Investigation On Dynamic Characteristic And Self-assembly Process Of Ellipsoidal Nanoparticles

As an original method to manufacture functional materials,nanoparticle self-assembly has been extensively applied in computer chip,thermoelectric conversion,electronic display screens and other fields.Anisotropic particles have received more concern in the field of self-assembly due to the adjustability of their interaction potential compare with isotropic colloid.However,the complex dynamic behavior of anisotropic particles brings many uncertain factors to the stability of self-assembly structures.For anisotropic particles,researchers focus on modifying the particle surface to change the interaction between particles,often ignoring the influence of particle diffusion characteristics on the self-assembly structure.In this work,the Brownian kinetic models of Ag nanoparticles with different shapes were structured by combining numerical simulation and theoretical calculation.The aspect ratios of ellipsoidal particles were 3 and 5,respectively.Compared with the results of spherical particles,the three types of particles have the same volume and mass.Based on the isotropic Brownian dynamics model,the translational and rotational friction coefficients of particles were decomposed along the long axis and the short axis according to the results of friction coefficients in the viscous hydrodynamics theory,and the correctness of the model was verified by the Stokes-Einstein relationship.Gay-Berne was used to describe the interaction potential between particles,and the well depth ratio was modified by correlating the well depth and the number of interaction sites in face to face and side to side according to the source of the Gay-Berne potential.Based on the established model,first of all,free Brownian motion is studied about the diffusion coefficient and characteristic time with respect to translation and rotation in earlier stage,found that for the same size particles of different shapes,with the increase of anisotropy rate,translational and rotational diffusion ability reduce particle dispersion,the ellipsoidal particles showed anisotropy over a short time,isotropic over a long period of time.By analyzing the self-assembly morphology and stability of the particles,it is found that the self-assembly structure of the spherical particles is the most stable although the well depth is the smallest.The stability of the ellipsoid with anisotropy rate of 5 is higher than that of the ellipsoid with anisotropy rate of 3.The Brownian force,friction force,van der Waals force are analyzed during self-assembly,also includes the nonlinear response between van der Waals force and Brownian force.Secondly,the variation of free energy along the different reaction coordinates were researched by means of the umbrella sampling and weighted histogram analysis,the effects of thermal disturbance and rotational diffusion on the stability of the self-assembly structure were quantified.It was found that rotational diffusion had no effect on the self-assembly process of spherical particles,but greatly reduced the stability of the self-assembly structure of ellipsoidal particles,the change caused by rotational diffusion accounts for about 50% of the total free energy In the following work,the configuration of the reaction is described by the angle and the distance between particles to calculate the free energy of different configurations in self-assembly path,so as to determine the optimal reaction path.Finally,the reaction time of different stages of the reaction process was calculated.It was found that the ellipsoid with the anisotropy rate of 3 had a clearer reaction path than the ellipsoid with the anisotropy rate of 5,and the reaction time from T configuration to face to face configuration is shorter,the ellipsoid with a length-diameter ratio of 3 took up 24% of the total time,while the ellipsoid with k=5 took up 33%.