Alkanethiol Mediated Release of Surface Bound Nanoparticles Fabricated by Nanosphere Lithography
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Alkanethiol Mediated Release of Surface Bound Nanoparticles Fabricated by Nanosphere Lithography Jing Zhao, Amanda J. Haes, * Xiaoyu Zhang, Shengli Zou, Erin M. Hicks, George C. Schatz, and Richard P. Van Duyne Northwestern University, Department of Chemistry, 2145 Sheridan Road, Evanston, Illinois 60208-3113 *Current address: Naval Research Laboratory, Washington D.C.
ABSTRACT This work presents an innovative approach to produce monodisperse solution-phase triangular silver nanoparticles with well-controlled geometry. Ag nanotriangles are fabricated by nanosphere lithography (NSL), functionalized with alkanethiol molecules and then released from the substrate into solution. The resulting single isolated nanoparticles are subsequently asymmetrically functionalized with alkanedithiol molecules to form dimer pairs. The optical properties of the Ag nanoparticles have been measured using UV-Vis spectroscopy while their structural properties have been characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Theoretical calculations based on Mie theory and the Discrete Dipole Approximation (DDA) method have been done to interpret the optical properties of the released Ag nanoparticles.
INTRODUCTION There is currently a great interest in studying the optical properties of metal particles in the nanoscale. Metal nanoparticles have distinctive optical properties that are not available in either isolated molecules or bulk solids. The most important optical excitation in noble metal nanoparticles is localized surface plasmon resonances (LSPR).1-3 When metal nanoparticles are excited by electromagnetic radiation, they exhibit collective oscillations of their conduction electrons known as localized surface plasmons. Recent investigations have revealed that the optical properties of the nanoparticles highly depend on their geometry, metal composition, and the surrounding dielectric environment.4,5 Traditionally, noble metal nanoparticles are produced by the reduction of a metal salt in solution. Alternative fabrication methods based on electron beam lithography (EBL),6 focused ion beam lithography (FIB),7 and nanosphere lithography (NSL)8-10 have been developed to fabricate nanoparticles confined on solid substrates. NSL inexpensively produces nanoparticle arrays with precisely controlled geometry and interparticle spacing. By changing the size of the nanospheres, deposited material and metal thickness, NSL has been developed to fabricate nanoparticles with tunable LSPR wavelengths (λmax).1,11 The optical properties of metal nanoparticles can be modeled by the classical
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electromagnetic approaches. Mie theory 12 is widely used to simulate the scattering of light by a spherical particle. A numerical method, discrete dipole approximation (DDA) method, has been used to model the optical properties of object with arbitrary shapes in various dielectric environments. In the DDA method, the particle of interest is represented by a polarizable cubic array. The induced
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