MoS2-Based Photodetectors Powered by Asymmetric Contact Structure with Large Work Function Difference

Abstract

Self-powered devices are widely used in the detection and sensing fields. Asymmetric metal contacts provide an effective way to obtain self-powered devices. Finding two stable metallic electrode materials with large work function differences is the key to obtain highly efficient asymmetric metal contacts structures. However, common metal electrode materials have similar and high work functions, making it difficult to form an asymmetric contacts structure with a large work function difference. Herein, Mo₂C crystals with low work function (3.8 eV) was obtained by chemical vapor deposition (CVD) method. The large work function difference between Mo₂C and Au allowed us to synthesize an efficient Mo₂C/MoS₂/Au photodetector with asymmetric metal contact structure, which enables light detection without external electric power. We believe that this novel device provides a new direction for the design of miniature self-powered photodetectors. These results also highlight the great potential of ultrathin Mo₂C prepared by CVD in heterojunction device applications.

Keywords: Asymmetric metal contacts; Chemical vapor deposition; Mo2C; MoS2; Photodetector.

Fig. 1

Schematic illustration (top) and calculated electronic band structure (bottom) of a monolayer MoS₂ and b monolayer Mo₂C. c Schematic diagram (top) and energy band diagram (bottom) of MoC₂/MoS₂/Au device

Fig. 2

a Schematic diagram of the CVD method to grown Mo₂C. In the type I method, copper foil is placed on molybdenum foil to directly grow molybdenum carbide. In the type II method, the molybdenum foil is placed near the copper foil. Copper evaporates at high temperatures and adsorbs on the molybdenum foil for the growth of Mo₂C. b Optical image of Mo₂C prepared by the type I method with a growth time of 20 min. c Schematic diagram of the type I Mo₂C growth process. d Optical image of Mo₂C prepared by the type II method. e Schematic diagram of the type II Mo₂C growth process. The black line represents graphene

Fig. 3

a High-resolution TEM image of Mo₂C, with space group Pbcn. b SAED pattern along the [$\overline{1}00$] zone axis. c Raman spectrum of MoS₂. d High-resolution TEM image of MoS₂, with space group P6₃/mmc. e SAED pattern along the [0001] zone axis. f Raman spectrum of MoS₂

Fig. 4

a Optical image of the Mo₂C/MoS₂/Mo₂C device. b Dark (black) and light (red) I–V curves of the Mo₂C/MoS₂/Mo₂C photodetector. The inset image shows the magnified dark I–V curve of the photodetector

Fig. 5

a Optical image and schematic illustration of the Mo₂C/MoS₂/Au device. b |I|–V curves of Mo₂C/MoS₂/Au devices with (red) and without (black) 80 mW cm⁻² white light irradiation. The inset image shows the magnified dark current. c Magnified I-V curves of Mo₂C/MoS₂/Au devices with a bias voltage range of − 0.1 to 0.3 mV. d Photocurrent response of self-powered Mo₂C/MoS₂/Au (red) and Au/MoS₂/Au (black) devices under 80 mW cm⁻² white light irradiation. The image in the inset shows the magnified dark current. e Transfer characteristic curves of the photodetector with and without white light irradiation

Fig. 6

Schematic band energy diagram of the Mo₂C/MoS₂/Au device

Fig. 7

a Photodetector responses to light with various wavelengths and energy density of 0.5 mW cm⁻². b Photodetector responses to 600 nm light of different intensities. c Response and recovery times of the photodetectors. The intensity of the incident light (600 nm) is 0.56 mW cm⁻². d Long-term performance of the Mo₂C/MoS₂/Au photodetector under illumination with 600 nm light