Constant current mode scanning tunneling

Fig. 26a. Scanning tunneling microscopic image of a 310nmx310nm area of bare Au film epitaxially grown on a mica substrate (Au mica) measured in air constant current mode bias (Vb) of + 50 mV, tunneling current (I,) of 10 nA. b STM image of a 3.5 nm x 3.5 nm area of a bare Au/mica film constant current mode Vb = 4- 4.9 mV I, = 3.0 nA [219]...

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). 

The step size is given by the size of the scan area and the pixel resolution of the image . Tire acquired signal - either the voltage needed to move the tip up and down in z direction (constant current mode) or the tunneling current itself (con-... 

The most common STM mode of operation is the constant current mode, where the bias voltage ( tens of millivolts to several volts) is held constant while the feedback circuit connected to the z piezo electrode moves the tip up and down to maintain a fixed pre-selected tunneling current (distance). At each point along each scan line in the x, y raster pattern, the current is measured and compared to the operator-chosen set-point current (typically tens of... 

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... 

Figure 10.11 Scanning tunneling microscope (STM) images and diagrams showing the process of controlling the initiation and termination of linear-chain polymerization with an STM tip. STM images were obtained in air at room temperature in constant-current mode.

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