Inelastic IETS

The well-known tetrahedral [Co(NCS)4]2 ion has continued to attract attention from analytical chemists, physical chemists, and spectroscopists. The inelastic electron tunneling (IET) spectrum of (Me4N)2[Co(NCS)4] was compared with IR and Raman spectra of the same complex.359 The vibrational bands due to the Me4N+ were prominent in all three spectra, but Coligand stretches were absent from the IET spectra. The lowest 4 42 4T2 electronic transition was strong in the IET spectrum but absent from the IR spectrum. The electric dipole allowed 4A2 4TX electronic transition was observed in both the IET and IR spectra and no fine structure was observed. Complex formation equilibria between Co11 and SCN- were studied calorimetri-... 

Section 5 is on one particular molecule, p-benzene dithiol. This is one of the most commonly studied molecules in molecular electronic transport junctions [7] (although it is also one of the most problematic). Section 6 discusses a separate measurement, inelastic electron tunneling spectroscopy [8, 9] (IETS). This can be quite accurate because it can be done on single molecules at low temperatures. It occurs because of small perturbations on the coherent transport, but it can be very indicative of such issues as the geometrical arrangement in the molecular transport junction, and pathways for electron transport through the molecular structure. 

Models are also required for analysis of the transport. For calculations of current/ voltage curves, current density, inelastic electron scattering, response to external electromagnetic fields, and control of transport by changes in geometry, one builds transport models. These are generally conceptual - more will be said below on the current density models and IETS models that are used to interpret those experiments within molecular transport junctions. 

It was noted early by Reed and others that the IETS spectrum could exhibit both absorption and emission peaks - that is, the plots of Fig. 9 could have positive excursions and negative excursions called peaks and dips. The simple analysis suggested in Fig. 9 implies that it should always be absorptive behavior, and therefore that there should always be a peak (a maximum, an enhancement) in the IETS spectrum at the vibrational resonances. It has been observed, however, that dips sometimes occur in these spectra. These have been particularly visible in small molecules in junctions, such as in the work of van Ruitenbeek [92, 109] (Fig. 12). Here, formal analysis indicates that, as the injection gap gets smaller, the existence of an inelastic vibrational channel does not contribute a second independent channel to the transport, but rather opens up an interference [100]. This interference can actually impede transport, resulting in a dip in the spectrum. Qualitatively, this occurs because the system is close to an electronic resonance without the vibrational coupling the conductance is close to g0, and the interference subtracts from the current. 

Fig. 3 Energy diagram for an M-A-M diode showing elastic and inelastic tunneling processes (top). The HOMO (n) and LUMO (71 ) orbital energies and a few vibrational levels are indicated. Applied bias energy (eV) is just sufficient to allow inelastic tunneling with excitation of the first vibrational level, eV = hv. Also shown (bottom) are the I(V) curve, conductance- / curve, and the IETS spectrum that would result from both elastic processes and the first inelastic channel. (Reproduced by permission of the American Chemical Society from [19])...

The width of the peaks in LETS depends upon the sharpness of the onset of the inelastic process, which in turn depends upon the thermal distribution of electron energies about EP. Thus, the IETS line width depends strongly on temperature and as shown by (3) [75]. Because of this, vibrational IETS provides infrared-quality resolution only when performed below 5 K. Electronic transitions are usually much broader than vibrational transitions therefore, electronic IETS is usually performed at liquid nitrogen temperature and slightly above (>77 K). An example of a system showing both vibrational and electronic IETS is presented in Fig. 5 [19]. 

In contrast to the minimal activity in infrared reflection studies the technique of inelastic electron tunneling spectroscopy (IETS) recently has contributed a large amount of information on monolayer adsorption of organic molecules on smooth metal oxide surfaces,Q),aluminum oxide layers on evaporated aluminum. These results indicate that a variety of organic molecules with acidic hydrogens, such as carboxylic acids and phenols chemisorb on aluminum Oxide overlayers by proton dissociation - 1 — and that monolayer coverage can be attained quite repro-ducibly by solution doping techniques. - The IETS technique is sensitive to both infrared and Raman modes. — However, almost no examples exist in which Raman il and or infrared spectra have been taken for an adsorbate/substrate system for which IETS spectra have been observed. 

Due to the high density of Rh atoms, no species of the form Rh(C0)2 were formed, however. This is expected to be the case on the (111) surface as well. Weak absorptions between 400 and 575 cm"l were seen and are indicative of metal-absorbate stretching and bending vibrations. Inelastic electron tunneling spectroscopic (IETS) measurements on alumina supported rhodium particles (47, 48, 49) add little new structural information... 

Section II will discuss the basic phenomena of inelastic tunneling from the viewpoint of the experimentalist. Section III will treat peak shapes, shifts, and widths. Section IV will deal with intensities and selection rules in IETS. Finally, Section V includes some recent applications of IETS to the fields of chemisorption and catalysis, and to the at first glance unrelated field of surface enhanced Raman spectroscopy. 

In other studies Allara (27) used FTIR reflection to study the interaction of the oxide film of aluminum with acetic acid in which th aluminum acetate carbonyl stretch peak was found at 1590 cm- > in excellent agreement with the 1589 cm-1 peak determined by inelastic electron tunnelling spectroscopy (IETS), another modern technique for vibrational spectra at metal interfaces. The spectra of acetic acid adsorbed on copper oxide or indium,oxide were about the same. 

In parallel, the related activity was in the field of single-electron shuttles and quantum shuttles [143-153]. Finally, based on the Bardeen s tunneling Hamiltonian method [154-158] and Tersoff-Hamann approach [159,160], the theory of inelastic electron tunneling spectroscopy (IETS) was developed [113-116,161-163],... 

ICS IETS IXS IR Indosyanine Green Inelastic Electron Tunneling Spectroscopy Inelastic X-ray Scattering Infrared... 

Fig. 13. Schematic illustration of the IET dissociation process, (a) Inelastic electrons are injected into the molecule through the adsorbate-induced resonance state, (b) The energy required for a dissociation can be supplied by single- or multiple-excitation processes.

The total current I through a molecule has an inelastic contribution /)ets from excitation of molecular vibrations (the main goal of IETS) (Fig. 11.25) and can have also an elastic contribution, which consists of two parts the off-resonance elastic ohmic IR (shown in the center part of Fig. 11.26) and a resonant elastic contribution Iomt between the Fermi59 level of the relevant metal electrode and an unoccupied molecular orbital of the molecule this part has been dubbed orbital-mediated tunneling (OMT) (Fig. 11.26) [11] ... 

One technique recently applied to UE [129,130] is inelastic electron tunneling spectroscopy (IETS) (Section 11.11). The IETS spectrum (dIz/dVz) measured at 4.2 K for a monolayer of fullerene-fczs-[12a.ethylthio-tetrakis(3,4-dibutyl-2-thiophene-5-ethenyl)-5-bromo-3,4-dibu-tyl-2-thiophene] malo-nate, 47, between Al and Pb electrodes [130] exhibited (i) the usual IETS spectrum of intramolecular vibrations and (ii) an elastic component, measured at slightly higher bias V, due to "orbital-mediated tunneling," that is, an... 

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