Paramagnetic Surfaces Research Articles

Resolving Chemical and Spatial Heterogeneities at Complex Electrochemical Interfaces in Li-Ion Batteries

The high specific capacities of Ni-rich transition-metal oxides have garnered immense interest for improving the energy density of Li-ion batteries (LIBs). Despite the potential of these materials, Ni-rich cathodes suffer from interfacial instabilities that lead to crystallographic rearrangement of the active material surface as well as the formation of a cathode electrolyte interphase (CEI) layer on the composite surface during electrochemical cycling. While changes in crystallographic structure can be detected with diffraction-based methods, probing the chemistry of the disordered, heterogeneous CEI layer is challenging. In this work, we use a combination of ex situ solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information, on the nanometer length scale, on the CEI deposited on LiNi0.8Mn0.1Co0.1O2 (NMC811) composite cathode films. XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI and paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in 13C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, we find that LiF, Li2CO3, and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ 1H and 19F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows spatially resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films.

Electronic correlations at paramagnetic (001) and (110) NiO surfaces: Charge-transfer and Mott-Hubbard-type gaps at the surface and subsurface of (110) NiO

We explore the interplay of electron-electron correlations and surface effects in the prototypical correlated insulating material, NiO. In particular, we compute the electronic structure, magnetic properties, and surface energies of the $(001)$ and $(110)$ surfaces of paramagnetic NiO using a fully charge self-consistent DFT+DMFT method. Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the $(001)$ and $(110)$ NiO surfaces being significantly different from that in bulk NiO. We obtain a sizeable reduction of the band gap at the surface of NiO, which is most significant for the $(110)$ NiO surface. This suggests a higher catalytic activity of the $(110)$ NiO surface than that of the $(001)$ NiO one. Our results reveal a charge-transfer character of the $(001)$ and $(110)$ surfaces of NiO. Most notably, for the $(110)$ NiO surface we observe a remarkable electronic state characterized by an alternating charge-transfer and Mott-Hubbard character of the band gap in the surface and subsurface NiO layers, respectively. This novel form of electronic order stabilized by strong correlations is not driven by lattice reconstructions but of purely electronic origin. We notice the importance of orbital-differentiation of the Ni $e_g$ states to characterize the Mott-Hubbard insulating state of the $(001)$ and $(110)$ NiO surfaces. The unoccupied Ni $e_g$ surface states are seen to split from the lower edge of the conduction band to form strongly localized states in the fundamental gap of bulk NiO. Our results for the surface energies of the $(001)$ and $(110)$ NiO surfaces show that the $(001)$ facet of NiO has significantly lower energy. This implies that the relative stability of different surfaces, at least from a purely energetic point of view, does not depend on the presence or absence of magnetic order in NiO.

Effect of Dusty Particles on Darcy-Brinkman Gravity-Driven Ferro-Thermal-Convection in a Ferrofluid Saturated Porous Layer with Internal Heat Source: Influence of Boundaries

In the present work, penetrative ferro-thermal-convection (FTC) via internal heating in a ferrofluid-saturated porous layer for different types of velocity, temperature and magnetic potential bounding surfaces, is investigated theoretically. A Galerkin-type based on weighted residual method has been used to obtain the eigenvalue problem. The stability of the system is notably influenced on porous medium via internal heating. It is demonstrated that paramagnetic surface with large magnetic susceptibility $$ (\chi > 0) $$ delay the FTC as compared to very low magnetic susceptibility $$ (\chi = 0) $$ as well as ferromagnetic ones. Increase in heat source strength, non-linearity of magnetization, dust particles and magnetic number hasten, while increase in viscosity ratio, concentration parameter and inverse Darcy number delay the onset. The rigid paramagnetic surfaces induct more stabilizing effect on FTC compared to very low magnetic susceptibility as well as ferromagnetic surfaces. In limiting cases, some results published previously are recovered from the present results.

Reply to 'Comment on "Angstrom-scale probing of paramagnetic centers location in nanodiamonds by 3He NMR at low temperatures"' by A. Shames, V. Osipov and A. Panich, Phys. Chem. Chem. Phys. 2018, 20, DOI.

Shames et al. made a comment on our article (DOI: 10.1039/C7CP05898E) stating that their experience in EPR studies of detonation nanodiamonds suggests the existence of two main types of paramagnetic center in detonation nanodiamonds which questions our results. In this reply we provide insights into why there is only one main type of paramagnetic centers detected in nanodiamonds used in this work, which validates the correctness of the proposed original method to determine the distances between paramagnetic centers and nanoparticle surfaces by 3He NMR.

Surface magnetism in O2 dissociation—from basics to application

First-principles calculations were performed to investigate magnetic phenomena in surface reactions involving O2. We present two magnetized surface cases: (1) oxidation of paramagnetic Ag, and magnetic properties of the high coverage oxide phase, which correspond to a magnetic impurity superlattice on paramagnetic surfaces and (2) oxidation of ferromagnetic Pt, represented by the Pt layer on M (M = Fe and Co) relevant to the oxidation reduction reaction (ORR) on Pt, in relation to both fundamental and application interests. In the first case, we found that the dissociative adsorption of O2, resulting in oxide phases in Ag(111), reveals interesting magnetic interactions. We note that the magnetic states are induced by the ferromagnetic superexchange interactions and Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions. Specifically, the superlattice structures with short O–O distances have an effective ferromagnetic superexchange and RKKY interaction. In the second case, we found that a magnetic moment is induced on the Pt layer by the M substrate. The spin polarization of Pt-d states is due to hybridization with M-d states. The d-band center (εd) of Pt (on M), is shifted downwards with respect to pure Pt. However, because of the spin polarization, the otherwise filled spin-down dzz orbital in paramagnetic pure Pt is shifted towards the Fermi level. This promotes πz↑–dzz↓ interactions, which influences the O2–Pt interaction at O2 far from the surface. Details and mechanisms of these two magnetic phenomena are discussed.

Spectral Functions of Isolated Ce Adatoms on Paramagnetic Surfaces

We report photoemission experiments revealing the full valence electron spectral function of Ce adatoms on Ag(111), W(110), and Rh(111) surfaces. A transfer of Ce 4f spectral weight from the ionization peak towards the Fermi level is demonstrated upon changing the substrate from Ag(111) to Rh(111). In the intermediate case of Ce on W(110) the ionization peak is found to be split. This evolution of the spectra is explained by means of first-principles theory, which clearly demonstrates that a reliable understanding of magnetic adatoms on metal surfaces requires simultaneous low and high energy spectroscopic information.

Structural, electronic and magnetic properties of the Mn–Ni(110) c(2 × 2) surface alloy

Using first-principles total energy method, we study the structural, the electronic and the magnetic properties of the MnNi(110) c(2 × 2) surface alloy. Paramagnetic, ferromagnetic, and antiferromagnetic surfaces in the top layer and the second layer are considered. It turns out that the substitutional alloy in the outermost layer with ferromagnetic surface is the most stable in all cases. The buckling of the Mn–Ni(110) c(2 × 2) surface alloy in the top layer is as large as 0.26 Å (1 Å = 0.1 nm) and the weak rippling is 0.038 Å in the third layer, in excellent agreement with experimental results. It is proved that the magnetism of Mn can stabilize this surface alloy. Electronic structures show a large magnetic splitting for the Mn atom, which is slightly higher than that of Mn–Ni(100) c(2 × 2) surface alloy (3.41 eV) due to the higher magnetic moment. A large magnetic moment for the Mn atom is predicted to be 3.81 μB. We suggest the ferromagnetic order of the Mn moments and the ferromagnetic coupling to the Ni substrate, which confirms the experimental results. The magnetism of Mn is identified as the driving force of the large buckling and the work-function change. The comparison with the other magnetic surface alloys is also presented and some trends are predicted.

Ab initiog ‐tensor calculation for paramagnetic surface states: hydrogen adsorption at Si surfaces

AbstractAb initio calculations of the electronic g ‐tensor of paramagnetic states at surfaces are presented taking the adsorption of hydrogen atoms at silicon surfaces as an example. We show that for silicon surfaces with different hydrogen coverages, the g ‐tensor is by far more characteristic than the hyperfine splitting of the Si dangling bonds or the adsorbed H atoms.This holds also in the case of powder spectra (e.g. amorphous or microcrystalline material) where only the angular average of the spectra is available from experiments. Hence, the ab initio calculation of the g ‐tensor should be in general a basic key to a better understanding of the microscopic structure of paramagnetic surfaces or interfaces (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-dependent electron excitation and emission in the neutralization of He+ ions at paramagnetic surfaces

The interaction of He+ ions with surfaces has been widely used as a tool to probe different properties of the surface. There are recent experiments that measure the spin polarization of electrons emitted during the neutralization of spin polarized He+ ions impinging on the Al(001), Au(001) and Cu(001) surfaces. The experimental results show that (i) the spin polarization of the total emitted electron yield is about 30% for the three surfaces, being the preferable spin state that of the electron bound to the He+ ion and (ii) the spin polarization of electrons emitted at higher energies is high (around 70% for Al(001) and 50% for Au(001) and Cu(001)) but decreases as electrons with lower energies are included. Here, we apply our local model to understand in a quantitative way these measurements. First, we calculate the energy distribution of electrons excited in the Auger neutralization of a He+ ion. Next, we describe the transport and escape of these excited electrons to obtain the energy distribution of emitted electrons. The theoretical results allow us to explain the similarities and the differences found in these surfaces.

Spin-dependent electron emission from metals in the neutralization ofHe+ions

paramagnetic surfaces, and to disentangle the role played by each of the involved mechanisms. We show that electron-electron scattering processes at the surface determine the spin-polarization of the total yield. High energy emitted electrons are the ones providing direct information on the He + ion neutralization process and on the electronic properties of the surface.

A Fully Relativistic Description of the Hyperfine Interaction for Paramagnetic Solids and Surfaces

A linear response formalism is presented that has been developed within the framework of spin-polarized relativistic multiple-scattering theory (SPR-KKR) and the Green's function technique to allow for a calculation of magnetic susceptibilities and Knight shifts. This very general approach allows to deal with paramagnetic as well as ferromagnetic solids and surfaces. In addition the approach is suited to deal with all spin and orbital contributions to the magnetic susceptibility and Knight shift on the same level. Furthermore, all relativistic effects are taken properly into account. The great flexibility of our approach will be demonstrated by calculations for the (001) surface of Pt.

A Fully Relativistic Description of the Hyperfine Interaction for Paramagnetic Solids and Surfaces

A Fully Relativistic Description of the Hyperfine Interaction for Paramagnetic Solids and Surfaces

De Haas-van Alphen Oscillations and Magnetic Properties of YbPb3and EuPb3

We observed the de Haas-van Alphen oscillations of a Pauli paramagnetic compound YbPb 3 and an antiferromagnetic compound EuPb 3 . Detected Fermi surfaces in YbPb 3 are three in number, which are well explained by the FLAPW band calculations. The cyclotron effective masses are in the range from 0.21 to 0.60 m 0 . The paramagnetic and antiferromagnetic Fermi surfaces were detected in EuPb 3 . The former Fermi surface is the same as in YbPb 3 , which is seen by magnetic breakdown. EuPb 3 is a divalent compound whose magnetic properties were studied by the specific heat, magnetic susceptibility and high-field magnetization experiments.

Magnetic consequences of NO chemisorption on (100) metal surfaces.

The magnetic properties of NO overlayers on a variety of ferromagnetic and paramagnetic (100) metal surfaces are calculated within the local-spin-density approximation. Bridge-site adsorption results in negligible spin densities in all cases on Rh, Pd, and Pt, and strongly suppresses the surface magnetic moments of Ni. Atop-site adsorption yields more complicated behavior, including residual NO magnetic moments on Pd (100) monolayers and induced ferromagnetism on thicker Pd and Pt slabs. \textcopyright{} 1996 The American Physical Society.

High-field magnetoresistance and de Haas-van Alphen effect in antiferromagnetic PrB6 and NdB6.

The transport properties and the de Haas--van Alphen (dHvA) effect have been measured for antiferromagnetic ${\mathrm{PrB}}_{6}$ and ${\mathrm{NdB}}_{6}$. The number of conduction electrons is approximately one per unit cell. The magnetoresistance shows the existence of open orbits, implying a multiply connected Fermi surface. The angular dependence of the magnetoresistance is roughly similar to that of the reference material, ${\mathrm{LaB}}_{6}$. The dHvA data in ${\mathrm{PrB}}_{6}$ show both paramagnetic and antiferromagnetic Fermi surfaces. The antiferromagnetic Fermi surface arises from new magnetic Brillouin zone boundaries and antiferromagnetic gaps introduced by the magnetic order, and the paramagnetic Fermi surface from magnetic breakdown through the small antiferromagnetic gaps in high field. Hybridization between the conduction electrons and the f electrons has been observed through the cyclotron masses, which in ${\mathrm{PrB}}_{6}$ are three times larger than the corresponding masses of ${\mathrm{LaB}}_{6}$. In ${\mathrm{NdB}}_{6}$ only the antiferromagnetic Fermi surface, quite different from those of ${\mathrm{LaB}}_{6}$ and ${\mathrm{PrB}}_{6}$, has been observed.

Theory of ortho-para conversion in hydrogen adsorbed on metal and paramagnetic surfaces at low temperatures.

In order to explain the experimental results on Cu(100), Ag(111), Ag thin films, graphite, and ${\mathrm{H}}_{2}$ bubbles in Cu, the ortho-para conversion rates of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ adsorbed on metal and paramagnetic surfaces at low temperatures have been considered. The conversion rates due to magnetic dipole-dipole, Fermi contact, and spin-orbit interaction between the conduction electrons, and nuclear spins of ${\mathrm{H}}_{2}$ (${\mathrm{D}}_{2}$) are calculated to elucidate the role of the metal surface. Although the rates on clean metal surfaces are found to be too slow to account for the observed rates on Ag, they may explain the catalytic conversion on ${\mathrm{H}}_{2}$ bubble surfaces at 1.3 K. Additionally, effects of impurities and defects on the surface are investigated by calculating the conversion rate in two-dimensional solid ${\mathrm{D}}_{2}$ (${\mathrm{H}}_{2}$) by emission of one (two) phonon(s). Fast conversion rates observed on Ag and graphite surfaces as well as on the surfaces of ${\mathrm{H}}_{2}$ bubbles may be accounted for by paramagnetic impurities or defects. On Grafoil, both in (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} commensurate and incommensurate solid phase, a temperature-independent conversion rate is predicted if the mobility of the molecules is high enough to prevent concentration gradients.

Theory of ortho ⤥ parahydrogen conversion on paramagnetic and antiferromagnetic surfaces

An attempt is made to establish a theory to explain the many facet experimental results of o ⤥ pH 2 conversion over paramagnetic and antiferromagnetic surfaces in the absence or presence of an externally applied field. The basis of the treatment invokes various nuclear relaxation mechanisms including that for nuclei of an antiferromagnetic above T N, and a postulate that there might exist pocketful regions in an antiferromagnetic which exhibits ferromagnetic ordering below T N.

Projected surface energy bands of bcc iron

We have projected Wood's calculated three-dimensional energy bands of paramagnetic iron to obtain band gaps in the two-dimensional energy bands for (001) and (110) paramagnetic iron surfaces. Extensive band gaps show the possibitlity that many surface states exist. The most important surface states probably exist well away from the center of the two-dimensional Brillouin zone, and may explain the existence of magnetically dead layers at surfaces of Fe and Ni.

Theoretical Rates and Correlation Functions in Ortho-ParaH2Conversion on Paramagnetic Surfaces

A theoretical study of ortho-para hydrogen conversion on paramagnetic surfaces is given. In the absorbed layer, hydrogen molecules are assumed to behave like plane rotators, and their translation at the surface is described by means of a two-dimensional diffusion equation. The spatial dependence of the dipolar matrix elements is found to be important only in the close vicinity of surface impurities. Ortho-para time-correlation functions are computed, and their behavior is compared to the nuclear relaxation case. The total conversion probability is calculated, and two extreme cases are distinguished: (i) The ortho-para conversion energy goes to the thermal bath and the conversion rate is proportional to the diffusion constant and (ii) the ortho-para conversion energy goes to the spin impurity system. In contrast to Leffler's results, the best catalysts are found to be those which have the shortest impurity relaxation times.

Guidelines for interpreting electron spin resonance spectra of paramagnetic species adsorbed on surfaces

Electron spin resonance (ESR) spectroscopy of paramagnetic solids and surfaces is discussed from the standpoint of the various kinds of information which ESR can provide, and how molecular structure theory is used in extracting this information from the observed spectra. The following topics are discussed: (1) Determination of magnetic constants, such as the electronic g-factors and nuclear hyperfine structure (hfs) constants, from the complex ESR lines of magnetically anisotropic molecules which are randomly oriented in polycrystalline or amorphous samples. (2) Qualitative and semiempirical theories of the electronic g-factors and hfs constants, which relate these quantities to the electronic structure of the paramagnetic species, its interaction with the surrounding solid or surface, and its ability to rotate or reorient about various molecular axes. (3) Spin-lattice relaxation with emphasis on its role in determining ESR line widths and the observability of ESR in a given system.