Time-Resolved Two-Photon Photoemission Study of Dodecanethiolate-Passivated Ag Nanoparticles

Abstract

Metallic nanoparticles are attracting much interest from the viewpoints of both fundamental and device physics, since they show the distinctive optical, physical, and chemical properties found in neither bulk nor molecular/ atomic systems. Among the nanoparticle systems, the surface-passivated nanoparticles are very stable and exhibit the closed-packed nanoparticle self-assemblies on the singlecrystalline substrates. Therefore, it is considered that they could be important constituents of future nanostructured devices, such as single electron device, catalyst, and ultra highly density memory. On the other hand, the transient dynamics of hot-electrons created by photoexcitation or inelastic scattering of other energenic particles play an important role in the various physical and chemical phenomena. The excited hot-electrons relax through a number of decay processes, such as electron–electron scattering, electron-phonon scattering, electron-impurity/ defect scattering, and scattering with other elementary excitations. The detailed understanding of hot-electron dynamics will provide not only information about the fundamental interactions in the many-body systems but also knowledge of the various optical, physical and chemical processes. In order to elucidate the intriguing properties of metallic nanoparticles and to develop the future devices, it is indispensable to understand the excited-electron dynamics as well as their electronic structures. In this work, we have carried out a femtosecond time-resolved two-photon photoemission (TR-2PPE) study of dodecanethiolate (DT)-passivated Ag nanoparticles on the highly oriented pyrolytic graphite (HOPG) substrates. Among the time-resolved spectroscopic methods, TR-2PPE spectroscopy is a unique method to observe directly the temporal evolution of excited-electrons that may not be easily accessible through optical techniques. The DT-passivated Ag nanoparticles were synthesized by the two-phase (water–toluene) reduction method. The detailed procedure is described elsewhere. The size and shape of the synthesized nanoparticles were characterized by ex-situ observations with transmission electron microscope (TEM). Figure 1 shows the TEM micrograph and the size distribution in diameter of DT-passivated Ag nanoparticles used in this work. As shown in Fig. 1, the mean diameter and standard deviation are dm 1⁄4 5:3 nm and 1⁄4 0:47 nm, respectively. The present samples have an extremely narrow size distribution in diameter ( 3:0 eV). The total energy resolution was about 150meV, and time resolution was about 10 fs. All measurements were performed at room temperature. Figure 2 shows the two-photon photoemission (2PPE) spectrum of the DT-passivated Ag nanoparticles on the HOPG substrate with dm 1⁄4 5:3 nm measured with photon energy of 3.3 eV at zero pump-probe delay, compared with that of HOPG substrate. These spectra are plotted as a function of intermediate-state energy relative to the Fermilevel. In Fig. 2, the intermediate-state energy of 3.3 eV corresponds to the highest intermediate-state, with onephoton energy (h 1⁄4 3:3 eV) above the Fermi-level. As shown in Fig. 2, the 2PPE spectral features are definitely different among the HOPG substrate and DT-passivated Ag nanoparticles on HOPG substrate. The 2PPE spectrum of Fig. 1. TEM micrograph and size distribution in diameter of dodecanethiolate-passivated Ag nanoparticles with mean diameter of dm 1⁄4 5:3 nm and standard deviation of 1⁄4 0:47 nm.