Dr. Comes Presents Invited Talks at UAB and the SPS Zone 6 Meeting

Dr. Comes was on the road last week, presenting an invited colloquium on Friday, April 7 at the University of Alabama-Birmingham on epitaxial oxide films and interfaces. Thanks to Prof. Mary Ellen Zvanut and the entire physics department for being such nice hosts! He followed that up with a trip to Emory University on Saturday the 8th with an invited talk at the Society of Physics Students Zone 6 meeting where he got to tour labs and learn about some of the best undergraduate research in the southeast.

Study on Interfacial Properties of SrTiO3/Ge Heterostructures Published in Applied Physics Letters

Our collaborative paper led by Dr. Scott Chambers at Pacific Northwest National Laboratory studying the electronic band alignment and interfacial behavior at the epitaxial SrTiO3/p-Ge interface has been published in Applied Physics Letters. Using molecular beam epitaxy of SrTiO3 was grown on a p-doped Ge substrate and characterized using x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy. Because STO is naturally n-type and Ge is doped p-type, this creates a pn junction that can be used for visible light photocatalysis. The STO layer may provide chemical protection for the Ge substrate, which has a small enough band gap to absorb all wavelengths of visible light.

Paper on Oxide-Water Surface Chemistry Published in Journal of Physical Chemistry Letters

Our paper exploring the surface reactivity of LaFeO3 thin films with water using ambient pressure x-ray photoelectron spectroscopy (AP-XPS) has been published in Journal of Physical Chemistry Letters! This work was led by Kelsey Stoerzinger at Pacific Northwest National Lab, who performed AP-XPS measurements on LaFeO3 epitaxial thin films with FeO2 and LaO surface terminations at the Advanced Light Source at Lawrence Berkeley National Lab. The films were synthesized by Dr. Comes while he was at PNNL. We find that the LaO terminated surface is significantly more reactive to gaseous water molecules, forming hydroxyl groups on the surface which were easily measured in situ using AP-XPS.

Paper on SrTiO3-LaCrO3 Superlattices Published in Chemistry of Materials

Our work exploring the role of interfacial defects on polarization and conductivity in SrTiO3-LaCrO3 superlattices has been published in Chemistry of Materials. In this work, we explore the effects of off-stoichiometry defects and oxygen vacancies on interfacial carriers and polarization in these superlattices. The work was a collaboration during Dr. Comes’ post-doctoral work at Pacific Northwest National Lab, with Steven Spurgeon leading electron microscopy efforts in collaboration with Demie Kepaptsoglou and Quentin Ramasse at the SuperSTEM, Peter Sushko performing density functional theory modeling, and Mark Engelhard performing depth-profile x-ray photoelectron spectroscopy measurements. The multi-dimensional analysis that we apply here should have broad applications to understanding the role of defects in the behavior of superlattice materials, which can be hard to characterize using traditional approaches.

Invited Talk at Electronic Materials and Application 2017

Dr. Comes will be presenting an invited talk on his work on the LaFeO3/n-SrTiO3 interface and on LaCrO3-SrTiO3 superlattices at the American Ceramics Society Electronic Materials and Applications conference this week at 9 am Friday, Jan 20. Portions of this work have been published in Physical Review Letters, Chemistry of Materials, and Advanced Materials Interfaces in the past year. We’re looking forward to a great conference!

Work on LaFeO3/n-SrTiO3 Interface Published in Physical Review Letters

Dr. Comes’ work on the LaFeO3/n-SrTiO3 interface has been published in Physical Review Letters here! In this work, which was done at Pacific Northwest National Laboratory in collaboration with Dr. Scott Chambers, we show that the band alignment between LaFeO3 (LFO) and Nb-doped SrTiO3 (STO) is relatively insensitive to the interfacial termination. We synthesized a series of LFO films on SrO- and TiO2-terminated STO to examine how changing the interface dipole affects band alignment, which is key to enabling next-generation photovoltaics and photocatalysts. Using in situ x-ray photoelectron spectroscopy, we were able to measure the valence band alignment and then determine the conduction band alignment through our measurement of the LFO band gap with ellipsometry. These results help to address some confusing results in the literature and will hopefully generate further study down the road.

Presenting at AVS Meeting and Southeastern Section of APS

Superlattice EELSDr. Comes will be presenting at the AVS meeting this Tuesday, Nov. 8, in Nashville, TN, discussing collaborative work with Prof. Chuck Fadley at UC Davis from his time at PNNL studying SrTiO3-LaCrO3 superlattices with standing-wave angle-resolved x-ray photoelectron spectroscopy. After the time in Nashville, he will move on to Charlottesville, VA to give an invited talk at the Southeastern section of the American Physical Society annual meeting on Friday, Nov. 11. This talk will focus on work exploring the SrTiO3-LaFeO3 interface that will be published soon in Phyiscal Review Letters and is available on arXiv now.

Paper on NdTiO3-SrTiO3 Interfaces Published in Physical Review Letters

A collaborative paper led by Bharat Jalan’s group at the University of Minnesota and our group at Pacific Northwest National Lab
exploring the effects of different defects at the interface between SrTiO3 (STO) and NdTiO3 (NTO) on the electronic behavior of the two-dimensional electron gas in this system has been published in Physical Review Letters! We find that the electronic behavior is highly dependent on both Nd:Ti stoichiometry and on the level of oxidation in the NTO layer. Somewhat surprisingly, our x-ray photoelectron spectroscopy measurements and density functional theory analysis indicate that oxygen interstitials may diffuse from the surface into the bulk of the NTO film, changing the Ti formal valence from 3+ to 4+. This has drastic effects on the electronic behavior of the conducting interface between STO and NTO, which will help us understand how to better engineer these materials going forward.