Small nanoparticles, globular proteins, viral capsids, and other nanoscopic biomolecules usually display dielectric properties that are different from those of the medium in which they are dispersed. These dielectric heterogeneities can significantly influence the surrounding ion distribution, which determines the self-assembly and colloidal stability of these nanoparticles in solution. Here, we study the impact of a dielectric discontinuity in the structural and thermodynamic properties of a spherical nanoparticle made of different dielectric materials when it is immersed in a charge-asymmetric 1:z supporting electrolyte. The mean electrostatic potential, integrated charge, and ionic profiles are analyzed as a function of both the salt concentration and the nanoparticle’s valence via Monte Carlo simulations and the nonlinear Poisson–Boltzmann theory. We observe that the electrostatic screening and charge neutralization near the surface of a nanoparticle increase when the nanoparticle’s dielectric permitt...

Polarization Effects of Dielectric Nanoparticles in Aqueous Charge-Asymmetric Electrolytes

Magneto-resistive anisotropy (AMR) has been studied in 45 nm thick La0.7−xPrxCa0.3MnO3 (LPCMO) manganite films (with Pr doping x between 0 and 0.40) deposited on LaAlO3 (LAO) and SrTiO3 (STO) substrates. The AMR in compressively strained films undergoes a sign change from positive to negative at low temperatures, whereas its sign does not change in films subjected to tensile strain. Temperature dependence of magnetization in a magnetic field applied parallel and perpendicular to the (100)-plane of the films shows that at low temperatures strain-induced rotation of the easy-axis magnetization determines the sign of the AMR. At higher temperatures near the TMI the sign of the AMR is the same in both LPCMO/LAO and LPCMO/STO films, suggesting the dominating influence of percolative transport in the plane of these films at these temperatures.

Mechanism of sign crossover of the anisotropic magneto-resistance in La0.7−xPrxCa0.3MnO3 thin films

In-plane angular dependent magneto-resistance has been studied in La0.3Pr0.4Ca0.3MnO3 (LPCMO) manganite thin films deposited on the (100) oriented NdGaO3, and (001) oriented SrTiO3 and LaAlO3 substrates. At temperatures where the electronic phase separation is the strongest, a metastable irreversible state exists in the films whose resistivity ρ attains a large time dependent value. The ρ decreases sharply with an increasing angle θ between the magnetic field and the current, and does not display an expected oscillatory cos2θ/sin2θ dependence for all films. The regular oscillations are recovered during repetitive sweeping of θ between 0° and 180°. We discuss possible factors that could produce these unusual changes in the resistivity.

Recovery of oscillatory magneto-resistance in phase separated La0.3Pr0.4Ca0.3MnO3 epitaxial thin films

The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral equation method used for computing static and dynamic polarizabilities. The capacitance is simply the dielectric permittivity multiplied by the area of the object and divided by the squared norm of the Neumann-Poincare operator eigenfunction corresponding to the largest eigenvalue. The norm of this eigenfunction varies slowly with shape thus enabling the definition of two scale-invariant shape factors and perturbative calculations of capacitance. The result is extended to a special class of capacitors in which the electrodes are the equipotential surfaces generated by the equilibrium charge on the object. This extention allows analytical expressions of capacitance for confocal spheroidal capacitors and finite cylinders. Moreover, a second order formula for thin constant-thickness capacitors is given with direct applications for capacitance of membranes in living cells and of supercapacitors. For axisymmetric geometries a fast and accurate numerical method is provided.

Applications of electrostatic capacitance and charging

The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral equation method used for computing static and dynamic polarizabilities. The capacitance is simply the dielectric permittivity multiplied by the area of the object and divided by the squared norm of the Neumann-Poincare operator eigenfunction corresponding to the largest eigenvalue. The norm of this eigenfunction varies slowly with shape thus enabling the definition of two scale-invariant shape factors and perturbative calculations of capacitance. The result is extended to a special class of capacitors in which the electrodes are the equipotential surfaces generated by the equilibrium charge on the object. This extension allows analytical expressions of capacitance for confocal spheroidal capacitors and finite cylinders. Moreover, a second order formula for thin constant-thickness capacitors is given with direct applications for capacitance of membranes in living cells and of supercapacitors. For axisymmetric geometries, a fast and accurate numerical method is provided.

Electrostatic force microscopy (EFM) allows measurement of tiny changes in tip-sample capacitance. When nanoobjects are studied by EFM, they only contribute a very small fraction of the total capacitance between the tip and the sample. We show that the analysis of 3D maps of the EFM signal allows extracting the contribution of the nanomaterial to the total capacitance. This opens the way to applications of EFM as a measure of the dielectric coefficient of electrically insulating nanomaterials or the quantum capacitance of conducting nanomaterials. We apply this method to study the charge response of magnetite, Fe3O4, nanoparticles.

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Nanoparticles charge response from electrostatic force microscopy

In our previous study anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) of epitaxial La2∕3Ca1∕3MnO3 (LCMO) thin films grown on SrTiO3(001) (STO) substrates were studied, and a phenomenological model in the high field limit was developed based on the 4/mmm point group. The derived longitudinal resistivity includes a four-fold as well as a two-fold symmetry term of the in-plane field angle, which can fit the experimental results well. In this study, to highlight the effects of misfit strain, AMR and PHE of LCMO thin films epitaxially grown on LaAlO3(001) substrates were studied, along either the [110] or the [100] direction. Both values are around a few percent, comparable to those measured in films on STO. Nevertheless, only tiny four-fold oscillations appear below the metal-insulator transition temperature Tp along the [110] direction, in contrast to the case of STO, where the four-fold term is prominent. The relationship between this four-fold symmetry and the misfit strain is then discussed in terms of the partial recovery of orbital magnetic moment. The mechanism for AMR and PHE in manganites then can be understood as an anisotropic percolation at metal-insulator transition resulting in the peak, and the spin-orbital coupling effect that accounts for the remnant far below Tp.

L. M. Cui

Papers

Nanoparticles charge response from electrostatic force microscopy

Anisotropic magnetoresistance and planar Hall effect in La2∕3Ca1∕3MnO3 thin films with misfit strain