|1Joint Research Center for Atom Technology (JRCAT), 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan
2Institute of Inorganic Synthesis, Yamanashi University, Takeda, Kofu 400-8511, Japan
3Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan
Observation of organic and biological materials at molecular level has recently attracted wide attention especially in surface science involving biomolecules such as DNA. The most powerful technique that is applicable to insulating materials is atomic force microscopy (AFM). There have been several studies demonstrating the possibility of molecular-level imaging of organic molecules  and DNA . However, the use of the contact or taping mode causes the local deformation of sample surfaces and the damage of the tip apex due to high loading and friction force between the tip and the sample surface. Such damage of both the tip apex and the sample surface prevent us from achieving higher-resolution imaging of molecular structures.
Recent progress of AFM in the noncontact mode using the frequency modulation (FM) technique allows us to image various surfaces with true atomic resolution [3,4]. This technique can be applied to weakly adsorbed organic and biological molecules on solid surfaces without any damage of both the sample surfaces and the tip apex. In this study, we have applied noncontact-AFM to high resolution imaging of self-assembled films of nucleic acid bases adsorbed on a graphite surface and DNA adsorbed on a mica surface in an ultrahigh vacuum (UHV).
The self-assembled films of the nucleic acid bases on graphite substrate were prepared by a molecular beam deposition under UHV condition and were annealed at about 330K to improve their crystallinity. The DNA was deposited on the freshly cleaved mica surface by placing a drop of DNA solution and then the sample was transferred into the UHV chamber. The noncontact-AFM measurement using Si tip was performed under the UHV at room temperature.
For the self-assembled films of the nucleic acid bases, we resolved not only the lattice images of molecular packing structures but also individual molecules. Point defect and domain structures were also imaged. The images of individual molecules revealed the fine structures which depended on the molecular structures of nucleic acid bases. These noncontact-AFM images were clearly different from those obtained by STM reported in previous studies [5,6].
Next we discussed the optimum condition for high resolution imaging by noncontact-mode AFM for the DNA sample. As a result, we found that the removal of the water layer covering on the sample surface by annealing was required for high-resolution imaging. Moreover, we resolved the right-handed helix turns in B-DNA.
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