Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy

Extending the source-sink potential method to include electron-nucleus coupling

The source-sink potential (SSP) method provides a simple tool for the qualitative analysis of the conductance of molecular electronic devices, and often analytical expressions for the conductance can be obtained. Here, we extend the SSP approach to account for decoherent, inelastic electron transport by including the non-adiabatic coupling between the electrons and the nuclei in the molecule. This coupling results in contributions to electron transport that can modify the qualitative structure-conductance relationships that we unraveled previously with SSP. In the approach proposed, electron-nucleus interactions are treated starting from the harmonic approximation for the nuclei, using a non-perturbative approach to account for the non-adiabatic coupling. Our method qualitatively describes experimentally observed phenomena and allows for a simple analysis that often provides analytical formulas in terms of the physical parameters of the junction, e.g., vibrational energies, non-adiabatic coupling, and molecule-contact coupling.

On-Surface Synthesis and Molecular Engineering of Carbon-Based Nanoarchitectures

On-surface synthesis via covalent coupling of adsorbed precursor molecules on metal surfaces has emerged as a promising strategy for the design and fabrication of novel organic nanoarchitectures with unique properties and potential applications in nanoelectronics, optoelectronics, spintronics, catalysis, etc. Surface-chemistry-driven molecular engineering (i.e., bond cleavage, linkage, and rearrangement) by means of thermal activation, light irradiation, and tip manipulation plays critical roles in various on-surface synthetic processes, as exemplified by the work from the Ernst group in a prior issue of ACS Nano. In this Perspective, we highlight recent advances in and discuss the outlook for on-surface syntheses and molecular engineering of carbon-based nanoarchitectures.

Photoinduced Charge Transfer in Single-Molecule p-n Junctions

We measured photoinduced charge separation in isolated individual C₆₀-tethered 2,5-dithienylpyrrole triad (C₆₀ triad) molecules with submolecular resolution using a custom-built laser-assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection, and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photoinduced charge separation was not detected for all C₆₀ triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. A photoinduced signal was not observed for dodecanethiol molecules in the surrounding matrix or for control molecules without C₆₀ moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photoinduced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, which can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.

Quantitative Understanding of van der Waals Interactions by Analyzing the Adsorption Structure and Low-Frequency Vibrational Modes of Single Benzene Molecules on Silver

The combination of a sub-Kelvin scanning tunneling microscope and density functional calculations incorporating van der Waals (vdW) corrections has been used successfully to probe the adsorption structure and low-frequency vibrational modes of single benzene molecules on Ag(110). The inclusion of optimized vdW functionals and improved C6-based vdW dispersion schemes in density functional theory is crucial for obtaining the correct adsorption structure and low-energy vibrational modes. These results demonstrate the emerging capability to quantitatively probe the van der Waals interactions between a physisorbed molecule and an inert substrate.

Electron-vibration interaction in multichannel single-molecule junctions

The effect of electron-vibration interaction in atomic-scale junctions with a single conduction channel was widely investigated both theoretically and experimentally. However, the more general case of junctions with several conduction channels has received very little attention. Here we study electron-vibration interaction in multichannel molecular junctions, formed by introduction of either benzene or carbon dioxide between platinum electrodes. By combining shot noise and differential conductance measurements, we analyze the effect of vibration activation on conductance in view of the distribution of conduction channels. Based on the shift of vibration energy while the junction is stretched, we identify vibration modes with transverse and longitudinal symmetry. The detection of different vibration modes is ascribed to efficient vibration coupling to different conduction channels according to symmetry considerations. While most of our observations can be explained in view of the theoretical models for a single conduction channel, the appearance of conductance enhancement, induced by electron-vibration interaction, at high conductance values indicates either unexpected high electron-vibration coupling or interchannel scattering.

Adsorption site determination of a molecular monolayer via inelastic tunneling

We have combined scanning tunneling microscopy with inelastic electron tunneling spectroscopy (IETS) and density functional theory (DFT) to study a tetracyanoethylene monolayer on Ag(100). Images show that the molecules arrange in locally ordered patterns with three nonequivalent, but undeterminable, adsorption sites. While scanning tunneling spectroscopy only shows subtle variations of the local electronic structure at the three different positions, we find that vibrational modes are very sensitive to the local atomic environment. IETS detects sizable mode frequency shifts of the molecules located at the three topographically detected sites, which permits us to determine the molecular adsorption sites through identification with DFT calculations.

Sensitizers in inelastic electron tunneling spectroscopy: a first-principles study of functional aromatics on Cu(111)

Low sensitivity is a key problem in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Using first-principles simulations, we predict different means to tune the IETS sensitivity of symmetrical functional aromatics on a Cu(111) surface. We show how the IET-spectra of phenyl-NO₂ compounds can be greatly enhanced as compared to pristine phenyl. More precisely, the NO₂ substituent qualifies as a sensitizer of low-frequency wagging modes, but also as a quencher of high-frequency stretching modes. At variance, the CO₂ substituent is found to suppress the whole IET-activity. The head-up (non-anchoring) and head-down (anchoring) configurations of the functional group lead to minor changes in the signals, nevertheless allowing access to discriminate configurational features. It is shown how to disentangle the electronic and steric effects of the substituent in the STM junction.

Electron transport through single π-conjugated molecules bridging between metal electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron-transport properties of single π-conjugated molecule junctions, which include benzene, fullerene, and π-stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single-molecule junctions. The effect of the metal-molecule contact on the conductance of the single π-conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π-stacked system using π-stacked aromatic molecules enclosed within self-assembled coordination cages is reviewed. The electron transport in the π-stacked systems is found to be efficient at the single-molecule level, thus providing insight into the design of conductive materials.

On the interpretation of IETS spectra of a small organic molecule

We have investigated vibrational spectra of nitrobenzene molecules adsorbed on Cu(111) by low temperature inelastic electron tunneling spectroscopy. This molecule, which should support 39 internal modes, only gives rise to seven peaks in the spectra. We outline a comparison with ensemble IR data and interpret the small number of vibrational peaks by the superposition of a multitude of almost isoenergetic vibrational modes. The non-detectability of further modes cannot be understood in terms of symmetry considerations. Additional modes in the spectra are attributed to external molecular-metal vibrations.

Electrons, photons, and force: quantitative single-molecule measurements from physics to biology

Single-molecule measurement techniques have illuminated unprecedented details of chemical behavior, including observations of the motion of a single molecule on a surface, and even the vibration of a single bond within a molecule. Such measurements are critical to our understanding of entities ranging from single atoms to the most complex protein assemblies. We provide an overview of the strikingly diverse classes of measurements that can be used to quantify single-molecule properties, including those of single macromolecules and single molecular assemblies, and discuss the quantitative insights they provide. Examples are drawn from across the single-molecule literature, ranging from ultrahigh vacuum scanning tunneling microscopy studies of adsorbate diffusion on surfaces to fluorescence studies of protein conformational changes in solution.

A first principle study of the structural, vibrational and electronic properties of tetrathiafulvalene adsorbed on Ag(110) and Au(110) surfaces

We have studied the adsorption properties of a charge donor organic molecule, tetrathiafulvalene (TTF), on the (110) surfaces of silver and gold by means of the generalized gradient approach of the density functional theory using periodic slab models. This molecule is the core building block of a host of molecular materials exhibiting extremely reach phase diagrams with a variety of ground states. The interfaces formed with metallic surfaces have received only limited attention, despite of their relevance. We have determined the stable adsorption sites for two unit cells representing high and low coverage, which are determinant for the adsorption properties of TTF on the surface. The preferential chemisorption is via the direct interaction of sulfur atoms with the Ag or Au atoms on top sites. All adsorbed TTF are more stable than gas phase TTF. The simulation of the vibrational spectra has permitted us to find the fingerprints of these structures to characterize them on this surface. The donor nature of TTF induces charge transfer to the metallic surfaces.

Inelastic electron tunneling spectroscopy of single-molecule junctions using a mechanically controllable break junction

We report the use of electrical measurements to identify simultaneously the number and type of organic molecules within metal-molecule-metal junctions. Our strategy combines analyses of single-molecule conductance and inelastic electron tunneling spectra, exploiting a nanofabricated mechanically controllable break junction. We found that the peak linewidth of the inelastic electron tunneling spectrum decreased as the modulation voltage and temperature decreased, and that the selection rule for inelastic electron tunneling spectroscopy agrees with that for Raman spectroscopy. Furthermore, the differential conductance curve of the single-molecule junction suggests that it has asymmetrical electrode-molecule coupling.

Manipulation and control of hydrogen bond dynamics in absorbed ice nanoclusters

Inelastic electron tunneling is used to explore the dynamics of ice nanoclusters adsorbed on Ag(111). The diffusion of entire nanoclusters or internal hydrogen bond rearrangement can be selectively controlled by injecting electrons either directly into the clusters themselves or indirectly ("indirect inelastic electron tunneling") into the substrate at distances of up to 20 nm from them; a reaction probability that oscillates with the tip-cluster lateral distance presents evidence that surface state electrons mediate the excitation. Density functional theory calculations reveal a strong sensitivity of the computed activation energies of the individual processes to the applied electrical field.

Molecular modeling of inelastic electron transport in molecular junctions

A quantum chemical approach for the modeling of inelastic electron tunneling spectroscopy of molecular junctions based on scattering theory is presented. Within a harmonic approximation, the proposed method allows us to calculate the electron-vibration coupling strength analytically, which makes it applicable to many different systems. The calculated inelastic electron transport spectra are often in very good agreement with their experimental counterparts, allowing the revelation of detailed information about molecular conformations inside the junction, molecule-metal contact structures, and intermolecular interaction that is largely inaccessible experimentally.

Electric field effect on the vibration of single CO molecules in a scanning tunneling microscope junction

A low-temperature scanning tunneling microscope (STM) and ab initio calculations were used to study the electric field effect on the vibration of single CO molecules in an STM junction at 13 K. The vibrational energy of CO molecules adsorbed on silver atoms, measured by STM-based inelastic electron tunneling spectroscopy, depends on the direction of the electric field applied between the STM tip and the silver species. This characteristic can be explained by the charge separation model. The electric field modifies the binding characteristics of CO on silver as a result of a change in the charged states of the species, which leads to an increase (or a decrease) of the energies of the hindered rotation and the CO stretch on silver.

Functional and spectroscopic measurements with scanning tunneling microscopy

Invented as a surface analytical technique capable of imaging individual atoms and molecules in real space, scanning tunneling microscopy (STM) has developed and advanced into a technique able to measure a variety of structural, functional, and spectroscopic properties and relationships at the single-molecule level. Here, we review basic STM operation and image interpretation, techniques developed to manipulate single atoms and molecules with the STM to measure functional properties of surfaces, local spectroscopies used to characterize atoms and molecules at the single-molecule level, and surface perturbations affecting surface coverage and surface reactions. Each section focuses on determining the identity and function of chemical species so as to elucidate information beyond topography with STM.

Reversible control of hydrogenation of a single molecule

Low-temperature scanning tunneling microscopy was used to selectively break the N-H bond of a methylaminocarbyne (CNHCH3) molecule on a Pt(111) surface at 4.7 kelvin, leaving the C-H bonds intact, to form an adsorbed methylisocyanide molecule (CNCH3). The methylisocyanide product was identified through comparison of its vibrational spectrum with that of directly adsorbed methylisocyanide as measured with inelastic electron tunneling spectroscopy. The CNHCH3 could be regenerated in situ by exposure to hydrogen at room temperature. The combination of tip-induced dehydrogenation with thermodynamically driven hydrogenation allows a completely reversible chemical cycle to be established at the single-molecule level in this system. By tailoring the pulse conditions, irreversible dissociation entailing cleavage of both the C-H and N-H bonds can also be demonstrated.

Imaging and vibrational spectroscopy of single pyridine molecules on Ag(110) using a low-temperature scanning tunneling microscope

A scanning tunneling microscope (STM) was used to extract the images of single, isolated pyridine molecules adsorbed on Ag(110) and to record their vibrational spectrum at 13 K. On the STM image, the pyridine molecule appears as an elongated protrusion along the [001] direction on top of a silver atom, indicating that it is bonded through its nitrogen lone pair electrons. STM inelastic electron tunneling spectroscopy of the adsorbed pyridine revealed C-D and C-H stretch modes at 282 and 378 meV, respectively.

Simulation of Single Molecule Inelastic Electron Tunneling Signals in Paraphenylene−Vinylene Oligomers and Distyrylbenzene[2.2]paracyclophanes

Inelastic resonances in the electron tunneling spectra of several conjugated molecules are simulated using the nonequilibrium Greens function formalism. The vibrational modes that strongly couple to the electronic current are different from the infrared and Raman active modes. Spatially resolved inelastic electron tunneling (IET) intensities are predicted. The simulated IET intensities for a large distyrylbenzene paracyclophane molecule are in qualitative agreement with recent experimental results.

Dynamics and Spectroscopy of Hydrogen Atoms on Pd{111}

Chemisorption of hydrogen on Pd{111} is a relatively simple, yet important surface chemical process. By using low-temperature scanning tunneling microscopy, tip-induced motion of adsorbed atomic hydrogen at 4 K has been observed at low coverage. The motion has been ascribed to excitation of vibrational modes that decay into translational modes; vibrational spectroscopy via inelastic electron tunneling corroborates this assignment, and the barrier to hydrogen atom motion has been determined. At higher coverages, tip-induced motion of vacancies in the hydrogen overlayer is observed, and the associated barrier has also been determined.

Excitation of molecular vibrational modes with inelastic scanning tunneling microscopy processes: examination through action spectra of cis-2-butene on Pd(110)

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection rules in terms of resonance tunneling.

Substrate-mediated intermolecular interactions: a quantitative single molecule analysis

Long-range intermolecular interactions mediated by the surface are believed to be responsible for many effects in surface science, including molecular ordering, formation of nanostructures, and aligning reactive intermediates in catalysis. Here, we use scanning tunneling microscopy to probe the weak substrate-mediated interactions in benzene overlayers on Au{111} at 4 K. Using an automated procedure to monitor single molecule motion, we are able to quantify the substrate-mediated interaction strength. We explain quantitatively both the kinetics of the benzene motion and the thermodynamics that determine the packing structures benzene adopts in this system in light of these substrate-mediated interactions.

Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis

Supported ruthenium metal catalysts have higher activity than traditional iron-based catalysts used industrially for ammonia synthesis. A study of a model Ru/C catalyst was carried out to advance the understanding of structure and reactivity correlations in this structure-sensitive reaction where dinitrogen dissociation is the rate-limiting step. Ru particles were grown by chemical vapor deposition (CVD) via a Ru(3)(CO)(12) precursor on a highly oriented pyrolytic graphite (HOPG) surface modified with one-atomic-layer-deep holes mimicking activated carbon support. Scanning tunneling microscopy (STM) has been used to investigate the growth, structure, and morphology of the Ru particles. Thermal desorption of dissociatively adsorbed nitrogen has been used to evaluate the reactivity of the Ru/HOPG model catalysts. Two different Ru particle structures with different reactivities to N(2) dissociation have been identified. The initial sticking coefficient for N(2) dissociative adsorption on the Ru/HOPG model catalysts is approximately 10(-6), 4 orders larger compared to that of Ru single-crystal surfaces.

Substrate-mediated interactions and intermolecular forces between molecules adsorbed on surfaces

Adsorbate interactions and reactions on metal surfaces have been investigated using scanning tunneling microscopy. The manners in which adsorbates perturb the surface electronic structure in their vicinity are discussed. The effects these perturbations have on other molecules are shown to be important in overlayer growth. Interactions of molecules with surface steps are addressed, and each molecule's electron affinity is shown to dictate its adsorption sites at step edges. Standing waves emanating from steps are demonstrated to effect transient molecular adsorption up to 40 A away from the step edge. Halobenzene derivatives are used to demonstrate how the surface is important in aligning reactive intermediates.

Single-molecule reaction and characterization by vibrational excitation

Controlled chemical reaction of single trans-2-butene molecules on the Pd(110) surface was realized by dosing tunneling electrons from the tip of a scanning tunneling microscope at 4.7 K. The reaction product was identified as a 1,3-butadiene molecule by inelastic electron tunneling spectroscopy. Threshold voltage for the reaction is approximately 365 mV, which coincides with the vibrational excitation of the C-H stretching mode. The reaction was ascertained to be caused by C-H bond dissociation by multiple vibrational excitations of the C-H stretching mode via inelastic electron tunneling process.