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Gas sensing research based on transition metal dichalcogenides (TMDs) suffers from irrecoverable responses. Prior attempts to improve the recoverability, such as applying heat to the material or using heterostructure designs, pose risks of damaging the surface or increasing sensor size. The ability to achieve repeatability is of the utmost importance in gas sensing research. In this study, we propose a novel approach that leverages the surface molecule trapping characteristic of TMDs to investigate gas adsorption on the material surface, which enables the repeatability of gas sensors. Our method involves regulating the gas sensing material surface by applying high concentrations of the target adsorbent to fill inactive sites on the material surface. To validate the effectiveness of our method, we introduced a two-trap model that combines physical and chemical adsorption. We applied various concentrations of the target adsorbent in different sequences, resulting in a highly linear relationship and repeatable outcomes. Our results demonstrate that there is a consistent relationship between the response and the target adsorbent, providing evidence that our method is capable of achieving repeatability for gas sensors. Therefore, our proposed method offers a promising solution to the challenge of irrecoverable responses and emphasizes the crucial importance of repeatability in the development of gas sensing technologies.

Quantum optics and photonic quantum-information technologies require emitters that have good stability and brightness, coupled with fabrication scalability and on-chip integrability. Most quantized emitters are presently based on 1D and 3D sources. Recently, monolayer transition metal dichalcogenides (TMDCs) hosting spatially localized excitons with narrow linewidths have garnered great interest. Advantages such as large binding energies and long room-temperature lifetimes of intralayer excitons suggest that TMDCs are promising candidates for use in optical devices. Here, we propose an emitter based on a 2D WSe2semiconductor monolayer integrated with a periodic 3D Si-based wrinkled pattern. Carriers confined within the wrinkled pattern can be electrically and optically pumped, and funneled, to boost emission from the 2D WSe2layer. This in turn acts as a monochromated quantum light source for the Si or any Si-based quantum optic and photonic information technologies. The brightness of the emission is enhanced by a factor greater than 40 compared with monolayer WSe2on conventional flat SiGe. Moreover, these monolayer 2D/3D semiconductor composite heterostructures are fully scalable and promisingly efficient chip-integrated emitters.

van der Waals heterojunctions with tunable polarity are being actively explored for more Moore and more-than-Moore device applications, as they can greatly simplify circuit design. However, inadequate control over the multifunctional operational states is still a challenge in their development. Here, we show that a vertically stacked InSe/SnS2van der Waals heterojunction exhibits type-II band alignment, and its polarity can be tuned by an external electric field and by the wavelength and intensity of an illuminated light source. Moreover, such SnS2/InSe diodes are self-powered broadband photodetectors with good performance. The self-powered performance can be further enhanced significantly with gas adsorption, and the device can be quickly restored to the state before gas injection using a gate voltage pulse. Our results suggest a way to achieve and design multiple functions in a single device with multifield coupling of light, electrical field, gas, or other external stimulants.

Layered materials are highly attractive in gas sensor research due to their extraordinary electronic and physicochemical properties. The development of cheaper and faster room-temperature detectors with high sensitivities especially in the parts per billion level is the main challenge in this rapidly developing field. Here, we show that sensitivity to NO2(S) can be greatly improved by at least two orders of magnitude using an n-type electrode metal. Unconventionally for such devices, the ln(S) follows the classic Langmuir isotherm model rather than Sas is for a p-type electrode metal. Excellent device sensitivities, as high as 13,000% for 9 ppm and 97% for 1 ppb NO2, are achieved with Mn electrodes at room temperature, which can be further tuned and enhanced with the application of a bias. Long-term stability, fast recovery, and strong selectivity toward NO2are also demonstrated. Such impressive features provide a real solution for designing a practical high-performance layered material-based gas sensor.

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2or WS2gas sensors, the MoS2/WS2van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.

The strong light–matter interaction and naturally passivated surfaces of van der Waals materials make heterojunctions of such materials ideal candidates for high-performance photodetectors. In this study, we fabricated SnS2/MoS2van der Waals heterojunctions and investigated their photoelectric properties. Using an applied gate voltage, we can effectively alter the band arrangement and achieve a transition in type II and type I junctions. It is found that the SnS2/MoS2van der Waals heterostructures are type II heterojunctions when the gate voltage is above −25 V. Below this gate voltage, the heterojunctions become type I. Photoelectric measurements under various wavelengths of incident light reveal enhanced sensitivity in the ultraviolet region and a broadband sensing range from 400 to 800 nm. Moreover, due to the transition from type II to type I band alignment, the measured photocurrent saturates at a specific gate voltage, and this value depends crucially on the bias voltage and light wavelength, providing a potential avenue for designing compact spectrometers.

The fabrication of graphene/SnS2van der Waals photodetectors and their photoelectrical properties are systematically investigated. It was found that a dry transferred graphene/SnS2van der Waals heterostructure had a broadband sensing range from ultraviolet (365 nm) to near-infrared (2.24 μm) and respective improved responsivities and photodetectivities of 7.7 × 103A/W and 8.9 × 1013jones at 470 nm and 2 A/W and 1.8 × 1010jones at 1064 nm. Moreover, positive and negative photoconductance effects were observed when the photodetectors were illuminated by photon sources with energies greater and smaller than the bandgap of SnS2, respectively. The photoresponsivity (R) versus incident power density (P) follows the empirical law R∝ Pinβ, with β > −1 for positive photoconductance effects and β < −1 for negative photoconductance effects. On the basis of the Fowler–Nordheim tunneling model and a Poisson and drift-diffusion simulation, we show quantitatively that the barrier height and barrier width of the heterostructure photodetector could be controlled by a laser and an external electrical field through a photogating effect generated by carriers trapped at the interface, which could be used to tune the separation and transport of photogenerated carriers. Our results may be useful for the design of high performance van der Waals heterojunction photodetectors.