Publications by authors named "Cesare Franchini"
Perovskites exhibit outstanding performance in applications such as photocatalysis, electrochemistry, or photovoltaics, yet their practical use is hindered by the instability of these materials under operating conditions, specifically caused by the segregation of alkali cations toward the surface. The problem arises from the bulk strain related to different cation sizes, as well as the inherent electrostatic instability of perovskite surfaces. Here, we focus on atomistic details of the surface-driven process of interlayer switching of alkali atoms at the inorganic perovskite surface.
View Article and Find Full Text PDFIn polarizable materials, electronic charge carriers interact with the surrounding ions, leading to quasiparticle behavior. The resulting polarons play a central role in many materials properties including electrical transport, interaction with light, surface reactivity, and magnetoresistance, and polarons are typically investigated indirectly through these macroscopic characteristics. Here, noncontact atomic force microscopy (nc-AFM) is used to directly image polarons in FeO at the single quasiparticle limit.
View Article and Find Full Text PDFQuantitative Measurement of Cooperative Binding in Partially Dissociated Water Dimers at the Hematite "R-Cut" Surface.
J Phys Chem C Nanomater Interfaces
October 2024
Water-solid interfaces pervade the natural environment and modern technology. On some surfaces, water-water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively.
View Article and Find Full Text PDFA Multitechnique Study of CH Adsorption on a Model Single-Atom Rh Catalyst.
J Phys Chem C Nanomater Interfaces
September 2024
Article Synopsis
- The study examines how carbon hydride (CH) interacts with a model catalyst, Rh/FeO(001), focusing on various coordination environments of rhodium (Rh) atoms.* -
- Researchers used surface-sensitive techniques and density functional theory (DFT) calculations to analyze how different Rh species react during the thermal evolution of the system.* -
- Results show that CH binds strongest to 2-fold coordinated Rh sites, but unexpected desorption occurs at lower temperatures due to Rh atoms migrating to less stable sites; 5-fold coordinated Rh sites exhibit a less pronounced interaction.*
Article Synopsis
- Understanding the local coordination of active sites is crucial for effectively modeling single-atom catalysts (SACs), but it's challenging with powder-based systems.
- This study explores how platinum (Pt) atoms interact with the (11̅02) facet of α-FeO, revealing that Pt modifies the lattice structure to achieve a favorable pseudolinear coordination with surface oxygen.
- The findings suggest that the linear O-Pt-O configuration is prevalent in reactive Pt-based SACs, striking a balance between stability and the ability to interact with gas-phase reactants; thus, extensive structural searches are essential for identifying realistic active site geometries.
Article Synopsis
- Recent findings suggest that high-temperature superconducting phases in hydrides under high pressure may allow for the possibility of near-ambient superconductivity.
- The study focuses on N-doped LuH hydride, using machine learning to show that nitrogen impurities can stabilize hydrogen molecules at ambient pressure, which are crucial for achieving low-temperature superconductivity.
Article Synopsis
- Researchers highlight the potential of using external stimuli to fine-tune the properties of smart materials, emphasizing strong interactions within single-phase materials.
- The study focuses on a layered mixed anion compound, MoBrO, known for its impressive electric-field switchable polarization and strong coupling between various physical properties.
- Findings suggest that MoBrO can enable advanced applications like energy harvesting and ultrafast control through property manipulation via electric fields and light, also showcasing a high photostrictive response.
Article Synopsis
- Polarons, resulting from strong electron-phonon interactions, and spin-orbit coupling, which occurs in materials with heavy atoms, both significantly affect how charge and spin behave in certain materials, especially in transition metal oxides.
- The study introduces a new compound, BaNaCaOsO, where these two effects, usually considered separate, are found to interact and create "spin-orbital bipolarons."
- As more electrons are added to BaNaCaOsO, it maintains its insulating properties with a stable Mott gap, preventing it from transitioning to a metallic state, even at high levels of electron doping.
Article Synopsis
- The study focuses on how single-atom catalysts coordinate reactants at their active sites, specifically using Rh adatoms and Rh dimers on FeO(001) when exposed to CO.
- Time-lapse scanning tunneling microscopy (STM) reveals that Rh atoms adapt their structure to form stable RhCO monocarbonyls and occasionally Rh(CO) gem-dicarbonyls, which require breaking down Rh dimers.
- The findings highlight the importance of minority species, often overlooked, in catalytic processes and demonstrate that the breakdown of dimers can lead to the formation of reactive configurations in single-atom catalysis.
Diffusion and Coalescence of Phosphorene Monovacancies Studied Using High-Dimensional Neural Network Potentials.
J Phys Chem C Nanomater Interfaces
December 2023
Article Synopsis
- This study examines how defects, specifically monovacancies, affect the properties of phosphorene, a two-dimensional material, using advanced molecular dynamics simulations.
- Researchers used high-dimensional neural network potentials to accelerate simulations while maintaining accuracy compared to traditional methods like density functional theory (DFT).
- Findings indicate that monovacancies are highly mobile, primarily moving in the zigzag direction, and can merge into more stable divacancies through various pathways, highlighting the complex nature of defect dynamics in two-dimensional materials.
Adsorption of 4-(,-Dimethylamino)-4'-nitrostilbene on an Amorphous Silica Glass Surface.
J Phys Chem C Nanomater Interfaces
November 2023
Article Synopsis
- - This study focuses on the adsorption properties of a specific compound, 4-(dimethylamino)-4'-nitrostilbene (DANS), on amorphous silica glass, using plane-wave density functional theory (DFT) calculations.
- - The research identifies dominant interactions during adsorption, including O-H···O hydrogen bonds between the compound’s nitro group and the silica’s hydroxyl groups, along with significant O-H···π interactions.
- - Both isomers of DANS prefer a parallel orientation when adsorbed on the silica surface, allowing strong interactions between their functional and aromatic groups with the glass.
A Multitechnique Study of CH Adsorption on FeO(001).
J Phys Chem C Nanomater Interfaces
September 2023
Article Synopsis
- The study investigates how ethene (C2H4) interacts with the FeO(001) surface under ultrahigh vacuum using various techniques like TPD and DFT computations.
- Ethene adsorbs on the surface, mainly at defects and structured sites, with a maximum capacity of about 4 molecules per specific unit cell and a desorption energy of 0.36 eV.
- The new analysis method based on equilibrium thermodynamics helps interpret the adsorption behavior and interactions of ethene on the FeO surface.
Article Synopsis
- The (111) facet of magnetite (FeO) has been widely researched, with ongoing debates about the true low-energy surface structures.
- Using density functional theory (DFT), three new surface reconstructions have been identified as more stable than the previously accepted structure, particularly in reducing environments.
- Microscopy techniques reveal a specific structure consisting of tetrahedral iron and 3-fold coordinated oxygen, which clarifies why certain areas are chemically inactive.
Combining first-principles density-functional calculations and Moriya's self-consistent renormalization theory, we explain the recently reported counterintuitive appearance of an ordered magnetic state in uniaxially strained Sr_{2}RuO_{4} beyond the Lifshitz transition. We show that strain weakens the quantum spin fluctuations, which destroy the static order, more strongly than the tendency to magnetism. A different rate of decrease of the spin fluctuations vs magnetic stabilization energy promotes the onset of a static magnetic order beyond a critical strain.
View Article and Find Full Text PDFArticle Synopsis
- * This study uses density functional theory and surface experiments to investigate how Rh, Pt, and Au metals interact with reduced TiO(110) surfaces, identifying different coupling behaviors influenced by polarons.
- * It finds that Pt and Au strongly interact with oxygen vacancy sites, resulting in charge transfer and metal reduction, while Rh has weaker interactions, indicating a complex relationship between adatoms, vacancies, and polarons essential for understanding single-atom catalysts.
Article Synopsis
- The study focuses on reducing the complexity of the four-point vertex function related to the functional renormalization group (FRG) flow in the two-dimensional t-t’ Hubbard model on a square lattice.
- Using a deep learning approach that employs a neural ordinary differential equation solver, the researchers effectively model the FRG dynamics and identify different magnetic and superconducting phases.
- The analysis reveals that only a few key modes are needed to represent the FRG dynamics, showcasing the potential of artificial intelligence to simplify and enhance our understanding of complex electron interactions in quantum field theory.
Article Synopsis
- Polarizable materials, like KTaO, are important in catalysis due to their ability to modify chemical reactivity through surface properties.
- The study highlights how KTaO's surfaces, which have different terminations, create unique environments for carbon monoxide (CO) molecules, affecting their charge and binding strength.
- Results indicate that the presence of excess charge on TaO terraces leads to stronger interactions with CO, demonstrating a link between adsorption states and ferroelectric polarization.
Article Synopsis
- - The common model for describing excess charge on ionic compound surfaces, known as the two-dimensional electron gas (2DEG), is shown to be an oversimplification as it does not account for more complex electronic states on polar surfaces.
- - Research combining scanning probe microscopy with density functional theory reveals that on the TaO surface of KTaO(001), charge density waves coexist with strongly-localized electron polarons and bipolarons, leading to varied electronic behavior.
- - These complex surface electronic states are more energetically favorable than the 2DEG model and influence surface properties, resulting in distinct spectroscopy signals and a reduction in ferroelectric distortions.
Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/FeO(001) "single-atom" catalyst.
View Article and Find Full Text PDFModeling polarons in density functional theory: lessons learned from TiO.
J Phys Condens Matter
March 2022
Article Synopsis
- Density functional theory (DFT) is widely used to analyze polarons in materials, particularly in rutile TiO, to ensure accurate predictions that match experimental results.
- The study examines how electronic correlation correction methods in the DFT + formalism influence charge localization, structural changes, and electronic features of polarons, comparing surface and subsurface layers.
- It highlights that local strain affects polaron behavior differently depending on their location, which impacts their stability and interactions, such as with CO adsorbates.
Article Synopsis
- The study addresses conflicting views on the quantum magnetic state of spin-orbit coupled d² double perovskites, focusing on whether their ground state features Janh-Teller-distorted quadrupoles or octupolar order.
- Through direct calculations and inelastic neutron scattering for the d² double perovskite series Ba₂MOsO₆ (with M being Ca, Mg, Zn), the researchers uncover that the ground state consists of ferro-ordered octupoles connected via superexchange interactions.
- Findings indicate the calculated ordering temperature aligns with observed trends in experiments, and slight changes in the cubic structure can significantly alter the magnetic excitations' characteristics.
Article Synopsis
- Oxygen exchange at interfaces of oxides is crucial for understanding catalytic activity and material degradation, but the atomic details are often unclear.
- New findings reveal that stable, defect-free surfaces can reactively exchange oxygen with water vapor quickly at temperatures below 70°C without altering their atomic structure.
- This exchange occurs mainly during the final stages of water desorption and involves a process where the stability of a specific hydrogen-oxygen complex offsets the energy costs of extracting lattice oxygen, offering insights valuable for various scientific fields.
Electronic State Unfolding for Plane Waves: Energy Bands, Fermi Surfaces, and Spectral Functions.
J Phys Chem C Nanomater Interfaces
June 2021
Article Synopsis
- Present day computing allows for advanced studies using density functional theory to investigate complex physical and chemical issues, typically requiring large supercells for accurate modeling.
- However, using supercells results in small Brillouin zones that complicate the analysis of electronic properties due to folded electronic states.
- The authors introduce a new unfolding scheme embedded in the Vienna Simulation Package (VASP) that simplifies this process, enabling easier computation of band structures, Fermi surfaces, and spectral functions while being applied to various complex scenarios, like the influence of doping in superconductors and interactions on material surfaces.
Article Synopsis
- This study examines how charge doping in the double perovskite BaNaCaOsO affects its structure and electronic properties, focusing on the substitution of Na with Ca.
- Through X-ray diffraction and absorption techniques, researchers found that this substitution causes a linear expansion in the crystal lattice and results in a decrease in the oxidation state of Os from 7+ to 6+.
- The findings demonstrate that the electron transfer occurs effectively due to the doping, influencing the local atomic structure and the energy levels of the Os-derived electronic states.