Nanoscale enhancement of photoconductivity by localized charge traps in the grain structures of monolayer MoS2

Abstract

We report a method for mapping the nanoscale anomalous enhancement of photoconductivity by localized charge traps in the grain structures of a molybdenum disulfide (MoS2) monolayer. In this work, a monolayer MoS2 film was laterally scanned by a nanoscale conducting probe that was used to make direct contact with the MoS2 surface. Electrical currents and noise maps were measured through the probe. By analyzing the data, we obtained maps for the sheet resistance and charge trap density for the MoS2 grain structures. The maps clearly show grains for which sheet resistance and charge trap density were lower than those of the grain boundaries. Interestingly, we found an unusual inverse proportionality between the sheet resistance and charge trap density in the grains, which originated from the unique role of sulfur vacancies acting as both charge hopping sites and traps in monolayer MoS2. In addition, under light illumination, the larger the trap density of a region was, the larger the photocurrent of the region was, indicating anomalous enhancement of the photocurrent by traps. Since our method provides valuable insights to understand the nanoscale effects of traps on photoconductive charge transport, it can be a powerful tool for noise studies and the practical application of two-dimensional materials.

Mapping the nanoscale effects of charge traps on electrical transport in grain structures of indium tin oxide thin films

“…In previous studies, it was reported that when electrical noises were generated by a small number of trap states with a similar trapping time, a noise PSD spectrum exhibited a scaling behavior of ∼1/ f 2 . 20–22 On the other hand, in the case of numerous trap states with random trapping time, a noise spectrum usually shows ∼1/ f behavior. 20–22 Our results with ∼1/ f behavior indicate that there were numerous trap states with different trapping times in our bulk ITO samples, as reported previously.…”

“… 23 Recently, we reported a scanning noise microscopy method that allows one to map the activities of electrical noise sources in lateral and vertical electrical channels at a nanoscale resolution. 20,24,25 In the conducting film, lateral transport is the dominant conduction mechanism between the electrodes. 25 However, such a method has not been utilized for ITO films.…”

“…2D materials such as graphene, reduced graphene oxide (RGO), and MoS 2 do not exhibit much variation in sheet conductance within a domain or grain. 20,24,25 Since our method provides valuable insights into the nanoscale charge transport and charge trap activities in ITO thin films, it can be a useful tool for the basic research and development of optoelectronic devices based on ITO.…”

Site-specific electrical contacts with the two-dimensional materials

“…The main contributions of the voltage drop are R S and R Contact . The resistance of the MoS 2 flake is around 1.6 × 10 3 to 6.4 × 10 5 Ω 58,59 , which is much smaller than the total resistance (>10 8 Ω). On the other hand, the interlayer resistance of the MoS 2 flake is at most 5.1 × 10 4 Ω 60 .…”

“…Spin-polarized DFT calculations are carried out using the Vienna ab initio simulation package (VASP) program package 58 within the projector augmented wave (PAW) to explore geometries and electronic properties of MoS 2 . The exchange-correlation functions are described with the generalized gradient approximation (GGA) in the form of the Perdew, Burke, and Ernzernhof (PBE) functional 59 . The DFT-D3 scheme of Grimme for the vdW correction 60 is applied on multiple layers of MoS 2 .…”