Monday, August 15, 2016
OT-SpectroscopyNOW blog-Phosphorene: Two-dimensional Raman
Raman spectroscopy and transmission electron microscopy have been used by an international team to investigate the phosphorus analogue of graphene, the two-dimensional phosphane, known as phosphorene.
Phosphorene has potential applications in a new class of semiconducting transistor for that perennial aspiration, the ever faster and more powerful computer of the future. Unfortunately, while phosphorene can conduct electrons its ability to do so is anisotropic, meaning it depends on which way you orient it relative to the system as to whether it does so or not. Thus, a quick and simple way to determine the orientation of the material was needed for experimental setups and now, a team comprising researchers from the Massachusetts Institute of Technology, the Rensselaer Polytechnic Institute (RPI) in Troy, New York state, Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania, has done just that. There approach accurately determines orientation by examining the interaction between light and electrons within phosphorene or other thin layers of black phosphorus.
Materials scientists have been studying phosphorene intently since it was first isolated in 2014. RPI's Vincent Meunier and his team confirmed the structure of phosphorene that same year. "This is a really interesting material because, depending on which direction you do things, you have completely different properties," explains Meunier, a phenomenon that might of course be exploited in devices. "But because it's such a new material, it's essential that we begin to understand and predict its intrinsic properties."
Meunier and colleagues have now built on the theoretical modelling and prediction of the properties of phosphorene using Rensselaer's supercomputer in the Center for Computational Innovations (CCI). On the basis of their calculations, they have home in on certain features of this novel material that will ultimately help physicists and materials scientists better understand it and thence technologists make use of those properties.
Writing in the journal ACS Nano Letters, the team initially set out to refine an existing technique for determining the orientation of the crystal using Raman spectroscopy. The team were reviewing their Raman data and spotted a few unexplained inconsistencies. So, they next turned to obtaining images of the orientation of their crystalline samples using Transmission Electron Microscopy (TEM), and lined these up with the "images" gleaned from the Raman results. As a topographic technique, TEM offers a definitive determination of the orientation of a crystal, but takes a lot more effort than recording a Raman spectrum. Nevertheless, the comparison revealed that electron-phonon interactions alone did not account for the orientation of the crystal. And the reason why led the way to yet another anisotropy of phosphorene - that of interactions between photons of light and electrons in the crystal.
The Raman spectrum should be intrinsic to the material and thus show the anisotropy of phosphorene. "But, it turns out that if you shine the light in different directions, you get different results, because the interaction between the light and the electrons in the material - the electron-photon interaction - is also anisotropic, but in a non-commensurate way," explains Meunier. The team suspected that phosphorene was anisotropic with respect to electron-photon interactions, but hadn't quite anticipated the significance of the property. "Usually electron-photon anisotropy doesn’t make such a big difference, but here, because we have such a particular chemistry on the surface and such a strong anisotropy, it's one of those materials where it makes a huge difference," Meunier adds.
Fundamentally, the discovery reveals a limitation in what current interpretation of Raman spectra can achieve in studying these materials. "It turns out that it's not so easy to use Raman vibrations to find out the direction of the crystal," Meunier explains. "But, and this is the beautiful thing, what we found is that the electron-photon interaction (which can be measured by recording the amount of light absorbed) - the interaction between the electrons and the laser - is a good predictor of the direction. Now you can really predict how the material will behave as a function of excitement with an outside stimulus."
Meunier worked with Mildred Dresselhaus of the Massachusetts Institute of Technology, as well as colleagues at Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania.