Colloidal crystals by electrospraying polystyrene nanofluids

Arnau Coll 0 Sandra Bermejo 0 David Hernndez 0 Luis Castaer 0 0 MNT, Electronic Engineering Department, Universitat Politcnica de Catalunya , Jordi Girona 1-3, Barcelona 08034, Spain 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 m 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. - 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 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 by application of a high electric field between an emitter, usually a thin needle, and a flat electrode. Above a given voltage threshold, a Taylor cone develops [22] and the liquid tip becomes unstable breaking into small droplets. The main application of electrospray is found in the ion source of mass spectrometers, although it has also been recently used as a nanoparticle deposition method [23-25], polymer thin film deposition [26], or to create photonic balls [27]. To our knowledge, electrospraying of nanofluids or colloidal solutions of nanometer-size spheres to produce full 3D self-assembled crystals has not been reported so far. A very comprehensive work on state-of-the-art colloidal crystals has recently been published [1] where a few indicators of the crystal quality produced by the various techniques are summarized and compared, namely the thickness, area, deposition time, and optical quality. We have drawn in Figure 1 a radial plot of selected information from Table one in [1] for some of the deposition techniques reported there. We have not included the indicators concerning four techniques, namely motor-drawing, sedimentation, cell confinement, and air-water interface due to the poor results compared to the rest. As can be seen in Figure 1, the most successful techniques exhibit good to poor optical quality, but deposition times are long and the crystal size is in the range of square millimeter of area. It can be seen that the only technique being able to provide wafer-size colloidal crystals (tens of square centimeter in area) in some minutes is the spin-coating technique. It can be seen from this plot that the combination of large area, tens of monolayers of thickness, range of minutes to fabricate, good or excellent optical quality of the crystals, and 3D order is difficult to achieve in most of the techniques. In Figure 1, we have highlighted the results that we have achieved with the technique we are describing in this paper: the electrospray. Using this technique, we were able to deposit up to tens of monolayers, in a few minutes, in square centimeter size, with 3D order, and with good quality. These remarkable results, which are described in the sections below, compare quite well with the other state-of-the-art techniques reported in Figure 1. Thus, we can claim to have achieved a good compromise between large area and low deposition time, achieving good quality of the colloidal nanostructures. In this work, the deposition conditions, such as flow rate, solution concentration, electrical potential, and distance between electrodes, are examined to find the optimal deposition conditions to create 3D self-assembly crystals. In the electrospraying deposition of particles on a substrate, several forces and physical phenomena are involved. In the short range, electrostatic forces are important, in addition to surface tension and capillarity, to explain particle adhesion to surfaces and particle chain, formation, or self-assembly. Coulombic and multipolar dielectrophoretic forces contribute to the total force acting on the particles, thereby affecting the adhesion regimes. The sign and magnitude of the dielectrophoretic component depends on the Claussius-Mossotty factor [28], which depends on the values of the permittivity of the particle and of the medium. In this work, we have observed a set of experimental conditions leading to net attractive forces between particles, so they aggregate in the three dimensions of the layer growth. Scanning electron microscope (SEM) images and optical measurements are also shown to demonstrate the quality of the fabricated colloidal crystals. Methods The electrospray setup consisted of an infusion pump from B. Braun SA (Melsungen, Germany), an OMNIFIX (Braun) 5-ml syringe, a Hamilton needle (600-m outer and 130-m inner diameter; Hamilton, Bonaduz, GR, Switzerland), and an Ultravolt high-voltage bipolar source, 15 kV to +15 kV (Ultravolt, Ronkonkoma, NY, USA). The deposition area was placed inside a glove box with controlled N2 atmosphere. Figure 2 shows schematically the experimental setup. In this work, silicon and (...truncated)