Search

Scanning tunneling microscopy (STM) at 5 K is used to study WSe2 layers grown on epitaxial graphene which is formed on Si-terminated SiC(0 0 0 1). Specifically, a partial hydrogenation process is applied to intercalate hydrogen at the SiC-graphene interface, yielding areas of quasi-free-standing bilayer graphene coexisting with bare monolayer graphene. We find that an abrupt and structurally perfect homojunction (band-edge offset ∼0.25 eV) is formed when WSe2 overgrows a lateral junction between adjacent monolayer and quasi-free-standing bilayer areas in the graphene. The band structure modulation in the WSe2 overlayer arises from the varying work function (electrostatic potential) of the graphene beneath. Scanning tunneling spectroscopy measurements reveal that this effect can be also utilized to create WSe2 quantum dots that confine either valence or conduction band states, in agreement with first-principles band structure calculations.

We present a combined high-resolution electron diffraction (SPALEED) and scanning tunneling microscopy study of the insulator/metal growth system NaCl/Cu(2 1 1) which is inherently unstable against lateral pattern formation on the nanometer scale. Cu(2 1 1) is a vicinal surface with (1 1 1) terraces and intrinsic (1 0 0) steps (step distance 6.25 A˚). This starting surface restructures upon NaCl deposition when the growth temperature exceeds ∼270 K. The initially flat surface is transformed into a periodic one-dimensional hill-and-valley structure consisting of (3 1 1) and (1 1 1) facets. NaCl grows selectively on (3 1 1) facets only, thereby creating a regular surface pattern with alternating stripes of bare Cu and chemically inert NaCl-covered areas. The lateral stripe spacing can be varied from ∼30 A˚ at 300 K to ∼230 A˚ at 600 K via the growth/annealing temperature. The present restructuring process is governed by the interplay between energetics and kinetics, namely by (1) the tendency to form (1 0 0)-terminated NaCl layers, (2) energetically favored interfacial matching between NaCl(1 0 0) and Cu(3 1 1), and (3) sufficient mobility of the substrate surface to allow for Cu mass transport. [Copyright &y& Elsevier]

The twofold decagonal Al71.8Ni14.8Co13.4(1 0 0 0 0) surface is investigated by using highly surface sensitive elastic helium atom scattering (HAS) and high resolution spot profile analyzing low energy electron diffraction (SPA-LEED). SPA-LEED and HAS data reveal diffraction patterns of the top most surface layer consistent with those of the surfaces generated by bulk truncations. Strong diffraction peaks corresponding to an 8 Å periodicity are observed. These peaks are broadened along the quasiperiodic surface direction, reflecting a reduced correlation length of ≈80 Å of the 8 Å periodicity within the quasicrystalline planes. In addition very weak streaks corresponding to a 16 Å periodicity are observed. [Copyright &y& Elsevier]

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment (θ = 0°), associated with flat bands in the Brillouin zone of the moiré pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS 2 /WSe 2 system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moiré unit cell of a θ = 0° MoS 2 /WSe 2 heterobilayer and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data. We further explore the role of a twist angle by showing experimental data for a twisted MoS 2 /WSe 2 heterobilayer structure with a twist angle of θ = 15°, which exhibits a moiré pattern but no confinement.

We created hexagonal rings on a semiconductor surface by atom manipulation in a scanning tunneling microscope (STM). Our measurements reveal the generic level structure of a quantum ring, including its single ground state and doubly degenerate excited states. The ring shape leads to a periodic potential modulation and thereby a perturbation of the level structure that can be understood in analogy to band gap formation in a one-dimensional periodic potential. The modulation can be enhanced or inverted by further adding or removing atoms with the STM tip. Our results demonstrate the possibility of designing and controlling electron dynamics in a tunable periodic potential, holding promise for the construction of two-dimensional artificial lattices on a semiconductor surface.

A two-dimensional (2D) heterobilayer system consisting of MoS 2 on WSe 2 , deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS 2 and WSe 2 . Significant energy shifts are observed in tunneling spectra observed at the maxima of the moiré corrugation, as compared with spectra obtained at corrugation minima, consistent with prior work. Furthermore, at the minima of the moiré corrugation, sharp peaks in the spectra at energies near the band edges are observed for spectra acquired at 5 K. The peaks correspond to discrete states that are confined within the moiré unit cells. Conductance mapping is employed to reveal the detailed structure of the wave functions of the states. For measurements at 80 K, the sharp peaks in the spectra are absent, and conductance maps of the band edges reveal little structure.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe 2 ) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe 2 /sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10 7 ), high current density (1-10 μA·μm -1 ), and good field-effect transistor mobility (∼30 cm 2 ·V -1 ·s -1 ) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

A scanning tunnelling microscope (STM) operated at 5 K was used to study dysprosium biphthalocyanine (DyPc 2 ) molecules adsorbed on the inert III-V semiconductor surface InAs(1 1 1)A. Orbital imaging and scanning tunnelling spectroscopy measurements reveal that the molecular electronic structure remains largely unperturbed, indicating a weak molecule-surface binding. The molecule adsorbs at the In vacancy site of the (2  ×  2)-reconstructed surface and is highly sensitive to current-induced excitations leading to random rotational fluctuations. Owing to the weak surface binding, individual molecules can be precisely repositioned and arranged by the STM tip via attractive tip-molecule interaction. In this way, DyPc 2 dimers of well-defined internal structure can be assembled which exist in two conformations of equivalent appearance. A binary switching between these two conformers can be induced by injecting electrons into one of the two molecules. The conformational change of the dimer proceeds via a concerted molecular rotation and minor lateral displacement. The synchronised switching observed here is attributed to steric interactions between the two molecules constituting the dimer.