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.