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Acetic acid vapor-deposited on ultrathin noncrystalline ice (NCI) and polycrystalline ice (PCI) films (less than 6 nm thick) under ultrahigh vacuum conditions has been investigated by using Fourier Transform Infrared Reflection-Absorption Spectroscopy. Pristine acetic acid deposited at 123 K (on a copper support) appears as an amorphous solid, which undergoes an irreversible phase transformation to a more structurally ordered (polycrystalline) form upon annealing to 153 K. Acetic acid is found to adsorb on NCI and PCI films initially through hydrogen bonding between CdO and dangling OH (of ice), followed by the formation of multilayers at 123 K. Thermal evolution studies of a low exposure of acetic acid on the ultrathin NCI and PCI films show that acetic acid undergoes coevaporation with water likely as an acetic acid hydrate at 155 K, which continues until the entire ice film has been exhausted at 165 K. Above 165 K, the remaining acetic acid solid appears to evaporate without undergoing the phase transformation, in contrast to the case of a high acetic acid exposure. Coevaporation of acetic acid with water is also found to proceed at a faster rate than the subsequent evaporation of acetic acid, which is consistent with the weaker interactions observed in the H-bonded acetic acid hydrate than that in acetic acid solid.

Monoshaped and monosized copper nanostructured particles have been prepared by potentiostatic electrochemical deposition on an ultrathin polypyrrole (PPY) film, electrochemically grown on a Si(100) substrate sputter-coated with a thin gold film or gold-film electrode (GFE). The crystal size and the number density of the copper nanocrystals have been examined by varying several deposition parameters, including the thickness of the gold film, the PPY film thickness, the applied potential, and the Cu2+ and the electrolyte concentrations for copper deposition. Optimal conditions for uniform growth ofnanocrystals well-dispersed on the GFE have been determined, along with insight into the mechanism of crystal growth. A minimum gold film thickness of 80 nm is required to eliminate the effects of the gold-silicon interface. The PPY film thickness and homogeneity principally affect the shape uniformity of the nanocrystals, while the copper deposition potential could be used to regulate the size and number density of the nanocrystals. Both the Cu2+ and electrolyte concentrations are also found to play important roles in controlling the electrodeposition of nanocrystal growth.

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2×1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2×1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2×1 reconstructed surface. The chemisorption of styrene on the 2×1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2×1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2×1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2×1 upon irradiation by lowenergy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.

The room-temperature adsorption of thiophene on Si(111)7X 7 has been investigated using photoemission with monochromatic synchrotron radiation at 340-eV photon energy. The presence of two C 1s photopeaks at room temperature is consistent with the observation of two molecular-desorption states and with an earlier proposed adsorption model of thiophene on Si(111)7X7 (that involves a surfaceparallel m-bonded geometry and a cr-bonded geometry through the sulfur atom). The C 1s photoemission spectra have also been collected after annealing the sample to different temperatures. Although the observed core-level chemical shifts and the temperature dependence of the two photopeaks are consistent with the thermal-desorption data in the framework of a simple charge-transfer model, correlation between the photopeaks and thermal-desorption data cannot be used to unambiguously identify the nature of the proposed adsorption structures for the adsorption states. Some of the potential difficulties involved in this type of common practice will be discussed.The studies of cyclic hydrocarbons on Si surfaces have attracted recent attention because of the unexpected surface reactivity observed. In particular, recent studies of Si(111)7X 7 carried out in our laboratory' and of Si(111)ZX1 by other groups have demonstrated the high reactivity of these semiconductor surfaces towards cyclic olefins. Not only adsorption of a series of homologous five-membered and six-membered cyclic olefins, including thiophene, furan, ' and benzene has been observed at room temperature (RT), strong evidence of catalytic dehydrogenation surface reactions of cyclohexene and cyclohexadienes as well as their methyl-substituted derivatives has also been obtained for Si(111)7X 7. Furthermore,~-conjugated heteroarornatic compounds such as thiophene and pyrrole have attracted practical interest as plausible candidates as the basic building blocks of the so-called conductive polymers, which are found to have valuable technological properties.Very recently, we reported a thermal desorption spectrometric (TDS) study of the room-temperature adsorption of thiophene and related five-membered cyclic olefins on clean and modified Si(111)7X 7 surfaces. ' By comparing the desorption profiles of these olefins, information about their interactions with the surface has been obtained. This work also provides an interesting comparison with the results of a series of photoemission and electron energy loss studies of Piancastelli et aI. , who reported desulfurization of thiophene ' and deoxygenation of furan on Si(111)2X1 for room-temperature exposure.At low temperature (85 K), however, no fragmentation was found and two adsorption states were reported.It was further proposed that the adsorption states for thiophene at low temperature involved a m-bonded geometry and a a. -bonded geometry resulting from the breakage of a C -H bond.In the case of roomtemperature adsorption of thiophene on Si(111)7X7, we have found two molecular desorption states which, unlike the proposed adsorptio...