Tamita Rakshit

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.

Enhanced sensitivity and selectivity of brush-like SnO2 nanowire/ZnO nanorod heterostructure based sensors for volatile organic compounds

Brush-like SnO2 nanowires have been grown by pulsed laser deposition on ZnO nanorods synthesized by the hydrothermal method. SnO2 nanowire/ZnO nanorod heterostructures have been used for sensing several volatile organic compounds (VOCs). The heterostructure sensor exhibits higher response compared to that of control ZnO nanorods. The potential barriers formed at the SnO2–ZnO and SnO2–SnO2 interface are proposed to be responsible for an improved sensing performance over the pure ZnO nanorods. The effect of the length of SnO2 nanowires on the performance of triethylamine, toluene, ethanol, acetic acid, acetone, and methanol sensing has been studied. It is found that the response to the VOCs greatly depends on the length of the brush-like SnO2 nanowires. The SnO2/ZnO heterostructures can be successfully used to discriminate acetone from other VOCs.

Nanostructures of Sr2+ doped BiFeO3 multifunctional ceramics with tunable photoluminescence and magnetic properties

Careful tuning of formation (calcination) temperature of Sr(2+) doped BiFeO(3) multiferroic ceramics results in tailorable particle morphologies ranging from spherical to pillar-like. Based on the minimization of Gibbs free energy approach, the dominant homogeneous mechanism for particle growth is suggested. The chemical substitution of a trivalent ion (Bi(3+)) by a divalent ion (Sr(2+)) causes the transformation of certain fraction of Fe(3+) to Fe(4+) and/or the appearance of oxygen vacancies. This has been respectively proved by the analysis of XPS and refinement of neutron diffraction data. Although significant modification in the particle morphology is observed, the crystal unit cell remains rhombohedral with a R3c space group but interesting variations in physical properties are achieved. O-vacancies induced strong and sharp photoluminescence activity in the IR region, similar to ZnO, is reported for the first time. This observation opens up a new application for multiferroic ceramics. SQUID M-H data confirms the straightening of the canted spin structure of BiFeO(3), which in turn results in magnetism similar to ferromagnetic materials. Findings of the magneto-dielectric effect are also discussed to claim the multiferroic nature of the sample.

Surface defects induced ferromagnetism in mechanically milled nanocrystalline ZnO

Bulk ZnO is a diamagnetic material but ferromagnetism (FM) has been observed by several groups in its nanostructures. In order to elucidate the room temperature (RT) FM of ZnO nanostructures, magnetic property of mechanically milled and subsequently annealed nano-ZnO powder has been investigated. Sample that has been milled and then annealed at 200 °C in ambient condition shows highest value of saturation magnetization (Ms), whereas lowest value of Ms has been noticed for the sample pre-annealed at 500 °C before milling. The variation of Ms with annealing temperatures closely resembles with the variation of average positron lifetime (τav) and S-parameter reported earlier for these nano-systems. It has also been found that Ms decreases systematically for increasing average grain size of the ZnO nanoparticles. Room temperature photoluminescence of the as-milled sample shows broad defect related emission centered ∼2.23 eV. Enhancement of such emission has been observed due to 200 °C annealing. Results altoge...

Role of Zn-interstitial defect states on d0 ferromagnetism of mechanically milled ZnO nanoparticles

Ferromagnetism in the nanostructures of undoped oxide semiconductors has become an exciting problem nowadays for its potential future applications in spintronics. In order to elucidate the room temperature d0 ferromagnetism of oxide semiconductors, we have investigated the changes in magnetic property of ZnO nanoparticles with the reduction of size by mechanical milling. We have observed that ferromagnetic ordering appears in the sample when the particle size decreases from 39 ± 1 nm to 30 ± 1 nm. This observation strongly supports the idea of the effect of specific grain boundaries in nanoparticles. The results of Raman scattering also support this observation. From photoluminescence spectra shifted green emissions have been found for ferromagnetic samples. This indicates clearly two different origins for green emissions that are strongly related to the changes in magnetic property. Observations from electron spin resonance spectra suggest that zinc related interstitial defects are significant to give rise to this ferromagnetic coupling. An impurity level formed by the interstitial defects at the surfaces could satisfy the Stoner criteria for the occurrence of band ferromagnetism for these samples.

Effect of SnO2 concentration on the tuning of optical and electrical properties of ZnO-SnO2 composite thin films

ZnO-SnO2 composite thin films have been deposited at 400 °C on glass substrates using targets of different SnO2 content (1 to 40 wt. %) by pulsed laser deposition technique. The structural, optical, and electrical properties of the composite films have been studied as a function of SnO2 content. It is revealed from X-ray diffraction analysis that films are crystalline in nature and the crystallite size decreases from 20–23 nm to 5–7 nm with increase of SnO2 content. X-ray photoelectron spectroscopy analysis indicates that Sn is predominantly doped into the ZnO lattice upto a SnO2 content of 15 wt. % in the composite. For higher concentration, a separate SnO2 phase is segregated in the composite. The band gap energy as well as the electrical conductivity can be tuned by varying the SnO2 content in the composite. Low temperature electrical conductivity measurements show three dominant conduction mechanisms in the temperature range of 20–300 K. At high temperature range of 200–300 K, thermal activation condu...

Growth and low-temperature photoluminescence properties of hybrid ZnO-SnO2 nanobelts

Hybrid ZnO–SnO2 nanobelts were synthesized through a simple thermal evaporation technique without using any catalyst. Detailed microstructural investigation showed that the nanobelts possessed doped/alloyed wurtzite (ZnO) and rutile (SnO2) structures. The diameter and length of the nanobelts were in the ranges 100–200 and 20–40 µm, respectively. Low-temperature photoluminescence properties of the hybrid ZnO–SnO2 nanostructured aggregate revealed a red shift of near-band-edge emission peaks of ZnO with increasing temperature. The method of synthesis offers a convenient and effective technique of producing hybrid ZnO–SnO2 nanobelts for gas sensing in the large quantity.

Growth and optical properties of La 0.7 Sr 0.3 MnO 3 /ZnO heterojunctions

LSMO/ZnO p-n hetero-junctions have been fabricated by pulsed laser deposition on p-Si (100) substrates. Grazing incidence XRD, FESEM, AFM, electrical I–V and photoluminescence study have been carried out to investigate the structural, film thickness, topographical, rectifying, photo-response and photoluminescence properties of the structures. The LSMO/ZnO junction, exhibits rectifying behavior for the heterojunction, shows an photocarrier injection effect illustrating the control over transport properties of LSMO in which electron injection decreases hole concentration following the photoexcitation of ZnO.