Femtosecond transient absorption dynamics of close-packed gold nanocrystal monolayer arrays q Sang-Kee Eah a , Heinrich M. Jaeger a , Norbert F. Scherer a,d , Xiao-Min Lin b,c, * , Gary P. Wiederrecht c a The James Franck Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA b Materials Science Division, Argonne National Laboratory, Building 200, 9700 South Cass Avenue, Argonne, IL 60439, USA c Chemistry Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA d Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, IL, 60637, USA Received 11 December 2003; in final form 17 January 2004 Published online: Abstract Femtosecond transient absorption spectroscopy is used to investigate hot electron dynamics of close-packed 6 nm gold nano- crystal monolayers. Morphology changes of the monolayer caused by the laser pump pulse are monitored by transmission electron microscopy. At low pump power, the monolayer maintains its structural integrity. Hot electrons induced by the pump pulse decay through electron–phonon (e–ph) coupling inside the nanocrystals with a decay constant that is similar to the value for bulk films. At high pump power, irreversible particle aggregation and sintering occur in the nanocrystal monolayer, which cause damping and peak shifting of the transient bleach signal. Published by Elsevier B.V. 1. Introduction Hot electron relaxation dynamics in nanoparticles has been an active research area because it can reveal the change of electronic and lattice vibrational properties when the size of materials approaches the nanometer scale [1–9]. Time-resolved femtosecond spectroscopy is the primary technique used to investigate these fast re- laxation dynamics. The principle of this technique is to excite electrons to higher energy levels through a short pump laser pulse. The excited nonthermalized electron distribution quickly relaxes on a femtosecond time scale into a Fermi–Dirac distribution with a higher electron temperature through electron–electron (e–e) interaction. The thermalized hot electron distribution then cools down through electron–phonon (e–ph) coupling, and eventually dissipates the excess energy through phonon– phonon (ph–ph) coupling, either within the nanoparticle or into the surrounding environment. The relaxation process can be monitored by measuring the transient absorption spectrum though a probe laser pulse that is variably delayed from the pump pulse. The change of electronic distribution will cause a change of the di- electric constant, which affects the transient absorption of the probe pulse. Most of the pump–probe experi- ments carried out so far use either diluted metal nano- particle colloids, or nanoparticles embedded in a dielectric matrix [1–12]. Aside from the potential prob- lem of large particle size distribution in these experi- ments, the optical density of these samples is typically quite low, and varies from one experiment to another. These issues have complicated the elucidation of the size range the e–ph coupling constant would deviate from its bulk value. Because of the finite particle size, the tran- sient absorption decay caused by e–ph coupling can be entangled with ph–ph coupling, especially with high q Work supported by the US Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38, National Science Foun- dation MRSEC Program under Award Number DMR-0213745, DOE Center for Nanoscale Materials, and by the University of Chicago – Argonne National Laboratory Consortium for Nanoscience Research (CNR). * Corresponding author. Fax: +1-630-252-9151. E-mail address: xmlin@anl.gov (X.-M. Lin). 0009-2614/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.cplett.2004.01.056 Chemical Physics Letters 386 (2004) 390–395 www.elsevier.com/locate/cplett