Microscopic basis for the band engineering of Mo1-xWxS2-based heterojunction

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS₂ from the edges of mechanically exfoliated multilayer flakes of WSe₂ or Nb_xMo_1-xS₂. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS₂ on the exfoliated flakes. For the WSe₂/MoS₂ sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the Nb_xMo_1-xS₂/MoS₂ in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS₂. The formation of a staggered gap band alignment of Nb_xMo_1-xS₂/MoS₂ is also supported by first-principles calculations.

Visualizing Large Facet-Dependent Electronic Tuning in Monolayer WSe2 on Au Surfaces

Two-dimensional transition metal dichalcogenides (TMDs) have shown great importance in the development of novel ultrathin optoelectronic devices owing to their exceptional electronic and photonic properties. Effectively tuning their electronic band structures is not only desired in electronics applications but also can facilitate more novel properties. In this work, we demonstrate that large electronic tuning on a WSe₂ monolayer can be realized by different facets of a Au-foil substrate, forming in-plane p-n junctions with remarkable built-in electric fields. This facet-dependent tuning effect is directly visualized by using scanning tunneling microscopy and differential conductance (dI/dV) spectroscopy. First-principles calculations reveal that the atomic arrangement of the Au facet effectively changes the interfacial coupling and charge transfer, leading to different magnitudes of charge doping in WSe₂. Our study would be beneficial for future customized fabrication of TMD-junction devices via facet-specific construction on the substrate.

Electronic Tuning in WSe2/Au via van der Waals Interface Twisting and Intercalation

The transition metal dichalcogenide (TMD)-metal interfaces constitute an active part of TMD-based electronic devices with optimized performances. Despite their decisive role, current strategies for nanoscale electronic tuning remain limited. Here, we demonstrate electronic tuning in the WSe₂/Au interface by twist engineering, capable of modulating the WSe₂ carrier doping from an intrinsic p-type to n-type. Scanning tunneling microscope/spectroscopy gives direct evidence of enhanced interfacial interaction induced doping in WSe₂ as the twist angle with respect to the topmost (100) lattice of gold changing from 15 to 0°. Taking advantage of the strong coupling interface achieved this way, we have moved a step further to realize an n-p-n-type WSe₂ homojunction. The intrinsic doping of WSe₂ can be recovered by germanium intercalation. Density functional theory calculations confirm that twist angle and intercalation-dependent charge transfer related doping are involved in our observations. Our work offers ways for electronically tuning the metal-2D semiconductor interface.

Electronic Properties and Carrier Dynamics at the Alloy Interfaces of WS2x Se2-2x Spiral Nanosheets

Electronic properties at the interfaces between different-composition domains of 2D-alloys are key for their optical, electronic, and optoelectronic applications. Understanding the interfacial electronic structures and carrier dynamics is essential for designing and fabricating devices that use these alloys. Here, WS_2x Se_2-2x spiral nanosheets are prepared using the physical vapor deposition method, and the nonlinear optical and electronic properties, as well as the carrier dynamics at the interfaces between the WS and WSe domains, are studied. Second-harmonic generation tests demonstrate that these nanosheets exhibit a very strong layer-dependent nonlinear optical effect. Atomic-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements reveal that S and Se atoms are non-uniformly distributed, forming WS domains, WSe domains, and defect-related areas. Atomic STM images and STS maps reveal enhanced local density of states by electron scattering at the WS/WSe interfaces, providing a detailed nanoscale interpretation of the S/Se-ratio-dependent lifetimes observed in pump-probe spectroscopy measurements. This work provides valuable interfacial characterization of 2D-alloy materials, using state-of-the-art methods with high temporal and spatial resolutions. The obtained insights are likely to be useful for prospective applications.

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

Electronic Structure of Quasi-Freestanding WS2/MoS2 Heterostructures

Growth of 2D materials under ultrahigh-vacuum (UHV) conditions allows for an in situ characterization of samples with direct spectroscopic insight. Heteroepitaxy of transition-metal dichalcogenides (TMDs) in UHV remains a challenge for integration of several different monolayers into new functional systems. In this work, we epitaxially grow lateral WS₂-MoS₂ and vertical WS₂/MoS₂ heterostructures on graphene. By means of scanning tunneling spectroscopy (STS), we first examined the electronic structure of monolayer MoS₂, WS₂, and WS₂/MoS₂ vertical heterostructure. Moreover, we investigate a band bending in the vicinity of the narrow one-dimensional (1D) interface of the WS₂-MoS₂ lateral heterostructure and mirror twin boundary (MTB) in the WS₂/MoS₂ vertical heterostructure. Density functional theory (DFT) is used for the calculation of the band structures, as well as for the density of states (DOS) maps at the interfaces. For the WS₂-MoS₂ lateral heterostructure, we confirm type-II band alignment and determine the corresponding depletion regions, charge densities, and the electric field at the interface. For the MTB, we observe a symmetric upward bend bending and relate it to the dielectric screening of graphene affecting dominantly the MoS₂ layer. Quasi-freestanding heterostructures with sharp interfaces, large built-in electric field, and narrow depletion region widths are proper candidates for future designing of electronic and optoelectronic devices.

Conductive Atomic Force Microscopy of Semiconducting Transition Metal Dichalcogenides and Heterostructures

Semiconducting transition metal dichalcogenides (TMDs) are promising materials for future electronic and optoelectronic applications. However, their electronic properties are strongly affected by peculiar nanoscale defects/inhomogeneities (point or complex defects, thickness fluctuations, grain boundaries, etc.), which are intrinsic of these materials or introduced during device fabrication processes. This paper reviews recent applications of conductive atomic force microscopy (C-AFM) to the investigation of nanoscale transport properties in TMDs, discussing the implications of the local phenomena in the overall behavior of TMD-based devices. Nanoscale resolution current spectroscopy and mapping by C-AFM provided information on the Schottky barrier uniformity and shed light on the mechanisms responsible for the Fermi level pinning commonly observed at metal/TMD interfaces. Methods for nanoscale tailoring of the Schottky barrier in MoS₂ for the realization of ambipolar transistors are also illustrated. Experiments on local conductivity mapping in monolayer MoS₂ grown by chemical vapor deposition (CVD) on SiO₂ substrates are discussed, providing a direct evidence of the resistance associated to the grain boundaries (GBs) between MoS₂ domains. Finally, C-AFM provided an insight into the current transport phenomena in TMD-based heterostructures, including lateral heterojunctions observed within Mo_xW_1-xSe₂ alloys, and vertical heterostructures made by van der Waals stacking of different TMDs (e.g., MoS₂/WSe₂) or by CVD growth of TMDs on bulk semiconductors.

Transition from Semimetal to Semiconductor in ZrTe2 Induced by Se Substitution

Two-dimensional layered transition-metal telluride can build stable metallic, metastable metallic, or semimetallic polymorphic crystal structures with enormous technological and scientific applications. Herein the hexagonal structures of zirconium ditelluride (ZrTe₂) and ZrTe_2(1-x)Se_2x (0 ≤ x ≤ 1) single crystals were selectively synthesized through the chemical vapor transport method. The electronic band structures were systematically studied through angle-resolved photoemission spectroscopy (ARPES) combined with first-principles density functional theory (DFT) calculations. The ARPES results suggested a clear electronic phase transition from a semimetal to a semiconductor in ZrTe_2(1-x)Se_2x with the x value changing. Compared with pristine ZrTe₂, the valence band splitting in ZrTe_2(1-x)Se_2x decreased at the Γ point due to the reduction of the spin-orbit interaction, whereas an indirect band gap opened in the vicinity of the Fermi level with the increase in Se concentration. Our DFT calculations further confirmed that the substituted Se atoms on Te sites could affect the band structure of ZrTe₂ to induce a distinct transition from semimetal to semiconductor, suggesting their high potential for valleytronics applications.

Reversible direct-indirect band transition in alloying TMDs heterostructures via band engineering

Alloying is a feasible and practical strategy to tune the electronic properties of 2D layered semiconductors. Here, based on first-principles calculations and analysis, we demonstrate band engineering through alloying W into a prototype MoS₂/MoSe₂ heterostructure. Especially, when the W compositions x  >  0.57 in Mo_1-x W _x S₂/MoSe₂, it exhibits remarkable and reversible direct- to indirect-gap transition. This is because for Mo_1-x W _x S₂/MoSe₂, the valence band maximum located at the K point originates from dominant MoSe₂, while the competing Γ state stems from the hybridization of both Mo_1-xW _x S₂ and MoSe₂, which is extremely sensitive to the interlayer coupling. Consequently, alloying in MoS₂ layer induces direct- to indirect-gap transition and gap increase due to the weakened p-d coupling. We also observe that whether initial alloying in MoS₂ or MoSe₂, the µMo-µW poor condition should always be used. Our findings are generally applicable and will significantly expand the band engineering to other alloying TMDs heterostructures.

Continuous Heteroepitaxy of Two-Dimensional Heterostructures Based on Layered Chalcogenides

The in-plane connection and layer-by-layer stacking of atomically thin layered materials are expected to allow the fabrication of two-dimensional (2D) heterostructures with exotic physical properties and future engineering applications. However, it is currently necessary to develop a continuous growth process that allows the assembly of a wide variety of atomic layers without interface degradation, contamination, and/or alloying. Herein, we report the continuous heteroepitaxial growth of 2D multiheterostructures and nanoribbons based on layered transition metal dichalcogenide (TMDC) monolayers, employing metal organic liquid precursors with high supply controllability. This versatile process can avoid air exposure during growth process and enables the formation of in-plane heterostructures with ultraclean atomically sharp and zigzag-edge straight junctions without defects or alloy formation around the interface. For the samples grown directly on graphite, we have investigated the local electronic density of states of atomically sharp heterointerface by scanning tunneling microscopy and spectroscopy, together with first-principles calculations. These results demonstrate an approach to realizing diverse nanostructures such as atomic layer-based quantum wires and superlattices and suggest advanced applications in the fields of electronics and optoelectronics.

Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides

The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals stacks, resulting in a 2D interface, or as stitched adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spin-orbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides MX ₂ (with M  =  Mo, W and X  =  S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.

Surface-mediated spin dynamics probed by optical-pump–probe scanning tunneling microscopy

In current materials science and technologies, surface effects on carrier and spin dynamics in functional materials and devices are of great importance.

Growth of 'W' doped molybdenum disulfide on graphene transferred molybdenum substrate

In the present study, a novel method has been carried out to grow tungsten (W) doped molybdenum disulfide (MoS₂) on the graphene transferred TEM grid in a chemical vapor deposition (CVD) setup. Tungsten trioxide (WO₃) has been used as a source for ‘W’ while ‘Mo’ has been derived from Mo based substrate. Different experimental parameters were used in this experiment. Higher gas flow rate decreases the size of the sample flake and on other side increases the dopant concentrations. The interaction mechanism between Mo, S, W and oxygen (O) have been explored. The influence of oxygen seems to be not avoidable completely which also imposes effective growth condition for the reaction of Mo with incoming sulfur atoms. The difference in the migration energies of Mo, WO₃, S clusters on the graphene and the higher reactivity of Mo clusters over other possibly formed atomic clusters on the graphene leads to the growth of W doped MoS₂ monolayers. Formation of MoS₂ monolayer and the nature of edge doping of ‘W’ is explained well with the crystal model using underlying nucleation principles. We believe our result provide a special route to prepare W doped MoS₂ on graphene substrate in the future.

Local Conduction in Mo xW1- xSe2: The Role of Stacking Faults, Defects, and Alloying

Here, we report on the surface conductivity of WSe₂ and Mo _xW_{1- x}Se₂ (0 ≤ x ≤ 1) crystals investigated with conductive atomic force microscopy. We found that stacking faults, defects, and chemical heterogeneities form distinct two-dimensional and one-dimensional conduction paths on the transition metal dichalcogenide surface. In the case of WSe₂, in addition to step edges, we find a significant amount of stacking faults (formed during the cleaving process) that strongly influence the surface conductivity. These regions are attributed to the alternation of the 2H and 3R polytypism. The stacking faults form regular 2D patterns by alternation of the underlying stacking order, with a periodicity that varies significantly between different regions and samples. In the case of Mo _xW_{1- x}Se₂, its conductivity has a localized nature, which depends on the underlying chemical composition and the Mo/W ratio. Segregation to W-rich and Mo-rich regions during the growth process leads to nonuniform conduction paths on the surface of the alloy. We found a gradual change of the conductivity moving from one region to the other, reminiscent of lateral band bending. Our results demonstrate the use of C-AFM as a nanoscopic tool to probe the electrical properties of largely inhomogeneous samples and show the complicated nature of the surface conductivity of TMDC alloys.

Re Doping in 2D Transition Metal Dichalcogenides as a New Route to Tailor Structural Phases and Induced Magnetism

Alloying in 2D results in the development of new, diverse, and versatile systems with prospects in bandgap engineering, catalysis, and energy storage. Tailoring structural phase transitions using alloying is a novel idea with implications in designing all 2D device architecture as the structural phases in 2D materials such as transition metal dichalcogenides are correlated with electronic phases. Here, this study develops a new growth strategy employing chemical vapor deposition to grow monolayer 2D alloys of Re-doped MoSe₂ with show composition tunable structural phase variations. The compositions where the phase transition is observed agree well with the theoretical predictions for these 2D systems. It is also shown that in addition to the predicted new electronic phases, these systems also provide opportunities to study novel phenomena such as magnetism which broadens the range of their applications.

A Simple Method for Synthesis of High-Quality Millimeter-Scale 1T' Transition-Metal Telluride and Near-Field Nanooptical Properties

The controlled synthesis of MoTe₂ and WTe₂ is crucial for their fundamental research and potential electronic applications. Here, a simplified ambient-pressure chemical vapor deposition (CVD) strategy is developed to synthesize high-quality and large-scale monolayer and few-layer 1T'-phase MoTe₂ (length ≈ 1 mm) and WTe₂ (length ≈ 350 µm) crystals by using ordinary salts (KCl or NaCl) as the growth promoter combining with low-cost (NH₄ )₆ Mo₇ O₂₄ ·4H₂ O and hydrate (NH₄ )₁₀ W₁₂ O₄₁ ·xH₂ O as the Mo and W sources, respectively. Atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy confirm the high-quality nature and the atomic structure of the as-grown 1T' MoTe₂ and WTe₂ flakes. Variable-temperature transport measurements exhibit their semimetal properties. Furthermore, near-field nanooptical imaging studies are performed on the 1T' MoTe₂ and WTe₂ flakes for the first time. The sub-wavelength effects of 1T'-phase MoTe₂ (λ_p ≈ 140 nm) and WTe₂ (λ_p ≈ 100 nm) are obtained. This approach paves the way for the growth of special transition-metal dichalcogenides materials and boosts the future polaritonic research of 2D telluride compounds.

Imaging of Interlayer Coupling in van der Waals Heterostructures Using a Bright-Field Optical Microscope

Vertically stacked atomic layers from different layered crystals can be held together by van der Waals forces, which can be used for building novel heterostructures, offering a platform for developing a new generation of atomically thin, transparent, and flexible devices. The performance of these devices is critically dependent on the layer thickness and the interlayer electronic coupling, influencing the hybridization of the electronic states as well as charge and energy transfer between the layers. The electronic coupling is affected by the relative orientation of the layers as well as by the cleanliness of their interfaces. Here, we demonstrate an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The color and brightness in such images are used here to identify mono- and few-layer crystals and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in heterobilayers composed of mechanically exfoliated flakes and as a function of the twist angle in atomic layers grown by chemical vapor deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterization of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth.

Isoelectronic Tungsten Doping in Monolayer MoSe2 for Carrier Type Modulation

Carrier-type modulation is demonstrated in 2D transition metal dichalcogenides as n-type monolayer MoSe₂ is converted to nondegenerate p-type monolayer Mo_1-x W_x Se₂ through isoelectronic doping. Although the alloys are mesoscopically uniform, the p-type conduction in monolayer Mo_1-x W_x Se₂ appears to originate from the upshift of the valenceband maximum toward the Fermi level at highly localized "W-rich" regions in the lattice.

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS₂ and WS₂. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.

Photocarrier dynamics in transition metal dichalcogenide alloy Mo0.5W0.5S2

We report a transient absorption study of photocarrier dynamics in transition metal dichalcogenide alloy, Mo0.5W0.5S2. Photocarriers were injected by a 400-nm pump pulse and detected by a 660-nm probe pulse. We observed a fast energy relaxation process of about 0.7 ps. The photocarrier lifetime is in the range of 50 - 100 ps, which weakly depends on the injected photocarrier density and is a few times shorter than MoS2 and WS2, reflecting the relatively lower crystalline quality of the alloy. Saturable absorption was also observed in Mo0.5W0.5S2, with a saturation energy fluence of 32 μJ cm(-2). These results provide important parameters on photocarrier properties of transition metal dichalcogenide alloys.