Self-Assembled Layers of Colloidal Crystals Submicron Spheres for Photonic Applications Edgar J. Cabrera, Sabine Portal, Esther Pascual and Enric Bertran Universitat de Barcelona, Department of Applied Physics and Optics and Institute of Nanoscience and Nanotechnology C/ Martí i Franquès, 1, 08028, Barcelona, Spain edgar.julian.cabrera@ub.edu Abstract— The use of large nanopatterned surfaces areas arouses a growing interest for the fabrication of templates, thin membranes, biomedical sensors and large-area photonic devices. Langmuir-Blodgett (LB) technique offers the possibility to deposit self-assembled monolayers of molecules and nanometric particles on surfaces exhibiting a structural order and uniformity at large scale. In this work, we studied the characteristics and properties of silica nanoparticles, produced by sol-gel process, for self-assembly in close-packed 2D crystal monolayers, deposited on glass and silicon substrates using the LB technique. The resulting monolayers were characterized by SEM (Scanning Electron Microscopy), AFM (Atomic Force Microscopy), optical measurements, transmittance and contact angle measurements. Index Terms: Colloidal Crystals, Langmuir-Blodgett (LB), Monolayer, Nanoparticles, Self-assembly, Sol-gel Process. I. INTRODUCTION The use of structures produced in the nanometer and sub- micron scale have been attracting the interest of the microelectronics and optical industries in the last years due to the improved physical, chemical, and biological properties obtained at these sizes [1 - 4]. The structures made with colloids are generally in the range of 50–500 nm [5]. Nowadays there are several methods for the formation of self-assembled monolayers (SAM) of molecules, particles and colloidal crystals. Some of these methods are vertical deposition method (VD) [6], gravitational sedimentation [7], spin-coating [8], Langmuir-Blodgett technique [9] and horizontal deposition [10]. However many of these methods, like VD method or gravitational sedimentation, lead to three- dimensional (3D) structures which are not suitable to fabricate quality thin films. The horizontal deposition and spin-coating many times require a surface modification step to cover all surface of the substrates or are difficult to reproduce, in order to obtain optimal results. The Langmuir-Blodgett (LB) technique is one of the oldest methods for manufacturing thin organized monolayers on a substrate that offers important advantages like the formation of self-assembled monolayers and multilayers (SAM), an accurate control of the deposited film thickness, a uniform deposition process, the creation of homogeneous or heterogeneous multilayers over the same substrate and does not require special pressure or temperature conditions which makes this technique cheaper and more versatile compared with other ones [11]. Fig. 1. The three basic types of transference for the LB structures: X, Y and Z type. In recent years the LB technique has become attractive as a nanofabrication technology due to the relative simplicity with which the conditions of self-assembly can be achieved and for the diverse applications in fields such as electronics, [12] electro-optics and biology [13, 14], in surface devices, artificial biological systems, surface acoustic waves devices and so on. The principle of the LB technique is based on the use of amphiphilic molecules or particles placed in a water-gas interphase in order to create a Langmuir monolayer; this monolayer can be transferred to a substrate preserving the characteristics it had in the water-gas interphase [15 - 18]. The structure of LB layers or multilayers of molecules and particles can be adapted to the material, substrate and deposition speed, and can be classified into three basic categories: X, Y and Z type [19, 20], each type correspond to the transfer only during upstroke, upstroke with downstroke and during downstroke, respectively (Fig. 1). In general the Y type is the most common type of multilayer deposition, due to the fact that the substrate can be used in both up and down directions. The main idea of this work was to create a LB ordered monolayers of silica particles, over glass and silicon substrates. The characterizations of the deposited silica monolayers over glass and silicon substrates by means of SEM, AFM, wettability and optical measurements were done in order to analyze their specific properties and characteristics. II. THEORETICAL BASES A. Surface tension In order to create a uniform and ordered monolayer in a surface, the molecules or particles must be compressed. The amount of energy per unit area required to reduce their surface 2012 International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale (3M-NANO) 29 August - 1 September 2012, Xi’an, China 978-1-4673-4589-7/12/$31.00 ©2012 IEEE 28