Colloidal crystals by electrospraying polystyrene nanofluids
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NANO EXPRESS
Open Access
Colloidal crystals by electrospraying polystyrene nanofluids Arnau Coll, Sandra Bermejo*, David Hernández and Luis Castañer
Abstract This work introduces the electrospray technique as a suitable option to fabricate large-scale colloidal nanostructures, including colloidal crystals, in just a few minutes. It is shown that by changing the deposition conditions, different metamaterials can be fabricated: from scattered monolayers of polystyrene nanospheres to self-assembled three-dimensional ordered nanolayers having colloidal crystal properties. The electrospray technique overcomes the main problems encountered by top-down fabrication approaches, largely simplifying the experimental setup. Polystyrene nanospheres, with 360-nm diameter, were typically electrosprayed using off-the-shelf nanofluids. Several parameters of the setup and deposition conditions were explored, namely the distance between electrodes, nanofluid conductivity, applied voltage, and deposition rate. Layers thicker than 20 μm and area of 1 cm2 were typically produced, showing several domains of tens of microns wide with dislocations in between, but no cracks. The applied voltage was in the range of 10 kV, and the conductivity of the colloidal solution was in the range of 3 to 4 mS. Besides the morphology of the layers, the quality was also assessed by means of optical reflectance measurements showing an 80% reflectivity peak in the vicinity of 950-nm wavelength. Keywords: Electrospray, Nanospheres, Colloidal crystal, Multilayer, Metamaterial
Background Self-assembly is a technological process resulting in an ordered structure of individual units without direct human intervention. Most often, this is the simplest technique to produce nanoscale structures, and this is the main reason of the recent wide interest, as revealed by comprehensive compilations. Some reviews [1-4] exhaustively describe the different existing technologies, mainly based on electrophoretic forces [5], capillary forces [6,7], dip coating [8,9], and ink-jet printing [10], among others. Top-down approaches, such as lithography or ion sputtering, have smaller chances to be able to produce largescale low cost materials than bottom-up wet methods, despite the limitations of techniques such as spinning or sedimentation. Mono- and multilayers of nanospheres have a huge number of promising electrical and optical applications [11-14]; some benefiting from the high surface-to-volume ratio to, for example, foster a new * Correspondence: [email protected] MNT, Electronic Engineering Department, Universitat Politècnica de Catalunya, Jordi Girona 1-3, Barcelona 08034, Spain
generation of ultrafast bulk battery electrodes [15], scaffolds of macroporous materials [16,17], while others benefit from the dimension of the periodicity of threedimensional (3D) structures making them suitable for photonic [18-20] or terahertz applications [21]. The technique used in this work is known as electrospray. It consists of producing a fine aerosol by dispersion of a liquid b
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