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Unveiling the origin of the capacity fade in MnO$_{2}$ zinc-ion battery cathodes through an analysis of the Mn vacancy formation
Authors:
Caio M. Miliante,
Kevin J. Sanders,
Liam J. McGoldrick,
Nicola Seriani,
Brian D. Adams,
Gillian R. Goward,
Drew Higgins,
Oleg Rubel
Abstract:
Currently explored rechargeable aqueous zinc-ion battery (RAZIB) cathode materials, such as $α$-MnO$_{2}$, suffer from severe capacity fade when cycling at rates appropriate for grid-scale operation. Mn dissolution has been previously identified as the cause of $α$-MnO$_{2}$ cathode degradation during RAZIB cycling, with conflicting evidence being found in support of the proposed Jahn-Teller effec…
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Currently explored rechargeable aqueous zinc-ion battery (RAZIB) cathode materials, such as $α$-MnO$_{2}$, suffer from severe capacity fade when cycling at rates appropriate for grid-scale operation. Mn dissolution has been previously identified as the cause of $α$-MnO$_{2}$ cathode degradation during RAZIB cycling, with conflicting evidence being found in support of the proposed Jahn-Teller effect-assisted charge disproportionation reaction as the mechanism behind Mn dissolution. In order to unveil the Mn dissolution mechanism in MnO$_{2}$ cathode cells under RAZIB operation conditions, the energetic feasibility for Mn vacancy formation was probed in both charged (MnO$_{2}$) and discharged (ZnMn$_{2}$O$_{4}$) phases of $α$ and $λ$ polymorphs of MnO$_{2}$ using density functional theory. The formation of a Mn vacancy, and consequently the dissolution of Mn as Mn$^{2+}_{(aq)}$, was found to be thermodynamically feasible for the $α$-ZnMn$_{2}$O$_{4}$ phase due to the energetically unfavourable Zn bent coordination formed during the Zn$^{2+}$ intercalation process, indicating that Mn dissolution is promoted by an unstable Zn coordination environment. The theoretical calculations were then corroborated by operando $^{1}$H nuclear magnetic resonance experiments which captured the Mn dissolution occurring throughout the RAZIB discharge, with subsequent electrochemical deposition of the Mn atoms on the electrode during charge. The combined computational and experimental analysis reveals the critical role of defect energetics and coordination environment in driving active material dissolution, and consequently capacity fade, with the proposed mechanism also relevant for understanding cathode degradation in other intercalating ion battery chemistries.
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Submitted 12 August, 2026; v1 submitted 13 February, 2026;
originally announced February 2026.
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Computational discovery of cathode materials for rechargeable aqueous zinc-ion batteries
Authors:
Caio Miranda Miliante,
Yuzhen Deng,
Brian D. Adams,
Drew Higgins,
Oleg Rubel
Abstract:
Rechargeable aqueous zinc-ion batteries (RAZIBs) attract major interest for deployment in grid-scale energy storage due to higher safety and lower cost when compared to lithium-ion batteries. However, currently studied cathode materials suffer from capacity fade when cycling at rates appropriate for grid-scale applications ($<$ C/2). To address the present limitation on cathode material availabili…
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Rechargeable aqueous zinc-ion batteries (RAZIBs) attract major interest for deployment in grid-scale energy storage due to higher safety and lower cost when compared to lithium-ion batteries. However, currently studied cathode materials suffer from capacity fade when cycling at rates appropriate for grid-scale applications ($<$ C/2). To address the present limitation on cathode material availability, more than 2000 previously synthesized oxides, chalcogenides, Prussian blue analogues, and polyanion materials were computationally screened for the discovery of highly stable RAZIB cathode materials. The structural, electrochemical, and chemical properties of the materials were respectively evaluated through an investigation of the available Zn$^{2+}$ percolation paths, the stability of the material in aqueous media under RAZIB operation conditions, and the attained transition metal oxidation state during cycling. The transition metal oxidation state and intercalating ion coordination environment were determined to govern the magnitude of the calculated Zn$^{2+}$ intercalation potential, with this finding guiding the development of batteries with high operation voltages. 12 materials previously unexplored as cathodes for RAZIBs were identified to have promising operational properties as cathodes, such as high Zn$^{2+}$ (de)intercalation potential, electrochemical stability, theoretical gravimetric capacity, and energy density. Finally, $α$-FePO$_{4}$ was experimentally tested as a RAZIB cathode, with a main redox peak observed from cyclic voltammetry matching previous results for amorphous FePO$_{4}$ as a cathode for RAZIB. However, the subpar charge storage performance highlights the necessity of further experimental investigations. Overall, the materials identified in this study present a guide for the experimental development of stable next-generation cathode materials for RAZIBs.
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Submitted 2 June, 2026; v1 submitted 10 February, 2026;
originally announced February 2026.
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Unfolding the kagome lattice to improve understanding of ARPES in CoSn
Authors:
Véronique Brouet,
Aaditya Vedant,
Francois Bertran,
Patrick Le Fèvre,
Oleg Rubel
Abstract:
Metallic kagome lattices are attracting significant attention as they provide a platform to explore the interplay between topology and magnetism. Angle-resolved photoemission spectroscopy (ARPES) plays a key role in unraveling their electronic structure. However, the analysis is often challenging due to the presence of multiple bands near the Fermi level. Indeed, each orbital generates three bands…
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Metallic kagome lattices are attracting significant attention as they provide a platform to explore the interplay between topology and magnetism. Angle-resolved photoemission spectroscopy (ARPES) plays a key role in unraveling their electronic structure. However, the analysis is often challenging due to the presence of multiple bands near the Fermi level. Indeed, each orbital generates three bands in a kagome lattice due to its three sites motif, which soon becomes complicated if many orbitals are present. To address this complexity, using ARPES matrix elements can be highly beneficial. First, band symmetry can be determined through selection rules based on light polarization. We emphasize that, in kagome lattices, as in all multi-site lattices, symmetry of the Bloch state is not only determined by the orbital character but also by the relative phase between the three sublattices. Additionally, interference between the three sublattices leads to a strong modulation of ARPES intensity across neighboring Brillouin zones. We show how unfolded band calculations capture these modulations, helping with band identification. We apply these ideas to CoSn, whose simple structure retains the key features of a kagome lattice. Using polarization dependent ARPES in several Brillouin zones, we isolate the dispersion of each band and discuss novel correlation effects, selectively renormalizing the bands crossing the Fermi level and shifting the others.
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Submitted 8 September, 2025; v1 submitted 2 June, 2025;
originally announced June 2025.
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Efficiency of band edge optical transitions of 2D monolayer materials: A high-throughput computational study
Authors:
A. F. Gómez-Bastidas,
Karthik Sriram,
A. C. Garcia-Castro,
Oleg Rubel
Abstract:
We performed high-throughput density functional theory calculations of optical matrix elements between band edges across a diverse set of non-magnetic two-dimensional monolayers with direct band gaps. Materials were ranked as potential optical emitters, leading to the identification of transition-metal nitrogen halides (ZrNCl, TiNBr, TiNCl) and bismuth chalcohalides (BiTeCl) with optical coupling…
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We performed high-throughput density functional theory calculations of optical matrix elements between band edges across a diverse set of non-magnetic two-dimensional monolayers with direct band gaps. Materials were ranked as potential optical emitters, leading to the identification of transition-metal nitrogen halides (ZrNCl, TiNBr, TiNCl) and bismuth chalcohalides (BiTeCl) with optical coupling comparable to or exceeding MoS$_2$. Despite strong in-plane dipole transitions, most two-dimensional materials underperform bulk semiconductors due to the absence of out-of-plane components. To elucidate interband transitions, we introduced the orbital overlap tensor and established a correlation between anomalous Born effective charges and optical coupling, linking charge redistribution to transition strength. We also identified chalcogen-mediated $d$-$d$ transition as a key mechanism enabling optical responses in transition-metal dichalcogenides. We derived an analytical radiative recombination model incorporating multi-valley effects and found that excitonic corrections are essential for accurate lifetime predictions. Some direct-gap materials exhibit dark excitons as their lowest-energy states, classifying them as quasi-direct band gap semiconductors, which is critical for tuning excitonic recombination dynamics.
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Submitted 22 February, 2025; v1 submitted 26 September, 2024;
originally announced September 2024.
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Defect tolerance of lead-halide perovskite (100) surface relative to bulk: band bending, surface states, and characteristics of vacancies
Authors:
Oleg Rubel,
Xavier Rocquefelte
Abstract:
We characterized the formation of vacancies at a surface slab model and contrasted the results with the bulk of lead-halide perovskites using cubic and tetragonal CsPbI$_3$ as representative structures. The defect-free CsI-terminated (100) surface does not trap charge carriers. In the presence of defects (vacancies), the surface is expected to exhibit $p$-type behavior. The formation energy of ces…
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We characterized the formation of vacancies at a surface slab model and contrasted the results with the bulk of lead-halide perovskites using cubic and tetragonal CsPbI$_3$ as representative structures. The defect-free CsI-terminated (100) surface does not trap charge carriers. In the presence of defects (vacancies), the surface is expected to exhibit $p$-type behavior. The formation energy of cesium vacancies $V_\text{Cs}^{-}$ is lower at the surface than in the bulk, while iodine vacancies $V_\text{I}^{+}$ have a similar energy (around 0.25$-$0.4 eV) within the range of chemical potentials compatible with solution processing synthesis conditions. Lead-iodine divacancies ($V_\text{PbI}^{-}$) are expected to dominate over lead-only vacancies at the surfaces. Major surface vacancies create shallow host-like energy states with a small Franck-Condon shift, making them electronically harmless (same as in bulk). The spin-orbit coupling contributes to the defect tolerance of lead-halide perovskite surfaces by causing delocalization of electronic states associated with $n$-type defects and retraction of lowest unoccupied states from the surface due to a mixing of Pb-$p_{x,y,z}$ orbitals. These results explain a high optoelectronic performance of two-dimensional structures, nanoparticles, and polycrystalline thin films of lead-halide perovskites despite the abundance of interfaces in these materials.
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Submitted 19 February, 2024; v1 submitted 9 September, 2023;
originally announced September 2023.
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Application of murexide as a capping agent for fabrication of magnetite anodes for supercapacitors: experimental and first-principle studies
Authors:
Coulton Boucher,
Igor Zhitomirsky,
Oleg Rubel
Abstract:
In this study, we investigate the effectiveness of murexide for surface modification of Fe$_3$O$_4$ nanoparticles to enhance the performance of multi-walled carbon nanotube-Fe$_3$O$_4$ supercapacitor anodes. Our experimental results demonstrate significant improvements in electrode performance when murexide is used as a capping or dispersing agent compared to the case with no additives. When murex…
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In this study, we investigate the effectiveness of murexide for surface modification of Fe$_3$O$_4$ nanoparticles to enhance the performance of multi-walled carbon nanotube-Fe$_3$O$_4$ supercapacitor anodes. Our experimental results demonstrate significant improvements in electrode performance when murexide is used as a capping or dispersing agent compared to the case with no additives. When murexide is used as a capping agent, we report a capacitance of 4.2 F cm$^{-2}$ from cyclic voltammetry analysis with good capacitance retention at high scan rate. From impedance measurements, we reveal a substantial decrease in the real part of impedance for samples prepared with murexide, indicating easier charge transfer at more negative electrode potentials, and reinforcing the role of murexide as a capping agent and charge transfer mediator. Density functional theory is used to investigate interactions between the murexide adsorbate and the Fe$_3$O$_4$ (001) surface, with a specific emphasis on adsorption strength, charge transfer, and electronic properties. This theoretical investigation uncovers a strong adsorption enthalpy of -4.5 eV, and allows us to identify the nature of chemical bonds between murexide and the surface, with significant charge transfer taking place between the Fe$_3$O$_4$ surface and murexide adsorbate. The transfer of electrons from the Fe$_3$O$_4$ surface to murexide is recognized as a vital component of the adsorption process. By examining the bonding nature of murexide on Fe$_3$O$_4$, this research study uncovers insights and proposes a novel bonding configuration of murexide that incorporates a combination of bridging and chelating bonding.
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Submitted 28 July, 2023;
originally announced July 2023.
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How to verify the precision of density-functional-theory implementations via reproducible and universal workflows
Authors:
Emanuele Bosoni,
Louis Beal,
Marnik Bercx,
Peter Blaha,
Stefan Blügel,
Jens Bröder,
Martin Callsen,
Stefaan Cottenier,
Augustin Degomme,
Vladimir Dikan,
Kristjan Eimre,
Espen Flage-Larsen,
Marco Fornari,
Alberto Garcia,
Luigi Genovese,
Matteo Giantomassi,
Sebastiaan P. Huber,
Henning Janssen,
Georg Kastlunger,
Matthias Krack,
Georg Kresse,
Thomas D. Kühne,
Kurt Lejaeghere,
Georg K. H. Madsen,
Martijn Marsman
, et al. (20 additional authors not shown)
Abstract:
In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a firs…
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In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).
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Submitted 26 May, 2023;
originally announced May 2023.
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Band unfolding with a general transformation matrix: from code implementation to interpretation of photoemission spectra
Authors:
Oleg Rubel,
Jean-Baptiste Moussy,
Paul Foulquier,
Véronique Brouet
Abstract:
Unfolding of a supercell band structure into a primitive Brillouin zone is important for understanding implications of structural distortions, disorder, defects, solid solutions on materials electronic structure. Necessity of the band unfolding is also recognised in interpretation of angle-resolved photoemission spectroscopy (ARPES) measurements. We describe an extension of the fold2Bloch package…
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Unfolding of a supercell band structure into a primitive Brillouin zone is important for understanding implications of structural distortions, disorder, defects, solid solutions on materials electronic structure. Necessity of the band unfolding is also recognised in interpretation of angle-resolved photoemission spectroscopy (ARPES) measurements. We describe an extension of the fold2Bloch package by implementing an arbitrary transformation matrix used to establish a relation between primitive cell and supercell. This development allows us to overcome limitations of supercells constructed exclusively by scaling of primitive cell lattice vectors. It becomes possible to transform between primitive and conventional cells as well as include rotations. The fold2Bloch is publicaly available from a GitHub repository as a FORTRAN code. It interfaces with the all-electron full-potential WIEN2k and the pseudopotential VASP density functional theory packages. The fold2Bloch is supplemented by additional pre- and post-processing utilities that aid in generating k points in the supercell (such that they later fall onto a desired path in the primitive Brillouin zone after unfolding) and plotting the unfolded band structure. We selected Sr$_2$IrO$_4$ as an illustrative example and, for the first time, present its properly unfolded band structure in direct comparison with ARPES measurements. In addition, critical importance of the band unfolding for interpretation of SrIrO$_3$ ARPES data is illustrated and discussed as a perspective.
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Submitted 6 January, 2023;
originally announced January 2023.
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Computational screening of cathode materials for Zn-ion rechargeable batteries
Authors:
Sriram Anand,
Caio Miranda Miliante,
Storm Gourley,
Brian D. Adams,
Drew Higgins,
Oleg Rubel
Abstract:
We propose a comprehensive set of indicators (including methods to obtain and analyse them) for computational screening of candidate cathode materials for rechargeable Zn-ion aqueous batteries relying on Zn$^{2+}$ intercalation processes. The indicators capture feasibility of Zn$^{2+}$ intercalation and transport within the material, the thermodynamic stability of charged and discharged material s…
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We propose a comprehensive set of indicators (including methods to obtain and analyse them) for computational screening of candidate cathode materials for rechargeable Zn-ion aqueous batteries relying on Zn$^{2+}$ intercalation processes. The indicators capture feasibility of Zn$^{2+}$ intercalation and transport within the material, the thermodynamic stability of charged and discharged material structures, electrochemical stability of the cathode material and electrolyte, volume expansion, and energy storage capacity. The approach was applied to well-known cathode materials ($α$-MnO$_2$ and V$_2$O$_5$) as well as some potential alternatives (MoS$_2$, ZrP$_2$O$_7$, MoO$_3$, and FeO$_2$) to demonstrate the screening workflow and the decision making process. We show that selection of cathode materials for Zn-ion aqueous rechargeable batteries is a multifaceted problem, and first principle calculations can help to narrow down the search. Despite us being unable to identify a particularly successful cathode material, tools and techniques developed in this work can be applied more broadly to screen a wider array of potential material compositions and structures, with the goal of identifying next generation cathode materials for aqueous rechargeable batteries with the intercalation energy storage mechanism not limited to Zn$^{2+}$ ions.
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Submitted 16 April, 2023; v1 submitted 10 December, 2022;
originally announced December 2022.
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Electrochemical stability of ZnMn2O4: Understanding Zn-ion rechargeable battery capacity and degradation
Authors:
Oleg Rubel,
Thuy Nguyen Thanh Tran,
Storm Gourley,
Sriram Anand,
Andrew Van Bommel,
Brian D. Adams,
Douglas G. Ivey,
Drew Higgins
Abstract:
We present a refined Mn-Zn-H$_2$O Pourbaix diagram with the emphasis on parameters relevant for the Zn/MnO$_2$ rechargeable cells. It maps out boundaries of electrochemical stability for MnO$_2$, ZnMn$_2$O$_4$, ZnMn$_3$O$_7$, and MnOOH. The diagram helps to rationalize experimental observation on processes and phases occurring during charge/discharge, including the position of charge/discharge red…
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We present a refined Mn-Zn-H$_2$O Pourbaix diagram with the emphasis on parameters relevant for the Zn/MnO$_2$ rechargeable cells. It maps out boundaries of electrochemical stability for MnO$_2$, ZnMn$_2$O$_4$, ZnMn$_3$O$_7$, and MnOOH. The diagram helps to rationalize experimental observation on processes and phases occurring during charge/discharge, including the position of charge/discharge redox peaks and capacity fade observed in rechargeable aqueous Zn-ion batteries for stationary storage. The proposed Pourbaix diagram is validated by observing the pH-dependent transformation of electrolytic manganese dioxide to hetaerolite and chalcophanite during discharge and charge, respectively. Our results can guide the selection of operating conditions (the potential range and pH) for existing aqueous Zn/MnO$_2$ rechargeable cells to maximise their longevity. In addition, the relation between electrochemical stability boundaries and operating conditions can be used as an additional design criterion in exploration of future cathode materials for aqueous rechargeable batteries.
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Submitted 3 June, 2022; v1 submitted 18 March, 2022;
originally announced March 2022.
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Length-gauge optical matrix elements in WIEN2k
Authors:
Oleg Rubel,
Peter Blaha
Abstract:
Hybrid exchange-correlation functionals provide superior electronic structure and optical properties of semiconductors or insulators as compared to semilocal exchange-correlation potentials due to admixing a portion of the non-local exact exchange potential from a Hartree-Fock theory. Since the non-local potential does not commute with the position operator, the momentum matrix elements do not ful…
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Hybrid exchange-correlation functionals provide superior electronic structure and optical properties of semiconductors or insulators as compared to semilocal exchange-correlation potentials due to admixing a portion of the non-local exact exchange potential from a Hartree-Fock theory. Since the non-local potential does not commute with the position operator, the momentum matrix elements do not fully capture the oscillator strength, while the length-gauge velocity matrix elements do. So far, length-gauge velocity matrix elements were not accessible in the all-electron full-potential WIEN2k package. We demonstrate the feasibility of computing length-gauge matrix elements in WIEN2k for a hybrid exchange-correlation functional based on a finite difference approach. To illustrate the implementation we determined matrix elements for optical transitions between the conduction and valence bands in GaAs, GaN, (CH$_3$NH$_3$)PbI$_3$ and a monolayer MoS$_2$. The non-locality of the Hartree-Fock exact exchange potential leads to a strong enhancement of the oscillator strength as noticed recently in calculations employing pseudopotentials [Laurien and Rubel: arXiv:2111.14772 (2021)]. We obtained an analytical expression for the enhancement factor in terms of the difference in eigenvalues not captured by the kinetic energy. It is expected that these results can also be extended to other non-local potentials, e.g., a many-body $GW$ approximation.
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Submitted 29 January, 2022; v1 submitted 30 December, 2021;
originally announced December 2021.
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Benchmarking exchange-correlation potentials with the mstar60 dataset: Importance of the nonlocal exchange potential for effective mass calculations in semiconductors
Authors:
Magdalena Laurien,
Oleg Rubel
Abstract:
The accuracy of effective masses predicted by density functional theory depends on the exchange-correlation functional employed, with nonlocal hybrid functionals giving more accurate results than semilocal functionals. In this article, we benchmark the performance of the Perdew-Burke-Ernzerhof (PBE), Tran-Blaha modified Becke-Johnson (TB-mBJ), and the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchang…
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The accuracy of effective masses predicted by density functional theory depends on the exchange-correlation functional employed, with nonlocal hybrid functionals giving more accurate results than semilocal functionals. In this article, we benchmark the performance of the Perdew-Burke-Ernzerhof (PBE), Tran-Blaha modified Becke-Johnson (TB-mBJ), and the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functionals and potentials for the calculation of effective masses with perturbation theory. We introduce the mstar60 dataset, which contains 60 effective masses derived from 18 semiconductors. The ratio between experimental and calculated effective masses is $1.70 \pm 0.20$ for PBE, $0.76 \pm 0.04$ for TB-mBJ, $0.99 \pm 0.04$ for HSE06. We reveal that the nonlocal exchange in HSE06 enlarges the optical transition matrix elements leading to the superior accuracy of the hybrid functional in the calculation of effective masses. The omission of nonlocal exchange in the transition operator for HSE leads to serious errors. For the semilocal PBE functional, the errors in the bandgap and the optical transition matrix elements partially cancel out in the calculation of effective masses. The TB-mBJ functional yields PBE-like matrix elements paired with realistic bandgaps leading to a consistent overestimation of effective masses. However, if only limited computational resources are available, experimental masses can be estimated by multiplying TB-mBJ masses with the factor of 0.76. We then compare effective masses of transition metal dichalcogenide bulk and monolayer materials: we show that changes in the matrix elements are important in understanding the layer-dependent effective mass renormalization.
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Submitted 6 May, 2022; v1 submitted 29 November, 2021;
originally announced November 2021.
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Band alignment of monolayer CaP$_3$, CaAs$_3$, BaAs$_3$ and the role of $p$-$d$ orbital interactions in the formation of conduction band minima
Authors:
Magdalena Laurien,
Himanshu Saini,
Oleg Rubel
Abstract:
Recently, a number of new two-dimensional (2D) materials based on puckered phosphorene and arsenene have been predicted with moderate band gaps, good absorption properties and carrier mobilities superior to transition metal dichalcogenides. For heterojunction applications, it is important to know the relative band alignment of these new 2D materials. We report the band alignment of puckered CaP…
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Recently, a number of new two-dimensional (2D) materials based on puckered phosphorene and arsenene have been predicted with moderate band gaps, good absorption properties and carrier mobilities superior to transition metal dichalcogenides. For heterojunction applications, it is important to know the relative band alignment of these new 2D materials. We report the band alignment of puckered CaP$_3$, CaAs$_3$ and BaAs$_3$ monolayers at the quasiparticle level of theory (G$_0$W$_0$), calculating band offsets for isolated monolayers according to the electron affinity rule. Our calculations suggest that monolayer CaP$_3$, CaAs$_3$ and BaAs$_3$ all form type-II (staggered) heterojunctions. Their quasiparticle gaps are 2.1 (direct), 1.8 (direct) and 1.5 eV (indirect), respectively. We also examine trends in the electronic structure in the light of chemical bonding analysis. We show that the indirect band gap in monolayer BaAs$_3$ is caused by relatively strong As $3p$ - Ba $5d$ bonding interactions that stabilize the conduction band away from the $Γ$ point between $Γ$ and $S$.
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Submitted 3 December, 2020;
originally announced December 2020.
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WloopPHI: A tool for ab initio characterization of Weyl semimetals
Authors:
Himanshu Saini,
Magdalena Laurien,
Peter Blaha,
Oleg Rubel
Abstract:
WloopPHI is a Python code that expands the features of WIEN2k, a full-potential all-electron density functional theory package, by the characterization of Weyl semimetals. It enables the calculation of the chirality (or "monopole charge") associated with Weyl nodes and nodal lines. The theoretical methodology for the calculation of the chirality is based on an extended Wilson loop method and a Ber…
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WloopPHI is a Python code that expands the features of WIEN2k, a full-potential all-electron density functional theory package, by the characterization of Weyl semimetals. It enables the calculation of the chirality (or "monopole charge") associated with Weyl nodes and nodal lines. The theoretical methodology for the calculation of the chirality is based on an extended Wilson loop method and a Berry phase approach. We validate the code using TaAs, which is a well-characterized Weyl semimetal, both theoretically and experimentally. Afterwards, we applied the method to the characterization of YRh$_6$Ge$_4$ and found two sets of Weyl points (ca. 0.2 eV below the Fermi energy) together with a topological nodal line (protected by mirror symmetry) crossing the Fermi energy and mapped their chiralities.
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Submitted 8 September, 2021; v1 submitted 18 August, 2020;
originally announced August 2020.
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Perturbation approach to ab initio effective mass calculations
Authors:
Oleg Rubel,
Fabien Tran,
Xavier Rocquefelte,
Peter Blaha
Abstract:
A degenerate perturbation $k\cdot p$ approach for effective mass calculations is implemented in the all-electron density functional theory (DFT) package WIEN2k. The accuracy is tested on major group IVA, IIIA-VA, and IIB-VIA semiconductor materials. Then, the effective mass in graphene and CuI with defects is presented as illustrative applications. For states with significant Cu-d character additi…
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A degenerate perturbation $k\cdot p$ approach for effective mass calculations is implemented in the all-electron density functional theory (DFT) package WIEN2k. The accuracy is tested on major group IVA, IIIA-VA, and IIB-VIA semiconductor materials. Then, the effective mass in graphene and CuI with defects is presented as illustrative applications. For states with significant Cu-d character additional local orbitals with higher principal quantum numbers (more radial nodes) have to be added to the basis set in order to converge the results of the perturbation theory. Caveats related to a difference between velocity and momentum matrix elements are discussed in the context of application of the method to non-local potentials, such as Hartree-Fock/DFT hybrid functionals and DFT+U.
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Submitted 7 July, 2020;
originally announced July 2020.
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Electronic properties of Pb-I deficient lead halide perovskites
Authors:
Chao Zheng,
Oleg Rubel,
Mikaël Kepenekian,
Xavier Rocquefelte,
Claudine Katan
Abstract:
The electronic structure evolution of deficient halide perovskites with a general formula $(A,A')_{1+x}M_{1-x}X_{3-x}$ was investigated using the density functional theory. The focus is placed on characterization of changes in the band gap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties…
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The electronic structure evolution of deficient halide perovskites with a general formula $(A,A')_{1+x}M_{1-x}X_{3-x}$ was investigated using the density functional theory. The focus is placed on characterization of changes in the band gap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties of the defect corresponding to a $M\!-\!X$ vacancy filled with an $A'$ cation. This defect "repels" electrons and holes producing no trap states and, in moderate quantities ($x\le0.1$), does not hinder charge transport properties of the material. This behavior is rationalized using a confinement model and provides an additional insight to the defect tolerance of halide perovskites.
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Submitted 24 December, 2019; v1 submitted 11 September, 2019;
originally announced September 2019.
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Stability of aziridinium lead iodide perovskite: ring strain and water vulnerability
Authors:
Kangze Ren,
Chao Zheng,
Michael A. Brook,
Oleg Rubel
Abstract:
Recently, an aziridinium lead iodide perovskite was proposed as a possible solar cell absorber material. We investigated the stability of this material using a density-functional theory with an emphasis on the ring strain associated with the three-membered aziridinium cation. It is shown that the aziridinium ring is prone to opening within the PbI$_3$ environment. When exposed to moisture, aziridi…
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Recently, an aziridinium lead iodide perovskite was proposed as a possible solar cell absorber material. We investigated the stability of this material using a density-functional theory with an emphasis on the ring strain associated with the three-membered aziridinium cation. It is shown that the aziridinium ring is prone to opening within the PbI$_3$ environment. When exposed to moisture, aziridinium lead iodide can readily react with water. The resultant product will not likely be a stoichiometric lead halide perovskite structure.
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Submitted 27 August, 2019;
originally announced August 2019.
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Unraveling the water degradation mechanism of CH$_3$NH$_3$PbI$_3$
Authors:
Chao Zheng,
Oleg Rubel
Abstract:
Instability of perovskite photovoltaics is still a topic which is currently under intense debate, especially the role of water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) in explicit…
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Instability of perovskite photovoltaics is still a topic which is currently under intense debate, especially the role of water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) in explicit water. It is found that the initial dissolution of MAPbI$_3$ is a complex multi-step process triggered by the departure of I$^-$ ion from the CH$_3$NH$_3$I-terminated surface. Reconstruction of the free energy landscape indicates a low energy barrier for water dissolution of MAPbI$_3$. In addition, we propose a two-step thermodynamic cycle for MAPbI$_3$ dissolution in water at a finite concentration that renders a spontaneity of the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI$_3$ dissolution in water explain why the water immediately destroys pristine MAPbI$_3$. The dissolution thermodynamics of all-inorganic CsPbI$_3$ perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI$_3$.
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Submitted 27 August, 2019; v1 submitted 29 May, 2019;
originally announced May 2019.
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Exploration of the bright and dark exciton landscape and fine structure of MoS$_2$ (using G$_0$W$_0$-BSE)
Authors:
Hongyu Yu,
Magdalena Laurien,
Zhenpeng Hu,
Oleg Rubel
Abstract:
Spectral ordering between dark and bright excitons in transition metal dichalcogenides is of increasing interest for optoelectronic applications. However, little is known about dark exciton energies and their binding energies. We report the exciton landscape including momentum-forbidden dark excitons of MoS$_{2}$ monolayer using single shot GW-Bethe Salpeter equation (G$_{0}$W$_{0}$-BSE) calculati…
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Spectral ordering between dark and bright excitons in transition metal dichalcogenides is of increasing interest for optoelectronic applications. However, little is known about dark exciton energies and their binding energies. We report the exciton landscape including momentum-forbidden dark excitons of MoS$_{2}$ monolayer using single shot GW-Bethe Salpeter equation (G$_{0}$W$_{0}$-BSE) calculations. We find the lowest-energy exciton to be indirect at ($\textrm K'_{v} \rightarrow \textrm K_{c}$) in agreement with recent GdW-BSE calculations [2D Mater. 6, 035003 (2019)]. We also find that by large, dark exciton binding energies ($E_b$) scale with the quasiparticle energies ($E_g$) according to the $E_b/E_g=0.25$ rule. Differences in exciton binding energies are explained using an orbital theory.
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Submitted 27 August, 2019; v1 submitted 17 April, 2019;
originally announced April 2019.
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Bimolecular theory of non-radiative recombination in semiconductors with disorder
Authors:
Oleg Rubel
Abstract:
The original Shockley-Read-Hall recombination statistics is extended to include recombination of localized excitations. The recombination is treated as a bimolecular process rather than a monomolecular recombination of excitons. The emphasis is placed on an interplay between two distinct channels of radiative recombination (shallow localized states vs extended states) mediated by trapping of photo…
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The original Shockley-Read-Hall recombination statistics is extended to include recombination of localized excitations. The recombination is treated as a bimolecular process rather than a monomolecular recombination of excitons. The emphasis is placed on an interplay between two distinct channels of radiative recombination (shallow localized states vs extended states) mediated by trapping of photogenerated charge carriers by non-radiative centers. Results of a numerical solution for a given set of parameters are complemented by an approximate analytical expression for the thermal quenching of the photoluminescence intensity in non-degenerate semiconductors derived in the limit of low pump intensities. The merit of a popular double-exponential empirical function for fitting the thermal quenching of the photoluminescence intensity is critically examined.
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Submitted 12 April, 2019;
originally announced April 2019.
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Electronic band structure of nitrogen diluted Ga(PAsN): Formation of the intermediate band, direct and indirect optical transitions, localization of states
Authors:
Maciej P. Polak,
Robert Kudrawiec,
Oleg Rubel
Abstract:
The electronic band structure of Ga(PAsN) with a few percent of nitrogen is calculated in the whole composition of Ga(PAs) host using the state-of-the-art density functional methods including the modified Becke-Johnson functional to correctly reproduce the band gap, and band unfolding to reveal the character of the bands within the entire Brillouin zone. As expected, relatively small amounts of ni…
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The electronic band structure of Ga(PAsN) with a few percent of nitrogen is calculated in the whole composition of Ga(PAs) host using the state-of-the-art density functional methods including the modified Becke-Johnson functional to correctly reproduce the band gap, and band unfolding to reveal the character of the bands within the entire Brillouin zone. As expected, relatively small amounts of nitrogen introduced to Ga(PAs) lead to formation of an intermediate band below the conduction band which is consistent with the band anticrossing model, widely used to describe the electronic band structure of dilute nitrides. However, in this study calculations are performed in the whole Brillouin zone and reveal the significance of correct description of the band structure near the edges of Brillouin zone, especially for indirect band gap P-rich host alloy, which may not be properly captured with simpler models. The theoretical results are compared with experimental studies, confirming their reliability. The influence of nitrogen on the band structure is discussed in terms of application of Ga(PAsN) in optoelectronic devices such as intermediate band solar cells and light emitters. It is found that Ga(PAsN) with low N and As concentration has a band structure suitable for integration in Si tandem solar cells, since the lattice mismatch between Si and Ga(PAsN) is small in this case. Moreover, it is concluded that P-rich Ga(PAsN) alloys with low N concentration have a promising band structure for two colour emitters. Additionally, the effect of nitrogen incorporation on the carrier localization is studied and discussed.
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Submitted 30 March, 2019;
originally announced April 2019.
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Pressure dependence of direct optical transitions in ReS2 and ReSe2
Authors:
R. Oliva,
M. Laurien,
F. Dybala,
J. Kopaczek,
Y. Quin,
S. Tongay,
O. Rubel,
R. Kudrawiec
Abstract:
We present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with th…
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We present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with those predicted by the calculations. We assign the transitions to the Z k-point of the Brillouin zone and other k-points located away from highsymmetry points. The origin of the pressure coefficients of the measured direct transitions is discussed in terms of orbital analysis of the electronic structure and van der Waals interlayer interaction. The anisotropic optical properties are studied at high pressure by means of polarization-resolved photoreflectance measurements.
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Submitted 7 December, 2018;
originally announced December 2018.
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Giant spontaneous Hall effect in a nonmagnetic Weyl-Kondo semimetal
Authors:
Sami Dzsaber,
Xinlin Yan,
Mathieu Taupin,
Gaku Eguchi,
Andrey Prokofiev,
Toni Shiroka,
Peter Blaha,
Oleg Rubel,
Sarah E. Grefe,
Hsin-Hua Lai,
Qimiao Si,
Silke Paschen
Abstract:
Nontrivial topology in condensed matter systems enriches quantum states of matter, to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry broken phases by Landau's order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion syst…
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Nontrivial topology in condensed matter systems enriches quantum states of matter, to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry broken phases by Landau's order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal Ce3Bi4Pd3 that is noncentrosymmetric but preserves time reversal symmetry. We attribute this finding to Weyl nodes - singularities of the Berry curvature - that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields, as well as its robust bulk nature may aid the exploitation in topological quantum devices.
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Submitted 1 February, 2021; v1 submitted 7 November, 2018;
originally announced November 2018.
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Stacking defects in GaP nanowires: Electronic structure and optical properties
Authors:
Divyanshu Gupta,
Nebile Isik Goktas,
Amit Rao,
Ray LaPierre,
Oleg Rubel
Abstract:
Formation of twin boundaries during the growth of semiconductor nanowires is very common. However, the effects of such planar defects on the electronic and optical properties of nanowires are not very well understood. Here, we use a combination of ab initio simulation and experimental techniques to study these effects. Twin boundaries in GaP are shown to act as an atomically-narrow plane of wurtzi…
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Formation of twin boundaries during the growth of semiconductor nanowires is very common. However, the effects of such planar defects on the electronic and optical properties of nanowires are not very well understood. Here, we use a combination of ab initio simulation and experimental techniques to study these effects. Twin boundaries in GaP are shown to act as an atomically-narrow plane of wurtzite phase with a type-I homostructure band alignment. Twin boundaries and stacking faults (wider regions of the wurtzite phase) lead to the introduction of shallow trap states observed in photoluminescence studies. These effects should have a profound impact on the efficiency of nanowire-based devices.
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Submitted 11 March, 2019; v1 submitted 2 October, 2018;
originally announced October 2018.
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Structural dynamics in hybrid halide perovskites: Bulk Rashba splitting, spin texture, and carrier localization
Authors:
Chao Zheng,
Shidong Yu,
Oleg Rubel
Abstract:
The extended carrier lifetime in hybrid halide perovskites was attributed to a quasi-indirect band gap that arises due to Rashba splitting in both conduction and valence band edges. In this paper, we present results for an effective relativistic band structure of (CH3NH3)PbI3 with the focus on the dispersion of electronic states near the band edges of (CH3NH3)PbI3 affected by thermal structural fl…
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The extended carrier lifetime in hybrid halide perovskites was attributed to a quasi-indirect band gap that arises due to Rashba splitting in both conduction and valence band edges. In this paper, we present results for an effective relativistic band structure of (CH3NH3)PbI3 with the focus on the dispersion of electronic states near the band edges of (CH3NH3)PbI3 affected by thermal structural fluctuations. We establish a relation between the magnitude of Rashba splitting and a deviation of Pb-atom from its centrosymmetric site position in the PbI6 octahedron. For the splitting energy to reach the thermal energy kT~26 meV (room temperature), the displacement should be of the order 0.3 Ang, which is far above the static displacements of Pb-atoms in the tetragonal phase of (CH3NH3)PbI3. The significant dynamic enhancement of the Rashba splitting observed at earlier simulation times (less than 2 ps) later weakens and becomes less than the thermal energy despite the average displacement of Pb-atoms remaining large (0.37 Ang). It is randomization of Pb-displacement vectors and associated cancelation of the net effective magnetic field acting on electrons at the conduction band edge is responsible for reduction of the Rashba splitting. The lattice dynamics also leads to deterioration of Bloch character for states in the valence band leading to subsequent localization of holes, which affects bipolar mobility of charge carriers in (CH3NH3)PbI3. These results call into question the quasi-indirect band gap as a reason for the long carrier lifetime observed in (CH3NH3)PbI3 at room temperature. An alternative mechanism involves dynamic localization of holes and their reduced overlap with electrons in reciprocal space.
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Submitted 4 November, 2018; v1 submitted 29 September, 2018;
originally announced October 2018.
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$μ$SR study of spin freezing and persistent spin dynamics in NaCaNi$_2$F$_7$
Authors:
Y. Cai,
M. N. Wilson,
A. M. Hallas,
L. Liu,
B. A. Frandsen,
S. R. Dunsiger,
J. W. Krizan,
R. J. Cava,
Y. J. Uemura,
O. Rubel,
G. M. Luke
Abstract:
A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($μ$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$μ$-F mu…
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A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($μ$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$μ$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ spins undergo spin freezing into a disordered ground state below 4~K, with a characteristic internal field strength of 140~G. Persistent Ni spin dynamics are present to our lowest temperatures (75~mK), a feature characteristic of many geometrically frustrated magnetic systems.
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Submitted 28 June, 2018;
originally announced June 2018.
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Kinetic mechanism for reversible structural transition in MoTe2 induced by excess charge carriers
Authors:
O. Rubel
Abstract:
Kinetic of a reversible structural transition between insulating (2H) and metallic (1T') phases in a monolayer MoTe2 due to an electrostatic doping is studied using first-principle calculations. The driving force for the structural transition is the energy gained by transferring excess electrons from the bottom of the conduction band to lower energy gapless states in the metallic phase as have bee…
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Kinetic of a reversible structural transition between insulating (2H) and metallic (1T') phases in a monolayer MoTe2 due to an electrostatic doping is studied using first-principle calculations. The driving force for the structural transition is the energy gained by transferring excess electrons from the bottom of the conduction band to lower energy gapless states in the metallic phase as have been noticed in earlier studies. The corresponding structural transformation involves dissociation of Mo-Te bonds (one per formula unit), which results in a kinetic energy barrier of 0.83 eV. The transformation involves a consecutive movement of atoms similar to a domain wall motion. The presence of excess charge carriers modifies not only the total energy of the initial and final states, but also lowers an energy of the transition state. An experimentally observed hysteresis in the switching process can be attributed to changes in the kinetic energy barrier due to its dependence on the excess carrier density.
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Submitted 25 April, 2018; v1 submitted 29 November, 2017;
originally announced November 2017.
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Aziridinium lead iodide: a stable, low bandgap hybrid halide perovskite for photovoltaics
Authors:
Chao Zheng,
Oleg Rubel
Abstract:
The low ionization energy of an $A$ site molecule is a very important factor, which determines the thermodynamical stability of $A$PbI$_3$ hybrid halide perovskites, while the size of the molecule governs the stable phase at room temperature and, eventually, the bandgap. It is challenging to achieve both a low ionization energy and the reasonable size for the PbI$_3$ cage to circumvent the stabili…
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The low ionization energy of an $A$ site molecule is a very important factor, which determines the thermodynamical stability of $A$PbI$_3$ hybrid halide perovskites, while the size of the molecule governs the stable phase at room temperature and, eventually, the bandgap. It is challenging to achieve both a low ionization energy and the reasonable size for the PbI$_3$ cage to circumvent the stability issue inherent to hybrid halide perovskites. Here we propose a new three-membered charged ring radical, which demonstrates a low ionization energy that renders a good stability for its corresponding perovskite and a reasonable cation size that translates into a suitable bandgap for the photovoltaic application. We use ab initio calculations to evaluate a polymorphism of the crystal structure of the proposed halide hybrid perovskite, its stability and electronic properties in comparison to the mainstream perovskites, such as the methylammonium and formamidinium lead iodide. Our results highlight the importance of van der Waals interactions for predicting a correct polymorphism of the perovskite vs hexagonal crystal structure.
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Submitted 15 November, 2017;
originally announced November 2017.
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Adsorption of maleic acid monomer on the surface of hydroxyapatite and TiO2: a pathway toward biomaterial composites
Authors:
Mitchell Albert,
Amanda Clifford,
Igor Zhitomirsky,
Oleg Rubel
Abstract:
Poly(styrene-alt-maleic acid) adsorption on hydroxyapatite and TiO2 (rutile) was studied using experimental techniques and complemented by \textit{ab initio} simulations of adsorption of a maleic acid segment as a subunit of the copolymer. Ab initio calculations suggest that the maleic acid segment forms a strong covalent bonding to the TiO2 and hydroxyapatite surfaces. If compared to vacuum, the…
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Poly(styrene-alt-maleic acid) adsorption on hydroxyapatite and TiO2 (rutile) was studied using experimental techniques and complemented by \textit{ab initio} simulations of adsorption of a maleic acid segment as a subunit of the copolymer. Ab initio calculations suggest that the maleic acid segment forms a strong covalent bonding to the TiO2 and hydroxyapatite surfaces. If compared to vacuum, the presence of a solvent significantly reduces the adsorption strength as the polarity of the solvent increases. The results of first-principle calculations are confirmed by the experimental measurements. We found that adsorbed poly(styrene-alt-maleic acid) allowed efficient dispersion of rutile and formation of films by the electrophoretic deposition. Moreover, rutile can be co-dispersed and co-deposited with hydroxyapatite to form composite films. The coatings showed an enhanced corrosion protection of metallic implants in simulated body fluid solutions, which opens new avenues for the synthesis, dispersion, and colloidal processing of advanced composite materials for biomedical applications.
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Submitted 7 August, 2018; v1 submitted 30 October, 2017;
originally announced November 2017.
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Alloying strategy for two-dimensional GaN optical emitters
Authors:
C. Pashartis,
O. Rubel
Abstract:
The recent progress in formation of two-dimensional (2D) GaN by a migration-enhanced encapsulated technique opens up new possibilities for group III-V 2D semiconductors with a band gap within the visible energy spectrum. Using first-principles calculations we explored alloying of 2D-GaN to achieve an optically active material with a tuneable band gap. The effect of isoelectronic III-V substitution…
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The recent progress in formation of two-dimensional (2D) GaN by a migration-enhanced encapsulated technique opens up new possibilities for group III-V 2D semiconductors with a band gap within the visible energy spectrum. Using first-principles calculations we explored alloying of 2D-GaN to achieve an optically active material with a tuneable band gap. The effect of isoelectronic III-V substitutional elements on the band gaps, band offsets, and spatial electron localization is studied. In addition to optoelectronic properties, the formability of alloys is evaluated using impurity formation energies. A dilute highly-mismatched solid solution 2D-GaN$_{1-x}$P$_x$ features an efficient band gap reduction in combination with a moderate energy penalty associated with incorporation of phosphorous in 2D-GaN, which is substantially lower than in the case of the bulk GaN. The group-V alloying elements also introduce significant disorder and localization at the valence band edge that facilitates direct band gap optical transitions thus implying the feasibility of using III-V alloys of 2D-GaN in light-emitting devices.
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Submitted 30 October, 2017; v1 submitted 14 July, 2017;
originally announced July 2017.
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Localization of electronic states in III-V semiconductor alloys: a comparative study
Authors:
C. Pashartis,
O. Rubel
Abstract:
Electronic properties of III-V semiconductor alloys are examined using first principles with the focus on the spatial localization of electronic states. We compare localization at the band edges due to various isovalent impurities in a host GaAs including its impact on the photoluminescence line widths and carrier mobilities. The extremity of localization at the band edges is correlated with the a…
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Electronic properties of III-V semiconductor alloys are examined using first principles with the focus on the spatial localization of electronic states. We compare localization at the band edges due to various isovalent impurities in a host GaAs including its impact on the photoluminescence line widths and carrier mobilities. The extremity of localization at the band edges is correlated with the ability of individual elements to change the band gap and the relative band alignment. Additionally, the formation energies of substitutional defects are calculated and linked to challenges associated with the growth and formability of alloys. A spectrally-resolved inverse participation ratio is used to map localization in prospective GaAs-based materials alloyed with B, N, In, Sb, and Bi for 1.55 $μ$m wavelength telecommunication lasers. This analysis is complemented by a band unfolding of the electronic structure and discussion of implications of localization on the optical gain and Auger losses. Correspondence with experimental data on broadening of the photoluminescence spectrum and charge carrier mobilities show that the localization characteristics can serve as a guideline for engineering of semiconductor alloys.
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Submitted 8 May, 2017; v1 submitted 24 December, 2016;
originally announced December 2016.
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Ionization energy as a stability criterion for halide perovskites
Authors:
Chao Zheng,
Oleg Rubel
Abstract:
Instability of hybrid organic-inorganic halide perovskites hinders their development for photovoltaic applications. First-principle calculations are used for evaluation of a decomposition reaction enthalpy of hybrid halide perovskites, which is linked to experimentally observed degradation of device characteristics. However, simple criteria for predicting stability of halide perovskites are lackin…
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Instability of hybrid organic-inorganic halide perovskites hinders their development for photovoltaic applications. First-principle calculations are used for evaluation of a decomposition reaction enthalpy of hybrid halide perovskites, which is linked to experimentally observed degradation of device characteristics. However, simple criteria for predicting stability of halide perovskites are lacking since Goldschmidt's tolerance and octahedral geometrical factors do not fully capture formability of those perovskites. In this paper, we extend the Born-Haber cycle to partition the reaction enthalpy of various perovskite structures into lattice, ionization, and molecularization energy components. The analysis of various contributions to the reaction enthalpy points to an ionization energy of a molecule and a cage as an additional criterion for predicting chemical trends in stability of hybrid halide perovskites. Prospects of finding new perovskite structures with improved chemical stability aimed for photovoltaic applications are discussed.
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Submitted 14 December, 2016;
originally announced December 2016.
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One-dimensional electron gas in strained lateral heterostructures of single layer materials
Authors:
O. Rubel
Abstract:
Confinement of the electron gas along one of the spatial directions opens an avenue for studying fundamentals of quantum transport along the side of numerous practical electronic applications, with high-electron-mobility transistors being a prominent example. A heterojunction of two materials with dissimilar electronic polarisation can be used for engineering of the conducting channel. Extension o…
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Confinement of the electron gas along one of the spatial directions opens an avenue for studying fundamentals of quantum transport along the side of numerous practical electronic applications, with high-electron-mobility transistors being a prominent example. A heterojunction of two materials with dissimilar electronic polarisation can be used for engineering of the conducting channel. Extension of this concept to single-layer materials leads to one-dimensional electron gas (1DEG). MoS2/WS2 lateral heterostructure is used as a prototype for the realisation of 1DEG. The electronic polarisation discontinuity is achieved by straining the heterojunction taking advantage of dissimilarities in the piezoelectric coupling between MoS2 and WS2. A complete theory that describes an induced electric field profile in lateral heterojunctions of two-dimensional materials is proposed and verified by first principle calculations.
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Submitted 23 March, 2017; v1 submitted 4 December, 2016;
originally announced December 2016.
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Configuration Dependence of Band Gap Narrowing and Localization in Dilute GaAs_{1-x} Bi_x Alloys
Authors:
Lars C. Bannow,
Oleg Rubel,
Phil Rosenow,
Stefan C. Badescu,
Jorg Hader,
Jerome V. Moloney,
Ralf Tonner,
Stephan W. Koch
Abstract:
Anion substitution with bismuth (Bi) in III-V semiconductors is an effective method for experimental engineering of the band gap Eg at low Bi concentrations, in particular in gallium arsenide (GaAs). The inverse Bi-concentration dependence of Eg has been found to be linear at low concentrations x and dominated by a valence band-defect level anticrossing between As and Bi occupied p levels. This de…
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Anion substitution with bismuth (Bi) in III-V semiconductors is an effective method for experimental engineering of the band gap Eg at low Bi concentrations, in particular in gallium arsenide (GaAs). The inverse Bi-concentration dependence of Eg has been found to be linear at low concentrations x and dominated by a valence band-defect level anticrossing between As and Bi occupied p levels. This dependence breaks down at high concentrations where empirical models accounting only for the As-Bi interaction are not applicable. Predictive models for the valence band hybridization require a first-principle understanding which can be obtained by density functional theory with the main challenges being the proper description of Eg and the spin-orbit coupling. By using an efficient method to include these effects, it is shown here that at high concentrations Eg is modified mainly by a Bi-Bi p orbital interaction and by the large Bi atom-induced strain. This points to the role of different atomic configurations obtained by varying the experimental growth conditions in engineering arsenide band gaps, in particular for telecommunication laser technology.
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Submitted 5 February, 2016;
originally announced February 2016.
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Robust Bloch character at the band edges of hybrid halide perovskites
Authors:
O. Rubel,
A. Bokhanchuk
Abstract:
The high-symmetry pseudocubic init cell is often used for modelling the electronic structure of halide perovskites. However, direct comparison of the band structure with more realistic low-symmetry phases is impeded by the zone folding. We utilize a spectral density technique in order to reduce the supercell band structure to a common Bloch basis. This allows us to compare the electronic structure…
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The high-symmetry pseudocubic init cell is often used for modelling the electronic structure of halide perovskites. However, direct comparison of the band structure with more realistic low-symmetry phases is impeded by the zone folding. We utilize a spectral density technique in order to reduce the supercell band structure to a common Bloch basis. This allows us to compare the electronic structure of high- and low-symmetry phases as well as investigate effects of structural and compositional disorder on states near to the band edges that govern transport, dissociation and recombination of optical excitations.
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Submitted 14 August, 2015;
originally announced August 2015.
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Unfolding the band structure of disordered solids: from bound states to high-mobility Kane fermions
Authors:
O. Rubel,
A. Bokhanchuk,
S. J. Ahmed,
E. Assmann
Abstract:
Supercells are often used in ab initio calculations to model compound alloys, surfaces and defects. One of the main challenges of supercell electronic structure calculations is to recover the Bloch character of electronic eigenstates perturbed by disorder. Here we apply the spectral weight approach to unfolding the electronic structure of group III-V and II-VI semiconductor solid solutions. The il…
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Supercells are often used in ab initio calculations to model compound alloys, surfaces and defects. One of the main challenges of supercell electronic structure calculations is to recover the Bloch character of electronic eigenstates perturbed by disorder. Here we apply the spectral weight approach to unfolding the electronic structure of group III-V and II-VI semiconductor solid solutions. The illustrative examples include: formation of donor-like states in dilute Ga(PN) and associated enhancement of its optical activity, direct observation of the valence band anticrossing in dilute GaAs:Bi, and a topological band crossover in ternary (HgCd)Te alloy accompanied by emergence of high-mobility Kane fermions. The analysis facilitates interpretation of optical and transport characteristics of alloys that are otherwise ambiguous in traditional first-principles supercell calculations.
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Submitted 11 September, 2014; v1 submitted 16 May, 2014;
originally announced May 2014.
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Lead monoxide $α$-PbO: electronic properties and point defect formation
Authors:
J. Berashevich,
O. Semeniuk,
O. Rubel,
J. A. Rowlands,
A. Reznik
Abstract:
The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline $α$-PbO platelets and the formation of the native point defects in $α$-PbO crystal lattice are studied using first principles calculations. The $α$-PbO lattice consists of coupled layers interaction between which is too low to produce high efficiency interlayer charge transfer. In practice, the po…
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The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline $α$-PbO platelets and the formation of the native point defects in $α$-PbO crystal lattice are studied using first principles calculations. The $α$-PbO lattice consists of coupled layers interaction between which is too low to produce high efficiency interlayer charge transfer. In practice, the polycrystalline nature of $α$-PbO causes the formation of lattice defects in such a high concentration that defect-related conductivity becomes the dominant factor in the interlayer charge transition. We found that the formation energy for the O vacancies is low, such vacancies are occupied by two electrons in the zero charge state and tend to donate their electrons to the Pb vacancies that leads to ionization of both vacancies.The vacancies introduce localized states in the band gap which can affect charge transport. The O vacancy forms a defect state at 1.03 eV above the valence band which can act as a deep trap for electrons, while the Pb vacancy forms a shallow trap for holes located just 0.1 eV above the valence band. Charge de-trapping from O vacancies can be accounted for the experimentally found dark current decay in ITO/PbO/Au structures.
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Submitted 14 December, 2012; v1 submitted 30 October, 2012;
originally announced October 2012.
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Single-carrier impact ionization favored by a limited band dispersion
Authors:
A. Darbandi,
O. Rubel
Abstract:
A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the conduction or valence band resulting in a restriction of kinetic energy for one of the charge carriers. This feature is not common to the majority of technologica…
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A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the conduction or valence band resulting in a restriction of kinetic energy for one of the charge carriers. This feature is not common to the majority of technologically relevant semiconductors, but it is observed in chalcogenides, such as Selenium and compound I2-II-IV-VI4 alloys.
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Submitted 15 October, 2012;
originally announced October 2012.