Optimizing Charge Injection across Transition Metal Dichalcogenide Heterojunctions: Theory and Experiment

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Forming Stable van der Waals Contacts between Metals and 2D Semiconductors

High-performing 2D electrical and optical devices can be realized by forming an ideal van der Waals (vdW) metal contact with weak interactions and stable interface states. However, the methods for applying metal contacts while avoiding damage from metal deposition present challenges in realizing a uniform, stable vdW interface. To overcome this problem, this study develops a method for forming vdW contacts using a sacrificial Se buffer layer. This study explores this method by investigating the difference in the Schottky barrier height between the vdW metal contact deposited using a buffer layer, a transferred metal contact, and a conventional directly deposited metal contact using rectification and photovoltaic characteristics of a Schottky diode structure with graphite. Evidently, the Se buffer layer method forms the most stable and ideal vdW contact while preventing Fermi-level pinning. A tungsten diselenide Schottky diode fabricated using these vdW contacts with Au and graphite as the top and bottom electrodes, respectively, exhibits excellent operation with an ideality factor of ≈1, an on/off ratio of > 107 , and coherent properties. Additionally, when using only the vdW Au contact, the electrical and optical properties of the device can be minutely modulated by changing the structure of the Schottky diode.

Localized Excitons in NbSe2-MoSe2 Heterostructures

Neutral and charged excitons (trions) in atomically thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe₂-MoSe₂ vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy, we discover a sharp emission feature, L1, that is localized at the NbSe₂-capped regions of MoSe₂. L1 is observed at energies below the commonly studied MoSe₂ excitons and trions and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. This PL feature is robust, observed in a variety of samples fabricated with different stacking geometries and cleaning procedures. Using first-principles calculations, we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe₂ and NbSe₂-MoSe₂ heterostructure. We discuss the implications of our studies for atomically thin optoelectronics devices with atomically sharp interfaces and tunable electronic structures.

Achieving a superlubricating ohmic sliding electrical contact via a 2D heterointerface: a computational investigation

Simultaneously achieving low friction and fine electrical conductance of sliding electrical contacts is a crucial factor but a great challenge for designing high-performance microscale and nanoscale functional devices. Through atomistic simulations, we propose an effective design strategy to obtain both low friction and high conductivity in sliding electrical contacts. By constructing graphene(Gr)/MoS₂ two-dimensional (2D) heterojunctions between sliding Cu surfaces, superlubricity can be achieved with a remarkably lowered sliding energy barrier as compared to that of the homogeneous MoS₂ lubricated Cu contact. Moreover, by introducing vacancy defects into MoS₂ and substituting Cu with active metal Ti, the Schottky and tunneling barriers can be substantially suppressed without losing the superlubricious properties of the tribointerface. Consequently, a high conductivity ohmic contact with low sliding friction could be realized in our proposed Ti-MoS_1.5-Gr-Ti system, which provides a potential strategy for tackling the well-known dilemma for high performance sliding electrical contacts.

Dielectric Properties of NbxW1-xSe2 Alloys

The growth of transition metal dichalcogenide (TMDC) alloys provides an opportunity to experimentally access information elucidating how optical properties change with gradual substitutions in the lattice compared with their pure compositions. In this work, we performed growths of alloyed crystals with stoichiometric compositions between pure forms of NbSe₂ and WSe₂, followed by an optical analysis of those alloys by utilizing Raman spectroscopy and spectroscopic ellipsometry.

One-Dimensional Edge Contacts to a Monolayer Semiconductor

Integration of electrical contacts into van der Waals (vdW) heterostructures is critical for realizing electronic and optoelectronic functionalities. However, to date no scalable methodology for gaining electrical access to buried monolayer two-dimensional (2D) semiconductors exists. Here we report viable edge contact formation to hexagonal boron nitride (hBN) encapsulated monolayer MoS₂. By combining reactive ion etching, in situ Ar⁺ sputtering and annealing, we achieve a relatively low edge contact resistance, high mobility (up to ∼30 cm² V^-1 s^-1) and high on-current density (>50 μA/μm at V_DS = 3V), comparable to top contacts. Furthermore, the atomically smooth hBN environment also preserves the intrinsic MoS₂ channel quality during fabrication, leading to a steep subthreshold swing of 116 mV/dec with a negligible hysteresis. Hence, edge contacts are highly promising for large-scale practical implementation of encapsulated heterostructure devices, especially those involving air sensitive materials, and can be arbitrarily narrow, which opens the door to further shrinkage of 2D device footprint.

Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces

Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated with the true contact area or the "contact quantity". Much of the physical understanding of the interfacial electrical contact quality was primarily based on Landauer's theory or Richardson formulation. However, a quantitative model directly connecting contact conductance to interfacial atomistic structures still remains absent. Here, we measure the atomic-scale local electrical contact conductance instead of local electronic surface states in graphene/Ru(0001) superstructure, via atomically resolved conductive atomic force microscopy. By defining the "quality" of individual atom-atom contact as the carrier tunneling probability along the interatomic electron transport pathways, we establish a relationship between the atomic-scale contact quality and local interfacial atomistic structure. This real-space model unravels the atomic-level spatial modulation of contact conductance, and the twist angle-dependent interlayer conductance between misoriented graphene layers.

Degenerately Doped Transition Metal Dichalcogenides as Ohmic Homojunction Contacts to Transition Metal Dichalcogenide Semiconductors

In search of an improved strategy to form low-resistance contacts to MoS₂ and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO₃/MoS₂ and MoO₂/MoS₂ bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS₂ very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS₂, resulting in a highly transparent contact region.

Epitaxial Growth of Two-Dimensional Metal-Semiconductor Transition-Metal Dichalcogenide Vertical Stacks (VSe2/MX2) and Their Band Alignments

Two-dimensional (2D) metal-semiconductor transition-metal dichalcogenide (TMDC) vertical heterostructures play a crucial role in device engineering and contact tuning fields, while their direct integration still challenging. Herein, a robust epitaxial growth method is designed to construct multiple lattice-matched 2D metal-semiconductor TMDC vertical stacks (VSe₂/MX₂, M: Mo, W; X: S, Se) by a two-step chemical vapor deposition method. Intriguingly, the metallic VSe₂ preferred to nucleate and extend from the energy-favorable edge site of the semiconducting MX₂ underlayer to form VSe₂/MX₂ vertical heterostructures. This growth behavior was also confirmed by density functional theory calculations of the initial adsorption of VSe₂ adatoms. In particular, the formation of Schottky-diode or Ohmic contact-type band alignments was detected for the stacks between VSe₂ and p-type WSe₂ or n-type MoSe₂, respectively. This work hereby provides insights into the direct integration, band-alignment engineering, and potential applications of such 2D metal-semiconductor stacks in next-generation electronics, optoelectronic devices, and energy-related fields.

Dielectric Environment-Robust Ultrafast Charge Transfer Between Two Atomic Layers

Understanding electron transfer across two-dimensional (2D) van der Waals (vdW) interfaces especially the effect of dielectric environment not only contributes to the rational design of high performance optoelectronic and photo/electrocatalytic devices but also unravels the nature of charge motion. Herein, we investigated the electron transfer process between two atomic thin layered materials coupled by vdW force at ultimate proximity. Despite their susceptible electronic properties, we show electron transfer at 2D vdW interface is robust and ultrafast (∼30 fs), regardless of the surrounding dielectrics and solvents. Considering the static energy landscape and dynamic nuclear rearrangements, our result suggests the electronic coupling at 2D vdW heterointerfaces is sufficiently strong such that electron transfers adiabatically in a barrierless and ultrafast manner where energetics and solvent relaxation are not that relevant. The robust ultrafast electron transfer against the variation of dielectric environment is highly encouraging for 2D optoelectronic and photo/electrocatalytic devices.

Band Alignment and the Built-in Potential of Solids

The built-in potential is of central importance to the understanding of many interfacial phenomena because it determines the band alignment at the interface. Despite its importance, its exact sign and magnitude have generally been recognized as ill-defined quantities for more than half a century. Here, we provide a common energy reference of bulk matter which leads to an unambiguous definition of the built-in potential and innate (i.e., bulk) band alignment. Further, we find that the built-in potential is explicitly determined by the bulk properties of the constituent materials when the system is in electronic equilibrium, while the interface plays a role only in the absence of equilibrium. Our quantitative theory enables a unified description of a variety of important properties of interfaces, ranging from work functions to Schottky barriers in electronic devices.

Three-Dimensional Atomistic Tomography of W-Based Alloyed Two-Dimensional Transition Metal Dichalcogenides

Increased interest in two-dimensional (2D) materials and heterostructures for use as components of electrical devices has led to the use of an atomically mixed phase between semiconducting and metallic transition metal dichalcogenides that exhibited enhanced interfacial characteristics. To understand the lattice structure and properties of 2D materials on the atomic scale, diverse characterization methods such as Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and X-ray photoemission spectroscopy (XPS) have been applied. However, determination of the exact chemical distribution, which is a critical factor for the interfacial layer, was hindered by limitations of these typical methods. In this work, atom-probe tomography (APT) was introduced for the first time to analyze the three-dimensional atomic distribution and composition variation of the atomic-scale multilayered alloy structure W _xNb_{(1- x)}Se₂. Composition profiles and theoretical calculations for each atom demonstrated the reaction kinetics and stoichiometric inhomogeneity of the W _xNb_{(1- x)}Se₂ layer. The role of the intermediate layer was investigated by fabrication of a WSe₂-based field-effect transistor. Introduction of W _xNb_{(1- x)}Se₂ between metallic NbSe₂ and semiconducting WSe₂ layers resulted in improved charge transport with lowering of the contact barrier.

A review on fabricating heterostructures from layered double hydroxides for enhanced photocatalytic activities

LDH is a controllable 2D material for fabricating heterostructures with another semiconductor.

Tunable Schottky contacts in MSe2/NbSe2 (M = Mo and W) heterostructures and promising application potential in field-effect transistors

MSe₂/NbSe₂ (M = Mo and W) heterostructures exhibit low and tunable Schottky barriers, indicating promising application potential in field-effect transistors.

Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons

Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (E_F). However, low-D metals possessing a limited density of states at E_F can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts.

A new insight for ohmic contacts to MoS2: by tuning MoS2 affinity energies but not metal work-functions

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently attracted tremendous interest for fundamental studies and applications.