Genki Yoshikawa

Growth of nanographite on Pt(111) and its edge state

The nanographite grains, the diameter of which was around 5nm, were formed on Pt(111) by exposing the Pt(111) substrate to benzene gas at room temperature and annealing it up to 850K. The increase of relative number of edge atoms enabled the observation of edge-derived electronic states. The measurement of ultraviolet photoelectron spectroscopy and near edge x-ray absorption fine structure on the nanographite revealed the appearance of the edge state located at the Fermi level.

Temperature and thickness dependence of molecular orientation of α-sexithienyl on Cu(111)

Atomic and electronic structures of α-sexithienyl(6 T) films grown on Cu(111) were studied by near edge x-ray absorption fine structure (NEXAFS) and Su200a1s x-ray photoemission spectroscopies. The polarization dependent NEXAFS indicated highly ordered arrays of 6 T molecules, with their molecular long axes parallel to the surface, at substrate temperatures lower than 300 K. On the other hand, their molecular long axes became normal to the surface at 360 K, while they initially adsorbed with their long axes parallel to the surface. The film thickness and substrate temperature dependence of the molecular orientation is discussed in terms of the van der Waals interaction between 6 T and Cu(111) and the kinetics of the growth process. It is demonstrated that the molecular orientation of the promising organic semiconductor 6 T can be controlled by substrate temperature.

Spontaneous aggregation of pentacene molecules and its influence on field effect mobility

The authors have found that pentacene molecules deposited on SiO2 substrates treated with self-assembled monolayers, such as hexamethyldisilazane or octadecyltrichlorosilane, aggregate spontaneously. In situ, real-time low-energy electron microscopy investigation of the morpho-logical changes in pentacene films deposited under ultrahigh vacuum reveals that the balance between pentacene film and substrate surface energies is an origin of this aggregation. With in situ atomic force microscopy - field effect transistor measurements, the authors demonstrate that field effect mobility is critically affected by this aggregation.

One-dimensional ordered structure of α-sexithienyl on Cu(110)

We have studied atomic structures of α-sexithienyl (6T) films grown on Cu(110) by near-edge x-ray absorption fine structure (NEXAFS). A one-dimensional (1D) ordered structure of 6T with its molecular long axis parallel to the Cu[001] direction could be fabricated by deposition at 300 K and subsequent annealing at 360 K. Polarization- and azimuth-dependent NEXAFS revealed the formation process of the 1D structure and showed the molecular orientation in the in-plane direction directly. We propose a method to obtain the orientation distribution function of molecules using NEXAFS.

Metal-induced gap states at well defined alkali-halide/metal interfaces.

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge x-ray absorption fine structure. An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by x-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states (MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.

Real-Time Observation and Control of Pentacene Film Growth on an Artificially Structured Substrate

Suppression of nucleation around a gold electrode during pentacene growth on a SiO2 channel is found by photoemission electron microscopy. Mass flow is driven by the difference between the molecular orientations on SiO2 and gold. The poor connectivity at the channel/electrode boundary causes degradation in the performance of a field-effect transistor, which is found to be improved by self-assembled monolayer treatment on the electrode (see figure; thickness in monolayers (ML)).

Molecular Chain Structure of α-Sexithienyl on Ag(110) Observed by Scanning Tunneling Microscopy

Self-assembled α-sexithienyl (6T) film on Ag(110) was investigated by scanning tunneling microscopy (STM). On a Ag(110) surface, 6T molecules formed a chain, tilted by 16° from the [110] direction of Ag(110), and the length of the chains reached over 100 nm. Single 6T molecules were resolved as alternate bright and dark rectangles with the molecular axes perpendicular to the [110] direction. The observed structure was interpreted well on the basis of a simple model of the 6T/Ag(110) interface structure.

Metal-induced gap states in epitaxial organic-insulator/metal interfaces

We have shown, both experimentally and theoretically, that the metal-induced gap states (MIGS) can exist in epitaxially grown organic insulator/metal interfaces. The experiment is done for alkane/Cu(001) with an element-selective near edge x-ray absorption fine structure (NEXAFS), which exhibits a pre-peak indicative of MIGS. An {it ab initio} electronic structure calculation supports the existence of the MIGS. When the Cu substrate is replaced with Ni, an interface magnetism (spin-polarized organic crystal at the interface) is predicted to be possible with a carrier doping.

Silicon on insulator for symmetry-converted growth

Integration of metals and semiconductors having three- or sixfold symmetry on device-oriented [i.e., (001)] silicon wafers, which have fourfold symmetry, has been a long-standing challenge. The authors demonstrate that, by using symmetry-converted (111) silicon on insulator, wurtzite-structure gallium nitride, which has threefold symmetry, can be integrated with Si(001). The stability of the symmetry-converted Si(111) layer makes this technique appealing to the commercial integration of wide-ranging important materials onto Si(001) base wafers.

Electronic properties of metal-induced gap states formed at alkali-halide/metal interfaces

The spatial distribution and site distribution of metal-induced gap states (MIGS) are studied by thickness-dependent near-edge x-ray absorption fine structure (NEXAFS) and by comparing the cation and anion-edge NEXAFS. The thickness-dependent NEXAFS shows that the decay length of MIGS depends on an alkali-halide rather than a metal, and it is larger for alkali-halides with smaller band gap energies. By comparing the Cl-edge and K-edge NEXAFS for