Elastic electron tunneling spectroscopy

Honciuc A, Metzger RM, Gong A, Spangler CW (2007) Elastic and inelastic electron tunneling spectroscopy of a new rectifying monolayer. J Am Chem Soc 129 8310-8319... 

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... 

Inelastic electron tunnelling spectroscopy (lETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidised metal from solution or vapour, and devices are completed by evaporation of a top electrode which is usually of lead, because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimise thermal broadening. Most electrons (>99%) pass through the device elastically, but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed the selection rule for lET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Keywords Inelastic Elastic Tunneling Spectroscopy Orbital mediated Vibrational Electronic Ionization levels Affinity levels... 

Thus far the discussion has centered on elastic tunneling, but consideration of inelastic processes may offer additional analytical opportunities. An energy scale of the relevant phenomena is presented in Table 2. Inelastic tunneling was first observed in metal-oxide-metal junctions. It was immediately developed as a technique for photon-free vibrational spectroscopy (lETS) where the tunneling electrons dissipate energy by coupling to vibra-... 

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