Hot Adsorbate-Induced Retardation of the Internal Thermalization of Nonequilibrium Electrons in Adsorbate-Covered Metal Nanoparticles

Abstract

Femtosecond transient absorption spectroscopy has been used to investigate the electron-electron scattering dynamics in sulfate-covered gold nanoparticles of 2.5 and 9.2 nm in diameter. We observe an unexpected retardation of the absolute internal thermalization time compared to bulk gold, which is attributed to a negative feedback by the vibrationally excited sulfate molecules. These hot adsorbates, acting as a transient energy reservoir, result from the back and forth inelastic scattering of metal nonequilibrium electrons into the π* orbital of the sulfate. The vibrationally excited adsorbates temporarily govern the dynamical behavior of nonequilibrium electrons in the metal by re-emitting hot electrons. In other terms, metal electrons reabsorb the energy deposited in the hot sulfates by a mechanism involving the charge resonance between the sulfate molecules and the gold NPs. The higher surface-to-volume ratio of sulfate-covered gold nanoparticles of 2.5 nm leads to a stronger inhibition of the internal thermalization. Interestingly, we also note an analogy between the mechanism described here for the slow-down of electron-electron scattering in metal nanoparticles by the hot adsorbates and the hot phonon-induced retardation of hot charge carriers cooling in semiconductors.Hybrid nanostructured materials can be seen as complex systems, which lie at the border of solid-state physics and supramolecular chemistry. Metal nanoparticles (NPs) protected with functional molecules can give a unique opportunity to investigate the coupling between electron transport dynamics and molecular structural changes beyond the Born-Oppenheimer approximation.As the size decreases to the nanoscale range (shorter than the electron mean free path), the electronic properties of the metal can fall under the influence of surface molecular species. For example, evidence appeared that adsorbates can modify the electronic properties of small metal nanoparticles 1-4 (below 20 nm) by decreasing the surface plasmon lifetimes. This additional damping channel, which leads to a homogeneous broadening of the surface plasmon band, is called chemical interface damping, i.e., metal electrons tunnel back and forth into empty electronic states of adsorbates. [5][6][7] The electronic structure of the gold/sulfate interface consists of a σ donation and a σ* and π* back-donation mechanism: The occupied σ orbitals of the sulfate give charge to the metal, while the unoccupied σ* and π* orbitals accept charge from the metal. 8 A key characteristic of the present system is the coupling between gold electrons and sulfate vibrations via the partial occupation of the sulfate π* orbital by the gold electrons. This charge resonance leads to a strong chemical interface damping of the surface plasmon as revealed by the nonclassical broadening of the surface plasmon band. 9 Thus, under equilibrium conditions, sulfate molecules affect the electronic properties of the gold nanoparticles by opening an additional damping channel for the surface plasmon. The next step w...

Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles

“…1, red solid line), a result that is consistent with previous reports (28)(29)(30). Within the theory of UCIS, an increased line width of the LSP absorbance after ligand exchange is expected due to dephasing of the plasmonic electrons as they transiently tunnel in and out of interfacial orbitals with metal and ligand character (17,19,20); however, this interfacial scattering processes is one of several possible contributions to the line width of the LSP (28). Inhomogeneous broadening due to polydispersity in size and shape is probably not a dominant factor in this line width.…”

“…Recently, Bauer and coworkers have described a phenomenological model in which the interface between the Au NP and an organic adlayer changes the rate at which plasmonic electrons equilibrate to form a thermal distribution by providing pathways for redistribution of the hot electrons' kinetic energy. The authors adopt the name "ultrafast chemical interface scattering (UCIS)" for this mechanism, as it is consistent with chemistryinduced localized surface plasmon (LSP) damping that has long been observed in small NPs in the colloid field (17)(18)(19)(20).…”

Ultrafast Electron Dynamics in Thiolate-Protected Plasmonic Gold Clusters: Size and Ligand Effect

“…41 The FWHM depends on many factors, such as the size of nanoparticles, polydispersity in size and shape and chemical bonding between the nanoparticles and ligand. 41 Although the ligand exchange can diphase the oscillations of the plasmonic electrons by adding an additional decay channel due to the transient in and out tunneling in interfacial orbitals with the NP and ligand, [77][78][79] the size of the NPs plays a dominant role in plasmonic damping (See Figure 3). As demonstrated in the Figure 3 and Figure S3, the FWHM of the smaller samples (Au279 (TBBT), Au329 (SC2Ph) and Au329 (SC6)) at a delay time of 0.5 ps were determined to be ~ 92 nm, which decreases with further increasing the size of the samples and reaches to ~ 53 nm for 13 nm-Au.…”

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…[152] Moreover, experimental evidence shows that CID by adsorbates is able to retard the thermalization process of hot charge carriers (picosecond regime) by the repeated back and forth transfer of hot electrons between the metal and the adsorbates. [165] Some factors need to be taken into account in order to increase efficiency by CID. [166] In contrast to the indirect transfer mechanism, the direct electron transfer requires not only an orbital overlap, but also strong hybridization between the metallic nanoparticle surface and the adsorbate.…”