Outreach to 5th Grade Students with St. Elmo Brady STEM academy

On Oct. 25, the University of Illinois AVS Student Chapter taught ~ 15 4th and 5th grade boys of the St. Elmo Brady STEM academy about vacuum science. The St. Elmo Brady STEM academy gives African-American 4th and 5th grade boys hands-on exposure to science, technology, engineering and mathematics. We showed them what happens to a ringing alarm clock, the speed at which a feather and magnet fall, Peeps, shaving cream, a candle, a balloon, and water in a vacuum and helped them to discover why these materials behave the way that they do in a vacuum. 

Tatyana Kuznetsova and Vladimir Grebennikov of the Russian Academy of Sciences Seminar

On Wednesday, Sept. 17, Drs. Kuznetsova and Grebennikov discussed their investigations into topological insulators using angle-resolved photoelectron spectroscpoy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS).

Bismuth tellurohalides including BiTeI belong to a family of layered polar semiconductors lacking inversion symmetry. Much attention has recently been paid to these compounds due to the strong spin-orbit interactions of electrons caused by Bi atoms. The strong correlation between the direction of motion and spin of charge carriers (called the Rashba splitting) in materials without spatial inversion can be used in spintronic devices. The Rashba effect leads to a shift of opposite spin-polarized bands in opposite directions on the momentum scale. In the semiconductor BiTeI, the Rashba effect can be used to control the spin of charge carriers using electric fields providing a pathway for the technological development of spintronic devices.

The near surface layer of BiTeI has been converted to the three-dimensional topological insulator state by doping the surface with Cs. We studied this system by angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy. We analyzed the electronic structure of the (0001) surface of BiTeI and its modification upon adsorption of Cs using ARPES. A strong shift of the electronic states at the (0001) BiTeI surface by several hundred meV to higher binding energies due to band bending effects as well as the Rashba splitting are observed. Both scanning tunneling microscopy (STM) and spectroscopy (STS) show unusual atomic structures of the surfaces terminated by both tellurium and iodine atoms. Our experiments have shown that the relaxation of the surface atoms in these compounds is significant and can be used to control surface states in topological insulators.

Michael V. Yakushev, University of Strathclyde UK Seminar

On Tues. Sept 16, Dr. Yakushev was able to teach us about the capabilities of optical spectroscopy including photoluminescence by describing his work with chalcopyrites and other semiconductors.

The chalcopyrite semiconductors based on CuInSe2 are key components in the absorber layer of solar cells which are amongst leading thin-film PV technologies in terms of efficiency and stability. These solar cells have been developed mostly using empirical approaches rather than scientific knowledge-based design: it worked first and was explained later. As a result the progress of the chalcopyrite-based solar cells shows a clear saturation towards not 30 %, a theoretical limit for single-junction solar cells, but 21 %. The problem seems to be that the materials are too complicated for an empirical design.

A solution could be in studying first high structural quality model materials and then using the obtained knowledge for technology grade materials. That is how all other successful semiconductor technologies have been developed. A signficant improvement in the quality of the chalcopyrites produced in Strathclyde facilitated the use of fine methods of optical and magneto-optical spectroscopy to study excitonic states and helped to determine a number of fundamental electronic properties.

The next step in the development of the chalcopyrites is the substitution of rare and expensive In and Ga with alternating Zn and Sn: Cu2ZnSn(SeS)4 with a kesterite structure. An encouraging efficiency in excess of 11 % has already been reported for solar cells based on this compound. However, the material might be far too complicated whereas some other compounds like Cu3BiS3 can probably offer a simpler solution.

Christian Schlepuetz Seminar

The past couple of years have marked the centennial anniversary of x-ray crystallography - a field of research that has seen a tremendous growth ever since its birth. Rather than going out of fashion eventually, it has helped to push the boundaries of our understanding of matter and materials steadily further. New x-ray techniques are continuously added to our set of available research tools, while established methods of crystallograpy are reaching new frontiers by taking advantage of the latest instrumentation developments.
The exploration of reciprocal space has taken a leap in terms of speed, accuracy, reliability, and the obtainable level of detail with the advent of modern single-photon-counting x-ray area detectors featuring high frame rates and zero readout noise. Each detector image represents a 2-dimensional slice through reciprocal space, and a single diffractometer scan extends this to a stack of slices probing a 3-dimensional volume. This allows for the rapid characterization of large volumes of reciprocal space, revealing all of the structural phases and their orientations present in a sample. The method is particularly powerful if not all the constituent phases and the corresponding locations of their diffraction signals are known, and aids in the discovery of unexpected phenomena or crystal structures.
In this talk, Christian gave a basic tutorial on how to navigate in reciprocal space and showed how to use a Pilatus 100K pixel detector to collect large volume data sets, which could then be processed to yield 3-dimensional reciprocal space maps (RSMs), high-quality powder diffraction data, or simultaneous measurements of pole figures for a whole range of 2-theta values. The capabilities of this approach were then highlighted using several topical research examples: The detailed investigation of the domain structure of multiferroic bismuth ferrite (BiFeO3) thin films and its phase transitions with temperature and film thickness was only possible due to the rapid collection of many RSMs, including those around the half-order film Bragg Peaks, which contain sensitive information about the oxygen octahedra rotation patterns in this material. Iron oxide (Fe2O3), as another example, is an attractive material for the photoelectrochemical (PEC) oxidation of water, particularly when it is heteroepitaxially grown on the facets of indium tin oxide (ITO) nanowires to form a core-shell structure with a large catalytic surface area, long optical absoption paths, and short charge transfer pathways. Simultaneous pole figure measurements of the ITO and Fe2O3 diffraction signals help to reveal the epitaxial relationship at the interface between the ITO nanowire facets and the Fe2O3 layer and provide information which can ultimately be used to guide the design and optimization of future PEC devices.

Engineering Open House 2014

UIUC AVS had a booth "Peeps in Spaaaaace" at Engineering Open House on Mar. 14-15, 2014.

Vacuums are more than your trusty Hoover! They are used in a wide variety of industries and can be used to illustrate many important physical concepts, such as pressure, phase changes, and acoustics. Have you ever wondered what would happen to a balloon in a vacuum? Could you hear an explosion in space? What happens to fire in the vacuum of space? Can you get a fireball in space? How can you get ice to crystallize at room temperature? What happens when you put Peeps and shaving cream in a vacuum?

Thank you to all who helped to man the booth!