Size Dependence of Investigations of Hot Electron Cooling Dynamincs in Metal/Adsorbates Nanoparticles
ABSTRACT The size dependence of electron-phonon coupling rate has been investigated by femtosecond transient absorption spectroscopy for gold nanoparticles (NPs) wrapped in a shell of sulfate with diameter varying from 1.7 to 9.2 nm. Broad-band spectroscopy gives an overview of the complex dynamics of nonequilibrium electrons and permits the choice of an appropriate probe wavelength for studying the electron-phonon coupling dynamics. Ultrafast experiments were performed in the weak perturbation regime (less than one photon in average per nanoparticle), which allows the direct extraction of the hot electron cooling rates in order to compare different NPs sizes under the same conditions. Spectroscopic data reveals a decrease of hot electron energy loss rates with metal/adsorbates nanosystem sizes. Electron-phonon coupling time constants obtained for 9.2 nm NPs are similar to gold bulk materials (a. 1 ps) whereas an increase of hot electron cooling time up to 1.9 ps is observed for sizes of 1.7 nm. This is rationalized by the domination of surface effects over size (bulk) effects. The slow hot electron cooling is attributed to the adsorbates-induced long-lived nonthermal regime, which significantly reduces the electron-phonon coupling strength (average rate of phonon emission).
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ABSTRACT: This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.Proceedings of the National Academy of Sciences 02/2013; DOI:10.1073/pnas.1222327110 · 9.81 Impact Factor
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ABSTRACT: Recently, it was reported that gold nanoprisms in a monolayer array on a quartz substrate were ejected in air when irradiated with femtosecond laser pulses near their surface plasmon absorption maximum. It was deduced from the measured reduction in particle thickness upon irradiation that the ejection mechanism involved ablation of surface atoms from the gold particle, which generates an intense pressure at the particle−substrate interface. The present study reports on this phenomenon when the substrate-bound nanoparticle is immersed in a liquid environment. In this system, it is found that the nanoparticle ejection requires less than one tenth the energy required if the system was irradiated in air. The ejected nanoparticle is also found to increase in thickness instead of the decrease observed in air. These results suggest another photoinitiated ejection mechanism, different from surface ablation, when the particles are surrounded by a liquid environment. From this and other spectroscopic and microscopic results on the ejected nanoprisms, we suggest a mechanism that involves energy transfer from the photoexcited nanoprism to the solvent within cavities and defects at the particle−substrate interface. The hot-solvent molecules result in an intense pressure at the particle−substrate interface, resulting in particle ejection. Ejection is proposed to consist of two processes, namely nanoparticle−substrate dissociation and nanoparticle solvation and diffusion away from the substrate. These two processes have independently been studied as a function of solvent property.The Journal of Physical Chemistry C 03/2007; 111(25). DOI:10.1021/jp070282q · 4.84 Impact Factor
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ABSTRACT: The integrity of a small model protein, the 36-residue villin headpiece HP36 attached to gold nanoparticles (AuNP) is examined and its response to laser excitation of the AuNPs is investigated. To that end, it is first verified by stationary IR and CD spectroscopy together with denaturation experiments that the folded structure of the protein is fully preserved when attached to the AuNP surface. It is then shown by time-resolved IR spectroscopy that the protein does not unfold even upon the highest pump fluences that lead to local temperature jumps in the order of 1000 K of the phonon system of the AuNPs, since that temperature jump persists for too short a time of a few nanoseconds only to be destructive. Judged from a blue shift of the amide I band, indicating destabilized or a few broken hydrogen bonds, the protein either swells, becomes more unstructured from the termini, and/or changes its degree of solvation. In any case, it recovers immediately after the excess energy dissipates into the bulk solvent. The process is entirely reversible for millions of laser shots without any indication of aggregation of the protein and/or the AuNPs and with only a minor fraction of broken protein-AuNP thiol-bonds. The work provides important cornerstones in designing laser pulse parameters for maximal heating with protein-capped AuNPs without destroying the capping layer.The Journal of Physical Chemistry B 03/2014; 118(28). DOI:10.1021/jp500845f · 3.38 Impact Factor