Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO(2) gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO(2) gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.

The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2 -), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= T M - S) is derived from a valence transition of Pt from an averaged-valence state of 2.5+ to a trapped-valence state of 2+ and 3+. The charge-ordering modes are considered to be −IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−I for the semiconducting phase below T M - S and −I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I− for the metallic phase above T M - S. 129I Mössbauer spectroscopic study is reported for a low-temperature insulating phase below 80 K. The low-temperature electronic structure is considered to be an alternate charge-ordering state with lattice distortions of IPt2+−Pt3+−I−Pt3+−Pt2+IPt2+−Pt3+−I−Pt3+−Pt2+I. The present binuclear platinum complex inherently possesses valence instability of the intermediate valence 2.5+. X-ray photoelectron spectroscopy and polarized reflection measurements are also reported.

It was found that a local charge-transfer excitation on the donor(D)-acceptor(A) pair leads to a semimacroscopic valence change from the quasi-ionic (D+A ) to quasineutral (D A ) states in organic molecular-compound TTF-chloranil with mixed DA stacks. Photogenerated neutralionic domain walls, which are responsible for anomalous photoconductivity, show the temporal annihilation due to the one-dimensional recombination.Various phenomena associated with the valence instability in solids have been of great interest with respect to electronic phase transitions. One of the prototypical examples is the neutral-ionic transition (NIT) observed in organic charge-transfer compounds. ' Several compounds with mixed stacks of donor (D) and acceptor (A) molecules are known to show the thermally or pressureinduced NIT from the quasineutral (DOA ) to the quasiionic (D+A ) state. ' The NIT has been attributed to the competition of the cost of energy for ionizing DoA stacks with the gain of Madelung energy in the ionic D+A lattice. ' Upon the NIT, the degree of charge transfer (CT) on the molecules shows a finite jump and the DA stacks in the ionic phase undergo the dimeric lattice distortion. Since the spin state in the ionic phase is approximately described as the S= -, ' Heisenberg chain, the lattice transition is quite analogous to the spin-Peierls transition. Such an electron (spin) -lattice interaction obviously plays an important role in promoting the NIT in real systems in addition to the above-mentioned electrostatic mechanism. In this paper, we report the observation that the valence instability in the DA compounds can be also caused by photoexcitation. The photogenerated CT excitations (D A ) or resultant charged species (D and A ) in the ionic DA lattice locally modify the Coulombic interaction and switch ofl' a channel of the collective spinlattice interaction. What we have observed in this study is the phenomenon that those local photoexcitations serve as nuclei which evolve into the neutral molecular domains over a semimacroscopic region.Among a number of CT compounds located near the neutral-ionic interface, tetrathiafulvalene (TTF)-chloranil (CA) was investigated here, since this is the most prototypical compound which shows the NIT not only by applying hydrostatic pressure above P, (equal to 11 kbar) but by lowering temperature below T, At ambient pressure, the first-order NIT is observed at 81 K in the cooling run (and at 84 K in the heating run) accompanying the stack dimerization in the ionic phase. To detect a change in molecular ionicity, optical spectra for local electronic excitations have been known to be very useful. 'In Fig. 1(a) reflectance spectra are plotted for the neutral and ionic phases taken on the (001) surface at 90 and 77 K, respectively, with the light polarization normal to the stack axis (a axis) (EJ a). The reflectance bands 13 and C are attributed to intramolecular transitions due to TTF+ molecules, while the band D is due to TTF molecules. As investigated in detail previously, 4 6 spectr...

Design of highly active nanoscale catalysts for electro-oxidation of small organic molecules is of great importance to the development of efficient fuel cells. Increasing steps on single-crystal Pt surfaces is shown to enhance the activity of CO and methanol electro-oxidation up to several orders of magnitude. However, little is known about the surface atomic structure of nanoparticles with sizes of practical relevance, which limits the application of fundamental understanding in the reaction mechanisms established on single-crystal surfaces to the development of active, nanoscale catalysts. In this study, we reveal the surface atomic structure of Pt nanoparticles supported on multiwall carbon nanotubes, from which the amount of high-index surface facets on Pt nanoparticles is quantified. Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly shows the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation. Here, we show that increasing surface steps on Pt nanoparticles of approximately 2 nm can lead to enhanced intrinsic activity up to approximately 200% (current normalized to Pt surface area) for electro-oxidation of methanol.