Publications

Publication Icon Publication (2021)

Using C-DFT to develop an e-ReaxFF force field for acetophenone radical anion

Katheryn Penrod, Maximiliano Burgess, Dooman Akbarian, Ismaila Dabo, William Woodward, Adri van Duin
Journal of Chemical Physics 155, 214104 (2021)

Abstract:
Increased electricity usage over the past several decades has accelerated the need for efficient high-voltage power transmission with reliable insulating materials. Cross-linked polyethylene (XLPE) prepared via dicumyl peroxide (DCP) cross-linking has emerged as the insulator of choice for modern power cables. Although DCP cross-linking generates the desired XLPE product in high yield, other by-products are also produced. One such by-product, acetophenone, is particularly intriguing due to its aromaticity and positive electron affinity. In this work, constrained density functional theory (C-DFT) was utilized to develop an e-ReaxFF force field suitable for describing the acetophenone radical anion. Initial parameters were taken from the 2021 Akbarian e-ReaxFF force field, which was developed to describe XLPE chemistry. Then, C-DFT geometry optimizations were performed wherein an excess electron was constrained to each atom of acetophenone. The resulting C-DFT energy values for the various electronic positions were added to the e-ReaxFF training set. Next, an analogous set of structures was energy-minimized using e-ReaxFF, and equilibrium mixture compositions for the two methods were compared at multiple temperatures. Iterative fitting against C-DFT energy data was performed until satisfactory agreement was achieved. To test force field performance, molecular dynamics simulations were performed in e-ReaxFF and the resulting electronic distributions were qualitatively compared to unconstrained-DFT spin density data. By expanding our e-ReaxFF force field for XLPE, namely, adding the capability to describe acetophenone and its interactions with an excess electron, we move one step closer to a comprehensive molecular understanding of XLPE chemistry in a high-voltage power cable
Publication Icon Publication (2021)

Quantifying multi-point ordering in alloys

James M. Goff, Bryant Y. Li, Susan B. Sinnott, Ismaila Dabo
Physical Review B 104, 054109 (2021)

Abstract:
A central problem in multicomponent lattice systems is to systematically quantify multipoint ordering. Ordering in such systems is often described in terms of pairs, even though this is not sufficient when three-point and higher-order interactions are included in the Hamiltonian. Current models and parameters for multipoint ordering are often only applicable for very specific cases or require approximating a subset of correlated occupational variables on a lattice as being uncorrelated. In this paper, cluster order parameters are introduced to systematically quantify arbitrary multipoint ordering motifs in substitutional systems through direct calculations of normalized cluster probabilities. These parameters can describe multipoint chemical ordering in crystal systems with multiple sublattices, multiple components, and systems with reduced symmetry. These are defined in this paper and applied to quantify four-point chemical ordering motifs in platinum/palladium alloy nanoparticles that are of practical interest to the synthesis of catalytic nanocages. Impacts of chemical ordering on nanocage stability are discussed. It is demonstrated that approximating four-point probabilities from superpositions of lower-order pair probabilities is not sufficient in cases where three- and four-body terms are included in the energy expression. Conclusions about the formation mechanisms of nanocages may change significantly when using common pair approximations.
Publication Icon Publication (2021)

Environmental impact of amino acids on stability of selenate-bearing hydrocalumite: Experimental and DFT studies

Mengmeng Wang, Hirofumi Akamatsu, Ismaila Dabo, Keiko Sasaki
Environmental Pollution 288, 117687 (2021)

Abstract:
Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca2Al-LDH) is expected to play an important role in the retention of SeO42−. Natural organic matters (NOMs) are known to be a risk to affect the transportation and mobility of undesirable chemical species in the pedosphere which receives the low level radioactive wastes (LLW). In the present work, five amino acids were selected as the simplified models of NOMs in the pedosphere to explore their effects on the stability of Ca2Al-LDH after immobilized SeO42− under alkaline conditions. As the loading amount of amino acids on Ca2Al-LDH increasing, release of SeO42− was enhanced in HGly, H2Asp, and H2Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO32− with SeO42− in a unit cell of LDH model. The intercalation of Asp2− and CO32− caused 003 peaks in XRD sharper and d003 decreased from 8.15 Å to 7.70 Å which is assigned to Ca2Al-LDH(Asp, CO3). In H2Cys series, the 003 peaks were kept broad and SeO42− was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO32− in the presence of H2Cys. Amino acids in the interlayer of Ca2Al-LDH have several possible configurations, where the most stable one is prone to be in a horizontal direction through hydrogen bonds and Ca–O chemical bonds. This provides an insight on the stability of selenate immobilized in hydrocalumite, which can be produced in cement disposing in the pedosphere for a long term of burying. Not only carbonate but also small molecular organic matters like amino acids possibly give environmental impact on the mobility of low level anionic radionuclides in LDH.
Publication Icon Publication (2021)

Single-step direct laser writing of multimetal oxygen evolution catalysts from liquid precursors

Shannon McGee, Yu Lei, James Goff, Collin Wilkinson, Nabila Nabi Nova, Cody Kindle, Fu Zhang, Kazunori Fujisawa, Edgar Dimitrov, Susan Sinnott, Ismaila Dabo, Mauricio Terrones, Lauren Zarzar
American Chemical Society (ACS) Nano 15, 9796-9807 (2021)

Abstract:
We investigate a laser direct-write method to synthesize and deposit metastable, mixed transition metal oxides and evaluate their performance as oxygen evolution reaction catalysts. This laser processing method enabled the rapid synthesis of diverse heterogeneous alloy and oxide catalysts directly from cost-effective solution precursors, including catalysts with a high density of nanocrystalline metal alloy inclusions within an amorphous oxide matrix. The nanoscale heterogeneous structures of the synthesized catalysts were consistent with reactive force-field Monte Carlo calculations. By evaluating the impact of varying transition metal oxide composition ratios, we created a stable Fe0.63Co0.19Ni0.18Ox/C catalyst with a Tafel slope of 38.23 mV dec–1 and overpotential of 247 mV, a performance similar to that of IrO2. Synthesized Fe0.63Co0.19Ni0.18Ox/C and Fe0.14Co0.46Ni0.40Ox/C catalysts were experimentally compared in terms of catalytic performance and structural characteristics to determine that higher iron content and a less crystalline structure in the secondary matrix decrease the charge transfer resistance and thus is beneficial for electrocatalytic activity. This conclusion is supported by density-functional theory calculations showing distorted active sites in ternary metal catalysts are key for lowering overpotentials for the oxygen evolution reaction.
Publication Icon Publication (2021)

Single-step synthesis of oxygen-doped hollow porous graphitic carbon nitride for photocatalytic ciprofloxacin decomposition

Chitiphon Chuaicham, Karthikeyan Sekar, Yihuang Xiong, Vellaichamy Balakumar, Yanisa Mittraphab, Kuniyoshi Shimizu, Bunsho Ohtani, Ismaila Dabo, Keiko Sasaki
Chemical Engineering Journal 425, 130502 (2021)

Abstract:
Pollutants degradation via visible-light driven photocatalysts have attracted interest as a potentially efficient and sustainable approach for wastewater treatment. In the present study, a series of oxygen-doped hollow porous surface graphitic carbon nitride (OCN) has been prepared by one-pot thermal polycondensation of melamine with different amounts of polyoxyethylene stearyl ether as the oxygen source and template. The prepared OCN samples were utilized for the photocatalytic ciprofloxacin (CIP) degradation, which is a pharmaceutical waste, under visible light irradiation. The highest degradation performance for CIP was obtained from the OCN sample with 1 mg polyoxyethylene stearyl, which was three times greater than that of pristine C3N4. The superior degradation performance of the OCN samples were observed due to the improved light absorption, less recombination rate of photogenerated electron and hole, and enhanced electron transportation, which was proven through the PL, photocurrent density, and EIS results. Thus, the proposed one-pot synthesis of OCN provides an effective method in producing potential photocatalysts for the removal of organic pollutants, such as discarded pharmaceuticals, in wastewater.
Publication Icon Publication (2021)

Ferroelectricity in boron-substituted aluminum nitride thin films

John Hayden, Mohammad Delower Hossain, Yihuang Xiong, Kevin Ferri, Wanlin Zhu, Mario Vincenzo Imperatore, Noel Giebink, Susan Trolier-McKinstry, Ismaila Dabo, Jon-Paul Maria
Physical Review Materials 5, 044412 (2021)

Abstract:
This manuscript demonstrates ferroelectricity in B-substituted AlN thin films and a complementary set of first-principles calculations to understand their structure-property relationships. Al(1–x)BxN films are grown by dual-cathode reactive magnetron sputtering on (110)W/(001) Al2O3 substrates at 300°C at compositions spanning x = 0 to x = 0.20. X-ray diffraction studies indicate a decrease in both the c and a lattice parameters with increasing B concentration, resulting in a decrease in unit cell volume and a constant c/a axial ratio of 1.60 over this composition range. Films with 0.02 ≤ x ≤ 0.15 display ferroelectric switching with remanent polarizations exceeding 125 μC/cm2 while maintaining band gap energies of > 5.2 eV. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of x > 0.15, c-axis orientation deteriorates and ferroelectric behavior is degraded. Density-functional theory calculations corroborate the structural observations and provide predictions for the wurtzite u parameter, polarization reversal magnitudes, and composition-dependent coercive fields.
Publication Icon Publication (2021)

Optimizing accuracy and efficacy on data-driven materials discovery for the solar production of hydrogen

Yihuang Xiong, Quinn T. Campbell, Julian Fanghanel, Catherine K. Badding, Huaiyu Wang, Nicole E. Kirchner-Hall, Monica J. Theibault, Iurii Timrov, Jared S. Mondschein, Kriti Seth, Rebecca Katz, Andres Molina Villarino, Betül Pamuk, Megan E. Penrod, Mohammed M. Khan, Tiffany Rivera, Nathan C. Smith, Xavier Quintana, Paul Orbe, Craig J. Fennie, Senorpe Asem-Hiablie, James L. Young, Todd G. Deutsch, Matteo Cococcioni, Venkatraman Gopalan, Héctor D. Abruña, Raymond E. Schaak, Ismaila Dabo
Energy & Environmental Science 14, 2335-2348 (2021)

Abstract:
The production of hydrogen fuels, via water splitting, is of practical relevance for meeting global energy needs and mitigating the environmental consequences of fossil-fuel-based transportation. Water photoelectrolysis has been proposed as a viable approach for generating hydrogen, provided that stable and inexpensive photocatalysts with conversion efficiencies over 10% can be discovered, synthesized at scale, and successfully deployed (Pinaud et al., Energy Environ. Sci., 2013, 6, 1983). While a number of first-principles studies have focused on the data-driven discovery of photocatalysts, in the absence of systematic experimental validation, the success rate of these predictions may be limited. We address this problem by developing a screening procedure with co-validation between experiment and theory to expedite the synthesis, characterization, and testing of the computationally predicted, most desirable materials. Starting with 70 150 compounds in the Materials Project database, the proposed protocol yielded 71 candidate photocatalysts, 11 of which were synthesized as single-phase materials. Experiments confirmed hydrogen generation and favorable band alignment for 6 of the 11 compounds, with the most promising ones belonging to the families of alkali and alkaline-earth indates and orthoplumbates. This study shows the accuracy of a nonempirical, Hubbard-corrected density-functional theory method to predict band gaps and band offsets at a fraction of the computational cost of hybrid functionals, and outlines an effective strategy to identify photocatalysts for solar hydrogen generation.
Publication Icon Publication (2021)

Extensive benchmarking of DFT+U calculations for predicting band gaps

Nicole E. Kirchner-Hall, Wayne Zhao, Yihuang Xiong, Iurii Timrov, Ismaila Dabo
Applied Sciences 11, 2395 (2021)

Abstract:
Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost.
Publication Icon Publication (2021)

Predicting the pseudocapacitance windows for MXene electrodes with voltage-dependent cluster expansion models

James Goff, Francisco Marques dos Santos Vieira, Nathan Keilbart, Yasuaki Okada, Ismaila Dabo
American Chemical Society (ACS) Applied Energy Materials 4, 3151-3159 (2021)

Abstract:
MXene transition-metal carbides and nitrides are of growing interest for energy storage applications. These compounds are especially promising for use as pseudocapacitive electrodes due to their ability to convert energy electrochemically at fast rates. Using voltage-dependent cluster expansion models, we predict the charge storage performance of MXene pseudocapacitors for a range of electrode compositions. M3C2O2 electrodes based on group-VI transition metals have up to 80% larger areal energy densities than prototypical titanium-based (e.g., Ti3C2O2) MXene electrodes. We attribute this high pseudocapacitance to the Faradaic voltage windows of group-VI MXene electrodes, which are predicted to be 1.2 to 1.8 times larger than those of titanium-based MXenes. The size of the pseudocapacitive voltage window increases with the range of oxidation states that are accessible to the MXene transition metals. By similar mechanisms, the presence of multiple ions in the solvent (Li+ and H+) leads to sharp changes in the transition-metal oxidation states and can significantly increase the charge capacity of MXene pseudocapacitors.
Publication Icon Publication (2021)

Data-driven analysis of the electronic-structure factors controlling the work functions of perovskite oxides

Yihuang Xiong, Weinan Chen, Wenbo Guo, Hua Wei, Ismaila Dabo
Physical Chemistry and Chemical Physics 23, 6880-6887 (2021)

Abstract:
Tuning the work functions of materials is of practical interest for maximizing the performance of microelectronic and (photo)electrochemical devices, as the efficiency of these systems depends on the ability to control electronic levels at surfaces and across interfaces. Perovskites are promising compounds to achieve such control. In this work, we examine the work functions of more than 1000 perovskite oxide surfaces (ABO3) using data-driven (machine-learning) analysis and identify the factors that determine their magnitude. While the work functions of the BO2-terminated surfaces are sensitive to the energy of the hybridized oxygen p bands, the work functions of the AO-terminated surfaces exhibit a much less trivial dependence with respect to the filling of the d bands of the B-site atom and of its electronic affinity. This study shows the utility of interpretable data-driven models in analyzing the work functions of cubic perovskites from a limited number of electronic-structure descriptors.
Publication Icon Publication (2021)

Environmental impact of amino acids on the release of selenate immobilized in hydrotalcite: Integrated interpretation with density-functional theory study

Mengmeng Wang, Hirofumi Akamatsu, Ismaila Dabo, Keiko Sasaki
Chemosphere 274, 129927 (2021)

Abstract:
The environmental impact of amino acids on the release of SeO42− immobilized into hydrotalcite (Mg2Al-LDH) which belongs to the layered double hydroxides (LDHs) family was investigated by experimental study and the observed layer structure of hydrotalcite was verified through density-functional theory (DFT) calculations. Glycine, l-cysteine, and l-aspartic acid, which have smaller molecular sizes, can release SeO42− largely due to intercalation, unstabilization of Mg2Al-LDH and simple dissolution, while l-tryptophan and l-phenylalanine caused limited SeO42− release due to their larger sizes and aromaticity. XRD patterns for the solid residues after intercalation of amino acids revealed that the layer distance of Mg2Al-LDH was partially expanded. The main peaks and shoulder features corresponding to d003 diffraction were well explained by DFT simulations using glycine as a model: the layer spacing of the main peak is responsible for the remaining SeO42− and singly stacked glycine molecule and the layer spacing of the shoulder peak was well explained by doubly stacked glycine molecules. Hydrogen bonds between amino acids and hydroxyl ions in the metallic layers of Mg2Al-LDH were responsible for the stable configuration of the intercalated Mg2Al-LDH. This study indicates potential limitations to the stability of low-level radioactive wastes of 79Se in repositories which are affected by smaller molecules of amino acids released through degradation of organic matters in the pedosphere.
Publication Icon Publication (2021)

A promising Zn-Ti layered double hydroxide/Fe-bearing montmorillonite composite as an efficient photocatalyst for Cr(VI) reduction: Insight into the role of Fe impurity in montmorillonite

Chitiphon Chuaicham, Yihuang Xiong, Karthikeyan Sekar, Weinan Chen, Li Zhang, Bunsho Ohtani, Ismaila Dabo, Keiko Sasaki
Applied Surface Science 546, 148835 (2021)

Abstract:
Zn-Ti layered double hydroxide/montmorillonite (ZTL/MT) and Fe-doped ZTL/MT (ZTL/Fe@MT20%) were prepared as photocatalyst for Cr(VI) reduction. The ZTL/Fe@MT20% exhibited the highest photocatalytic activity, suggesting that a combination of ZTL and Fe-bearing MT could enhance photocatalytic activity by suppressing the recombination of photogenerated electron-hole pairs. On the basis of electronic-structure calculations, a photocatalytic mechanism for Cr(VI) reduction and the effect of Fe3+ on the structure of MT in ZTL/MT composites is proposed by density functional theory approach, whereby Fe impurities in MT generate a new mid-gap state that induce a Z-scheme heterojunction with ZTL, leading to reduced recombination of the photogenerated charge carriers in ZTL. These results establish that MT can not only act as a support material but also play an important role for photocatalytic reaction when Fe impurities are present in its structure.
Publication Icon Publication (2020)

Photophysics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions

Shruti Subramanian, Quinn T Campbell, Simon Moser, Jonas Kiemle, Philipp Zimmermann, Paul Seifert, Florian Sigger, Deeksha Sharma, Hala Al-Sadeg, Michael Labella III, Dacen Waters, Randall M Feenstra, Roland J Koch, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Ismaila Dabo, Alexander Holleitner, Thomas E Beechem, Ursula Wurstbauer, Joshua A Robinson
American Chemical Society (ACS) Nano 14, 16663-16671 (2020)

Abstract:
Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10× larger photocurrent is extracted at the EG/MoS2 interface when compared to the metal (Ti/Au)/MoS2 interface. This is supported by semi-local density functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ∼2× lower than that at Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle-resolved photoemission spectroscopy with spatial resolution selected to be ∼300 nm (nano-ARPES) and DFT calculations. A bending of ∼500 meV over a length scale of ∼2–3 μm in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.
Publication Icon Publication (2020)

First-principles study and experimental characterization of metal incorporation in germanium telluride

Kayla A. Cooley, Nathan D. Keilbart, James G. Champlain, Laura B. Ruppalt, Timothy N. Walter, Ismaila Dabo, Suzanne E. Mohney
Journal of Applied Physics 128, 225306 (2020)

Abstract:
Germanium telluride is a well-known phase change material (PCM) used in non-volatile memory cells and radio frequency switches. Controlling the properties of GeTe for improved PCM device performance has sometimes been achieved by doping and/or alloying with metals, often at concentrations greater than 10 at. % and using non-equilibrium methods. Since switching PCMs between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of metal-incorporated GeTe (Ge(0.5−x)MxTe0.5) films is also critical to practical implementation of these materials in electronic and optoelectronic devices. In this work, we use both density-functional theory and experimental characterization methods to probe the solubility and critical properties of Ge(0.5−x)MxTe0.5 films. Using first-principles calculations, we determine the enthalpy of formation for GeTe with 2.08, 4.17, and 6.25 at. % of Cu, Fe, Mn, Mo, and Ti and show trends between the stability of the Ge(0.5−x)MxTe0.5 systems and the atomic position, composition, and distribution of the metal atoms in the GeTe matrix. Out of all the studied systems, Mo was the only metal to cluster within GeTe. Analysis of the Ge–Te bond lengths and volumes of the Ge(0.5−x)MxTe0.5 supercells shows that increasing the atomic concentration (2.08, 4.17, 6.25 at. %) of the different metals causes varied distortions of the crystal structure of GeTe that are accompanied by significant changes in the projected density of states. Computational predictions concerning metal solubility and the effect of metal incorporation on critical properties of GeTe are compared to experimental results in the literature (Cu, Mn, Mo, and Ti) and to transmission electron microscopy and transport data from newly characterized co-sputtered Ge(0.5−x)FexTe0.5 films. The computational predictions of decreasing solubility (Mn > Cu, Fe > Ti, Mo) shows good agreement with experimental observations (Mn, Cu > Fe > Ti, Mo), and Ge(0.5−x)FexTe0.5 films exhibited increased crystallization temperatures from pure GeTe.
Publication Icon Publication (2020)

Colloidal nanoparticles of a metastable copper selenide phase with near-infrared plasmon resonance

Robert Lord, Julian Fanghanel, Cameron Holder, Ismaila Dabo, Raymond Schaak
Chemistry of Materials 32, 10227-10234 (2020)

Abstract:
Copper selenide nanoparticles are important materials with desirable properties for a broad scope of applications. Copper selenides are additionally known to adopt several different crystal structures and compositions. Here, we report the direct solution-phase synthesis of colloidal Cu2–xSe nanoparticles that adopt a structure distinct from other known copper selenide phases. This Cu2–xSe phase was determined to be structurally related to weissite Cu2–xTe, which is a layered compound containing alternating Cu-rich and Cu-deficient layers sandwiching distorted hexagonal layers of chalcogen atoms. When the weissite-like Cu2–xSe nanoparticles were annealed in solution, they converted to the more stable cubic berzelianite phase, indicating that they are metastable. Optical characterization of the weissite-like Cu2–xSe nanoparticles showed a broad plasmon absorption band centered around 1550 nm, and the position of this absorption band shifted only slightly when subjected to reductive and oxidative conditions.
Publication Icon Publication (2020)

Phase-selective solution synthesis of perovskite-related cesium cadmium chloride nanoparticles

Cameron Holder, Julian Fanghanel, Yihuang Xiong, Ismaila Dabo, Raymond Schaak
Inorganic Chemistry 59, 11688-11694 (2020)

Abstract:
All-inorganic metal halide perovskite-related phases are semiconducting materials that are of significant interest for a wide range of applications. Nanoparticles of these materials are particularly useful because they permit solution processing while offering unique and tunable properties. Of the many metal halide systems that have been studied extensively, cesium cadmium chlorides remain underexplored, and synthetic routes to access them as nanoscale materials have not been established. Here we demonstrate that a simple solution-phase reaction involving the injection of a cesium oleate solution into a cadmium chloride solution produces three distinct cesium cadmium chlorides: hexagonal CsCdCl3 and the Ruddlesden–Popper layered perovskites Cs2CdCl4 and Cs3Cd2Cl7. The phase-selective synthesis emerges from differences in reagent concentrations, temperature, and injection rates. A key variable is the rate at which the cesium oleate solution is injected into the cadmium chloride solution, which is believed to influence the local Cs:Cd concentration during precipitation, leading to control over the phase that forms. Band structure calculations indicate that hexagonal CsCdCl3 is a direct band gap semiconductor while Cs2CdCl4 and Cs3Cd2Cl7 have indirect band gaps. The experimentally determined band gap values for CsCdCl3, Cs2CdCl4, and Cs3Cd2Cl7 are 5.13, 4.91, and 4.70 eV, respectively, which places them in a rare category of ultrawide-band-gap semiconductors.
Publication Icon Publication (2020)

Antisymmetry: Fundamentals and applications

Hari Padmanabhan, Jason M. Munro, Ismaila Dabo, Venkatraman Gopalan
Annual Review of Materials Research 50, 255-281 (2020)

Abstract:
Symmetry is fundamental to understanding our physical world. An antisymmetry operation switches between two different states of a trait, such as two time states, position states, charge states, spin states, or chemical species. This review covers the fundamental concepts of antisymmetry and focuses on four antisymmetries, namely, spatial inversion in point groups, time reversal, distortion reversal, and wedge reversion. The distinction between classical and quantum mechanical descriptions of time reversal is presented. Applications of these antisymmetries—in crystallography, diffraction, determining the form of property tensors, classifying distortion pathways in transition state theory, finding minimum energy pathways, diffusion, magnetic structures and properties, ferroelectric and multiferroic switching, classifying physical properties in arbitrary dimensions, and antisymmetry-protected topological phenomena—are described.
Publication Icon Publication (2020)

Optimized utilization of COMB3 reactive potentials in LAMMPS

Robert Slapikas, Ismaila Dabo, Susan B Sinnott
Journal of Chemical Physics 152, 224702 (2020)

Abstract:
An investigation to optimize the application of the third-generation charge optimized many-body (COMB3) interatomic potential and associated input parameters was carried out through the study of solid–liquid interactions in classical molecular dynamics simulations. The rates of these molecular interactions are understood through the wetting rates of water nano-droplets on a bare copper (111) surface. Implementing the Langevin thermostat, the influence of simulation time step, the number of atoms in the system, the frequency at which charge equilibration is performed, and the temperature relaxation rate are all examined. The results indicate that time steps of 0.4 fs are possible when using longer relaxation times for the system temperature, which is almost double the typical time step used for reactive potentials. The use of the charge equilibration allows for a fewer atomic layers to be used in the Cu slab. In addition, charge equilibrium schemes do not need to be performed every time step to ensure accurate charge transfer. Interestingly, the rate of wetting for the nanodroplets is dominantly dependent on the temperature relaxation time, which is predicted to significantly change the viscosity of the water droplets. This work provides a pathway for optimizing simulations using the COMB3 reactive interatomic potential.
Publication Icon Publication (2020)

Achieving minimum heat conductivity by ballistic confinement in phononic metalattices

Weinan Chen, Disha Talreja, Devon Eichfeld, Pratibha Mahale, Nabila Nabi Nova, Hiu Y Cheng, Jennifer L Russell, Shih-Ying Yu, Nicolas Poilvert, Gerald Mahan, Suzanne E Mohney, Vincent H Crespi, Thomas E Mallouk, John V Badding, Brian Foley, Venkatraman Gopalan, Ismaila Dabo
American Chemical Society (ACS) Nano 14, 4235-4243 (2020)

Abstract:
Controlling the thermal conductivity of semiconductors is of practical interest in optimizing the performance of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an effective means of achieving such control; it has been proposed that the thermal conductivity of silicon could be reduced to 1 W/m/K using this approach and that a minimum in the heat conductivity would be reached for some optimal size of the inclusions. Yet the experimental verification of this design rule has been limited. In this work, we address this question by studying the thermal properties of silicon metalattices that consist of a periodic distribution of spherical inclusions with radii from 7 to 30 nm, embedded into silicon. Experimental measurements confirm that the thermal conductivity of silicon metalattices is as low as 1 W/m/K for silica inclusions and that this value can be further reduced to 0.16 W/m/K for silicon metalattices with empty pores. A detailed model of ballistic phonon transport suggests that this thermal conductivity is close to the lowest achievable by tuning the radius and spacing of the periodic inhomogeneities. This study is a significant step in elucidating the scaling laws that dictate ballistic heat transport at the nanoscale in silicon and other semiconductors.
Publication Icon Publication (2020)

Energy-resolved distribution of electron traps for O/S-doped carbon nitrides by reversed double-beam photoacoustic spectroscopy and the photocatalytic reduction of Cr(VI)

Chitiphon Chuaicham, Sekar Karthikeyaa, Radheshyam R Pawar, Yihuang Xiong, Ismaila Dabo, Bunsho Ohtani, Yoonyoung Kim, Jun Tae Song, Tatsumi Ishihara, Keiko Sasaki
Chemical Communications 56, 3793 (2020)

Abstract:
We report for the first time to our knowledge the identification of heteroatom-doped and undoped C3N4 with the energy-resolved distribution of electron traps (ERDT) near the conduction band bottom position (CBB) using reversed double-beam photoacoustic spectroscopy. The ERDT/CBB pattern is used to classify the type of elemental doping in C3N4, related to photocatalytic efficiency.
Publication Icon Publication (2020)

Effects of surface charge and cluster size on the electrochemical dissolution of platinum nanoparticles using COMB3 and continuum electrolyte models

James M. Goff, Susan B. Sinnott, Ismaila Dabo
Journal of Chemical Physics 152, 064102 (2020)

Abstract:
We study the site-dependent dissolution of platinum nanoparticles under electrochemical conditions to assess their thermodynamic stability as a function of shape and size using empirical molecular dynamics and electronic-structure models. The third-generation charge optimized many-body potential is employed to determine the validity of uniform spherical representations of the nanoparticles in predicting dissolution potentials (the Kelvin model). To understand the early stages of catalyst dissolution, implicit solvation techniques based on the self-consistent continuum solvation method are applied. It is demonstrated that interfacial charge and polarization can shift the dissolution energies by amounts on the order of 0.74 eV depending on the surface site and nanoparticle shape, leading to the unexpected preferential removal of platinum cations from highly coordinated sites in some cases.
Publication Icon Publication (2020)

Spectroscopic and first-principles investigations of iodine species incorporation into ettringite: Implications for iodine migration in cement waste forms

Binglin Guo, Yihuang Xiong, Weinan Chen, Sarah A Saslow, Naofumi Kozai, Toshihiko Ohnuki, Ismaila Dabo, Keiko Sasaki
Journal of Hazardous Materials 389, 121880 (2020)

Abstract:
Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca6Al2(OH)12(SO4)3∙26H2O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (129I) that persists as iodide (I–) and iodate (IO3–) in the cementitious nuclear waste repository. However, the structural information and immobilization mechanisms of iodine species in ettringite remain unclear. The present results suggested minimal I– incorporation into ettringite (0.05 %), whereas IO3– exhibited a high affinity for ettringite via anion substitution for SO42– (96 %). The combined iodine K-edge extended X-ray absorption fine structure (EXAFS) spectra and first-principles calculations using density functional theory (DFT) suggested that IO3– was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO3– for SO42– was energetically favorable by –0.41 eV, whereas unfavorable substitution energy of 4.21 eV was observed for I– substitution. Moreover, the bonding charge density analysis of the substituted IO3– and I– anions into the ettringite structure revealed the interaction between intercalated ions with the structural water molecules. These results provided valuable insight into the long-term stabilization of anionic iodine species and their migration in cementitious nuclear waste repository or alkaline environments.
Publication Icon Publication (2020)

Tuning triplet-pair separation versus relaxation using a diamond anvil cell

Grayson S Doucette, Haw-Tyng Huang, Jason M Munro, Kyle T Munson, Changyong Park, John E Anthony, Timothy Strobel, Ismaila Dabo, John V Badding, John B Asbury
Cell Reports Physical Science 1, 100005 (2020)

Abstract:
A tradeoff exists between triplet-pair separation versus relaxation that can limit the ability to utilize singlet fission for enhancing solar cell efficiency beyond the Shockley-Queisser limit. Here, we show that this tradeoff can be avoided in crystalline environments by studying a functionalized pentacene compressed in a diamond anvil cell. We demonstrate, using ultrafast transient absorption spectroscopy, that there is a “sweet spot” where the rate of triplet-pair separation can be accelerated by nearly an order of magnitude without causing fast excited state relaxation. X-ray diffraction and computational modeling allow us to quantify the corresponding increase of intermolecular coupling. Our findings suggest that increased coupling enhances excited state relaxation but that crystalline environments can suppress these relaxation processes in pentacene derivatives. The combination of these effects leads to the sweet spot and informs efforts to enhance triplet-pair separation rates in amorphous systems such as polymers.
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Vibrational probe of the origin of singlet exciton fission in TIPS-pentacene solutions

Christopher Grieco, Grayson S Doucette, Kyle M Munson, John R Swartzfager, Jason M Munro, John E Anthony, Ismaila Dabo, John B Asbury
Journal of Chemical Physics 151, 154701 (2019)

Abstract:
We use native vibrational modes of the model singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) to examine the origins of singlet fission in solution between molecules that are not tethered by a covalent linkage. We use the C—H stretch modes of TIPS side groups of TIPS-Pn to demonstrate that singlet fission does not occur by diffusive encounter of independent molecules in solution. Instead, TIPS-Pn molecules aggregate in solution through their TIPS side groups. This aggregation breaks the symmetry of the TIPS-Pn molecules and enables the formation of triplets to be probed through the formally symmetry forbidden symmetric alkyne stretch mode of the TIPS side groups. The alkyne stretch modes of TIPS-Pn are sensitive to the electronic excited states present during the singlet fission reaction and provide unique signatures of the formation of triplets following the initial separation of triplet pair intermediates. These findings highlight the opportunity to leverage structural information from vibrational modes to better understand intermolecular interactions that lead to singlet fission.
Publication Icon Publication (2019)

First-principles simulations of electrified interfaces in electrochemistry

Stephen E Weitzner and Ismaila Dabo,
in Heterogeneous catalysts: emerging techniques for design, characterization and applications
edited by W. Y. Teoh, A. Urakawa, Y. H. Ng, P. H.-L. Sit (Wiley, 2019).

Publication Icon Publication (2019)

Probing the pseudocapacitance and energy-storage performance of RuO2 facets from first principles

Nathan D Keilbart, Yasuaki Okada, Ismaila Dabo
Physical Review Materials 3, 085405 (2019)

Abstract:
The energy density of ruthenia (RuO2) pseudocapacitor electrodes is critically dependent on their surface structure. To understand this dependence, we simulate the electrochemical response of RuO2(110), RuO2(100), and RuO2(101) in aqueous environments using a self-consistent continuum solvation (SCCS) model of the solid-liquid interface. The insertion of protons into the RuO2(110) sublayer is found to profoundly affect the voltage-dependent characteristics of the system, leading to a sharp transition from a battery-type to capacitor-type response. The calculated charge-voltage properties for RuO2(101) are in qualitative agreement with experiment, albeit with a pseudocapacitance that is significantly underestimated. In contrast, the RuO2(100) facet is correctly predicted to be pseudocapacitive over a wide voltage window, with a calculated pseudocapacitance in close agreement with experimental voltammetry. These results establish the SCCS model as a reliable approach to predict and optimize the facet-dependent pseudocapacitance of polycrystalline systems.
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Understanding the influence of defects and surface chemistry on ferroelectric switching: A ReaxFF investigation of BaTiO3

Dooman Akbarian, Dundar E Yilmaz, Ye Cao, Panchapakesan Ganesh, Ismaila Dabo, Jason Munro, Renee Van Ginhoven, Adri CT van Duin
Physical Chemistry and Chemical Physics 21, 18240-18249 (2019)

Abstract:
Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- as well as temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and reduction in the polarization and the curie temperature due to the OVs; (3) domain wall interaction with surface chemistry to influence ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.
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Electrochemical stability and light-harvesting ability of Si photoelectrodes in aqueous environments

Quinn Campbell and Ismaila Dabo
Journal of Chemical Physics 151, 044109 (2019)

Abstract:
We consider the factors that affect the photoactivity of silicon electrodes for the water-splitting reaction using a self-consistent continuum solvation (SCCS) model of the solid-liquid interface. This model allows us to calculate the charge-voltage response, Schottky barriers, and surface stability of different terminations while ac- counting for the interactions between the charge-pinning centers at the surface and the depletion region of the semiconductor. We predict that the most stable oxidized surface does not have a favorable Schottky barrier, which further explains the low solar-to-hydrogen performance of passivated silicon electrodes.
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Topological control of water reactivity on glass surfaces: Evidence of a chemically stable intermediate phase

Collin J Wilkinson, Karan Doss, Seung Ho Hahn, Nathan D Keilbart, Arron R Potter, Nicholas J Smith, Ismaila Dabo, Adri CT van Duin, Seong Han Kim, John C Mauro
Journal of Physical Chemistry Letters 10, 3955-3960 (2019)

Abstract:
Glass surfaces are of considerable interest due to their disproportionately large influence on the performance of glass articles in many applications. However, the behavior of glass surfaces has proven difficult to model and predict due to their complex structure and interactions with the environment. Here, the effects of glass network topology on the surface reactivity of glasses have been investigated using reactive and nonreactive force field-based molecular dynamics simulations as well as density functional theory. A topological constraint-based description for surface reactivity is developed, allowing for improved understanding of the physical and chemical origins of surface reactivity. Results show evidence for the existence of a chemically stable intermediate phase on the surface of the glass where the glass network is mechanically isostatic.
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MXene electrode materials for electrochemical energy storage: first-principles and grand canonical Monte Carlo simulations

Yasuaki Okada, Nathan D Keilbart, James M Goff, Shin’ichi Higai, Kosuke Shiratsuyu, Ismaila Dabo
MRS Advances 4, 1-9 (2019)

Abstract:
MXenes are a novel class of two dimensional materials, discovered by Barsoum and Gogotsi [M. Naguib, J. Come, B. Dyatkin, V. Presser, P. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochemistry Communications 16, 61-64 (2012); B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nature Reviews Materials vol. 2, 16098 (2017)]. Their large specific surface area and the tunability of their physicochemical properties as a function of the transition metal and surface terminal group make them a unique design platform for various applications, a primary example of which is pseudocapacitive energy storage. However, there is still incomplete understanding of how the transition metal chemistry and stoichiometry, and the surface termination affect charge storage mechanisms in MXene. In this study, we have performed systematic first-principles calculations for bulk MXene and found that the atomic charge of the metal cations, which is related to their valence, decreases across the d-electron metal series. Electronic-structure indicators of performance are examined to understand the energy storage behavior, whereby charges are stored between the terminal groups and adsorbing cations. Importantly, we found that the differential Bader charges show good agreement with theoretical capacitances and are useful in predicting charge storage trends in MXene-based pseudocapacitors. Furthermore, we have performed first-principles and grand canonical Monte Carlo calculations for the slab systems, finding that the solvent plays a critical role in enhancing the pseudocapacitive response.
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Influence of surface restructuring on the activity of SrTiO3 photoelectrodes for photocatalytic hydrogen reduction

Yihuang Xiong and Ismaila Dabo
Physical Review Materials 3, 065801 (2019)

Abstract:
Perovskite photoelectrodes are being extensively studied in search for photocatalytic materials that can produce hydrogen through water splitting. The solar-to-hydrogen efficiency of these materials is critically dependent on the electrochemical state of their surface. Here, we develop an embedded quantum-mechanical approach using the self-consistent continuum solvation (SCCS) model to predict the relation between band alignment, electrochemical stability, and photocatalytic activity taking into account the long-range polarization of the semiconductor electrode under electrical bias. Using this comprehensive model, we calculate the charge-voltage response of various reconstructions of a solvated SrTiO3 surface, revealing that interfacial charge trapping exerts primary control on the electrical response and surface stability of the photoelectrode. Our results provide a detailed molecular-level interpretation of the enhanced photocatalytic activity of SrTiO3 upon voltage-induced restructuring of the semiconductor-solution interface.
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BaZrSe3: Ab-initio study of anion substitution for bandgap tuning in a chalcogenide material

Marc Ong, David Guzman, Quinn Campbell, Ismaila Dabo, Jishi A Radi
Journal of Applied Physics 125, 235702 (2019)

Abstract:
Recently, transition metal perovskite chalcogenide materials have been proposed as possible candidates for solar cell applications. In this work, we provide accurate theoretical calculations for BaZrS3 and two phases of SrZrS3, which have been recently synthesized and their optical properties elaborated. In this study, we consider the substitution of S in BaZrS3 with Se to form BaZrS3. Evolutionary methods are used to find the optimal structure of this compound, and accurate calculations of its optoelectronic properties are presented. Using phonon frequency calculations and ab initio molecular dynamics, we assess the stability of this compound. We find that BaZrS3 is likely to be stable under typical conditions, with a low band gap and high optical absorption coefficients. This suggests that BaZrS3 could be useful for solar cell applications.
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Implementation of distortion symmetry for the nudged elastic band method with DisPy

Jason M Munro, Vincent S Liu, Venkatraman Gopalan, Ismaila Dabo
Nature Partner Journal (NPJ) Computational Materials Science 5, 52 (2019)

Abstract:
The nudged elastic band (NEB) method is a commonly used approach for the calculation of minimum energy pathways of kinetic processes. However, the final paths obtained rely heavily on the nature of the initially chosen path. This often necessitates running multiple calculations with differing starting points in order to obtain accurate results. Recently, it has been shown that the NEB algorithm can only conserve or raise the distortion symmetry exhibited by an initial pathway. Using this knowledge, symmetry-adapted perturbations can be generated and used as a tool to systematically lower the initial path symmetry, enabling the exploration of other low-energy pathways that may exist. The group and representation theory details behind this process are presented and implemented in a standalone piece of software (DiSPy). The method is then demonstrated by applying it to the calculation of ferroelectric switching pathways in LiNbO3. Previously reported pathways are more easily obtained, with new paths also being found which involve a higher degree of atomic coordination.
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New frontiers for the Materials Genome Initiative

Juan de Pablo​, Nicholas E Jackson, Michael A Webb, Long-Qing Chen, ​Joel Moore, Dane Morgan, Tresa Pollock, Darrell Schlom, Eric Toberer, James Analytis, Ismaila Dabo, Dean DeLongchamp, Greg Fiete, Greg Grason, Geoffroy Hautier, Yifei Mo, Krishna Rajan, Evan Reed, Efrain Rodriguez, Vladan Stevanovic, Jin Suntivich, Katsuyo Thornton, Ji-Cheng Zhao
Nature Partner Journal (NPJ) Computational Materials Science 5, 41 (2019)

Abstract:
The Materials Genome Initiative (MGI) advanced a new paradigm for materials discovery and design, namely that the pace of new materials deployment could be accelerated through complementary efforts in theory, computation, and experiment. Along with numerous successes, new challenges are inviting researchers to refocus the efforts and approaches that were originally inspired by the MGI. In May 2017, the National Science Foundation sponsored the workshop “Advancing and Accelerating Materials Innovation Through the Synergistic Interaction among Computation, Experiment, and Theory: Opening New Frontiers” to review accomplishments that emerged from investments in science and infrastructure under the MGI, identify scientific opportunities in this new environment, examine how to effectively utilize new materials innovation infrastructure, and discuss challenges in achieving accelerated materials research through the seamless integration of experiment, computation, and theory. This article summarizes key findings from the workshop and provides perspectives that aim to guide the direction of future materials research and its translation into societal impacts.
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Voltage-dependent reconstruction of layered Bi2WO6 and Bi2MoO6 photocatalysts and its influence on charge separation for water splitting

Quinn Campbell, Daniel Fisher, Ismaila Dabo
Physical Review Materials 3, 015404 (2019)

Abstract:
We study the surface stability and electronic characteristics of the layered bismuth-oxide Bi2WO6 and Bi2MoO6 photocatalysts, which belong to the Aurivillius (Bi2An−1BnO3n+3) perovskite series and have been proposed as efficient visible-light absorbers. We present a Newton–Raphson optimization of the equilibrium charge distribution at the semiconductor–solution interface using the self-consistent continuum solvation (SCCS) model, and we extend previous surface-energy determination methods to layered materials that exhibit alternating terminations. Our computational analysis provides a detailed description of the charged interface under controlled pH and applied voltage, and highlights the competing structural and electrical factors that underlie the facet-dependent photocatalytic activity of layered Bi2An−1BnO3n+3 compounds.
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First-principles investigation of BiVO3 for thermochemical water splitting

Marc Ong, Quinn Campbell, Ismaila Dabo, Radi A Jishi
International Journal of Hydrogen Energy 44, 1425 (2019)

Abstract:
Thermochemical water splitting is a promising clean method of hydrogen production of high relevance in a society heavily reliant on fossil fuels. Using evolutionary methods and density functional theory, we predict the structure and electronic properties of BiVO3. We build on previous literature to develop a framework to evaluate the thermodynamics of thermochemical water splitting cycles for hydrogen production. We use these results to consider the feasibility of BiVO3 as a catalyst for thermochemical water splitting. We show that for BiVO3, both the thermal reduction and gas splitting reactions are thermodynamically favorable under typical temperature conditions. We predict that thermochemical water splitting cycles employing BiVO3 as a catalyst produce hydrogen yields comparable to those of commonly used catalysts.
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Voltage effects on the stability of Pd ensembles in PdAu/Au(111) surface alloys

Stephen E Weitzner and Ismaila Dabo
Journal of Chemical Physics 150, 041715 (2019)

Abstract:
The catalytic performance of multimetallic electrodes is often attributed to a beneficial combination of ligand, strain, and ensemble effects. Understanding the influence of the electrochemical environment on the stability of the alloy surface structure is thus a crucial component to the design of highly active and durable electrocatalysts. In this work, we study the effects of an applied voltage to electrocatalytic Pd–Au/Au(111) surface alloys in contact with a model continuum electrolyte. Using planewave density functional theory, two-dimensional cluster expansions are parameterized and used to simulate dilute Pd–Au surface alloys under electrochemical conditions via Metropolis Monte Carlo within an extended canonical ensemble. While Pd monomers are stable at all potentials considered, different extents of surface electrification are observed to promote the formation of Pd dimers and trimers, as well as clusters of Pd monomers. We find that the relative proportion of monomer, dimer, and trimer surface fractions is in good agreement with in situ scanning tunneling microscopy measurements. The further development and refinement of the approaches described herein may serve as a useful aid in the development of next-generation electrocatalysts.
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Conjugated block copolymers as model systems to examine mechanisms of charge generation in donor-acceptor materials

Melissa P Aplan, Christopher Grieco, Youngmin Lee, Jason M Munro, Jennifer L. Gray, Qing Wang, Zach D Seibers, Brooke Kuei, Josh H Litofsky, S Michael Kilbey, Ismaila Dabo, John B Asbury, Enrique D Gomez
Advanced Functional Materials 29, 1804858 (2018)

Abstract:
Fully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.
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Revealing the importance of energetic and entropic contributions to the driving force for charge photogeneration

Melissa P Aplan, Jason M Munro, Youngmin Lee, Alyssa N Brigeman, Christopher Grieco, Qing Wang, Noel C Giebink, Ismaila Dabo, John B Asbury, Enrique D Gomez
American Chemical Society (ACS) Applied Materials and Interfaces 10, 39933-39941 (2018)

Abstract:
Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor–acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor–acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.
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Discovering minimum energy pathways via distortion symmetry groups

Jason M Munro, Hirofumi Akamatsu, Haricharan Padmanabhan, Vincent S Liu, Yin Shi, Long-Qing Chen, Brian K VanLeeuwen, Ismaila Dabo, Venkatraman Gopalan
Physical Review B 98, 085107 (2018)

Abstract:
Physical systems evolve from one state to another along paths of least energy barrier. Without a priori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However, in many cases, the user has to repeatedly provide initial guess paths, thus implying that the reliability of the final result is heavily user-dependent. Recently, the idea of “distortion symmetry groups” as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method, which we name the distortion symmetry method, is shown to lead to the discovery of previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. These provide novel physical insights into the nucleation of switching pathways at experimentally observed domain walls in Ca3Ti2O7, as well as how polarization switching can proceed without reversing magnetization in BiFeO3. Furthermore, we demonstrate how symmetry-breaking from a highly symmetric pathway can be used to probe the non-Ising (Bloch and Néel) polarization components integral to transient states involved in switching in PbTiO3. The distortion symmetry method is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials.
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Spatio-temporal symmetry: Crystallographic point groups with time translations and time inversion

Vincent S Liu, Brian K VanLeeuwen, Jason M Munro, Haricharan Padmanabhan, Ismaila Dabo, Venkatraman Gopalan, Daniel B Litvin
Acta Crystallographica A74, 399-402 (2018)

Abstract:
The crystallographic symmetry of time-periodic phenomena has been extended to include time inversion. The properties of such spatio-temporal crystal- lographic point groups with time translations and time inversion are derived and one representative group from each of the 343 types has been tabulated. In addition, stereographic symmetry and general-position diagrams are given for each representative group. These groups are also given a notation consisting of a short Hermann–Mauguin magnetic point-group symbol with each spatial operation coupled with its associated time translation.
Publication Icon Publication (2017)

Voltage-dependent cluster expansion for electrified solid-liquid interfaces: Application to the electrochemical deposition of transition metals

Stephen E Weitzner and Ismaila Dabo
Physical Review B 96, 205134 (2017)

Abstract:
The detailed atomistic modeling of electrochemically deposited metal monolayers is challenging due to the complex structure of the metal-solution interface and the critical effects of surface electrification during electrode polarization. Accurate models of interfacial electrochemical equilibria are further challenged by the need to include entropic effects to obtain accurate surface chemical potentials. We present an embedded quantum-continuum model of the interfacial environment that addresses each of these challenges and study the underpotential deposition of silver on the gold (100) surface. We leverage these results to parametrize a cluster expansion of the electrified interface and show through grand canonical Monte Carlo calculations the crucial need to account for variations in the interfacial dipole when modeling electrodeposited metals under finite-temperature...
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Triplet transfer mediates triplet pair separation during singlet fission in 6,13-bis(triisopropylsilylethynyl)-pentacene

Christopher Grieco, Grayson S Doucette, Jason M Munro, Eric R Kennehan, Youngmin Lee, Adam Rimshaw, Marcia M Payne, Nichole Wonderling, John E Anthony, Ismaila Dabo, Enrique D Gomez, John B Asbury
Advanced Functional Materials 1703929 (2017)

Abstract:
Triplet population dynamics of solution cast films of isolated polymorphs of 6, 13-bis (triisopropylsilylethynyl) pentacene (TIPS-Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS-Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin-forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS-Pn molecules, which are correlated with the relative...
Publication Icon Publication (2017)

Quantum-continuum calculation of the surface states and electrical response of silicon in solution

Quinn Campbell and Ismaila Dabo
Physical Review B 95, 205308 (2017)

Abstract:
A wide range of electrochemical reactions of practical importance occur at the interface between a semiconductor and an electrolyte. We present an embedded density- functional theory method using the recently released self-consistent continuum solvation (SCCS) approach to study these interfaces. In this model, a quantum description of the surface is incorporated into a continuum representation of the bending of the bands within the electrode. The model is applied to understand the electrical response of silicon electrodes in solution, providing microscopic insights into the low-voltage region, where surface states determine the electrification of the semiconductor electrode.
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Quantum-continuum simulation of the electrochemical response of pseudocapacitor electrodes under realistic conditions

Nathan D Keilbart, Yasuaki Okada, Aion Feehan, Shin'ichi Higai, Ismaila Dabo
Physical Review B 95, 115423 (2017)

Abstract:
Pseudocapacitors are energy-storage devices characterized by fast and reversible redox reactions that enable them to store large amounts of electrical energy at high rates. We simulate the response of pseudocapacitive electrodes under realistic conditions to identify the microscopic factors that determine their performance, focusing on ruthenia (RuO2) as a prototypical electrode material. Electronic-structure methods are used together with a self-consistent continuum solvation (SCCS) model to build a complete data set of free energies as the surface of the charged electrode is gradually covered with protons under applied voltage. The resulting data set is exploited to compute hydrogen-adsorption isotherms and charge-voltage responses by means of grand-canonical sampling, finding close agreement with experimental voltammetry. These simulations reveal that small...
Publication Icon Publication (2017)

A silicon microwire under a three-dimensional anisotropic tensile stress

Xiaoyu Ji, Nicolas Poilvert, Wenjun Liu, Yihuang Xiong, Hiu Yan Cheng, John V Badding, Ismaila Dabo, Venkatraman Gopalan
Applied Physics Letters 110, 091911 (2017)

Abstract:
Three-dimensional tensile stress, or triaxial tensile stress, is difficult to achieve in a material. We present the investigation of an unusual three-dimensional anisotropic tensile stress field and its influence on the electronic properties of a single crystal silicon microwire. The microwire was created by laser heating an amorphous silicon wire deposited in a 1.7 μm silica glass capillary by high pressure chemical vapor deposition. Tensile strain arises due to the thermal expansion mismatch between silicon and silica. Synchrotron X-ray micro- beam Laue diffraction (μ-Laue) microscopy reveals that the three principal strain components are +0.47% (corresponding to a tensile stress of +0.7 GPa) along the fiber axis and nearly isotropic +0.02%(corresponding to a tensile stress of +0.3 GPa) in the cross- sectional plane. This effect was accompanied with a reduction of 30 meV in the band gap.
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Solution-synthesized In4SnSe4 semiconductor microwires with a direct band gap

Du Sun, Yihuang Xiong, Yifan Sun, Ismaila Dabo, Raymond E Schaak
Chemistry of Materials 29, 1095-1098 (2017)

Abstract:
Semiconductor materials having direct band gaps that overlap well with the solar spectrum are important for a variety of applications in solar energy conversion and optoelectronics. Here, we identify the ternary chalcogenide In4SnSe4 as a direct band gap semiconductor having a band gap of approximately 1.6 eV. In4SnSe4, which contains isolated tetrahedral [SnIn4] clusters embedded in an In–Se framework, was synthesized by precipitation from solution at 300° C. The In4SnSe4 product consists of microwires having lengths of approximately 5–20 μm and widths of approximately 100–400 nm. Band structure calculations predict a direct electronic band gap of approximately 2.0 eV. Diffuse reflectance UV–visible spectroscopy qualitatively validates the predicted direct band gap, yielding an observed optical band gap of 1.6 eV.
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Quantum-continuum simulation of underpotential deposition at electrified metal-solution interfaces

Stephen E Weitzner and Ismaila Dabo
Nature Partner Journal (NPJ) Computational Materials 3, 1-7 (2017)

Abstract:
The underpotential deposition of transition metal ions is a critical step in many electrosynthetic approaches. While underpotential deposition has been intensively studied at the atomic level, first-principles calculations in vacuum can strongly underestimate the stability of underpotentially deposited metals. It has been shown recently that the consideration of co-adsorbed anions can deliver more reliable descriptions of underpotential deposition reactions; however, the influence of additional key environmental factors such as the electrification of the interface under applied voltage and the activities of the ions in solution have yet to be investigated. In this work, copper underpotential deposition on gold is studied under realistic electrochemical conditions using a quantum– continuum model of the electrochemical interface. We report here on the influence of...
Publication Icon Publication (2017)

Single-crystal silicon optical fiber by direct laser crystallization

Xiaoyu Ji, Shiming Lei, Shih-Ying Yu, Hiu Yan Cheng, Wenjun Liu, Nicolas Poilvert, Yihuang Xiong, Ismaila Dabo, Suzanne E Mohney, John V Badding, Venkatraman Gopalan
American Chemical Society (ACS) Photonics 4, 85-92 (2017)

Abstract:
Semiconductor core optical fibers with a silica cladding are of great interest in nonlinear photonics and optoelectronics applications. Laser crystallization has been recently demonstrated for crystallizing amorphous silicon fibers into crystalline form. Here we explore the underlying mechanism by which long single-crystal silicon fibers, which are novel platforms for silicon photonics, can be achieved by this process. Using finite element modeling, we construct a laser processing diagram that reveals a parameter space within which single crystals can be grown. Utilizing this diagram, we illustrate the creation of single- crystal silicon core fibers by laser crystallizing amorphous silicon deposited inside silica capillary fibers by high-pressure chemical vapor deposition. The single-crystal fibers, up to 5.1 mm long, have a very well-defined core/cladding interface and a chemically pure...
Publication Icon Publication (2016)

Removal mechanism of high concentration borate by co-precipitation with hydroxyapatite

Keiko Sasaki, Kenta Toshiyuki, Keiko Ideta, Hajime Miki, Tsuyoshi Hirajima, Jin Miyawaki, Mitsuhiro Murayama, Ismaila Dabo
Journal of Environmental Chemical Engineering 4, 1092-1101 (2016)

Abstract:
Co-precipitation of borate in a wide range of concentration with hydroxyapatite (HAp) was investigated using Ca(OH)2 as a mineralizer in the presence of phosphate. The sorption data of borate was fitted to Freundlich model. The maximum sorption density of B/Ca to maintain a mono-phase of HAp was found around 0.40. In higher B concentrations, borate was still immobilized, however, the crystalization of hydroxyapatite was inhibited, where the solid residues were accompanied with amorphous CaB2O4, as well as HAp. Based on 11 B-NMR and elemental analysis for solid residues in addition to solution chemistry, the removal mechanism of high concentration borate can be explained.
Publication Icon Publication (2015)

First-principles photoemission spectroscopy and orbital tomography in molecules from Koopmans-compliant functionals

Ngoc Linh Nguyen, Giovanni Borghi, Andrea Ferretti, Ismaila Dabo, Nicola Marzari
Physical Review Letters 114, 166405 (2015)

Abstract:
The determination of spectral properties from first principles can provide powerful connections between microscopic theoretical predictions and experimental data, but requires complex electronic-structure formulations that fall outside the domain of applicability of common approaches, such as density-functional theory. We show here that Koopmans-compliant functionals, constructed to enforce piecewise linearity and the correct discontinuity derivative in energy functionals with respect to fractional occupation—ie, with respect to charged excitations—provide molecular photoemission spectra and momentum maps of Dyson orbitals that are in excellent agreement with experimental ultraviolet photoemission spectroscopy and orbital tomography data. These results highlight the role of Koopmans-compliant functionals as accurate and inexpensive quasiparticle...
Publication Icon Publication (2015)

Koopmans-compliant self-interaction corrections

Nicolas Poilvert, Giovanni Borghi, Ngoc Linh Nguyen, Nathan D Keilbart, Kevin Wang, Ismaila Dabo
Advances in Atomic, Molecular, and Optical Physics 64, 105-127 (2015)

Abstract:
Self-interaction is a central problem for the accuracy of density-functional approximations in describing the electronic structure of atoms and molecules. In this work, we discuss the different types of self-interaction errors commonly encountered in density- functional calculations, providing precise definitions for each of them. Based upon these definitions, we derive an orbital-dependent density-functional method, called the Koopmans- compliant approach, which simultaneously corrects the different self-interaction errors, by enforcing piecewise linearity with respect to fractional particle counts and by imposing the correct asymptotic behavior of the one-electron potential in approximate energy functionals. We illustrate the very good performance of this new method in predicting the electronic properties of atoms and molecules, while preserving or improving the prediction of total...
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Koopmans-compliant functionals and their performance against reference molecular data

Giovanni Borghi, Andrea Ferretti, Ngoc Linh Nguyen, Ismaila Dabo, Nicola Marzari
Physical Review B 90, 075135 (2014)

Abstract:
Koopmans-compliant functionals emerge naturally from extending the constraint of piecewise linearity of the total energy as a function of the number of electrons to each fractional orbital occupation. When applied to approximate density-functional theory, these corrections give rise to orbital-density-dependent functionals and potentials. We show that the simplest implementations of Koopmans' compliance provide accurate estimates for the quasiparticle excitations and leave the total energy functional almost or exactly intact, ie, they describe correctly electron removals or additions, but do not necessarily alter the electronic charge density distribution within the system. Additional Koopmans-compliant functionals can be constructed that modify the potential energy surface, starting, eg, from Perdew-Zunger corrections. These functionals become exactly one-electron self...
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Bridging density-functional and many-body perturbation theory: Orbital-density dependence in electronic-structure functionals

Andrea Ferretti, Ismaila Dabo, Matteo Cococcioni, Nicola Marzari
Physical Review B 89, 195134 (2014)

Abstract:
Energy functionals which depend explicitly on orbital densities, rather than on the total charge density, appear when applying self-interaction corrections to density-functional theory; this is, eg, the case for Perdew-Zunger and Koopmans-compliant functionals. In these formulations the total energy is not invariant under unitary rotations of the orbitals, and local, orbital-dependent potentials emerge. We argue that this is not a shortcoming, and that instead these potentials can provide, in a functional form, a simplified quasiparticle approximation to the spectral potential, ie, the local, frequency-dependent contraction of the many-body self-energy that is sufficient to describe exactly the spectral function. As such, orbital-density-dependent functionals have the flexibility to accurately describe both total energies and quasiparticle excitations in the electronic-structure problem. In addition, and...
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Chemisorbed molecules under potential bias: detailed insights from first-principles vibrational spectroscopies

Nicéphore Bonnet, Ismaila Dabo, Nicola Marzari
Electrochimica Acta 121, 210-214 (2014)

Abstract:
The electrochemical factors that control the interaction of a chemisorbed molecule under potential bias are identified by considering the vibrational response of carbon monoxide on a platinum electrode from first principles. Using three complementary approaches to simulate the potential bias, it is shown that the frequency shifts upon electrode bias are electrostatic in nature and give rise to the vibrational Stark effect, in which the first and second-order responses are determined by the dipole moment and the capacitance of the system, respectively. These results are rationalized by examining the central role of electrostatic screening by the metal at the atomic scale.
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Piecewise linearity and spectroscopic properties from Koopmans-compliant functionals

Ismaila Dabo, Andrea Ferretti, Nicola Marzari
First Principles Approaches to Spectroscopic Properties of Complex Materials. Topics in Current Chemistry, vol 347. Springer, Berlin, Heidelberg (2014)

Abstract:
Density-functional theory is an extremely powerful and widely used tool for quantum simulations. It reformulates the electronic-structure problem into a functional minimization with respect to the charge density of interacting electrons in an external potential. While exact in principle, it is approximate in practice, and even in its exact form it is meant to reproduce correctly only the total energy and its derivatives, such as forces, phonons, or dielectric properties. Quasiparticle levels are outside the scope of the theory, with the exception of the highest occupied state, since this is given by the derivative of the energy with respect to the number of electrons. A fundamental property of the exact energy functional is that of piecewise linearity at fractional occupations in between integer fillings, but common approximations do not follow such piecewise behavior, leading to electron delocalization.
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Donor and acceptor levels of organic photovoltaic compounds from first principles

Ismaila Dabo, Andrea Ferretti, Cheol-Hwan Park, Nicolas Poilvert, Yanli Li, Matteo Cococcioni, Nicola Marzari
Physical Chemistry Chemical Physics 15, 685-695 (2013)

Abstract:
Accurate and efficient approaches to predict the optical properties of organic semiconducting compounds could accelerate the search for efficient organic photovoltaic materials. Nevertheless, predicting the optical properties of organic semiconductors has been plagued by the inaccuracy or computational cost of conventional first-principles calculations. In this work, we demonstrate that orbital-dependent density-functional theory based upon Koopmans' condition [Phys. Rev. B, 2010, 82, 115121] is apt for describing donor and acceptor levels for a wide variety of organic molecules, clusters, and oligomers within a few tenths of an electron-volt relative to experiment, which is comparable to the predictive performance of many-body perturbation theory methods at a fraction of the computational cost.
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Role of electronic localization in the phosphorescence of iridium sensitizing dyes

Burak Himmetoglu, Alex Marchenko, Ismaila Dabo, Matteo Cococcioni
Journal of Chemical Physics 137, 154309 (2012)

Abstract:
In this work we present a systematic study of three representative iridium dyes, namely, Ir (ppy) 3, FIrpic, and PQIr, which are commonly used as sensitizers in organic optoelectronic devices. We show that electronic correlations play a crucial role in determining the excited- state energies in these systems, due to localization of electrons on Ir d orbitals. Electronic localization is captured by employing hybrid functionals within time-dependent density- functional theory and with Hubbard-model corrections within the Δ-SCF approach. The performance of both methods are studied comparatively and shown to be in good agreement with experiment. The Hubbard-corrected functionals provide further insight into the localization of electrons and on the charge-transfer character of excited-states. The gained insight allows us to comment on envisioned functionalization strategies to improve...
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Resilience of gas-phase anharmonicity in the vibrational response of adsorbed carbon monoxide and breakdown under electrical conditions

Ismaila Dabo
Physical Review B 86, 035139 (2012)

Abstract:
In surface catalysis, the adsorption of carbon monoxide on transition-metal electrodes represents the prototype of strong chemisorption. Notwithstanding significant changes in the molecular orbitals of adsorbed CO, spectroscopic experiments highlight a close correlation between the adsorbate stretching frequency and equilibrium bond length for a wide range of adsorption geometries and substrate compositions. In this work, we study the origins of this correlation, commonly known as Badger's rule, by deconvoluting and examining the contributions from the adsorption environment to the intramolecular potential using first-principles calculations. Noting that intramolecular anharmonicity is preserved upon CO chemisorption, we show that Badger's rule for adsorbed CO can be expressed solely in terms of the tabulated Herzberg spectroscopic constants of isolated CO...
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First-principles simulation of arsenate adsorption on the surface of hematite

Marc Blanchard, Guillaume Morin, Michele Lazzeri, Etienne Balan, Ismaïla Dabo
Geochimica et Cosmochimica Acta 86, 182-195 (2012)

Abstract:
Recent experimental studies revealed an unprecedented bimodal distribution of arsenate at the hematite (1 1¯ 2) surface with a simultaneous adsorption of inner-sphere and outer-sphere complexes. In the present study, first-principles calculations based on density-functional theory were performed to provide detailed insights into the structural and electronic properties of such inner-sphere and outer-sphere adsorption complexes on two hydroxylated terminations of the hematite (1 1¯ 2) surface. For bidentate corner-sharing complexes, the predicted most stable adsorption configurations display interatomic distances in good agreement with EXAFS-derived data (ie As–Fe distances of∼ 3.3 Å). Our calculations also suggest that edge-sharing bidentate complexes can form on ideal (1 1¯ 2) hematite surfaces and do not necessarily involve step edges. These edge-sharing...
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Revised self-consistent continuum solvation in electronic-structure calculations

Oliviero Andreussi, Ismaila Dabo, Nicola Marzari
Journal of Chemical Physics 136, 064102 (2012)

Abstract:
The solvation model proposed by Fattebert and Gygi [J. Comput. Chem. 23, 662 (2002) 10.1002/jcc. 10069] and Scherlis et al.[J. Chem. Phys. 124, 074103 (2006) 10.1063/1.2168456] is reformulated, overcoming some of the numerical limitations encountered and extending its range of applicability. We first recast the problem in terms of induced polarization charges that act as a direct mapping of the self-consistent continuum dielectric; this allows to define a functional form for the dielectric that is well behaved both in the high-density region of the nuclear charges and in the low-density region where the electronic wavefunctions decay into the solvent. Second, we outline an iterative procedure to solve the Poisson equation for the quantum fragment embedded in the solvent that does not require multigrid algorithms, is trivially parallel, and can be applied to any Bravais...
Publication Icon Publication (2011)

Electronic levels and electrical response of periodic molecular structures from plane-wave orbital-dependent calculations

Yanli Li and Ismaila Dabo
Physical Review B 84, 155127 (2011)

Abstract:
Abstract Plane-wave electronic-structure predictions based upon orbital-dependent density- functional theory (OD-DFT) approximations, such as hybrid density-functional methods and self-interaction density-functional corrections, are severely affected by computational inaccuracies in evaluating electron interactions in the plane-wave representation. These errors arise from divergence singularities in the plane-wave summation of electrostatic and exchange interaction contributions. Auxiliary-function corrections are reciprocal-space countercharge corrections that cancel plane-wave singularities through the addition of an auxiliary function to the point-charge electrostatic kernel that enters into the expression of interaction terms. At variance with real-space countercharge corrections that are employed in the context of density-functional theory (DFT), reciprocal-space corrections are...
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Koopmans’ condition for density-functional theory

Ismaila Dabo, Andrea Ferretti, Nicola Poilvert, Yanli Li, Nicola Marzari, Matteo Cococcioni
Physical Review B 82, 115121 (2010)

Abstract:
Abstract In approximate Kohn-Sham density-functional theory, self-interaction manifests itself as the dependence of the energy of an orbital on its fractional occupation. This unphysical behavior translates into qualitative and quantitative errors that pervade many fundamental aspects of density-functional predictions. Here, we first examine self-interaction in terms of the discrepancy between total and partial electron removal energies, and then highlight the importance of imposing the generalized Koopmans' condition—that identifies orbital energies as opposite total electron removal energies—to resolve this discrepancy. In the process, we derive a correction to approximate functionals that, in the frozen-orbital approximation, eliminates the unphysical occupation dependence of orbital energies up to the third order in the single-particle densities. This non-Koopmans correction brings...
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Ab-initio electrochemical properties of electrode surfaces

Ismaila Dabo, Yanli Li, Nicephore Bonnet, Nicola Marzari
in Fuel Cell Science: Theory, Fundamentals, and Biocatalysis
edited by A. Wieckowski and J. K. Nørskov, 415-43 (Wiley, 2010)

Abstract:
The electrical response of electrochemical convertors (eg, fuel cells, batteries, electrochemical capacitors) is to a large extent governed by polarization and charge separation phenomena that take place at the electrode–electrolyte double-layer interface in the presence of an applied voltage. Therefore, predicting the electrical properties of electrochemical systems from first principles entails understanding the influence of the macroscopic voltage on the microscopic electronic and ionic charge distributions at the surface of the electrode. Despite considerable progress in large-scale quantum chemistry calculations, determining the potential dependence of the interfacial charge represents a challenging problem, due to the prohibitively large length scales that characterize electrostatic screening in the electrolyte. To overcome these limitations, we present a model of the electrode–electrolyte interface.
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Quantum-Espresso: a modular and open-source software project for quantum simulations of materials

Paolo Giannozzi, Stefano Baroni, Nicola Bonini, Matteo Calandra, Roberto Car, Carlo Cavazzoni, Davide Ceresoli, Guido L Chiarotti, Matteo Cococcioni, Ismaila Dabo, Andrea Dal Corso, Stefano De Gironcoli, Stefano Fabris, Guido Fratesi, Ralph Gebauer, Uwe Gerstmann, Christos Gougoussis, Anton Kokalj, Michele Lazzeri, Layla Martin-Samos, Nicola Marzari, Francesco Mauri, Riccardo Mazzarello, Stefano Paolini, Alfredo Pasquarello, Lorenzo Paulatto, Carlo Sbraccia, Sandro Scandolo, Gabriele Sclauzero, Ari P Seitsonen, Alexander Smogunov, Paolo Umari, Renata M Wentzcovitch
Journal of Physics: Condensed Matter 21, 395502 (2009)

Abstract:
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic- structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel...
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Electrostatics in periodic boundary conditions and real-space corrections

Ismaila Dabo, Boris Kozinsky, Nicholas E Singh-Miller, Nicola Marzari
Physical Review B 77, 115139 (2008)

Abstract:
We address periodic-image errors arising from the use of periodic boundary conditions to describe systems that do not exhibit full three-dimensional periodicity. The difference between the periodic potential, as straightforwardly obtained from a Fourier transform, and the potential satisfying any other boundary conditions can be characterized analytically. In light of this observation, we present an efficient real-space method to correct periodic-image errors, based on a multigrid solver for the potential difference, and demonstrate that excellent convergence of the energy with respect to cell size can be achieved in practical calculations. Additionally, we derive rapidly convergent expansions for determining the Madelung constants of point-charge assemblies in one, two, and three dimensions.
Publication Icon Publication (2007)

Vibrational recognition of adsorption sites for CO on platinum and platinum-ruthenium surfaces

Ismaila Dabo, Andrzej Wieckowski, Nicola Marzari
Journal of the American Chemical Society 129, 11045-11052 (2007)

Abstract:
We have studied the vibrational properties of CO adsorbed on platinum and platinum− ruthenium surfaces using density-functional perturbation theory within the Perdew− Burke− Ernzerhof generalized-gradient approximation. The calculated C− O stretching frequencies are found to be in excellent agreement with spectroscopic measurements. The frequency shifts that take place when the surface is covered with ruthenium monolayers are also correctly predicted. This agreement for both shifts and absolute vibrational frequencies is made more remarkable by the frequent failure of local and semilocal exchange-correlation functionals in predicting the stability of the different adsorption sites for CO on transition metal surfaces. We have investigated the chemical origin of the C−O frequency shifts introducing an orbital-resolved analysis of the force and frequency density of states, and assessed the effect of donation and backdonation on the CO vibrational frequency using a GGA + molecular U approach. These findings rationalize and establish the accuracy of density-functional calculations in predicting absolute vibrational frequencies, notwithstanding the failure in determining relative adsorption energies, in the strong chemisorption regime.