We present the results of ultrafast optical experiments on silver quantum dot superlattices. Dramatic changes in the electron dynamics occur as a function of interparticle spacing due to enhanced dipolar coupling and, most importantly, electron delocalization

Averitt, R. D. (Richard D.)

Beverly, K. C.

Demsar, J. (Jure)

Heath, J. R.

Taylor, Antoinette J.,

Woode, E. K. (Ebenezer K.)

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Approved for public release: distribution is unlimited. Title: Author@). Submitted to. ULTRAFAST ELECTRON DYNAMICS IN SILVER NANOPARTICLE ARRAYS RICHARD D. AVERITT, EBENEZER K. WOODE, JURE BEVERLY, J.R. HEATH, DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY DEMSAR, ANTOINETTE J. TAYLOR MST-10; K.C. CLEO/QELS 2003 BALTIMORE, MD JUNE 2-6, 2003 Los Alamos NATIONAL LABORATORY Los Alamos National Laboratory, an affirmative actionlequal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, orto allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish: as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (8100) . r Ultrafast Electron Dynamics in Silver Nanoparticle Arrays Abstract: We present the results of ultrafast optical experiments on silver quantum dot superlattices. Dramatic changes in the electron dynamics occur as a function of interparticle spacing due to enhanced dipolar coupling and, most importantly, electron delocalization. 02002 Optical Society o f  Amcrica OClS codes: (320.71 30) Ultrafast Processes in Condensed Matter (160.3900) Metals The metal-insulator (MI) transition remains the most enigmatic problem in condensed matter physics and is dccply connected to unsolved problems in many materials [ I ] .  Metallic quantum dot (QD) superlattices scrvc ;is ;I motlcl system to investigatc the MI transition since, to a large extent, the two competing effects that drive a metallic syslcm insulating can be independently controlled. These effects are electron correlation and disorder. The nnturc o f  this quanluni phase transition in QD arrays has been investigated in detail [2]. Using femtosecond optical spcctroscopy. we have observed dramatic changes in the electron dynamics as a function of interpartick spacing due to cnlinnccd dipolar coupling and. for arrays with the least disorder, electron delocalization. The Ag nanoparticles (nominal diameter 2R = 7.5 nm) were synthesized in chloroform using an invcrsc miccllc technique. The hexagonally close-packed arrays were formed on a Langmuir trough. Films wcre prepared by transfer to an MgO substrate. To prevent degradation, the samples wcre kept under vacuum. The photoinduced change in reflectivity (AWR) was measured using a Ti:AI2O3 oscillator producing 20-fs pulses at I .5 eV with iin 80 MHz repetition rate. To minimize photodegradation, fluences below 3~J/Clll? were used. I E & x .- .- - 48mNlm - - - -  14mNhn 5 2 -I- 37mNlm c '1- Fig I : (a)  Spectrum for arrays consisting of 7.5 nm diameter heptanethiol coated nanoparticles. (b) Photoinduced chanycs in retlcctivity for the arrays in (a). In going from 14 mN/m to 48 mN/m Dd2R decreases from -1.16 to -1.10 wherc I),, I S  thc average interparticle spacing. Figurc I(a) shows the transmission as a function of wavelength for a series of QD arrays at different compressions. The plasmon resonance is shifted from 400 nm (i.e. the plasmon resonance for isolated nanoparticles) to -520 nm due to dipolar coupling. With compression thcre is a strong increase in the absorption, primarily due to an incrcascd polarizability of the QD arrays. Figure I(b) shows the AWR dynamics for the same arrays. With compression, thc magnitude of AWR increases, and thcre is an enhancement of the - 2ps oscillation that is related to surface oscillations of the individual nanoparticles [3]. In spite of the strong coupling in the most compressed films, the changes in the electron dynamics are minimal - we ascribe this to disorder in the QD arrays duc to the large size distribution of the nanoparticles. -4 !iV /so -6J ,  . , . , 400 SO0 600 700 800 900 0 s 10 1s 20 2s Wavelength (nm) Time (ps) Fig 7:  (a) Spectrum for arrays consisting of 7.5 nni diameter dodecanethiol coated nanoparticles. (b) Photoinduced changcs in reflectivity for the arrays in (a). In going from I6 mN/m to 43 mN/m DJ2R decreases from - I .4 to - 1.2. Figure 2(a) shows the transmission for a scries of QD arrays with a narrower size distribution. For the most compressed film a large increase in the absorption on the low energy side of the resonance is observed. Figure 2(b) shows the corresponding dynamics. Initially, with compression there is an increase in AIUR for the QD arrays as in Fig. 1 .  However, a decrease in the magnitude of ARIR occurs from 30 to 40 mN/m. In addition, there is a x phasc shift in the 2 ps oscillations. With a further compression to 43 mN/m there is a dramatic changc in thc A W <  dynamics - the signal is negative and increases in magnitude due to electron delocalization in the arrays. Finally, ;is shown in the inset, an oscillation with a period of 12 ps is observed. This is likely due to the coherent oscillation ol‘ii phonon mode of the Q D  array. [ I ]  1’. P. Edwards. et al.. Solid State Phys. 52. 229 (1999). [?.I K. C. Beverly. et al., Proc. Nal. Acad. Sci. 99. 6456 (2002) and references therein [31 C. Voisin. er al.. J. Phys. Chem. B 105. 2264 (2001). 

Ultrafast electron dynamics in silver nanoparticle arrays

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Ultrafast electron dynamics in silver nanoparticle arrays

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