Hexagonal molybdenum diselenide (MoSe2) multilayers were grown by chemical vapor deposition (CVD). A relatively high pressure (>760 Torr) was used during the CVD growth to achieve multilayers by creating multiple nuclei based on the two-dimensional crystal growth model. Our CVD-grown multilayer MoSe2 thin-film transistors (TFTs) show p-type-dominant ambipolar behaviors, which are attributed to the formation of Se vacancies generated at the decomposition temperature (650 °C) after the CVD growth for 10 min. Our MoSe2 TFT with a reasonably high field-effect mobility (10 cm2/V · s) exhibits a high photoresponsivity (93.7 A/W) and a fast photoresponse time (τrise ~ 0.4 s) under the illumination of light, which demonstrates the practical feasibility of multilayer MoSe2 TFTs for photodetector applications.

Local-gate multilayer MoS2 phototransistors exhibit a photoresponsivity of up to 342.6 A W(-1) , which is higher by 3 orders of magnitude than that of global-gate multilayer MoS2 phototransistors. These simulations indicate that the gate underlap is critical for the enhancement of the photoresponsivity. These results suggest that high photoresponsivity can be achieved in indirect-bandgap multilayer MoS2 phototransistors by optimizing the optoelectronic design.

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Molybdenum disulfide (MoS) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Indirect bandgap of multilayer molybdenum disulfide has been recognized as a major hindrance to high responsivity of MoS2 phototransistors. Here, to overcome this fundamental limitation, we propose a structural engineering of MoS2 via nano-patterning using block copolymer lithography. The fabricated nanoporous MoS2, consisting of periodic hexagonal arrays of hexagon nanoholes, includes abundant edges having a zigzag configuration of atomic columns with molybdenum and sulfur atoms. These exposed zigzag edges are responsible for multiple trap states in the bandgap region, as confirmed by photo-excited charge-collection spectroscopy measurements on multilayer nanoporous MoS2 phototransistors, showing that in-gap states only near the valence band can result in a photogating effect. The effect of nano-patterning is therefore to significantly enhance the responsivity of multilayer nanoporous MoS2 phototransistors, exhibiting an ultra-high photoresponsivity of 622.2 A W−1. Our nano-patterning of MoS2 for photosensing application paves a route to structural engineering of two-dimensional materials for highly sensitive and responsive optoelectronic devices.

We investigate bilayer phosphorene field-effect transistors (FETs) by self-consistent atomistic quantum transport simulations. Despite a penalty in electrostatic control for multiple layers, 10-nm-channel bilayer phosphorene FETs can exhibit excellent device characteristics, such as I on > 3 mA/µm, large current ratio (>10 7 ), and small subthreshold swing (SS) of 66 mV/dec, with a double-gate device structure. While the scaling of gate dielectric monotonically enhances the overall performance of this device, channel length can only be scaled down to ∼8 nm due to significant short-channel effects. We benchmark bilayer phosphorene FETs against bilayer MoS 2 and WSe 2 FETs along with a monolayer phosphorene device, which reveals that bilayer phosphorene FETs have favorable switching characteristics over other similar 2-D bilayer semiconductor devices, making both monolayer and bilayer phosphorene attractive for future switching applications. Our simulation results not only provide the performance and scaling limit of bilayer phosphorene FETs but also create irreplaceable insights into proper device design and parameter optimizations.Index Terms-Bilayer Phosphorene, Field-Effect Transistor, quantum transport, non-equilibrium Green's function.

chemical doping, elemental doping, and electrostatic gating techniques. The photoresponsive performances of PN-junction photodiodes have thus been effectively increased compared to intrinsic 2D material-based photodiodes. [18,19] Although PN-homojunction photodiodes exhibit better optical properties than intrinsic 2D materials photodiodes, there are many challenges present in fabricating the junctions. Chemical doping techniques are highly dependent on environmental conditions, [12,13] the elemental doping method causes limited vertical junction formation which leads to lower charge mobility, [5] and the electrostatic doping method has led to devices with slow photoresponses. [14] Heterojunction photodiodes with dissimilar materials and unequal bandgap have also been studied, using MoS 2 /Si, [20,21] MoS 2 /GaN, [22] MoS 2 / graphene, [23,24] MoS 2 /Black Phosphorous (BP), [6,25] and MoS 2 /pentacene, [26,27] and show improved electrical and optical properties compared to homojunction photodiodes. However, despite notable performance improvements in terms of photoresponsivity and sensitivity, homo-and heterojunction photodiodes are limited by low carrier mobility, difficult processing techniques, and unstable photoresponsive behavior. [18,19] In these designs, the chosen fabrication technique also has a significant influence on the device performance.Many studies have also been reported on Schottky barrier phototransistors, which incorporate a gate terminal to a metal-semiconductor-metal (M-S-M) design to obtain reliable electrical and optical properties along with a stable photoresponse and easier processing methods. [28,29] The gating effect is a key device function in these devices as it tunes the Schottky barrier, which controls charge transfer in the device. [30,31] The forward and reverse saturation currents can be determined by modulating the Schottky barrier by applying positive, negative, or zero bias. [28] However, electrical characteristics are often limited by the interface properties of the materials used, and more generally by fabrication techniques. The investigation of the Schottky contact between metals and MoS 2 is now gaining interest due to possible applications in photodetectors, biosensors, photovoltaics, as well as field-effect transistors (FETs).In this work, we introduce a unique design for MoS 2 photodiodes. This design uses a multilayer MoS 2 semiconductor layer 2D materials, specifically MoS 2 semiconductors, have received tremendous attention for photo-sensing applications due to their tunable bandgap and low noise levels. A unique photodetector using multilayer MoS 2 as the semiconductor channel, in which the gate electrode of the device is permanently connected to the grounded source electrode to introduce rectification, is reported. The proposed grounded-gate photodiode exhibits high photoresponsivity of 1.031 A W −1 , excellent photodetectivity (>6 × 10 10 jones), and highly stable rise/fall time response (100-200 ms) under illumination of visible light (at the wavelengths of 405,...

Woong Choi, Sunkook Kim, Y. Yoon, and co‐workers present local‐gate multilayer MoS2 phototransistors with outstanding photoresponsivity of up to 342.6 A W−1, which is 3 orders of magnitude higher than that of global‐gate multilayer MoS2 phototransistors, as described on page 2224. These results suggest that high photoresponsivity can be achieved in indirect‐bandgap multilayer MoS2 phototransistors by optimizing the optoelectronic design.