The wondrous nature of graphene is one that I’m sure all reader’s have heard of previously. Thermal and electric conductivity; physical strength and malleability; and intrinsic chemical stability. But when it comes down to the energy bands of graphene we hit a snag. It is described as a zero band gap semiconductor meaning the difference in energy between the conducting and valence band is pretty much zero. As semiconductors go this means graphene is difficult to use as it has limited electronic applications and really acts too much like a normal conductor for a semiconductor circuit and a bit too much like a semiconductor when a normal conductor is needed. Quantum dots, which I have written on before, do have a band gap engineered into them due to their much smaller size and fewer available energy levels to form the bands.
To probe and understand the energy levels in these graphene dots a process called two photon absorption was used. Normally when electrons and photons interact it is said that an electron can totally absorb just one photon at one instant. This means that if there was an electron in the ground state and an energy level 2 Arbitrary Units above it, and light was shone with each photon having 1 Arbitrary Unit, the photon could not reach the higher energy level. This turns out not to be true as occasionally an electron simultaneously absorbs 2 photons and makes the seemingly impossible leap. When this was induced in the graphene it was found that the spectra for the two photon absorption was actually red shifted. This is because all quantum dots from any semiconductor grow when the photon is absorbed, but graphene was found to grow many orders of magnitude bigger than any previously explored semiconductor dot. When the photons are released the shrinking of the quantum dot means that the surface is accelerating away and due to its size this acceleration is great enough to red shift the emitted photons. This strange combination of effects is what makes physics such an interesting subject to study, where everything is interconnected and the Doppler effect stretches from stars to quantum particles.