Kian Soon Yong

Dissociative adsorption of pyrrole on Si(111)-(7×7)

Pyrrole adsorption on Si(111)-(7×7) has been investigated using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy, scanning tunneling microscopy (STM), and theoretical calculations. Compared to physisorbed pyrrole, chemisorption leads to the appearance of N–Si and Si–H vibrational features, together with the absence of N–H stretching mode. This clearly demonstrates the dissociative nature of pyrrole chemically binding on Si(111)-(7×7) through the breakage of N–H bond. Based on STM results, the resulting fragments of pyrrolyl and H atom are proposed to bind with an adatom and an adjacent rest atom, respectively. The STM images further reveal that the adsorption is site selective. The faulted center adatoms are most favored, followed by unfaulted center adatoms, faulted corner adatoms, and unfaulted corner adatoms. In addition, the chainlike pattern of reacted adatoms was observed, implying the possible existence of attractive interaction between adsorbed pyrrolyl a...

Cycloaddition chemistry of thiophene on the silicon (111)-7×7 surface

The mechanism of the cycloaddition reaction between thiophene and Si(111)-7×7 has been investigated using thermal desorption spectroscopy (TDS), high resolution electron energy loss spectroscopy (HREELS), electronic electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), and PM3 semiempirical calculations. The results show that thiophene binds on Si(111)-7×7 through a [4+2] cycloaddition reaction between the 2, 5 C-atoms of thiophene and the adjacent adatom-rest atom pairs on Si(111)-7×7. Semiempirical PM3 calculations based on the cluster model further confirm this di-σ bonding configuration. A stepwise surface diradical mechanism has been proposed to account for the regioselective [4+2] cycloaddition reaction.

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.

Adsorption-Induced Desorption of Benzene on Si(111)-7 × 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.

Formation of order molecular nanostructures on the Si(111)-(7×7) surface by patterned assembly

The well-defined and patterned copper clusters formed on the Si(111)-(7×7) surface have been employed as a template for selective binding of molecules, forming ordered molecular nanostructures. Scanning tunneling microscopic studies show that thiophene molecules preferentially bind to the copper clusters through the S–Cu interaction involving S lone-pair electrons. Large-scale two-dimensional thiophene molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on Si(111) surface.

Surface phase transition of Cu/Si(1 1 1)-(5 × 5) by scanning tunnelling microscopy and photoemission study

The phase transition process from the Si(1 1 1)-(7 × 7) surface to the Cu/Si(1 1 1)-(5 × 5) surface structure has been studied by scanning tunnelling microscopy and synchrotron radiation photoemission spectroscopy. The nucleation and growth of Cu/Si(1 1 1)-(5 × 5) on the Si(1 1 1)-(7 × 7) surface progress gradually with the increase in Cu coverage. Cu deposition on the Si(1 1 1)-(7 × 7) surface at room temperature may only involve the saturation of the surface dangling bonds, whereas a new surface phase of Cu/Si(1 1 1)-(5 × 5) is formed upon annealing, which saturates at a Cu coverage of 0.9 ML. Our experiments clearly show the surface phase transition process of the (5 × 5) structure as a function of the Cu coverage and provide useful insight into the Cu/Si(1 1 1)-(5 × 5) structure.

Formation of copper clusters on a thiophene mediated Si(111)-(7×7) surface via molecular anchors

Thiophene selectively binds to the pair of adjacent Si adatom and Si rest atom on the Si(111)-(7×7) surface, leading to the covalent attachment of C–S–C linkages onto the surface. Cu atoms are found to preferentially adsorb onto the S atoms of the functional C–S–C groups in the formation of copper nanoclusters on the thiophene-mediated Si(111)-(7×7) surface.

Molecular anchor Cu–S formed on a thiophene mediated Si(111)-(7×7) surface

Thiophene molecule selectively binds to the adjacent adatom-rest atom pair on the Si(111)-(7x7) surface through its alpha-carbon atoms, leading to the covalent attachment of a C-S-C linkage and remaining C=C (beta-carbon) bond onto the surface. Photoemission studies show that Cu atom readily adsorbs onto the S atom of the functional group to form the Cu-S molecular anchor in two forms: one points away from the thiophene C=C group; the other points toward the C=C group.