rXXXX American Chemical Society A dx.doi.org/10.1021/jp112129k | J. Phys. Chem. C XXXX, XXX, 000–000 ARTICLE pubs.acs.org/JPCC Plasmonic Enhancement of Nonradiative Charge Carrier Relaxation and Proposed Effects from Enhanced Radiative Electronic Processes in Semiconductor-Gold Core-Shell Nanorod Arrays Erik C. Dreaden, † Svetlana Neretina, †,‡ Wei Qian, † Mostafa A. El-Sayed,* ,† Robert A. Hughes, ‡,§ John S. Preston, §,|| and Peter Mascher || † Laser Dynamic Laboratory, Department of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States § Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario L8S 4M1, Canada ) Department of Engineering Physics, McMaster University, Hamilton, Ontario L8S 4L7, Canada ’ INTRODUCTION The resonant interaction of electromagnetic waves with electron oscillations at the surfaces of nanoscale metals is known as localized surface plasmon resonance (LSPR). LSPR is deter- mined by the nanoparticle’s material, as well as its size, shape, and dielectric environment, and results in intense absorption/scatter- ing and the generation of highly polarized near-fields that dephase on the femtosecond time scale. 1-4 Absorbed energy can be dissipated through electron collisions, 5 lattice vibra- tions, 2,6 or resonant coupling to neighboring electronic systems. LSPR coupling interactions have been shown to enhance the absorption cross sections of molecules 7 and semiconductor nanoparticles 8 in their vicinity, as well as to enhance the optical activity of surface adsorbates. 9 Their interactions with radiative electronic systems have been shown to enhance molecular fluorescence intensity, 10 Raman scattering, 11-13 second- and third-harmonic generation (SHG and THG), 14,15 hyper-Rayleigh scattering, 16 and hyper-Raman scattering. 17 The rational design of photovoltaic and photodiode devices incorporating plasmonic elements has also resulted in improved performance 18,19 and recently allowed for ultrafast electron spin manipulation in colloidal core-shell nanoparticles. 20 LSPR coupling with excitonic systems exhibiting well-defined band structure involves increasingly complex and varied interac- tions and has been shown to enhance F€ orster resonance energy transfer (FRET) efficiency between semiconductor quantum dots (QDs), 21 as well as resonant energy transfer between metal- lic and semiconducting nanoparticles. 22,23 In general, plasmon- exciton interactions in solid-state excitonic systems are charac- terized by (i) enhanced quantum efficiency, 8,24,25 (ii) exciton energy shift, 26,27 (iii) increased radiative decay rate, 26,28,29 and (iv) emission polarized parallel with resonantly coupled LSPR modes. 24,30 These phenomena require that the exciton energy overlap with that of the LSPR 25,31 and that the transition dipole moment of the exciton lie collinear with the dipole moment of the LSPR. 24,32 Received: December 21, 2010 Revised: February 16, 2011 ABSTRACT: Plasmonic field enhancement of nonradiative exciton relaxation rates in vertically aligned arrays of high aspect ratio CdTe-Au core-shell nanorods was investigated by transient absorption spectroscopy, computational electromag- netics, and kinetic modeling. Increasing shell thickness in the high aspect ratio nanorods was found to result in dramatic differences in polarization-dependent nonradiative exciton re- laxation rates, which we attribute to differing mechanisms of plasmonic field enhancement associated with predominant ground- or excited-state absorption processes. These results are compared with previous investigations of low aspect ratio CdTe-Au core-shell nanorods, and overall conclusions regarding plasmonic enhancement of nonradiative relaxation rates in this system are presented. We propose that when the resonantly coupled dipolar plasmon field of the shell is polarized parallel to the ground-state absorption transition moment of the core, Auger recombination dominates carrier relaxation and slower second-order decay kinetics are observed. When contributions of the resonantly coupled plasmon field are nondipolar or orthogonal to the ground-state absorption transition moment of the core, excited-state absorption processes are believed to dominate and increasingly rapid first-order relaxation kinetics are observed. We find that these processes can vary greatly, depending on shell thickness and the orientation of the array, but are insensitive to aspect ratio. These investigations have significant implications in the design of photovoltaic and optoelectronic devices incorporating anisotropic plasmonic elements.