Hot Electrons at the Interface between Silver Nanoparticles and a Graphite Substrate

The interface between nano-sized precious metal clusters such as Silver (Ag) and a semiconductor such as graphite (Gr) is called a heterojunction (Ag/Gr). Heterojunctions have great promise in improving solar energy conversion due to their unique and enhanced optical, electronic, and chemical properties. When excited with laser pulses, electrons in the system acquire a mean energy higher than its thermal equilibrium value and are referred to as "hot electrons". In fact, graphitic materials are model systems for the study of hot electron dynamics. An ineffective screening within the layers of graphite allows the hot electrons to reach temperatures comparable to that in the solar photosphere!

In a study supported by the Center for Chemistry at the Space-Time Limit recently published in the Journal of the American Chemical Society, Hrvoje Petek and his group modified the Gr surface with Ag nanoclusters (NC)s to investigate how the excitation of the plasmonic resonance of the Ag/Gr heterojunction affects the generation, spatial distributions, and relaxation processes of hot electrons. Plasmonic resonance is a prominent feature of precious-metal nanoparticles; it is a sharp and intense absorption band in the visible range that arise from a collective resonant oscillation of the free electrons of the conduction band of the metal called plasmon.

The method that was used to create and probe the energy, space, and time evolution of the coherent polarization field of the plasmons and the hot electron distribution is called 2-Photon Photoemission Spectroscopy (2PPS). The technique utilizes femtosecond (10-15s) laser pulses to first excite electrons from occupied states below the Fermi level (EF) into an intermediate state, and then after a time delay, the excited electrons are photoemitted into a vacuum state (EV) by a second pulse. The 2PP spectra provide information about state populations and coherent polarizations between the optically coupled states.

2-photon photoemission spectroscopy of Ag/Gr

Compared to the 2PP spectrum of bare graphite (Figure 1a), the enhanced 2PP spectra of Ag/Gr (Figure 1b, top and bottom) exhibit three new features:

(i) a waning photoelectron intensity above EV (blue arrow);
(ii) a new broad peak at a final state energy of ∼6.65 eV above the Fermi level (red arrow); and
(iii) a Fermi edge at 2hv + EF final state energy (black vertical line).

2-Photon Photoemission Spectra of graphite and graphite decorated with silver nanoclusters and photoelectron yield with coverage

In addition, with increasing Ag coverage, a substantial intensity enhancement is observed, in particular for the p- polarized excitation as shown in Figures 1b and 1c, which quantifies the enhancements by integrating the 2PP yield with respect to energy for different Ag coverages.

The relative enhancement of the 2PP signal as a function of hv at a constant fluence (al) is used to determine the frequency of the Ag/Gr plasmon resonance. A plot of al against hv shows no relative plasmonic enhancement for the s-polarization; however, there is a pronounced resonance at hv = 3.6 eV for the p-polarization, which is attributed to the excitation of the ⊥-plasmon mode of the Ag/Gr heterojunctions by the surface normal component of the p-polarized incident light. An upper limit for the plasmon dephasing time of 4.7 fs is calculated from the width of the resonance.

Furthermore, the interface state (IFS) of Ag/Gr which is responsible for the peak at ∼6.65 eV in the figure above is assigned to an occupied state of the heterojunction at 0.2 eV below EF. The IFS gradually appears upon formation of Ag nanoclusters on Gr, and there is no other distinct feature in the 2PP spectra that could be attributed to the Ag nanoclusters. The surface assignment to a chemisorption-induced IFS was confirmed by single-photon photoemission spectroscopy as well as Density Functional Theory calculations and is consistent with the observed charge transfer from the Ag NPs to Gr that was deduced from X-ray photoelectron spectroscopy in other studies.

Hot electron dynamics

The hot electron signal in the 2PP spectra of Ag/Gr does not reveal the origin of the hot electrons; do they emanate from the Ag clusters, interface, graphite substrate, or some combination thereof?

Interferometric two-pulse correlation (I2PC) measurements provide the answer, however. The measurements find identical hot electron lifetimes for the bare Gr and Ag/Gr surfaces with the distinct energy dependence of Gr, indicating that the hot electrons emanate from Gr rather than the Ag NPs or their interface, and therefore demonstrate that there is an efficient resonant transfer channel from Ag plasmons.

2PP measurements suggest that metal-induced IFSs with density just below the Fermi level are general features of metal chemisorption on semiconductor surfaces, which could be important in plasmonically enhanced photocatalysis. Ultrafast pump−probe measurements show that hot electrons, which could energize chemical reactions, are generated by a resonant energy transfer from Ag plasmons, and they establish that the hot electrons are defined by the properties of the Gr substrate.

The mechanisms for plasmonically enhanced photocatalysis have long been a mystery, because most experiments detect only the reaction products, rather than how the chemicals are energized. The study provides the first direct probe of the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.