Ken Ichi Shudo

Ultrafast Dynamics of Surface-Enhanced Raman Scattering Due to Au Nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SERS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (∼10 nm in diameter) by using sensitive pump-probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C═C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SERS and may also find application in nanofabrication.

Light-induced monolayer modification of chlorinated silicon (111) surfaces studied with a scanning tunneling microscope

Abstract Chlorine adsorbed silicon (111) surfaces are studied with a scanning tunneling microscope before and after weak irradiation of ultraviolet, visible and infrared light. We found that di- and tri- chlorinated silicon adatoms are desorbed out after ultraviolet light irradiation, resulting in the formation of a 7×7 surface structure resembling the so-called rest-atom layer . On the other hand, mono-chlorinated silicon adatoms are hardly moved by irradiation. The modification induced by the visible and infrared light are qualitatively similar to that by ultraviolet light. The obtained surfaces are different from the chlorinated surfaces after annealing. These results indicate that the modification is initiated by light-induced electronic excitation at the surface.

High reaction selectivity on UV-laser-induced desorption from chlorinated Si(111) 7*7 studied by scanning tunnelling microscopy

Chlorinated Si(111) 7*7 surfaces were examined with scanning tunnelling microscope images after pulsed laser irradiation at 266 and 355 nm with low laser fluence, where the thermal effect can be ignored. From the surface irradiated by the 266 nm laser, dichloride and trichloride species are desorbed, while monochloride species remain on the surface. This desorption selectivity was not observed at 355 nm. These results give useful information to elucidate the formation mechanism of a stripe pattern observed after 266 nm laser irradiation on a Cl-saturated Si(111) 7*7 surface.

TWO-PHOTON PHOTOEMISSION FROM BENZENE ADSORBED ON CU(111)

Abstract We have observed occupied and excited states induced by benzene adsorption on Cu(111) by means of two-photon photoemission. The benzene-derived occupied state is observed as a clear peak at 0.23 eV below E f , in between the peak of the copper surface state and the stepwise Fermi edge. The occupied state is attributed to the bonding orbital between benzene and copper substrate. The excited state is at 1.0 eV above E f . The occupied state and the excited state arise from bonding and antibonding orbitals formed by hybridization of the copper surface state orbital and benzene e 2u π * orbital.

Terahertz-Field-Induced Nonlinear Electron Delocalization in Au Nanostructures

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond time scale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field of THz pulses; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.

The local electronic properties and formation process of titanium silicide nanostructures on Si(001)-(2 × 1)

Titanium silicide island formation on an Si(001)-(2 × 1) surface was studied by means of scanning tunneling microscopy (STM) in situ at high temperature. Just after the start of annealing at 873 K, homogeneous nucleation occurs on the terrace, while preferential growth at the step edges was observed upon prolonged annealing. As the titanium silicide islands grow, multiple steps are formed nearby. The island size distribution was analyzed at several temperatures. Two types of TiSi2 structures, namely C49 and C54, were identified from the scanning tunneling spectroscopy (STS) spectra, in accordance with first-principles calculations. There was a critical island size for the transformation of C49–C54.

Laser-induced desorption from silicon (111) surfaces with adsorbed chlorine atoms

We have studied the initial stage of the laser-induced reaction of silicon surfaces with adsorbed chlorine atoms in ultrahigh vacuum, by measuring the species desorbing from the surfaces. In particular, our studies have focused on photo-chemical etching without laser-induced thermal heating. We found that the primary species desorbing from Cl-saturated Si(111) surfaces is the molecule and that the desorption efficiency with 2.3, 3.5 and 4.7 eV photons is significantly enhanced with respect to that for 1.2 eV photons. The results of previous STM studies are discussed and a possible mechanism for the photo-chemical etching is proposed.

Thermal desorption process of bromide on Si(111) studied by highly sensitive mass spectroscopy

Thermal desorption kinetics of bromine adsorbed onto Si(111) was studied using a highly sensitive mass spectrum measurement system. Temperature programmed desorption measurements simultaneously monitored SiBr+, SiBr2+ and SiBr3+. Low-energy electron diffraction and an isothermal desorption method identified desorption spectral features associated with the surface structures. As the temperature increased to ~600 K, silicon polybromides composed of Si adatoms desorbed with desorption barriers of 0.8 eV for SiBrn (n≥3) and 1.0 eV for SiBr2. The desorption of SiBrn (n≥3) originated from silicon polybromide clusters, which emerged at ~550 K. When the transition of surface structure occurred from 7×7 into 1×1 at ~800 K, different SiBr2 desorption barriers were observed for the growing 1×1 domain and the disappearing 7×7 domain. After reconstruction around 900 K, the recombinative desorption, which is caused by the diffusion of the atoms on the surface, had a desorption barrier of 2.6 eV for SiBr2 from the stable Br-terminated 1×1 surface.

Reaction kinetics in the rapid oxide growth on Si(001)-(2×1) probed with reflectance difference spectroscopy

We investigated the NO adsorption process on Si(001)-(2×1) and the oxygen adsorption process on potassium-covered Si(001)-(2×1) by reflectance difference spectroscopy (RDS). In both cases, the time courses that deviated from a simple Langmuirian kinetics could not be well fitted with a single exponential function, indicating the involvement of two different processes. In NO adsorption, a highly coordinated nitrogen adsorption site (N≡Si3) might play a role in the initial reaction process, producing an inhomogeneous strain on the first layer of oxynitride. In potassium-assisted oxidation, a sudden decrease in RD intensity just after oxygen exposure is associated with a reaction of oxygen with a potassium film, and the subsequent oxidation is slightly enhanced by the potassium–oxygen complex.

Thermal Effect of Picosecond-Pulsed Laser Irradiation on Cl-Adsorbed Si(111) Surface

The effect of 800 nm picosecond-pulsed laser irradiation on the local adsorbate structure of the Cl/Si(111) surface was investigated using a highly sensitive mass spectrum measurement system. It was found from the desorption profile obtained after laser irradiation at room temperature that the concentration of polychlorides and silicon chloride clusters were increased on the surface. Irradiation to the surface at about 150 K causes decomposition of tetrachloride, fragments of which are trapped by defects. As desorptions exhibit strong anisotropy to laser polarization and cooling the substrate suppresses desorption, it is suggested that the process is inequilibriumly coupled with the substrate lattice due to very short pulse excitation.