A 15-laboratory round-robin inter-comparison of ultrafast Terahertz and Microwave time-domain measurements will be presented. Detailed time-to-frequency domain spectral analyses to extract photo-generated carrier conductivity/mobility of high-mobility Perovskite film samples were examined. Experiments comparing non-contact Terahertz to Hall conductivity measurements conducted at NIST for bulk semiconductors (Float Zone Si, n/p-doped Si, GaAs, ZnTe, GaP, etc.) will be briefly shown. An overview of ultrafast terahertz studies of conducting polymers, 2D monolayer and exciton “multiplication” in multilayer transition metal dichalcogenide films (MoTe2, MoS2) will be given.
Exciton dissociation at organic semiconductor donor-acceptor (D-A) heterojunctions is critical for the performance of organic photovoltaic (OPV) structures. Interfacial charge separation and recombination processes control device efficiency. We have investigated these fundamental interfacial issues using time-resolved two-photon photoemission (TR-2PPE), coupled with the formation of well-controlled D-A structures by organic molecular beam epitaxy. The interfacial electronic and molecular structure of these model interfaces was well-characterized using scanning tunneling microscopy and ultraviolet photoemission. Exciton dissociation dynamics were investigated by using a sub-picosecond pump pulse to create Pc π→π* transitions, producing a population of singlet (S1) Pc excitons. The subsequent decay dynamics of this population was monitored via photoemission with a time-delayed UV pulse. For CuPcC60 interfaces, S1 exciton population decay in the interfacial CuPc layer was much faster than decay in the bulk due to interfacial charge separation. The rate constant for exciton dissociation was found to be ≈ 7 x 10 12 sec-1 (≈ 140 fs). Excitons that lose energy via intersystem crossing (ISC) to triplet levels dissociate approximately 500 to 1000 times slower. The dependence of exciton dissociation on separation was also studied. Exciton dissociation falls of rapidly with distance from the interface. Dissociation from the 2nd, and subsequent, layers of H2Pc is reduced by at least a factor of 10 from that in the interfacial layer. Finally, investigations of the relative efficiency for interfacial exciton dissociation by alternative acceptors based on perylene cores, (perylene tetracarboxylic dianhydride, or PTCDA) compared to fullerene-based acceptors such as C60 will also be discussed.
Photoemission spectra of self-assembled monolayers of para-phenylene-ethynylene thiols chemisorbed on gold have
been measured. Three compounds were studied: 4,4'-bis(phenylethynyl)benzenethiol, 4-(phenylethynyl)benzenethiol,
and benzenethiol. The monolayer spectra were interpreted with the aid of gas-phase photoemission spectra of 4,4'-
bis(phenylethynyl)benzenethiol and benzenethiol. The work function of the monolayer-covered surface and alignment
of the highest occupied π-state of the monolayer relative to the Fermi level of the substrate were determined from the
monolayer spectra. The work function of the monolayer-covered surface decreased relative to that of the bare substrate
by about an electronvolt. The shift is attributed to changes in the charge distribution at the interface associated with
chemisorption. No statistically significant trend in the work function shift was observed with respect to oligomer
length. This observation points to a near-constant interface dipole and the role of the gold-sulfur bond in determining
energy-level alignment.
Photoelectron spectroscopy was employed to follow the development of the Ti 3d spectral intensity within approximately 3 eV of the Fermi level (EF) in Nd1-xSrxTiO3 as a function of x. The total intensity in this region is found to correlate linearly with composition as the nominal Ti valence is varied from plus 3 to plus 4. Changes in the spectral structure in this binding energy region are discussed in terms of the one-electron spectral function. For x less than 0.25, (semiconducting compositions) only incoherent intensity associated with the lower Hubbard band is present. Additional intensity, attributed to a coherent quasiparticle contribution, appears at the composition of the metal-insulator transition, x approximately 0.25. The relative intensities of these two components are determined as a function of x, and connections with theoretical models and experimental results on related systems are discussed.
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