Qing-Min Xu

Studies of the effects of hydrogen bonding on monolayer structures of C18H37X (X=OH, SH) on HOPG

Abstract Based on Scanning tunneling microscopy (STM) observations of 1-C 18 H 37 X (X=OH, SH) adsorbed on HOPG, the hydrogen bonded complexes were studied using density functional theory (DFT) and molecular mechanics (MM). At B3LYP/6-31g and B3LYP/6-311g** levels, molecular dimers connected by hydrogen bond were obtained. The models of 1-C 18 H 37 X monolayer were optimized by consistent valence force field (CVFF), whose structures and interaction energies agree well with DFT calculations. It is noticed that due to the stronger hydrogen bond interactions between C 18 H 37 OH molecules, C 18 H 37 OH adsorbed on HOPG shows only one kind of molecular arrangement, while C 18 H 37 SH exhibits two kinds of packing induced by alkyl–alkyl and adsorbate–substrate interactions.

Scanning Tunneling Microscopy Characterization of Aromatic Molecules Stabilized by a Buffer Layer of Alkane Derivatives

Using lamellae of 1-octadecanol and stearic acid as a buffer layer, aromatic molecules such as copper phthalocyanine (CuPc) and nitrobenzene were immobilized on the surface of highly oriented pyrolytic graphite (HOPG), and been characterized by scanning tunneling microscopy (STM). These molecules crystallized in two dimensions on top of the monolayer of alkane derivatives, and high-resolution images of these molecules were obtained. An additional advantage is that the buffer layer underneath does not change the electronic properties of the immobilized molecules. The mechanism of the immobilization of the buffer layer is considered to be the cutting down of charge transfer between the aromatic molecules and graphite. These results proved that this strategy could be used as a general method to immobilize organic species on a substrate surface.

Surface Structure of Heterogeneous Catalysts: Cinchona and Tartaric Acid on Solid Surface

Enantioselective hydrogenation by heterogeneous catalysts has gained great importance in organic synthesis, chemical engineering and pharmaceutical industry. However, the mechanism of enantioselectivity is not well understood due to the absence of evidence at the molecular level. Recently, the great advance on this topic has been made by applying scanning tunneling microscopy (STM) and other surface characterization techniques to get direct evidence about the enantioselective mechanism of catalysts. Here, the progress on the adsorption mode of chiral modifiers such as cinchona and tartaric acid and the related enantioselective mechanism is reviewed from the viewpoint of surface science. Particularly, some STM works on this topic are summarized. Finally, several key issues for further investigation are outlined.

Tuning molecular orientation with STM at the solid/liquid interface

Triptycene molecular orientation has been tuned with a STM tip at a Cu(111) surface in solution from flat, to tilt, to vertical. The tuning is completely bias dependent and reversible. The study is important in the fields of nanoscience and technology.