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Experimental Techniques - Scanning Tunneling Microscope

To use this technique, substrates must be relatively flat and electrically conducting. To obtain an image of a substrate, the substrate is moved until it is approximately 0.3 – 1.0 nm away from a probe tip usually made of W or a Pt-Ir alloy.

The tip is attached to a piezodrive. There are three mutually perpendicular piezoelectric transducers: x piezo, y piezo, and z piezo. These transducers expand or contract in a controlled way upon application of an applied voltage. By applying a saw tooth voltage signal on the x piezo and a slower voltage ramp on the y piezo, the tip can be made to raster in a controlled fashion in the xy plane.

Using the z piezo, the tip and substrate can be brought close enough to each other so that the electronic wave functions in the tip overlap with the electronic wave functions in the substrate. Under these conditions, an applied bias voltage causes current to flow. Such a current is quantum mechanical phenomenon and is called tunneling current.

The tunneling current is amplified and changed to a voltage by a high-gain, low noise preamplifier, and then compared to a reference value. The difference is amplified and used as a feedback signal to drive the z piezo. The phase of the amplifier is chosen to provide negative feedback. When the tunneling current is larger than the reference value, the z piezo tends to withdraw the tip from the sample and vice versa. Therefore an equilibrium z position is established and kept during the whole scanning process.

The voltage on the z piezo is recorded, changed to height value (in nanometers) using the calibration values, then stored to provide a constant current contour of the sample. The magnitude of the current depends strongly on the separation between the tip and the sample, which results a 0.01 nm vertical resolution. The lateral resolution is determined by the single atom at the end of the tip in ideal case to be 0.1 nm.


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