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Coherent acoustic phonon dynamics of gold nanorods and theirenvironmental effects by transient absorption spectroscopy(Kwansei Gakuin Univ.) ○Li Wang, Syouhei Takeda, Naoto Tamai[Introduction]

Metal nanoparticles have attracted much interest due to the unique properties ofsurface plasmon resonance and surface enhanced electric field. With the light irradiation, therelaxation of excited plasmon modes follows the processes of electron-electron scattering,
electron-phonon coupling and coherent acoustic phonon vibration combined with heat dissipation tothe surroundings. A frequency range from 1 GHz to 1 THz of acoustic phonon vibration have been
observed for small sized metal nanoparticles. This kind of mechanical property of metalnanostructures can be applied in ultrasensitive mass sensors and ultrafast sound sources. In this report,
ultrafast dynamics of gold nanorods (Au NRs) and nanospheres (Au NSs) dispersed in poly(vinylalcohol) (PVA) films were investigated at various temperatures. Extensional mode of coherent acoustic
phonon vibration for Au NRs and breathing mode for Au NSs were observed. The effects ofenvironments on coherent acoustic phonon vibration for Au NRs were discussed in terms of themechanical interaction of the polymer matrix and the liquid-solid coupling of the solution.

[Experiments]

Five kinds of Au NRs were gifted from Dai-Nippon-Toryo Co., Ltd and fourkinds of Au NSs were commercially purchased from Aldrich. Concentrated Au NRs (or NSs) inaqueous solution was dispersed in 2 wt% PVA solutions and spin-casted on glass slides, which left
various molar ratios of Au NR to PVA (1 to 400~40,000). Transient absorption (TA) spectroscopy wasperformed with a conventional pump-probe method described previously. [1][Results and Discussion] A representative TEM image of Au NRs is shown in Fig. 1a. The widthand length of Au NRs are around 9 nm and 46 nm by fitting the histograms for over 100 nanorods with
Lognormal

distributionfunctions.

Theextinction spectra of Au NRs with 46-nm
length in PVA matrix and aqueous solutions areshown in Fig. 1(b). Both transverse- andlongitudinal- surface plasmon modes for Au
NRs in PVA matrix are red-shifted as comparedwith those for Au NR solutions due to theFig. 1 A TEM image of Au NRs with 9-nm width and 46-nm
length (a) and normalized extinction spectra of NRs embedded
in PVA matrix and dispersed in aqueous solution (b).

difference of the refractive index
between PVA matrix (n = 1.53) andaqueous solution (n = 1.33).
Time dependences of bleaching peakwavelengths can be obtained byroughly

fittingsomeofGauss

functions to TA spectra of Au NR andNS samples. The period of coherent
acousticphonon

vibration

wasestimated by fitting a damped cosine

Fig. 2 Experimental periods of coherent phonon vibration of Au NRs in
PVA matrix (square), in water (circle) and in vacuum (triangle) wereplotted as a function of Au NR length (a). The simulated periods based on
pile-soil interaction model are plotted by the solid lines corresponding to
20, 10 and 5 nm diameter of Au NRs. Oscillation periods of Au NRs in
PVA matrix (46-nm length) and calculated periods by using Young’s
modulus E[100] of bulk Au were plotted as a function of temperature (b).

function to the decay profile ofbleaching peaks. The oscillation periods of Au NRs in PVA matrices become smaller than thecalculations while those of Au NRs in aqueous solution present a bit larger values in Fig. 2a. It is veryclear that the oscillation periods of Au NRs are shifted with the surrounding environments. The
environmental effects on coherent acoustic phonon vibration for Au NRs were discussed in terms ofthe mechanical interaction of the polymer matrix and the liquid-solid coupling of the solution.[2,3] A
simplified model of pile-soil interaction was introduced for Au NRs in PVA matrix. By considering ofthe elastic and viscous properties of PVA matrix, the oscillation period of Au NR was found to beaffected by their mechanical interaction. The calculated periods were in good agreement with theexperimental data as shown by the solid lines in Fig. 2a.
In Fig. 2b, the oscillation periods of Au NRs were experimentally 15 % decreasing from 40.5 ps to34.3 ps with the decrease of temperature from 294 K to 10 K, while the period of extensional mode fora 46-nm rod was only 4 % decreased by using the elastic property of bulk Au with [100] growth
direction. The elastic property of PVA is also changed with the temperature. Therefore, thecontribution from PVA matrix should be considered for the temperature effect on the periods of Au
NRs.

[References][1] Wang, L.; Kiya, A.; Okuno, Y.; Niidome, Y.; Tamai, N., J. Chem. Phys. 2011, 134, 054501.
[2] Zijlstra, P.; Tchebotareva, A. L.; Chon, J. W. M.; Gu, M.; Orrit, M., Nano Lett. 2008, 8, 3493-3497.
[3] Chakraborty, D.; van Leeuwen, E.; Pelton, M.; Sader, J. E., J. Phys. Chem. C 2013, 117, 8536-8544.