Session X14: Transition Metal Dichalcogenides: Growth and Synthesis

van der Waals Heterostructures Grown by MBE

In this work, we demonstrate the high-quality MBE heterostructure growth of various layered 2D materials by van der Waals epitaxy (VDWE). The coupling of different types of van der Waals materials including transition metal dichalcogenide thin films (e.g., WSe$_{2}$, WTe$_{2}$, HfSe$_{2})$, insulating hexagonal boron nitride (h-BN), and topological insulators (e.g., Bi$_{2}$Se$_{3})$ allows for the fabrication of novel electronic devices that take advantage of unique quantum confinement and spin-based characteristics. The relaxed lattice-matching criteria of van der Waals epitaxy has allowed for high-quality heterostructure growth with atomically abrupt interfaces, allowing us to couple these materials based primarily on their band alignment and electronic properties. We will discuss the impact of sample preparation, surface reactivity, and lattice mismatch of various substrates (sapphire, graphene, TMDs, Bi$_{2}$Se$_{3})$ on the growth mode and quality of the films and will discuss our studies of substrate temperature and flux rates on the resultant growth and grain size. Structural and chemical characterization was conducted via reflection high energy electron diffraction (RHEED, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM/S), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Experimentally determined band alignments have been determined and compared with first-principles calculations allowing the design of novel low-power logic and magnetic memory devices. Initial results from the electrical characterization of these grown thin films and some simple devices will also be presented. These VDWE grown layered 2D materials show significant potential for fabricating novel heterostructures with tunable band alignments and magnetic properties for a variety of nanoelectronic and optoelectronic applications.   

Direct growth of single- and few-layer MoS$_{\mathrm{2}}$ on h-BN by CVD method

As a promising candidate for the next-generation electronics, large-scale single- and few-layer molybdenum disulfide (MoS$_{\mathrm{2}})$ grown by CVD method is an important advancement towards technological implementation of this material. However, the choice of substrate can significantly affect the performance of MoS$_{\mathrm{2}}$ based devices. An attractive insulating substrate or mate for MoS$_{\mathrm{2}}$ (and related materials such as graphene) is hexagonal boron nitride (h-BN). Stacked heterostructures of MoS$_{\mathrm{2}}$ and h-BN have been produced by manual transfer methods, but a more efficient and scalable assembly method is needed. Here we demonstrate the direct growth of single- and few-layer MoS$_{\mathrm{2}}$ on h-BN by chemical vapor deposition (CVD) method. The growth mechanisms for single- and few-layer samples are found to be distinct, and for single-layer samples low relative rotation angles (\textless 5\textdegree ) between the MoS$_{\mathrm{2}}$ and h-BN lattices prevail. In addition, MoS$_{\mathrm{2}}$ directly grown on h-BN maintains its intrinsic 1.89 eV bandgap. Our CVD synthesis method presents a viable path towards high-quality MoS$_{\mathrm{2}}$ based field effect transistors in a controllable and scalable fashion.   

Spiral Growth of Few-Layer MoS2 by Chemical Vapor Deposition

Monolayer and few-layer transition metal dichalcogenide MoS2 are grown by chemical vapor deposition on SiO2/Si substrates using MoO3 and S powder as precursors. Before growth, the substrates are pretreated with perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt to promote nucleation. Monolayer MoS2 islands are triangularly shaped with sizes ranging from a few to tens of micrometers, which also exhibits the characteristic Raman bands at 403.36 and 385.05 cm-1 corresponding to the A1g and E2g modes, respectively. Atomic force microscopy imaging further confirms the monolayer thickness to be 0.8 nm. For few-layer MoS2 films, triangular spirals are observed with both left- and right-handed chirality. Raman spectra showed interesting features of these growth spirals, the details of which will be presented at the meeting.   

Controlled synthesis of single-layer MoSe$_{2}$ nanostructures

Group VIB transition metal dichalcogenides (TMD), such as MoSe$_{2}$, WS$_{2}$, etc., are a family of layered materials with weak van der Waals (vdW) interaction between neighboring layers. A transition from indirect to direct bandgap semiconductor takes place for most of these materials when they become single layer (SL), and the values of these direct band gap are comparable to visible light. This makes SL TMDs attractive candidates for 2D electronic and optoelectronic devices. Though epitaxial SL TMDs have been successfully prepared, there is controversy in their growth conditions and in their edge structures. Moreover, some intriguing theoretical predictions about the finite-size effect on SL TMDs are still awaiting experimental proof. Here we report systematic studies of the thermodynamics/ kinetics of SL MoSe$_{2}$ formation on a vdW surface (namely, highly oriented pyrolytic graphite) using molecular beam epitaxy (MBE). We also report the controlled creation of various nanostructures of MBE SL MoSe$_{2}$. The edge properties and the influence of the in-plane confinement on the electronic structure are addressed by in-situ STM and STS. Theoretical calculations have been carried out to help understanding the experimental discoveries.   

Large Area Synthesis of 2D Materials

Transition metal dichalcogenides (TMDs) have generated significant interest for numerous applications including sensors, flexible electronics, heterostructures and optoelectronics due to their interesting, thickness-dependent properties. Despite recent progress, the synthesis of high-quality and highly uniform TMDs on a large scale is still a challenge. In this talk, synthesis routes for WSe2 and MoS2 that achieve monolayer thickness uniformity across large area substrates with electrical properties equivalent to geological crystals will be described. Controlled doping of 2D semiconductors is also critically required. However, methods established for conventional semiconductors, such as ion implantation, are not easily applicable to 2D materials because of their atomically thin structure. Redox-active molecular dopants will be demonstrated which provide large changes in carrier density and workfunction through the choice of dopant, treatment time, and the solution concentration. Finally, several applications of these large-area, uniform 2D materials will be described including heterostructures, biosensors and strain sensors.   

Thermal stability of MBE-grown epitaxial MoSe2 and WSe2 thin films

Layered transition metal dichalcogenides (TMDs) draw much attention, because of its unique optical properties and band structures depending on the layer thicknesses. However, MBE growth of epitaxial films demands information about thermal stability of stoichiometry and related electronic structure for high temperature range. We grow epitaxial MoSe2 and WSe2 ultrathin films by using molecular beam epitaxy (MBE). We characterize stoichiometry of films grown at various growth temperature by using various methods, XPS, EDX, and TOF-MEIS. We further test high temperature stability of electronic structure for those films by utilizing in-situ ellipsometry attached to UHV chamber. We discuss threshold temperatures up to 700\textasciitilde 1000$^{\mathrm{o}}$C, at which electronic phases changes from semiconductor to metal due to selenium deficiency. This information can be useful for potential application of TMDs for fabrication of Van der Waals multilayers and related devices.   

Synthesis and Optical Control of Circular Polarization in monolayer Tungsten Disulfide.

The unique electronic band structure in single layer WS$_{\mathrm{2}}$ provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL). We investigate the circularly polarized PL in WS$_{\mathrm{2}}$ monolayers synthesized using chemical vapor deposition (CVD). The resulting polarization is strongly dependent on the sample preparation. As-grown CVD WS$_{\mathrm{2}}$ (still on the growth substrate) exhibits low polarized emission, regardless of laser excitation or laser power. Removing WS$_{\mathrm{2}}$ from the growth substrate and repositioning on the same substrate significantly impacts the optical properties. In transferred films, the excitonic state is optically controlled via high-powered laser exposure such that subsequent PL is solely from either the charged exciton state or the neutral exciton state. Neutral excitonic emission exhibits zero polarization whereas the trion polarization can exceed 25{\%} at room temperature. The removal process may modify the strain, sample-to-substrate distance, and chemical doping in the WS$_{\mathrm{2}}$ monolayer, and work is underway to determine how these factors influence the valley populations. These results demonstrate a new method to control the excitonic state and PL polarization in monolayer WS$_{\mathrm{2}}$. .   

Atomically flat Ge buffer layers and alternating shutter growth of CaGe$_{\mathrm{2}}$ for large area germanane

Germanane (GeH), which is converted from CaGe$_{\mathrm{2}}$ by soaking in HCl acid, has recently attracted interest because of its novel properties, such as large band gap (1.56eV), spin orbit coupling and predictions of high mobility (18000 cm$^{\mathrm{2}}$/Vs). Previously CaGe$_{\mathrm{2}}$ was successfully grown on Ge(111) substrates by molecular beam epitaxy (MBE) growth. But there were cracks between \textmu m-sized islands, which is not desirable for scientific study and application, and limits the material quality. By growing atomically flat Ge buffer layers and using alternating shutter MBE growth, we are able to grow crack-free, large area films of CaGe$_{\mathrm{2}}$ films. Reflection high energy electron diffraction (RHEED) patterns of Ge buffer layer and CaGe$_{\mathrm{2}}$ indicates high quality two dimensional surfaces, which is further confirmed by atomic force microscopy (AFM), showing atomically flat and uniform Ge buffer layer and CaGe$_{\mathrm{2}}$. The appearance of Laue oscillation in X-ray diffraction (XRD) and Kiessig fringes in X-ray reflectivity (XRR) proves the uniformity of CaGe$_{\mathrm{2}}$ film and the smoothness of the interface. The high quality of CaGe$_{\mathrm{2}}$ film makes it promising to explore novel properties of GeH.   

Synthesis and Properties of Group IV Graphane Analogues

Similar to how carbon networks can be sculpted into low-dimensional allotropes such as fullerenes, nanotubes, and graphene with fundamentally different properties, it is possible to create similar ligand terminated sp$^{3}$-hybridized honeycomb graphane derivatives containing Ge or Sn that feature unique and tunable properties. Here, we will describe our recent success in the creation of hydrogen and organic-terminated group IV graphane analogues, from the topochemical deintercalation of precursor Zintl phases, such as CaGe$_{2}$. We will discuss how the optical, electronic, and thermal properties of these materials can be systematically controlled by substituting either the surface ligand or via alloying with other Group IV elements. Additionally, we have also developed an epitopotaxial approach for integrating precise thicknesses of germanane layers onto Ge wafers that combines the epitaxial deposition of CaGe$_{2}$ precursor phases with the topotactic interconversion into the 2D material. Finally, we will describe our recent efforts on the synthesis and crystal structures of Sn-containing graphane alloys in order to access novel topological phenomena predicted to occur in these graphanes.