Coherent Electron Transfer at the Ag/Graphite Heterojunction Interface

Abstract

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 σ symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10 fs completing the coherent transfer.

Coherent Two-Dimensional Multiphoton Photoelectron Spectroscopy of Metal Surfaces

“…Because the OBE approach has proven useful to interpret, simulate, and analyze ITR-mPP data [1,41,50,51,63,86], in this section we present an OBE simulation of the coherent electronic response of Ag(111) surface, by calculating the 2PP signal as the delay is varied interferometrically between identical pump and probe pulses [ Fig. 3(a)].…”

“…Interferometric time-resolved multi-photon photoemission (ITR-mPP) is an effective multidimensional spectroscopy for evaluating quantum coherence in solids and solid surfaces; it has been developed and applied to study of dephasing of surface and bulk bands in metals [1,48,49], the coherence in interfacial charge transfer processes [50,51], and the formation of transient excitons at Ag(111) surface [33,[52][53][54]. Cui et al [33] developed and applied the multidimensional capabilities of ITR-mPP [41,55] to record movies of the coherent transformation of the transient exciton formed by resonant two-photon excitation of the n = 1 image potential (IP) state Shockley surface (SS) state transition of Ag(111) by collecting photoelectron energy (E), and parallel momentum (k || ) distributions with an imaging electron spectrometer, while scanning the optical delay time (t) between identical pump and probe pulses with ~50 as (sub-optical cycle) resolution.…”

Plasmon-Induced Direct Hot-Carrier Transfer at Metal–Acceptor Interfaces

“…Fortunately, recent experiments at metal-semiconductor [24][25][26][27] and metal-molecule 28,29 interfaces have provided strong evidence for another carrier-transfer mechanism that can operate simultaneously, termed direct transfer. This process is also referred to as coherent charge transfer, 25,26 plasmon-induced interfacial charge-transfer transition when the acceptor is a semiconductor, 24 and chemical interface damping in the case of adsorbed molecules. 2,[30][31][32] In this case, surface plasmons interact directly with empty acceptor states and dephase by directly injecting carriers into the acceptor (Figure 1b).…”

Attosecond-fast internal photoemission

“…With the obtained cut-off bandwidth of 3.3 PHz (1 PHz = 10 15 Hz) for the charge transfer rate, this semimetalsemiconductor interface represents a functional solid-state interface offering the speed and design space required for future light-wave signal processing.The transfer of charge via internal photoemission at a solid-state interface is a fundamental process with direct relevance in ultrafast optoelectronics [24] and the transduction of light to chemical or electrical energy in light-harvesting [811]. Various solid state-based interfaces have been investigated to study the ultimate speed of this fundamental process using optical methods [2,3,12,13]. Time constants for the charge transfer in the attosecond domain (∼100 as) have so far only been observed in photoemission from metal surfaces or atoms into vacuum [6,7,14] or from atoms/molecules to metals [1517]; part of these experiments infer the charge transfer rates based on the uncertainty principle [12,18].…”

“…The ideal system to achieve attosecond-fast charge separation at a solid-state interface resembles the photoelectric effect, so external photoemission from a metal into vacuum [6,7]: It is strongly asymmetric with only one side optically absorbing; it has an atomically sharp interface, reducing the electron transfer distance; in addition to its external counterpart it has a strong built-in electric field that promotes fast transfer of electrons into the electronabsorbing half-space, i.e., the acceptor material. Electron absorption at such an extended acceptor might preserve electronic coherence but hinders the carrier wavefunction to sling back to the donor material [2,3,8,10].The 2D semimetal graphene grown epitaxially on the wide-bandgap semiconductor 4H silicon carbide (SiC) represents such an ideal system (Fig. 1a, [5]).…”