Langmuir monolayer properties of 4-methylbenzenethiol capped gold nanoparticles Sasha Y. Heriot a, * , Jose-Maria Pedrosa b , Luis Camacho c , Tim H. Richardson a a Department of Physics and Astronomy, The University of Sheffield, Sheffield, S3 7RH, UK b Departamento de Ciencias Ambientales, Universidad Pablo de Olavide, Ctra. Utrera Km 1, 41013 Sevilla, Spain c Departamento de Quı ´mica Fı ´sica y Termodina ´mica Aplicada, Universidad de Co ´rdoba, 14014 Co ´rdoba, Spain Available online 30 August 2005 Abstract The properties of 4-methylbenzenethiol capped gold nanoparticles have been investigated at the air – water interface. Langmuir isotherms and compression cycles show that a stable monolayer is formed on the water surface. In situ UV-visible spectroscopy confirms that the plasmon absorption band is present which is characteristic of small metallic particles. The monolayer was imaged using real time Brewster Angle Microscopy (BAM). Above surface pressures of 5 mN m À 1 the monolayer was virtually homogeneous. During decompression, the monolayer fractured into rod-like structures. These rods were initially orientated parallel to the movable barriers containing the monolayer and as the area was increased, their orientation randomized. During subsequent compressions the rods recombined to recreate a homogeneous monolayer. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticle; Langmuir; BAM; Monolayer; Air – water interface 1. Introduction Metallic nanoparticles have attracted much interest in recent years due to their optical and electrical properties that differ significantly from the bulk material [1–5]. They have potential to be used in a diverse range of applications including electronic devices [6,7], catalysis [8,9] and che- mical sensors [6,10,11]. In particular, gold nanoparticles stabilized by organic molecules are proving to be of great interest. This family of nanoparticles is less prone to aggregation compared to other nanoparticles and is extremely stable at room temperature. They are prepared using simple wet chemistry techniques that allow some control of the size distribution of the nanoparticles [12]. Finally, and most importantly for the work presented here, their properties can be controlled by careful selection of the stabilizing ligand. Due to their wide range of potential applications, much work has been conducted in an attempt to form ordered structures of these nanoparticles. Techniques that have previously been employed include self-assembly [13] and layer-by-layer deposition [14]. There has, however, been very little work conducted using Langmuir techniques and previous work has concentrated mainly on nanoparticles stabilized with long alkyl chains [15–17]. There are many advantages of using Langmuir methods. Langmuir deposi- tion not only allows some control of the structure in the x – y plane, but also in the z -direction. The substrates do not have to be specially treated, and film formation can be relatively fast compared to other techniques [18]. The only require- ment is that the nanoparticles have the ability to form a stable layer at the air–water interface. Traditionally, only amphiphilic monomer materials such as fatty acids [19,20] would be considered as having potential for Langmuir studies. However this view has been revised in recent years. A wide range of polymers [21,22] and even materials as diverse as fullerenes [23] have been shown to form stable Langmuir monolayers, and by careful selection of the stabilizing ligand, this is also true for gold nanoparticles. The gold nanoparticles in this work are stabilized by a short organic molecule, 4-methylbenzenethiol. A schematic diagram of the nanoparticles is shown in Fig. 1. The outer 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.08.001 * Corresponding author. Tel.: +44 114 2224539; fax: +44 114 2223555. E-mail address: s.heriot@sheffield.ac.uk (S.Y. Heriot). Materials Science and Engineering C 26 (2006) 154 – 162 www.elsevier.com/locate/msec