Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations

Abstract

Control of nanocrystal (NC) crystallinity currently raises great interest because of its potential benefits in both physics modeling and technological applications. Advances in methods for synthesizing perfect single-crystalline NCs are recent, so that the effect of crystallinity on NC properties has received only limited study and still needs to be properly investigated. Here, we report that crystallinity of gold NCs dramatically modifies their vibrations. Using low-frequency Raman scattering, we clearly demonstrate that single-domain NCs vibrate differently than their multiply twinned counterparts, through the splitting of the quadrupolar vibrations, which is only observed for the former. Using the resonant ultrasound approach, we calculate the vibrational frequencies of a gold sphere and show that elastic anisotropy induces a lift of degeneracy of the quadrupolar mode in good agreement with our experimental measurements. These findings open up challenging perspectives on using Raman spectroscopy to characterize nanocrystallinity.crystallinity ͉ elastic anisotropy ͉ quadrupolar mode ͉ Raman spectroscopy C rystallinity was recently shown to have an effect on the reactivity, the electron-phonon interaction, and the mechanical properties of silver nanocrystals (NCs), in this way demonstrating perfect nanocrystallinity to be a relevant issue in tailoring NCs' properties (1). From a theoretical point of view, atomistic simulations have pointed out some specific effects of morphology on phonons in nanoscopic metal grains (2), showing that realistic nanograin models display complex vibrational properties. As a counterpart to the simulations, very few experimental investigations have been reported that attempt to demonstrate such a dependence of vibrational properties on the degree of crystallinity of metal NCs (1, 3). This is partially due to the difficulty in tailoring the crystallinity of NCs, as mentioned above, but also to the technical limitation in detecting and identifying the related experimental features. The present article reports that vibrational properties of noble-metal NCs dramatically depend on their nanocrystallinity and clearly demonstrates the relevance of elastic anisotropy in small NCs. These findings make it possible to use vibrational spectroscopy as an alternative or complementary nondestructive tool to the electron microscopy techniques for characterizing nanocrystallinity and also provide interesting insights for understanding, at the nanoscale, how vibrations are affected by the presence of twinning planar defects in metal NCs. Results and DiscussionIn this work, we use a one-step one-phase synthetic route (4) to produce colloidal solutions containing gold NCs. Several samples are synthesized by depositing the NCs on a substrate of highly oriented pyrolitic graphite (HOPG). These samples mainly contain single-domain gold NCs with a size distribution slightly varying from one sample to the other. In these samples, NCs tend to spontaneously self-organize because of their very narrow size dist...

Size and nanocrystallinity controlled gold nanocrystals: synthesis, electronic and mechanical properties

“…1a), 26 revisiting the organometallic method developed by Stucky et al 16 It has been demonstrated that nanocrystallinity segregation occurs by allowing such a colloidal solution under toluene saturation over 7 days. Indeed, low frequency Raman spectra show that the polycrystalline NPs are still dispersed in the solution whereas single-domain self-assemble into a 3D superlattice during the destabilization of the colloidal solution ( Fig.…”

“…6,[22][23][24][25]38,[43][44][45][46][47][48] These lattice vibrations can be monitored either in the frequency domain (using low-frequency Raman spectroscopy), allowing the observation of the NP quadrupolar vibration modes, or in the time domain (through pump-probe spectroscopy), enabling the investigation of the NP radial breathing modes (RBMs). 26,44,[49][50][51] How these ultrafast relaxation processes in metal NPs are affected by nanocrystallinity is still a matter of debate.…”

“…concentration and nature of reagents) is still lacking. [22][23][24][25][26][27] Due to the limitations in synthesis control, only a few studies have been reported concerning the role of nanocrystallinity in the properties of metal NPs. [22][23][24]27,28 This difficulty in synthesis is attributed to the low stacking-fault energy of gold (≈40 mJ m −2 ), which implies that planar crystal defects in this material are easily formed.…”

Phonon interaction with coupled photonic-plasmonic modes in a phoxonic cavity

“…32 for a list of references), in general by pump-probe experiments [33][34][35][36] or by Raman scattering. [37][38][39] In a recent work 32 we extended such an investigation to Au nanoparticles separated from a Au film by a thin dielectric. This allows very confined LSP in the region separating each nanoparticle from the metal film which shows a high sensitivity to the vibrations of the metallic particles.…”

High-order vibration modes of bimetallic Ag-Au nanoparticles embedded in glass

“…Under these conditions, the Raman activity of selected vibration modes of the nanoparticles is strongly enhanced so that intense signals can be obtained from very diluted nanoparticle assemblies 1,2 . Metallic nanoparticles have allowed significant progress in the understanding of nanoparticle modes like shape anisotropy effects from Ag nanocolumns and nanolentils 3 and elastic anisotropy effects in Au nanocrystals 2,4 . Concomitantly, the theoretical description of the Raman spectra from nanoparticle vibrations has significantly improved over the years: while the free sphere model of H. Lamb 5 , combined with appropriate Raman selection rules 6 , initially provided satisfactory predictions of the mode frequencies 7 , coupling with the vibrations inside a surrounding matrix in the case of embedded nanoparticles have been well accounted for 8,9 .…”