Volume 19 (1) 2013

Monte Carlo simulations of the fcc phase of polydisperse hard spheres near close packing are reported. An experimental equation of state (EoS) is determined numerically in the NpT ensemble with variable shape of the periodic box. The close packing volume extrapolated from the obtained data shows a good agreement with earlier experiments performed by other methods. A new theoretical EoS, based on the free volume approximation, is proposed. The modified EoS fits experimental data for polydisperse hard spheres better than the approximation used before.Pozna

Monte Carlo simualtionsof the hard square-triangle fluid close to melting

The system of hard squares and triangles of equal sides, for which the free volumeapproximation predicts entropic phase separation in the solid phase, is studied in the densefluid region, close to melting. The standard Monte Carlo simulations are inconclusiveconcerning the possibility of the phase separation in the dense fluid. By applying correlatedmotions of the particles, we present a direct evidence that, in contrast to the dense solidphase, the squares and triangles mix in the fluid phase.Pozna

High Partial Auxeticity Induced by Nanochannels in [111]-Direction in a Simple Model with Yukawa Interactions

Computer simulations using Monte Carlo method in the isobaric-isothermal ensemble were used to investigate the impact of nanoinclusions in the form of very narrow channels in the [ 111 ] -direction on elastic properties of crystals, whose particles interact via Yukawa potential. The studies were performed for several selected values of Debye screening length ( ( κ σ ) − 1 ). It has been observed that introduction of the nanoinclusions into the system reduces the negative value of Poisson’s ratio towards [ 110 ] [ 1 1 ¯ 0 ] , maintaining practically constant values of Poisson’s ratio in the directions [ 100 ] and [ 111 ] . These studies also show that concentration of particles forming the nanoinclusions in the system has a significant effect on the value of Poisson’s ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction. A strong (more than fourfold) decrease of Poisson’s ratio in this direction was observed, from − 0.147 ( 3 ) (system without inclusions) to − 0.614 ( 14 ) (system with nanoinclusions) at κ σ = 10 when the inclusion particles constituted about 10 percent of all particles. The research also showed an increase in the degree of auxeticity in the system with increasing concentration of nanoinclusion particles for all the screening lengths considered

Inorganic salts direct the assembly of charged nanoparticles into composite nanoscopic spheres, plates, or needles

Oppositely charged, nanoionic nanoparticles can act as "universal surfactants" regulating the growth of ionic microcrystals. This phenomenon derives from a subtle interplay between crystal growth and cooperative electrostatic adsorption of the nanoparticles onto crystal faces. In addition to the electrostatic interactions acting in the system, the nature of salts is also important in the sense that for the same Debye screening length, different salts can mediate formation of markedly different assemblies including supraspheres, nanoneedles, or nanoplates. The method can be further extended to coat non-ionic crystals with appropriately functionalized nanoparticles

Auxeticity of Yukawa Systems with Nanolayers in the (111) Crystallographic Plane

Elastic properties of model crystalline systems, in which the particles interact via the hard potential (infinite when any particles overlap and zero otherwise) and the hard-core repulsive Yukawa interaction, were determined by Monte Carlo simulations. The influence of structural modifications, in the form of periodic nanolayers being perpendicular to the crystallographic axis [111], on auxetic properties of the crystal was investigated. It has been shown that the hard sphere nanolayers introduced into Yukawa crystals allow one to control the elastic properties of the system. It has been also found that the introduction of the Yukawa monolayers to the hard sphere crystal induces auxeticity in the [ 11 1 ¯ ] [ 112 ] -direction, while maintaining the negative Poisson’s ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction, thus expanding the partial auxeticity of the system to an additional important crystallographic direction

Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles

In traditional photoconductors(1-3), the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material's conductivity to increase(4). Such positive photoconductance is observed in both bulk and nanostructured(5,6) photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles' surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs)(7,8) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states(9,10). The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates

The dependence between forces and dissipation rates mediating dynamic self-assembly

Dynamic self-assembly (DySA) outside of thermodynamic equilibrium underlies many forms of adaptive and intelligent behaviors in both natural and artificial systems. At the same time, the fundamental principles governing DySA systems remain largely undeveloped. In this context, it is desirable to relate the forces mediating self-assembly to the nonequilibrium thermodynamics of the system -specifically, to the rate of energy dissipation. In this paper, numerical simulations are used to calculate dissipation rates in a prototypical, magneto-hydrodynamic DySA system, and to relate these rates to dissipative forces acting between the system's components. It is found that (i) dissipative forces are directly proportional to the gradient of the dissipation rate with respect to a coordinate characterizing the steady-state assemblies, and (ii) the constant of proportionality linking these quantities is a characteristic time describing the response of the system to small, externally applied perturbations. This relationship complements and extends the applicability of Prigogine's minimal-entropy-production formalism