Christoph Bubeck

Layer-by-Layer Assembly and UV Photoreduction of Graphene–Polyoxometalate Composite Films for Electronics

Graphene oxide (GO) nanosheets and polyoxometalate clusters, H(3)PW(12)O(40) (PW), were co-assembled into multilayer films via electrostatic layer-by-layer assembly. Under UV irradiation, a photoreduction reaction took place in the films which converted GO to reduced GO (rGO) due to the photocatalytic activity of PW clusters. By this means, uniform and large-area composite films based on rGO were fabricated with precisely controlled thickness on various substrates such as quartz, silicon, and plastic supports. We further fabricated field effect transistors based on the composite films, which exhibited typical ambipolar features and good transport properties for both holes and electrons. The on/off ratios and the charge carrier mobilities of the transistors depend on the number of deposited layers and can be controlled easily. Furthermore, we used photomasks to produce conductive patterns of rGO domains on the films, which served as efficient microelectrodes for photodetector devices.

Third-harmonic generation in polyphenylacetylene: Exact determination of nonlinear optical susceptibilities in ultrathin films

Third-harmonic generation measurements have been performed at λ0=1064 nm on ultrathin films of polyphenylacetylene. The nonlinear susceptibility χxxx x(3)(−3ω; ω, ω, ω) was determined to be (7.0±1.0)×10−13 esu with the phase of χ(3) equal to 152°±5° indicating that two- and three-photon resonances contribute to the observed nonlinearity. The influence of back-reflection effects was estimated experimentally by measurements in different optical configurations. We found the experimental data could only be explained by using an evaluation procedure including all bound waves in the layer system.

Preparation and characterization of monolayers and multilayers of preformed polymers

Abstract In an attempt to study which factors determine the transferability of monolayers of preformed polymers from the air-water interface onto substrates we investigated flexible polymers (poly(octadecylmethacrylates) (PODMAs)) and α-helical polymers (polyglutamates). Pressure-area isotherms show the formation of a liquid-analogous state which depends on temperature and side chain “impurity”. Y-mode Langmuir-Blodgett multilayers of these polymers can be formed with a constant transfer ratio under conditions at which a more or less liquid-analogous state exists. Polarized IR spectra suggest that the polyglutamate α helices in the multilayer are oriented with the main axis parallel to the transfer direction and that carbon side chains are practically randomly oriented around the α-helical cylinder. In PODMA multilayers the side chains are perpendicular to the film. In both cases the side chains seem to interdigitate.

Linear and non-linear optical properties of substituted polyphenylacetylene thin films

Optical third-harmonic generation (THG) at 1064 nm and wavelength dispersed degenerate four-wave mixing (DFWM) have been performed on substituted polyphenylacetylenes (PPAS). Strong changes in the linear and nonlinear optical properties can be achieved by varying the substituent. Resonant values of chi (3)(-3 omega ; omega , omega , omega ) up to 10-11 esu and chi (3)(- omega ; omega , - omega , omega ) of more than 10-9 esu with ultra-fast response times were observed. A linear relationship between chi (3)(- omega ; omega , - omega , omega ) and the absorption coefficient alpha was found. This is compatible with an inhomogeneous broadening of the absorption band or with phase-space filling by one-dimensional excitons. Model calculations are presented that relate the modulus and phase of chi (3)(-3 omega , omega , omega , omega ) to the linear optical properties of PPA and indicates a direct two-photon interaction with the absorption band. This may be due to a non-centrosymmetric electronic structure of the PPA backbone.

Poly(p-phenylenevinylene) derivatives: new promising materials for nonlinear all-optical waveguide switching

Several new derivatives of poly(p-phenylenevinylene) (PPV) are investigated regarding their linear and nonlinear optical material and waveguide properties, including their nonlinear photonic bandgap properties that are induced by photoablated periodic Bragg gratings. The new materials were prepared by means of the polycondensation route, which yields polymers with excellent solubilities and film-forming properties. Comparative data suggest that the new polycondensation-type MEH-PPV (completely soluble, strictly linear and fully conjugated), in particular, is the most promising polymer under investigation to fulfill the requirements for all-optical switching in planar waveguide photonic bandgap structures. UV-photobleaching techniques and photoablation in the UV, VIS, and near-infrared ranges at different pulse durations are investigated. Homogeneous submicrometer gratings that serve as Bragg reflectors have been fabricated in MEH-PPV thin films by application of these methods. The great potential of this type of materials for nonlinear all-optical switching applications that arises from their unique optical properties and their patterning behavior is discussed in detail. Numerical simulations of a switching device based on gap-soliton formation in a nonlinear periodic waveguide structure with the newly obtained material data have been carried out. We show that one can expect photonic bandgap all-optical switching in MEH-PPV planar waveguides. Device performance considering different grating parameters is discussed.

Molecular weight dependence of birefringence of thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene]

We prepared thin films of the conjugated model polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) by the spin coating of polymer samples whose weight-average molecular weight was varied from 104–106 g/mol. Waveguide prism coupling and polarized optical transmission and reflection spectroscopy were used to determine the refractive index, birefringence, and intrinsic absorption coefficient of each film. We show that these optical constants significantly depend on the molecular weight of the polymers. With decreasing molecular weight, the birefringence is strongly reduced and a nearly isotropic refractive index is approached. We conclude that the molecular weight primarily determines the extent of polymer chain alignment in the film plane.

Preparation and Nonlinear Optics of Monodisperse Oligo(1,4-phenyleneethynylene)s

Oligo(1,4-phenyleneethynylene)s 1a−e, with solubilizing propoxy side chains, were prepared by use of Hagihara−Sonogashira coupling reactions. The synthetic strategy was based on a building block system and on the use of trimethylsilyl and triisopropylsilyl protecting groups that could be cleaved selectively. The extension of the conjugation with an increasing number of repeat units provokes a bathochromic shift of the long wavelength absorption and a superlinear increase of the second hyperpolarizability |γ|. The corresponding third harmonic generation (THG) measurements were performed using polystyrene matrices and variable laser wavelengths. We conclude that the conjugation length is much larger than 5 repeat units.

Resonant degenerate four wave mixing and scaling laws for saturable absorption in thin films of conjugated polymers and Rhodamine 6G

Abstract Wavelength dispersed degenerate four wave mixing (DFWM) experiments were performed with thin films of poly(3-decyl-thiophene), poly ( p -phenylene vinylene) and Rhodamine 6G. At the low energy side of the main absorption bands the resonant χ( 3 ) (-ω; ω, ω, -ω) values scale differently with the absorption coefficient α. In Rhodamine 6G, χ( 3 ) is proportional to α 2 due to saturable absorption in a two level system. In the case of polymers with a conjugated one-dimensional π-electron system a linear relationship between χ( 3 ) and α is found. This is attributed to phase-space filling effects by one-dimensional excitons.

Picosecond degenerate four-wave mixing in colloidal solutions of gold nanoparticles at high repetition rates

Time-resolved degenerate four-wave mixing experiments on a colloidal solution of 50-nm gold particles in acetone, using a high-repetition-rate laser source, are presented. The signal is found to depend strongly on the repetition rate of the incident radiation and above a frequency of 300 kHz is impossible to measure.

Fourier transform IR spectroscopy of protonated cadmium arachidate layers in different positions of a perdeuterated multilayer

Abstract The grazing incidence reflection IR spectra of protonated cadmium arachidate monolayers and bilayers at different positions in a deuterated multilayer on a gold surface were studied. In this way the degree of disorder in a single layer can be determined qualitatively as a function of distance from the substrate surface. The characteristic ratios of the CH stretching band intensities are interpreted in terms of increased disorder or tilt of the first monolayer on the substrate and the uppermost layers of the stack.