Scanning Tunneling Microscope
A Brief History of the Scanning Tunneling Microscope
- Invented, 1981, by Gerd Bennig and Heinrich Rohrer at
IBM's Zurich Research Laboratories.
- 1983, first images revealing atomic details were made
of gold surfaces, Au(110).
- 1986, Bennig and Rohrer win a Nobel Prize for their invention.
- 1993, Davidson College Physics Department purchases a
Burleigh Instructional STM for the mere price of $11,000.
Tunneling and Other Quantum Mechanical Words
The Scanning Tunneling Microscope
- A metal tip scans a microscopic area of the sample surface
at a distance of around 10 Angstroms-- just close enough for the electron
clouds to overlap slightly.
- The tip, made of PtIr or electrochemically etched Tungsten,
usually has one atom which protrudes further than the rest of the atoms.
This one atom tip facilitates a tunneling current on the order of one nanoamp.
|A one atom tip||Energy diagram showing tunneling between the tip|
the sample surface
- A bias voltage is applied to facilitate tunneling in
one direction in order to measure the tunneling current.
- The needle is scanned across the surface of the sample
by applying a voltage to different areas of a cylindrically shaped
- The tip can operate in one of two modes:
- Constant Current Mode- The piezzo-electic device moves
the tip toward and away from the surface to maintain a constant reference
- Constant Height Mode- The tip is maintained at a constant
height and variations in in the tunneling current are measured, thus mapping
- Since the Schrodinger equation in the space between adjacent
wells is an exponentially decaying function, the probability of encoutering
electrons and thus the tunneling current fall of exponentially with increasing
seperation between the surface and the tip.
- This exponentially changing function allows the STM to
give very precise measurements within 0.1 Angstroms.