Coherent Electron Transfer at the $\mathrm{Ag}/\mathrm{Graphite}$ Heterojunction Interface

Charge transfer in transduction of light to electrical or chemical energy at heterojunctions of metals with semiconductors or semimetals is believed to occur by photogenerated hot electrons in metal undergoing incoherent internal photoemission through the heterojunction interface. Charge transfer, however, can also occur coherently by dipole coupling of electronic bands at the heterojunction interface. Microscopic physical insights into how transfer occurs can be elucidated by following the coherent polarization of the donor and acceptor states on the time scale of electronic dephasing. By time-resolved multiphoton photoemission spectroscopy (MPP), we investigate the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a $\sigma$ symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10 fs completing the coherent transfer.

Figure 1

The incoherent internal photoemission (a) and coherent (b) photoinduced charge transfer schemes at the metal-semiconductor interface and their detection by 2PP spectroscopy. (c) The $k_{\parallel }$ band structure of Gr, with the Ag chemisorption induced occupied IFS. The green arrows indicate the MPP excitation involving the resonant two-photon transition from IFS to ILB. The wavy arrows indicate the phonon assisted electron scattering from the ${\pi }^{*}$ band of Gr. The widths of the bands convey their perpendicular dispersions. (d) The four-level excitation scheme in 3PP process including the initial state “0” (IFS), the virtual intermediate state “1”, the penultimate state “2” (ILB), and the final photoemission continuum “3”. The $T_1^2$ represent the effective hot electron lifetime and $T_2^{02}$ the polarization dephasing time between the IFS and ILB.

Figure 2

MPP spectra with $p$- (a) and $s$-polarized (b) $\hslash \omega =2.0\mathrm{eV}$ excitation obtained at the Gr surface for sequential Ag deposition. The blue arrow marks the new state, i.e., the ILB. (c) The normalized $s$- and $p$-polarized spectra at $\mathrm{Ag}/\mathrm{Gr}$ with $\hslash \omega =2.14\mathrm{eV}$ and $I=0.34\mathrm{mJ}/{\mathrm{cm}}^2$ excitation, and their difference spectrum (red); the inset shows the Gaussian fitting of the IPS and ILB components of the difference spectrum. (d) The log-log plots of photoelectron yields for the $p$- and $s$-polarized spectra, the IPS, and ILB components vs $I$ for $\hslash \omega =2.14\mathrm{eV}$. The dashed lines indicate slopes of $3.7\pm 0.1$ and $4.2\pm 0.2$ for the $p$- and $s$-polarized spectra, and $3.3\pm 0.1$ and $3.1\pm 0.1$ for the IPS and ILB.

Figure 3

(a) A series of normalized 3PP spectra at $\mathrm{Ag}/\mathrm{Gr}$ surface excited with $p$-polarized $\hslash \omega =1.79\text{−}2.24\mathrm{eV}$ (693–553 nm) light. The inset shows details of the IPS and ILB peaks near the IFS-ILB resonance. The blue dashed line traces the resonance behavior of the ILB. (b) The plots of the $E_{\text{final}}$ of IPS and ILB vs $\hslash \omega$; the slopes of $\sim 1$ and intercepts confirm them to be penultimate states in 3PP with energies of 3.60 and 4.0 eV, respectively. (c) The plot of intensity ratio between ILB and secondary electron yield vs $\hslash \omega$.

Figure 4

(a)–(d) The spatial distributions (side view) of the bands involved in the optical transitions. (e) The calculated transition dipole moment amplitudes, $|T_{n^{\prime }n}^i|$ (broadened with Gaussian functions) from the initial IFS to unoccupied states at the $\mathrm{\Gamma }$ point for an ${\mathrm{Ag}}_3$ array on a ten-layer-thick Gr stack, as shown in the inset. The bottom axis represents the total photon energy and the upper axis the energy of coupled states relative to $E_F$.

Figure 5

(a) Interferogram of the photoelectron counts vs the $E_{\text{final}}$ and pump-probe time delay $\tau$ measured with $p$-polarized $\hslash \omega =2.05\mathrm{eV}$ light for $\mathrm{Ag}/\mathrm{Gr}$ at $k_{\parallel }=0{\AA }^{-1}$. The data in the dashed rectangle are enlarged in the lower panel. (b) 2D photoelectron spectra obtained by Fourier transforming the interferogram in (a). The vertical dashed lines indicate the photon energies, and the black dashed lines indicate the slope of 2. (c) Left panel: experimental I2PC trace obtained at the cross section in (a) at 6.1 eV. Right panel: simulated I2PC trace with the four-level OBE approach [Fig. 1]. The I2PCs are decomposed into phase average (yellow), the envelopes of the $1\omega$ (red), $2\omega$ (cyan), and $3\omega$ (pink) oscillatory components, which are used to fit the polarization and population decay parameters [41].