Scanning tunneling microscopy of DNA: Atom-resolved imaging, general observations and possible contrast mechanism

We have shown that it is possible to image DNA with atomic resolution using scanning tunneling microscopy (STM), [R. J. Driscoll, M. G. Youngquist, and J. D. Baldeschwieler, Nature 346, 294 (1990)]. Here we describe that data together with our general observations on STM of DNA in ultrahigh vacuum. We also suggest a possible contrast mechanism for DNA imaging by STM based on wave function orthogonality requirements between a molecule and its substrate. Topographic images are presented which resolve atomic features in addition to the double helical structure and nucleotide pairs of the DNA molecule. Comparisons of experimental STM profiles and modeled contours of the van der Waals surface of A-DNA show excellent correlation. Successive scans show that the imaging is nondestructive and reproducible. For this study, double-stranded DNA was deposited on highly oriented pyrolitic graphite without coating, shadowing, or chemical modification.