Ying Hui Cai

Selective attachment of benzaldehyde on Si(100)-2×1: Structure, selectivity, and mechanism

The interaction of benzaldehyde with the Si(100) surface has been investigated as a model system for understanding the interaction of conjugated pi-electron systems with semiconductor surfaces. Vibrational features of chemisorbed benzaldehyde unambiguously demonstrate that the carbonyl group directly interacts with the Si surface dangling bonds, evidenced in the disappearance of the C=O stretching mode around 1713 cm(-1) coupled with the retention of all vibrational signatures of its phenyl ring. X-ray photoemission spectroscopy shows that both C 1s and O 1s binding energies of the carbonyl group display large downshifts by 1.9 and 1.3 eV, respectively. Vibrational and electronic results show that the covalent attachment of benzaldehyde on Si(100) occurs in a highly selective manner through the direct interaction of both C and O atoms of the carbonyl group with a Si=Si dimer to form a four-membered Si-C-O-Si ring at the interface, leaving a nearly unperturbed phenyl ring protruding into vacuum. This conclusion is further confirmed by the observation of a predominant protrusion for benzaldehyde adsorbed on Si(100)-2 x 1 in scanning tunneling microscopy experiments, consistent with the predication of density-functional theory calculation.

Enabling enelike reactions on Si(111)-7×7 through tuning organic molecular structures

We previously demonstrated that acetonitrile (N[triple bond]C-CH(3)) binds to the adjacent adatom-rest atom pair of Si(111)-7x7 through a [2+2]-like cycloaddition reaction, forming a (Si)N=C(Si)-CH(3)-like surface species [Tao et al., J. Phys. Chem. B 106, 3890 (2002)]. Current investigation clearly showed that chloroacetonitrile (N[triple bond]C-CH(2)Cl), propargyl chloride (HC[triple bond]C-CH(2)Cl), and 3-chloropropionitrile (N[triple bond]C-CH(2)-CH(2)Cl) react with the surface via enelike reactions, concurrently involving N[triple bond]C/C[triple bond]C as well as the breakage of the C-Cl/C-H bond. Further separation of the unsaturated bond (C[triple bond]C) from the C-Cl bond using CH(2) spacers in 5-chloro-1-pentyne (CH[triple bond]C-CH(2)CH(2)CH(2)Cl) would direct the reaction to a [2+2]-like cycloaddition. These experimental results clearly suggest the possibility of controlling the surface reaction pathways by tuning the organic molecular structures. This strategy can be useful in designing and fabricating functional molecular templates on Si(111)-7x7.