Propagation of Conformation in the Surface-Aligned Dissociation of Single CH3SSCH3Molecules on Au(111)

Long-range migration of H-atoms from electron-induced dissociation of HS on Si(111)

The electron-induced dissociation of chemisorbed HS to give recoiling H-atoms was investigated on a Si(111)-7 × 7 surface at 270 K by scanning tunnelling microscopy and modelled by density functional theory. Two different H-atom migratory pathways were identified: 'short-range' (S-R; 37%) and 'long-range' (L-R; 42%). In S-R reaction the H-atom recoiled by only 4 Å whereas in L-R the average H-recoil distance was 17 Å extending up to 72 Å. Chemisorbed H-atoms were not detected in the remaining 22% of dissociative events. Excitation involved three successive events, e^-+ HS. Molecular dynamics calculations of S-R and L-R recoil of H-atoms were performed using a model based on electron-induced H ⋅ S repulsion. In S-R the repulsion gave the H-atom sufficient energy to dissociate HS, but not enough to result in capture of the H-atom by the adjacent rest Si-atom. In L-R a higher translational energy of the recoiling H, above 0.2 eV, caused the H-atom to 'bounce' off surface atoms and migrate L-R. The finding that H-atom L-R migration followed the ballistics and 'bounce' mechanism is indicative of the generality of this mode of L-R recoil.

Bond selectivity in electron-induced reaction due to directed recoil on an anisotropic substrate

Bond-selective reaction is central to heterogeneous catalysis. In heterogeneous catalysis, selectivity is found to depend on the chemical nature and morphology of the substrate. Here, however, we show a high degree of bond selectivity dependent only on adsorbate bond alignment. The system studied is the electron-induced reaction of meta-diiodobenzene physisorbed on Cu(110). Of the adsorbate's C-I bonds, C-I aligned 'Along' the copper row dissociates in 99.3% of the cases giving surface reaction, whereas C-I bond aligned 'Across' the rows dissociates in only 0.7% of the cases. A two-electronic-state molecular dynamics model attributes reaction to an initial transition to a repulsive state of an Along C-I, followed by directed recoil of C towards a Cu atom of the same row, forming C-Cu. A similar impulse on an Across C-I gives directed C that, moving across rows, does not encounter a Cu atom and hence exhibits markedly less reaction.

Retention of chirality in electron-induced reactions

Two enantiomers were observed by Scanning Tunneling Microscopy (STM) when meta-iodopyridine was physisorbed on a 4.6 K Cu(110) surface. The chirality of the reagent was retained in the products of the electron-induced reaction. Dynamical calculations showed this to be a consequence of the reaction occurring on one side of the mirror plane.

Self-assembled monolayers of CH3S from the adsorption of CH3SSCH3 on Au(111)

By performing density functional theory calculations, we have examined the adsorption of dimethyl disulfide (CH3SSCH3) on Au(111). In addition, we have also charted the reaction paths leading to dissociation of the S-S bond and the formation of thiolate-adatom species. Further, we have simulated the scanning tunneling microscopic (STM) images of the key adsorption intermediates and products and compared them with the experimental data available in the literature. In summary, our computations show that CH3SSCH3 adsorbs on Au(111) non-dissociatively with a moderate adsorption energy 0.42 eV for the trans-conformation which is 0.24 eV more stable than its cis-isomer. When the adsorbed trans-CH3SSCH3 dissociates at low temperature, the two CH3S fragments are retained by the closest neighboring sites and the trans-conformation of them prior to S-S dissociation can be preserved. The activation barrier to rotating the CH3S fragments on Au(111) is 0.17 eV so a moderate temperature rise can facilitate the transformation of the trans-conformation and generate a mixture of the trans- and cis-conformation. Our calculations also show that intrinsic defects such as gold adatoms (Auad) are active in CH3SSCH3 dissociation in two mechanisms: a direct mechanism in which CH3SSCH3 dissociation drives Auad formation and CH3S-Auad formation simultaneously, with leftover vacancies as by-products; a sequential mechanism in which Auad is present in advance and subsequently interacts with a nearby CH3S fragment to form CH3S-Auad. The respective activation barriers are 0.15 and 0.32 eV for the trans- and cis-CH3S-Auad-SCH3 complex along the direct mechanism, and 0.21 and 0.18 eV along the sequential mechanism.

Molecular calipers control atomic separation at a metal surface

If a molecule controls the length of some other moiety, it can be termed a "molecular caliper". Here we image individual molecular calipers of this type by scanning tunneling microscopy. These consist of linear polymers of p-diiodobenzene, (pDIB)n, of varying length, 0.7-2.9 nm, physisorbed on Cu(110) at 4.6 K. Through electron-induced reaction these chemically imprint their terminal I-atoms on the copper, 0.7 nm further apart than their initial separations. The physisorbed monomer or polymer, therefore, constitutes a molecular-caliper with variable terminal I..I separation. The localized nature of the I-atom reaction at the copper surface relative to the parent molecule, constitutes a novel finding reported here. It ensures that the separation of the I-atoms in the physisorbed molecular caliper correlates with their subsequent separation when chemisorbed at the surface.

STM imaging ortho- and para-fluorothiophenol self-assembled monolayers on Au(111)

Self-assembled monolayers (SAMs) of para- and ortho-fluorothiophenol (p- and o-FTP) spontaneously formed on Au(111) substrate have been contrasted through investigation by a scanning tunneling microscope (STM) at room temperature. High-resolution STM imaging reveals that p-FTP adopts a 6 x radical3R30 degrees molecule arrangement containing six molecules. Two different kinds of p-FTP molecule dimer line structures have been formed on Au(111) by intermolecular pi-pi stacking along 112 substrate directions, besides a single p-FTP molecule line. In contrast, o-FTP molecules self-assemble into a much looser wave-like SAM, which can be described as a 5 x 3 radical3R30 degrees structure containing two molecules. Periodic density functional theory (DFT) calculations for the two systems suggest that these kinds of FTP molecules preferentially take the asymmetrical positions between 3-fold face-centered cubic (fcc) hollow and bridge sites on Au(111), tilting from the substrate surface. Theoretical simulation gives apparent average tilted angles of 58 degrees and 68 degrees for p-FTP and o-FTP with respect to the surface normal, respectively. This simulation shows that o-FTP is more inclined to lie down toward the Au(111) surface compared to p-FTP. The difference between p-FTP and o-FTP SAM structures can be qualitatively understood in terms of the variation of intermolecular dipole-dipole orientation. This suggests that, besides well-known Au-S and pi-pi interactions, electrostatic interactions including dipole-dipole, quadrupole-quadrupole, and dipole-quadrupole interactions might also play an important role in influencing the SAM structures formed by aromatic thiols with a permanent dipole moment.

Nonlocal dissociative chemistry of adsorbed molecules induced by localized electron injection into metal surfaces

We present a novel approach to surface chemistry studies using scanning tunneling microscopy (STM), where dissociation of molecules adsorbed on metal surfaces is induced nonlocally in a 10-100 nm radius around the STM tip by hot electrons that originate from the STM tip and transport on the surface. Nonlocal molecular excitation eliminates the influence of the STM tip on the outcome of the electron-induced chemical reaction. The spatial attenuation of the nonlocal reaction is used as a direct measure of hot-electron transport on the surface.

Methanethiolate Adsorption Site on Au(111): A Combined STM/DFT Study at the Single-Molecule Level

The chemisorptive bonding of methanethiolate (CH(3)S) on the Au(111) surface has been investigated at a single-molecule level using low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT). The CH(3)S species were produced by STM-tip-induced dissociation of methanethiol (CH(3)SH) or dimethyl disulfide (CH(3)SSCH(3)) at 5 K. The adsorption site of an isolated CH(3)S species was assigned by comparing the experimental and calculated STM images. We conclude that the S-headgroup of chemisorbed CH(3)S adsorbs on the 2-fold coordinated bridge site between two Au atoms, consistent with theoretical predictions for CH(3)S on the nondefective Au(111) surface. Our assignment is also supported by the freezing of the tip-induced rotational dynamics of a single CH(3)SH molecule upon conversion to CH(3)S via deprotonation.