Archive for the ‘Journal Articles’ Category

So I decided to write some more since no one is in lab and I have reactions setup, so time to blog I guess.

4) Freshman year

So at this point, you still want to be a chemist, after going through everything you have back in high school. Good for you! I highly recommend taking intro chem at your respective college, because AP Chemistry leaves a lot to be desired. Mainly the intro lab techniques (titration, gravimetry, etc) are very important and build a solid foundation. I dont care what school you went to, but going straight into organic chemistry is not a good idea, simply because of the lab component you’re missing. I’ve taught people who didnt take freshman chem lab, and oye, their technique later on is SEVERELY lacking.

If you’ve declared a chemistry major, take the intro chem for chemistry majors. This is always an intensive course,and way more than anything taught at the AP level. It builds solid foundations and will help you out for the rest of your college career as a chemist. Back in undergrad, I took intro chem for the chem majors, which was crosslisted as ‘honors’ chem, and it was tough. Our exams were a lot more in depth and we covered topics at a more in depth level, to when pchem came, it was a synch.

I also TAed an intensive intro chem class here as a grad student, and it’s well worth it. The topics they cover are very advanced and the students learned more than their regular intro chem counterparts.

If you can, start looking and reading various research descriptions in your department. A lot will still be over your head, but if you impress your chem prof, he/she might offer you a research position. I got one straight off the bat my freshman year, so I was very lucky in that sense. But dedication is always good and you should try for it.

Another note: APPLY for REUs. I applied for them starting my freshman year, and was also very lucky to get one, but this research experience is always paid for and you get to travel, and it’s always good if you’re applying for grad school later on. Make sure you apply to many of them, because as a freshman, you’ll be extremely lucky to get into a program right off the bat.

What classes to take freshman year? Intro chem, calculus, physics (and if you want intro bio).  Take the physics for physics majors (if they have that option), and at a minimum take physics with calculus. This will be really good preparation for physical chemistry later on.

5) Sophomore year

Now sophomore year. You’re taking organic chemistry in the minimum, and well, I hated organic, but it’s a good skills to have, especially since the chem GRE is basically 90% organic later on. Some colleges just have calculus, but you want to take as much math as you can. I was a physics major in undergrad as well, so I had a bajillion math classes, but in the minimum, to prepare you for the rigors of physical chemistry, you want partial differential equations, vector calculus,  and linear algebra/matrices (so that’s three classes beyond your usual calc 1 and 2 requirement). These will be very important in quantum chem. Eigenvalues, eigenvectors, all that, very good for pchem.

At this point, if you didnt get a research position, you will want to get one. Talk to professors. Email them (but not in an obnoxious spammy way) and attend seminars. These will be over your head, but your enthusiasm will be duly noted by your professors.

Also, dont be annoying to your grad TAs. I had several freshmen/sophomores who wanted to do research in my lab group. There were ones I wanted, there were ones who just solicited their services. It really depends on your TA as to what approach they like. I prefer  doing the whole: “So, I’m looking for an undergrad lackey” invitation to the students in my classes who are good. Some will just accept unsolicited offers, but oh well.

Again, APPLY to REUs. So, I’ll write an entry about REUs and successful applications later on. I was extremely lucky in that I got into an REU every summer, so I think I have how to apply for those down pat.

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Soluble Direct-Band-Gap Semiconductors LiAsS2 and NaAsS2: Large Electronic Structure Effects from Weak AsS Interactions and Strong Nonlinear Optical Response

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.

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Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States
Joseph P. Heremans,1,2* Vladimir Jovovic,1 Eric S. Toberer,3 Ali Saramat,3 Ken Kurosaki,4 Anek Charoenphakdee,4 Shinsuke Yamanaka,4 G. Jeffrey Snyder, Science 321, 554 (2008)

The Fermi surface changes as a result of doping with Tl.
The Fermi surface changes as a result of doping with Tl.

Oh thermoelectrics, how I love thee. For those of you who dont know, thermoelectricity is the phenomenon of converting heat into electricity. For all our energy problems right now, this research could prove to be useful, if the zT (figure of merit for thermoelectrics) can break 2. I believe that a value of 3 or higher would increase the Carnot efficiency of thermoelectric (TE) modules for power generation, to make it a viable technology. I know the navy has a considerable amount invested as they’re planning to use it submarines. Automotive companies are also looking at it to make more efficient combustion engines, as the heat of the engine (around 600K) is the range where a good number of materials have a zT of around 1 or so.

I like the paper a lot. I respect the people who did it tremendously, but one of my problems is this. The parent compound is PbTe. Lead telluride. So many people do PbTe in search of better thermoelectrics. If you do a SciFinder scholar search, there are almost 100 or more PbTe based systems for thermoelectrics. As a synthetic chemist, I like new compounds. I personally believe, if we’re to break the zT barrier of 2 (in bulk materials) that we need to look at other compounds. For instance, beta-Zn4Sb3, also from the Snyder group is an amazing thermoelectric material as it’s an PGEC (phonon glass-electron crystal). Clathrates are also like that, and have the added bonus of ‘rattlers’ inside their cages to lower thermal conductivity.

PbTe is a cubic structure. Therefore, it’s Fermi surface is usually isotropic, which is, not that interesting. Tl-doping forces a distortion in the density of states by moving the Fermi level near a peak. Whenever Ef (the Fermi level) is in a high point in k-space, it’s bound to distort and usually a pseudo gap will appear (see Gruner or your condensed matter physics texts, as this is a known phenomenon on charge density waves and superconductors). This is what they basically did to PbTe. They made it’s Fermi surface interesting.

One interesting experiment that could come from this paper is a pair distribution function (PDF) analysis of the bulk. PbTe is cubic, like I said earlier, and electronic distortions still /should/ show up somehow. If you look at the PDF on this material, will the peaks show the same cubic ordering or will it change? Will the bond distances from Pb-Te be constant or will they have a greater range? Considering the size of Tl, I’m guessing there will be a greater range (duh), but it would be neat to model it experimentally.

More problems. How much better is Tl in PbTe for practical uses. Tl is notoriously toxic and the feasibility of actually using these in TE modules is IMHO, slim to nil. Let’s try to make something more environmentally friendly, or at least more environmentally neutral, so that when we save energy by being more efficient with thermoelectrics, we dont end up killing everyone around it.

However, I do like this paper in that they didnt resort to ‘low dimensionality’ or rather, nanostructuring to get it. As much as nano is a buzzword, I dont really like nano, so for that, this paper is awesome.

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