Materials Science (cond-mat.mtrl-sci)

Abstract: Organic photovoltaics (OPVs) are promising candidates for solar-energy conversion, with device efficiencies continuing to increase. However, the precise mechanism of how charges separate in OPVs is not well understood because low dielectric constants produce a strong attraction between the charges, which they must overcome to separate. Separation has been thought to require energetic offsets at donor-acceptor interfaces, but recent materials have enabled efficient charge generation with small offsets, or with none at all in neat materials. Here, we extend delocalised kinetic Monte Carlo (dKMC) to develop a three-dimensional model of charge generation that includes disorder, delocalisation, and polaron formation in every step from photoexcitation to charge separation. Our simulations show that delocalisation dramatically increases charge-generation efficiency, partly by enabling excitons to dissociate in the bulk. Therefore, charge generation can be efficient even in devices with little to no energetic offset, including neat materials. Our findings demonstrate that the underlying quantum-mechanical effect that improves the charge-separation kinetics is faster and longer-distance hops between delocalised states, mediated by hybridised states of exciton and charge-transfer character.

Abstract: Microplasmas can be used for a wide range of technological applications and to improve our understanding of fundamental physics. Scanning electron microscopy, on the other hand, provides insights into the sample morphology and chemistry of materials from the mm-down to the nm-scale. Combining both would provide direct insight into plasma-sample interactions in real-time and at high spatial resolution. Up till now, very few attempts in this direction have been made, and significant challenges remain. This work presents a stable direct current glow discharge microplasma setup built inside a scanning electron microscope. The experimental setup is capable of real-time in-situ imaging of the sample evolution during plasma operation and it demonstrates localized sputtering and sample oxidation. Further, the experimental parameters such as varying gas mixtures, electrode polarity, and field strength are explored and experimental $V$-$I$ curves under various conditions are provided. These results demonstrate the capabilities of this setup in potential investigations of plasma physics, plasma-surface interactions, and materials science and its practical applications. The presented setup shows the potential to have several technological applications, e.g., to locally modify the sample surface (e.g., local oxidation and ion implantation for nanotechnology applications) on the $\mu$m-scale.

Abstract: The study of ion dissolution from metal surfaces has a long-standing history, wherein the gradual dissolution of solute atoms with increasing electrode potential, leading to their existence as ions in the electrolyte with integer charges, is well-known. However, our present work reveals a more intricate and nuanced physical perspective based on comprehensive first-principles/continuum calculations. We investigate the dissolution and deposition processes of 22 metal elements across a range of applied electrode potentials, unveiling diverse dissolution models. By analyzing the energy profiles and valence states of solute atoms as a function of the distance between the solute atom and metal surface, we identify three distinct dissolution models for different metals. Firstly, solute atoms exhibit an integer valence state following an integer-valence jump, aligning with classical understandings. Secondly, solute atoms attain an eventual integer valence, yet their valence state increases in a non-integer manner during dissolution. Lastly, we observe solute atoms exhibiting a non-integer valence state, challenging classical understandings. Furthermore, we propose a theoretical criterion for determining the selection of ion valence during electrode dissolution under applied potential. These findings not only contribute to a deeper understanding of the dissolution process but also offer valuable insights into the complex dynamics governing metal ion dissolution at the atomic level. Such knowledge has the potential to advance the design of more efficient electrochemical systems and open new avenues for controlling dissolution processes in various applications.

Abstract: Molecular hydrogen is at the core of hydrogen energy applications and has the potential to significantly reduce the use of carbon dioxide emitting energy processes. However, hydrogen gas storage is a major bottleneck for its large-scale use as current storage methods are energy intensive. Among different storage methods, physisorbing molecular hydrogen at ambient pressure and temperatures is a promising alternative - particularly thanks to tuneable lightweight nanomaterials and high throughput screening methods. Nonetheless, understanding hydrogen adsorption in well-defined nanomaterials remains experimentally challenging and reference information is scarce despite the proliferation of works predicting hydrogen adsorption. In this work, we focus on Li, Na, Ca, and K, decorated graphene sheets as substrates for hydrogen adsorption and compute the most accurate adsorption energies available to date using quantum diffusion Monte Carlo (DMC). Building on our previous insights at the density functional theory (DFT) level, we find that a weak covalent chemisorption of molecular hydrogen, known as Kubas binding, is feasible on Ca decorated graphene according to DMC, in agreement with DFT. This finding is in contrast to previous DMC predictions of the 4H$_2$/Ca$^+$ gas cluster where chemisorption is not favoured. However, we find that the adsorption energy of hydrogen on metal decorated graphene according to a widely-used DFT method is not fully consistent with DMC and the discrepancies are not systematic. The reference adsorption energies reported herein can be used to find better work-horse methods for application in large-scale modelling of hydrogen adsorption. Furthermore, the implications of this work affect strategies for finding suitable hydrogen storage materials and high-throughput methods.

Abstract: It has been observed in fossil tracks and experiments in the layered silicate mica muscovite the transport of charge through the cation layers sandwiched between the layers of tetrahedra-octahedra-tetrahedra. A classical model for the propagation of anharmonic vibrations along the cation chains has been proposed based on first principles and empirical functions. In that model, several propagating entities have been found as kinks or crowdions and breathers, both with or without wings, the latter for specific velocities and energies. Crowdions are equivalent to moving interstitials and transport electric charge if the moving particle is an ion, but they also imply the movement of mass, which was not observed in the experiments. Breathers, being just vibrational entities, do not transport charge. In this work, we present a semiclassical model obtained by adding a quantum particle, electron or hole to the previous model. We present the construction of the model based on the physics of the system. In particular, the strongly nonlinear vibronic interaction between the nuclei and the extra electron or hole is essential to explain the localized charge transport, which is not compatible with the adiabatic approximation. The formation of vibrational localized charge carriers breaks the lattice symmetry group in a similar fashion to the Jahn-Teller Effect, providing a new stable dynamical state. We study the properties and the coherence of the model through numerical simulations from initial conditions obtained by tail analysis and other means. We observe that although the charge spreads from an initial localization in a lattice at equilibrium, it can be confined basically to a single particle when coupled to a chaotic quasiperiodic breather. This is coherent with the observation that experiments imply that a population of charge is formed due to the decay of potassium unstable isotopes.

Abstract: This article provides a review of recent developments in the field of 3d transition metal (TM) catalysts for different reactions including oxygen-based reactions such as Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER). The spin moments of 3d TMs can be exploited to influence chemical reactions, and recent advances in this area, including the theory of chemisorption based on spin-dependent d-band centers and magnetic field effects, are discussed. The article also explores the use of scaling relationships and surface magnetic moments in catalyst design, as well as the effect of magnetism on chemisorption and vice versa. In addition, recent studies on the influence of a magnetic field on the ORR and OER are presented, demonstrating the potential of ferromagnetic catalysts to enhance these reactions through spin polarization.

Abstract: Transition metal dichalcogenides exhibit giant spin-orbit coupling, and intriguing spin-valley effects, which can be harnessed through proximity in van der Waals (vdW) heterostructures. Remarkably, due to the prismatic crystal field, the Zeeman-type band splitting of valence bands reach values of several hundreds of meV. While this effect is suppressed in the commonly studied hexagonal (H)-stacked bilayers due to the presence of inversion symmetry, the recent discovery of sliding ferroelectricity in rhombohedral (R-)stacked MX$_2$ bilayers (M=Mo, W; X=S, Se) suggests that the Zeeman effect could be present in these non-centrosymmetric configurations, making it even more intriguing to investigate how the spin-resolved bands would evolve during the phase transition. Here, we perform density functional theory calculations complemented by symmetry analysis to unveil the evolution of ferroelectricity during sliding and the behavior of Zeeman splitting along the transition path. While the evolution of the out-of-plane component of the electric polarization vector resembles the conventional ferroelectric transition, we observe significant in-plane components parallel to the sliding direction, reaching their maximum at the intermediate state. Moreover, we demonstrate that the R-stacked bilayers exhibit persistent Zeeman-type band splitting throughout the transition path, allowed by the lack of inversion symmetry. Further analysis of different stacking configurations generated by sliding along various directions confirms that the Zeeman effect in MX$_2$, primarily arising from the polarity of prismatic ligand coordination of the metal atom, is remarkably robust and completely governs the spin polarization of bands, independently of the sliding direction. This resilience promises robust spin transport in vdW based MX$_2$ bilayers, opening new opportunities for ferroelectric spintronics.

Abstract: The structure of the excess proton in liquid water has been the subject of lively debate from both experimental and theoretical fronts for the last century. Fluctuations of the proton are typically interpreted in terms of limiting states referred to as the Eigen and Zundel species. Here we put these ideas under the microscope taking advantage of recent advances in unsupervised learning that use local atomic descriptors to characterize environments of acidic water combined with advanced clustering techniques. Our agnostic approach leads to the observation of only a single charged cluster and two neutral ones. We demonstrate that the charged cluster involving the excess proton, is best seen as an ionic topological defect in water's hydrogen bond network forming a single local minimum on the global free-energy landscape. This charged defect is a highly fluxional moiety where the idealized Eigen and Zundel species are neither limiting configurations nor distinct thermodynamic states. Instead, the ionic defect enhances the presence of neutral water defects through strong interactions with the network. We dub the combination of the charged and neutral defect clusters as ZundEig demonstrating that the fluctuations between these local environments provide a general framework for rationalizing more descriptive notions of the proton in the existing literature.

Abstract: In-plane anomalous Hall effect (IPAHE) is a unconventional anomalous Hall effect (AHE) with the Hall current flows in the plane spanned by the magnetization or magnetic field and the electric field. Here, we predict a stable two-dimensional ferromagnetic monolayer $\mathrm{Mn}_{3}\mathrm{Si}_{2}\mathrm{Te}_{6}$ with collinear ordering of Mn moments in the basal plane. Moreover, we reveal that the monolayer $\mathrm{Mn}_{3}\mathrm{Si}_{2}\mathrm{Te}_{6}$ possesses a substantial periodic IPAHE due to the threefold rotational symmetry, which can be switched by changing the magnetization orientation by external magnetic fields. In addition, we briefly discuss the impacts of moderate strains on the electronic states and AHE, which lead to a near quantized Hall conductivity. Our work provides a potential platform for realizing a sizable and controllable IPAHE that greatly facilatates the application of energy-efficient spintronic devices.

Abstract: In this study, we present the fabrication and characterization of Ni/LiNbO3/Ni trilayers using RF sputtering. These trilayers exhibit thick Ni layers (10 microns) and excellent adherence to the substrate, enabling high magnetoelectric coefficients. By engineering the magnetic anisotropy of Nickel through anisotropic thermal residual stress induced during fabrication, and by selecting a carefully chosen cut angle for the LiNbO3 substrate, we achieved a self-biased behavior. We demonstrate that these trilayers can power medical implant devices remotely using a small AC magnetic field excitation, thereby eliminating the need for a DC magnetic field and bulky magnetic field sources. The results highlight the potential of these trilayers for the wireless and non-invasive powering of medical implants. This work contributes to the advancement of magnetoelectric materials and their applications in healthcare technology.

Abstract: The article [Desmarais \textit{et al.} Phys Rev. B. \textbf{107} 224430 (2023)] provided a generalization of the ``\textit{modern theory of orbital magnetization}'' to include non-local Hamiltonians (e.g. hybrid functionals of the generalized Kohn-Sham theory) for magnetic response properties. The formulation was employed for the calculation of the optical rotatory power of periodic systems, where results indicated inequivalence between sampling of direct and reciprocal spaces for those calculations far from the complete basis set limit. We show that such results can be explained by the fact that the reported calculations correspond to an approximate treatment of the action of the $\boldsymbol{\nabla_k}$ operator on Bloch orbitals. In the article, calculations are missing a hidden ``response'' contribution to the reciprocal-space derivatives. The missing response term is shown to (generally) affect the results of calculations of not only magnetic, but also electric response properties, within the context of the ``\textit{modern theory of polarization}''. Necessary conditions are provided to permit avoiding the calculation of the hidden response term.

Abstract: We demonstrate the application of multislice ptychography to a twisted hexagonal boron nitride (h-BN) heterointerface from a single-view data set. The propagation from the top flake, through the interface, to the bottom flake is visualized from separate slices of the reconstruction. The depth resolution of the reconstruction is determined to be 2.74 nm, which is a significant improvement over the aperture-limited depth resolution of 6.74 nm. This is attributed to the diffraction signal extending beyond the aperture edge with the depth resolution set by the curvature of the Ewald sphere. Future advances to this approach could improve the depth resolution to the sub-nanometer level and enable the identification of individual dopants, defects and color centers in twisted heterointerfaces and other materials.