Constant current/height mode, scanning tunneling

In constant-height mode, the tunneling current is monitored as the tip is scanned parallel to the surface. 

Figure Bl.19.2. The two modes of operation for scanning tunnelling microscopes (a) constant current and (b) constant height. (Taken from [214], figure 1.)...

The constant height mode of operation results in a faster measurement. In this analysis, the tip height is maintained at a constant level above the surface and differences in tunneling current are measured as the tip is scanned across the surface. This approach is not as sensitive to surface irregularities as the constant current mode, but it does work well for relatively smooth surfaces. 

Two different modes of imaging have been developed the constant current mode and the constant height mode. In the first experiment, the tip is scanned across the surface at constant tunnel current /T, maintained at a pre-set value by continuously adjusting the vertical tip position with the feedback voltage Vz. In the case of an electronically homogeneous surface, constant current essentially means constant tip-sample distance z (Fig. 7). 

Constant Height Mode Alternatively, on (atomically) flat surfaces, the tip-sample distance can be kept constant by disabling the feedback (Fig. 10.5b). The variations of the tunneling current are then recorded. Constant height mode is often used to atomically resolve surfaces and does not give any direct topographic information. However, it is particularly useful, when high scan speeds are desired (e.g. for video STM). 

Q.29.12 A scanning tunneling microscope is set to operate in constant height mode at 0.9 nm above the surface with a tip bias of 0.01 V. A scan is performed in the +x direction. What topography do the following measured currents (in picoamperes) suggest ... 

Fig. 2. (A) Fourier filtered ex-situ STM image, taken in constant-height mode, of a thermally oxidized HOPG electrode modified with K2[Fe(Cl J)5 [4-(aminomethyl)pyridine]] using DCC as a coupling reagent. Tip bias = -50 mV, tunneling current =1.0 nA, scan rate = 30.5 Hz. (B) Structure and proposed binding mode of the covalently immobilized iron complex.

Fig. 7.2. Scheme of scanning tunneling microscopy in the constant height mode (left) and the constant current mode (right) X, Y and Z designate the respective piezodrives, dashed line indicates tip position... 

For a smooth surface, it is also possible to keep the tip height constant above the surface. The variation of the tunneling current reflects the small atomic corrugation of the surface. The constant height mode has no fundamental difference to the constant current mode. However, the tip could be crashed if the surface corrugation is big. On the other hand, the STM can scan very fast in this mode for research of surface dynamic processes. 

Scanning Tunneling Microscope, Fig. 2 Basic operation mode in STM constant current mode and constant height mode (Hansma and Tersoff 1987)... 

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