The scanning tunneling microscope was developed at IBM Zürich in 1981 by Gerd Binning and Heinrich Rohrer who shared the Nobel Prize for physics in 1986 for the microscope. STM uses a single atom tip to attain atomic resolution and an extremely fine conducting probe is held about an atom’s diameter from the sample. Electrons tunnel between the surface and the tip produces an electrical signal. When the tip slowly scans across the surface, it is raised and lowered in order to keep the signal constant and to maintain the distance. This enables it to follow even the smallest details of the surface being scanned.
Tungsten is the commonly used tip because electro-chemical etching techniques can be used to create very sharp tips. Classically, when an object hits a potential that it doesn’t have enough energy to pass, it will never go though that potential wall but always bounces back. In quantum mechanics when a particle hits a potential that it doesn’t have enough energy to pass, when inside the square well, the wave function dies off exponentially. If the well is short enough, there will be a noticeable probability of finding the particle on the other side.
Quantum Tunneling
The finite square well potential is a good approximation for looking at electrons on conducting slabs with a gap between them. So if the tip is brought close enough to the surface, a tunneling current can be created, even though there is a break in the circuit. The size of the gap in practice is on the order of a couple of Angstroms and the current is very sensitive to the gap distance. To get the distance between the tip and the sample down to a couple of Angstroms where the tunneling current is at a measurable level, STMs use feedback servo loops and converse piezoelectricity. If we can get a single atom at the tip, the vast majority of the current will run through it and thus give us atomic resolution. A STM does not measure nuclear position directly; rather it measures the electron density clouds on the surface of the sample. Most common use for STM is the measuring of high precision optical components and disk drive surface roughness of machined or ground surfaces. By measuring variations in current, voltage, tip/surface separation, and their derivatives, the electronic properties of different materials can be studied. One such element studied was the bucky ball (C60). When we press down on a bucky ball by 1/10th nm, it lowers the resistance of the bucky ball by 100 times
Vibration-Isolation
The original STM design had the tunnel unit with permanent magnets levitated on a superconducting lead bowl and used 20 L of liquid helium per hour. The presently widely used vibration protection is simple with a stack of metal plates separated by an ultra high vacuum compatible rubber spacer called viton.