Title: Brownian dynamics simulations of adsorption and ordering of charged colloidal nanoparticles on an oppositely charged surface
Presenter: Jennifer Luna Singh
Affiliation: Rice University / AFRL
Abstract: Bottom-up self assembly of two-dimensional nanoparticle arrays promises to revolutionize device fabrication, such as enabling print-on-demand nano-bio devices on flexible substrates. Presently, a quantitative understanding of the relationship between the structure of the nanoparticle film (close packed, aligned, random, and defect density) and assembly conditions remain elusive. Previous two-dimensional simulations have shown that tuning particle and surface potentials, screening lengths, particle concentrations, and surface patterns can lead to particle ordering. However, identifying a priori the appropriate experimental conditions to observe in-plane disorder-order and order-order transitions remains a substantial challenge. To understand how processing variables impact assembly and particle surface mobility, we discuss Brownian dynamics simulations of absorption and ordering of electrostatically stabilized spheres with various ratios of the particle-particle repulsion (?p) to the particle-surface attraction (?w). Analysis of the orientation correlation function for varying particle-surface attraction follows the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory of phase transition. For ratios of - ?w/?p between 0.01 and 20, ordering occurs at high bulk particle concentration (10%) and intermediate temperature (1-10 kT). The final surface coverage depends on particle-particle repulsion, ranging from 15% for low inverse screen lengths (?a=2) to 60% for higher inverse screen lengths (?a=20). Decreased bulk concentration, temperature, or attractive wall potential results in less adsorption leading to disordered arrays. Detailed Voronoi analysis reveals movement and defect annihilation as adsorption occurs. Better understanding of the phase transition between liquid, hexatic, and crystalline phase structures are expected to provide guidelines to experimental conditions necessary to create high resolution patterns, and smaller devices via printing of nanoparticle based inks.