Average Interparticle Distance Research Articles

Interpenetration of polymeric microgels at ultrahigh densities

Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance as can be smaller than the initial unperturbed particle diameter σ0, due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕeff above close packing (ϕcp), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕcp using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕeff. We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕeff ≫ ϕcp, a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.

Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophene-Based Ligands: Morphology and Optical Responses.

The assembly of Ag and Au nanoparticles (NPs) into nanoparticulate arrays mediated by terpyridine (tpy), 4'-(2-thienyl)terpyridine (T-tpy), and short α,ω-bis(tpy)oligothiophene ligands has been accomplished at the interface between the Ag or Au NP hydrosol and a solution of the molecular species in dichloromethane. The relationship between the morphology and the optical responses of the arrays has been investigated by advanced methods of TEM (transmission electron microscopy) image analysis and surface plasmon extinction (SPE) spectra. It has been established that the size of islands of closely spaced NPs rather than the average interparticle distance affects the extent of delocalization of the surface plasmon excitations and thus also the SPE spectra. Furthermore, the structure of surface-adsorbate complexes formed in these arrays has been investigated by SERS spectral measurements carried out as a function of the excitation wavelength. Photoinduced charge transfer (CT) transitions from the neutral Ags0 and Aus0 adsorption sites on metal NPs to antibonding orbitals of the adsorbates have been identified for Ag/tpy, Ag/T-tpy, Au/tpy, and Au/T-tpy nanoparticulate arrays. Although the surface-adsorbate complexes displaying a photoinduced CT are known for Ag NPs, the Aus0 surface complexes with this CT are newly reported. Bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). The match between the interparticle distances within the NP islands and the lengths of the oligomers molecules indicates that the molecules act as interparticle linkers. In this case, unequivocal spectral marker band evidence of the Ags0 as well as Aus0 surface complex formation has not been obtained.

Tunable Electromagnetic Enhancement of Gold Nanoparticle Arrays

In this paper, triblock copolymer polyisoprene-block-polystyrene-block-poly(2-vinylpyridine) (PI-b-PS-b-P2VP) micelles containing HAuCl4 were spin-coated on silicon wafers followed by calcination to form gold nanoparticle arrays. Subsequently the surface optical performances of poly(3-hexylthiophene) (P3HT)-coated Au nanoparticle arrays were investigated. The particle size and the interparticle distance of the gold nanoparticle arrays could be controlled by adjusting the molar ratio of HAuCl4 precursor to vinyl pyridine units in PI-b-PS-b-P2VP and the spin speed during spin-coating. The results demonstrated that Au nanoparticle arrays with large nanoparticle size were able to produce strong electromagnetic field enhancement. Furthermore, the ratio of average particle size to average interparticle distance increased with decreasing spin speed, resulting in strong electromagnetic field enhancement for metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS).

Reducing light-scattering losses in nanocolloids by increasing average inter-particle distance

First, we present a brief review of the theory of the complex effective refractive index of a disordered suspension of nanoparticles that take into account the effects of scattering by the particles. We present numerical examples of the dependence of the scattering losses in nanocolloids with the concentration of non-absorbing and weakly absorbing nanoparticles. Then, we explore a way to reduce scattering losses in colloidal suspensions of non-absorbing nanoparticles. Finally, we provide some physical insight into the dependence of scattering losses with the particle concentration.

Non-Born-Oppenheimer calculations of the rovibrational spectrum of H2 excited to the second rotational level

Quantum mechanical, non-relativistic, non-Born-Oppenheimer (non-BO) calculations are performed for the rovibrational spectrum of H2 excited to the second rotational level. The non-BO wave functions of the considered states are expanded in terms of all-particle explicitly correlated Gaussian functions. The dissociation energies and rovibrational transition energies are calculated and compared with experimental values and values obtained in calculations performed by others. The average interparticle distances are calculated and compared with the corresponding values for HD. They show that H2 is a more “diffuse molecule”. The nuclear-nuclear correlation functions are calculated and plotted to visualize the “non-BO molecular structure” of H2.

Dispersed SiC nanoparticles in Ni observed by ultra-small-angle X-ray scattering

A metal–ceramic composite, nickel reinforced with SiC nanoparticles, was synthesized and characterized for its potential application in next-generation molten salt nuclear reactors. Synchrotron ultra-small-angle X-ray scattering (USAXS) measurements were conducted on the composite. The size distribution and number density of the SiC nanoparticles in the material were obtained through data modelling. Scanning and transmission electron microscopy characterization were performed to substantiate the results of the USAXS measurements. Tensile tests were performed on the samples to measure the change in their yield strength after doping with the nanoparticles. The average interparticle distance was calculated from the USAXS results and is related to the increased yield strength of the composite.

Molecularly Rigid Microporous Polyamine Captures and Stabilizes Conducting Platinum Nanoparticle Networks.

A molecularly rigid polyamine based on a polymer of intrinsic microporosity (PIM-EA-TB) is shown to capture and stabilize platinum nanoparticles during colloid synthesis in the rigid framework. Stabilization here refers to avoiding aggregation without loss of surface reactivity. In the resulting rigid framework with embedded platinum nanoparticles, the volume ratio of platinum to PIM-EA-TB in starting materials is varied systematically from approximately 1.0 to 0.1 with the resulting platinum nanoparticle diameter varying from approximately 4.2 to 3.1 nm, respectively. Elemental analysis suggests that only a fraction of the polymer is "captured" to give nanocomposites rich in platinum. A transition occurs from electrically conducting and electrochemically active (with shorter average interparticle distance) to nonconducting and only partially electrochemically active (with longer average interparticle distance) polymer-platinum composites. The conducting nanoparticle network in the porous rigid macromolecular framework could be beneficial in electrocatalysis and in sensing applications.

Effect of filler gradation on creep response of asphalt mix

Smaller fractions of aggregates in asphalt mix are known as fillers. Although substantial literature dealing with, (i) the response of asphalt mastic due to load and (ii) estimation of optimal quantity of filler for different types of fillers, is available, literature on the effect of gradation of filler (derived from same natural source) on the response of asphalt mix is rather scanty. This issue has been explored in the present work. In this study, granite stone dust is used as filler. Three different gradations of filler (G1, G2 and G3) are prepared from natural filler (G0) by varying the size proportions of the filler particles. Indirect tensile strength (IDT) and creep tests are conducted on filler–binder mix and also on asphalt mix. The results show that gradation of filler affects the creep response of the filler–binder mix as well as that of the asphalt mix. It is observed that the IDT value varies from about 555 to 665 kPa across the four fillers (G0, G1, G2 and G3) for 5.5% of asphalt content. Likewise, the deformation value in the creep test (with 500N creep loading for 120s) varies from about 0.26–1.09 mm across the four fillers. The average inter-particle distance between the filler particles is hypothesised as a possible indicator so as to explain the variations observed in the test results.

Charge asymmetry and rovibrational excitations of HD+

ABSTRACTAverage values of the interparticle distances for rovibrationally excited HD+ are calculated using a method where the Born–Oppenheimer (BO) approximation is not assumed. The difference between the proton–electron and deuteron–electron distances is used to describe the charge asymmetry in the system. All-particle one-centre explicitly correlated Gaussian functions are used in the calculations of the HD+ rovibrational states. In this work, the non-BO method is extended to calculate the rovibrational states corresponding to the total rotational quantum number of two (N = 2). The algorithms for calculating the Hamiltonian and overlap matrix elements, and the matrix elements of the analytical energy gradient determined with respect to the exponential parameters of the Gaussians, are presented. The gradient is employed in the variational optimisation of the parameters, which is key in obtaining very accurate rovibrational energies in the calculations. The algorithm for calculating the average interparticle distances is also shown. The charge asymmetry of HD+ near the dissociation limit occurs, as expected, with the electron preferentially being near to the deuteron. The asymmetry for a particular vibrational level increases with rotational excitations. The rovibrational transition energies are also calculated and compared with available experimental data.

Determination of the effective anisotropy constant of CoFe2O4 nanoparticles through the T-dependence of the coercive field

We present a systematic study of the coercive field of CoFe2O4–SiO2 nanocomposites. The samples were prepared via the sol-gel method by using the Tetraethyl Orthosilicate as starting reagent. Results of X-ray diffraction, transmission electron microscopy, and X-ray fluorescence confirm the dispersion of the magnetic nanoparticles inside the silica matrix. In addition, the shift in the maximum of Zero-Field-Cooled curves observed by varying the weight ratio of CoFe2O4 nanoparticles to the precursor of silica is consistent with the increasing of average interparticle distances. Because our samples present a particle size distribution, we have used a generalized model which takes account such parameter to fit the experimental data of coercive field extracted from the magnetization curves as a function of applied field. Unlike most of the coercive field results reported in the literature for this material, the use of this model provided a successful description of the temperature dependence of the coercive field of CoFe2O4 nanoparticles in a wide temperature range. Surprisingly, we have observed the decreasing of the nanoparticles anisotropy constant in comparison to the bulk value expected for the material. We believe that this can be interpreted as due to both the migration of the Co2+ from octahedral to tetrahedral sites.

Enhanced Collective Magnetic Properties in 2D Monolayers of Iron Oxide Nanoparticles Favored by Local Order and Local 1D Shape Anisotropy.

Magnetic nanoparticle arrays represent a very attractive research field because their collective properties can be efficiently modulated as a function of the structure of the assembly. Nevertheless, understanding the way dipolar interactions influence the intrinsic magnetic properties of nanoparticles still remains a great challenge. In this study, we report on the preparation of 2D assemblies of iron oxide nanoparticles as monolayers deposited onto substrates. Assemblies have been prepared by using the Langmuir-Blodgett technique and the SAM assisted assembling technique combined to CuAAC "click" reaction. These techniques afford to control the formation of well-defined monolayers of nanoparticles on large areas. The LB technique controls local ordering of nanoparticles, while adjusting the kinetics of CuAAC "click" reaction strongly affects the spatial arrangement of nanoparticles in monolayers. Fast kinetics favor disordered assemblies while slow kinetics favor the formation of chain-like structures. Such anisotropic assemblies are induced by dipolar interactions between nanoparticles as no magnetic field is applied and no solvent evaporation is performed. The collective magnetic properties of monolayers are studied as a function of average interparticle distance, local order and local shape anisotropy. We demonstrate that local control on spatial arrangement of nanoparticles in monolayers significantly strengthens dipolar interactions which enhances collective properties and results in possible super ferromagnetic order.

Total and correlation energy of the uniform polarized electron gas at finite temperature: Direct path integral simulations

The uniform electron gas (UEG) at finite temperature has recently attracted substantial interest due to the experimental progress in the field of warm dense matter. To explain the experimental data accurate theoretical models for high density plasmas are needed which crucially depend on treatment of quantum effects in electron-electron interaction as well as in the interaction of electrons with uniform positive background. To comply with these requirements we have developed the new quantum path integral model of the UEG and present the results of related direct path integral Monte-Carlo (DPIMC) simulations. Contrary to the known in literature approaches treating the electron-background interaction classically our simulations take into account the quantum effects in this interaction. We have observed very good agreement with known in literature results only up to moderate densities when the ratio of the average interparticle distance to the Bohr radius is of order four (rs ≥ 4) and observe deviations for higher densities. At very high electron density (rs ≈ 1) presented in literature approaches as well as our simulations are problematic due to the strong degeneracy of electrons and increasing fermion sign problem.

Dense Deposition of Gold Nanoclusters Utilizing a Porphyrin/Inorganic Layered Material Complex as the Template.

We examined the deposition of gold clusters through the reduction of a gold precursor sensitized by nonaggregated, assembled porphyrin molecules on an inorganic layered material surface in order to develop a novel strategy for constructing assemblies of gold clusters. Visible light irradiation on nonaggregated, assembled porphyrin on the inorganic surface in the presence of the gold precursor and an electron donor induced the deposition of gold NPs on the surface of the inorganic layered material. Uniform gold clusters, with an average diameter of 1.5 nm, were deposited on the surface without aggregation. The average interparticle distance between adjacent gold clusters (center to center) was 2.3 nm, which agrees well with the average intermolecular distance of the nonaggregated, assembled porphyrin molecules on the inorganic surface. Thus, the generated gold clusters appear to reflect the nonaggregated, assembled structure of the porphyrin molecules on the inorganic surface. This method, termed the photosensitized template reduction (PTR) method, is a useful and novel technique for the deposition of metal nanoparticles on the surfaces of supporting materials.

Observation of the Rayleigh-Bénard convection cells in strongly coupled Yukawa liquids

Using “first principles” molecular dynamics simulation, we report for the first time the formation of Rayleigh-Bénard convection cells (RBCC) in two-dimensional strongly coupled Yukawa liquids, characterized by coupling strength Γ (ratio of average potential energy to kinetic energy per particle) and screening parameter κ (ratio of average inter-particle distance to Debye length). For typical values of (Γ, κ), existence of a critical external temperature difference is demonstrated, beyond which RBCC are seen to set in. Beyond this critical external temperature difference, the strength of the maximum convective flow velocity is shown to exhibit a new, hitherto unsuspected linear relationship with external temperature difference and with a slope independent of (Γ, κ). The time taken for the transients to settle down (τs) to a steady state RBCC is found to be maximum close to the above said critical external temperature difference and is seen to reduce with increasing external temperature difference. For the range of values of (Γ, κ) considered here, τs ≈ 10 000–20 000 ωpd−1, where ωpd is dust plasma frequency. As Γ is increased to very high values, due to strong coupling effects, cells are seen to be in a transient state without attaining a steady state for as long as 100 000 ωpd−1, even for a very high external temperature difference. Role of system size, aspect ratio, and dust-neutral collisions has also been addressed.

Quantum Effects of Nonlocal Plasmons in Epsilon-Near-Zero Properties of a Thin Gold Film Slab

Dispersion properties of metals and propagation of quantum plasmons in the high photon energy range are studied. The nonlocal dielectric permittivity of a metal is determined by the quantum plasma effects and is calculated by taking into account collisions between free charge carriers and the lattice. The properties of epsilon-near-zero material are investigated in a thin gold film slab. The spectrum and the damping rate of the quantum plasmons are obtained for a wide range of energies, and the electron’s wave function is calculated in both classical and quantum limits. It is shown that the quantum plasmons exist with a propagation length of 1–10 nm, which strongly depends on the electron energy. The propagation length is found to be much larger than the propagation length in the classical regime, where the former is comparable to the atomic radius and the average inter-particle distance. It is found that the spatial localization of the electron wave function is extended due to the quantum effects. It is also shown that the damping of electromagnetic waves in the high photon energy range decreases when the photon energy decreases which is opposite to the conclusions obtained from the classical Drude model in this range.

Morphological and Electrical Properties of Stretched Nanoparticle Layers

To examine perspectives of nanoparticle films in the role of active elements in strain sensors, morphological and electrical properties of self-assembled Au nanoparticle monolayer prepared by modified Langmuir-Schaefer technique onto supporting Mylar foil were studied under elongation. Along the probing of electrical response (characterized by the gauge factor of about 60), the small-angle x-ray scattering (SAXS) characterization assessed an average interparticle distance change, which was shown to vary proportionally to the substrate elongation. The approach allowed to unambiguously address the mechanism of the deformation-resistivity transduction.

Solid state plasmas

Magnetic fusion devices operate at regimes characterized by extremely high temperatures and low densities, for which the charged particles motion is well described by classical mechanics. This is not true, however, for solid-state metallic objects: their density approaches 1028 m−3, so that the average interparticle distance is shorter than the de Broglie wavelength, which characterizes the spread of the electron wave function. Under these conditions, the conduction electrons behave as a true quantum plasma even at room temperature. Here, we shall illustrate the impact of quantum phenomena on the electron dynamics in metallic objects of nanometric size, particularly thin metallic films excited by short laser pulses. Further, we will discuss more recent results on regimes that involve spin and relativistic effects.

Influence of dipolar interactions on the angular-dependent coercivity of nickel nanocylinders

In this study the influence of dipolar interactions on the orientation-dependent magnetization behavior of an ensemble of single-domain nickel nanorods was investigated. The rods were synthesized by electrodeposition of nickel into porous alumina templates. Some of the rods were released from the oxide and embedded in gelatine hydrogels (ferrogel) at a sufficiently large average interparticle distance to suppress dipolar interactions. By comparing the orientation-dependent hystereses of the two ensembles in the template and the gel-matrix it could be shown that the dipolar interactions in the template considerably alter the functional form of the angular-dependent coercivity. Analysis of the magnetization curves for an angle of 60° between the rod-axes and the field revealed a significantly reduced coercivity of the template compared to the ferrogel, which could be directly attributed to a stray field induced magnetization reversal of a steadily increasing number of rods with increasing field strength. The magnetization curve of the template could be approximated by a weighted linear superposition of the hysteresis branches of the ferrogel. The magnetization reversal process of the rods was investigated by analyzing the angular-dependent coercivity of the non-interacting nanorods. Comparison of the functional form with analytical models and micromagnetic simulations emphasized the assumption of a localized magnetization reversal. Additionally, it could be shown that the nucleation field of rods with diameters in the range 18–29 nm tends to increase with increasing diameter.

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies.

Interphase vs confinement in starch-clay bionanocomposites.

Starch-clay bionanocomposites containing 1-10% of natural montmorillonite were elaborated by melt processing in the presence of water. A complex macromolecular dynamics behavior was observed: depending on the clay content, an increase of the glass transition temperature and/or the presence of two overlapped α relaxation peaks were detected. Thanks to a model allowing the prediction of the average interparticle distance, and its comparison with the average size of starch macromolecules, it was possible to associate these phenomena to different populations of macromolecules. In particular, it seems that for high clay content (10%), the slowdown of segmental relaxation due to confinement of the starch macromolecules between the clay tactoïds is the predominant phenomenon. While for lower clay contents (3-5%), a significant modification of chain relaxation seems to occur, due to the formation of an interphase by the starch macromolecules in the vicinity of clay nanoparticles coexisting with the bulk polymer.