Surface Analysis of WSe₂ Crystals: Spatial and Electronic Variability

Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe₂ is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe₂ characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry. The STM images reveal the presence of atomic-size imperfections and a variation in the electronic structure caused by the presence of defects and impurities below the detection limit of XPS. Both STS and photoemission reveal a spatial variation in the Fermi level position. The analysis of the core levels indicates the presence of different doping levels. The presence of a large concentration of defects and impurities has a strong impact on the electronic properties of the WSe₂ surface. Our findings demonstrate that the growth of controllable and high quality two-dimensional materials at nanometer scale is one of the most challenging tasks that requires further attention

Interface between Graphene and SrTiO₃(001) Investigated by Scanning Tunneling Microscopy and Photoemission

Graphene, grown by chemical vapor deposition, is transferred onto Nb-doped SrTiO₃(001) surface and the interface properties are characterized by scanning tunneling microscopy and photoemission spectroscopy. Charge doping of graphene changes from n- to p-type with vacuum annealing and correspondingly opposite space charge regions are observed in SrTiO₃ substrate. Formation of an ordered surface reconstruction of the SrTiO₃ substrate underneath the graphene is observed. The surface restructuring can be measured in scanning tunneling microscopy because the graphene closely follows to the substrate topography. This causes at the atomic-level a wavy graphene morphology on the SrTiO₃ (001)-c(6 × 2) surface reconstruction. Prolonged high temperature (above 700 °C) vacuum annealing causes formation of hexagonal holes with ‘armchair’ edges in the graphene and an eventual disappearance of the graphene. Etching of the graphene is assumed to be caused by reaction with released substrate oxygen

Surface Defects on Natural MoS₂

Transition metal dichalcogenides (TMDs) are being considered for a variety of electronic and optoelectronic devices such as beyond complementary metal-oxide-semiconductor (CMOS) switches, light-emitting diodes, solar cells, as well as sensors, among others. Molybdenum disulfide (MoS₂) is the most studied of the TMDs in part because of its availability in the natural or geological form. The performance of most devices is strongly affected by the intrinsic defects in geological MoS₂. Indeed, most sources of current transition metal dichalcogenides have defects, including many impurities. The variability in the electrical properties of MoS₂ across the surface of the same crystal has been shown to be correlated with local variations in stoichiometry as well as metallic-like and structural defects. The presence of impurities has also been suggested to play a role in determining the Fermi level in MoS₂. The main focus of this work is to highlight a number of intrinsic defects detected on natural, exfoliated MoS₂ crystals from two different sources that have been often used in previous reports for device fabrication. We employed room temperature scanning tunneling microscopy (STM) and spectroscopy (STS), inductively coupled plasma mass spectrometry (ICPMS), as well as X-ray photoelectron spectroscopy (XPS) to study the pristine surface of MoS₂(0001) immediately after exfoliation. ICPMS used to measure the concentration of impurity elements can in part explain the local contrast behavior observed in STM images. This work highlights that the high concentration of surface defects and impurity atoms may explain the variability observed in the electrical and physical characteristics of MoS₂

Defect-Dominated Doping and Contact Resistance in MoS₂

Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS₂ dominate the metal/MoS₂ contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS₂ can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS₂ since the same metal in contact with MoS₂ can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition

Contact Metal–MoS₂ Interfacial Reactions and Potential Implications on MoS₂‑Based Device Performance

Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS₂ using electron beam deposition under two different reactor base pressures to determine the contact metal–MoS₂ interface chemistry and its dependence on the reactor ambient. The high work function metal Au does not react with MoS₂ regardless of reactor ambient. In contrast, interfacial reactions between MoS₂ and another high work function metal, Ir, are observed when it is deposited under both high vacuum (HV, ∼ 1 × 10^–6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10^–9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS₂ when the contact metal is deposited under UHV conditions. In addition, Sc is rapidly oxidized on the MoS₂ surface, whereas Cr is only partially oxidized when deposited under HV conditions. This indicates that deposition chamber ambient can affect the contact metal chemistry in addition to the chemistry present at the contact metal–MoS₂ interface. These results elucidate the true chemistry of some contact metal–MoS₂ interfaces and its dependence on the deposition ambient, and highlight the need to consider the chemical states present at the interface and their impact on contact resistance with MoS₂

Direct Observation of Interlayer Hybridization and Dirac Relativistic Carriers in Graphene/MoS₂ van der Waals Heterostructures

Artificial heterostructures assembled from van der Waals materials promise to combine materials without the traditional restrictions in heterostructure-growth such as lattice matching conditions and atom interdiffusion. Simple stacking of van der Waals materials with diverse properties would thus enable the fabrication of novel materials or device structures with atomically precise interfaces. Because covalent bonding in these layered materials is limited to molecular planes and the interaction between planes are very weak, only small changes in the electronic structure are expected by stacking these materials on top of each other. Here we prepare interfaces between CVD-grown graphene and MoS₂ and report the direct measurement of the electronic structure of such a van der Waals heterostructure by angle-resolved photoemission spectroscopy. While the Dirac cone of graphene remains intact and no significant charge transfer doping is detected, we observe formation of band gaps in the π-band of graphene, away from the Fermi-level, due to hybridization with states from the MoS₂ substrate

Influence of Hydroxyls on Pd Atom Mobility and Clustering on Rutile TiO₂(011)‑2 × 1

Understanding agglomeration of late transition metal atoms, such as Pd, on metal oxide supports, such as TiO₂, is critical for designing heterogeneous catalysts as well as for controlling metal/oxide interfaces in general. One approach for reducing particle sintering is to modify the metal oxide surface with hydroxyls that decrease adatom mobility. We study by scanning tunneling microscopy experiments, density functional theory (DFT) calculations, and Monte Carlo (MC) computer simulations the atomistic processes of Pd sintering on a hydroxyl-modified TiO₂(011)-2 × 1 surface. The formation of small 1–3 atom clusters that are stable at room temperature is achieved on the hydroxylated surface, while much larger clusters are formed under the same conditions on a hydroxyl-free surface. DFT shows that this is a consequence of stronger binding of Pd atoms adjacent to hydroxyls and increased surface diffusion barriers for Pd atoms on the hydroxylated surface. DFT, kinetic MC, and ReaxFF-based NVT-MC simulations show that Pd clusters larger than single Pd monomers can adsorb the hydrogen from the oxide surface and form Pd hydrides. This depletes the surface hydroxyl coverage, thus allowing Pd to more freely diffuse and agglomerate at room temperature. Experimentally, this causes a bimodal cluster size distribution with 1–3 atom clusters prevalent at low Pd coverage, while significantly larger clusters become dominant at higher Pd concentrations. This study demonstrates that hydroxylated oxide surfaces can significantly reduce Pd cluster sizes, thus enabling the preparation of surfaces populated with metal clusters composed of single to few atoms

Defects and Surface Structural Stability of MoTe₂ Under Vacuum Annealing

Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe₂ with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe₂. Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature. Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a “wagon wheel” morphology after 400 °C annealing for 15 min. Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain centers. To prevent extensive Te loss at higher temperatures, where Mo₆Te₆ nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe₂ is tested. With this passivation strategy, the structural stability of MoTe₂ is greatly enhanced up to 500 °C without apparent structural defects

Hole Contacts on Transition Metal Dichalcogenides: Interface Chemistry and Band Alignments

MoO_<i>x</i> shows promising potential as an efficient hole injection layer for p-FETs based on transition metal dichalcogenides. A combination of experiment and theory is used to study the surface and interfacial chemistry, as well as the band alignments for MoO_<i>x</i>/MoS₂ and MoO_<i>x</i>/WSe₂ heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. A Mo⁵⁺ rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoO_<i>x</i> contacts on MoS₂ and WSe₂

Al₂O₃ on Black Phosphorus by Atomic Layer Deposition: An <i>in Situ</i> Interface Study

<i>In situ</i> “half cycle” atomic layer deposition (ALD) of Al₂O₃ was carried out on black phosphorus (“black-P”) surfaces with modified phosphorus oxide concentrations. X-ray photoelectron spectroscopy is employed to investigate the interfacial chemistry and the nucleation of the Al₂O₃ on black-P surfaces. This work suggests that exposing a sample that is initially free of phosphorus oxide to the ALD precursors does not result in detectable oxidation. However, when the phosphorus oxide is formed on the surface prior to deposition, the black-P can react with both the surface adventitious oxygen contamination and the H₂O precursor at a deposition temperature of 200 °C. As a result, the concentration of the phosphorus oxide increases after both annealing and the atomic layer deposition process. The nucleation rate of Al₂O₃ on black-P is correlated with the amount of oxygen on samples prior to the deposition. The growth of Al₂O₃ follows a “substrate inhibited growth” behavior where an incubation period is required. <i>Ex situ</i> atomic force microscopy is also used to investigate the deposited Al₂O₃ morphologies on black-P where the Al₂O₃ tends to form islands on the exfoliated black-P samples. Therefore, surface functionalization may be needed to get a conformal coverage of Al₂O₃ on the phosphorus oxide free samples