Plasmon-enhanced Raman scattering of coaxial hybrid nanowires made with light-emitting polymer and gold Se Hee Park, a Dong Hyuk Park, a Young Ki Hong, a Seok Ho Lee, a Dae-Chul Kim, b Krishna Prasad Dhakal, b Jinsoo Joo a * and Jeongyong Kim b * We report on the plasmon-enhanced Raman scattering of coaxial hybrid nanowires (NWs) made with light emitting poly(3- methylthiophene) (P3MT) and gold (Au) core or coating. Absorption spectra of coaxial hybrid NWs showed the absorption peaks because of localized surface plasmon (SP) excitation at ~ 567 and ~ 610 nm, in addition to the p–p* transition peaks of P3MT. Raman spectra of a single strand of coaxial hybrid NW and plain P3MT NW was obtained by using three different wavelengths of laser excitation of 514, 633, and 785 nm. We found that Raman intensities of hybrid coaxial NWs were enhanced by 4 to 15 times over those of plain P3MT NWs with the Raman excitation wavelengths close to the observed SP energies of coaxial NWs. We attributed the observed enhancement of the Raman signal to the resonance of the incident laser with the matching SP energies, rather than the possible doping level change, in hybrid coaxial NWs. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: surface plasmon; nanowire; coaxial hybrid; poly(3-methylthiophene); resonant Raman scattering Introduction Surface plasmons (SPs) are energy quanta of the collective oscilla- tions of conduction electrons confined to the surfaces of metallic nanostructures. [1] When the energy and the momentum of incident electromagnetic fields match with those of SPs, strong polarization is induced at the metallic nanostructure and the amplitude of the electromagnetic fields in close vicinity to the metallic nanostructure is greatly enlarged. [1,2] This phenomenon is due to the excitation of SPs and causes the enhancement of light absorption and scattering cross-sections. [3] In recent years, it has been reported that SP excitation enhances the optical properties such as absorption, [4–6] scattering, [7–9] and fluorescence [10,11] of metallic or hybrid nanostructures. These effects have been used in biological and optical applications, such as chemical and biological sensors, [12,13] solar cells, [14,15] and light- emitting devices. [16,17] For organic solar cells, the enhancement of absorption of nanostructures because of SP has been applied to increase photocurrent, which is the result of increasing photogen- erated charge carriers because of SP resonance in the active layer. [14,15] For light-emitting devices, SP coupling with a light emitter has been applied to enhance the emission efficiency of the devices. [16,17] We previously reported on significant enhance- ments of the photoluminescence (PL) intensity from a single strand of hybrid metal/polymer nanotubes. [18–20] This phenomenon was explained by charge and/or energy transfer and the excitation of SPs between the light-emitting polymer and the metal coating layer. Raman spectroscopy has been very useful in determining the structural details of p-conjugated polymers by identifying the chemical composition and molecular bonding. [21–23] Recently, Raman spectra of isolated single poly(3-methylthiophene) (P3MT) nanotubes provided the direct measurement of structural properties of individual P3MT nanotubes such as the doping level and the conjugation length. [22] Raman spectrum is known to be closely related to the excitation of SP, which is responsible for the phenomenon called the surface-enhanced Raman scattering. [23] In this study, we investigated the optical properties, especially the plasmon-enhanced Raman spectra of single coaxial hybrid nano- wires (NWs), using three different wavelengths of laser excitations. These coaxial NWs were fabricated through electrochemical and galvanostatic methods and consist of a core and coating of Au and P3MT. For comparison, we made plain P3MT NWs by the same method and performed the similar Raman and PL measurements. In the results, we found that the Raman spectra and PL intensity of the hybrid NWs were greatly enhanced over plain P3MT NWs, when excited with a laser of which the wavelength is close to the SP energies of the coaxial NW observed in UV/Vis absorption spectra. Experimental Sample preparation The plain P3MT NWs were electrochemically synthesized using an anode alumina oxide (Al 2 O 3 ) nanoporous template with a diame- ter of ~200 nm and thickness of ~60 mm (Whatman International, Ltd). For the metallic contact of the electrode, the gold (Au) was * Correspondence to: Jeongyong Kim, Department of Physics, University of Incheon, Incheon 406-772, Korea. E-mail: jeongyong@incheon.ac.kr * Jinsoo Joo, Department of Physics, Korea University, Seoul 136-713, Korea. E-mail: jjoo@korea.ac.kr a Department of Physics, Korea University, Seoul 136-713, Korea b Department of Physics, University of Incheon, Incheon 406-772, Korea J. Raman Spectrosc. (2012) Copyright © 2012 John Wiley & Sons, Ltd. Research Article Received: 11 April 2011 Revised: 3 October 2011 Accepted: 28 October 2011 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jrs.3121