Adsorbate-mediated relaxation dynamics of hot electrons at metal/organic interfaces

Hot-electron dynamics at metal surfaces are crucial for charge transport properties and chemical reactions at metal/organic interfaces. In order to study the influence of electron-donating adsorbates on hot-electron lifetimes we performed femtosecond time-resolved two-photon photoemission measurements of tetrathiafulvalene (TTF)-covered Au(111) surfaces. The electron-donating nature of TTF provides density of states near the Fermi energy ($E_F$) increasing the possibility for electron-electron scattering events. This leads to an additional fast electron-electron scattering assisted hot-electron relaxation channel for low intermediate state energies ($E_i-E_F$) at the TTF/Au(111) interface with hot-electron lifetimes up to $340\mathrm{fs}$. However, suppressing the electron donation via intercalation of electron-accepting tetracyanoquinodimethane molecules causes quenching of this fast electron relaxation channel. This results in an asymptotic increase of the hot-electron lifetimes for infinitesimal small intermediate state energies up to about $1000\mathrm{fs}$ as predicted by the pure Landau theory of Fermi liquids.

Figure 1

TR-2PPE measurements of the TTF/Au(111) interface at different coverages. (a) At $1.0\pm 0.1\mathrm{ML}$ coverage hot electrons can be observed in an extended energy range of about $1\mathrm{eV}$. Their lifetime increases up to some hundred femtoseconds for decreasing intermediate-state energy, whereas an unoccupied interfacial electronic state (IS) features no significant lifetime. (b) The adsorption of a second and third TTF layer has no significant influence on the hot-electron lifetimes but reduces the 2PPE signal intensity.

Figure 2

Characterizing the hot-electron dynamics at the TTF/Au(111) interface. (a) Averaged XC curves for different intermediate-state energies fitted by a monoexponential decay function. (b) Energy-dependent hot-electron lifetimes for the TTF/Au(111) interfaces and alkali-metal-covered Au(111) films (data adopted from Ref. [19]).

Figure 3

(a) UPS spectra of the bare and TTF-covered Au(111) surface. For the bare Au(111) the Shockley surface state (SS) is observed, while the state labeled as HOMO* is the result of a hybridization between the SS and the TTF highest occupied molecular orbital. The inset shows a 2PPE spectrum of 1 ML TTF/Au(111) recorded with 2.05 and 4.10 eV photons. Based on photon energy dependent measurements the photoemission peaks labeled as ${\mathrm{IS}}_1$ and ${\mathrm{IS}}_2$ can be assigned to TTF-derived unoccupied electronic states located at 2.25 eV (${\mathrm{IS}}_1$) and 3.22 eV (${\mathrm{IS}}_2$) above $E_F$. ${\mathrm{IPS}}_1$ is attributed to the first image potential state located 3.65 eV above $E_F$ (corresponding to 0.5 eV below the vacuum energy). (b) 2PPE data of the mixed TCNQ/TTF/Au(111) monolayer after annealing to 370 K recorded at different photon energies. On the basis of the photon energy dependent 2PPE measurements (see inset) the strong photoemission feature can be assigned to the ${\mathrm{IPS}}_1$.

Figure 4

Hot-electron dynamics at the TCNQ-covered TTF/Au(111) interface. Two-dimensional representations of TR-2PPE measurements for a TCNQ-covered TTF/Au(111) interface with $1$ ML coverage (a) before and (b) after annealing up to $370\mathrm{K}$.

Figure 5

Comparison of resulting intermediate-state energy dependent hot-electron dynamics. Resulting intermediate state energy dependent hot-electron lifetimes for TTF/Au(111) at different coverages and TCNQ intercalated TTF/Au(111) before and after the annealing step up to $370\mathrm{K}$. For comparison, hot-electron dynamics were also investigated for the inverse evaporation sequence: TTF on TCNQ on Au(111) after annealing up to $370\mathrm{K}$.