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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

Inelastic electron tunneling spectroscopy (IETS) [R. C. Jaklevic and J. Lambe, Phys. Rev. Lett. 17, 1139 (1966); R. G. Keil et al., Appl. Spectrosc. 30, 1 (1976); K. W. Hipps and U. Mazur, J. Phys. Chem. 97, 7803 (1993); U. Mazur et al., Anal. Chem. 64, 1845 (1992); P. K. Hansma, Tunneling Spectroscopy (Plenum, New York, 1982)] measurements are performed on Si nanowire (NW)/SiO2/Al NW tunnel junctions. The tunnel junction area is 50 120 nm and tunneling occurs across a 10 nm thick SiO2 layer. IETS measurements are performed at 300 K for ammonium hydroxide (NH4OH), acetic acid (CH3COOH), and propionic acid (C3H6O2) molecules. The I–V, dI/dV–V, and d2 I/dV2 –V characteristics of the tunnel junction are measured before and after the adsorption of molecules on the junction using vapor treatment or immersion. Peaks can be observed in the d2 I/dV2 –V characteristics in all the cases following molecules adsorption. These peaks may be attributed to vibrational modes of N–H and C–H bonds.

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complimented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending the model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of the CO frustrated translation in response to the change of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM and IETS-STM images can deliver information about intramolecular charge transfer for molecules deposited on a~surface. To demonstrate this, we extended our numerical model by taking into the account the electrostatic force acting between the decorated tip and surface Hartree potential., 5 pages, 4 figures

Using the nonequilibrium Green’s function (NEGF) approach, we develop a microscopic ac transport theory in the presence of electron-phonon interaction. Taking into account the self-consistent Coulomb interaction, the displacement current is included. This ensures that our theory satisfies the current-conserving and gauge-invariant conditions. Importantly, the inclusion of self-consistent Coulomb interaction naturally connects the NEGF formalism to the density functional theory (DFT). This allows us to calculate the self-consistent Hamiltonian using DFT within the NEGF framework, which paves the way for first principles ac transport calculation of nanoelectronic devices in the presence of electron-phonon interaction. It is known that the inelastic electron tunneling spectroscopy (IETS) is a powerful tool in studying the inelastic dc quantum transport in molecular devices. The basic idea of IETS is to obtain the information of vibrational spectrum of molecular devices by measuring the second derivative of the dc current with respect to the bias voltage. In the ac transport, we find that the phonon spectrum and electron-phonon coupling strength can be obtained from the second derivative of the admittance with respect to the frequency which is the working principle of the inelastic electron admittance spectroscopy (IEAS). Hence we propose to use IEAS to probe the effect of the phonon in ac transport. As an example, dynamic conductance of a quantum dot is discussed in detail and the concept of IEAS is demonstrated.

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface.

The magnetic properties of molecular structures can be tailored by chemical synthesis or bottom-up assembly at the atomic scale. We used scanning tunneling microscopy to study charge and spin transfer in individual complexes of transition metals with the charge acceptor, tetracyanoethylene (TCNE). The complexes were formed on a thin insulator, Cu2N on Cu(100), by manipulation of individual atoms and molecules. The Cu2N layer decouples the complexes from Cu electron density, enabling direct imaging of the TCNE molecular orbitals as well as spin-flip inelastic electron tunneling spectroscopy. Results were obtained at low temperature down to 1 K and in magnetic fields up to 7 T in order to resolve splitting of spin states in the complexes. We also performed spin-polarized density functional theory calculations to compare with the experimental data. Our results indicate that charge transfer to TCNE leads to a change in spin magnitude, Kondo resonance, and magnetic anisotropy for the metal atoms.

We use inelastic electron tunneling spectroscopy first-principles simulations to identify the different chemical bonds present at metal-molecule junctions. We unambiguously identify the nature of these bonds from two distinctive features in the calculated spectra: (i) the presence (or absence) of active vibrational modes and (ii) the dependence of vibrational frequencies on electrode separation. We use this method to present a study of the vibrational properties of alkanes bound to the electrodes via highly conducting Au-C links. In the experiment, these links were formed from molecules synthesized with trimethyl-tin (SnMe3) terminations, where the SnMe3 groups were removed in situ at the junction, in a process involving both breaking and formation of bonds [Cheng, Z.-L.; Skouta, R.; Vázquez, H.; Widawsky, J. R.; Schneebeli, S.; Chen, W.; Hybertsen, M. S.; Breslow, R.; Venkataraman, L. Nat. Nanotechnol. 2011, 6, 353-357]. We obtain the vibrational fingerprint of these direct Au-alkane 0links and extend this study to the other scenario considered in that paper (bonding via SnMe2 groups), which may be relevant under other experimental conditions. We also explore the effect of deuteration on inelastic electron tunneling spectroscopy (IETS). Complete deuteration of the molecules diminishes the differences of the spectra corresponding to the two bonding geometries, making identification more difficult. IETS of an isolated SnMe3 fragment provides an additional basis for comparison in the characterization of the molecular junction., H.V. acknowledges financial support from the Academy of Sciences of the Czech Republic through the J.E. Purkyně fellowship. G.F., D.S.-P., A.A., and T.F. acknowledge the support of the Basque Departamento de Educacion and the UPV/EHU (Grant No. IT-756-13) and the Spanish Ministerio de Economia y Competitividad (Grant No. FIS2013-48286-C2-2-P and Grant No. MAT2013-46593-C6-2-P). D.S.-P. and T.F. acknowledge the European Union FP7-ICT project PAMS (Contract No. 610446).

This thesis describes the experimental study of electron-boson interactions in superconductors by means of inelastic electron tunneling spectroscopy performed with a scanning tunneling microscope (STM) at temperatures below 1 K. This new approach allows the direct measurement of the Eliashberg function of conventional superconductors as demonstrated on lead (Pb) and niobium (Nb). Preparative experiments on unconventional iron-pnictides are presented in the end.

We investigate the origin of purported "spin accumulation" signals observed in local "three-terminal" (3T) measurements of ferromagnet/insulator/n-Si tunnel junctions using inelastic electron tunneling spectroscopy (IETS). Voltage bias and magnetic field dependences of the IET spectra were found to account for the dominant contribution to 3T magnetoresistance signals, thus indicating that it arises from inelastic tunneling through impurities and defects at junction interfaces and within the barrier, rather than from spin accumulation due to pure elastic tunneling into bulk Si as has been previously assumed.

We investigated the localized electronic properties of nanoporous gold films by using an ultra-high vacuum scanning tunneling microscope at low temperature (4.2 K). Second derivative scanning tunneling spectroscopy shows the plasmon peaks of the nanoporous gold films, which are excited by inelastic tunneling electrons. We propose that the nanorod model is appropriate for nanoporous gold studies at the nanometer-scale. These results are supported by a 3D electron tomography analysis and theoretical calculations of nanoporous gold with ellipsoid shape., Comment: 6 pages, 3 figures. This is the authors' version. The published, high resolution version of this paper, Copyright (2014) by the American Physical Society, can be found at http://journals.aps.org/prb/

Recent experimental conductance measurements performed on paramagnetic molecular adsorbates on a superconducting surface, using superconducting scanning tunneling microscopy techniques, are theoretically investigated. For low temperatures, we demonstrate that tunneling current assisted excitations of the local magnetic moment cannot occur for voltage biases smaller than the superconducting gap of the scanning tunneling microscope. The magnetic moment is only excited for voltages corresponding to the sum of the superconducting gap and the spin excitation energies. In excellent agreement with experiment, we show that pumping into higher excitations give additional current signatures by accumulation of density in the lower ones. Using external magnetic fields, we Zeeman split possible degeneracy and thereby resolve all excitations comprised in the magnetic moment., Comment: 6 pages, 4 figures, submitted

The morphology of tetracyanoquinodimethane (TCNQ) and of benzoic acid deposited from solutions onto atomically smooth native oxide surfaces of Al(111) films was observed by atomic force microscopy (AFM). A surface height analysis (bearing analysis) of the AFM images of TCNQ-deposited atomically smooth oxidized Al films gave a bimodal height distribution of the adsorbed TCNQ on the surfaces. The height distribution showed that TCNQ is adsorbed on the oxide surface as both a uniform film on a nanometer scale and micrometer-sized particles with the height ranging from 10 to 100 nm above the surface. The large particles are easily identified by AFM as microcrystallites of neutral TCNQ and they have many morphologies. The uniform film with the nanometer scale height distribution results predominantly from corrosion of the oxide surface by the TCNQ anion formation reaction. The volumes of the large particles and of the thin uniform film were obtained separately from the analysis of the bearing histogram and bearing area curve. Generally, the volume (approx. 2 μm 3 for a 30×30-μm 2 area) of the thin uniform film is 4–5 times larger than the volume of one monolayer of TCNQ or TCNQ salt. Benzoic acid treated surfaces, on the other hand, did not show a bimodal distribution nor large particles, and surface roughness is consistent with monolayer formation of aluminum benzoate. Discrepancies in inelastic electron tunneling spectroscopy and X-ray photoelectron spectroscopy results for TCNQ adsorption on alumina were discussed.

Inelastic electron tunneling spectroscopy (IETS) was performed on single molecules with a variable temperature scanning tunneling microscope. The peak intensity, width, position, and line shape of single molecule vibrational spectra were studied as a function of temperature, modulation bias, bias polarity, and tip position for the (C–H,C–D) stretching vibration of acetylene (C2H2,C2D2) on Cu(001). The temperature broadening of vibrational peaks was found to be a consequence of Fermi smearing as in macroscopic IETS. The modulation broadening of vibrational peaks assumed the expected form for IETS. Extrapolation of the peak width to zero temperature and modulation suggested an intrinsic width of ∼4 meV due primarily to instrumental broadening. The inelastic tunneling cross section at negative bias was reduced by a factor of 1.7 for the C–H stretch mode. Low energy modes of other molecules did not show such a reduction. There was no evidence of a tip-induced Stark shift in the peak positions. The spatial var...

非弾性電子トンネル分光法 (IETS) の原理と特徴, 実験方法と測定装置及び各種分野における研究成果を総説した. この分光法は, 金属/絶縁層/金属 (MIM) 構造の接合に流れるトンネル電流を極低温において検出し解析することによって, 絶縁層とその表面に存在する化学種の回転・振動及び電子励起スペクトルを測定する方法である. IETSは高感度な表面分析法であり, 単分子層以下の表面吸着種や数ナノメートルの絶縁層薄膜の詳細な振動スペクトルの測定ができる. MIM構造の接合は金属酸化物触媒や電子素子あるいはセンサーなどのモデルシステムである. アルミナとマグネシアの表面状態, これらの金属酸化物上における有機酸, アルコール, フェノール, エステル, アミン, アミドやニトリルなどの吸着種の状態分析と表面反応, 絶縁層上の金属を通しての分子の注入, アルミナとマグネシアに担持された各種遷移金属の触媒作用, シリコンやゲルマニウムとその酸化物の蒸着薄膜の状態分析, 金属錯体や多環式化合物の電子遷移と共鳴現象, 走査トンネル顕微鏡との組み合わせによる新しい計測法などの分野における研究成果を評論した.

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).

For the first time orbital mediated tunneling spectroscopy (OMTS) is reported for the same species, vanadyl phthalocyanine (VOPc), in both the tunnel diode and STM environments. IETS and OMTS for a junction of the form [Al−Al2O3−VOPc−Pb] show both the expected bias symmetry of vibrational and electronic inelastic electron tunneling spectroscopy (IETS) bands and the highly bias asymmetric OMTS band associated with tunneling via direct occupation of the Pc ring π* orbital. STM studies of submonolayer coverage of VOPc on Au(111) produce well-resolved submolecular images of the molecule. Current imaging tunneling spectroscopy (CITS) allows one to easily identify VOPc islands, and the resulting I−V curves can be used to generate dI/dV spectra for VOPc and for bare gold regions. An intense band seen near +1.4 V bias (Au+) is identified as the same OMTS band as seen in the tunnel diode: resonance-like tunneling through the Pc ring π* orbital.

Electrons from a scanning tunneling microscope were used to break a C-H bond in a single acetylene (HCCH) molecule adsorbed on Cu(001) at 9 K. The product ethynyl (CCH) was characterized by imaging, inelastic electron tunneling spectroscopy, and variable temperature measurements of the rotation rate. Significant changes in the vibrational spectra and bonding geometry accompanied HCCH dissociation. The ethynyl was further dehydrogenated to form dicarbon (CC). Dissociation studies of the isotopes HCCD and DCCD permitted unambiguous identification of the reaction products.

In this, the second in a series of three articles, inelastic electron tunneling (IET), and Fourier transform MRAIR spectra are obtained for a series of aliphatic di-carboxylic acids and oxo-substituted aliphatic di-carboxylic acids. The acids are chosen in order to investigate the effect of the internal carbonyl group location within the molecule itself on spectral intensities. The oxo-substituted di-carboxylic acids produce a carbonyl absorption in the region 1700–1760 cm−1 for IET spectra. These are attributed to the internal carbonyl group present in these molecules, since this vibration is absent in the corresponding spectra for the non-oxo-substituted di-carboxylic acids. A comparison of MRAIR spectra for samples prepared with and without a lead cover film shows that both carbonyl, and C–H mode intensities for the MRAIR samples are attenuated to various degrees by the presence of a lead cover film depending on their location within the molecules. This indicates steric hindrance of these groups by the lead cover film, which causes the molecular backbone to be canted at the alumina surface. Evidence for this is obtained from the unusually high intensity of the asymmetric carboxylate ion stretch in the MRAIR spectra for samples with a lead cover film.

In this paper, the last in the series of three articles, inelastic electron tunneling (IET) spectra are obtained for a series of aliphatic dicarboxylic acids of increasing chain length with one of the methylene groups substituted by a carbonyl group which is not involved in bonding with the alumina surface. The effect of surface coverage on the relative intensity of the carbonyl mode is discussed. For lower surface coverages, the number of possible configurations for the adsorbed molecules increases, and in certain cases, the carbonyl group is located closer to the lead cover film thereby attenuating its intensity. The effect of changing location of the carbonyl group within the molecule (and hence, the tunnel barrier) and increasing chain length on the relative intensity of the carbonyl mode is also discussed.

In the first of a series of three articles, evidence for attenuation of the carbonyl vibrational mode intensity in inelastic electron tunneling spectroscopy (IETS), and multiple reflection absorption infrared spectroscopy (MRAIRS) is presented. The work may have considerable impact on the interpretation of IET and MRAIR spectra of certain polymers adsorbed on alumina of interest in the field of adhesion. For a better comparison between the two techniques, MRAIR spectra are recorded for compounds adsorbed on conventional oxidized aluminum mirror samples and on samples prepared with thin-film lead cover films to form planar aluminum/alumina/compound/lead structures similar to the tunnel junctions used for IETS. The compounds chosen for the study are all carboxylic acids, known to adsorb on alumina by coordinate bonding of the carboxylate anion in acid–base reactions. The acids contain non-bonding carbonyl groups on the carbon backbone of the molecules whose intensities are investigated. When the compounds are constrained by the above sample geometry, similar carbonyl attenuation is observed in both IETS and MRAIRS, but to varying degrees depending on the geometry of the adsorbed molecules. The effect, which disappears in MRAIR spectra for samples without the lead cover films, can be explained by consideration of the deformation of the molecules by the lead cover film, which results in steric hindrance of the carbonyl groups and a corresponding reduction in their activity in accordance with the orientational selection rules for both IETS and MRAIRS. This and further spectroscopic evidence for deformation of the molecules by the lead cover film is discussed.

A scanning tunneling microscope (STM) operating at 9 K in ultrahigh vacuum was used to initiate a bimolecular reaction between isolated hydrogen sulfide and dicarbon molecules on the Cu(001) surface. The reaction products ethynyl (CCH) and sulfhydryl (SH) were identified by inelastic electron tunneling spectroscopy (STM-IETS) and by sequentially removing hydrogen atoms from an H2S molecule using energetic tunneling electrons. For comparison, the thermal diffusion and reaction of H2S and CC at 45 K and H2O and CC at 9 K were also observed.

Molecular magnetic tunnel junctions are fabricated and inelastic electron tunneling spectroscopy (IETS) provides unambiguous experimental evidence of the existence of molecular species in the devices. Tunneling spectroscopy is also used to investigate the spin-polarized inelastic tunneling processes in the molecular device. The results show that inelastic scattering due to molecular vibrations is likely the main cause of the observed JMR bias-dependence.

Method of mapping an inelastic electron tunneling signal observed by scanning tunneling microscope (STM) with feedback active is presented, where the influence of the modulation voltage to the z-piezo of STM is minimized. With this method, identification of hexanethiol molecules embedded into the matrix of its isotope is demonstrated.

Single-molecule vibrations of four isotopes of CO on Cu(001) and Cu(110) at 8 K have been measured by inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM-IETS). While the low energy hindered rotation exhibits strong intensity, the C-O stretch approaches the present detection limit of STM-IETS. By performing vibrational microscopy, the spatial distribution of the hindered rotation intensity is found to be more localized than the topographic depression in constant current images.

A relatively simple and inexpensive ac signal regulator is described which facilitates constant resolution inelastic electron tunneling spectroscopy (IETS). Constant resolution is achieved by maintaining an approximately constant ac modulation voltage across IET junctions during spectral scans. The regulator circuit is based upon a field-effect transistor optoisolator with appropriate feedback control acting as a voltage comparator. It is modular in design and can easily be added in the signal path of existing IET spectrometers. A complete schematic diagram of the circuit is provided as well as a discussion on the theory of operation. IET spectra obtained from tunnel junctions with various degrees of nonlinear conductance-voltage behavior are presented with, and without, the circuit. Analysis of these spectra shows that the regulator increases the spectrometer’s signal-to-noise ratio, produces no distortion and, in the case of severely nonlinear junctions, reveals spectral features at mid to high bias, wh...

We report on an IETS (inelastic electron tunneling spectroscopy) technique which has successfully resolved 40 of the 46 C60 intramolecular vibrational modes and ∼50 of the 122 modes of C70. By layering thin, thermally evaporated films of germanium and fullerene molecules, composite multilayer tunnel barriers are formed which appear to produce enhanced IET results. IETS is in general not selection-rule restricted and has the potential to present all non-degenerate vibrational modes.

The design and performance of a variable-temperature scanning tunneling microscope (STM) is presented. The microscope operates from 8 to 350 K in ultrahigh vacuum. The thermally compensated STM is suspended by springs from the cold tip of a continuous flow cryostat and is completely surrounded by two radiation shields. The design allows for in situ dosing and irradiation of the sample as well as for the exchange of samples and STM tips. With the STM feedback loop off, the drift of the tip–sample spacing is approximately 0.001 A/min at 8 K. It is demonstrated that the STM is well-suited for the study of atomic-scale chemistry over a wide temperature range, for atomic-scale manipulation, and for single-molecule inelastic electron tunneling spectroscopy (IETS).

In a recent experiment, scanning tunneling microscopy has been used to obtain a direct probe of the magnetic interaction in linear manganese chains arranged by atomic manipulation on thin insulating copper nitride islands grown on Cu(001). The local spin excitation spectra of these chains have been measured with inelastic electron tunneling spectroscopy. Analyzing the spectroscopic results with a Heisenberg Hamiltonian the interatomic coupling strength within the chains has been obtained. It has been found that the coupling strength depends on the deposition sites of the Mn atoms on the islands. In this contribution, we perform ab initio calculations for different arrangements of infinite Mn chains on CuN in order to understand the influence of the environment on the value of the magnetic interactions.

The reversible rotation of a single isolated acetylene molecule between two diagonal sites on the Cu(100) surface at 8 K was induced and monitored with tunneling electrons from a scanning tunneling microscope (STM). Excitation of the C---H (C---D) stretch mode of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ ( ${\mathrm{C}}_{2}{\mathrm{D}}_{2}$) at 358 meV (266 meV) led to a 10-fold (60-fold) increase in the rotation rate. This increase is attributed to energy transfer from the C---H (C---D) stretch mode to the hindered rotational motion of the molecule. Inelastic electron tunneling spectroscopy with the STM provides the energies of the stretch modes and allows a quantitative determination of the inelastic tunneling and coupling probabilities.

Vibrational spectra for a single molecule adsorbed on a solid surface have been obtained with a scanning tunneling microscope (STM). Inelastic electron tunneling spectra for an isolated acetylene (C 2 H 2 ) molecule adsorbed on the copper (100) surface showed an increase in the tunneling conductance at 358 millivolts, resulting from excitation of the C-H stretch mode. An isotopic shift to 266 millivolts was observed for deuterated acetylene (C 2 D 2 ). Vibrational microscopy from spatial imaging of the inelastic tunneling channels yielded additional data to further distinguish and characterize the two isotopes. Single-molecule vibrational analysis should lead to better understanding and control of surface chemistry at the atomic level.

We have studied the UV-vis and vibrational spectra of the brevetoxins PbTx-2, PbTx-3, PbTx-9, and 2, 3, 27, 28, 41, 43-hexahydro-(reduced) PbTx-3. The absorption at 194 nm (ε ≈ 18 000 L mol−1 cm−1) of PbTx-2 and PbTx-3 is assigned to the Π–Π* transition of the lactone conjugated double bonds. The n-Π* transition of the lactone is seen at 239 nm (∈ ≈ 2 800 L mol−1 cm−1). The absorption band at 208 nm for PbTx-2 is assigned to the conjugated double bonds of the aldehyde function located on the K-ring side chain. The inelastic electron tunneling spectra of PbTx-2, PbTx-3, PbTx-9, and fully reduced PbTx-3 are presented and the vibrational bands are assigned. We show that inelastic electron tunneling spectroscopy can be used to monitor and measure the vibrational fingerprint of the brevetoxins at a level as low as 1 ng.

IET spectra are presented for the first time for inorganic monolayers adsorbed on artificial tunnel barriers. They are obtained from hydrolyzed triethoxysilane (TES) monolayers introduced onto ultra-thin sputtered germania films incorporated in Al/germania/TES/Pb tunnel junctions. The minimum thickness required for germania film continuity is determined to be ∼ 1.5nm using a reliable method which is described. Three techniques are then used to dope the germania surface with TES; these are by exposure to: (i) TES vapor under a dry nitrogen atmosphere, (ii) TES vapor under a water vapor/nitrogen atmosphere, and (iii) aqueous acidic hydrolyzed TES solution. Vibrational spectra obtained using (i) and (iii) are similar; in both cases strong Si H stretching and bending modes are present at approximately 2000–2200 cm −1 , and 900 cm −1 respectively. However, only very weak features due to residual CH species are present. These results confirm that the TES hydrolyses to a large degree and condenses on the germania surface. Spectra obtained using (ii) are similar to those obtained by the other two methods but also contain vibrational modes due to Si O Si stretching vibrations, which indicates that the TES not only hydrolyzes and condenses on the surface, but also polymerizes to form a hydrogenated polysiloxilane matrix. In contrast, spectra obtained for TES adsorbed on alumina using the same three techniques indicate that the TES neither hydrolyzes, condenses, nor polymerizes unless introduced from an aqueous acidic solution.

To investigate zinc dithiophosphate (ZnDTP) films on iron oxide surfaces by inelastic electron tunneling spectroscopy (IETS), it was necessary to define the conditions required to generate films of usable resistances. In this paper, the results of electrical contact resistance (ECR) measurements are reported on two different ferrous tribocouples sliding in oils containing commercial ZnDTPs of different types (primary, secondary, and aryl).

The rotational and vibrational transitions of a hydrogen molecule weakly adsorbed on the Au(110) surface at 10 K were detected by inelastic electron tunneling spectroscopy with a scanning tunneling microscope. The energies of the j=0 to j=2 rotational transition for para-H(2) and HD indicate that the molecule behaves as a three-dimensional rigid rotor trapped within the tunnel junction. An increase in the bond length of H(2) was precisely measured from the downshift in the rotational energy as the tip-substrate distance decreases.

We performed first-principles simulations of Inelastic Electron Tunneling Spectroscopy (IETS) for horizontally lying individual molecules that form popular donor-acceptor pairs (the TTF donor and its possible partner acceptors TCNE, TCNQ and DCNQI) on Cu(100). We find that the highest frequency C-H stretching modes are highly active for the (electron-rich) donor molecule but inactive for the (electron-poor) acceptors. We explain this contrasting response by the spatial extension of sp(3) rehybridization upon adsorption: the donor molecule entirely deforms into sp(3) while the acceptors rehybridize only at their outer ends leaving the central spacer unaffected. The sp(3)-induced buckling permits in-plane vibration modes to overlap with the π-type tunneling states and hence to be detected in IETS. In addition, the IET-spectra of a family of cyano-group acceptors, TCNE, TCNQ and DCNQI, show a recurring pattern of signals from vibrations involving their common CN outer ends plus a set of compound-dependent signals arising from the spacing moiety. The IET-response of individual chemical groups thus adds up for these flat-lying acceptor molecules, evidencing a sum rule that may facilitate their identification.

Inelastic electron tunneling spectroscopy (IETS) probes the energies of atomic and molecular excitations in a tunnel junction. When the electron energy reaches the excitation threshold, a new conductance channel opens, leading to a step in the differential conductance (dI=dV). IETS measurements were first carried out in planar tunnel junctions probing vibrations [1] and magnetic excitations [2] of large ensembles of molecules or atoms. A major breakthrough was achieved when performing IETS with the scanning tunneling microscope (STM). This was first demonstrated for molecular vibrations [3], identifying the molecules and their isotopes [4]. A few years later, this was followed

We have developed an efficient and accurate formalism which allows the simulation at the ab initio level of inelastic electron tunneling spectroscopy data under a scanning tunneling microscope setup. It exploits fully the tunneling regime by carrying out the structural optimization and vibrational mode calculations for surface and tip independently. The most relevant interactions in the inelastic current are identified as the inelastic tunneling terms, which are taken into account up to lowest order, while all other inelastic contributions are neglected. As long as the system is under tunneling regime conditions and there is no physisorbed molecule on the surface or tip apex, this lowest order in inelastic tunneling (LOIT) approach reduces drastically the computational cost compared to related approaches while maintaining a good accuracy. Adopting the wide-band limit for both tip and surface further reduces calculation times significantly, and is shown to give similar results to when the full energy dependence of the Green's functions is taken into account. The LOIT is applied to the Cu(111)+CO system probed by a clean and a CO contaminated tip to find good agreement with previous works. Different parameters involved in the calculations such as basis sets, k sampling, tip-sample distance, or temperature, among others, are discussed in detail. © 2013 American Physical Society., J.C. acknowledges financial support from the Spanish Ministry of Innovation and Science under Contract No.MAT2010-18432.

The density of two dimensional electron gas (2DEG) in the channel of AlGaN/GaN HEMT is often altered by trapped charges on the surface or in the bulk of the heterostructure, limiting the device performance at high frequencies. The existing methods, such as photoionization spectroscopy, Deep Level Transient Spectroscopy (DLTS) etc., mainly focus on deep level traps in GaN, providing little information about traps in the AlGaN layer. In this work, we use Inelastic Electron Tunneling Spectroscopy (IETS), in conjunction with the Frenkel-Poole (F-P) trap energy extraction method developed by Yeh at el, to examine electrically active traps in the AlGaN barrier layer. The results indicate that traps exist in the AlGaN bulk, as well as at the AlGaN/GaN interface. The trap energy levels are within a 0.5eV band below the conduction band edge (Ec) of AlGaN in the sample that we studied.

Complexes obtained by the ligation of nitric oxide (NO) to metalloporphyrins represent important model systems with biological relevance. Herein we report a molecular-level investigation of surface-confined cobalt tetraphenyl porphyrin (Co-TPP) species and their interaction with NO under ultrahigh vacuum conditions. It is demonstrated that individual NO adducts can be desorbed using the atomically sharp tip of a scanning tunneling microscope, whereby a writing process is implemented for fully saturated regular metalloporphyrin arrays. The low-energy vibrational characteristics of individual Co-TPP-nitrosyl complexes probed by inelastic electron tunneling spectroscopy (IETS) reveal a prominent signature at an energy of ~/=31 meV. Using density functional theory-based IETS simulations-the first to be performed on such an extensive interfacial nanosystem-we succeed to reproduce the low-frequency spectrum for the NO-ligated complex and explain the absence of IETS activity for bare Co-TPP. Moreover, we can conclusively assign the IETS peak of NO-Co-TPP to a unique vibration mode involving the NO complexation site, namely, the in-plane Co-N-O rocking mode. In addition, we verify that the propensity rules previously designed on small aromatic systems and molecular fragments hold true for a metal-organic entity. This work notably permits one to envisage IETS spectroscopy as a sensitive tool to chemically characterize hybrid interfaces formed by complex metal-organic units and gaseous adducts.

Melamine on Cu(001) is mechanically unstable under the current of a scanning tunneling microscope tip and can switch among configurations. However, these are not equally accessible, and the switching critical current depends on the bias polarity. In order to explain such rich phenomenology, we have developed a scheme to evaluate the evolution of the reaction paths and activation barriers as a function of bias, which is rooted in the nonequilibrium Green's function method implemented within density functional theory. This, combined with the calculation of the inelastic electron tunneling spectroscopy signal, allows us to identify the vibrational modes promoting the observed molecular conformational changes. Finally, once our ab initio results are used within a resonance model, we are able to explain the details of the switching behavior, such as its dependence on the bias polarity, and the noninteger power relation between the reaction rate constants and both the bias voltage and the electric current.

Individual Fe atoms on a Cu2N/Cu(100) surface exhibit strong magnetic anisotropy due to the crystal field. Using atom manipulation in a low-temperature STM we demonstrate that the anisotropy of one Fe atom is significantly influenced by local strain due to a second Fe atom placed nearby. Depending on the relative positions of the two atoms on the Cu2N lattice we can controllably enhance or reduce the uniaxial anisotropy. We present a model that explains the observed behavior qualitatively in terms of first principles., Comment: Three figures, one table

We have developed a new scanning-tunneling-microscopy--based spectroscopy technique to characterize infrared (IR) absorption of submonolayers of molecules on conducting crystals. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by IR light from a tunable laser source. Using this technique, we obtain the IR absorption spectra of [121]tetramantane and [123]tetramantane on Au(111). Significant differences between the IR spectra for these two isomers show the power of this new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy (STM) images look quite similar. Furthermore, the new technique was found to yield significantly better spectral resolution than STM-based inelastic electron tunneling spectroscopy, and to allow determination of optical absorption cross sections. Compared to IR spectroscopy of bulk tetramantane powders, infrared scanning tunneling microscopy (IRSTM) spectra reveal narrower and blueshifted vibrational peaks for an ordered tetramantane adlayer. Differences between bulk and surface tetramantane vibrational spectra are explained via molecule-molecule interactions.

The interaction between a single magnetic atom and the metal environment (including a magnetic field) is analyzed by introducing an ionic Hamiltonian combined with an effective crystal-field term, and by using a Green-function equation of motion method. This approach describes the inelastic electron tunneling spectroscopy and the Kondo resonances as due to atomic spin fluctuations associated with electron co-tunneling processes between the leads and the atom. We analyze in the case of Fe on CuN the possible spin fluctuations between states with S = 2 and 3/2 or 5/2 and conclude that the experimentally found asymmetries in the conductance with respect to the applied bias, and its marked structures, are well explained by the 2↔3/2 spin fluctuations. The case of Co is also considered and shown to present, in contrast with Fe, a resonance at the Fermi energy corresponding to a Kondo temperature of 6 K. Fil: Goldberg, Edith Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química (i); Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Flores, F.. Universidad Autónoma de Madrid; España

Inelastic Electron Tuneling Spectroscopy (IETS) is a powerful spectroscopy that allows one to investigate the nature of local excitations and energy transfer in the system of interest. We study IETS for Topological Insulators (TI) and investigate the role of inelastic scattering on the Dirac node states on the surface of TIs. Local inelastic scattering is shown to significantly modify the Dirac node spectrum. In the weak coupling limit, peaks and steps are induced in second derivative $d^2I/dV^2$. In the strong coupling limit, the local negative U centers are formed at impurity sites, and the Dirac cone structure is fully destroyed locally. At intermediate coupling resonance peaks emerge. We map out the evolution of the resonance peaks from weak to strong coupling, which interpolate nicely between the two limits. There is a sudden qualitative change of behavior at intermediate coupling, indicating the possible existence of a local quantum phase transition. We also find that even for a simple local phonon mode the inherent coupling of spin and orbital degrees in TI leads to the spin polarized texture in inelastic Friedel oscillations induced by local mode., Comment: 5 pages, 5 figures

We address local inelastic scattering from vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks/dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy, $d^2I/dV^2$, using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative $U$-center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative $U$ formation and its evolution from weak to strong coupling. The negative $U$-site effectively acts as local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac cone like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining., 7 pages, 5 figures

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.

The characterization and manipulation of deposited magnetic clusters or molecules on surfaces is a prerequisite for their future utilization. In recent years techniques like spin-flip inelastic electron tunneling spectroscopy using a scanning tunneling microscope proved to be very precise in determining e.g. exchange constants in deposited finite spin chains in the meV range. In this article we tackle the problem numerically by investigating the transition from where a pure spin Hamiltonian is sufficient to the point where the interaction with the surface significantly alters the magnetic properties. To this end we study the static, i.e. equilibrium impurity magnetization of antiferromagnetic chains for varying couplings to a conduction electron band of a metal substrate. We show under which circumstances the screening of a part of the system enables one to deduce molecular parameters of the remainder from level crossings in an applied field., Comment: 7 pages, 7 figures