Investigation of tunneling current and local contact potential difference on the TiO₂(110) surface by AFM/KPFM at 78 K

Abstract

We propose a new multi-image method for obtaining the frequency shift, tunneling current and local contact potential difference (LCPD) on a TiO(110) surface with atomic resolution. The tunneling current image reveals rarely observed surface oxygen atoms contrary to the conventional results. We analyze how the surface and subsurface defects affect the distribution of the LCPD. In addition, the subsurface defects are observed clearly in the tunneling current image, in contrast to a topographic image. To clarify the origin of the atomic contrast, we perform site-dependent spectroscopy as a function of the tip-sample distance. The multi-image method is expected to be widely used to investigate the charge transfer phenomena between the nanoparticles and surface sites, and it is useful for elucidating the mechanisms of catalytic reactions.

Multi-Channel Exploration of O Adatom on TiO2(110) Surface by Scanning Probe Microscopy

“…In the V LCPD image (see Figure 5 c), the imaging contrast was the reverse of that in the image, except for the O ad . The relative value of V LCPD between the Ti 5c and O 2c sites was approximately 28 mV from the line profile (not shown here), and this was in good agreement with our previous studies [ 18 , 19 ]. V LCPD had a higher value on the O ad than on the proximate Ti 5c rows, and the relative value of CPD between the O ad and Ti 5c row was about 75 mV.…”

“…Thus, tunneling current dramatically decreases, and the contrast of O ad becomes a dark spot in < I t > image. The bright spot marked by an oval circle is due to subsurface defects, which was not probed in Figure 4 a, ever reported by our group or other groups [ 19 , 22 ]. Figure 4 c is the ∆ f image and Figure 4 d is the corresponding tunneling current image, recorded in the constant height mode.…”

“…CPD originates from the difference of the work functions and is specifically referred to as the local CPD (LCPD) in atomic-resolution KPFM [ 11 , 12 ]. To date, different modes of KPFM have been successfully used to simultaneously measure surface structures and LCPD, and the surface potential of TiO 2 (110) was measured [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Local density of states (LDOS) gives significant information of the electronic structure of the surface, which is measured by scanning tunneling microscopy (STM).…”

“…Simultaneous measurement of tunneling current and LCPD is useful to explore the surface properties and surface reaction process, but it cannot be achieved by the conventional KPFM due to the regulation of DC bias voltage. To simultaneously characterize the electronic structure and LCPD, we previously proposed a method to achieve the frequency shift (∆ f ), average tunneling current (< I t >) and LCPD by the KPFM without DC bias voltage feedback, and this method was successfully performed on the rutile TiO 2 (110) surface [ 19 ]. Hence, based on AFM, multi-channel exploration has the potential to explore the charge transfer of the interface and to unravel physical features for understanding the surface catalytic process.…”

Increase of power conversion efficiency in dye-sensitized solar cells through ferroelectric substrate induced charge transport enhancement

“…Variations of surface potential over the scan area were observed from both samples, which might be caused by an AC voltage generally applied to the tip while scanning. KPFM though is not for a quantitative measurement of surface potential 28 , the results clearly confirmed that the electric field originating from the LiNbO 3 is significantly large and can penetrate through the ITO layer and TiO 2 film. In other words, the TiO 2 film is “immersed” in an electric field in view of the spontaneous polarization of LiNbO 3 .…”