This is my last day in the lab. Well, at least for this summer. We've (James and I) have made quite a bit of progress on Dr. Krebs' current project. Honestly, leaving today is a bit bittersweet. Part of me really, really wants to go home and have some actual vacation time, because I know that all disappears when grad school happens, but part of mewantsto just stay put and live in my new apartment. The latter is probably explained by Jennifer's closer abode to myself out here.
So this is our lab. It's actually a pretty small room but it's definitely has a homey feeling to it. The table to the right is our laser set up area, along with a cryostat chamber (hard to see) and monochrometer hooked up to a PMT (photomultiplyer tube; the blue box). James' fume hood in the back is very easy to see, as well as part of his chemistry table in the bottom left-hand corner.The furnace, which isn't visible in the picture to the right
but is visible to the left, is used to heat all of our samples. It can safely go up to about 1,200 degrees Celsius, but we usually keep it below 900 so we don't completely melt our samples.
Basically the point of this summer.... rather, the point of this whole project is to use PLZT, a known ferroelectric, and dope it with nanoparticles to observe a stimulated photocurrent
using a visible spectrum. To anyone who understands that.... Bravo. To the majority of those who don't, myself partially included, don't be afraid. This explanation won't hurt a bit.A ferroelectric material is something that has an inherent molecular dipole. This means that the crystal structure is not completely symmetric. Because of this dissymmetry, if a large amount of these crystal structures are aligned the same way the material as a whole obtains different attributes. The most important is an electric field that is created in the material. This electric field is a direct consequence of the dipoles aligning in the material. When the majority of the dipoles are aligned in the same orientation, the material is said to be poled. So with the material poled and this internal electric field in it, one is able to stimulate a weak
photocurrent from it. The way this happens is as such. When a particle (yes, I said a particle) of light hits the material,
an electron gains energy. If this energy is enough to bump it into a different energy state (technically it's bridging the band gap between the conduction and valence bands, but this is an easier, albeit flawed, way of looking it), the electron creates a hole in the previous energy state. Due to the electric field in the material, the electron has a good chance of being pulled away from the nucleus it is "attached" to, with the hole it created before going in the opposite direction. When a lot of light hits the material, this effect is brought on to a measurable scale with many electrons going one way, while the absences of electrons are going the opposite way, thus creating a current. This is called a photocurrent.
Nanoparticles... well, to be honest, I don't completely understand. What I do know is that they are groups of molecules that are bunched together and then doped into things. Alone in a solution of hexane or whathaveyou, they are able to make wonderful luminescing liquids. Our goal is to dope, or put a very small percentage into the material, PLZT with various nanoparticles, from Prof. Plass' chemistry lab upstairs, and hopefully find that the conductance in the films have increased.
PLZT, or Lead (Pb) Lanthanum (La) Zirconium (Zr) Titanium (Ti) oxide, is a well known ferroelectric. James made a fresh batch this summer so we know who to blame if we can't get anything to work. In the PLZT, James refluxed in some copper sulfide nanoparticles for the first trails, and then cadmium selenide for the later trials. We hope that the nanoparticles will
increase the electron-hole pairs created by the photoelectric effect (the long explanation about photocurrents is basically called the photoelectric effect.... kinda).
To test if the nanoparticles are in there we set up the laser table for spectroscopy measurements. The final table we built is pictured to the right. We set a laser up to hit the sample after traveling 15 meters due to the mirrors, and we measure the spectrum that the sample gives off after being excited by the laser. James also can look for the particles in the data given by an XRD, or an x-ray diffraction. This will let us see what the sample is made up of. If we see a signature that looks like cadmium selenide or copper sulfide then we know they're in the film.
Another series of tests we do on the samples are electrical tests. We deposit the PLZT onto a special type of substrate, ITO coated glass. The ITO, indium tin oxide, film is a wonderful conductor and allows us to make capacitors out of our films. Well, that's what we wanted. These films are actually somewhat conductive and thus don't make good capacitors. None the less we can run our hysteresis loop tests (not even going to try to explain that in depth here) which basically shows how poled the sample is, and our IV curve tests. The IV curve simply plots voltage vs. current. This means that as you put a voltage across the sample you can plot how the current will rise or fall. Because the film acts like a basic linear resistor, we can use the relationship V= IR. Thus, the IV curve can tell us what the resistance is of the film.
That basically sums up what we've been doing in the lab for the last ten weeks. It's been fun yet frustrating at the same time. It has been a wonderful experience and I hope to continue next summer again. Up ahead are a few more pictures I took of the lab. Thanks for listening.
This is the three stage student-made vacuum pump system for the cryostat.
Evidence that we did a bit of math over the summer!
The ever-present dartboard. I got a bulls-eye!
James eating cheez-its.
Lasers are cool!
