Scanning Tunnel Microscope

A Scanning Tunneling Microscope is an instrument to image a surface with atomic resolution. The imaging technique is based on the quantum tunneling. The tunneling current has an exponential dependence to the tip-sample distance which is used to obtain information about the surface. Not only structural but also electronic properties as e.g. the local density of states or the work function can be determined by STM.

Sample preparation

View into the STM chamber while flashing a sample

New samples are brought to the STM via a sample lock without breaking the ultra high vacuum in the STM chamber. For an STM measurement, the sample surface needs to be clean. This can be achieved by etching outside the vacuum or by ion bombardment and thermal treatment inside the STM chamber. Therefore an Argon sputter gun was installed which can also be used for cleaning the STM tip. After Argon sputtering the samples are annealed and flashed by an electron impact heating.

Measurements

In Constant Height mode of operation the scanner of STM moves the tip only in plane and the variation of the tunneling current is observed. In this mode no adjusting is needed, thus higher scan speeds can be obtained but a crashing of the tip due to surface roughness is possible.

Secondly, the Constant Current mode where the tunneling current is kept constant by a feedback loop. This feedback loop applies a voltage to the piezoelectric height control for a vertical tip displacement, which reflects the surface topography. The scan speed is limited by the feedback loop.

Dimers on Si(100)
Herringbone reconstruction of Au(111)