Selective measurement of the electronic states of the steps on rutile TiO₂ (15 15 4) surface by optical second harmonic spectroscopy

Abstract

We have obtained optical second harmonic spectra of stepped rutile TiO 2 (15 15 4) adsorbed with formic acid HCOOH. We have achieved selective measurement of the electronic states of the bunched steps on this surface in an optical configuration with nearly normal incident laser illumination. For the polarization combinations S in-S out and P in-S out, the SH intensity was higher near the second harmonic photon energy 3.6 eV and also above 4.45 eV. The terraces and steps were found to show remarkably different contributions to the optical second harmonic spectra at 45• incidence.

Selective observation of local carrier dynamics at step bunches on vicinal TiO2(110) by time-resolved pump-probe second harmonic generation method

“…One is exactly the same sample as was used in our earlier study. 34 This was prepared by annealing of the as-delivered sample at 800 • C in an O 2 gas for 2 h. Before and after the annealing, the sample went through chemical etching by a 5% HF aqueous solution. The other sample was prepared just by HF etching of the as-received wafer.…”

“…By choosing a proper vicinal surface of a centrosymmetric single crystal such as TiO 2 (110), it is basically feasible to selectively extract the electronic information of regularly aligned steps. 29,[32][33][34][35] For vicinal TiO 2 (110), the step selectivity of SHG can be described phenomenologically as below. When the electric fields of fundamental and reflected SHG light are set almost parallel to the terrace plane of vicinal TiO 2 (110), surface SHG from the (110) terraces is forbidden since the C 2v symmetry of the (110) plane does not have broken symmetry in the direction parallel to the terrace.…”

“…However, the number of research groups handling the latter case is limited. 29,[32][33][34][35] The first selective detection of the pure step contribution through SHG was done by Rasing and his co-workers for vicinal Si (110) under S in-S out polarization geometry. 32 In spite of the great potential of this discovery, it has not triggered much attention to further application of this SHG property as a new surface analytical method.…”

“…We have casted new light on this issue and have developed the surface probe experimentally analyzing local density of states (LDOS) at step sites using vicinal TiO 2 (110) by incorporating the step-selective scheme into SHG spectroscopy. 33,34 In our previous report, we discussed a difference in electronic properties between (110) terraces and step bunches formed on TiO 2 (15 15 4) in terms of the LDOS distribution near the band-gap regions by using the above-mentioned method. 34 The result indicated that the density of states of the valence and/or conduction bands for the step bunches is distributed energetically closer to the gap regions than that for the (110) terraces.…”

“…33,34 In our previous report, we discussed a difference in electronic properties between (110) terraces and step bunches formed on TiO 2 (15 15 4) in terms of the LDOS distribution near the band-gap regions by using the above-mentioned method. 34 The result indicated that the density of states of the valence and/or conduction bands for the step bunches is distributed energetically closer to the gap regions than that for the (110) terraces. We suggested that this difference can perturb carrier dynamics in the vicinity of the step bunches and may result in different reactivity between the steps and terraces on TiO 2 (15 15 4).…”