Published March 7, 2007 | Version v1
Journal Article Open

Time-resolved spectroscopy of silver nanocubes: Observation and assignment of coherently excited vibrational modes

Abstract

The response of single crystal, cubic silver particles to ultrafast laser-induced heating has been examined experimentally and theoretically. The transient absorption traces display clear modulations due to coherently excited vibrational modes. Nanocube samples with edge lengths smaller than 50nm show a single modulation, whereas samples larger than 50nm show two vibrational modes. The results are compared to finite element calculations, where the cubes are modeled as having cubic crystal symmetry with the principal axes parallel to the sides of the particle. The action of the laser pulse is treated in two ways, first, as creating a uniform initial strain. In this case the predominant mode excited is the breathing mode. The period of this mode is in reasonable agreement with the vibrational periods measured for the smaller cubes and with the higher frequency modulation observed for the larger cubes. A nonuniform initial strain is also considered, which could arise from nonuniform heating for particles larger than the optical skin depth of the metal. In this case the predominant mode excited is a nontotally symmetric mode. The calculated periods from this analysis are in reasonable agreement with the lower frequency modulations observed for the larger samples. The results from this study show that, to within the accuracy of these measurements, the elastic constants of cubic silver nanoparticles are the same as bulk silver.

Acknowledgement

This work has been supported in part by a DARPA-DURINT subcontract from Harvard University and the David and Lucile Packard Foundation. One of the authors (Y.X.) is an Alfred P. Sloan Research Fellow and a Camille Dreyfus Teacher Scholar. Another authors (G.V.H.) acknowledges the support of the NSF (Grant No. CHE02-36279), and the Petroleum Research Fund (Grant No. PRF-39761AC) administered by the American Chemical Society. The work in Australia was supported by the Particulate Fluids Processing Centre and the Australian Research Council grants scheme. The authors would also like to thank Prashant Kamat and Istvan Robel for their help in collecting the time and wavelength resolved data in Fig. 3.

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Additional details

Identifiers

ISSN
1089-7690

Funding

Defense Advanced Research Projects Agency
David and Lucile Packard Foundation
Alfred P. Sloan Foundation
Camille and Henry Dreyfus Foundation
National Science Foundation
CHE02-36279
American Chemical Society
Petroleum Research Fund PRF-39761AC
Australian Research Council

Dates

Accepted
2007-03-07
published print
Accepted
2007-03-07
published online