Imperial College London

ProfessorAronWalsh

Faculty of EngineeringDepartment of Materials

Chair in Materials Design
 
 
 
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Contact

 

+44 (0)20 7594 1178a.walsh Website

 
 
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Location

 

2.10Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

559 results found

Ogawa K, Abe R, Walsh A, 2024, Band Gap Narrowing by Suppressed Lone-Pair Activity of Bi3., J Am Chem Soc, Vol: 146, Pages: 5806-5810

Post-transition metal cations with a lone pair (ns2np0) electronic configuration such as Pb2+ and Bi3+ are important components of materials for solar-to-energy conversion. As in molecules like NH3, the lone pair is often stereochemically active in crystals, associated with distorted coordination environments of these cations. In the present study, we demonstrate that suppressed lone pair stereochemical activity can be used as a tool to enhance visible light absorption. Based on an orbital interaction model, we predict that a centrosymmetric environment of the cations limits the orbital interactions with anions, deactivates the lone pair, and narrows the band gap. A high-symmetry Bi3+ site is realized by isovalent substitutions with Y3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi2YO4X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y-O compounds. The introduction of Bi3+ to the undistorted Y3+ site in Bi2YO4X results in a narrowed band gap, as predicted theoretically and confirmed experimentally. The orbital interaction controlled by site symmetry engineering offers a pathway for the further development of post-transition metal compounds for optoelectronic applications.

Journal article

Li M, Li Z, Liu M, Fu H, Qi F, Lin FR, Walsh A, Jen AK-Yet al., 2024, A Hole-Selective Self-Assembled Monolayer for Both Efficient Perovskite and Organic Solar Cells., Langmuir, Vol: 40, Pages: 4772-4778

Self-assembled monolayers (SAMs) emerging as promising hole-selective layers (HSLs) are advantageous for facile processability, low cost, and minimal material consumption in the fabrication of both perovskite solar cells (PSCs) and organic solar cells (OSCs). However, owing to the different nature between perovskites and organic semiconductors, few SAMs were reported to effectively accommodate both PSCs and OSCs at the same time. In this regard, a universally applicable SAM that can accommodate both perovskites and organic semiconductors could be desirable for simplifying cell manufacturing, especially from an industrial perspective. In this work, we designed a SAM, TDPA-Cl by introducing chlorinated phenothiazine as the headgroup and linking with anchor phosphonic acid through a butyl chain. The resulting dense SAM was carefully characterized in terms of molecular bonding, surface morphology, and packing density, and its functions in OSCs and PSCs were discussed from the aspects of interactions with the absorber layer, energy level alignment, and charge-selective dipoles. The PM6:Y6-based OSCs with TDPA-Cl SAM as the HSL showed a superior performance to those with PEDOT:PSS. Furthermore, the universality was proved with an efficiency of 17.4% in the D18:Y6 system. In PSCs, the TDPA-Cl-based devices delivered a better performance of 22.4% than the PTAA-based devices (20.8%) with improved processability and reproducibility. This work represents a SAM with reasonably good compromise between the differing requirements of OSCs and PSCs.

Journal article

Tort R, Bagger A, Westhead O, Kondo Y, Khobnya A, Winiwarter A, Davies BJV, Walsh A, Katayama Y, Yamada Y, Ryan MP, Titirici M-M, Stephens IELet al., 2024, Correction to "Searching for the Rules of Electrochemical Nitrogen Fixation"., ACS Catal, Vol: 14, Pages: 3169-3170, ISSN: 2155-5435

[This corrects the article DOI: 10.1021/acscatal.3c03951.].

Journal article

Aggarwal S, Kavanagh SR, Woo YW, Verga LG, Ganose AM, Walsh Aet al., 2024, PyTASER: Simulating transient absorption spectroscopy(TAS) for crystals from first principles, Journal of Open Source Software, Vol: 9, Pages: 5999-5999

Journal article

Li M, Li Z, Fu H, Yu R, Jiang W, Qi F, Lin FR, Chen G, Walsh A, Jen AKYet al., 2024, Molecularly Engineered Self-Assembled Monolayers as Effective Hole-Selective Layers for Organic Solar Cells, ACS Applied Energy Materials, Vol: 7, Pages: 1306-1312

Self-assembled monolayers (SAMs) are an emerging class of hole-selective layers (HSLs) to replace the conventional poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) in organic solar cells (OSCs). Despite the wide use of SAMs, it is difficult to directly establish a feedback loop between material design and OSC performance as the SAM quality will also affect the OSC performance and was frequently neglected. In this work, we designed a series of carbazole-derived SAMs by engineering the halogen substituents and the alkyl linker lengths. A SAM stacking model was established to evaluate the SAM qualities in terms of surface morphology, molecular bonding, and packing quality. Consequently, the iodinated carbazole SAM with C2 spacer (Cz-I-2) showed the highest molecular regularity, the top OSC performance in the PM6:Y6 system, and good universal applicability with a power conversion efficiency (PCE) of 18.1% in the D18:Y6 system. It can be concluded that SAMs should benefit from OSC performance by achieving suitable energy level alignment, high packing regularity, and enhanced interactions with adjacent layers. Our work provides insight into designing SAMs for effective HSLs in efficient OSCs.

Journal article

Back S, Aspuru-Guzik A, Ceriotti M, Gryn'ova G, Grzybowski B, Gu GH, Hein J, Hippalgaonkar K, Hormázabal R, Jung Y, Kim S, Kim WY, Moosavi SM, Noh J, Park C, Schrier J, Schwaller P, Tsuda K, Vegge T, von Lilienfeld OA, Walsh Aet al., 2024, Accelerated chemical science with AI., Digit Discov, Vol: 3, Pages: 23-33

In light of the pressing need for practical materials and molecular solutions to renewable energy and health problems, to name just two examples, one wonders how to accelerate research and development in the chemical sciences, so as to address the time it takes to bring materials from initial discovery to commercialization. Artificial intelligence (AI)-based techniques, in particular, are having a transformative and accelerating impact on many if not most, technological domains. To shed light on these questions, the authors and participants gathered in person for the ASLLA Symposium on the theme of 'Accelerated Chemical Science with AI' at Gangneung, Republic of Korea. We present the findings, ideas, comments, and often contentious opinions expressed during four panel discussions related to the respective general topics: 'Data', 'New applications', 'Machine learning algorithms', and 'Education'. All discussions were recorded, transcribed into text using Open AI's Whisper, and summarized using LG AI Research's EXAONE LLM, followed by revision by all authors. For the broader benefit of current researchers, educators in higher education, and academic bodies such as associations, publishers, librarians, and companies, we provide chemistry-specific recommendations and summarize the resulting conclusions.

Journal article

Baldwin WJ, Liang X, Klarbring J, Dubajic M, Dell'Angelo D, Sutton C, Caddeo C, Stranks SD, Mattoni A, Walsh A, Csányi Get al., 2024, Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains., Small, Vol: 20

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases.

Journal article

Walsh A, 2024, Open computational materials science., Nat Mater, Vol: 23, Pages: 16-17

Journal article

Tolborg K, Walsh A, 2023, Low-Cost Vibrational Free Energies in Solid Solutions with Machine Learning Force Fields., J Phys Chem Lett, Vol: 14, Pages: 11618-11624

The rational design of alloys and solid solutions relies on accurate computational predictions of phase diagrams. The cluster expansion method has proven to be a valuable tool for studying disordered crystals. However, the effects of vibrational entropy are commonly neglected due to the computational cost. Here, we devise a method for including the vibrational free energy in cluster expansions with a low computational cost by fitting a machine learning force field (MLFF) to the relaxation trajectories available from cluster expansion construction. We demonstrate our method for two (pseudo)binary systems, Na1-xKxCl and Ag1-xPdx, for which accurate phonon dispersions and vibrational free energies are derived from the MLFF. For both systems, the inclusion of vibrational effects results in significantly better agreement with miscibility gaps in experimental phase diagrams. This methodology can allow routine inclusion of vibrational effects in calculated phase diagrams and thus more accurate predictions of properties and stability for mixtures of materials.

Journal article

Bagger A, Walsh A, 2023, Metal–organic framework electrocatalysis: More than a sum of parts?, APL Energy, Vol: 1

<jats:p>The ever cheapening renewable energy calls for an effective means of storing and using electricity. Electrocatalysis is key for transforming electricity into chemical bonds. However, electrolysis is limited by the catalyst at the electrodes. In this work, we explore metal–organic frameworks (MOFs) as potential electrocatalysts. We investigate MOF-525, consisting of Zr nodes and tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers. We show using density functional theory simulations that metal incorporation in the ligand changes the reactivity in an electrochemical environment. Furthermore, we find that the MOF-derived porphyrin structure has a similar catalytic performance to the MOF itself for the hydrogen evolution, oxygen reduction, and CO2 reduction reactions. Our findings highlight the challenge of using and reporting catalysis from complex hybrid materials, such as MOFs.</jats:p>

Journal article

Mączka M, Ptak M, Gągor A, Zaręba JK, Liang X, Balčiu Nas S, Semenikhin OA, Kucheriv OI, Gural'skiy IA, Shova S, Walsh A, Banys JR, Šimėnas Met al., 2023, Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites., Chem Mater, Vol: 35, Pages: 9725-9738, ISSN: 0897-4756

Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.

Journal article

Tort R, Bagger A, Westhead O, Kondo Y, Khobnya A, Winiwarter A, Davies BJV, Walsh A, Katayama Y, Yamada Y, Ryan MP, Titirici M-M, Stephens IELet al., 2023, Searching for the rules of electrochemical nitrogen fixation, ACS Catalysis, Vol: 13, Pages: 14513-14522, ISSN: 2155-5435

Li-mediated ammonia synthesis is, thus far, the only electrochemical method for heterogeneous decentralized ammonia production. The unique selectivity of the solid electrode provides an alternative to one of the largest heterogeneous thermal catalytic processes. However, it is burdened with intrinsic energy losses, operating at a Li plating potential. In this work, we survey the periodic table to understand the fundamental features that make Li stand out. Through density functional theory calculations and experimentation on chemistries analogous to lithium (e.g., Na, Mg, Ca), we find that lithium is unique in several ways. It combines a stable nitride that readily decomposes to ammonia with an ideal solid electrolyte interphase, balancing reagents at the reactive interface. We propose descriptors based on simulated formation and binding energies of key intermediates and further on hard and soft acids and bases (HSAB principle) to generalize such features. The survey will help the community toward electrochemical systems beyond Li for nitrogen fixation.

Journal article

Wildman EJ, Lawrence GB, Walsh A, Morita K, Simpson S, Ritter C, Stenning GBG, Arevalo-Lopez AM, Mclaughlin ACet al., 2023, Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO., Nat Commun, Vol: 14

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.

Journal article

Liga SM, Kavanagh SR, Walsh A, Scanlon DO, Konstantatos Get al., 2023, Mixed-Cation Vacancy-Ordered Perovskites (Cs<inf>2</inf>Ti<inf>1-x</inf>Sn<inf>x</inf>X<inf>6</inf>; X = I or Br): Low-Temperature Miscibility, Additivity, and Tunable Stability, Journal of Physical Chemistry C, Vol: 127, Pages: 21399-21409, ISSN: 1932-7447

Lead toxicity and poor stability under operating conditions are major drawbacks that impede the widespread commercialization of metal-halide perovskite solar cells. Ti(IV) has been considered as an alternative species to replace Pb(II) because it is relatively nontoxic and abundant and its perovskite-like compounds have demonstrated promising performance when applied in solar cells (η > 3%), photocatalysts, and nonlinear optical applications. Yet, Ti(IV) perovskites show instability in air, hindering their use. On the other hand, Sn(IV) has a similar cationic radius to Ti(IV), adopting the same vacancy-ordered double perovskite (VODP) structure and showing good stability in ambient conditions. We report here a combined experimental and computational study on mixed titanium-tin bromide and iodide VODPs, motivated by the hypothesis that these mixtures may show a stability higher than that of the pure titanium compositions. Thermodynamic analysis shows that the cations are highly miscible in these vacancy-ordered structures. Experimentally, we synthesized mixed titanium-tin VODPs as nanocrystals across the entire mixing range x (Cs2Ti1-xSnxX6; X = I or Br), using a colloidal synthetic approach. Analysis of the experimental and computed absorption spectra reveals weak hybridization and interactions between Sn and Ti octahedra with the alloy absorption being essentially a linear combination of the pure Sn and Ti compositions. These compounds are stabilized at high percentages of Sn (x of ∼60%), as expected, with bromide compositions demonstrating greater stability compared to the iodides. Overall, we find that these materials behave akin to molecular aggregates, with the thermodynamic and optoelectronic properties governed by the intraoctahedral interactions.

Journal article

Wang X, Kavanagh SR, Scanlon DO, Walsh Aet al., 2023, Four-electron negative- U vacancy defects in antimony selenide, Physical Review B, Vol: 108, ISSN: 2469-9950

The phenomenon of negative-U behavior, where a defect traps a second charge carrier more strongly than the first, has been established in many host crystals. Here, we report the case of four-carrier transitions for both vacancy defects in Sb2Se3. A global structure searching strategy is employed to explore the defect energy landscape from first principles, revealing large atomic reconfigurations which facilitate a major charge redistribution. A thermodynamic analysis of the accessible charge states reveals a four-electron negative-U transition (Δq=4) for both VSe and VSb which, combined with previous calculations for antisites and interstitials, now demonstrates amphoteric behavior for all intrinsic point defects in Sb2Se3, with an impact on its usage in solar cells. The unusual behavior is facilitated by valence alternation, a reconfiguration of the local bonding environments, characteristic of both Se and Sb.

Journal article

Wang X, Li Z, Kavanagh SR, Ganose AM, Walsh Aet al., 2023, Correction: Lone pair driven anisotropy in antimony chalcogenide semiconductors, Physical Chemistry Chemical Physics, Vol: 25, Pages: 25055-25055, ISSN: 1463-9076

Correction for 'Lone pair driven anisotropy in antimony chalcogenide semiconductors' by Xinwei Wang et al., Phys. Chem. Chem. Phys., 2022, 24, 7195-7202, https://doi.org/10.1039/D1CP05373F.

Journal article

Onwuli A, Hegde AV, Nguyen KVT, Butler KT, Walsh Aet al., 2023, Element similarity in high-dimensional materials representations, Digital Discovery, Vol: 2, Pages: 1558-1564

The traditional display of elements in the periodic table is convenient for the study of chemistry and physics. However, the atomic number alone is insufficient for training statistical machine learning models to describe and extract composition-structure-property relationships. Here, we assess the similarity and correlations contained within high-dimensional local and distributed representations of the chemical elements, as implemented in an open-source Python package ElementEmbeddings. These include element vectors of up to 200 dimensions derived from known physical properties, crystal structure analysis, natural language processing, and deep learning models. A range of distance measures are compared and a clustering of elements into familiar groups is found using dimensionality reduction techniques. The cosine similarity is used to assess the utility of these metrics for crystal structure prediction, showing that they can outperform the traditional radius ratio rules for the structural classification of AB binary solids.

Journal article

Liang X, Klarbring J, Baldwin WJ, Li Z, Csanyi G, Walsh Aet al., 2023, Structural Dynamics Descriptors for Metal Halide Perovskites, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 127, Pages: 19141-19151, ISSN: 1932-7447

Journal article

Mosquera-Lois I, Kavanagh SR, Klarbring J, Tolborg K, Walsh Aet al., 2023, Imperfections are not 0 K: free energy of point defects in crystals, CHEMICAL SOCIETY REVIEWS, Vol: 52, Pages: 5812-5826, ISSN: 0306-0012

Journal article

Ogawa K, Tolborg K, Walsh A, 2023, Models of oxygen occupancy in lead phosphate apatite Pb10(PO4)6O, ACS Energy Letters, Vol: 8, Pages: 3941-3944, ISSN: 2380-8195

Lead phosphate apatite, the parent compound of the proposed room-temperature superconductor LK-99, features a [Pb10(PO4)6]II scaffold with a charge-compensating oxide ion. This O–II occupies a 4e site in the P63/m unit cell, with 25% probability on average. We model the occupancy of this site from substoichiometric (x = 0) to superstoichiometric (x = 4) regimes in Pb10(PO4)6Ox. Doping is predicted by adjusting the oxygen composition within the ⟨0001⟩ channel, with evidence for strong O–O correlation. This behavior introduces a sensitivity to the crystal growth and annealing conditions, with an opportunity for novel functionality to emerge.

Journal article

Grosso BF, Davies DW, Zhu B, Walsh A, Scanlon DOet al., 2023, Accessible chemical space for metal nitride perovskites, CHEMICAL SCIENCE, ISSN: 2041-6520

Journal article

Huang J, Shin S-J, Tolborg K, Ganose A, Krenzer G, Walsh Aet al., 2023, Room-temperature stacking disorder in layered covalent-organic frameworks from machine-learning force fields, MATERIALS HORIZONS, Vol: 10, Pages: 2883-2891, ISSN: 2051-6347

Journal article

Krenzer G, Klarbring J, Tolborg K, Rossignol H, McCluskey AR, Morgan BJ, Walsh Aet al., 2023, Nature of the Superionic Phase Transition of Lithium Nitride from Machine Learning Force Fields, CHEMISTRY OF MATERIALS, Vol: 35, Pages: 6133-6140, ISSN: 0897-4756

Journal article

Ogawa K, Suzuki H, Walsh A, Abe Ret al., 2023, Orbital Engineering in Sillen-Aurivillius Phase Bismuth Oxyiodide Photocatalysts through Interlayer Interactions, CHEMISTRY OF MATERIALS, Vol: 35, Pages: 5532-5540, ISSN: 0897-4756

Journal article

Tolborg K, Walsh A, 2023, Models of orientational disorder in hybrid organic-inorganic piezoelectric materials, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 11, Pages: 8885-8891, ISSN: 2050-7526

Journal article

Li Z, Park J-S, Ganose AMM, Walsh Aet al., 2023, From Cubic to Hexagonal: Electronic Trends across Metal Halide Perovskite Polytypes, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 127, Pages: 12695-12701, ISSN: 1932-7447

Journal article

Shin S-J, Gittins JWW, Golomb MJJ, Forse ACC, Walsh Aet al., 2023, Microscopic Origin of Electrochemical Capacitance in Metal-Organic Frameworks, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 145, Pages: 14529-14538, ISSN: 0002-7863

Journal article

Nicolson A, Breternitz J, Kavanagh SR, Tomm Y, Morita K, Squires AG, Tovar M, Walsh A, Schorr S, Scanlon DOet al., 2023, Interplay of Static and Dynamic Disorder in the Mixed-Metal Chalcohalide Sn<sub>2</sub>SbS<sub>2</sub>I<sub>3</sub>, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 145, Pages: 12509-12517, ISSN: 0002-7863

Journal article

Son J, Ma S, Jung Y-K, Tan J, Jang G, Lee H, Lee CU, Lee J, Moon S, Jeong W, Walsh A, Moon Jet al., 2023, Unraveling chirality transfer mechanism by structural isomer-derived hydrogen bonding interaction in 2D chiral perovskite, NATURE COMMUNICATIONS, Vol: 14

Journal article

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