Puspen Mondal

Photoluminescence study of β-Ga2O3 nanostructures annealed in different environments

β-Ga2O3 nanostructures (nanowires, nanoribbons, and nanosheets) were synthesized via vapor transport method on gold coated silicon substrate in N2 ambient and these β-Ga2O3 nanostructures grown on silicon substrates were taken as the starting material to study the effect of annealing in the different environments (oxygen, water vapour, and ammonia solution) on the structural front and photoluminescence (PL) properties. The PL spectra of β-Ga2O3 nanostructures exhibit a UV-blue emission band whose intensity is strongly affected by the annealing in different environments. Annealing modifies the surface of the nanostructures by creating surface states which quench the PL by creating competitive nonradiative paths. This study also indicates the dominance of the formation of water induced surface states over ammonia induced surface states. The irreversible nature of these defects significantly affects the applicability of this system in moist high temperature environments.

Superconducting properties of nanocrystalline Nb3Al in Nb–Al matrix

We present the results of x-ray diffraction and magnetization measurements on the nanocrystalline as-cast compound Nb3Al. X-ray diffraction shows the presence of the A15 Nb3Al phase embedded in a matrix of a bcc Nb–Al solid solution. The average grain size of Nb3Al estimated from the line broadening is about 35nm. Magnetization measurements give a lower limit of the superconducting transition temperature to be about 16.8K, which is amongst the highest known TC for the as-cast sample of Nb3Al. In the superconducting state, the sample shows anomalous thermomagnetic history effects in the temperature and field variation of magnetization.

Vortex Matter in Highly Strained Nb_{75}Zr_{25}: Analogy with Viscous Flow of Disordered Solids

We present the results of magnetization and magneto-transport measurements in the superconducting state of an as-cast Nb75Zr25 alloy. We also report the careful investigation of the microstructure of our sample at various length scales by using optical, scanning electron and transmission electron microscopies. The information of microstructure is used to understand the flux pinning properties in the superconducting state within the framework of collective pinning. The magneto-transport measurements show a non-Arrhenius behaviour of the temperature and field dependent resistivity in the flux flow region. This non-Arrhenius behaviour is understood in terms of a model, which was originally proposed for viscous flow of disordered solids and is popularly known in the literature as the ‘shoving’ model. The activation energy for flux flow is obtained from magneto-transport measurements and is assumed to be mainly the elastic energy stored in the flux-line lattice. The critical current density estimated from magnetization measurements is moderately high of the order of 108 Am−2 at a temperature of 2K. The scaling of pinning force density with respect to reduced field indicates the presence of two pinning mechanisms of different origins. The elastic constants of the flux-line lattice are estimated from magnetization measurements and are used to estimate the length scale of vortex lattice movement, or the volume displaced by the flux-line lattice, during flux flow. It appears that the vortex lattice displacement estimated from elastic energy considerations is of the same order of magnitude as that of the flux-bundle hopping length when a finite resistance appears during flux flow. Our results could provide possible directions for establishing a framework where vortex matter and glass forming liquids or amorphous solids can be treated in a similar manner for understanding the phenomenon of viscous flow in disordered solids or more generally the pinning and depinning properties of elastic manifolds in random media. PACS numbers: 74.25.Uv, 74.25.Wx

A study of growth and thermal dewetting behavior of ultra-thin gold films using transmission electron microscopy

The growth and solid-state dewetting behavior of Au thin films (0.7 to 8.4 nm) deposited on the formvar film (substrate) by sputtering technique have been studied using transmission electron microscopy. The size and number density of the Au nanoparticles (NPs) change with an increase in the film thickness (0.7 to 2.8 nm). Nearly spherical Au NPs are obtained for 6 nm show capability to be used as a...

Effect of isochronal annealing on phase transformation studies of iron oxide nanoparticles

The effect of isochronal annealing on the phase transformation in iron oxide nanoparticles is reported in this work. Iron oxide nanoparticles were successfully synthesized using an ash supported technique followed by annealing for 2 h at various temperatures between 300 and 700°C. It was observed using X-ray diffraction (XRD) and transmission electron microscopy (TEM) that as-grown samples have mixed phases of crystalline haematite (α-Fe2O3) and a minor phase of either maghemite (γ-Fe2O3) or magnetite (Fe3O4). On annealing, the minor phase transforms gradually to haematite. The phase transformation is complete at annealing temperature of 442°C as confirmed by differential scanning calorimetric (DSC) analysis. The unresolved phases in XRD were further analysed and confirmed to be maghemite from the X-ray absorption near edge structure (XANES) studies. The magnetic measurements showed that at room temperature nano- α-Fe2O3 is weak ferromagnetic, and its magnetization is larger than the bulk value. The mixed phase sample shows higher value of magnetization because of the presence of ferromagnetic γ-Fe2O3 phase.

Fabrication of tungsten Fresnel zone plates for hard x-rays using wet etching

A novel and scalable method to fabricate tungsten Fresnel zone plate (FZP) structures with a high aspect ratio using the wet etching method is presented. FZPs with an outermost zone width of 370 nm and a height of 1.1 μm were fabricated in tungsten on a 35 μm polyimide film. They were designed for a focal length of 50 cm at 8 keV such that the radius is 100 μm for 142 zones, with an opaque center zone. The fabrication method presented is a simple single step resist process. The testing of FZP was carried out at the Indus-2 synchrotron source, where focusing was observed as per design parameters. The transmission characteristics of polyimide coupled with the phase shifting properties of tungsten make the FZP useful also at lower energies. An improvement in the zone plate structural parameters and hence its performance has also been discussed.

Phase transformation of [Co/Ti]x10 multilayer under swift heavy ion irradiation

Swift heavy ions induced structural and magnetic modifications of Co/Ti multilayers have been studied with 120 MeV Ag9+ ions at different ion fluences. The decrease in 1st Bragg peak intensity of X-ray reflectivity spectra confirms an increase in Co-Ti mixing at the interfaces, whereas X-ray diffraction (XRD) measurements reveal amorphization of the crystalline Co layer. XRD also indicates a shift in peak position towards a lower angle, confirming the development of tensile stress upon irradiation whereas an increase in the peak width suggests the reduction in the grain size. Vibrating sample magnetometer measurements show a reduction in Mr/Ms due to Co-Ti interface mixing and formed bcc-Co3Ti metastable phase. The X-ray absorption fine structure (XAFS) technique has been utilized to obtain variation in Co-Co and Co-Ti bond distances as a function of ion fluences. Quantitative estimation of Co3Ti phase generated due to ion irradiation has also been obtained using Co K-edge XAFS fitting.

Effect of intercropping on the Helicoverpa armigera (Hubner) incidence and yield in chickpea

Field experiment was conducted during rabi 2007–08 and 2008–09 at the Rama Krishna Mission Divyayan KVK, Ranchi to assess the performance of intercrops against gram pod borer, Helicoverpa armigera (Hubner). The lowest incidence was recorded when chickpea was intercropped with linseed at the ratio of 2:2 (6.02 larvae/m row length) followed by chickpea + mustard at 2:2 (6.45 larvae/m row length). The equivalent yield of chickpea was highest in intercropping with coriander at 2:2 ratio (14.19 q/ha) followed by chickpea+ coriander (2:1) (12.28 q/ha), and chickpea+ lentil at 2:2 ratio (10.92 q/ha).

Fabrication of high aspect ratio submicron gratings on ∼100nm titanium membranes using electron beam lithography

We have developed a new technique for fabrication of 1:1 submicron gratings by means of single step electron beam lithography coupled with thin film deposition. The edge profile and the width ratio (line-to-groove) are easily controlled. Metal like gold or copper transmission gratings can be produced by an additional lift-off process. A 30kV electron beam was used to generate 500nm lines on standalone ∼2.3μm thick PMMA/Titanium. The dimensions of fabricated features together with their surface morphology and profiles were investigated by optical microscope and SEM. Low current slow writing coupled with special development is optimized to write structures smaller than the thickness of the resist membrane.

Experimental realization of Talbot array illumination for a 2-dimensional phase grating

We present the result of a multiple phase level Talbot array illuminator (TAI) based on two dimensional phase grating. The grating was fabricated on glass by a Deep Reactive Ion Etching (DRIE) system with optimized gas combination recipe. The opening ratio of the grating is ∼0.71 with a phase step of 11π. The experimental study on Talbot array illuminations (TAIs) of this fabricated device in near-field region was carried out using a He-Ne laser. The TAIs were observed at the positions 2.81 mm, 8.59 mm, 14.28 mm, and 20.57 mm along the light propagation direction. The computer simulations of wave field distribution in near field region at different fractional Talbot distances have been generated. These simulated results are in reasonably good agreement with our experimental observed results. Our analysis on Talbot array illumination (TAIs) shows that the multiple harmonic phase modulations are transformed into intensity modulations at 1/8, 3/8, 5/8, and 7/8 fractional Talbot position. We have observed ...