Power/Performance Bits: Oct. 8

How light interacts with gold nanostructures
With the potential to possibly increase the efficiency of solar cells and photo detectors, University of Manchester researchers have discovered that graphene can be used to investigate how light interacts with nano-antennas.

The team, which also included researchers from Freie Universität Berlin and Imperial College London, have shown that graphene can be used to investigate how light interacts with gold nanostructures of different shape, size and geometry.

This interaction, through plasmon resonance, is the same phenomenon that gives color to the gothic stained glass rose window of Notre-Dame de Paris. When light shines on a metal particle smaller than the wavelength of the light, the electrons in the particle start to move back and forth along with the light wave, which causes an increase in the electric field at the surface of the particle.

When two such particles are brought close to each other, the oscillating electrons in the two particles interact with each other, forming an even higher electric field between the two particles, resulting in a coupling between the two particles. It has proven to be difficult to experimentally observe and measure the magnitude of this coupling and resulting electric field.

Wavy CNT forests draw heat effectively
Georgia Tech researchers have found that “waviness” in forests of vertically-aligned carbon nanotubes dramatically reduces their stiffness, answering a long-standing question surrounding the tiny structures. Instead of being a detriment, the waviness may make the nanotube arrays more compliant and therefore useful as thermal interface material for conducting heat away from future high-powered integrated circuits.

Measurements of nanotube stiffness, which is influenced by a property known as modulus, had suggested that forests of vertically-aligned nanotubes should have a much higher stiffness than what scientists were actually measuring. The reduced effective modulus had been blamed on uneven growth density, and on buckling of the nanotubes under compression. But based on experiments, scanning electron microscope (SEM) imaging and mathematical modeling, the Georgia Tech researchers found that kinked sections of nanotubes may be the primary mechanism reducing the modulus.

They found that the effective modulus remained low – as much as 10,000 times less than expected – regardless of whether the nanotube sandwiches were compressed or pulled apart which suggests growth issues, or buckling, could not fully account for the differences observed.

In looking for potential explanations, the researchers examined the carbon nanotubes using scanning electron microscopes located in Georgia Tech’s Institute for Electronics and Nanotechnology facilities. At magnification of 10,000 times, they saw the waviness in sections of the nanotubes.

Also, under compression, the nanotubes contact one another, influencing nanotube behavior. These observations were modeled mathematically to help explain what was being seen across the different conditions studied.

Though the loss of modulus might seem like a problem, it actually may be helpful in thermal management applications, the researcher believe. The compliance of the nanotubes allows them to connect to a silicon integrated circuit on one side, and be bonded to a copper heat spreader on the other side. The flexibility of the nanotubes allows them to move as the top and bottom structures expand and contract at different rates due to temperature changes.

The carbon nanotubes act like springs between the silicon chip and the copper heat spreader and can conduct lots of heat because of good thermal properties. At the same time, they are supple and compliant. Carbon nanotubes have extraordinarily high thermal conductivity, as much as ten times that of copper, making them ideal for drawing heat away from the chips.

This montage includes images of carbon nanotube forests. New research explains why the CNT forests have less stiffness than expected. (Source: Georgia Tech)

As the demand for heat removal from chips continues to increase, industry has been looking for new materials and new techniques to add to their toolbox for heat transfer. Different approaches will be needed for different devices, and the researchers believe this provides the industry with a new way to address the challenge.

~Ann Steffora Mutschler