Rubber is a good thermal insulator, but not a very good electrical conductor; copper, on the other hand, is a great electrical conductor, but it dissipates heat. A material that is a good electrical conductor and a thermal insulator is something new. Researchers in the Nojeh Nanostructure Group are looking for nano-materials that can be used as efficient electron sources. While they are experimenting with nano-materials they can find unpredicted and surprising results. Recently they have found that carbon nanotube forests can act as heat traps or thermal insulators while also conducting electricity.
Growing Carbon Nanotube Forests (photo of lab and tubes)In the Nojeh Nanostructure Group researchers grow their own nanotube forests for study. Carbon nanotubes are not expensive to grow. They require a source of hydrocarbon– this lab uses ethylene– an inert gas like helium, and the ability to precisely control a heat source. The tubes are grown on a small sheet and gases flow passed the sheet and passed a heated zone. By controlling the flow of gases and the heated area, the height, density and verticality of the nanotube forest can be controlled. Researchers are also able to produce single walled tubes, double walled tubes (a tube inside a tube) or multi-walled tubes.
Parham Yaghoobi, a Ph.D. student in Dr. Nojeh’s Lab, studies the interactions of light with nano-materials, particularly photo emission, the emission of electrons when photons are applied to a material. A material that emits a lot of electrons is considered a good electron source, and is necessary in the production of many things. Historically, cathode ray tube televisions and scanning electron microscopes have both needed a material and process that can generate electrons. Today it’s computer chips that require this technology.
Quantum Efficiency
Interested in the nanotube forest’s capacity for electron emission, Parham began his experiments using ultraviolet light, a light with a highly energetic photon. Emission is measured by comparing the number of electrons emitted in relation to the number of photons put in. This ratio is known as quantum efficiency. Specifically designed to be an electron source, semi-metals have the highest quantum efficiency. However, semi-metals are very complex structures that need to be produced in a vacuum (very expensive) and are highly volatile (they do not last very long), so they are not commercially attractive as an electron source. Solid metals, on the other hand, can operate in low vacuum conditions and are indeed less volatile, but they have a much lower quantum efficiency than semi-metals.
Through his research, Parham has found that nanotubes have a quantum efficiency that compares or is better than that of solid metals, and actually approaches the quantum efficiency of semi-metals. Plus, nanotubes are robust, they can operate in low vacuum conditions and they are inexpensive to produce.
Visible Light
After assessing ultraviolet light, Parham went on to investigate the quantum efficiency of nanotubes using the less energetic photons of visible light. He and his fellow researchers discovered that if they focused the light with a lens, they could see electron emission from the nanotubes. At first their results were ambiguous because it seemed 
as though the electrons were not energetic enough to kick out an electron from the nanotube.“We were using visible light, not very much power, and so we thought the nanotubes weren’t heating up to the temperatures required to kick out an electron. You usually need about 2000-3000 degrees to kick out an electron. The laser power we were using was only a few hundred milliwatts.” But as their measurements were refined Parham discovered that electrons were in fact being emitted under these conditions.
Heat Trap
Parham found that the thermal conductivity– how fast nanotubes can dissipate heat– drops off radically as the temperature increases. What is so surprising about these findings is that carbon nanotubes are known for being very good heat dissipaters at room temperature, On a nanotube forest chip, as laser energy increases, a small glow appears belying an area that is not conducting heat away. That little spot is actually very hot; using only a ten milliwatt laser it climbs as high as 3000 degrees, temperatures hot enough to kick out electrons. The team has tested several kinds of carbon nanotubes, and while the causes are still unknown, trapping heat seems to be true in each case. This is a new physical property of nanotubes that carries allot of potential for scientific discovery as well as practical application.
Read More: Nojeh Nanostructures Group