We have investigated the effects of thermal annealing on ex-situ chemically vapor deposited submonolayer graphene islands on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum scanning tunneling microscopy. Low-temperature annealed graphene islands on Cu foil (at ∼430 °C) exhibit predominantly striped Moiré patterns, indicating a relatively weak interaction between graphene and the underlying polycrystalline Cu foil. Rapid high-temperature annealing of the sample (at 700-800 °C) gives rise to the removal of Cu oxide and the recovery of crystallographic features of the copper that surrounds the intact graphene. These experimental observations of continuous crystalline features between the underlying copper (beneath the graphene islands) and the surrounding exposed copper areas revealed by high-temperature annealing demonstrates the impenetrable nature of graphene and its potential application as a protective layer against corrosion.

Articles you may be interested inElectronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111) J. Appl. Phys. 113, 053706 (2013); 10.1063/1.4789353 Incorporation, valence state, and electronic structure of Mn and Cr in bulk single crystal β-Ga2O3

Single crystals of transition metal (TM) doped β–Ga2O3, a wide gap semiconductor system of interest for transparent conductive oxide and diluted magnetic semiconductor applications, have been studied in the dilute, non-interacting limit (≤0.06 cation %). Based on optical absorption, particle induced x-ray emission, and Rutherford backscattering measurements, Mn does not incorporate as well as Cr, and Mn degrades the crystal quality. Using superconducting quantum interference device (SQuID) magnetometry, a Brillouin type paramagnetic magnetization is observed for Mn or Cr doped crystals with an effective number of Bohr magnetons per TM ion of 5.88 ± 0.1 or 3.95 ± 0.1, respectively. A trace ferromagnetic signal is consistent with a very small concentration of secondary phases in the Mn-doped crystal. The position of the edge in x-ray absorption near edge structure (XANES) measurements suggests that the Cr takes the 3+ valence, while a mixture of Mn2+ and Mn3+ are present; based on the absence of a prominent pre-edge feature in the XANES, both TM predominantly occupy an octahedral site in β–Ga2O3. Density functional theory (DFT) results, optical absorption and SQuID data are consistent with this assignment. While the Cr-doped crystal is conductive, the Mn-doped crystal is insulating, which is consistent with the Mn2+/Mn3+ mixed valence, assuming the Fermi level is pinned mid-gap at the Mn 2+/3+ transition level, which is predicted by DFT to be 1.8 eV above the valence band maximum.

Graphene is nature's ideal two‐dimensional conductor and is comprised of a single sheet of hexagonally packed carbon atoms. Since the first electrical measurements made on graphene, researchers have been trying to exploit the unique properties of this material for a variety of applications that span numerous scientific and engineering disciplines. In order to fully realize the potential of graphene, large scale synthesis of high quality graphene and the ability to control the electronic properties of this material on a nanometer length‐scale are necessary and remain key challenges. This article will review the efforts at the Center for Nanoscale Materials that focus on the atomic‐scale characterization and modification of graphene via scanning tunneling microscopy and its synthesis on various materials (SiC, Cu(111), Cu foil, etc.). These fundamental studies explore growth dynamics, film quality, and the role of defects. The chemical modification of graphene following exposure to atomic hydrogen will also be covered, while additional emphasis will be made on graphene's unique structural properties.

Molecular diffusion, motion, and conformation are critical to chemical and biological processes. Concurrently, understanding how chirality affects these processes has become a critical challenge for various applications in the pharmaceutical and food industries ranging from drug catalysis to novel sensing. Here, we present a unique way of transferring the chirality of simple amino acids, L-and D-alanine, to large-scale chiral networks on Cu(111). We further utilize the unique geometry of the chiral network as a scaffolding to isolate individual molecules within a 1.2 nm hexagonal pore. These hexagonal pores act as single molecule "race tracks" where excess alanine molecules trapped at the perimeter are observed to hop between six distinct locations around the perimeter. Scanning tunneling microscopy (STM) as well as density functional theory (DFT) calculations have been utilized to directly track, influence, and probe this molecular motion confined to self-assembled, chiral, hexagonal pores which also form quantum corrals.

Gallium selenide ͑Ga 2 Se 3 ͒ is a III-VI chalcogenide material whose intrinsic cation vacancy ordering into one dimensional chains has been predicted to result in a one dimensional band at the valence-band maximum ͑VBM͒. The electronic structure of Ga 2 Se 3 thin films on Si͑001͒:As is studied with angle-resolved photoemission spectroscopy. The integrated density of states and the dispersion exhibit good agreement with that predicted by published density-functional theory results. Consistent with a one dimensional state, the electronic states at the VBM show no dispersion perpendicular to the vacancy chains. The Se-Ga bond length from extended x-ray absorption fine structure is 2.34 Å. Low-energy electron-diffraction results indicate that the surface is characterized by nanometer-scale ͑111͒ facets, consistent with previous scanning tunneling microscopy results.

Chromium-doped gallium sesquiselenide, Cr:Ga2Se3, is a member of a new class of dilute magnetic semiconductor exploiting intrinsic vacancies in the host material. The correlation among room-temperature ferromagnetism, surface morphology, electronic structure, chromium concentration and local chemical and structural environments in Cr:Ga2Se3 films grown epitaxially on silicon is investigated with magnetometry, scanning tunneling microscopy, photoemission spectroscopy and x-ray absorption spectroscopy. Inclusion of a few percent chromium in Ga2Se3 results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment ≥ 4µB/Cr. The intrinsic-vacancy structure of defected-zinc-blende β−Ga2Se3 enables Cr incorporation in a locally octahedral site without disrupting long-range order, determined by x-ray absorption spectroscopy, as well as strong overlap between Cr 3d states and the Se 4p states lining the intrinsic vacancy rows, observed with photoemission. The highest magnetic moment per Cr is observed near the solubility limit of roughly one Cr per three vacancies. At higher Cr concentrations, islanded, metallic films result, with a magnetic moment that depends strongly on surface morphology. The effective valence is Cr 3+ in laminar films, with introduction of Cr 0 upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism.

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