Self-assembled multi-layer simple cubic photonic crystals of oppositely charged colloids in confinement

Designing and fabricating self-assembled open colloidal crystals have become one major direction in soft matter community because of many promising applications associated with open colloidal crystals. However, most of the self-assembled crystals found in experiments are not open but close-packed. Here by using computer simulation, we systematically investigate the self-assembly of oppositely charged colloidal hard spheres confined between two parallel hard walls, and we find that the confinement can stabilize multi-layer NaCl-like (simple cubic) open crystals. The maximal layers of stable NaCl-like crystal increases with decreasing the inverse screening length. More interestingly, at finite low temperature, the large vibrational entropy can stabilize some multi-layer NaCl-like crystals against the most energetically favoured close-packed crystals. In the parameter range studied, we find upto 4-layer NaCl-like crystal to be stable in confinement. Our photonic calculation shows that the inverse 4-layer NaCl-like crystal can already reproduce the large photonic band gaps of the bulk simple cubic crystal, which open at low frequency range with low dielectric contrast. This suggests new possibilities of using confined colloidal systems to fabricate open crystalline materials with novel photonic properties

Soft colloids in confinements

Colloids are ubiquitous in nature, e.g. dust, milk or blood and also exist as daily life products, e.g paints, jelly or whipped cream. Thus, better understanding of behaviors of colloidal systems can draw paths to advancement in technologies and innovation. By confining colloidal dispersions, the systems exhibit richer phase behaviors compared to their three-dimensional bulk counterparts. For example, in the simplest model of hard spheres system, the confinement induces a phase transition between triangular(Δ) and square(□) symmetries with a sequence of ...nΔ → (n+1)□ → (n+1)Δ... ,where n is the number of crystal layers, which is a function of confinement height out of the FCC phase of the bulk system. Throughout this thesis, we study soft colloidal systems in topographical confinement. In chapter 1, we provide general backgrounds of colloidal systems and the simulation method we applied, i.e. Monte Carlo simulation method. In chapter 2, the system of confined oppositely charged colloids are studied in their phase behaviour. The oppositely charged colloids have been reported to form FCC and BCC phases in bulk systems. By confining the suspension between two parallel hard walls, we demonstrated a novel phase of NaCl-like (Simple Cubic structure) open crystals which can be stabilized up to several layers. The maximum number of layer of the phase is an inverse function with the inverse screening length. Moreover, at finite low temperature, the multi-layer NaCl-like crystals can be stabilized against the most energetically favored close-packed crystals by large vibrational entropy. We found up to 4-layer NaCl-like crystal as a stable phase in this confined system, in the range of parameters studied. The photonic properties calculation shows that the inverse 4-layer NaCl-like crystal can already reproduce the large photonic band gaps of the bulk simple cubic crystal, which open at low frequency range with low dielectric contrast. This lights up new possibilities of using confined colloidal systems to fabricate open crystalline materials with novel photonic properties. In chapter 3, the nucleation of the novel multi-layer simple cubic crystal we found in confined oppositely charged colloidal systems in chapter 2 is studied. By implementing umbrella sampling Monte Carlo method, we calculated Gibbs free-energy barriers in supersaturation region 0.33 ≤ β|Δµ| ≤ 0.47. We also fit the numerical results with the classical nucleation theory assuming spherical and cylindrical shapes of the nucleus. The fitting with assuming cylindrical configuration shows better alignment which is caused by the surface term differences of both shapes wherein the cylindrical surface term the confinement parameter is included while the spherical one is only a function of cluster size and its density. Then, we switch our attention to the system of like-charge repulsive system in chapter 4. By comparing the simulation results and experimental data, we found FCC-BCC reentrance which is different from the confined hard-sphere system in the regime where the BCC lattice is the stable phase in the bulk system. The numerical results show that this reentrance is governed by the entropy and can be stacked up to 14 layers whereat the FCC phase becomes rare. This can pave a novel pathway of bottom-up approach for fabricating micro- or macroscale colloidal crystal films with highly complicated nanostructures. Last but not least in chapter 5, we exploit the confinement to eliminate vacancies formed in colloidal crystals of magnetic dipoles systems confined between a flat wall and a curved wall. The illustration of the trajectory of a single vacancy shows that the vacancies travel through the phase space and merge with the perturbed region induced by the curved wall at last. The traveling time to the curved wall is system size dependent with no correlation with the initial position of the vacancy in the crystal. We found that at the curved wall with radius of the curvature of 4.5σ, the vacancy can diffuse fastest. For the two-vacancies case, the vacancies bind to each other and diffuse towards the curved wall.Doctor of Philosoph