BaSnO3 is a high mobility, wide bandgap semiconductor that is promising for high electron mobility transistors and other devices. However, it is hard to modulation dope across the interface because of the low effective mass and band filling of the Sn 5s orbitals. Enter SrNbO3, which has low electron affinity and will readily donate electrons across an interface to BaSnO3. Using MBE to grow these heterostructures and in situ XPS to measure charge transfer, we’ve estimated that we can get more than 1014 electrons/cm2 into BaSnO3, which is more than a 10x improvement over past materials work with other donor materials. DFT modeling predicted carrier concentrations quite well. It’s a very promising first step towards engineering higher carrier concentrations in stannates with a lot more work to come.
This work was supported by our Air Force Office of Scientific Research (AFOSR) Young Investigator project and Suresh’s Alabama EPSCOR Graduate Research Scholars fellowship.
Our paper on the growth of SrNbO3 by hybrid molecular beam epitaxy is out in APL Materials today. This work was led by former student Suresh Thapa as part of his Ph.D. and is the first demonstration of SrNbO3 by MBE. Suresh used a new kind of metal-organic precursor to grow these materials, which are very promising as interfacial donors. He did a great job with this challenging project and even built a DFT model in collaboration with Dr. Marcelo Kuroda to compare our experimental results with theoretical expectations!
One of the biggest challenges with d1 transition metal oxides is preserving a stable surface that doesn’t over-oxidize. To do that, we also developed a growth recipe for SrHfO3 that is critical if the samples are going to be accurately characterized. Our former postdoc Sydney Provence and current Ph.D. student Patrick Gemperline made big contributions to that part of the project. Our in situ XPS measurements from the figure above show that the capping layer preserves free electrons at the Fermi level, which is critical for use in interfacial heterostructures. Steven Spurgeon and Bethany Matthews from PNNL did a great job with STEM to show that we can preserve crystalline SrNbO3 down to 2 unit cells with a cap after atmospheric exposure. Steve Heald from the Advanced Photon Source conducted X-ray absorption spectroscopy on the capped samples to measure the valence after atmospheric exposure, which showed that the cap preserves the Nb4+ charge state.
Thanks to Air Force Office of Scientific Research (AFOSR) for funding support on the project through the Young Investigator Program as well as the Alabama EPSCOR Graduate Research Scholars Program for supporting Suresh’s studies in his last year.
Our lab will be starting a new project thanks to $750k in research funding from the Department of Energy EPSCoR Laboratory Partnership program in collaboration with Prof. Wencan Jin. The project focuses on the synthesis of 3d and 5d transition metal oxides for renewable energy and spintronic applications with in situ X-ray diffraction and spectroscopy during film growth. We will be working with Dr. Dillon Fong at Argonne National Lab and the Advanced Photon Source, Dr. Jurek Sadowski at Brookhaven National Lab and the NSLS-II, and Dr. Steven Spurgeon, our long-time electron microscopy collaborator at Pacific Northwest National Lab.
We will be supporting a post-doctoral scholar through this project to start in the next few months. Interested researchers should contact Dr. Comes by email. Contact info can be found under the “About” page on this website. An ad will be posted in the coming days.
Congratulations Rajendra! His Ph.D. thesis focused on the synthesis of LaFeO3 heterostructures by MBE for use as oxygen evolution reduction catalysts. Through collaborations with Dr. Byron Farnum’s lab group he showed that LaFeO3/LaNiO3 heterostructures exhibit greater catalytic performance than either material by itself due to the favorable band alignment at the interface. Rajendra is starting work with Micron in Boise, ID in September to work with their thin films group. Way to go!
“The International Workshop on Oxide Electronics series has become an important venue to discuss recent advances and emerging trends in this developing field. The aim of the workshop is to provide an interdisciplinary forum for researchers – theorists as well as experimentalists – on understanding the fundamental electronic and structural properties and also on the design, synthesis, processing, characterization, and applications of (epitaxial) functional oxide materials. Results of critical scientific importance as well as studies revealing the technological potential of functional oxide thin films to create devices with enhanced performance will be showcased.”
Abstract submission is now open on the website and are due June 15th. We look forward to seeing you in Maine!
Suresh and Miles are the first two FINO Lab Ph.D. graduates! Both defended their dissertations over the last week and did a great job.
Miles’dissertation is entitled “Synthesis and Characterization of Mn-based Spinels Grown by Molecular Beam Epitaxy.” He has written two first author papers and has another in preparation on Mn spinels for water-splitting catalysis applications on our NSF project with Prof. Byron Farnum’s group. He was the first Ph.D. student to join the group and helped build the group’s MBE and XPS. He’ll be moving on to an industrial job over the summer that is being finalized soon. Congrats Miles!
Suresh did the first in situ studies on films grown by hybrid MBE, including SrTiO3 and SrNbO3. His thesis is entitled “Surface and Interfacial Studies of Perovskite Oxides Grown by Hybrid Molecular Beam Epitaxy” and was supported by our AFOSR grant. He has written a first author paper, has another in review, and one more in preparation. Suresh will move on to a job with Intel in Hillsboro, OR over the summer. Way to go Suresh!
Our collaborative paper with Prof. Byron Farnum’s group in the Department of Chemistry has been published in ACS Catalysis. In this work, Miles Blanchet from FINO Lab grew a series of MnFe2O4 and Fe3O4 samples by MBE on Nb-doped SrTiO3 and performed in situ XPS measurements. Alex Bredar from Prof. Farnum’s group performed cyclic voltammetry studies to examine the use of the materials for the oxygen reduction reaction (ORR). Mn-based spinels have generated significant interest for their use in ORR and this work is the first to examine epitaxial films for ORR to attempt to explain the role that Mn plays in the catalysis. The work was funded by NSF through our collaborative grant.
Our collaborative paper with Dr. Eren Suyolcu of Cornell University and Prof. Gennady Logvenov of Max Planck Institute for Solid State Research is out today in the Journal of Vacuum Science and Technology A. In this work, Eren performed in situ experiments examining the surface reconstructions of Sr-doped La2CuO4 films grown by MBE. Using reflection high energy electron diffraction (RHEED), he tracked how the surface evolves during a shuttered growth routine in ozone and vacuum environments. Ph.D. student Patrick Gemperline applied the principle component analysis and K-means clustering codes developed in our lab by former postdoc Sydney Provence to track the evolution of these surface reconstructions and found that they match quite well to the shuttering times, supporting the conclusions from Eren’s experiments. Our contributions to the work were supported by our AFOSR Young Investigator grant.
Our collaborative work with Prof. Byron Farnum’s group is out today in the Journal of Materials Chemistry A. This work was co-led by Andricus Burton in the Farnum group and Rajendra Paudel in FINO Lab and examines the role of film thickness on catalytic performance of LaFeO3 catalysts for the oxygen evolution reaction. It draws on Prof. Comes’ previouswork on the band alignment of LaFeO3 with n-doped SrTiO3 to show how a combination of interfacial electronic reconstruction and the intrinsic high resistivity of LaFeO3 produce a sweet spot for catalysis at ~6 nm thickness. Electron microscopy work from Drs. Steven Spurgeon and Bethany Matthews as well as DFT modeling from Dr. Michel Sassi at Pacific Northwest National Laboratory helped to explain the complex physics and chemistry that occurs at the buried interface and film surface. The work is funded as part of the collaborative NSF grant between our two labs at Auburn.
Our group’s review article in the Journal of Materials Research has been chosen as the winner of the Early Career Scholar in Materials Science Prize by the journal editors with the Materials Research Society! This paper was the result of hard work by graduate students Suresh Thapa, Rajendra Paudel, Miles Blanchet, and Patrick Gemperline, who were co-first authors on it and wrote it during the height of the COVID pandemic in the spring and summer of 2020. Thanks to the many Zoom calls and revisions from everyone it is now a valuable resource for new researchers looking to integrate X-ray photoelectron spectroscopy into film growth! The video above is an interview with Dr. Comes and Prof. Gary Messing of Penn State University, who is the Editor-in-Chief of the journal. The press release from JMR is here.