X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometer (SSIMS) have been applied to characterize Si(111) surface treated with 40% NH4F solution. As-treated Si(111) surface is predominately terminated with monohydrides and free of contamination. The dynamics of etching process on ultra-clean atomically flat hydrogen-terminated Si(111) surface in 40% NH4F solution has been examined at various potentials including the open circuit potential (OCP) by using in situ electrochemical scanning tunneling microscopy (ECSTM). Two distinct mechanisms are observed:  (1) dihydride terminated step silicon atoms are dissolved much faster than the defect-free monohydride terminated ones; (2) dihydrides in the corner of {110} zigzag enclosed characteristic triangular pits lead to fast erosion of silicon atoms. Our results explain dynamic mechanics in formation of triangular pits on atomically flat silicon (111) surface in solution.

The adsorption of chloride on Cu(110) single-crystal surface has been investigated in hydrochloric acid aqueous solution by using in situ scanning tunneling microscopy (STM). A (1 × 1) structure of Cu(110) substrate lattice was visualized at the potentials negative than -450 mV vs SCE, where chloride anions were desorbed. In the potential range from -400 mV to -150 mV vs SCE, two main complex adlayers with 3-fold and 4-fold periodicity along [11 h0] direction of the substrate were observed. These structures with long-range periodicity are attributed to ordered chloride adlayers on unreconstructed Cu( 110)-(1 × 1) structure. The different corrugation heights and periodic modulations along [11 h0] direction observed in STM images indicate that chloride anions are located at nonequivalent binding sites. The average measured lattice constants for the two adlayers have the same size as the substrate lattice along [001] direction and have 4 and 5 times the lattice spacing of the underlying copper substrate along [11 h0] direction. Models are proposed to interpret these two structures to (4 × 1) and (5 × 1) chloride adlayers containing three and four chloride anions, respectively.

In situ real-time and electrochemical attenuated total reflection Fourier transform infrared ͑ATR-FTIR͒ spectroscopy has been employed to study the dependence of hydride structures of an Si͑111͒ surface in diluted NH 4 F solution on etching time and applied potential. The in situ real-time ATR-FTIR result shows that the Si͑111͒ surface remains H terminated in diluted NH 4 F solution, and the hydride on Si͑111͒ has a strong interaction with water molecules under cathodic potential control. Potential dependence of the in situ electrochemical ATR-FTIR spectra reveals that Si͑111͒ surface is free of oxide at the open circuit potential ͑OCP͒ and SiHO 3 is formed only at potentials positive of the OCP. Infrared roughness factors, defined as the ratio of monohydride at steps or dihydride over the monohydride on the ͑111͒ plane, suggest that monohydride chain steps are the dominating defects on the Si͑111͒ surface in NH 4 F at cathodic electrode potential. Horizontal ͑DЈ͒ and vertical ͑D͒ dihydrides are predominant on the Si͑111͒ surface leading to roughened morphology via oxidation at anodic potential.