Growth Of Transition Metal Dichalcogenides Research Articles

Vertical Chemical Vapor Deposition Growth of Highly Uniform 2D Transition Metal Dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention due to their physical and chemical properties that make them promising in electronics and optoelectronics. Because of the difficulties in controlling concentrations of solid precursors and spatially nonuniform growth dynamics, it is challenging to grow 2D TMDCs over large areas with good uniformity and reproducibility so far, which significantly hinders their practical use. Here we report a vertical chemical vapor deposition (VCVD) design with gaseous precursors to grow monolayer TMDCs with a uniform density and high quality over the whole substrate and with excellent reproducibility. Such a gaseous VCVD design can well control the three key parameters in TMDC growth, including precursor concentration, gas flow, and temperature, which cannot be done in a currently widely used horizontal CVD system with solid precursors. Statistical results show that VCVD-grown monolayer TMDCs including MoS2 and WS2 are of high uniformity and quality on substrates over centimeter size. We also fabricated multiple van der Waals heterostructures by one-step transfer of VCVD-grown TMDCs, owning to their good uniformity. This work sheds light on the growth of 2D materials with high uniformity on a large-area substrate, which can be used for the wafer-scale fabrication of 2D materials and their heterostructures.

Large area, patterned growth of 2D MoS2 and lateral MoS2–WS2 heterostructures for nano- and opto-electronic applications

The patterned growth of transition metal dichalcogenides (TMDs) and their lateral heterostructures is paramount for the fabrication of application-oriented electronics and optoelectronics devices. However, the large scale patterned growth of TMDs remains challenging. Here, we demonstrate the synthesis of patterned polycrystalline 2D MoS2 thin films on device ready SiO2/Si substrates, eliminating any etching and transfer steps using a combination of plasma enhanced atomic layer deposition (PEALD) and thermal sulfurization. As an inherent advantage of ALD, precise thickness control ranging from a monolayer to few-layered MoS2 has been achieved. Furthermore, uniform films with exceptional conformality over 3D structures are obtained. Finally, the approach has been leveraged to obtain in-plane lateral heterostructures of 2D MoS2 and WS2 thin films over a large area which opens up an avenue for their direct integration in future nano- and opto-electronic device applications.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe2 layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe2 thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

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.

Atomistic mechanisms of seeding promoter-controlled growth of molybdenum disulphide

Seeding promoters facilitate the nucleation and growth of transition metal dichalcogenides in chemical vapor deposition (CVD). However, sophisticated roles of seeding promoter remain unclear. Here, adopting triangular-shaped crystal violet (CV) consisting of nonpolar and polar parts as the seeding promoter, we study the role of seeding promoter for the growth of molybdenum disulfide (MoS2). We systematically control the geometrical configuration of CV on SiO2/Si substrate by changing the solvent polarity and find that it strongly affects the growth of monolayer or multilayer MoS2 domains via CVD. Monolayer MoS2 domains were predominantly grown on randomly lying-down CV configurations on SiO2/Si substrate, whereas multilayer MoS2 domains are synthesized at concentrated polar parts in CV micelle on the substrate. Density functional theory calculations reveal that the initial nucleation step for the MoS2 growth is the adsorption of S on CV and the most favourable S adsorption site is the polar part in CV. Furthermore, it is found that the CV adsorption to SiO2 is mediated by the polar CV part and additionally strengthened in the lying-down CV configuration. Enhancing the thermal stability as well as hindering the re-aggregation of CV at high temperature, the lying down CV configuration allows the predominant growth of monolayer MoS2. This work provides a general framework to understand the growth of MoS2 from aromatic seeding promoters.

Mo Concentration Controls the Morphological Transitions from Dendritic to Semicompact, and to Compact Growth of Monolayer Crystalline MoS2 on Various Substrates.

The domain morphology in the growth of transition-metal dichalcogenides (TMDCs) is mostly triangular but rarely dendritic. Here, we report a robust chemical vapor deposition method to fabricate atomic-thin 2H-phase MoS2 dendrites on several single-crystalline substrates with different lattice structures, such as rutile-TiO2(001), SrTiO3(001), and sapphire(0001). It is found that by tuning the concentration of Mo adatoms, the morphology of MoS2 domains on these substrates evolves from tridentate dendrites at a low Mo concentration to semicompact fractal domains at an intermediate Mo concentration, and to a compact triangular shape at a high Mo concentration. First-principles calculations reveal that the edge diffusion barrier of Mo is comparable to the attachment barrier, inhibiting fast Mo atom diffusion along the edge. Kinetics Monte Carlo simulations with varying Mo concentrations well reproduce the experimental results. Our combined experimental and theoretical analyses evidently show that the growth of MoS2 dendritic domains at a low Mo concentration is a nonequilibrium process, which is dominated by the kinetics of Mo adatoms. Our study presents an effective route to control the morphology of TMDCs by simply tuning the transition-metal adatom concentration.

Sulfur‐Mastery: Precise Synthesis of 2D Transition Metal Dichalcogenides

AbstractChemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS2 flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS2 flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS2 atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.

(Invited) The Role of Defects in Van Der Waals Epitaxy of Transition Metal Dichalcogenides

Direct growth of transition metal dichalcogenides (TMDs) such as WS2 and WSe2 on graphene and hexagonal boron nitride (hBN) is of interest for the fabrication of 2D heterostructures as an alternative to exfoliation and 2D layer stacking. This can be challenging, however, given the low surface energy of the basal plane in 2D layered materials which inhibits nucleation and lateral growth of TMD domains. In addition, it is equally probable for TMD domains to orient in two rotational directions (referred to as 0o and 180o domains) on pristine surfaces of hBN and graphene which gives rise to inversion domain boundaries (IDBs) in coalesced films. However, the presence of defects in hBN and graphene can dramatically alter the energy landscape providing a route to control nucleation and epitaxy. Our recent studies have focused on the epitaxial growth of WSe2, WS2 and related TMD monolayer films by gas source chemical vapor deposition (CVD) on hBN and epitaxial graphene. The CVD process is carried out in a cold-wall reactor using metal hexacarbonyls and hydride chalcogen precursors in a hydrogen carrier gas. In the case of epitaxial graphene, nucleation of WS2 occurs primarily at step edges and terrace defects resulting in different WS2 rotational orientations. In the case of WSe2 growth on hBN, however, single atom vacancies in the hBN were found to act as sites for metal atom trapping which facilitates the nucleation of WSe2 domains on the surface. In this case, the TMD nucleation density can be controlled by manipulating the density of surface atom vacancies which can be achieved through plasma irradiation and annealing in NH3. In addition, the metal atoms break the surface symmetry which leads to a preferred orientation (~95%) for WSe2 domains on hBN. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe2 monolayer films on hBN were achieved which exhibit optical and transport properties superior to comparable films grown on sapphire substrates. The results demonstrate the important role of defects in nucleation and epitaxial growth of 2D heterostructures.

Mechanism of substrate-induced anisotropic growth of monolayer WS2 by kinetic Monte Carlo simulations

Controlled anisotropic growth of two-dimensional materials provides an approach for the synthesis of large single crystals and nanoribbons, which are promising for applications as low-dimensional semiconductors and in next-generation optoelectronic devices. In particular, the anisotropic growth of transition metal dichalcogenides induced by the substrate is of great interest due to its operability. To date, however, their substrate-induced anisotropic growth is typically driven by the optimization of experimental parameters without uncovering the fundamental mechanism. Here, the anisotropic growth of monolayer tungsten disulfide on an ST-X quartz substrate is achieved by chemical vapor deposition, and the mechanism of substrate-induced anisotropic growth is examined by kinetic Monte Carlo simulations. Results show that, besides the variation of substrate adsorption, the chalcogen to metal (C/M) ratio is a major contributor to the large growth anisotropy and the polarization of undergrowth and overgrowth; either perfect isotropy or high anisotropy can be expected when the C/M ratio equals 2.0 by properly controlling the linear relationship between gas flux and temperature.

Epitaxial growth of TiSe2/TiO2 heterostructure

Here we report that TiSe2 thin films can be epitaxially grown on TiO2 substrates despite different lattice symmetry between the two materials. The TiSe2 thin films can be prepared on TiO2 via molecular beam epitaxy (MBE) in two ways: by conventional co-deposition using selenium and titanium sources, and by evaporating just selenium on reconstructed surfaces of TiO2. Both growth methods yield crystalline thin films with similar electronic band structures. TiSe2 films on TiO2 substrates exhibit large electron doping and a lack of charge density wave (CDW) order, which is different from both bulk single crystal TiSe2 and TiSe2 thin films on graphene. These phenomena can be explained by selenium vacancies in the TiSe2 films, which naturally occur when these films are grown on TiO2 substrates. Our successful growth of transition metal dichalcogenide (TMDC) films on a transition metal oxide (TMO) substrate provides a platform to further tune the electrical and optical properties of TMDC thin films.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire.

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS2) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS2 grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS2-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS2, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS2 growth on sapphire was further developed to the rational synthesis of an in-plane MoS2-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS2 parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS2 was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.

MoS2 synthesis by gas source MBE for transition metal dichalcogenides integration on large scale substrates

We present in this paper the use of Gas Source Molecular Beam Epitaxy for the large-scale growth of transition metal dichalcogenides. Fiber-textured MoS2 co-deposited thin films (down to 1 MLs) are grown on commercially 200 mm wafer size templates where MX2 crystalline layers are achieved at temperatures ranging from RT to 550 °C. Raman Spectroscopy and photoluminescence measurements along with X-Ray Photoelectron Spectroscopy show that a low growth rate is essential for complete Mo sulfurization during MoS2 co-deposition. Finally, cross-section Transmission Electron Microscopy investigations are discussed to highlight the influence of SiO2 and Al2O3 used surfaces on MoS2 deposition.

Substrate dependent electronic structure variations of van der Waals heterostructures of MoSe2 or MoSe2(1−x)Te2x grown by van der Waals epitaxy

Substrate induced variation of the electronic structure of MoSe2 monolayers is studied. MoSe2 is directly grown by van der Waals epitaxy on MoS2 and highly ordered pyrolytic graphite (HOPG). In this article, we give a review of growth of transition metal dichalcogenides (TMDCs) by van der Waals epitaxy and discuss previously found evidence for the modifications of the electronic structure of TMDCs by van der Waals substrates. Using angle resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy we determine the dispersion of the valence band and the band gap, respectively. For MoSe2 on graphite the valence band resembles that expected for free standing single layer MoSe2, however, the substrate induces a narrowing of the overall band gap. For MoSe2 on MoS2 evidence for hybridization of the valence band maximum between the monolayer and the substrate are presented. Such hybridization, results in an alignment of the valence band maximum (VBM) of MoSe2 with the binding energy of the VBM of the MoS2-substrate at the Γ-point. Furthermore, the VBM at the Γ-point is very close in energy to that of the VBM at the K-point. The effective electron mass around the Γ-point is also much decreased for MoSe2 on MoS2- compared to HOPG-substrates or free standing MoSe2 monolayers. This indicates that in TMDC heterostructures interlayer interactions significantly modify the electronic structure and the resulting properties are in between those of free-standing monolayer and bulk materials. In an attempt to control the electronic states incorporation of Te by formation of MoSe2(1−x)Te2x has been investigated. While shifts in the core level position are observed, the VBM states are found to be very similar to those of pure MoSe2.

(Invited) Low-Temperature Growth of Transition Metal Dichalcogenides (TMDs) Layered Materials Toward Phase-Engineered Hybrid Films: From Controllable Growth to Material Characterizations and Device Applications

Novel condensed matter systems can be understood as new compositions of elements or old materials in new forms. According to the definition, various new condensed matter systems have been developed or are under development in the recent years. 2D layered materials, including graphene, transition metal dichalcogenides (TMDs) allow the scaling down to atomically thin thicknesses and possess unique physical properties under dimensionality confinement. Chemical vapor deposition (CVD) process is the most popular approach for all kind of 2D materials due to its high yield and quality. Nevertheless, the need for high temperature and the relatively long process time within each cycle hinders for commercial development in terms of production cost. However, the transfer procedure has become one of the major limitations of the overall performance. In the first part of my talk, I will present several approaches, including ultra-fast microwave heating, plasma selective reaction, controlled segregation process by laser irradiation and metal vapers-assisted growth processes developed in my lab these years to directly grow graphene with controllable thicknesses on arbitrary substrates without any extra transfer process. In the 2nd part of my talk, I will also introduce the development of various methods, including laser irradiation assisted-selenization (LIAS) process, fast microwave annealing and plasms enhance selenization processes on different two-dimensional transition metal dichalcogenides materials in my lab. The detailed formation mechanisms, microstructures and physical performances as well as its applications in gas sensors, photodetector and water splitting behaviors based on these two 2D materials were reported and investigated.

Concurrent Synthesis of High‐Performance Monolayer Transition Metal Disulfides

To date, the chemical vapor deposition (CVD) approach has been widely used for the growth of transition metal dichalcogenides (TMDs). However, the reported CVD methods to synthesize TMDs cannot be used to grow more than one type of TMDs. This work reports a promising CVD technique to concurrently synthesize multiple monolayer transition metal disulfides once. The optoelectrical characterization and high‐resolution transmission electron microscopy show the high quality of monolayer crystals, and, more importantly, there is no mixing between different precursors during the growth process, which has been investigated by considering the gas flow dynamics and concentration distribution of precursors in our setup. This strategy indicates the promising future for the batch production of 2D materials and the concurrent synthesis techniques in standard state‐of‐the‐art complementary metal‐oxide‐semiconductor (CMOS) fabrication technology.

Selective and confined growth of transition metal dichalcogenides on transferred graphene

We demonstrate confinement of CVD grown MoS2 to a patterned graphene area, forming a vertically stacked 2D heterostructure.

Metal Induced Growth of Transition Metal Dichalcogenides at Controlled Locations.

Metal induced nucleation is adopted to achieve the growth of transition metal dichalcogenides at controlled locations. Ordered arrays of MoS2 and WS2 have successfully been fabricated on SiO2 substrates by using the patterned Pt/Ti dots as the nucleation sites. Uniform MoS2 monolayers with the adjustable size up to 50 μm are grown surrounding these metal patterns and the mobility of such layer is about 0.86 cm2/V·s. The crystalline flakes of WS2 are also fabricated extending from the metal patterns and the electron mobility of these flakes is up to 11.36 cm2/V·s.

Monolayer Single-Crystal 1T′-MoTe2 Grown by Chemical Vapor Deposition Exhibits Weak Antilocalization Effect

Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T'-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion [ Qian et al. Science 2014 , 346 , 1344 - 1347 ]. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T'-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin-orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T'-MoTe2 and implementation in next-generation nanoelectronic devices.

Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides.

The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them. Triangular single-crystals of molybdenum disulfide (MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains. Grain boundaries in the polycrystalline graphene were also visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels. Moreover, we demonstrate that this method can be extended to determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties.

Lithography-free plasma-induced patterned growth of MoS2 and its heterojunction with graphene.

Application-oriented patterned growth of transition metal dichalcogenides (TMDCs) and their heterojunctions is of critical importance for sophisticated, customized two-dimensional (2D) electronic and optoelectronic devices; however, it is still difficult to fabricate these patterns in a simple, clean, and high controllability manner without using optical lithography. Here, we report the direct synthesis of patterned MoS2 and graphene-MoS2 heterojunctions via selective plasma treatment of a SiO2/Si substrate and chemical vapor deposition of MoS2. This method has multiple merits, such as simple steps, a short operating time, easily isolated MoS2 layers with clean surfaces and controllable locations, shapes, sizes and thicknesses, which enable their integration into the device structure without using a photoresist. In addition, we demonstrate the direct growth of patterned graphene-MoS2 heterojunctions for the fabrication of transistor. This study reveals a novel method to fabricate and use patterned MoS2 and graphene-MoS2 heterojunctions, which could be generalized to the rational design of other 2D materials, heterojunctions and devices in the future.