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Inelastic Electron Tunnelling Spectroscopy (IETS) provides information concerning the vibrational and excitation modes present in molecular species adsorbed onto an electrically insulating substrate. Our objective is not to cover the entire literature to date. Rather we hope to stimulate an interest in IETS among chemists by outlining some of the systems of chemical interest already studied; and to provide sufficient background information to guide the construction and operation of an inelastic electron tunnelling spectrometer and interpretation of the data.

Inelastic electron tunneling spectra have been obtained for some aminosilanes (3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, the mixed and separated isomers of aminophenyltrimethoxysilane and N-2-aminoethyl-3-aminopropyl trimethoxysilane) adsorbed on aluminium oxide. The silanes have been applied from the vapour and from solutions in either benzene, water or acidic aqueous alcohol. Spectra indicate diferent levels of hydrolysis and condensation for the amines, which depend upon the doping conditions, and that no isotopic exchange occurs when the doping medium is D2O. The mode of attachment to the oxide surface is different for the three isomers of aminophenyltrimethoxysilane.

We have made a series of Al-Al 2 O 3 -benzoic acid-Pb inelastic electron tunneling spectroscopy (IETS) junctions on substrates roughened by varying thicknesses of CaF 2 . The vibrational peaks due to the molecular monolayer of benzoic acid disappear as the substrates are made rougher. We speculate on the cause of this disappearance.

The vibrational spectra of phthalic, isophthalic and terephthalic acid adsorbed onto the alumina surface have been measured by using inelastic electron tunneling spectroscopy. The tunneling spectra of these acids were obtained by liquid-phase doping with water or various alcohol solutions. The detailed assignments for these spectra are given, together with a comparison of their infrared and Raman spectra. It has been found that both phthalic and isophthalic acid are adsorbed onto the alumina surface mainly as the dicarboxylate ions. However, a comparison between the tunneling spectra of terephthalic and p-acetylbenzoic acid shows that terephthalic acid is adsorbed predominantly as the monocarboxylate ion onto the surface when doped with alcohol solutions. Hydrogen bonding is observed among the adsorbed hydrogen terephthalate ions. These results show a difference in their orientation and different probabilities of chemical interaction between the alumina surface and the carboxyl groups of the adsorbed isom...

The vibrational spectra of HCOOH (DCOOD) molecules adsorbed on Al oxide tunnel barriers are studied by Inelastic Electron Tunneling Spectroscopy. The (Al/Al oxide/Pb) tunnel junctions are prepared in situ in an ultrahigh vacuum cryostat. It is concluded that the formate adsorbs with a preferred orientation on microcrystalline Al oxide, whereas on amorphous Al oxide there is a rather random orientation.

We wish to discuss the application of IETS to the observation of electronic, rather than vibrational transitions. This extension of the method seemed interesting for several reasons.

An experimental and theoretical study has been carried out on the process of inelastic tunneling between metal films. The experimental results show that this process gives rise to the observation of vibrational spectra for molecules contained in the junction region. The phenomena have been studied for a number of molecular species and at temperatures ranging from 1 to 300\ifmmode^\circ\else\textdegree\fi{}K. The principal effect of temperature is to broaden the observed spectral lines which can be accounted for quantitatively by considering the thermal smearing of the electron distribution in metals. Two detailed interaction mechanisms are discussed which can give rise to infrared dipole or Raman selection rules. None of the experiments carried out thus far can distinguish between these two processes, because of the low symmetry of the molecules used. In addition to the vibrational spectra, it appears that it should be possible to observe electronic transitions as well.

The inelastic tunneling of electrons in a metal-insulator-metal junction has been shown to be a spectroscopic method for studying the vibrational modes of the whole system. In the present paper we consider the possibility of deducing precise information from this spectroscopy. The low-voltage part of the spectrum (i.e., the $\frac{{d}^{2}I}{d{V}^{2}}\ensuremath{-}\mathrm{v}\mathrm{s}\ensuremath{-}V$ characteristic) gives information about the phonons of the electrodes. The phonon density, which is deduced for a Mg electrode, is critically compared with the density deduced from neutron scattering. The range of this phonon probe is then studied by tunneling into multilayer electrodes. The 40-90-meV range of the characteristic of a Mg-Pb junction exhibits a specific structure due to the lattice vibrations of the insulator. This structure is compared with the infrared spectrum and the phonon density of states of MgO, as well as with a theoretical calculation of the tunneling current in the transfer-Hamiltonian formalism. From the fit obtained, it is deduced that the 30-\AA{}-thick insulator, grown on Mg, is an oxide, in contrast with the insulator grown on Al, which was previously deduced to be a hydroxide. At higher energies (100-500 meV), the vibrational spectrum of molecules contained in the insulator region is observed. The identification of the lines is shown to be accurate and it gives precise information on these molecules, especially about their chemical binding with the insulator. This last point could be important in the future for studying the problem of adsorption on solid surfaces.

It is pointed out that inelastic electron tunneling spectroscopy (IETS) is a useful technique for the study of vibrational modes of molecules adsorbed on the surface of the insulating layer in a metal-insulator-metal tunnel junction. A detailed study of electron tunneling in aluminum-aluminum oxide-gold (Al-Al2O3-Au) tunnel junctions has been conducted with an overall goal of exploring possible mechanisms in such a thin film device useful for chemical sensing. For an understanding of the electronic effects in the presence of chemicals in a junction, the electronic surface properties of the metallic films in a clean tunnel junction must be established. The present investigation is concerned with room temperature observations of electronic states on the surface of highly oriented gold film in Al-Al2O3-Au junctions.

Inelastic electron tunneling (IET) spectra of clean ultrahigh vacuum (UHV) prepared (Al/Al oxide/Pb) tunnel junctions are discussed. Microcrystalline Al oxide is shown to grow on these Al films. This is in contrast to the formation of amorphous Al oxide in the common high vacuum (HV) preparation process. The IET-spectra of UHV prepared tunnel junctions are free of peaks due to contaminations. Conclusions concerning the growth of oxides on Al films are drawn.

A comparison is made of inelastic electron tunneling spectroscopy results for the chemisorption of formic acid into into two different surfaces. Details of the spectra are used to confirm that one surface is alumina and the other aluminum. A formate ion spectrum is obtained in both instances but the aluminum surface shows both greater spectral intensity and a slight shift to higher energies for the peaks. In the case of the aluminum surface the peak intensities are the same for both bias polarities as expected in the case of a single homogeneous barrier. For the alumina surface, on the other hand, there is an asymmetry in peak intensity due to the two insulator barriers.

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.

Low energy inelastic electron tunneling spectra are presented for thermally formed oxides on copper films. Thinner oxide tunnel barriers are observed to contain interfacial gradients as evidenced by spectral intensity asymmetries between forward and reverse bias, whereas thicker oxides appear to be more uniform in composition. Spectral results are in good qualitative agreement with reported studies of bulk cuprous oxides, and the semiconducting nature of the oxide layers is reflected in a negative temperature coefficient of resistivity of the tunnel junctions.

Strictly speaking, adhesion is an interfacial phenomenon and if the events at the interface can be understood then this information can be used to tailor the interface. However, it should be noted that the practical adhesion (e.g. of films and coatings) depends on interfacial interactions plus a number of other factors. So the study of interfaces is necessary and important but by no means sufficient. Most commonly when one speaks of coating adhesion, he is referring to adhesion in the mechanical sense (force or the work required to disrupt the coating); whereas the term “resist adhesion” as used in lithography has special meaning and it will also be discussed. in the last decade or so, the availability of sophisticated surface analytical tools has been a boon in understanding adhesion on a microscopic level. Particularly, ESCA * and inelastic electron tunneling spectroscopy (IETS) have been very helpful in unravelling interfacial interactions, and examples will be cited to illustrate their usefulness. the areas for future research include, inter alia , microscopic understanding of the effect of moisture in disrupting otherwise “healthy” bonds.

An outline of the history of inelastic tunneling must begin with the experiments of IVAN GIAEVER with superconducting metals which established the physical reality of the metal-insulator-metal tunneling method. Early attempts to observe electrode band structure effects at higher voltages were not successful but led to the discovery of inelastic tunneling spectroscopy of organic molecules, oxides, and electrode phonons. The study of a variety of organic species has been an active area of experimentation. Observations of electronic transitions, plasmons quantum light emission are of current interest. Possible future developments will be discussed.

The nature of the supported zirconium complex has been determined by investigating its behavior as a function of temperature from 300 to 475 K and also its interactions with D2, D2O and H2O. Additionally, results have been obtained for C2H2, C2H4 and C3H6 on alumina supported Zr(BH4)4.

Applications of inelastic electron tunneling spectroscopy to the study of biological molecules will be reviewed. Experiments include work on amino acids, purine and pyridine bases, nucleosides, nucleotides, DNA, RNA, proteins and other selected compounds. Detailed studies of mononucleotides using a series of derivatives with selected bases have established that the tunneling spectra resolve small differences in chemical structure as well as clearly distinguishing different bases. The technique has also been extended to polynucleotides and may be of help in sequence studies. Applications include studies on ultraviolet radiation damage and a series of results on bases, mononucleotides, and polynucleotides will be prepared. Results and problems associated with applying the tunneling technique to the full DNA molecule and other large molecules will also be analyzed. Applications to other relevant materials such as long chain fatty acids will also be discussed

Inelastic electron tunneling spectroscopy,(1,2) also known as IETS or tunneling spectroscopy, is a sensitive technique for measuring the vibrational spectra of molecules. At present, it is particularly well suited to measuring the vibrational spectra of a monolayer or submonolayer of molecules adsorbed on aluminum or magnesium oxide or on metal particles or metal complexes that are themselves supported on aluminum or magnesium oxide. In the future, it may be possible to extend it to a much wider range of systems with the use of mechanically adjusted tunnel junctions.

The inelastic current in Inelastic Electron Tunneling Spectroscopy (IETS) is due to the interaction of the tunneling electron with the vibrational modes of the molecule. The intensity of IETS spectrum should depend on the details of this interaction. Previously this interaction was described by making use of fictitious ‘partial charges’ located on each atom of the vibrating molecule. We report here a ‘first principle’ approach to the calculation of relative IETS peak intensities. Initial calculations have been made for Ethylene molecule. The present calculation employs a more realistic description of the molecular charge distribution and its vibrational properties. The charge density is obtained using linear combination of atomic orbital method. The polarization vectors and frequencies for the vibrating molecule is obtained using a force constant model. The electron-molecule interaction is dependent on the gradient of the molecular charge distribution with respect to nuclear displacements and consists of nuclear, Coulomb, and exchange terms. Our results for Ethylene molecule indicate that the interaction can not be described in terms of the partial charges located on each atom.

In this chapter, we review the application of tunneling spectroscopy to the study of biological molecules. These studies have included molecules with a wide range of molecular weights and include amino acids, bases, nucleotides, nucleic acids, and proteins. These compounds represent the important building blocks of biological materials, and detailed descriptions of them can be found in standard texts on biochemistry.(1.2) Using inelastic electron tunneling spectroscopy (IETS) techniques to examine these molecules involves the same problems as encountered in many other IETS studies, although resolution problems associated with molecules of large molecular weight play an increasingly important role.

Molecular monolayers can be chemisorbed on the surface of oxidized evaporated thin metal films. The vibrational spectra of these molecular monolayers can be studied by inelastic electron tunneling spectroscopy (IETS) and, under certain circumstances, surface enhanced (SER) Raman spectroscopy. Tunnel junctions, formed by evaporating a second metal electrode on top of the molecular monolayer, provide a portable and durable model system for the study of, for example, chemisorption, catalysis, and lubrication, as well as an aid in the study of the tunneling and Raman scattering processes themselves. Advances in Raman instrumentation have made it possible to detect unenhanced molecular monolayers- avoiding some of the inherent disadvantages of tunneling and surface enhanced Raman spectroscopies.

Three kinds of new techniques of the identification and characterization of chemical species in solids and liquids are presented: 1) inelastic electron tunneling spectroscopy, 2) in-house extended X-ray absorption fine structure spectrometry, and 3) solution photoemission spectroscopy.

A brief review is given of Inelastic Electron Tunneling Spectroscopy (IETS) as it is applied to investigations both of surface chemical phenomena as well as heterogeneous catatlysis. The use of IETS to probe the vibrational structure of admolecules is compared with alternate electron scattering methodologies, namely, Field Emission Energy Distribution (FEED) measurements and Inelastic Electron Scattering Spectroscopy (IESS). Both the advantages and the disadvantages of each type of measurement are emphasized.

The present review reports the coordinated application of three spectroscopic methods (Raman, infrared (IR) and inelastic electron tunneling spectroscopy (IETS)) in the study of the conformation of Ach and some analogues (β-MeAch, Mu and Nic) in solid state, aqueous solution and in interaction with a surface. Useful correlated information is obtained by Raman and IR spectroscopies on the conformational possibilities of these molecules in transition from solid state to aqueous solution. With this information in hand as well as on the basis of Raman and IR study of the nonenzymatic hydrolysis of Ach, the first detailed experimental investigation of the interaction of Ach and β-MeAch adsorbed on a surface (Al2O3) is realised by the IETS method. The results are used to discuss an interaction analogous to that of Ach with receptor and another one analogous to that of Ach and AchE.

Over the past few years, vibrational tunneling spectroscopy has become an important surface and interface spectroscopy with applications to the study of adsorption phenomena and heterogeneous catalysis. But in these fields, the understanding of the electronic states of molecules is also of fundamental importance. What information can inelastic electron tunneling spectroscopy (IETS) provide?

The modes of simple carboxylic acids, when adsorbed on alumina are not all undisputably identified. The spectra of marked acetic acid CH3C18O18OH and CD3COOD adsorbed on alumina allow us to clarify the assignments.

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.

Inelastic electron tunneling spectra of nucleosides and nucleotides are presented which show that detailed information on the vibrational spectra of these molecules can be obtained from a few micrograms of material. A series of studies on adenine derivatives demonstrates that unambiguous identification of a large number of slightly different derivatives can be carried out by means of this technique.

This report describes the research efforts in five different areas, all related to a better understanding of solid surfaces: (1) The analysis of surface structures by low-energy electron diffraction (LEED); (2) Other research related to LEED such as an improved method of measuring experimentally the angle of incidence of the electron beam in a LEED apparatus; (3) An examination of desorption phenomena including a theoretical treatment of thermal desorption mass spectrometry (TDS); (4) Model calculations concerning the dependence of the spectral intensity upon adsorbate concentration in inelastic electron tunneling spectroscopy (IETS); and (5) Chemisorption and surface reactions. (Author)

The intensity of Raman scattering from molecules in solution suggests that the observation of Raman scattering from molecular monolayers should be very difficult given the small number of molecules involved. However, recent experimental work on Raman scattering from molecules adsorbed on the surface of metal electrodes in electrochemical cells has shown that strong Raman scattering can be obtained from a monolayer of adsorbed molecules.1,2,3 It has been estimated that the Raman cross section of a pyridine molecule adsorbed on an electrochemically prepared Ag surface in a KC1 solution is six orders of magnitude larger than for the same molecule in solution.2 Although this large enhancement has stirred considerable experimental and theoretical interest, its microscopic origins remain unknown.4,5,6

Publisher Summary This chapter presents an overview of spectroscopic methods used in polymer science. Of the physical methods used to analyze molecular structure, infrared and Raman spectroscopy are amongst the oldest. In the field of polymers, infrared has long been used for identification and compositional analysis, and it still maintains a prominent position in spite of the availability of newer techniques. Studies on the structure of polymer crystals in the solid state, and chain conformation in both the solid and liquid states, are appearing at an increasing rate, and are tending more to complement studies by other techniques. Inelastic Electron Tunneling Spectroscopy (IETS) is a new technique that provides a versatile and sensitive method for measuring the vibrational spectrum of a molecular species adsorbed on the surface of a metal oxide.. The chapter elaborates those aspects of the basic principles and experimental techniques of inelastic neutron scattering that are useful for the investigation of polymers. An overview of Rayleigh–Brillouin scattering in polymers is also presented in the chapter.

A scanning tunneling microscope (STM) can probe the inelastic spin excitations of single magnetic atoms in a surface via spin-flip assisted tunneling. A particular and intriguing case is the Mn dimer case. We show here that the existing theories for inelastic transport spectroscopy do not explain the observed spin transitions when both atoms are equally coupled to the STM tip and the substrate. The hyperfine coupling to the nuclear spins is shown to lead to a finite excitation amplitude, but the physical mechanism leading to the large inelastic signal observed is still unknown. We discuss some other alternatives that break the symmetry of the system and allows for larger excitation probabilities., 4 pages, 2 figures

Molecular junctions have been characterized to determine the influence of the metal contact formation in the electron transport process through a single molecule. With inelastic electron tunneling spectroscopy and first-principles calculations, the vibration modes of a carbon monoxide molecule have been surveyed as a function of the distance from a copper electrode with unprecedented accuracy. We observe a continuous but nonlinear blue shift of the frustrated rotation mode in tunneling with decreasing distance followed by an abrupt softening upon contact formation. This indicates that the presence of the metal electrode sensibly alters the structural and conductive properties of the junction even without the formation of a strong chemical bond.

We have designed and constructed a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), taking extreme measures to isolate the microscope from acoustic, vibrational, and electronic noise. We combined a 4 K STM with line-of-sight dosing to enable one to position the crystal surface in front of an impinging molecular beam as in scattering experiments. Due to the mechanical stability of the instrument and the minimal thermal drift associated with working at 4 K we are able to locate and to image repeatedly isolated adsorbates and atomic-scale structures, such as step edges, for extended periods days. The instrument has been designed for the topographic and spectroscopic characterization of atoms and molecules on metal and semiconductor surfaces, for the investigation of the mechanism by which the STM images adsorbates on surfaces, and for inelastic electron tunneling spectroscopy of single molecules.

Inelastic electron tunneling spectroscopy (IETS) has been used to determine the molecular species which occur on an aluminum oxide surface during metallic corrosion by carbon tetrachloride. IETS spectra were obtained for CCl4 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 the molecular species CCl4, AlCl, AlCl2, AlCl3, ⋅CCl3, C2Cl6, and the complex CCl+3[AlCl4]−. These species are those in the reaction sequence proposed by Stern and Uhlig for the corrosion of aluminum by CCl4. The results do not provide information on whether the reaction is chemical or electrochemical in nature. The fact that no modes were observed which would reflect oxygen–carbon bonding, and the presence of a large number of aluminum–chlorine modes suggest that the corrosion mechanism is by reaction of the solvent with exposed aluminum atoms.

The relations of line peak intensity and width in IETS with the modulation parameter W = eV s KT is presented to a great extent of W. The line quality factor Q (the ratio of line peak intensity to width) is introduced, as a parameter for appraising line shape quality. In accordance with the relation of quality factor Q with W, the optimum measurement condition in IETS have been obtained.