Yan Jun Li

For the first time, the charge states of adsorbed oxygen adatoms on the rutile TiO₂(110)-1×1 surface are successfully measured and deliberately manipulated by a combination of noncontact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultrahigh vacuum and interpreted by extensive density functional theory modeling. Several kinds of single and double oxygen adatom species are clearly distinguished and assigned to three different charge states: O_ad<sup

The controversial issue of the origin of the p(2 x 1) reconstruction of the Si(001) surface observed in recent low temperature scanning tunneling microscopy experiments is clarified here using 5 K noncontact atomic force microscopy. The c(4 x 2) phase is observed at separations corresponding to weak tip-surface interactions, confirming that it is the ground state of the surface. At larger frequency shifts the p(2 x 1) phase of symmetric dimers is observed. By studying the interaction of a reacti

We study a low-temperature on-surface reversible chemical reaction of oxygen atoms to molecules in ultrahigh vacuum on the semiconducting rutile TiO₂(110)-(1 × 1) surface. The reaction is activated by charge transfer from two sources, natural surface/subsurface polarons and experimental Kelvin probe force spectroscopy as a tool for electronic charge manipulation with single electron precision. We demonstrate a complete control over the oxygen species not attainable previously, allowin

Titanium dioxide (TiO₂) is of considerable interest as a photocatalyst and a catalyst support. Surface hydroxyl groups (OH) are the most common adsorbates on the TiO₂ surface and are believed to play crucial roles in their applications. Although the characteristics of bridging hydroxyl (OH_br) have been well understood, the adsorption structure and charged states of terminal hydroxyl (OH_t) have not yet been experimentally elucidated at an atomic scale.

Atomic force microscopy (AFM) has been developed as a powerful tool to study surface topography and surface chemistry with atomic resolution, whereas straightforward identification of surface point defect/adsorbate is far from trivial in AFM imaging due to its complex mechanism. In this work, we successfully demonstrate a reliable way to discriminate surface point defect/adsorbate on a rutile TiO2 surface by AFM. Each surface point defect/adsorbate can positively respond to the applied bias volt

We investigate the surface potential distribution on a TiO₂ (110)-1 × 1 surface by Kelvin probe force microscopy (KPFM) and atom-dependent bias-distance spectroscopic mapping. The experimental results demonstrate that the local contact potential difference increases on twofold-coordinated oxygen sites, and decreases on OH defects and fivefold-coordinated Ti sites. We propose a qualitative model to explain the origin of the surface potential of TiO₂ (110). We qualitatively c

We have developed high-speed phase-modulation atomic force microscopy (PM-AFM) in a constant-amplitude (CA) mode. Using this imaging mode, we have theoretically demonstrated that energy dissipation due to tip–sample interaction can be obtained from the excitation amplitude of a cantilever. Moreover, we have found that the photothermal excitation method is better than the acoustic excitation method for cantilever oscillation in liquids. For the first time, we have demonstrated that a homebuilt hi

Clarifying the atomic configuration of step edges on a rutile TiO2 surface is crucial for understanding its fundamental reactivity, and the direct observation of atomic step edges is still a challenge. AFM is a powerful tool for investigating surface structures with true atomic resolution, and it provides the opportunity to resolve the real structure of step edges with improved techniques. In this work, we successfully imaged the atomic configuration of 001 and 1-11 step edges on the surface of

Single-atom catalysis of carbon monoxide oxidation on metal-oxide surfaces is crucial for greenhouse recycling, automotive catalysis, and beyond, but reports of the atomic-scale mechanism are still scarce. Here, using scanning probe microscopy, we show that charging single gold atoms on oxidized rutile titanium dioxide surface, both positively and negatively, considerably promotes adsorption of carbon monoxide. No carbon monoxide adsorption is observed on neutral gold atoms. Two different carbon

The charge state of Au nanoclusters on oxidized/reduced rutile TiO₂ (110) surfaces were investigated by a combination of non-contact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultra-high vacuum. We found that the Au nanoclusters supported on oxidized/reduced surfaces had a relatively positive/negative charge state, respectively, compared with the substrate. In addition, the distance dependence of LCPD verified the contrast observed in the KPFM images. The

The authors have developed phase modulation atomic force microscopy in constant excitation mode capable of simultaneously imaging the topography and energy dissipation of a sample surface in a liquid. This setup utilizes a fast, low-cost sample-and-hold technique to analyze the oscillation signals of a cantilever. The proposed circuitry allows us to measure the local energy dissipated by the tip-sample interaction during imaging. The energy dissipation image exhibits a material-specific contrast

In this study, we have systematically characterized and reversibly manipulated the subsurface hydrogen (Hsub) on rutile TiO2(110)-(1 × 1) by a combination of noncontact atomic force microscopy, scanning tunneling microscopy, and Kelvin probe force microscopy at 78 K. Four different configurations of the Hsub, including the monomer, dimer, trimer, and tetramer, are clearly characterized and discriminated by simultaneous atomic force microscopy and scanning tunneling microscopy measurements. Speci