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.

Microstructure of vacuum-deposited tin thin films

Abstract: Tin thin films of equal thickness were vacuum deposited on NaCl substrates at different deposition rates both in high vacuum (2×10−5 Torr) and poor vacuum (1×10−3 Torr). These films were examined by electron microscopy and the grain size and density in the films were determined. It was found that both in poor and high vacuum, as the deposition rate was increased the grain size decreased and grain density increased. However, the deposition rate must be increased by an order of magnitude in order to observe large changes in the grain size and density. It was also observed that the films prepared in poor vacuum, especially at low deposition rates, consist of islands with jagged edges. These observations have been interpreted on the basis of nucleation theory and the effect of adsorbed/chemisorbed gases on island growth in these films.

Semiconducting behaviour of thin bismuth films vacuum-deposited at different substrate temperatures

Abstract: Thin bismuth films (thickness 25 nm) have been vacuum-deposited onto glass substrates at different substrate temperatures in a vacuum of 2×10−5 torr. The resistance of the films has been measured as a function of temperaturein situ during and after annealing. It is found that the resistance of all the annealed films decreases with increasing temperature thus showing a semiconducting type of behaviour. The films do not show a resistivity minimum observed in thicker films [1]. The absence of a resistivity minimum is attributed to the thinness of the films and consequent larger energy band gap and smaller grain size.

Thermoelectric Properties of SnzCo4Sb12-yTey Skutterudites

Abstract: Sn-filled and Te-doped CoSb3 skutterudites (Snz Co4Sb12-yTey) were synthesized by encapsulated induction melting, and their thermoelectric properties were investigated from 300 K to 700 K. Single delta-phase was successfully obtained by the subsequent heat treatment at 823 K for 6 days. Intrinsic CoSb3 showed n-type conductivity from 300 K to 400 K and there is a transition to p-type conductivity above 400 K. Sn-filled Sn zCo4Sb12 showed p-type conductivity and Sn-filled/Te-doped SnzCo4Sb12-yTey showed n-type conductivity from 300 K to 700 K, indicating that Te atoms substituting for Sb atoms acted as electron donors. Thermal conductivity was considerably reduced by filling and doping due to the phonon scattering. Sn filling effect and Te doping effect are examined and compared in this study

Electrical conductivity, defect density and structure of obliquely vacuum-deposited tin antimonide alloy thin films

Abstract: Thin films of tin antimonide alloy have been vacuum deposited on glass substrates at room temperature at different angles of deposition. These films have been heat-treated in situ and their electrical resistance has been continuously monitored during the heating-cooling cycle. From the resistance against temperature data during heat-treatment, “initial lattice distortion energy spectra” of these films have been determined using Vand's theory. It has been found that for angles of deposition below 50°, the defect density increases with increasing angle of deposition. At higher angles of deposition, the resistance against temperature behaviour during heat-treatment is different. This is attributed to the columnar structure of the film. It is also found that preferential decay energies of the defects exist and these are 1.45 and 1.80 eV.

Effect of oxygen adsorption on the electrical resistance of thin films

Abstract: Thin films of of varying thicknesses deposited by the flash evaporation method showed considerable change in resistance with time when exposed to oxygen or atmosphere. The change in resistance is found to decrease with increase in thickness and with increase in substrate temperature. The observed behaviour is explained by an oxygen adsorption model, which is further supported by the x-ray photoelectron spectroscopy (XPS) of the films.

A review on thermoelectric renewable energy: Principle parameters that affect their performance

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.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

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.

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

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...

Functional inks and printing of two-dimensional materials.

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.

Effect of magnetic dipolar interactions on nanoparticle heating efficiency: Implications for cancer hyperthermia

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.