Mahesh Kumar

Bio: Mahesh Kumar is an academic researcher from Indian Institute of Technology, Jodhpur. The author has contributed to research in topics: Molecular beam epitaxy & Heterojunction. The author has an hindex of 29, co-authored 204 publications receiving 4864 citations. Previous affiliations of Mahesh Kumar include Indian Institutes of Technology & Indian Institute of Technology Delhi.

Electronic states in spherical GaN nanocrystals embedded in various dielectric matrices: The k ⋅ p-calculations

Abstract: Using the envelope-function approximation, the single-particle states of electrons and holes in spherical GaN nanocrystals embedded in different amorphous dielectric matrices (SiO2, Al2O3, HfO2 and Si3N4) have been calculated. Ground state energies of electrons and holes in GaN nanocrystals are determined using the isotropic approximation of the k ⋅ p -Hamiltonian. All the ground state energies are found to increase with lowering the nanocrystal size and are proportional to the R−n, where R is the nanocrystal radius, n =1.5-1.9 for electrons and 1.7-2.0 for holes. The optical gap of GaN nanocrystals changes from 3.8 to 5 eV for the nanocrystal radius ranging from 3 to 1 nm.

High Tolerance of BST Thin Film Based Varactor Under Neutron Irradiation

Abstract: We report neutron irradiation resistance of the barium strontium titanate (BST) thin-film-based varactor under different neutron fluences. BST thin films were deposited on sapphire substrate by RF magnetron sputtering and gold interdigital capacitor (IDC) electrodes were patterned with the photolithography technique. Capacitance-voltage ( $C$ – $V$ ) and leakage current characteristics were measured at several neutron fluences. A small change in capacitance is observed at different fluences. The changes are attributed to the radiation-induced defect dipoles and other charged defects, which serve as effective domain wall pinning sites. We supplemented this macroscopic $C$ – $V$ behavior of BST varactor with investigation through X-ray diffraction and atomic force microscopy for their structural and morphological changes, respectively. This paper shows that BST varactor can be used in neutron environment due to its high tolerance.

Wurtzite InN nanodots on Si(100) by molecular beam epitaxy

Abstract: Studies were carried on the growth behavior of InN nanodots by plasma assisted molecular beam epitaxy on bare Si(100) substrates and their structural, optical, electrical properties. The growth was carried out by two different methods such as, (i) mono-step growth process at a low temperature and a (ii) bi-step growth process with the combination of low and high temperatures for the formation of single crystalline nanodots with well defined crystallographic facets due to cluster migration. Low temperature photoluminescence shows a free excitonic (FE) luminescence at 0.80 eV. The Raman spectroscopy and X-ray diffraction studies reveal that the nanodots as well as the film were of wurtzite structure and strain free.

Pt Nanoparticles on Vertically Aligned Large-Area MoS2 Flakes for Selective H2 Sensing at Room Temperature

Abstract: The increasing demand of hydrogen (H2) as an alternative clean fuel emboldened the parallel development of extremely sensitive room-temperature H2 sensors for safety purposes. Molybdenum disulfide (MoS2) is an intriguing material, exhibiting a high chemical sensing ability. However, usage of MoS2 in H2 sensors has been limited and usually suffers from low sensitivity and selectivity, especially at room temperature. In this work, we report a highly sensitive and selective H2 sensor based on Pt nanoparticle-functionalized vertically aligned large-area MoS2 flakes. The fabricated Pt@MoS2 sensor exhibits a high sensitivity value of 23%, excellent reproducibility, fast response, and complete recovery at room temperature. The superior response of the sensor is attributed to the spillover effect and adsorption sites distributed on the Pt surface and the MoS2–Pt interface. The influence of operating temperature on the sensing performance is also investigated. Density functional theory calculations validate our experimental results and demonstrate higher adsorption of H2 for Pt@MoS2 leading to improved and selective H2 response. This study offers Pt nanoparticle-sensitized MoS2 as a potential candidate for the development of low-power and room-temperature H2 sensors for near future hydrogen vehicles and related technologies.

Association of linear type pelvic traits with testicular characteristics and reproductive capability of breeding dairy bulls.

Abstract: Associations of pelvic linear type traits (PLT) on production and reproduction of cows were widely documented, in contrast, importance of PLT on bulls' reproductive ability, semen quality, semen cryo-preservability, frozen semen doses (FSD) production etc. inadequately reported. Present study was conducted on Frieswal bulls (N=378, age: 6-91months, m) to assess age-wise growth dynamics of pelvic dimensions, classifying age into 6 groups (6m, 7-12m, 13-24m, 25-36m, 37-48m, >48m). Iso-age group bulls' (N=100; 25-36m) records were analyzed after classifying seasons (summer/rainy/winter) of semen collection, if pelvic morphometric traits had any practical associations with testicular traits, semen quality and FSD production or not. A discriminant function was developed based on PLT, which showed poor predictive ability to distinguish FSD/Non-FSD category bulls. Age significantly influenced growth and dimensions of gonadal and external pelvic morphometry traits. The dimensions of PLT increased at higher rate up to 36m age and thereafter enhancement became insignificant. PLT were positively (P<0.01) associated with testicular/scrotal traits. Among PLT studied, pelvic triangle area (PTA) was the strongest discriminating variable to distinguish between good and poor breeding bulls. Bulls of larger PTA (≥1000cm2 ) at average 30m age, produced relatively inferior quality semen viz. lower volume (-3%), sperm concentration (-48x106 /mL), motility, semen quality index, total sperm counts/ejaculate (-601x106 ), motile sperm counts/ejaculate (-474x106 ) and total post-thaw motile sperm counts (-226x106 ) than bulls of lesser PTA (<1000cm2 ). It was concluded that external PTA could be a useful addition to the bulls' breeding soundness evaluation.

Perspective on the Development of Lead-free Piezoceramics

Abstract: A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Abstract: Single-phased ferroelectromagnet BiFeO3 ceramics with high resistivity were synthesized by a rapid liquid phase sintering technique. Saturated ferroelectric hysteresis loops were observed at room temperature in the ceramics sintered at 880 °C for 450 s. The spontaneous polarization, remnant polarization, and the coercive field are 8.9 μC/cm2, 4.0 μC/cm2, and 39 kV/cm, respectively, under an applied field of 100 kV/cm. It is proposed that the formation of Fe2+ and an oxygen deficiency leading to the higher leakage can be greatly suppressed by the very high heating rate, short sintering period, and liquid phase sintering technique. The latter was also found effective in increasing the density of the ceramics. The sintering technique developed in this work is expected to be useful in synthesizing other ceramics from multivalent or volatile starting materials.

Room-temperature gas sensing of ZnO-based gas sensor: A review

Abstract: Novel gas sensors with high sensing properties, simultaneously operating at room temperature are considerably more attractive owing to their low power consumption, high security and long-term stability. Till date, zinc oxide (ZnO) as semiconducting metal oxide is considered as the promising resistive-type gas sensing material, but elevated operating temperature becomes the bottleneck of its extensive applications in the field of real-time gas monitoring, especially in flammable and explosive gas atmosphere. In this respect, worldwide efforts have been devoted to reducing the operating temperature by means of multiple methods In this communication, room-temperature gas sensing properties of ZnO based gas sensors are comprehensively reviewed. Much more attention is particularly paid to the effective strategies that create room-temperature gas sensing of ZnO based gas sensors, mainly including surface modification, additive doping and light activation. Finally, some perspectives for future investigation on room-temperature gas-sensing materials are discussed as well.