Optical-Phonon-Limited High-Field Transport in Layered Materials

An optical-phonon-limited velocity model has been employed to investigate high-field transport in a selection of layered 2-D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities, and intrinsic cutoff frequencies as a function of carrier density have been predicted, thus providing a benchmark for the optical-phonon-limited high-field performance limits of these materials. The optical-phonon-limited carrier velocities for a selection of multi-layers of transition metal dichalcogenides and black phosphorus are found to be modest compared to their n-channel silicon counterparts, questioning the utility of biasing these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency, high-power devices, subject to the experimental realization of high carrier densities, primarily due to its large optical-phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with the predicted values. The temperature dependence of the measured v(sat) is discussed and compared with the theoretically predicted dependence over a range of temperatures.

Evolution of Jahn-Teller distortion, transport and dielectric properties with doping in perovskite NdFe1-xMnxO3 (0 <= x <= 1) compounds

We have carried out dielectric and transport measurements in NdFe1-xMnxO3 (0 <= x <= 1) series of compounds and studied the variation of activation energy due to a change in Mn concentration. Despite similar ionic radii in Mn3+ and Fe3+, large variation is observed in the lattice parameters and a crossover from dynamic to static Jahn-Teller distortion is discernible. The Fe/Mn-O-Fe/Mn bond angle on the ab plane shows an anomalous change with doping. With an increase in the Mn content, the bond angle decreases until x = 0.6; beyond this, it starts rising until x = 0.8 and again falls after that. A similar trend is observed in activation energies estimated from both transport and dielectric relaxation by assuming a small polaron hopping (SPH) model. Impedance spectroscopy measurements delineate grain and grain boundary contributions separately both of which follow the SPH model. Frequency variation of the dielectric constant is in agreement with the modified Debye law from which relaxation dispersion is estimated.

Perturbation of nucleo-cytoplasmic transport affects size of nucleus and nucleolus in human cells

Size regulation of human cell nucleus and nucleolus are poorly understood subjects. 3D reconstruction of live image shows that the karyoplasmic ratio (KR) increases by 30-80% in transformed cell lines compared to their immortalized counterpart. The attenuation of nucleo-cytoplasmic transport causes the KR value to increase by 30-50% in immortalized cell lines. Nucleolus volumes are significantly increased in transformed cell lines and the attenuation of nucleo-cytoplasmic transport causes a significant increase in the nucleolus volume of immortalized cell lines. A cytosol and nuclear fraction swapping experiment emphasizes the potential role of unknown cytosolic factors in nuclear and nucleolar size regulation.

Tailored electrical conductivity, electromagnetic shielding and thermal transport in polymeric blends with graphene sheets decorated with nickel nanoparticles

Electromagnetic interference shielding (EMI) materials were designed using PC (polycarbonate)/SAN poly(styrene-co-acrylonitrile)] blends containing few-layered graphene nanosheets decorated with nickel nanoparticles (G-Ni). The graphene nanosheets were decorated with nickel nanoparticles via the uniform nucleation of the metal salt precursor on graphene sheets as the substrate. In order to localize the nanoparticles in the PC phase of the PC/SAN blends, a two-step mixing protocol was adopted. In the first step, graphene sheets were mixed with PC in solution and casted into a film, followed by dilution of these PC master batch films with SAN in the subsequent melt extrusion step. The dynamic mechanical properties, ac electrical conductivity, EMI shielding effectiveness and thermal conductivity of the composites were evaluated. The G-Ni nanoparticles significantly improved the electrical and thermal conductivity in the blends. In addition, a total shielding effectiveness (SET) of -29.4 dB at 18 GHz was achieved with G-Ni nanoparticles. Moreover, the blends with G-Ni exhibited an impressive 276% higher thermal conductivity and 29.2% higher elastic modulus with respect to the neat blends.

Localized Charge Carrier Transport Properties of Zn1-x Ni (x) O/NiO Two-Phase Composites

We report the localized charge carrier transport of two-phase composite Zn1-x Ni (x) O/NiO (0 a parts per thousand currency sign x a parts per thousand currency sign 1) using the temperature dependence of ac-resistivity rho (ac)(T) across the N,el temperature T (N) (= 523 K) of nickel oxide. Our results provide strong evidence to the variable range hopping of charge carriers between the localized states through a mechanism involving spin-dependent activation energies. The temperature variation of carrier hopping energy epsilon (h)(T) and nearest-neighbor exchange-coupling parameter J (ij)(T) evaluated from the small poleron model exhibits a well-defined anomaly across T (N). For all the composite systems, the average exchange-coupling parameter (J (ij))(AVG) nearly equals to 70 meV which is slightly greater than the 60-meV exciton binding energy of pure zinc oxide. The magnitudes of epsilon (h) (similar to 0.17 eV) and J (ij) (similar to 11 meV) of pure NiO synthesized under oxygen-rich conditions are consistent with the previously reported theoretical estimation based on Green's function analysis. A systematic correlation between the oxygen stoichiometry and, epsilon (h)(T) and J (ij)(T) is discussed.

Insights into the Structural Dynamics of Nucleocytoplasmic Transport of tRNA by Exportin-t

Exportin-t (Xpot) transports mature 5'- and 3'-end processed tRNA from the nucleus to the cytoplasm by associating with a small G-protein Ran (RAs-related nuclear protein), in the nucleus. The release of tRNA in cytoplasm involves RanGTP hydrolysis. Despite the availability of crystal structures of nuclear and cytosolic forms of Xpot, the molecular details regarding the sequential events leading to tRNA release and subsequent conformational changes occurring in Xpot remain unknown. We have performed a combination of classical all-atom and accelerated molecular dynamics simulations on a set of complexes involving Xpot to study a range of features including conformational flexibility of free and cargo-bound Xpot and functionally critical contacts between Xpot and its cargo. The systems investigated include free Xpot and its different complexes, bound either to Ran (GTP/GDP) or tRNA or both. This approach provided a statistically reliable estimate of structural dynamics of Xpot after cargo release. The mechanistic basis for Xpot opening after cargo release has been explained in terms of dynamic structural hinges, about which neighboring region could be displaced to facilitate the nuclear to cytosolic state transition. Post-RanGTP hydrolysis, a cascade of events including local conformational change in RanGTP and loss of critical contacts at Xpot/tRNA interface suggest factors responsible for eventual release of tRNA. The level of flexibility in different Xpot complexes varied depending on the arrangement of individual HEAT repeats. Current study provides one of the most comprehensive and robust analysis carried out on this protein using molecular dynamics schemes.

Theoretical insights on the electro-thermal transport properties of monolayer MoS2 with line defects

Two dimensional (2D) materials demonstrate several novel electrical, mechanical, and thermal properties which are quite distinctive to those of their bulk form. Among many others, one important potential application of the 2D material is its use in the field of energy harvesting. Owing to that, here we present a detailed study on electrical as well as thermal transport of monolayer MoS2, in quasi ballistic regime. Besides the perfect monolayer in its pristine form, we also consider various line defects which have been experimentally observed in mechanically exfoliated MoS2 samples. For calculating various parameters related to the electrical transmission, we employ the non-equilibrium Green's function-density functional theory combination. However, to obtain the phonon transmission, we take help of the parametrized Stillinger-Weber potential which can accurately delineate the inter-atomic interactions for the monolayer MoS2. Due to the presence of line defects, we observed significant reductions in both the charge carrier and the phonon transmissions through a monolayer MoS2 flake. Moreover, we also report a comparative analysis showing the temperature dependency of the thermoelectric figure of merit values, as obtained for the perfect as well as the other defective 2D samples. (C) 2016 AIP Publishing LLC.

Growth of Silicon Carbide Bulk Crystals by Physical Vapor Transport Method and Modeling Efforts in the Process Optimization

Silicon carbide bulk crystals were grown in an induction-heating furnace using the physical vapor transport method. Crystal growth modeling was performed to obtain the required inert gas pressure and temperatures for sufficiently large growth rates. The SiC crystals were expanded by designing a growth chamber having a positive temperature gradient along the growth interface. The obtained 6H-SiC crystals were cut into wafers and characterized by Raman scattering spectroscopy and X-ray diffraction, and the results showed that most parts of the crystals had good crystallographic structures.

Thermoelastic Stresses in SiC Single Crystals Grown by the Physical Vapor Transport Method

A finite element-based thermoelastic anisotropic stress model for hexagonal silicon carbide polytype is developed for the calculation of thermal stresses in SiC crystals grown by the physical vapor transport method. The composite structure of the growing SiC crystal and graphite lid is considered in the model. The thermal expansion match between the crucible lid and SiC crystal is studied for the first time. The influence of thermal stress on the dislocation density and crystal quality is discussed.

Thermal transport in high porosity cellular metal foams

The heat dissipation capability of highly porous cellular metal foams with open cells subject to forced air convection is studied using a combined experimental and analytical approach. The cellular morphologies of six FeCrAlY (an iron-based alloy) foams and six copper alloy foams with a range of pore sizes and porosities are quantified with the scanning electronic microscope and image analysis. Experimental measurements on pressure drop and heat transfer for copper foams are carried out. A numerical model for forced convection across open-celled metal foams is subsequently developed, and the predictions are compared with those measured. Reasonably good agreement with test data is obtained, given the complexity of the cellular foam morphology and the associated momentum/energy transport. The results show that cell size has a more significant effect on the overall heat transfer than porosity. An optimal porosity is obtained based on the balance between pressure drop and overall heat transfer, which decreases as the Reynolds number is increased.

Correlation of transport and magnetic critical currents in melt-processed YBa2Cu3O7-δ thick films

Transport critical current measurements have been carried out on melt-processed thick films of YBa2Cu3O7-δ on yttria-stabilized zirconia in fields of up to 8 T both within grains and across grain boundaries. These measurements yield Jc values of ∼3000 A cm-2 at 4.2 K and zero magnetic field and 400 A cm -2 at 77 K and zero magnetic field, taking the entire sample width as the definitive dimension. Optical and scanning electron microscopy reveals that the thick-film grains consist typically of a central "hub" region ∼50 μm in diameter, which is well connected to radial subgrains or "spokes" which extend ∼1 mm to define the complete grain structure. Attempts have been made to correlate the transport measurements of inter- and intra-hub-and-spoke (H-S) critical current with values of this parameter derived previously from magnetization measurements. Analysis of the transport measurements indicates that current flow through H-S grains is constrained to paths along the spokes via the grain hub. Taking the size of the hub as the definitive dimension yields an intra-H-S grain Jc of ∼60 000 A cm-2 at 4.2 K and 0 T, which is in reasonable agreement with the magnetization data. Experiments in which the hub is removed from individual grains confirm that this feature determines critically the J c of the film.

Modeling Ammonothermal Growth of GaN Single Crystals: The Role of Transport

Single crystal gallium nitride (GaN) is an important technological material used primarily for the manufacture of blue light lasers. An important area of contemporary research is developing a viable growth technique. The ammonothermal technique is an important candidate among many others with promise of commercially viable growth rates and material quality. The GaN growth rates are a complicated function of dissolution kinetics, transport by thermal convection and crystallization kinetics. A complete modeling effort for the growth would involve modeling each of these phenomena and also the coupling between these. As a first step, the crystallization and dissolution kinetics were idealized and the growth rates as determined purely by transport were investigated. The growth rates thus obtained were termed ‘transport determined growth rates’ and in principle are the maximum growth rates that can be obtained for a given configuration of the system. Using this concept, a parametric study was conducted primarily on the geometric and the thermal boundary conditions of the system to optimize the ‘transport determined growth rate’ and determine conditions when transport might be a bottleneck.