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Understanding the operation mode of a two-dimensional (2D) material-based field-effect transistor (FET) is one of the most essential issues in the study of electronics and physics. The existing Schottky barrier FET model for devices with global back gate and metallic contacts overemphasizes the metal/2D contact effect, and the widely observed residual conductance cannot be explained by this model. Here, an accumulation-mode (ACCU) FET model, which directly reveals 2D channel transport properties, is developed based on a partial top-gate MoS FET with metallic contacts and a channel thickness of 0.65-118 nm. The operation mechanism of an ACCU-FET is validated and clarified by carefully performed capacitance measurements. A depletion capacitance-quantum capacitance transition is observed. After the analysis of the MoS ACCU-FET, we have confirmed that most 2D FETs show an accumulation-mode behavior. The universal thickness scaling rule of 2D-FETs is then proposed, which provides guidance for future research on 2D materials.

Although MoS 2 field-effect transistors (FETs) with high-k dielectrics are promising for electron device applications, the underlying physical origin of interface degradation remains largely unexplored. Here, we present a systematic analysis of the energy distribution of the interface state density (D it ) and the quantum capacitance (C Q ) in a dual-gate monolayer exfoliated MoS 2 FET. The C Q analysis enabled us to construct a D it extraction method as a function of E F . A band tail distribution of D it with the lowest value of 810 11 cm -2 eV -1 suggests that D it is not directly related to the sharp peak energy distribution of the S vacancy. Therefore, the Mo-S bond bending related to the strain at the interface or the surface roughness of the SiO 2 /Si substrate might be the origin. It is also shown that ultra-thin 2D materials are more sensitive to interface disorder due to the reduced density of states. Since all the constituents for the measured capacitance are well understood, I-V characteristics can be reproduced by utilizing the drift current model. As a result, one of the physical origins of the metal/insulator transition is suggested to be the external outcome of interface traps and quantum capacitance.

2D materials are promising to overcome the scaling limit of Si field-effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (D it ) are presented for both n-and p-MoS 2 FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h-BN as well as high-k oxides. For n-MoS 2 , D it around the mid-gap is drastically reduced to 5 × 10 11 cm −2 eV −1 for the heterostructure FET with h-BN from 5 × 10 12 cm −2 eV −1 for the high-k top-gate. On the other hand, D it remains high, ≈10 13 cm −2 eV −1 , even for the heterostructure FET for p-MoS 2 . The systematic study elucidates that the strain induced externally through the substrate surface roughness and high-k deposition process is the origin for the interface degradation on conduction band side, while sulfur-vacancy-induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.

The development of high‐performance catalysts with high activity, selectivity, and stability are essential for the practical applications of H2O2 electrosynthesis technology, but it is still formidably challenging. It is reported that the low‐coordinated structure of Pd sites in amorphous PdSe2 nanoparticles (a‐PdSe2 NPs) can significantly boost the electrocatalytic synthesis of H2O2. Detailed investigations and theoretical calculations reveal that the disordered arrangement of Pd atoms in a‐PdSe2 NPs can promote the activity, while the Pd sites with low‐coordinated environment can optimize the adsorption toward oxygenated intermediate and suppress the cleavage of O–O bond, leading to a significant enhancement in both the H2O2 selectivity and productivity. Impressively, a‐PdSe2 NPs/C exhibits high H2O2 selectivity over 90% in different pH electrolytes. H2O2 productivities with ≈3245.7, 1725.5, and 2242.1 mmol gPd−1 h−1 in 0.1 m KOH, 0.1 m HClO4, and 0.1 m Na2SO4 can be achieved, respectively, in an H‐cell electrolyzer, being a pH‐universal catalyst for H2O2 electrochemical synthesis. Furthermore, the produced H2O2 can reach 1081.8 ppm in a three‐phase flow cell reactor after 2 h enrichment in 0.1 m Na2SO4, showing the great potential of a‐PdSe2 NPs/C for practical H2O2 electrosynthesis.