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Inelastic Electron Tunneling Spectroscopy with the Scanning Tunneling Microscope (STM-IETS) is a powerful technique used to characterize vibration and spin at the single molecule level. While IETS lacks hard selection rules, historically it has been assumed that vibrational overtones are rarely seen or even absent. Here we provide direct experimental evidence that the hindered rotation overtone excitation of CO molecules on Ag(110) can be detected with STM-IETS by isotope substitution. We also show that the anharmonicity of the overtone excitation can be characterized and compared between unique adsorption sites, and find evidence of anisotropy in the vibrational anharmonicity for CO adsorbed on the [1 1 0] step edge.

Graphene phonons are excited by the local injection of electrons and holes from the tip of a scanning tunneling microscope. Despite the strong graphene-Ru(0001) hybridization, monolayer graphene unexpectedly exhibits pronounced phonon signatures in inelastic electron tunneling spectroscopy. Spatially resolved spectroscopy reveals that the strength of the phonon signal depends on the site of the moiré lattice with a substantial red-shift of phonon energies compared to those of free graphene. Bilayer graphene gives rise to more pronounced spectral signatures of vibrational quanta with energies nearly matching the free graphene phonon energies. Spectroscopy data of bilayer graphene indicate moreover the presence of a Dirac cone plasmon excitation.

To overcome the numerous challenges for the miniaturization of complementary metal–oxide–semiconductor (CMOS) technology, diverse materials have been examined using various characterization methods. Inelastic electron tunneling spectroscopy (IETS) has been regarded as a critical measurement technique to scrutinize materials. This is because the high sensitivity of IETS can provide important information regarding the material structures and molecular compositions (bonding structures, impurities, defects, and traps) in metal–oxide–semiconductor (MOS) structures, as well as the molecular conformation/isomerization, and contact geometry in molecular devices. This review introduces the background of IETS and recent applications of IETS on thin gate dielectrics, large DNA molecules, functional molecules, and diatomic molecules.

This thesis focuses on possible platforms for a bottom-up approach towards realizing and characterizing atomically assembled magnetic and electronic artificial lattices. For this, we make use of the scanning tunneling microscope (STM), which provides a local probe of the magnetic and electronic properties of the sample and allows for the atom-by-atom construction of extended lattices. On the one hand, to address avenues for constructing extended spin lattices, we study single Fe atoms coordinated on the four-fold symmetric nitrogen binding site of the Cu2N/Cu3Au surface—a system which permits large-scale atomic assembly, and allows for independent access to both the orbital and spin degrees of freedom. On the other hand, we investigate the viability of laterally confined vacuum resonances on the chlorinated Cu(100) surface as a basis for constructing electronic lattices. We atomically assemble dimers and trimers of various geometries to determine the tight-binding parameters, and as a proof of concept, experimentally realize a looped Su-Schrieffer–Heeger chain using this platform. These studies are made possible by means of a low-temperature, ultra-high vacuum STM, which allows for atom manipulation and, via spectroscopic techniques, permits us to locally probe the sample density of states and detect inelastic excitations of the spin and orbital angular momentum.

Compared to intramolecular interactions, intermolecular interactions are relatively weak but they lay the foundation for research involving molecular recognition, self-assembly and surface adsorption in chemical and physical systems. This dissertation describes three different experimental approaches based on the detection of molecular vibrations to provide direct insights into intermolecular interactions at the sub-Ångström spatial resolution with a home built sub-Kelvin scanning tunneling microscope (STM).First, the intermolecular interaction can be evaluated by measuring the coupled vibrational mode of two interacting molecules with STM inelastic electron tunneling spectroscopy (IETS). The measurement of intermolecular coupled vibrations with tunable vertical and lateral displacements offers a direct assessment of the short range intermolecular repulsion in three dimensions.Second, the self-assembled molecular bonding structures can be imaged by the inelastic tunneling probe (itProbe). In itProbe, a carbon monoxide (CO) molecule is transferred onto the STM tip. The hindered translation energy of the CO-tip varies when it is positioned over different locations of the self-assembled structures. By recording the intensity variations of the inelastic electron tunneling signal caused by the energy shift, the geometric structure of each molecule and intermolecular interactions can be imaged in real space.Third, a sample surface can be considered as a giant molecule with infinite mass, and the binding strength of an adsorbed molecule on the surface is reflected in the external vibrations. The combination of the itProbe and IETS enable the determination of the adsorption configuration and low energy external vibrational modes of individual physisorbed benzene on an inert metal for the first time.All the approaches mentioned above rely on resolving fine spectral features induced by intermolecular interactions. The unprecedented spectral sensitivity, energy resolution and thermal stability achieved in 600 mK create the opportunity to study these weak intermolecular interaction effects at the single molecule level, which are otherwise obscured at higher temperatures.

The power of rotational spectroscopy has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space has been limited. Using a scanning tunneling microscope (STM) and inelastic electron tunneling spectroscopy (IETS), we were able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on surfaces at 10 K. The j=0 to j=2 rotational transition for para-H2 and HD were observed by STM-IETS. It is also found that the rotational energy is very sensitive to its local environment, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Due to this high sensitivity, the spatial variation in the potential energy surface can be quantified by the rotational and vibrational energies of the trapped H2. The ability of the tip to drag along a hydrogen molecule as it scans over another adsorbed molecule combined with the sensitivity of the hydrogen rotational excitation recorded by IETS to its immediate environment lead to the implementation of rotational spectromicroscopy. Hydrogen rotational spectroscopy and microscopy provides novels approach toward visualizing and quantifying the intermolecular interaction as well as the intermediate processes of chemical reactions.

International audience; The vibrational excitation related transport properties of a manganese phthalocyanine molecule suspended between the tip of a scanning tunneling microsope (STM) and a surface are investigated by combining the local manipulation capabilities of the STM with inelastic electron tunneling spectroscopy. By attachment of the molecule to the probe tip, the intrinsic physical properties similar to those exhibited by a free standing molecule become accessible. This technique allows one to study locally the magnetic properties, as well as other elementary excitations and their mutual interaction. In particular a clear correlation is observed between the Kondo resonance and the vibrations with a strong incidence of the Kondo correlation on the thermopower measured across the single-molecule junction

Single-molecule magnets (SMMs) incorporate key properties that make them promising candidates for the emerging field of spintronics. The challenge to realize ordered SMM arrangements on surfaces and at the same time to preserve the magnetic properties upon interaction with the environment is a crucial point on the way to applications. Here we employ inelastic electron tunneling spectroscopy (IETS) to address the magnetic properties in single Fe4 complexes that are adsorbed in a highly ordered arrangement on graphene/Ir(111). We are able to substantially reduce the influence of both the tunneling tip and the adsorption environment on the Fe4 complex during the measurements by using appropriate tunneling parameters in combination with the flat-lying Fe4H derivative and a weakly interacting surface. This allows us to perform noninvasive IETS studies on these bulky molecules. From the measurements we identify intermultiplet spin transitions and determine the intramolecular magnetic exchange interaction constant on a large number of molecules. Although a considerable scattering of the exchange constant values is observed, the distribution maximum is located at a value that coincides with that of the bulk. Our findings confirm a retained molecular magnetism of the Fe4H complex at the local scale and evaluate the influence of the environment on the magnetic exchange interaction.

We use inelastic electron tunneling spectroscopy (IETS) first-principles simulations to identify and characterize the different vibrational modes of single conjugated molecules bonded to Au metal electrodes. The molecules are polyphenyls (with 1 to 4 benzene units) bonded to Au via highly conducting direct Au-C bonds. The short molecule shows near resonant elastic transmission, with a crossover to tunneling for the longer backbones. The calculated inelastic spectra exhibit dips in the IETS signal of the short molecule and peaks for the longer molecules. We characterize the symmetry of vibrational modes and scattering states and discuss the changes with increasing length, where the inelastic signal of different modes can be amplified, quenched or present a crossover as more benzene units are added to the molecular backbone. This analysis rationalizes the observed trends as a function of molecular length and illustrates the role of electronic and vibrational properties on Au-C bonded molecular junctions.

Water splitting is an essential process for converting light energy into easily storable energy in the form of hydrogen. As environmentally preferable catalysts, Cu-based materials have attracted attention as water-splitting catalysts. To enhance the efficiency of water splitting, a reaction process should be developed. Single-molecule junctions (SMJs) are attractive structures for developing these reactions because the molecule electronic state is significantly modulated, and characteristic electromagnetic effects can be expected. Here, water splitting is induced at Cu-based SMJ and the produced hydrogen is characterized at a single-molecule scale by employing electron transport measurements. After visible light irradiation, the conductance states originate from Cu/hydrogen molecule/Cu junctions, while before irradiation, only Cu/water molecule/Cu junctions were observed. The vibration spectra obtained from inelastic electron tunneling spectroscopy combined with the first-principles calculations reveal that the water molecule trapped between the Cu electrodes is decomposed and that hydrogen is produced. Time-dependent and wavelength-dependent measurements show that localized-surface plasmon decomposes the water molecule in the vicinity of the junction. These findings indicate the potential ability of Cu-based materials for photocatalysis.

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.).

Ion conducting materials are critical components of batteries, fuel cells, and devices such as memristive switches. Analytical tools are therefore sought that allow the behavior of ions in solids to be monitored and analyzed with high spatial resolution and in real time. In principle, inelastic tunneling spectroscopy offers these capabilities. However, as its spectral resolution is limited by thermal softening of the Fermi-Dirac distribution, tunneling spectroscopy is usually constrained to cryogenic temperatures. This constraint would seem to render tunneling spectroscopy useless for studying ions in motion. We report here the first inelastic tunneling spectroscopy studies above room temperature. For these measurements, we have developed high-temperature-stable tunnel junctions that incorporate within the tunnel barrier ultrathin layers for efficient proton conduction. By analyzing the vibrational modes of O-H bonds in BaZrO3-based heterostructures, we demonstrate the detection of protons with a spectral resolution of 20 meV at 400 K (FWHM). Overturning the hitherto existing prediction for the spectral resolution limit of 186 meV (5.4 kBT at 400 K), this resolution enables high-temperature tunneling spectroscopy of ion conductors. With these advances, inelastic tunneling spectroscopy constitutes a novel, valuable analytical tool for solid-state ionics., 5 figures in main text, 7 figures in supporting info

We present the first experimental determination of the intra-atomic exchange energy between the inner 4f and the outer 6s5d shells in rare-earth elements. Inelastic electron tunneling spectroscopy on individual rare-earth atoms adsorbed on metal-supported graphene reveals an element-dependent excitation, with energy between 30 and 170 meV, linearly increasing with the spin angular momentum of the 4f shell. This observation is possible owing to the strong spin polarization of the outer shells, characteristic of rare-earth adatoms on graphene. This polarization gives rise to a giant magnetoresistance of up to 75% observed for Dy on graphene/Ir(111) single-atom magnets. Density functional theory calculations of the 6s5d shell spin polarizations and of their intra-atomic exchange constants with the 4f shell yield exchange energies in agreement with the experimental values. These results prove that the description of the spin dynamics in RE considering only the 4f - 5d interaction is oversimplified. A more realistic treatment requires us to consider a multishell intra-atomic exchange in which both 6s and 5d shells are taken into account, with the 4f - 6s contribution possibly prevailing over the 4f - 5d one. Our findings are important for the general understanding of magnetism in rare earths, whether they are in bulk compounds or as surface adsorbed atoms and clusters. The results presented here also push for a revision of the description of the spin dynamics in rare-earth-based systems.

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.

The electrical control and readout of molecular spin states are key for high-density storage. Expectations are that electrically-driven spin and vibrational excitations in a molecule should give rise to new conductance features in the presence of magnetic anisotropy, offering alternative routes to study and, ultimately, manipulate molecular magnetism. Here, we use inelastic electron tunneling spectroscopy to promote and detect the excited spin states of a prototypical molecule with magnetic anisotropy. We demonstrate the existence of a vibron-assisted spin excitation that can exceed in energy and in amplitude a simple excitation among spin states. This excitation, which can be quenched by structural changes in the magnetic molecule, is explained using first-principles calculations that include dynamical electronic correlations., This work was supported by the Agence Nationale de la Recherche (grants No. ANR-13-BS10-0016, ANR-11-LABX-0058 NIE and ANR-10-LABX-0026 CSC) and by the Agencia Española de Investigación (grants Nos. MAT2016-78293-C6-1-R and MDM-2016-0618). D.-J.C. and N.L. thank the MICINN (project RTI2018-097895-B-C44). M.-L.B. thanks the national computational center CINES and TGCC (GENCI project: A0030807364).

Inelastic Electron Tunneling Spectroscopy (IETS) is a powerful tool to study the properties of molecular junctions. In particular, it is considered useful for extracting information on electron transport pathways. We explore the limits of this approach by comparing computed interatomic transmission pathways with IETS intensities for different molecular junctions, employing a new efficient implementation for evaluating IETS intensities via the mode-tracking algorithm. We find that while a correlation be- tween pathways and IETS intensities indeed holds when vibrations are clearly localized on atoms off the transport pathway, there is no such correlation for molecules with less localized vibrations, even if transport pathways only sample part of the molecule, and even if a statistical analysis over the vibrational modes is made. This could indicate that the significance of IETS signals for transport pathways is limited to molecules with localized vibrational modes.

Triangular zigzag nanographenes, such as triangulene and its π‐extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high‐spin networks with long‐range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on‐surface synthesis and a proof‐of‐principle experimental study of magnetism in covalently bonded triangulene dimers. On‐surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4‐phenylene spacer. The chemical structures of the dimers have been characterized by bond‐resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling., The on‐surface synthesis of covalently bonded triangulene dimers with or without a 1,4‐phenylene spacer was achieved on Au(111). Scanning tunneling spectroscopy measurements revealed collective magnetism in the dimers in the form of singlet–triplet spin excitations, demonstrating efficient and tunable intertriangulene magnetic coupling.

Two low-energy excitations of a single water molecule are observed via inelastic electron tunneling spectroscopy, where a significant enhancement is achieved by attaching the molecule to the tip apex in a scanning tunneling microscope. Density functional theory simulations and quantum mechanical calculations of an asymmetric top are carried out to reveal the origin of both excitations. Variations in tunneling junction separation give rise to the quantum confinement effect on the quantum state of a water molecule in the tunneling junction. Our results demonstrate a potential method for measuring the dynamic behavior of a single molecule confined in a tunneling junction, where the molecule–substrate interaction can be purposely tuned.

Inelastic electron tunneling spectroscopy with a scanning tunneling microscope is a powerful method to excite and detect vibrational quanta with atomic resolution. The focus of this review article is on the local spectroscopy of graphene phonons. The experimental observation of their spectroscopic signatures together with theoretical modeling highlight the importance of the graphene–surface as well as the graphene–tip hybridization, the electron– phonon coupling strength, phonon-mediated tunneling, local doping profiles, and the phonon density of state for the measured signal. Meanwhile, a comprehensive understanding of the underlying mechanisms has been attained and justifies an overview on available findings.

Herein, we discuss the STM-IETS (scanning tunneling microscopy-inelastic electron tunneling spectroscopy) technique in the following order. After briefly mentioning conventional IETS (inelastic electron tunneling spectroscopy), STM-IETS experimental results are introduced focusing on similarities and differences to conventional IETS. The working principle behind STM-IETS for the detection of vibrational modes is considered by reviewing recent progress in STM-IETS theoretical calculations. In addition, experimental setups to improve the quality of the IET (inelastic electron tunneling) spectrum, including low-temperature measurements, electronics (especially the use of the lock-in amplifier), and IET signal mapping are reviewed. With this information, we discuss in detail STM-IETS measurements performed on an alkanethiol self-assembled monolayer (SAM) formed on an Au(111) surface. This molecule is often employed as a standard sample for the examination of IETS observations with atom-scale electrodes, which are used in single-molecule electronics investigations. STM-IETS reveals not only C − H stretching mode, which often appears as a prominent feature in IET spectra, but also other vibrational features in the so-called fingerprint region including vibrational modes which are beneficial for distinguishing functional groups. A comparison with recent calculations shows excellent agreement. In addition, partial deuteration of the molecule can provide more information about the site of the molecule where the excitation of the vibrational mode occurs. A selection rule or propensity for IETS detection is then discussed on the basis of a combination of these experimental investigations and theoretical simulations.

The adsorption behavior of single CO molecules at 4 K bound to Au adatoms on a Ag(001) metal surface is studied with scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS). In contrast to earlier observations two different binding configurations are observed—one on top of a Au adatom and the other one adsorbed laterally to Au on Ag(001). Moreover, IETS reveals different low-energy vibrational energies for the two binding sites as compared to the one for a single CO molecule bound to Ag(001). Density functional theory (DFT) calculations of the adsorption energies, the diffusion barriers, and the vibrational frequencies of the CO molecule on the different binding sites rationalize the experimental findings.