Computational Study of MoS₂/HfO₂ Defective Interfaces for Nanometer-Scale Electronics

Atomic structures and electronic properties of MoS₂/HfO₂ defective interfaces are investigated extensively for future field-effect transistor device applications. To mimic the atomic layer deposition growth under ambient conditions, the impact of interfacial oxygen concentration on the MoS₂/HfO₂ interface electronic structure is examined. Then, the effect on band offsets (BOs) and the thermodynamic stability of those interfaces is investigated and compared with available relevant experimental data. Our results show that the BOs can be modified up to 2 eV by tuning the oxygen content through, for example, the relative partial pressure. Interfaces with hydrogen impurities as well as various structural disorders were also considered, leading to different behaviors, such as n-type doping, or introducing defect states close to the Fermi level because of the formation of hydroxyl groups. Then, our results indicate that for a well-prepared interface the electronic device performance should be better than that of other interfaces, such as III–V/high-κ, because of the absence of interface defect states. However, any unpassivated defects, if present during oxide growth, strongly affect the subsequent electronic properties of the interface. The unique electronic properties of monolayer-to-few-layered transition-metal dichalcogenides and dielectric interfaces are described in detail for the first time, showing the promising interfacial characteristics for future transistor technology

<i>In Situ</i> TEM Characterization of Shear-Stress-Induced Interlayer Sliding in the Cross Section View of Molybdenum Disulfide

The experimental study of interlayer sliding at the nanoscale in layered solids has been limited thus far by the incapability of mechanical and imaging probes to simultaneously access sliding interfaces and overcome through mechanical stimulus the van der Waals and Coulombic interactions holding the layers in place. For this purpose, straightforward methods were developed to achieve interlayer sliding in molybdenum disulfide (MoS₂) while under observation within a transmission electron microscope. A method to manipulate, tear, and slide free-standing atomic layers of MoS₂ is demonstrated by electrostatically coupling it to an oxidized tungsten probe attached to a micromanipulator at a bias above ±7 V. A first-principles model of a MoS₂ bilayer polarized by a normal electric field of 5 V/nm, emanating from the tip, demonstrates the appearance of a periodic negative sliding potential energy barrier when the layers slide into the out-of-registry stacking configuration, hinting at electrostatic gating as a means of modifying the interlayer tribology to facilitate shear exfoliation. A method to shear focused ion beam prepared MoS₂ cross section samples using a nanoindenter force sensor is also demonstrated, allowing both the observation and force measurement of its interlayer dynamics during shear-induced sliding. From this experiment, the zero normal load shear strength of MoS₂ can be directly obtained: 25.3 ± 0.6 MPa. These capabilities enable the site-specific mechanical testing of dry lubricant-based nanoelectromechanical devices and can lead to a better understanding of the atomic mechanisms from which the interlayer tribology of layered materials originates

Charge Mediated Reversible Metal–Insulator Transition in Monolayer MoTe₂ and W_<i>x</i>Mo_1–<i>x</i>Te₂ 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 W_<i>x</i>Mo_1–<i>x</i>Te₂. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T′) of monolayer W_<i>x</i>Mo_1–<i>x</i>Te₂ 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 W_<i>x</i>Mo_1–<i>x</i>Te₂ 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

Air Stable p‑Doping of WSe₂ by Covalent Functionalization

Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe₂ <i>via</i> NO_<i>x</i> chemisorption at 150 °C is explored, with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe_{2–<i>x</i>–<i>y</i>}O_<i>x</i>N_<i>y</i> species both on the surface and interface between layers upon chemisorption reaction. <i>Ab initio</i> simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe₂:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼10¹⁹ cm^–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe₂ thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe₂. More generally, the work presents a platform for manipulating the electrical properties and band structure of TMDCs using covalent functionalization