Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO2 surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL. Under the flat-band condition, which is realized in nanometer-sized photocatalysts, the carriers have a longer lifetime on the anatase surface than the rutile one, naturally explaining the higher photocatalytic activity for anatase than rutile.

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The structure of the Si(111}&3&&3-Gasurface has been studied by dynamic analysis of lowenergy electron-diffraction curves of intensity versus energy (I-V). It has been found that a structure in which Ga atoms are located on the second layer of Si atoms well explains the experimentally obtained I-V curves. In this geometry, a large deformation of the surface layer results from the stable adsorption of Ga atoms. The optimum configuration is close to that proposed by Northrup in his study of the Si(111}&3 & &3-Al surface.Among the reconstructed surface structures which are formed on the (111) surface of crystals with fcc or diamond structures, the~3 X &3 structure is the most frequently observed, and many experimental and theoretical studies related to it have already been published. ' However, in only a few cases has the geometry of the surface been successfully determined. In most cases, this is mainly due to inadequate use of the methods of measurement. In the case of low-energy electron-difFraction (LEED) studies, sufficient data to determine the surface geometry can be obtained by measuring the intensityenergy (I-V) curves of diffracted beams. Shortcomings of LEED analyses include the necessity of complicated calculations and the considerable computing time required to perform this kind of calculation. ' However, recent advances in computer technology enable us to study reconstructed surface structures whose surface unit cells contain fairly large numbers of atoms.In this study we report the results of dynamic analysis of the I-V curves of LEED from the Si(111)&3Xv 3-Ga surface.We used n-type silicon wafers of high resistivity ( & 10 kQcm), cut to within 1' parallel to the (111) plane, as substrates. A molecular beam of Ga from a Knudsen cell was impinged on the silicon surface. The saturation coverage of Ga atoms for the +3X +3 structure has been determined as -, ' of a monolayer by quadrupole mass spectroscope observation of the increase in intensity of the desorption Aux of Ga atoms from the Si surface after the completion of the &3 X &3 phase at high substrate temperatures, and by observing the break points in curves of Auger-electron-spectroscopy signal versus deposition time. ' %'hen -, ' of a monolayer of Ga atoms was deposited at 740 K, an unambiguous Si(111)/3 X &3 structure was observed. I-V curves from the Si(ill)v 3Xv 3 surface have been obtained by measuring the brightness of the bright spots on the Auorescent screen by means of a spot photometer. The calculations of I-V curves were performed using a renormalized forward-scattering algorithm. The scattering matrices for nonequivalent composite layers were calculated by a matrix inversion formalism, and the intensity of the beams rejected from a stack of these composite layers 0 0 0 0 0 o Ga FIG. 1. Top and side views of the surface structural model. was obtained by the renormalized forward scattering perturbation method. The computer programs developed by Pendry and by Tong and van Hove were modi6ed to apply to cases in which the substrate layers c...

The probe effect, the effect of parameters in scanning tunneling microscopy (STM) measurement, on the Si͑100͒ surface with two competing phases in delicate balance, was investigated systematically by reexamining its influence on the Si͑100͒ dimer flip-flop motions at 5 and 80 K. On the basis of the results, the complex array of the phenomena of the Si͑100͒ surface structures was comprehended. The phase transition between c͑4 ϫ 2͒ and p͑2 ϫ 2͒ structures below ϳ40 K was studied by STM, as well as by low-energy electron diffraction, and the appearance of the p͑2 ϫ 2͒ structure at a reduced probe effect was confirmed. In these investigations, a phase with long-range ordering of the c͑4 ϫ 2͒ and p͑2 ϫ 2͒ structures was observed.

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