Crystallization of hard-sphere colloids: deviations from classical nucleation theory

The creation of three-dimensional ordered colloidal crystals, for application in a range of nanotechnologies, has been a goal for many researchers in the past few years. The main difficulty in creating macroscopic sized crystals of densely packed colloidal particles is that colloidal particles always have a range of particle sizes - ie, they are polydisperse. This paper studied the crystallization kinetics of a hard-sphere colloid with a well defined Gaussian polydispersity. The authors find that crystallization occurs in two stages, and does not follow the simple classical nucleation picture. The paper discusses the implications of these results for research into colloidal crystals as possible nano-materials

Small changes in particle-size distribution dramatically delay and enhance nucleation in hard sphere colloidal suspensions

We present hard-sphere crystallization kinetics for three samples with small differences in polydispersity. We show that an increase in polydispersity of 1% is sufficient to cause dramatic changes in the crystallization kinetics: Crystallization is delayed by almost one decade in time and quantitative and qualitative changes in the crystallization scenario are observed. Surprisingly the nucleation rate density is enhanced by almost a factor of 10. We interpret these results in terms of polydispersity limited growth, where local fractionation processes lead to a delayed but faster nucleation

Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition

We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies

Preparation and characterization of particles with small differences in polydispersity

Colloidal particles are widely used both in fundamental research and in materials science. One important parameter influencing the physical properties of colloidal materials is the particle size distribution (polydispersity) of the colloidal particles. Recent work on colloidal crystallization has demonstrated that even subtle changes in polydispersity can have significant effects. In this study we present centrifugation techniques for subtly manipulating the width and the shape of the particle size distribution, for polydispersities less than 10%. We use scanning electron microscopy as well as dynamic and static light scattering to characterize the particle size distributions. We compare the results and highlight the difficulties associated with the determination of accurate particle size distributions

Non-ohmic conduction in doped polyacetylene at low temperatures

The electrical conductivity of a iodine-doped polyacetylene is measured as a function of the electric field Escr at temperatures between 4K and 0.3K. We find that after an initial non-linear behavior sgr increases linearly with Escr in agreement with a theoretical description based on variable-range hopping conduction. The non-linear rise at low fields depends on the iodine concentration. In heavily doped samples the increase is small and varies as Escr2, whereas in less conductive samples a large change is observed at 0.3K which varies approximately as log Escr for fields from 1 V/m to 150 V/m

High-θ polyacetylene: DC conductivity between 14 mK and 300 K

Highly stretch-oriented polyacetylene (6.5:1) yields conductivities σ of typically 20 000–100 000 Ω−1cm−1 at room temperature when highly doped with iodine. Between T = 300 K and T = 14 mK, σ decreases monotonically by about a factor of 5 for fresh samples. Above 400 mK the temperature dependence for fresh samples is fitted by the SHENG formula and can be interpreted within a phenomenological model. On fresh samples, MONTGOMERY measurements of the conductivities parallel (σı) and perpendicular (σσ) to the stretching axis show a temperature independent anisotropy A=σı/σσ of about 25 indicating a common limiting mechanism for both, σı and σσ. Deliberate oxygen ageing drastically changes σ(T) and results in a temperature dependence of A

Numerical calculation for large positive magnetoresistance ratios I insulating materials

Numerical calculations are presented for predicting the large positive magnetoresistance ratios R(B)/R(0) which are often observed in insulating materials. The magnetic field causes shrinkage of the localized electronic wave function, resulting in less overlapping between the hopping sites and hence a relatively large increase of the resistance in the insulating material. Two specific cases are considered: (a) an insulating 3D sample that exhibits a “Mott” variable-range hopping law in its zero-field resistance and (b) an insulating 3D sample that exhibits an “Efros–Shklovskii” variable-range hopping law in its zero field resistance. The numerical calculations are tabulated. The general 3D “soft gap” case is also discussed