Alain Mermet

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

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.

Simultaneous Growths of Gold Colloidal Crystals

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or supracrystals. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air-liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties.

Crystallinity Dependence of the Plasmon Resonant Raman Scattering by Anisotropic Gold Nanocrystals

Au nanocrystals (NCs) with different crystalline structures and related morphologies are unselectively synthesized using an organometallic route. The acoustic vibrations of these NCs are studied by plasmon mediated low-frequency Raman scattering (LFRS). A splitting of the quadrupolar vibration mode is pointed out in the LFRS spectrum. Comparison of the measured frequencies with calculations and careful examination of the NCs morphologies by transmission electron microscopy ascertain this splitting as being an effect of crystallinity. The excitation dependence of the LFRS spectra is interpreted by the shape-selection of the NCs via plasmon-vibration coupling. These results give new insights into the crystallinity influence on both the vibrations of the NCs and their coupling with plasmons and demonstrate the relevance of elastic anisotropy in monodomain NCs.

Crystallinity Segregation upon Selective Self-Assembling of Gold Colloidal Single Nanocrystals

Spontaneous separation of single from polycrystalline 5 nm gold nanocrystals (NCs) is observed in colloidal solution. This segregation takes place upon self-assembling of single crystalline NCs at the air-solvent interface and in precipitated superlattices. Polycrystalline NCs are observed to remain in the suspension. Transmission electron microscopy analysis of the size distribution of NCs issued from the different populations indicates that the NC size does not change from each other, excluding therefore any size segregation in this process. Using both low-frequency Raman scattering and X-ray diffraction provides reliable characterization of nanocrystallinity for each population of NCs, thus confirming the crystallinity segregation. The single crystalline NCs are found by electron diffraction to self-assemble into close-packed superlattices with long-range translational and orientational ordering, while polycrystalline NCs behave like spheres with no preferential orientation. The face-to-face orientational ordering, which is only observed for single crystalline NCs, supports the relevance of the specific crystallinity-related morphologies of these NCs in their better ability to self-assemble. Exploiting this spontaneous segregation would open up a simple alternative to other demanding routes for controlling crystallinity of nanocrystals and optimizing their properties for potential applications.

Permanent densification of compressed silica glass: a Raman-density calibration curve

Raman scattering experiments have been carried out to study persistent densification in SiO(2) glass following hydrostatic compression at room temperature. A new relationship linking selective Raman parameters to the degree of densification in the glass has been developed here. This approach will allow quantification of the residual densification in silica following microindentation experiments, with the goal being the development of a constitutive law for amorphous silica.

Progressive transformations of silica glass upon densification.

The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

Precise frequency shift of NIR luminescence from bismuth-doped Ta2O5–GeO2 glass via composition modulation

Bismuth-doped glasses have received much attention in the last decade due to their broadband near-infrared (NIR) emission. Their optical properties are sensitive to composition and there are a few reports that qualitatively describe the dependence of these properties on their compositions. Yet the actual role of bismuth, as the matrix composition changes, remains fundamentally unclear. In this work, we investigate optical properties of bismuth-doped germanate glasses with different compositions and interpret their dependences in terms of their microscopic structure using Raman and Fourier transform infrared (FTIR) spectra. Beyond qualitative descriptions, quantitative empirical prediction laws are established on the grounds of the relationships between compositions, structures and properties of bismuth-doped germanate glasses. These findings enable precise predictions of the frequency shift of NIR luminescence from bismuth-doped glasses and a better understanding of the nature of bismuth NIR luminescent centers.

Homogeneity of bismuth-distribution in bismuth-doped alkali germanate laser glasses towards superbroad fiber amplifiers

Compared to rare-earth doped glasses, bismuth-doped glasses hold promise for super-broadband near-infrared (NIR) photoemission and potential applications in optical amplification. However, optically active bismuth centers are extremely sensitive to the properties of the surrounding matrix, and also to processing conditions. This is strongly complicating the exploitation of this class of materials, because functional devices require a very delicate adjustment of the redox state of the bismuth species, and its distribution throughout the bulk of the material. It also largely limits some of the conventional processing routes for glass fiber, which start from gas phase deposition and may require very high processing temperature. Here, we investigate the influence of melting time and alkali addition on bismuth-related NIR photoluminescence from melt-derived germanate glasses. We show that the effect of melting time on bismuth-related absorption and NIR photoemission is primarily through bismuth volatilization. Adding alkali oxides as fluxing agents, the melt viscosity can be lowered to reduce either the glass melting temperature, or the melting time, or both. At the same time, however, alkali addition also leads to increasing mean-field basicity, what may reduce the intensity of bismuth-related NIR emission. Preferentially using Li2O over Na2O or K2O presents the best trade-off between those above factors, because its local effect may be adverse to the generally assumed trend of the negative influence of more basic matrix composition. This observation provides an important guideline for the design of melt-derived Bi-doped glasses with efficient NIR photoemission and high optical homogeneity.

In situ Brillouin study of sodium alumino silicate glasses under pressure.

The in situ elastic and plastic behaviors of sodium aluminosilicate glasses with different degrees of depolymerization were analyzed using Brillouin spectroscopy. The observed elastic anomaly progressively vanished with depolymerization. The densification process appears to be different from that observed in pure silica glass. In the plastic regime of densified glasses hysteresis loops were observed and related to modification of the local silicon environment facilitated by the addition of sodium.

Simple model for the vibrations of embedded elastically cubic nanocrystals

Laboratoire de Physico-Chimie des Mat´eriaux Luminescents, Universit´e de Lyon,Universit´e Claude Bernard Lyon 1, UMR 5620 CNRS, 69622 Villeurbanne, FranceThe purpose of this work is to calculate the vibrational modes of an elastically anisotropic sphereembedded in an isotropic matrix. This has important application to understanding the spectra oflow-frequency Raman scattering from nanoparticles embedded in a glass matrix. First some lowfrequency vibrational modes of a free cubically elastic sphere are found to be nearly independent ofone combination of elastic constants. This is then exploited to obtain an isotropic approximationfor these modes which enables to take into account the surrounding isotropic matrix. This methodis then used to quantatively explain recent spectra of gold and copper nanocrystals in glasses.