Solution processed, vertically stacked hetero-structured diodes based on liquid-exfoliated WS2 nanosheets: from electrode-limited to bulk-limited behavior

Ultrahigh Photosensitivity Based on Single-Step Lay-on Integration of Freestanding Two-Dimensional Transition-Metal Dichalcogenide

Two-dimensional transition-metal dichalcogenides have attracted significant attention because of their unique intrinsic properties, such as high transparency, good flexibility, atomically thin structure, and predictable electron transport. However, the current state of device performance in monolayer transition-metal dichalcogenide-based optoelectronics is far from commercialization, because of its substantial strain on the heterogeneous planar substrate and its robust metal deposition, which causes crystalline damage. In this study, we show that strain-relaxed and undamaged monolayer WSe2 can improve a device performance significantly. We propose here an original point-cell-type photodetector. The device consists in a monolayer of an absorbing TMD (i.e., WSe2) simply deposited on a structured electrode, i.e., core-shell silicon-gold nanopillars. The maximum photoresponsivity of the device is found to be 23.16 A/W, which is a significantly high value for monolayer WSe2-based photodetectors. Such point-cell photodetectors can resolve the critical issues of 2D materials, leading to tremendous improvements in device performance.

Quantitative analysis of printed nanostructured networks using high-resolution 3D FIB-SEM nanotomography

Networks of solution-processed nanomaterials are becoming increasingly important across applications in electronics, sensing and energy storage/generation. Although the physical properties of these devices are often completely dominated by network morphology, the network structure itself remains difficult to interrogate. Here, we utilise focused ion beam - scanning electron microscopy nanotomography (FIB-SEM-NT) to quantitatively characterise the morphology of printed nanostructured networks and their devices using nanometre-resolution 3D images. The influence of nanosheet/nanowire size on network structure in printed films of graphene, WS2 and silver nanosheets (AgNSs), as well as networks of silver nanowires (AgNWs), is investigated. We present a comprehensive toolkit to extract morphological characteristics including network porosity, tortuosity, specific surface area, pore dimensions and nanosheet orientation, which we link to network resistivity. By extending this technique to interrogate the structure and interfaces within printed vertical heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

Solution-Processed Heterojunction Photodiodes Based on WSe2 Nanosheet Networks

Solution-processed photodetectors incorporating liquid-phase-exfoliated transition metal dichalcogenide nanosheets are widely reported. However, previous studies mainly focus on the fabrication of photoconductors, rather than photodiodes which tend to be based on heterojunctions and are harder to fabricate. Especially, there are rare reports on introducing commonly used transport layers into heterojunctions based on nanosheet networks. In this study, a reliable solution-processing method is reported to fabricate heterojunction diodes with tungsten selenide (WSe2 ) nanosheets as the optical absorbing material and PEDOT: PSS and ZnO as injection/transport-layer materials. By varying the transport layer combinations, the obtained heterojunctions show rectification ratios of up to ≈104 at ±1 V in the dark, without relying on heavily doped silicon substrates. Upon illumination, the heterojunction can be operated in both photoconductor and photodiode modes and displays self-powered behaviors at zero bias.

Sustainable Synthesis of Highly Biocompatible 2D Boron Nitride Nanosheets

2D ultrafine nanomaterials today represent an emerging class of materials with very promising properties for a wide variety of applications. Biomedical fields have experienced important new achievements with technological breakthroughs obtained from 2D materials with singular properties. Boron nitride nanosheets are a novel 2D layered material comprised of a hexagonal boron nitride network (BN) with interesting intrinsic properties, including resistance to oxidation, extreme mechanical hardness, good thermal conductivity, photoluminescence, and chemical inertness. Here, we investigated different methodologies for the exfoliation of BN nanosheets (BNNs), using ball milling and ultrasound processing, the latter using both an ultrasound bath and tip sonication. The best results are obtained using tip sonication, which leads to the formation of few-layered nanosheets with a narrow size distribution. Importantly, it was observed that with the addition of pluronic acid F127 to the medium, there was a significant improvement in the BN nanosheets (BNNs) production yield. Moreover, the resultant BNNs present improved stability in an aqueous solution. Cytotoxicity studies performed with HeLa cells showed the importance of taking into account the possible interferences of the nanomaterial with the selected assay. The prepared BNNs coated with pluronic presented improved cytotoxicity at concentrations up to 200 μg mL^-1 with more than 90% viability after 24 h of incubation. Confocal microscopy also showed high cell internalization of the nanomaterials and their preferential biodistribution in the cell cytoplasm.