Scanning Tunneling Microscopy
Fun facts
- High vacuum is not necessary, but often used.
- Performed on conducting and semi-conducting materials.
Principle
A voltage is applied on the probe tip and it is brought close to the surface of the sample. The probe measures the tunneling current that passes from the sample to the tip. How can there be a tuneling current? In classical physics, if a particle would encounter an energy barrier higher than its own energy, it would bounce back. But tunneling is a quantum effect. When an electron meets an energy barrier higher than its own energy it will penetrate the energy barrier, and if the barrier is thin enough (which in this case means a small distance between the probe and the suface) it will have a chance to go through the barrier. This probability of tunneling decreases exponentially with the distance from the tip of the probe to the surface. The tunneling current is also dependent on the density of states of the molecules on the surface of the sample.
The tunneling current is therefore dependent on the density of states at the sample surface (work function) and exponentially dependent of the distance.
Operation modes
- Constant z, changes in tunneling current is monitored. Interpreted as either change in work function or probe-sample separation.
- Tunneling current pre-selected and varying z to keep electron field emission current constant. Changes in z interpreted as change in topography.
- Scan bias voltage at each x-y pixel. Obtain local value for voltage dependence of i(V) for both given tip-sample separation and x-y location. Slope of i(V) can be interpreted in terms of local density of states. Results in excellent atomic resolution of surface.
