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The hydrogen-bonding networks of water have strong intra- and intermolecular vibrational coupling which influences the energy dissipation and proton transfer in water. Disentangling and quantitative characterization of different coupling effects in water at a single-molecular level still remains a great challenge. Using tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on low-temperature scanning tunneling microscopy, we report the direct quantitative assessment of the intermolecular coupling constants of the OH-stretch vibrational bands of an isolated water tetramer adsorbed on a Au(111)-supported NaCl(001) bilayer film. This is achieved by distinguishing various coupled modes of the H-bonded O–H stretching vibrations through tip-height dependent IET spectra. In contrast, such vibrational coupling is negligible in the half-deuterated water tetramer owing to the large energy mismatch between the OH and OD stretching modes. Not only do these findings advance our understanding on the effects of local environment on the intermolecular vibrational coupling in water, but also open up a new route for vibrational spectroscopic studies of extended H-bonded network at the single-molecular level.

We study the magnetic anisotropy of a single iron phthalocyanine (FePc) molecule on a Cu(110) (2 × 1)-O by using inelastic electron tunneling spectroscopy (IETS) with low-temperature scanning tunneling microscopy. Two inelastic excitations derived from the splitting of the molecular triplet spin state appear as two pairs of steps symmetrically with respect to zero sample voltage. We measured IETS spectra with external magnetic fields perpendicular and parallel to the molecular plane, and we analyzed the spectral evolution with the effective spin Hamiltonian approach. We determined all parameters related with magnetic anisotropy at a single-molecule level, both the easy- and hard-magnetization directions, zero-field splitting constant, D = - 4.0 meV and E = 1.1 meV, the Lande g-tensor gxx, gyy, gzz=(1.82, 2.02, 2.34), and the constant of spin-orbit coupling λ = - 19.1 meV. We stress that the symmetry breaking caused by the adsorption of FePc on the oxidized Cu(110) significantly impacts the magnetic anisotropy.

We study inelastic electron tunneling through a molecular junction using the non-equilibrium Green function (NEGF) formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage when plotted against the latter. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative d(current)/d(voltage) and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed., 41 pages, 9 figures

Vibronic states are observed in single C(60) and C(70) molecules by scanning tunneling microscopy. When single fullerene molecules are adsorbed on a thin layer of Al(2)O(3) grown on a NiAl(110) substrate, equally spaced features are observed in the differential conductance (dI/dV), which are clearly resolved in d(2)I/dV(2) spectra. These features are attributed to the vibronic states of the molecule. The vibronic progressions are sensitive to the molecular orientations and can have different spacings in different electronic bands of the same molecule. For C(60,) these vibronic states are associated with the intramolecular A(g) and H(g) vibrational modes. Vibronic states are not resolved in molecules adsorbed on the metal surface. However, inelastic electron tunneling spectroscopy exhibits a vibrational mode at 64 meV for C(60) and 61 meV for C(70) adsorbed on NiAl(110).

The ability to probe individual atoms and molecules have made it possible to reveal properties which otherwise would be hidden in the study of an ensemble of atoms and molecules. The scanning tunneling microscope (STM) with its unmatched spatial resolution and versatility literally allows us to touch atoms and molecules one at a time and to carry out experiments which previously were only imagined. One of the great attributes of the STM is that it provides a real space view of the individual molecules and the atomic landscape of their environment, thus removing many of the uncertainties surrounding the nature of the system under study. Combining its imaging, manipulation, spectroscopic characterization, and chemical modification capabilities, the STM has enabled direct visualization of chemistry by revealing the fundamental properties of atoms and molecules and their interactions with each other and the environment. While femtosecond lasers have made it possible to study chemistry at the temporal limit, the STM provides an understanding of chemistry at the spatial limit.

Destructive quantum interference in single molecule electronics is an intriguing phe- nomenon; however, distinguishing quantum interference effects from generically low transmission is not trivial. In this paper, we discuss how quantum interference ef- fects in the transmission lead to either low current or a particular line shape in current-voltage curves, depending on the position of the interference feature. Sec- ondly, we consider how inelastic electron tunneling spectroscopy can be used to probe the presence of an interference feature by identifying vibrational modes that are se- lectively suppressed when quantum interference effects dominate. That is, we expand the understanding of propensity rules in inelastic electron tunneling spectroscopy to molecules with destructive quantum interference., Comment: 19 pages, 6 figures

We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption.

Electronic devices composed of single molecules constitute the ultimate limit in the continued downscaling of electronic components. A key challenge for single-molecule electronics is to control the temperature of these junctions. Controlling heating and cooling effects in individual vibrational modes, can in principle, be utilized to increase stability of single-molecule junctions under bias, to pump energy into particular vibrational modes to perform current-induced reactions or to increase the resolution in inelastic electron tunneling spectroscopy by controlling the life-times of phonons in a molecule by suppressing absorption and external dissipation processes. Under bias the current and the molecule exchange energy, which typically results in heating of the molecule. However, the opposite process is also possible, where energy is extracted from the molecule by the tunneling current. Designing a molecular 'heat sink' where a particular vibrational mode funnels heat out of the molecule and into the leads would be very desirable. It is even possible to imagine how the vibrational energy of the other vibrational modes could be funneled into the 'cooling mode', given the right molecular design. Previous efforts to understand heating and cooling mechanisms in single molecule junctions, have primarily been concerned with small models, where it is unclear which molecular systems they correspond to. In this paper, our focus is on suppressing heating and obtaining current-induced cooling in certain vibrational modes. Strategies for cooling vibrational modes in single-molecule junctions are presented, together with atomistic calculations based on those strategies. Cooling and reduced heating are observed for two different cooling schemes in calculations of atomistic single-molecule junctions., Comment: 18 pages, 6 figures

We have examined the effects of intermolecular interactions on the inelastic electron tunneling spectroscopy (IETS) of model systems: a pair of benzenethiol or a pair of benzenedithiol sandwiched between gold electrodes. The dependence of the IETS on the mutual position of and distance between the paired molecules has been predicted and discussed in detailed. It is shown that, although in most cases, there are clear spectral fingerprints present which allow identification of the actual structures of the molecules inside the junction. Caution must be exercised since some characteristic lines can disappear at certain symmetries. The importance of theoretical simulation is emphasized.

Inelastic electron tunneling spectroscopy (IETS) of a single hydrogen atom on the Cu(100) surface in a scanning tunneling microscopy (STM) configuration has been investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. The electron-vibration interaction is treated at the level of lowest order expansion. Our calculations show that the single peak observed in the previous STM-IETS experiments is dominated by the perpendicular mode of the adsorbed H atom, while the parallel one only makes a negligible contribution even when the STM tip is laterally displaced from the top position of the H atom. This propensity of the IETS is deeply rooted in the symmetry of the vibrational modes and the characteristics of the conduction channel of the Cu-H-Cu tunneling junction, which is mainly composed of the 4s and 4pz atomic orbitals of the Cu apex atom and the 1s orbital of the adsorbed H atom. These findings are helpful for deepening our understanding of the propensity rules for IETS and promoting IETS as a more popular spectroscopic tool for molecular devices.

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.

Single 4,7,12,15-tetrakis[2.2]paracyclophane were deposited on NiAl(110) surface at 11 K. Two adsorbed species with large and small conductivities were detected by the scanning tunneling microscope (STM). Their vibrational properties were investigated by inelastic electron tunneling spectroscopy (IETS) with the STM. Five vibrational modes were observed for the species with the larger conductivity. The spatially resolved vibrational images for the modes show striking differences, depending on the coupling of the vibrations localized on different functional groups within the molecule to the electronic states of the molecule. The vibrational modes are assigned on the basis of ab initio calculations. No IETS signal is resolved from the species with the small conductivity.

The authors have studied adsorption of acetylene on Cu(110) by means of low-temperature scanning tunneling microscopy. Adsorbed molecules preferentially aggregate at 40 K to yield dimer, trimer, and larger islands on the surface. Isolated species (monomer) adsorbs on the fourfold hollow site with approximately sp3 rehybridization as characterized by inelastic electron tunneling spectroscopy. Tunneling electron induces an acetylene molecule to migrate along the trough of Cu(110). The migration proceeds in two steps: the molecule first hops to the adjacent long-bridge site and then to the next fourfold site. The voltage and current dependencies of the hopping probability show that the migration is induced by inelastic electron tunneling that causes vibrational excitation of mainly C-H stretch mode.

Using a perturbative approach to simple model systems, we derive useful propensity rules for inelastic electron tunneling spectroscopy (IETS) of molecular wire junctions. We examine the circumstances under which this spectroscopy (that has no rigorous selection rules) obeys well defined propensity rules based on the molecular symmetry and on the topology of the molecule in the junction. Focusing on conjugated molecules of C(2h) symmetry, semiquantitative arguments suggest that the IETS is dominated by a(g) vibrations in the high energy region and by out of plane modes (a(u) and b(g)) in the low energy region. Realistic computations verify that the proposed propensity rules are strictly obeyed by medium to large-sized conjugated molecules but are subject to some exceptions when small molecules are considered. The propensity rules facilitate the use of IETS to help characterize the molecular geometry within the junction.

A scanning tunneling microscope was used to perform inelastic electron tunneling spectroscopy on single Cu(II) etioporphyrin-I molecules. The resulting vibrational spectra showed an increase in ac tunneling conductance at 359 mV. This change was attributed to the vibrational excitation of the methene bridge C–H stretch mode. Vibrational microscopy was used to image the spatial distribution of the inelastic tunneling channels involved in this excitation.

Inelastic electron tunneling spectroscopy has been applied to study the adsorption of a series of phenols on plasma‐grown aluminium oxide. Vapor phase doping was used and spectra recorded for o‐, m‐, p‐chlorophenol, p‐fluorophenol, p‐bromophenol, o‐, m‐, p‐cresol, and phenol. The high resolution spectra obtained at 2 K allow most of the possible modes to be identifield. The relative strengths of these modes are compared in terms of the proposed molecular orientation of the absorbates in an effort to observe the predicted directional sensitivity of the tunneling process. No systematic line intensity variation is evident, and it is postulated that for large molecules this may be because of the complexity of the motions involved in the vibrational modes. It is also suggested that the contouring of the lead electrode round the adsorbate has the important effect of obscuring the directional sensitivity of the tunneling process. An attempt has also been made further to characterize the chemical nature of the oxide and its surface.

Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH or OD groups present on alumina. These results support the hypothesis that the alkoxy groups exchange with OH groups on the surface to form siloxane bonds, and that these bonds are stabilized with trialkoxysilanes. For monoalkoxysilanes, either the bonds do not form so readily, or they are not stabilized by further condensation of the adsorbed species. Instead, the monoalkoxysilanes appear to catalyze further oxidation of the alumina surface.Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH or OD groups present on alumina. These results support the hypothesis that the alkoxy groups exchange with OH groups on the surface to form siloxane bonds, and that these bonds are stabilized with trialkoxysilanes. For monoalkoxysilanes, either the bonds do not form so readily, or they are not stabilized by further condensation of the adsorbed species. Instead, the monoalkoxysilanes appear to catalyze further oxidation of the alumina surface.

Inelastic electron tunneling spectroscopy (IETS) has been used to determine the molecular species which occur on an aluminum oxide surface during metallic corrosion by uninhibited trichloroethylene. IETS spectra were obtained for trichloroethylene adsorbed on aluminum oxide and the observed vibration modes were assigned by comparison with infrared and Raman frequencies. Modes were observed which could be associated with AlO–CH2–CH3, or a similar molecular species, and with Al–Cl stretch vibrations. A reaction sequence consistent with the results has been developed. According to this model the corrosive attack is by chlorine ions formed by the reaction of trichloroethylene at AlOH (or AlO) surface sites. The chlorine can initiate corrosive attack by reaction with exposed aluminum atoms at oxygen vacancy sites.

Inelastic electron tunneling spectroscopy (IETS) has been employed to study the vibrational structure of ethanol reacting with a clean alumina surface as a function of temperature from 295 to 575 K. The spectra for the surface species up to approximately 450 K correspond to infrared spectra of aluminum ethoxide, Al(OC2H5)3. Between 450 and 475 K the structure changes from an ethoxide to an acetate, the most obvious spectral change being the appearance of a new peak at approximately 1585 cm−1 (197 meV) which is characteristic of the asymmetric OCO stretching mode of various inorganic acetates. No further changes are observed above 475 K. Above 370 K, there is also a significant increase in the total concentration of adsorbed species. This is due to a partial dehydration or dehydroxylation of the surface, thus freeing active sites which had previously been blocked by water or hydroxyl groups formed during the initial stages of ethanol adsorption. This dehydration or dehydroxylation of the surface is simulta...

We use inelastic electron tunneling spectroscopy to study catalytically induced hydrogenation of several unsaturated dicarboxylates chemisorbed on undried, hydrated alumina. Using isotope substitution we estabilish that hydroxyl groups on the alumina are the source of hydrogen. Fumarate and maleate hydrogenate completely at room temperature forming a succinate species. Gutaconate, hydromuconate, and muconate hydrogenate in a way that is exponentially temperature dependent, only slightly pressure dependent, and time independent in the range of a few minutes to an hour. We have investigated the temperature dependence over the range 25–400 °C, and we believe it is related to the thermal dehydration mechanism in alumina. From the variation in temperature dependence among the surface species we develop an index of the relative ease of hydrogention that correlates well with double bond distance from the surface and with bond resonance stabilization. Species whose double bonds are constrained to be closer to the...

Raman spectra have been obtained from molecular monolayers of nitrobenzoic acid and aminobenzoic acid on AlOx and in Ag and Sn tunnel junctions. The large electromagnetic field enhancement necessary for surface enhanced Raman scattering experiments from roughened noble metal surfaces is replaced by the enhanced sensitivity of a cooled, optical multichannel detector. The behavior of the Raman intensity for a molecule adsorbed on a nonresonant metallic substrate has been studied and compared with theory. The evolution of the vibrational spectrum of the adsorbate during the fabrication of an inelastic electron tunneling spectroscopy junction has been observed and the dominant spectral changes assigned to the initial adsorption of the molecule on the oxide surface.

Thin films of silicon monoxide have been deposited on alumina surfaces and examined by inelastic electron tunneling spectroscopy. A strong absorption peak at 2210 cm−1 reveals the formation of SiH bonds in the presence of trace amounts of water. It is inferred that a reaction between SiO and –OH groups to yield SiH bonds is catalyzed by the alumina surface.

The adsorption on aluminum oxide of acetaldehyde (CH3CHO) and acetic acid (CH3COOH) has been studied by utilizing inelastic electron tunneling spectroscopy (IETS) to probe the vibrational structures of the surface species over the spectral range between 240 and 4000 cm−1. Results for temperatures between 295 and 475 K are compared with our previous IETS results for ethanol (CH3CH2OH) adsorption on alumina up to 575 K. Although all three hydrocarbons form acetate complexes with the surface under certain conditions, slight differences in bonding and structure are evident in the IET spectra. The formation of some bulk aluminum hydroxide is observed also in certain cases. Observed variations in surface concentrations, bulk hydration, and the nature of surface hydroxyl groups are related to previous experimental results. Mechanisms are substantiated for the initial adsorption and subsequent reaction of each hydrocarbon with the surface.