Scanning current versus distance measurement

Another novel method described recently involves the use of the electron-stimulated desorption (ESD, EID) of species from a surface. Lichtmann and Campuzano [239] used a scanning electron beam to induce desorption of F+ ions from a fluoridised molecule and obtained traces of F+ current (measured in a mass spectrometer) versus distance scanned across the sample. Only qualitative results were obtained but this again illustrates the strength of the scanning technique for boundary diffusion studies. 

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). 

Fig. 17 SECM TG-SC images of ORR activity measured on Pd-Au-Co arrays in 0.5 M H2SO4. Tip-to-sample distance = 30 pm, tip current = -160 nA, scan rate = 50 pm each 0.2 s, sample potential, Es = 0.2 V (a), 0.4 V (b), 0.6 V (c) and 0.75 V versus the hydrogen reference electrode (HRE). Wm is the atomic ratio of metal M in the spot. Taken from [80] with permission of the American Chemical Society...

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