Abbas Ebnonnasir

Tunable MoS2 bandgap in MoS2-graphene heterostructures

Using density functional theory calculations with van der Waals corrections, we investigated how the interlayer orientation affects the structure and electronic properties of MoS2-graphene bilayer heterostructures. Changing the orientation of graphene with respect to MoS2 strongly influences the type and the value of the electronic bandgap in MoS2, while not significantly altering the binding energy between the layers or the interlayer spacing. We show that the physical origin of this tunable bandgap arises from variations in the S–S interplanar distance (MoS2 thickness) with the interlayer orientation, variations which are caused by electron transfer away from the Mo–S bonds.

Moiré superstructures of graphene on faceted nickel islands.

Using scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations, we investigated the morphology and electronic structure of monolayer graphene grown on the (111) and (110) facets of three-dimensional nickel islands on highly oriented pyrolytic graphite substrate. We observed graphene domains exhibiting hexagonal and striped moiré patterns with periodicities of 22 and 12 Å, respectively, on (111) and (110) facets of the Ni islands. Graphene domains are also observed to grow, as single crystals, across adjacent facets and over facet boundaries. Scanning tunneling spectroscopy data indicate that the graphene layers are metallic on both Ni(111) and Ni(110), in agreement with the calculations. We attribute this behavior to a strong hybridization between the d-bands on Ni and the π-bands of carbon. Our findings point to the possibility of preparing large-area epitaxial graphene layers even on polycrystalline Ni substrates.

Growth structure and work function of bilayer graphene on Pd(111)

Using in situ low-energy electron microscopy and density functional theory, we studied the growth structure and work function of bilayer graphene on Pd(111). Low-energy electron diffraction analysis established that the two graphene layers have multiple rotational orientations relative to each other and the substrate plane. We observedheterogeneous nucleationandsimultaneousgrowthofmultiple,facetedlayerspriortothecompletionof secondlayer.Weproposethatthefacetedshapesareduetothezigzag-terminatededgesboundinggraphenelayers growing under the larger overlying layers. We also found that the work functions of bilayer graphene domains are higher than those of monolayer graphene, and depend sensitively on the orientations of both layers with respect to the substrate. Based on first-principles simulations, we attribute this behavior to oppositely oriented electrostatic dipoles at the graphene/Pd and graphene/graphene interfaces, the strengths of which depend on the orientations of the two graphene layers.

Kinetics of monolayer graphene growth by segregation on Pd(111)

Using in situ low-energy electron microscopy and density functional theory calculations, we follow the growth of monolayer graphene on Pd(111) via surface segregation of bulk-dissolved carbon. Upon lowering the substrate temperature, nucleation of graphene begins on graphene-free Pd surface and continues to occur during graphene growth. Measurements of graphene growth rates and Pd surface work functions establish that this continued nucleation is due to increasing C adatom concentration on the Pd surface with time. We attribute this anomalous phenomenon to a large barrier for attachment of C adatoms to graphene coupled with a strong binding of the non-graphitic C to the Pd surface.

Orientation-dependent binding energy of graphene on palladium

Using density functional theory calculations, we show that the binding strength of a graphene monolayer on Pd(111) can vary between physisorption and chemisorption depending on its orientation. By studying the interfacial charge transfer, we have identified a specific four-atom carbon cluster that is responsible for the local bonding of graphene to Pd(111). The areal density of such clusters varies with the in-plane orientation of graphene, causing the binding energy to change accordingly. Similar investigations can also apply to other metal substrates and suggests that physical, chemical, and mechanical properties of graphene may be controlled by changing its orientation.

Growth and characterization of epitaxial Zr(0001) thin films on Al2O3(0001)

The authors report the growth of epitaxial Zr(0 0 0 1) thin films on Al2O3(0 0 0 1) substrates at a temperature of 700 °C via dc magnetron sputtering in an ultrahigh vacuum deposition system equipped with facilities for chemical vapor deposition, low-energy electron diffraction, and Auger electron spectroscopy. Zr layers with a nominal thickness of ∼220 nm are deposited at a rate of ∼0.06 nm/s in 10 mTorr Ar atmosphere. In situ Auger electron spectra of the as-deposited film surface reveal the presence of a Zr peak at 145 eV and Hf peak at 172 eV, the latter due to the presence of Hf impurities in the Zr sputter target. In situ low-energy electron diffraction patterns acquired from the Zr sample show sixfold symmetric spots with an in-plane lattice spacing of 0.31 ± 0.02 nm, characteristic of Zr(0 0 0 1)–(1 × 1) surface. Cross-sectional transmission electron microscopy images reveal columnar growth and the formation of a crystalline, 22 ± 8 nm thick, interfacial layer. Energy dispersive x-ray spectra obta...

STRONGLY AND WEAKLY INTERACTING CONFIGURATIONS OF HEXAGONAL BORON NITRIDE ON NICKEL

Using density functional theory calculations with van der Waals corrections, we have investigated the stability and electronic properties of monolayer hexagonal boron nitride (hBN) on the Ni(111) surface. We have found that hBN can bind either strongly (chemisorption) or weakly to the substrate with metallic or insulating properties, respectively. While the more stable configuration is the chemisorbed structure, many weakly bound (physisorbed) states can be realized via growth around an hBN nucleus trapped in an off-registry position. This finding provides an explanation for seemingly contradictory sets of reports on the configuration of hBN on Ni(111).