UC Berkeley

The properties of interfacial electronic states in device-relevant materials are investigated

using time- and angle-resolved two photon photoemission (TPPE) spectroscopy.

Angle- and time-resolved TPPE is used to investigate electronic states in the buffer layer

of 4H-SiC(0001). An image potential state (IPS) series is observed on this strongly surface-bound buffer layer, and dispersion measurements indicated free-electron-like behavior for all states in this series. These results are compared with TPPE taken on bilayer graphene, which also shows the existence of a free-electron-like IPS series. Lifetimes for the n = 2, and n = 3 states are obtained from time-resolved TPPE; slightly increased lifetimes are observed in the bilayer graphene sample for the n = 2 the n = 3 states. Despite the large band gap of graphene at the center of the Brillouin zone, the lifetime results demonstrate that the graphene layers do not behave as a simple tunneling barrier, suggesting that the buffer layer and graphene overlayers play a direct role in the decay of IPS electrons. Ultrafast response of the room temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr][NTf2]) to a photoinjected electron is investigated in few-monolayer films using time- and angle-resolved two-photon photoemission spectroscopy. A delocalized precursor state and a localized solvated state are resolved at early times, but after 200 fs only a single solvated state is observed. The dynamics of film response to this solvated state are shown to depend significantly on film temperature and thickness. Population lifetime measurements demonstrate that the RTIL film can significantly affect the coupling between solvated state and metal substrate, as the solvated state’s average lifetime increases from 90 ± 20 fs in 1 ML films to 195 ± 83 ps in 3 ML films. Additionally, a temperature dependence of the time-dependent binding energy shift of the solvated state after c.a. 500 fs is attributed to a phase change occurring between the two temperature regimes that were investigated. Results from xenon overlayer experiments suggest that the solvation process occurs near the surface of the RTIL film. Finally, film degradation is found to be present, suggesting that the observed solvation response could involve a radical species. Valuable information about two very different interfacial materials is collected using TPPE, proving the power and versatility of the technique. The results presented here show that interfacial states can be used to monitor a variety of physical phenomena, from corrugation in graphitic materials to solvation and degradation in RTILs.