-gallery (WG) microcavity due to the nanostructure size and its structure, besides the CdS band-edge emission. By simply changing the excitation power from 0.25 to 1.36 mW, we demonstrate that the typical Sn-doped CdS nanowire with the weight ratio of 10:1 of CdS and SnO2, the emission color can change from red to orange to yellow to green. In view of the stable properties and large spectral range, the Sn-doped CdS nanowires are very promising potential candidates in nanoscale optoelectronic devices.

Resonance plasmonic coupling: selective enhancement of band edge emission over trap state emission of CdSe quantum dots.

The photoluminescence properties of quantum dots (QDs) are often enhanced by eliminating surface trap states through chemical methods. Alternatively, a physical approach is presented here for improving photoluminescence purity in QDs by employing frequency-specific plasmon resonance coupling. Emitter-bound plasmonic hybrids are designed by electrostatically binding negatively charged QDs in water to positively charged gold nanoparticles having a thin polymer coating. Herein, two types of QDs are used: (i) bare CdSe, which exhibits both band edge and trap state emission, and (ii) CdSe overcoated with a ZnS shell (CdSe/ZnS) devoid of trap state emission. Tuning the extinction spectrum of the plasmonic system to match the band edge emission of CdSe enables the selective enhancement of band edge emission over trap state emission. Excellent match in the extinction spectrum of the gold nanoparticle with both, experimentally calculated photoluminescence enhancement factor and theoretically calculated radiative rate enhancement signifies the role of frequency-specific plasmon resonance coupling. Plasmon-coupled photoluminescence of CdSe/ZnS is further investigated by varying the number density of emitter on the surface of plasmonic nanoparticle. An enhancement in the photoluminescence is observed at a lower emitter density of CdSe/ZnS and the photoluminescence enhancement factor closely follows the plasmon resonance. However, photoluminescence quenching occurs with an increase in CdSe/ZnS due to plasmon-assisted nonradiative energy transfer between nearby QDs, as indicated by a red shift in the PL maximum. These studies establish that resonance plasmonic coupling is a convenient physical strategy for tuning the intrinsic photoluminescence properties of QDs for various optoelectronic applications.

ZnSeTe quantum dots modified with zinc chloride toward bright trap-state emission.

ZnSeTe quantum dots (QDs) attract growing interest owing to their low threats to health and the environment. They are widely applied as emitters in displays and lighting devices. Previous findings have indicated that inorganic halides are excellent candidates for surface ligands on QDs. By incorporating inorganic halides during the synthesis process, the photoluminescence (PL) intensity and quantum yield (QY) of QDs can be significantly enhanced. However, the alteration of surface states in QDs induced by zinc halide modification and the mechanism of formation of trap-state radiative recombination processes have been less discussed. Herein, we proposed a synthesis strategy for ZnSeTe/ZnSe/ZnSeS/ZnS core/shell/shell/shell QDs modified with ZnCl2, and by comparing the morphology and elemental composition of QDs with different amounts of ZnCl2 added, we revealed the regulatory mechanism of nanocrystal growth in the presence of ZnCl2. QDs with modification of ZnCl2 exhibited broad yellow fluorescence, distinct from the intrinsic blue emission. Through spectroscopic and surface ligand analyses, we attributed this yellow emission to the intermediate state energy levels caused by the defects on the surface. Finally, we used the QDs with broad linewidth emission to fabricate a simple white-light-emitting diode (WLED). This work provided new insights into the role of inorganic ligands and the use of a single emitting material in solid-state lighting devices.

Blue CdSe/CdS core/crown nanoplatelet light-emitting diodes obtained via a design-of-experiments approach.

Obtaining efficient blue emission from CdSe nanoplatelets (NPLs) remains challenging due to charge trapping and sub-bandgap emission. Thanks to a design-of-experiments (DoE) approach, we significantly improved the NPL synthesis, obtaining precise control over the lateral aspect ratio (length/width). We raised the photoluminescence quantum efficiency up to 66% after growth of a CdS crown, with complete elimination of trap-state emission. Using these 3.5 monolayer, blue-emitting CdSe/CdS core/crown NPLs (λ = 460 nm), we fabricated light-emitting diodes (LEDs) with narrowband (16 nm) blue electroluminescence, an external quantum efficiency of 1.3% and low turn-on voltage of 2.9 V after DoE optimization. Our findings show that NPLs are a promising system to obtain LEDs that emit a saturated blue color.

Intrinsic Trapping and Recombination Dynamics in Low‐Dimensional Bismuth Sulfide Nanocrystals

AbstractBismuth sulfide (Bi2S3) possesses bandgap of 1.30–1.45 eV, perfectly suitable for light‐harvesting materials in solar cells. Carrier dynamics in the Bi2S3 absorber material are determinant for energy conversion performance of solar cells, yet the carrier dynamics and associated recombination mechanism are rarely studied. Taking advantage of spectroscopic study on Bi2S3 nanocrystals, here new findings are uncovered inherent from the unique crystal structure that causes the energy loss in Bi2S3 materials. One is the trap state emission at near‐infrared region, the other is the ultrafast intrinsic trapping in picosecond scale without saturation at 1019 cm−3 carrier density and various decay rates by adjusting excitation fluence. These observations indicate that photoinduced carriers are likely to be intrinsically trapped due to lattice distortion and display different recombination pathways. This study provides fundamental understanding on the detailed carriers dynamics of Bi2S3 nanocrystals and new insights into the optimization of Bi2S3‐based photovoltaic devices.

Reactive grid-assisted co-sputtering of titanium and chromium in a pure nitrogen atmosphere: Uniformity, optics, and structure of the Ti–Cr–N films

This study provides the results of the first attempt to deposit thin films by reactive grid-assisted co-sputtering with the deployment of small grounded grids having two varying apertures for enabling the transfer of particles. The diameter of all the grids utilized was 50 mm, and their apertures' diameters were changed from 5 mm to 15 mm. Furthermore, the ratios of the grid area to the chamber wall area were lower than 0.0161, preventing the formation of ion-rich sheaths and their adverse impact upon discharge and plasma stability. Accordingly, Ti 1 Cr 1-x N films (0.88 < x < 0.97) with low chromium contents were deposited on soda-lime glass substrates by using pure nitrogen as the sputtering gas. The grazing incidence X-ray diffraction patterns of the films, the average thicknesses of which were lower than 50 nm, showed no sign of crystallinity in the films. Ranging from 3.26 to 3.79 eV, the optical bandgap of the films changed by altering the apertures’ size; chromium content, internal stress, and quantum confinement effect appeared to be the main contributing factors. The photoluminescence intensities appertaining to trap state emissions reflected a decreasing trend by increasing the chromium content, which can be ascribed to the capability of the chromium particles included in the surface, structure, and grain boundaries of the films to prevent photoinduced electron-hole pairs from recombination. It was found that not only is the grid-assisted co-sputtering method an effective tool whereby the doping range of conventional co-sputtering methods can be significantly extended but also it can cover a spectrum of surface morphologies. However, in comparison with the conventional co-sputtering method, the utilization of the grids with fairly small apertures can limit the thickness uniformity of the films; this effect may be attenuated by changing the geometry and design of grids.

Excited-State Charge Transfer and Extended Charge Separation within Covalently Tethered Type-II CdSe/CdTe Quantum Dot Heterostructures: Colloidal and Multilayered Systems.

We used N,N'-dicyclohexylcarbodiimide (DCC) coupling chemistry to synthesize (1) heterostructures of CdSe and CdTe quantum dots (QDs) in colloidal dispersions and (2) heterostructures of CdSe and CdTe QDs, as well as CdS and CdSe QDs, immobilized on metal oxide thin films. The DCC-mediated formation of amide bonds between terminal carboxylic acid and amine groups of ligands on different QDs drove the formation of heterostructures. This cross-linking mechanism selectively yields heterostructures and prohibits the undesired formation of homostructures consisting of just one type of QD. Products of adsorption, ligand-exchange, and covalent-coupling reactions were characterized by transmission electron microscopy and ATR-FTIR, 1H NMR, electronic absorption, steady-state emission, and time-resolved emission spectroscopy. Ground-state absorption spectra of constituent QDs were unperturbed upon incorporation into heterostructures, enabling control over electronic properties. Heterostructures of CdSe and CdTe QDs exhibit type-II interfacial energetic offsets that promote charge separation following excitation of either QD. Indeed, photoexcited CdTe QDs transferred electrons to CdSe, and photoexcited CdSe QDs transferred holes to CdTe, on time scales of 10-100 ns, as evidenced by dynamic quenching of band-edge and trap-state emission. Mixed dispersions of noninteracting QDs did not undergo excited-state charge transfer. Constructing heterostructures on TiO2 thin films introduced an additional charge-transfer pathway, electron transfer from QDs to TiO2, which occurred on subnanosecond time scales and enabled extended spatial separation of photogenerated electrons and holes. Our results reveal that carbodiimide coupling chemistry can be used to tether colloidal QDs selectively and covalently to each other, yielding dispersed or immobilized heterostructures with programmable compositions and energetic offsets that can undergo efficient excited-state interfacial electron transfer.

Optical investigation of starch capped cadmium sulfide nanoparticles

Green synthesis using soluble starch as stabilizing agent in aqueous medium is an approach towards to develop an eco friendly method for synthesis of nanoparticles. It has advantages over conventional method involving toxic reagents. Cadmium sulfide is a suitable material among the II-VI compounds for optoelectronic and solar applications. We report a study on the optical properties of CdS nanocrystals prepared by green synthesis method. Cadmium sulfide nanoparticles were prepared using starch as stabilizing agent. The sample was characterized by X-ray diffraction, Scanning electron microscope, UV–Visible absorption spectroscopy, and photoluminescence spectroscopy. From the study, the blue shift in absorption wavelength with respect to bulk CdS indicate the increased in effective energy band gap and formation of nano sized CdS particles. The estimated size confirms the weak confinement regime as the size comparable to exciton Bohr radius. Photoluminescence spectrum shows band edge emission and deep trap states emission. The CdS nanoparticles exhibit photo catalytic activity against methyl orange under visible light irradiation. The 75% photo degradation was observed for 90 min irradiation time.

Crystallographic and electro-optic analysis of pure and Cu/Mn-doped Cd0.6Zn0.4O ternary alloy: Role of the defect states and imperfection density

Pure and Cu/Mn-doped Cd0.4Zn0.6O (0 ≤ x ≤ 1) ternary alloy is prepared by co-precipitation and further the variation of dopant concentration has been studied. The diffractogram of the undoped sample has peaks of both rocksalt (RS) CdO and wurtzite (WZ) ZnO phase. The individual and simultaneous doping of Cu and Mn elements has significantly altered the intensity, FWHM, and developments of defect states in Cd0.4Zn0.6O alloy. TEM microscope has investigated the crystalline size and interplanar spacing that is slightly decreased with dopant concentration which lies within the range ~ 38–30 nm. UV–Vis spectroscopy analyzed a decreasing trend in bandgap energy with dopant concentration. IN PL, the near band edge and deep trap state emission is observed at 382, 417, 448, 520 and 580 nm wavelengths. The resistivity, carrier concentration, and mobility are also computed which proposed that Cu/Mn-doped Cd0.4Zn0.6O alloy could be the better TCO with new opportunities for material science researchers.

One-Pot Phosphine-Free Route for Single-Component White Light Emitting CdSexSy Alloy Nanocrystals

In an enduring quest to develop highly efficient display and lighting devices over the decades, semiconductor nanocrystals have recently emerged as favorable candidates. However, developing a prudent and viable methodology to fabricate high-quality semiconductor nanocrystals exhibiting tunable luminescence, including single-component white light emission, is challenging. Herein, we report a facile one-pot synthetic route to fabricate white light emitting CdSexSy nanocrystals demonstrating high quantum yield using CdO, S powder, and Se powder as precursors. A phosphine-free route was adopted, employing paraffin oil as the reducing agent and solvent for the fabrication. The optical properties can be effectively tailored by controlling the reaction time and the Se/S molar ratio. The pristine CdSexSy nanocrystals exhibited a broad visible range emission from 400 to 750 nm. The CdSexSy nanocrystals (Se/S = 0.4) displayed white light emission with Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of (0.30, 0.31) and quantum yield of 50 ± 3%. Moreover, by manipulating the Se/S ratio effectively, the band-edge (∼400–450 nm) to trap-state (∼550–750 nm) emission was tuned, and the distinct shades of white light were obtained. The white light emission was retained in nanocrystals-embedded poly(methyl methacrylate) (PMMA) thin film and also in the hydrogel matrix. Furthermore, white light emission characteristics from CdSexSy nanocrystals (Se/S = 0.4), PMMA thin film, and thin film coated white light emitting devices were found to be similar even after months of fabrication, indicating the high stability of the nanocrystals and sustainability of the materials derived from nanocrystals. The one-pot, low-cost fabrication route demonstrated in the present study offers the promising scope of white light emitting CdSexSy nanocrystals in solid-state display applications.

Plasmon-Induced Trap State Emission from Single Quantum Dots.

Charge carriers trapped at localized surface defects play a crucial role in quantum dot (QD) photophysics. Surface traps offer longer lifetimes than band-edge emission, expanding the potential of QDs as nanoscale light-emitting excitons and qubits. Here, we demonstrate that a nonradiative plasmon mode drives the transfer from two-photon-excited excitons to trap states. In plasmonic cavities, trap emission dominates while the band-edge recombination is completely suppressed. The induced pathways for excitonic recombination not only shed light on the fundamental interactions of excitonic spins, but also open new avenues in manipulating QD emission, for optoelectronics and nanophotonics applications.

(Zn + Co) co-doped CdO thin films with improved figure of merit values and ferromagnetic orderings with low squareness ratio well suited for optoelectronic devices and soft magnetic materials applications

The present study focuses on the suitability of CdO thin films towards optoelectronic and spintronic devices through (Zn + Co) co-doping. Para to ferromagnetic transition mediated by oxygen vacancies takes place in the CdO:Zn thin films through Co doping which was acknowledged from the photoluminescence studies. Defect level peaks observed in the PL spectra at 411, 460 and 494 nm are associated with metal interstitials (Cdi, Zni, Coi), singly ionized oxygen vacancies (Vo+) and oxygen vacancies (Vo), respectively. The deep-level or trap state emission due to the ionized oxygen vacancies is observed at 521 nm. XRD analysis indicate that the undoped, Zn-doped (CdO:Zn) and (Zn + Co) co-doped CdO thin films have cubic structure with (1 1 1) plane being the preferential one. The crystallite size of pure CdO decreases with Zn-doping and with Co co-doping it increases. Cauliflower shaped nanostructures are evinced for the (Zn + Co) co-doped films. Increased optical transparency and decreased sheet resistance were observed for the (Zn + Co) co-doped films. Increased Haacke’s quality factor observed for the (Zn + Co) co-doped films confirmed their potential for optoelectronic devices.

Investigating in novel CdZnSe/CdSe/CdZnSe quantum well with 97% PLQY

Thick-shell not only provides colloidal quantum dots (QDs) stability and workability, but also plays a significant role in passivation of QDs surface defects and change of electron transport characteristics of QDs. Here, the dedicated design and synthesis of high luminescence CdZnSe/CdSe/CdZnSe quantum wells are reported by precisely controlled shell growth. By tuning the thicknesses of the CdSe and CdZnSe layers, photoluminescence quantum yields (PLQY) can be attained a maximum of 97% as the CdSe shell thickness is 4 monolayers and the CdZnSe shell thickness is 10 monolayers. These gradient thick shell CdZnSe/CdSe/CdZnSe quantum wells confirm the suppression of surface trap-state emission in gradient core/shell structures. Superior optical properties render CdZnSe/CdSe/CdZnSe quantum wells suitable for use in solid-state lightings.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η2σ2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

Photoluminescence imaging of modern paintings: there is plenty of information at the microsecond timescale

Luminescent emission from artworks is usually employed by conservators and restorers as a visual and quick method for assessing the presence of protective varnishes and restoration materials, as well to evaluate the painting conservation status. Besides these mere qualitative observations, the analysis of both spectral and temporal features of the optical emission from a painting can provide useful information about the pictorial materials employed by artists. In this work we propose Multi-spectral Time-gated Photoluminescence Imaging to differentiate between luminescent pigments, on the basis of their different emission spectra and lifetimes. We present the application of the approach on two modern paintings from Fondazione Maimeri Private Collection, on which we differently probe the fast-living emission from surface varnish and lake pigment and the long-living trap state emissions from different semiconductor pigments. With the aid of the time gated detection we record the trap state emission of semiconductor pigments, otherwise covered by the intense fluorescence of the organic compound. The method is complemented with Macro-XRF analysis, to confirm the nature of the identified inorganic pigments, as zinc white and cadmium yellow. The protocol represents a rapid, non-invasive and reliable way for assessing the identification of luminescent pigments and their spatial distribution.

Synchrotron Deep-UV Photoluminescence Imaging for the Submicrometer Analysis of Chemically Altered Zinc White Oil Paints.

Zinc oxide (ZnO) is a II-VI semiconductor that has been used for the last 150 years as an artists' pigment under the name of zinc white. Oil paints containing zinc white are known to be prone to the formation of zinc carboxylates, which can cause protrusions and mechanical failure. In this article, it is demonstrated how a multispectral synchrotron-based deep-UV photoluminescence microimaging technique can be used to show the distribution of zinc soaps on the submicrometer scale and how this information is used to further the understanding of zinc white degradation processes in oil paint. The technique is based on the luminescence of zinc soaps in the near-UV (∼3.65 eV) upon excitation in the deep-UV (4.51 eV), involving transitions that are argued to subsequently involve ligand-to-metal and metal-to-ligand charge transfer with intermediate structural reconfiguration. Because the primary emission peak lies at a higher energy than the band gap of ZnO (3.3 eV), the signal can easily be isolated from the pigment's very intense band gap and trap state emission by employing a multispectral acquisition approach. Moreover, analysis at such short wavelengths, in combination with a UV-transparent optical setup, allows for lateral resolution on the order of 200 nm to be obtained. The unprecedented capabilities of the microimaging technique are illustrated by showing its application to the study of a historical cross section from an early 20th century painting by Piet Mondrian. Revealing the submicrometer distribution of crystalline zinc soaps in this cross section provides new insights that suggest that microfissures, the starting points of paint delamination, are the result of an overall expansion of a heavily saponified zinc white layer.

A Mechanochemical Route for ZnS Nanocrystals, and Batch Sorting along Size Distribution

The assistances of sodium dodecyl benzene sulfonate (SDBS) and aging treatment were introduced to further improve the room-temperature mechanochemical synthesis of the quantum-sized zinc sulfide (ZnS) nanocrystals. As a result, a green strategy for synthesizing the monodisperse nanocrystals with tunable size and crystallinity was developed, holding convenient, highly efficient and low pollution. Size evolution shows a gradually increasing trend along the aging-temperature. A model that the independent reaction cells constructed by SDBS-wrapped reactant packages (solid state vesicles, SSVs) for the confined growth of ZnS nanocrystals was proposed to access the formation mechanism of ZnS quantized crystal in a solid-state synthesis system. The band gaps and band-edge luminescent emissions of as-prepared ZnS nanocrystals experienced the size-dependent quantum confinement effect, while the trap-state emissions exhibited the lattice integrity-dependence. Furthermore, ZnS quantum-sized nanocrystals with narrower size distribution can be obtained by a batch-sorting process through adjusting the centrifugal speed.

Luminescence Change of CdS and CdSe Quantum Dots on a Ag Film.

Enhanced luminescence of an emitter on a Ag film is usually ascribed to the resonant surface plasmons. In these studies, the solid cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dot/silver (QD/Ag) hybrids were prepared, and the luminescence characteristics of these QD/Ag hybrids were measured. It is found that the enhancement of the trap state emission (TSE) is related to the QD size. The TSE features of the annealed QD/Ag hybrids are insensitive to the morphology of the Ag film. We used the wet and dry methods to separate the QD and Ag components and found that the photoluminescence (PL) of the QD component was permanently changed from the initial state. The PL modification is ascribed to the Ag+ doping effect rather than the surface plasmons. This doping method uses pure Ag as the Ag+ ion source. In this case, though the CdS and CdSe QD/Ag hybrids are the solid state, the cation exchange between Ag+ and Cd2+ ions can still occur on the QD surface. Even a small amount of Ag can efficiently influence the luminescence of the QDs embedded in the poly(methyl methacrylate) matrix. A hypothetical model was proposed to explain the PL modification of the QD/Ag hybrid with and without annealing. Using this dry method for doping, the transparent luminescence label can be prepared easily, and the doped QDs can be further doped with Ag+ dopants.

Conversion of the yellow to blue emission of CdSe quantum dots (QDs) via ZnSe shell growth

In this research, CdSe QDs were synthesized via a microwave approach. ZnSe shell was grown on the CdSe QDs using a simple, room temperature and rapid photochemical approach. Synthesized QDs were characterized by means of the different analyses such as XRD, FESEM, TEM, EDAX, UV–Vis and PL. CdSe synthesized QDs indicated a broad band (FWHM of about 120 nm) yellow surface trap states emission between 450 and 800 nm. By successful growth of the ZnSe shell broad band trap states emission of the CdSe QDs was completely converted to a narrow band and high intense blue emission with FWHM of about 40 nm which is a unique result for water soluble QDs.

Temporal evolution of white light emitting CdS core and Cd1-xZnxS graded shell quantum dots fabricated using single step non-injection technique

White light emitting quantum dot (WQD) materials are being investigated due to their possible applications in the light emitting devices as single layer white light emitters and these have major advantage of lower re-absorption and self-absorption over multilayer emitters of different sized quantum dots (QDs). In this paper, white light emitting CdS core and Cd1-xZnxS graded shell based quantum dots have been synthesized using a single step non-injection technique. Detailed analysis of optical and structural properties has been carried out with QDs growth time. The photoluminescence (PL) consists a narrow band edge emission and a broad (in the range of 440–800 nm) trap state emission which results in white light emission and the quantum efficiency (QE) improves from ∼12% to 34% as growth time increases from 5 min to 30 min. The average fluorescence lifetime of band edge emission and trap state emission varies in the range of ∼2–5 ns and ∼250–550 ns respectively with QDs growth time. We have obtained a series of white light whose color coordinates are very close to the standard white light (CIE-1931); hence these QDs can be a good candidate for white light applications.