Scanning constant height mode

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.)... 

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 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). 

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 ... 

The data provided is only for a single scan in the x-direction so it is not possible to determine from this data whether the variation suggests regular conical variation (like an egg crate pattern) or some other periodic pattern. In addition because the current falls exponentially with increasing distance between the STM tip and the conductive surface, the constant height mode is less sensitive to detecting variations in surface topography as the surface falls away from the tip. 

Nearly all of the published SECM images are made in the constant-height mode, where the probe is scanned at a constant reference plane above the... 

Constant height mode of operation is reserved for small scan areas and atomically smooth surfaces. The use of little or no feedback during scanning in constant height mode makes it possible to scan at much higher rates than in constant current mode, which is limited by the feedback circuitry. 

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... 

Instrumentation. A cantilever with a sharp tip interacting with the surface under investigation is used. The actual bending of the cantilever is measured with a laser beam deflected from a mirror-like surface spot on the back of the cantilever towards a position-sensitive photodetector. The measured signal is used to control the piezo actuators. A constant force mode in which the cantilever-surface distance is kept at a preset interaction force and a constant height mode of scanning operation are possible. The principle of operation is schematically outlined in Fig. 7.9. 

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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