Dr. Comes and undergraduate student Will Bowers took a road trip to Milledgeville, GA before Thanksgiving break to attend the Southeastern Section of the American  Physical Society meeting. Will presented his poster studying the preparation and surface properties of Nb-doped SrTiO3 substrates done in collaboration with former REU student Patrick Gemperline. Congrats Will! Dr. Comes organized an invited speaker session on thin film surfaces and interfaces and organized the Auburn Physics recruiting table.

Paper on LaFeO3/n-SrTiO3 Published in Physical Review Materials

A paper led by Steven Spurgeon and Peter Sushko from Pacific Northwest National Lab building Dr. Comes’ previous work on the LaFeO3/Nb-doped SrTiO3 interface has just been published in Physical Review Materials. We examine the interface between the two materials using scanning transmission electron microscopy and complementary density functional theory modeling. In the work we find that ion intermixing and the surface termination of the substrate strongly affect the electronic structure of the resulting material. For a TiO2-terminated substrate there is electron accumulation at the nominally positively charged LaO-TiO2 interface. Meanwhile, for an SrO-terminated substrate intermixing removes an energetically unfavorable band alignment. These results go a long way to explaining some surprising observations from Dr. Comes’ initial work that was published in 2016 in Physical Review Letters. You can read more about the work in the PNNL news highlight here .

Dr. Comes Presenting Talks at International Workshop on Oxide Electronics and University of Alabama

Dr. Comes will be sharing some of his postdoctoral work at the International Workshop on Oxide Electronics on Wed, Sep 27 in Chicago. He’ll be sharing work on the LaFeO3/n-SrTiO3 interface that was published last year in Physical Review Letters and recently submitted work led by Dr. Steven Spurgeon using electron microscopy and density functional theory modeling performed by Dr. Peter Sushko that explains the unusual results we initially observed.

A week later, Dr. Comes will be in Tuscaloosa, AL to visit the University of Alabama Department of Physics and give a colloquium. This is a great opportunity to learn more about condensed matter research in the state and discuss ways that the two football rivals can collaborate in the laboratory.

Paper on MBE-grown Oxide Nanocomposites Published in Advanced Materials Interfaces

Dr. Comes’ paper in collaboration with Steven Spurgeon and Daniel Perea at Pacific Northwest National Laboratory has been published in Advanced Materials Interfaces. In this paper we demonstrate the first growth of an epitaxial oxide nanocomposite by molecular beam epitaxy (MBE). Almost all of the previous work had focused on depositing materials by pulsed laser deposition, with separate sources of spinel material (in this case NiFe2O4) and perovskite material (LaFeO3 here). When we do the growth by MBE, instead we have to provide elemental La, Fe, Ni, and O and allow the phases to form on their own. We find that the same thermodynamics apply, but that the kinetics of how the nanocomposites grow are a bit different. Using state-of-the-art atom probe tomography and scanning transmission electron microscopy measurements of the same region of a sample, we are able to understand how the phases segregate and get a 3-dimensional model of what forms at the end. This should pave the way for studies of new combinations of materials and new morphologies of these interesting nanocomposites.

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.