As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS₂) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS₂ is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS₂. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS₂ transistors. The generation and transport of photocurrent in multilayer MoS₂ are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS₂, the PVE at the MoS₂-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS₂, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS₂ lattice.

Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, I_on/I_off. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given I_on/I_off as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS₂ films. A difference in I_on/I_off by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in I_on/I_off is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, I_on/I_off approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of I_on/I_off with t. This excellent agreement confirms that multilayer-MoS₂ films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate I_on/I_off. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

In this paper, capacitive-type humidity sensors were prepared by sequentially drop-coating the aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinyl pyrrolidone–reduced graphene oxide (PVP-RGO) nanocomposites onto interdigitated electrodes. Significant improvements in both sensitivity and linearity were achieved for the ZnO/PVP-RGO sensors compared with the PVP-RGO/ZnO, PVP-RGO, and ZnO counterparts. Moreover, the produced ZnO/PVP-RGO sensors exhibited rather small hysteresis, fast response-recovery time, and long-term stability. Based on morphological and structural analyses, it can be inferred that the excellent humidity sensing properties of the ZnO/PVP-RGO sensors may be attributed to the high surface-to-volume ratio of the multilayer structure and the supporting roles of the PVP-RGO nanocomposites. The results in this work hence provide adequate guidelines for designing high-performance humidity sensors that make use of the multilayer structure of semiconductor oxide materials and PVP-RGO nanocomposites.

Chemical vapor deposition (CVD) is the most efficient method to grow large-area two dimensional (2D) transition metal dichiacogenides (TMDCs) in high quality. Monolayer molybdenum disulfide (MoS₂) and seed-assistant are the mostly selected 2D TMDC and growth strategy for such CVD processes, respectively. Though the advantages of seed catalysts in facilitating the nucleation, achieving higher yield and better repeatability, as well as their effects on the morphologies of as-grown MoS₂ have been studied, the influence of seeding promoters on both optical and electrical properties of as-grown monolayer MoS₂ is not known comprehensively, which is indeed critical for understanding fundamental physics and developing practical application of such emerging 2D semiconductors. In this report, we systematically investigated the effect of different seeding promoters on the properties of CVD-grown monolayer MoS₂. It is found that different seed molecules lead to different impacts on the optical and electrical properties of as-grown monolayer MoS₂. Among three different seed catalysts (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), copper phthalocyanine (CuPc), and crystal violet (CV)), PTAS performs better in obtaining large area monolayer MoS₂ with good optical quality and high electrical mobility than the other two. Our work gives a guide for modifying the properties of as-grown monolayer MoS₂ and other 2D transition metal dichalcogenides in seeding promoters-assisted synthesis process.