Moire Superstructure Research Articles

Honeycomb on Square Lattices: Geometric Studies and Strain Analysis of Moiré Structures at a Symmetry-Mismatched Interface

Moire superstructures are common for epitaxial graphene on transition metal surfaces and can strongly influence the mechanical and electronic properties of the overgrown film. To date, most of the ...

Orientational and directional locking of colloidal clusters driven across periodic surfaces

When particles are driven across crystalline surfaces, their trajectories do not necessarily follow the applied force but become locked to the substrate lattice directions. Such directional locking, being relevant for bottom-up nanodevice assembly1,2 and particle sorting3–6, has been intensively studied for isolated or single particles3–11. Here we experimentally study the motion of extended colloidal clusters sliding over a periodically corrugated surface. We observe that both their orientational and centre-of-mass motions become locked into directions not coinciding with the substrate symmetry but determined by the geometrical moire superstructure formed by the cluster and substrate lattices. In general, such moire superstructures are not strictly periodic, which leads to competing locking directions depending on cluster size. Remarkably, we uncover a dependence of directional locking on the higher Fourier components of the surface corrugation profile, which can be tuned on atomic surfaces via the external load12,13. This allows for an unprecedented control of cluster steering relevant for nanomanipulations on surfaces. Colloidal clusters are shown to undergo directional locking when driven across a patterned surface. The role of the Fourier components of the particle–surface interaction suggests a means of leveraging this behaviour for nanoscale manipulation.

Template Assisted Nucleation of Cobalt and Gold Nano-clusters on an Ultrathin Iron Oxide Film

Being the basic building blocks for nano-magnetic and nano-catalytic devices, regular arrays of nano-clusters play a crucial role in modern nanotechnology. One of the possible fabrication methods of periodic nanostructures consists in exploiting nano-patterned substrates as templates for the self-assembly of the deposited atoms. Here, we have investigated the templating properties of a Moire superlattice formed at the interface between a FeO(111)-like ultrathin film and a Ni/Fe(001) substrate. Co and Au, representative of elements with high and low oxygen affinity, respectively, have been deposited on the iron oxide film. Scanning tunneling microscopy reveals that Co nucleates preferentially along the corrugated regions of the Moire superstructure, forming stripes with high aspect ratio. On the other hand, Au atoms nucleate randomly distributed three-dimensional islands on the FeO(111) surface.

Periodic Modulation of Graphene by a 2D-FeO/Ir(111) Moiré Interlayer

Ultrathin films of iron oxide form a two-dimensional (2D) FeO layer on Ir(111). Because of difference in lattice constant between 2D-FeO and Ir(111), a moire superstructure is formed. The 2D-FeO/Ir(111) structure is examined by soft X-ray photoelectron spectroscopy, X-ray photoemission diffraction, scanning tunneling microscopy, and low-energy electron diffraction. A 2D-FeO layer may also be grown by iron intercalation and subsequent oxidation underneath a graphene layer on Ir(111). Thus, the graphene can be decoupled from the metal by the 2D-FeO layer. Changes in the graphene C 1s binding energy can be mainly explained by shifts in the Fermi level of graphene as a consequence of interface band alignment for weak interactions between graphene and the substrate. A shift of C 1s to lower binding energy, for graphene supported on FeO/Ir(111), is a consequence of the dipole moment in the 2D-FeO layer normal to the Ir(111) surface. Broadening of the C 1s peak is consistent with a locally varying 2D-FeO dipole ...

H2O on Graphene/Ir(111): A Periodic Array of Frozen Droplets

Scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS) show that deposition of water molecules onto epitaxial graphene on Ir(111) leads to the formation of an extended and well ordered array of amorphous water clusters. We trace the evolution of this cluster phase as dependent on water exposure and deposition temperature. The formation of separated clusters is due to binding energy differences within the moire superstructure.

Lattice‐Contraction‐Induced Moiré Patterns in Direction‐Controlled Epitaxial Graphene on Cu(111)

Contrary to the general understanding, direction controlled epitaxial graphenes with an identical rotational angle (R θ = R0ˆ) is reported. Despite the identical Rθ, various Moire patterns with different periodicity are observed, which cannot be understood by conventional Rθ dependence. A new method is suggested for practical analysis, in which degree of lattice contraction of EG results in the various Moire superstructures.

Anomalous moiré pattern of graphene investigated by scanning tunneling microscopy: Evidence of graphene growth on oxidized Cu(111)

The growth of graphene on oriented (111) copper films has been achieved by atmospheric pressure chemical vapor deposition. The structural properties of as-produced graphene have been investigated by scanning tunneling microscopy. Anomalous moire superstructures composed of well-defined linear periodic modulations have been observed. We report here on comprehensive and detailed studies of these particular moire patterns present in the graphene topography revealing that, in certain conditions, the growth can occur on the oxygen-induced reconstructed copper surface and not directly on the oriented (111) copper film as expected.

The classical molecular dynamics simulation of graphene on Ru(0001) using a fitted Tersoff interface potential

The accurate molecular dynamics simulation of weakly bound adhesive complexes, such as supported graphene (gr), is challenging because of the lack of an adequate interface potential. Instead of the widely used Lennard‐Jones potential for weak and long‐range interactions, we use a newly parameterized Tersoff potential for gr/Ru(0001) system. The new interfacial force field provides adequate moire superstructures in accordance with scanning tunneling microscopy images and with density functional theory (DFT) results. In particular, the corrugation of ξ ≈ 1.0 ± 0.2 Å is found that is somewhat smaller than found by DFT approaches (ξ ≈ 1.2 Å) and is close to scanning tunneling microscope measurements (ξ ≈ 0.8 ± 0.3 Å). The new potential could open the way toward large‐scale simulations of supported gr with adequate moire supercells in many fields of gr research. Moreover, the new interface potential might provide a new strategy in general for obtaining accurate interaction potentials for weakly bound adhesion in large‐scale systems in which atomic dynamics is inaccessible yet by accurate DFT calculations. Copyright © 2013 John Wiley & Sons, Ltd.

Adsorption and Diffusion of 4d and 5d Transition Metal Adatoms on Graphene/Ru(0001) and the Implications for Cluster Nucleation

To explore the possibility of using the graphene moire superstructure formed on Ru(0001) (g/Ru(0001)) as a template to self-assemble super-lattices of metal nanoparticles as model catalysts, it is desirable to know the minimum-energy adsorption sites, adsorption energies, and diffusion properties of small metal species on this surface. Toward that end, density functional theory calculations have been carried out to investigate the adsorption and diffusion of 18 4d (Y–Ag) and 5d (La–Au) transition metal adatoms on g/Ru(0001), using small surface models representing different regions of the g/Ru(0001) surface. For each adatom, adsorption is the strongest in the fcc region and the weakest in the mound region of the moire. Diffusion within the fcc region is facile for most adatoms, but an additional barrier is imposed by the corrugation of the graphene moire for traversing between neighboring fcc regions. Overall, the earlier 4d and 5d metal adatoms have stronger adsorption energies and higher diffusion barriers on g/Ru(0001) than the later ones. The results are then interpreted to provide a better understanding of the conditions necessary to achieve dense super-lattices of monodisperse metal clusters on g/Ru(0001).

Moiré superstructures of silicene on hexagonal boron nitride: A first-principles study

Abstract Using first-principles calculations, we predicted hexagonal boron nitride (h-BN) with flat surface is an ideal substrate for silicene. Van der Waals interactions hold silicene and h-BN together, forming silicene/BN moire superstructures. The moire superstructures open a band gap of about 30 meV at the Dirac point of silicene at equilibrium distance. The band gap is almost independent of the rotation angle between the two lattices, but can be effectively tuned by changing the interlayer spacing. The high Fermi velocity of silicene is well preserved in these superstructures. These features are helpful in achieving applications of silicene in nanoscale electronic devices.

First-principles study of the adsorption of Au atoms and Au2and Au4clusters on FeO/Pt(111)

Adsorption of Au atoms and Au-2 and Au-4 clusters on Pt(111)-supported bilayer FeO film were studied using first-principles density functional theory. For atom adsorption on the FeO/Pt(111) coincidence lattice, two types of adsorption were found, in agreement with previous studies. One is that the Au atom binds with one Fe cation lifted from underneath to the top of the oxygen layer (flipped adsorption), and the other is that the Au atom adsorbs directly on the oxygen layer (direct adsorption). Flipped adsorption was found to be energetically most favored in all domains of the Moire superstructure, while direct adsorption was either metastable or unstable. The charge of the Au adatom is negative for the flipped adsorption and positive for the direct adsorption. For the Au2 and Au4 clusters, the global most stable adsorption configurations on the FeO/Pt(111) were explored, and the results indicated that one-dimensional (dimer) or two-dimensional (Au-4) upright configurations of direct adsorption were energetically preferred over the flat or three-dimensional ones throughout the Moire superstructure. Moreover, it was found that the configurations of Au-2 dimers and Au-4 clusters on FeO/Pt(111) is tightly related to the interfacial interaction including two effects: the direction-dependent reactivity of a planar Au cluster and the interfacial electrostatic interaction between the Au clusters and the FeO/Pt(111) support. The role of relativistic effects in the configuration of Au clusters and the interaction with FeO/Pt(111) were illustrated. The evolution of small Au cluster configuration on FeO/Pt(111) was discussed.

Impact of Atomic Oxygen on the Structure of Graphene Formed on Ir(111) and Pt(111)

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer graphite (MG or graphene) grown on Pt(111) and Ir(111) has been studied using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy. For comparison, the adsorption of atomic oxygen on highly oriented pyrolytic graphite has been studied under the same conditions. Graphene oxidation predominantly occurs through the formation of epoxy groups and causes atomic-scale buckling of the graphene lattice, as evidenced by an sp2-to-sp3 bonding transformation. The different parts of the graphene/metal moire superstructure show different oxidation dynamics, with the initial formation of epoxy groups in the more bonding “pores”. Upon O adsorption, the nearest C neighbors of epoxy groups get engaged in a stronger bonding with the substrate. As a result, the pores of the graphene mesh become attracted and effectively pinned to the substrate by the O atoms. A ...

Origin of Moiré structures inC60on Pb(111) and their effect on molecular energy levels

Low-energy electron diffraction LEED indicates that the monolayer structure of C60 on Pb 111 comprises two coexisting incommensurate structures with nonsymmetry epitaxial rotations near 20° relative to the Pb 111 lattice. These structures are observed in scanning tunneling microscopy STM as Moire superstructures having periods of about 46 A and 34 A. The Moire images and LEED patterns are consistent with two higher-order commensurate HOC structures that were identified using the hexagonal number sequence method. These structures are close to predictions from the Novaco-McTague theory of epitaxial rotation, assuming a weakly corrugated substrate potential. As a consequence of the fullerenes within the Moire structures having different local environments, the energetic alignment of the molecular resonances is also modulated, with shifts measured by tunneling spectroscopy of up to 20 meV.

Moiré Patterns in Superimposed Nanoporous Thin Films Derived from Block-Copolymer Assemblies

Nanoscale rotation moire patterns are observed in double-layer nanoporous thin films produced with use of block-copolymer self-assembly. Periodic hexagonal moire superstructures appear when the films possessing long-range order are superimposed at small misorientation angles. Overlapping films misoriented by angles close to 30° generate aperiodic quasi-crystal-like superstructures with 5-fold symmetries. The stacking of the disordered nanoporous films produces labyrinth-like patterns. Block-copolymer moire superstructures are promising as nanolithography masks with controllable morphology and periodicity.

Fermi Gap Stabilization of an Incommensurate Two-Dimensional Superstructure

The compressed, incommensurate approximately (9.5 x 9.5) moire superstructure of the Ag monolayer on Cu(111) displays a filled surface state band with a Fermi energy gap at the Brillouin zone boundary. By contrast, the surface band is gapless for the less compressed, commensurate (9 x 9) moire of two Ag layers. A simple estimate of the energy gain rendered by opening this gap gives a value similar to the elastic energy change required to modify the commensurate structure, thereby suggesting that the approximately (9.5 x 9.5) incommensurate phase is stabilized by such a gap opening. The possible presence of a charge density wave state is discussed.

Integration of point-contact microscopy and atomic-force microscopy: Application to characterization of graphite/Pt(111)

The electrical current through the point-contact junction of an AFM tip is used to image the surfaces of bulk graphite (HOPG) and the surface of a graphitized carbon monolayer on Pt(111) under ultra-high-vacuum (UHV) conditions. Lattice-resolved images are obtained simultaneously in topography, lateral friction, and contact current channels. Lattice resolution in current maps persisted up to 0.9 mA and pressures of up to 5 GPa. In both bulk graphite and the case of graphitized carbon monolayer on Pt(111), the current images show only one maximum per unit cell. In addition, the contact current images of the graphite monolayer reveal local conductivity variations. We observed local conductivity variations in the form of moire superstructures resulting from high order commensurability with the Pt lattice. (c) 1999 The American Physical Society.