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Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions Shijing Tan, Liming Liu, Yanan Dai, Jindong Ren, Jin Zhao, and Hrvoje Petek J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01079 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017
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Submitted to JACS
Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions Shijing Tan,† Liming Liu,‡ Yanan Dai,† Jindong Ren,† Jin Zhao,†‡ Hrvoje Petek*,† †
Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh, Pittsburgh, PA 15260, USA ‡
ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
Abstract: Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we combine Ag nanoparticles with graphite (Gr) to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. Tuning the wavelength of p-polarized femtosecond excitation pulses we find enhancement of 2PP yields by two orders-of-magnitude, which we attribute to excitation of a surface normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from: i) coherent two-photon absorption of an occupied interface state 0.2 eV below Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer; and ii) hot electrons in the π*-band of graphite, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the 1
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interface state photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to the population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.
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Introduction Heterojunctions composed of nanometer size metallic clusters on semiconductor substrates have attracted intense interest for their potential for enhancing of solar energy conversion.1-6 Metallic clusters act as optical antennas with large absorption cross sections that concentrate light on the nanometer scale in the plasmonic field. In heterojunctions combining metals with semiconductors or semimetals, the plasmonic field of metal nanoparticles is modified through screening by the real and virtual excitations of the interface and substrate.7 Within a few femtoseconds, the coherent plasmon field dephases through momentum scattering into single-particle excitations creating an inhomogeneous distribution of hot electrons in the metal clusters, at the interface, or in the substrate.8-10 The primary hot electrons can rapidly transfer by tunneling through barriers or resonant channels among the cluster-interface-substrate subsystems. The electronic system first thermalizes to a hot Fermi-Dirac distribution through electron-electron (e-e) processes on the femtosecond timescale, and subsequently equilibrates with the lattice through electron-phonon (e-p) processes on the picosecond timescale.11-12 Because the generation, transport, and relaxation processes of hot electrons occur on the ultrafast temporal and nanometer spatial scales in heterogeneous media, it is poorly understood.13-14 Nevertheless, during their brief existence, hot electrons can act as energetic reagents that catalyze chemical reactions,15-16 as evidenced by indirect action spectra of chemical activity.17-18 To realize efficient plasmon-enhanced hot electron chemistry, however, requires a deep understanding of elementary hot electron processes at catalytic interfaces.19 The metal/semiconductor heterojunction interface has unique electronic, optical, and chemical properties that differ from the component materials. Moreover, it is the locus of the static charge accumulation through chemisorption, and dynamic charge density fluctuations of the plasmonic field
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on the femtosecond timescale. Thus, in the presence of chemical coupling at a metal/semiconductor heterojunction, it is necessary to consider how the band alignment, charge redistribution, and interfacial potentials affect the light-induced responses.5, 20-21 The role of the chemical interface in the plasmon-enhanced hot electron generation, however, has hardly been explored. Here, we investigate the hot electron generation and dynamics in Ag nanocluster decorated highly ordered pyrolytic graphite (HOPG) heterojunction by time-resolved two-photon photoemission (2PP) spectroscopy. 2PP measurements provide a direct method to create and probe the energy, space and time evolution of the coherent polarization fields and hot electron distributions on the time scale of electron dephasing, thermalization, and energy relaxation.22-25 The ultrafast 2PP studies are augmented by atomic scale characterization of Ag cluster distributions by scanning tunneling microscopy (STM), and calculations of the interfacial electronic structure by density functional theory (DFT). Graphitic
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quasi-two-dimensional materials.26-32 Optical pump-probe measurements of Gr with 4.7 eV we see that for Ag/Gr there is an additional contribution to 1PP spectra at 0.2 eV below EF, which is not found for the clean Gr surface and corresponds to the IFS feature in the 2PP spectra of Figure 4. Although the 1PP signal from IFS is weak, it is clearly visible because the density of states of Gr is minimum at EF. Finally, we note that the escape depth for 4.5-9.0 eV electrons in Ag is several nm, and therefore photoemission from the Ag/Gr interface can be observed through the ~1 nm thick Ag clusters. To get further insight into the origin of IFS, we performed DFT calculations for an Ag trimer (Ag3) on a ten-layer thick Gr substrate, as shown in Figure 5, and Ag monolayer and bilayer films on three-layer Gr substrate (supporting Information Figures S5 and S6). The partial density of states
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(PDOS) in Figure 5a clearly shows that for Ag3 chemisorption-induced charge transfer introduces a new occupied state with a binding energy of 0 to -0.25 eV below EF. Very similar results are obtained for the Ag monolayer and bilayer. Plotting the spatial distribution of the chemisorption-induced PDOS in Figure 5b, c indicates a delocalized charge distribution at the interface, where 0.05 electron/Ag atom is transferred mostly to the two topmost layers of the Gr substrate (also, supporting information Figure S6). Thus, the electronic structure calculations for Ag cluster and films provide robust evidence for a chemisorption-induced state at ~0.2 eV below EF in agreement with 2PP spectra. The existence of IFS is consistent with the observed charge transfer from Ag nanoparticles to Gr that was deduced from x-ray photoelectron spectroscopy,68 as well as similar, but more deeply bound (-1.2 eV) state we found for Ag nanoclusters on TiO2.54 Although the DOS of IFS may not be large, the field strength of the ⊥-plasmon resonance is maximum at the interface; this enables 2PP from the IFS to be preferentially enhanced by the plasmonic field. Similar features have been previously observed in plasmonically enhanced 2PP spectra of Ag/Al2O3, Ag/Gr, and Ag/TiO2 suggesting that they are common to Ag cluster chemisorption on Gr, semiconductor, and oxide substrates.51-54 In the case of Ag/Al2O3,52 the corresponding peak was attributed to two-photon excitation of the occupied Shockley surface state from Ag(111) facets on Ag nanoclusters. This assignment, however, cannot be correct, because nanoconfinement pushes the Shockley surface (SS) state above EF, where it is unoccupied.67 For a flat Ag(111) surface with large atomically flat terraces the band minimum of SS is at 60 meV below EF. STM measurements find that when the area of Ag(111) terraces becomes
