Subhasis Ghosh

Single-Electron Transfer Driven Cyanide Sensing: A New Multimodal Approach

A new SET-driven reaction-based strategy is reported for sensing of cyanide with indicators having low LUMO levels. The cyanide-specific reaction produces an air-stable radical anion marker and by virtue of its spin, charge, and the SOMO-LUMO-based electronic transition generates multimodal signal outputs. High selectivity and sensitivity (0.2-16 microM) were observed when compared to other reducing anions. This new indicator system exhibits regenerability and dip-stick sensing, and fabrication of an electronic sensing device for cyanide is demonstrated.

Direct current electrical characterization of ds-DNA in nanogap junctions

Measurements of DNA conductivity, hybridization, and melting using electronic means can have wide applications in molecular electronics and biological sensors. We have fabricated nanogap break-junctions by electromigration through thin gold-on-titanium films. 18-mer thiolated ds-DNA molecules were covalently attached between the electrodes and dc electrical measurements were done. The conductance was measured through the molecule before and after a temperature ramp from 300 to 400 K. A dramatic decrease in conductance was observed, analogous to an electrical fuse, possibly attributed to complete or partial denaturing of the ds-DNA molecules bridging the nanogaps. We also show evidence that the dc resistance of dry DNA strands of the same length decreases with increasing guanine-cytosine content in the sequence with values ranging from 10 M Ω to 2 G Ω. These findings can have important consequences in DNA-based molecular electronics and direct label-free detection of DNA hybridization.

Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis

Band gap engineering offers tunable optical and electronic properties of semiconductors in the development of efficient photovoltaic cells and photocatalysts. Our study demonstrates the band gap engineering of ZnO nanorods to develop a highly efficient visible-light photocatalyst. We engineered the band gap of ZnO nanorods by introducing the core/shell geometry with Ag2S sensitizer as the shell. Introduction of the core/shell geometry evinces great promise for expanding the light-harvesting range and substantial suppression of charge carrier recombination, which are of supreme importance in the realm of photocatalysis. To unveil the superiority of Ag2S as a sensitizer in engineering the band gap of ZnO in comparison to the Cd-based sensitizers, we also designed ZnO/CdS core/shell nanostructures having the same shell thickness. The photocatalytic performance of the resultant core/shell nanostructures toward methylene blue (MB) dye degradation has been studied. The results imply that the ZnO/Ag2S core/shell nanostructures reveal 40- and 2-fold enhancement in degradation constant in comparison to the pure ZnO and ZnO/CdS core/shell nanostructures, respectively. This high efficiency is elucidated in terms of (i) efficient light harvesting owing to the incorporation of Ag2S and (ii) smaller conduction band offset between ZnO and Ag2S, promoting more efficient charge separation at the core/shell interface. A credible photodegradation mechanism for the MB dye deploying ZnO/Ag2S core/shell nanostructures is proposed from the analysis of involved active species such as hydroxyl radicals (OH(•)), electrons (e(-)(CB)), holes (h(+)(VB)), and superoxide radical anions (O2(•-)) in the photodegradation process utilizing various active species scavengers and EPR spectroscopy. The findings show that the MB oxidation is directed mainly by the assistance of hydroxyl radicals (OH(•)). The results presented here provide new insights for developing band gap engineered semiconductor nanostructures for energy-harvesting applications and demonstrate Ag2S to be a potential sensitizer to supersede Cd-based sensitizers for eco-friendly applications.

Schottky energy barrier and charge injection in metal/ copper-phthalocyanine/metal structures

We present experimental results on current injection from different metal electrodes into copper–phthalocyanine (Cu–Pc). The current–voltage (J–V) characteristics and current injected at the contact are investigated as a function of Schottky energy barrier, thickness of organic semiconductor, and temperature. These results are interpreted using a consistent description of J–V characteristics through the injection limited current in the case of high Schottky energy barriers and space charge limited current in the case of low Schottky energy barrier.

Zinc oxalate nanorods: a convenient precursor to uniform nanoparticles of ZnO

Nanorods of zinc oxalate dihydrate have been synthesized using the reverse micellar route. These nanorods were decomposed at 450 °C in air to obtain nanoparticles of zinc oxide. Transmission electron microscopy shows the nanorods to be 120 nm in diameter and 600 nm in length. The ZnO nanoparticles are 55 nm in diameter. The photoluminescence studies show two peaks at 370 and 403 nm which can be ascribed to free excitonic transition and donor-acceptor pair transition respectively. The temperature dependent PL intensity shows an anomalous non-monotonous temperature dependence probably due to two different optical processes.

Electric-field-induced conductance transition in 8-hydroxyquinoline aluminum (Alq3)

We report an electric-field-induced conductance transition from an insulating state to a conducting state in a thin layer of Alq3 sandwiched between two metal electrodes. Field-induced switching behavior with a high on-off ratio of ∼105 is observed in the devices, in which the cathode electrode is Al and the anode electrode is varied including Al, Au, and indium tin oxide. The switching behavior is absent in devices in which both electrodes have a high work function, indicating that efficient electron injection has an important role in the electric-field-induced switching behavior of Alq3-based single-layer devices.

Low field electron mobility in GaN

Temperature and doping dependencies of electron mobility in GaN have been calculated using an iterative technique. The following scattering mechanisms, i.e., impurity, polar optical phonon, acoustic phonon, piezoelectric, and electron plasmon are included in the calculation. Ionized impurity scattering has been treated beyond the Born approximation using a phase-shift analysis. The compensation ratio is used as a parameter with a realistic charge neutrality condition. Comparisons with experimental data confirm the present calculation over a wide range of temperatures and electron concentrations. Mobility and electron concentration data from Hall measurements reveal a degenerate layer at the GaN-substrate interface. This degenerate layer affects the bulk mobility and electron concentration and needs to be accounted for in order to extract reliable experimental values of the bulk electron mobility.

Capacitance-voltage characteristics of organic Schottky diode with and without deep traps

Capacitance based spectroscopic techniques have been used to characterize defects in organic Schottky diode based on copper phthalocyanine. Deep traps in organic thin films introduced by varying growth conditions have been identified and characterized by voltage and temperature dependence of capacitance. These results are interpreted using a consistent modelling of capacitance of organic Schottky diode with and without deep traps.

Thickness dependence of space charge limited current and injection limited current in organic molecular semiconductors

We report the experimental investigations on space charge limited current (SCLC) and injection limited current (ILC) in copper phthalocyanine (CuPc), sandwiched between two metal electrodes. Thickness dependence of current-voltage characteristics of SCLC and ILC is accurately reproduced by the electric field and temperature dependent charge carrier mobility, without invoking charge density dependent mobility. These results are interpreted using a consistent description of SCLC and ILC, based on a unified model of hopping transport within Gaussian density of states in CuPc.

Nanometer scale electrode separation (nanogap) using electromigration at room temperature

Pairs of electrodes with nanometer separation (nanogap) are achieved through an electromigration-induced break-junction (EIBJ) technique at room temperature. Lithographically defined gold (Au) wires are formed by e-beam evaporation over oxide-coated silicon substrates silanized with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and then subjected to electromigration at room temperature to create a nanometer scale gap between the two newly formed Au electrodes. The MPTMS is an efficient adhesive monolayer between SiO/sub 2/ and Au. Although the Au wires are initially 2 /spl mu/m wide, gaps with length /spl sim/1 nm and width /spl sim/5 nm are observed after breaking and imaging through a field effect scanning electron microscope. This technique eliminates the presence of any residual metal interlink in the adhesion layer (chromium or titanium for Au deposition over SiO/sub 2/) after breaking the gold wire, and it is much easier to implement than the commonly used low-temperature EIBJ technique which needs to be executed at 4.2 K. Metal-molecule-metal structures with symmetrical metal-molecule contacts at both ends of the molecule are fabricated by forming a self-assembled monolayer of -dithiol molecules between the EIBJ-created Au electrodes with nanometer separation. Electrical conduction through single molecules of 1,4-Benzenedimethanethiol (XYL) is tested using the Au/XYL/Au structure with chemisorbed gold-sulfur coupling at both contacts.