Prediction and observation of crystal structures of oppositely charged colloids

We studied crystal structures in mixtures of large and small oppositely charged spherical colloids with size ratio 0.31 using Monte Carlo simulations and confocal microscopy. We developed an interactive method based on simulated annealing to predict new binary crystal structures with stoichiometries from 1 to 8. Employing these structures in Madelung energy calculations using a screened Coulomb potential, we constructed a ground-state phase diagram, which shows a remarkably rich variety of crystals. Our phase diagram displays colloidal analogs of simple-salt structures and of the doped fullerene C60 structures, but also novel structures that do not have an atomic or molecular analog. We found three of the predicted structures experimentally, which provides confidence that our method yields reliable results.     

Evidence for out-of-equilibrium crystal nucleation in suspensions of oppositely charged colloids

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.     

Epitaxial crystal growth of gadolinium on tungsten

Field electron microscopy is used to measure activation energies for multilayer diffusion of gadolinium over several different surfaces of tungsten and to prepare crystal layers of gadolinium by epitaxy on tungsten substrates. Nucleation, crystal growth and epitaxial relations are described.     

Sticky model of charged colloids

Summary We use the Sticky Electrolyte Model, which we solved before in PY/MSA, to obtain the equation of state of a charge colloid taking into account both sticky and charge contribution. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Noncentral forces in crystals of charged colloids

The elastic properties of fcc crystals consisting of charge stabilized colloidal particles are determined from real space imaging experiments using confocal microscopy. The normal modes and the force constants of the crystal are obtained from the fluctuations of the particles around their lattice sites using the equipartition theorem. We show that the Cauchy relation is not fulfilled and that only noncentral many-body forces can account for the elastic properties.     

Three-dimensional binary superlattices of oppositely charged colloids

We report the equilibrium self-assembly of binary crystals of oppositely charged colloidal microspheres at high density. By varying the magnitude of the charge on near equal-sized spheres we show that the structure of the binary crystal may be switched between face-centered cubic, cesium chloride, and sodium chloride. We interpret these transformations in terms of a competition between entropic and Coulombic forces.     

Virial expansion for charged colloids and electrolytes

Using a field-theoretic approach, we derive the first few coefficients of the exact low-density (“virial”) expansion of a binary mixture of positively and negatively charged hard spheres (two-component hard-core plasma, TCPHC). Our calculations are nonperturbative with respect to the diameters d₊ and d_- and charge valences q₊ and q_- of positive and negative ions. Consequently, our closed-form expressions for the coefficients of the free energy and activity can be used to treat dilute salt solutions, where typically d ₊∼d _- and q ₊∼q _-, as well as colloidal suspensions, where the difference in size and valence between macroions and counterions can be very large. We show how to map the TCPHC on a one-component hard-core plasma (OCPHC) in the colloidal limit of large size and valence ratio, in which case the counterions effectively form a neutralizing background. A sizable discrepancy with the standard OCPHC with uniform, rigid background is detected, which can be traced back to the fact that the counterions cannot penetrate the colloids. For the case of electrolyte solutions, we show how to obtain the cationic and anionic radii as independent parameters from experimental data for the activity coefficient. Received 6 September 2001 / Received in final form 20 May 2002 Published online 24 September 2002     

Transport in charged colloids driven by thermoelectricity

We study the thermal diffusion coefficient D{T} of a charged colloid in a temperature gradient, and find that it is to a large extent determined by the thermoelectric response of the electrolyte solution. The thermally induced salinity gradient leads in general to a strong increase with temperature. The difference of the heat of transport of coions and counterions gives rise to a thermoelectric field that drives the colloid to the cold or to the warm, depending on the sign of its charge. Our results provide an explanation for recent experimental findings on thermophoresis in colloidal suspensions.     

Direct numerical simulations of electrophoresis of charged colloids

We propose a numerical method to simulate electrohydrodynamic phenomena in charged colloidal dispersions. This method enables us to compute the time evolutions of colloidal particles, ions, and host fluids simultaneously by solving Newton, advection-diffusion, and Navier-Stokes equations so that the electrohydrodynamic couplings can be fully taken into account. The electrophoretic mobilities of charged spherical particles are calculated in several situations. The comparisons with approximation theories show quantitative agreements for dilute dispersions without any empirical parameters; however, our simulation predicts notable deviations in the case of dense dispersions.     

Critical adsorption of polyelectrolytes onto charged spherical colloids

The adsorption of a flexible polyelectrolyte in a salt solution onto an oppositely charged spherical surface is investigated. An analytical solution is derived, which is valid for any sphere radius and consistently recovers the result of a planar surface in the limit of large sphere radii, by substituting the Debye-Hückel potential via the Hulthén potential. Expressions for critical quantities such as the critical radius and the critical surface charge density are provide. A comparison of our theoretical results with experiments and computer simulations yields remarkable good agreement.     

Direct measurement of three-body interactions amongst charged colloids

Three-body interactions amongst three charged colloidal particles are measured in a deionized aqueous solution. Two of the particles are confined to an optical line trap while the third one is approached by means of a focused laser beam. From the observed particle configurations we extract the three-body potential which is found to be attractive and roughly of the same magnitude and range as the pair interactions. In addition, numerical calculations are performed which show qualitative agreement with the experimental results.     

Computer simulations of charged colloids in alternating electric fields

We briefly review recent theoretical and simulation studies of charged colloidal dispersions in alternating electric fields (AC fields). The response of single colloid to an external field can be characterized by a complex polarizability, which describes the dielectric properties of the colloid and its surrounding electrical double layer. We present computer simulation studies of single spherical colloid, using a coarse-grained mesoscale approach that accounts in full for hydrodynamic and electrostatic interactions as well as for thermal fluctuations. We investigate systematically a number of controlling parameters, such as the amplitude and frequency of the AC-fields. The results are in good agreement with recent theoretical predictions.     

Epitaxial crystal growth by sputter deposition: Applications to semiconductors. Part 2

Sections I through IV, presented in Part 1 of this review, included discussions on experimental techniques, the physics of ion/surface interactions, and ion bombardment effects on film nucleation and growth kinetics. In this section, the literature on the growth of epitaxial semiconductor films is reviewed in detail in this section. The discussion is divided into separate subsections on elemental semiconductors, III–V, II–VI, IV–VI, “other” semiconductors, and metastable semiconducting alloys. In each case, an attempt was made to provide a critical analysis of the present understanding and state of the art. Early work on sputter deposition is described in a series of general review articles by Francombe^223–225 as well as in more recent reviews by Greene²²⁶ and Greene and Eltokhy²²⁷ directed specifically towards semiconductors.     

Complexation and overcharging of polyelectrolyte stars and charged colloids

We examine the complexation behavior of polyelectrolyte stars on oppositely charged colloidal particles with similar sizes by means of computer simulations employing the molecular dynamics approach. In particular the overcharging phenomenon is considered and its dependence on the charge and functionality of the stars. The complexes thus formed are a realization of inverse patchy particles (Bianchi et?al 2011 Soft Matter 7 8313) for which both the number of patches and the total charge can be tuned.     

Epitaxial crystal growth by sputter deposition: Applications to semiconductors. Part I

The process of sputtering may be defined as the ejection of target particles due to the impingement of energetic projectile particles. The use of sputtered species as source material to deposit thin films was first reported in the literature in 1852¹ and has since enjoyed several periods of scientific and commercial interest interspersed with periods of disrepute. However, it is only recently that sufficient understanding of the complex processes occurring during, and simultaneously with. ion bombardment of solid surfaces has developed to result in the ability to reproducably and controllably use sputter deposition to grow high quality single crystal semiconducting thin films. The evolution of the branch of science concerned with ion‐surface interactions has been facilitated by the parallel development of ultra high vacuum technology and highly sensitive microanalytic techniques for identifying the state of scattered particles, sputtered species, and implanted material.     

Epitaxial growth of a graphene single crystal on the Ni(111) surface

The thermally controlled synthesis of graphene from propylene molecules on the Ni(111) surface in ultrahigh vacuum is studied by scanning tunneling microscopy and density functional theory. It is established that the adsorption of propylene on Ni(111) atomic terraces at room temperature results in the dehydration of propylene molecules with the formation of single-atomic carbon chains and in the complete dissociation of propylene at the edges of atomic steps with the subsequent diffusion of carbon atoms below the surface. The annealing of such a sample at 500°С leads to the formation of multilayer graphene islands both from surface atomic chains and by the segregation of carbon atoms collected in the upper nickel atomic layers. The process of formation of an epitaxial graphene monolayer until the complete filling of the nickel surface is controllably observed. Atomic defects seen on the graphene surface are interpreted as individual nickel atoms incorporated into graphene mono- or bivacancies.     

Mayer function perturbation theory of effective interaction of charged colloids

In this paper, the effective interaction between charged colloids has been studied based on the standard Mayer function perturbation theory. With the formalism developed in this paper, the effective interaction as a function of Mayer functions and the correlation functions of the homogeneous microions is obtained. The asymptotic behaviour of the effective interaction at large distance is analysed in detail. It is found that at large distance the effective interaction is Yukawa like, provided the bare charge is replaced by the renormalised one. Exact expressions for the renormalised charge and the decay length as functions of the short-range part of the Mayer function and that of the correlation function of the homogeneous microions are obtained. With perturbation methods, it is easy to see how the effective interaction at large distance is affected by microion correlations and nonlinearity.     

Disappearance of the gas-liquid phase transition for highly charged colloids

We calculate the full phase diagram of spherical charged colloidal particles using Monte Carlo free energy calculations. The system is described using the primitive model, consisting of explicit hard-sphere colloids and point counterions in a uniform dielectric continuum. We show that the gas-liquid critical point becomes metastable with respect to a gas-solid phase separation at colloid charges Q > or =20 times the counterion charge. Approximate free energy calculations with only one and four particles in the fluid and solid phases, respectively, are used to determine the critical line for highly charged colloids up to Q=2000. We propose the scaling law T*(c) approximately Q(1/2) for this critical temperature.     

Nonlinear screening and gas-liquid separation in suspensions of charged colloids

We calculate phase diagrams of charged colloidal spheres (valency Z and radius a) in a 1:1 electrolyte from multicentered nonlinear Poisson-Boltzmann theory. Our theory takes into account charge renormalization of the colloidal interactions and volume terms due to many-body effects. For valencies as small as Z = 1 and as large as 10(4) we find a gas-liquid spinodal instability in the colloid-salt phase diagram provided Z lambdaB/a > or similar 24+/-1, where lambdaB is the Bjerrum length.     

Microscopics of complexation between long DNA molecules and positively charged colloids

Extensive atomic force and electron microscopy reveal a new, generic DNA-colloid complex with a fixed number of DNA bases per colloid. The fiber shaped complex is stable in the presence of excess colloids in the solution. As more DNA is added to the solution and the ratio between colloids and DNA approaches the fiber's stoichiometry, the system undergoes a sharp coagulation transition. The system is restabilized at even higher DNA concentrations through localization of small colloid clusters on extensive DNA networks.