arXiv subjects

Ryan B. Comes

Publications and source records attributed to Ryan B. Comes.

At least 19 recordsLinked to original sources

arXiv2026

Born-Qualified: An Autonomous Framework for Deploying Advanced Energy and Electronic Materials

Autonomous science is transforming how we discover materials and chemical systems for advanced energy technologies. However, many initially promising systems never reach deployment. This "valley of death" stems from optimization that prioritizes laboratory metrics over industrial viability. We propose a new strategy: "born-qualified" autonomous development, which embeds manufacturability, cost, and durability constraints from the outset. This approach is enabled by four pillars, including the development of multi-objective metrics, causal models, a modular infrastructure, and embedding manufacturing in the discovery loop. Realizing this vision will require sustained, community-wide commitment, but the potential return on that investment is commensurate with the scale of the challenge.

cond-mat.mtrl-sci↗

arXiv2025

Band Alignment Tuning from Charge Transfer in Epitaxial SrIrO$_3$/SrCoO$_3$ Superlattices

Understanding charge transfer at oxide interfaces is crucial for designing materials with emergent electronic and magnetic properties, especially in systems where strong electron correlations and spin-orbit coupling coexist. SrIrO$_3$/SrCoO$_3$ (SIO/SCO) superlattices offer a unique platform to explore these effects due to their contrasting electronic structures and magnetic behaviors. Building on past theory based on continuity of O 2p band alignment, we employ density functional theory (DFT) to model electron transfer from Ir to Co across the SIO/SCO interface. To characterize these effects, we synthesized epitaxial SIO/SCO superlattices via molecular beam epitaxy. Structural and transport measurements confirmed high crystallinity, metallic behavior, and suppression of Kondo scattering that has been reported in uniform SIO films. Further characterization via X-ray absorption spectroscopy (XAS) revealed orbital anisotropy and valence changes consistent with interfacial charge transfer. Co K- and L$_{2,3}$-edge and Ir L$_2$-edge spectra verified electron donation from Ir to Co, stabilizing the perovskite SCO phase and tuning the electronic structure of SIO via hole-doping. O K-edge XAS showed band alignment shifts in the SIO layer consistent with DFT predictions. Our work here provides a pathway for engineering oxide heterostructures with tailored magnetic and electronic properties.

cond-mat.mtrl-sci↗

arXiv2025

New Materials, New Functionalities: Molecular Beam Epitaxy of Ultra-High Conductivity Oxides

Understanding fundamental properties of materials is necessary for all modern electronic technologies. Toward this end, the fabrication of new ultrapure thin film materials is critical to discover and understand novel properties that can allow further development of technology. Oxide materials are a vast material class abound with diverse properties, and, therefore, harnessing such phases is critical for realizing emerging technologies. Pushing forward, however, requires understanding basic properties of insulating, semiconducting and metallic oxides, as well as the more complex phases that arise out of strong electronic correlations unique to this class of materials. In this review, we will focus on one of the unique aspects of oxides: the ultra-high conductivity metallic state, which can be a critical component for future all-oxide microelectronics such as low-loss interconnects and gate-metals, spintronics, as well as future quantum technologies that employ emergent magnetic or superconducting ground states. Like most oxides, a critical challenge to understanding and ultimately integrating high-conductivity metals into new technologies is the ability to synthesize high-quality materials. Therefore, we will frame the discussions in the context of epitaxial film growth via molecular beam epitaxy (MBE), which has provided insights into the electronic behavior of specific materials while providing samples with unprecedented quality. We will highlight and underscore how MBE has enabled developments and deeper understanding of their properties and how it plays a critical role in the future of this unique class of materials.

cond-mat.mtrl-sci↗

arXiv2025

Electron magnetization effects on carbonaceous dusty nanoparticles grown in $Ar/C_2H_2$ capacitively coupled nonthermal plasma

Carbonaceous dusty nanoparticles spontaneously grow in nonthermal plasmas from a gas mixture of argon and acetylene. These particles levitate and grow within the bulk plasma for a duration known as the growth cycle ($T_c$), after which they gradually move away. In experiments operating at 500 milliTorr, the particles reach a maximum radius of approximately 250 nm for $T_c \sim$ 121 s. However, the introduction of weak magnetic fields reduces both the maximum radius and $T_c$. The modified electron Hall parameter ($H_e'$), which quantifies the degree of electron magnetization, increases linearly with the magnetic field strength, transitioning from unmagnetized electrons ($H_e' < 1$) to magnetized electrons ($H_e' > 1$). $T_c$ gradually decreases to around 40 s until $H_e' \sim 1$ at approximately 330 Gauss, after which it remains roughly constant for fields up to about 1020 Gauss. Additionally, with increasing magnetic field strength, the dust growth rate initially decreases to $H_e' \sim 1$, then increases slightly again. These results demonstrate that the onset of electron magnetization at can control the growth of nanoparticles from chemical precursors in nonthermal plasmas, which is relevant for industrial applications.

physics.plasm-ph↗

arXiv2025

Deep learning with reflection high-energy electron diffraction images to predict cation ratio in Sr$_{2x}$Ti$_{2(1-x)}$O$_{3}$ thin films

Machine learning (ML) with in situ diagnostics offers a transformative approach to accelerate, understand, and control thin film synthesis by uncovering relationships between synthesis conditions and material properties. In this study, we demonstrate the application of deep learning to predict the stoichiometry of Sr$_{2x}$Ti$_{2(1-x)}$O$_{3}$ thin films using reflection high-energy electron diffraction images acquired during pulsed laser deposition. A gated convolutional neural network trained for regression of the Sr atomic fraction achieved accurate predictions with a small dataset of 31 samples. Explainable AI techniques revealed a previously unknown correlation between diffraction streak features and cation stoichiometry in Sr$_{2x}$Ti$_{2(1-x)}$O$_{3}$ thin films. Our results demonstrate how ML can be used to transform a ubiquitous in situ diagnostic tool, that is usually limited to qualitative assessments, into a quantitative surrogate measurement of continuously valued thin film properties. Such methods are critically needed to enable real-time control, autonomous workflows, and accelerate traditional synthesis approaches.

cond-mat.mtrl-sci↗

arXiv2025

Improvement of Data Analytics Techniques in Reflection High Energy Electron Diffraction to Enable Machine Learning

Perovskite oxides such as LaFeO$_3$ are a well-studied family of materials that possess a wide range of useful and novel properties. Successfully synthesizing perovskite oxide samples usually requires a significant number of growth attempts and a detailed film characterization on each sample to find the optimal growth window of a material. The most common real-time \textit{in situ} diagnostic technique available during molecular beam epitaxy (MBE) synthesis is reflection high-energy electron diffraction (RHEED). Conventional use of RHEED allows a highly experienced operator to determine growth rate by monitoring intensity osciallations and make some qualitative observations during growth, such as recognizing the sample has become amorphous or recognizing that large islands have formed on the surface. However, due to a lack of theoretical understanding of the diffraction patterns, finer, more precise levels of observations are challenging. To address these limitations, we implement new data analytics techniques in the growth of three LaFeO$_3$ samples on Nb-doped SrTiO$_3$ by MBE. These techniques improve our ability to perform unsupervised machine learning using principal component analysis (PCA) and k-means clustering by using drift correction to overcome sample or stage motion during growth and intensity transformations that highlight more subtle features in the images such as Kikuchi bands. With this approach, we enable the first demonstration of PCA and k-means across multiple samples, allowing for quantitative comparison of RHEED videos for two LaFeO$_3$ film samples. These capabilities set the stage for real-time processing of RHEED data during growth to enable machine learning-accelerated film synthesis.

cond-mat.mtrl-sci↗

arXiv2024

Revealing the Evolution of Order in Materials Microstructures Using Multi-Modal Computer Vision

The development of high-performance materials for microelectronics, energy storage, and extreme environments depends on our ability to describe and direct property-defining microstructural order. Our present understanding is typically derived from laborious manual analysis of imaging and spectroscopy data, which is difficult to scale, challenging to reproduce, and lacks the ability to reveal latent associations needed for mechanistic models. Here, we demonstrate a multi-modal machine learning (ML) approach to describe order from electron microscopy analysis of the complex oxide La$_{1-x}$Sr$_x$FeO$_3$. We construct a hybrid pipeline based on fully and semi-supervised classification, allowing us to evaluate both the characteristics of each data modality and the value each modality adds to the ensemble. We observe distinct differences in the performance of uni- and multi-modal models, from which we draw general lessons in describing crystal order using computer vision.

cond-mat.mtrl-sci↗

arXiv2024

Strain Effects in SrHfO$_{3}$ Films Grown by Hybrid Molecular Beam Epitaxy

Perovskite oxides hetero-structures are host to a large number of interesting phenomena such as ferroelectricity and 2D-superconductivity. Ferroelectric perovskite oxides have been of significant interest due to their possible use in MOSFETs and FRAM. SrHfO$_3$ (SHO) is a perovskite oxide with pseudo-cubic lattice parameter of 4.1 $\mathring{A}$ that previous DFT calculations suggest can be stabilized in a ferroelectric P4mm phase, similar to STO, when stabilized with sufficient compressive strain. Additionally, it is insulating, possesses a large band gap, and a high dielectric constant, making it an ideal candidate for oxide electronic devices. In this work, SHO films were grown by hybrid molecular beam epitaxy with a tetrakis(ethylmethylamino)hafnium(IV) source on GdScO$_3$ and TbScO$_3$ substrates. Equilibrium and strained SHO phases were characterized using X-ray diffraction, X-ray absorption spectroscopy, and scanning transmission electron microscopy to determine the perovskite phase of the strained films, with the results compared to density functional theory models of phase stability versus strain. Contrary to past reports, we find that compressively-strained SrHfO$_3$ undergoes octahedral tilt distortions and most likely takes on the I4/mcm phase with the a$^0$a$^0$c$^-$ tilt pattern.

cond-mat.mtrl-sci↗

arXiv2024

Comparing growth of titania and carbonaceous dusty nanoparticles in weakly magnetised capacitively coupled plasmas

This study compares the growth cycles and spatial distribution of dust cloud for titania and carbonaceous dusty nanoparticles in capacitively coupled radiofrequency plasmas, with and without the presence of a weak magnetic field of approximately 500 Gauss. Findings on cycle time, growth rate, and spatial distribution of dust cloud are discussed. The growth of nanoparticles in these plasmas is cyclic, with particles reaching their maximum size and subsequently moving out of the plasma, followed by the generation of a new particle growth cycle. The presence of the magnetic field speeds up the growth cycle in both plasma. The magnetic field also makes the spatial distribution of the two dust cloud different from each other. Langmuir probe measurement of the background plasma parameters such as electron temperature and floating potential reveal radial variations in floating potential but not electron temperature. Furthermore, the magnetic field changes the radial variation of floating potential. These measurements, however, are not sufficient to explain why the two dust clouds appear differently. It is possible that the differences occur due to a gradient in the radial distribution of the magnetic field.

physics.plasm-ph↗

arXiv2024

Strain-dependent Insulating State and Kondo Effect in Epitaxial SrIrO$_{3}$ Films

The large spin-orbit coupling in iridium oxides plays a significant role in driving novel physical behaviors, including emergent phenomena in the films and heterostructures of perovskite and Ruddlesden-Popper iridates. In this work, we study the role of epitaxial strain on the electronic behavior of thin SrIrO$_3$ films. We find that compressive epitaxial strain leads to metallic transport behavior, but a slight tensile strain shows gapped behavior. Temperature-dependent resistivity measurements are used to examine different behaviors in films as a function of strain. We find Kondo contributions to the resistivity, with stronger effects in films that are thinner and under less compressive epitaxial strain. These results show the potential to tune SrIrO$_3$ into Kondo insulating states and open possibilities for a quantum critical point that can be controlled with strain in epitaxial films.

cond-mat.mtrl-sci↗

arXiv2023

Introducing dusty plasma particle growth of nanospherical titanium dioxide

In dusty plasma environments, the spontaneous growth of nanoparticles from reactive gases has been extensively studied for over three decades, primarily focusing on hydrocarbons and silicate particles. Here, we introduce the growth of titanium dioxide, a wide band gap semiconductor, as dusty plasma nanoparticles. The resultant particles exhibited a spherical morphology and reached a maximum homogeneous radius of 230 $\pm$ 17 nm after an elapsed time of 70 seconds. The particle grew linearly and the growth displayed a cyclic behavior; that is, upon reaching their maximum radius, the largest particles fell out of the plasma, and a new growth cycle immediately followed. The particles were collected after being grown for different amounts of time and imaged using scanning electron microscopy. Further characterization was carried out using energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy to elucidate the chemical composition and crystalline properties of the maximally sized particles. Initially, the as-grown particles after 70 seconds exhibited an amorphous structure. However, annealing treatments at temperatures of 400 $^\circ$C and 800 $^\circ$C induced crystallization, yielding anatase and rutile phases, respectively. Notably, annealing at 600 $^\circ$C resulted in a mixed phase of anatase and rutile. These findings open new avenues for a rapid and controlled growth technique of titanium dioxide as dusty plasma.

physics.plasm-ph↗

arXiv2023

Advances in Complex Oxide Quantum Materials Through New Approaches to Molecular Beam Epitaxy

Molecular beam epitaxy (MBE), a workhorse of the semiconductor industry, has progressed rapidly in the last few decades in the development of novel materials. Recent developments in condensed matter and materials physics have seen the rise of many novel quantum materials that require ultra-clean and high-quality samples for fundamental studies and applications. Novel oxide-based quantum materials synthesized using MBE have advanced the development of the field and materials. In this review, we discuss the recent progress in new MBE techniques that have enabled synthesis of complex oxides that exhibit "quantum" phenomena, including superconductivity and topological electronic states. We show how these techniques have produced breakthroughs in the synthesis of 4d and 5d oxide films and heterostructures that are of particular interest as quantum materials. These new techniques in MBE offer a bright future for the synthesis of ultra-high quality oxide quantum materials.

cond-mat.mtrl-sci↗

arXiv2022

Jahn-Teller-driven Phase Segregation in Mn$_{x}$Co$_{3-x}$O$_{4}$ Spinel Thin Films

Transition metal spinel oxides comprised of Earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding of functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely-controlled materials to explore the variations reported in the literature. In this work, Mn$_{x}$Co$_{3-x}$O$_{4}$ samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Films were characterized via in situ X-ray photoelectron spectroscopy, X-ray diffraction, scanning transmission electron microscopy, and polarized K-edge X-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn$^{3+}$ and Mn$^{4+}$ character with evidence of phase segregation tendencies with increasing Mn content and increasing Mn$^{3+}$ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn$^{4+}$ and Jahn-Teller active Mn$^{3+}$ octahedra, respectively. Our results help to explain the reported differences across samples in these promising materials for renewable energy technologies.

cond-mat.mtrl-sci↗

arXiv2022

Band-Engineered LaFeO$_{3}$-LaNiO$_{3}$ Thin Film Interfaces for Electrocatalysis of Water

Transition metal oxides have generated significant interest for their potential as catalysts for the oxygen evolution reaction (OER) in alkaline environments. Iron and nickel-based perovskite oxides have proven particularly promising, with catalytic over-potentials rivaling precious metal catalysts when the alignment of the valence band relative to the OER reaction potential is tuned through substitutional doping or alloying. Here we report that engineering of band alignment in LaFeO$_{3}$/LaNiO$_{3}$ (LFO/LNO) heterostructures via interfacial doping yields greatly enhanced catalytic performance. Using density functional theory modeling, we predict a 0.2 eV valence band offset (VBO) between metallic LNO and semiconducting LFO that significantly lowers the barrier for hole transport through LFO compared to the intrinsic material and make LFO a p-type semiconductor. Experimental band alignment measurements using in situ X-ray photoelectron spectroscopy of epitaxial LFO/LNO heterostructures agree quite well with these predictions, producing a measured VBO of 0.3(1) eV. OER catalytic measurements on the same samples in alkaline solution show an increase in catalytic current density by a factor of ~275 compared to LFO grown on n-type Nb-doped SrTiO$_{3}$. These results demonstrate the power of tuning band alignments through interfacial band engineering for improved catalytic

cond-mat.mtrl-sci↗

arXiv2021

Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$

Nanocrystalline MnFe$_{2}$O$_{4}$ has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been lightly studied as highly ordered thin film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$ spinel oxide films were grown on (001) and (111) oriented Nb:SrTiO$_{3}$ perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). HRXRD and XPS show synthesis of pure phase materials while STEM and RHEED analysis demonstrate island-like growth of (111) surface terminated pyramids on both (001) and (111) oriented substrates, consistent with the literature and attributed to lattice mismatch between the spinel films and perovskite substrate. Cyclic voltammograms under an N$_{2}$ atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$. Under O$_{2}$ atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, diffusion limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe$_{2}$O$_{4}$ reported in the literature where diffusion limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N$_{2}$, in addition to low conductivity through the MnFe$_{2}$O$_{4}$ film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials.

cond-mat.mtrl-sci↗

arXiv2021

Surface Stability of SrNbO$_{3+δ}$ Grown by Hybrid Molecular Beam Epitaxy

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO$_{3}$ is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d$^{1}$ Nb$^{4+}$ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO$_{3}$ thin films were grown using hybrid MBE for the first time using a tris(diethalamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO$_{3}$ substrates. Varying thicknesses of insulating SrHfO$_{3}$ capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO$_{3}$ films to preserve the metastable surface. Grown films were transferred in vacuo for X-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by X-ray absorption near edge spectra illustrates the SrHfO$_{3}$ capping plays an important role in preserving the Nb 4d$^{1}$ metastable charge state in atmospheric conditions.

cond-mat.mtrl-sci↗

arXiv2021

Thickness Dependent OER Electrocatalysis of Epitaxial LaFeO$_{3}$ Thin Films

Transition metal oxides have long been an area of interest for water electrocatalysis through the oxygen evolution and oxygen reduction reactions. Iron oxides, such as LaFeO$_{3}$, are particularly promising due to the favorable energy alignment of the valence and conduction bands comprised of Fe$^{3+}$ cations and the visible light band gap of such materials. In this work, we examine the role of band alignment on the electrocatalytic oxygen evolution reaction (OER) in the intrinsic semiconductor LaFeO$_{3}$ by growing epitaxial films of varying thicknesses on Nb-doped SrTiO$_{3}$. Using cyclic voltammetry and electrochemical impedance spectroscopy, we find that there is a strong thickness dependence on the efficiency of electrocatalysis for OER. These measurements are understood based on interfacial band alignment in the system as confirmed by layer-resolved electron energy loss spectroscopy and electrochemical Mott-Schottky measurements. Our results demonstrate the importance of band engineering for the rational design of thin film electrocatalysts for renewable energy sources.

cond-mat.mtrl-sci↗

arXiv2021

Correlating surface stoichiometry and termination in SrTiO$_{3}$ films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO$_{3}$ (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy and low energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich-Schwoebel barrier consistent with an A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO$_{2}$ termination, or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

cond-mat.mtrl-sci↗