Shining a Light on Phosphorene's Crystal Structure

A measurement method's poor performance leads to a new technique that exploits that failure

Illustration: Matthew Cherny
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Black phosphorus—sometimes referred to as phosphorene in a nod to its 2-D cousin, graphene—has been on a research tidal wave since 10- to 20-atom-thick sheets of the material were first exfoliated back in 2014.

The research community has been excited by a number of its properties, like its tunable band gap, which opens up all sorts of photonic applications. But they have also perceived its intrinsically strong in-plane anisotropy, which means its properties are dependent on the orientation of the crystal, as being at once a strength and a weakness.

Now a joint research project involving scientists from MIT, Tohoku University in Japan, Oak Ridge National Laboratory, the University of Pennsylvania, and Rensselaer Polytechnic Institute in New York, have developed a method for determining the orientation of the crystal that should make it easy to deduce the properties of a given sample of black phosphorous and help pave the way for greater use of phosphorene.

“This is a really interesting material because, depending on which direction you do things, you have completely different properties,” said Vincent Meunier, head of the Physics, Applied Physics, and Astronomy Department at Rensselaer and a leader of the team that developed the new method, in a press release.

In research described in the journal ACS Nano Letters, the researchers first turned to Raman spectroscopy as a jumping off point to determine the anisotropy of the crystal.

Raman spectroscopy involves shining a single color of light on a material and, by measuring how the light interacts with the material, deducing information on the material’s composition and properties. In their particular take on Raman spectroscopy, the researchers used a laser to measure the vibrations of the crystal when energy passed through it. The vibrations are the interactions that occur between electrons and phonons, which are the two main elementary particles in solids. These electron-phonon interactions are also anisotropic, so that once you know their direction you can predict the orientation of the crystal.

It all would have ended there happily, especially since Raman spectroscopy is a fairly simple technique. However, the researchers noticed a number of inconsistencies that required further investigation. To dig deeper, they turned to Transmission Electron Microscopy (TEM). TEM involves shooting a beam of electrons through a very thin specimen of a material; as the electrons interact with the material, an image is formed that is then magnified. This is a more complicated microscopy technique, but it yields a definitive determination on the orientation of the crystal.

By comparing the TEM’s results with those from the Raman spectroscopy, it became clear that Raman spectroscopy was not accurately predicting the orientation of the crystal. But the reason why the two results didn’t quite jibe led them to a new understanding: The interactions between photons and electrons in the crystal are also anisotropic.

“In Raman you use a laser to impart energy into the material, and it starts to vibrate in ways that are intrinsic to the material, and which, in phosphorene, are anisotropic,” said Meunier in the press release. “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.”

While it was fairly predictable that phosphorene would be anisotropic in its photon-electron interactions, what the researchers couldn’t have guessed was just how important this property was.

“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 said. Meunier added:

It turns out that it’s not so easy to use Raman vibrations to find out the direction of the crystal. 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.

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