Two-photon photoemission spectroscopy for silver nanoparticles on a hydrogen-terminated Si(1 1 1) surface: Metal nanoparticle-enhanced photoemission

Confined Hot Electron Relaxation at the Molecular Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster and Layered C60

“…As mentioned above, the PIE structure peaking just below E F (in final energy) can be observed for all plasmonic Ag n NCs ( n ≥ 9). According to various literature on 2PPE spectroscopy, Ag NPs consisting of a large number of Ag atoms (a few to several nm ≈ n = 10 2 –10 3 ) deposited on a substrate also provide a broad spectral feature such as PIE just below the E F . − , In this study, the plasmonic behavior depending on hv at n ≥ 9 (Figures a,b) clearly shows that the valence Ag 5 s electrons fill up to around the E F even at such small Ag n NCs. As is known, Ag n NCs at a low temperature (in case of calculation (0 K) or rare gas matrix experiment (∼10 K)) exhibit molecular-like optical properties possessing sharp and discrete absorption bands characteristic to each Ag n NC. ,,, In this study, the temperature of the deposited Ag n NCs is 300 K (25 meV), while the difference in the total energy between possible isomers is calculated to be a few to several tens of meV.…”

“…The polarization selectivity reveals that plasmon-induced excitation is likely to be involved in the PIE process; the plasmonic excitation of the deposited Ag n NC is expected to exhibit p-polarization selectivity owing to the electric field being in the normal direction, while the electron density oscillation that is parallel to the surface is screened by its image dipole formed inside the surface. [31][32][33]68,69 Furthermore, the plasmon-induced hot electrons generally relax via an electron−electron scattering process, 1−12 resulting in exponential-like energy distributions similar to those observed in the low-energy region in Figure 4a,b, which is schematically illustrated in Figure 3c. In fact, the I p/s value corresponding to the hot electrons gradually decreases from 8 to 6.5 eV (Figure 4b) and from 6.5 to 5.5 eV (Figure 4d) in the final energy because the selectivity associated with ppolarized probe photons decreases owing to the multiple scattering of hot carriers.…”

“…The polarization selectivity reveals that plasmon-induced excitation is likely to be involved in the PIE process; the plasmonic excitation of the deposited Ag n NC is expected to exhibit p-polarization selectivity owing to the electric field being in the normal direction, while the electron density oscillation that is parallel to the surface is screened by its image dipole formed inside the surface. − ,, …”

“…Therefore, the large I p/s values confirm that majority of the excited hot photocarriers are initiated by the plasmonic excitation of Ag 21 NCs exhibiting selectivity for p-polarized pump photons. Consistently, a similar spectral feature and enhanced hot electrons have been observed previously for Ag NPs having diameters of a few to 10 nm that are embedded on inorganic insulators or semiconductor substrates ( e.g ., Al 2 O 3 , TiO 2 , H–Si(111), and graphite). − , …”

“…The plasmonic response of noble metal NPs/NCs, relevant to their ultrafast hot carrier dynamics, has been studied by time-resolved (TR)-optical/photoemission spectroscopy using femtosecond light sources. ,− In particular, two-photon photoemission (2PPE) spectroscopy is an incisive probe of negative hot carriers (electrons) generated through plasmonic excitation coherently with the two photons. − Recent studies using 2PPE spectroscopy have unveiled effective hot electron generation mediated by the plasmonic excitation of Ag NCs on inorganic substrates, where Ag NCs were formed on the surface by an atomic thermal deposition, resulting in a broad size distribution of Ag NCs. − However, the broad size distribution makes the discussion of electron dynamics ambiguous owing to heterogeneous contributions to the energy relaxation and by obscuring the size-selective plasmonic behavior of metal NCs, which has been investigated in a gas phase and in a rare gas matrix. ,,− For a molecularly well-defined system to enhance the CT process after plasmonic excitation, it is preferable to form a molecular heterointerface combining low ionization energy ( E i ) species and high electron affinity (EA) species to reduce the barrier to the CT process, as electron transfer easily occurs in alkali-halogen thermal reactions without a reaction barrier . A simple estimate can be obtained by considering the energetics of the CT when a molecular organic substrate is coupled with the size-selected noble metal NCs.…”