WS2 Nanotube Transistor for Photodetection and Optoelectronic Memory Applications.

Nanotube and nanowire transistors hold great promises for future electronic and optoelectronic devices owing to their downscaling possibilities. In this work, a single multi-walled tungsten disulfide (WS2) nanotube is utilized as the channel of a back-gated field-effect transistor. The device exhibits a p-type behavior in ambient conditions, with a hole mobility µp ≈  1.4 cm2V-1s-1 and a subthreshold swing SS ≈ 10 V dec-1. Current-voltage characterization at different temperatures reveals that the device presents two slightly different asymmetric Schottky barriers at drain and source contacts. Self-powered photoconduction driven by the photovoltaic effect is demonstrated, and a photoresponsivity R ≈ 10 mAW-1 at 2 V drain bias and room temperature. Moreover, the transistor is tested for data storage applications. A two-state memory is reported, where positive and negative gate pulses drive the switching between two different current states, separated by a window of 130%. Finally, gate and light pulses are combined to demonstrate an optoelectronic memory with four well-separated states. The results herein presented are promising for data storage, Boolean logic, and neural network applications.

Memory effect and coexistence of negative and positive photoconductivity in black phosphorus field effect transistor for neuromorphic vision sensors.

Black phosphorus (BP) field-effect transistors with ultrathin channels exhibit unipolar p-type electrical conduction over a wide range of temperatures and pressures. Herein, we study a device that exhibits mobility up to 100 cm2 V-1 s-1 and a memory window up to 1.3 µA. Exposure to a supercontinuum white light source reveals that negative photoconductivity (NPC) and positive photoconductivity (PPC) coexist in the same device. Such behavior is attributed to the chemisorbed O2 molecules, with a minor role of physisorbed H2O molecules. The coexistence of NPC and PPC can be exploited in neuromorphic vision sensors, requiring the human eye retina to process the optical signals through alerting and protection (NPC), adaptation (PPC), followed by imaging and processing. Our results open new avenues for the use of BP and other two-dimentional (2D) semiconducting materials in transistors, memories, and neuromorphic vision sensors for advanced applications in robotics, self-driving cars, etc.

Multilayer WS2 for low-power visible and near-infrared phototransistors.

Mechanically exfoliated multilayer WS2 flakes are used as the channel of field effect transistors for low-power photodetection in the visible and near-infrared (NIR) spectral range. The electrical characterization as a function of the temperature reveals devices with n-type conduction and slightly different Schottky barriers at the drain and source contacts. The WS2 phototransistors can be operated in self-powered mode, yielding both a current and a voltage when exposed to light. The spectral photoresponse in the visible and the NIR ranges shows a high responsivity (4.5 µA/W) around 1250 nm, making the devices promising for telecommunication applications.

Field enhancement induced by surface defects in two-dimensional ReSe2 field emitters.

The field emission properties of rhenium diselenide (ReSe2) nanosheets on Si/SiO2 substrates, obtained through mechanical exfoliation, have been investigated. The n-type conduction was confirmed by using nano-manipulated tungsten probes inside a scanning electrode microscope to directly contact the ReSe2 flake in back-gated field effect transistor configuration, avoiding any lithographic process. By performing a finite element electrostatic simulation of the electric field, it is demonstrated that the use of a tungsten probe as anode, at a controlled distance from the ReSe2 emitter surface, allows the collection of emitted electrons from a reduced area that furtherly decreases by reducing the tip-sample distance, i.e. allowing a local characterization of the field emission properties. Experimentally, it is shown that the turn-on voltage can be linearly reduced by reducing the cathode-anode separation distance. By comparing the measured current-voltage characteristics with the numerical simulations, it is also shown that the effective field enhancement on the emitter surface is larger than expected because of surface defects. Finally, it is confirmed that ReSe2 nanosheets are suitable field emitters with high time stability and low current fluctuations.

Temperature-Dependent Conduction and Photoresponse in Few-Layer ReS2.

The electrical behavior and the photoresponse of rhenium disulfide field-effect transistors (FETs) have been widely studied; however, only a few works have investigated the photocurrent as a function of temperature. In this paper, we perform the electrical characterization of few-layer ReS2-based FETs with Cr-Au contacts over a wide temperature range. We exploit the temperature-dependent transfer and output characteristics to estimate the effective Schottky barrier at the Cr-Au/ReS2 interface and to investigate the temperature behavior of parameters, such as the threshold voltage, carrier concentration, mobility, and subthreshold swing. Through time-resolved photocurrent measurements, we show that the photocurrent increases with temperature and exhibits a linear dependence on the incident light power at both low and room temperatures and a longer rise/decay time at higher temperatures. We surmise that the photocurrent is affected by the photobolometric effect and light-induced desorption of adsorbates which are facilitated by the high temperature and the low pressure.

Subthreshold Current Suppression in ReS2 Nanosheet-Based Field-Effect Transistors at High Temperatures.

Two-dimensional rhenium disulfide (ReS2), a member of the transition-metal dichalcogenide family, has received significant attention due to its potential applications in field-effect transistors (FETs), photodetectors, and memories. In this work, we investigate the suppression of the subthreshold current during the forward voltage gate sweep, leading to an inversion of the hysteresis in the transfer characteristics of ReS2 nanosheet-based FETs from clockwise to anticlockwise. We explore the impact of temperature, sweeping gate voltage, and pressure on this behavior. Notably, the suppression in current within the subthreshold region coincides with a peak in gate current, which increases beyond a specific temperature but remains unaffected by pressure. We attribute both the suppression in drain current and the presence of peak in gate current to the charge/discharge process of gate oxide traps by thermal-assisted tunnelling. The suppression of the subthreshold current at high temperatures not only reduces power consumption but also extends the operational temperature range of ReS2 nanosheet-based FETs.

Electric Transport in Few-Layer ReSe2 Transistors Modulated by Air Pressure and Light.

We report the fabrication and optoelectronic characterization of field-effect transistors (FETs) based on few-layer ReSe2. The devices show n-type conduction due to the Cr contacts that form low Schottky barriers with the ReSe2 nanosheet. We show that the optoelectronic performance of these FETs is strongly affected by air pressure, and it undergoes a dramatic increase in conductivity when the pressure is lowered below the atmospheric one. Surface-adsorbed oxygen and water molecules are very effective in doping ReSe2; hence, FETs based on this two-dimensional (2D) semiconductor can be used as an effective air pressure gauge. Finally, we report negative photoconductivity in the ReSe2 channel that we attribute to a back-gate-dependent trapping of the photo-excited charges.