Materials chemistry was in its infancy when I started my independent research efforts in India after returning from the United States in the late 1950s. I have investigated phase transformations of TiO2 and CsCl, and also carried out defect calculations. While working on rare-earth oxides, I made TbO2 and PrO2 by a simple solution route; this is probably an early example of chimie douce (soft chemistry). I started working on transition-metal oxides by building simple instruments, including a thermobalance and furnaces. In 1987, we were able to fully characterize the first liquid N2 superconductor (YBa2Cu3O7) using a home-built AC susceptometer. Oxides have been of great interest to me because of the variety of phenomena exhibited by them. I have studied various aspects of transition-metal oxides, including metal-insulator transitions, colossal magnetoresistance, and multiferroics. The last two decades have included research on synthesis, characterization, and properties of various nanomaterials, and in particular, two-dimensional nanosheets (graphene and its inorganic analogues). Two-dimensional sheets and other nanomaterials have been covalently cross-linked to derive new materials with novel properties. Work on water splitting and reduction of CO2, besides using aliovalent anion substitution to generate novel inorganics (Zn2NF in place of ZnO), has also been conducted.

Harvesting energy from otherwise wasted resources has been intensively investigated as a promising technology especially for enabling the deployment of autonomous wireless-sensor networks for the Internet of Things. Multi-stimulus energy harvesting, simultaneously from different energy sources, provides an attractive opportunity to amplify the power density of harvesters, thereby extending their potential for self-powered devices. In this article, we review recent and ongoing research efforts aimed at enhancing the energy-harvesting performance of magnetoelectric (ME) composite harvesters employing dual stimuli, mechanical vibrations, and magnetic fields. After a brief introduction to vibration, magnetic field, and dual-mode energy harvesting, we survey the key materials utilized for ME energy harvesting. We then focus on progress in this area and discuss relevant ideas to realize electromechanical and magnetoelectric coupling for harvesting energy from mechanical vibrations and magnetic fields simultaneously. We provide perspectives and future directions as well.

Magnetic field has been used to trigger biofilm formation. Iron oxide nanoparticles were attached to bacterial cells and cells were aggregated by application of magnetic field. Artificial cellular crowding triggered quorum sensing and led to the formation of biofilm at the sub-threshold population. Aggregation process was monitored by studying temporal dynamics of capacitance and conductance profiles. Capacitive profile exhibited a plateau upon introduction of magnetic field which was retained even after field was removed. This hysteresis property signified biofilm initiation in response to artificial crowding. This work demonstrates how synthetic biology is enabled by including nanoparticles in the interactome.

Manipulating the thermal conductivity of solids is important for practical applications. Due to the fact that phonons in thermoelectric materials have longer mean free paths (MFPs) than electrons, strengthening phonon scattering to reduce lattice thermal conductivity (κlat) becomes the most straightforward and effective approach to enhance the thermoelectric figure of merit, ZT, which determines the maximum device efficiency. Phonons have a wide range of MFPs in semiconductors, and different dimensions of lattice defects can be targeted to scatter particular phonons with distinct relaxation times. Designing hierarchical nano-microstructures, spanning from point defects to volume defects, would be beneficial to achieve low κlat via a full spectrum of phonon scattering. Herein, we review the formation and underlying mechanisms for lattice defects and highlight the role of all-scale hierarchical nano-microstructure on phonon engineering. Existing challenges in simulations are also discussed.

The human body is a challenging platform for energy harvesting. For thermoelectrics, the small temperature differences between the skin and air necessitate materials with low thermal conductivities in order to maintain useful output powers. For kinetic harvesting, human motion is not strongly tonal, the frequencies are very low, and the accelerations are modest. Kinetic harvesting can be split into two categories—inertial, in which human motion excites an inertial mass–the motion of which is transduced to electricity, and clothing integrated, in which the harvesting material is integrated with a garment or other flexible wearable system. In the first case, key issues are the electromechanical dynamics of the system and materials with improved electromechanical transduction figures of merit. In the second case, materials that provide flexibility, stretchability, and comfort are of primary importance.

Research in thermoelectric (TE) quantum structures was greatly propelled by the prediction in the early 1990s of a significant boost in TE efficiency by quantum size effects. Recently, research interest has shifted from quantum size effects in conventional semiconductors toward new types of quantum materials (e.g., topological insulators [TIs], Weyl and Dirac semimetals) characterized by their nontrivial electronic topology. Bi2Te3, Sb2Te3, and Bi2Se3, established TE materials, are also TIs exhibiting a bulk bandgap and highly conductive and robust gapless surface states. The signature of the nontrivial electronic band structure on TE transport properties can be best verified in transport experiments using nanowires and thin films. However, even in nanograined bulk, the typical peculiarities in the transport properties of TIs can be seen. Finally, the remarkable transport properties of Dirac and Weyl semimetals are discussed.