Nanosphere Lithography: Synthesis and Application of Nanoparticles with Inherently Anisotropic Structures and Surface Ch
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Nanosphere Lithography: Synthesis and Application of Nanoparticles with Inherently Anisotropic Structures and Surface Chemistry Christy L. Haynes, Amanda J. Haes, and Richard P. Van Duyne Department of Chemistry, Northwestern University Evanston, IL 60208-3113, U.S.A. ABSTRACT Early work with size-tunable periodic particle arrays (PPAs) fabricated by nanosphere lithography (NSL) demonstrated that the localized surface plasmon resonance (LSPR) could be tuned throughout the visible region of the spectrum. The LSPR is sensitive to changes in nanoparticle aspect ratio and local dielectric environment. This property has recently been exploited to develop a novel method of measuring surface-enhanced Raman scattering (SERS) excitation profiles. Single layer PPAs consist of size-tunable anisotropic nanoparticles that can be modified to exhibit anisotropic surface chemistry. This work demonstrates multiple schemes for PPA modification using self-assembled monolayers and colloid decoration. Nanoparticle anisotropy can be further exploited with the recent combination of NSL and reactive ion etching (RIE); this extends the two-dimensional PPA structure into the third dimension INTRODUCTION In recent years, it has become possible to investigate the size dependent nature of chemical and physical properties in the nanoscale regime. While "top-down" nanoarchitecture techniques are the natural extension from previous capabilities, "bottom-up" approaches are gaining popularity.[1] This paper discusses the application of the "bottom-up" nanofabrication strategy nanosphere lithography (NSL) to create structurally anisotropic nanoparticles. Deckman et al pioneered the NSL technique wherein the natural self assembly of a monolayer of nanospheres forms a hexagonally close-packed crystal.[2] Deposition of material through this colloidal crystal mask, with subsequent removal of the nanospheres, results in an array of evenly spaced, homogeneous nanoparticles known as a periodic particle array (PPA) (Figure 1).[3] The dimension of these truncated tetrahedral nanoparticles can be tuned by choice of nanosphere diameter and deposition thickness (b). While it is possible to study the behavior of any chemical Contact Mode AFM Image
b a 2.3µm D=400 nm Sphere Mask b=50 nm Ag
Figure 1. AFM image of Ag PPA and graphical illustration of a single nanoparticle. C6.3.1
or physical property as a function of material size regime, the optical properties of metallic nanoparticles hold special significance in the field of surface-enhanced Raman scattering (SERS), and thus, are the focus of this work. SERS was first witnessed in 1974 by Fleishmann et al.[4] However, the true significance of the amplified Raman signals was not understood until 1977 when Jeanmaire and Van Duyne attributed the large signals to an electromagnetic enhancement.[5] Until recently, most experimental work in SERS was devoted to determining how the 106 SERS enhancement is divided between the chemical (CHEM) and the electromagnetic (EM) enhancement mechanisms. The CHEM is based on the
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