S. J. Diol

Ultrafast electron dynamics in two dimensional layered systems: two-photon photoemission studies of SnS2

Abstract We performed femtosecond two-photon photoemission studies of SnS 2 , and measured the lifetime of electrons in the conduction band at different excitation energies. The lifetime is short (⩽ 60 fs) even at the conduction band minimum. In addition, the inverse lifetime shows a linear energy dependence. The experimental results are interpreted as due to the electron-plasmon interaction in layered two dimensional materials, which causes electron-hole recombination through the emission of a plasmon. These results are discussed in the context of the electron dephasing processes involved in the barrier crossing dynamics for electron transfer from physisorbed molecular donors at semiconductor surfaces (dye sensitization) under barrierless conditions.

The effect of disorder on excited state dynamics in organic molecular films

We have investigated the dynamics of highly excited electron states in thin films of N,N′-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP), a well-known organic photoconductor, with time-resolved photoemission spectroscopy. We observe an extremely fast relaxation process of photoexcitations with a typical lifetime of 30 fs in DPEP films and attribute it to a rapid internal conversion process from S2 and S1 levels to the vibrational manifolds of S1 and S0 states. Interestingly, the relaxation rate is almost twice as fast at low excitation energies in polycrystalline DPEP films as it is in less-ordered DPEP films. We explain this difference by fast transitions within the manifold of extended states that are shown to form in ordered DPEP films.

MORPHOLOGY AND EXCITED STATE DYNAMICS IN THIN ORGANIC MOLECULAR FILMS PROBED BY FEMTOSECOND TIME-RESOLVED PHOTOEMISSION SPECTROSCOPY

We have studied the lifetimes of the excited electron states of thin N,N′bis(phenethyl)perylene-3,4:9,10-bis(dicarboximide) (DPEP) films, prepared in situ, using femtosecond time-resolved photoemission spectroscopy. DPEP is an organic compound similar to photoreceptor materials widely used in many imaging applications. By controlling the evaporation conditions, we have been able to grow films of different morphologies, and found that the relaxation dynamics depends on the morphology. We have investigated two distinct films characterized by very different absorption spectra. We have found that for the film with absorption maximum at 500 nm, a typical lifetime is 55 fs at 2.1 eV above the highest occupied molecular orbital level. For the other film with absorption maximum at 630 nm, the relaxation rate is almost twice as fast, resulting in a lifetime of 30 fs at the same energy. We attribute the extremely short lifetimes to a rapid internal conversion process from S2 and S1 levels to the vibrational manifol...

Reverse surface photovoltaic effects in GaAs surface quantum wells

We observed that GaAs surface quantum wells on undoped AlGaAs substrates experienced a reverse surface photovoltage (RSPV) that caused the bands to bend upward when illuminated by femtosecond laser pulses for time-resolved two-photon photoemission spectroscopy (TRPES). The RSPV effect is created by trapping of photoexcited electrons in the quantum well. Secondary illumination creates a surface photovoltage opposite in magnitude that flattens the bands. The secondary illumination does not affect the width of the energy spectrum or the ultrafast dynamics with respect to the conduction band minimum. A simple model based on the recombination current from the AlGaAs fits the data excellently.

Photogenerated hot electron dynamics at GaAs (100) surfaces

MBE grown GaAs (100) surface quantum wells were studied using time-resolved two-photon photoemission spectroscopy where the electron distribution was directly determined. Relaxation lifetimes from 50 to 450 fs were measured and were found to be dependent on the energy of the excited electron distribution. Radiative lifetimes at the semiconductor - liquid interface were obtained using time-correlated single-photon counting. From the concentration dependence and other studies, we estimate that the electron transfer cross section is comparable to the molecular cross sections, i.e. electron transfer approaches the adiabatic limit and conditions are optimal for hot electron capture. This work provides parameters to make the hot electron transfer channel competitive with electron relaxation enabling a future generation of solar cells exploiting hot electrons.

Super-Radiant Decay in Two-Dimensional Layered Semiconductors

Geometry confinement of excitons in quasi-two dimensional semiconductors, leads to a shrinkage of the exciton Bohr radius, resulting in an increase of the oscillator strength and binding energy.1,2 This effect is evidenced by the nonlinear absorption and the strong luminescence observed in multiquantum wells (MQW) and semiconductor superlattices,3,4 and harbors a large number of potential applications.5 The theoretical study of Hanamura6 shows that the formation of excitonic polaritons in two-dimensional (2D) systems leads to extremely fast superradiant decay due to the 2D confinement, and dynamics on the order of a picosecond for GaAs MQW and subpicosecond for CdS MQW were predicted.6 The 2D confinement and fast decay makes the excitons deviate from ideal bosons. The resultant increase of the third-order susceptibility thus extends the potential application of quasi-2D semiconductors to fast response nonlinear optical devices.6

Hot Electron Reaction Dynamics at GaAs(100) Surface Quantum Wells

The interfacial charge transfer and competing dynamics, relevant to field accelerated electrons, have been characterized. We have measured electron relaxation as a function of energy at high quality GaAs (100) surface quantum wells (QW). It is found to depend on QW doping. In addition, in situ studies of electron transfer from these QWs to outer-sphere acceptors at liquid contacts indicate that the electron capture cross-section is comparable to the molecular cross-section. These combined studies provide parameters to make the hot electron transfer channel competitive with electron relaxation, thereby supporting the hot electron model.