Scanning tunneling microscope imaging of (CH3S)2 on Cu(111)

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (n_S)) to the LUMO-derived MO (the antibonding orbital localized on the S-S bond (σ*_SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule-metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes.

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.

Role of molecular orbitals near the fermi level in the excitation of vibrational modes of a single molecule at a scanning tunneling microscope junction

Inelastically tunneled electrons from the tip of a scanning tunneling microscope were used to induce S-S bond dissociation of a (CH(3)S)(2) and lateral hopping of a CH(3)S on Cu(111) at 4.7 K. Both experimental results and theoretical calculations confirm that the excitation mechanism of the vibrationally induced chemistry reflects the projected density of states of molecular orbitals that appear near the Fermi level as a result of the rehybridization of the orbitals between the adsorbed molecules and the substrate metal atoms.

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.

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

Single CH3SSCH3 molecules adsorb on the Au(111) surface in a trans-conformation. Subsequent electron-induced dissociation of CH3SSCH3 cleaves the S-S bond and produces a pair of CH3S fragments. The dissociation process was found to preserve the conformation of the parent molecule with a high probability of 75% and imprint it in the relative position and orientation of the product CH3S species on the surface. The high probability of the conformation-retaining scenario is likely due to surface alignment of the S-S bond-breaking reaction coordinate.

Solvent Effect on Self-Assembled Structures of 3,8-Bis-hexadecyloxy-benzo[c]cinnoline on Highly Oriented Pyrolytic Graphite

3,8-Bis-hexadecyloxy-benzo[c]cinnoline (BBC16) self-assembled into two structures at highly oriented pyrolytic graphite (HOPG) surface: one was formed by molecules with a V-like configuration (C2v symmetry) and the other by molecules with a Z-like configuration (C(s) symmetry). The self-assembled structures could be tweaked by the solvents used. In the self-assembled monolayers (SAMs) on HOPG, the BBC16 molecule adopted the V-like configuration in polar solvents and the Z-like configuration in nonpolar solvents. Moreover, the solvent viscosity, solvent dissolvability of BBC16, and substrate temperature also played some roles in tuning the two-dimensional self-assembled structures.