Manabu Ohtomo

Contracted Interlayer Distance in Graphene/Sapphire Heterostructure

Direct growth of graphene on insulators is expected to yield significant improvements in performance of graphene-based electronic and spintronic devices. In this study, we successfully reveal the atomic arrangement and electronic properties of a coherent heterostructure of single-layer graphene and α-Al2O3(0001). The analysis of the atomic arrangement of single-layer graphene on α-Al2O3(0001) revealed an apparentcontradiction. The in-plane analysis shows that single-layer graphene grows not in a single-crystalline epitaxial manner, but rather in polycrystalline form, with two strongly pronounced preferred orientations. This suggests relatively weak interfacial interactions are operative. However, we demonstrate that unusually strong physical interactions between graphene and α-Al2O3(0001) exist, as evidenced by the small separation between the graphene and the α-Al2O3(0001) surface. The interfacial interaction is shown to be dominated by the electrostatic forces involved in the graphene π-system and the unsaturated electrons of the topmost O layer of α-Al2O3(0001), rather than the van der Waals interactions. Such features causes graphene hole doping and enable the graphene to slide on the α-Al2O3(0001) surface with only a small energy barrier despite the strong interfacial interactions.

Precise control of single- and bi-layer graphene growths on epitaxial Ni(111) thin film

In situ analysis was performed on the graphene growth in ultrahigh vacuum chemical vapor deposition by exposing the epitaxial Ni(111) thin film to benzene vapor at 873 K. It is shown that the highly uniform single- and bi-layer graphenes can be synthesized by the control of benzene exposure in the range of 10–105 langmuirs, reflecting a change in the graphene growth-rate by three orders of magnitude in between the first and second layer. Electron energy loss spectroscopy measurements of single- and bi-layer graphenes indicates that the interface interaction between bi-layer graphene and Ni(111) is weakened in comparison with that between single-layer graphene and Ni(111). It is also clarified from the micro-Raman analysis that the structural and electrical uniformities of the graphene film transformed on a SiO2 substrate are improved remarkably under the specific exposure conditions at which the growths of single- and bi-layer graphenes are completed.

Contact-induced spin polarization in graphene/h-BN/Ni nanocomposites

Atomic and electronic structure of graphene/Ni(111), h-BN/Ni(111) and graphene/h-BN/Ni(111) nanocomposites with different numbers of graphene and h-BN layers and in different mutual arrangements of graphene/Ni and h-BN/Ni at the interfaces was studied using LDA/PBC/PW technique. Using the same technique corresponding graphene, h-BN and graphene/h-BN structures without the Ni plate were calculated for the sake of comparison. It was suggested that C-top:C-fcc and N-top:B-fcc configurations are energetically favorable for the graphene/Ni and h-BN/Ni interfaces, respectively. The Ni plate was found to induce a significant degree of spin polarization in graphene and h-BN through exchange interactions of the electronic states located on different fragments.

Contact-induced spin polarization of monolayer hexagonal boron nitride on Ni(111)

Hexagonal boron nitride (h-BN) is a promising barrier material for graphene spintronics. In this Letter, spin-polarized metastable de-excitation spectroscopy (SPMDS) is employed to study the spin-dependent electronic structure of monolayer h-BN/Ni(111). The extreme surface sensitivity of SPMDS enables us to elucidate a partial filling of the in-gap states of h-BN without any superposition of Ni 3d signals. The in-gap states are shown to have a considerable spin polarization parallel to the majority spin of Ni. The positive spin polarization is attributed to the π-d hybridization and the effective spin transfer to the nitrogen atoms at the h-BN/Ni(111) interface.

Spin orientation transition across the single-layer graphene/nickel thin film interface

The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.

The adsorption of h-BN monolayer on the Ni(111) surface studied by density functional theory calculations with a semiempirical long-range dispersion correction

The geometric and spin-resolved electronic structure of a h-BN adsorbed Ni(111) surface has been investigated by density functional theory calculations. Two energy minima (physisorption and chemisorption) are obtained when the dispersive van der Waals correction is included. The geometry of N atom on top site and B atom on fcc site is the most energetically favorable. Strong hybridization with the ferromagnetic Ni substrate induces considerable gap states in the h-BN monolayer. The induced π* states are spin-polarized.

Atomic structure and physical properties of fused porphyrin nanoclusters

The atomic and electronic structures, mechanical properties and potential barriers of formation of a set of meso–meso β–β fused porphyrin/metalloporphyrin nanopages, nanotapes, nanotubes and 2D nanofabrics were studied by GGA LC-DFT technique using cluster and PBC models. The porphyrin pages of the nanoclusters are connected with each other by graphene fragments formed by meso–meso β–β links. Fusion of all the edges of six porphyrin/metalloporphyrin units produces a novel ~ 1 nm sized molecule of cubic symmetry with a hollow cage inside. It was found that all studied nanoclusters are metastable with formation energies 0.36–7.57 kcal/mol per atom. Under applied mechanical stress, the nanoclusters exhibit superelastic and ultrastrong properties with binding graphene fragments being the weakest links for mechanical rupture. Depending on the spin-dependent reaction pathways, the hollow caged nanoclusters exhibit almost zero or low potential energy barriers (1–10 kcal/mol) during the initial stages of self-assembly. All nanoclusters exibit the main features of the electronic structures of the parent porphyrins, in particular the nature of HOMO/LUMO states and the relative energetic positions of the metal d states. The induced curvature of the hollow cage nanoclusters leads to admixture of more than 2% of the dπ⊥ states to the dσ energy region and formation of vacant superatomic molecular orbitals of d character in cubic ligand field. The Fe-derived hollow-caged nanoclusters reveal extremely high spin states with small energy differences between ferromagnetic and antiferromagnetic configurations, which can be utilized for quantum holonomic computations.

Graphene nanoribbon field-effect transistors fabricated by etchant-free transfer from Au(788)

We report etching-free and iodine-free transfer of highly aligned array of armchair-edge graphene nanoribbons (ACGNRs) and their field-effect transistor (FET) characteristics. They were prepared by on-surface polymerization on Au(788) templates. The ACGNRs were mechanically delaminated and transferred onto insulating substrates with the aid of a nano-porous support layer composed of hydrogen silsesquioxane (HSQ). The key process in the mechanical delamination is the intercalation of octanethiol self-assembled monolayers (SAMs), which penetrate the HSQ layer and intercalate between the ACGNRs and Au(788). After the transfer, the octanethiol SAMs were removed with Piranha solution, enabling the reuse of the Au single crystals. The FETs fabricated with the transferred ACGNR array showed ambipolar behavior when the channel length was as long as 60 nm. Quasi-one-dimensional conductivity was observed, which implies a good alignment of GNRs after the transfer. In contrast, short-channel ACGNR FETs (channel length ∼20 nm) suffer from a geometry-dependent short-channel effect. This effect is more severe in the FETs with ACGNRs parallel to the channel, which is an ideal geometry, than in ones perpendicular to the channel. Since the ID-VD curve is well fitted by the power-law model, the short-channel effect likely stems from the space-charge limited current effect, while the wide charge-transfer region in the GNR channel can be another possible cause for the short-channel effect. These results provide us with important insights into the designing short-channel GNR-FETs with improved performance.We report etching-free and iodine-free transfer of highly aligned array of armchair-edge graphene nanoribbons (ACGNRs) and their field-effect transistor (FET) characteristics. They were prepared by on-surface polymerization on Au(788) templates. The ACGNRs were mechanically delaminated and transferred onto insulating substrates with the aid of a nano-porous support layer composed of hydrogen silsesquioxane (HSQ). The key process in the mechanical delamination is the intercalation of octanethiol self-assembled monolayers (SAMs), which penetrate the HSQ layer and intercalate between the ACGNRs and Au(788). After the transfer, the octanethiol SAMs were removed with Piranha solution, enabling the reuse of the Au single crystals. The FETs fabricated with the transferred ACGNR array showed ambipolar behavior when the channel length was as long as 60 nm. Quasi-one-dimensional conductivity was observed, which implies a good alignment of GNRs after the transfer. In contrast, short-channel ACGNR FETs (channel lengt...

Orientation Control and Angle-resolved Photoemission Studies of Organic Epitaxial Thin Films on Bismuth-terminated Silicon Surfaces

Orientation Control and Angle-resolved Photoemission Studies of Organic Epitaxial Thin Films on Bismuth-terminated Silicon Surfaces Manabu OHTOMO1,2,†, Yuya TSUCHIDA1, Tetsuya HASEGAWA2 and Toshihiro SHIMADA1 1Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo-shi, Hokkaido 0608628, Japan 2Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo 731, Bunkyo-ku, Tokyo 1130033, Japan Present address: Advanced Science Research Center, Japan Atomic Energy Agency, Shirakata-Shirane 24, Tokai-mura, Naka-gun, Ibaraki 3191195, Japan