Differential conductance tunneling spectroscopy

Instrumentation. An STM is equipped with suitable additional electronics to generate the desired bias modulation and to detect the modulation of the tunneling current [72]. Differential Conductance Tunneling Spectroscopy data that was obtained for a platinum film electrode have been interpreted in terms of step density and... 

Fig. 1.21. Tunneling spectroscopy in classic tunneling junctions, (a) If both electrodes are metallic, the HV curve is linear, (b) If one electrode has an energy gap, an edge occurs in the HV curve, (c) If both electrodes have energy gaps, two edges occur. A "negative differential conductance" exists. (After Giaever and Megerle, 1961).

A variation of STM, called scanning tunneling spectroscopy (STS), can, in principle, also provide chemical information. In STS, the applied bias is modulated (e.g., 10 mV at 10 kHz) as it is slowly swept, keeping the tip position essentially constant while the variation in the tunneling current is measured. The quantity di yj JdV) (the differential tunneling conductance), or as it is usually plotted (dii JdV)/(ii iJV) is related to the density of states in the sample. This technique has not yet been applied widely for in situ electrochemical studies, because of difficulties with separating the desired effects from simple modulation of the faradaic component. 

Figure 3.23 displays the tunneling spectroscopy measurements that were carried out on this sample. The differential conductance dZ/dU is a direct measure of the local density of states. Within the error of the measurement there is no difference between the first and the second ML of GdFe2. This observation reflects the identical geometric arrangement of the alloy in both layers. 

In a tunneling spectroscopy experiment, the tip is kept at a fixed position and the change in tunnehng current is recorded while changing the tunneling gap voltage. As outlined above, the differential conductance (d/t/dVb) recorded in this way reflects the LDOS of occupied or unoccupied states and provides valuable information on the local electronic structure of the surface [54]. 

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