We have used high-resolution X-ray photoelectron spectroscopy (XPS) to investigate the molecular dependence on sulfur chemical states of organosulfur monolayers of alkanethiol, dialkyl disulfide, monosulfide, thiophene, and aromatic thiols on a Au(111) surface. When monosulfides or thiophenes adsorbed on the Au surface, the S(2p) peaks appeared between 161 and 164 eV, and peak fitting revealed that these organosulfurs exhibited almost the same peaks as those of alkanethiol and dialklyl disulfide monolayers. The monolayer made from monosulfide with shorter alkyl chains exhibited almost the same S(2p) XPS spectrum as the typical alkanethiol or dialkyl disulfide monolayers. Another S(2p3/2) peak appeared for aromatic derivatized thiol SAMs at around 161 eV, in addition to strong doublet S(2p) peak observed at 162.0 and 163.3 eV in the S(2p) spectra. The 161 eV peak was observed even in the S(2p) XPS spectra of alkanethiol or dialkyl disulfide monolayers, at the initial stage of monolayer growth or after low-temperature (∼ 100 °C) annealing of low molecular density alkanethiol SAMs. We consider that this 161 eV peak can be formed without molecular decomposition as well as due to the atomic sulfur produced by C-S cleavage.

In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way.

Scanning force microscopy (SFM) is becoming a powerful technique with great potential both for imaging and for control of domain structures in ferroelectric materials at the nanometer scale. Application of SFM to visualization of domain structures in ferroelectric thin films is described. Imaging methods of ferroelectric domains are based on the detection of surface charges in the noncontact mode of SFM and on the measurement of the piezoelectric response of a ferroelectric film to an external field applied by the tip in the SFM contact mode. This latter mode can be used for nondestructive evaluation of local ferroelectric and piezoelectric 101 102 GRUVERMAN, AUCIELLO & TOKUMOTO properties and for manipulation of domains of less than 50 nm in diameter. The effect of the film thickness and crystallinity on the imaging resolution is discussed. Scanning force microscopy is shown to be a technique well suited for nanoscale investigation of switching processes and electrical degradation effects in ferroelectric thin films.

We have studied electrical conduction of conjugated molecules with phenyl rings embedded into alkanethiol self-assembled monolayers (SAMs), to investigate the molecular structural effect on the electrical conduction. Scanning tunneling microscope (STM) images of this surface revealed that the conjugated molecules with phenyl rings adsorbed mainly on defects and domain boundaries of the pre-assembled alkanethiol (nonanethiol C9) SAM and formed conjugated domains. In the case of conjugated molecules with one or three methylene groups between the sulfur and phenyl rings, the measured height of the conjugated molecular domains depended on their lateral sizes, while a strong dependence was not observed in the case of conjugated molecules without a methylene group. By analyzing size dependence on the height of the conjugated molecular domain, we could evaluate the electronic conductivity of the molecular domains. As a result of the analysis, to increase the vertical conduction of the molecular domains, one methylene group was found to be necessary between the sulfur and aromatic phenyl rings. Local barrier heights on the conjugated molecular domains in all the cases were larger than on the C9 SAM surface, suggesting that the increase in the vertical conductivitity is not likely to be due to the lowering of the local barrier height, but can be attributed to the conjugated molecular adsorption. X-ray photoelectron spectra (XPS) and ultraviolet light photoelectron spectra (UPS) revealed that the carrier density among conjugated molecular SAMs does not depend on the number of methylene groups between the sulfur and phenyl rings, suggesting that the higher vertical conduction of conjugated molecules with one methylene group can probably be attributed to higher transfer probability of carriers during the STM measurements.

The electrical conduction of self-assembled monolayers (SAMs) made from conjugated molecules was measured using conductive atomic force microscopy (AFM), with a focus on the molecular structural effect on the electrical conduction. For phenylene oligomer SAMs, resistances through the monolayers increased exponentially with increases in molecular length and the decay constants of transconductance β were ca. 0.35 to 0.5 Å-1. Using an insertion technique into insulative alkanethiol SAMs, we successfully obtained single molecular resistance of terphenyl methanethiol at ca. 5.4 × 1010 Ω. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with increases in the applied load up to 15 nN. When two or three methylene spacers were introduced between the sulfur and terphenyl groups in a series of terphenyl derivatized thiols, the monolayer resistances and β values increased extraordinarily. One explanation is that the addition of methylene spacers changed the location of the molecular orbital as a result of MOPAC calculation.