Synchrotron-induced surface-photovoltage saturation at intercalated Na/WSe2 interfaces

Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides

Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance.

P-Terminated InP (001) Surfaces: Surface Band Bending and Reactivity to Water

Stable InP (001) surfaces are characterized by fully occupied and empty surface states close to the bulk valence and conduction band edges, respectively. The present photoemission data show, however, a surface Fermi level pinning only slightly below the midgap energy which gives rise to an appreciable surface band bending. By means of density functional theory calculations, it is shown that this apparent discrepancy is due to surface defects that form at finite temperature. In particular, the desorption of hydrogen from metalorganic vapor phase epitaxy grown P-rich InP (001) surfaces exposes partially filled P dangling bonds that give rise to band gap states. These defects are investigated with respect to surface reactivity in contact with molecular water by low-temperature water adsorption experiments using photoemission spectroscopy and are compared to our computational results. Interestingly, these hydrogen-related gap states are robust with respect to water adsorption, provided that water does not dissociate. Because significant water dissociation is expected to occur at steps rather than terraces, surface band bending of a flat InP (001) surface is not affected by water exposure.

Surface Analysis of WSe2 Crystals: Spatial and Electronic Variability

Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe₂ is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe₂ characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry. The STM images reveal the presence of atomic-size imperfections and a variation in the electronic structure caused by the presence of defects and impurities below the detection limit of XPS. Both STS and photoemission reveal a spatial variation in the Fermi level position. The analysis of the core levels indicates the presence of different doping levels. The presence of a large concentration of defects and impurities has a strong impact on the electronic properties of the WSe₂ surface. Our findings demonstrate that the growth of controllable and high quality two-dimensional materials at nanometer scale is one of the most challenging tasks that requires further attention.

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.