Manuela Schiek

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.

Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons

We report photoelectron emission from the apex of a sharp gold nanotaper illuminated via grating coupling at a distance of 50 μm from the emission site with few-cycle near-infrared laser pulses. We find a fifty-fold increase in electron yield over that for direct apex illumination. Spatial localization of the electron emission to a nanometer-sized region is demonstrated by point-projection microscopic imaging of a silver nanowire. Our results reveal negligible plasmon-induced electron emission from the taper shaft and thus efficient nanofocusing of few-cycle plasmon wavepackets. This novel, remotely driven emission scheme offers a particularly compact source of ultrashort electron pulses of immediate interest for miniaturized electron microscopy and diffraction schemes with ultrahigh time resolution.

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.

Surface bound organic nanowires

The results of a comparative study of nanowires grown on single crystalline substrates from para-hexaphenylene, α-sexithiophene, and 5,5′-Di-4-biphenyl-2,2′-bithiophene are presented. Due to their interesting optical properties such nanowires are of importance for future integrated optoelectronic devices. From atomic force microscopy and polarized far-field optical microscopy data, it is deduced that epitaxy and electrostatic interactions determine the microscopic growth mechanism, whereas kinetics ascertains the macroscopic habit. Understanding such basic growth principles for these systems allows one to predict qualitatively nanowire surface growth from other conjugated molecules and thus allows for a sophisticated design of new devices.

Directed self-assembled crystalline oligomer domains on graphene and graphite

We observe the formation of thin films of fibre-like aggregates from the prototypical organic semiconductor molecule para-hexaphenylene (p-6P) on graphite thin flakes and on monolayer graphene. Using atomic force microscopy, scanning electron microscopy, x-ray diffraction, polarized fluorescence microscopy, and bireflectance microscopy, the molecular orientations on the surface are deduced and correlated to both the morphology as well as to the high-symmetry directions of the graphitic surface: the molecules align with their long axis at ±11° with respect to a high-symmetry direction. The results show that the graphene surface can be used as a growth substrate to direct the self-assembly of organic molecular thin films and nanofibres, both with and without lithographical processing.

Photoelectrical Stimulation of Neuronal Cells by an Organic Semiconductor–Electrolyte Interface

As a step toward the realization of neuroprosthetics for vision restoration, we follow an electrophysiological patch-clamp approach to study the fundamental photoelectrical stimulation mechanism of neuronal model cells by an organic semiconductor-electrolyte interface. Our photoactive layer consisting of an anilino-squaraine donor blended with a fullerene acceptor is supporting the growth of the neuronal model cell line (N2A cells) without an adhesion layer on it and is not impairing cell viability. The transient photocurrent signal upon illumination from the semiconductor-electrolyte layer is able to trigger a passive response of the neuronal cells under physiological conditions via a capacitive coupling mechanism. We study the dynamics of the capacitive transmembrane currents by patch-clamp recordings and compare them to the dynamics of the photocurrent signal and its spectral responsivity. Furthermore, we characterize the morphology of the semiconductor-electrolyte interface by atomic force microscopy and study the stability of the interface in dark and under illuminated conditions.

Stability of organic nanowires

The morphological stability of organic nanowires over time and under thermal load is of major importance for their use in any device. In this study the growth and stability of organic nanowires from a naphthyl end-capped thiophene grown by organic molecular beam deposition is investigated via atomic force microscopy (AFM). Aging experiments under ambient conditions already show substantial morphological changes. Nanoscopic organic clusters, which initially coexist with the nanowires, vanish within hours. Thermal annealing of nanowire samples leads to even more pronounced morphology changes, such as a strong decrease in nanowire number density, a strong increase in nanowire height, and the formation of new types of crystallites. This happens even before sublimation of organic material starts. These experiments also shine new light on the formation process of the nanowires.

Revealing the recombination dynamics in squaraine-based bulk heterojunction solar cells

We combine steady-state with transient optoelectronic characterization methods to understand the operation of photovoltaic devices based on a benchmark model squaraine blended with a fullerene acceptor. These devices suffer from a gradual decrease in the fill factor when increasing the active layer thickness and incident light intensity. Using transient photocurrent, transient photovoltage, and bias-assisted charge extraction measurements, we show that the fill factor deteriorates due to slow charge carrier collection competing with bimolecular recombination. Under normal operating conditions, we find a bimolecular recombination rate constant of ∼10–17 m3 s−1, which corresponds to a reduction of one to two orders of magnitude compared to the Langevin model.

Organic Nanofibers from PPTPP

The growth of 2,5-Di-4-biphenyl-thiophene (PPTPP) on the dielectric substrates NaCl, KCl, KAP, muscovite mica, and phlogopite mica is investigated by atomic force microscopy (AFM) and fluorescence microscopy. In all cases fibers are formed with several ten nanometers height and several hundred nanometers width, respectively. Only for PPTPP on muscovite mica the fibers are mutually parallel aligned along a single substrate direction, i.e. along muscovite 〈110〉. This uniaxial growth is explained by an electrostatic interaction between the molecules and surface electric fields in combination with epitaxy. The various growth directions on other substrates are dictated by epitaxy alone.

Printed second harmonic active organic nanofiber arrays

Organic nanofibers from semiconducting conjugated molecules are well suited to meet refined demands for advanced applications in future optoelectronics and nanophotonics. In contrast to their inorganic counterparts, the properties of organic nanowires can be tailored at the molecular level by chemical synthesis. Recently we have demonstrated the complete route from designing hyperpolarizabilities of individual molecules by chemically functionalizing para-quaterphenylene building blocks to the growth and optical characterization of nonlinear, optically active nanoaggregates. For that we have investigated nanofibers as grown via organic epitaxy. In the present work we show how chemically changing the functionalizing end groups leads to a huge increase of second order susceptibility, making the nanofibers technologically very interesting as efficient frequency doublers. For that the nanofibers have to be transferred either as individual entities or as ordered arrays onto specific target substrates. Here, we study the applicability of contact printing as a possible route to non-destructive nanofiber transfer.