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

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 electronphonon 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 temperaturedependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling. two-temperature model | dissipation T his paper describes an observed dependence of two physical properties-the electronic heat capacity and the electronphonon coupling magnitude-of small (diameter of ∼3.5 nm), completely absorptive plasmonic gold nanoparticles (Au NPs) on the surface chemistry of the NPs. The electronic heat capacity, quantified by the coefficient γ, is the amount of energy required to change the temperature of a population of electrons in the NP. The magnitude of the electron-phonon coupling, quantified by the coefficient g, is a measure of the rate of exchange of energy between the electrons and the lattice. Upon photoexcitation of the Au NPs at the absorption maximum of their plasmon resonance, the plasmonic electron oscillation in the NPs rapidly decoheres to form a thermally equilibrated population of hot electrons (1-8). Within the commonly used "two-temperature model (2TM)," the rate of subsequent cooling of these electrons is dictated by the initial electronic temperature of the particle and the efficiency of dissipation of electronic energy into available vibrational modes. These properties, in turn, depend on the electronic heat capacity of the system and the magnitude of electron-phonon coupling. Our observed dependence of the coefficients γ and g on the chemical structure of the organic adlayer therefore implies that electronic states that participate in the electron cooling process are not only those of the gold lattice, but also orbitals at the metal-organic interface-that is, the surface chemistry of the NPs influences the rate of hot electron cooling.Interest in using metal and semiconductor NPs as optical and/or electronic components within energy conversion systems (9, 10) has inspired studies of ene...

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