I. L. Isaev

General principles of the crystallization of nanostructured disperse systems

Regularities of the self-organization of ensembles of nanoparticles into crystalline structures in metal lyosols are studied by the Brownian dynamics. Van der Waals, elastic, and electrostatic interparticle interactions, as well as the action of dissipative and fluctuation forces are taken into account in the model. Pair interaction potentials for the polydisperse ensembles of particles are analyzed and the correlations between the pattern of potential curves and the type of formed structures (from crystalline to fractal) are revealed. The effect of interparticle tangential friction on the degree of ordering of formed structures is studied. It is demonstrated that this factor can determine the degree of ordering of nanoparticle aggregates. An essentially new approach to describing elastic interactions of nanoparticles of polymer-containing sols is implemented.

The origin of anomalous enhancement of electromagnetic fields in fractal aggregates of metal nanoparticles

It is shown that the anisotropy of the environment of metal nanoparticles with plasmon absorption in fractal aggregates is the most important and universal characteristic underlying their unique electrodynamic properties. It is noted that it is this morphological feature, but not the fractal distribution of particles in aggregates as such, that plays the dominant role in the manifestation of the enhancement of a local field. In this case, fractal aggregates possess the ability to enhance local electromagnetic fields only owing to their inherent local anisotropy; macroscopic characteristics of aggregates do not markedly influence their electrodynamic interactions with the external field. The quantitative characteristic of local anisotropy is introduced. Statistical correlation between the factor of local anisotropy and fractal dimension D of aggregates is established within the range 1.6 < D < 2.8. It is disclosed that the local anisotropy is independent of the fractal dimension within the wide range (1.6 < D < 2.5) except for the range D > 2.5 corresponding to aggregates with close-packed particles where the factor of local anisotropy tends toward zero. Strong correlation in the spatial arrangement of particles with the largest local anisotropy of the environment in aggregates and the strength of the local electromagnetic field is established using aggregates of silver nanoparticles as an example for the spectrum in the visible range; a polarization dependence of this correlation is revealed. It is noted that parameters of local anisotropy can be used to determine the degree of imperfection of colloidal crystals via optical methods.

Experimental manifestations of the correlation between the local structure of silver nanoparticle aggregates and their absorption spectra

Experimental data testifying in favor of the correlation between electrodynamic characteristics of fractal aggregates and the structure of the local environment of particles are discussed. One of the possible variants of the control of local structure of disordered silver nanoparticle aggregates placed into a polymer matrix via a significant decrease in its volume is proposed and realized. An indirect method for the detection of the variations in the local structure with the use of plasmon absorption spectra is proposed and substantiated. The evolution of local anisotropy of the environment of the particles of loose aggregates during their compaction is studied. Differences in the absorption spectra of silver nanoaggregates in hydrosols with various concentrations of water-soluble polymer that determine the properties of the local structure of aggregates are discussed, these differences being manifested upon the formation of fragments with quasi-ordered arrangement of particles and in the absence of such fragments.

Defects of colloidal crystals

Reasons for the appearance of defects of various types in crystalline colloidal structures formed during the self-organization of the ensembles of spherical nanoparticles are analyzed using lyosols of metal nanoparticles stabilized with polymers as examples. Quantitative characteristic of the degree of imperfection of colloidal crystals is proposed and the procedures for the minimization of the degree of imperfection are discussed. Order-disorder phase transitions of colloidal crystals are studied.

Simulation of the growth of nanoparticle aggregates reproducing their natural structure in disperse systems

A three-dimensional continuum model of the generation of aggregates of spherical particles is developed that allows us to reproduce natural conditions of structure formation in real disperse systems with an arbitrary viscosity of a dispersion medium. The exchange between translational and rotational degrees of freedom for aggregates, subaggregates, and single particles at an arbitrary interparticle interaction potential, as well as at the preset functions of the distribution over particle sizes and thicknesses of the adsorption layer, is taken into account in the model. The interaction potential of the particles includes electrostatic (among them, screened), elastic, and van der Waals interactions and allows for the effect of external electric, magnetic, and gravitational fields. Structural and statistical characteristics of aggregates formed at different stages of particle coagulation are studied, and the quasi-ordering of single subaggregates in the elastic adsorption layer of particles is revealed. The model can be applied to the study of optical absorption by large aggregates of nanoparticles with natural structure and the kinetics of sol aggregation as functions of the properties of the adsorption layer of particles and the action of external physical factors.

Effect of defects of plasmon resonance colloidal crystals on their extinction spectra

The effect of defects such as vacancies, dislocations, and interstices on the extinction spectra of colloidal crystals consisting of plasmon resonance silver nanoparticles is investigated. The possibility of using extinction spectra for the express monitoring of individual and combined defects is discussed.

Effect of Electron Tunneling on the Crystallization of Nanostructured Metal Sols

Regularities of the self-organization of bimodal ensembles of nanoparticles into crystalline structures in metal lyosols are studied by the Brownian dynamics method. Differences in the thicknesses of adsorption layers, van der Waals, elastic, and electrostatic interactions, and random and dissipative forces, as well as variations in interparticle electrostatic interactions due to electron tunneling are taken into account in the model. Pair potentials for polydisperse ensembles of particles are analyzed and the correlation between the pattern of potential curves and the type of forming structures is revealed. The possibility of the selective coagulation of particles with different sizes by virtue of dissimilar charging at the moment of their approach is demonstrated. The kinetics of this process is investigated. Specific features of the crystallization of bimodal sols are discussed using real gold organosols as examples.

Variations arising in extinction spectra of nanoparticle aggregates upon deformation during deposition on planar dielectric substrate

Variations in the extinction spectra of aggregates of plasmon-resonant nanoparticles are studied at different stages of their planar deformation during interaction with dielectric substrates. Deformation and extinction spectra are calculated by the Brownian dynamics and coupled dipole methods, respectively. The effect of planar deformation of the aggregates on the pattern of the extinction contour in the range of the plasmon absorption band is analyzed by the example of aggregated silver hydrosols. It is shown that the extinction spectra of three-dimensional nanoparticle aggregates formed in bulk and those deformed upon deposition onto a substrate are markedly different. The calculation results are confirmed by experimental data.

Variation of extinction spectra of plasmon resonance colloidal crystals upon structural transitions

The effect of the temperature of a silver sol on its extinction spectra during formation of aggregates of plasmon resonance nanoparticles with varying degree of order and the effect exerted on these spectra by changes in the structure of a colloidal crystal during its heating, which lowers elastic properties of a polymer adsorption layer of particles are studied.

Evolution of extinction spectra of plasmon resonance nanocolloids in the process of their crystallization

Changes in extinction spectra of nanostructured colloids of plasmon resonance particles are studied at different stages of their crystallization. The spontaneous crystallization is calculated by the method of Brownian dynamics, and the extinction spectra are calculated by the method of coupled dipoles. Based on the example of silver sols, we analyzed how separate parameters of the crystallizing disperse system affect the shape of the extinction contour in the plasmon absorption band.