Bhagyashree A. Chalke

Tunable thermal conductivity in defect engineered nanowires at low temperatures

We measure the thermal conductivity (κ) of individual InAs nanowires (NWs), and find that it is three orders of magnitude smaller than the bulk value in the temperature range of 10–50 K. We argue that the low κ arises from the scattering of phonons in the random superlattice of twin defects oriented perpendicular to the axis of the NW. We observe a significant electronic contribution arising from the surface accumulation layer, which gives rise to the tunability of κ with the application of an electrostatic gate and a magnetic field. Our devices and measurements of κ at different carrier concentrations and magnetic field offer a means to study unique aspects of nanoscale thermal transport.

Organosilane oxidation with a half million turnover number using fibrous nanosilica supported ultrasmall nanoparticles and pseudo-single atoms of gold

The combination of ultrasmall nanoparticles and pseudo-single atoms of gold (Au) and fibrous nanosilica (KCC-1) functionalized with 3-aminopropyltriethoxysilane (APTS) enabled the design of KCC-1-APTS/Au nanocatalysts with very high turnover numbers (TONs). KCC-1-APTS/Au catalysed the oxidation of organosilanes to silanols, with a TON of approximately half a million (591 000 for dimethylphenyl silane as a model substrate). Additionally, the figure-of-merit (FOM), which provides an integrated view of the rate of the reaction, the energy required, the reaction scale and the recyclability of the catalysts, was 633 mmol h−1 K−1. KCC-1-APTS/Au also catalysed two additional challenging reactions, the alcoholysis of silane and the hydrosilylation of aldehydes, with very high TONs. These characteristics make KCC-1-APTS/Au a versatile nanocatalyst.

Free-standing semipolar III-nitride quantum well structures grown on chemical vapor deposited graphene layers

We report the synthesis and optical characterization of semipolar-oriented III-nitride quantum well (QW) structures obtained by growth on chemical vapor deposited graphene layers using metalorganic vapor phase epitaxy. Various multi-quantum well stacks of GaN(QW)/AlGaN(barrier) and InGaN (QW)/GaN (barrier) were grown. Growth on graphene not only helps achieve a semipolar orientation but also allows facile transfer of the QW multilayer stack to other cheap, flexible substrates. We demonstrate room-temperature photoluminescence from layers transferred to flexible Kapton films.

Comparison of GaN nanowires grown on c-, r- and m-plane sapphire substrates

Gallium nitride nanowires were grown on c-plane, r-plane and m-plane sapphire substrates in a showerhead metalorganic chemical vapor deposition system using nickel catalyst with trimethylgallium and ammonia as precursors. We studied the inuence of carrier gas, growth temperature, reactor pressure, reactant ow rates and substrate orientation in order to obtain thin nanowires. The nanowires grew along the and axes depending on the substrate orientation. These nanowires were further characterized using x-ray diraction, electron microscopy, photoluminescence and Raman

Light matter interaction in WS2 nanotube-graphene hybrid devices

We study the light matter interaction in WS2 nanotube-graphene hybrid devices. Using scanning photocurrent microscopy, we find that by engineering graphene electrodes for WS2 nanotubes we can improve the collection of photogenerated carriers. We observe inhomogeneous spatial photocurrent response with an external quantum efficiency of ∼1% at 0 V bias. We show that defects play an important role and can be utilized to enhance and tune photocarrier generation.

Rheology of hydrating cement paste: Crossover between two aging processes

Abstract The roles of applied strain and temperature on the hydration dynamics of cement paste are uncovered in the present study. We find that the system hardens over time through two different aging processes. The first process dominates the initial period of hydration and is characterized by the shear stress σ varying sub-linearly with the strain-rate γ ; during this process the system is in a relatively low-density state and the inter-particle interactions are dominated by hydrodynamic lubrication. At a later stage of hydration the system evolves to a high-density state where the interactions become frictional, and σ varies super-linearly with γ ; this is identified as the second process. An instability, indicated by a drop in σ, that is non-monotonic with γ and can be tuned by temperature, separates the two processes. Both from rheology and microscopy studies we establish that the observed instability is related to fracture mechanics of space-filling structure.

Superconductivity in immiscible Nb–Cu nanocomposite films

Superconductivity in granular films is controlled by the grain size and the inter-grain coupling. In a two-component granular system formed by a random mixture of a normal metal (N) and a superconductor (S), the superconducting nano-grains may become coupled through S-N weak links, thereby affecting the superconducting properties of the network. We report on the study of superconductivity in immiscible Nb-Cu nanocomposite films with varying compositions. The microstructure of the films revealed the presence of phase separated, closely spaced, nano-grains of Nb and Cu whose sizes changed marginally with composition. The superconducting transition temperature (Tc0) of the films decreased with increasing concentration of Cu with a concomitant decrease in the upper critical field (Hc2) and the critical current (Ic). Our results indicate the presence of superconducting phase fluctuations in all films with varying Nb:Cu content which not only affected the temperature for the formation of a true phase coherent superconducting condensate in the films but also other superconducting properties.

Growth of high-quality Bi 2 Sr 2 CaCu 2 O 8+δ whiskers and electrical properties of resulting exfoliated flakes

In this work, we demonstrate a simple technique to grow high-quality whiskers of Bi2 Sr2 CaCu2 O8+δ – a high Tc superconductor. Structural analysis shows the single-crystalline nature of the grown whiskers. To probe electrical properties, we exfoliate these whiskers into thin flakes (~50 nm thick) using the scotch-tape technique and develop a process to realize good electrical contacts. We observe a superconducting critical temperature, Tc, of 86 K. We map the evolution of the critical current as a function of temperature. With 2-D materials emerging as an exciting platform to study low-dimensional physics, our work paves the way for future studies on two-dimensional high-Tc superconductivity.

Self-assembly of metal nanorod arrays into hierarchical clusters and evolution of their hydrophobic behaviour

Electrochemically grown copper nanorod arrays form clusters when they are dried after etching. By optimizing the aspect ratio and the drying conditions, we can tune the interplay between the two competing forces: elastic force due to bending and the surface tension from the evaporating internanorod liquid and consequently get clusters of varied sizes and shapes. Static contact angle and contact angle measurements on a tilt were done for the clusters, and it was observed that the hydrophobicity and the pinning properties of the clustered surfaces depend on three main physical parameters-rod length, single cluster size and cluster size by inter-cluster separation ratio. The clusters are double hierarchical structures where air traps in between the clusters (microstructures) determine the static contact angle and wetting of the nanorods (nanostructures) contained within each single cluster determines the pinning.