@article{wu_mirrielees_irving_2022, title={Defect Chemistry of Halogen Dopants in ZnSe}, volume={13}, ISSN={["1948-7185"]}, DOI={10.1021/acs.jpclett.2c01976}, abstractNote={Halogen dopants in ZnSe have been a research focus for quantum applications utilizing excitonic emissions, wherein point defects play a critical role. To provide a full first-principles perspective on the defect chemistries of halogen-doped ZnSe, Cl- and F-doped ZnSe were explored via hybrid functional density functional theory calculations involving all possible isolated defects and defect-defect complexes. Cl and F both exhibit more complicated defect chemistries than just forming a shallow substitutional donor on the Se site. For Cl, the complex of Cl substituting for Se with a neighboring Zn vacancy was also found to be prevalent. For F, its interstitial in the Zn tetrahedron was found to be stable in addition to the complex of such interstitial with an adjacent F atom substituting for Se. The explicitly simulated emission photoluminescence lineshapes of the self-activated centers exhibited both a peak value and a broad line width consistent with the experiment.}, number={35}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Wu, Yifeng and Mirrielees, Kelsey J. and Irving, Douglas L.}, year={2022}, month={Sep}, pages={8380–8385} }
 @article{baker_irving_2022, title={Inverse Materials Design of Doping Strategies with AI, Thermodynamics, and Density Functional Theory}, ISSN={["1543-1851"]}, DOI={10.1007/s11837-021-05087-x}, journal={JOM}, author={Baker, Jonathon N. and Irving, Douglas L.}, year={2022}, month={Jan} }
 @article{wu_mirrielees_irving_2022, title={On native point defects in ZnSe}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0092736}, abstractNote={Aiming at a fundamental understanding of the defect chemistry of pure ZnSe for optical and quantum applications, systematic density functional theory calculations with hybrid exchange-correlation functionals were performed to build an accurate database of native defects in ZnSe, including isolated defects and first nearest-neighbor defect–defect complexes. From the defect formation energies, zinc vacancy is found to be the most prevalent defect as the Fermi level approaches the conduction band edge, while zinc interstitial in the selenium tetrahedron and selenium vacancy become the most prevalent defects as the Fermi level approaches the valence band maximum. The divacancy complex, consisting of first nearest-neighboring zinc and selenium vacancies, is also found to have a favorable binding energy across the entire bandgap. Its formation energy is, however, always higher than either the isolated zinc or selenium vacancy, meaning it will never be the predominant defect in equilibrium. Finally, a point defect with extended spin coherence in Fluorine-implanted ZnSe was recently discovered, and it was found to exhibit a broad emission peak centered at 2.28 eV. The identity of this defect was determined to be either zinc vacancy or its associated complex according to the electron paramagnetic resonance measurements. Explicit simulations of the optical signatures of all zinc vacancy-related native defects were conducted here, showing that both zinc vacancy and divacancy are the most likely native defect contributors to that peak.}, number={23}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Mirrielees, Kelsey J. and Irving, Douglas L.}, year={2022}, month={Jun} }
 @article{al-tawhid_kanter_hatefipour_irving_kumah_shabani_ahadi_2022, title={Oxygen Vacancy-Induced Anomalous Hall Effect in a Nominally Non-magnetic Oxide}, volume={9}, ISSN={["1543-186X"]}, url={https://doi.org/10.1007/s11664-022-09941-9}, DOI={10.1007/s11664-022-09941-9}, abstractNote={The anomalous Hall effect, a hallmark of broken time-reversal symmetry and spin-orbit coupling, is frequently observed in magnetically polarized systems. Its realization in non-magnetic systems, however, remains elusive. Here, we report on the observation of anomalous Hall effect in nominally non-magnetic KTaO3. Anomalous Hall effect emerges in reduced KTaO3 and shows an extrinsic to intrinsic crossover. A paramagnetic behavior is observed in reduced samples using first principles calculations and quantitative magnetometry. The observed anomalous Hall effect follows the oxygen vacancy-induced magnetization response, suggesting that the localized magnetic moments of the oxygen vacancies scatter conduction electrons asymmetrically and give rise to anomalous Hall effect. The anomalous Hall conductivity becomes insensitive to scattering rate in the low temperature limit (T<5 K), implying that the Berry curvature of the electrons on the Fermi surface controls the anomalous Hall effect. Our observations describe a detailed picture of many-body interactions, triggering anomalous Hall effect in a non-magnetic system.}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Al-Tawhid, Athby H. and Kanter, Jesse and Hatefipour, Mehdi and Irving, Douglas L. and Kumah, Divine P. and Shabani, Javad and Ahadi, Kaveh}, year={2022}, month={Sep} }
 @article{mirrielees_baker_bowes_irving_2021, title={Computational approaches to point defect simulations for semiconductor solid solution alloys}, volume={154}, ISBN={1089-7690}, url={https://doi.org/10.1063/5.0041127}, DOI={10.1063/5.0041127}, abstractNote={Despite their technological importance, studying the properties of alloys with first principles methods remains challenging. In cases of AlxGa1-xN and BaxSrx-1TiO3 (BST), whose most important properties are governed by point defects, explicit simulation can be a computationally demanding task due to the random occupation of Al and Ga on cation sites in AlGaN and Ba and Sr on A-sites in BST. In this work, interpolation between end member compounds is used as a first approximation to defect properties and concentrations in intermediate alloy compositions in lieu of explicit simulation. In AlGaN, the efficacy of Si and Ge as dopants for n-type Al-rich AlGaN is explored by considering self-compensating defects such as multi-donor vacancy complexes and Si and Ge DX configurations. In BST, variation of the high temperature defect chemistry of Mg and Fe is examined. The approach presented here is expected to be generally appropriate for first approximation of defect properties in semiconductors and dielectrics where the alloy is a random solid solution of the end members.}, number={9}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Mirrielees, Kelsey J. and Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2021} }
 @article{ryu_bowes_mcgarrahan_irving_dickey_2021, title={Fermi level pinning in Co-doped BaTiO3: Part I. DC and AC electrical conductivities and degradation behavior}, volume={105}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.18042}, DOI={10.1111/jace.18042}, abstractNote={Abstract We explore the synergistic effects of co‐doping BaTiO 3 with a judicious combination of acceptors and donors to control the point defect chemistry and electrical properties, with the goal of simultaneously limiting the electronic and ionic conductivities over broad temperature and oxygen partial pressure ( p O 2 ) ranges. Specifically, we compare the temperature‐ and p O 2 ‐dependent electrical properties of BaTiO 3 ceramics acceptor‐doped with either Mn or Mg and co‐doped with a Y donor. This study, which is the first of a two‐part series, presents the electrical properties as a function of p O 2 , temperature, and time, focusing on the grain‐interior electrical response. The DC and AC electrical conductivity measurements reveal that co‐doping with Mn and Y can result in (1) increased electrical resistivity over a broad temperature range, (2) p O 2 ‐independent electrical conductivity in oxidizing conditions, and (3) improved time‐dependent dielectric degradation resistance. These behaviors are attributed to a Fermi level pinning effect, as is explained in the companion paper, which presents complementary density functional theory (DFT)‐based grand‐canonical defect chemistry models. The collective experimental and computational studies demonstrate that the p O 2 ‐independent electrical conductivity in the Mn and Y co‐doped BaTiO 3 is attributed to a Fermi level pinning mechanism arising from the multivalent Mn dopant, and the background reservoir of positive charge provided by the predominant substitution of Y on the Ba sites. The enhanced degradation resistance is attributed to a reduced oxygen vacancy concentration relative to the other doping chemistries.}, number={1}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Ryu, Gyung Hyun and Bowes, Preston C. and McGarrahan, John R. and Irving, Douglas L. and Dickey, Elizabeth C.}, year={2021}, month={Jul} }
 @article{bowes_ryu_baker_dickey_irving_2021, title={Fermi level pinning in Co-doped BaTiO3: Part II. Defect chemistry models}, volume={7}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.17938}, DOI={10.1111/jace.17938}, abstractNote={Abstract A first‐principles informed grand canonical defect chemistry model capable of accounting for non‐stoichiometry and partial equilibration of different sub‐lattices is developed and used to study Mg and Mn doped, and (Mg+Y) and (Mn+Y) co‐doped BaTiO 3 to elucidate the role of Mn and Y in improving the resistivity and resistance degradation of BaTiO 3 as observed by Ryu et al. in Part I of this series of papers. The model qualitatively captures the behavior of the samples in all conditions, reproducing the observed carrier plateau and increased resistivity of (Mn+Y) co‐doped BaTiO 3 , and expected trends in the concentrations of free oxygen vacancies with doping. These trends reflect the observed differences in degradation characteristics, and help explain the substantially improved degradation resistance of the (Mn+Y) co‐doped samples. Our model adds to the mechanism proposed by Yeoh et al. that the Fermi level is pinned by the multivalent character of Mn Ti in (Mn+Y) co‐doped BaTiO 3 by giving insight into the role of barium vacancies, the site preferences of the dopants, and defect complexes in this mechanism. These insights provide a set of criteria in the search for sets of co‐dopants with similar behaviors.}, number={11}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Ryu, Gyung Hyun and Baker, Jonathon N. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2021}, month={Jul} }
 @article{bowes_wu_baker_irving_2021, title={Modeling the spatial control over point defect spin states via processing variables}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0039972}, DOI={10.1063/5.0039972}, abstractNote={Contemporary models that are used to search for solid-state point defects for quantum-information applications tend to focus on the defect’s intrinsic properties rather than the range of conditions in which they will form. In this work, a first-principles based multi-scale device model is used to explore how the conditions (i.e., growth temperature, doping concentration, unintentional impurity concentration) influence the formation of a neutral aluminum vacancy complexed with an oxygen impurity at a neighboring nitrogen site v Al-1O N in an Si/Mg:AlN homojunction. Varying the donor (Si) concentration is predicted to lead to the greatest change in both the maximum height and shape of the (v Al-1O N) 0 profile. The shape is found to depend on the acceptor (Mg) concentration as well, and a critical ratio between the acceptor and unintentional impurities below which the (v Al-1O N) 0 center would not form was identified. A detailed analysis of the electrostatic potential, electric field, and defect chemistry obtained with the model was used to reveal the underlying causes of these changes. These results show the potential of varying processing parameters to manipulate the local electronic structure as a means to control the properties of point defects for quantum-information applications.}, number={22}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Jun} }
 @article{mirrielees_dycus_baker_reddy_collazo_sitar_lebeau_irving_2021, title={Native oxide reconstructions on AlN and GaN (0001) surfaces}, volume={129}, ISSN={["1089-7550"]}, DOI={10.1063/5.0048820}, abstractNote={Properties of AlN/GaN surfaces are important for realizing the tunability of devices, as the presence of surface states contributes to Fermi level pinning. This pinning can influence the performance of high electron mobility transistors and is also important for passivation of the surface when developing high-power electronic devices. It is widely understood that both AlN and GaN surfaces oxidize. Since there are many possible reconstructions for each surface, it is a challenge to identify the relevant surface reconstructions in advance of a detailed simulation. Because of this, different approaches are often employed to down select initial structures to reduce the computational load. These approaches usually rely on either electron counting rules or oxide stoichiometry, as both of these models tend to lead to structures that are energetically favorable. Here we explore models from these approaches but also explore a reconstruction of the (0001) surface directly observed using scanning transmission electron microscopy with predictive density functional theory simulations. Two compositions of the observed surface reconstruction—one which obeys oxide stoichiometry and one which is cation deficient and obeys electron counting—are compared to reconstructions from the previous work. Furthermore, surface states are directly calculated using hybrid exchange-correlation functionals that correct for the underestimation of the bandgaps in AlN and GaN and improve the predicted positions of surface states within the gap. It is found that cation deficiency in the observed reconstruction yields surface states consistent with the experiment. Based on all of these results, we provide insight into the observed properties of oxidized AlGaN surfaces.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, author={Mirrielees, Kelsey J. and Dycus, J. Houston and Baker, Jonathon N. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and LeBeau, James M. and Irving, Douglas L.}, year={2021}, month={May} }
 @article{wu_bowes_baker_irving_2021, title={Photochromism of UV-annealed Fe-doped SrTiO3}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0068523}, DOI={10.1063/5.0068523}, abstractNote={High-temperature annealing coupled with above bandgap UV illumination is an emerging approach to manipulate defect chemistries and resultant properties of electroceramics. To explore defect-processing-property relationships in these materials, an advanced multiphysics and multiscale model has been developed, which involves (a) high-fidelity first principles simulations of defect energies, (b) grand canonical thermodynamics of defect equilibria, (c) UV-perturbed defect formation energies from Shockley–Read–Hall generation and recombination, and (d) finite-element analyses of electrostatic potential and defect redistribution. Using this model, bottom-up insights into defect mechanisms associated with the UV-induced brown photochromism of Fe-doped SrTiO3 at high temperatures are provided. It is found that UV illumination leads to dissociation of the FeTi-vO complex and reduction in the oxygen vacancy concentration through exchange with the gas reservoir. Changes to these defect populations cause reionization of the FeTi defect from −1 to 0 charge state to maintain charge neutrality. This collectively gives rise to an increased concentration of FeTi0, which is the source of brown chromism. In addition, this model reproduces the experimentally observed electrical resistance degradation of samples annealed in this manner due to the increasing hole concentration in the material with time. The present model itself offers a route to guide and facilitate future efforts in this field.}, number={26}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Dec} }
 @article{wu_irving_2021, title={Prediction of chemical ordering in refractory high-entropy superalloys}, volume={119}, ISSN={["1077-3118"]}, DOI={10.1063/5.0059453}, abstractNote={Refractory high-entropy superalloys (RHESs) exhibit impressive nanostructure-property relationships and have promise in next-generation high-temperature structural applications, which has motivated extensive research into these materials. The design space, however, is compositionally vast and complex due to the presence of multiple phases that differ in the composition and chemical order. To address these obstacles in a computationally efficient manner, an advanced approach combining the mean-field density functional theory with parameters determined using machine learning tools has been developed. This approach was implemented here to investigate AlMo0.5NbTa0.5TiZr, which exhibits a nanostructure consisting of cuboidal BCC precipitates coherently embedded within the B2 matrix. It was found that Al and Zr were responsible for the formation of the B2 matrix. In addition, the matrix and the precipitate were found to have very different elastic characteristics. The matrix has a small elastic moduli and large anisotropy while the precipitate is elastically stiff and nearly isotropic. Beyond the current findings, the parameters for the mean field approach are given in the supplementary material and these can be used in future efforts to predict chemical orders, phase partitioning, and elastic properties of RHESs as a function of chemical composition.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Irving, Douglas L.}, year={2021}, month={Sep} }
 @article{washiyama_mirrielees_bagheri_baker_kim_guo_kirste_guan_breckenridge_klump_et al._2021, title={Self-compensation in heavily Ge doped AlGaN: A comparison to Si doping}, volume={118}, ISSN={["1077-3118"]}, DOI={10.1063/5.0035957}, abstractNote={Self-compensation in Ge- and Si-doped Al0.3Ga0.7N has been investigated in terms of the formation of III vacancy and donor-vacancy complexes. Both Ge- and Si-doped AlGaN layers showed a compensation knee behavior with impurity compensation (low doping regime), compensation plateau (medium doping regime), and self-compensation (high doping regime). A maximum free carrier concentration of 4–5 × 1019 cm−3 was obtained by Ge doping, whereas Si doping resulted in only half of that value, ∼2 × 1019 cm−3. A DFT calculation with the grand canonical thermodynamics model was developed to support the hypothesis that the difference in self-compensation arises from the difference in the formation energies of the VIII-n•donor complexes relative to their onsite configurations. The model suggested that the VIII-2•donor and VIII-3•donor complexes were responsible for self-compensation for both Ge- and Si-doped AlGaN. However, a lower free carrier concentration in Si-doped samples was due to a high VIII-3•Si concentration, resulting from a lower energy of formation of VIII-3•Si.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Washiyama, Shun and Mirrielees, Kelsey J. and Bagheri, Pegah and Baker, Jonathon N. and Kim, Ji-Hyun and Guo, Qiang and Kirste, Ronny and Guan, Yan and Breckenridge, M. Hayden and Klump, Andrew J. and et al.}, year={2021}, month={Jan} }
 @article{baker_bowes_harris_collazo_sitar_irving_2020, title={Complexes and compensation in degenerately donor doped GaN}, volume={117}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0013988}, DOI={10.1063/5.0013988}, abstractNote={Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions.}, number={10}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2020}, month={Sep} }
 @article{li_li_irving_varga_vitos_schonecker_2020, title={Ductile and brittle crack-tip response in equimolar refractory high-entropy alloys}, volume={189}, ISSN={["1873-2453"]}, DOI={10.1016/j.actamat.2020.03.004}, abstractNote={Understanding the strengthening and deformation mechanisms in refractory high-entropy alloys (HEAs), proposed as new high-temperature material, is required for improving their typically insufficient room-temperature ductility. Here, density-functional theory simulations and a continuum mechanics analysis were conducted to systematically investigate the competition between cleavage decohesion and dislocation emission from a crack tip in the body-centered cubic refractory HEAs HfNbTiZr, MoNbTaVW, MoNbTaW, MoNbTiV, and NbTiVZr. This crack-tip competition is evaluated for tensile loading and a totality of 15 crack configurations and slip systems. Our results predict that dislocation plasticity at the crack tip is generally unfavorable – although the competition is close for some crack orientations, suggesting intrinsic brittleness and low crack-tip fracture toughness in these five HEAs at zero temperature. Fluctuations in local alloy composition, investigated for HfNbTiZr, can locally reduce the resistance to dislocation emission for a slip system relative to the configuration average of that slip system, but do not change the dominant crack-tip response. In the case of single-crystal MoNbTaW, where an experimental, room-temperature fracture-toughness value is available for a crack on a {100} plane, theoretical and experimental results agree favorably. Factors that may limit the agreement are discussed. We survey the effect of material anisotropy on preferred crack tip orientations, which are found to be alloy specific. Mixed-mode loadings are found to shift the competition in favor of cleavage or dislocation nucleation, depending on crack configuration and amplified by the effect of material anisotropy on crack tip stresses.}, journal={ACTA MATERIALIA}, author={Li, Xiaoqing and Li, Wei and Irving, Douglas L. and Varga, Lajos K. and Vitos, Levente and Schonecker, Stephan}, year={2020}, month={May}, pages={174–187} }
 @article{wu_bowes_baker_irving_2020, title={Influence of space charge on the conductivity of nanocrystalline SrTiO3}, volume={128}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0008020}, DOI={10.1063/5.0008020}, abstractNote={A grand canonical multiscale space-charge model has been developed to study and predict the electrical properties of polycrystalline perovskites with complex defect chemistries. This model combines accurate data from hybrid exchange-correlation functional density functional theory calculations (defect formation energies, resultant grand canonical calculations of defect concentrations, and ionization states) with finite-element simulation of the electric field and its coupling to defect redistribution and reionization throughout the grain. This model was used to simulate the evolution of the oxygen partial pressure-dependent conductivity of polycrystalline acceptor-doped strontium titanate as the grain size decreases, and the results were compared to previous experiments. These results demonstrate that as the grain size is reduced from the microscale to nanoscale, the experimentally observed disappearance of ionic conductivity and forward shift of the oxygen partial pressure of the n–p crossover are successfully reproduced and explained by the model. Mechanistically, the changes to conductivity stem from the charge transfer from the grain boundary core into the grain interior, forming a space-charge layer near the grain boundary core that perturbs the local defect chemistry. The impact of the grain size on the electrical conductivity and the underlying defect chemistry across the grain are discussed. In addition to the findings herein, the model itself enables exploration of the electrical response of polycrystalline semiconductor systems with complex defect chemistries, which is critical to the design of future electronic components.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Jul} }
 @article{bowes_baker_irving_2020, title={Site preference of Y and Mn in nonstoichiometric BaTiO3 from first principles}, volume={4}, ISSN={["2475-9953"]}, url={https://doi.org/10.1103/PhysRevMaterials.4.084601}, DOI={10.1103/PhysRevMaterials.4.084601}, abstractNote={$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_{3}$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\ensuremath{\circ}}\mathrm{C}$.}, number={8}, journal={PHYSICAL REVIEW MATERIALS}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Aug} }
 @article{bowes_baker_irving_2020, title={Survey of acceptor dopants in SrTiO3: Factors limiting room temperature hole concentration}, volume={103}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.16784}, DOI={10.1111/jace.16784}, abstractNote={Abstract Unintentional impurities often found in strontium titanate (doped or undoped) have hindered efforts to study individual impurities experimentally. To fill this gap, a computational survey of acceptor‐type point defects of common intentional or unintentional impurities (Al, Cu, Fe, K, Mg, Mn, N, Na, Ni, and Zn) is presented. Utilizing defect formation energies from density functional theory using hybrid exchange correlation functionals in a grand canonical model of the defect chemistry, the equilibrium Fermi level ( μ e ) was calculated as a function of processing conditions for pure SrTiO 3 , SrTiO 3 individually doped with each impurity, and SrTiO 3 co‐doped with Al and N. Above a certain concentration, each impurity reduced the maximum predicted hole concentration relative to the intrinsic case. Al, Mg, Zn, K, and Na exhibited similar trends and behaved more like ideal acceptors while N, Ni, Fe, Mn, and Cu were all unique and pinned μ e near or above the mid‐gap in most conditions. Al/N:SrTiO 3 also exhibited similar trends at 800°C for all Al/N ratios, but more variation at 25°C. Additionally, the behavior of Al:SrTiO 3 was not recovered until Al/N = 10 4 . This suggests that to achieve SrTiO 3 with free holes at room temperature, the concentration of most impurities must be controlled.}, number={2}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Feb}, pages={1156–1173} }
 @article{baker_bowes_harris_irving_2019, title={An informatics software stack for point defect-derived opto-electronic properties: the Asphalt Project}, volume={9}, ISSN={["2159-6867"]}, url={http://dx.doi.org/10.1557/mrc.2019.106}, DOI={10.1557/mrc.2019.106}, abstractNote={Computational acceleration of performance metric-based materials discovery via high-throughput screening and machine learning methods is becoming widespread. Nevertheless, development and optimization of the opto-electronic properties that depend on dilute concentrations of point defects in new materials have not significantly benefited from these advances. Here, the authors present an informatics and simulation suite to computationally accelerate these processes. This will enable faster and more fundamental materials research, and reduce the cost and time associated with the materials development cycle. Analogous to the new avenues enabled by current first-principles-based property databases, this type of framework will open entire new research frontiers as it proliferates.}, number={3}, journal={MRS COMMUNICATIONS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Sep}, pages={839–845} }
 @article{wu_irving_2019, title={Finite temperature elastic properties of equiatomic CoCrFeNi from first principles}, volume={162}, ISSN={["1872-8456"]}, DOI={10.1016/j.scriptamat.2018.11.010}, abstractNote={The finite temperature elastic properties of the equiatomic CoCrFeNi medium-entropy alloy has been studied by density functional theory. Besides atomic vibrations and electronic free energy, the predictive model developed here includes contributions from spin fluctuations (SFs) in determining the elastic properties of CoCrFeNi. Including SFs changes the magnitude of the temperature derivatives of the poly-crystal elastic moduli, resulting in a close agreement between simulation and experimentally measured trends. How the single-crystal elastic moduli depend on SFs and how these dependencies influence changes in the poly-crystal elastic moduli are analyzed systematically. Finally, the elemental sources to the simulated trends are identified.}, journal={SCRIPTA MATERIALIA}, author={Wu, Yifeng and Irving, Douglas L.}, year={2019}, month={Mar}, pages={176–180} }
 @article{baker_bowes_harris_irving_2019, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3 (vol 124, 114101, 2018)}, volume={125}, ISSN={["1089-7550"]}, DOI={10.1063/1.5084251}, abstractNote={First Page}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jan} }
 @article{harris_gaddy_collazo_sitar_irving_2019, title={Oxygen and silicon point defects in Al0.65Ga0.35N}, volume={3}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.3.054604}, abstractNote={The formation energies of oxygen and silicon impurities have been examined explicitly in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ using hybrid exchange-correlation density-functional theory simulations. Both impurities were initialized in on-site substitutional and off-site DX configurations in a range of charge states. The ${\mathrm{O}}_{\mathrm{N}}^{+1}$ donor was found to always relax into an on-site configuration, and its formation energy is relatively independent of local chemistry (the configuration of Al and Ga atoms surrounding the defect). By contrast, the ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ acceptor almost always relaxes into a DX configuration, with a formation energy that is strongly dependent on local chemistry. The differences in formation energy of distinct ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ defect configurations are understood through the interplay of two qualitative trends in the types of nearest-neighbor bonds (O-Al or O-Ga), as well as the subtler influence of the lengths of the O-Al bonds. Knowledge of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ formation energies as well as the relative frequencies of sites with different local chemistry allows one to compute the relative site occupancies of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$. Because the thermodynamic transition levels associated with different defect configurations are unique, the ${\mathrm{O}}_{\mathrm{N}}$ DX transition is associated with multiple defect levels. ${\mathrm{Si}}_{\mathrm{III}}$, where III represents the group III cation of Al or Ga, provides an interesting counterexample. ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is predicted to be the dominant charge state across the entire band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$, and little dependence of the formation energy on the composition of nearby cation sites was found. This is explained by the fact that the first-nearest neighbors are all of the same species (N), so the local environment is similar to a bulk III nitride, in which on-site ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is stable across the same Fermi level range (i.e., below the band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$). Thus, the trends in the energetics of ${\mathrm{O}}_{\mathrm{N}}$ and ${\mathrm{Si}}_{\mathrm{III}}$ in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ are both determined by the chemistry of the four nearest-neighbor sites surrounding the defect site.}, number={5}, journal={PHYSICAL REVIEW MATERIALS}, author={Harris, Joshua S. and Gaddy, Benjamin E. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2019}, month={May} }
 @article{bowes_wu_baker_harris_irving_2019, title={Space charge control of point defect spin states in AlN}, volume={115}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5099916}, DOI={10.1063/1.5099916}, abstractNote={One barrier to developing quantum information systems based on impurity point defects is that the desirable spin states of the defects are often unstable for Fermi levels obtained at increased impurity concentrations. The space charge induced band bending near the interface of Si/Mg aluminum nitride (AlN) homojunction is investigated computationally as a method to control the concentration, spin state, and position of such point defects. This is done by solving Poisson's equation with the charge density described by a grand canonical defect chemistry model informed by hybrid-functional density functional theory (DFT) calculations. Previous experimental works have found unintentional carbon and oxygen impurities pervade AlN homojunctions. First principles calculations have predicted the neutral complex between an aluminum vacancy and oxygen impurity on a neighboring nitrogen site (vAl-1ON)0 has a spin triplet configuration, which is stable in a region when the Fermi level is below midgap. From defect equilibrium simulations considering 602 possible defects, vAl-1ON was found to be unstable on the Mg-doped side of the homojunction and isolated oxygen impurities are preferred. On the Si-doped side, vAl-1ON forms but as (vAl-1ON)–2, not (vAl-1ON)0. This makes vAl-1ON a prototypical test case for the proposed strategy. Simulations of the Si/Mg:AlN homojunction showed (vAl-1ON)0 is stabilized within 6 nm of the interface in the Si-doped portion. This result indicates space charge induced band bending enables control over the concentration, spin state, and position of point defects, which is critical to realizing point defect based quantum information systems.}, number={5}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jul} }
 @article{long_cai_baker_bowes_bayer_wang_wang_chen_randall_irving_et al._2018, title={Conductivity of iron‐doped strontium titanate in the quenched and degraded states}, volume={102}, ISSN={0002-7820 1551-2916}, url={http://dx.doi.org/10.1111/jace.16212}, DOI={10.1111/jace.16212}, abstractNote={Abstract}, number={6}, journal={Journal of the American Ceramic Society}, publisher={Wiley}, author={Long, Daniel M. and Cai, Biya and Baker, Jonathon N. and Bowes, Preston C. and Bayer, Thorsten J.M. and Wang, Jian‐Jun and Wang, Rui and Chen, Long‐Qing and Randall, Clive A. and Irving, Douglas L. and et al.}, year={2018}, month={Dec}, pages={3567–3577} }
 @article{bryan_bryan_washiyama_reddy_gaddy_sarkar_breckenridge_guo_bobea_tweedie_et al._2018, title={Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5011984}, DOI={10.1063/1.5011984}, abstractNote={In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.}, number={6}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Isaac and Bryan, Zachary and Washiyama, Shun and Reddy, Pramod and Gaddy, Benjamin and Sarkar, Biplab and Breckenridge, M. Hayden and Guo, Qiang and Bobea, Milena and Tweedie, James and et al.}, year={2018}, month={Feb} }
 @article{li_irving_vitos_2018, title={First-principles investigation of the micromechanical properties of fcc-hcp polymorphic high-entropy alloys}, volume={8}, ISSN={2045-2322}, url={http://dx.doi.org/10.1038/S41598-018-29588-Z}, DOI={10.1038/S41598-018-29588-Z}, abstractNote={High-entropy alloys offer a promising alternative in several high-technology applications concerning functional, safety and health aspects. Many of these new alloys compete with traditional structural materials in terms of mechanical characteristics. Understanding and controlling their properties are of the outmost importance in order to find the best single- or multiphase solutions for specific uses. Here, we employ first-principles alloy theory to address the micro-mechanical properties of five polymorphic high-entropy alloys in their face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. Using the calculated elastic parameters, we analyze the mechanical stability, elastic anisotropy, and reveal a strong correlation between the polycrystalline moduli and the average valence electron concentration. We investigate the ideal shear strength of two selected alloys under shear loading and show that the hcp phase possesses more than two times larger intrinsic strength than that of the fcc phase. The derived half-width of the dislocation core predicts a smaller Peierls barrier in the fcc phase confirming its increased ductility compared to the hcp one. The present theoretical findings explain a series of important observations made on dual-phase alloys and provide an atomic-level knowledge for an intelligent design of further high-entropy materials.}, number={1}, journal={Scientific Reports}, publisher={Springer Science and Business Media LLC}, author={Li, Xiaoqing and Irving, Douglas L. and Vitos, Levente}, year={2018}, month={Jul} }
 @article{baker_bowes_irving_2018, title={Hydrogen solubility in donor-doped SrTiO3 from first principles}, volume={113}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5047793}, DOI={10.1063/1.5047793}, abstractNote={Hydrogen contamination of strontium titanate (STO) during processing and usage is a known problem. However, it is relatively little-studied due to the difficulty in quantifying the amount of hydrogen that dissolves in the lattice. Here, we use hybrid exchange-correlation density functional theory calculations as input to a grand canonical thermodynamics framework to estimate hydrogen solubility and site preferences in donor-doped STO. Our results provide clear theoretical evidence that hydrogen contamination in donor-doped STO occurs at a low enough level to essentially ignore. But, this simple conclusion belies hydrogen's rich behavior; unlike many dopants, it is able to easily change its incorporation site in response to changes in processing conditions. Overall, the findings are consistent with prevailing wisdom and suggest that the presented first principles approach could be used for systematic exploration of hydrogen's impact as a function of doping and processing in this and other wide bandgap materials.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @article{eldred_harris_baker_irving_lebeau_2018, title={Imaging of Single Vacancies and Dopant Complexes in Aluminum Nitride}, volume={24}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927618000600}, DOI={10.1017/S1431927618000600}, abstractNote={To enable efficient aluminum nitride UV LEDs and power electronics, charge carrier concentration must be carefully engineered via dopant control. Silicon is the most studied and promising dopant in AlGaN systems, however it has been demonstrated that charge carrier concentration drops as Al content increases [1,2]. The prevailing explanation for the decrease in charge carrier concentration is that other defects compensate Si donors at high Si concentrations. The exact distribution and nature of the defects responsible have not been identified, however it is theorized that a complex with cation vacancies contribute to the effect [3]. The purpose of this study is to determine a method of identifying and mapping the vacancies and other defects in the material for the purpose of studying defect formation and confirming the statistical models used to guide growth techniques. In this presentation, we will discuss how high precision and accuracy scanning transmission electron microscopy (STEM) can be used to detect and map dopant complexes formed during metal-organic chemical vapor deposition crystal growth, specifically cation vacancies complexed with cation substitution dopants. We will present the approach to model the structure using DFT calculations in the Vienna ab initio Simulation Package (VASP 5.3.4). Specifically, formation energies determined using the grand canonical formalism are used to determine the relative concentrations of the different potential configurations of the Si Al -V Al complexes. Multislice simulations are then performed using the various calculated defect coordinates as a function of sample thicknesses and operating condition. To observe the effect of the defect, intensities and positions are then extracted from ADF STEM images as shown in Figure 1. ADF STEM with a detector inner semiangle of 28 mrad was used to maximize the signal from the weakly scattering aluminum and nitrogen. For a Si-vacancy complex, the relative intensity of the column decreases by an amount}, number={S1}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Eldred, Tim B. and Harris, Joshua S. and Baker, Jonathon N. and Irving, Douglas L. and LeBeau, James M.}, year={2018}, month={Aug}, pages={22–23} }
 @article{bowes_baker_harris_behrhorst_irving_2018, title={Influence of impurities on the high temperature conductivity of SrTiO3}, volume={112}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5000363}, DOI={10.1063/1.5000363}, abstractNote={In studies of high temperature electrical conductivity (HiTEC) of dielectrics, the impurity in the highest concentration is assumed to form a single defect that controls HiTEC. However, carrier concentrations are typically at or below the level of background impurities, and all impurities may complex with native defects. Canonical defect models ignore complex formation and lump defects from multiple impurities into a single effective defect to reduce the number of associated reactions. To evaluate the importance of background impurities and defect complexes on HiTEC, a grand canonical defect model was developed with input from density functional theory calculations using hybrid exchange correlation functionals. The influence of common background impurities and first nearest neighbor complexes with oxygen vacancies (vO) was studied for three doping cases: nominally undoped, donor doped, and acceptor doped SrTiO3. In each case, conductivity depended on the ensemble of impurity defects simulated with the extent of the dependence governed by the character of the dominant impurity and its tendency to complex with vO. Agreement between simulated and measured conductivity profiles as a function of temperature and oxygen partial pressure improved significantly when background impurities were included in the nominally undoped case. Effects of the impurities simulated were reduced in the Nb and Al doped cases as both elements did not form complexes and were present in concentrations well exceeding all other active impurities. The influence of individual impurities on HiTEC in SrTiO3 was isolated and discussed and motivates further experiments on singly doped SrTiO3.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Bowes, Preston C. and Baker, Jonathon N. and Harris, Joshua S. and Behrhorst, Brian D. and Irving, Douglas L.}, year={2018}, month={Jan} }
 @article{baker_bowes_harris_irving_2018, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3}, volume={124}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5044746}, DOI={10.1063/1.5044746}, abstractNote={Barium titanate (BTO) and strontium titanate (STO) are often treated as close analogues, and models of defect behavior are freely transferred from one material to the other with only minor modifications. On the other hand, it is often reported that B-site vacancies (vB) are the dominant metal vacancy in BTO, while A-site vacancies (vA) dominate in STO. This difference precludes the use of analogous defect models for BTO and STO, begging the question: how similar are the defect chemistries of the two materials? Here, we address this question with density functional theory calculations using a state-of-the-art hybrid exchange correlation functional, which more accurately describes the electronic structure and charge localization than traditional functionals. We find that vA is the dominant metal vacancy in STO but that different combinations of vA, vB, and vB-vO complexes are present in BTO depending on processing and doping. Mechanistically, this occurs for two reasons: thermodynamic differences in the accessible processing conditions of the two materials and energy differences in the bonds broken when forming the vacancies. These differences can also lead to widely differing responses when impurity dopants are intentionally added. Therefore, the response of metal vacancy behavior in BTO and STO to the inclusion of niobium and iron, two typical dopants in these systems, is examined and compared.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @article{harris_baker_gaddy_bryan_bryan_mirrielees_reddy_collazo_sitar_irving_2018, title={On compensation in Si-doped AlN}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5022794}, DOI={10.1063/1.5022794}, abstractNote={Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified VAl+nSiAl complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Harris, Joshua S. and Baker, Jonathon N. and Gaddy, Benjamin E. and Bryan, Isaac and Bryan, Zachary and Mirrielees, Kelsey J. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2018}, month={Apr} }
 @article{alden_harris_bryan_baker_reddy_mita_callsen_hoffmann_irving_collazo_et al._2018, title={Point-Defect Nature of the Ultraviolet Absorption Band in AIN}, volume={9}, ISSN={["2331-7019"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85047735459&partnerID=MN8TOARS}, DOI={10.1103/physrevapplied.9.054036}, abstractNote={We present an approach, where point defects and defect complexes are identified using power dependent photoluminescence excitation spectroscopy, impurity data from SIMS and DFT-based calculations accounting for the total charge balance in the crystal. Employing the capabilities of such experimental-computational approach, in this work, the UVC absorption band at 4.7 eV, as well as the 2.7 eV and 3.9 eV luminescence bands in AlN single crystals grown via physical vapor transport (PVT) are studied in detail. Photoluminescence excitation spectroscopy measurements demonstrate the relationship between the defect luminescent bands centered at 3.9 eV and 2.7 eV to the commonly observed absorption band centered at 4.7 eV. Accordingly, the thermodynamic transition energy for the absorption band at 4.7 eV and the luminescence band at 3.9 eV is estimated at 4.2 eV, in agreement with the thermodynamic transition energy for the C N– point defect. Finally, the 2.7 eV PL band is the result of a donor-acceptor pair transition between the V N and C N point defects since nitrogen vacancies, is predicted to be present in the crystal in concentrations similar to carbon employing charge balance constrained DFT calculations. Power dependent photoluminescence measurements reveal the presence of the deep donor state with a thermodynamic transition energy of 5.0 eV, which we hypothesize to be nitrogen vacancies in agreement with predictions based on theory. The charge state, concentration and type of impurities in the crystal is calculated considering a fixed amount of impurities and using a density functional theory (DFT) based defect solver, which considers their respective formation energies and the total charge balance in the crystal. The presented results show that nitrogen vacancies are the most likely candidate for the deep donor state involved in the donor acceptor pair transition with peak emission at 2.7 eV for the conditions relevant to PVT growth.}, number={5}, journal={PHYSICAL REVIEW APPLIED}, author={Alden, D. and Harris, J. S. and Bryan, Z. and Baker, J. N. and Reddy, P. and Mita, S. and Callsen, G. and Hoffmann, A. and Irving, D. L. and Collazo, R. and et al.}, year={2018}, month={May} }
 @article{dycus_mirrielees_grimley_kirste_mita_sitar_collazo_irving_lebeau_2018, title={Structure of Ultrathin Native Oxides on III-Nitride Surfaces}, volume={10}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.8b00845}, abstractNote={When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure is often elusive through the use of indirect diffraction methods or techniques that probe only the outermost layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this study, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of ultrathin native oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra-octahedra cation-oxygen units, in an arrangement similar to bulk θ-Al2O3 and β-Ga2O3. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimensional nitrides.}, number={13}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2018}, month={Apr}, pages={10607–10611} }
 @article{li_schonecker_li_varga_irving_vitos_2018, title={Tensile and shear loading of four fcc high-entropy alloys: A first-principles study}, volume={97}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.97.094102}, abstractNote={Ab initio density-functional calculations are used to investigate the response of four face-centered-cubic (fcc) high-entropy alloys (HEAs) to tensile and shear loading. The ideal tensile and shear ...}, number={9}, journal={PHYSICAL REVIEW B}, author={Li, Xiaoqing and Schonecker, Stephan and Li, Wei and Varga, Lajos K. and Irving, Douglas L. and Vitos, Levente}, year={2018}, month={Mar} }
 @article{tian_wang_harris_irving_zhao_vitos_2017, title={Alloying effect on the elastic properties of refractory high-entropy alloys}, volume={114}, ISSN={["1873-4197"]}, DOI={10.1016/j.matdes.2016.11.079}, abstractNote={Ab initio total energy calculations are used to determine the elastic properties of TiZrVNb, TiZrNbMo and TiZrVNbMo high-entropy alloys in the body centered cubic (bcc) crystallographic phase. Calculations are performed using the Vienna Ab initio Simulation Package and the Exact Muffin-Tin Orbitals methods, and the compositional disorder is treated within the frameworks of the special quasi-random structures technique and the coherent potential approximation, respectively. Special emphasis is given to the effect of local lattice distortion and trends against composition. Significant distortion can be observed in the relaxed cells, which result in an overlap of the first and second nearest neighbor (NN) shells represented in the histograms. When going from the four-component alloys TiZrVNb and TiZrNbMo to the five-component TiZrVNbMo, the changes in the elastic parameters follow the expected trends, except that of C44 which decreases upon adding equiatomic Mo to TiZrVNb despite of the large shear elastic constant of elemental Mo. Although the rule of mixtures turns out to be a useful tool to estimate the elastic properties of the present HEAs, to capture the more delicate alloying effects one needs to resort to ab initio results.}, journal={MATERIALS & DESIGN}, author={Tian, Li-Yun and Wang, Guisheng and Harris, Joshua S. and Irving, Douglas L. and Zhao, Jijun and Vitos, Levente}, year={2017}, month={Jan}, pages={243–252} }
 @article{baker_bowes_long_moballegh_harris_dickey_irving_2017, title={Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles}, volume={110}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4978861}, DOI={10.1063/1.4978861}, abstractNote={To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Long, Daniel M. and Moballegh, Ali and Harris, Joshua S. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2017}, month={Mar}, pages={122903} }
 @article{dycus_mirrielees_grimley_dhall_kirste_mita_sitar_collazo_irving_lebeau_2017, title={Structure and Chemistry of Oxide Surface Reconstructions in III-Nitrides Observed using STEM EELS}, volume={23}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927617007887}, DOI={10.1017/S1431927617007887}, abstractNote={.}, number={S1}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Dhall, Rohan and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2017}, month={Jul}, pages={1444–1445} }
 @article{xu_bowes_grimley_irving_lebeau_2017, title={Utilizing High-temperature Atomic-resolution STEM and EELS to Determine Reconstructed Surface Structure of Complex Oxide}, volume={23}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927617008649}, DOI={10.1017/S1431927617008649}, abstractNote={Surface reconstructions are conventionally characterized by scanning tunneling microscopy (STM), electron diffraction, or X-ray photoelectron spectroscopy (XPS) [1]. These methods indirectly determine the surface structure and chemistry, which requires subsequent structural refinement and confirmation by density functional theory (DFT). Complex oxide surface reconstructions not only involve atomic structural rearrangement, but also changes in surface stoichiometry or charge state. As the underlying crystal structure becomes more complicated, the surface tends to restructure into a variety of structures, affected by numerous factors such as surface polarization, temperature, and partial pressure. In the case when the surface structure changes with temperature, above indirect methods become limited [2]. A direct real-space characterization using scanning transmission electron microscopy (STEM) is thus ideal. However, it is still challenging to reveal surface atomic structure at high temperatures, particularly when additional surface chemical structure characterization is needed while maintaining the atomic-resolution.}, number={S1}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Xu, Weizong and Bowes, Preston C. and Grimley, Everett D. and Irving, Douglas L. and LeBeau, James M.}, year={2017}, month={Jul}, pages={1596–1597} }
 @article{wang_huang_bayer_moballegh_cao_klein_dickey_irving_randall_chen_et al._2016, title={Defect chemistry and resistance degradation in Fe-doped SrTiO3 single crystal}, volume={108}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2016.02.022}, DOI={10.1016/j.actamat.2016.02.022}, abstractNote={Defect chemistry and transport in Fe-doped SrTiO3 single crystal are studied to understand its resistance degradation mechanism. The temporal evolution of electric conductivity under a voltage stress was obtained computationally by solving the transport equations for ionic and electronic defects coupled with the defect reaction equilibrium equations. The computational results are compared to the corresponding experimental measurement under similar conditions. It is shown that the local electron and hole concentrations are controlled by the local electronic defect equilibria rather than by their quasi-steady state diffusional transport. It is the electric field-induced migration of oxygen vacancies and the subsequent instantaneous reestablishment of the local defect equilibria that lead to the resistance degradation. The resistance degradation behavior and the defect distributions under a long-term voltage stress are strongly influenced by the sample-annealing oxygen partial pressure, degrading electric field, and temperature. The present study contributes to the understanding of resistance degradation mechanism and provides guidance to improve the lifetime and reliability of wide band-gap semiconducting capacitors.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Wang, Jian-Jun and Huang, Hou-Bing and Bayer, Thorsten J. M. and Moballegh, Ali and Cao, Ye and Klein, Andreas and Dickey, Elizabeth C. and Irving, Douglas L. and Randall, Clive A. and Chen, Long-Qing and et al.}, year={2016}, month={Apr}, pages={229–240} }
 @article{niu_zaddach_koch_irving_2016, title={First principles exploration of near-equiatomic NiFeCrCo high entropy alloys}, volume={672}, ISSN={["1873-4669"]}, DOI={10.1016/j.jallcom.2016.02.108}, abstractNote={High entropy alloy NiFeCrCo was systematically studied in the range of near-equal atomic concentrations, i.e., 10–40 at.%, by first-principles tools and high throughput calculations. Enthalpy of mixing, lattice parameter (a0), bulk modulus (B), and shear modulus (G) were calculated by the exact muffin-tin orbital method combined with coherent potential approximation (EMTO-CPA) for over 2700 compositions of the NiFeCrCo alloy as a single-phase solid solution in paramagnetic state. It was found that certain elements have the most significant influence on each property, namely, Cr on enthalpy of mixing, Co on a0, Fe on B, Co on G, and Cr on the ratio of B/G. An equation to predict the enthalpy of mixing by use of binary enthalpy data was evaluated and was found to have a good accuracy with a root-mean-square deviation (RMSD) of 42 meV per formula unit in the prediction. A similar equation to predict bulk modulus with weighted contribution from first–shell interaction is proposed and tested on all alloys. This equation was also found to be accurate with a RMSD of 6 GPa. Finally, it was found that shear moduli of all tested alloys are largely dependent on C44, while the concentration of Co has a noticeable control on C44. Spin polarized calculations were performed for a select group of alloys with both EMTO-CPA and the Vienna ab-initio Simulation Package (VASP) with special quasi-random structure models for comparison. Good agreement was found between these methods.}, journal={JOURNAL OF ALLOYS AND COMPOUNDS}, author={Niu, C. and Zaddach, A. J. and Koch, C. C. and Irving, D. L.}, year={2016}, month={Jul}, pages={510–520} }
 @article{xu_bowes_grimley_irving_lebeau_2016, title={In-situ real-space imaging of single crystal surface reconstructions via electron microscopy}, volume={109}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.4967978}, DOI={10.1063/1.4967978}, abstractNote={Here, we report a high temperature in-situ atomic resolution scanning transmission electron microscopy (STEM) study of single crystal surface structure dynamics. With the approach, we gain direct insight into a double layer reconstruction that occurs on the polar SrTiO3 (110) surface. We find that structural details of this reconstruction can be directly attributed to charge redistribution and the thermal mismatch between the surface and the bulk material. Periodic surface defects, similar to dislocations, are found, which act to relieve stress as the temperature is lowered. Combining STEM observations, electron energy loss spectroscopy, and density functional theory, we highlight the combined role of lattice misfit and charge compensation to determine the structure and chemistry of the observed polar surface reconstruction.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Xu, Weizong and Bowes, Preston C. and Grimley, Everett D. and Irving, Douglas L. and LeBeau, James M.}, year={2016}, month={Nov} }
 @article{lebeau_sang_dycus_nui_irving_2016, title={Probing the subtleties of atomic distortions through accurate STEM imaging and density functional theory}, volume={22}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927616007844}, DOI={10.1017/S1431927616007844}, abstractNote={While aberration correction has unlocked previously inaccessible information at the atomic scale, real-space measurements of atomic displacements and distances have remained largely qualitative for scanning transmission electron microscopy (STEM). In particular, accuracy and precision in STEM is significantly hampered by the presence of sample drift and scan distortion. While sample drift is usually the dominate consideration, the scan system calibration is rarely perfectly orthonormal, which can introduce significant errors. These limitations have obscured the capabilities to directly characterize minute atomic structure changes that can ultimately control properties. In this talk, we will show how revolving scanning transmission electron microscopy (RevSTEM) is able to achieve highly accurate imaging on a routine basis [1]. The method uses a series of fast-acquisition STEM images, but with the scan coordinates rotated between successive frames. This, however, only accounts for the drift introduced distortion. Equipped with the Atom Column Indexing approach [2], we will introduce a method to very accurately and precisely correct the global scan coordinate system, to ensure a near-perfectly orthonormal scan. We will demonstrate the stability of this correction and that the combination of RevSTEM with such a corrected scan system is capable of achieving sub-picometer accuracy of real-space crystallographic measurements in STEM [3]. Multiple case studies will be presented to demonstrate the power of this new technique in combination with density functional theory. For example, we will show how picometer precise measurements enable the capability to directly observe static atomic displacements within a complex oxide and chalcogenide solid solutions [3, 4]. These subtle distortions will be examined using density functional theory to investigate the role of local chemical fluctuations and the influence of bond strengths. With the level of accuracy afforded by this imaging approach we will show how concepts used in analyzing total diffraction data, such as the pair distribution function (PDF), can be applied to STEM data. To do so, we will introduce the projected-PDF to analyze correlation between displacements spanning the entire image [4]. Furthermore, we will also show how RevSTEM images}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={LeBeau, James and Sang, Xiahan and Dycus, J. Houston and Nui, Changning and Irving, Douglas}, year={2016}, month={Jul}, pages={1400–1401} }
 @article{zaddach_niu_oni_fan_lebeau_irving_koch_2016, title={Structure and magnetic properties of a multi-principal element Ni-Fe-Cr-Co-Zn-Mn alloy}, volume={68}, ISSN={["1879-0216"]}, DOI={10.1016/j.intermet.2015.09.009}, abstractNote={A nanocrystalline alloy with a nominal composition of Ni20Fe20Cr20Co20Zn15Mn5 was produced by mechanical alloying and processed using annealing treatments between 450 and 600 °C for lengths from 0.5 to 4 h. Analysis was conducted using x-ray diffraction, transmission electron microscopy, magnetometry, and first-principles calculations. Despite designing the alloy using empirical high-entropy alloy guidelines, it was found to precipitate numerous phases after annealing. These precipitates included a magnetic phase, α-FeCo, which, after the optimal heat treatment conditions of 1 h at 500 °C, resulted in an alloy with reasonably good hard magnetic properties. The effect of annealing temperature and time on the microstructure and magnetic properties are discussed, as well as the likely mechanisms that cause the microstructure development.}, journal={INTERMETALLICS}, author={Zaddach, A. J. and Niu, C. and Oni, A. A. and Fan, M. and LeBeau, J. M. and Irving, D. L. and Koch, C. C.}, year={2016}, month={Jan}, pages={107–112} }
 @article{bayer_wang_carter_moballegh_baker_irving_dickey_chen_randall_2016, title={The relation of electrical conductivity profiles and modulus data using the example of STO:Fe single crystals: A path to improve the model of resistance degradation}, volume={117}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2016.07.024}, DOI={10.1016/j.actamat.2016.07.024}, abstractNote={Resistance degradation in perovskites is characterized by an increase in current over time with applied electric field. This behavior can be simulated and spatially resolved conductivity profiles can be measured, but some inconsistencies remain. A new approach to address these problems is presented that utilizes time-resolved impedance spectroscopy with an applied DC voltage to provide new insight into the resistance degradation phenomenon. In particular, this method allows the in-situ acquisition of spatio-temporal variations in conductivity. In SrTiO3 a single bulk-dominated maximum of the imaginary part of the modulus M″ transitions to two maxima during degradation, reflecting the hole conductivity in the anode region and the electron conductivity in the cathode region. To clarify the influence of conductivity profiles on impedance data, the reversed route is presented by using simulated conductivity profiles to calculate impedance data. It will be emphasized that this methodology is not limited to the perovskite system considered here, but can be adapted to any kind of system characterized by a spatially varying conductivity.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Bayer, Thorsten J.M. and Wang, Jian-Jun and Carter, Jared J. and Moballegh, Ali and Baker, Jonathon and Irving, Douglas L. and Dickey, Elizabeth C. and Chen, Long-Qing and Randall, Clive A.}, year={2016}, month={Sep}, pages={252–261} }
 @article{youssef_zaddach_niu_irving_koch_2015, title={A Novel Low-Density, High-Hardness, High-entropy Alloy with Close-packed Single-phase Nanocrystalline Structures}, volume={3}, ISSN={["2166-3831"]}, DOI={10.1080/21663831.2014.985855}, abstractNote={A low-density, nanocrystalline high-entropy alloy, Al20Li20Mg10Sc20Ti30 was produced by mechanical alloying. It formed a single-phase fcc structure during ball milling and transformed to single-phase hcp upon annealing. The alloy has an estimated strength-to-weight ratio that is significantly higher than other nanocrystalline alloys and is comparable to ceramics. High hardness is retained after annealing.}, number={2}, journal={MATERIALS RESEARCH LETTERS}, author={Youssef, Khaled M. and Zaddach, Alexander J. and Niu, Changning and Irving, Douglas L. and Koch, Carl C.}, year={2015}, pages={95–99} }
 @article{dycus_harris_sang_fancher_findlay_oni_chan_koch_jones_allen_et al._2015, title={Accurate Nanoscale Crystallography in Real-Space Using Scanning Transmission Electron Microscopy}, volume={21}, ISSN={["1435-8115"]}, DOI={10.1017/s1431927615013732}, abstractNote={Abstract}, number={4}, journal={MICROSCOPY AND MICROANALYSIS}, author={Dycus, J. Houston and Harris, Joshua S. and Sang, Xiahan and Fancher, Chris M. and Findlay, Scott D. and Oni, Adedapo A. and Chan, Tsung-ta E. and Koch, Carl C. and Jones, Jacob L. and Allen, Leslie J. and et al.}, year={2015}, month={Aug}, pages={946–952} }
 @article{sang_grimley_niu_irving_lebeau_2015, title={Direct observation of charge mediated lattice distortions in complex oxide solid solutions}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4908124}, abstractNote={Using aberration corrected scanning transmission electron microscopy combined with advanced imaging methods, we directly observe atom column specific, picometer-scale displacements induced by local chemistry in a complex oxide solid solution. Displacements predicted from density functional theory were found to correlate with the observed experimental trends. Further analysis of bonding and charge distribution was used to clarify the mechanisms responsible for the detected structural behavior. By extending the experimental electron microscopy measurements to previously inaccessible length scales, we identified correlated atomic displacements linked to bond differences within the complex oxide structure.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Sang, Xiahan and Grimley, Everett D. and Niu, Changning and Irving, Douglas L. and LeBeau, James M.}, year={2015}, month={Feb} }
 @article{sachet_shelton_harris_gaddy_irving_curtarolo_donovan_hopkins_sharma_sharma_et al._2015, title={Dysprosium-doped cadmium oxide as a gateway material for mid-infrared plasmonics}, volume={14}, ISSN={1476-1122 1476-4660}, url={http://dx.doi.org/10.1038/NMAT4203}, DOI={10.1038/nmat4203}, abstractNote={The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet-visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate 'defect equilibrium engineering'. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm(2) V(-1) s(-1) for carrier densities above 10(20) cm(-3). Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.}, number={4}, journal={Nature Materials}, publisher={Springer Science and Business Media LLC}, author={Sachet, Edward and Shelton, Christopher T. and Harris, Joshua S. and Gaddy, Benjamin E. and Irving, Douglas L. and Curtarolo, Stefano and Donovan, Brian F. and Hopkins, Patrick E. and Sharma, Peter A. and Sharma, Ana Lima and et al.}, year={2015}, month={Feb}, pages={414–420} }
 @article{dycus_harris_sang_fancher_findlay_oni_chan_koch_jones_allen_et al._2015, title={Highly Accurate Real Space Nanometrology Using Revolving Scanning Transmission Electron Microscopy}, volume={21}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927615012003}, DOI={10.1017/S1431927615012003}, abstractNote={, Accurately determining crystallography at the nanoscale provides key understanding of materials behavior. X-ray and neutron based diffraction methods provide highly accurate and precise measurements, but are typically limited in their application for nanoscale materials by poor spatial sensitivity. On the other hand, scanning transmission electron microscopy (STEM) is capable of spatial resolutions below an angstrom, making atomic scale analysis routine. Moreover, high-angle annular dark-field STEM produces images that are directly interpretable with intensities scaling to the atomic number and total number of atoms in a column [1-2]. While, real-space distance measurements are possible with STEM, the effects of thermal drift and scan distortion hinder accurate metrology. In this talk, we will combine revolving STEM (RevSTEM) with a method for scan distortion correction to show accurate and precise real space length measurements for a nanostructured Bi 2 Te 3-x Se x alloy. We will show the effects of thermal drift can be corrected via measuring the drift parameters from multiple frames in an image series [3]. B y using <100> silicon as a reference standard, we correct the effects from distortions introduced from the scan system, which can then be used for imaging samples of unknown crystallography. The atom columns in drift corrected image series are then indexed and assigned to a matrix representation, which yields information such as the lattice parameters on a unit cell-by-unit cell basis, shown in Figure 1a [4]. To validate the accuracy of the technique, samples of pure Bi 2 Te 3 and Bi 2 Se}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Dycus, J. H. and Harris, J. S. and Sang, X. and Fancher, C. M. and Findlay, S. D. and Oni, A. A. and Chan, T. E. and Koch, C. C. and Jones, J. L. and Allen, L. J. and et al.}, year={2015}, month={Aug}, pages={2245–2246} }
 @article{lu_mily_irving_maria_brenner_2015, title={New Method for Extracting Diffusion-Controlled Kinetics from Differential Scanning Calorimetry: Application to Energetic Nanostructures}, volume={119}, ISSN={1932-7447 1932-7455}, url={http://dx.doi.org/10.1021/ACS.JPCC.5B03317}, DOI={10.1021/ACS.JPCC.5B03317}, abstractNote={A new expression is derived for interpreting differential scanning calorimetry curves for solid-state reactions with diffusion-controlled kinetics. The new form yields an analytic expression for temperature at the maximum peak height that is similar to a Kissinger analysis, but that explicitly accounts for laminar, cylindrical, and spherical multilayer system geometries. This expression was used to analyze two reactive multilayer nanolaminate systems, a Zr/CuO thermite and an Ni/Al aluminide, that include systematically varied layer thicknesses. This new analysis scales differential scanning calorimetry (DSC) peak temperatures against sample geometry, which leads to geometry-independent inherent activation energies and prefactors. For the Zr/CuO system, the DSC data scale with the square of the bilayer thickness, while, for the Ni/Al system, the DSC data scale with the thickness. This suggests distinct reaction mechanisms between these systems.}, number={25}, journal={The Journal of Physical Chemistry C}, publisher={American Chemical Society (ACS)}, author={Lu, Shijing and Mily, Edward J. and Irving, Douglas L. and Maria, Jon-Paul and Brenner, Donald W.}, year={2015}, month={Jun}, pages={150610143303004} }
 @article{sang_grimley_niu_irving_lebeau_2015, title={Revealing Unit Cell Level Distortions in Random Oxide Solid Solutions by Scanning Transmission Electron Microscopy and the Projected Pair Distribution Function}, volume={21}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927615006984}, DOI={10.1017/S1431927615006984}, abstractNote={Picometer-scale lattice distortions critically determine a range of properties in oxide solid solutions including ferroelectricity, piezoelectricity, high temperature superconductivity}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Sang, Xiahan and Grimley, Everett D. and Niu, Changning and Irving, Douglas L. and LeBeau, James M.}, year={2015}, month={Aug}, pages={1239–1240} }
 @article{niu_zaddach_oni_sang_hurt_lebeau_koch_irving_2015, title={Spin-driven ordering of Cr in the equiatomic high entropy alloy NiFeCrCo}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4918996}, abstractNote={Spin-driven ordering of Cr in an equiatomic fcc NiFeCrCo high entropy alloy (HEA) was predicted by first-principles calculations. Ordering of Cr is driven by the reduction in energy realized by surrounding anti-ferromagnetic Cr with ferromagnetic Ni, Fe, and Co in an alloyed L12 structure. The fully Cr-ordered alloyed L12 phase was predicted to have a magnetic moment that is 36% of that for the magnetically frustrated random solid solution. Three samples were synthesized by milling or casting/annealing. The cast/annealed sample was found to have a low temperature magnetic moment that is 44% of the moment in the milled sample, which is consistent with theoretical predictions for ordering. Scanning transmission electron microscopy measurements were performed and the presence of ordered nano-domains in cast/annealed samples throughout the equiatomic NiFeCrCo HEA was identified.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Niu, C. and Zaddach, A. J. and Oni, A. A. and Sang, X. and Hurt, J. W., III and LeBeau, J. M. and Koch, C. C. and Irving, D. L.}, year={2015}, month={Apr} }
 @article{bryan_bryan_gaddy_reddy_hussey_bobea_guo_hoffmann_kirste_tweedie_et al._2014, title={Fermi level control of compensating point defects during metalorganic chemical vapor deposition growth of Si-doped AlGaN}, volume={105}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4903058}, DOI={10.1063/1.4903058}, abstractNote={A Fermi-level control scheme for point defect management using above-bandgap UV illumination during growth is presented. We propose an extension to the analogy between the Fermi level and the electrochemical potential such that the electrochemical potential of a charged defect in a material with steady-state populations of free charge carriers may be expressed in terms of the quasi-Fermi levels. A series of highly Si-doped Al0.65Ga0.35N films grown by metalorganic chemical vapor deposition with and without UV illumination showed that samples grown under UV illumination had increased free carrier concentration, free carrier mobility, and reduced midgap photoluminescence all indicating a reduction in compensating point defects.}, number={22}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Zachary and Bryan, Isaac and Gaddy, Benjamin E. and Reddy, Pramod and Hussey, Lindsay and Bobea, Milena and Guo, Wei and Hoffmann, Marc and Kirste, Ronny and Tweedie, James and et al.}, year={2014}, month={Dec} }
 @article{gaddy_paisley_maria_irving_2014, title={Overcoming the polarization catastrophe in the rocksalt oxides MgO and CaO}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.125403}, abstractNote={Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.}, number={12}, journal={PHYSICAL REVIEW B}, author={Gaddy, Benjamin E. and Paisley, Elizabeth A. and Maria, Jon-Paul and Irving, Douglas L.}, year={2014}, month={Sep} }
 @article{sang_grimley_niu_irving_lebeau_2014, title={Putting a New Spin on Scanning Transmission Electron Microscopy}, volume={20}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927614002426}, DOI={10.1017/S1431927614002426}, abstractNote={Local atomic scale ordering and structural distortions can significantly modify material properties. Aberration correction dramatically improves spatial resolution into the sub-Angstrom regime, unlocking information about material defects previously beyond reach. While STEM has proven essential to the atomic scale characterization of materials, for example at defects, interfaces, or even in perfect crystals, measurement of atomic displacements and distances has remained challenging due to the presence of sample drift. Distortion proportional to the rate of sample drift during image acquisition hinders the accurate measurement or even representation of atomic structure. Though modern STEM installations are optimized to reduce vibration, air flow/fields, and temperature fluctuations, sample drift generally remains.}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Sang, Xiahan and Grimley, Everett D. and Niu, Changning and Irving, Douglas L. and LeBeau, James M.}, year={2014}, month={Aug}, pages={140–141} }
 @article{paisley_gaddy_lebeau_shelton_biegalski_christen_losego_mita_collazo_sitar_et al._2014, title={Smooth cubic commensurate oxides on gallium nitride}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4861172}, abstractNote={Smooth, commensurate alloys of ⟨111⟩-oriented Mg0.52Ca0.48O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, Elizabeth A. and Gaddy, Benjamin E. and LeBeau, James M. and Shelton, Christopher T. and Biegalski, Michael D. and Christen, Hans M. and Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and et al.}, year={2014}, month={Feb} }
 @article{gaddy_bryan_bryan_xie_dalmau_moody_kumagai_nagashima_kubota_kinoshita_et al._2014, title={The role of the carbon-silicon complex in eliminating deep ultraviolet absorption in AlN}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4878657}, abstractNote={Co-doping AlN crystals with Si is found to suppress the unwanted 4.7 eV (265 nm) deep ultraviolet absorption associated with isolated carbon acceptors common in materials grown by physical vapor transport. Density functional theory calculations with hybrid functionals demonstrate that silicon forms a stable nearest-neighbor defect complex with carbon. This complex is predicted to absorb at 5.5 eV and emit at or above 4.3 eV. Absorption and photoluminescence measurements of co-doped samples confirm the presence of the predicted CN-SiAl complex absorption and emission peaks and significant reduction of the 4.7 eV absorption. Other sources of deep ultraviolet absorption in AlN are also discussed.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and Kinoshita, Toru and et al.}, year={2014}, month={May} }
 @article{gaddy_kingon_irving_2013, title={Effects of alloying and local order in AuNi contacts for Ohmic radio frequency micro electro mechanical systems switches via multi-scale simulation}, volume={113}, ISSN={["0021-8979"]}, DOI={10.1063/1.4804954}, abstractNote={Ohmic RF-MEMS switches hold much promise for low power wireless communication, but long-term degradation currently plagues their reliable use. Failure in these devices occurs at the contact and is complicated by the fact that the same asperities that bear the mechanical load are also important to the flow of electrical current needed for signal processing. Materials selection holds the key to overcoming the barriers that prevent widespread use. Current efforts in materials selection have been based on the material's (or alloy's) ability to resist oxidation as well as its room-temperature properties, such as hardness and electrical conductivity. No ideal solution has yet been found via this route. This may be due, in part, to the fact that the in-use changes to the local environment of the asperity are not included in the selection criteria. For example, Joule heating would be expected to raise the local temperature of the asperity and impose a non-equilibrium thermal gradient in the same region expected to respond to mechanical actuation. We propose that these conditions should be considered in the selection process, as they would be expected to alter mechanical, electrical, and chemical mechanisms in the vicinity of the surface. To this end, we simulate the actuation of an Ohmic radio frequency micro electro mechanical systems switch by using a multi-scale method to model a current-carrying asperity in contact with a polycrystalline substrate. Our method couples continuum solutions of electrical and thermal transport equations to an underlying molecular dynamics simulation. We present simulations of gold-nickel asperities and substrates in order to evaluate the influence of alloying and local order on the early stages of contact actuation. The room temperature response of these materials is compared to the response of the material when a voltage is applied. Au-Ni interactions are accounted for through modification of the existing Zhou embedded atom method potential. The modified potential more accurately captures trends in high-temperature properties, including the enthalpy of mixing and melting temperatures. We simulate the loading of a contacting asperity to several substrates with varying Ni alloying concentrations and compare solid solution strengthening to a phase-separated system. Our simulations show that Ni concentration and configuration have an important effect on contact area, constriction resistance, thermal profiles, and material transfer. These differences suggest that a substrate with 15 at. % Ni featuring phase segregation has fewer early markers that experimentally have indicated long-term failure.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Gaddy, Benjamin E. and Kingon, Angus I. and Irving, Douglas L.}, year={2013}, month={May} }
 @article{freeze_ji_kingon_irving_2013, title={Impact of Joule heating, roughness, and contaminants on the relative hardness of polycrystalline gold}, volume={25}, DOI={10.1088/0953-8984/25/47/472202}, abstractNote={Asperities play a central role in the mechanical and electrical properties of contacting surfaces. Changes in trends of uniaxial compression of an asperity tip in contact with a polycrystalline substrate as a function of substrate geometry, compressive stress and applied voltage are investigated here by implementation of a coupled continuum and atomistic approach. Surprisingly, an unmodified Au polycrystalline substrate is found to be softer than one containing a void for conditions of high stress and an applied voltage of 0.2 V. This is explained in terms of the temperature distribution and weakening of Au as a function of temperature. The findings in this communication are important to the design of materials for electrical contacts because applied conditions may play a role in reversing relative hardness of the materials for conditions experienced during operation.}, number={47}, journal={Journal of Physics. Condensed Matter}, author={Freeze, C. R. and Ji, X. Y. and Kingon, A. I. and Irving, D. L.}, year={2013} }
 @article{zaddach_niu_koch_irving_2013, title={Mechanical Properties and Stacking Fault Energies of NiFeCrCoMn High-Entropy Alloy}, volume={65}, ISSN={1047-4838 1543-1851}, url={http://dx.doi.org/10.1007/S11837-013-0771-4}, DOI={10.1007/S11837-013-0771-4}, number={12}, journal={JOM}, publisher={Springer Science and Business Media LLC}, author={Zaddach, A. J. and Niu, C. and Koch, C. C. and Irving, D. L.}, year={2013}, month={Oct}, pages={1780–1789} }
 @article{gaddy_bryan_bryan_kirste_xie_dalmau_moody_kumagai_nagashima_kubota_et al._2013, title={Vacancy compensation and related donor-acceptor pair recombination in bulk AlN}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4824731}, abstractNote={A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Kirste, Ronny and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and et al.}, year={2013}, month={Oct} }
 @article{dongare_lamattina_irving_rajendran_zikry_brenner_2012, title={An angular-dependent embedded atom method (A-EAM) interatomic potential to model thermodynamic and mechanical behavior of Al/Si composite materials}, volume={20}, ISSN={0965-0393 1361-651X}, url={http://dx.doi.org/10.1088/0965-0393/20/3/035007}, DOI={10.1088/0965-0393/20/3/035007}, abstractNote={A new interatomic potential is developed for the Al/Si system in the formulation of the recently developed angular-dependent embedded atom method (A-EAM). The A-EAM is formulated by combining the embedded atom method potential for Al with the Stillinger–Weber potential for Si. The parameters of the Al/Si cross-interactions are fitted to reproduce the structural energetics of Al/Si bulk alloys determined based on the results of density functional theory calculations and the experimentally observed mixing behavior of the AlSi liquid alloy at high temperatures. The ability to investigate the thermodynamic properties of the Al/Si system is demonstrated by computing the binary phase diagram of the Al–Si system as predicted by the A-EAM potential and comparing with that obtained using experiments. The ability to study the mechanical behavior of the Al/Si composite systems is demonstrated by investigating the micromechanisms related to dynamic failure of the Al/Si nanocomposites using MD simulations.}, number={3}, journal={Modelling and Simulation in Materials Science and Engineering}, publisher={IOP Publishing}, author={Dongare, Avinash M and LaMattina, Bruce and Irving, Douglas L and Rajendran, Arunachalam M and Zikry, Mohammed A and Brenner, Donald W}, year={2012}, month={Feb}, pages={035007} }
 @article{collazo_xie_gaddy_bryan_kirste_hoffmann_dalmau_moody_kumagai_nagashima_et al._2012, title={On the origin of the 265 nm absorption band in AlN bulk crystals}, volume={100}, ISSN={["1077-3118"]}, DOI={10.1063/1.4717623}, abstractNote={Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, Ramon and Xie, Jinqiao and Gaddy, Benjamin E. and Bryan, Zachary and Kirste, Ronny and Hoffmann, Marc and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and et al.}, year={2012}, month={May} }
 @article{paisley_losego_gaddy_tweedie_collazo_sitar_irving_maria_2011, title={Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions}, volume={2}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms1470}, abstractNote={Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type—precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance. Property coupling by heteroepitaxy is severely limited in material combinations with highly dissimilar bonding. This report presents a chemical boundary condition methodology to actively engineer two-dimensional film growth in such systems that otherwise collapse into island formation and rough morphologies.}, journal={NATURE COMMUNICATIONS}, author={Paisley, Elizabeth A. and Losego, Mark. D. and Gaddy, Benjamin E. and Tweedie, James S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L. and Maria, Jon-Paul}, year={2011}, month={Sep} }
 @article{crill_ji_irving_brenner_padgett_2010, title={Atomic and multi-scale modeling of non-equilibrium dynamics at metal–metal contacts}, volume={18}, ISSN={0965-0393 1361-651X}, url={http://dx.doi.org/10.1088/0965-0393/18/3/034001}, DOI={10.1088/0965-0393/18/3/034001}, abstractNote={A coarse graining method that introduces Joule heating and improves heat transport in a classical molecular dynamics simulation is reviewed, and two example sets of simulations, opening of gold–gold nano-asperity contacts and nano-asperity sliding at loaded copper–aluminum interfaces are discussed. For the gold contact, dislocations nucleate from the edges of where the asperity contacts the substrates and move along the close-packed planes, resulting in stacking faults that form two subsurface Thompson tetrahedra. For a null voltage, a nanowire with a diameter much smaller than the initial contact area is created when the two tetrahedra are completed, and as the wire yields the partial dislocations retreat to the surface. Opening with Joule heating enhances dislocation mobility and intransient subsurface plasticity. Constant current simulations show melting and boiling of the nanowires depending on the voltage cap. Sliding of an aluminum asperity on copper with a null voltage shows dislocation formation in the copper and aluminum, while heating from an applied voltage eliminates damage in the copper. Sliding with a copper asperity enhances plastic damage in the copper substrate compared with the aluminum asperity, while Joule heating enhances aluminum pile-up in front of the copper asperity due to plowing.}, number={3}, journal={Modelling and Simulation in Materials Science and Engineering}, publisher={IOP Publishing}, author={Crill, J W and Ji, X and Irving, D L and Brenner, D W and Padgett, C W}, year={2010}, month={Mar}, pages={034001} }
 @article{purohit_sun_irving_scattergood_brenner_2010, title={Computational study of the impurity induced reduction of grain boundary energies in nano- and bi-crystalline Al-Pb alloys}, volume={527}, ISSN={["0921-5093"]}, DOI={10.1016/j.msea.2009.11.034}, abstractNote={Abstract Segregation of impurities with limited solubilities to grain boundaries can slow or even eliminate grain growth in nanocrystalline materials. Due to a very limited miscibility Pb is a potential candidate for thermodynamically stabilizing nanocrystalline Al. To investigate this we have used atomic modeling to characterize the structure and energy of substitutional Pb defects in bulk Al, in Al bi-crystals and in an Al nanocrystal. Monte Carlo simulations using a modified embedded-atom method (MEAM) potential fit to the results of density functional theory (DFT) calculations predict the formation of Pb clusters, in agreement with prior experiments. In addition, the simulations show strong segregation of Pb atoms to grain boundaries, a result that supports prior suggestions that Pb is distributed along grain boundaries in nanocrystals created by ball milling. Analysis of the enthalpies for Pb defects using MEAM and DFT calculations suggests that Pb impurities can help stabilize nanocrystalline Al against grain growth.}, number={7-8}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Purohit, Y. and Sun, L. and Irving, D. L. and Scattergood, R. O. and Brenner, D. W.}, year={2010}, month={Mar}, pages={1769–1775} }
 @inbook{sinnott_heo_brenner_harrison_irving_2010, title={Computer Simulation of Nanometer-Scale Indentation and Friction'}, ISBN={9783642025242}, booktitle={Handbook of Nanotechnology, Third Edition}, publisher={New York:  Springer}, author={Sinnott, S. B. and Heo, S. J. and Brenner, D. W. and Harrison, J. A. and Irving, D. L.}, year={2010}, pages={955–1012} }
 @inbook{irving_2010, title={Understanding Metal/Metal Electrical Contact Conductance from the Atomic to Continuum Scales}, ISBN={9780470587140}, DOI={10.1002/9780470890905.ch4}, abstractNote={This chapter contains sections titled: Introduction Factors That Influence Contact Resistance Computational Considerations Selected Case Studies Concluding Remarks Acknowledgments References}, publisher={Oxford:  Wiley-Blackwell}, author={Irving, Douglas L.}, year={2010} }
 @article{irving_padgett_brenner_2009, title={Coupled molecular dynamics/continuum simulations of Joule heating and melting of isolated copper-aluminum asperity contacts}, volume={17}, number={1}, journal={Modelling and Simulation in Materials Science and Engineering}, author={Irving, D. L. and Padgett, C. W. and Brenner, D. W.}, year={2009} }
 @article{sun_irving_zikry_brenner_2009, title={First-principles investigation of the structure and synergistic chemical bonding of Ag and Mg at the Al vertical bar Omega interface in a Al-Cu-Mg-Ag alloy}, volume={57}, ISSN={["1873-2453"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65849380009&partnerID=MN8TOARS}, DOI={10.1016/j.actamat.2009.04.006}, abstractNote={Density functional theory was used to characterize the atomic structure and bonding of the Al | Ω interface in a Al–Cu–Mg–Ag alloy. The most stable interfacial structure was found to be connected by Al–Al bonds with a hexagonal Al lattice on the surface of the Ω phase sitting on the vacant hollow sites of the Al {1 1 1} matrix plane. The calculations predict that when substituted separately for Al at this interface, Ag and Mg do not enhance the interface stability through chemical bonding. Combining Ag and Mg, however, was found to chemically stabilize this interface, with the lowest-energy structure examined being a bi-layer with Ag atoms adjacent to the Al matrix and Mg adjacent to the Ω phase. This study provides an atomic arrangement for the interfacial bi-layer observed experimentally in this alloy.}, number={12}, journal={ACTA MATERIALIA}, author={Sun, Lipeng and Irving, Douglas L. and Zikry, Mohammed A. and Brenner, D. W.}, year={2009}, month={Jul}, pages={3522–3528} }
 @article{elkhodary_sun_irving_brenner_ravichandran_zikry_2009, title={Integrated experimental, atomistic, and microstructurally based finite element investigation of the dynamic compressive behavior of 2139 aluminum}, volume={76}, number={5}, journal={Journal of Applied Mechanics: Transactions of the ASME}, author={Elkhodary, K. and Sun, L. P. and Irving, D. L. and Brenner, D. W. and Ravichandran, G. and Zikry, M. A.}, year={2009} }
 @article{irving_padgett_guo_mintmire_brenner_2009, title={Multiscale Modeling of Metal-Metal Contact Dynamics Under High Electromagnetic Stress: Timescales and Mechanisms for Joule Melting of Al-Cu Asperities}, volume={45}, ISSN={["0018-9464"]}, DOI={10.1109/tmag.2008.2008544}, abstractNote={An analysis and initial results from a multiscale continuum-atomistic simulation of the Joule heating and melting of Cu-Al asperity contacts is presented. An analytic expression is given for the time needed to reach the Al melting point for an asperity as a function of the voltage drop and the asperity contact area. The coupled continuum-atomistic simulations capture the initial stages of the formation of Al-Cu alloys that arises from the solvation of Cu atoms into the Al melt. Implications of these results for understanding contacts in electromagnetic launchers are discussed.}, number={1}, journal={IEEE TRANSACTIONS ON MAGNETICS}, author={Irving, Douglas L. and Padgett, Clifford W. and Guo, Yin and Mintmire, John W. and Brenner, Donald W.}, year={2009}, month={Jan}, pages={331–335} }
 @article{purohit_jang_irving_padgett_scattergood_brenner_2008, title={Atomistic modeling of the segregation of lead impurities to a grain boundary in an aluminum bicrystalline solid}, volume={493}, ISSN={["1873-4936"]}, DOI={10.1016/j.msea.2007.05.128}, abstractNote={Using Monte Carlo simulations, new insights into the atomic segregation of lead (Pb) impurities to a Σ5 〈1 0 0〉 {2 1 0} tilt aluminum (Al) grain boundary have been obtained. Interatomic interactions in the Al–Pb alloy system were described using a modified embedded atom method potential with parameters that fit to the results of density functional calculations. The simulations predict segregation of Pb impurities along the Al grain boundary prior to the formation of Pb clusters. Analyses of grain boundary energies for varying concentrations of Pb suggests that grain boundaries in Al can be thermodynamically stabilized by Pb impurities with respect to a dilute solid solution of Pb in Al.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Purohit, Y. and Jang, S. and Irving, D. L. and Padgett, C. W. and Scattergood, R. O. and Brenner, D. W.}, year={2008}, month={Oct}, pages={97–100} }
 @article{irving_padgett_brenner_2008, title={Coupled molecular dynamics/continuum simulations of Joule heating and melting of isolated copper–aluminum asperity contacts}, volume={17}, ISSN={0965-0393 1361-651X}, url={http://dx.doi.org/10.1088/0965-0393/17/1/015004}, DOI={10.1088/0965-0393/17/1/015004}, abstractNote={Atomic-level dynamics of Joule heating, melting and plastic dynamics at loaded nanometer-scale Cu and Al asperity contacts are modeled using an ad hoc coupling between a numerical solution to a heat transport equation, a virtual resistor network for describing electric current flow and a molecular dynamics simulation using the embedded atom method. Under constant voltage conditions the simulations demonstrate the formation of an Al melt that removes faceting from a Cu asperity via surface disordering at the melt–solid interface. Constant current simulations demonstrate initial disordering of both copper and aluminum at the interface. Flow from the aluminum melt increases the contact area, which lowers the resistance and drops the voltage to below that needed for melting. For the system with a loaded copper asperity, the interface recrystallizes and the dynamics transition from molten flow to plastic damage via dislocation emission. For an aluminum asperity, the asperity remains disordered after the voltage drop and no dislocation emission occurs into the copper or aluminum substrate.}, number={1}, journal={Modelling and Simulation in Materials Science and Engineering}, publisher={IOP Publishing}, author={Irving, D L and Padgett, C W and Brenner, D W}, year={2008}, month={Nov}, pages={015004} }
 @article{jang_purohit_irving_padgett_brenner_scattergood_2008, title={Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations}, volume={56}, ISSN={["1359-6454"]}, DOI={10.1016/j.actamat.2008.05.024}, abstractNote={Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.}, number={17}, journal={ACTA MATERIALIA}, author={Jang, S. and Purohit, Y. and Irving, D. L. and Padgett, C. and Brenner, D. and Scattergood, R. O.}, year={2008}, month={Oct}, pages={4750–4761} }
 @article{irving_brenner_2008, title={MEMS Lubrication: An atomistic perspective of a bound+ mobile lubricant'}, volume={1052}, journal={Materials Research Society Symposium Proceedings}, author={Irving, D. L. and Brenner, D. W.}, year={2008}, pages={29–34} }
 @article{jang_purohit_irving_padgett_brenner_scattergood_2008, title={Molecular dynamics simulations of deformation in nanocrystalline Al-Pb alloys}, volume={493}, ISSN={["0921-5093"]}, DOI={10.1016/j.msea.2007.05.130}, abstractNote={Abstract A modified embedded-atom method (MEAM) potential was developed and used for molecular dynamics (MD) straining simulations of Al–Pb alloys with a grain size of 10 nm and Pb content up to 3 at.%. Monte Carlo (MC) simulations done at 300 K indicated that all the Pb is segregated to the grain boundaries in these alloys. As the Pb content increases, partial dislocation nucleation at grain boundaries is suppressed, and the plastic strain is accommodated by mechanisms other than dislocation slip. The increasing Pb content was accompanied by a reduction in the yield and peak stress values.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Jang, S. and Purohit, Y. and Irving, D. and Padgett, C. and Brenner, D. and Scattergood, R. O.}, year={2008}, month={Oct}, pages={53–57} }
 @article{purohit_irving_scattergood_brenner_2008, title={Prediction of Energies of <100> Tilt Boundaries in Al-Pb Alloy}, volume={1056E}, journal={Materials Research Society Symposium Proceedings}, author={Purohit, Y. and Irving, D. L. and Scattergood, R. O. and Brenner, D. W.}, year={2008}, pages={1056–HH01-105610} }
 @article{rezvanian_brown_zikry_kingon_krim_irving_brenner_2008, title={The role of creep in the time-dependent resistance of Ohmic gold contacts in radio frequency microelectromechanical system devices}, volume={104}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2953072}, DOI={10.1063/1.2953072}, abstractNote={It is shown that measured and calculated time-dependent electrical resistances of closed gold Ohmic switches in radio frequency microelectromechanical system (rf-MEMS) devices are well described by a power law that can be derived from a single asperity creep model. The analysis reveals that the exponent and prefactor in the power law arise, respectively, from the coefficient relating creep rate to applied stress and the initial surface roughness. The analysis also shows that resistance plateaus are not, in fact, limiting resistances but rather result from the small coefficient in the power law. The model predicts that it will take a longer time for the contact resistance to attain a power law relation with each successive closing of the switch due to asperity blunting. Analysis of the first few seconds of the measured resistance for three successive openings and closings of one of the MEMS devices supports this prediction. This work thus provides guidance toward the rational design of Ohmic contacts with ...}, number={2}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Rezvanian, O. and Brown, C. and Zikry, M. A. and Kingon, A. I. and Krim, J. and Irving, D. L. and Brenner, D. W.}, year={2008}, month={Jul}, pages={024513} }
 @article{irving_devine_sinnott_2007, title={Calculated Optical Absorption and Emissions from Poly p-Phenylene Oligomers}, volume={126}, journal={Journal of Luminescence}, author={Irving, D. L. and Devine, B. D. and Sinnott, S. B.}, year={2007}, pages={278} }
 @article{irving_devine_sinnott_2007, title={Computational study of steric effects on the optical properties of oligomers}, volume={126}, ISSN={0022-2313}, url={http://dx.doi.org/10.1016/j.jlumin.2006.07.006}, DOI={10.1016/j.jlumin.2006.07.006}, abstractNote={The optical properties of a variety of copolymer structures and their derivatives are determined from semi-empirical quantum chemical calculations. Possible candidates are found for organic light emitting diode and thin photovoltaic film applications. The largest blue shifts in the absorption spectrum are seen for derivatives that cause the oligomers under consideration to deviate from their unsubstituted planar configurations. This is primarily predicted to occur in systems with large steric interactions. Optical emissions are also predicted based on time dependent density functional calculations of excited states of these materials optimized with configuration interaction Hartree–Fock methods.}, number={2}, journal={Journal of Luminescence}, publisher={Elsevier BV}, author={Irving, Douglas L. and Devine, Bryce D. and Sinnott, Susan B.}, year={2007}, month={Oct}, pages={278–288} }
 @article{brenner_irving_kingon_krim_padgett_2007, title={Multiscale analysis of liquid lubrication trends from industrial machines to micro-electrical-mechanical systems}, volume={23}, ISSN={["0743-7463"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548529657&partnerID=MN8TOARS}, DOI={10.1021/la701280k}, abstractNote={An analytic multiscale expression is derived that yields conditions for effective liquid lubrication of oscillating contacts via surface flow over multiple time and length scales. The expression is a logistics function that depends on two quantities, the fraction of lubricant removed at each contact and a scaling parameter given by the logarithm of the ratio of the contact area to the product of the lubricant diffusion coefficient and the cycle time. For industrial machines the expression confirms the need for an oil mist. For magnetic disk drives, the expression predicts that existing lubricants are sufficient for next-generation data storage. For micro-electrical-mechanical systems, the expression predicts that a bound + mobile lubricant composed of tricresyl phosphate on an octadecyltrichlorosilane self-assembled monolayer will be effective only for temperatures greater than approximately 200 K and up to approximately MHz oscillation frequencies.}, number={18}, journal={LANGMUIR}, author={Brenner, D.W. and Irving, D.L. and Kingon, A.I. and Krim, Jacqueline and Padgett, C.W.}, year={2007}, month={Aug}, pages={9253–9257} }
 @article{irving_brenner_2006, title={Diffusion on a self-assembled monolayer: Molecular modeling of a bound plus mobile lubricant}, volume={110}, ISSN={["1520-5207"]}, DOI={10.1021/jp0609840}, abstractNote={The diffusion of tricresyl phosphate molecules on an octadecyltrichlorosilane self-assembled monolayer (SAM) was characterized using molecular dynamics simulations. The simulations predict that when placed on the top of a close-packed SAM, the molecules remain mobile on the surface with an isotropic diffusion activation energy of approximately 9 kJ/mol. In contrast, an anisotropic barrier that results from chain tilt within the SAM is predicted for diffusion into a defect created by reducing the alkane chain length within a cylinderical region of the surface. Once in the defect, the molecules become trapped by embedding part of the molecule into the side of the SAM.}, number={31}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Irving, Douglas L. and Brenner, Donald W.}, year={2006}, month={Aug}, pages={15426–15431} }
 @article{hu_irving_sinnott_2006, title={Ion deposition induced modification of carbon nanopeapods: A computational study}, volume={422}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/j.cplett.2006.02.049}, DOI={10.1016/j.cplett.2006.02.049}, abstractNote={Classical, many-body molecular dynamics simulations are used to study polyatomic ion beam deposition on carbon nanopeapods (C60-filled, single-walled carbon nanotubes). The results confirm the effectiveness of ion bombardment in inducing chemical modifications of nanopeapods. The simulations show how the coalescence of C60 molecules starts with the damage of fullerenes and intermolecular bridging. In addition, the cross-linking of C60 molecules to the tube wall is also predicted. This cross-linking results in changes in the electronic band structure as compared to the unmodified nanopeapod, which is investigated using first principles density functional theory.}, number={1-3}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Hu, Yanhong and Irving, Douglas L. and Sinnott, Susan B.}, year={2006}, month={Apr}, pages={137–141} }
 @article{irving_sinnott_wood_2006, title={Relative stabilities of Ag multilayers on GaAs and GaSb determined from ab initio calculations}, volume={74}, DOI={10.1103/physrevb.74.195403}, abstractNote={Experimental studies have shown that flat metallic overlayers can be formed on semiconducting substrates by use of a two-step deposition process in systems traditionally known to be nonwetting. Specifically, atomically smooth monolayers are formed on semiconducting substrates when the equivalence of a critical thickness of metal is deposited at low temperatures and subsequently annealed, which is in contrast to the three-dimensional islands that form in room temperature growth. Here, ab initio density functional theory calculations are used to study the energy associated with adding new layers of Ag to the $\mathrm{Ag}∕\mathrm{Ga}\mathrm{As}$ and the $\mathrm{Ag}∕\mathrm{Ga}\mathrm{Sb}$ systems. The results predict a shift in the critical thickness for $\mathrm{Ag}∕\mathrm{Ga}\mathrm{As}$ compared to that of $\mathrm{Ag}∕\mathrm{Ga}\mathrm{Sb}$, which is in agreement with experimental findings. The role of charge spilling and quantization in the fluctuations of the adhesion energy is also explored for these systems. It is found that charge spilling saturates for a coverage of three monolyers and greater. Beyond this point, the main contribution to adhesion energy fluctuation is attributed to structural changes due to the strain induced by the presence of the interface.}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Irving, D. L. and Sinnott, S. B. and Wood, R. F.}, year={2006}, pages={195403} }
 @article{abdelmaksoud_lee_padgett_irving_brenner_krim_2006, title={STM, QCM, and the windshield wiper effect: A joint theoretical-experimental study of adsorbate mobility and lubrication at high sliding rates}, volume={22}, ISSN={["0743-7463"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247572834&partnerID=MN8TOARS}, DOI={10.1021/la061797w}, abstractNote={We have observed that when mobile adsorbed films of benzene, tricresyl phosphate, and tertiary-butyl phenyl phosphate are present on the surface electrode of a quartz crystal microbalance (QCM), oscillation of the QCM produces clearer scanning tunneling microscope (STM) images of the electrode surface. This is in contrast to an immobile overlayer of iodobenzene, where oscillation of the QCM does not affect image quality. This observation is attributed to a "windshield wiper effect", where at MHz frequencies the tip motion maintains a region of the surface where the absorbate concentration is reduced, which leads to a clearer image. A straightforward model is presented that supports this conclusion and that provides guidelines for effective lubrication of contacts operating at MHz frequencies.}, number={23}, journal={LANGMUIR}, author={Abdelmaksoud, M. and Lee, S. M. and Padgett, C. W. and Irving, D. L. and Brenner, D. W. and Krim, J.}, year={2006}, month={Nov}, pages={9606–9609} }
 @article{irving_sinnott_lindner_2004, title={Interaction of functionalized benzene molecules with carbon nanopores}, volume={389}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/j.cplett.2004.03.073}, DOI={10.1016/j.cplett.2004.03.073}, abstractNote={Benzene and its substituted analogs are considered hazardous because of their high risk to public health and the environment. Porous carbon materials are widely studied for their potential ability to act as a filter of carcinogenic materials. This study uses first-principles calculations to explore the selective adsorption of benzene and benzenes with substituents varying in electronegativity and size in a graphitic slit and (9,9) carbon nanotubes. The results show that the molecular adhesion energies are dependent on the bulkiness of substituent but are much less dependent on their electronegatives.}, number={1-3}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Irving, Douglas L. and Sinnott, Susan B. and Lindner, Angela S.}, year={2004}, month={May}, pages={96–100} }
 @inproceedings{alden_bryan_gaddy_bryan_callsen_koukitu_kumagai_hoffmann_irving_sitar_et al., title={On the origin of the 4.7 eV absorption and 2.8 eV emission bands in bulk AlN substrates}, volume={72}, number={5}, booktitle={Wide bandgap semiconductor materials and devices 17}, author={Alden, D. and Bryan, Z. and Gaddy, B. E. and Bryan, I. and Callsen, G. and Koukitu, A. and Kumagai, Y. and Hoffmann, A. and Irving, D. L. and Sitar, Z. and et al.}, pages={31–40} }