Investigation of the Layer-by-Layer Assembly of Colloid Crystals on a Templated Substrate
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Investigation of the Layer-by-Layer Assembly of Colloid Crystals on a Templated Substrate Sonia Grego, Thomas Jarvis, Brian Stoner, and Jay Lewis MCNC Research & Development Institute 3021 Cornwallis Road Research Triangle Park, NC 27709, U.S.A. ABSTRACT We investigated the fabrication of a 3D colloid crystal assembled in a layer-by-layer fashion on a template substrate. This method of assembling an ordered structure offers flexibility in the choice of crystal orientation, because of the template, and the capability to insert a “defect” layer to introduce defect modes in the stop bands of the crystal. The assembly of layers of 4.4 µm polystyrene microspheres on the patterned (100) substrates was studied using a combination of evaporation and chemical binding. It was observed that the ordering process depends on the physical parameters of the template such as well size and depth. A good adhesion of the beads to the template required chemical functionalization of the substrate. INTRODUCTION Colloidal self-assembly of crystalline structures has generated significant interest in recent years for photonic and sensing applications. For device applications, it would be beneficial to develop a method to fabricate colloid crystals with controlled “defects” and engineered crystal structure. Recent works from different groups have proposed a method for site-selective self-assembly [1] and a method for the introduction of a 2D hexagonally packed “defect” inside a 3D colloid crystal ([2],[3]). Growth of crystals that have a different packing than the usual (111)-oriented fcc substrate has been obtained on a template using sedimentation [4], convective assembly [5], [6], hydrodynamic flow within confined spaces [7] and other methods. Despite these developments, a method that allows site selection, defect insertion and arbitrary crystal configuration in one process has not been reported yet. Working towards this goal we investigated the feasibility of a layer-by-layer approach to the deposition of ordered arrays of identical spheres on a template substrate. We propose a concept wherein the first layer of microspheres is deposited following the template substrate and robustly adheres to it. A second deposition of bead suspension creates a second layer that uses the first layer as a template and so on. The number of layers required to clearly observe a stop band and the effect of a defect is expected to be approximately 15-20 layers ([2],[7]), so a relatively rapid deposition method must be used. Good adhesion between the first layer and the substrate and between layers was defined by the uppermost layer’s ability to withstand deposition of subsequent layers.Good order of the bead array according to the template and good adhesion to it were competing requirements. We investigated the effect of the combination of surface relief, chemical bonds and evaporation forces on the order and robustness of microspheres arrays.
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EXPERIMENTAL DETAILS Template fabrication We fabricated a (100) template structure using conventiona
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