The unique properties of two-dimensional (2D) materials make them promising candidates for chemical and biological sensing applications. However, most 2D material sensors suffer from extremely long recovery time due to the slow molecular desorption at room temperature. Here, we report an ultrasensitive p-type molybdenum ditelluride (MoTe) gas sensor for NO detection with greatly enhanced sensitivity and recovery rate under ultraviolet (UV) illumination. Specifically, the sensitivity of the sensor to NO is dramatically enhanced by 1 order of magnitude under 254 nm UV illumination as compared to that in the dark condition, leading to a remarkable low detection limit of 252 ppt. More importantly, the p-type MoTe sensor can achieve full recovery after each sensing cycle well within 160 s at room temperature. Finally, the p-type MoTe sensor also exhibits excellent sensing performance to NO in ambient air and negligible response to HO, indicating its great potential in practical applications, such as breath analysis and ambient NO detection. Such impressive features originate from the activated interface interaction between the gas molecules and p-type MoTe surface under UV illumination. This work provides a promising and easily applicable strategy to improve the performance of the gas sensors based on 2D materials.

van der Waals (vdW) p–n heterojunctions formed by two-dimensional nanomaterials exhibit many physical properties and deliver functionalities to enable future electronic and optoelectronic devices. In this report, we demonstrate a tunable and high-performance anti-ambipolar transistor based on MoTe2/MoS2 heterojunction through in situ photoinduced doping. The device demonstrates a high on/off ratio of 105 with a large on-state current of several micro-amps. The peak position of the drain–source current in the transfer curve can be adjusted through the doping level across a large dynamic range. In addition, we have fabricated a tunable multivalue inverter based on the heterojunction that demonstrates precise control over its output logic states and window of midlogic through source–drain bias adjustment. The heterojunction also exhibits excellent photodetection and photovoltaic performances. Dynamic and precise modulation of the anti-ambipolar transport properties may inspire functional devices and applications of two-dimensional nanomaterials and their heterostructures of various kinds.

Energy band engineering is of fundamental importance in nanoelectronics. Compared to chemical approaches such as doping and surface functionalization, electrical and optical methods provide greater flexibility that enables continuous, reversible, and in situ band tuning on electronic devices of various kinds. In this report, we demonstrate highly effective band modulation of MoTe2 field-effect transistors through the combination of electrostatic gating and ultraviolet light illumination. The scheme can achieve reversible doping modulation from deep n-type to deep p-type with ultrafast switching speed. The treatment also enables noticeable improvement in field-effect mobility by roughly 30 and 2 times for holes and electrons, respectively. The doping scheme also provides good spatial selectivity and allows the building of a photo diode on a single MoTe2 flake with excellent photo detection and photovoltaic performances. The findings provide an effective and generic doping approach for a wide variety of 2D materials.

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van der Waals heterostructures based on two-dimensional (2D) materials have attracted tremendous attention for their potential applications in optoelectronic devices, such as solar cells and photodetectors. In addition, the widely tunable Fermi levels of these atomically thin 2D materials enable tuning the device performances/functions dynamically. Herein, we demonstrated a MoTe 2 /BP heterostructure, which can be dynamically tuned to be either p−n or p−p junction by gate modulation due to compatible band structures and electrically tunable Fermi levels of MoTe 2 and BP. Consequently, the electrostatic gating can further accurately control the photoresponse of this heterostructure in terms of the polarity and the value of photoresponsivity. Besides, the heterostructure showed outstanding photodetection/voltaic performances. The optimum photoresponsivity, external quantum efficiency, and response time as a photodetector were 0.2 A/W, 48.1%, and 2 ms, respectively. Our study enhances the understanding of 2D heterostructures for designing gatetunable devices and reveals promising potentials of these devices in future optoelectronic applications.

Understanding and engineering the interface between metal and two-dimensional materials are of great importance to the research and development of nanoelectronics. In many cases the interface of metal and 2D materials can dominate the transport behavior of the devices. In this study, we focus on the metal contacts of MoTe (molybdenum ditelluride) FETs (field effect transistors) and demonstrate how to use post-annealing treatment to modulate their transport behaviors in a controlled manner. We have also carried out low temperature and transmission electron microscopy studies to understand the mechanisms behind the prominent effect of the annealing process. Changes in transport properties are presumably due to anti-site defects formed at the metal-MoTe interface under elevated temperature. The study provides more insights into MoTe field effect devices and suggests guidelines for future optimizations.

The discovery of atomically thin two-dimensional materials enables building numerous van der Waals heterostructures with original and promising properties for potential electronic and optoelectronic applications. Among them, the antiambipolar characteristic is one of the most appealing ones, which refers to the inverse “V” shape of the transfer curve of the heterojunction. As a result, it is expected to implement various important logic functions, such as double-frequency and multivalue. In this work, we modulated an ambipolar MoTe2/MoS2 heterojunction to show prominent antiambipolar behavior by simply annealing the device at elevated temperature. The on-off ratio and on-state current of the antiambipolar characteristic can be tuned as large as 106 and approximately microamperes, respectively, by optimizing the annealing temperature. Furthermore, we preliminarily demonstrated a self-powered photodetector and a ternary inverter based on this device. The photodetector showed a short-current circuit and an open-circuit voltage of 0.4 μA and 7.5 mV, respectively, at incident light intensity of 2.54 mW/cm2, and gate tunable photocurrent ranging from 0 to 380 pA under zero source-drain bias. The ternary inverter can output three distinct values varying on the order of subvolt as the input voltage (gate bias) ranges from −60 V to 60 V.

We developed a new way to enhance the photoresponsivity of a van der Waals heterojunction p-n diode using surface acoustic waves (SAWs). The diode was constructed on top of a piezoelectric LiNbO3 substrate and composed of p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS2) flakes that partly overlapped with each other. This layout facilitated the applied SAWs to rapidly drive carriers out of the depletion region. In this structural design, SAWs promoted the separation of photogenerated carriers, and thus greatly increased the photocurrent. The measured photocurrent for the device with SAWs was about 103 times higher than that of the device without SAWs. The device using SAWs showed a photoresponsivity as high as 2.17 A W-1 at the wavelength of 582 nm. This excellent performance was attributed to the SAWs suppressing electron-hole recombination in the device under light illumination. Our device exhibits promise as a high-performance photodetector and reveals new possibilities for acoustic devices in optoelectronics.