NEW THEORY EXPLAINS ELECTRONIC AND THERMAL BEHAVIOR OF NANOTUBES

Recently researchers made an important theoretical breakthrough in the understanding of energy dissipation and thermal breakdown in metallic carbon nanotubes. Their discovery will help move nanotube wires from laboratory to marketplace.We all know about the remarkable electrical and mechanical properties of metallic carbon nanotubes make them promising candidates for interconnects in future nanoscale electronic devices. But, like tiny metal wires, nanotubes grow hotter as electrical current is increased. At some point, a nanotube will burn apart like an element in a blown fuse. So, heat dissipation is a fundamental problem of electronic transport at the nanoscale.

To fully utilize nanotubes as interconnects, CNT must be characterized properly and there by getting their behavior and operating limits.Up to now, no coherent interpretation had been proposed that reconciled heat dissipation and electronic transport, and described thermal effects in metallic carbon nanotubes under electronic stress. Now in this research some theoretical results not only reproduce experimental data for electronic transport, they also explain the odd behavior of thermal breakdown in these nanotubes. For example, in both theory and experiment, the shorter the nanotube, the larger the current that can be carried before thermal breakdown occurs. Also, the longer the nanotube, the faster the rise in temperature as the threshold current for thermal heating is reduced.In nanotubes, heat generated by electrical resistance creates atomic vibrations in the nanostructure, which causes more collisions with the charge carriers. The additional collisions generate more heat and more vibrations, followed by even more collisions in a vicious cycle that ends when the nanotube burns apart, breaking the circuit.Short nanotubes can carry more current before burning apart because they dissipate heat better than longer nanotubes. Although the entire nanotube experiences resistance heating, the electrical contacts at each end act as heat sinks, which in short nanotubes are relatively close to one another, leading to efficient heat removal. This phenomenon also explains why the highest temperature always occurs in the middle of the nanostructure. In another important finding, researchers have revised the common belief that charge carriers go ballistic in short metallic nanotubes having high currents.

Researchers had previously thought that charge carriers traveled from one terminal to the other like a rocket; that is, without experiencing collisions.They have shown that the high current level in short metallic nanotubes is not due to ballistic transport, but to reduced heating effects. Owing to their large concentration, the charge carriers collide efficiently among themselves, which prevent them from going ballistic. Even in short nanostructures, the current level is determined by a balance between the attractive force of the external electric field and the frictional force caused by the nanotube thermal vibrations. The collisions among charge carriers help the energy transfer to the nanotubes which results in heat dissipation.

DNA-WRAPPED CARBON NANOTUBES SERVE AS SENSORS IN LIVING CELLS

Single-walled carbon nanotubes wrapped with DNA can be placed inside living cells and detect trace amounts of harmful contaminants using near infrared light, report researchers at the University of Illinois at Urbana-Champaign. Their discovery opens the door to new types of optical sensors and biomarkers that exploit the unique properties of nanoparticles in living systems.This is the first nanotube-based sensor that can detect analytes at the subcellular level, said Michael Strano, a professor of chemical and biomolecular engineering at Illinois. They also showed for the first time that a subtle rearrangement of an adsorbed biomolecule can be directly detected by a carbon nanotube.

At the heart of the new detection system is the transition of DNA secondary structure from the native, right-handed "B" form to the alternate, left-handed "Z" form.It was observed that the thermodynamics that drive the switching back and forth between these two forms of DNA structure would modulate the electronic structure and optical emission of the carbon nanotube. To make their sensors, the researchers begin by wrapping a piece of double-stranded DNA around the surface of a single-walled carbon nanotube, in much the same fashion as a telephone cord wraps around a pencil. The DNA starts out wrapping around the nanotube with a certain shape that is defined by the negative charges along its backbone.When the DNA is exposed to ions of certain atoms - such as calcium, mercury and sodium - the negative charges become neutralized and the DNA changes shape in a similar manner to its natural shape-shift from the B form to Z form. This reduces the surface area covered by the DNA, perturbing the electronic structure and shifting the nanotube's natural, near infrared fluorescence to a lower energy.The change in emission energy indicates how many ions bind to the DNA. Removing the ions will return the emission energy to its initial value and flip the DNA back to the starting form, making the process reversible and reusable. The researchers demonstrated the viability of their measurement technique by detecting low concentrations of mercury ions in whole blood, opaque solutions, and living mammalian cells and tissues - examples where optical sensing is usually poor or ineffective. Because the signal is in the near infrared, a property unique to only a handful of materials, it is not obscured by the natural fluorescence of polymers and living tissues. The nanotube surface acts as the sensor by detecting the shape change of the DNA as it responds to the presence of target ions.