5 research outputs found
Morphological and Structural Investigation of Sexithiophene Growth on KCl (100)
The morphology and structure of sexithiophene deposited
on KCl
(100) substrates was investigated by scanning force microscopy and
specular X-ray diffraction measurements. Two different needle-like
structures with {010} and {4Ì…11} contact planes have been observed
as well as islands of almost upright standing sexithiophene molecules
with a {100} contact plane. Furthermore an azimuthal alignment of
all three crystal orientations was observed by X-ray diffraction pole
figure measurements, and the growth directions reflect the 4-fold
rotational symmetry of the substrate surface. In addition the analysis
of crystals with {4Ì…11} and {100} contact planes unveiled that
they share a common crystallographic direction which is explained
by ledge directed epitaxy
The Epitaxial Growth of Self-Assembled Ternaphthalene Fibers on Muscovite Mica
The
morphology and structure of 2,2′:6′,2″-ternaphthalene
(NNN) deposited on muscovite mica(001) substrates was investigated
by scanning force microscopy (SFM) and specular X-ray diffraction
measurements. Consistently, both methods reveal the coexistence of
needle-like structures with a {111} contact plane and {001} orientated
island-like crystallites, which are built up by almost upright standing
NNN molecules. Both orientations are characterized by a well-defined
azimuthal alignment relative to the substrate surface, which is analyzed
by X-ray diffraction pole figure (XRD-PF) measurements. Based on XRD-PF
and SFM analysis, the azimuthal alignment of {001} orientated crystallites
is explained by ledge-directed epitaxy along the fibers’ sidewalls.
These fibers are found to orient along two dominant directions, which
is verified and explained by a doubling of the energetically preferred
molecular adsorption site by mirror symmetry of the substrate surface.
The experimental findings are confirmed by force-field simulations
and are discussed based on a recently reported growth model
Interface Properties of Organic <i>para</i>-Hexaphenyl/α-Sexithiophene Heterostructures Deposited on Highly Oriented Pyrolytic Graphite
It
was recently reported, that heterostructures of <i>para</i>-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite
mica exhibit the intriguing possibility to prepare lasing nanofibers
of tunable emission wavelength. For p-6P/6T heterostructures, two
different types of 6T emission have been observed, namely, the well-known
red emission of bulk 6T crystals and additionally a green emission
connected to the interface between p-6P and 6T. In this study, the
origin of the green fluorescence is investigated by photoelectron
spectroscopy (PES). As a prerequisite, it is necessary to prepare
structurally similar organic crystals on a conductive surface, which
leads to the choice of highly oriented pyrolytic graphite (HOPG) as
a substrate. The similarity between p-6P/6T heterostructures on muscovite
mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning
force microscopy (SFM), and optical spectroscopy. PES measurements
show that the interface between p-6P and 6T crystals is sharp on a
molecular level without any sign of interface dipole formation or
chemical interaction between the molecules. We therefore conclude
that the different emission colors of the two 6T phases are caused
by different types of molecular aggregation
Epitaxy of Rodlike Organic Molecules on Sheet Silicatesî—¸A Growth Model Based on Experiments and Simulations
During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule−surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model
Color Tuning of Nanofibers by Periodic Organic–Organic Hetero-Epitaxy
We report on the epitaxial growth of periodic para-hexaphenyl (<i>p</i>-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of <i>p</i>-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the <i>p</i>-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and <i>p</i>-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of <i>p</i>-6P spacer thickness (Δ<i>d</i><sub><i>p</i>–6P</sub>) we have determined a critical value of Δ<i>d</i><sub><i>p</i>–6P </sub>≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and <i>p</i>-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from <i>p</i>-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/<i>p</i>-6P multilayer structures depends on the <i>p</i>-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue <i>via</i> white to the yellow-green spectral range