Immobilizing Au Nanoparticles on SiO 2 Surfaces Using Octadecylsiloxane Monolayers Roland Resch, † Sheffer Meltzer, † Thomas Vallant, ‡ Helmuth Hoffmann, ‡ Bruce E. Koel,* ,† Anupam Madhukar, † Aristides A.G. Requicha, † and Peter Will † Laboratory for Molecular Robotics, University of Southern California, 37th Street, University Park, Los Angeles, California 90089-0482, and Institute for Inorganic Chemistry, University of Technology, Getreidemarkt, A-1060 Vienna, Austria Received September 11, 2000. In Final Form: March 27, 2001 Gold nanoparticles were immobilized on silicon dioxide (SiO2) surfaces by embedding them in a layer of octadecylsiloxane (ODS) and also in SiO2 layers created by deposition and oxidation of ODS layers. The ODS and the SiO2 layers form selectively around the nanoparticles. Analysis of the particles after several deposition/oxidation cycles showed a decrease in measured height in accordance with the number of deposited layers. Following one embedding cycle, the nanoparticles could not be moved with the tip of a scanning force microscope operated in dynamic mode, whereas they moved in a controlled fashion prior to the deposition of any layer. Depositing a second set of particles onto an ODS layer in contact with the substrate allowed us to construct a two-particle column by placing one particle directly on top of another one. 1. Introduction The synthesis, physical characterization, and manipu- lation of nanometer-scale particles are very active research areas today. Nanoparticles may be synthesized out of a variety of materials, 1,2 in very well-defined sizes, and with a variety of properties (e.g., metallic, conductive, semi- conductive, insulating, or magnetic). 3-5 They have po- tential applications that range from optoelectronic devices 6 to biological sensors. 7 For example, nanoparticles with a diameter of 1-2 nm have been proposed as a basis for future electronic digital circuits based on single-electron tunneling. 8 In this and many other examples, potential applications require positioning of nanoparticles in com- plex two- and even three-dimensional structures. Several methods ranging from self-assembly to nanomanipulation with a scanning probe microscope (SPM) have been proposed to achieve this goal. 9-16 A crucial issue is the positional stability of particles on a surface, or in a structure, after they have been positioned. Surface dif- fusion and movement of the particles due to mechanical stress or electric field have to be eliminated. One useful approach to particle stabilization involves the use of self-assembled monolayers (SAMs) to surround the particles. SAMs have attracted considerable interest during the last 2 decades and are formed spontaneously by immersing substrates in dilute solutions of amphilic molecules in appropriate solvents. 17,18 A variety of film/ substrate systems are known to lead to the formation of these highly ordered monolayer films. 19-22 Among these systems, alkylsiloxane SAMs on hydroxylated surfaces have proven to be particularly versatile. 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