Nanoparticle Mediated Colloidal Epitaxy Yields Crack-free Colloidal Crystals
We have performed a fundamental study of the epitaxial assembly of crack-free 3-D colloidal crystals containing very low vacancy concentrations from binary mixtures of colloidal microspheres and highly charged nanoparticles into patterned substrates created by focused ion beam milling. This work builds directly upon our pioneering studies of the phase behavior of such mixtures, as previously reported in the Proc. Nat. Acad. Sci., 2001 and Langmuir, 2001. Specifically, we investigate the effect of template pitch on colloidal epitaxy, and demonstrate the first example of epitaxial assembly that yields mechanically robust, dried epitaxially grown crystals. Our novel approach relies on nanoparticle-mediated colloidal assembly, which offers two distinct advantages: (1) nanoparticle stabilization of the colloidal microspheres induces a hard-sphere-like response thereby allowing such species to pack into a nearly touching network in the wet state, and (2) in situ nanoparticle gelation post assembly allows these crystals to be dried without crack formation or stacking faults.
Laser scanning confocal microscopy was used to directly observe microsphere structural evolution during sedimentation, nanoparticle gelation, and subsequent drying. After microsphere settling, the nanoparticle solution surrounding the colloidal crystal was gelled in situ by introducing ammonia vapor, which increased the pH and enabled drying with minimal microsphere rearrangement. By infilling the dried colloidal crystals with an index-matched fluorescent dye solution, we generated full 3-D reconstructions of their structure including defects as a function of initial suspension composition and pitch of the patterned features. Through proper control over these important parameters, 3-D colloidal crystals were created with low defect densities suitable for use as templates for photonic crystals and photonic band gap materials.
Self-assembled colloidal crystals are of significant interest as route to create photonic band gap materials, however most techniques for colloidal assembly result in highly cracked colloidal crystals which furthermore have a low degree of crystallographic orientation with respect to the substrate. Because photonic properties are highly sensitive to the crystallographic orientation of and defects such as cracks in the colloidal crystal, colloidal self-assembly has not been perceived to be practical for most applications. The colloidal epitaxy route using binary mixtures of colloidal microspheres and highly charged nanoparticles we have discovered generates highly oriented, low defect density colloidal crystals which may enable the colloidal crystal route to photonic band gap devices including 3-D waveguides and optical cavities to be a reality.
Senior FS-MRL PIs: P. Braun, J. Lewis