Voltage tunneling spectroscopy

Hugelmann, R, and W. Schindler, In-situ voltage tunneling spectroscopy at electrochemical interfaces, J. Phys. Chem. B, Vol. 109, 2005 pp. 6262-6267. 

The ESTM experiment provides actually five measurable quantities tunnelling current, / at the applied voltage, U, and three dimensions, x, y, z. The standard STM can therefore easily be modified by recording the local l-Uy U-zy or /-z characteristics (z is vertical distance of the tip from the electrode surface). Plot of dl/dU or d//dz versus x and y brings additional information on the electronic and chemical surface properties (local work functions, density-of-states effects, etc.), since these manifest themselves primarily as l-U dependences. The mentioned plots are basis of the scanning tunnelling spectroscopy (STS). 

Apart from the more conventional transport measurements of molecular junctions at constant bias voltage, alkane(di)thiols-based molecular junctions were also characterized by transition voltage spectroscopy [258, 259], AC voltage modulation [260], and inelastic electron tunneling spectroscopies [261],... 

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... 

Fig. 21. Current-voltage I v.r. V traces taken with scanning tunneling spectroscopy (STS) on individual nanotubes of various outer diameters (1) dt = 8.7 nm, (2) dt = 4.0 nm, and (3) dt = 1.7 nm. The top inset shows the conductance v.r. voltage plot for data taken on the 1.7 nm nanotube. The bottom inset shows an I-V trace taken on a gold surface under the same conditions [182]. 

It is possible to obtain chemical information from STM when it is used in the spectroscopic mode, for example by measuring at a fixed distance the tunneling current / as a function of the voltage over the gap (I/V spectroscopy). This method of measurement is called scanning tunneling spectroscopy (STS) [49],... 

Fig. 14.1. Electronics for local tunneling spectroscopy. By using an op-amp with FET input stage as the isolation amplifier to the high-voltage amplifier for the z piezo, the holding time on the capacitor can be as long as 100 sec. The values of R and C show typical ranges.

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