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STMs can be designed to scan a sample in two different modes: constant-height or constant-current mode.
Constant-height mode:
The tip goes in a horizontal plane above the sample and the tunneling current varies according to the topography and the local surface electronic properties of the sample.
The tunneling current measured at each point on the sample's surface constitute the data set, i.e. the topographic image.
Constant-current mode:
STMs use feedback to keep the tunneling current constant by adjusting the height of the scanner at each measurement point. The motion of the scanner constitutes the data set.
Each mode has his own features: constant-height mode is faster because the system doesn't have to move the scanner up and down, but it provides useful information only for relatively smooth surfaces; constant-current mode can measure irregular surfaces with high precision, but the measurement takes more time. |
STM tecniques
