Photodetectors Based on Two-Dimensional Materials and Their van der Waals Heterostructures

Two-dimensional (2D) layered materials have garnered increasing interest in the past few years due to their unique structures and novel properties. These 2D layered materials with atomic thicknesses cover metals, semiconductors, and insulators, including graphene, black phosphorus (BP), transition metal dichalcogenides (TMDs) and hexagonal boron nitride (BN). Their bandgaps are usually tunable by changing the number of layers and the thicknesses. These 2D material are also sensitive to changes in the surrounding environment, e.g. changes in temperature, pressure, and illumination. Particularly, most 2D materials have high absorption coefficients. Owing to their excellent performance in electronics and optoelectronics and their potential for further development, many optoelectronic devices based on 2D materials, such as photodetectors, have been manufactured and widely used. In this paper, the latest progress of photodetectors based on 2D materials has been outlined. We introduce some 2D materials and their preparation methods, and the mechanisms of photodetectors based on 2D materials, i.e. photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, have been discussed. Next, we summarize the parameters used to evaluate the performance of photodetectors, including photoresponsivity, external quantum efficiency, internal quantum efficiency, photoconductive gain, signal-to-noise ratio, noise-equivalent power, response time, cutoff frequency, linear dynamic range, and specific detectivity. We also report some recent studies on photodetectors based on 2D materials; among the 2D materials used in these studies, graphene, TMDs, and BP are the most widely used. Many methods have been proposed to improve the performances of photodetectors based on 2D materials, such as doping, designing novel structures, changing the dielectric layer, modifying the contact between channel and electrodes, controlling the surface and the interface, etc. Compared to single 2D materials, heterostructures composed of different 2D materials are more promising for use in photodetectors because they combine materials with different properties, which makes it possible to obtain photodetectors with desired and enhanced performances. Thus, we present some van der Waals heterojunctions and their applications in photodetectors. Finally, we provide a brief summary of the full article and an outlook for future development.