Hot Electron Relaxation Dynamics of Gold Nanoparticles Embedded in MgSO 4 Powder Compared To Solution: The Effect of the Surrounding Medium Stephan Link, ² Akihiro Furube, ‡,§ Mona B. Mohamed, ² Tsuyoshi Asahi, ‡ Hiroshi Masuhara, ‡ and Mostafa A. El-Sayed* ,² Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565-0871, Japan ReceiVed: August 27, 2001; In Final Form: NoVember 19, 2001 To test the influence of the surrounding medium on the relaxation dynamics of the plasmon band bleach recovery of gold nanoparticles after excitation with femtosecond laser pulses, we embedded 14.5 and 12.1 nm colloidal gold nanoparticles (synthesized electrochemically) in MgSO 4 powder and investigated these samples by femtosecond diffuse reflectance spectroscopy. By measuring the relaxation dynamics over a wide range of excitation energies, we found that the fast decay component is slower by about a factor of 2 for the particles in the MgSO 4 powder compared to those in solution while no significant change in the slow decay component is observed. In agreement with this observation, we found that adding solvent to the particles embedded in the powder caused a decrease in the relaxation time from about 10 ps to 5 ps for the fast decay component. This leads to the conclusion that the electron-phonon relaxation in these gold nanoparticles depends on the chemical nature and/or physical phase (solid vs solution) of the surrounding medium. A discussion of this in terms of the type of phonon involved, and the nature of the electron-phonon and phonon- phonon relaxation processes is discussed. To our knowledge, this also presents the first time that a transient bleach could be observed by diffuse reflectance spectroscopy. I. Introduction Ultrafast dynamics of photoexcited noble metal nanoparticles is attracting great interest lately. 1-26 Many femtosecond pump- probe experiments of colloidal solution systems and of particles incorporated in optically transparent glasses have been carried out in order to investigate the relaxation processes of electrons with a non-Fermi distribution and the subsequent thermalization through electron-electron, electron-phonon, and phonon- phonon interactions. Most of the experimental effort has been devoted to the size 6,7,16,23,25,26 and shape 9 dependence of the electron-phonon coupling in small metal particles. Independent of the metal and its size and shape, it was found that the electron-phonon relaxation time increases with increasing excitation power in the high perturbation regime and a second, much longer decay component can then be observed. 5,7,14,15 This second component has been assigned to phonon-phonon relaxation between the metal particles and the surrounding medium and was found to be on the order of about 100 ps. For example, Hamanaka et al. 1 observed a decay of the plasmon band broadening of 6 nm silver nanoparticles with relaxation times of 2-3 and 200 ps. Perner et al. 2,3 measured relaxation times of 4 and 200 ps for 30 nm gold particles embedded in a sol-gel matrix after excitation at 400 nm. Inouye et al. 4 measured gold nanoparticles with a diameter of 7.6 nm in a SiO 2 glass matrix and found decay times of 2.8 and 120 ps. Ahmadi et al. 5-7 reported decay times of 2.5 and >50 ps for the electron-phonon and phonon-phonon relaxation times in 30 nm colloidal gold particles. Zhang and co-workers 10-13 observed slightly longer relaxation times of 7 and 400 ps for 15 nm gold nanoparticles in water when probed at 790 nm after excitation with 390 nm pulses. Furthermore, they reported a reduction of the decay times for the electron-phonon interac- tions from 7 to 3.5 ps when the solvent was changed from water to cyclohexane, suggesting that the electronic relaxation is sensitive to the surface environment. A similar dependence of the relaxation dynamics on the surrounding environment was found by Bigot and co-workers 18,19 for 6.5 nm silver nanopar- ticles. The electron-phonon relaxation increased from 0.77 to 1.4 ps when the matrix was changed from alumina to glass. This was explained in terms of the higher thermal conductivity of the alumina matrix. Except for these two studies, the main focus of the ultrafast electron dynamics has so far mainly been on the size-dependence of the electron-phonon relaxation under low-intensity excitation conditions. Here we report on the electron cooling of gold nanoparticles in different media after high-intensity excitation with an amplified Ti:Sapphire laser. Under these conditions, the temperature change of the gold lattice is on the order of several tens of degrees. The transfer of the excitation energy to the surrounding medium, which acts as an energy sink, might therefore become an important factor in the observed relaxation dynamics, in contrast to the low excitation intensity experiments reported in the literature. We have therefore investigated the cooling dynamics of gold nanoparticles embedded in a MgSO 4 powder and compared the results with those obtained for the same particles in solution. This represents not only a change in the chemical nature of the surrounding species but also a change in the physical phase of the medium (liquid vs solid medium). We chose MgSO 4 powder as a surrounding medium because * Corresponding author ² Georgia Institute of Technology. ‡ Osaka University. § Current address: Photoreaction Control Research Center, National Institute of Advanced Industrial Science and Technology. 945 J. Phys. Chem. B 2002, 106, 945-955 10.1021/jp013311k CCC: $22.00 © 2002 American Chemical Society Published on Web 01/03/2002