Sesha Vempati

Bio: Sesha Vempati is an academic researcher. The author has contributed to research in topics: Surface plasmon & Percolation. The author has an hindex of 2, co-authored 2 publications receiving 21 citations.

Non-universal behavior of leaky surface waves in a one dimensional asymmetric plasmonic grating

Abstract: We report on a non-universal behavior of leaky surface plasmon waves on asymmetric (Si/Au/analyte of different height) 1D grating through numerical modelling. The occurrence of the leaky surface wave was maximized (suppressing the Fabry–Perot cavity mode), which can be identified in a reflection spectrum through characteristic minimum. Beyond a specific analyte height (h), new sets of surface waves emerge, each bearing a unique reflection minimum. Furthermore, all of these minima depicted a red-shift before saturating at higher h values. This saturation is found to be non-universal despite the close association with their origin (being leaky surface waves). This behavior is attributed to the fundamental nature and the origin of the each set. Additionally, all of the surface wave modes co-exit at relatively higher h values.

Non-universal behavior well above the percolation threshold and thermal properties of core-shell-magnetite-polymer fibers

Abstract: Aligned nanocomposite electrospun fibers are investigated for electrical (dc, ac, and dielectric) and thermal properties (10−40 wt.%). This nanocomposite consists of poly(methylmethacrylate) (PMMA) grafted core-shell magnetite (CSM) nanoparticles and PMMA. Electrical properties as a function of tunnel gap (in between the CSM nanoparticles) are studied as the polymer shell stays intact and the “cores” do not touch each other well above the percolation threshold. The depleted improvement in dc conductivity (σdcwt%) with increasing wt.% (the improvement percentage: σdc10 to σdc20 ≈ 100%; σdc20 to σdc30 ≈ 40% and σdc30 to σdc40 ≈ 1.2%) affirms the non-touching “cores” of CSM. Interestingly, the observed ac conductive behavior (0.1−13 MHz) in the high end of the frequency range is in clear contrast to that of a typical percolating system, in fact it does not explicitly follow the universal power law. Some of the obtained critical exponents are not accommodated by the universal theory and significantly differen...

Review of one-dimensional and two-dimensional nanostructured materials for hydrogen generation

Abstract: Hydrogen is an attractive alternative to fossil fuels in terms of environmental and other advantages. Of the various production methods for H2, photocatalysis requires further development so that it can be applied economically on an industrial scale. One- and two-dimensional nanostructures in both pristine and modified forms have shown great potential as catalysts in the generation of H2. We review here recent developments in these nanostructure catalysts and their efficiency in the generation of H2 under UV/visible/simulated solar light. Despite much research effort, many photocatalysts do not yet meet the practical requirements for the generation of H2, such as visible light activity. H2 production is dependent on a variety of parameters and factors. To meet future energy demands, several challenges in H2 production still need to be solved. We address here the factors that influence the efficiency of H2 production and suggest alternatives. The nanostructures are classified based on their morphology and their efficiency is considered with respect to the influencing parameters. We suggest effective ways of engineering catalyst combinations to overcome the current performance barriers.

Enhanced photocatalytic activity of homoassembled ZnO nanostructures on electrospun polymeric nanofibers: A combination of atomic layer deposition and hydrothermal growth

Abstract: We report on the synthesis and photocatalytic activity (PCA) of electrospun poly(acrylonitrile) (PAN) nanofibrous mat decorated with nanoneedles of zinc oxide (ZnO). Apart from a detailed morphological and structural characterization, the PCA has been carefully monitored and the results are discussed elaborately when juxtaposed with the photoluminescence. The present hierarchal homoassembled nanostructures are a combination of two types of ZnO with diverse optical qualities, i.e. (a) controlled deposition of ZnO coating on nanofibers with dominant oxygen vacancies and significant grain boundaries by atomic layer deposition (ALD), and (b) growth of single crystalline ZnO nanoneedles with high optical quality on the ALD seeds via hydrothermal process. The needle structure (∼25 nm in diameter with an aspect ratio of ∼24) also supports the vectorial transport of photo-charge carriers, which is crucial for high catalytic activity. Furthermore, it is shown that enhanced PCA is because of the catalytic activity at surface defects (on ALD seed), valence band, and conduction band (of ZnO nanoneedles). PCA and durability of the PAN/ZnO nanofibrous mat have also been tested with aqueous solution of methylene blue and the results showed almost no decay in the catalytic activity of this material when reused.

One Dimensional Plasmonic Grating: High Sensitive Biosensor

Abstract: We report a simple 1D grating device fabrication on ∼50 nm gold (Au) film deposited on glass, which is employed as a high performance refractive index (RI) sensor by exploiting the surface plasmon polaritons (SPP) excited by the grating device along the Au/analyte interface. A finite element analysis (FEA) method is employed to maximize the sensitivity of the sensor for a fixed period and thickness of a gold film and its close correspondence with experiment has given the insight for high sensitivity and enhanced transmission. Significantly, in the context of economic design and performance, it is shown that an optimally designed and fabricated 1D grating can be as sensitive as 524 nm/RIU (linearity RI = 1.33303 to 1.47399), which is remarkably higher than existing reports operating in a similar wavelength region.

An Optimal Au Grating Structure for Light Absorption in Amorphous Silicon Thin Film Solar Cell

Abstract: Effect of different gold (Au) grating structures on light absorption in solar cell is investigated by finite elemental analysis using COMSOL multiphysics-RF module. The geometry of the solar cell consists of a 50-nm Au film on the substrate of amorphous silicon (a-Si). An optimum value of the slit width (w) of the Au grating has been obtained whereas periodicity of the grating structure remained the same. The periodicity in the grating device was chosen in such a way that the excitation of the surface plasmon polritons (SPPs) lies in the IR or NIR region where most of the spectrometers work well in practical life. Far-field transmission spectra were extracted from the grating device when illuminating with p-polarized light through the substrate side. Near-field plots of the Fano resonance (dip) associated with the excitation of the surface plasmon polritons (SPPs) were carefully examined to understand the underlying physics. It was deduced from the results that a grating device with slit width of 250–350 nm is the most efficient which reveals the fact that such device offers intermediate scattering from the grating structure and supports fundamental plasmonic mode. Hence, such devices absorb more light being most efficiently and find application in solar cell.