arXiv subjects

Jan P. Hofmann

Publications and source records attributed to Jan P. Hofmann.

7 recordsLinked to original sources

arXiv2026

Physics-informed automated surface reconstructing via low-energy electron diffraction based on Bayesian optimization

Low-energy electron diffraction (LEED) is a cornerstone technique for determining surface atomic structures[heldStructureDeterminationLowenergy2025], yet the quantitative analysis of electron diffraction intensity as a function of incident electron energy -- that is, LEED-\textit{I(V)} analysis -- remains a complex inverse problem. In this work, we tackle quantitative LEED-\textit{I(V)} analysis based on physics-informed Bayesian optimization (BO). By embedding multiple scattering LEED forward models directly into a trust-region BO loop, our approach simultaneously optimizes both structural and experimental parameters, adaptively adjusting trust regions for efficient exploration of complex non-convex parameter spaces without manual intervention. The robustness and scalability of the approach are demonstrated using the Ag(100)-(1$\time$1) and Fe\textsubscript{2}O\textsubscript{3}(1$\overline{1}$02)-(1$\time$1) surfaces as examples. Our work establishes a general framework for solving inverse problems in various characterization techniques, unlocking a physics-informed efficient, reproducible, and autonomous paradigm.

physics.comp-ph↗

arXiv2025

Plasma engineered Hydroxyl Defects in NiO a DFTSupported-Spectroscopic Analysis of Oxygen Hole States and Implications for Water Oxidation

Controlling lattice oxygen reactivity in earth abundant OER catalysts requires precise tuning of defect chemistry in the oxide lattice. Here, we combine DFT+U calculations with plasma assisted synthesis to show how O2 and H2O in the discharge govern vacancy formation, electronic structure, and catalytic predisposition in NiO thin films. Oxygen rich plasmas generate isolated and clustered Ni vacancies that stabilize oxygen ligand hole states and produce shallow O 2p Ni 3d hybrid levels, enhancing Ni O covalency. In contrast, introducing H2O during growth drives local hydroxylation that compensates vacancy induced Ni3+ centers, restoring Ni2+ like coordination, suppressing deep divacancy derived in gap states, and introducing shallow Ni O H derived valence-band tails. EXAFS confirms that hydroxylation perturbs only the local environment while preserving the medium-range NiO lattice, and Ni L-edge spectroscopy shows a persistent but redistributed ligand-hole population. These complementary vacancy and hydroxylation driven pathways provide a plasma controlled route to pre define electronic defect landscapes in NiO and to tune its activation toward OER relevant NiOOH formation.

cond-mat.mtrl-sci↗

arXiv2025

Redox Chemistry of LiCoO$_2$, LiNiO$_2$, and LiNi$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ Cathodes: Deduced via XPS, DFT+DMFT, and Charge Transfer Multiplet Simulations

Understanding the evolution of the physicochemical bulk properties during the Li deintercalation (charging) process is critical for optimizing battery cathode materials. In this study, we combine X-ray photoelectron spectroscopy (XPS), density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, and charge transfer multiplet (CTM) model simulations to investigate how hybridization between transition metal (TM) 3d and oxygen 2p orbitals evolves with Li deintercalation. Based on the presented approach combining theoretical calculations and experimental studies of pristine and deintercalated cathodes, two important problems of ion batteries can be addressed: i) the detailed electronic structure and involved changes with deintercalation providing information of the charge compensation mechanism, and ii) the precise experimental analysis of XPS data which are dominated by charge transfer coupled to final-state effects affecting the satellite structure. As main result for the investigated Li TM oxides, it can be concluded that the electron transfer coupled to the Li$^{+}$-ion migration does not follow a rigid band model but is modified due to changes in TM 3d and O 2p states hybridization. Furthermore, this integrated approach identifies the 2p XPS satellite peak intensity of TM as an effective indicator of the redox chemistry. With that the redox chemistry of cathodes can be deduced, thus offering a foundation for designing more efficient battery materials.

cond-mat.mtrl-sci↗

arXiv2025

DFT-Guided Operando Raman Characterization of Ni-Based Phases Relevant to Electrochemical Systems

We present a phase-resolved investigation of Ni-based oxides and hydroxides relevant to the oxygen evolution reaction (OER), combining ground-state DFT+U calculations with operando and in situ Raman spectroscopy, supported by high-resolution TEM. Five crystalline phases-cubic and hexagonal NiO, monoclinic and trigonal Ni(OH)2, and NiOOH-are systematically characterized in terms of their vibrational and electronic structure. Although the DFT models are idealized (0 K, defect-free, no solvation), they serve as clean, phase-specific references for interpreting complex experimental spectra. Cubic NiO is confirmed to be dynamically and electronically stable, consistent with dominant Raman modes observed experimentally. Despite dynamic instabilities in phonon dispersions, hexagonal NiO is structurally verified via TEM, suggesting substrate- or defect-stabilized metastability. Ni(OH)2 polymorphs are both vibrationally stable semiconductors, with the trigonal phase exhibiting stronger spin polarization. NiOOH exhibits spin-polarized electronic states across the Brillouin zone, consistent with its asymmetric band structure under ferromagnetic ordering. Independently, phonon calculations reveal soft modes near the Gamma-point, indicating dynamic instability under idealized conditions, yet operando Raman spectra align closely with calculated zone-center modes. However, introducing 0.03 Angstrom symmetry-breaking displacements relaxes the NiOOH lattice off its saddle point, removing imaginary phonon modes and stabilizing the phase. This integrated framework demonstrates how idealized DFT can reveal intrinsic fingerprints that anchor the interpretation of vibrational and electronic responses in catalytically active, dynamically evolving Ni-based materials.

cond-mat.mtrl-sci↗

arXiv2023

Giant proton transmembrane transport through sulfophenylated graphene in a direct methanol fuel cell

An ideal proton exchange membrane should only permeate protons and be leak tight for fuels. Graphene is impermeable to water and poorly conducting to protons. Here, we chemically functionalized monolayer graphene to install sulfophenylated sp3 dislocations by diazotization. Selective to protons, transmembrane areal conductances are up to ~50 S/cm2, which is ~5000 fold higher than in pristine graphene. Mounted in a direct methanol fuel cell, sulfophenylated graphene resulted in power densities up to 1.6 W/mg or 123 mW/cm under standard cell operation (60 C), a value ~two-fold larger than micron-thick films of Nafion 117. The combination of sp3 dislocations and polar groups, therefore, allow the creation of hydrophilic ion paths through graphene and unveils a novel route to rationalize transmembrane hydron transport through 2D materials.

physics.chem-ph↗

arXiv2023

Electrical characterization of the azimuthal anisotropy of $(\mathrm{Ni}_x\mathrm{Co}_{1-x})\mathrm{B}$-based ferromagnetic nanotubes

We report on the structural, electric and magnetic properties of $(\mathrm{Ni}_x\mathrm{Co}_{1-x})\mathrm{B}$ ferromagnetic nanotubes, displaying azimuthal magnetization. The tubes are fabricated using electroless plating in polycarbonate porous templates, with lengths several tens of micrometers, diameters from 100nm to 500nm and wall thicknesses from 10nm to 80nm. The resistivity is $\sim 1.5\times10^{-6}\mathrm{Ω/m}$, and the anisotropic magnetoresistance~(AMR) of 0.2-0.3%, one order of magnitude larger~(resp. smaller) than in the bulk material, which we attribute to the resistance at grain boundaries. We determined the azimuthal anisotropy field from M(H) AMR loops of single tubes contacted electrically. Its magnitude is around 10mT, and tends to increase with the tube wall thickness, as well as the Co content. However, surprisingly it does not dependent much on the diameter nor on the curvature.

cond-mat.mes-hall↗

arXiv2022

Band energy diagrams of n-GaInP/n-AlInP(100) surfaces and heterointerfaces studied by X-ray photoelectron spectroscopy

Lattice matched n-type AlInP(100) charge selective contacts are commonly grown on n-p GaInP(100) top absorbers in high-efficiency III-V multijunction solar or photoelectrochemical cells. The cell performance can be greatly limited by the electron selectivity and valance band offset at this heterointerface. Understanding of the atomic and electronic properties of the GaInP/AlInP heterointerface is crucial for the reduction of photocurrent losses in III-V multijunction devices. In our paper, we investigated chemical composition and electronic properties of n-GaInP/n-AlInP heterostructures by X-ray photoelectron spectroscopy (XPS). To mimic an in-situ interface experiment with in-situ stepwise deposition of the contact material, 1 nm - 50 nm thick n-AlInP(100) epitaxial layers were grown on n-GaInP(100) buffer layer on n-GaAs(100) substrates by metal organic vapor phase epitaxy. We observed (2x2)/c(4x2) low-energy electron diffraction patterns with characteristic diffuse streaks along the [01-1] direction due to P-P dimers on both AlInP(100) and GaInP(100) as-prepared surfaces. Atomic composition analysis confirmed P-rich termination on both surfaces. Angle-resolved XPS measurements revealed a surface core level shift of 0.9 eV in P 2p peaks and the absence of interface core level shifts. We assigned the surface chemical shift in the P2p spectrum to P-P bonds on a surface. We found an upward surface band bending on the (2x2)/c(4x2) surfaces most probably caused by localized mid-gap electronic states. Pinning of the Fermi level by localized electronic states remained in n-GaInP/n-AlInP heterostructures. A valence band offset of 0.2 eV was derived by XPS and band alignment diagram models for the n-n junctions were suggested.

cond-mat.mtrl-sci↗