Dixit, Ambesh

Two-dimensional layered transition metal dichalcogenide exhibit important characteristics such as suitable bandgap, high absorption coefficient, and favourable electron transport properties for their uses in nano-electronic such as ultrathin solar cells. We adopted a hybrid simulation approach, where density functional calculations are performed for optoelectronic properties of semiconductor materials and macroscopic device simulation is carried out to evaluate photovoltaic response. We investigated electronic and optical properties of bulk WS2, WSe2 and noticed very high absorption coefficient, making them suitable absorber materials for solar cell. Further, the electronic and optical properties of an ultrathin WSSe Janus layer are investigated using density functional theory and noticed low reflectance and high bandgap, supporting its usefulness as a buffer layer for W(S/Se)2 absorbers. The computed density functional results for W(S/Se)2 and Janus WSSe are used to simulate the photovoltaic response of WSSe/W(S/Se)2 solar cell using macroscopic device simulation. The photovoltaic performance of a single junction solar cell is optimized for W(S/Se)2 absorbers and WSSe Janus buffer materials. The effect of absorber layer thickness, carrier concentration, and contact work function is evaluated to understand the solar cell performance. We noticed that interface recombination speed between absorber and buffer layer and minority carrier lifetime are affecting the solar cell performance. The maximum efficiency of about ~17.73% and 18.87% is noticed for optimized WSSe/WS2 and WSSe/WSe2 solar cell. The present study will provide a new approach to design, develop, and optimize a solar cell and evaluate the impact of different materials parameters on solar cell performance.

We investigated the effect of Au nanoparticles in Zinc o xide semiconductor matrix for enhanced absorption. Apart from ZnO optical absorption at 360 n m, the matrix also shows plasmonic absorption at 560 nm.

Thermal emittance is an important parameter for the solar thermal collectors as thermal radiative losses from the solar thermal collector increase to the fourth power of temperature. This should be minimized using infrared reflectors in designing spectrally selective absorber coatings for solar thermal applications. The thermal emittance of zirconium (Zr) film as an infrared reflector has been investigated for the use in the spectrally selective absorber. The Zr metallic films are deposited using DC magnetron sputtering process on stainless steel and glass substrates and the deposition process has been optimized to achieve the minimum thermal emittance. The effect of structural, microstructural and surface morphological properties of Zr films is investigated on the emittance of fabricated structures. The X-ray diffraction analysis revealed that the Zr film coatings consist of both cubic and hexagonal Zr crystallographic phase. The optimized deposition time and temperature showed 0.12 and 0.14 emittance values for Zr film coatings on stainless steel and glass substrates respectively.

Zirconium carbide-nitride absorber (ZrC-ZrN) and zirconium reflector tandem structures were prepared on glass and aluminum substrate, in conjunction with zirconium oxide (ZrOx) anti-reflection coatings, using DC/RF magnetron sputtering system. The solar absorption properties of zirconium carbide-nitride absorber layers were optimized by controlling nitrogen flow during synthesis process for optimal solar thermal response. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements were carried out on fabricated absorber-reflector tandem structures to understand the structure-property correlation. We absorbed strong dependence of zirconium nitride fraction on solar thermal performance. We observed enhanced solar absorptance α ∼ 0.86 and thermal emittance ϵ ∼ 0.05 at room temperature for structures fabricated with optimized synthesis parameters. The optimized structures are stable up to 150 °C in the air without any significant degradation in their solar performance.

Tetragonal BaTiO3 bulk samples are prepared using the solid-state route in conjunction with intermediate high-temperature annealing steps. The (002) and (200) X-ray diffraction peaks near 2Ɵ~45° and 310, 520, and 720 cm−1 characteristic vibrational modes in Raman spectroscopic measurements confirm the tetragonal crystallographic structure of BaTIO3 bulk samples. The 1100°C annealed BaTiO3 sample showed optimal tetragonality ~1.016 and the same is used for BaTiO3–acrylonitrile butadiene rubber (NBR) composites at different BaTiO3 loading fractions in parts per hundred (PHR). These BaTiO3/NBR composite systems exhibit dual band microwave resonance, widening the operating window for microwave absorption applications. Eighty PHR BaTiO3/NBR composite exhibits microwave reflection losses (RL) at 9.5 and 16.5 GHz with ~−9 and ~−18 dB reflection losses, respectively. The onset of dual band is attributed to the ferroelectric-induced dipolar relaxation at 9.5 GHz and its second-order resonance at 16.5 GHz in such composite systems.

The cubic double perovskites A2ZnWO6 (A = Ba, Sr and Ca) are studied to understand the effect of A cation site, using the density functional theory (DFT) based full potential augmented plane wave method (FP-LAPW) with GGA and mBJ exchange correlation potentials. The structural robustness and stability are investigated using the bond lengths and the total energy. The band structure and density of states suggest that all these cubic double perovskites are indirect wide band gap semiconductors. The band gap varies from 3.90 eV (2.97 eV) for Ba2ZnWO6 system to 3.40 eV (2.8 eV) for Ca2ZnWO6 system using mBJ (GGA) exchange correlation potentials. Our studies suggest that A cation site modification has a strong effect on physical and electronic properties, in contrast to the structural robustness. The lattice parameter decreases from 8.19 Å to 7.9 Å from Ba to Ca at alkali cation site and the electronic band gap variation follows the common cation rule. The charge densities show enhanced localization of charges near the zinc and oxygen sites with increasing alkali cation atomic radii. In addition, we discuss the impact of A cation site modification on the dielectric and optical properties for A2ZnWO6 double perovskites.

BiFeO3 thin films are deposited on FTO coated glass substrates using a simple sol-gel process, limiting thickness about 70 nm and Ag/BiFeO3/FTO RRAM devices are prepared. The devices showed low-voltage bipolar switching with the maximum Ion/Ioff ratio ∼450, and low set and reset voltages ∼1.1 V and −1.5 V, respectively. The devices are stable against on-off cycles with ∼104 s retention time without any significant degradation. The variations in the set and reset voltages are 0.4 V and 0.6 V, respectively. We found that ohmic and trap-controlled space charge limited conductions are responsible for low and high resistance states, respectively. The resistive switching mechanism is attributed to the formation and rupturing of the metal filament during the oxidation and reduction of Ag ions for the set and reset states. The devices showed strong robustness against environmental conditions even after ten months from their synthesis and first measurements, exhibiting good reproducibility, retention and endurance.

We study the behavior of the exchange bias (EB) and the blocking temperature in an antiferromagnetic FeVO 4 –ferrimagnetic Fe 3 O 4 nanocomposite system upon annealing in Ar atmosphere. Surprisingly, the blocking temperature of post-annealed samples increased to ∼ 50 K, more than two-fold compared the Néel temperature (T N = 22 K) of individual FeVO 4 nanoparticles. This significant enhancement of the blocking temperature was accompanied by the corresponding increase of EB, from ∼ 50 Oe in as-prepared samples to ∼ 110 Oe in post-annealed samples. The temperature dependence of EB can be described by two approximately linear regions with different slopes, with an inflection point at T ∼ 21 K coinciding with the Néel temperature of FeVO 4 nanoparticles. The region above the inflection point with non-zero EB is characterized by a weaker temperature dependence and is expanded well beyond T N . The x-ray photoemission spectroscopy measurements indicate that the surface of post-annealed Fe 3 O 4 particles becomes oxygen deficient, which leads to a modification of the electronic, magnetic and morphological properties of the FeVO 4 /Fe 3 O 4 interface. We associate this unusual behavior with a magnetic proximity effect, in which the ordering temperature of the antiferromagnetic FeVO 4 nanoparticles and the corresponding exchange bias are strongly affected by the adjacent ferrimagnetic Fe 3 O 4 layer.

Low energy optical modes of MBE-grown In1-xGaxN thin films with different values of x are investigated using Raman spectroscopy. We also studied the influence of Raman excitation frequency using red and green lasers on scattering intensity of various Raman modes. For those In1-xGaxN alloys whose bandgap energy is close to the red laser, a huge enhancement in the intensities of ALO) mode and its 2ALO) replica is observed when excited with red laser as compared to the green laser excitation. We found that the energies of longitudinal optical modes (ALO) and 2ALO)) vary nonlinearly unlike the Emode with increasing Ga atomic fraction. A Raman mode 540 cm-1 was observed in all In1-xGaxN films with low energy red laser excitation but was absent with green laser excitation. We attribute this mode to ATO) mode of the underneath GaN buffer layer. © 2013 A. Dixit et al.

We report here on the complex magnetic structure and magnetocapacitance in NiF 2 , a non-oxide multifunctional system. It undergoes an anti-ferromagnetic transition near 68.5 K, superimposed with canted Ni spin driven weak ferromagnetic ordering, followed by a metastable ferromagnetic phase at or below 10 K. Our density functional calculations account for the complex magnetic structure of NiF 2 deduced from the temperature and the field dependent measurements. Near room temperature, NiF 2 exhibits a relatively large dielectric response reaching >10 3 with a low dielectric loss of <0.5 at frequencies >20 Hz. This is attributed to the intrinsic grain contribution in contrast to the grain boundary contribution in most of the known dielectric materials. The response time is 10 μs or more at 280 K. The activation energy for such temperature dependent relaxation is ~500 meV and is the main source for grain contribution. Further, a large negative magneto capacitance >90% is noticed in 1 T magnetic field. We propose that our findings provide a new non-oxide multifunctional NiF 2 , useful for dielectric applications.