Fabrication of large binary colloidal crystals with a NaCl structure

Abstract

Binary colloidal crystals offer great potential for tuning material properties for applications in, for example, photonics, semiconductors and spintronics, because they allow the positioning of particles with quite different characteristics on one lattice. For micrometer-sized colloids, it is believed that gravity and slow crystallization rates hinder the formation of high-quality binary crystals. Here, we present methods for growing binary colloidal crystals with a NaCl structure from relatively heavy, hard-spherelike, micrometer-sized silica particles by exploring the following external fields: electric, gravitational, and dielectrophoretic fields and a structured surface (colloidal epitaxy). Our simulations show that the free-energy difference between the NaCl and NiAs structures, which differ in their stacking of the hexagonal planes of the larger spheres, is very small (Ϸ0.002 kBT). However, we demonstrate that the fcc stacking of the large spheres, which is crucial for obtaining the pure NaCl structure, can be favored by using a combination of the above-mentioned external fields. In this way, we have successfully fabricated large, 3D, oriented single crystals having a NaCl structure without stacking disorder.colloidal materials ͉ photonic crystals ͉ self-assembly ͉ surface patternings (epitaxy) R ecently, examples have been shown of the tunability of binary colloidal crystals in several binary systems of nanoparticles (1-5). This tunability of the material properties offers great potential for applications in materials science. However, in many cases, the absolute size of the building blocks is of crucial importance for the functionality of the material. For example, to obtain a photonic band gap in the visible or near-infrared range, which is important for applications in telecommunications, the lattice spacings have to be of the order of 0.1 to 1 m. Unfortunately, for larger colloids, gravity and slow crystallization rates hinder the formation of binary crystals (6-8). We recently found in calculations that a so-called inverse photonic crystal based on the binary sodium chloride has a larger band gap than that of an inverse fcc crystal which until now has been the structure almost exclusively focused on in methods using selfassembly (see SI 1 in the SI Text and Fig. S1). The finding of this property of the NaCl binary lattice motivated us to focus on this binary crystal, but the methods we present for the manipulation of the growth of high-quality binary crystals from dispersions of micrometer-sized spheres are quite general (see also SI 2 in the SI Text and Fig. S2). For instance, the results presented in this paper also directly indicate how to grow other binary crystals as discussed in the conclusion section. We explore here several combinations of external fields to optimize the number of small particles in the NaCl structure and to avoid stacking faults. The binary NaCl crystal structure consists of spheres of two different radii ( L and S ), both ordered in a fcc structure, where the small particle...

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

“…There is very good agreement if the renormalized value R ′ for the depletion bubble radius is taken (diamonds) while the agreement deteriorates for the low-density-expression R (circles). This demonstrates that the analytical expression (21) incorporates the basic physics principles.…”

“…Sediments of binary charged mixtures are commonly used to determine the phase behavior [19][20][21][22] . In highly deionized charged colloidal mixtures, the sediment 23,24 can split into separated layers due to counterion lifting, a phenomenon referred to as "colloidal brazil nut effect".…”

Binary colloidal structures assembled through Ising interactions

“…Multi-component colloidal systems [26][27][28][29][30][31][32][33][34][35] (that is, 'colloidal alloys') as well as systems of particles with non-trivial internal structure, such as Janus particles [36][37][38][39] and particles with shape anisotropy [40][41][42] , are currently being explored due to their ability to tailor the particle interactions and enable the assembly of diverse crystalline structures. Current methods for assembling ordered structures with two or three particle types include kinetic techniques based on controlled drying [26][27][28][29][30][31] and thermodynamic techniques [32][33][34][35]43,44 that rely on spontaneous assembly into minimum free-energy configurations.…”

“…Current methods for assembling ordered structures with two or three particle types include kinetic techniques based on controlled drying [26][27][28][29][30][31] and thermodynamic techniques [32][33][34][35]43,44 that rely on spontaneous assembly into minimum free-energy configurations. Kinetic techniques using evaporation of a liquid meniscus have demonstrated a large variety of crystalline colloidal alloy structures [26][27][28] .…”

“…The challenges there are to understand the necessary particle interactions, fabricate particles that manifest those interactions and develop techniques for avoiding metastable states in order to realize the equilibrium phase of interest. Ionic assembly techniques that are based on fixing the charges of two different particle types (one of each polarity) may offer the requisite programmability 32,33 ; however, these systems are difficult to anneal due to the narrow window for adjusting the temperature in aqueous fluids.…”