Buddha Deka Boruah

Highly Dense ZnO Nanowires Grown on Graphene Foam for Ultraviolet Photodetection

Growth of highly dense ZnO nanowires (ZnO NWs) is demonstrated on three-dimensional graphene foam (GF) using resistive thermal evaporation technique. Photoresponse of the as-grown hybrid structure of ZnO NWs on GF (ZnO NWs/GF) is evaluated for ultraviolet (UV) detection. Excellent photoresponse with fast response and recovery times of 9.5 and 38 s with external quantum efficiency of 2490.8% is demonstrated at low illumination power density of 1.3 mW/cm(2). In addition, due to excellent charge carrier transport, mobility of graphene reduces the recombination rate of photogenerated charge carriers, hence the lifetime of photogenerated free charge carriers enhances in the photodetectors.

Few-layer graphene/ZnO nanowires based high performance UV photodetector

A graphene and zinc oxide nanowires (G/ZnO NWs) based ultraviolet (UV) photodetector presents excellent responsivity and photocurrent gain with detectivity. Graphene due to higher charge carrier transport mobility induces faster response to UV illumination at the interface between ZnO and graphene with improved response and decay times as compared to a ZnO NWs device alone. A linear increase is revealed for both the responsivity and photocurrent gain of the G/ZnO NWs device with the applied bias. These results suggest that the G/ZnO NWs device exhibits great promise for highly efficient UV photodetectors.

Energy-Efficient Hydrogenated Zinc Oxide Nanoflakes for High-Performance Self-Powered Ultraviolet Photodetector.

Light absorption efficiency and doping induced charge carrier density play a vital role in self-powered optoelectronic devices. Unique vanadium-doped zinc oxide nanoflake array (VZnO NFs) is fabricated for self-powered ultraviolet (UV) photodetection. The light harvesting efficiency drastically improved from 84% in ZnO NRs to 98% in VZnO NFs. Moreover, the hydrogenation of as-synthesized VZnO (H:VZnO) NFs displayed an outstanding increase in response current as compared to pristine structures. The H:VZnO NFs device presents an extraordinary photoelastic behavior with faster photodetection speed in the order of ms under a low UV illumination signal. Excellent responsivity and external quantum efficiency with larger value of specific detectivity of H:VZnO NFs device promises an outstanding sensitivity for UV signal and self-powered high-performance visible-blind photodetector.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron-hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods

Cobalt integrated zinc oxide nanorod (Co-ZnO NR) array is presented as a novel heterostructure for ultraviolet (UV) photodetector (PD). Defect states in Co-ZnO NRs surface induces an enhancement in photocurrent as compared to pristine ZnO NRs PD. Presented Co-ZnO NRs PD is highly sensitive to external magnetic field that demonstrated 185.7% enhancement in response current. It is concluded that the opposite polarizations of electron and holes in the presence of external magnetic field contribute to effective separation of electron-hole pairs that have drifted upon UV illumination. Moreover, Co-ZnO NRs PD shows a faster photodetection speed (1.2 s response time and 7.4 s recovery time) as compared to the pristine ZnO NRs where the response and recovery times are observed as 38 and 195 s, respectively.

A flexible ternary oxide based solid-state supercapacitor with excellent rate capability

A porous vanadium doped-zinc–nickel–cobalt ternary oxide (VZnNiCo) nanostructure is presented for a high performance flexible supercapacitor (SC). The symmetric solid-state SC demonstrated a remarkable enhancement in the capacitive response as compared to SCs based on its individual oxide constituents e.g. NiCo2O4//NiCo2O4 or ZnNiCo//ZnNiCo. A synergistic redox reaction from all the ions was offered to increase the capacitive response and moreover, direct growth of the unique porous nanostructure on a flexible current collector exhibited excellent ion-diffusion efficiency with a high electrochemically active surface area. The flexible SC demonstrated excellent capacitive response stability towards large mechanical deformations for a much higher scan rate up to 5000 mV s−1 in addition to high cycling stability. Hence, the present work opens novel avenues towards exploiting hybrid flexible energy storage devices that are mechanically robust with a highly stable capacitive ability at a much higher scan rate.

ZnO quantum dots and graphene based heterostructure for excellent photoelastic and highly sensitive ultraviolet photodetector

Heterostructures comprised of zinc oxide quantum dots (ZnO QDs) and graphene are presented for ultraviolet photodetectors (UV PD). Graphene–ZnO QDs–graphene (G–ZnO QDs–G) based PD demonstrated an excellent UV photoresponse with outstanding photoelastic characteristics when illuminated for several cycles with a periodicity 5 s. PD demonstrated faster detection ability with the response and recovery times of 0.29 s in response to much lower UV illumination. A direct variation in photoresponse is revealed with the bias voltage as well as UV illumination intensity. A drastic reduction in the dark current is noticed due to potential barrier formation between adjacent ZnO QDs and the recombination rate reduces by directly transferring photogenerated charge carriers from ZnO QDs to graphene for enhanced the charge mobility.

Nickel hydroxide coated carbon nanoparticles mediated hybrid three-dimensional graphene foam assembly for supercapacitor

A binder-free novel three-dimensional assembly comprising graphene, carbon nanoparticles and nickel hydroxide is presented as a supercapacitor electrode. The combined effect of three-dimensional graphene for excellent charge conduction and nickel hydroxide coating on carbon nanoparticles for high surface area was exploited to obtain excellent supercapacitance. The resulting structure presented an ultrahigh 4667 F g−1 specific capacitance at a 2 A g−1 discharge rate with 98.6% high charge retention for more than 1000 charge–discharge cycles. In addition, the presented assembly demonstrated very high power and energy densities.

Flexible Array of Microsupercapacitor for Additive Energy Storage Performance Over a Large Area

An on-chip microsupercapacitor (MSC) pattern is obtained by layer-by-layer spray deposition of both manganese dioxide (MnO2) nanoparticle-coated carbon nanotubes (MnO2-CNTs) and MnO2 nanosheet-decorated reduced graphene oxide (MnO2-rGO) on mechanically robust, flexible polyethylene terephthalate. Layer-by-layer patterning of MSC electrodes offers rapid in-plane diffusion of electrolyte ions in electrodes the layered electrode and hence ultrahigh capacitance and energy density of 7.43 mF/cm2 (32300 mF/cm3) and 0.66 μW h/cm2 (2870 μW h/cm3), respectively, are obtained. A robust electrochemical response was measured under multiple bending of the solid-state flexible MSC as well as under repetitive cycles (∼5000).

Influence of charge traps in carbon nanodots on gas interaction

The nonlinear electrical characteristic of carbon nanodots (CNDs) has revealed important physical phenomena of charge trapping playing a dominant role in surface interactions. Functional groups on the surface of CNDs attract ambient water molecules which in turn act as charge traps and give rise to electrical hysteresis that plays a dominant role in understanding charge transport in CNDs on surface interactions. Hysteresis in the current-voltage response is further utilized to study the interaction of the CNDs with nitrogen dioxide gas as an external stimuli. The hysteresis area is observed to be dependent on the time of gas interaction with the CNDs, therefore revealing the interaction mechanism of the CNDs with the gas.