Optical Second Harmonic Generation from Periodic Steps on Rutile TiO2(110)

Abstract

We have succeeded in observing optical second harmonic generation (SHG) from steps on the TiO2 surfaces adsorbed with HCOOH. The samples were single crystals of rutile TiO2 with (110), (15 15 4), (13 9 0), and (671) faces. These samples underwent annealing in O2 gas, HF etching, and immersion into HCOOH aqueous solution to adsorb HCOOH molecules on their surfaces. Their surfaces were characterized by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM), and were confirmed to have regularly aligned steps created by the miscut of the samples. SH intensity as a function of the sample rotation angle was measured in air with the incident angle of 2• at the SH photon energy of 2ω = 4.66 eV. The SH intensity from the surfaces with steps were 2 to 3 orders of magnitude stronger than that from the (110) surface. The SH intensity patterns from these stepped surfaces were anisotropic. The result indicates that the contribution of the surface steps was observed selectively.

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

“…3. As already reported in our earlier work,35 the annealed sample surface is made up of multilayer step bunches (>10 ML) and flat (110) terraces [Fig. 3(a)].…”

“…Our previous rotational SHG measurement for TiO 2 (110) at the SHG photon energy of 4.66 eV showed that quadrupole bulk SHG from the (110) face is almost at the noise level for the nearly normal incidence. 35 Analogously, the corresponding bulk contribution from the vicinal (110) face in question is expected to be as small. In fact, the clearly different SFG responses between annealed and unannealed TiO 2 (15 15 4) in Fig.…”

“…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.…”

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

“…In our previous work,21 we succeeded in detecting the signals of the steps periodically aligned on the TiO 2 surfaces selectively by means of second harmonic generation (SHG)13–20 with nearly normal incident fundamental excitation. We found that both high index TiO 2 (15 15 4) and (671) surfaces with steps have large nonlinear susceptibility elements $\chi_{xxx}^{(2)}$ and $\chi_{xyy}^{(2)}$ , with x representing the [0 0 1] and y representing the [1 1 0] directions.…”