Advances in graphene/molybdenum dichalcogenide-based van der Waals heterostructure photodetectors

Abstract

Graphene is widely used in photodetection because of its high carrier mobility and wide spectral absorption range. However, its high dark current caused by its low light absorption severely limits its performance. Molybdenum dihalide (MoX2, X=S, Se and Te) has a high absorption coefficient, which can compensate for the high dark current in graphene-based photodetectors and result in outstanding photoelectronic properties of those based on a graphene/MoX2 van der Waals heterostructure (vdWH). In this review, we firstly review working principles, performance indicators, and structures of photodetectors. After that, the significance of graphene/MoX2 vdWH photodetectors is highlighted from the fundamental perspective. Preparation methodologies and performance enhancement strategies of graphene/MoX2 vdWH photodetectors are correspondingly summarized. In the end, we highlight the current challenges and future directions of the graphene/MoX2 vdWH photodetectors. This review will guide the design of high-performance vdWH photodetectors.

1. Introduction

Photodetectors can convert the absorbed optical signal into electrical signal[1], which are widely used in military reconnaissance[2], medical imaging[3], astronomy[4], optical communication[5], fire warning[6], temperature detection[7], etc. Based on the working mechanism, photodetectors can be categorized into photon detectors[8] and thermal detectors[9]. The thermal detectors can convert the infrared radiation energy induced by temperature change into electricity, which includes pneumatic detector[10], thermocouple[11], thermistor[12] and pyroelectric detector[13,14]. Thermal detectors can be used in fire alarming[15], temperature detection[16], rocket engine[17], thermal imaging[18], etc. However, their application is limited by temperature fluctuation noise arising from the radiating background. This drawback can be overcome by photon detectors[19]. Photon detectors are radiation detectors based on either external[2,20] or internal photoelectric effects[21,22](Fig. 1). Modern photodetectors are mostly based on the internal photoelectric effect, which are solid state electronic devices made of semiconductor materials and also known as semiconductor photodetectors[23]. In semiconductor photodetectors, two-dimensional (2D) material-based photodetectors[23] have attracted extensive interest because of their ultra-wide spectral response range from ultraviolet to terahertz, ultra-speedy photoresponsivity, and high spatial resolution for imaging[24].

The signal conversion process from optical signal to electrical signal in 2D material-based photodetectors can be separated into in 3 steps. (1) When the incident photons are absorbed by the photosensitive materials after the light trapping process, the photoexcited electrons transit from the valence band (VB) to the conduction band (CB) of the photosensitive materials and left holes in the VB. In this step, materials with high absorption coefficient and wide spectrum absorption range play important roles in improving the performance of 2D material-based photodetectors. (2) Under the applied external electric field or the internal electric field formed at the heterostructure interface, the photoinduced electron-hole pairs can be separated, and different carriers move to the corresponding electrodes along the conduction path. In this step, high carrier mobility (μ) of the material can contribute to high photoelectronic performance. (3) After the charge carriers arrive at the electrodes, in accordance with configurations of different electrodes, charge carriers can be extracted and conducted to the external circuit and generate the photocurrent[25]. Therefore, it can be seen that high-performance photodetectors require materials with high μ, high absorption coefficient, and wide spectrum absorption range[26].

Graphene, as one of the most important 2D layered materials, is a honeycomb crystal with a closed-packed hexagonal structure of single carbon atom[29–31]. Graphene has a Dirac coni...