Taizo Ohgi

Au particle deposition onto self-assembled monolayers of thiol and dithiol molecules

Abstract Gold depositions on self-assembled monolayers (SAMs) of thiol (HS(CH 2 ) 7 CH 3 ) and dithiol (HS(CH 2 ) n SH, n =6, 8, 10) molecules were investigated by a scanning tunneling microscope in air. In the case of thiols, evaporated gold atoms were found to penetrate and form monoatomic height islands under the SAMs. On the contrary, gold deposition onto the dithiols resulted in the formation of the gold particles on top of the SAMs. Each Au particle is estimated to have ∼200 atoms and this size is independent of Au coverage, at least below 1 ML. The electrical properties such as Coulomb blockade were also studied by scanning tunneling spectroscopy.

Observation of Au deposited self-assembled monolayers of octanethiol by scanning tunneling microscopy

Au deposited self-assembled monolayers (SAMs) of octanethiol molecules have been studied by scanning tunneling microscopy. We have observed ordered structures of the molecules on both the original terraces and subsequently grown Au islands after the Au deposition. These results indicate that Au atoms penetrate through and form islands underneath the SAMs. At the initial stage of Au deposition, islands with a monatomic height grow and become larger as more Au atoms are evaporated onto the surface. The number of islands remains constant as the Au coverage increases up to approximately 0.5 ML. Above this coverage, the islands on each terrace abruptly coalesce into one network structure. The second layer starts to form after coalescence, before the first layer fully covers the surface. This unique island growth is not seen in the normal homoepitaxial growth of Au on Au(111), and is presumably attributed to both the high nucleation density of deposited atoms caused by SAMs and the relatively high diffusion of adatoms along island step edges.

Scanning tunneling microscopy and X-ray photoelectron spectroscopy of silver deposited octanethiol self-assembled monolayers

Ag deposited self-assembled monolayers (SAMs) of octanethiol (CH 3 (CH 2 ) 7 SH) have been studied using scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). At the initial stage of Ag deposition, monatomic height islands, ∼7 x 10 1 cm 2 in density, grow at the SAMs/Au(1 I I) interface and become larger as more Ag atoms are deposited up to a full monolayer coverage of Ag. The differences of the nucleation density and the growth property between Ag and Au islands can be attributed to the higher mobility of Ag atoms and the difference of the molecular packing on these islands. XPS analysis of this structure (SAMs/Ag monolayer/Au) shows that the Ag 3d 5.2 binding energy is shifted -0.3 eV with respect to bulk Ag, the C Is binding energy is ∼0.3 eV higher than that before Ag deposition, and the S 2p 3 2 binding energy exhibits little shift before and after deposition. The origin of the shift can be explained by the change of the dipole at the interface and the electrical isolation of alkyl chains from the surroundings.

Discovery of Carbon Nanowires Formed on a Carbon-Doped Ni(111) Substrate by a Bulk-to-Surface Precipitation Process

We have discovered a synthesis method for carbon nanowires on a flat single-crystal graphite (0001) surface on a carbon-doped Ni(111) substrate using only heat treatment in ultrahigh vacuum. It should be noted that this new carbon nanowire synthesis method requires no external carbon-containing source. The growth mechanism utilizes a bulk-to-surface precipitation process of internal carbon atoms that were doped in a pure Ni(111) substrate in advance. Nanometer-scale morphology and chemistry of the carbon nanowires have been clarified by low-energy electron diffraction combined with Auger electron spectroscopy (LEED/AES), scanning tunneling microscopy (STM) and field-emission scanning Auger microcopy (FE-SAM).

STM induced photon emission from adsorbed porphyrin molecules on a Cu(100) surface in ultrahigh vacuum

Photon emission induced by tunneling electrons has been observed from a submonolayer of Cu-tetra-[3,5-di-t-butylphenyl]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface in ultrahigh vacuum. Near-field photons generated in a nanometer-scale region with Cu-TBPP molecules were collected effectively through the apex of a conductive optical fiber tip. The photon emission mechanism can be attributed to the inelastic tunneling events involving the tip, the Cu-TBPP molecules, and the Cu(100) substrate.

Light Emission from Porphyrin Molecules Induced by a Scanning Tunneling Microscope

Positioning of a scanning tunneling microscope (STM) tip above Cu-tetra-(3,5-di-tertiary-butyl-phenyl)-porphyrin (Cu-TBPP) molecules on Cu(100) is found to induce plasmon-mediated emission and molecular luminescence when bias voltages are above ~ 2.3 V. Optical spectra acquired at a low current of 0.2 nA suggest not only the enhancement effect of the molecules on light emission but also new features associated with the molecules. The quantum efficiency of such light emission excited by inelastic tunneling is on the order of 10-6 photons per electron.

Electrochemical potential arrangement of nanoclusters weakly coupled with metal surface

We investigated the electrochemical potential arrangement of Au nanoclusters, 1–3 nm in diameter, weakly coupled with bulk Au surface through tunneling junctions. The measurement of the Coulomb staircase by scanning tunneling spectroscopy and the statistical analysis for clusters reveal that [μ(0)+μ(1)]/2, where μ(n) is the electrochemical potential of the cluster when the number of excess electrons changes between n−1 and n, distributes around the Fermi level of the bulk Au electrode with the standard deviation σ of 30–70 meV. The spacing ΔE of the equally spaced electrochemical potentials decreases with increasing cluster size, which leads to the breakdown of the charge neutrality of the clusters below ΔE∼0.3 eV due to the competition between σ and ΔE.

Characteristics of Indium Tin Oxide Films Deposited by DC and RF Magnetron Sputtering.

Conductive and transparent indium tin oxide (ITO) films with a thickness of 100 nm were deposited onto glasses and Si(100) wafers by direct current (DC) and radio frequency (RF) magnetron sputtering. The formation and the annealing effect of films were studied by the measurements of resistivity, optical-transmission, X-ray diffraction and scanning tunneling microscopy (STM). Experimental studies indicated those films deposited by DC sputtering in a 1% O2 in an O2/Ar gas mixture, without annealing, have the lowest resistivity and the highest transmission. In addition, the films deposited by RF sputtering in a 3% O2 in an O2/Ar gas mixture, with annealing in air at 300°C for 2 h, have better resistivity and transmission.

Octanedithiol layer as tunneling barrier

Abstract Au deposited octanedithiol/Au(111) samples provide homogeneous Au nanoclusters, well defined tunneling barriers and atomically flat substrate, which enables us to observe single electron tunneling effects and the kinetic energy shifts of the photoelectrons on the same sample in tunneling spectroscopy and photoelectron spectroscopy, respectively. Through the manipulation technique and the observation of Coulomb staircase by tunneling microscopy and spectroscopy, it is shown that this system provides the uniform resistance (∼140 MΩ ) between clusters and Au(111) surface.

Metal Atomic Chains on the Si(100) Surface

The selection of a single-domain Si(100)2×1 surface enables us to make an indium atomic chain over 70 nm in length. Such self-assembled chains can be extended by atomic manipulation using a scanning tunneling microscope tip, as briefly demonstrated in this work on In/Si(100). The advantage of a single-domain Si(100) surface over a double-domain one for the growth of long chains is rationalized, and the mechanism behind the atomic manipulation is noted.