Avijit Chowdhury

Ambient condition oxidation of zinc foil in supersaturated solution for shape tailored ZnO nanostructures: low cost candidates for efficient electron emitter and UV-detector

A well-ordered array of ZnO nanostructures is realized by a one pot ambient condition approach where the nanoforms are spontaneously formed over zinc foil in alkaline supersaturated solution containing zinc salt. Shape of nanostructure, a key issue for practical application, can be tailored by only manipulating the counter anion of the zinc salt used. The synthesized array nanostructures exhibit great promise for their application as electron field emitters as well as UV photo-detectors. A comparative study reveals that nanospikes having emission beneficial ultra-sharp tips with large inter-tip separation, shows the highest field emission current. Nanowire arrays with high surface defect constitution were observed to show the highest photocurrent gains as high as 1.1 × 103. Nanostructures organized over substrate for direct integration into nanodevices and their performance optimization through structure tailoring will be boosted by this chemical method.

Transient electroluminescence from rubrene light-emitting diodes using double voltage pulse

Light-emitting diodes using a dye-insulating matrix blend of rubrene and arachidic acid have been fabricated using Langmuir-Blodgett film deposition technique. Transient electroluminescence (EL) measurements were performed by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to study the effect of accumulated charges during the first pulse separated from that of the injected carriers, which are predominant during the second pulse. During the first voltage pulse, intrinsically accumulated charges recombine to generate an instantaneous EL peak, which is absent during the second pulse. A constant EL was observed during both the pulses. We also have observed an overshoot in EL in the falling edges of mainly negative voltage pulses.

Tuning of electrical conductivity and hysteresis effect in poly(methyl methacrylate)–carbon nanotube composite films

Reproducible current vs. voltage hysteresis has been observed in single-walled carbon nanotubes (SWCNTs) embedded in poly (methyl methacrylate) (PMMA) measured under various experimental conditions. We have observed a stable hysteresis loop along anticlockwise direction in positive bias region and the width of the hysteresis was reduced/quenched when the film was exposed to monochromatic light. By varying the SWCNTs content in PMMA composite thin films, the electrical conductance behavior of the Al/PMMA-SWCNTs/ITO sandwich structure can be tuned in a controlled manner. The conductivity increases with SWCNTs concentration. The hysteresis effects of the composite films have been attributed to the injection of electrons from the electrode to the trap site located in the PMMA and SWCNTs layer. Such nanocomposites are expected to be useful for various nanotechnology-based devices.

Electric field induced tunable bistable conductance switching and the memory effect of thiol capped CdS quantum dots embedded in poly(methyl methacrylate) thin films

Experimental results on conductivity switching and appearance of hysteresis in the current–voltage characteristics of thiol (benzyl mercaptan)-capped CdS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix are presented in this manuscript. The dependence of switching behavior has been studied under different conditions such as sample cell temperature, scan speed of voltage, illumination of light of various intensities, film thickness, size of thiol-capped CdS QDs etc. The voltage at which the conductivity switching occurs i.e. threshold voltage (Vth) appearing in the current–voltage characteristic curve, decreases with the increasing sample cell temperature and also with increasing intensity of photoexcitations but Vth increases with the increasing scan rate of voltage and size of thiol-capped CdS QDs. Between forward and reverse bias sweeps, the switching events exhibit hierarchy of hysteresis loops. The area within the hysteresis loops decreases with increasing sample cell temperature and ultimately disappear at higher temperatures. The switching events can be tuned by changing the external parameter (experimental conditions) such as sample cell temperature, scan speed of bias voltage, irradiation of light, size of thiol-capped CdS QDs etc. The switching mechanism and appearance of hysteresis have been attributed to an electric field-induced charge transfer from the polymer to the thiol-capped CdS QDs. The nanocomposites of PMMA and thiol-capped CdS nanocrystallites may be suitable for polymer based memory devices.

Nanostructured organic–inorganic heterojunction diodes as gas sensors

Nanostructured organic–inorganic heterojunction devices were fabricated by employing n-type zinc sulphide (ZnS) grown on ITO by RF magnetron sputtering and thin film of zinc phthalocyanine (ZnPc) deposited on the top of the ZnS layer by thermal evaporation. On the top of the ZnPc layer, an Al film was coated to form an electrode. Thin films of ZnS and ZnPc were characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). Current–voltage (I–V) characteristics of the fabricated ITO/ZnS/ZnPc/Al hetero-structures were studied at different temperatures (30 to 90 °C) in vacuum and diode-like behaviour was observed. In the case of single layer devices ITO/ZnS/Al and ITO/ZnPc/Al diode-like behavior was not obtained. The effects of donor ammonia and acceptor oxygen gases on the electrical properties of single layer devices ITO/ZnS/Al, ITO/ZnPc/Al and ITO/ZnS/ZnPc/Al heterojunctions were studied in order to investigate the utility of these devices as gas sensors. A drastic increase in forward current of the heterojunction has been noticed in the presence of donor ammonia gas containing water vapor, while a decrease in forward current has been noticed in the presence of acceptor oxygen gas. This change in forward current (diode current) in the presence of gases (under doped conditions) has been explained by the creation of charge carriers inside the ZnPc layer and hence bending of ZnPc bands. The threshold voltage (Vth) of the junction diodes has been found to depend on the temperature and the presence of donor and acceptor gases. Our experimental results suggest that this heterojunction diode could be a novel candidate for gas sensors.

Alternating current light-emitting devices based on Langmuir–Blodgett films of a porphyrin derivative: space charges in device operation

Light-emitting devices based on a molecularly thin film of a porphyrin derivative have been fabricated and operated under sinusoidal alternating voltage. The general frequency response of luminance of such devices has been studied and compared with device characteristics under direct current mode. Space charge-assisted electron injection has been found to be operative under ac modes. Transient characteristics of electroluminescence have been studied to explain the frequency response in these devices. Frequency response of luminance further showed that ac light of 400 kHz can be obtained from these molecularly thin devices.

Transient characteristics of light-emitting devices based on Langmuir–Blodgett films of a porphyrin derivative

Transient electroluminescence responses of light-emitting devices based on Langmuir-Blodgett films of a porphyrin derivative have been studied. A sequence of two rectangular voltage pulses separated by a delay has been applied. This has enabled us to study the effect of accumulated/trapped charges during the first pulse, while their relaxation dynamics have been studied during the second pulse. The separation time between the two pulses has been varied to study the relaxation of the accumulated charges in the device. The amplitude and polarity of the applied voltage pulse have been varied to study the barrier modification and space-charge limited electron injection in the device.

Spectroscopic Studies on the Effect of Some Ferrocene Derivatives in the Formation of Silver Nanoparticles.

Silver nanoparticles were prepared by microwave assisted method using silver nitrate as precursor in the presence of some ferrocene derivatives. The formation of the silver nanoparticles was monitored using UV-Vis spectroscopy. The UV-Vis spectroscopy revealed the formation of silver nanoparticles by exhibiting typical surface plasmon absorption band. The position of plasmon band (406-429 nm) was observed to depend on the nature of a particular ferrocene derivative used. TEM images indicated that the nanoparticles were spherical in shape and well-dispersed. Quantum dots (3.2 nm) were prepared by using ferrocenecarboxylic acid. The surface plasmon absorption band has shown red shift with increasing concentration of ferrocene derivative. For different duration of microwave heating time, intensity of absorption spectra in general was found to increase except in presence of ferrocene carbaldehyde where it decreased. Time-dependent spectra have indicated almost stable position of the surface plasmon band with increasing time of observation confirming that the as prepared silver nanoparticles did not aggregate with lapse of time.

Spectacular oscillations in the dark and photocurrent in thiol-capped CdS quantum dots embedded in PMMA matrix

In this work, experimental results of low-frequency, time-dependent current oscillations (under both dark and photoexcitation) in the sample cells of thiol-capped CdS quantum dots (QDs) embedded in poly(methyl methacrylate) (PMMA) matrix have been presented. Dark and photocurrent oscillatory behaviours have been studied under diverse conditions, such as sample cell temperature, applied bias voltage across the electrodes and concentration of thiol-capped CdS QDs. Oscillation amplitude gradually increases with applied bias voltage at a particular temperature and also with increasing sample cell temperature. The period of oscillation (i.e., frequency) can be tuned over a wide range either by changing the sample temperature, applied bias voltage, or varying concentration of thiol-capped CdS QDs. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in these semiconductors. Such current oscillations are possibly associated with the formation of some kind of charge density waves in the PMMA films embedded with thiol-capped CdS QDs.