Tarun K. Bera 1, Jung-Hwan Song, Dr. 2, Arthur J. Freeman, Prof. 2, Joon I. Jang, Dr. 2, John B. Ketterson, Prof. 2, Mercouri G. Kanatzidis, Prof. Dr. 1 * Angewandte Chemie Int. Ed. 41, 7828 (2008)
So this stuff is really cool, at least I think so anyway. This comes from the Kanatzidis, Ketterson and Freeman groups at Northwestern University. So, what did they make exactly? Bera et al. synthesized two new semiconducting chalcogenides, LiAsS2 and NaAsS2, that has the strongest nonlinear optical response. The previous record was held by a silver compound, but these materials has at least ten times stronger response than that!
Oh noes, it has Arsenic! Big deal. It’s in a 3+ oxidation state, but the other pnictides, particularly Sb and Bi and P all in 3+ states are just as toxic as arsenic. That’s one of the problems with chemistry is that it gets a bad rap, but I digress, back to the awesomeness of this paper.
So what is needed for a good NLO material. There’s a paper from the Poeppelmeier group also at Northwestern University (first author P. Shiv Halasyamani, look it up, I’m too lazy to find the link and put it here) that talks about what is needed for good nonlinear optic materials. Mainly, it requires a noncentrosymmetric space group.
Now, NaAsS2, I looked it up in the Find It database and Pearson’s crystal database typically crystallizes in a centrosymmetric space group. Basically, that sucks and wont get any NLO response. Using a polychalcogenide flux, a new form of this material was synthesized by the Kanatzidis group crystallizing in a NONcentrosymmetric space group.
This is good, but another thing that is needed for a strong NLO response is polarizability of the atoms. If you look at the crystal structure presented in the paper, arsenic forms tetrahedral chains with sulfur that are really big electron cloud wise and hence has a strong polarizability. But let’s use different atoms instead and we too can get an Angewandte Paper!
Bzzt! Wrong! Phosphorous (the element above arsenic) is simply too small and will NOT produce tetrahedral chains that are noncentrosymmetric (again I looked this up). Antimony (the element below arsenic) is simply too big, and will provide a large coordination sphere for sulfur to go around. So really, you’ve optimized the tetrahedral geometry of the arsenic-sulfur units.
Now, this paper, I think is extra sooper awesome because it combines solid state chemistry, with solid state physics. These two things go hand in hand, and the Freeman group (one of the Gods of electronic structure calculation) used full potential linearized augmented plane wave (FLAPW) method for the calculations.
What the heck does that mean? Well, it’s simple really. For the atoms in the crystal lattice, they use plane wave equations of the form psi = e^(ikx) and take into account the full potential of the electrons for each atom. What happens in the interstitial spaces? Well for that they assume that the electron acts as a ‘free electron’ as if it’s in a free electron gas, so your psi = Asin(x) + Bcos(y) equation from your elemental quantum class should fit. It’s a really neat technique, and the added awesomeness is that for the exchange correlation term in the Kohn-Sham equation that they use is using screen-exchange localized density approximation (sx-LDA).
sx-LDA simply uses a local density approximation for your electrons (it’s a good DFT method), but is better at calculating bandgaps for compounds due to the Lagrange parameters in the equations actually having more physical meaning. This is a gross oversimplification, but let’s move on.
So, what is the take home message, after getting through the nitty gritty. Arsenic is awesome. It’s the perfect size to form large enough tetrahedral units to be quite polarizable and have the noncentrosymmetry required to make a good NLO material. These materials have the strongest second-harmonic generation response, beating out the previous standard, and can be synthesized in a facile manner using a polychalcogenide flux method.
what is polychalcogenide flux method? I think I’ll separate that in another entry. But I still have stacks of papers to read so I’ll do that later.