Scanning Tunneling Microscopy

The scanning tunneling microscope (STM) works by scanning an extremely sharp tip over a conductive surface. Due to the quantum mechanical tunnel effect, electrons can "jump" from the last couple of atoms of the tip to the surface, through the vacuum. This happens when the vacuum gap is of the order of only one or a few atoms.

In STM, the application of a finite bias voltage between tip and sample, combined with the constant tunneling of electrons through the gap, results in a steady flow of electrons from tip to sample or vice versa: a current. Typically, we maintain a bias voltage of approximately 1 or 2 volt, and maintain a tiny tunneling current of the order of just several picoampere or nanoampere.

When we perform STM in current feedback mode, it means that we keep the tip-sample distance approximately the same by actively retracting the tip whenever the current increases beyond a certain setpoint, and moving it closer to the surface when the current drops below. When combined with horizontally moving over the surface, we can then actively trace the height profile of the surface, down to the atom.

The simulations below show the effects of feedback settings and loop gain on STM scanning. These simulations were made in Mathematica, with the aid of MathemaTB. Here, the tip is moved over the surface and the current is calculated by summing over the negative exponentials of the distances of the lowest tip atom to all atoms on the surface. The black dots represent the trace of the tip height, while the blue dots represent the magnitude of the tunneling current.

The effect of lowering the bias voltage is similar to that of increasing the current setpoint: it brings the tip closer to the surface. Scanning close to the surface enables features to be resolved more sharply, but it is also easier for the tip to crash into something.

Setting a larger bias voltage or lower current setpoint results in the tip tracing out the surface at a safer distance. The drawback is that the features are not as well-resolved anymore.

Maintenance

The life of an experimentalist is not always as shiny and glamorous as it seems. All that glitters is not gold (except for our surfaces, those are actually pure gold). Doing beautiful experiments, making great new discoveries and publishing in high-impact journals is only possible when everything is working well. But a scanning tunneling microscope is a complicated machine, as is all its peripheral machinery. So a significant portion of the work of a scientist comprises troubleshooting, repairing, maintaining and engineering. The images below give an impression of the repairing/engineering work I have been doing.