Constant-height mode

Figure Bl.19.13. (a) Tliree STM images of a Pt(l 11) surface covered witli hydrocarbon species generated by exposure to propene. Images taken in constant-height mode. (A) after adsorption at room temperature. The propylidyne (=C-CH2-CH2) species that fomied was too mobile on the surface to be visible. The surface looks similar to that of the clean surface. Terraces ( 10 mn wide) and monatomic steps are the only visible features. (B) After heating the adsorbed propylidyne to 550 K, clusters fonn by polymerization of the C H...

Fig. 4. Operational modes for stm. (a) Constant height mode, (b) Constant current mode (18).

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 ate measured as the tip is scaimed 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 working modes are used for the STM first, the constant height-mode, in which the recorded signal is the tunneling current versus the position of the tip over the sample, and the initial height of the STM tip with respect to the sample surface is kept constant (Fig. 22(a)). In the constant currentmode, a controller keeps the measured tunneling current constant. In order to do that, the distance between tip and sample must be adjusted to the surface structure and to the local electron density of the probed sample via a feedback loop (Fig. 22(b)). 

FIG. 14 Constant height mode gray-scale image of a 5/xm-diameter pore in a polycarbonate membrane obtained with a 3 fim pipette tip. The filling DCE solution contained 10 mM TBATPBCl. The aqueous phase contained 0.4mM TEACl + lOmM LiCl. The scale bar corresponds to 10/xm. The tip scan speed was 10/xm/s. (Reprinted with permission from Ref. 30. Copyright 1998 American Chemical Society.)... 

FIG. 1. Schematic representation of the (left) constant current, and (right) constant height modes of operation of an STM. (From Ref. 38.)... 

Alternatively, the STM may be operated in the constant-height mode, whereby the z-position is kept constant, and direct variations in the tunneling current are recorded as surface corrugation. Generally, constant-current operation yields better resolution, and the constant-height mode might be used for faster data acquisition. 

One of the main advantages of contact SFM is the high spatial resolution. Variations in height as small as 0.01 nm can be easily detected in the constant height mode. Figure 8a shows as an example the image of the smectic structure of per-... 

Figure 7.22 STM images in constant height mode of (left) individual oxygen atoms on Ru(0001) at low coverage. The middle and right images shows how oxygen atoms order into islands at higher coverages (0.09 and 0.12 ML respectively). The images are part of a video sequence which shows that the O atoms exhibit considerable mobility at room temperature (courtesy of J. Wintterlin [64]).

Two different operation modes are commonly used. In the topographic mode, images are created by scanning the tip in the xy plane and recording the position required to keep It constant. A three dimensional map z(x, y, It = const.) is recorded. In the constant height mode, the probe scans the surface while the signal at the z-scanner is kept constant, and a three-dimensional image It(x,p,z = const.) is created. 

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). 

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