Avijit Ghosh

Phonon assisted photoluminescence and surface optical mode of Zn embedded ZnO nanostructure

Zn embedded ZnO nanostructure thin films were prepared using a cost-effective novel vacuum-carbon-arc combined with the vacuum annealing technique. The photoluminescence spectra of the samples exhibit a strong near-band-edge emission around 377 nm and a weak defect band in the visible range accompanied by some multiphonon peaks at a higher energy. The shift in the optical absorption band arises due to potential fluctuation of the carrier density. Surface optical phonon mode predominant Raman spectra are observed, which is attributed to the surface potential (due to defects) present in the system. The predicted surface phonon frequency based on the dielectric continuum approach is in good agreement with that observed experimentally.

Microstructural aspects for defect emission and E2high phonon mode of ZnO thin films

The ZnO thin films were prepared using a cost-effective vacuum-carbon-arc technique combined with thermal oxidation under ambient conditions from zinc nanoparticles in carbon matrix. The microstructural analysis of grazing incidence x-ray diffraction pattern using Rietveld method was investigated systematically under the influence of different annealing temperature. Photoluminescence spectroscopic properties of the samples studied at room temperature exhibits the dependence of violet emission accompanying with blue and green emission on oxidation temperature. The origin of defect emission mechanism through the microstructure of ZnO thin films was analyzed. The anharmonic properties of the upper E2 phonon mode were investigated using Raman scattering spectroscopy at room temperature. The downshift of phonon frequency and linewidth broadening of high frequency E2 phonon mode were analyzed through defect state according to annealed conditions.

Compatibility study of thin passivation layers with hydrazine for silicon-based MEMS microthruster

In this work, the compatibility studies of silicon and its different multilayer structures with hydrazine for possible applications to MEMS have been reported. Grazing incidence x-ray diffraction patterns of the r.f. sputtered Si/SiO2/Si3N4 stack layer show preferably oriented crystalline structure after hydrazine treatment at different temperatures. The Fourier transform infrared spectroscopic measurement reveals that local bonding of the constituent atoms of the surface layers, where Si–O bond is replaced partially by Si–N bond while treated. Further, the surface morphology carried out by atomic force microscopy exhibits the tendency of reducing surface roughness with the increase in temperature during hydrazine treatment. From the axisymmetric drop shape analysis (ADSA), it is observed that static contact angle changes slightly for different wettability nature of solid surface due to aggregation of crystallites in the valley of the surface fluctuation and anisotropic modification in preferred orientation of the film surface. On the basis of equation of state theory with approximation of solid surface–liquid, interfacial energy was applied to determine the solid surface free energy providing the limited variation in different stack layers. Lastly, the J–V characteristic of the stack layer treated by hydrazine at different temperatures shows multiple current conduction regions with the same current density for varying electric field. Therefore, among various single or multilayer silicon-based thin film combinations, the Si/SiO2/Si3N4 stack layer is the most promising passivation layer for hydrazine-based MEMS applications.

Optical emission and absorption spectra of Zn–ZnO core-shell nanostructures

The core-shell Zn–ZnO nanostructures were fabricated from Zn-powder embedded in graphite (i.e. carbon matrix) in a thin-films form by an inexpensive vacuum arc technique followed by laser ablation. The grazing incidence X-ray diffraction pattern shows that intensity of Zn-peak decreases, and subtle ZnO-peak increasing with the increase in laser power. The high resolution transmission electron microscopic study clearly exhibits the formation of a core-shell nanostructure as fabricated by laser ablation. The emission characteristics of laser ablated (with different powers) samples show a strong exciton peak at 388 nm, and a few more weak peaks (due to weak defect states in the visible range). The optical absorption spectra were obtained from the excitonic peaks (from 344 nm to 317 nm) on decreasing laser power. These peaks occur due to the coupling of exciton absorption (from ZnO shell layer) and core metal interband absorption. The Zn–ZnO core-shell nanostructure is useful for nanophotonic applications.

Effect of Co doping on optical behaviors in ZnO nanorods

The stokes shift in Co-doped ZnO nanorods are investigated through optical emission and absorption spectroscopic study. High resolution x-ray diffraction and transmission electron microscopic study reveal that well crystalline as well as rod-like nanostructures. The red shifts in UV emission due to strong exciton-phonon coupling with enhanced defect emission are employed by photoluminescence spectroscopy. The optical absorption spectroscopy indicates the blue shifting of absorption edge owing to potential fluctuation of carrier density through incorporation of Co in ZnO host. Thus the exhibited a red shift in energy for band-edge emission relative to the absorption edge, could be significant for tailoring the optical behavior of nanocrystals for specific applications by controlling their size and structure.

SAVITR: A System for Real-time Location Extraction from Microblogs during Emergencies

We present SAVITR, a system that leverages the information posted on the Twitter microblogging site to monitor and analyse emergency situations. Given that only a very small percentage of microblogs are geo-tagged, it is essential for such a system to extract locations from the text of the microblogs. We employ natural language processing techniques to infer the locations mentioned in the microblog text, in an unsupervised fashion and display it on a map-based interface. The system is designed for efficient performance, achieving an F-score of 0.81, and is approximately two orders of magnitude faster than other available tools for location extraction.

Growth and microstructure for visible emission and surface optical phonon mode of Zn–ZnO nanostructure

The Zn–ZnO nanostructured thin films were prepared in carbon matrix using a cost-effective vacuum-carbon arc method. On increasing graphitization with ZnO, the grazing incidence X-ray diffraction pattern showed that the intensity of the ZnO peak increases, whereas that of the Zn peak decreases. X-ray line profile analysis and transmission electron microscopy were employed to investigate the microstructural evolution of Zn–ZnO nanostructure during vacuum arc processing. A growth mechanism is proposed for the Zn–ZnO nanostructure when reaction with carbon-containing gas inside the reactor wall takes place. Detailed studies of photoluminescence bands clearly exhibit the intensity variation of violet and blue-green bands on increasing graphitization ratio. Using the dielectric continuum approach, surface optical phonon modes of the Zn–ZnO nanostructure were studied for different synthesized samples.