Nanoscale electronic devices like field-effect transistors have long promised to provide sensitive, label-free detection of biomolecules. In particular, single-walled carbon nanotubes have the requisite sensitivity to detect single molecule events and sufficient bandwidth to directly monitor single molecule dynamics in real time. Recent measurements have demonstrated this premise by monitoring the dynamic, single-molecule processivity of three different enzymes: lysozyme, protein Kinase A, and the Klenow fragment of DNA polymerase I. In each case, recordings resolved detailed trajectories of tens of thousands of individual chemical events and provided excellent statistics for single-molecule events. This electronic technique has a temporal resolution approaching 1 microsecond, which provides a new window for observing brief, intermediate transition states. In addition, the devices are indefinitely stable, so that the same molecule can be observed for minutes and hours. The extended recordings provide new insights into rare events like transitions to chemically-inactive conformations.
As electronic devices shrink to the one-dimensional limit, unusual device physics can result, even at room temperature. Nanoscale conductors like single-walled carbon nanotubes (SWNTs) are particularly useful tools for experimentally investigating these effects. Our characterization of point defects in SWNTs has focused on these electronic consequences. A single scattering site in an otherwise quasi-ballistic SWNT introduces resistance, transconductance, and chemical sensitivity, and here we investigate these contributions using a combination of transport and scanning probe techniques. The transport measurements determine the two-terminal contributions over a wide range of bias, temperature, and environmental conditions, while the scanning probe work provides complementary confirmation that the effects originate at a particular site along the conduction path in a SWNT. Together, the combination proves that single point defects behave like scattering barriers having Poole-Frenkel transport characteristics. The Poole-Frenkel barriers have heights of 10 – 30 meV and gate-dependent widths that grow as large as 1 μm due to the uniquely poor screening in one dimension. Poole-Frenkel characteristics suggest that the barriers contain at least one localized electronic state, and that this state primarily contributes to conduction under high bias or high temperature conditions. Because these localized states vary from one device to another, we hypothesize that each might be unique to a particular defect’s chemical type.
Dual color four-wave-mixing is used to visualize individual gold nanowires and single carbon nanotubes. The
strong nonlinear signals, which are detected at the anti-Stokes frequency, originate from the electronic response
of the nanostructures. In gold nanowires, the collective electron motions produce detectable coherent anti-Stokes signals that can be used to study the orientation and relative strength of the structure's plasmon resonances. In single walled carbon nanotubes, coherent anti-Stokes contrast can be used to map the orientation of the electronic resonances in single tubes. Coherent anti-Stokes imaging of the material's electronic response allows the first
close-ups of the coherent nonlinear properties of individual structures and molecules.
A variety of noise measurements have been accomplished on electronic devices incorporating individual single-walled carbon nanotubes. Noise with a 1/f frequency dependence and noise attributed to two-level fluctuators are independently measured, even when these two components occur simultaneously in the same device. We demonstrate the importance of isolating these two components before attempting quantitative analysis, and then proceed to characterize devices as a function of temperature and processing history. High temperature desorption, surface passivation with polymer, and encapsulation in SiO2 films followed by forming gas annealing are three different process pathways which failed to substantially decrease the noise in these devices. In all of the devices measured, the 1/f noise components are found to only weakly decrease with temperature and to be practically independent of processing history. The two-level fluctuators, on the other hand, appear to be thermally activated and their contribution to the total noise typically increases with different processing steps.