NEW METHODS FOR DEPOSITING AND IMAGING MOLECULES IN SCANNING-TUNNELING-MICROSCOPY

Methods and apparatus are described to deposit and image molecules by scanning tunneling microscopy (STM) under an inert atmosphere. Three methods of applying molecules have been evaluated: equilibrium adsorption from the vapor phase, sublimation, and electrospraying. Using these methods, a variety of organic and biopolymer molecules have been deposited and imaged on graphite and on gold (111), grown epitaxially on mica. Compared with alternatives, such as the use of high vacuum apparatus or glove boxes, these procedures offer some important advantages: they are inexpensive, convenient, and more rapid. Mercaptoethanol, ethanolamine, ethanol, acetic acid, and water produce two-dimensional crystalline adlayers on gold substrates, when they are introduced into the scanning cell as vapors. These adlayers are assumed to involve hydrogen bonding of the molecules to an oxide of gold formed on the surface. Electrospraying protein solutions on gold surfaces yielded images of individual protein molecules with lateral dimensions close to those measured by X-ray analysis, and thicknesses of 0.6-1.3 nm. In the case of metallothionein, the known internal domain structure of the molecule was reproducibly observed. No detailed internal structure could be resolved in the other examples examined.