A novel digestive ripening process is shown to narrow the particle size distribution from a highly polydisperse dodecanethiol ligated gold colloid. Unlike the Ostwald ripening process, the digestion occurs through transferring materials from large particles to small particles. Temperature-induced size segregation can further select the particle sizes. By using these two methods, highly ordered superlattices using nanocrystals as building blocks can be synthesized directly from a polydisperse colloid. 相似文献
Summary: The theory of lamellar superstructures of an ABC 3‐miktoarm star terpolymer in the conditions corresponding to the strong segregation limit for linear ABC triblock terpolymer has been developed. According to the particular molecular topology (namely, the common junction point for all three blocks), the system cannot avoid volume interactions between monomers of different blocks even in this limit. Hence, in the lamellar superstructure, there exists the so‐called “mixed” domain containing monomers of all three blocks but formed mainly of the block with the lowest degree of incompatibility. It is shown that unfavorable volume interactions in this domain are minimized by the increase of the interfacial area per ABC molecule which makes the mixed domain narrow. This leads to an unusual behavior of the period of the superstructure which decreases with an increase in the length of the block with the lowest incompatibility. However, in the case of a “synchronous” increase in the size of the branches of the ABC 3‐miktoarm star terpolymer, the period of the superstructure increases similarly to that for a linear ABC triblock terpolymer.
Rényi statistical entropy as a means to quantify mixing in two‐dimensional binary systems is presented. The use of Rényi entropies in defining the scale and intensity of segregation for mixing quality determination is analyzed. Finally, the relationship between the mixing process and the structures generated in the system is explored by using the Rényi entropy as an easy and computationally efficient method to calculate the system spectrum of fractal dimensions.
A series of PdxNi100−x nanoparticles were prepared by the co-precipitation method and analyzed using a temperature-programmed surface reaction (TPSR) of their methanation reactions. ESCA measurement suggested that the as-prepared Pd-Ni alloys had Pd-core/Ni-shell structure. Surface Pd segregation occurred during H2 reduction and resulted in a surface composition close to the nominal value. The TPSR experiments were performed by pre-adsorption of CO with H2 to form methane. The peak temperature of methanation increased as Pd content increased, indicating that a methanation reaction is favored on Ni and Ni-rich alloy nanoparticles. For physical mixtures of Pd and Ni nanoparticles, methanation behaviors is similar to those of alloy nanoparticles; but the methanation temperatures of physical mixtures are always higher than those of alloy nanoparticles. This may be due to the formation of a Pd-enriched alloy surface layer during reduction in H2 at 400 °C, or because the CO molecules adsorbed on the Pd sites spill over onto the Ni sites for methanation. Using TPSR technique and measuring methanation temperature, the top-most surface of such bimetallic nanoparticles can be probed. 相似文献
We solve analytically the Schrödinger equation taking into account the shape changes of GaInAs/GaAs quantum wells due to indium segregation during the MBE growth by using transfer matrix method. The indium compositional profiles of the quantum wells are provided using the phenomenological model. The fundamental transition energy, binding energy and oscillator strength of excitons as a function of indium segregation coefficient R and well width are studied. For narrow wells (less than 40 ML), the exciton binding energy and oscillator strength decrease, but for wide wells (larger than 40 ML), increase with increasing the segregation coefficient R. It is shown that indium segregation degrades the optical properties and results in a blue-shift of exciton transition energy in GaInAs/GaAs quantum wells. 相似文献
A theory based on a solution of the inelastic Enskog equation that goes beyond the weak dissipation limit is used to determine
the thermal diffusion factor of a binary granular mixture under gravity. The Enskog equation that aims to describe moderate
densities neglects velocity correlations but retains spatial correlations arising from volume exclusion effects. As expected,
the thermal diffusion factor provides a segregation criterion that shows the transition between the Brazil-nut effect (BNE)
and the reverse Brazil-nut effect (RBNE) by varying the parameters of the system (masses, sizes, composition, density and
coefficients of restitution). The form of the phase diagrams for the BNE/RBNE transition is illustrated in detail in the tracer
limit case, showing that the phase diagrams depend sensitively on the value of gravity relative to the thermal gradient. Two
specific situations are considered: i) absence of gravity, and ii) homogeneous temperature. In the latter case, after some
approximations, our results are consistent with previous theoretical results derived from the Enskog equation. Our results
also indicate that the influence of dissipation on thermal diffusion is more important in the absence of gravity than in the
opposite limit. The present analysis, which is based on a preliminary short report of the author (Phys. Rev. E 78, 020301(R) (2008)), extends previous theoretical results derived in the dilute limit case. 相似文献