Adsorption-induced desorption of benzene on Si(111)-7 x 7 by substrate-mediated electronic interactions.

The process of benzene adsorption on an adjacent adatom-rest atom pair on Si(111)-7 x 7 at room temperature was studied using in-situ scanning tunneling microscopy (STM). Both adsorption and desorption of benzene were observed to take place mostly at adjacent sites during the process. DFT calculation results show that the bond length between the rest atom and the carbon atom in a pre-adsorbed benzene molecule increases due to the charge transfer from a neighboring rest atom in response to an approaching benzene molecule. Such increase in the bond length, when coupled resonantly to the C-Si thermal vibration, could result in bond breakage and desorption of the adsorbate. The studies provide evidence for the desorption of a chemisorbed benzene caused by an adsorbing benzene at a neighboring site through a substrate-mediated electronic interaction.

Studies of chemisorbed tetracene on si(111)-7x7.

The chemisorption of tetracene on the Si(111)-7x7 surface was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. On the basis of the STM results and dimension analysis, two types of binding configurations were proposed. One of the configurations involves the di-sigma reaction between two C atoms of an inner ring with an adatom-rest atom pair on the substrate to give rise to an unsymmetrical butterfly structure. Tetracene in another configuration possesses four C-Si bonds that are formed via di-sigma reactions between the C atoms at the terminal rings with two center adatom-rest atom pairs within one-half of the surface unit cell. Besides, two other binding modes were proposed based on the dimension compatibility between the tetracene C and the substrate Si dangling bonds even though their identifications through the STM images are nonexclusive. Structural modeling and adsorption energies calculations were carried out using the DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of tetracene in the different binding configurations as observed experimentally were discussed.

Binding of glycine and L-cysteine on Si(111)-7 x 7.

The adsorption of glycine and l-cysteine on Si(111)-7 x 7 was investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The observation of the characteristic vibrational modes and electronic structures of NH3+ and COO- groups for physisorbed glycine (l-cysteine) demonstrates the formation of zwitterionic species in multilayers. For chemisorbed molecules, the appearance of nu(Si-H), nu(Si-O), and nu(C=Omicron) and the absence of nu(O-H) clearly indicate that glycine and l-cysteine dissociate to produce monodentate carboxylate adducts on Si(111)-7 x 7. XPS results further verified the coexistence of two chemisorption states for each amino acid, corresponding to a Si-NH-CH2-COO-Si [Si-NHCH(CH2SH)COO-Si] species with new sigma-linkages of Si-N and Si-O, and a NH2-CH2-COO-Si [NH2CH(CH2SH)COO-Si] product through the cleavage of the O-H bond, respectively. Glycine/Si(111)-7 x 7 and l-cysteine/Si(111)-7 x 7 can be viewed as model systems for further modification of Si surfaces with biological molecules.

Selective attachment of benzaldehyde on Si(100)-2 x 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.

Selective attachment of 1,4-benzenedimethanethiol on the copper mediated Si(111)-(7 x 7) surface through S-Cu linkage.

The well-defined and patterned copper clusters formed on the Si(111)-(7 x 7) surface have been employed as a template for selective binding of 1,4-benzenedimethanethiol (HS-CH2-C6H4-CH2-SH, 1,4-BDMT), to form ordered molecular nanostructures. Scanning tunneling microscopic (STM) studies showed that each 1,4-BDMT molecule preferentially binds to two neighboring copper atoms within one copper cluster through the S-Cu interaction with its molecular plane parallel to the surface, whereas some 1,4-BDMT bond to individually adsorbed copper atoms, resulting in an upright configuration. Large-scale two-dimensional molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on the Si(111)-(7 x 7) surface.

Formation of molecular templates containing cumulative double bonds (C=C=C) at organic/silicon hybrid interfaces.

The cumulative double bond (C=C=C), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acetylethyne to Si(111)-7 x 7. The experimental observation of the characteristic vibrational modes and electronic structures of the C=C=C group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal O and C atoms of acetylethyne and the neighboring Si adatom-rest atom pair, consistent with the prediction of density functional theory calculations. Scanning tunneling microscopy images further reveal that the molecules selectively bind to the adjacent adatom-rest atom pairs on Si(111)-7 x 7.