Frank Balzer

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.

From clusters to fibers: parameters for discontinuous para-hexaphenylene thin film growth.

All relevant steps of discontinuous thin film growth of para-hexaphenylene on muscovite mica (0 0 1) from wetting layer over small and large clusters to nanofibers are observed and investigated in detail by a combined polarized fluorescence and atomic force microscopy study. From a variation of film thickness and surface temperature, we determine effective activation energies for cluster growth of 0.17 eV, for nanofiber length growth of 0.46 eV, for width growth of 0.19 eV, and for height growth of 0.07 eV. The corresponding exponential prefactors for the nanofiber growth are 1 x 10(9), 6 x 10(4), and 3 x 10(2) nm. Polarized fluorescence studies reveal that nanofibers grow along the grooves of the mica surface and that they do not change direction if they cross an even number of mica surface steps, while they change direction by 120 degrees for an odd number of steps. These results are taken as an input for a model of the unidirectional growth process on mica. Absolute parameters allowing one to grow nanofibers of predetermined morphology via organic molecular beam epitaxy are also given.

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.

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.

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.

Size dependent optical second harmonic generation from surface bound Na clusters: comparison between experiment and theory

Abstract Optical second harmonic (SH) generation from large Na clusters bound to dielectric surfaces is reported for laser excitation with λ = 660 nm. A pronounced maximum of SH yield is found as a function of increasing (via growth) and decreasing (via thermal desorption) cluster size, which results from SH enhancement by collective surface plasmon excitation. Experimental measurements, performed both at λ = 660 nm and λ = 1064 nm are found to be in good agreement with predictions derived from the electromagnetic field enhancement theory, thereby suggesting that χ (2) for large Na aggregates is independent of cluster size. The observation of SH as a function of surface temperature shows that thermal desorption results in a strongly changing size distribution for clusters remaining on the surface that differs significantly from the distribution imposed by adsorption processes.