Prof. Dr. David Egger
Academic Research Focus
- Simulation based materials discovery
- Simulation of energy materials
- Development of computational methods
- Simulation of energy materials
- Development of computational methods
Fields of Application
Batteries, Data Science / Machine Learning, Semiconductor Materials
Publications
25.
F P Delgado, F Simoes, L Kronik, W Kaiser, D A Egger
Machine-Learning Force Fields Reveal Shallow Electronic States on Dynamic Halide Perovskite Surfaces Journal Article
In: Acs Energy Letters, 2025, ISSN: 2380-8195.
@article{nokey,
title = {Machine-Learning Force Fields Reveal Shallow Electronic States on Dynamic Halide Perovskite Surfaces},
author = {F P Delgado and F Simoes and L Kronik and W Kaiser and D A Egger},
url = {\<Go to ISI\>://WOS:001514111900001},
doi = {10.1021/acsenergylett.5c01519},
issn = {2380-8195},
year = {2025},
date = {2025-06-23},
journal = {Acs Energy Letters},
abstract = {Previous studies indicated that defects in halide perovskites can generate shallow electronic states, which are crucial for their performance in devices. However, how shallow states persist amid pronounced atomic dynamics on halide perovskite surfaces remains unknown. We reveal that electronic states at surfaces of prototypical CsPbBr3 are energetically distributed at room temperature, akin to well-passivated inorganic semiconductors, despite the presence of undercoordinated atoms and cleaved bonds. Notably, approximately 70% of surface-state energies appear within 0.2 eV of the valence-band edge. Although deep states can still form, they are rarely energetically isolated and are less likely to act as traps. Accelerating first-principles calculations via machine learning, we show that the unique atomic dynamics in halide perovskites render the formation of deep electronic states at their surfaces unlikely. These findings reveal the microscopic mechanism behind the low density of deep states at dynamic halide perovskite surfaces, which is key to their device performance.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Previous studies indicated that defects in halide perovskites can generate shallow electronic states, which are crucial for their performance in devices. However, how shallow states persist amid pronounced atomic dynamics on halide perovskite surfaces remains unknown. We reveal that electronic states at surfaces of prototypical CsPbBr3 are energetically distributed at room temperature, akin to well-passivated inorganic semiconductors, despite the presence of undercoordinated atoms and cleaved bonds. Notably, approximately 70% of surface-state energies appear within 0.2 eV of the valence-band edge. Although deep states can still form, they are rarely energetically isolated and are less likely to act as traps. Accelerating first-principles calculations via machine learning, we show that the unique atomic dynamics in halide perovskites render the formation of deep electronic states at their surfaces unlikely. These findings reveal the microscopic mechanism behind the low density of deep states at dynamic halide perovskite surfaces, which is key to their device performance.
24.
D Cahen, Y Rakita, D A Egger, A Kahn
Surface Defects Control Bulk Carrier Densities in Polycrystalline Pb-Halide Perovskites Journal Article
In: Advanced Materials, vol. 36, no. 50, pp. 2407098, 2024, ISSN: 0935-9648.
@article{nokey,
title = {Surface Defects Control Bulk Carrier Densities in Polycrystalline Pb-Halide Perovskites},
author = {D Cahen and Y Rakita and D A Egger and A Kahn},
url = {https://doi.org/10.1002/adma.202407098},
doi = {https://doi.org/10.1002/adma.202407098},
issn = {0935-9648},
year = {2024},
date = {2024-12-01},
journal = {Advanced Materials},
volume = {36},
number = {50},
pages = {2407098},
abstract = {Abstract The (opto)electronic behavior of semiconductors depends on their (quasi-)free electronic carrier densities. These are regulated by semiconductor doping, i.e., controlled ?electronic contamination?. For metal halide perovskites (HaPs), the functional materials in several device types, which already challenge some of the understanding of semiconductor properties, this study shows that doping type, density and properties derived from these, are to a first approximation controlled via their surfaces. This effect, relevant to all semiconductors, and already found for some, is very evident for lead (Pb)-HaPs because of their intrinsically low electrically active bulk and surface defect densities. Volume carrier densities for most polycrystalline Pb-HaP films (\<1 µm grain diameter) are below those resulting from even \< 0.1% of surface sites being electrically active defects. This implies and is consistent with interfacial defects controlling HaP devices in multi-layered structures with most of the action at the two HaP interfaces. Surface and interface passivation effects on bulk electrical properties are relevant to all semiconductors and are crucial for developing those used today. However, because bulk dopant introduction in HaPs at controlled ppm levels for electronic-relevant carrier densities is so difficult, passivation effects are vastly more critical and dominate, to first approximation, their optoelectronic characteristics in devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract The (opto)electronic behavior of semiconductors depends on their (quasi-)free electronic carrier densities. These are regulated by semiconductor doping, i.e., controlled ?electronic contamination?. For metal halide perovskites (HaPs), the functional materials in several device types, which already challenge some of the understanding of semiconductor properties, this study shows that doping type, density and properties derived from these, are to a first approximation controlled via their surfaces. This effect, relevant to all semiconductors, and already found for some, is very evident for lead (Pb)-HaPs because of their intrinsically low electrically active bulk and surface defect densities. Volume carrier densities for most polycrystalline Pb-HaP films (<1 µm grain diameter) are below those resulting from even < 0.1% of surface sites being electrically active defects. This implies and is consistent with interfacial defects controlling HaP devices in multi-layered structures with most of the action at the two HaP interfaces. Surface and interface passivation effects on bulk electrical properties are relevant to all semiconductors and are crucial for developing those used today. However, because bulk dopant introduction in HaPs at controlled ppm levels for electronic-relevant carrier densities is so difficult, passivation effects are vastly more critical and dominate, to first approximation, their optoelectronic characteristics in devices.
23.
E Sirotti, L I Wagner, C-M Jiang, J Eichhorn, F Munnik, V Streibel, M J Schilcher, B März, F S Hegner, M Kuhl, T Höldrich, K Müller-Caspary, D A Egger, I D Sharp
Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3 Journal Article
In: Advanced Energy Materials, vol. 14, no. 42, pp. 2402540, 2024, ISSN: 1614-6832.
@article{nokey,
title = {Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3},
author = {E Sirotti and L I Wagner and C-M Jiang and J Eichhorn and F Munnik and V Streibel and M J Schilcher and B M\"{a}rz and F S Hegner and M Kuhl and T H\"{o}ldrich and K M\"{u}ller-Caspary and D A Egger and I D Sharp},
url = {https://doi.org/10.1002/aenm.202402540},
doi = {https://doi.org/10.1002/aenm.202402540},
issn = {1614-6832},
year = {2024},
date = {2024-11-01},
journal = {Advanced Energy Materials},
volume = {14},
number = {42},
pages = {2402540},
abstract = {Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.
22.
W O’leary, M Grumet, W Kaiser, T Bučko, J L M Rupp, D A Egger
In: Journal of the American Chemical Society, vol. 146, no. 39, pp. 26863-26876, 2024, ISSN: 0002-7863.
@article{nokey,
title = {Rapid Characterization of Point Defects in Solid-State Ion Conductors Using Raman Spectroscopy, Machine-Learning Force Fields, and Atomic Raman Tensors},
author = {W O’leary and M Grumet and W Kaiser and T Bu\v{c}ko and J L M Rupp and D A Egger},
url = {https://doi.org/10.1021/jacs.4c07812},
doi = {10.1021/jacs.4c07812},
issn = {0002-7863},
year = {2024},
date = {2024-10-02},
journal = {Journal of the American Chemical Society},
volume = {146},
number = {39},
pages = {26863-26876},
abstract = {The successful design of solid-state photo- and electrochemical devices depends on the careful engineering of point defects in solid-state ion conductors. Characterization of point defects is critical to these efforts, but the best-developed techniques are difficult and time-consuming. Raman spectroscopy─with its exceptional speed, flexibility, and accessibility─is a promising alternative. Raman signatures arise from point defects due to local symmetry breaking and structural distortions. Unfortunately, the assignment of these signatures is often hampered by a shortage of reference compounds and corresponding reference spectra. This issue can be circumvented by calculation of defect-induced Raman signatures from first principles, but this is computationally demanding. Here, we introduce an efficient computational procedure for the prediction of point defect Raman signatures in solid-state ion conductors. Our method leverages machine-learning force fields and “atomic Raman tensors”, i.e., polarizability fluctuations due to motions of individual atoms. We find that our procedure reduces computational cost by up to 80% compared to existing first-principles frozen-phonon approaches. These efficiency gains enable synergistic computational\textendashexperimental investigations, in our case allowing us to precisely interpret the Raman spectra of Sr(Ti0.94Ni0.06)O3-δ, a model oxygen ion conductor. By predicting Raman signatures of specific point defects, we determine the nature of dominant defects and unravel impacts of temperature and quenching on in situ and ex situ Raman spectra. Specifically, our findings reveal the temperature-dependent distribution and association behavior of oxygen vacancies and nickel substitutional defects. Overall, our approach enables rapid Raman-based characterization of point defects to support defect engineering in novel solid-state ion conductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The successful design of solid-state photo- and electrochemical devices depends on the careful engineering of point defects in solid-state ion conductors. Characterization of point defects is critical to these efforts, but the best-developed techniques are difficult and time-consuming. Raman spectroscopy─with its exceptional speed, flexibility, and accessibility─is a promising alternative. Raman signatures arise from point defects due to local symmetry breaking and structural distortions. Unfortunately, the assignment of these signatures is often hampered by a shortage of reference compounds and corresponding reference spectra. This issue can be circumvented by calculation of defect-induced Raman signatures from first principles, but this is computationally demanding. Here, we introduce an efficient computational procedure for the prediction of point defect Raman signatures in solid-state ion conductors. Our method leverages machine-learning force fields and “atomic Raman tensors”, i.e., polarizability fluctuations due to motions of individual atoms. We find that our procedure reduces computational cost by up to 80% compared to existing first-principles frozen-phonon approaches. These efficiency gains enable synergistic computational–experimental investigations, in our case allowing us to precisely interpret the Raman spectra of Sr(Ti0.94Ni0.06)O3-δ, a model oxygen ion conductor. By predicting Raman signatures of specific point defects, we determine the nature of dominant defects and unravel impacts of temperature and quenching on in situ and ex situ Raman spectra. Specifically, our findings reveal the temperature-dependent distribution and association behavior of oxygen vacancies and nickel substitutional defects. Overall, our approach enables rapid Raman-based characterization of point defects to support defect engineering in novel solid-state ion conductors.
21.
X Zhu, D A Egger
The Effect of Overdamped Phonons on the Fundamental Band Gap of Perovskites Journal Article
In: arXiv preprint arXiv:2406.05201, 2024.
@article{nokey,
title = {The Effect of Overdamped Phonons on the Fundamental Band Gap of Perovskites},
author = {X Zhu and D A Egger},
year = {2024},
date = {2024-06-07},
journal = {arXiv preprint arXiv:2406.05201},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
20.
F S Hegner, A Cohen, S S Rudel, S M Kronawitter, M Grumet, X Zhu, R Korobko, L Houben, C-M Jiang, W Schnick, G Kieslich, O Yaffe, I D Sharp, D A Egger
The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2 Journal Article
In: Advanced Energy Materials, vol. 14, no. 19, pp. 2303059, 2024, ISSN: 1614-6832.
@article{nokey,
title = {The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2},
author = {F S Hegner and A Cohen and S S Rudel and S M Kronawitter and M Grumet and X Zhu and R Korobko and L Houben and C-M Jiang and W Schnick and G Kieslich and O Yaffe and I D Sharp and D A Egger},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.202303059},
doi = {https://doi.org/10.1002/aenm.202303059},
issn = {1614-6832},
year = {2024},
date = {2024-05-17},
journal = {Advanced Energy Materials},
volume = {14},
number = {19},
pages = {2303059},
abstract = {Abstract Ternary nitride semiconductors are rapidly emerging as a promising class of materials for energy conversion applications, offering an appealing combination of strong light absorption in the visible range, desirable charge transport characteristics, and good chemical stability. In this work, it is shown that finite-temperature lattice dynamics in CuTaN2 \textendash a prototypical ternary nitride displaying particularly strong visible light absorption \textendash exhibit a pronounced anharmonic character that plays an essential role in defining its macroscopic optoelectronic and thermal properties. Low-frequency vibrational modes that are Raman-inactive from symmetry considerations of the average crystal structure and unstable in harmonic phonon calculations are found to appear as intensive Raman features near room temperature. The atomic contributions to the anharmonic vibrations are characterized by combining Raman measurements with molecular dynamics and density functional theory calculations. This analysis reveals that anharmonic lattice dynamics have large ramifications on the fundamental properties of this compound, resulting in uniaxial negative thermal expansion and the opening of its bandgap to a near-optimal value for solar energy harvesting. The atomic-level understanding of anharmonic lattice dynamics, as well as the finding that they strongly influence key properties of this semiconductor at room temperature, have important implications for design of new functional materials, especially within the emerging class of ternary nitride semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ternary nitride semiconductors are rapidly emerging as a promising class of materials for energy conversion applications, offering an appealing combination of strong light absorption in the visible range, desirable charge transport characteristics, and good chemical stability. In this work, it is shown that finite-temperature lattice dynamics in CuTaN2 – a prototypical ternary nitride displaying particularly strong visible light absorption – exhibit a pronounced anharmonic character that plays an essential role in defining its macroscopic optoelectronic and thermal properties. Low-frequency vibrational modes that are Raman-inactive from symmetry considerations of the average crystal structure and unstable in harmonic phonon calculations are found to appear as intensive Raman features near room temperature. The atomic contributions to the anharmonic vibrations are characterized by combining Raman measurements with molecular dynamics and density functional theory calculations. This analysis reveals that anharmonic lattice dynamics have large ramifications on the fundamental properties of this compound, resulting in uniaxial negative thermal expansion and the opening of its bandgap to a near-optimal value for solar energy harvesting. The atomic-level understanding of anharmonic lattice dynamics, as well as the finding that they strongly influence key properties of this semiconductor at room temperature, have important implications for design of new functional materials, especially within the emerging class of ternary nitride semiconductors.
19.
T Miyagawa, M Grumet, N Krishnan, C R Baecker, W Kaiser, D A Egger
Ion Migration in Anharmonic Solid-State Ion Conductors from Machine-Learning Molecular Dynamics Journal Article
In: Proceedings of 24th International Conference on Solid State Ionics (SSI24), 2024.
@article{nokey,
title = {Ion Migration in Anharmonic Solid-State Ion Conductors from Machine-Learning Molecular Dynamics},
author = {T Miyagawa and M Grumet and N Krishnan and C R Baecker and W Kaiser and D A Egger},
url = {https://www.nanoge.org/proceedings/SSI24/65c112123ff63c57b8cfdeea},
year = {2024},
date = {2024-04-10},
urldate = {2024-04-10},
journal = {Proceedings of 24th International Conference on Solid State Ionics (SSI24)},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
18.
T Miyagawa, N Krishnan, M Grumet, C R Baecker, W Kaiser, D A Egger
Accurate description of ion migration in solid-state ion conductors from machine-learning molecular dynamics Journal Article
In: Journal of Materials Chemistry A, vol. 12, no. 19, pp. 11344-11361, 2024, ISSN: 2050-7488.
@article{nokey,
title = {Accurate description of ion migration in solid-state ion conductors from machine-learning molecular dynamics},
author = {T Miyagawa and N Krishnan and M Grumet and C R Baecker and W Kaiser and D A Egger},
url = {http://dx.doi.org/10.1039/D4TA00452C},
doi = {10.1039/D4TA00452C},
issn = {2050-7488},
year = {2024},
date = {2024-04-09},
journal = {Journal of Materials Chemistry A},
volume = {12},
number = {19},
pages = {11344-11361},
abstract = {Solid-state ion conductors (SSICs) have emerged as a promising material class for electrochemical storage devices and novel compounds of this kind are continuously being discovered. High-throughout approaches that enable a rapid screening among the plethora of candidate SSIC compounds have been essential in this quest. While first-principles methods are routinely exploited in this context to provide atomic-level details on ion migration mechanisms, dynamic calculations of this type are computationally expensive and limit us in the time- and length-scales accessible during the simulations. Here, we explore the potential of recently developed machine-learning force fields for predicting different ion migration mechanisms in SSICs. Specifically, we systematically investigate three classes of SSICs that all exhibit complex ion dynamics including vibrational anharmonicities: AgI, a strongly disordered Ag+-conductor; Na3SbS4, a Na+ vacancy conductor; and Li10GeP2S12, which features concerted Li+ migration. Through systematic comparison with ab initio molecular dynamics data, we demonstrate that machine-learning molecular dynamics provides very accurate predictions of the structural and vibrational properties including the complex anharmonic dynamics in these SSICs. The ab initio accuracy of machine-learning molecular dynamics simulations at relatively low computational cost opens a promising path toward the rapid design of novel SSICs.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Solid-state ion conductors (SSICs) have emerged as a promising material class for electrochemical storage devices and novel compounds of this kind are continuously being discovered. High-throughout approaches that enable a rapid screening among the plethora of candidate SSIC compounds have been essential in this quest. While first-principles methods are routinely exploited in this context to provide atomic-level details on ion migration mechanisms, dynamic calculations of this type are computationally expensive and limit us in the time- and length-scales accessible during the simulations. Here, we explore the potential of recently developed machine-learning force fields for predicting different ion migration mechanisms in SSICs. Specifically, we systematically investigate three classes of SSICs that all exhibit complex ion dynamics including vibrational anharmonicities: AgI, a strongly disordered Ag+-conductor; Na3SbS4, a Na+ vacancy conductor; and Li10GeP2S12, which features concerted Li+ migration. Through systematic comparison with ab initio molecular dynamics data, we demonstrate that machine-learning molecular dynamics provides very accurate predictions of the structural and vibrational properties including the complex anharmonic dynamics in these SSICs. The ab initio accuracy of machine-learning molecular dynamics simulations at relatively low computational cost opens a promising path toward the rapid design of novel SSICs.
17.
F S Hegner, A Cohen, S S Rudel, S M Kronawitter, M Grumet, X Zhu, R Korobko, L Houben, C-M Jiang, W Schnick, G Kieslich, O Yaffe, I D Sharp, D A Egger
The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2 Journal Article
In: Advanced Energy Materials, vol. 14, no. 19, pp. 2303059, 2024, ISSN: 1614-6832.
@article{nokey,
title = {The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2},
author = {F S Hegner and A Cohen and S S Rudel and S M Kronawitter and M Grumet and X Zhu and R Korobko and L Houben and C-M Jiang and W Schnick and G Kieslich and O Yaffe and I D Sharp and D A Egger},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.202303059},
doi = {https://doi.org/10.1002/aenm.202303059},
issn = {1614-6832},
year = {2024},
date = {2024-03-30},
journal = {Advanced Energy Materials},
volume = {14},
number = {19},
pages = {2303059},
abstract = {Abstract Ternary nitride semiconductors are rapidly emerging as a promising class of materials for energy conversion applications, offering an appealing combination of strong light absorption in the visible range, desirable charge transport characteristics, and good chemical stability. In this work, it is shown that finite-temperature lattice dynamics in CuTaN2 \textendash a prototypical ternary nitride displaying particularly strong visible light absorption \textendash exhibit a pronounced anharmonic character that plays an essential role in defining its macroscopic optoelectronic and thermal properties. Low-frequency vibrational modes that are Raman-inactive from symmetry considerations of the average crystal structure and unstable in harmonic phonon calculations are found to appear as intensive Raman features near room temperature. The atomic contributions to the anharmonic vibrations are characterized by combining Raman measurements with molecular dynamics and density functional theory calculations. This analysis reveals that anharmonic lattice dynamics have large ramifications on the fundamental properties of this compound, resulting in uniaxial negative thermal expansion and the opening of its bandgap to a near-optimal value for solar energy harvesting. The atomic-level understanding of anharmonic lattice dynamics, as well as the finding that they strongly influence key properties of this semiconductor at room temperature, have important implications for design of new functional materials, especially within the emerging class of ternary nitride semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ternary nitride semiconductors are rapidly emerging as a promising class of materials for energy conversion applications, offering an appealing combination of strong light absorption in the visible range, desirable charge transport characteristics, and good chemical stability. In this work, it is shown that finite-temperature lattice dynamics in CuTaN2 – a prototypical ternary nitride displaying particularly strong visible light absorption – exhibit a pronounced anharmonic character that plays an essential role in defining its macroscopic optoelectronic and thermal properties. Low-frequency vibrational modes that are Raman-inactive from symmetry considerations of the average crystal structure and unstable in harmonic phonon calculations are found to appear as intensive Raman features near room temperature. The atomic contributions to the anharmonic vibrations are characterized by combining Raman measurements with molecular dynamics and density functional theory calculations. This analysis reveals that anharmonic lattice dynamics have large ramifications on the fundamental properties of this compound, resulting in uniaxial negative thermal expansion and the opening of its bandgap to a near-optimal value for solar energy harvesting. The atomic-level understanding of anharmonic lattice dynamics, as well as the finding that they strongly influence key properties of this semiconductor at room temperature, have important implications for design of new functional materials, especially within the emerging class of ternary nitride semiconductors.
16.
M Righetto, S Caicedo-Dávila, M T Sirtl, V J Y Lim, J B Patel, D A Egger, T Bein, L M Herz
Alloying Effects on Charge-Carrier Transport in Silver–Bismuth Double Perovskites Journal Article
In: The Journal of Physical Chemistry Letters, vol. 14, no. 46, pp. 10340-10347, 2023.
@article{nokey,
title = {Alloying Effects on Charge-Carrier Transport in Silver\textendashBismuth Double Perovskites},
author = {M Righetto and S Caicedo-D\'{a}vila and M T Sirtl and V J Y Lim and J B Patel and D A Egger and T Bein and L M Herz},
url = {https://doi.org/10.1021/acs.jpclett.3c02750},
doi = {10.1021/acs.jpclett.3c02750},
year = {2023},
date = {2023-11-10},
journal = {The Journal of Physical Chemistry Letters},
volume = {14},
number = {46},
pages = {10340-10347},
abstract = {Alloying is widely adopted for tuning the properties of emergent semiconductors for optoelectronic and photovoltaic applications. So far, alloying strategies have primarily focused on engineering bandgaps rather than optimizing charge-carrier transport. Here, we demonstrate that alloying may severely limit charge-carrier transport in the presence of localized charge carriers (e.g., small polarons). By combining reflection\textendashtransmission and optical pump\textendashterahertz probe spectroscopy with first-principles calculations, we investigate the interplay between alloying and charge-carrier localization in Cs2AgSbxBi1\textendashxBr6 double perovskite thin films. We show that the charge-carrier transport regime strongly determines the impact of alloying on the transport properties. While initially delocalized charge carriers probe electronic bands formed upon alloying, subsequently self-localized charge carriers probe the energetic landscape more locally, thus turning an alloy’s low-energy sites (e.g., Sb sites) into traps, which dramatically deteriorates transport properties. These findings highlight the inherent limitations of alloying strategies and provide design tools for newly emerging and highly efficient semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Alloying is widely adopted for tuning the properties of emergent semiconductors for optoelectronic and photovoltaic applications. So far, alloying strategies have primarily focused on engineering bandgaps rather than optimizing charge-carrier transport. Here, we demonstrate that alloying may severely limit charge-carrier transport in the presence of localized charge carriers (e.g., small polarons). By combining reflection–transmission and optical pump–terahertz probe spectroscopy with first-principles calculations, we investigate the interplay between alloying and charge-carrier localization in Cs2AgSbxBi1–xBr6 double perovskite thin films. We show that the charge-carrier transport regime strongly determines the impact of alloying on the transport properties. While initially delocalized charge carriers probe electronic bands formed upon alloying, subsequently self-localized charge carriers probe the energetic landscape more locally, thus turning an alloy’s low-energy sites (e.g., Sb sites) into traps, which dramatically deteriorates transport properties. These findings highlight the inherent limitations of alloying strategies and provide design tools for newly emerging and highly efficient semiconductors.
15.
S Caicedo-Dávila, A Cohen, S G Motti, M Isobe, K M Mccall, M V Kovalenko, O Yaffe, L M Herz, D H Fabini, D A Egger
Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites Journal Article
In: arXiv preprint arXiv:2310.03408, 2023.
@article{nokey,
title = {Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites},
author = {S Caicedo-D\'{a}vila and A Cohen and S G Motti and M Isobe and K M Mccall and M V Kovalenko and O Yaffe and L M Herz and D H Fabini and D A Egger},
url = {https://arxiv.org/abs/2310.03408},
doi = {https://doi.org/10.48550/arXiv.2310.03408},
year = {2023},
date = {2023-10-05},
journal = {arXiv preprint arXiv:2310.03408},
abstract = {Metal halide perovskites have shown great performance as solar energy materials, but their outstanding optoelectronic properties are paired with unusually strong anharmonic effects. It has been proposed that this intriguing combination of properties derives from the "lone pair" electrons of the octahedral metal cations, but the precise impact of this chemical feature remains unclear. Here we show that in fact a lone pair of electrons is not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in this class of materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and first-principles molecular dynamics calculations to directly contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds which bear close structural similarity but with the former lacking lone pairs on the octahedral metal. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint to detect anharmonicity and reveal that low-frequency tilting occurs irrespective of lone pair presence. This work highlights the key role of structure in perovskite lattice dynamics, providing important design rules for the emerging class of soft perovskite semiconductors for optoelectronic and light-harvesting devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Metal halide perovskites have shown great performance as solar energy materials, but their outstanding optoelectronic properties are paired with unusually strong anharmonic effects. It has been proposed that this intriguing combination of properties derives from the "lone pair" electrons of the octahedral metal cations, but the precise impact of this chemical feature remains unclear. Here we show that in fact a lone pair of electrons is not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in this class of materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and first-principles molecular dynamics calculations to directly contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds which bear close structural similarity but with the former lacking lone pairs on the octahedral metal. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint to detect anharmonicity and reveal that low-frequency tilting occurs irrespective of lone pair presence. This work highlights the key role of structure in perovskite lattice dynamics, providing important design rules for the emerging class of soft perovskite semiconductors for optoelectronic and light-harvesting devices.
14.
M J Schilcher, D J Abramovitch, M Z Mayers, L Z Tan, D R Reichman, D A Egger
Correlated Anharmonicity and Dynamic Disorder Control Carrier Transport in Halide Perovskites Journal Article
In: arXiv preprint arXiv:2305.13682, 2023.
@article{nokey,
title = {Correlated Anharmonicity and Dynamic Disorder Control Carrier Transport in Halide Perovskites},
author = {M J Schilcher and D J Abramovitch and M Z Mayers and L Z Tan and D R Reichman and D A Egger},
url = {https://arxiv.org/abs/2305.13682},
doi = {https://doi.org/10.48550/arXiv.2305.13682},
year = {2023},
date = {2023-05-23},
journal = {arXiv preprint arXiv:2305.13682},
abstract = {Halide pervoskites are an important class of semiconducting materials which hold great promise for optoelectronic applications. In this work we investigate the relationship between vibrational anharmonicity and dynamic disorder in this class of solids. Via a multi-scale model parameterized from first-principles calculations, we demonstrate that the non-Gaussian lattice motion in halide perovskites is microscopically connected to the dynamic disorder of overlap fluctuations among electronic states. This connection allows us to rationalize the emergent differences in temperature-dependent mobilities of prototypical MAPbI3 and MAPbBr3 compounds across structural phase-transitions, in agreement with experimental findings. Our analysis suggests that the details of vibrational anharmonicity and dynamic disorder can complement known predictors of electronic conductivity and can provide structure-property guidelines for the tuning of carrier transport characteristics in anharmonic semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Halide pervoskites are an important class of semiconducting materials which hold great promise for optoelectronic applications. In this work we investigate the relationship between vibrational anharmonicity and dynamic disorder in this class of solids. Via a multi-scale model parameterized from first-principles calculations, we demonstrate that the non-Gaussian lattice motion in halide perovskites is microscopically connected to the dynamic disorder of overlap fluctuations among electronic states. This connection allows us to rationalize the emergent differences in temperature-dependent mobilities of prototypical MAPbI3 and MAPbBr3 compounds across structural phase-transitions, in agreement with experimental findings. Our analysis suggests that the details of vibrational anharmonicity and dynamic disorder can complement known predictors of electronic conductivity and can provide structure-property guidelines for the tuning of carrier transport characteristics in anharmonic semiconductors.
13.
S A Seidl, X Zhu, G Reuveni, S Aharon, C Gehrmann, S Caicedo-Dávila, O Yaffe, D A Egger
Anharmonic Fluctuations Govern the Band Gap of Halide Perovskites Journal Article
In: arXiv preprint arXiv:2303.01603, 2023.
@article{nokey,
title = {Anharmonic Fluctuations Govern the Band Gap of Halide Perovskites},
author = {S A Seidl and X Zhu and G Reuveni and S Aharon and C Gehrmann and S Caicedo-D\'{a}vila and O Yaffe and D A Egger},
url = {https://arxiv.org/abs/2303.01603},
doi = {https://doi.org/10.48550/arXiv.2303.01603},
year = {2023},
date = {2023-03-02},
journal = {arXiv preprint arXiv:2303.01603},
abstract = {We determine the impact of anharmonic thermal vibrations on the fundamental band gap of CsPbBr3, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase-transitions, cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We determine the impact of anharmonic thermal vibrations on the fundamental band gap of CsPbBr3, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase-transitions, cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.
12.
G Reuveni, Y Diskin-Posner, C Gehrmann, S Godse, G G Gkikas, I Buchine, S Aharon, R Korobko, C C Stoumpos, D A Egger
Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals Journal Article
In: arXiv preprint arXiv:2211.06904, 2022.
@article{nokey,
title = {Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals},
author = {G Reuveni and Y Diskin-Posner and C Gehrmann and S Godse and G G Gkikas and I Buchine and S Aharon and R Korobko and C C Stoumpos and D A Egger},
url = {https://arxiv.org/abs/2211.06904},
doi = {https://doi.org/10.48550/arXiv.2211.06904},
year = {2022},
date = {2022-11-13},
journal = {arXiv preprint arXiv:2211.06904},
abstract = {We show that formamidinium lead bromide is unique among the halide perovskite crystals because its inorganic sub-lattice exhibits intrinsic local static disorder that co-exists with a well-defined average crystal structure. Our study combines THz-range Raman-scattering with single-crystal X-ray diffraction and first-principles calculations to probe the inorganic sub-lattice dynamics evolution with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal's structural dynamics and phase transitions at higher temperatures.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We show that formamidinium lead bromide is unique among the halide perovskite crystals because its inorganic sub-lattice exhibits intrinsic local static disorder that co-exists with a well-defined average crystal structure. Our study combines THz-range Raman-scattering with single-crystal X-ray diffraction and first-principles calculations to probe the inorganic sub-lattice dynamics evolution with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal's structural dynamics and phase transitions at higher temperatures.
11.
J D Ziegler, K-Q Lin, B Meisinger, X Zhu, M Kober-Czerny, P K Nayak, C Vona, T Taniguchi, K Watanabe, C Draxl, H J Snaith, J M Lupton, D A Egger, A Chernikov
Excitons at the Phase Transition of 2D Hybrid Perovskites Journal Article
In: ACS Photonics, 2022.
@article{nokey,
title = {Excitons at the Phase Transition of 2D Hybrid Perovskites},
author = {J D Ziegler and K-Q Lin and B Meisinger and X Zhu and M Kober-Czerny and P K Nayak and C Vona and T Taniguchi and K Watanabe and C Draxl and H J Snaith and J M Lupton and D A Egger and A Chernikov},
url = {https://doi.org/10.1021/acsphotonics.2c01035},
doi = {10.1021/acsphotonics.2c01035},
year = {2022},
date = {2022-10-18},
journal = {ACS Photonics},
abstract = {2D halide perovskites are among intensely studied materials platforms profiting from solution-based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical, as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase transitions in 2D perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in 2D perovskites.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
2D halide perovskites are among intensely studied materials platforms profiting from solution-based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical, as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase transitions in 2D perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in 2D perovskites.
10.
S Liu, M W Heindl, N Fehn, S Caicedo-Dávila, L Eyre, S M Kronawitter, J Zerhoch, S Bodnar, A Shcherbakov, A Stadlbauer, G Kieslich, I D Sharp, D A Egger, A Kartouzian, F Deschler
Optically Induced Long-Lived Chirality Memory in the Color-Tunable Chiral Lead-Free Semiconductor (R)/(S)-CHEA4Bi2BrxI10–x (x = 0–10) Journal Article
In: Journal of the American Chemical Society, vol. 144, no. 31, pp. 14079-14089, 2022, ISSN: 0002-7863.
@article{nokey,
title = {Optically Induced Long-Lived Chirality Memory in the Color-Tunable Chiral Lead-Free Semiconductor (R)/(S)-CHEA4Bi2BrxI10\textendashx (x = 0\textendash10)},
author = {S Liu and M W Heindl and N Fehn and S Caicedo-D\'{a}vila and L Eyre and S M Kronawitter and J Zerhoch and S Bodnar and A Shcherbakov and A Stadlbauer and G Kieslich and I D Sharp and D A Egger and A Kartouzian and F Deschler},
url = {https://doi.org/10.1021/jacs.2c01994},
doi = {10.1021/jacs.2c01994},
issn = {0002-7863},
year = {2022},
date = {2022-07-27},
journal = {Journal of the American Chemical Society},
volume = {144},
number = {31},
pages = {14079-14089},
abstract = {Hybrid organic\textendashinorganic networks that incorporate chiral molecules have attracted great attention due to their potential in semiconductor lighting applications and optical communication. Here, we introduce a chiral organic molecule (R)/(S)-1-cyclohexylethylamine (CHEA) into bismuth-based lead-free structures with an edge-sharing octahedral motif, to synthesize chiral lead-free (R)/(S)-CHEA4Bi2BrxI10\textendashx crystals and thin films. Using single-crystal X-ray diffraction measurements and density functional theory calculations, we identify crystal and electronic band structures. We investigate the materials’ optical properties and find circular dichroism, which we tune by the bromide\textendashiodide ratio over a wide wavelength range, from 300 to 500 nm. We further employ transient absorption spectroscopy and time-correlated single photon counting to investigate charge carrier dynamics, which show long-lived excitations with optically induced chirality memory up to tens of nanosecond timescales. Our demonstration of chirality memory in a color-tunable chiral lead-free semiconductor opens a new avenue for the discovery of high-performance, lead-free spintronic materials with chiroptical functionalities.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hybrid organic–inorganic networks that incorporate chiral molecules have attracted great attention due to their potential in semiconductor lighting applications and optical communication. Here, we introduce a chiral organic molecule (R)/(S)-1-cyclohexylethylamine (CHEA) into bismuth-based lead-free structures with an edge-sharing octahedral motif, to synthesize chiral lead-free (R)/(S)-CHEA4Bi2BrxI10–x crystals and thin films. Using single-crystal X-ray diffraction measurements and density functional theory calculations, we identify crystal and electronic band structures. We investigate the materials’ optical properties and find circular dichroism, which we tune by the bromide–iodide ratio over a wide wavelength range, from 300 to 500 nm. We further employ transient absorption spectroscopy and time-correlated single photon counting to investigate charge carrier dynamics, which show long-lived excitations with optically induced chirality memory up to tens of nanosecond timescales. Our demonstration of chirality memory in a color-tunable chiral lead-free semiconductor opens a new avenue for the discovery of high-performance, lead-free spintronic materials with chiroptical functionalities.
9.
C Gehrmann, S Caicedo-Dávila, X Zhu, D A Egger
Transversal Halide Motion Intensifies Band-To-Band Transitions in Halide Perovskites Journal Article
In: Advanced Science, vol. n/a, no. n/a, pp. 2200706, 2022, ISSN: 2198-3844.
@article{nokey,
title = {Transversal Halide Motion Intensifies Band-To-Band Transitions in Halide Perovskites},
author = {C Gehrmann and S Caicedo-D\'{a}vila and X Zhu and D A Egger},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/advs.202200706},
doi = {https://doi.org/10.1002/advs.202200706},
issn = {2198-3844},
year = {2022},
date = {2022-04-04},
journal = {Advanced Science},
volume = {n/a},
number = {n/a},
pages = {2200706},
abstract = {Abstract Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites (HaPs) exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the finite-temperature optoelectronic properties in the prototypical cubic CsPbBr3, using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with HaPs enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. The authors find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr3 but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. The authors conclude that the dynamic structural flexibility in HaPs and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites (HaPs) exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the finite-temperature optoelectronic properties in the prototypical cubic CsPbBr3, using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with HaPs enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. The authors find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr3 but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. The authors conclude that the dynamic structural flexibility in HaPs and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.
8.
T M Brenner, M Grumet, P Till, M Asher, W G Zeier, D A Egger, O Yaffe
Anharmonic Lattice Dynamics in Sodium Ion Conductors Journal Article
In: arXiv preprint arXiv:2203.07955, 2022.
@article{nokey,
title = {Anharmonic Lattice Dynamics in Sodium Ion Conductors},
author = {T M Brenner and M Grumet and P Till and M Asher and W G Zeier and D A Egger and O Yaffe},
url = {https://arxiv.org/abs/2203.07955},
doi = {https://doi.org/10.48550/arXiv.2203.07955},
year = {2022},
date = {2022-03-15},
journal = {arXiv preprint arXiv:2203.07955},
abstract = {We employ THz-range temperature-dependent Raman spectroscopy and first-principles lattice-dynamical calculations to show that the undoped sodium ion conductors Na3PS4 and isostructural Na3PSe4 both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice -- Na+ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na3PSe4 shows almost purely displacive character with the soft modes disappearing in the cubic phase as the change of symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na3PS4 instead shows order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining active in Raman in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice -- mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity in these Na+ ion conductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We employ THz-range temperature-dependent Raman spectroscopy and first-principles lattice-dynamical calculations to show that the undoped sodium ion conductors Na3PS4 and isostructural Na3PSe4 both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice -- Na+ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na3PSe4 shows almost purely displacive character with the soft modes disappearing in the cubic phase as the change of symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na3PS4 instead shows order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining active in Raman in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice -- mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity in these Na+ ion conductors.
7.
X Zhu, S Caicedo-Dávila, C Gehrmann, D A Egger
Probing the Disorder inside the Cubic Unit Cell of Halide Perovskites from First-Principles Journal Article
In: arXiv preprint arXiv:2111.14668, 2021.
@article{nokey,
title = {Probing the Disorder inside the Cubic Unit Cell of Halide Perovskites from First-Principles},
author = {X Zhu and S Caicedo-D\'{a}vila and C Gehrmann and D A Egger},
doi = {arXiv:2111.14668v1},
year = {2021},
date = {2021-11-29},
journal = {arXiv preprint arXiv:2111.14668},
abstract = {Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr3. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr3 and quantify the important local structural fluctuations on an atomic scale.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr3. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr3 and quantify the important local structural fluctuations on an atomic scale.
6.
C Gehrmann, S Caicedo-Dávila, X Zhu, D A Egger
The Effect of Dynamic Structural Flexibility in Halide Perovskites Journal Article
In: J. Mater. Sci., 2021.
@article{nokey,
title = {The Effect of Dynamic Structural Flexibility in Halide Perovskites},
author = {C Gehrmann and S Caicedo-D\'{a}vila and X Zhu and D A Egger},
year = {2021},
date = {2021-11-01},
journal = {J. Mater. Sci.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
5.
M J Schilcher, P J Robinson, D J Abramovitch, L Z Tan, A M Rappe, D R Reichman, D A Egger
The Significance of Polarons and Dynamic Disorder in Halide Perovskites Journal Article
In: ACS Energy Letters, pp. 2162-2173, 2021.
@article{,
title = {The Significance of Polarons and Dynamic Disorder in Halide Perovskites},
author = {M J Schilcher and P J Robinson and D J Abramovitch and L Z Tan and A M Rappe and D R Reichman and D A Egger},
url = {https://doi.org/10.1021/acsenergylett.1c00506},
doi = {10.1021/acsenergylett.1c00506},
year = {2021},
date = {2021-05-17},
journal = {ACS Energy Letters},
pages = {2162-2173},
abstract = {The development of halide perovskite semiconductors led to various technological breakthroughs in optoelectronics, in particular in the areas of photovoltaics and light-emitting diodes. Additionally, the study of their fundamental properties has uncovered intriguing puzzles that demand explanation. Polaronic effects associated with the coupling of electrons and holes to polar lattice vibrations are often invoked as a microscopic mechanism to explain various unusual experimental observations. While some form of polaronic behavior undoubtedly exists in these systems, several assumptions underlying standard models used to describe a polaron mechanism appear to be strongly violated in these materials. In this Perspective, we investigate the role of polaronic effects in halide perovskites and summarize signatures and failures of the polaron picture to explain physical characteristics of the materials. We highlight the importance of the complementary dynamic disorder concept that can rationalize various key properties of halide perovskites for which standard polaron and band-theory pictures of carrier transport fail.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The development of halide perovskite semiconductors led to various technological breakthroughs in optoelectronics, in particular in the areas of photovoltaics and light-emitting diodes. Additionally, the study of their fundamental properties has uncovered intriguing puzzles that demand explanation. Polaronic effects associated with the coupling of electrons and holes to polar lattice vibrations are often invoked as a microscopic mechanism to explain various unusual experimental observations. While some form of polaronic behavior undoubtedly exists in these systems, several assumptions underlying standard models used to describe a polaron mechanism appear to be strongly violated in these materials. In this Perspective, we investigate the role of polaronic effects in halide perovskites and summarize signatures and failures of the polaron picture to explain physical characteristics of the materials. We highlight the importance of the complementary dynamic disorder concept that can rationalize various key properties of halide perovskites for which standard polaron and band-theory pictures of carrier transport fail.
4.
C Griesser, H Li, E-M Wernig, D Winkler, Shakibi N Nia, T Mairegger, T Götsch, T Schachinger, A Steiger-Thirsfeld, S Penner, D Wielend, D A Egger, C Scheurer, K Reuter, J Kunze-Liebhäuser
True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity Journal Article
In: ACS Catalysis, pp. 4920-4928, 2021.
@article{,
title = {True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity},
author = {C Griesser and H Li and E-M Wernig and D Winkler and Shakibi N Nia and T Mairegger and T G\"{o}tsch and T Schachinger and A Steiger-Thirsfeld and S Penner and D Wielend and D A Egger and C Scheurer and K Reuter and J Kunze-Liebh\"{a}user},
url = {https://pubs.acs.org/doi/abs/10.1021/acscatal.1c00415},
doi = {10.1021/acscatal.1c00415},
year = {2021},
date = {2021-04-07},
urldate = {2021-04-07},
journal = {ACS Catalysis},
pages = {4920-4928},
abstract = {Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as −1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as −1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.
3.
T M Brenner, C Gehrmann, R Korobko, T Livneh, D A Egger, O Yaffe
Anharmonic host-lattice dynamics enable fast ion conduction in superionic AgI Journal Article
In: Physical Review Materials, vol. 4, no. 11, pp. 115402, 2020.
@article{,
title = {Anharmonic host-lattice dynamics enable fast ion conduction in superionic AgI},
author = {T M Brenner and C Gehrmann and R Korobko and T Livneh and D A Egger and O Yaffe},
url = {https://link.aps.org/doi/10.1103/PhysRevMaterials.4.115402},
doi = {10.1103/PhysRevMaterials.4.115402},
year = {2020},
date = {2020-11-30},
urldate = {2020-11-30},
journal = {Physical Review Materials},
volume = {4},
number = {11},
pages = {115402},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
2.
J D Ziegler, J Zipfel, B Meisinger, M Menahem, X Zhu, T Taniguchi, K Watanabe, O Yaffe, D A Egger, A Chernikov
Fast and Anomalous Exciton Diffusion in Two-Dimensional Hybrid Perovskites Journal Article
In: Nano Letters, vol. 20, no. 9, pp. 6674-6681, 2020, ISSN: 1530-6984.
@article{,
title = {Fast and Anomalous Exciton Diffusion in Two-Dimensional Hybrid Perovskites},
author = {J D Ziegler and J Zipfel and B Meisinger and M Menahem and X Zhu and T Taniguchi and K Watanabe and O Yaffe and D A Egger and A Chernikov},
url = {https://doi.org/10.1021/acs.nanolett.0c02472},
doi = {10.1021/acs.nanolett.0c02472},
issn = {1530-6984},
year = {2020},
date = {2020-08-10},
journal = {Nano Letters},
volume = {20},
number = {9},
pages = {6674-6681},
abstract = {Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and light-emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are barely explored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime, we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton\textendashphonon coupling, highlighting two-dimensional hybrids as promising platforms for basic research and optoelectronic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and light-emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are barely explored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime, we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton–phonon coupling, highlighting two-dimensional hybrids as promising platforms for basic research and optoelectronic applications.
1.
M Asher, D Angerer, R Korobko, Y Diskin-Posner, D A Egger, O Yaffe
Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors Journal Article
In: Advanced Materials, vol. 32, no. 10, pp. 1908028, 2020, ISSN: 0935-9648.
@article{,
title = {Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors},
author = {M Asher and D Angerer and R Korobko and Y Diskin-Posner and D A Egger and O Yaffe},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.201908028},
doi = {10.1002/adma.201908028},
issn = {0935-9648},
year = {2020},
date = {2020-01-31},
journal = {Advanced Materials},
volume = {32},
number = {10},
pages = {1908028},
abstract = {Abstract The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.