Now odd is too small a word for a material seemingly set on winning all the records a material can win. In the first part of our series, we looked at what graphene is and how it was discovered. In part two, we explored the different techniques we can use to make graphene.
But what is it that makes this material so remarkable? Here are 10 of the strangest facts about graphene. Strength "The most amazing thing to me about graphene is its strength. This is a sheet of atoms that you can pick up. That blows my mind.
A sheet of atoms that you can pick up: say it out loud to yourself a couple of times. Everyone you ask about graphene's amazing properties says the same thing: it is really hard to pick one feature when the material is so astonishing. So let's consider a few more of them. No band gap Graphene has no band gap. A band gap is the gap between the energy of an electron when it is bound to an atom, and the so-called conduction band, where it is free to move around. An electron can't have an energy level between those two states.
This makes graphene a wonderful candidate for use in photovoltaic PV cells , for instance, because it can absorb photons with energy at every frequency — photons of different frequencies of light are converted to electrons with matching energy levels. A material with a band gap can't convert wavelengths of light that correspond to the forbidden energy states of the electrons.
No band gap means everything is accepted. This opens the tantalising possibility of highly efficient PV cells, but it's a problem if you want to use graphene in transistors, where you need the band gap to provide the isolation necessary if you want it to act as a switch that can be turned off.
It is possible to induce a small band gap in graphene by doping it. This is good enough for very fast amplifiers for radio work, but for transistors that make efficient logic circuitry you need a bigger gap. Ballistic conduction Yes, this is a really weird one: ambient temperature "unimpeded" conduction of electrons.
It was observed in multi-walled carbon nanotubes at least as far back as , and since graphene is basically an unzipped carbon nanotube, it does it too. The most amazing thing to me about graphene is its strength. The hexagonal lattice has the longest "mean free path" of any known material — of the order of microns. This is the distance an electron can travel freely without bumping into anything, or having its path disrupted by scattering; the things that induce resistance.
When the mean free path is longer than the dimensions of the material, you get ballistic transport. In graphene, the mean free path is of the order of 65 microns — long enough that electronic components could be made that would operate at ambient temperatures with virtually no resistance. But while many patents have been filed for all sorts of applications -- from bendable computer screens and solar cells to long-life batteries -- so far turning the ideas into materials or practical products has proven difficult.
Putting strong materials to use. The researchers at MIT used computer models to see if it might be possible to make two-dimensional flakes of graphene into three-dimensional structures.
To do this they needed the flakes to fuse, something they achieved through applying heat and pressure in cycles hundreds of times until the flakes formed a stable, integrated form. The researchers then looked to biological materials, including butterfly wings, coral and sea urchins, for naturally occurring geometric shapes that could be a template for the new graphene material.
One shape they observed at the microscopic level was the "gyroid," a structure with a continuous surface that is also porous, a bit like a sponge. Gyroid shapes have an enormous surface area in proportion to their volume. Fused graphene flakes that are arrayed in this geometry, the researchers found, formed an unusually light yet strong material. Similarly, a piece of paper is flimsy until it is rolled into a tube and stood on end.
Since graphene, for now, is prohibitively expensive and difficult to manufacture, other materials such as polymers or metals could instead take advantage of the inherent strength of the gyroid geometry.
Qin plans to experiment with cellulose and silk -- organic materials with high carbon content. Similar to diamond — which is a three-dimensional carbon crystal where every atom is connected to four neighbours — these strong bonds lend the structure significant robustness.
Specifically, graphene has incredible tensile strength, especially on a small scale. This means that compared to a thin string of crystalised steel just a few micrometres across, graphene is more than six times harder to rip apart. Tests with other, less ideal forms of steel have previously suggested it could be hundreds of times stronger. The basic hexagonal shape of graphene forms the basis of a fullerenes, which is a hollow molecular structure made up of 60 or more carbon molecules, like those beautiful carbon spheres called ' buckyballs '.
The nanoscale mesh on these structures is so flexible, it can be rolled into hollow cylinders, making them potentially useful containers on a molecular scale.
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