No abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are the subject of intense investigation for applications in optics, electronics, catalysis, and energy storage. Their optical and electronic properties can be significantly enhanced when encapsulated in an environment that is free of charge disorder. Because hexagonal boron nitride (h-BN) is atomically thin, highly-crystalline, and is a strong insulator, it is one of the most commonly used 2D materials to encapsulate and passivate TMDCs. In this report, we examine how ultrathin h-BN shields an underlying MoS 2 TMDC layer from the energetic argon plasmas that are routinely used during semiconductor device fabrication and post-processing. Aberrationcorrected Scanning Transmission Electron Microscopy is used to analyze defect formation in both the h-BN and MoS 2 layers, and these observations are correlated with Raman and photoluminescence spectroscopy. Our results highlight that h-BN is an effective barrier for short plasma exposures (< 30 secs) but is ineffective for longer exposures, which result in extensive knock-on damage and amorphization in the underlying MoS 2.

A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm−2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film.

Monolayer MoS2 crystals investigated in this work were grown via chemical vapor deposition on Si/SiO2 substrates. Using a wet KOH etch, these crystals were transferred onto the edge of a freshly cleaved p-Si/SiO2 wafer where they formed mechanically robust heterojunctions at the p-Si/MoS2 interface. Electrical characterization of the device across the junction yielded an asymmetric I–V response similar to that of a p-n diode. The I–V response was electrostatically tunable via an ionic liquid gel gate. This is the first report demonstrating reversible gate control of the p-Si/MoS2 diode current by several orders of magnitude while lowering its turn-on voltage. Fermi energy level shifts within the MoS2 bandgap by the gate was believed to be responsible for the observed effects. The ease of fabrication, low operating voltages (&lt;±2 V), and moderately high throughput currents (∼1 µA) are attractive features of this diode, especially for use in sensors and power saving electronics.

A chemical vapor-deposited monolayer MoS 2 crystal was crossed with an electro-spun PEDOT-PSS nano-ribbon under ambient conditions. The current-voltage (I-V) curve measured across the hetero-junction was nonlinear and asymmetric, similar to a diode. Under thermal equilibrium, electrons flowed from the MoS 2 conduction band into the PEDOT-PSS LUMO level. This occurred via band bending that established a constant Fermi level across the interface. Consequently, a potential barrier was formed that restricted the current. Under normal operation in air, the diode turn-on voltage was 0.1 V and the rectification ratio at ± 1 V was 20. The thermionic emission Schottky junction model was employed for data analysis. The ideality factor was 1.9, and height of the barrier was 0.18 eV. The easy fabrication, low turn-on voltage and high rectification ratio could make this diode useful in cheap, low-power-consumption signal rectifiers.