Abstract
We used the high-resolution Inelastic X-ray Scattering beamline of the Advanced Photon Source at Argonne National Laboratory to measure the terahertz spectrum of pure water and a dilute aqueous suspension of 15 nm diameter spherical Au nanoparticles (Au-NPs). We observe that, despite their sparse volume concentration of about 0.5%, the immersed NPs strongly influence the collective molecular dynamics of the hosting liquid. We investigate this effect through a Bayesian inference analysis of the spectral lineshape, which elucidates how terahertz transport properties of water change upon Au-NP immersion. In particular, we observe a nearly complete disappearance of the longitudinal acoustic mode and a mildly decreased ability to support shear wave propagation.
Highlights
The collective terahertz dynamics of disordered materials has been the focus of thorough investigation in the last five decades, there are still many unexplored areas
We found that its scattering signal had a nearly Lorentzian profile with 1.6 meV width, which was adequate to resolve the inelastic modes of both pure water and suspension spectra even at the lowest Q, i.e., 3 nm−1
The broad high-energy shoulder in the pure water is customarily assigned to the longitudinal acoustic (LA) mode, and its phenomenology was thoroughly investigated by Inelastic X-ray Scattering (IXS) [24,25,29]
Summary
The collective terahertz dynamics of disordered materials has been the focus of thorough investigation in the last five decades, there are still many unexplored areas. The influence of floating colloids on acoustic propagation is reasonably understood for lengthscales in a continuum and small colloid diameters [6,7,8,9,10], no comparable knowledge was achieved beyond these approximations Filling this void seems especially compelling for mesoscopic acoustic waves, which have wavelengths and periods respectively matching first neighboring particle–particle separations and “in cage” rattling periods, and are directly coupled with microscopic degrees of freedom of the suspension. These collective excitations are probed by terahertz spectroscopic methods such as Inelastic X-ray (IXS [11]) and Neutron Scattering (INS [12,13]). The coverage of an expanded Q-range (3 nm−1 ≤ Q ≤ 21 nm−1) enabled these measurements to achieve a more extensive mapping of the complex multi-scale behavior of the samples
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