Arup Ghorai

Hydrothermal growth of few layer 2H-MoS2 for heterojunction photodetector and visible light induced photocatalytic applications

The present low yielding growth techniques of semiconducting 2H phase molybdenum disulfide (MoS2) hamper its widespread applications. In this article, we report a novel hydrothermal chemical approach to synthesize micron sized few layer 2H-MoS2 on a large scale. Sodium molybdate and ammonium thiocyanate have been used as precursors to obtain template-free 2H-MoS2 in solution. Detailed microscopic and spectroscopic characterizations reveal that the bottom-up synthesized few layer MoS2 flakes are highly crystalline having the hexagonal 2H phase. Photodetector devices comprising a p-type silicon (p-Si)/n-MoS2 heterostructure have been fabricated for the first time using solution processed 2H-MoS2 synthesized by the bottom up approach. The heterojunction diode exhibits a high rectification ratio (>103) with broad band photoresponse over the visible range. Because of the visible light photoresponse, as-synthesized MoS2 along with reduced graphene oxide (MoS2–RGO hybrids) have been utilized to study the potential of this two dimensional (2D) heterostructure for visible light driven photocatalytic Rhodamine B dye degradation. This study demonstrates the potential of solution processed MoS2 for integration with silicon and growth of 2D heterostructures for visible light induced multifunctional applications.

Highly Luminescent WS2 Quantum Dots/ZnO Heterojunctions for Light Emitting Devices

Sonication induced vertical fragmentation of two-dimensional (2D) WS2 nanosheets into highly luminescent, monodispered, zero-dimensional (0D) quantum dots (QDs) is reported. The formation of 0D structures from 2D sheets and their surface/microstructure characterization are revealed from their microscopic and spectroscopic investigations. Size dependent optical properties of WS2 nanostructures have been explored by UV-vis absorption and photoluminescence spectroscopy. Interestingly, it is observed that, below a critical dimension (∼2 nm), comparable to the Bohr exciton radius, the tiny nanocrystals exhibit strong emission. Finally, the electroluminescence characteristics are demonstrated for the first time, by forming a heterojunction of stabilizer free WS2 QDs and ZnO thin films. The signature of white light emission in the light emitting device is attributed to the adequate intermixing of emission characteristics of WS2 QDs and ZnO. The observation of white electroluminescence may pave the way to fabricate prototype futuristic efficient light emitting devices.

Superior heterojunction properties of solution processed copper-zinc-tin-sulphide quantum dots on Si

CZTS nanocrystals have been synthesized via a new facile and environmentally friendly route using olive oil at a relatively low temperature. Nanocrystals synthesized using olive oil have a smaller average size in comparison to those synthesized with a conventional solvent-like ethylenediamine. Nanocrystals with an average diameter of 40, 20 and 6 nm have been extracted from the olive oil at different centrifugation speeds of 500, 1000 and 2000 rpm, respectively. The photovoltaic characteristics of p-CZTS/n-Si heterojunctions fabricated using the synthesized colloidal quaternary nanocrystals are demonstrated. The device fabricated with smallest sized CZTS nanocrystals, having an average diameter of ∼6 nm, exhibits an enhancement in power conversion efficiency of 61% in comparison to that of the device fabricated with the nanocrystals of 40 nm in diameter. A lower reflectance and higher minority carrier life time along with a larger surface-to-volume ratio resulted in an enhanced power conversion efficiency for smaller sized CZTS nanocrystals.