The flat-band potential (UFB) of n-Si electrodes in concentrated hydrogen halide (HX) solutions, as determined from Mott-Schottky plots and the onset potential of photocurrent, shifts largely toward the negative with the decreasing electronegativity of halogen atoms. XPS studies have shown the presence of halogen atoms on the Si electrodes which were beforehand immersed in the HX solutions. The shift in UFB can be explained as due to changes in surface potential by formation of Si-halogen surface termination bonds.

Photoelectrochemistry I Solar cells I Small metal particle I Porous silicon I Surface termination bondStructural control of semiconductor surfaces on an atomic scale and that on a nanometer scale are both important for efficient solar energy conversion. The former is important for achieving low surface carrier recombination, high catalytic activity for interfacial reactions, formation of high energy barriers, etc. The latter is important for simultaneous achievement of all these requirements at one surface, which are in general incompatible with each other for homogeneous surfaces. The structural control on both scales can lead to ideal semiconductor electrodes. Some examples are shown for efficient solar cells and solar-to-chemical conversion by use of metal nano-particles, nanoporous Si layers, and introduction of surface-band asymmetry by formation of Si-halogen termination bonds. * Corresponding

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Electrodeposition of Ni on atomically flat H-terminated Si͑111͒ surfaces in aqueous acidic solutions containing Ni 2ϩ ions produced rodlike Ni clusters 20-80 nm wide, 5-20 nm high, and more than 1 m long, nearly in the ͗1 1 2͘ direction. Such aligned Ni nanorods were formed where the H-Si͑111͒ surface had regularly arranged step lines, indicating that the morphology of the Si surface plays an important role in the Ni-rod formation. Detailed analyses of atomic force microscopy images revealed that the Ni nanorods started to grow from dihydride step edges of the H-Si͑111͒ surface and extended along lines connecting the dihydride step edges. The cross section of the Ni rods suggested that they grew epitaxially on H-Si͑111͒.Structural control of semiconductor surfaces on nanometer scales constitutes the basis of future devices such as quantum confined optical and electronic devices, and has recently been attracting keen attention in the field of semiconductor sciences and technologies. The control in a size region from a few to several tens of nanometers is beyond the control size of the conventional photolithographic techniques, which is in a range of a hundred nanometers or more. Atomic scale fabrication by means of scanning probe microscopy ͑SPM͒ is a powerful technique, 1 but has, unfortunately, the severe limit of not being adaptable to mass production, which is eventually necessary for practical application. A number of studies have been reported on formation of nanostructures by depositing metals on semiconductor surfaces under ultrahigh vacuum ͑UHV͒ conditions. 2,3 However, the UHV techniques also seem to be difficult to adapt to mass production.Chemical and electrochemical methods to produce nanosized structures at solid-liquid interfaces by making use of self-organizing abilities of molecular systems 4-8 are one promising approach to achieve the above target. They have strong merits in that they are easily adapted to mass production, and nanostructures made by them have high stability and self-restoration ability. A number of studies have been reported on electrochemical nanorod formation by use of porous templates 9 or surfactants. 10 Recently, Hara et al. reported 4 that rodlike Cu clusters were formed without any template when they were electrochemically deposited on H-terminated p-type Si͑111͒ surfaces, although the detailed mechanism of the nanorod formation was not clarified.In this paper, we report that electrochemical deposition of Ni on H-terminated n-type Si͑111͒ surfaces also produces nanosized rods, similar to Cu. This is, to our knowledge, the second example of the unique phenomenon of electrochemical metal nanorod formation with no template. The investigation on the formation mechanism in the present work has revealed that the Ni nanorods tend to extend along lines connecting dihydride step edges of the H-Si͑111͒ surface, suggesting a possibility of shape control of metal nanorods.n-Type Si͑111͒ wafers ͑resistivity 10-15 ⍀ cm͒ with vicinal surfaces tilting toward the ͗1 1 2͘ direction were obt...