Strong Interparticle Interactions Research Articles

Magnetic Force Microscopy Studies of Superparamagnetic Nanoparticles

We have tested ability of superparamagnetic nanoparticles embedded in matrix to be detected by the magnetic force microscopy (MFM). The sample was characterized by X-ray diffraction and by further magnetic studies such as the temperature and field dependence of magnetization (M) that served as the starting point for an optimal setup for the MFM imaging. To demonstrate the magnetic origin of the phase contrast, the images were taken both with the magnetic and non-magnetic probes. Because of the strong inter-particle interactions arising from the small distances among the particles, only the clusters of the magnetic nanoparticles have been imaged. The optimization of scanning parameters that depend on the sample properties, as well as the possibility to measure the contrast arising from the single particle embedded in matrix will be discussed.

CONVERGENCE OF ITERATIVE SOLVERS FOR NON-LINEAR STEP-AND-FLASH IMPRINT LITHOGRAPHY SIMULATIONS

The paper presents the analysis of the iterative solvers utilized to solve the non-linear problemof Step-and-Flash Imprint Lithography (SFIL) a modern patterning process. The simulationsconsists in solving molecular statics problem for the polymer network, with quadratic potentials.The model distinguishes the strong interparticle interactions between particles forminga polymer network, and weak interactions between remaining particles. It also allows for largedeformations, which all together implies the non-linear model. To illustrate the convergenceof the iterative solvers, we present snapshots of the deformation of the sample being subjectto the iterative solution. We claim that the animation is an interesting way of illustratingthe convergence of the iterative solvers.

Resonance Elastic Light Scattering and Plasmonic Phenomena in Glutathione-Mediated Gold Nanoparticle Assembly

The resonance elastic light scattering (RELS) spectroscopy has been applied to study the interactions of glutathione (GSH) with citrate-capped gold nanoparticles (AuNP). A strong dependence of GSH-induced gold nanoparticle assembly on nanoparticle size has been found. The RELS intensity measurements were carried out in a short reaction time (τ = 60 s) after injection of GSH. The strong interparticle interactions between AuNP are due to zwitterionic forces and intermolecular hydrogen bonding. The AuNP assembly has been confirmed using the surface plasmon UV-Vis spectroscopy. A fast initial adsorption rate of GSH on a clean Au surface, v = 2.4 ng cm-2 s-1, has been measured using a nanogravimetric flow-cell with piezoelectric QC/Au resonator. The adsorption/desorption processes have been evaluated. The RELS spectroscopy has been found as a very sensitive tool for studying interparticle interactions and its applications can be extended to monitor reactivities and assembly of other monolayer-protected metal clusters.

Elaboration of superparamagnetic cobalt–ferrite nanocomposites from films of chitosan chelates

AbstractCobalt–ferrite nanocomposites were synthesized from polymeric films of chelates of Co(II) and Fe(III) ions within a chitosan matrix by a solid‐state coprecipitation reaction with weight content ratios of chitosan to cobalt–ferrite of 50/50 and 25/75 w/w. Morphological and crystalline studies of the composites were performed by high‐resolution transmission electron microscopy, X‐ray diffraction, and selected area electron diffraction with a nanobeam diffraction probe. The results show nanoparticles around 4 nm with a spinel structure, consistent with the cobalt–ferrite phase. The magnetic behavior was evaluated with curves of the applied‐field‐dependent magnetization [M(H)] and the temperature‐dependent magnetization [M(T)]. Both the M(H) and M(T) curves showed typical superparamagnetic behavior, depicting an absence of hysteretic characteristics and the characteristic peak at blocking temperature in the zero‐field‐cooled curve. There was also evidence of strong interparticle and intraparticle interactions, which suggested magnetic frustration in the particle magnetic moment alignment with the applied field. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Mössbauer spectra of hematite and magnetite nanoparticles in polymer composites

It is experimentally demonstrated that the shape of Mössbauer spectra of polymer composites based on single-domain magnetic particles depends more on an inter-particle interaction than on superparamagnetic properties of a separate particle. The characteristic features of the spectra of such systems with strong inter-particle interactions are separated, such as an asymmetric shape of lines with sharp outer and smeared inward sides and presence of central anomalously splitted doublet. It is shown that the three-level relaxation model, which takes into account the precession and diffusion of magnetization of single-domain particles between two local energy minima and one absolute energy maximum, allows to describe all characteristic features of the spectra of such systems.

Magnetic Behavior of O-Carboxymethylchitosan Bounded With Iron Oxide Particles

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Two magnetic chitosan derivatives of O-carboxymethylchitosan bounded with iron oxide particles have been prepared by <emphasis emphasistype="boldital">in situ</emphasis> coprecipitation and incorporation methods. The investigations were focused on structural and magnetic aspects—morphology of iron oxide particles and overall magnetic behavior of these two magnetic chitosan microspheres. It has been shown that the magnetic properties of the sample prepared by the incorporation method are roughly the same as those of a powder of <formula formulatype="inline"><tex Notation="TeX">$\gamma$</tex></formula>-Fe<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex></formula>O<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula>/Fe<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula>O<formula formulatype="inline"> <tex Notation="TeX">$_{4}$</tex></formula> nanoparticles and both are dominated by strong magnetostatic interparticle interactions. In turn, the sample prepared by the <emphasis emphasistype="boldital">in situ</emphasis> method is more susceptible to the temperature increase (resulted in predominant superparamagnetic behavior at 300 K) which is related to smaller particle sizes and better separation of particles with chitosan molecules. </para>

Shear rheology of carbon nanotube suspensions

The shear rheology of carbon nanotube suspensions is reviewed from the perspective of colloid and polymer science. In the semi-dilute to concentrated regimes, the nature of the equilibrium or quiescent state is often dominated by nanotube entanglement and strong attractive inter-particle interactions that favor the formation of a disordered network or gel. The strength of this network with respect to the applied stress dictates the development of mesoscale structural anisotropy, first through a global yield stress and then through a critical stress for homogenization. For concentrated suspensions, the nematic liquid-crystalline order anticipated for high-aspect-ratio rigid rods has been observed in a few select scenarios. The opportunity for deeper theoretical insight is emphasized and intuitive physical arguments are offered that might serve as a foundation for future study.

Effects of Interparticle Interaction in Ferromagnetic Nanoparticle Systems

The effect of interparticle interaction in ferromagnetic nanoparticle systems is reviewed focusing on metallic nanoparticles in an insulating host. Strong dipolar interparticle interactions may give rise to a superspin glass phase with dynamical properties radically different from the superparamagnetic relaxation displayed by non-interacting or only weakly interacting nanoparticles. Strong exchange coupling in nanoparticle systems with a volume concentration above the threshold for geometrical percolation induces ferromagnetism.

Optimal Surface-Electrode Trap Lattices for Quantum Simulation with Trapped Ions

Trapped ions offer long internal state (spin) coherence times and strong interparticle interactions mediated by the Coulomb force. This makes them interesting candidates for quantum simulation of coupled lattices. To this end, it is desirable to be able to trap ions in arbitrary conformations with precisely controlled local potentials. We provide a general method for optimizing periodic planar radio-frequency electrodes for generating ion trapping potentials with specified trap locations and curvatures above the electrode plane. A linear-programming algorithm guarantees globally optimal electrode shapes that require only a single radio-frequency voltage source for operation. The optimization method produces final electrode shapes that are smooth and exhibit low fragmentation. Such characteristics are desirable for practical fabrication of surface-electrode trap lattices.

Dissipation-induced coherence and stochastic resonance of an open two-mode Bose-Einstein condensate

We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate, experimentally measured via the contrast in an interference experiment, as well as the response to an external driving becomes maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. This can be understood as a stochastic resonance of the many-particle system. Even stronger effects are observed when dissipation acts in concurrence with strong interparticle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence significantly. Our theoretical results are backed by Monte Carlo simulations, which show a good qualitative agreement and provide a microscopic explanation for the observed stochastic resonance effect.

Thermodynamic Measurements in a Strongly Interacting Fermi Gas

Strongly interacting Fermi gases provide a clean and controllable laboratory system for modeling strong interparticle interactions between fermions in nature, from high temperature superconductors to neutron matter and quark-gluon plasmas. Model-independent thermodynamic measurements, which do not require theoretical models for calibrations, are very important for exploring this important system experimentally, as they enable direct tests of predictions based on the best current non-perturbative many-body theories. At Duke University, we use all-optical methods to produce a strongly interacting Fermi gas of spin-1/2-up and spin-1/2-down 6Li atoms that is magnetically tuned near a collisional (Feshbach) resonance. We conduct a series of measurements on the thermodynamic properties of this unique quantum gas, including the energy E, entropy S, and sound velocity c. Our model-independent measurements of E and S enable a precision study of the finite temperature thermodynamics. The E(S) data are directly compared to several recent predictions. The temperature in both the superfluid and normal fluid regime is obtained from the fundamental thermodynamic relation T=∂ E/∂ S by parameterizing the E(S) data using two different power laws that are joined with continuous E and T at a certain entropy S c, where the fit is optimized. We observe a significant change in the scaling of E with S above and below S c. Taking the fitted value of S c as an estimate of the critical entropy for a superfluid-normal fluid phase transition in the strongly interacting Fermi gas, we estimate the critical parameters. Our E(S) data are also used to experimentally calibrate the endpoint temperatures obtained for adiabatic sweeps of the magnetic field between the ideal and strongly interacting regimes. This enables the first experimental calibration of the temperature scale used in experiments on fermionic pair condensation, where the ideal Fermi gas temperature is measured before sweeping the magnetic field to the strongly interacting regime. Our calibration shows that the ideal gas temperature measured for the onset of pair condensation corresponds closely to the critical temperature T c estimated in the strongly interacting regime from the fits to our E(S) data. We also calibrate the empirical temperature employed in studies of the heat capacity and obtain nearly the same T c. We determine the ground state energy by three different methods, using sound velocity measurements, by extrapolating E(S) to S=0 and by measuring the ratio of the cloud sizes in the strongly and weakly interacting regimes. The results are in very good agreement with recent predictions. Finally, using universal thermodynamic relations, we estimate the chemical potential and heat capacity of the trapped gas from the E(S) data.

Dissipation Induced Coherence of a Two-Mode Bose-Einstein Condensate

We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate as well as the response to an external driving show a pronounced stochastic resonance effect: Both quantities become maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. Even stronger effects are observed when dissipation acts in concurrence with strong interparticle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence significantly.

Strong repulsive interactions in polyelectrolyte-liposome clusters close to the isoelectric point: A sign of an arrested state

Charged colloidal particles whose interacting potential is governed by a short-range attraction and a long-range screened electrostatic repulsion contributions form aggregates whose shape, size, and overall charge are sensitively dependent on the balance between attraction and repulsion. In some cases, this class of colloidal systems shows an equilibrium cluster phase, where particles associate and dissociate reversibly into clusters. When the aggregation of the charged particles is induced by adding an oppositely charged polyion, very close to the isoelectric condition, the interaggregate interactions become very strong and a dynamical arrested state seems to occur. We provide some experimental evidences of this structural arrest in a colloid system composed by vesicles built up by a cationic lipid stuck together by an oppositely charged linear polyion, by means of the combined use of static and dynamic light scattering technique complemented by laser Doppler electrophoretic measurements. Our results show that the second virial coefficient, which is related to the potential of mean force between two adjacent aggregates, markedly increases in the vicinity of the isoelectric point. We interpret this increase as a print of strong interparticle interactions, yielding to a dynamical arrested state via cluster growth.

Spectroscopic diagnostics of plasma with strong interparticle interactions

Taking as examples the emission spectra of low-temperature dense plasma of organic composition obtained in jets of capillary and slot discharges with an evaporating wall, it is shown that some lines of singly ionized carbon that are broadened under the conditions of significant screening of plasma microfields can be used as reliable “standards” for determination of the electron concentration.

Strategies for Dispersing Nanoparticles in Polymers

AbstractControlling the dispersion of nanoparticles in polymeric matrices is the most significant impediment in the development of high-perform ance polymer nanocomposite ma te rials and results primarily from the strong interpar ticle interactions between the nanopar ticles. This review examines the theoretical and experimental strategies employed in developing appropriate chemical and physical methods to achieve controlled dispersion of nanopar ticles. Methods to characterize the state of dispersion, including force and electron micros copy, and scattering, electrical, and mechanical spectroscopy, are considered with special emphasis on achieving quantitative meas ures of the dispersion. Some of the outstanding issues, such as long-term aging and the implication for the dispersion of nanopar ticles, development of high-throughput methods for rapid screening, and methods for in-line monitoring, are also discussed.

Effect of interacting nanoparticles on the ordered morphology of block copolymer/nanoparticle mixtures

AbstractWe investigated the effect of hard additives, that is, magnetic nanoparticles (NPs) and metal NPs, on the ordered morphology of block copolymers by varying the NP concentration. To characterize the structural changes of a block copolymer associated with different NP loadings, small‐angle X‐ray scattering and transmission electron microscopy were performed. Monodisperse maghemite (γ‐Fe2O3) NPs (7 nm in diameter) and silver (Ag) NPs (6 nm in diameter) with surfaces modified with oleic acids were synthesized, and a cylinder‐forming poly(styrene‐block‐isoprene) diblock copolymer was used as a structure‐directing matrix for the NPs. As the NP concentration increased, domains of NP aggregates were observed for both magnetic and metal NPs. In the case of mixtures of cylinder‐forming poly(styrene‐block‐isoprene) and Ag NPs with weak particle–particle interactions, random aggregates of Ag NPs were observed, and the ordered morphology of the block copolymer lost its long‐range order with an increase in the NP concentration. However, regular, latticelike aggregates obtained with γ‐Fe2O3 NPs, because of the strong interparticle interactions, induced an intriguing morphological transformation from hexagonal cylinders to body‐centered‐cubic spheres via undulated cylinders, whereas the neat block copolymer did not show such a morphological transition over a wide range of temperatures. The interplay between magnetic NPs and the block copolymer was also tested with magnetic NPs of different sizes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3571–3579, 2006

Superferromagnetic domain state of a discontinuous metal insulator multilayer

Polarized neutron reflectivity PNR and magnetometry studies have been performed on the granular multilayer Co80Fe201.3 nm /A l2O3 3n m10. Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth.

Non-Hamiltonian dynamics of grains with spatially varying charges

Dynamics of particles with spatially varying charges is studied. It is shown that the motion cannot be described in terms of the Hamiltonian formalism. Strong interparticle interactions give rise to unusual features of the dynamics, e.g., they can cause self-sustaining nonlinear oscillations triggered at finite frictional dissipation. The comprehensive analysis of the non-Hamiltonian dynamics is carried out analytically and numerically by employing a one-dimensional chain of charged particles as a model system. Analysis of available experimental data is performed and implications to strongly coupled complex (dusty) plasmas are discussed.

Structural and magnetic characterization of norbornene–deuterated norbornene dicarboxylic acid diblock copolymers doped with iron oxide nanoparticles

A series of iron oxide doped norbornene (NOR)/deuterated norbornene dicarboxylic acid (NORCOOH) diblock copolymers were synthesized and characterized by X-ray photoelectron spectroscopy (XPS), small angle neutron scattering (SANS) and superconducting quantum interference device (SQUID) experiments. γ-Fe 2O 3 nanoparticles were synthesized within the microdomains of diblock copolymers with volume fractions of NOR/NORCOOH 0.64/0.36, 0.50/0.50 and 0.40/0.60. A spherical nanoparticle morphology was displayed in the polymer with 0.64/0.36 volume fraction. Polymers with 0.50/0.50 and 0.40/0.60 volume fractions exhibited interconnected metal oxide nanostructures. The observed changes in the shape and peak positions of the small-angle neutron scattering profiles of polymers after metal doping were related to the scattering from the metal oxide particles and to the possible deformed morphologies due to the strong interparticle interactions between metal particles, which may influence the polymer microphase separation. The combined scattering from both polymer domains and magnetic particles was depicted in SANS profiles of metal oxide doped polymers. γ-Fe 2O 3 containing block copolymers were superparamagnetic at room temperature. An increase in the blocking temperature ( T b) of interconnected nanoparticles was observed and was related to the interparticle interactions, which depends on the average distance ( d) between particles and individual particle diameter (2 R). The sample with volume fraction of 0.4/0.6 have the lowest d/(2 R) ratio and exhibit the highest T b at 115 K.

Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-size effects. The discrete dipole approximation (DDA), which is a finite-element method, describes the experimental results well even at d < 20 nm, including particles that have metallic bridges.