Materials Science
AI-Driven Expansion and Application of the Alexandria Database
Théo Cavignac, Jonathan Schmidt, Pierre-Paul De Breuck et al. (+9 more)
We present a novel multi-stage workflow for computational materials discovery that achieves a 99% success rate in identifying compounds within 100 meV/atom of thermodynamic stability, with a threefold improvement over previous approaches. By combining the Matra-Genoa generative model, Orb-v2 universal machine learning interatomic potential, and ALIGNN graph neural network for energy prediction, we generated 119 million candidate structures and added 1.3 million DFT-validated compounds to the ALEXANDRIA database, including 74 thousand new stable materials. The expanded ALEXANDRIA database now contains 5.8 million structures with 175 thousand compounds on the convex hull. Predicted structural disorder rates (37-43%) match experimental databases, unlike other recent AI-generated
datasets. Analysis reveals fundamental patterns in space group distributions, coordination environments, and phase stability networks, including sub-linear scaling of convex hull connectivity. We release the complete
dataset, including sAlex25 with 14 million out-of-
equilibrium structures containing forces and stresses for training universal force fields. We demonstrate that fine-tuning a GRACE model on this data improves
benchmark accuracy. All data, models, and workflows are freely available under Creative Commons licenses.
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All authors Théo Cavignac, Jonathan Schmidt, Pierre-Paul De Breuck, Antoine Loew, Tiago F. T. Cerqueira, Hai-Chen Wang, Anton Bochkarev, Yury Lysogorskiy, Aldo H. Romero, Ralf Drautz, Silvana Botti, Miguel A. L. Marques
Materials Science · Cross-listed from physics.app-ph
Solvent-Directed Femtosecond Laser Ablation: Tuning Phase and Defect Engineering in Hybrid CdPS3/CdS Nanostructures
Andrei Ushkov, Nadezhda Belozerova, Gleb Tikhonowski et al. (+8 more)
The limited visible-light absorption of wide-bandgap van der Waals
crystals fundamentally restricts their utility in
solar energy conversion. Here, we report a surfactant-free, solvent-directed
laser synthesis strategy to engineer the phase and optoelectronic properties of Cadmium Phosphorus Trisulfide (CdPS3). By exploiting the non-
equilibrium thermodynamics of femtosecond pulsed
laser ablation in
liquid (fs-PLAL), we demonstrate a tunable transition from the stoichiometric ternary phase to a highly active binary-rich
heterostructure. While ablation in water preserves the monoclinic CdPS3
lattice, the reducing environment of isopropanol triggers the formation of CdS
quantum dots and metallic cadmium defect sites. This solvent-induced phase engineering transforms the ultraviolet-active host into a robust visible-light photocatalyst. The resulting hybrid CdPS3/CdS nanocolloids exhibit superior charge separation efficiency, driven by Schottky-like metal-
semiconductor junctions, achieving ~ 90% degradation of Methylene Blue under 532 nm irradiation within 30 minutes. This work establishes fs-PLAL as a scalable defect-engineering tool for complex ternary layered materials, offering a new design of high-performance metal-thiophosphate-based photocatalysts.
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All authors Andrei Ushkov, Nadezhda Belozerova, Gleb Tikhonowski, Stepan Klimov, Alexander Syuy, Sergey V. Bazhenov, Sergey Novikov, Vladimir G. Leiman, Aleksey Arsenin, Gleb I. Tselikov, Valentyn Volkov
Materials Science · Cross-listed from cs.LG
Contrast transfer functions help quantify neural network out-of-distribution generalization in HRTEM
Luis Rangel DaCosta and Mary C. Scott
Neural networks, while effective for tackling many challenging scientific tasks, are not known to perform well out-of-distribution (OOD), i.e., within domains which differ from their training data. Understanding neural network OOD generalization is paramount to their successful deployment in experimental workflows, especially when ground-truth knowledge about the experiment is hard to establish or experimental conditions significantly vary. With inherent access to ground-truth information and fine-grained control of underlying distributions, simulation-based data curation facilitates precise investigation of OOD generalization behavior. Here, we probe generalization with respect to imaging conditions of neural network
segmentation models for high-resolution transmission electron microscopy (HRTEM) imaging of nanoparticles, training and measuring the OOD generalization of over 12,000 neural networks using synthetic data generated via random structure sampling and multislice simulation. Using the HRTEM contrast transfer function, we further develop a framework to compare information content of HRTEM
datasets and quantify OOD domain shifts. We demonstrate that neural network
segmentation models enjoy significant performance stability, but will smoothly and predictably worsen as imaging conditions shift from the training distribution. Lastly, we consider limitations of our approach in explaining other OOD shifts, such as of the atomic structures, and discuss complementary techniques for understanding generalization in such settings.
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Materials Science
First-Principles Investigation of Mechanical, Lattice Dynamical, and Thermodynamic Properties of BaTiO3 Polymorphs
Arpon Chakraborty, M. N. H. Liton, M. S. I. Sarker et al. (+2 more)
BaTiO3 (BTO) is one of the most interesting classes of perovskite materials. The present study has been complied to explore some physical properties such as mechanical, vibrational, thermo-physical, and temperature dependent thermodynamic properties of BaTiO3 polymorphs comprehensively using first principles calculations based on density functional theory (DFT). All the polymorphs are found to be mechanically stable. The polymorphs are elastically anisotropic, machinable and have high hardness and toughness. The cubic phase possesses brittle nature while the other phases show ductile character. The high melting point of the polymorphs reveals that they can be used in tough situations. Also, three of the polymorphs can be used as thermal barrier coating. Moreover, we have also calculated the
lattice dynamics and found improved results compared to the available results in the literature. In addition, the temperature and pressure dependent thermodynamic parameters of the polymorphs are evaluated and analyzed for the first time using the quasi-harmonic Debye model. The thermodynamic properties suggested that all phases would be good choices for application in the fields of automobiles, cooling systems, thermal electronic devices, thermal exchangers, and space crafts.
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All authors Arpon Chakraborty, M. N. H. Liton, M. S. I. Sarker, M. M. Rahman, M. K. R. Khan
Materials Science · Cross-listed from cond-mat.mes-hall
Orbital magnetization from parallel transport of Bloch states
Johannes Mitscherling, Jan Priessnitz and Libor Šmejkal
Quantum geometric formulations of linear and nonlinear responses can be constructed from a single building block in the form of a gauge-invariant interband transition operator. Here, we identify a second building block for
quantum geometry: a band-resolved adiabatic connection operator that captures the noncommutativity between band projectors and their momentum derivatives. The band-resolved adiabatic connection operator, first introduced in the theory of adiabatic driving, serves as a generalized angular momentum within the state
manifold of single bands, and we employ it to reformulate expressions for the band-resolved orbital
magnetic moment. This form provides a complementary
geometric interpretation alongside the multiband separation between energetic- and
quantum-state properties by the two-state Berry
curvature. Our formalism allows us to present formulas valid for both nondegenerate and degenerate bands, thereby removing the limitations of the common Bloch-state formula. We illustrate our theory by calculating a large orbital magnetization emerging without
spin-orbit coupling in a
spin-compensated, noncoplanar anomalous
Hall magnet with degenerate bands.
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Materials Science
Phonon-assisted tunneling in Jahn-Teller E$ \times $e impurity centers in crystals
V. Hizhnyakov
Tunnel transitions between various distorted states of
impurity centers in
crystals with the E$ \times $e Jahn-Teller effect are considered, taking into account changes in the dynamics of
phonons caused by this effect. Both linear and quadratic vibrational interactions are taken into account. It was found that
phonon scattering accompanying tunneling leads not only to a broadening of the energy spectrum of the transitions, but also to a deterioration in resonance. The results obtained are consistent with measurements of ultrasonic attenuation in Al2O3:Ni, GaAs:Mn and in GaAs:Cu doped
crystals. A notable feature of E$ \times $e tunneling is the existence of a range of values for quadratic vibration interaction, in which the tunneling remains coherent at sufficiently high temperatures.
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Materials Science
A Unified Symmetry Classification of Magnetic Orders via Spin Space Groups: Prediction of Coplanar Even-Wave Phases
Ziyin Song, Ziyue Qi, Chen Fang et al. (+2 more)
Spin space groups (SSGs) impose fundamentally different constraints on
magnetic configurations in real and reciprocal spaces. As a consequence, the correspondence between real-space and momentum-space
spin arrangements is far richer than traditionally assumed. Building on the complete enumeration of SSGs, we develop a systematic,
symmetry-based framework that classifies all possible
spin arrangements allowed by these groups. This unified approach naturally incorporates conventional
magnetic orders, altermagnetism, and p-wave magnetism as distinct
symmetry classes. Crucially, our classification predicts a variety of novel
magnetic phases, highlighted by the discovery of the coplanar even-wave magnet: a state that is non-collinear in real space but hosts a collinear even-wave
spin polarization in k-space. Analysis of a minimal model reveals that this phase is characterized by non-quantized
spin polarization and exhibits a novel mechanism for
symmetry-enforced zero polarization on non-degenerate bands. Extending the framework from bulk
crystals to layer SSGs appropriate for two-dimensional systems, we further predict layered counterparts and provide
symmetry guidelines for designing
bilayer coplanar p-wave and even-wave magnets. We further validate this finding through first-principles calculations and propose CoCrO4 as a promising candidate for its experimental realization, thereby demonstrating the completeness and predictive power of the SSG-based classification of
magnetic orders.
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All authors Ziyin Song, Ziyue Qi, Chen Fang, Zhong Fang, Hongming Weng
Materials Science
Low Resistance Non-Alloyed Ohmic Contacts to High Al Composition n-type AlGaN
Joseph E. Dill, Xianzhi Wei, Changkai Yu et al. (+6 more)
Ohmic contacts to high (>70%) Al content n-type Al$_x$Ga$_{1-x}$N ultra-wide bandgap
semiconductor layers in nitride electronic and photonic devices are typically fabricated by a lift-off process and high temperature ($>700^\circ$C) thermal alloying. These conditions often result in significant structural deformations of the fabricated structures and impose a harsh thermal budget on all other aspects of the device. Here, we report the fabrication of non-alloyed as-deposited ohmic contacts to 71% n+AlGaN ($E_\text{g}\sim5.4$~eV) with a free carrier concentration of roughly $7\times 10^{19}$~cm$^{-3}$ and a resistivity of 4 - 5.5 m$\Omega$cm (among the lowest reported for Al$_{0.71}$Ga$_{0.29}$N) with linear $I-V$ characteristics and a contact resistivity of $\rho_\text{c}=(4.4\pm1.0)\times10^{-4}$~$\Omega$cm$^2$ (measured at zero voltage). Contacts with this quality are formed by two separate fabrication schemes: (i) metal-first patterning, and (ii) lift-off with an oxygen asher descum prior to metal deposition. Given the low threading dislocation density in the single-
crystal AlN substrate used for epitaxy, the smooth morphology of the contacted epitaxial surface, and the non-alloyed nature of the contacts, this contact resistivity is attributed purely to thermionic field emission through the metal-
semiconductor junction. Contact resistivity extraction at low current injection enables us to model these results using a thermionic field-emission model of contact resistivity, yielding a barrier height for Ti/Al$_{0.71}$Ga$_{0.29}$N of $(0.81\pm0.02)$ eV.
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All authors Joseph E. Dill, Xianzhi Wei, Changkai Yu, Akhansha Arvind, Shivali Agrawal, Debaditya Bhattacharya, Keisuke Shinohara, Debdeep Jena, Huili Grace Xing
Materials Science
An accurate alternative to hybrid functionals for germanium: DFT+$\alpha$
Abdulgaffar Abdurrazaq, Ruggero Lot, Antoine Jay et al. (+5 more)
The accuracy of bulk property predictions in density functional theory (DFT) calculations depends on the choice of exchange-correlation functional. While the Perdew-Burke-Ernzerhof (PBE) functional systematically overestimates
lattice parameters and strongly underestimates electronic band gaps, hybrid functionals such as Heyd-Scuseria-Ernzerhof (HSE) offer better overall agreement across a broad range of materials. Using germanium as a critical test case, we challenge the ability of both functionals to capture
semiconductor properties. Although HSE improves PBE's gap error, it fails to reproduce germanium's correct $\Gamma$-L indirect and $\Gamma$-$\Gamma$ band gaps simultaneously. Noting that the PBE underestimated energy separation between the 4p valence-band maximum and 4s conduction-band minimum causes unphysical $sp$ mixing, we propose DFT+$\alpha$, a semi-empirical correction scheme applied selectively to 4s-like orbitals. For germanium, DFT+$\alpha$ restores the proper ordering and orbital character of the band edges and yields accurate
lattice constant, bulk modulus, elastic constants and
phonon frequencies at a fraction of hybrid-functional computational cost.
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All authors Abdulgaffar Abdurrazaq, Ruggero Lot, Antoine Jay, Gabriela Herrero-Saboya, Nicolas Richard, Layla Martin-Samos, Anne Hémeryck, Stefano de Gironcoli
Materials Science
Molybdenum Disulfide Bilayers Hybridized with Reduced Graphene Oxide Nanosheets for Enhanced Hydrogen Evolution Reaction
Bruno Ferreira Brischi, Gabriela Frajtag, João Pedro da Silva Mariano et al. (+6 more)
Two-dimensional (2D) molybdenum disulfide (MoS2) nanosheets have attracted
attention as a promising and cost-effective alternative catalyst for the hydrogen
evolution reaction (HER). However, their aggregation and poor conductivity limit their catalytic activity. Consequently, researchers are exploring ways to improve the conductivity and density of electroactive edges of MoS2 through hybridization with akin advanced 2D materials. Here, reduced
graphene oxide (rGO) nanosheets are added to a
liquid-phase-exfoliated MoS2 dispersion to create drop-casting hybrid MoS2@rGO electrodes for HER. The findings reveal the formation of ~1.1 nm-thick MoS2
bilayers, with the catalytic performance of MoS2@rGO dependent on the degree of
graphene oxide reduction. The rGO moderate reduction prevents the MoS2
bilayers from restacking, improves conductivity, retains oxygenated groups that enhance interlayer spacing, and exposes more electroactive edges. When hybridized with rGO crafted at optimal reduction time, the hybrid system eclipses the efficiency of pristine MoS2
bilayers by attaining the lowest overpotential (~70 mV) and the lowest Tafel slope (~46 mV dec-1). This shows that MoS2
bilayers hybridized with rGO offer a promising method to outperform the electrocatalytic efficiency of MoS2-based electrodes. These findings expand opportunities for future strain engineering, defect engineering, and high-end twist-angle assemblies for hybrid systems combining MoS2
bilayers and rGO.
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All authors Bruno Ferreira Brischi, Gabriela Frajtag, João Pedro da Silva Mariano, Laís Fernanda Medeiros Ruela, Gustavo Duarte Verçosa, Camila Desiderio Fernandes, Murilo Santhiago, Letícia Mariê Minatogau Ferro, Leandro Merces
Materials Science
Computational Screening and Discovery of Silver-Indium Halide Double Salts
Christos Tyrpenou, G. Krishnamurthy Grandhi, Paola Vivo et al. (+2 more)
Perovskite-inspired materials have emerged as promising candidates for both outdoor and indoor photovoltaic applications owing to their favorable optoelectronic properties and reduced toxicity. Here, we employ the experimentally realized AgBiI$_4$ double salt as a structural prototype and replace Bi$^{3+}$ with In$^{3+}$ to design a novel lead-free halide compound, AgInI$_4$. First-principles calculations predict that AgInI$_4$ is both chemically and dynamically stable, exhibiting a direct band gap of 1.72 eV, comparable to its bismuth analogue. However, its predicted photovoltaic performance, evaluated using the spectroscopic limited maximum efficiency metric, is lower under both
solar and LED illumination. This reduction arises primarily from
symmetry-forbidden optical transitions and the absence of Bi-derived 6s$^2$ lone-pair states at the valence band maximum. High-throughput screening of the Ag-In-I ternary phase-space reveals several more stable and metastable compounds that fall into two structural families: tetrahedrally and octahedrally coordinated, with characteristic band gaps near 3.0 eV and 2.0 eV, respectively. Despite multiple synthetic attempts, the predicted AgInI$_4$ phase could not be experimentally realized, underscoring the challenges of stabilizing indium-based halide double salts. While these materials are unlikely to serve as efficient photovoltaic absorbers, their tunable band gaps and stability make them promising candidates for charge transport and other optoelectronic applications.
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All authors Christos Tyrpenou, G. Krishnamurthy Grandhi, Paola Vivo, Mikaël Kepenekian, George Volonakis
Materials Science · Cross-listed from physics.app-ph
Reducing dislocation defect levels via sub-melt nanosecond pulsed-laser induced densification of diamond
Adam H. Khan, Tae Sung Kim, Gabe Guss et al. (+4 more)
Dislocations and polishing-induced defect networks in synthetic diamond generate local strain fields that broaden Raman features and limit optical, thermal, and electronic performance. Sub-melt
laser annealing has emerged as a route to repair near-surface defects without graphitization, yet quantitative evidence of densification, defect depletion, and property recovery remains limited. Here, we show that nanosecond pulsed-
laser annealing (PLA) can relax dislocation-associated strain in single-
crystal CVD diamond by compacting and reorganizing the damaged near-surface region. Single- and two-pulse PLA were applied, and structural
evolution was quantified using co-registered ISO 25178 white-light interferometry, depth-resolved Raman spectroscopy, and cross-sectional STEM with
geometric phase analysis (GPA). Across a 5x6 grid(n = 30), responsive regions show large reductions in local slope (Sdq 45-65%), developed area (Sdr 60-90%), height spread (Sp, Sz 30-65%), void volume (Vv 57-60%), and roughness amplitude (Sa, Sq 48-57%), consistent with densification of ~4-6.5 nm. Raman profiling reveals narrowing of the diamond line and improved spectral uniformity to depths of ~2-3 {\mu}m, indicating relaxation of dislocation-mediated strain beyond the compaction layer. STEM-GPA strain maps confirm smoother strain fields, reduced hotspots, and redistribution of localized strain concentrations after PLA. These results show that sub-melt PLA reduces dislocation-driven strain by compacting surface-connected free volume and reorganizing defect networks. The approach provides a scalable path to upgrade industrial-grade diamond including homoepitaxial, heteroepitaxial, and polycrystalline CVD to low-defect, device-ready surfaces relevant to high-power electronics, photonics, and
quantum substrates.
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All authors Adam H. Khan, Tae Sung Kim, Gabe Guss, Ted A. Laurence, Sonny S. Ly, Thejaswi U. Tumkur, Afaq H. Piracha
Materials Science · Cross-listed from physics.app-ph
Electron Emission Yield Datasets Under Electron Impact From Surfaces Characterized In Situ by XPS or AES
M.Belhaj and S. dadouch
We present the measurement, characterization, and calibration procedures used to produce a series of
datasets for various conductive and semiconductive materials. The data, provided, include emission yields as a function of incident electron energy together with surface composition obtained from X-Ray Photoelectron Spectroscopy or Auger Electron Spectroscopy analyses. Initial
datasets cover copper and gold, with additional materials to be released on arXiv.
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Materials Science
Engineering Topological Bands in Strained Covalent Organic Frameworks
Rebecca Peake, Zoé Truyens, Jan Mol et al. (+4 more)
The tunability of covalent organic frameworks (COFs) opens opportunities to engineer topological electronic phases, including topological
insulators (TIs) and higher-order topological
insulators (HOTIs)–materials that host in-gap states localized at their edges, hinges, or corners. Here we explore how chemically feasible perturbations can drive triazine-based COFs (CTFs) into topological regimes. Using a tight-binding model on the Honeycomb
lattice inspired by the frontier electronic states of CTFs, we show that introducing an effective uniaxial strain–implemented as a modulation of electron hopping on a subset of bonds–can generate a series of distinct topological band structures. This effect can be realized in practice through chemical substitution of linkers along the strained bonds. First-principles calculations demonstrate that replacing biphenyl with pyrene linkers drives a CTF to the brink of a HOTI phase, suggesting a viable route toward topological band-structure engineering in COFs.
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All authors Rebecca Peake, Zoé Truyens, Jan Mol, Christian B Nielsen, David Beljonne, David Cornil, Owen Benton
Materials Science
Observation of Raman anomaly and characterization of magnetic phases in van der Waals ferromagnet Fe$_5$GeTe$_2$
Sreelakshmi M. Nair, Aabhaas Vineet Mallik and R. S. Patel
Two-dimensional (2D) van der Waals (vdW) ferromagnet Fe$_5$GeTe$_2$ has garnered significant interest due to its high Curie temperature (T$_C$), large saturation magnetization, and complex
magnetic behavior arising, in part, from multiple inequivalent iron sites and vacancies. While several aspects of its complex
magnetic and structural characteristics have been examined through careful experiments and first principles studies, much of it remains debatable. In this study, we present one of the first comprehensive temperature-dependent Raman spectrum for bulk Fe$_5$GeTe$_2$ and in the process reveal an interesting peak shift anomaly at 150 K. We discuss the possible relationship of this Raman anomaly with the anomalous
lattice expansion reported earlier for this material at around 110 K. The impact of the anomalous
lattice expansion on the
magnetic anisotropy in this van der Waals material is also revealed by an isothermal magnetization analysis. These findings will prove crucial for the use of Fe$_5$GeTe$_2$ in high-performance spintronic devices.
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Materials Science
Nonequilibrium Photocarrier and Phonon Dynamics from First Principles: a Unified Treatment of Carrier-Carrier, Carrier-Phonon, and Phonon-Phonon Scattering
Stefano Mocatti, Giovanni Marini, Giulio Volpato et al. (+2 more)
We develop a first-principles many-body framework to describe the dynamics of photocarriers and
phonons in
semiconductors following ultrafast excitation. Our approach incorporates explicit ab initio light-matter coupling and first-principles collision integrals for carrier-carrier, carrier-
phonon, and
phonon-
phonon scattering. It also yields time-dependent quasiparticle and
phonon frequency renormalizations, along with light-induced coherent atomic motion. The equations of motion are solved in a maximally localized Wannier basis, ensuring gauge-consistent scattering integrals and ultradense momentum sampling, thereby enabling direct comparison with pump-probe experiments. The method can be coupled to constrained density-functional theory to access light-induced structural phase transitions at longer times after the light pulse. We showcase the capabilities and predictive power of this framework on MoS$_2$ and h-BN monolayers. For MoS$_2$, we resolve photoinduced renormalizations of electronic and
lattice properties, ultrafast carrier relaxation, hot-
phonon dynamics, and displacive coherent atomic motion. Including carrier-carrier scattering is crucial to obtain realistic photocarrier equilibration times, while omitting
phonon-
phonon scattering leads to incorrect long-time
lattice thermalization and a factor of two larger A$_{1g}$ coherent
phonon damping time. For h-BN, we quantify photoinduced changes in the electronic, optical, and
lattice responses in quasi-
equilibrium, demonstrating a fluence-dependent enhancement of screening and melting of excitonic features.
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All authors Stefano Mocatti, Giovanni Marini, Giulio Volpato, Pierluigi Cudazzo, Matteo Calandra
Materials Science
MBE obtained n-CdO:Eu p-Si heterojunctions – electron beam induced profiling, electrical and structural properties
E Przezdziecka, I Perlikowski, S Chusnutdinow et al. (+7 more)
The present investigation reports on the fabrication and characterization of heterojunctions based on in situ Eu doped CdO layers, which were deposited on p-type silicon using the
plasma assisted molecular beam epitaxy (PA MBE) method. The structural and optical properties of the cadmium
oxide (CdO) films were investigated using X ray diffraction and
Fourier transform infrared spectroscopy (FTIR). The CdO Eu films are polycrystalline. The electrical properties of the p n heterojunction composed of transparent n CdO Eu and p Si
semiconductors were investigated by current voltage and electron beam-induced current (EBIC) measurements. Current voltage measurements demonstrate good junction characteristics with a rectifying ratio of 20 . EBIC measurements allowed us to calculate the
diffusion length of minority carriers and the precise location of the depleted area at the CdO and Si interfaces.
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All authors E Przezdziecka, I Perlikowski, S Chusnutdinow, A Wierzbicka, A Adhikari, M Stachowicz, R Jakiela, E Zielony, P Wojnar, A Kozanecki
Materials Science · Cross-listed from cond-mat.supr-con
Michał J. Winiarski, Dominik Walczak, Szymon Królak et al. (+3 more)
We report the synthesis and physical properties of a polycrystalline, hexagonal boride YRu$_3$B$_2$. Our resistivity and heat capacity measurements indicate that YRu$_3$B$_2$ is a weakly coupled
superconductor, with critical temperature $T_c$ = 0.63 K and upper critical field $\mu_0 H_{c2}$ (0)=0.11 T. Density functional theory calculations, together with chemical-bonding analysis, reveal that the electronic states at and near the
Fermi energy level are dominated by the Ru kagome sublattice.
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All authors Michał J. Winiarski, Dominik Walczak, Szymon Królak, Duygu Yazici, Robert J. Cava, Tomasz Klimczuk
Materials Science · Cross-listed from cond-mat.mes-hall
Single-Particle X-ray Scattering Reveals a High Local Supersaturation of Precursors as the Origin of CoO Assembly Formation
Sani Y. Harouna-Mayer, Lars Klemeyer, Cecilia A. Zito et al. (+24 more)
Single-particle small-angle X-ray scattering (SP-SAXS) enables quantitative morphological analysis by recording diffraction snapshots from isolated particles using X-ray free-electron
laser (XFEL) pulses. Unlike conventional X-ray techniques, which average over the entire illuminated sample volume, SP-SAXS resolves low-contrast, less abundant, or transient species within heterogeneous particle populations that would otherwise remain hidden. Here, we apply SP-SAXS to investigate the solvothermal formation of CoO nanocrystal assemblies from a Co(acac)$_3$ precursor in benzyl alcohol. The single-particle data reveal amorphous, uniform-density Co(acac)$_2$ spheres as transient intermediates that directly crystallize into cavernous CoO nanocrystal assemblies, which explains why CoO forms as hierarchical aggregates rather than as isolated nanocrystals. These results demonstrate that SP-SAXS provides a powerful framework for disentangling morphological heterogeneity in nanoparticle formation processes.
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All authors Sani Y. Harouna-Mayer, Lars Klemeyer, Cecilia A. Zito, Johan Bielecki, Xuemei Cheng, Davide Derelli, Armando D. Estillore, Tjark L.R. Groene, Lukas V. Haas, Romain Letrun, Chan Kim, Jayanath C.P. Koliyadu, Abhishek Mall, Parichita Mazumder, Diogo V.M. Melo, Adam R. Round, Amit K. Samanta, Abhisakh Sarma, Zhou Shen, Xiao Sun, Patrik Vagovic, Tamme Wollweber, Richard Bean, Jochen Küpper, Henry N. Chapman, Dorota Koziej, Kartik Ayyer
Materials Science
Exogenous Metal Cations in the Synthesis of CsPbBr3 Nanocrystals and their Interplay with Tertiary Amines
Zhanzhao Li, Luca Goldoni, Ye Wu et al. (+9 more)
Current syntheses of CsPbBr3 halide perovskite nanocrystals (NCs) rely on over-stoichiometric amounts of Pb2+ precursors, resulting in unreacted lead ions at the end of the process. In our synthesis scheme of CsPbBr3 NCs we replaced excess Pb2+ with different exogenous metal cations (M) and investigated their effect on the synthesis products. These cations can be divided into two groups: group 1 delivers monodisperse CsPbBr3 cubes capped with oleate species (as for the case when Pb2+ is used in excess) and with photoluminescence
quantum yield (PLQY) as high as 90% with some cations (for example with M= In3+); group 2 yields irregularly shaped CsPbBr3 NCs with broad size distributions. In both cases, the addition of a tertiary ammonium cation (didodecylmethyl ammonium, DDMA+) during the synthesis, after the nucleation of the NCs, reshapes the NCs to monodisperse truncated cubes. Such NCs feature a mixed oleate/DDMA+ surface termination with PLQY values up to 90%. For group 1 cations, this happens only if the ammonium cation is directly added as a salt (DDMA-Br) while for group 2 cations this happens even if the corresponding tertiary amine (DDMA) is added, instead of DDMA-Br. This is attributed to the fact that only group 2 cations can facilitate the protonation of DDMA by the excess oleic acid present in the reaction environment. In all cases studied, the incorporation of M cations is marginal and the reshaping of the NCs is only transient: if the reactions are run for a long time the truncated cubes evolve to cubes.
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All authors Zhanzhao Li, Luca Goldoni, Ye Wu, Muhammad Imran, Yurii P. Ivanov, Giorgio Divitini, Juliette Zito, Iyyappa Rajan Panneerselvam, Dmitry Baranov, Ivan Infante, Luca De Trizio, Liberato Manna
Materials Science
Core@Shell AgBr@CsPbBr3 Nanocrystals as Precursors to Hollow Lead Halide Perovskite Nanocubes
Zhanzhao Li, Yurii P. Ivanov, Anna Cabona et al. (+7 more)
We report the synthesis of colloidal core@shell AgBr@CsPbBr3 nanocubes by a one-pot approach, where the nucleation and growth of AgBr nanocrystals occurs rapidly after the injection of chemical precursors. This is immediately followed by the overgrowth of CsPbBr3, delivering AgBr@CsPbBr3 nanocubes of several tens of nanometers in size, with the volume of the AgBr core being only a small fraction of the overall nanocrystal volume. The formation of a core@shell geometry is facilitated by the epitaxial compatibility between AgBr and CsPbBr3 along multiple crystallographic directions. Exchange with Cl-ions leads to Ag@CsPbCl3 nanocubes, whereas exchange with I-ions leads to hollow CsPbI3 nanocubes, due to selective etching of the AgBr (or Ag) core region by the I-ions diffusing in the nanocubes. These hollow CsPbI3 nanocubes can then be converted into hollow CsPbBr3 and CsPbCl3 nanocubes by halide exchange. The optical emission properties of the hollow CsPbX3 (X=Cl, Br, I) nanocubes are in line with those expected from large, non-hollow halide perovskite nanocrystals, indicating that the small hollow region in the cubes has no major influence on their optical properties.
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All authors Zhanzhao Li, Yurii P. Ivanov, Anna Cabona, Andrea Fratelli, Stefano Toso, Saptarshi Chakraborty, Giorgio Divitini, Ilka Kriegel, Sergio Brovelli, Liberato Manna