Size-Dependent Ultrafast Electronic Energy Relaxation and Enhanced Fluorescence of Copper Nanoparticles Qusai Darugar, Wei Qian, and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400 Marie-Paule Pileni Laboratoire des Mate ´ riaux Me ´ soscopiques et Nanome ´ triques, UMR CNRS 7070, UniVersite ´ Pierre et Marie Curie, BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France, and School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400 ReceiVed: August 12, 2005; In Final Form: October 25, 2005 The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles. I. Introduction The study of electronic and optical properties of metallic nanoparticles is presently one of the most active areas of nanoscience and technology. The results have attracted more and more attention from experimentalists, theorists, and tech- nologists during the past decade. 1-12 There are several motiva- tions from both scientific and technological points of view that drive the intensive investigations of both static and dynamic electronic/optical properties of metallic nanoparticles. First, it is possible to discover new physical phenomena in metallic nanoparticles. Metallic nanoparticles have electronic structures that are in transition between atomiclike and bulklike structures. Many phenomena related to the properties of the electronic wave function, such as electronic and thermal transport, 13-15 the inter- action processes of the elementary excitations, 16,17 and the cou- pling between elementary excitations with the environment, 18,19 will be different in metallic nanoparticles with respect to those present in the corresponding isolated atoms and bulk metal. Second, the metallic nanoparticles have numerous potential applications ranging from optical waveguides to biosensors. 20-22 The basis for most of the potential applications of metallic nanoparticles is their unique localized surface plasmon resonance (LSPR) and scattering due to the excitation of collective oscillations of the electrons in the conduction band. For example, the LSPR oscillations can tremendously enhance the local electromagnetic field, making it possible to use surface-enhanced spectroscopy for single-molecule detection. 23,24 Physical properties of nanoparticles could be tailored by simply controlling their size and/or shape. 10,25,26 This fact led to intense research to understand the dependence of the physical properties on the size and shape. 10,25,27-33 For steady-state ab- sorption spectra, it is well-known that both the LSPR frequency and the line width depend on the nanoparticle size and shape and this dependence could be explained using classical elec- trodynamics theory. 25 According to this theory, if the metallic particle size is comparable to or smaller than the mean free path of the conduction electrons in the bulk, then the electrons will be scattered by the surface, 25 and then both the dielectric func- tion of particle that determines the LSPR frequency and the phase-coherence time of the collective excitation that determines the LSPR line width become size- and shape-dependent. With the advancement of ultrafast laser spectroscopy, it is possible to study the nonequilibrium relaxation dynamics of excited conduction electrons in metallic films and nano- particles. 13,34-40 The large difference in the heat capacity of the electrons and the lattice makes it possible to create a significant temperature rise in the electrons with respect to the ionic lattice in metallic nanoparticles by irradiating them with a sufficiently short laser pulse. Before irradiation, the electrons are located in energy states below the Fermi level. Immediately upon irra- diation, the energy is transferred to the electrons by absorption of photons via interband and intraband transitions. This quasi- instantaneous excitation of multiple electrons is a coherent collective excitation process, where the phase memory is conserved between the electromagnetic field and the newly occupied electronic states. According to Fermi-Dirac statistics, the excited electrons are in a nonthermal distribution. The * Author to whom correspondence should be addressed. Phone: (404) 894-0292. Fax: (404) 385-0294. E-mail: mostafa.el-sayed@ chemistry.gatech.edu. 143 J. Phys. Chem. B 2006, 110, 143-149 10.1021/jp0545445 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2005