Subash Adhikari

ZnO nanowire arrays on 3D hierachical graphene foam: biomarker detection of Parkinson's disease.

We report that vertically aligned ZnO nanowire arrays (ZnO NWAs) were fabricated on 3D graphene foam (GF) and used to selectively detect uric acid (UA), dopamine (DA), and ascorbic acid (AA) by a differential pulse voltammetry method. The optimized ZnO NWA/GF electrode provided a high surface area and high selectivity with a detection limit of 1 nM for UA and DA. The high selectivity in the oxidation potential was explained by the gap difference between the lowest unoccupied and highest occupied molecular orbitals of a biomolecule for a set of given electrodes. This method was further used to detect UA levels in the serum of patients with Parkinsons disease (PD). The UA level was 25% lower in PD patients than in healthy individuals. This finding strongly implies that UA can be used as a biomarker for PD.

Charge Transport in MoS2/WSe2 van der Waals Heterostructure with Tunable Inversion Layer

Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS2/WSe2 heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe2 due to its relatively low doping concentration compared to that of MoS2, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS2 side, while charges are fully depleted on the whole WSe2 side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.

Photocurrent Switching of Monolayer MoS2 Using a Metal–Insulator Transition

We achieve switching on/off the photocurrent of monolayer molybdenum disulfide (MoS2) by controlling the metal-insulator transition (MIT). N-type semiconducting MoS2 under a large negative gate bias generates a photocurrent attributed to the increase of excess carriers in the conduction band by optical excitation. However, under a large positive gate bias, a phase shift from semiconducting to metallic MoS2 is caused, and the photocurrent by excess carriers in the conduction band induced by the laser disappears due to enhanced electron-electron scattering. Thus, no photocurrent is detected in metallic MoS2. Our results indicate that the photocurrent of MoS2 can be switched on/off by appropriately controlling the MIT transition by means of gate bias.

A High‐On/Off‐Ratio Floating‐Gate Memristor Array on a Flexible Substrate via CVD‐Grown Large‐Area 2D Layer Stacking

Memristors such as phase-change memory and resistive memory have been proposed to emulate the synaptic activities in neuromorphic systems. However, the low reliability of these types of memories is their biggest challenge for commercialization. Here, a highly reliable memristor array using floating-gate memory operated by two terminals (source and drain) using van der Waals layered materials is demonstrated. Centimeter-scale samples (1.5 cm × 1.5 cm) of MoS2 as a channel and graphene as a trap layer grown by chemical vapor deposition (CVD) are used for array fabrication with Al2 O3 as the tunneling barrier. With regard to the memory characteristics, 93% of the devices exhibit an on/off ratio of over 103 with an average ratio of 104 . The high on/off ratio and reliable endurance in the devices allow stable 6-level memory applications. The devices also exhibit excellent memory durability over 8000 cycles with a negligible shift in the threshold voltage and on-current, which is a significant improvement over other types of memristors. In addition, the devices can be strained up to 1% by fabricating on a flexible substrate. This demonstration opens a practical route for next-generation electronics with CVD-grown van der Waals layered materials.

Determining the Fermi level by absorption quenching of monolayer graphene by charge transfer doping

While optical properties of graphene in the visible region are solely defined by the frequency-independent fine structure constant, an onset of absorption has been observed in the infrared region due to Pauli blocking of interband transitions. Here, we report a complete absorption quenching in the infrared region by coating graphene with bis(trifluoromethanesulfonyl)amine (TFSA), an optically transparent p-type chemical dopant. The Fermi level downshift due to TFSA doping results in enhanced transmission in the infrared region proportional to the doping concentration. An absorption quenching onset method, developed in our work, to extract the Fermi level shift in pristine and doped graphene agrees with values extracted from Raman G-band and 2D-band shifts, Hall measurements and the binding energy shift observed in X-ray photo-electron spectroscopy. Performing simple UV-visible transmittance spectroscopy to obtain the absorption quenching onset of graphene also allows detection of environmental and substrate effects via Fermi level shift. Our method opens up the practical implementation of this unique phenomenon of graphene in future optoelectronic devices.