Fabrication of high-quality non-close-packed 2D colloid crystals by template-guided Langmuir–Blodgett particle deposition

Abstract

We present a new method of fabricating highly-ordered two-dimensional (2D) colloid crystals with non-closed-packed (NCP) symmetries. In this method, using the Langmuir–Blodgett (LB) monolayer deposition technique, we transfer a Langmuir monolayer of colloidal particles constructed at the air–water interface onto a substrate that contains micro-fabricated topological patterns. We demonstrate that by using this template-guided LB deposition method, near perfect single 2D colloid crystal domains of the order of a hundred micrometres can be easily fabricated under typical LB processing conditions. We investigate the effects of various control parameters (such as the initial particle density at the air–water interface and the substrate lifting speed during the LB particle deposition process) on the density of the deposited particles in the resultant LB monolayer; the final density of the particles deposited on the patterned surface is found to be systematically lower than the particle packing density of the initial Langmuir monolayer, and this dilation is an increasing function of the LB deposition speed. On the length scales larger than approximately a hundred micrometres, we typically observe the formation of stripe patterns in the template-guided LB particle monolayer film, which (we believe) indicates that the contact line of the water meniscus is not stationary. Rather, its position undulates periodically due to the water evaporation during the LB deposition, contrary to what has been commonly assumed in many previous models describing the LB (dip-coating) processes. We present a simple theoretical model, which by taking into account the effects of the evaporation-induced subphase water flow and the particle concentration gradient around the meniscus contact line, can explain all the above-stated experimental observations. Finally, we provide experimental evidence that under high-humidity conditions in which water evaporation is suppressed, the pattern-guided LB deposition technique can indeed produce a high-quality 2D colloid crystal structure that is homogeneous throughout the entire area of the micro-patterned region of the substrate.