Examining Possible Exciton Existence In Anatase

Materials that we develop in laboratories are all well and good but there is always something interesting about the behaviour of materials that have developed normally in nature.

Thank you to Saphira Minerals for this image

The example being looked at today is a mineral called anatase (here seen growing on a piece of quartz) which you can hopefully see has an almost iridescent property. Of course what else would scientists do but try and shine a light through it. Now anatase is actually the ore of aluminium (along with rutile and brookite) and is incredibly good at converting or dissipating the energy from light entering it. Despite it being the focus of study for quite some time the exact excitation mechanisms are still poorly understood. This study aims to clear some of this up by investigating whether the electron and hole pairs in the crystal remain separated or whether they combine into an exciton pseudoparticle.

In order to see whether an exciton is likely to be produced accurate knowledge of anatase’s band gap is required. To do this, pure and single crystals of the titanium oxide, at just 20 K, were induced into the photoelectric effect (or the scattering of the valence electrons depending on perspective). The photons responsible were of energy 128 eV (making them low energy X-rays), and the results were that the band energies were found to lie at about -4.0 eV and -3.5 eV. Further experiments and theoretical predictions were carried out and it is believed that the proper resonance does form in order for an exciton to develop. There is the question, however, of whether this is really representative of the behaviour of anatase at all. The scattering of excitons at impurities magnified by being at room temperature and standard pressure would mean that these excitons may just be a product of the extreme conditions this experiment is done in. It may be possible that a use could be found for anatase such as an ultraviolet controlled switch or a circuit segment that converted optical excitation, through excitons, into mechanical strain. The hope is that many of the unique effects remain when more moderate conditions are met which will mean the variety of applications is much greater.

Paper links: Strongly bound excitons in anatase TiOsingle crystals and nanoparticles

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