Chung-Kai Fang

Interface-Induced Ordering of Gas Molecules Confined in a Small Space

The thermodynamic properties of gases have been understood primarily through phase diagrams of bulk gases. However, observations of gases confined in a nanometer space have posed a challenge to the principles of classical thermodynamics. Here, we investigated interfacial structures comprising either O2 or N2 between water and a hydrophobic solid surface by using advanced atomic force microscopy techniques. Ordered epitaxial layers and cap-shaped nanostructures were observed. In addition, pancake-shaped disordered layers that had grown on top of the epitaxial base layers were observed in oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. The structures are further stabilized by interfacial water. Here we show that gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The crystalline solid surface may even induce a solid-gas state when the gas-substrate interaction is significantly stronger than the gas-gas interaction. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions.

Nucleation processes of nanobubbles at a solid/water interface.

Experimental investigations of hydrophobic/water interfaces often return controversial results, possibly due to the unknown role of gas accumulation at the interfaces. Here, during advanced atomic force microscopy of the initial evolution of gas-containing structures at a highly ordered pyrolytic graphite/water interface, a fluid phase first appeared as a circular wetting layer ~0.3 nm in thickness and was later transformed into a cap-shaped nanostructure (an interfacial nanobubble). Two-dimensional ordered domains were nucleated and grew over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that interfacial nanobubbles and fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature, explaining their high stability and their roles in boundary slip and bubble nucleation. These ordered domains may be the interfacial hydrophilic gas hydrates and/or the long-sought chemical surface heterogeneities responsible for contact line pinning and contact angle hysteresis. The gradual nucleation and growth of hydrophilic ordered domains renders the original homogeneous hydrophobic/water interface more heterogeneous over time, which would have great consequence for interfacial properties that affect diverse phenomena, including interactions in water, chemical reactions, and the self-assembly and function of biological molecules.

Photoemission studies of ion bombardment effect on SnO2 surfaces

Abstract Polycrystalline SnO2 surfaces have been studied by X-ray and He I photoelectron spectroscopies (XPS and UPS). To examine ion bombardment effects, SnO2 surfaces were sputtered by Ar+, He+ and H+ ions, after they had been cleaned by heating at 1300 K for 5 h. Surface stoichiometry was determined by the O(1s)/Sn(3d 5 2 ) XPS peak ratio. UPS probes directly the density of occupied valence bands. Instead of a minimum in the O/Sn ratio for sputtering energies on SnO2(110) surfaces [7] we observed that the preferential sputtering effect in favour of oxygen increases with sputtering energy and ion mass. Electronic properties such as Sn2+ defect states, O(2p)-Sn(5p) hybridized electrons, O(2p) lone pair orbitals, work function changes and the bending of the valence band edge toward the Fermi level were demonstrated to have a strong correlation with oxygen loss under ion bombardment.

Hydrogen adsorption on Re/W alloy surfaces studied by photoemission

UV and X-ray photoelectron spectroscopy (UPS and XPS) were used to study polycrystalline ReW alloy surfaces. The Re/W alloy surface was prepared by heating a Re sputter-coated tungsten substrate. Auger electron spectroscopy (AES) was used to check the metal interdiffusion when heating the Re-coated sample. The width of the valence band of HeI UPS spectra was used to calculate the work function changes for different metal surface concentrations. Results indicate that valence band features of pure rhenium and tungsten appear in the spectrum obtained from the ReW alloy surface. The work function increases with substrate work function for the H2 chemisorbed ReW alloy surface.

Lateral Force Microscopy of Interfacial Nanobubbles: Friction Reduction and Novel Frictional Behavior

Atomic force microscopy is used to conduct single-asperity friction measurements at a water-graphite interface. Local mapping of the frictional force, which is based on the degree of the cantilever twisting, shows nearly friction-free when a tip scans over a nanobubble. Surprisingly, apart from being gapless, the associated friction loop exhibits a tilt in the cantilever twisting versus the tip’s lateral displacement with the slope depending on the loading force. The sign of the slope reverses at around zero loading force. In addition, the measured normal and lateral tip-sample interactions exhibit unison versus tip-sample separation. Theoretical analysis, based on the balance of forces on the tip originated from the capillary force of the nanobubble and the torsion of the cantilever, offers quantitative explanations for both the tilted friction loop and the unison of force curves. The analysis may well apply in a wider context to the lateral force characterization on cap-shaped fluid structures such as liquid droplets on a solid substrate. This study further points to a new direction for friction reduction between solids in a liquid medium.

Scanning Tunneling Microscope Study of Structural Transformations on One Monolayer Pb/Si(111)

With a variable‐temperature scanning tunneling microscope (STM), we study structural phase transformations for one monolayer Pb/Si(111) from 170 K up to room temperature. On the 1×1 structure, Pb adatoms are located at the T1 site and the coverage is exactly 1 ML. The 1×1 phase undergoes a reversible phase transition into a 7 × 3 phase upon cooling below ∼260 K. Our study of the phase transition indicates that there is no coverage change across the transition and that the transition temperature decreases with the decreasing domain size. At room temperature, we find that some defects in the 1×1 phase can induce formation of a small region of alternating trimer domains around them. This may be a precursor to the formation of an entire region of a so‐called “incommensurate” phase, which is composed of alternating domains of two different trimer structures. The 7 × 3 and the “incommensurate” phases are basically the distorted 1×1 structure with Pb adatoms slightly displaced from the T1 site. The relationship ...