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Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.

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.

MoOx 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 MoOx/MoS2 and MoOx/WSe2 heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. A Mo(5+) rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoOx contacts on MoS2 and WSe2.

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.

Heterostructures of dissimilar 2D materials are potential building blocks for novel materials and may enable the formation of new (photo)electronic device architectures. Previous work mainly focused on supporting graphene on insulating wide-band gap materials, such as hex-BN and mica. Here we investigate the interface between zero-band gap semiconductor graphene and band-gap semiconductor MoS2 as a potential building block for entirely 2D-material based semiconducting devices. We show that solution transfer results in water trapping at the interface which may be removed by annealing to ~300 °C in a vacuum. After removal of the water, by high temperature annealing, ultraflat graphene is obtained on MoS2 with only a very weak moiré pattern observable in scanning tunneling microscopy images due to lattice mismatch and random alignment of graphene with respect to the MoS2 substrate. Photoemission spectroscopy indicates interface dipole formation, p-type doping of graphene by ~0.09 eV downward shift of the Fermi-level below the Dirac point, and a negative space charge region in bulk MoS2. Interestingly, valence band spectra of the graphene covered MoS2 surface indicate a band gap narrowing of the MoS2 surface by ~0.1 eV. This band gap reduction at the surface is further evidence that interlayer van der Waals interactions critically influence the band structure of 2D-layered dichalcogenides and suggest that interfacing with dissimilar van der Waals materials allows tuning of their electronic properties.

Self-assembly of terephthalic acid (TPA), vacuum deposited on Pt(111) supported graphene, has been investigated by scanning tunneling microscopy (STM). TPA organizes in an ordered 3 × 4 superstructure with respect to the graphene lattice. This structure is a consequence of hydrogen-bonded TPA chains that arrange in a commensurate overlayer on graphene. Due to the polycrystalline nature of graphene on Pt(111), the TPA layer exhibits various grain boundaries and dislocations. Molecular resolved STM imaging has been used to characterize these defect structures in the TPA monolayer.

Integrating graphene into device architectures requires interfacing graphene with dielectric materials. However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating continuous graphene/dielectric interfaces challenging. Here, we show that yttria (Y(2)O(3))--a high-κ dielectric--can form a complete monolayer on platinum-supported graphene. The monolayer interacts weakly with graphene, but is stable to high temperatures. Scanning tunnelling microscopy reveals that the yttria layer exhibits a two-dimensional hexagonal lattice rotated by 30° relative to the hexagonal graphene lattice. X-ray photoemission spectroscopy measurements indicate a shift of the Fermi level in graphene on yttria deposition, which suggests that dielectric layers could be used for charge doping of metal-supported graphene.