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Triangulene nanographenes are open-shell molecules with predicted high spin state due to the frustration of their conjugated network. Their long-sought synthesis became recently possible over a metal surface. Here, we present a macrocycle formed by six [3]triangulenes, which was obtained by combining the solution synthesis of a dimethylphenyl-anthracene cyclic hexamer and the on-surface cyclodehydrogenation of this precursor over a gold substrate. The resulting triangulene nanostar exhibits a collective spin state generated by the interaction of its 12 unpaired π-electrons along the conjugated lattice, corresponding to the antiferromagnetic ordering of six S=1 sites (one per triangulene unit). Inelastic electron tunneling spectroscopy resolved three spin excitations connecting the singlet ground state with triplet states. The nanostar behaves close to predictions from the Heisenberg model of an S=1 spin ring, representing a unique system to test collective spin modes in cyclic systems., We acknowledge funding from Agencia Estatal de Investigación (PID2019-107338RB, FIS2017-83780-P, MDM-2016-0618), from the Xunta de Galicia (Centro singular de investigación de Galicia, accreditation 2019–2022, ED431G 2019/03), the Basque Government (Grant IT1255-19), the European Union H2020 program (FET Open project SPRING #863098), the European Regional Development Fund and the Basque Departamento de Educación for the PhD scholarship no. PRE_2020_2_0049 (S.S.).

A broad range of materials is currently being studied for possible use as the insulating layer in next generation metal-oxide-semiconductor transistors. Inelastic electron tunneling spectroscopy (IETS) has become a powerful tool to characterize both the structural and electrical properties of the resulting device structures made from these materials. IETS can address issues related to reactions and intermixing at interfaces, as well as properties related to carrier mobility, such as phonon modes and charge traps, for structures that are difficult to characterize accurately by other techniques.

Recent experiments, by Kirtley and Hansma, in inelastic tunneling spectroscopy (IETS) of the vibrational excitation of molecular impurities trapped in the barrier of a metal-insulator-metal diode exhibited a marked asymmetry with respect to reversal of bias. It is shown that the standard, linear response formulation of the transfer Hamiltonian theory of inelastic tunneling cannot accommodate such an asymmetry. The alternatives to the ad-hoc “golden-rule” type theory proposed by Kirtley, Scalapino, and Hansma of inelastic tunneling are critically reviewed. A new theory based on Feuchtwang's general many-body tunneling theory is developed. It is shown that this theory exhibits all the qualitative features of the golden-rule type analysis. The theory suggests that the differential tunneling conductivity exhibits a more complex structure than predicted by the golden rule. This contrasts with the analysis of Caroli et al., which formally agrees with the results of the golden-rule type theory. It is suggested that the golden-rule type theory of inelastic tunneling may possibly fail to provide a satisfactory phenomenological description of IETS.

The simplest component of molecular electronics consists of a single-molecule transport junction: a molecule sandwiched between source and drain electrodes, with or without a third gate electrode. In this Concept article, we focus on how molecules control transport in metal-electrode molecular junctions, and where the molecular signatures are to be found. In the situation where the molecule is relatively short and the gap between injection energy and molecular eigenstates is large, transport occurs largely by elastic tunneling, stochastic switching is common, and the vibronic signature can be found using inelastic electron tunneling spectroscopy (IETS). As the energy gaps for injection become smaller, one begins to see stronger molecular signatures - these include Franck-Condon-like structures in the current/voltage characteristic and strong vibronic interactions, which can lead to hopping behavior at the polaron limit. Conformational changes induced by the strong electric field lead to another strong manifestation of the molecular nature of the junction. We overview some of this mechanistic landscape, focusing on significant effects of switching (both stochastic and controlled by the electric field) and of molecular vibronic coupling.

Inelastic electron tunneling spectroscopy is studied for Pb/InSe/Pb tunnel junctions at liquid helium temperature in the normal state of Pb under an external magnetic field. The insulating barrier of InSe, a layered semiconducting compound, is cleaved from a single crystal along the layer. Inelastic electron tunneling spectra for the junctions show phonon structures of indium selenide as well as those of the lead electrodes. Die inelastische Elektronentunnelspektroskopie wird auf Pb/InSe/Pb-Tunnelubergange bei der Tem- peratur des flussigen Heliums fur den normalleitenden Zustand von Pb angewandt. InSe, ein Halbleiter mit Schichtstruktur, wird der Schicht entlang gespalten, und eine InSe-dielektrische Barriere wird hergestellt. Die inelastischen Elektronentunnelspektren fur die Ubergange zeigen die Phonen- strukturen des InSe.

Inelastic electron tunneling spectroscopy is used to compare the adsorption onto aluminium oxide of poly(ethylene glycol) of relative molar mass 400 (PEG 400 ) with poly(ethylene glycol) that has been coordinated with zinc chloride. This work carried out on a low-molecular-weight analogue of a polymer electrolyte reveals that the steric hinderance brought about by the coordination of zinc chloride with poly(ethylene glycol) changes the mechanism by which the polymer bonds to an aluminium oxide surface. The zinc bonds both to atmospheric water and to the ether oxygens in the polymer, which restricts the conformational freedom of the polymer. This restriction causes the attachment of the polymer to the aluminium oxide substrate to be primarily by hydrogen bonding, which reveals itself through the appearance of an extra peak in the inelastic electron tunneling spectrum at ∼440 meV. The implication is that when monolayer coatings of PEG 400 /ZnCl 2 are adsorbed on aluminium oxide, the coordination of the zinc with the polymer imposes a structure upon the latter.

Vibrational spectra of thin (0.2-1.0 nm) films of evaporated germanium on alumina surfaces have been measured by inelastic electron tunneling spectroscopy. The tunneling spectra of the evaporated Ge films were compared with those of the Si films. The tunneling spectra were also compared with the vibrational spectra of surface species on crystalline Ge formed from reactions with atomic hydrogen, water, and oxygen measured by high-resolution electron energy loss spectroscopy and multiple internal reflection infrared spectroscopy, and they showed the formation of germanium hydride species. Monohydride is predominantly formed in films prepared both in high vacuum (10 -6 Torr) and in an atmosphere of H 2 O (10 -5 Torr)

The basic theoretical and experimental concepts required for an understanding of inelastic electron tunneling spectroscopy (IETS) are presented. While most of the applications of IETS to date have centered on surface chemical analysis, the thrust of the present review is to present IETS as an alternative molecular spectroscopy. Comparison of IETS, IR, and Raman data obtained in the vibrational region of the spectrum and of IETS and absorption and reflectance data taken in the electronic region of the spectrum will be made. The difference in selection rules in IETS and in optical spectroscopy is emphasized. Numerous examples of optically forbidden transitions observed as strong bands in IETS are presented

The sections in this article are Introduction Vibrational Spectroscopic Techniques for the Study of Adsorbed Species on Single-Crystal Metal Surfaces Vibrational Energy Loss Spectroscopy Reflection–Absorption Infrared Spectroscopy Infrared–Visible Sum Frequency Generation Spectroscopy Inelastic Electron Tunneling Spectroscopy Raman Spectroscopy Interpretation of Vibrational Spectra of Adsorbed Species on Single-Crystal Metal Surfaces Symmetry Considerations Site Symmetries on Single-Crystal Metal Surfaces Symmetry and Vibrational Properties of the Overall Surface Species Absorption Mechanisms and Selection Rules Dipolar Mechanism Impact Mechanism Polarizability Mechanism Resonance Mechanism Group Frequency and Intensity Analysis of Model C2-Hydrocarbon Systems A Group Frequency/Intensity Correlation Chart for C2-Hydrocarbon Ligands in Metal Complexes and Clusters An Exploration of the CC Internuclear Distance/CC Bond-Stretching Wavenumber (dCC/νCC) Relationship Worked Example π-C2H4 Complexes Di-σ-C2H4 and di-σ*-C2H4 Complexes Ethylidyne Species Ethyl Surface Group Conclusion Acknowledgments

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.

Aluminum nitride films ranging in thickness from 0.4 to 25 nm were prepared by reactive ion-beam sputter deposition, utilizing a pure aluminum metal target and both pure nitrogen and 25% hydrogen-enriched nitrogen plasma. The 0.4-1.5-nm AlN films were studied by inelastic electron tunneling spectroscopy. Complementary vibrational techniques, specular reflectance and absorbance FT-IR spectroscopy, were used to analyze the thicker insulating films. The principal component of the insulating layers is AlN. Al-H, Al-N{sub 2}, and NH{sub 2} surface species are also present as minor components. Tunneling spectroscopy showed superior sensitivity over the optical methods in identifying various components of the AlN films. The quality of the tunneling barrier is enhanced by the presence of H{sub 2} during the AlN deposition.

Inelastic electron tunneling spectroscopy (IETS) has been used to study two classes of adhesion promoting compounds adsorbed on alumina: trialkoxysilanes and phosphonic acids. It has been shown that both types of compounds can condense directly with hydroxyl groups on the surface. Condensation is through the alkoxy groups in the case of the silanes and through the hydroxyl groups in the case of the acids. The silanes can further condense to form surface siloxanes. The absence of bands characteristic of the phosphoryl group in the spectra of the acids is indicative of resonance bond formation followed by ionic bonding to the surface.

The adsorption mechanism of aminophenyltrimethoxysilane adsorbed from the vapor on plasma grown aluminum oxide and effects of high humidity on the adsorbed silane were studied by inelastic electron tunneling spectroscopy (IETS). The IETS spectrum of aminophenyltrimethoxysilane contains the bands due to phenyl rings, methyl groups and amino groups. The NH2 deformation mode is shifted to lower frequencies, which may imply that the silane is chemisorbed through the amino group. By exposing the sample to water vapor, the methoxy group hydrolizes and the resultant methanol is removed from the varrier region indicated by a significant loss of intensity of the CH3 stretching and deformation modes. A loss of intensity of the NH2 deformation mode was also observed after the exposure.

Inelastic electron tunneling spectroscopy (IETS) at liquid He temperature is used to study the vibrational spectrum of the polymethine dye NH-rhodanine merocyanine (NH-RMc). The dye interlayers are prepared by in situ sublimation of the molecules onto the Al2O3 layer of (AlAl2O3Pb) tunneling sandwiches. An interpretation of the most prominent vibration bands is given. By comparison with infrared data it is concluded that the dye molecules are arranged in the film with their polymethine chain parallel to the substrate. Mit Hilfe der inelastischen Elektronen-Tunnelspektroskopie (IETS) wird das Schwingungsspektrum des Polymethin-Farbstoffes NH-Rhodanin-Merocyanin (NH-RMc) bei der Temperatur des flussigen Heliums gemessen. Durch in-situ Sublimation liegt der Farbstoff als Zwischenschicht auf der Al2O3-Barriere in einer (AlAl2O3Pb)-Tunnelanordnung vor. Die wichtigsten Schwingungsbanden werden interpretiert. Durch Vergleich mit dem Infrarotspektrum folgt, das die Farbstoffmoletule in der Schicht mit ihrer Polymethinkette parallel zum Substrat angeordnet sind.

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.

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.