Seigi Mizuno

Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire

Epitaxial chemical vapor deposition (CVD) growth of uniform single-layer graphene is demonstrated over Co film crystallized on c-plane sapphire. The single crystalline Co film is realized on the sapphire substrate by optimized high-temperature sputtering and successive H(2) annealing. This crystalline Co film enables the formation of uniform single-layer graphene, while a polycrystalline Co film deposited on a SiO(2)/Si substrate gives a number of graphene flakes with various thicknesses. Moreover, an epitaxial relationship between the as-grown graphene and Co lattice is observed when synthesis occurs at 1000 °C; the direction of the hexagonal lattice of the single-layer graphene completely matches with that of the underneath Co/sapphire substrate. The orientation of graphene depends on the growth temperature and, at 900 °C, the graphene lattice is rotated at 22 ± 8° with respect to the Co lattice direction. Our work expands a possibility of synthesizing single-layer graphene over various metal catalysts. Moreover, our CVD growth gives a graphene film with predefined orientation, and thus can be applied to graphene engineering, such as cutting along a specific crystallographic direction, for future electronics applications.

Catalytic Growth of Graphene: Toward Large-Area Single-Crystalline Graphene.

For electronic applications, synthesis of large-area, single-layer graphene with high crystallinity is required. One of the most promising and widely employed methods is chemical vapor deposition (CVD) using Cu foil/film as the catalyst. However, the CVD graphene is generally polycrystalline and contains a significant amount of domain boundaries that limit intrinsic physical properties of graphene. In this Perspective, we discuss the growth mechanism of graphene on a Cu catalyst and review recent development in the observation and control of the domain structure of graphene. We emphasize the importance of the growth condition and crystallinity of the Cu catalyst for the realization of large-area, single-crystalline graphene.

Observation of anomalous LEED patterns from Li adsorbed Cu(001): 2 × 1, 3 × 3 and 4 × 4

Lithium adsorption on Cu(001) at 300 K has been studied with low-energy electron diffraction (LEED) and Auger electron spectroscopy. The LEED patterns show a sequential change with increasing Li coverage; 2 × 1, 3 × 3 and 4 × 4. The 2 × 1 LEED pattern has weak streaky spots denoted by a matrix of 12θ 0−14θ 2 in the coverage range of 0.375 ⩽ Θ ⩽ 0.4. The 4 × 4 pattern disappears upon further Li deposition and the surface becomes disordered. The features and behavior of the streaky spots are very similar to those observed for K or Cs adsorption on Ni(110) or Cu(110) at room temperature, and this leads to a structure model for 2 × 1; the Cu(001) surface undergoes a missing row type restructuring induced by Li adsorption. Li adatoms are located in missing rows formed by themselves resulting in the streaky spots. Possible structure models for the 3 × 3 and 4 × 4 structures are proposed and they are assigned to ordered surface-alloys of LiCu on Cu(001).

Composite surface structures formed by restructuring-type adsorption of alkali-metals on fcc metals

Abstract Recent studies of alkali-metal (AM) adsorption on fcc metal surfaces, such as Cu, Ag, Au, Ni, Pd and Al, have revealed the formation of stable complex-structures involving considerable restructuring of substrate surface atoms, in contrast to adlayer formation which has been considered likely. In this review article, such structures, referred to as composite surface structures, are described in detail. In such adsorption, the restructuring occurs at room or slightly higher temperatures, whereas AM adlayers are formed at low temperatures. Recent advances in experimental techniques and theoretical calculations have made it possible to determine complicated composite surface structures. In restructuring-type adsorption, substrate surface atoms are replaced, migrated and rearranged to form ordered composite surface structures in collaboration with AM atoms. Thermal activation is required for the formation of the composite surface structures. We have classified the composite surface structures into nine groups based on the fcc low-index planes (001), (110) and (111). In each class a notation is used to identify and, we name each structural group. From the notation, AM coverages in the structural groups can be obtained. Structural parameters of each group are summarized in tables. Conditions and reasons for the formation of the composite surface structures are described. Similar composite surface structures are formed by deposition of metals other than AM in four groups, and their structures are compared with AM induced ones. Common features of the composite surface structures are summarized in the final section.

Growth and electronic transport properties of epitaxial graphene on SiC

With the aim of developing a single-crystal graphene substrate indispensable to graphenes practical applications, we are investigating the structural and physical properties of graphene epitaxially grown on SiC by thermal decomposition. We grow monolayer and bilayer graphene uniformly on a micrometre scale on the Si face of SiC in an Ar environment and in ultra-high vacuum, respectively. Epitaxial bilayer graphene, even if uniform in thickness, contains two types of domains with different stacking orders. We compare the transport properties of monolayer and bilayer graphene using top-gate Hall bar devices. Quantum Hall effects are observed in monolayer graphene and a band gap is electrically detected in bilayer graphene. The monolayer and bilayer graphene show quite different transport properties, reflecting their electronic structures.

Formation of a linear LiOH compound on Cu(001): reaction of H2O with Li adatoms at low coverages

The reaction of H2O with Li adatoms at low coverages on Cu(001) has been studied at room temperature by using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and work-function measurement. The coverage (θ) of Li was 0.125 except for the experiment of the work function (θ = 0.25). The O 1s XPS spectra from the H2O exposed Li/Cu(001) surface showed a single peak at 531.5 eV. It was found by comparing the area of the O 1s peaks of H2O/Li/Cu(001) with that of (√2 × 2√2) R45°OCu(001) that the stoichiometry of oxygen to Li is 1:1. HREELS spectra showed a strong peak at 600 cm−1 and small peaks at 3600, 1300 and 1100 cm−1. The work function increased with increasing H2O exposure. These observations and the results of previous studies lead to the conclusion that the reaction product as a result of interaction of H2O with Li adatoms on Cu(001) is a linear triatomic molecule of LiOH which sits on the surface upright with Li down. The reaction scheme is expressed as Li(a) + H2O(a) → LiOH(a) + H(a), where (a) denotes adspecies. The strong 600 cm−1 peak in HREELS spectra is assigned to the Li-OH stretching mode. The intense loss-peak indicates the LiOH molecule formed on Cu(001) has an ionic-bond character. In fact the effective dynamic charge of the Li-OH stretching vibration is estimated to be ∼ 0.5e, and this is larger than that of the Li stretching vibration in the Li/Cu(001) system, ∼ 0.3e. Force constants of the LiOH admolecule are estimated. A transition state for the reaction is proposed.

Photostimulated desorption of NO chemisorbed on Pt(100) at 193 nm

Photostimulated desorption of NO chemisorbed on a Pt(100) surface at 80 and 300 K has been studied with an ArF excimer laser (λ=193 nm) and a positive‐ion measurement system. NO+ is the only ion species observed when a NO‐saturated Pt(100) surface is irradiated with laser light. The NO+ yield is proportional to the third power of laser fluence. The translational‐energy distribution of the NO+ ions is independent of laser fluence. We propose a two‐step model as the most probable NO+ formation mechanism. The first step is desorption of neutral NO induced by valence‐electron excitation in chemisorbed NO with one‐photon absorption. Then, the desorbed NO is ionized in the vicinity of the surface via the two‐photon nonresonant ionization process. Relatively large desorption cross sections estimated from the decay of the NO+ yield support the above NO+ formation mechanism.

Surface structure of Cu(001)-c(2 x 2)-Mg: a tensor low energy electron diffraction analysis and a first-principles calculation

A c(2×2) structure formed by adsorption of Mg atoms on Cu(0 0 1) at room temperature was determined by a tensor low-energy electron diffraction (LEED) analysis and a first-principles total-energy calculation. Both studies conclude that in the c(2×2) structure every second Cu atom in the first layer is substituted by Mg. Structural parameters obtained by the LEED analysis and the first-principles calculation are in good agreement and the interlayer distance between Mg and the first Cu layer is determined to be 0.55 and 0.60 A, respectively. The interlayer distance between the first and second Cu layers is contracted by about 5% from the bulk value. The calculation shows that the total adsorption energy at substitutional sites is 0.26 eV per Mg atom larger than that at hollow sites. The reason for large stability of the substitutional c(2×2) structure is examined and discussed theoretically.

Surface Crystal Structure of Magnetite Fe3O4(110)

The surface crystal structure of magnetite Fe3O4(110) was studied by low-energy electron diffraction (LEED). A clean surface was obtained after sputtering and annealing at 840 K. The clear LEED patterns show fractional order spots corresponding to a (3×1) surface reconstruction with missing spots. The missing spots indicate a glide plane symmetry of the (3×1) surface. Moreover, the LEED patterns have two-fold rotational symmetry, the surface structure should be p2mg-(3×1) or double domain of p1g1-(3×1). The same glide plane symmetry of the reconstructed surface structure and the ideal surface structure of Fe3O4(110) shows a strong relation between surface and bulk structures. We propose one possible model that corresponds to the p2mg-(3×1).

Determination of the c(2×2) structure formed on Cu(001) upon Li adsorption: a low-energy electron diffraction analysis

Abstract We have determined the c(2 × 2) structure formed on Cu(001) upon Li adsorption at 180 K by low-energy electron diffraction analysis. It is found that an overlayer of Li atoms sitting on the fourfold hollow sites (coverage 0.5) is preferred with a Cu-Li interlayer spacing of 1.96 ± 0.08 A. The radius of the Li atom is 92% of the Li metallic one. The interlayer spacing between the first and second layers of Cu(001) is 1.81 ± 0.04 A, and this is the same value of spacing of the clean Cu(001) surface.