The Relationship between Growth Speed and Ambient Humidity in Convective Self-assembly
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The Relationship between Growth Speed and Ambient Humidity in Convective Selfassembly Hans D. Robinson, Kai Chen and Stefan V. Stoianov Department of Physics, Virginia Tech, Blacksburg, VA, 24060 ABSTRACT We present a variation of a standard convective self-assembly technique, where the drying meniscus is restricted by a straight-edge located apprR[LPDWHO\ȝm above the substrate adjacent to the drying zone. We find this technique to yield films at roughly twice the growth rate compared to the standard technique. We attribute this to differing local evaporation rates in the two cases. We also investigate how the crystal growth rate depends on ambient relative humidity and find a clear linear dependency, which we attribute to the length of the drying zone being constant over a wide range of humidity. INTRODUCTION Coatings of colloidal particles, consisting of single or multiple layers of densely packed nanoparticles, can be fabricated through a number of different methods1-9. In applications where the goal is to produce thin films where polycrystalline ordering of the lattice is sufficient, convective self-assembly (CSA), also known as evaporation-induced self-assembly10-12, is probably the fastest and most convenient technique to implement. In one standard version of this technique, illustrated in Figure 1(a), a plate is placed at an acute angle immediately above the substrate, and a IHZȝOof nanoparticle suspension is placed in the corner formed by the plate and the substrate. The plate is then withdrawn at a velocity vw, dragging the suspension and a thin wetting film attached to the suspension with it. The drying of the film induces a flow of solvent towards is far edge. Particles are pulled along with the flow from the suspension, which drives the growth of a thin colloidal crystal. In our version of the technique, illustrated in Figure 1(b), the meniscus of the solvent is restricted by placing a straight-edge, (here formed the lowest edge of the top plate,) above the substrate just before the drying zone of the film. This can be accomplished by running the setup just described in reverse so that the upper contact line of the meniscus is attached along the bottom edge of the angled plate. We will call this growth mode Restricted Meniscus Convective Self-Assembly (RMCSA) to distinguish it from the conventional configuration, where the upper contact line is free to attach anywhere along the flat side of the plate. We want to point it out that the setups for RMCSA and conventional CSA are the same except for the withdrawal direction of the top plate. EXPERIMENTAL Au pellets (99.999% pure) and Ni pellets (99.98% pure) were obtained from Kurt J. Lesker (Clairton, PA). Surfactant free white carboxyl polystyrene latex nanosphere suspension (784nm diameter, 4.2 % w/v) was purchased from Invitrogen (Carlsbad, CA). Unless otherwise noted, the nanoparticles were concentrated by centrifugation to 21% w/v prior to use. Plain precleaned glass slides and all chemicals were purchased from Fisher Scientific (Pi
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