Simple Vapor-Phase Synthesis of Single-Crystalline Ag Nanowires and Single-Nanowire Surface-Enhanced Raman Scattering

Abstract

We report vapor-phase synthesis of single-crystalline freestanding Ag nanowires (NWs) and polarized surface-enhanced Raman scattering (SERS) of a single NW. To the best of our knowledge, vapor-phase synthesis of single-crystalline Ag NWs and polarized SERS on an individual NW have not yet been reported.Silver has the highest electrical and thermal conductivity among all metals. In addition, because of the nature of its optical constants, silver shows the most effective SERS among all metals in the visible region. 1 Since the intensity of the SERS signal depends strongly on the detailed morphology of the Ag nanostructures, it is most important to have a well-defined and well-characterized system with a clean surface for the preparation of reliable sensors based on SERS detection. Current fabrication processes for the production of SERSactive substrates, however, are often irreproducible to a certain degree, thereby leading to inconsistent optical properties and, thus, significantly fluctuating enhancement factors for substrates prepared by apparently the same procedure. Irreproducible SERS enhancement would be detrimental to quantitative and reliable sensing, which would be most critical for medical diagnosis.As a first step toward fabrication of well-controlled nanobiosensors employing SERS technique, we searched for a method to synthesize well-characterized free-standing single-crystalline Ag NWs with a clean surface. This led us to the adoption of vaporphase synthesis rather than solution phase. While vapor-phase synthesis is one of the most widely used methods for the synthesis of 1D nanostructures, it has been used mainly for the synthesis of semiconductor NWs and nanotubes. Only a few metallic NWs have been synthesized through vapor phase, 2 and most of the reported methods for the synthesis of Ag NWs are wet chemical methods involving templates, surfactants, or capping agents. 3 Our synthetic method is unique in that it uses only a single reactant, Ag 2 O, without using any templates or catalysts. In a typical synthesis, 0.2 g of Ag 2 O powder was placed in an alumina boat in the middle of a 1 in. diameter horizontal quartz tube furnace. The NWs were grown at a few centimeters downstream from the precursor on a Si substrate. At high temperature (T 1 ) 900-1000°C ), the precursor vapor was carried downstream by the flow of 500 sccm of Ar at 5-10 Torr to a lower temperature zone (T 2 ) 500°C), where Ag NWs were grown. The noncatalytic growth of the NWs and the preferred formation of large particles over NWs at higher precursor temperatures demonstrated that the Ag NWs were formed by a vapor-solid mechanism. 2a A representative SEM image in Figure 1a shows good density of straight NWs tens of micrometers long on a Si substrate. The insets are SEM and TEM images of Ag NWs, showing that the tips of NWs are round, and NWs have clean surfaces and diameters of 80-150 nm.The XRD patterns of as-grown NW ensembles (Supporting Information Figure S1) are indexed perfectly to the face-centered cubic (fcc) crystal str...

Room-Temperature Chemical Synthesis of Silver Telluride Nanowires

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Creating Well-Defined Hot Spots for Surface-Enhanced Raman Scattering by Single-Crystalline Noble Metal Nanowire Pairs

“…25,26 At the atomic level, however, they still contain inhomogeneities of the surface structure, i.e., they are not well-defined yet on the atomic scale. 27 In contrast to the past SERS experiments, we employed singlecrystalline noble metal nanowires that have atomically smooth surfaces 28 as building blocks for an optimal SERS active system. Herein, we carried out a systematic study of noble metal nanowire pairs in order to construct a well-defined "metal/ molecule/metal" structure that could increase the SERS activity to a very high extent.…”

Bimetallic Nanocobs: Decorating Silver Nanowires with Gold Nanoparticles

“…[20,36] Although single nanowire-SERS activity can in fact be achieved with a proper selection of growth modes and polarization conditions, the overall SERS activity is generally compromised due to reduced interaction with the laser beam because of the absorption shift associated with longitudinal resonances far into the infrared region. [28,37] The close match of the lattice spacings for silver and gold and their tendency towards mutual displacement are favorable for alloyed nanoparticles, but not desirable for core/shell nanoparticles, which are composed of separated layers of distinctive elements. [30,[38][39][40] A simple bimetallic core/shell without an interlayer, or the use of silicon oxide as an interlayer prevents the utilization of this ''inner shell'' space as a prospective, highly desirable hot spot for analyte molecules, which can be created by growing gold nanoparticles on functionalized surfaces, as demonstrated by Halas.…”