A scanning tunneling microscope suitable for biological applications

We have designed and built a scanning tunneling microscope (STM) suitable for biological applications. A sound-isolated chamber contains the STM unit; the chamber can be evacuated or filled with inert gas, after the sample and tip are loaded, in order to reduce contamination on sample and tip. The STM unit is mounted inside two separable cylindrical supports. The lower cylinder contains the sample holder mounted on top of a step-motor controlled x-y-z stage, while the upper one contains the piezoelectric tip scanner. An I/V converter preamplifier mounted inside the x-y-z stage and a completely digitized feedback circuit, together with a full software package in UNIX X-window environment and data processing, allow fast sample-tip approach and reduced acquisition time. Atomic resolution images of highly oriented pyrolytic graphite (HOPG) in air and reproducible large scan on a platinum grating are routinely obtained. Images of human liver ferritin deposited on an activated gold substrate have also been obtained.