Tunneling inelastic

Troisi A, Beebe JM, Picraux LB, Zee RDv, Stewart DR, Ratner MA, Kushmerick JG (2007) Tracing electronic pathways in molecules by using inelastic tunneling spectroscopy. Proc Natl Acad Sci USA 104(36)44255-14259... 

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])...

Rahimi M, Hegg M (2009) Probing charge transport in single-molecule break junctions using inelastic tunneling. Phys Rev B 79 081404(R)... 

Solomon GC, Gagliardi A, Pecchia A, Frauenheim T, Di Carlo A, Reimers JR, Hush NS (2006) Understanding the inelastic-tunneling spectra of alkanedithiols on gold. J Chem Phys 124 094704... 

Figure 2. A schematic view of the steps involved in making an inelastic tunnel junction. In practice, the mask touches the substrate. The lead mask is held 0.1 mm from the substrate by dimpling it with a center punch. Reproduced with permission from "Tunneling Spectroscopy", Plenum Press, 1982.

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-... 

Inelastic tunnelling electrons can also be used to selectively induce either the translation over a metallic surface or desorption from the metallic surface of individual molecules, as has been shown for NH3 on Cu(lOO) surfaces (Pascual et al, 2003). Activation of either the stretching vibration of ammonia ( 408 meV) leading to lateral translation on the surface, or the inversion of its pyramidal structure (umbrella mode s(NH3) 139 meV) leading to desorption, can be achieved by adjusting 7t and Vt. 

Figure 3.44. Dissociation of 02 adsorbed on Pt(lll) by inelastic tunneling of electrons from a STM tip. (a) Schematic ID PES for chemisorbed Of dissociation and illustrating different types of excitations that can lead to dissociation, (b) Schematic picture of inelastic electron tunneling to an adsorbate-induced resonance with density of states pa inducing vibrational excitation (1) competing with non-adiabatic vibrational de-excitation that creates e-h pairs in the substrate (2). (c) Dissociation rate Rd for 0 as a function of tunneling current I at the three tip bias voltages labeled in the figure. Solid lines are fits of Rd a IN to the experiments with N = 0.8, 1.8, and 3.2 for tip biases of 0.4, 0.3, and 0.2 V, respectively and correspond to the three excitation conditions in (a). Dashed lines are results of a theoretical model incorporating the physics in (a) and (b) and a single fit parameter. From Ref. [153].

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

Figure I. (a) Schematic diagram of a metal/insulator/metal tunnel junction with a variable applied d.c. bias voltage, (b) Partial schematic energy band diagram under zero applied bias conditions, where j and s are the mean barrier height and thickness respectively, (c) Corresponding energy-band diagram where applied dx. bias V is sufficient to excite a vibrational mode in the barrier thus producing an inelastic tunneling current.

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. 

If we neglect the energy dependence of the inelastic tunneling matrix elements within a particular band s, we can write for the inelastic tunneling current (2) ... 

Figure 8. Inelastic tunneling cross section, from the theory of Kirtley, Scalapino, and Hansma (KSH), for a point dipole as a function of position in a ISA thick barrier, for 3 different vibrational energy losses. One electrode is taken to be at OA,...

The applications of inelastic tunneling presented in Sec. V point up both the strong and weak points of this spectroscopy. Inelastic electron tunneling is sensitive, has good resolution, does not require large capital investment, has a wide spectral range, is sensitive to all surface vibrations, and can be used on oxide and supported metal catalysts. However, a counter-electrode must be used, single crystal metal surfaces cannot be used, and spectra must be run at low temperatures. 

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