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International audience; The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V 0 = 67.65(3) Å 3 , K 0 = 170(3)GPa, at zero pressure.

Excitons in semiconductor quantum dots (QDs) feature high values of the two-photon absorption cross-sections (TPACSs), enabling applications of two-photon-excited photoluminescence (TPE PL) of QDs in biosensing and nonlinear optoelectronics. However, efficient TPE PL of QDs requires high-intensity laser fields, which limits these applications. There are two possible ways to increase the TPE PL of QDs: by increasing their photoluminescence quantum yield (PLQY) or by further increasing the TPACS. Plasmonic nanoparticles (PNPs) may act as open nanocavities for increasing the PLQY via the Purcell effect, but this enhancement is strictly limited by the maximum possible PLQY value of 100%. Here we directly investigated the effect of PNPs on the effective TPACS of excitons in QDs. We have found that effective TPACS of excitons in a QD–PMMA thin film can be increased by a factor of up to 12 near the linearly excited gold nanorods (GNRs). Using gold nanospheres (GNSs), in which plasmons cannot be excited in the infrared range, as a control system, we have shown that, although both GNSs and GNRs increase the recombination rate of excitons, the TPACS is increased only in the case of GNRs. We believe that the observed effect of TPACS enhancement is a result of the nonlinear interaction of the plasmons in GNRs with excitons in QDs, which we have supported by numerical simulations. The results show the way to the rational design of the spectral features of plasmon–exciton hybrids for using them in biosensing and nonlinear optoelectronics., V. K. acknowledges support from the Russian Science Foundation (Grant No. 18-72-10143) for the part of the study related to the investigation of the effects of PNPs on the effective TPACS of excitons in QDs and numerical simulations of these effects. Support from the Ministry of Education and Science of the Russian Federation (Grant No. 14.Y26.31.0011) for the part of this study related to the synthesis and functionalisation of the QDs and engineering of hybrid QD-GNR materials is also acknowledged. Y. R. acknowledges the support from the Basque Government (grant no. IT1164-19). Y. R. and M. G. acknowledge the support from the Spanish MINECO (PID2019-111772RB-I00). I. N. acknowledges the Ministry of Higher Education, Research and Innovation of the French Republic and Université de Reims Champagne-Ardenne.

Semiconductor quantum dots (QDs) are known for their ability to exhibit multiphoton emission caused by recombination of biexcitons (BX). However, the quantum yield (QY) of BX emission is low due to the fast Auger process. Plasmonic nanoparticles (PNPs) provide an attractive opportunity to accelerate BX radiative recombination. Here, we demonstrate the PNPs induced distance-controlled enhancement of BX emission of single QDs. Studying the same single QD before and after its integration with the PNPs, we observed a plasmon-mediated increase in the QY of BX emission. Remarkably, the enhancement of BX emission remains pronounced even at distances of 170 nm. We attribute this effect to efficient coupling, which results in the trade-off between resonance energy transfer from QD to gold nanorods and the Purcell effect at small QD-PNP separations and the predominant influence of the Purcell effect at longer distances. Our findings constitute a reliable approach to managing the efficiency of multiexciton emission over a wide span of distances, thus paving the way for new applications., The authors acknowledge the financial support from the Ministry of Education and Science of the Russian Federation (Grant 14.Y26.31.0011). Y.R. acknowledges support from MINECO (Ministerio de Economiá y Competitividad, Spain), Project Fis2016.80174-P (PLASMOQUANTA).

Semiconductor quantum dots (QDs) are known for their unique photophysical properties and, in particular, their ability to multiphoton emission caused by recombination of biexcitons. However, the luminescence quantum yield of biexciton states is relatively low due to the fast Auger non-radiative process. Plasmonic nanoparticles can significantly accelerate the radiative rate of QDs. In this study we demonstrate the distance-controlled enhancement of the biexciton emission of single CdSe/ZnS/CdS/ZnS QDs due to their coupling with gold nanorods. We explain this enhancement as the distancedependent trade-off between the energy transfer and the Purcell effect. Our findings constitute a reliable approach to managing the efficiency of multiphoton emission over a wide span of distances.

International audience; Gold nanoparticles are prime candidates for cancer thermotherapy. However, while the ultimate target for nanoparticle-mediated photothermal therapy is the cancer cell, heating performance has not previously been evaluated in the tumoral environment. A systematic investigation of gold nanostar heat-generating efficiency in situ is presented: not only in cancer cells in vitro but also after intratumoral injection in vivo. It is demonstrated that (i) in aqueous dispersion, heat generation is governed by particle size and exciting laser wavelength; (ii) in cancer cells in vitro, heat generation is still very efficient, but irrespective of both particle size and laser wavelength; and (iii) heat generation by nanostars injected into tumors in vivo evolves with time, as the nanostars are trafficked from the extracellular matrix into endosomes. The plasmonic heating response thus serves as a signature of nanoparticle internalization in cells, bringing the ultimate goal of nanoparticle-mediated photothermal therapy a step closer.

International audience; Investigations of the ultrafast acoustic response of metal nanosystems yield important information on the validity of continuous elastic mechanics at the nanoscale and also provide an optical way to probe nanoobject morphologies. In this context, we used femtosecond time-resolved pump-probe spectroscopy to study two classes of bimetallic nanoparticles: chemically synthesized AuAg nanospheres in water in the 20-45 nm size range, both with alloyed and segregated core-shell morphologies, and mass-selected glass-embedded PtAu core-shell nanospheres in the very small size range (2.3-2.5 nm), synthesized by physical methods. The analysis of the corresponding breathing mode periods demonstrates validity of the predictions of the continuous elastic model for bimetallic nanoobjects with the investigated sizes, morphologies and composition. Moreover, discrimination of nanoparticles internal structure (alloy or core-shell) by measurement of their acoustic response is also demonstrated.

International audience; The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.