Swati Nagpal

Ultraviolet electroluminescence from zinc oxide nanorods/deoxyribonucleic acid hybrid bio light-emitting diode

Ultraviolet (UV) light-emitting diode using salmon deoxyribonucleic acid (sDNA)- cetyltrimethylammonium complex as an electron blocking layer and zinc oxide (ZnO) nanorods as emissive material was fabricated. UV emission, which was blue shifted up to 335 nm with respect to the band edge emission of 390 nm, was observed. This blue shift was caused due to accumulation of electrons in the conduction band of ZnO because of a high potential barrier existing at the sDNA/ZnO interface. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Study of CdTe quantum dots grown using a two-step annealing method

High size dispersion, large average radius of quantum dot and low-volume ratio has been a major hurdle in the development of quantum dot based devices. In the present paper, we have grown CdTe quantum dots in a borosilicate glass matrix using a two-step annealing method. Results of optical characterization and the theoretical model of absorption spectra have shown that quantum dots grown using two-step annealing have lower average radius, lesser size dispersion, higher volume ratio and higher decrease in bulk free energy as compared to quantum dots grown conventionally.

Improving the efficiency of a poly(3-hexylthiophene)-CuInS2 photovoltaic device by incorporating graphene nanopowder

Abstract. In the present work, the effect of incorporation of graphene on the poly(3-hexylthiophene) (P3HT):CuInS2 quantum dot (CIS QD)-based solar cell has been studied. For this purpose, the concentration of graphene is varied from 0 to 0.01% w/w in P3HT–CIS (1:0.5) film. It is found that graphene does not deteriorate the absorption of the composite film. It assists in dissociating the photogenerated excitons (both in P3HT and QDs) owing to its two-dimensional structure and high electron affinity as is evident by photoluminescence (PL) quenching. At 0.01% w/w concentration of graphene about ∼95% of PL is quenched. The electrical characteristics show that the incorporation of graphene enhances the efficiency of the device by establishing interconnected conducting pathways in the volume of polymer matrix. The maximum efficiency is observed to be 1.5% at 0.005% w/w content of graphene. However, at higher concentration, i.e., 0.01% w/w, the device starts deteriorating.

Development of Low Size Dispersion, High Volume Fraction and Strong Quantum Confined CdSxSe1-x Quantum Dots Embedded in Borosilicate Glass Matrix and Study of their Optical Properties

Quantum Dots (QDs) of CdSxSe1-x embedded in borosilicate glass matrix (BGM) have been grown using colored glass filter (RG695). Double-Step (DS) annealing method was adopted in which nucleation is achieved at a lower temperature (475°C) without any crystallization. To obtain crystallization on these nucleation centers, the annealing temperature is raised to 575°C at which the nucleation rate is negligible. QDs of various average radii and volume fractions are grown by varying the annealing duration from 3 to 11hrs. QDs corresponding to higher annealing duration are found to have low size dispersion (SD) and high volume fraction but weak quantum confinement, while, the QDs corresponding to lower annealing durations have high quantum confinement due to their much lower radii as compare to Bohr exciton radius, their SD is high and volume fraction low. For nonlinear optical applications the SD must be low and volume fraction should be high. Attempt has been made to optimize the two parameters. Further it has been concluded that there is no contribution of the band edge recombination to the PL and the origin of the PL is due to shallow traps existing in the volume of the QDs. Studies of absorption and PL have also been made on the samples aged for 18, 24 and 36 months. It is found that the effect of aging is to increase the absorption coefficient, reduce the shallow trap centers and reduce the SD.

Estimation of Incubation Time for the Erasure Process in Phase Change Materials for Optical Memories

A general approach has been given to estimate incubation time (τ) which was experimentally observed for the first time by Sugiyama et al. for InTe-based optical memories. Prior to this observation it was established that τ was dependent upon the frequency factor (k 0 ), the Johnson-MehlAvrami exponent (n) and the activation energy required for crystallization (E a ). In the present work it is shown that interfacial strain energy (σ); E a and the change in Gibbs free energy (AC,.) play vital roles in deciding τ. It has also been shown that for the case of laser induced (a → c) transitions only the grain boundary nucleation process explains the incubation time observed. Calculations have been presented for a typical Sb 2 Te 3 system and τ has been found to decrease with increase in σ and to increase with increase in E a .

GROWTH DYNAMICS OF II–VI COMPOUND SEMICONDUCTOR QUANTUM DOTS EMBEDDED IN BOROSILICATE GLASS MATRIX

Wide bandgap II–VI semiconductor quantum dots embedded in glass matrix have shown great potential for opto-electronic device applications. The current problem is to achieve low size dispersion, high volume fraction, and better control over the size of the quantum dots in glass matrix. In this work, a modified growth method has been proposed to achieve a greater control over the size of quantum dots, to reduce their size dispersion and to increase their volume fraction. A theoretical model has been developed to quantitatively estimate the various parameters of the quantum dots. The effects of aging on various parameters of quantum dots in Semiconductor-Doped Glass (SDG) samples have also been discussed in the present work.

Quantum-dot temperature profiles during laser irradiation for semiconductor-doped glasses

Temperature profiles around laser irradiated CdX (X=S, Se, and Te) quantum dots in borosilicate glasses were theoretically modeled. Initially the quantum dots heat up rapidly, followed by a gradual increase of temperature. Also it is found that larger dots reach higher temperatures for the same pulse characteristics. After the pulse is turned off, the dots initially cool rapidly, followed by a gradual decrease in temperature.

Temperature profiles for laser-induced heating of nanocrystals embedded in glass matrices

Quantum confined nanostructures are very important because of their application towards optoelectronic devices. Commercial colored glass filters, which have large semiconductor particles, are being used to manufacture nanocrystals by suitable heat treatments. The progress in this area has been hampered by high size dispersion of these dots in the glass matrix which leads to reduction in higher order susceptibility thereby reducing non-linearity. In the present paper attempt has been made to theoretically model the temperature profiles of a laser irradiated CdS doped Borosilicate sample. Laser being used has a beam diameter of 1.5 mm and energy for 10 nsec pulse is 10 mJ. Two different particle radii of 5 nm and 10 nm have been considered. It is found that larger particles reach higher temperatures for the same pulse characteristics. This is because smaller particles have larger surface to volume ratio and hence dissipates out heat faster to the surrounding. Hence bigger particles will reach dissolution temperature faster than smaller particle and particle beyond a certain size should dissolve in the glass matrix when a sample is heat treated by laser. This could lead to a reduction in size dispersion of the nanocrystals. Also photodarkening effect found in semiconductor doped glasses is a big handicap for practical application of these materials in fast optical switching and non-linear optical devices. Photodarkening effect has been established to be a photochemical effect and it is important to study the temperature profiles around a particle since it will effect the impurity migration.

New model for optimization of erasure time in reversible optical memories including shape factor, temperature, and mass effects on incubation time in various nucleation processes

Work is in progress for improving the switching times of erasable as well as DOW type of optical memories. As it has been already established that the erasure time (ayieldsc) is dominated by incubation time ((tau) ), hence (tau) needs to be explored more in depth. In the present work the mass, molecular diameter and density effects have been shown in addition to the temperature effects. The earlier calculations could only estimate the value of (tau) using a range of critical radii values, whereas the present model can be used to calculate the exact values of critical radii using shape factor calculations for grain boundary, grain edge and grain corner nucleation separately. Knowing the material parameter of a system, one can calculate variation of (tau) with temperature, mass or composition.