Carbon nanotubes are cylindrical carbon molecules with unique properties that make them potentially useful for devices such as cell phones, computers, and personal handheld electronics. Nanotubes, measuring about one-thousandth of the diameter of a human hair, cannot be seen with the naked eye, yet they are one of the strongest materials known to humankind. In fact, their size is so small that their material properties have only been explored in bundles. This led to inaccuracy and misinterpretation of experimental data, which prevented large-scale integration of carbon nanotubes into electronic devices.
Our research has focused on improving fundamental understanding of the structural, mechanical, and electrical transport behavior of these nanomaterials under various conditions. To accomplish this, we developed novel experimental tools to identify, manipulate, and measure these materials properties at the nanoscale under extreme heat while stretching them. Although we expected to confirm that carbon nanotubes are rigid materials, our data indicate that they show superplastic behavior at very high temperatures.
|Figure 1. A schematic drawing shows our multiwalled nanotube experimental setup to manipulate an individual nanotube. The gold contact on one end of the nanotube is attached to a piezoelectric manipulator that allows us to stretch the tube in a precise manner while simultaneously determining its electro-mechanical behavior. This nanomanipulating device can also heat the nanotube to extremely high temperatures by applying a high bias (V) on both ends of nanotubes. The electrical conductance (I~V curve) of carbon nanotubes can be measured simultaneously.|
Relevance to PLS Research Themes
Nanoscale materials often have new and technologically useful properties. By studying material in this extreme state of matter, we have an opportunity to explore and discover fundamental materials properties relevant to LLNL’s national security mission.
Carbon nanotubes are broadly targeted for applications relevant to national security and defense, such as in biosensing and radiation detection applications. Hence, the physical properties of these nanotubes under extreme conditions are of great relevance to fundamental science and also to essential missions of LLNL. Examples of projects that are directly relevant include carbon nanotube lipid bilayer membranes for pathogen detection and carbon nanotube mixture explosives for photographic ignition of targets.
Major Accomplishments in 2005
Despite over a decade of studies, the manipulation of individual carbon nanotubes and the accurate determination of their electrical and mechanical properties remain grand challenges. To solve this problem, we have developed, in collaboration with Boston College, a manipulator that employs a piezoelectric device to precisely stretch the nanotube (Figure 1). We have also developed a resistance-heating method to heat individual nanotubes or nanowires to extremely high temperatures while stretching them. This allows us to investigate the mechanical and electrical transport behavior of these 1D nanostructured materials under previously inaccessible conditions.
Using this new setup, we observed the stretched nanotubes at atomic resolution using a transmission electron microscope equipped with an in situ digital camera. We investigated the mechanical properties and electrical conductance of a number of single-, double-, and multiwalled carbon nanotubes. Single-walled nanotubes are a one-atom-thick layer in the form of a cylinder, while double- and multi-walled nanotubes contain two or more layers rolled into a tube shape.
|Figure 2. These sequential high-resolution transmission electron microscopy images show a single-walled carbon nanotube being stretched from 24 to 91 nm. Our experiments demonstrate that, by heating the nanotube to more than 3600 °F, it becomes 280% longer than its original form, and its diameter has shrunk by 15 times. Kinks are frequently observed during the stretch.|
Contrary to the belief that carbon nanotubes are rigid when stretched at room temperature, we have discovered a superplastic behavior in all three types of nanotubes while they are deformed at temperatures over several thousand degrees Fahrenheit. We believe that nanotube plasticity is mediated by a new physical mechanism, called kink nucleation, and motion inside nanotube walls. Because of drastic dimensional changes during elongation of carbon nanotubes (Figure 2), we further observed a large adjustable semiconducting band gap in carbon nanotubes. This allows us to tune the electronic properties of individual carbon nanotubes by straining, thereby providing a new route to investigate the fundamental electromechanical behavior of carbon nanotubes.
Carbon nanotubes were previously thought to be a class of rigid materials that could not sustain stretch under normal conditions. Our discovery of superplastic behavior in carbon nanotubes at high temperatures opens the door for their potential applications as strengthening agents in ceramics, hightemperature alloys, and nanocomposites that are subjected to extreme loading or heating conditions. The super-strains we discovered can be employed to tune the electrical properties of carbon nanotubes for insulating, semiconducting, metallic, or even superconducting applications. The physical properties observed at high temperatures also provide a foundation for a worldwide computational endeavor to understand the unique and versatile properties of carbon nanotubes.
Huang, J. Y., Y. M. Wang, et al., “Superplastic Carbon Nanotubes,” Nature 439, 281 (2006).
Huang, J. Y., et al., “Atomic-scale Imaging of Wall-bywall Breakdown and Concurrent Transport Measurements in Multiwall Carbon Nanotubes,” Phys. Rev. Lett. 94, 236802 (2005).
Yinmin (Morris) Wang
The nanomanipulation device we developed provides a platform to investigate the fundamental electromechanical behavior of many dimensionally small nanostructured materials under extreme conditions. Our discovery of the unexpected properties of carbon nanotubes is of particular importance because nanotubes and nanowires will likely be used to build innovative functional nanodevices with applications in radiation and biological agent detection, water desalination, and pressure diagnostics. As we continue to investigate the fundamental physical properties of carbon nanotubes, we will start to incorporate them into various detection nanodevices that are relevant to our national security missions. For example, single-walled carbon nanotubes can be used as piezoresistive strain gages for pressure sensing of materials at extreme conditions.