Publications

Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2019)

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.
Publication Icon Publication (2018)

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.
Publication Icon Publication (2018)

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.
Publication Icon Publication (2018)

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.
Publication Icon Publication (2018)

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...
Publication Icon Publication (2017)

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.
Publication Icon Publication (2017)

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.
Publication Icon Publication (2017)

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.
Publication Icon Publication (2017)

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...
Publication Icon Publication (2014)

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...
Publication Icon Publication (2014)

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.
Publication Icon Publication (2014)

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.
Publication Icon Publication (2013)

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.
Publication Icon Publication (2012)

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...
Publication Icon Publication (2010)

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...
Publication Icon Publication (2008)

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.