Horst-Günter Rubahn

Dipole-assisted self-assembly of light-emitting p-nP needles on mica

We report on dipole-assisted, self-assembled formation of p-6P and p-5P needles on cleaved and heated mica (0001) surfaces. Low-energy electron diffraction (LEED) reveals that the needles are single crystalline with the (111) face parallel to the surface, consisting of parallel stacks of laying molecules oriented along the direction of microscopic dipoles on the mica surface. They have submicrometer cross-sectional dimensions and lengths as large as millimeters. Moreover, due to the strong dipole confinement of individual molecules, the needles form large domains with parallel oriented entities. A pronounced optical dichroism agrees with the findings from LEED.

Gain amplification and lasing properties of individual organic nanofibers

We study gain and lasing processes in individual self-assembled organic nanofibers grown on mica substrates. The gain-induced response of the nanofibers is found to depend sensitively on the fiber structure. In homogeneous fibers where no coherent optical feedback is present, high net optical gain (of up to 103cm−1) results in spectral narrowing at the material gain peaks. In the case of strong optical feedback, which occurs in long nanofibers with randomly distributed scattering centers, gain is in turn responsible for low-threshold coherent random laser action.

Isolated hexaphenyl nanofibers as optical waveguides

Laser-supported, dipole-assisted self-assembly results in blue-light guiding nanostructures, namely single-crystalline nanofibers of hexaphenyl molecules. The nanofibers are up to 1 mm long, extremely well-aligned to each other and their cross sections can be tuned to span the range from nonguiding to guiding single optical modes at λ=425.5 nm. An analytical theory for such organic waveguides can reproduce quantitatively the experimentally observed behavior. From the measured damping of propagating, vibrationally dressed excitons the imaginary part of the dielectric function of isolated nanoscaled organic aggregates is determined.

Organic Molecular Nanotechnology

A new route to bottom-up organic nanotechnology is presented. Molecular building blocks with specific optoelectronic properties are designed and grown via directed self-assembly arrays of morphologically controlled light-emitting organic nanofibers on template surfaces. The fibers can be easily transferred from the growth substrate to device platforms either as single entities or as ordered arrays. Due to the extraordinary flexibility in the design of their optoelectronic properties they serve as key elements in next-generation nanophotonic devices.

Organic surface-grown nanowires for functional devices

Discontinuous organic thin film growth on the surface of single crystals results in crystalline nanowires with extraordinary morphological and optoelectronic properties. By way of being generated at the interface of organic and inorganic materials, these nanowires combine the advantages of flexible organic films with the defectless character of inorganic crystalline substrates. The development of destruction-free transfer and direct growth methods allows one to integrate the organic nanowires into semiconductor, metallic electronic or photonic platforms. This article details the mechanisms that lead to the growth of these nanowires and exemplifies some of the linear as well as non-linear photonic properties, such as optical wave guiding, lasing and frequency conversion. The article also highlights future potential by showing that organic nanowires can be integrated into optoelectronic devices or hybrid photonic/plasmonic platforms as passive and active nanoplasmonic elements.

Exposure to silver nanoparticles induces size- and dose-dependent oxidative stress and cytotoxicity in human colon carcinoma cells

The antimicrobial properties of silver nanoparticles (AgNPs) have made these particles one of the most frequently utilized nanomaterials in consumer products; therefore, a comprehensive understanding of their toxicity is necessary. In particular, information about the cellular uptake and size dependence of AgNPs is insufficient. In this study, we evaluated the size-dependent effects of AgNPs by treating the human LoVo cell line, an intestinal epithelium model, with spherical AgNPs of well-defined sizes (10, 20, 40, 60 and 100nm). The cellular uptake was visualized by confocal laser scanning microscopy, and various cytotoxicity parameters were analyzed in a size- and dose-dependent manner. In addition, the cellular proteomic response to 20 and 100nm AgNPs was investigated to increase the understanding of potential mechanisms of action. Our data indicated that cellular uptake and toxicity were regulated by size; smaller particles easily penetrated the cells, and 100nm particles did not. It was hypothesized that this size-dependent effect resulted from the stimulation of a signaling cascade that generated ROS and inflammatory markers, leading to mitochondrial dysfunction and subsequently inducing apoptosis. By contrast, the cell proliferation, was independent of AgNPs particle size, indicating a differentially regulated, ROS-independent pathway.

Nanofibers from functionalized para-phenylene molecules

Tens to hundreds of micrometers long organic nanofibers have been generated from methoxy functionalized quaterphenylene molecules. The mutual alignment of the fibers is similar to that of previously reported nanofibers from para-hexaphenylene, and they emit intense, blue light centered at 400 nm with well resolved higher order vibronic peaks. The morphology is slightly different from that of para-hexaphenylene nanofibers, reflecting the different molecular structure. This study demonstrates that it is possible to generate organic nanofibers from artificially functionalized molecules and thus opens up the route to dedicated applications in new microdevices.

Mapping surface plasmon polariton propagation via counter-propagating light pulses

In an interferometric time-resolved photoemission electron microscopy (ITR-PEEM) experiment, the near-field associated with surface plasmon polaritons (SPP) can be locally sensed via interference with ultrashort laser pulses. Here, we present ITR-PEEM data of SPP propagation at a gold vacuum interface recorded in a counter-propagating pump-probe geometry. In comparison to former work this approach provides a very intuitive real-time access to the SPP wave packet. The quantitative analysis of the PEEM data enables us to determine in a rather direct manner the propagation characteristics of the SPP.

Nonlinear optics of hexaphenyl nanofibers

Abstract The nonlinear optical response of films of needle-shaped para-hexaphenyl nanoaggregates on mica surfaces is investigated. Two-photon luminescence as well as optical second harmonic generation (SHG) are observed following excitation with femtosecond pulses at 770 nm. Polarization dependent measurements reveal that the nonlinear optical transition dipole moment is oriented with an angle of 75° with respect to the needles long axes. The absolute value of the macroscopic second-order susceptibility, averaged over a size distribution of p-6P nanoaggregates, is estimated to be of the order of 6×10−10 esu.

Chain length dependence of the structure of alkane thiol films on Au(111)

Abstract We report the results of a detailed LEED (low energy electron diffraction) study of the chemisorption of alkane thiols [CH 3 (CH 2 ) n −1 ]SH ( n = 4–10,12,18) on single crystalline Au(111) surfaces. Upon vapor deposition the organic films form initially low coverage structures which can be characterized by disulfides, the carbon axes of which are oriented parallel to the surface plane. The length of the unit cells increases linearly with increasing length of the disulfides for n = 4–12. A further increase in coverage results in intermediate chain length dependent structures ( n = 9–12) and eventually in a close-packed c(4√3 × 2√3)R30° structure ( n = 10–12). This structure, which is characterized by molecules with their carbon axes oriented upright with respect to the surface plane, is independent of chain length. It can also be obtained via organic film growth in an ethanolic solution. Low coverage and high coverage structures can be mutually transformed via the deposition or desorption of molecules.