A VERY-LOW CURRENT SCANNING TUNNELING MICROSCOPE

The applications of the scanning tunneling microscope (STM) in air are usually restricted to good conducting materials as clean metals. dope d and passivated semiconductors, or to some molecular adsorbates depos ited onto graphite. In order to study poor conducting materials as bio logical molecules, we have built a very low current STM. This instrume nt can routinely be operated at 0.1 pA while having a bandwidth of 7 k Hz. The advantages of using very low currents are illustrated by imagi ng 5-nm-thick purple membranes. These membranes can only be imaged at currents smaller than 2 pA. (C) 1995 American Institute of Physics.