Archive | Solid-State Physics RSS feed for this section

Graphene and DNA sequencing

8 Nov

One of the most exciting applications for graphene is DNA sequencing. Some clever folks at the Kavli Institute of Nanoscience are using graphene to read genetic codes in an entirely new way. They drew a little hole in a sheet of graphene, called a nanopore, and then they use that graphene to separate two liquid chambers. This whole is so small that things go through it one at a time. By monitoring the voltage changes across the sheet of graphene as molecules pass through the nanopore, they can get a pretty good idea of what is passing through, nucleotides for example. The DNA strand gets pulled through the nanopore like a thread in sewing, and the DNA sequence can be read as it passes through. It’s so simple! Incredible.

This could allow forensics analysts to do DNA testing in a matter of hours, instead of days. Which would mean a world of difference to real life detectives, and faster-moving tv detective drama plots for the rest of us.

Graphene and DNA sequencing article here.

Also, here is a video if you want to learn a little more about graphene: how to make it (the simple version), and some of its amazing properties and applications.



Graphene is the next buzz word

7 Nov

Carbon just keeps on surprising us. It turns out that one of the building blocks of life of earth, the carbon atom, has some of the most amazing properties we’ve ever seen. Pack it in tight with a lot of pressure and heat and you get diamond, a little looser structure will give you graphite. Cook it just right so that it builds itself up atom by atom in a straw-like structure and you have carbon nanotubes that can be twisted together like a rope to form a material stronger than steel, flexible and otherwise amazing (learn more about carbon nanotubes here). Take an atom thick layer of carbon atoms bound in a hexagonal pattern and you have graphene: a material with extraordinary properties, and the buzz word of the future.

Graphene is strong, flexible and it conducts electricity magnificently. It isn’t a metal, and it doesn’t have a ton of extra electrons floating around inside it like a metal does. Graphene is more like a superconductor, with its conducting electrons in discreet energy bands. But the lowest of these energy bands reaches all the way down to the highest energy of electrons in bound states, making it an amazing conductor. As is, graphene is too good a conductor. We need to be able to manipulate the properties of graphene in such a way as to create a gap between the band of energies of the bound electrons and the band of energies of the conducting electrons, a band gap. If we do that, and it’s easy enough to create and collapse the band gap at will, then we have a switch… and we are in business… the electronics business to be precise. We’ve made electronics about as small and fast as we can using current technologies. Everyone is looking for the next big thing in electronics. And graphene-based transistors could be it.

Two separate groups have published papers in the past couple weeks showing a tunable band gap in graphene. There’s still a lot of work to be done, since graphene is tricky to make exactly right. Nevertheless, it’s pretty exciting. Get a patent as soon as you can, people!

You’ll find the papers here and here.


24 Oct

This is pretty amazing. Let me tell you a bit about what is going on here. Superconductors do not like magnetic fields. If possible, the superconductor will expel the magnetic field from its interior. This is called the Meissner Effect. But in this case, the superconductor is a super-thin wafer of ceramic on top of a sapphire substrate. It is so thin that the magnetic field does penetrate the superconductor, but in discreet quantities called flux tubes. The flux tubes penetrate weaknesses in the superconductor, like grain boundaries. Any spacial movement of the superconductor would change the position of the flux tubes. The flux tubes really want to stay in the grain boundaries, so the entire superconductor is locked in place. Strongly.

Image courtesy of Tel-Aviv University


Applied magnetic field enhances superconductivity

22 Sep

Metals conduct electricity better at low temperatures. Superconductivity is a phenomenon exhibited by some materials where the resistance of that material drops to zero, meaning that it conducts electricity very well, below a certain critical temperature. And I mean VERY well…like electrical current can run around forever in there. (Picture shows levitation of a magnet due to an induced magnetic field by the superconductor beneath).

Superconductivity is a very useful feature, but the problem is that the critical temperature is usually extremely low. It is a bit impractical to work with something that has to be kept at such a low temperature. There are high-temperature superconductors, but we are talking 90K (-300 degrees F),which is still pretty cold. Scientists have been looking for ways to increase the critical temperature of materials to make superconductivity more applicable.

For a minute, let’s look at the theory. There are a couple different ways of explaining the phenomenon of superconductivity. BCS theory describes it as a “condensation” of elections into bound, boson-like, states due to electron-phonon (quantum mechanical characterization of vibration) interaction. Which pretty much means that the electrons pair up because of the attractions and repulsions caused by lattice vibrations. You can kind of think of it classically in that the electron will pull the positive parts of the lattice towards itself, causing a net positive density around the first electron, which then attracts another electron. These are known as Cooper pairs–Cooper is the “C” in BCS.

According to BCS theory, applied magnetic fields should lower the superconductivity by disrupting these pairs of electrons. It would break up the symmetry. There have been some cases where an applied magnetic field slightly enhances the superconductivity, but never actually raises the critical temperature. It was thought that the magnetic field suppressed the negative effects of paramagnetic impurities in the material, causing the slight increase in superconductivity. This paper, however, shows that applied magnetic field can actually raise the critical temperature of the material, while the presence of impurities still decreases it, which completely flies in the face of accepted theory.