Suvra Prakash Mondal

CdS-Decorated ZnO Nanorod Heterostructures for Improved Hybrid Photovoltaic Devices

Cadmium sulfide (CdS)-decorated zinc oxide (ZnO) nanorod heterostructures have been grown by a combination of hydrothermal and pulsed laser deposition techniques. Hybrid photovoltaic devices have been fabricated with CdS modified and unmodified ZnO nanorods blended separately with regioregular poly(3-hexylthiophene) (P3HT) polymer as the active layer. The solar cell performance has been studied as a function of ZnO concentration and the casting solvent (chlorobenzene, chloroform, and toluene) in the unmodified ZnO:P3HT devices. The power conversion efficiency is found to be enhanced with the increase of ZnO concentration up to a certain limit, and decreases at a very high concentration. The surface modification of ZnO nanorods with CdS leads to an increase in the open circuit voltage and short-circuit current, with enhanced efficiency by 300% over the unmodified ZnO:P3HT device, because of the cascaded band structure favoring charge transfer to the external circuit.

Transparent and flexible resistive switching memory devices with a very high ON/OFF ratio using gold nanoparticles embedded in a silk protein matrix

The growing demand for biomaterials for electrical and optical devices is motivated by the need to make building blocks for the next generation of printable bio-electronic devices. In this study, transparent and flexible resistive memory devices with a very high ON/OFF ratio incorporating gold nanoparticles into the Bombyx mori silk protein fibroin biopolymer are demonstrated. The novel electronic memory effect is based on filamentary switching, which leads to the occurrence of bistable states with an ON=OFF ratio larger than six orders of magnitude. The mechanism of this process is attributed to the formation of conductive filaments through silk fibroin and gold nanoparticles in the nanocomposite. The proposed hybrid bio-inorganic devices show promise for use in future flexible and transparent nanoelectronic systems.

Characteristics of CdS nanowires grown in a porous alumina template using a two-cell method

CdS nanowires have been grown by a two-cell chemical method using a porous alumina template. Field-emission scanning and transmission electron microscopy show the growth of nanowires of diameters ranging from 100 to 110 nm and 50 to 60 nm for two different templates. The surface topography of the template filled with CdS has been studied using atomic force microscopy. The microstructural properties of the nanowires have been studied by high resolution x-ray diffraction. UV–visible spectroscopy of nanowires exhibits a clear blue shift due to quantum confinement. The photoluminescence spectra show green emission due to the defect states of the nanowires and the resonance Raman spectra revealed the stoichiometric phase of CdS with good crystallinity.

Enhanced broadband photoresponse of Ge/CdS nanowire radial heterostructures

CdS/Ge nanowire heterojunction has been grown by chemical deposition of CdS on Ge nanowire templates. Transmission electron micrographs show the growth of core-shell Ge/CdS nanowire radial heterostructures. Raman spectra reveal the confinement of phonons in nanocrystalline CdS shell grown on Ge nanowires. A diodelike behavior in I-V characteristics of Ge/CdS heterojunction is observed due to the formation of rectifying junction between CdS shells and Ge nanowire cores. An improved photocurrent spectrum of Ge/CdS heterojunction nanowires with broadband response from visible to near-IR region is demonstrated.

Multifunctional white-light-emitting metal-organic gels with a sensing ability of nitrobenzene.

In this study, three novel luminescent nanofibrous metal-organic gels (MOGs) have been synthesized by the reaction of 1,3,5-tris(3-pyridylmethoxyl)benzene (L) with chloride salts of Cd(II), Hg(II), and Cu(II). The metal-ligand coordination, intermolecular π-π stacking and several other weak interactions found to play an important role in the formation of nanofibrous materials. The gel materials are characterized by rheology, diffuse reflectance spectra and various microscopic techniques such as TEM, FESEM, and AFM. The gels MOG-1 and MOG-2 were found to exhibit significant white photoluminescence, whereas the MOG-3 exhbits green emission upon excitation at 325 nm. Furthermore, the MOG-1 has shown its application as a chemosensor for the remarkable detection of nitroaromatics such as nitrobenzene (NB), 2,4-dinitrophenol (DNP). The significant quenching response for NB and DNP is attributed to the strong charge-transfer interactions between the electron-deficient aromatic ring of NB and the electron rich aromatic group of L in MOG-1. The crystal structure of Cd(II) complex of L reveals the formation one-dimensional network which contains strong π-π interactions within and between the networks and these strong π-π interactions generate the free charge carrier in all these nanofibrous gels.

Photoelectrochemical and photosensing behaviors of hydrothermally grown ZnO nanorods

ZnO nanorods have been grown on indium-tin-oxide coated glass substrates by a low cost chemical process. Current-voltage characteristics have been studied using ZnO nanorods as photoanode in an electrochemical cell. The flat band voltage shift and depletion width of ZnO nanorods/electrolyte interface have been estimated from Mott-Schottky (MS) characteristics. The electrochemical impedance measurements have been carried out to study the charge transport mechanism at the semiconductor-electrolyte interface under dark and white light (100 mW/cm2) illumination. The doping concentration of nanorods has been extracted from MS plot. Photoresponse behavior of ZnO nanorods is found to be enhanced than seed layers with the incident of white light. Spectral dependent photovoltage of ZnO nanorods has been carried out using monochromatic light of wavelength 250–600 nm. The photopotential recovery time has been estimated for nanorods and seed layers. The stability of ZnO nanorods as a photoanode has been investigated.

Dielectric and transport properties of carbon nanotube-CdS nanostructures embedded in polyvinyl alcohol matrix

Multiwalled carbon nanotube-CdS/polyvinyl alcohol (MWCNT-CdS/PVA) composites have been grown by a simple chemical process on one-dimensional templates. The plane-view transmission electron micrographs clearly indicate the formation of nanocrystalline CdS on the nanotube surfaces. The superior dielectric behavior of the MWCNT-CdS nanostructures over MWCNT and PVA host matrices has been demonstrated. The dc and ac transport properties of CdS carbon nanotube-insulating polymer nanocomposites have been studied using impedance spectroscopy. An enhancement in optical band gap of nanocomposites over the bulk CdS has been observed due to the quantum confinement effect in CdS nanocrystals.

Optical and dielectric properties of junctionlike CdS nanocomposites embedded in polymer matrix

CdS nanocomposites have been grown in a polymer (polyvinyl alcohol) matrix using a simple chemical bath deposition process. Transmission electron micrographs of nanocomposites grown at different solution temperatures revealed the formation of isolated as well as junctionlike structures. X-ray and selected area electron diffraction patterns show that the nanocomposites are polycrystalline with cubic CdS phase. Optical band gaps of nanocomposite films are found to decrease (3.26–2.86eV) with the increase in bath temperature from 70to90°C. Photoluminescence spectra show strong green emission attributed to the Cd2+ or Cd+ ion-related recombination via moderately deep trap states. The nanocomposites show an enormous enhancement of dielectric constant in polyvinyl alcohol matrix over a frequency range of 40Hz–10MHz.

Memory effect in a junction-like CdS nanocomposite/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylene-vinylene] heterostructure

The operation of a nonvolatile memory device is demonstrated using junction-like CdS nanocomposites embedded in a polymer matrix. The capacitance-voltage characteristics of Al/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]/CdS nanocomposites in a polyvinyl alcohol matrix/indium tin oxide device exhibit hysteresis, which is attributed to the trapping, storage, and emission of holes in the quantized valence band energy levels of isolated CdS nanoneedles. The characteristics at different operating frequencies show that the hysteresis is due to trapping of charge carriers in CdS nanocomposites rather than in the interfacial states. The memory behavior in the inorganic/organic heterostructure is explained on the basis of a simple energy band diagram.

Efficient control of ultrafast optical nonlinearity of reduced graphene oxide by infrared reduction

Simultaneous occurrence of saturable absorption nonlinearity and two-photon absorption nonlinearity in the same medium is well sought for the devices like optical limiter and laser mode-locker. Pristine graphene sheet consisting entirely of sp2-hybridized carbon atoms has already been identified having large optical nonlinearity. However, graphene oxide (GO), a precursor of graphene having both sp2 and sp3-hybridized carbon atom, is increasingly attracting cross-discipline researchers for its controllable properties by reduction of oxygen containing groups. In this work, GO has been prepared by modified Hummers method, and it has been further reduced by infrared (IR) radiation. Characterization of reduced graphene oxide (RGO) by means of Raman spectroscopy, X-ray photoelectron spectroscopy, and UV-Visible absorption measurements confirms an efficient reduction with infrared radiation. Here, we report precise control of non-linear optical properties of RGO in femtosecond regime with increased degrees of IR reduction measured by open aperture z-scan technique. Depending on the intensity, both saturable absorption and two-photon absorption effects are found to contribute to the non-linearity of all the samples. Saturation dominates at low intensity (∼127 GW/cm2) while two-photon absorption becomes prominent at higher intensities (from 217 GW/cm2 to 302 GW/cm2). The values of two-photon absorption co-efficient (∼0.0022–0.0037 cm/GW for GO, and ∼0.0128–0.0143 cm/GW for RGO) and the saturation intensity (∼57 GW/cm2 for GO, and ∼194 GW/cm2 for RGO) increase with increasing reduction, indicating GO and RGO as novel tunable photonic devices. We have also explained the reason of tunable nonlinear optical properties by using amorphous carbon model.