Scanned conductance microscopy of carbon nanotubes and lambda-DNA

We have devised a scanned probe technique based on electrostatic force microscopy capable of probing the conductance of samples without requiring attached leads. We demonstrate that, using our technique, conductance can be probed on length scales as small as 0.4 mum. To demonstrate the utility of our technique, we use it to probe the conductance of DNA, a subject that has been the focus of intense debate with reported results ranging from metallic to insulating. In contrast to conducting single-wall carbon nanotubes, used as a control, individual strands of lambda-DNA, a widely studied sequence, are found to be insulating on the length scale probed.