Scanning Tunneling Microscopy

Scanning tunneling microscopy is a microscopical technique that allows the investigation of electrically conducting surfaces down to the atomic scale. In the following I will give a very short overview of the basic principle.

STM: The Basics
Color-Images: Technique

Principle of the STM
Figure 1: Basic principle of the Scanning Tunneling Microscope

In the scanning tunneling microscope the sample is scanned by a very fine metallic tip. The tip is mechanically connected to the scanner, an XYZ positioning device realized by means of piezoelectric materials.
The sample is positively or negatively biased so that a small current, the "tunneling current" flows if the tip is in contact to the sample. This feeble tunneling current is amplified and measured. With the help of the tunneling current the feedback electronic keeps the distance between tip and sample constant. If the tunneling current exceeds its preset value, the distance between tip and sample is decreased, if it falls below this value, the feedback increases the distance.
The tip is scanned line by line above the sample surface following the topography of the sample.

Tunneling Current:
The reason for the extreme magnification capabilities of the STM down to the atomic scale can be found in the physical behavior of the tunneling current:
the tunneling contact

exponential law of the tunneling current
Figure 2a:
tip-sample tunneling contact

Figure 2b:
exponential behavior of the tunneling current I with distance d

The tunneling current flows across the small gap that separates the tip from the sample, a case that is forbidden in classical physics but that can be explained by the better approach of quantum mechanics: the electrons are "tunneling" across the gap.
The tunneling current I has a very important caracteristic: it exhibits an exponentially decay with an increase of the gap d:

I= K*U*e -(k*d)       k and K are constants, U is the tunneling bias

.Very small changes in the tip-sample separation induce large changes in the tunneling current!

This has the consequences that:

  • The tip-sample separation can be controlled very exactly
  • The tunneling current is only carried by the outermost atom of the tip; the atoms that are second nearest carry only an negligible amount of the current: The sample surface is scanned with a single atom!

Tunneling Tip:
The question I am asked most is: "How do you obtain these tips with only one atom at the top?" In reality is relatively easy to obtain such tips by etching or tearing a thin metal wire. I often use the following comparison: Imagine pouring a bucket of sand on the floor. If you examine the resulting conic heap in most cases you will find a single grain of sand that represents the outermost peak. Very seldomly you will have several grains exactly representing the peak together. Now take the heap of sand for the tip and remember the exponential decay of the tunneling current. The tunneling current is carried and the sample surface scanned only by this outermost grain of sand. Many of the tips that work fine in an STM will look blunt
when imaged in an optical microscope. Only if you want so investigate a sample with a large corrugation as for instance the islands or crystallites shown in some images, you need a tip that is microscopically as well as atomically sharp.