Helium balloons?

Have you seen helium balloons (the balloons that float)? Did you know that helium can be used for other things, as well? There is only a small amount of helium on the planet. We use helium in our experiments to measure things at low-temperatures. Liquid nitrogen can only cool things to around 77  K. Liquid helium, however, can cool things much more (to less than 0.2 K; 0 K is the lowest temperature and it is impossible to actually reach (this can be proven)). Some things are easier to measure at low-temperatures (or are impossible to measure at high temperatures). Save the helium!

Comic from Maki Naro

Raman is not just salty pasta. What is Raman Spectroscopy?

This video from Mustard Tree TV describes what Raman spectroscopy is and some basics of how it works.We use Raman spectroscopy to determine what compounds are in our samples. For our material system, it is actually more effective at distinguishing the compounds than x-ray diffraction. I am currently making a high-temperature stage to allow me to measure the Raman scattering at high temperatures in different environments. This should tell me how stable my films (thin layers of my material, CZTS) are at different temperatures and in different environments. Raman spectroscopy is also very sensitive to the sample surface and doesn't destroy the sample. Raman spectroscopy is a really cool technique!

In x-ray crystallography, Bragg-ing is not a bad thing

Ok. That's a fairly bad pun. Seriously, though the father-son team of William Lawrence and William Henry Bragg are inseparable from the modern understanding of x-ray diffraction. In a previous post, I had talked about how we use x-ray diffraction. I just came across a series of comic strips that describe many other ways of using x-ray diffraction. Many of the techniques discussed in these comic strips discuss techniques that are used to study single-crystalline samples (samples with only one crystal in the entire chunk--just think the placement of a single atom fully describes the location of all other atoms in the single crystal). Some people in our research group investigate thin films of single-crystalline material using similar techniques that are best for these really small samples. These comics were made by Maki Naro for the Boxplot blog in Popular Science.

Also, here is in interview of the daughter of Bragg, Jr. about her father and grandfather's discoveries.

Go forth and make something!

Hey, this girl is really building stuff! She has a Youtube channel where she talks about building things. Many (not all) scientists and engineers began by building things. We often still do (just finished making a coat rack). Be sure to ask your parents for permission before building anything (trust me on this)!

Here is a link to Sylvia's website: http://sylviashow.com/

Scientists and engineers, understandably, build things all the time. In our lab, we are designing, machining, and building a high-temperature Raman spectroscopy stage. Regularly, experiments require us to wire basic circuits. Since we are exploring new things, it is sometimes impossible (or prohibitively expensive) to buy something to do what is needed to test a hypothesis.

It is a fun challenge to think up ways to use existing things in new ways!

Why Statistics?

Science is one way people make sense of the world around us.  We look very closely at Nature, and try to describe precisely what happens--without extra, unnecessary aspects--and without missing necessary aspects.

In order to look very closely at how the Universe behaves, we look at very small particles, waves, and phenomena.  Indeed, we must look at things that are so small, they seem to have very little in common with the way we see our macroscopic world unfolding.  Our real life experiences, in the macroscopic world, are made up of incomprehensibly tiny activities on the microscopic scale.  Statistics allows us to unify these two perspectives, which is incredibly powerful (as well as profound!).

An example of this is a volume of gas particles.  For simplicity, we will consider a gas at low pressure, so that the particles effectively never collide (these days, this is routine to achieve in the lab--I do it every day).  Every particle in the system will be in a slightly different state--maybe this one's rotating this way, that one's vibrating that way, this one's going really fast in that direction, etc.  In a macroscopic system, there are simply too many individual particles and possible states to keep track of.  With statistics, we can describe them in spite of this baffling complexity!

Let's look at the speeds of argon atoms (argon makes up about 1% of the air we breathe).  In our example below, a system of 56 argon atoms is organized by color/speed and position.  Moving from left to right, and from red to purple, atom speed is increasing (the labels have units of meters per second).  Most atoms are in the middle (yellow), and have relatively moderate speed.  There are also statistical outliers that have very slow speeds (red), and very fast speeds (purple).  The beauty and power of statistics is that it doesn't matter if we have 56 atoms or 56x(10^23) atoms-- arranging them this way will always approach the shape of the curve with almost perfect precision.  The size of the system doesn't affect its behavior, and time doesn't either!  If we look at a single particle at different times, it's speed distribution will also follow that line (speeding up and slowing down as it bumps into neighbors who had been going faster and slower, respectively).  [Note that in our example, we've greatly reduced the pressure, so these energy-exchanging collisions are so rare that colors/speeds are effectively constant in time.]

This demonstrates that although we can't actually arrange atoms into neat little rows and count their speed distributions, we can do this in our minds, and it's a valuable trick we can use to understand our world, and sometimes even control it!

[download this free software if the demo doesn't load]


Details and Warnings:
The example is for argon at room temperature (300 K) and a pressure of 5x(10^-10) atm, and our box has edges of ~17 micrometers.  In order to see things happen, we have to slow time by a factor of 10^8 (so 1 demo second = 10^-8 nature seconds), and blow up the atoms to ~3,600 times their real size.  The balls are just over-sized place-markers, so when they appear to pass through one another without colliding, it's because the atoms are actually much smaller, and so have plenty of room to fly by without colliding.  Also, real atoms don't bounce off walls the way basketballs bounce on flat surfaces.  In reality, they bounce off walls the way a basketball would bounce off a bunch of packed together basketballs.  (After all, the walls are just more atoms!)

Why would a materials researcher use a particle accelerator?

Good question!

While you may have heard that particle accelerators smash particles together, they also release radiation that can be used to study materials. When a charged particle (see What is an electron? What is charge?) is accelerated (it's path is bent along a curve or its speed changes), it releases radiation. This radiation tends to be very bright in a synchrotron and it can be of high energies. This radiation can be passed through a monochromator, which can select out light having only a single energy (only one color). This light can then be used in experiments to image, diffract off of, scatter off of, or be absorbed by samples. We use light diffracted from our sample to determine which crystal structures (phases) are present in our samples. By understanding the conditions under which certain phases are present in our samples and the ways in which different phases transform into one another, we should be able to design more-efficient and less-expensive solar cells, cutting down on unwanted phases in our samples.

How do you use x-rays?

Photo courtesy of Microsoft Clip Art

Superman sees through walls with x-ray vision. When you break a bone, a doctor x-rays the area to see that the bone is broken. There are a lot of uses of x-rays other than looking at bones. We often hear the word "x-ray" but what does it mean? X-rays are a form of electromagnetic radiation (light energy) that have a wavelength of around 1 Å.

Courtesy of Wikipedia Commons

Who cares? Scientists care. That wavelength is of similar size to the spacing of atoms, which allows the waves to be used to probe how the atoms are arranged. This will give you what is known as a diffraction pattern. X-rays interfere with each other to redirect into only certain directions, which are related to the crystal structure. This results in a pattern like that below, which can be attributed to specific compounds in the sample and can be used to determine what compounds are present in the sample.

Diffraction pattern of sample annealed at 200 C showing GaAs and AgInSe2 phases present. (HAII project, funding from NSF)

The material used in this project, Cu2ZnSn(S,Se)4, presents several challenges to using x-ray diffraction to identify the phases present in films. For example, since the lattice spacing in Cu2ZnSnS4 and ZnS are excessively similar (the same if measured using conventional equipment), it is impossible to tell using x-ray diffraction if these two compounds coexist in a film. However, x-ray diffraction is still a reliable method to distinguish many other phases present and can be used for high-temperature measurements. Using a different x-ray source, it may be possible to distinguish more phases.