Ionic surfactants, which are widely used to stabilize nanomaterials in dispersions, can drastically alter the nanomaterial’s photophysical properties. Here, we use femtosecond optical spectroscopy to study the dynamics of excitons and charges in few-layer flakes of the two-dimensional semiconductor MoS2. We compare samples obtained via exfoliation in water with different amounts of adsorbed sodium cholate, obtained by repeated washing of the dried flakes. We find that the femtosecond dynamics is remarkably stable against the surfactant adsorption, with a slight increase of the initial exciton quenching occurring during the first few picoseconds as the only appreciable effect.
We perform two-colour pump probe experiments on metals and superconductors using synchronized 10-fs pulses
generated by optical parametric amplifiers, tunable from the visible to the near-infrared, mapping with unprecedented
detail the energy equilibration dynamics of the free electron gas. In gold films we observe dramatic changes of the
differential reflectivity spectrum on the 100-fs timescale, corresponding to the establishment of the thermal electron
distribution, with dynamics dictated by excess energy. In high-Tc cuprate superconductors we observe fast electron
relaxation, attributed to a strong electron-phonon coupling which may play a role in the superconductivity mechanism.
Single-walled carbon nanotubes dispersed in a polymer matrix are studied via cw photomodulation spectroscopy. The spectrum is dominated by a modulation of the absorption lineshape, which we assigned to electro-absorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation and the dominance of the contribution by low energies point to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.
A fundamental aim in organic electronics is an implementation of a polymeric laser diode. For this purpose, an essential step towards this goal is the understanding and fabrication of feedback structures for conjugated polymer lasers. In this work we will present the realization and characterization of a directly imprinted distributed feedback structure into a highly luminescent conjugated polymer, poly(2-methoxy-5(2-ethyl-hexyloxy)-1,4-phenylevevinylene, which was utilized as a model polymer for conjugated laser materials. The presented method permits to directly imprint a distributed feedback grating into the conjugated polymer. Such distributed feedback structures may be fabricated either in the substrate or in the active polymer layer itself. Therefore we chose two accomplished techniques from soft lithography, which has proven itself as a useful tool to pattern a wide variety of materials. By the combination of these two techniques, namely micrmolding in capillaries and liquid embossing, we created a novel technique. We called this new technique liquid imprinting. Under photo-excitation of a frequency doubled Nd:YAG laser, our corrugated distributed feedback structures showed lasing. Because of the inexpensiveness and repeatability, this approach is a very promising candidate for the mass production of conjugated polymer laser devices.
The origin of a broad low-energy photo-luminescence (PL) and electro-luminescence (EL) band emerging upon oxidative degradation of hihgly emissive polyfluorenes (PFs) has recently been identified as the emission from on-chain keto defects acting as exciton and/or charge traps. In this work we compare several polyfluorenes with respect to their stability upon thermal degradation, and their stability upon fabrication and operation of PF-based polymer light emitting devices (PLEDs). We show that in addition to the keto emission a second type of defect emission, which is related to the deposition of the metal electrode, can also affect the color purity of PF-PLEDs. Investigated materials are a poly(9,9 dialkylfluorene) with hexahydrofarnesyl sidechains (PF111/12) a poly(9,9 dialkylfluorene) with ethyl-hexyl sidechains (PF 2/6) and two different slightly branched spiro-PFs with and without triphenylamine endcappers, respetively. We find significant differences in the spectral stability of the polymers which may on the one hand be explained by a difference of the chemical stability of the polymers but to some extent must be explained withiin the picture of excited energy migration. Regarding a comparison of the polymers, the end-capped spiro-type PF shows an overall improved performance compared to the other investigated polymers provided that the evaporation process of the metal cathode of an PLED is well controlled to avoid the formation of emissive defects at the interface.