My areas of expertise bridge several areas of physics and chemistry. Beginning my career in organic semiconductor processing and device physics, I have found amazing utility in the electronic and chemical aspects of interface formation and operation, thin film growth and processing and vacuum based methods. Think of the organic light emitting devices now found in the most impressive displays and many interfaces which must satisfy power, performance and durability requirements. Vacuum science played a huge role in the development of this and many other advanced device architectures. In vacuum, the ambient variables may be completely controlled for, ensuring a robust testing of physical and chemical hypotheses. Whether we’re testing a newly synthesized molecular material or optimizing an oxide thin film growth process, we seek approaches to add depth to the analysis in ways that are fundamental in nature.
Here is an example of how one of my favorite vacuum science methods, photoelectron spectroscopy (PES) in its pure form, can provide new information on an important material that is changing under the X-rays. From PES, methyl ammonium lead triiodide (an important solar material) composition is measured repeatedly over 40 hours. During that time the I concentration is observed to decrease relative to Pb (left to right in the figure). However, the electronic properties are invariant as evidenced by the lack of shift in the valence band edge. It is not until a significant fraction of I (as well as C and N) has left the material that the system begins to transform into lead iodide. Electronic structure modeling shows that these types of defects fall outside the bandgap and self compensate. This is the first experimental observation of high defect tolerance for this material, which is helping to explain its incredible performance in solar cells. Steirer et al. ACS Energy Lett., 2016, 1 (2), pp 360–366