DNA Molecular Electronics

   Charge transport through the DNA molecule has attracted interest for nanoscale electronics, due to the molecule's stacked pi-to-pi interactions. The stacked aromatic bases overlap just as they do in organic compounds capable of sustaining appreciable conduction (e.g. TTF-TCNQ). The question is not as much whether DNA is an insulator or not (in our experiments it likely is, since the resistivity increases on decreasing the temperature), but whether the molecule's conductivity, however low, can be reliably quantified. We also investigate whether the charge transport mechanisms can be related to known mechanisms. Our experiments are performed on lithographic Au electrodes, fabricated either by photolithography or electron beam lithography (an example of each is depicted, I-V data at room temperature, on lambda-DNA).

   DNA differs from many polymers in its variability: base pair sequence, length, attachment to electrodes, and environment all determine its charge transport properties. Our measurements are performed on DNA strands aligned between Au electrodes on SiO2 surfaces, rinsed to elminate salts and dried. The DNA is hence for example likely not in the biologically relevant B-form. We have found values around 10 Tera-Ohm per lambda-DNA strand (16 micron long). More importantly, the shape of the current-voltage characterisitics is sensitive to modifications such as ligation gaps in the phosphate backbone: an incompletely ligated population leads to a conductivity gap of about 3 eV around zero applied bias. The examples shown here correspond to fully ligated phosphate backbones, and are appreciably linear. The origin of the conductivity gap in the incompletely ligated DNA can lie in an energy barrier, in interband tunneling or in a Coulomb blockade effect. Further, our experiments on incompletely ligated DNA, on various contacting methods, and on single-stranded DNA also show that the complete double helical structure plays an important role in electronic transport.