electron diffraction

Diffraction of Electrons.

According to the poll of the greatest physicists conducted by The New York Times, the experiment with electron diffraction is one of the most astonishing studies in the history of science.

There is a source that emits a stream of electrons onto photosensitive screen. And there is an obstruction in the way of these electrons, a copper plate with two slits. What kind of picture can be expected on the screen if the electrons are imagined as small charged balls? Two strips illuminated opposite to the slits. In fact, the screen displays a much more complex pattern of alternating black and white stripes. This is due to the fact that, when passing through the slit, electrons begin to behave not as particles, but as waves (just like the photons, or light particles, which can be waves at the same time). These waves interact in space, either quenching or amplifying each other, and as a result, a complex pattern of alternating light and dark stripes appears on the screen. At the same time, the result of this experiment does not change, and if electrons pass through the slit not as one single stream, but one by one, even one particle can be a wave. Even a single electron can pass simultaneously through both slits (and this is also one of the main postulates of the Copenhagen interpretation of quantum mechanics, when particles can simultaneously display both their “usual” physical properties and exotic properties as a wave).

But what about the observer? The observer makes this complicated story even more confusing. When physicists, during similar experiments, tried to determine with the help of instruments which slit the electron actually passes through, the image on the screen had changed dramatically and became a “classic” pattern with two illuminated sections opposite to the slits and no alternating bands displayed. Electrons did not seem to show their wave nature under the watchful eye of observers. Is this some kind of a mystery? There is a more simple explanation: no observation of a system can be carried out without physically impacting it.

Determining the structures of nanocrystalline pharmaceuticals by electron diffraction

Reliable information about the structure of pharmaceutical compounds is important for patient safety, for the development of related drugs and for patenting purposes. However, working out the structures of pharmaceuticals can be tough. The individual molecules can pack together in the solid in different ways to form different polymorphs, and pertinent properties such as stability, bioavailability or how fast they dissolve in the stomach can vary from one polymorph to another. Single crystals (as used in standard X-ray diffraction experiments) therefore might not be representative of the bulk sample, or indeed might not even be available.

Moreover, the compounds themselves can be damaged by the high energy of the X-radiation used. As electrons are less damaging than X-rays by several orders of magnitude, using electron diffraction should be an attractive alternative, particularly when only nanometre-sized crystals are available. Cooling the sample to liquid-nitrogen temperatures (‘cryo-cooling’) can also help to minimize radiation damage, but the compound might change structure on cooling, so the structure that is obtained is not actually that of the material as taken by the patient at room temperature.

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High-resolution electron diffraction patterns recorded at 1.8 K with a color translation technique which permits quantitative evaluation of characteristic parameters of single-crystal films and deposited thin layers at liquid helium temperatures. This illustrates the potential of cryoelectron microscopy and diffraction in the study of novel electron optical phenomena, including direct observation of Josephson junction devices and other critical components of superconducting computers under cryogenic operating conditions. (Courtesy of Humberto Fernández-Morán)


Meet Artoni “Toni” Ang:

1) What do you do?

I’m a Ph.D. student working on surface science. I have been studying the atomic and electronic structures of the Si(110) surface. We use surface sensitive techniques like reflection high energy electron diffraction (RHEED) and angle resolved photoelectron spectroscopy (ARPES) to study the atomic and electronic band structures of various surface reconstructions on Si(110). The high hole mobility of the Si(110) surface and its highly anisotropic structure has recently made it an attractive FET (field-effect transistor) substrate material and a template for self-assembled 1D metal growth.

2) Where do you work?

I work in the Surface and Materials Science laboratory of the Nara Institute of Science and Technology (NAIST), Japan. Our laboratory is fully equipped to investigate different physical and chemical properties of atomically clean surfaces, this is mostly done in the UHV surface science complex in our lab, which has various surface sensitive experiments all connected by a UHV transfer system (H. Yamatani et al. Surf. Sci. 601 5284 (2007)). This allows us to investigate surfaces using various experimental techniques in-situ, without exposing these surfaces to air.

3) Tell us about the photos!

[Left:] A photo of me taken in the middle of repairs of our ARPES chamber’s sample 5-axis manipulator. Several months of repairs had to be done because a screw thread got damaged inside. Then we had to make sure everything is aligned, followed by 2 weeks of baking the chamber to get good UHV conditions.

[Right:] A new hobby I picked up in Japan– skiing. This photo was taken in Nagano, the “Japanese Alps”. I now plan on skiing every winter season in as many mountains as I can. It’s fun and easy to learn, but physically demanding and a little painful at first. I try to do as much strengthening and conditioning during off-seasons to prepare for the next skiing season.

When you’re at the top of the mountain looking down, everything else doesn’t matter anymore. All you’re thinking about is how to get down as fast as possible and in one piece. It’s just you, your skis, the beautiful snowy scenery and gravity. Gravity can be a bitch sometimes though.

4) Tell us about your academic career path so far. 

I took my B.S. and M.S. degrees in Physics from the Ateneo de Manila University. I then enrolled in the Nara Institute of Science and Technology for my Ph.D. in Materials Science. Next, I plan to work as a postdoc or a researcher, to investigate novel low-dimensional electronic structures.

5) Anything else you’d like to share?

As scientists, we spend most of our time on our desks in front of a computer or in a lab doing experiments. Being physically fit goes a long way, not only in being healthy, but also in being more productive at work. Shaky hands and tired eyes are never good for experiments, so you also have to consider being fit and rested as part of your job as a scientist. Pick up a hobby, get some exercise, and spend time with friends and family, anything to get your mind and body rested.