Scanning tunneling microscopy
Constant current mode.
In this mode, a sharp, conductive, typically metallic probe (tip) is brought so close to the (conductive) sample that the electron “jumping” through the vacuum, i.e. tunneling from the tip to the sample and in the opposite direction, becomes possible. By applying certain so-called bias potential between the tip and the sample, one obtains tunneling current flowing between the two. The value of tunneling current is exponentially dependent on the tip-sample separation: the closer the tip to the surface, the higher the current, and vice versa. If a certain value of tunneling current is maintained by a feedback loop which keeps the tip-sample separation constant, by adjusting the tip-sample separation while scanning the tip across the sample surface, the tip follows the topography of the sample. The signal from the feedback loop is digitized and processed by a computer, revealing the information about sample topography, ultimately with atomic precision.
Constant height mode.
In this mode, the probe is scanned across the sample at the constant height, without any feedback or position adjustment. The tunneling current variations are thus proportional to the sample topography. This mode must be used with caution due to the risk of crashing the tip into the surface if the latter is tilted, or into the surface features if the surface is mesoscopically rough.
In this mode, tunneling current in the region between V and V+dV is registered and its value is used for imaging. Thus spatial arrangement of energy states contributing to the energy (V, V+dV) may be determined. Typically performed with a lock-in amplifier.
*schematics coming soon*
In this mode, a quartz probe with a tuning fork geometry and a metallic tip in the end oscillates at a resonant frequency and an amplitude of 1-2 nm. The metallic tip is brought in the tunneling contact with the surface only at the a certain part of oscillation, while the rest of the time the tip is further away from the surface. This way, the tip is less likely to pick up adsorbates from the surface and the images are less noisy and better resolved.
Non-contact Atomic Force Imaging
In this mode, a quartz probe with a tuning fork geometry is brought into resonance by oscillating the scanner around the resonance frequency. When close enough to surface, typically 2-3 Å away, the interaction between the sharp tip monted on the probe’s free prong and the surface starts to affect the resonance frequency of the probe. These changes may be detected due to the piezoelectric effect in the quartz.