Raman spectroscopy of a single gold nanoparticle dimer

This paper reports the first measurement of ultra-low frequency Raman scattering by a single nano-system (an isolated gold nanoparticle embedded in a polymer matrix or a dimer of two such nanoparticles). This spectroscopy... [ view full abstract ]

This paper reports the first measurement of ultra-low frequency Raman scattering by a single nano-system (an isolated gold nanoparticle embedded in a polymer matrix or a dimer of two such nanoparticles). This spectroscopy approach, avoiding the inhomogeneous broadening effects affecting experiments on nanoparticle assemblies, yields both the frequencies and damping rates of the detected acoustic modes, allowing a detailed analysis of the opto-mechanical response of a dimer of two close nanoparticles.

The Raman spectra of single dimers of close nanoparticles considerably differ from those of isolated gold nanoparticles, dominated by their quadrupolar vibration mode. Indeed, the frequency of this mode is modified, and additional modes become observable at both lower and higher frequencies. These observations are ascribed to mechanical (i.e., hybridization of the vibrational modes of nanoparticles in a dimer) and opto-mechanical (i.e., the fact that vibrations which do not affect the optical response of an isolated nanoparticle may modify that of a dimer, thus becoming Raman active) coupling effects. In particular, the two ultra-low frequency modes appearing in the Raman spectra of single dimers are interpreted as out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. In the high frequency range, the dimer shape is also shown to enable Raman scattering by high angular momentum vibration modes, with an efficiency increasing with decreasing interparticle distance.

These results show that a gold dimer (or oligomer) constitutes a simple opto-mechanical resonator whose properties can be controlled by adjusting the morphology and number of composing nanoparticles, as well as the elastic properties of their surrounding matrix.