Session V26: Surface Studies of Beyond Graphene Materials (STM/ARPES)

Variable Temperature Scanning Tunneling Microscopy of WTe$_{\mathrm{2,}}$ MoTe$_{\mathrm{2}}$ and alloyed MoWTe$_{\mathrm{2}}$

The transition metal dicalcogenides MoTe2 and WTe2 grow in a Van der Waals layered structure and can be produced down to monolayer thickness. These materials exhibit multiple crystal structures with drastically differing electronic properties including semiconductor (2H) and metal (1T'). Nanoscale phase engineering has been proposed as a way to create a variety of device architectures. This phase engineering can be achieved by strain, chemical doping or alloying. Alloying in particular has been proposed as a facile technique to continuously tune the structural phase of the resultant material and thus lower the barrier for transitions between the insulating and metallic states. In this study we use variable temperature scanning tunneling microscopy to image both parent compounds MoTe2, WTe2 and alloyed crystals MoWTe2. Using dI/dV spectroscopy we determine the nature of the insulating and metallic states of both the parent compounds as well as use this technique to characterize the properties of the alloyed material.   

Local Spectroscopic Characterization of Spin and Layer Polarization in WSe$_2$

Semiconducting transition metal dichalcogenides, such as WSe$_2$, exhibit very strong spin-orbit coupling (SOC) at certain band extrema due to large in-plane dipole moments formed by their heavy constituent atoms. The strong SOC links the spin and valley degrees of freedom in monolayers. In bilayers, interlayer hopping is suppressed by this SOC, leading to a spontaneous layer polarization and a coupling of the layer pseudospin with the spin and valley degrees of freedom. We examine these effects by tracking allowed and forbidden electronic scattering pathways in monolayer and bilayer WSe$_2$ using scanning tunneling spectroscopy. Specifically, we observe a strong suppression of intervalley scattering in both monolayer and bilayer WSe$_2$ indicative of these band polarizations.   

Resolving 2D Amorphous Materials with Scanning Probe Microscopy

Novel two-dimensional (2D) materials have garnered significant scientific interest due to their potential technological applications. Alongside the emphasis on crystalline materials, such as graphene and hexagonal BN, a new class of 2D amorphous materials must be pursued. For amorphous materials, a detailed understanding of the complex structure is necessary. Here we present a structural study of 2D bilayer silica on Ru(0001), an insulating material which is weakly coupled to the substrate. Atomic structure has been determined with a dual mode atomic force microscopy (AFM) and scanning tunneling microscopy (STM) sensor in ultra-high vacuum (UHV) at low temperatures, revealing a network of different ring sizes. Liquid AFM measurements with sub-nanometer resolution bridge the gap between clean UHV conditions and the environments that many material applications demand. Samples are grown and characterized in vacuum and subsequently transferred to the liquid AFM. Notably, the key structural features observed, namely nanoscale ring networks and larger holes to the substrate, show strong quantitative agreement between the liquid and UHV microscopy measurements. This provides direct evidence for the structural stability of these silica films for nanoelectronics and other applications.   

Measurement of electrostatic potential variations between 2D materials using low-energy electron microscopy

Among the many properties that evolve as isolated 2D materials are brought together to form a heterostructure, rearrangement of charges between layers due to unintentional doping results in dipole fields at the interface, which critically affect the electronic properties of the structure. Here we report a method for directly measuring work function differences, and hence electrostatic potential variations, across the surface of 2D materials and heterostructures thereof using low energy electron microscopy (LEEM). Study of MoSe$_2$ grown by molecular beam epitaxy on epitaxial graphene on SiC with LEEM reveals a large work function difference between the MoSe$_2$ and the graphene, indicating charge transfer between the layers and a subsequent dipole layer. In addition to quantifying dipole effects between transition metal dichalcogenides and graphene, direct imaging of the surface, diffraction information, and the spectroscopic dependence of electron reflectivity will be discussed.   

Scanning tunneling spectroscopy of tungsten disulfide

Atomically thin layers of tungsten disulfide possess interesting optoelectronic properties characterized by strong photoluminescence. Here we perform scanning tunneling microscopy and spectroscopy measurements of 2H WS$_{2}$ on silicon oxide substrates to understand how electronic properties are affected by defects and substrate-induced disorder. Specifically, the electronic property of tungsten disulfide is probed as a function of gate-induced carrier density.   

Structure and Electronic Properties of Single- to Few Layers Molybdenum Disulfide Films

Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS) we have investigated the electronic properties of mono- to few layers molybdenum disulfide films grown on HOPG using ambient pressure chemical vapor deposition (APCVD). Atomic force microscopy and STM show that this growth technique produces crystalline triangular and hexagonal islands with varying thicknesses in 1ML increments. The films exhibited a suppression of quasiparticle band-gap as a function of layer number as measured by local spectroscopy. Changes in the valence band edge were supported by photoemission electron microscopy (PEEM) measurements. We also report on a strain-induced contraction of the quasiparticle band-gap in proximity to grain boundaries and defects.   

Scanning tunneling microscopy on CVD grown lateral graphene molybdenum disulfide heterostructures

We investigate the interface of single layer graphene, molybdenum disulfide lateral heterostructures using scanning tunneling microscopy (STM). Samples are fabricated using chemical vapor deposition to deposit graphene, photolithography to pattern graphene and metal-organic chemical vapor deposition to grow molybdenum disulfide in patterned areas. The lateral junction of the two materials allows investigation of structural and electronic properties at the interface of the two materials, an interface usually buried in conventional stacked heterostructures. STM is used to image the stitching of the two materials with nanoscale resolution. STM is also used to perform local spectroscopy, probing the local density of states on an atomic scale across the junction. Interesting phenomena such as the charge transfer and atomic bonding are investigated. The spatially changing chemical potential between the two materials is also examined at different gate voltages.   

Monolayer MoS$_{\mathrm{2}}$ on HOPG Studied by Scanning Tunneling Microscopy / Spectroscopy

Chemical Vapor Deposition (CVD) is a promising way to prepare 2D material such as graphene and MoS$_{\mathrm{2}}$ for $\mu $m-scale. In this report, we deposit monolayer MoS$_{\mathrm{2}}$ by CVD method on HOPG to create the heterojunction. We observe that, the alignment of triangle MoS$_{\mathrm{2}}$ islands shows the tendency that they have some preferred directions from AFM morphology. From STM atomic resolution images, the moir\'{e} superstructures analysis could summarize that the MoS$_{\mathrm{2}}$ lattice tends to have a small angle with graphite's lattice. On the other hand, we also take the tunneling spectra from the different moir\'{e} domains and the moir\'{e} hills, moir\'{e} volleys of the single moir\'{e} domain. The results reveal the extraordinary states, which appear in the band gap range of MoS$_{\mathrm{2}}$. We consider these states are the consequence of hybridized of two layers and be detected from the interlayer space.   

STM/STS Study of Surface Modification Effect on Bandgap Structure of Ti$_{2}$C with -OH, -F, and -H.

In this presentation, we present Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) study of bandgap structures of surface-modified Ti$_{2}$C with -OH, -F, and -O in atomic scale. Since the discovery of new two dimensional (2D) materials like graphene, various 2D materials including transition metal dichalcogenide (TMD) have been intensively investigated. There are, however, still scientific issues to apply them to the device fabrications for controlling the appropriate bandgap structure with high field effect mobility. Recently another 2D materials of transition metal carbide (TMC), Ti$_{2}$CT$_{x}$ with modifiable surface group T$_{x\, }$(-OH, -F, and -O) was suggested. [S. Lai et. al, Nanoscale (2015), DOI: 10.1039/C5NR06513E]. This 2D material shows that the mobility at room temperature is less sensitive to the measured transport bandgap, which can imply that Ti$_{2}$CT$_{x}$ can be a strong candidate of 2D TMC for application to the future electronic devices. Surface modification on the electronic structure of Ti$_{2}$C by -OH, -F, and -O is, therefore, investigated by STM and STS in atomic scale. More scientific results will be further discussed in the presentation.   

An ARPES investigation of band evolution of MoS$_{\mathrm{2}}$ in presence of high pressure hydrogen gas

The monolayer MoS$_{\mathrm{2}}$, has a large direct band gap and spin band splitting in K-point which make it a good candidate for several applications such as solar cell, valley Hall transistor and so on. When it has more than two layers, turns into a semiconductor with indirect band gap. Theoretical predictions have revealed that the number of layers is directly related to number of bands. Also, it was recently reported that the resistivity of MoS$_{\mathrm{2}}$ decreases when exposed to high pressure hydrogen gas for few hours. To investigate the evolution of energy bands as a function of high pressure hydrogen exposure, we performed angle resolved photoemission spectroscopy (ARPES) experiment on pristine and hydrogen treated bulk MoS$_{\mathrm{2}}$. Our result, is suggestive for quantum well state in the treated sample case, and impurity state induced by sulphur vacancy between valence and conduction band at K-point. We argue that the impurity state depending on momentum mediate decrease in resistivity.   

Investigation of the Spatially Resolved Electronic Structure of Single Layer WS2 on Transition Metal Oxide Surfaces

The family of semiconducting single layer (SL) transition metal dichalcogenides (TMDs) have lately been intensely studied, owing to the strong coupling between spin and valley degrees of freedom as well as the presence of strongly bound excitons. The choice of supporting substrate is known to strongly influence these properties. We set out to investigate the electronic properties of CVD grown SL WS2 transferred onto the dielectric oxide materials SrTiO3 and TiO2. By using a combination of photoemission electron microscopy (PEEM) and angle-resolved photoemission (ARPES) with micrometer focus we obtain simultaneous spatial, momentum and energy-resolved information about SL WS2 on a polar (SrTiO3) and a nonpolar (TiO2) surface for the first time.   

Graphene protected surface state on Ir(111) with adsorbed lithium

It is well known that electronic surface states (SS) get strongly perturbed upon the chemical adsorption of very small amount of adsorbates. Adsorption of lithium atoms on Ir(111) is no exception to that rule. Iridium SS gets strongly perturbed and is practically eradicated - it can not be seen as a sharp peak in the ARPES measurement. However, if the system is prepared with graphene on top of Ir/Li system, the iridium SS reappears. We present a combined experimental and theoretical study of the described system. Using the density functional theory calculations for large unit cells with disordered lithium atoms geometries on the (111) surface of iridium we were able to reproduce the results of the ARPES measurements - showing clearly that the SS signal is strongly suppressed when lithium is adsorbed, while it is almost unchanged when lithium is intercalated (i.e. with graphene on top of it). Looking at the projected density of states we constructed a rather simple model explaining this behavior which seems to be general.   

Semiconducting graphene and its incommensurate SiC interface

The development of a viable form of semiconducting graphene has been the goal since the onset of graphene research. Using improved growth techniques, we show that the first epitaxial graphene layer grown on SiC(0001) (the buffer layer) is semiconducting. With ARPES, we found that the buffer layer has a band gap $> 0.5$eV. At present, no existing theory explains the observed band structure. This is in part due to a corresponding lack of detailed structural studies of the buffer. Using SXRD, we show that the buffer layer is not the commensurate $(6 \sqrt{3} \times 6 \sqrt{3})R30^{\circ}$ structure assumed for the past four decades. Rather, it is tensile strained and interacts with a strongly modulated SiC interface layer. The buffer-interface layer pair is well ordered, yet incommensurate with bulk SiC. We also find that the buffer evolves during the growth process and reverts to a near commensurate phase with a large RMS roughness when a monolayer forms. These structural changes correspond with changes in the band structure that demonstrate the importance of the incommensurate phase in producing semiconducting graphene.   

Band parameters of 2D semiconductor heterostructures determined by micro-ARPES

Heterostructures made by stacking monolayers of different 2D materials can have unique properties, such as hosting long-lived polarized interlayer excitons. Understanding these depends on knowledge of the band parameters of both the separate monolayers and the hetero-bilayer. Interlayer hybridization can also produce distinct electronic structure dependent on the relative monolayer crystal orientation. The most powerful technique for determining such properties is angle-resolved photoemission (ARPES), which can now be applied to micron-scale samples at the Spectromicroscopy Elettra Trieste beamline. Using this new facility, combined with careful sample design, we have studied heterostructures of WSe2, MoSe2, WS2 and graphene.~ We determined band offsets, effective masses, and spin-orbit splittings with an energy resolution \textless 50 meV. Interestingly, the bands near the gamma-point in hetero-bilayers oriented near zero degrees are not a superposition of those in the isolated monolayers, but exhibit an additional higher band. However, the valence band edge remains at the K-point, which together with the band offsets is consistent with measurements of strong luminescence from interlayer excitons in MoSe2/WSe2.   

Angle- and spin-resolved photoemission spectroscopy study of monolayer semiconducting transition metal dichalcogenides

Monolayer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, and so on. In particular, the multiple degrees of freedom in thess materials (e.g. spin, valley and layer) are coupled with each other, providing various ways to control their properties. Here we report the electronic and spin structural studies of a monolayer semiconducting transition metal dichalcogenide thin film using Angle-resolved photoemission spectroscopy (ARPES) and Spin-Resolved ARPES.