PRECISION HEIGHT MEASUREMENTS OF FREEZE-FRACTURE REPLICAS USING THE SCANNING TUNNELING MICROSCOPE

We propose a general model for the interaction between a scanning tunneling microscope tip and a surface being imaged in air. The van der Waals force and a liquid bridge formed from condensation cause a net attraction between the tip and sample. If the sample is flexible or weakly bound to the scanning tunneling microscopy (STM) base our model predicts this attraction can lead to a significant amplification of the height of surface features. STM images of a freeze-fracture replica of a mica surface clearly show the amplification. Freshly made replicas of an atomically smooth mica surface had a standard deviation in their heights of 0.51 nm, but this increased to 0.61 nm after five days in a Petri dish and to 1.64 nm after one day of exposure to air. Methods of further reducing the amplification are discussed.