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V. Damodara Das

Bio: V. Damodara Das is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Thin film & Electrical resistivity and conductivity. The author has an hindex of 20, co-authored 89 publications receiving 1145 citations.


Papers
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TL;DR: In this article, the authors measured the resistance of lead telluride and lead selenide films at different air pressures and found that resistance of the as-grown films was of the order of a kilohm but increased with increasing pressure.
Abstract: Lead telluride and lead selenide films were prepared by the thermal evaporation technique on clean glass substrates held at room temperature in a vacuum of 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ Torr. The resistance of the films was measured at different air pressures. The resistance of the as-grown films was found to be of the order of a kilohm but increased with increasing pressure. A resistance maximum was observed at a particular air pressure after which the resistance decreased with increasing pressure. An indirect method, namely, the measurement of the conductivity variation as a function of air pressure, has been used to estimate the extent of air adsorption and the adsorption isotherms have been drawn on this basis. The dependence of adsorption on thickness is also discussed.

20 citations

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TL;DR: The presence of a thin film of water vapor on the substrate surface impedes the agglomeration rate to a great extent while the presence of adsorbed substrate-surface contaminants decreases the energy for surface migration of islands of copper thereby increasing the aggLomersation rate.
Abstract: The post-deposition dc electrical-resistance increase of island copper films deposited on glass substrates at room temperature and at a pressure of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ torr is studied. Films in the resistance range 10\char21{}95 M\ensuremath{\Omega}/\ensuremath{\square} were studied under different conditions to ascertain the role of residual gases and substrate-surface contaminants on the agglomeration rate. Mobility coalescence of small islands of Cu giving rise to an agglomerated film structure is assumed to explain the post-deposition resistance increase. It was found that the logarithm of the normalized resistance [ln(R/${R}_{0}$); ${R}_{0}$ is the initial resistance of the film at a time t=0] varies linearly as the logarithm of the time elapsed after the cessation of deposition. The constant of proportionality in the above relationship, termed the agglomeration rate m, shows an interesting oscillatory dependence on the initial resistance of the film and the condition of study. Nearly all the maxima and minima of m for two of the conditions studied in detail as a function of the initial resistance occur at about the same initial resistance values with the films having almost the same structure at these points. An argument is put forward based on the oscillatory nature of the effective tunneling barrier as a function of the island size to explain this unusual behavior. It is found that the presence of a thin film of water vapor on the substrate surface impedes the agglomeration rate to a great extent while the presence of adsorbed ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$ decreases the energy for surface migration of islands of copper thereby increasing the agglomeration rate.

20 citations

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TL;DR: In this paper, the size-dependent phase transition is explained by taking into account the varying surface and intergrain surface energy contributions to the total energy of the stable phase as a function of thickness and the difference in specific surface and integrain surface energies of the two phases.

19 citations

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TL;DR: In this paper, photoelectrochemical properties of thin polycrystalline thin films of n-CdSe 0.5 Te 0.5 alloy were characterized by X-ray diffraction and optical absorption studies, and it was found that the films are n-type and show photoactivity in 2 M Kl + 20 mM I 2 /C electrolyte.

19 citations

Journal ArticleDOI
TL;DR: In this article, the Jain-Verma theory of carrier energy dependent relaxation time was applied to the thermoelectric data of thin films to understand the nature of scattering mechanisms in this material.

17 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided, as well as the prospects for the optimization and their applications are also discussed.
Abstract: Developing thermoelectric materials with superior performance means tailoring interrelated thermoelectric physical parameters – electrical conductivities, Seebeck coefficients, and thermal conductivities – for a crystalline system. High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desirable for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. In general, research in recent years has focused on developing thermoelectric structures and materials of high efficiency. The importance of this parameter is universally recognized; it is an established, ubiquitous, routinely used tool for material, device, equipment and process characterization both in the thermoelectric industry and in research. In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided. The prospects for the optimization of thermoelectric materials and their applications are also discussed.

663 citations

Journal ArticleDOI
25 Jul 1996-Nature
TL;DR: In this article, the primordial fireball was simulated using a neutron-induced nuclear reaction to heat small regions of liquid 3He above the superfluid transition temperature, leading to the formation of a random network of vortices (the superfluid analogue of cosmic strings).
Abstract: TOPOLOGICAL defects formed during a rapid symmetry-breaking phase transition in the early Universe1,2 could be responsible for seeding large-scale structure, for the anisotropy of the microwave background radiation, and for the predominance of matter over antimatter3,4. The theory describing this cosmological phase transition is formally analogous to that describing the transition to the superfluid state in liquid 3He, so that in principle the process of cosmological defect formation can be modelled in the laboratory. Here we report the results of an experiment in which the 'primordial fireball' is mimicked using a neutron-induced nuclear reaction (n + 3He → p + 3He + 0.76 MeV) to heat small regions of superfluid 3He above the superfluid transition temperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompanied by the formation of a random network of vortices (the superfluid analogue of cosmic strings). We monitor the evolution of this defect state by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid second-order phase transition, and give quantitative support to the Kibble–Zurek mechanism5,6 of cosmological defect formation.

411 citations

Journal ArticleDOI
TL;DR: In this article, the authors characterized the optical absorption properties of Ge2Sb2Te5 in its amorphous, face-centered-cubic, and hexagonal phases, and explained the origins of inconsistent or unphysical results in previous reports.
Abstract: Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorpho...

387 citations

Journal ArticleDOI
TL;DR: In this review, the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them are summarized and perspectives on their research and technological future prospects are presented.
Abstract: Graphene and related two-dimensional materials provide an ideal platform for next generation disruptive technologies and applications. Exploiting these solution-processed two-dimensional materials in printing can accelerate this development by allowing additive patterning on both rigid and conformable substrates for flexible device design and large-scale, high-speed, cost-effective manufacturing. In this review, we summarise the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them. We also present our perspectives on their research and technological future prospects.

371 citations

Journal ArticleDOI
TL;DR: Analysis of ferromagnetic resonance data shows that high particle concentrations correlate with increasing chain length producing decreasing SLP, and a theoretical model describing dipole interactions valid for the linear response regime is proposed, predicting optimum particle sizes for hyperthermia to about 30% smaller than those previously predicted, depending on the nanoparticle parameters and chain size.
Abstract: Nanostructured magnetic systems have many applications, including potential use in cancer therapy deriving from their ability to heat in alternating magnetic fields. In this work we explore the influence of particle chain formation on the normalized heating properties, or specific loss power (SLP) of both low- (spherical) and high- (parallelepiped) anisotropy ferrite-based magnetic fluids. Analysis of ferromagnetic resonance (FMR) data shows that high particle concentrations correlate with increasing chain length producing decreasing SLP. Monte Carlo simulations corroborate the FMR results. We propose a theoretical model describing dipole interactions valid for the linear response regime to explain the observed trends. This model predicts optimum particle sizes for hyperthermia to about 30% smaller than those previously predicted, depending on the nanoparticle parameters and chain size. Also, optimum chain lengths depended on nanoparticle surface-to-surface distance. Our results might have important implications to cancer treatment and could motivate new strategies to optimize magnetic hyperthermia.

323 citations