A great challenge is presented when metals have contact with a 2D semiconducting material because the contact resistances (Rc) induced at the metal‐graphene interfaces hinder the performances of 2D devices, and therefore low resistance Ohmic contacts need to be developed to achieve unique and high performance of the 2D devices. This study demonstrates that edge‐contacted graphene devices of multiple stacked 2D hetero‐structures with hexagonal boron nitride (hBN) exhibit superior performances in carrier transport across channel and contact regions, compared to surface‐contacted devices. In surface‐contacted graphene devices, Rc and contact resistivity (ρc) are calculated by applying the modified transfer length (LT*) obtained from the contact‐end‐resistance method, while Rc and ρc in edge‐contacted graphene devices are estimated by replacing the LT* with the thickness of graphene. The edge‐contacted device is fabricated via a controlled plasma etching that allows each layer of graphene and hBN consisting hetero‐structures to be removed evenly at a uniform speed. Four‐point probe measurements are conducted in addition to transmission line method and confirms that ρc is lower for edge contact than surface contact. ρc of a graphene edge‐contacted device (≈10 Ω µm2) is much lower than that of a surface‐contacted device (≈230 Ω µm2).

Recently, 2D materials have been intensively investigated for their novel nanoelectronic applications; among these materials, tungsten diselenide (WSe 2 ) is attracting substantial research interest due to its high mobility, sizable bandgap, and ambipolar characteristics. However, Fermi-level pinning (FLP) at the metal-semiconductor contact is a critical issue preventing further integration of WSe 2 to complementary metal-oxide-semiconductor (CMOS) technology. In this study, a facile doping method of oxygen (O 2 ) plasma treatment and an aging effect to overcome the FLP of WSe 2 field-effect transistors (FETs) are utilized. After aging, a reduction is observed in FLP on oxidized WSe 2 FETs, along with a decrease in pinning factor (S) for holes from −0.06 to −0.36. Further, the field-effect mobility of high-(Pd) and low-(In) work-function contacted WSe 2 devices indicates the presence of more improvement in highwork-function metal-contacted p-type WSe 2 FETs, which further strengthens the Fermi level de-pinning behavior attributed to the O 2 plasma and aging processes. The existence of different tunneling behaviors of Pd and In devices also confirms the effect of O 2 plasma doping into WSe 2 FETs. Ultimately, this work demonstrates a simple and efficient method for achieving the de-pinning of Fermi-levels and modulating FLP of 2D FETs.

Even though atomically thin 2D semiconductors have shown great potential for next‐generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high‐mobility field‐effect transistors (FETs) by introducing few‐layer PdSe2 flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect‐free van der Waals (vdW) stackings at the Sb–PdSe2 heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on‐current density reaching 96 µA µm−1, and high electron mobility of 383 cm2 V−1 s−1. Furthermore, metal–insulator transition (MIT) is observed in the PdSe2 FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe2 vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T0, and ultrahigh electron mobility of 2,184 cm2 V−1 s−1 at 10 K. These findings are expected to be significant for developing high mobility 2D‐based quantum devices.

Palladium diselenide (PdSe 2 ), as an emerging twodimensional (2D) layered material, is gaining growing attention in nanoelectronics and optoelectronics due to its thickness-dependent band gap, high carrier mobility, and good air stability. However, its asymmetric pentagon structure is inclined to breed defects. Herein, the intrinsic Se vacancy-induced trap states and their influence on the hopping transport in PdSe 2 are systematically investigated. We provide direct evidence that Se vacancies exist in the fresh PdSe 2 samples, which results in the localized trapping states inside the band gap. For the few-layer PdSe 2 , at 77 K, the trap density (D it ) near the midgap is about 2.2 × 10 13 cm −2 eV −1 , whereas at 295 K, the D it value increases to ∼7.1 × 10 13 cm −2 eV −1 . By comparison, the multilayer PdSe 2 shows nonobvious temperature-dependent trap behaviors with almost unchanged D it values of ∼8.1 × 10 12 cm −2 eV −1 at midgap in the temperature range between 77 and 295 K. Thus, trap states in the few-layer PdSe 2 are more vulnerable to temperature effect. Transport measurements demonstrated that both few-layer and multilayer PdSe 2 field-effect transistor (FET) devices show n-type dominant ambipolar behaviors. The electron mobility in the multilayer PdSe 2 FET is nearly 15-fold higher than that in the few-layer PdSe 2 FET at 315 K, probably owing to the decreased effective mass and suppression of charge impurity scattering in the thicker channel material. However, both FET devices exhibit variable-range hopping over a temperature range from 77 to 240 K and thermally activated hopping at temperatures above 240 K. The hopping transport mechanism is strongly associated with the Se vacancy-induced localized states with poor screening and strong potential fluctuations. This study reveals the important role of structural defects in tailoring and improving the charge transport properties of PdSe 2 .

Carrier–carrier interactions or disorders strongly affect the quantum localization–delocalization of carriers which leads to the metal to insulator transition (MIT) in two-dimensional (2D) systems. However, the physical origin of MIT in 2D systems remains controversial. Here, we report the MIT in black phosphorus (BP) nanosheet-based devices with and without the encapsulation of hexagonal boron nitride (hBN). In hBN encapsulated BP devices, we perform critical scaling analysis to elucidate the microscopic origin of 2D MIT by considering the significant role of carrier–carrier interactions over disorder. We find the critical exponents zν = 2.49 ± 0.05 and zν = 2.65 ± 0.06 in metallic and insulting phases, respectively, supporting the quantum percolation (zν = 7/3). In hBN unencapsulated BP devices, the Mott variable range hopping dominates the charge transport in the insulating phase, suggesting that the disorder plays a significant role over the carrier–carrier interactions. The extracted conductivity exponent of 1.31 ± 0.01 at 10 K approaches the 2D percolation exponent value of 4/3, which supports the classical percolation-based 2D MIT. Our findings pave the way toward the utilization of BP with and without hBN encapsulation as a model system with which to study the 2D MIT as well as various classical and quantum transports in 2D nanoelectronic devices.