Scanning tunneling microscopy of freeze-fracture replicas of biomembranes

Under ambient conditions STM measurements of feature heights on biological and other soft materials are often much larger than expected and vary from point to point on the surface: STM imaging of these samples is also accompanied by deformations of the surface that would not be expected from the conventional picture of noncontact STM imaging through a vacuum gap. To explain these observations we have developed a two spring model for the interaction between the tip and sample that suggests that a fluid meniscus couples the tip to the sample leading to large height amplifications and the possibility of damage to the surface. To test this theory we imaged platinum carbon replicas of cadmium arachidate multilayer Langmuir-Blodgett films under a dry nitrogen environment, exposed to humid air, and returned to a nitrogen environment. Feature heights increased significantly in the humid environment, but were reversible upon return to a dry nitrogen environment.