Functionalization of small, rod like organic molecules can be used to optimize organic devices. Here we report on nanofiber formation and thin film growth of a methoxy-functionalized para-quaterphenylene (1,4'''-Dimethoxy- 4,1':4',1'':4'',1'''-quaterphenylene, MOP4) on prototypical dielectric substrates such as muscovite mica, phlogopite mica, highly ordered pyrolytic graphite (HOPG), and on the alkali halide NaCl. The nanofibers consist of lying molecules, the films of upright standing ones. The grown samples are characterized by polarized optical microscopy (fluorescence, birefringence, bireflectance), by atomic force microscopy (AFM), and by Kelvin probe force microscopy (KPFM) to gain insight into their structure and epitaxial relation with the substrates.
Environmentally stable, non-toxic squarylium dyes with strong absorption maxima in the red and near infrared
spectral region are known for almost fifty years. Despite the fact that their optoelectronic properties distinguish
them as promising materials for organics based photovoltaic cells, they have regained attention only very
recently. For their application in heterojunction solar cells knowledge of their nanoscopic morphology as well
as nanoscopic electrical properties is paramount. In this paper thin films from two different squarylium dyes,
from squarylium (SQ) and from hydroxy-squarylium (SQOH) are investigated. The thin films are either solution
casted or vacuum sublimed onto substrates such as muscovite mica, which are known to promote self-assembly
into oriented, crystalline nanostructures such as nanofibers. Local characterization is performed via (polarized)
optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Kelvin probe
force microscopy (KPFM).
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.
Nanofibers from light-emitting organic molecules such as para-phenylenes have already demonstrated a promising
application potential in nanophotonic devices and can act as waveguides or nanolasers. Here, the basic mechanisms
for self-assembly of three different green- and green/blue-light emitting thiophene/phenylene co-oligomers
into nanofibers are investigated. Under well defined conditions in high vacuum the molecules are deposited on
cleaved mica surfaces. The effect of substrate surface energy as well as epitaxy on the overall film morphology
is studied and significant differences between different co-oligomers are found.
Aligned ensembles of nanoscopic nanofibers from organic molecules such as para-phenylenes for photonic applications
can be fabricated by self-assembled molecular growth on a suited dielectric substrate. Epitaxy together
with alignment due to electric surface fields determines the growth directions. In this paper we demonstrate
how aligned growth along arbitrary directions can be realized by depositing the molecules on a micro-structured
and gold covered Silicon surface, consisting of channels and ridges. For the correct combination of ridge width
and deposition temperature fibers grow perpendicular to the ridge edge, emitting light polarized along the ridges
after UV excitation.
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.
Nanofibers from symmetrically and unsymmetrically functionalized p-quaterphenylenes are fabricated by a bottom-up process on muscovite mica. The symmetrically functionalized p-quaterphenylenes emit intense, polarized blue light after unpolarized UV-excitation. Upon implementing electron push-pull functional groups like chlor and methoxy groups to the molecular building block new properties of the nanoaggregates have been generated: the nanofibers exhibit increased non-linear optical properties, acting, e.g., as frequency doublers after
excitation with NIR femtosecond laser pulses. Depending on the
growth conditions the chloro-methoxy-p-quaterphenylene forms
either parallel nanofibers or nano-branches on a muscovite mica
substrate, adding another degree of freedom for the design of, e.g.,
resonator structures.
The synthesis of molecules consisting of various combinations of phenylene and thiophene groups and the subsequent
vacuum growth of needle-shaped nanoaggregates on specific surfaces allows us a systematic investigation of
the transition between single parallel and multiple aligned needle ('nanofiber') growth. The former growth mode
is observed for blue light emitting phenylene fibers, whereas the latter growth mode appears for single crystalline
fibers made from green- and orange-light emitting oligo-thiophenes and thiophene/phenylene co-oligomers. In all
cases the tailored bottom-up growth results in strongly polarized light emission along specific surface directions.
The results are compared to those found for nanoaggregates made from less rod-like organic molecules, namely
rubrene and POPOP.
It has been shown recently, that organic nanofibers grown from para-hexaphenyl and from α-sexithiophene molecules can be used as a new type of nanoscopic waveguides. Their growth is due to a self-assembly process, thus large quantities of aligned nanofibers can be fabricated simultaneously. Because of the growth mechanism of the nanofibers, their widths and heights are limited to a few 100 nm and a few 10 nm, respectively. In this paper we show how this kind of control has been obtained via modification of the bare muscovite surface before organic molecule deposition. Introducing e.g. a thin layer of Au islands before nanofiber growth results in an up to 15-fold increase in height, whereas the mean width and the optical properties of the fibers remain almost unchanged. Au films of varying thickness lead to tailor-made height profiles along the fiber. Using atomic force microscopy the details of these Au/organic heterostructures are examined and the growth is compared to growth on untreated mica. By scratching the fibers with an AFM tip grating structures have been written into the fibers.
Single crystalline organic nanoaggregates from organic semiconductors such as para-hexaphenyl and sexithiophene might become building blocks for a new type of organic electronic and optoelectronic devices. For the performance of such devices detailed knowledge about the mechanisms responsible for formation and for alignment of the aggregates on the growth substrate is important. On muscovite mica long, mutually parallel fibers of para-hexaphenyl grow, whereas on alkali halides mainly two different orientations, on phlogopite mica three different orientations exist. For sexithiophene on muscovite mica depending on the growth temperature either three equivalent aggregate orientations exist, or a single one dominates. The interplay between epitaxy and dipole assisted alignment on different growth substrates favors either unidirectional or multidirectional growth.
Light-emitting nanofibers grown from organic molecules such as para-hexaphenyl or substituted para-quaterphenyl have extraordinary morphological, optical and electrical properties that make them interesting candidates as key elements in future electronics and photonics. These fibers are generated in a self assembly fashion on template substrates. In order to integrate them into more complex structures, a transfer from the growth substrate is necessary. In this paper we show results from optical and morphological measurements on nanofibers transferred onto semiconductors, kept freely floating in solution and frozen in gel. The former investigations allow us to study with nanometric resolution via an atomic force microscope the deformability of nanofibers. The latter studies, based on single photon as well as confocal two-photon microscopy, provide three-dimensional optical images and also the angular distribution of light emitted from individual aggregates. It is observed that waveguiding affects the spatial emission characteristics.
The growth of nanoscopic oligophenylene and oligothiophene aggregates on muscovite mica by vacuum deposition has been investigated. In the case of para-phenylenes a dipole assisted self assembly generation of needle-like aggregates is observed on mica. At optimum fiber growth temperature phenylene aggregates grow in most cases without a layer of upright oriented molecules. In contrast, vacuum deposition of oligothiophenes results simultaneously in fibers of laying molecules as well as islands of upright molecules. Since both phenylenes and thiophenes are strongly polarizable but differ in the lattice parameters of the resulting crystalline overlayers a direct comparison between the two classes of molecules allows us to study the role of epitaxy on the growth of nanoaggregates. Besides straight
aggregates we also observe thiophene rings on water and methanol treated mica surfaces, which consist of radially oriented, laying molecules.
Nanofibers made from organic molecules such as para-hexaphenyl allow guiding of electromagnetic waves. Since they possess nanometric widths and heights but macroscopic lengths they represent the smallest possible optical waveguides. Recently, gain enhancement has been observed, pointing to possible applications as nanolasers. Owing to their self assembly growth mode on mica substrates the nanofibers posses well-defined morphology. In order to implement these aggregates into new optical devices or to enhance feedback and thus build up a resonator structure a defined cutting of the end faces is necessary. This article presents results from the first experimental studies in this direction. Irradiation with UV laser pulses of 193 nm at a fluence of 100 mJ/cm2 removes the fibers completely without damaging the substrate. In addition, the fibers can be cut in any orientation relative to their long axes. The quality of the ablation process in terms of readsorbed debris and steepness of cutting is investigated by atomic force and scanning electron as well as fluorescence microscopy.
In this work we exploit growth as well as linear and nonlinear
optical properties of long, parallel single-crystalline
hexaphenylene (p-6P) nanofibers grown on mica surfaces. Typical
widths and heights of these needle-like structures are a few 10 -
400 nanometers, whereas lengths of up to one millimeter can be
achieved. The nanofibers allow us to perform experiments at either
densely packed, well-aligned bunches of aggregates or at isolated
entities. Linear optical properties are probed by local
spectroscopy using a fiber-optic spectrometer and by guiding
UV-light through individual fibres and relating the waveguiding
efficiency to their morphology using atomic force and fluorescence
microscopy. Results are compared with an analytical theory. As
nonlinear optical probes we use two-photon luminescence as well as
optical second harmonic generation induced by ultrashort
laserpulses in the near-infrared spectral range.
KEYWORDS: Nanofibers, Luminescence, Spectroscopy, Near field optics, Atomic force microscopy, Fluorescence spectroscopy, Near field scanning optical microscopy, Scanning tunneling microscopy, Molecules, Mica
Nanoscaled photonic devices rely on a thorough understanding of
the influence of microscopic morphological changes on the
optoelectronic properties. Here, we investigate as a model system
organic nanofibers from para-phenylene molecules, which provide
high flexibility in terms of controlled growth manipulation, while
on the other hand showing self assembled multiplication of
individual entities. Examples on selective spectroscopy, scanning
fluorescence optical microscopy and waveguiding of individual
nanofibers as well as arrays of nanofibers are given. Both the
linear optical properties as well as the waveguiding efficiency
are strongly related to the nanofibers morphology, which turn out
to be an interesting benchmark system for the investigation of the
applicability of a variety of optical methods in the nanodomain.
Blue light-emitting aggregates of para-hexaphenyl molecules
('needles') are generated via dipole-assisted self assembly on
single crystalline mica substrates. By applying atomic force
microscopy and fluorescence microscopy we deduce size
distributions as well as height-width correlations of individual
aggregates. We find growth of densely-packed, needle-like
structures even in the initial growth stage at low surface
temperatures. However, long (up to one millimeter) and sparsely
distributed, individually addressable needles grow only at high
substrate temperatures and low adsorption rates. For a given
sample at constant deposition conditions the height of the needles
seems to be independent of width. This opens up the possibility to
control the morphology of individual nanostructured aggregates.
In this work we investigate with the help of low energy electron scattering, force microscopy and optical spectroscopy the growth of ultrathin p-nP (n=4,5,6) films on mica surfaces. We find conditions under which we obtain on a nanometric scale spatially localized emission and conditions under which the emission is spatially non-localized. The latter case results from continuous films of upright molecules (i.e. with transition dipole moments oriented parallel to the surface normal) whereas the former case is obtained if - under certain growth conditions - films consisting of high-aspect ratio 'needles' of laying molecules with well-oriented transition dipole moments along the surface have been grown. Because of a strong interaction between substrate surface dipoles and induced dipoles along the long molecular axes, the needles form macroscopic domains of almost perfectly mutually parallel aligned entities. The quality of the alignment depends on the length of the molecules.
Crystallographic unit cells of vacuum grown ultrathin films of blue-light emitting para-phenylene oligomers on alkali halides, on mica and on Au(111) have been determined via low energy electron diffraction (LEED). On the alkali halides the growth of continuous single crystalline films with either standing or laying molecules dominates. On mica, single-crystalline aggregates (needles) of laying molecules are grown. As the chain length of the molecules increases the mutual order of the needles increases. For p-6P the parallel orientation of the needles is strictly determined by the orientation of surface dipole fields in large dipole domains. A combination of LEED structural results with optical and morphological information from fluorescence microscopy and from atomic force microscopy allows us to deduce subtle details of the organic film aggregates. E.g., bright fluorescence spots could be assigned to nanoscaled gaps in the needles.
A systematic investigation has been performed in order to determine the unit cells of ultrathin p-nP films grown under surface science conditions on dielectrics. We have employed low energy electron diffraction (LEED) as well as polarization dependent linear absorption spectroscopy on films deposited onto alkali halide (NaCl, KCl, NaF, LiF, KBr) and mica(0001) single crystals at various substrate temperatures. Surface unit cells of the films are determined as a function of chain length n(n=3-6) and deposition parameters. Linear optical spectroscopy reveals a strong dichroism and allows us to distinguish between laying and standing molecules on the substrate. In contrast to the alkali halides, we observe on mica at elevated surface temperatures the growth of single crystalline needles, the orientation of which is controlled by the presence of surface dipoles on the cleaved mica surface.
Ablation yields and thresholds for 193 nm UV laser ablation of ultrathin HfO2 are presented. The single shot threshold fluence increases approximately linearly with HfO2 thickness form 28 nm to 120 nm. Due to the logarithmic dependence of ablation depth on fluence this result with increasing layer thickness in an exponential increase of fluence necessary for clean ablation of the whole layer. The observed ablation depth for fixed HfO2 thickness can be reproduced phenomenologically by taking ablation from the HfO2 film as well as the quartz substrate into account. As a first approach to a quantitative understanding we calculate numerically the heat evolution in the layered system and identificate ablation with the onset of melting of the absorbing layer. Whereas the ablation curve for a 74 nm thick film can be reproduced that way, this is not the case for the case for the overall thickness dependence of the ablation threshold. This points to possible finite size effects for the phonon-phonon scattering rate in the thin dielectric layers.
In this work we study metal surface-induced changes of lifetime and transition frequency of alkali atoms and clusters, deposited onto nanoscaled insulator-metal systems. The systems are made of rough metallic surfaces (characterized by atomic force microscopy), onto which ultrathin organic films as spacer layers (characterized by LEED) are epitaxially grown. We observe an unusually small red shift of the transition frequency of Na atoms of a few hundred Megahertz, which is due to the interaction with the metal surface. This is explained by the nonlocal response of the surface, i.e., the excitation of multipole surface plasmons (MSPs) in the selvedge region of the metal surface, which is influenced by surface roughness. The MSPs should become observable also via linear optical methods such as attenuated total reflection spectroscopy. As a first step in this direction, we present linear extinction spectra of alkali cluster films that are grown on top of organic spectra layers of different length. Due to the interaction with the gold films a red shift of the dipole plasmon resonance is observed, which increases with decreasing chain length.
We investigate via optical (viz. linear extinction and nonlinear second harmonic generation) and non-optical methods (viz. helium atom scattering) rough alkali cluster films, which are formed on dielectric substrates. The application of nonlinear optical methods to these systems allows us to obtain real-time dynamical information on the time-constants for laser-induced desorption and for the decay of the elementary optical surface plasmon excitation into single electron excitations and finally into lattice oscillations of the clusters and the substrate. The combination of linear and nonlinear optics also enables us to deduce structural information about the morphology of the cluster films, which--in the submonolayer regime--is complementary to information deduced from atom scattering data.
In this work we present experimental data that show that the roughness of a metal surface strongly influences the metal induced optical transition frequency shift of alkali atoms that are adsorbed close (a few tens of Angstroms) to the metal. The metal induced changes of electronic lifetime depend on distance (alpha) d4, suggesting surface electron hole pair excitation to be the dominant relaxation mechanism for electronically excited Na atoms at distances between 24 and 32 angstroms from a rough Au surface. The nonlinear response of metal surfaces is also well known to be enhanced by surface roughness. It has been anticipated that this enhancement should be most pronounced for a third order nonlinear optical process. Here, we present data of strong enhancement of (chi) (3)eff for rough metal surfaces. The surfaces consist of large alkali metal clusters, adsorbed on dielectrics. By changing the cluster size distribution we are able to study the third order nonlinearity as a function of shape of all the alkali protrusions.
The photodesorption of Na atoms from Na clusters deposited on dielectric surfaces is investigated via pulsed laser excitation and cw two-photon laser-induced fluorescence detection. The combination of pulsed excitation and cw detection within the focii of two counterpropagating lasers provides high spatial resolution (allowing one to obtain accurate angular and kinetic energy distributions) while at the same time preserving the high temporal resolution given by the pulsed laser. Moreover, spatial overlap of desorption and detection lasers on the surface facilitates to follow directly the conversion of initial surface plasmon excitation into bond-breaking in the clusters or phonon excitation in the substrate. It is found that the final step of photodesorption of Na from large Na clusters bound to mica surfaces can be described via multiphonon scattering of the desorbing particles from the surface of the clusters (with nearly zero initial kinetic energy). By use of lithium fluoride as the supporting substrate it is observed that the activation energy for photodesorption from the clusters depends on the Debye temperature of the supporting substrate. This emphasizes the need to include coupling between clusters and substrate in order to understand the overall desorption process.
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