Scanning Tunneling Microscopy(STM)

 

Introduction

 

The scanning Tunneling Microscope(STM) was invented in the early 1980s by Binning and Rohrer who were awarded the 1986 Nobel Prize in Physics.

In Scanning Tunneling Microscopy(STM),a sharp metal tip (W, Pt–Ir) is brought sufficiently close to proximity of a conducting sample, and a bias is applied, so that their electron-wave functions can overlap and electrons tunnel between the two.

Fig: Schematic diagram of STM instrument

The working of STM

 

In STM once the gap between the tip and sample is about as small as the diameter of an atom, a tunnelling current flows in the range of pico and nano amperes. The magnitude of the current is very sensitive to the size of the gap, changing by a factor of 10 when the distance changes by 100 pm. The metal tip is scanned backward and forward across the solid, and the steep variation of the tunneling current with distance gives an image of the atoms on the surface. The image is usually formed by keeping a constant tunneling current and measuring the distance, thus creating contours of constant density of states on the surface. By changing the sign of the potential, the tunneling direction reverses, and thus STM can map either occupied or unoccupied density of states. The map thus illustrates features due to both the topography and to the electronic structure, and can illustrate the positions of individual atoms.

Fig: Tunneling current flows

 

Tunneling current It

It ∝ (V/d)exp(-Aφ^1/2d)

A = 1.025 (eV)^-1/2Å^-1

φ ~ 4 – 5 eV

d decreases by 1 Å,

It will be increased by ~10 times.

Conclusion

In this technique care should be taken to keep the noise signal ratio on a low level. Also the response time of the feedback has to be minimized without loosing accuracy.