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Electrical Conducting Properties of DNA Molecules in a Metal (tip)/DNA/highly Oriented Pyrolytic Graphite Configuration

We have performed the electrical characterization of DNA molecules in the structure of metal (tip)/DNA/highly oriented pyrolytic graphite (HOPG) using a conducting probe atomic force microscope (CP-AFM). In air, the resistance of DNA is strongly dependent on the contact area of the tip when the tip force is between 0 and 50 nN. At a tip force of 40 nN, the resistances of DNA [Poly(dA–dT)·Poly(dA–dT) and Poly(dG–dC)·Poly(dG–dC)] were found to be between 1 and 5 MΩ. Under vacuum, the resistance of Poly(dA–dT)·Poly(dA–dT) was found to be 0.1 to 0.5 GΩ, approximately one thousand times larger than that in air. By comparing the I–V curves of the system on tip/DNA/HOPG and those of the system on tip/SiO2 (thickness of approximately 1 nm)/Si, it was determined that tunneling current existed between the tip and HOPG in the experiment.