Nadim Ferdous Hoque

Vertically aligned VO2(B) nanobelt forest and its three-dimensional structure on oriented graphene for energy storage

Assembling two-dimensional (2D) nanomaterials into an ordered forest structure that provides an easily accessible large surface area and/or chemically active 2D edges will present numerous opportunities, including as electrodes for energy storage. We report a densely packed vertically aligned VO2(B) nanobelt (NB) based forest structure synthesized by a solvothermal method using a vertically oriented graphene (VOG) network as the underlying support. We further expanded this forest structure into a folded three-dimensional (3D) forest structure using VOG-covered metallic foam as the scaffold. To demonstrate its potential, this free-standing 3D ordered structure built from 2D nanomaterials was directly used as the electrode for lithium-ion batteries. Excellent performance was confirmed by a stable discharge capacity of 178 mA h g−1 at a current density of 10 A g−1 (or a rate of 59 C) and 100 mA h g−1 at 27 A g−1 (300 C), contributed by both lithium ion intercalation into the crystal lattice and surface-related pseudocapacitance. A high cycling stability over 2000 cycles under a high current density was also demonstrated. We expect that our method can be expanded to synthesize 2D sheet based forest structures of other layered oxides and hydroxides by using VOG as a versatile platform for numerous applications such as energy storage and catalytic energy conversion.

Electrical oscillation generation with current-induced resistivity switching in VO2 micro-channel devices

ABSTRACT We report large amplitude modulation waveforms as large as ~ 10 V using vanadium dioxide micro-channel devices operating under current-controlled conditions. The self-sustained electrical oscillations were generated by controlling the applied current in the negative differential resistance region of the investigated devices. An appropriate value of internal capacitance was achieved as parasitic capacitance in the device structure to stabilize the electrical oscillations. This eliminates the need of an external pulsed source or any external passive component connected to the micro-channel devices. Amplitude and frequency of the oscillation were tuned by illuminating the device micro-channel with an external laser. An equivalent circuit model was developed to simulate the waveforms. A good agreement between experiment and simulation was verified. Graphical Abstract

Effects of Moisture-Based Grain Boundary Passivation on Cell Performance and Ionic Migration in Organic–Inorganic Halide Perovskite Solar Cells

Because of the polycrystalline nature, grain boundaries (GBs) in hybrid perovskite thin films play critical roles in determining the charge collection efficiency of perovskite solar cells (PSCs), material stability, and in particular the ion migration, considering their relatively soft ionic bonds with low formation energy. Different GB passivation methods are being studied, and introducing PbI2-rich phase at GBs in methylammonium lead iodide (MAPbI3) has been found to be useful. In this study, combining macroscale measurements with tip-based microscopic probing that includes scanning Kelvin probe microscopy for surface potential mapping and conductive atomic force microscopy for charge transport mapping, we revealed the effects of PbI2-rich phase at GBs, which was introduced in moisture-assisted synthesis of MAPbI3 thin films. It was found that PbI2 passivation of GBs could change the surface potential and charge carrier screening and significantly retard current conduction at the GB while enhancing conduction through the grain interior. Inhibition of ion migration at GBs, as well as enhanced PSC device performance, is reported.