Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C–O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state

Ligand Field Effect at Oxide–Metal Interface on the Chemical Reactivity of Ultrathin Oxide Film Surface

Ultrathin oxide film is currently one of the paramount candidates for a heterogeneous catalyst because it provides an additional dimension, i.e., film thickness, to control chemical reactivity. Here, we demonstrate that the chemical reactivity of ultrathin MgO film grown on Ag(100) substrate for the dissociation of individual water molecules can be systematically controlled by interface dopants over the film thickness. Density functional theory calculations revealed that adhesion at the oxide–metal interface can be addressed by the ligand field effect and is linearly correlated with the chemical reactivity of the oxide film. In addition, our results indicate that the concentration of dopant at the interface can be controlled by tuning the <i>drawing effect</i> of oxide film. Our study provides not only profound insight into chemical reactivity control of ultrathin oxide film supported by a metal substrate but also an impetus for investigating ultrathin oxide films for a wider range of applications

Enhancement of Inelastic Electron Tunneling Conductance Caused by Electronic Decoupling in Iron Phthalocyanine Bilayer on Ag(111)

The effect of electronic decoupling on the inelastic electron tunneling process of iron phthalocyanine (FePc) molecules on Ag(111) was investigated using scanning tunneling microscopy (STM). A drastic difference in the inelastic electron tunneling to individual FePc molecules was found for the first and the second layer molecules grown on Ag(111). The spectrum of the first layer molecule is essentially structureless, whereas the second layer molecules provide giant conductance changes reaching several tens % due to the vibrational excitations. This is the first clear example to demonstrate, by using inelastic tunneling spectroscopy with STM, the enhancement of vibrational inelastic tunneling driven through the electronic decoupling of the molecules from the substrate

Two-Dimensional Superstructure Formation of Fluorinated Fullerene on Au(111): A Scanning Tunneling Microscopy Study

A two-dimensional fluorinated fullerene (C₆₀F₃₆) superstructure has been successfully formed on Au(111) and was investigated using scanning tunneling microscopy (STM) and density functional theory calculations. Although there exist three isomers (<i>C</i>₃, <i>C</i>₁, and <i>T</i>) in our molecular source, STM images of the molecules in the well-ordered region all appear identical, with 3-fold symmetry. This observation together with the differences in the calculated lowest unoccupied molecular orbital (LUMO) distribution among the three isomers suggests that a well-ordered monolayer consists of only the <i>C</i>₃ isomer. Because of the strong electron-accepting ability of C₆₀F₃₆, the adsorption orientation can be explained by localized distribution of its LUMO, where partial electron transfer from Au(111) occurs. Intermolecular C–F···π electrostatic interactions are the other important factor in the formation of the superstructure, which determines the lateral orientation of C₆₀F₃₆ molecules on Au(111). On the basis of scanning tunneling spectra obtained inside the superstructure, we found that the LUMO is located at 1.0 eV above the Fermi level (<i>E</i>_F), while the highest occupied molecular orbital (HOMO) is at 4.6 eV below the <i>E</i>_F. This large energy gap with the very deep HOMO as well as uniform electronic structure in the molecular layer implies a potential for application of C₆₀F₃₆ to an electron transport layer in organic electronic devices

Substituent Effect on the Intermolecular Arrangements of One-Dimensional Molecular Assembly on the Si(100)-(2×1)‑H Surface

The effect of methyl substitution in styrene molecules on the spatial arrangement of molecules in a one-dimensional (1-D) molecular assembly on the Si(100)-(2×1)-H surface has been studied using a scanning tunneling microscope (STM) at 300 K. Styrene molecules form well-defined 1-D molecular assemblies through a chain reaction mechanism along the dimer row direction, where the phenyl rings are separated by distances equal to that of the interdimer distance in a row and aligned parallel to each other. We observed that the substitution in a phenyl ring has no observable effect on the adsorption sites, configurations, and stacking of phenyl rings along the dimer row. In contrast, the methyl substitution at α site (α-methylstyrene) results in a 1-D assembly where the adsorption sites are similar to that of styrene but the adsorbed molecules are arranged in alternate geometrical configurations along the dimer row. In the case of β-methylstyrene, the adsorption sites (diagonal silicon atoms in a dimer row) and the geometrical configurations of adsorbed molecules along the dimer row are different from that of styrene. These results suggest that the selective arrangement of the molecules in a 1-D assembly can be achieved by inducing a steric hindrance through substitution at specific sites of the reacting molecule

Ordering of Molecules with π‑Conjugated Triangular Core by Switching Hydrogen Bonding and van der Waals Interactions

Using alkoxylated derivatives of triangular dehydrobenzo[12]­annulene (DBA) as building blocks, we demonstrate control of a formation of 2D molecular networks on Au(111). The tunability of intermolecular interactions by substituting alkoxy groups can improve the homogeneity of the 2D molecular network by restricting the number of polymorphs, and it can induce domain-specific chirality. The π-conjugated triangular core of each alkoxylated DBA derivative is locked on a specific site on the Au(111) surface by the interaction between the oxygen atoms of the molecule and the gold (Au) surface atoms, and the relative importance of intermolecular hydrogen bonding versus van der Waals interactions depends on the length of the alkoxy groups. Such tunable intermolecular interactions balanced with surface–molecule interaction may eventually enable control of the formation of 2D molecular networks. These results could lead to potential applications in tailoring 2D molecular networks or allow the use of these networks as templates. The 2D molecular networks are investigated using scanning tunneling microscopy, and modeling is based on density functional theory calculations

Dispersive Electronic States of the π‑Orbitals Stacking in Single Molecular Lines on the Si(001)-(2×1)‑H Surface

One-dimensional (1D) molecular assemblies have been considered as one of the potential candidates for miniaturized electronic circuits in organic electronics. Here, we present the quantitative experimental measurements of the dispersive electronic feature of 1D benzophenone molecular assemblies on the Si(001)-(2×1)-H. The well-aligned molecular lines and their certain electronic state dispersion were observed by scanning tunneling microscopy (STM) and angle-resolved ultraviolet photoemission spectroscopy (ARUPS), respectively. Density functional theory (DFT) calculations reproduced not only the experimental STM image but also the dispersive features that originated from the stacking phenyl π-orbitals in the molecular assembly. We obtained the effective mass of 2.0<i>m</i>_e for the hole carrier along the dispersive electronic state, which was comparable to those of the single-crystal molecules widely used in organic electronic applications. These results ensure the one-dimensionally delocalized electronic states in the molecular lines, which is requisitely demanded for a charge-transport wire

Spectroscopic Identification of Ag-Terminated “Multilayer Silicene” Grown on Ag(111)

The electronic structure of the outermost layer of “multilayer silicene” was investigated by metastable atom electron spectroscopy (MAES). It is usually difficult to elucidate the electronic structure of an ultrathin film grown on a solid substrate excluding the contribution from the substrate, especially such as “multilayer silicene” grown on a Ag(111) substrate. MAES used in this study thus provides a proper solution because the excitation source, He*­(2³S) atom, cannot penetrate through the first layer. Comparing the MAES spectra of “multilayer silicene” and of the Si(111)√3 × √3-Ag surface where the Ag atoms are arranged to form a superlattice on the (111) surface of the Si diamond crystal, we find that these spectra are essentially identical to each other. This result indicates that the so-called “multilayer silicene” is actually not multilayered, i.e., a stack of honeycomb lattice layers

Confinement of the Pt(111) Surface State in Graphene Nanoislands

We present a combined experimental and theoretical study of electron confinement in graphene nanoislands (GNs) grown on a Pt(111) substrate using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We observed standing wave patterns in the STM images of GNs, and the bias dependency of the standing wave pattern was reproduced by considering free electrons with an effective mass of <i>m</i>* ≈ (0.27 ± 0.03)<i>m</i>_e. Because the effective mass of Pt is <i>m</i>* = 0.28<i>m</i>_e, our results reveal that the electron confinement is due to the effect of the Pt substrate rather than the massless Dirac electrons of graphene. Our calculated maps of the local density of states (LDOS) for the GNs confirm that the electronic properties of the confinement may be described in terms of electrons with an effective mass. The DFT-calculated charge distribution for graphene on the Pt system also shows a clear hybridization between the p_<i>z</i> orbitals of both the first layer of the Pt substrate and the carbon atoms