Electron-microscope images of the life and death of a nanotube device provide a better understanding of how such devices can fail.
Carbon nanotubes are seen by many as promising building blocks in the construction of future electronic devices and circuits. Their small size and high surface-to-volume ratio makes them particularly suitable for high-precision optical, mechanical and chemical sensor applications. But these same traits also make nanotubes vulnerable to degradation and failure. Writing in Applied Physical Letters, Thomas Yuzvinsky and colleagues1 present a sequence of electron microscope images that follow the evolution and breakdown of a multi-walled carbon nanotube as an electrical current is passed through it. As well as providing striking detail for one mechanism of nanotube failure, the results also demonstrate how nanotubes conduct electricity through their cross-sectional structure.
By mounting nanotubes between gold contacts grown on a silicon nitride membrane that was thin enough (20 nm) to be transparent to the beam of a transmission electron microscope, Yuzvinsky et al. obtained real-time images of the nanotubes as an electrical current was passed through them. In a typical case, at voltage and current less than 1.7 V and 190 A, respectively, they found no visible change to either the nanotube or its surrounds. As the voltage was slowly increased, they found that the heat generated in the nanotube caused gold nanoparticle contaminants, present on the nanotube and supporting membrane as a result of the growth of the contacts, to evaporate. Yet, despite this requiring a temperature of around 1,200 K, the authors found no evidence of degradation or failure in the nanotube at that point.
Not until the voltage was increased still further did Yuzvinsky et al. begin to observe the onset of current-induced damage in the nanotube. At voltages above 1.9 V, the authors found that holes began to form in the supporting membrane, enabling them to collect higher-resolution images of the nanotube, which itself began to thin. These images along with electrical measurements collected at the same time reveal that the nanotubes erode one wall at a time. Moreover, by modelling the electrical measurements, which consist of a series of discrete steps in current as each wall erodes under the application of a constant high voltage, the authors show that the current flows evenly through all layers of the nanotube — in contrast to earlier data suggesting that, at low temperatures, multi-walled carbon nanotubes conduct electricity only in their outer walls2.