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Journal Articles Nanoscale Year : 2017

Environmental effects on the natural vibrations of nanoplatelets: a high pressure study

Abstract

Resonant acoustic modes from ultrathin CdS colloidal nanoplatelets (NPLs) are probed under high pressure using low frequency Raman spectroscopy. In particular we focus on the characterization of the recently evidenced mass load effect that is responsible for a significant downshift of the NPL breathing frequency due to the inert mass of organic ligands. We show that a key parameter in the observation of the mass effect is whether the surrounding medium is able to support THz acoustic wave propagation, at a frequency close to that of the inorganic vibrating core. At low pressures, surface organic molecules show a single particle-like behavior and a strong mass effect is observed. Upon pressure loading the ligands are compacted together with the surrounding medium and slowly turned into a solid medium that supports THz acoustic phonons. We observe a continuous transition towards a fully embedded NPL with a frequency close to that of a freely vibrating slab and a progressive loss of the mass effect. The quality factor of the detected vibration significantly decreases as a result of the appearance of a "phonon-like" behavior of the environment at the origin of damping and energy dissipation. Resonant acoustic modes arise from a structure when the phonon wavelength becomes comparable to the confinement dimension. These modes have been investigated in the past few years for various geometries of nanomaterials such as spherical metallic, 1,2 semiconducting 3,4 and dielectric NPs. 5 Low frequency Raman experiments have essentially focused on nano-objects embedded in solid environments such as glassy matrices, which aroused theoretical studies to understand the coupling with the matrix. 6,7 This coupling essentially results in a damping of the modes i.e., a broadening of the Raman peaks without significant shifting of the resonance frequency as compared to a free vibration. The damping mainly originates from energy dissipation in the matrix due to the emission of THz acoustic waves. Inorganic materials can accommodate acoustic phonons with relatively high frequencies. For instance in inorganic glass, 8 metallic, 9 CdS and CdSe systems, the maximum of acoustic phonon frequency reaches several THz at the maximum of the dispersion relations. For this reason, experimental Raman spectra of embedded NPs (position and width of the resonances) have been so far well explained by core-shell 10 or core-matrix 11 models. However, the recent finding of a mass load effect from CdS and CdSe NPLs loaded with oleic acids at their surfaces suggests that the sound propagation within the organic molecules should be neglected. This is the basis of the mass loaded model where only the inertial mass of the ligands is taken into account to explain the lowering of the NPL breathing frequencies. 12 The discovery of this effect with two-dimensional nano-objects was recently extended to QDs explaining the temperature dependence of acoustic vibrations of CdSe and CdSe-CdS core-shell nanocrystals 13 and the modulation of QD acoustic vibration frequencies through the choice of surface ligands. 14 It can also provide insights into more complex geometries such as nanorod heterostructures. 15 Resonant acoustic modes in finite size structures arise when the phonon wavelength matches the confinement dimension. In the case of nanomaterials with a 3D confinement , such as nanoparticles, breathing acoustic modes with an angular momentum ℓ = 0 have been extensively studied in particular because they are Raman active. The breathing motion of a free sphere involves a radial displacement of the surface with a frequency given by ν 3D ' 0:9=D ffiffiffiffiffiffiffiffiffiffiffiffi C 11 =ρ p where D, C 11 and ρ are the sphere diameter, elastic constant and density, respectively. 1D confinement geometries such as
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Dates and versions

hal-02107265 , version 1 (23-04-2019)

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Adrien Girard, Jeremie Margueritat, Lucien Saviot, Denis Machon, Benoit Mahler, et al.. Environmental effects on the natural vibrations of nanoplatelets: a high pressure study. Nanoscale, 2017, 9 (19), pp.6551-6557. ⟨10.1039/c7nr00731k⟩. ⟨hal-02107265⟩
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