Sachindra Nath Das

ZnO single nanowire-based UV detectors

In this report, ZnO single nanowire (NW)-based devices were fabricated on the same nanowire by e-beam lithography so that both sides had Ohmic contact and one side had Schottky contact. Information about the mechanism for low-power UV detection by these devices was unambiguously provided by I-V measurements. Adsorption and desorption of oxygen molecules at the NW surface are responsible for the UV detection by the device with Ohmic contacts on both sides. Barrier height modulations and interface states are responsible for UV detection by the device with Schottky contact on one side.

Biomimetic hierarchical ZnO structure with superhydrophobic and antireflective properties

A two step method, with a combination of top-down and bottom-up approaches, was developed for the fabrication of ZnO based hierarchical structures with nanorods on microcraters. A layer of well c-axis aligned, transparent, conductive ZnO thin film was deposited by pulsed DC sputtering on a Corning glass substrate. The microcraters were created with anisotropic etching on the as-deposited ZnO thin film. ZnO nanorods were then synthesized onto the etched film by means of metal organic chemical vapor deposition. The resulting hierarchical film exhibits a high water contact angle (>160 degrees) with a low contact angle hysteresis (2 degrees) and low reflection over a wide spectral range. This biomimetic material may find potential applications in many industrial fields, e.g., self-cleaning, solar cells, displays.

Junction properties of Au/ZnO single nanowire Schottky diode

In this study, we have analyzed the Au/ZnO single nanowire based Schottky diode by investigating temperature dependent current voltage and x-ray photoelectron spectroscopy (XPS) measurements. The calculated barrier height of the Schottky diodes by using the thermionic emission model is in good agreement with the value obtained from the XPS measurements but lower than the theoretically predicted value. The ionization of interface states has been considered for explaining this discrepancy.

Programmable Direct-Printing Nanowire Electronic Components

In order for recently developed advanced nanowire (NW) devices(1-5) to be produced on a large scale, high integration of the separately fabricated nanoscale devices into intentionally organized systems is indispensible. We suggest a unique fabrication route for semiconductor NW electronics. This route provides a high yield and a large degree of freedom positioning the device on the substrate. Hence, we can achieve not only a uniform performance of Si NW devices with high fabrication yields, suppressing device-to-device variation, but also programmable integration of the NWs. Here, keeping pace with recent progress of direct-writing circuitry,(6-8) we show the flexibility of our approach through the individual integrating, along with the three predesigned N-shaped sites. On each predesigned site, nine bottom gate p-type Si NW field-effect transistors classified according to their on-current level are programmably integrated.

Controllable resistance switching behavior of NiO/SiO2 double layers for nonvolatile memory applications

Resistive switching characteristics of the double layer (NiO/SiO2) were studied for possible nonvolatile memory applications. The effect of SiO2 thickness variation in the memory device was investigated. A repeatable resistance switching behavior was observed with on/off ratio 105. The operation voltage of the device depended on the thickness of SiO2 layer and it increases with increasing SiO2 thickness. High-resolution transmission electron microscopy analyses revealed that the formation/rapture of Ni filament like percolation path inside SiO2 layer is responsible for the current transport mechanism.

Influence of surface morphology on the optical property of vertically aligned ZnO nanorods

In this letter we have studied an influence of surface morphology on the optical property of vertically aligned ZnO nanorods. At low temperature the near band edge excitonic emission shows a strong dependence on surface morphology. A prominent and well resolved near band edge photoluminescence (PL) peak was obtained for nanowires with decreasing diameter and thus assigned due to the contributions to the optical properties of individual nanorods. Depending on surface morphology, the difference in low temperature PL property is attributed to the tailing of the density of states due to the potential fluctuations in randomly distributed intrinsic defects.

SURFACE MODIFICATION OF HYDROTHERMALLY GROWN ZnO NANOSTRUCTURES WITH PROCESS PARAMETERS

ZnO nanorods (NRs) were hydrothermally synthesized by using equimolar zinc nitrate hydrate (Zn(NO3)2 ⋅ 6H2O) and hexamethylenetetramine (C6H12N4) solutions. The shape of the nanostructures, obtained by aqueous method, was greatly influenced by the growth temperature and the molar concentrations. NRs grown at higher temperature (90°C) have rounded tips, whereas nanostructures of hexagonal flat-end shape were obtained at 75°C. Hardly any nanostructures were observed by further reducing the temperature to 60°C. In addition, solutions with higher molarity favored the appearance of nanoflowers. Scattered ZnO NRs were observed on silicon substrate, whereas aligned ZnO nanowires (NWs) 50–70 nm in diameter were obtained at 75°C by introducing sputtered ZnO film as a seed layer. High-resolution transmission electron microscopy (HRTEM) confirmed the growth of ZnO nanowires along [001] direction. A band-edge luminescence along with a broad visible spectrum was observed for the ZnO nanowires.

Synthesis of ZnO Nanowire by MOCVD Technique: Effect of Substrate and Growth Parameter

ZnO nanostructures have been studied extensively in the past few years due to their funda‐ mental and technological importance [1-5]. In particular, it is a wide-direct-band gap (~3.37 eV), II–VI semiconductor with many potential applications such as nanolaser arrays [2,6], gas sensors [7-10], field emission devices [11-12] and luminescent materials [13-16]. A great deal of attention has been focused on one-dimensional nanostructures because of their supe‐ rior properties such as high surface-to-volume ratio, high crystalline quality, and quantum confinement effects, which are suitable for various electronics and optoelectronics applica‐ tions. Up to now, many ZnO configurations have been reported such as nanobelts, nano‐ wires [10–11], nanoneedles [3], nanotetrapods [9], nanocombs [17] and so on. It has also been suggested that ZnO nanostructures are probably the most abundant forms of any known materials. Therefore, proper control of processing parameters is essential to reproduce the desired nanostructures. Synthesis of aligned one-dimensional ZnO nanowire arrays has at‐ tracted much attention, which includes the techniques of metal-organic chemical vapour deposition (MOCVD), vapor-liquid-solid (VLS), pulsed laser deposition method (PLD) and solution based chemical technique [21-24]. A solution-based technique is an attractive ap‐ proach for the growth of ZnO nanostructures because of its simplicity and low cost [25, 26]. In addition, this method allows large-scale growth of aligned ZnO nanowires on an arbitra‐ ry substrate, which promotes the integration of ZnO nanomaterials for various applications. But the main drawback of chemical process is the poor quality of nanowire. On the other hand, physical process are of particular interest since they have many advantages such as the ability to fabricate nanostructures of better quality, well controllable configurations, and good reproducibility. From a physical point of view, the synthesis methods for one-dimen‐

Self-regulating pseudo-monolayer printing of percolating networks of ZnO nanostructures for macroelectronics

A patterned dry transfer printing technique that can generate monolayer-like percolating networks of ZnO nanorods (NRs) has been developed. The method relies on the relative adhesion strength between NR–NR and NR–substrate, as well as soft and elastomeric nature of polydimethylsiloxane (PDMS) stamp material. When the NR–substrate adhesion is stronger than the rod–rod interaction, which is the usual case due to a large difference in the contact area, the printing leads to a monolayer-like percolating network of NRs on substrate. The method exploits the contact area difference between NR–NR and NR–substrate, which is inherent in the systems involving high aspect ratio nanostructures on a soft stamp, without considering the complex and elaborate tailoring of the surface chemistry or energetics. When the stamp has multilayer stacks of nanostructures, the monolayer-like printing can be repeated many times, possibly on a large area substrate, due to the self-regulating printing characteristics. The printed percolating network of semiconductor nanostructures have been used as active channels in thin film transistors, where the better gate coupling due to the pseudo-monolayer leads to higher-performance devices compared to other configurations of nanostructures. This self-regulating, patterned dry transfer printing method may enable high-performance macroelectronics with various functional nanostructured materials that have high aspect ratios.

Ethanolic leaf extract of Coccinia grandis is effective against both drug resistant and drug sensitive clinical isolates of Indian Kala-azar

The emergence of resistance to the current available drugs used for treatment against Indian Kala-azar (KA) or Visceral Leishmaniasis makes the control strategy inadequate for the disease. This grave epidemiological situation directed researches towards alternative treatments including herbal therapy. In this background, the aim of the present study was to evaluate the antileishmanial activity of the leaves of Coccinia grandis (a tropical vine) against both the Sodium Stibo Gluconate (SSG) sensitive and resistant as well as Miltefosine (MIL) sensitive and resistant field isolates of Leishmania donovani. The cytotoxicity effect of ethanolic extract of leaves of C. grandis (Cg-LE) against the clinical isolates of L. donovani was checked both in promastigotes and intracellular amastigotes stages. In both sensitive and resistant promastigotes, Cg-LE stimulated reactive oxygen species generation and apoptosis. Parasites infected macrophages showing enhanced nitric oxide production after Cg-LE treatment suggested the leishmanicidal activity of the leaf extract. Furthermore, Cg-LE treatment led to mitochondrial membrane damage and DNA fragmentation in promastigotes. The present study is very encouraging for the fact that Cg-LE showed promising antileishmanial activity against both SSG and MIL drug resistant clinical isolates of Indian KA.