Session L30: Transition Metal Dichalcogenides: Synthesis and Characterization

Single-crystalline transition-metal dichalcogenide thin films grown by molecular-beam epitaxy

Emerging properties of 2D materials such as graphene and monolayer transition-metal dichalcogenides (TMDCs) have attracted considerable attention both from fundamental and applied viewpoints. To date, most of the researches have been performed on mechanically-exfoliated nano-thick crystals from top-down approach, while bottom-up approach by thin film growth technique has been of growing significance to further exploration of physical properties of TMDCs including their heterostructures as well as to practical device applications. One of promising routes to well-controlled growth of high-quality large-area TMDC thin films is to use chemical-vapor deposition. We have instead chosen to use molecular-beam epitaxy with a view to future heterostructure fabrication, and recently succeeded in growing single-crystalline TMDC thin films on insulating substrates, both semiconducting and metallic ones, with desired thickness and crystallographic orientation. In the presentation, we will discuss a detailed growth process essential for stabilizing single-crystalline phase, in particular for WSe2 and TiSe2, together with their electrical transport properties.   

Growth of TiSe2 Thin Films Using Chemical Vapor Transport

TiSe$_2$ is a member of transition metal dichalcogenide family of layered van-der-Waals materials that exhibits some distinctive electronic and optical properties due to the presence of excitonic condensate and strong electron-phonon coupling. This makes TiSe$_2$ a candidate for electronic, thermoelectric, and energy applications. We report on the growth of TiSe$_2$ thin films using chemical vapor transport with I$_2$ as a transport agent. Atomic force microscopy and transmission electron microscopy are used to determine the structure of grown films confirming single-crystalline layered structure of the films. Kelvin probe microscopy and resistivity measurement show transition into charge density wave state at temperatures below 180 K. Somewhat lower CDW transition temperature than in single crystals suggests carrier doping due to presence of intrinsic defects.   

Van der Waals growth of monolayer MoS$_{\mathrm{\mathbf{2}}}$\textbf{ and its heterostructures}

Monolayer MoS$_{\mathrm{2}}$ as one kind of TMDs two-dimensional (2D) crystals, is a direct bandgap semiconductor and has attracted much research interests beyond graphene. We report the van der Waals (vdW) growth of monolayer MoS$_{\mathrm{2}}$ by chemical vapor deposition (CVD) in our developed 3-zones furnace with high quality. The integrated flexible TFTs based on our CVD MoS$_{\mathrm{2}}$ have been fabricated with the device structure compatible with the traditional semiconductor fabrication process. The uniformity of the continuous MoS$_{\mathrm{2}}$ film leads to the stable performance over a centimeter scale even under uniaxial strain 1{\%} with mobilities of 14 cm$^{\mathrm{2}}$v$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ and on/off ratio higher than 10$^{\mathrm{5}}$. The excellent performance of those devices suggests that they are promising candidates for flexible and integrable electronics devices in future. The epitaxial growth of MoS$_{\mathrm{2}}$ on WS$_{\mathrm{2}}$ via a two-step CVD growth approach was also reported to create bilayers of vdW heterostructures with clean interface and strong interlayer coupling. It was demonstrated that our epitaxial growth of MoS$_{\mathrm{2}}$ /WS$_{\mathrm{2}}$ heterostructures has strong interlayer coupling and reveals more efficient interlayer charge transfer and spatially separated exciton recombination than the transferred heterostructures. References: 1. \textbf{Adv. Electron. Mater. }2016, 2: 1500379. 2. \textbf{Adv. Mater.} 2016, 28: 1950. 3. \textbf{J. Am. Chem. Soc. }2015, 137 (50): 15632. 4. \textbf{ACS Nano} 2014, 8: 6024.   

Scanning Tunneling Microscopy of Transition Metal Dichalcogenide Monolayers and Heterostructures

Monolayer transition metal dichalcogenides (TMDCs) have emerged as a new class of materials with interesting optoelectronic properties. In this talk, I will describe scanning tunneling microscopy and spectroscopy measurements of various TMDCs and their heterostructures on silicon oxide substrates. First, I will describe spectroscopy measurements on the gate dependence of the bandstructure in low doped monolayer $MoS_2$ films. These measurements show a continuous shift in valence and conduction band edges as defects are charged, allowing us to quantitatively estimate defect concentration. Next, I will discuss the nature of electrical contact between metals and TMDC layers. Atomically resolved spectroscopy at the edge of metal deposited contacts as well as few layer graphite contacts to highly doped $MoS_2$ reveals the presence of metal induced gap states (MIGS), the first experimental observation of MIGS in a two-dimensional semiconductor. We show that MIGS dominate contact properties in the highly doped limit. Third, I will discuss the atomic-scale structural and electronic properties of lateral and vertical heterogeneous stacks of TMDCs such as $MoS_2$, $WSe_2$ and $WS_2$. Finally, I will discuss how one can distinguish intrinsic and extrinsic defects in the TMDCs, and the correlation between defect concentration and optical properties in these materials.   

Uniform wafer-scale growth of stencil templated, high-quality monolayer MoS$_{2}$

With the widespread interest in transition metal dichalcogenides and the recent focus on two-dimensional (2D) vertically stacked heterostructures, a need for an inexpensive and reliable method of producing clean, high-quality, patterned 2D materials has emerged. Here, we report on a templated MoS$_{2}$ growth technique by metal sulfurization where Mo is deposited through a SiN stencil onto highly-crystalline sapphire substrates. After sulfurization, the resulting MoS$_{2}$ films are shown to be high-quality with thicknesses that can be tuned layer-by-layer---down to a single layer---through manipulation of the initial Mo deposition time. The quality of these films is confirmed through scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopy. This facile growth technique results in templated, high-quality MoS$_{2}$ films with centimeter-scale uniformity, feature sizes down to 100 nm, and offers both a means to easily probe MoS$_{2}$ growth dynamics and a route to 2D stacked heterostructures with arbitrary geometry and pristine interfaces. We will discuss potential applications of this novel growth technique for the development of TMD heterostructures and alloys.   

Deterministic and Scalable Growth of Electrically Self-Contacted 2D Materials

We will present a chemical vapor deposition process to selectively grow 2D materials in a deterministic manner around lithographically defined bulk metallic patterns which concurrently provide as-grown electrical contact to the material. With this process, monolayer films, with lateral extent of up to hundreds of microns are controllably grown on and around patterned regions of transition metals. The materials display strong luminescence, monolayer Raman signatures, and relatively large crystal domains. In addition to producing high optical quality monolayer material, the metallic patterns remain conductive providing as-grown metallic contacts to the material. The ability to controllably process 2D material into working devices with any scalability is still one of the largest impediments in the field. This is the first report of using bulk metallic patterns resulting in as-grown, self-forming, electrical contact to the monolayer material, providing a simple, scalable, and reproducible method for creating as-grown two-dimensional materials-based devices with broad implications for basic research and industrial applications.   

Optimization of a growth process for as-grown 2D materials-based devices

We will present the effects of varying key parameters of a deterministic growth method for producing self-contacted 2D transition metal dichalcogenides. Chemical vapor deposition is used to grow a film of 2D material nucleated around and seeded from metallic features prepared by photolithography and sputtering on a Si/SiO2 substrate prior to growth. We will focus on a particular method of growing variable MoS2 based device structures. The goal of this work is to arrive at robust platform for growing a variety of device structures by systematically altering parameters such as the amount of reactants used, the heat of the substrate and oxide powder, and the flow rate of argon gas used. These results will help advance a comprehensive process for the scalable production of as-grown, complex, 2D materials-based device architectures.   

Mapping the Electronic Structure of Two-Dimensional WS$_{\mathrm{2}}$ Heterostructures with Spatially Resolved ARPES at the MAESTRO Facility

Single-layer (SL) semiconducting transition metal dichalcogenides (TMDCs) such as WS$_{\mathrm{2}}$ exhibit strong spin-orbit coupling around the valence band maximum and a direct band gap that is highly sensitive to the dielectric properties of the surrounding medium. High-resolution angle-resolved photoemission spectroscopy (ARPES) studies of these properties are lacking for TMDCs on truly insulating supports such as oxides or hexagonal boron nitride (hBN), which form the basis of a wide range of high performance two-dimensional (2D) heterostructure devices. Here, we use the new microARPES capability with spatial resolution on the order of 10 $\mu $m at the MAESTRO facility at the Advanced Light Source (ALS) to spatially map the electronic structure of micron-sized SL WS$_{\mathrm{2}}$ heterostructures with transition metal oxides and hBN. We directly observe dramatic changes in the SL WS$_{\mathrm{2}}$ band structure and the gap around the valence band maximum when we vary the substrate or the charge carrier concentration in WS$_{\mathrm{2}}$. These findings are discussed in relation to how we can achieve control of the spin and optical properties of such 2D materials.   

Molecular beam epitaxy growth of monolayer niobium diselenide flakes

The recent studies on two-dimensional metals, in particular, the metallic transition metal dichalcogenides (TMDCs), have shown that 2D metals are essentially different from bulk metals, leading to discoveries of interesting phenomena arising from the low dimensionally. Although researches on 2D metals are emerging, difficulty in the sample preparation has been the bottleneck. In this presentation, we report a molecular beam epitaxy (MBE) growth of monolayer NbSe$_{2}$ flakes on the exfoliated hexagonal boron nitride (hBN) substrate. AFM observation have shown that grown NbSe$_{2}$ are monolayer with triangle shapes with an average lateral size of ca. 200 nm. Grown NbSe$_{2}$ crystals show only two crystal orientations, where 60 degree rotation of one orientation corresponds to the other orientation. Unlike the ultrathin triangular crystals of NbSe$_{2}$ grown on hBN substrate, particulate products, with average diameter of 100 nm, form on sapphire substrates, indicating that the atomic flatness of hBN substrates plays important roles in the growth of monolayer NbSe$_{2}$.   

Concurrent Chemical Vapor Deposition Synthesis of Multiple Transition Metal Disulfides

Recently, transition metal disulfides have received tremendous attention due to their exceptional optical and electrical properties. Many techniques have been explored to obtain monolayer TMD and chemical vapor deposition synthesis using transition metal oxide and chalcogenide solid precursors is the most common method used in laboratories now. However, the quantity of solid precursors used is usually surplus giving rise to chemical reactions between precursors in each of their crucibles, as a result of precursors' diffusion at growth temperature. Hence, a CVD setup is normally dedicated for the growth of only one type of TMD to avoid cross-contamination (except for hetero-structures synthesis), and it is impossible to grow multiple monolayer TMD in one synthesis step. Here, we report a new technique to synthesize MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayer films in one CVD process. We first disperse a minuscule amount of metal oxide precursor on targeted substrates, which were then loaded to the furnace in slanting position, rather than horizontal, followed by a sulfur annealing to concurrently grow monolayer MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ on separate substrates. The synthesized TMD films exhibit good properties as confirmed by Raman, PL, XPS, STEM analyses, and electrical measurements.   

Large scale synthesis of niobium disulfide as a transparent transition metal dichalcogenide electrode.

Atomically thin transition metal dichalcogenides(TMDC) semiconductor such as MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ is considered as a promising candidate for future flexible and transparent electronic devices. However, direct metal contact to atomically thin transition metal dichalcogenides(TMDC) semiconductor shows high contact resistance, which suppress electrical performance like electron mobility. NbS$_{\mathrm{2}}$, one of the transition metal dichalcogenides(TMDC) conductor, is an important material because it is expected to form schottky barrier-free contact with transition metal dichalcogenides(TMDC) semiconductor. Here, we synthesize large scale niobium disulfide film as a transparent transition metal dichalcogenide electrode. Synthesized NbS$_{\mathrm{2}}$ film shows corresponding Raman shift and binding energy and has good crystallinity. NbS$_{\mathrm{2}}$ film can be easily patterned and shows uniform conductivity in large area. Large scale transparent NbS$_{\mathrm{2}}$ electrode applied to large scale MoS$_{\mathrm{2}}$ grown by chemical vapor deposition on quartz substrate. Ion-gel gated MoS$_{\mathrm{2}}$ transistor which uses NbS$_{\mathrm{2}}$ as an electrode shows 10$^{\mathrm{4}}$ on/off ratio and 1\textasciitilde 5cm$^{\mathrm{2}}$/Vs electron mobility which is better than metal contact MoS$_{\mathrm{2}}$ transistor.   

Synthesis of Large-area Crystalline MoTe$_{2}$ Atomic layer from Chemical Vapor Deposition

The controlled synthesis of highly crystalline large-area molybdenum ditelluride MoTe$_{2}$ atomic layers is crucial for the practical applications of this emerging material. Here we develop a novel approach for the growth of large-area, uniform and highly crystalline few-layer MoTe$_{2}$ film via chemical vapour deposition (CVD). Large-area atomically thin MoTe$_{2}$ film has been successfully synthesized by tellurization of a MoO$_{3}$ film. The as-grown MoTe$_{2}$ film is uniform, stoichiometric, and highly crystalline. As a result of the high crystallinity, the electronic properties of MoTe$_{2}$ film are comparable with that of mechanically exfoliated MoTe$_{2}$ flakes. Moreover, we found that two different phases of MoTe$_{2}$ (2H and 1T') can be grown depending on the choice of Mo precursor. Since the MoTe$_{2}$ film is highly homogenous, and the size of the film is only limited by the substrate and CVD system size, our growth method paves the way for large-scale application of MoTe$_{2}$ in high performance nanoelectronics and optoelectronics.   

Understanding chemical vapor deposition (CVD) growth of MoS$_{\mathrm{2}}$ layers by ReaxFF-molecular dynamics simulations

Recently, mono-layered MoS$_{\mathrm{2}}$ has been widely studied for the next generation of electronic devices. A fundamental understanding of the CVD growth of MoS$_{\mathrm{2}}$ layer is the key to manufacturing a high quality of MoS$_{\mathrm{2}}$-based devices. However, reaction kinetics of the CVD growth of the MoS$_{\mathrm{2}}$ layer has not been fully understood; and synthesis of uniform mono-layered MoS$_{\mathrm{2}}$ up to the wafer-scale is still challenging. This is primarily due to the complexity of the CVD processes ($i.e.,$ intermediate structures from MoO$_{\mathrm{3}}$ to MoS$_{\mathrm{2}}$ phases). Reactive molecular dynamic (MD) simulations can provide atomistic-scale insights into complex surface reactions during the CVD growth. For this reason, our work focuses on developing a ReaxFF reactive force field for MoO$_{\mathrm{3}}$/MoS$_{\mathrm{2}}$/S interactions and performing massively parallel MD simulations of the sulfidation of MoO$_{\mathrm{3}}$ systems. Our goal is to clarify the reaction mechanism of the sulfidation of MoO$_{\mathrm{3}}$ clusters, and provide a theory-supported rational design for not only MoS$_{\mathrm{2}}$-based applications but also for synthesis of other two-dimensional materials.