Heteroepitaxy of Organic Nanofibers: Example of

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Heteroepitaxy of Organic Nanofibers: Example of
Article
pubs.acs.org/crystal
Heteroepitaxy of Organic Nanofibers: Example of Ternaphthalene on
p‑Hexaphenyl
Clemens Simbrunner,*,† Günther Schwabegger,† Roland Resel,‡ Theo Dingemans,⊥ Francesco Quochi,§
Michele Saba,§ Andrea Mura,§ Giovanni Bongiovanni,§ and Helmut Sitter†
†
Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstrasse 69, A-4040 Linz, Austria
Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria
⊥
Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands
§
Dipartimento di Fisica, Universita di Cagliari, SLACS-INFM/CNR, I-09042 Monserrato, Cagliari, Italy
‡
S Supporting Information
*
ABSTRACT: Nowadays heteroepitaxy is well understood and
investigated for inorganic compounds. In contrast, the epitaxial
growth of organic−organic multilayer structures is rarely
reported. By a comprehensive comparison between experiments and simulations, we demonstrate that highly anisotropic, needle-shaped p-hexaphenyl (p-6P) crystallites can
efficiently act as an organic template and that the epitaxial
overgrowth by 2,2′:6′,2″-ternaphthalene (NNN) yields a high
molecular order and optical anisotropy of the nucleated NNN
crystallites. It is shown that surface corrugations formed by the
p-6P template are responsible for a parallel molecular
alignment and a geometrical adoption of the herringbone
stacking sequence of NNN. On the basis of the obtained results, it can be concluded that, in contrast to inorganic heteroepitaxy,
lattice matching plays a minor role, whereas a geometrical adoption of the molecular stacking is directly connected with an
optimized adsorption energy. In that sense, polarization-dependent photoluminescence studies prove a significantly increased
optical anisotropy of NNN crystallites, when a p-6P template layer is inserted between NNN and the muscovite mica substrate.
The organic interlayer is also responsible for the formation of a different NNN contact plane and the suppression of islandshaped crystal morphologies which are a fingerprint for standing molecular configurations. Consequently, only highly anisotropic,
lying molecular orientations are obtained, which is relevant for the design of future organic-based optoelectronic devices.
■
INTRODUCTION
Rod-like small organic molecules are well-known for their
tendency to form highly anisotropic crystal shapes which are
frequently called nanofibers or nanoneedles.1−7 In particular phexaphenyl (p-6P) when deposited on muscovite mica has
been recognized as a promising material combination for the
fabrication of highly parallel-oriented, blue fluorescent nanofibers, which can act as optical waveguides8,9 or random
lasers.10
Moreover, it has been recognized that p-6P is an ideal model
compound with respect to its geometry and ability to adsorb
onto a muscovite mica surface.11 As sketched in Figure 1a(i) a
tilted adsorption geometry of molecules, e.g., sexi-thiophene
(6T),4,12 relative to the mirror plane of muscovite mica leads to
a reduced anisotropy due to the generation of mirror symmetric
adsorption sites.11 In contrast, p-6P (the molecular structure is
sketched in Figure 1b) adsorbs with its long molecular axis
(LMA) approximately normal to the mirror symmetry
plane.11,13 As indicated in Figure 1a(ii), the latter configuration
leads to a coincidence with the mirror symmetric twin
molecules, which are characterized by their LMA*. As the
© 2014 American Chemical Society
optical transition dipole is aligned along the LMA, a parallel
molecular orientation is key to the fabrication of photonic
devices yielding highly polarized emission or adsorption.14
On the basis of these considerations, it can be understood
that the fabrication of organic nanofibers requires a precise
control on the molecular adsorption and LMA orientation. As
sketched in Figure 1a(iii), it has been demonstrated that
organic−organic heteroepitaxy of 6T/p-6P nanofibers allows
forcing of a realignment of 6T molecules parallel to the LMA of
p-6P within an organic template fiber.15 On the basis of
structural analysis and polarization-dependent photoluminescence, it has been shown that the LMA of 6T aligns parallel to
p-6P molecules when deposited on top of a p-6P template.15,16
Besides a precise control of the crystallographic and
structural order, mainly tunable electronic properties and in
particular the energy level alignment at the organic−organic
heterojunctions are critical not only to improve the device
Received: July 2, 2014
Revised: September 22, 2014
Published: October 6, 2014
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general concept of overgrown p-6P fibers for the fabrication of
organic−organic heterojunctions.
■
EXPERIMENTAL SECTION
Synthesis of 2,2′:6′,2″-Ternaphthalene (NNN). NNN was
prepared using standard Suzuki cross-coupling procedures.27,28 This
all-aromatic compound could be obtained in high yield by coupling 2
equiv of 2-naphthaleneboronic acid (1) with 1 equiv of 2,6dibromonaphthalene (2).25 The final product, NNN, was obtained
as a colorless product, which appears to be highly insoluble in
common solvents and could only be recrystallized from 1,2,4trichlolorobenzene (colorless platelets).
Sample Preparation. All samples have been fabricated on
muscovite mica (001) substrates (SPI - Structure Probe, Inc.).
Muscovite mica is a representative of sheet silicate minerals and
provides a layered structure of aluminum silicate sheets weakly bound
by layers of potassium ions. Each layer is characterized by a high
symmetry direction identified by parallel aligned surface grooves.
Between the individual sheets, the high symmetry direction alternates
by 120° leading to a periodic αβαβ stacking sequence along the (001)
direction.13 Immediately after cleaving, the mica substrates were
transferred to the hot wall epitaxy (HWE) chamber.
The HWE technique was applied for the deposition of the organic
material, which allows one to perform the growth process close to
thermodynamic equilibrium and as a further consequence relatively
high vapor pressure of the organic deposit in the substrate region can
be achieved. Therefore, the requirements concerning vacuum
conditions are reduced as compared to, for example, molecular
beam epitaxy.29 The source materials p-6P and NNN were purified
twice by thermal sublimation before they were filled into the quartz
tube of the HWE reactor. The deposition was performed thereafter
under a base pressure of 9 × 10−6 mbar.
For the deposition of NNN/p-6P heterostructures, p-6P nanofibers
were fabricated by the deposition of p-6P (TCI) source material. The
optimized evaporation temperature for p-6P is given at 240 °C leading
to a nominal growth rate of 2 nm/min. It has to be stated that the
nominal layer thickness is defined as the average fiber height within the
present article. In order to avoid temperature gradients during growth
and to reduce adsorbed species on the surface, the substrate was
preheated at 120 °C for 30 min. After p-6P was deposited for 30 min
(≅60 nm fiber height), the sample was transferred in air to a HWE
chamber providing NNN as the source material. After the evacuation
of the chamber, the p-6P template was preheated at 80 °C for 20 min.
Subsequently, NNN was deposited for 20 min at 80 °C.
Morphological Investigation. Scanning force microscopy (SFM)
studies of the deposited organic films were performed using a Digital
Instruments Dimension 3100 in the tapping mode. The 10 × 10 μm2
images were acquired at scan speeds of 4−6 μm/s using SiC tips
exhibiting a cone angle of 40°. Nominal values for resonance frequency
and tip radius are 325 kHz and 10 nm, respectively. For better
readability, height information on the SFM image was normalized (h =
0 nm) to the mean height value correlated with the muscovite mica
substrate surface.
X-ray Diffraction Experiments. X-ray diffraction (XRD)
measurements were carried out on a Philips X’pert X-ray
diffractometer using Cr Kα radiation (= 2.29 Å) and a secondary
graphite monochromator. Specular scans were performed in Bragg−
Brentano configuration by varying the z-component of the scattering
vector q. Consequently it is possible to detect lattice planes which are
parallel to the sample surface. X-ray diffraction pole figure measurements were performed in a Schultz reflective geometry.30 Pole figures
are acquired by measuring at a constant length of q and only varying its
direction. The unit cell parameters of NNN, which are used for
analysis are defined by a = 8.1478 Å, b = 5.978 Å, c = 19.452 Å, and β
= 94.58°.31 For the analysis of p-6P/NNN bilayers, the following unit
cell parameters of p-6P were used: a = 8.091 Å, b = 5.568 Å, c = 26.241
Å, and β = 98.17°.32,33
Force Field Simulations. In order to obtain the data for the
presented force field calculations a self-written C program was
Figure 1. (a) Sketched influence of substrate surface mirror symmetry
on the molecular adsorption. (i) In general, mirror symmetry leads to
a doubling of the energetically equivalent molecular adsorption sites.
Consequently, the orientation of the long molecular axis (LMA) and
LMA*, which is defined by the orientation of the mirrored twin
molecule, is aligned not parallel, e.g., sexi-thiophene (6T) deposited on
muscovite mica. (ii) In contrast, the molecular adsorption of phexaphenyl (p-6P) represents an extraordinary configuration. As the
LMA is oriented normal to the mirror plane, the LMA* coincides. (iii)
By using p-6P nanofibers for organic−organic heteroepitaxy 6T
molecules can be forced to follow the molecular alignment within the
organic template. (b) The chemical structure of 2,2′:6′,2″-ternaphthalene (NNN) and p-hexaphenyl (p-6P). The orientation of the long
molecular axis (LMA) is also indicated.
performance but also to enlarge the scope of possible
applications.17,18 In that sense, recent results based on the
6T/p-6P system have demonstrated a successful implementation of type-I heterojunctions19 to tune the emission color20
and to broaden the wavelength range21 of highly polarized
fluorescent organic nanofibers.
On the basis of these results, the question arises if the chosen
concept of organic−organic heteroepitaxy on p-6P nanofibers
can be extended to a wider spectrum of molecular species. A
positive answer would certainly open novel perspectives for the
realization and improvement of functional devices. In that sense
a possible utilization of molecular engineering to tune the
material properties22,23 could further broaden the covered
lasing range or allow the implementation of photovoltaic
devices by moving to type-II organic−organic heterojunctions.24 Consequently, the main motivation of the present
paper is to discuss the potential of heteroepitaxy on p-6P
template fibers as a general concept for the fabrication of
organic−organic heterojunctions.
When taking a look at the terminating surface of a p-6P
(111̅) stack, strong surface corrugation becomes visible, which
is generated by edge-on and tilted p-6P molecules. On the basis
of this observation, it can be assumed that not only 6T but in
general chain-like carbon-based molecules should have a strong
affinity to adsorb along these corrugations and consequently
parallel to the p-6P molecules of the organic template. In order
to verify the latter hypothesis, we have selected NNN as a
proper candidate. On the one hand, analogous to 6T4,12 the
epitaxial growth of NNN on plain muscovite mica (001) yields
the formation of multidirectional oriented, fiber-like crystallites.25 On the other hand, NNN is characterized by a chain-like
molecular structure. As indicated in Figure 1b, the molecule is
built up by three naphthalene units, which should also
complement and contrast reported investigations on thiophenes.15,16,26
On the basis of a combined experimental and theoretical
approach, the epitaxial growth of NNN on p-6P template fibers
is investigated. All methods provide a consistent picture and
serves as basis to answer the raised question concerning a
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compiled. The algorithm is based on force field parameters, which are
taken from the universal force field.34 Moreover, a dominant van der
Waals interaction was assumed and is expressed by a Lennard−Jones
6-12 type potential. The atomic distances for the p-6P surface are
deduced from its equilibrium bulk structure.33
For the presented adsorption data of NNN, the binding energy for
each combination of φx = {− 180°, −175°, ...180°}; φy={−20°, −16°,
...20°}; φz = {0°, 5°, ..180°} was optimized. This has been done by the
following algorithm. (1) The molecule is positioned at h = 10 Å above
the p-6P (111)̅ template layer. (2) The molecule is moved within the
substrate surface unit cell (100 × 100 points) and for each position the
molecular adsorption energy is calculated, yielding Ead(φx, φy, φz, x, y,
h). (3) In order to optimize the distance to the substrate surface, the
molecule is continuously approached with steps of Δh = −0.1 Å, and
the described procedure for calculating Ead(φx, φy, φz, x, y, h) is
repeated. The algorithm is stopped if all values Ead(φx, φy, φz, x, y, h)
have passed a minimum in adsorption energy versus adsorption height
h. (4) Finally Ead(φx, φy, φz) is obtained by searching the minimum of
Ead for the lateral degrees of freedom (x, y, h).
A detailed discussion concerning the force field simulations can be
found in the Supporting Information.
Polarization Dependent Photoluminescence. Samples were
excited at 370 nm wavelength by a frequency-doubled Ti:sapphire
oscillator operating at 82 MHz repetition frequency. The optical
emission was analyzed by a rotating linear polarizer, and emission
spectra were measured using a single 500 mm grating spectrometer
equipped with a liquid-N2-cooled CCD. The optical detection
apparatus placed after the linear polarizer was assessed to have a
negligible polarization sensitivity.
■
the p-6P template fibers, height levels at about 185 nm can be
interpreted as fingerprints for nearly parallel oriented, rectangularshaped NNN crystallites, which have nucleated on top of p-6P
nanofibers. As analogous film morphologies are also reported for 6T/
p-6P nanofibers,15 film morphology already hints toward the successful
implementation of organic−organic heteroepitaxy as a means to gain
control of the azimuthal molecular adsorption.
Polarization Dependent Optics. In order to substantiate
morphological investigations, polarization-dependent photoluminescence (PL) measurements were chosen as a next step. Polarization
resolved PL allows for analyzing the molecular orientation on the basis
of Malus’ law; that is, the emitted PL intensity is proportional to
cos2(Δφ), where Δφ is the angle between the in-plane component of
the molecular transition dipole axis and the polarizer axis. On the basis
of the latter concept, the degree of optical anisotropy should be
compared between NNN deposited on plain muscovite mica and p-6P
template fibers.
PL intensity spectrograms versus wavelength and polarizer axis’
angle are reported in Figure 3. Panels a and b relate to NNN fibers
directly deposited on muscovite mica, whereas panels c and d relate to
NNN on p-6P template fibers. The zero angle of the polarization was
set in correspondence to the minimum emission intensity. The
emission intensity reaches its maximum value when the polarizer is
turned by 90°. PL spectra measured at 0° and 90° are shown in the top
panels. NNN fibers exhibit a complex vibronic emission structure.
Owing to the herringbone packing, implying side-by-side interactions
between nearest neighboring molecules, NNN crystals are inferred to
be H aggregates. The weak band at ≈375 nm wavelength is thus
attributed to the 0−0 (pure electronic) transition, while the peaks at
≈397 nm, ≈424 nm, and ≈455 nm are vibronic replicas generated by
the aromatic C−C stretching mode. Additional replicas are present in
between the main vibronic peaks, arising from additional phonon
modes weakly coupled to the electronic transitions.
The pure NNN PL spectrum measured at 90°, SNNN(λ), and the
one of p-6P measured instead in a reference p-6P nanofibers’ sample at
the same polarization angle, Sp‑6P(λ), are used as references in the
analysis of the angle dependence of the NNN and p-6P emissions. For
each polarization angle, the overall PL spectrum is fitted with a linear
combination of the two reference spectra:
EXPERIMENTAL RESULTS
Epitaxial Growth of NNN on p-6P Nanofibers. In a first step
≈60 nm height p-6P template fibers were grown on muscovite mica
(001), and subsequently NNN was deposited for 20 min at the same
substrate temperature.
A scanning force microscopy (SFM) analysis of the fabricated
organic−organic heterostructure is reported in Figure 2. Whereas the
S(λ) = ANNNS NNN(λ) + A p ‐ 6pSp ‐ 6p(λ)
and the fit values of the ANNN, Ap−6P weights are reported in the PL
polar plots (Ap−6P = 0 for NNN fibers on muscovite mica). It turns out
that the overall azimuthal order of NNN on muscovite mica is weak
and that p-6P template fibers tend to align NNN molecules parallel to
the p-6P ones, thereby increasing their azimuthal order. Assuming (i)
that the optical transition dipoles are parallel to the LMAs and (ii) that
zero angle of the polarization coincides with the mirror symmetry
plane of muscovite mica, a quantitative analysis of the polar plots of
the PL intensities is done by fitting the following function to the data:
Figure 2. Scanning force microscopy (SFM) image of NNN/p-6P
nanofibers fabricated on muscovite mica. As indicated by the extracted
height level histogram, three characteristic structural morphologies can
be observed and are labeled by arrows. The area marked by p-6P (61
nm) can be correlated to p-6P nanofiber templates. Height levels,
which are labeled as NNN (185 nm), indicate a preferred nucleation of
NNN crystallites on top of p-6P, whereas no organic material is
observed in between the fibers.
f (ϕ) = A[cos2(ϕ − δ) + cos2(ϕ + δ)]
= A[1 − cos(2δ) + 2 cos(2δ) cos2(ϕ)]
where δ represents the angle between the LMA and the mirror
symmetry plane of muscovite mica. The signal amplitude (A) and δ
are free parameters of the fit. For NNN deposited on muscovite mica,
best fit yiels δNNN,mica = 51°. This result of the fit nicely agrees with the
±49° LMA(*)NNN orientations deduced from structural investigations
reported elsewhere.25 As for NNN deposited on p-6P template fibers,
the highly polarized p-6P emission is best fitted with δp−6P = 74°, and
is in very good agreement with the value of ≈80° inferred by XRD
analysis;13 the smaller angle value δNNN,p‑6P = 61° obtained for NNN is
attributed to the poor fit of the NNN/p-6P emission spectral function,
which is in turn traced back to the fact that the NNN and p-6P spectra
used for spectral decomposition were measured in different samples. A
possible explaination for the observed deviation could originate from
different crystal orientations35 of NNN on p-6P template fibers and
plain mica, NNN/mica was used as reference sample. Indeed, a
sample morphology of NNN deposited on plain muscovite mica (001)
was dominated by multidirectional nanofibers and island-like
structures,25 the SFM analysis of the fabricated p-6P/NNN bilayers
now reveals only the presence of highly parallel crystallites. As
indicated in the right panel of Figure 2, three peaks appear at 0 nm
(muscovite mica substrate), 61 nm (p-6P) and 185 nm (NNN) when
extracting a height histogram from the SFM data. For a better
comparison, corresponding areas are labeled in the SFM image by
white circles. Whereas the height levels at ≈61 nm can be attributed to
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Figure 3. Polarization-dependent photoluminescence (PL) intensity spectograms versus wavelength and polarizer axis’ angle are reported in panel
(a, b) for NNN fibers deposited on plain muscovite mica and (c, d) on p-6P template fibers. PL spectra acquired for minimum intensity (0°) and
maximum intensity (90°) are extracted and plotted by black solid lines in the top panel of (a, c). The spectral emission for the p-6P/NNN bilayer
structure (c, d) is characterized by contributions from both, p-6P template fibers and NNN crystallites. In order to separate both components, the
spectral emission is fitted (red solid line) by weighted superposition of the p-6P (blue line) and NNN (green line) reference spectra. The obtained fit
values of the NNN and p-6P weights are reported in the PL polar plots in panel (b, d). Whereas the PL of NNN on plain mica (b) is characterized
by relatively weak polarization, the p-6P/NNN heterostructure reveals high polarization for both molecular components.
different contact plane is found by structural investigations, which is
discussed in the next section. Moreover, PL measurements verify an
increased optical anisotropy of NNN/p-6P bilayers in comparison to
NNN fibers fabricated on plain muscovite mica, which can be stated by
|45° − δNNN,mica| < |45° − δNNN,p‑6p|. Similar values for δp‑6P and
δNNN,p‑6P further underline a nearly parallel orientation of both
molecular species, forced by organic−organic heteroepitaxy.
Structural Investigations. In the next step, X-ray diffraction
(XRD) was chosen, and Θ/2Θ scans were acquired on NNN/p-6P
organic−organic heterostructures. As an example, an obtained
diffraction pattern for 120 nm/60 nm height NNN/p-6P nanofibers
is reported in Figure 4a. The obtained pattern is dominated by a series
of {00.2n} diffraction peaks (indicated by arrows and a black solid
circle) stemming from the muscovite mica (001) substrate.
In contrast to the specular XRD scan for NNN deposited on plain
mica,25 no diffraction peak at qz = 0.324 Å−1 can be observed anymore.
Consequently, no traces of {001}NNN oriented NNN crystallites are
found. As such crystallites would be characterized by an island-shaped
surface morphology, the obtained XRD data are perfectly consistent
with SFM analysis presented in Figure 2. In contrast, a peak arises at qz
= 1.38 Å−1 which is characteristic for {111}̅ oriented p-6P crystallites,13
and a small shoulder becomes visible in between the muscovite mica
(004)M and p-6P (111)̅ p‑6P diffraction peak at qz = 1.31 Å−1. The
observed position correlates well with the {110}NNN diffraction peak of
NNN as indicated by the arrow in the bottom part of Figure 4. In
contrast, no contributions of the {111}NNN diffraction peak (qz = 1.36
Å−1) can be detected anymore, which hints at a different adsorption
mechanism and orientational geometry of NNN as compared to plain
muscovite mica.25 A more detailed analysis can be found in Supporting
Information.
In order to study the azimuthal alignment of the organic crystallites,
XRD-PFs were acquired, and two representative diffraction patterns
are reported in Figure 4b,c. Whereas the XRD-PF in Figure 4b was
taken with a maximum sensitivity to {111}NNN diffraction peaks (q =
1.36 Å−1), {201}NNN netplanes (q = 1.61 Å−1) are probed in the polefigure of Figure 4c. For both measurements, the acceptance angle of
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the XRD setup additionally allows the detection of diffraction peaks,
which stem from p-6P crystallites. In particular, the setup used for
collecting the data presented in Figure 4b is also sensitive to diffraction
intensities which originate from scattering at their {111̅}p‑6P (q = 1.38
Å−1) netplanes. In contrast, contributions of {203̅}p‑6P diffraction peaks
(q = 1.63 Å−1) can be found in the XRD-PF presented in Figure 4c.
Moreover, diffraction peaks originating from the muscovite mica (001)
and indicated by red circles allow an unique determination of the
substrate’s alignment.
Before discussing the diffraction pattern obtained by XRD-PF
measurements, the main properties of a (111̅) oriented p-6P crystallite
should be summarized: The fast growth direction or long needle axis
(LNA) of the generated nanofibers is characterized by the zone axis of
the p-6P (001) low energy plane and (111̅) contact plane and
consequently defined by the [11̅0] crystallographic direction.14 In
contrast, the long molecular axis (LMA) can be approximated by the
[302] direction of p-6P.14 The LMA is slightly tilted by ≈5° out of the
contact plane, and p-6P molecules do not pack perfectly orthogonal to
the LNA but are tilted by ≈15°.13 Both properties are indicated in
Figure 5 by a sketch of the p-6P fiber. The red box symbolizes a
perfect cuboid oriented along the LNA to stress the generated oblique
angles of the p-6P molecules within the stack, indicated by yellowshaded objects. Further details including a graphical sketch of the
molecular alignment and geometry of p-6P (111)̅ and NNN (110) are
summarized in the Supporting Information.
In a further step, Figure 4 visualizes the geometrical alignment of
poles (red/blue squares) scanned by XRD-PF measurements. Whereas
poles stemming from scattering at {111}̅ netplanes are located at Ψ =
0° and Ψ = 69.8°, surface normals derived from {203̅} planes cut the
upper hemisphere at Ψ = 55.5°. Crystallographic directions
representing the LMA [302] and LNA [11̅0] orientation are also
indicated by red crosses. Beside each polar plot the corresponding real
space model of the p-6P stack is sketched.
When comparing the simulated pole distribution in Figure 5 (1)
with the experimental results a nice correlation with the observed
diffraction peaks can be observedmarked by blue ovals (1) in Figure
4b,c. Whereas the (1̅11) diffraction peak is located on the left side of
the muscovite mica [11̅ 0̅ ]M crystallographic direction in Figure 4b,
traces from scattering at (203̅) planes can be found on the opposite
side as shown in Figure 4c.
In a next step the p-6P crystal is rotated by 180° around the growth
direction. Both the obtained polar plot as well as the crystal sketch are
presented in Figure 5 (2). When comparing the obtained diffraction
Figure 4. (a) Specular X-ray diffraction (XRD) scan, acquired on
NNN/p-6P nanofibers deposited on muscovite mica (001). The
obtained diffraction pattern is dominated by a series of {00.2n}
diffraction peaks stemming from the muscovite mica (001) substrate
(indicated by arrows and a black solid circle). In comparison to NNN
on plain muscovite mica, no traces of {001}NNN-oriented NNN
crystallites can be observed at qz = 0.324 Å−1. Diffraction peaks at qz =
1.38 Å−1 and 1.31 Å−1 can be attributed to {111}̅ p‑6P and {110}NNN
crystallites, respectively. The panels below depict XRD-PF measurements probing the orientation of {111}NNN (b) and {201}NNN
netplanes (c). The acceptence angle of XRD setup allows a
simultanous sensitivity to diffraction at {111}̅ p‑6P (b) and {203}̅ p‑6P
(c) netplanes. Diffraction spots, which allow a unique determination of
the muscovite mica substrate’s orientation, are marked in (b) by red
circles.
Figure 5. Simulated orientation of {111̅} and {203̅} poles (red/blue rectangles) and ⟨302⟩ (LMA), ⟨11̅0⟩ (LNA) directions (red/blue crosses),
which can be obtained by 180° rotation of the p-6P stack either along the growth- (vertical) or LNA-direction (horizontal). A 180° rotation along
the LNA flips the upper and lower hemisphere and the stack changes from a (111̅) to a (1̅1̅1) orientation, which is indicated by a blue box of the
corresponding modeled stack. To underline that also all detected poles in the upper hemisphere change their sign, blue symbols are chosen in the
corresponding pole figures.
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molecules are tilted by ≈14° out of the contact plane and do not pack
perfectly orthogonal to the LNA axis but are tilted by an angle of
≈22°. A more detailed discussion can be found in the Supporting
Information. In order to distinguish the modeled NNN stack from p6P* crystallites, tilted NNN molecules are typified by cyan-shaded
objects in Figure 6.
pattern in Figure 4b,c with the simulated alignment of (1̅11) and
(203̅) poles, it can be concluded that in contrast to the previously
discussed crystal orientation (1), much weaker diffraction patterns,
marked by blue ovals (2), can be correlated. At this point it should be
stated that the intensity distribution is consistently observed for {111̅}
and {203̅} diffraction peaks in Figure 4b−c and also in agreement with
reported XRD-PFs in the literature.13,15 Consequently, we exclude
possible alignment effects being responsible for the obtained intensity
distribution and correlate the observed intensity variation with a non
equal fraction of both crystal geometries. Indeed, neither the
muscovite mica (001) substrate surface11,14 nor the (111̅) oriented
p-6P substrate surface unit cell provide 2-fold rotational symmetry
which explains the observed behavior. Nevertheless, as XRD-PFs
underline the presence of both crystal configurations, the adsorption
energy of type (2) crystals seems only slightly different in contrast to
type (1).
Analogous to the previously discussed rotational operation, the
crystal type (2) is now rotated by 180° around the LNA orientation.
The resulting pole Figure (1*) and model stack is presented in the
bottom right panel of Figure 5. As a 180° rotation along the LNA flips
the upper and lower hemisphere, the p-6P stack changes from a (111̅)
to a (11̅ 1̅ ) orientation. For a bettervisualization, also a blue box is
chosen for the corresponding modeled stack. Certainly, also detectable
poles in the upper hemisphere are subject to this operation which
becomes visible by blue symbols and an inversion of the corresponding
indices in Figure 5 (1*). As the LNA represents the zone axis of the p6P contact- and low-plane, it is aligned at Ψ = 90° and consequently
not subject to any changes. In contrast, the ≈5° tilt of the LMA
relative to the contact plane is responsible for the fact that the [302]
crystallographic direction now points in the direction of the lower
hemisphere. The latter observation is also indicated in the sketched
model stack in Figure 5 (1*).
Taking a closer look to the original crystal configuration in Figure 5
(1) and the previously discussed geometry (1*) further reveals an
interesting geometrical correlation. Obviously, the double rotational
operation led to the generation of the mirror symmetric crystal type
where the mirror plane is spanned up by the growth direction and the
vertically aligned LNA axis. The latter observation also explains the
chosen labeling by an asterisk to indicate the mirror symmetric crystal
types. The simulated pole figures further reveal that mirror symmetry
is also reflected by the alignment of the netplanes and crystallographic
orientations. At this point it should be stated that the generation of
mirror symmetric crystallites by a double rotation additionally requires
the presence of inversion symmetry, which becomes visible by an
alternating blue/red color of the mirror symmetric poles. As both
discussed organic crystal types, namely, p-6P and NNN, possess an
inversion center, and analogous considerations also hold true for
NNN.
A comparison of the observed diffraction pattern (marked by oval
1*) in Figure 4b,c and the geometrical alignment of the p-6P poles
presented in Figure 5 (1*) again reveals a nice geometric overlap.
Additionally and in contrast to crystal types (2), also the detected
diffraction intensities correlate between (1) and (1*) crystal types.
The latter observation can be explained by the presence of a mirror
symmetry plane along [11̅ 0̅ ]M for an α-terminated surface,13,14 which
is indicated by a vertical solid line in Figure 4b,c. Consequently,
energetically equivalent adsorption sites must be found for crystals of
type (1) and (1*). Certainly, analogous considerations hold true for
crystals of type (2) and (2*), which can be verified by comparing the
acquired XRD-PFs with Figure 5 (2, 2*). In order to increase
readability, only one crystal type will be discussed in the next
paragraphs, and it should only be stated that a mirror symmetric twin
exists, for both NNN and p-6P, which is characterized by an inverse
orientation, e.g., (1) p-6P (111̅) and (1*) p-6P (1̅1̅1). A detailed
analysis can be found in Supporting Information.
In a final step, the azimuthal alignment of a (110) oriented NNN
crystallites on top of a (1̅1̅1) p-6P* template should be discussed.
Analogous to previous analyses for p-6P, the LMA of NNN (110) can
be approximated by the [101̅] crystallographic direction, whereas the
LNA is characterized by [11̅0].25 In the latter configuration, NNN
Figure 6. Sketch of (110) oriented NNN crystallites (red box) on top
of a (1̅1̅1) p-6P* template (blue box). Configuration (a) is
characterized by a parallel orientation of [11̅0]NNN relative to
[110̅ ]p-6P. Panel (b) sketches the crystal alignment obtained by a
180° rotation along the growth direction. As indicated by the cyanshaded objects, additionally to a tilt out of the contact plane, the LMA
of NNN is not perfectly orthogonal to the LNA but ≈22° tilted.
Simulated orientations of {201} and {111} poles (red/blue hexagons)
and [101̅] (LMA), [11̅0] (LNA) directions (red crosses) of NNN are
also indicated. The blue cross marks the molecular orientation of a
(1*) p-6P* crystallite. As underlined by the graphical sketch of the p6P* template (blue) and NNN crystal (red), only one configuration of
the NNN crystallite (b) fits the template concerning the molecular tilt
out of the contact plane.
Again, simulated poles for two, azimuthally 180° rotated
configurations of NNN crystallites are presented in Figure 6a,b.
Whereas poles stemming from scattering at {111} netplanes of NNN
are located at Ψ = 13.8° and Ψ = 74.1°, diffraction patterns attributed
to {201} planes are located at Ψ = 54.5°. Crystallographic directions
representing the LMA [101̅] and LNA [11̅0] orientation are also
indicated by red crosses.
By comparing the diffraction pattern, which was acquired with
maximum sensitivity to (201) diffraction peaks, presented in Figure 4c,
with the simulated poles in Figure 6 at Ψ = 54.5° it can be concluded
that also NNN crystallites follow 1-fold symmetry as discussed for the
p-6P template fibers. Strong diffraction peaks stemming from organic
crystallites can only be detected in the lower part of Figure 4c and are
marked by (1, 1*).
NNN crystal types (2) do not follow the template geometry, which
can be underlined by taking a closer look to the LMA orientations of
p-6P* [3̅02̅] and NNN [101̅], indicated in the simulated pole figure of
Figure 6a. Whereas the corresponding red (NNN) and blue (p-6P*)
crosses are aligned in an azimuthally nearly parallel configuration
(indicated by a green line), they are located in an opposite sector
which correlates with a different molecular tilt angle out of the contact
plane. This is also sketched by the real space model in Figure 6a.
In contrast, NNN crystal types (1) which correlate with strong
diffraction peaks in Figure 4b−c lead to an alignment of both
crystallographic directions within the same sector of the pole figure
(indicated by a green oval in Figure 6b). The latter observation
correlates with a perfect geometrical adoption of the NNN molecules
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Figure 7. (a) Simulated top view onto the molecular alignment of a NNN/p-6P* heterostructure. In order to obtain a parallel alignment of the long
molecular axes (LMA), the long needle axes (LNA) are characterized by a tilt of ≈3.5° relative to each other. Corresponding surface unit cells and
vectors are also indicated for both crystal types. (b) Simulated side view onto a heterostructure demonstrating the herringbone stacking at the NNN/
p-6P* interface.
6P (111̅)15 prevents the nucleation of standing molecular
configurations at their side walls.
(II) We demonstrated by using a combined approach of
polarization-dependent optics and XRD-PF analysis that NNN
molecules tend to adopt the geometrical molecular alignment
of the p-6P template layer, which is characterized by
(1) A preferred azimuthal parallel molecular alignment
In order to investigate the molecular adsorption mechanism
in more detail and to estimate the potential of organic−organic
heteroepitaxy concerning a precise control on the azimuthal
molecular order, force field simulations were performed. In
particular, the adsorption energy Ead of a rigid, single NNN
molecule on a p-6P (111̅) terminated surface is probed as a
function of three lateral (x,y,z) and angular degrees of freedom
(φx, φy, φz). While φz is representative for the azimuthal
adsorption geometry of NNN, φx describes the herringbone
angle and φy the molecular inclination relative to the substrate
surface, respectively. A graphical representation of a NNN
molecule and the orientation of the chosen rotational axes is
indicated in the bottom right of Figure 8a.
On the basis of the performed simulations, Figure 8a depicts
a color-coded representation of Ead versus φz and φx. As an
adsorbed NNN molecule is characterized by 2-fold symmetry,
the angular range for φz is selected from 0° to 180°, whereas a
missing mirror plane explains the presentation of φx by
choosing the full angular range. As indicated by the blue shaded
areas, which characterize energetical preferable adsorption
geometries, pronounced minima for Ead can be found at φz =
105°. It should be stressed that these minima are independent
from the probed herringbone angle φx (dotted white line).
For a better visualization, the bottom panel of Figure 8a
depicts the most favorable Ead versus φz, yielding a deep global
minimum at φz = 105°. As sketched in Figure 8b(A), φz is
defined relative to the LNA of the p-6P stack, namely, the
crystallographic [11̅0] orientation. Consequently, the energetic
minimum coincides with a parallel alignment of the NNN’s
LMA relative to the p-6P molecules. The latter observation is
perfectly consistent with polarization-dependent PL and XRDPF analysis and can be interpreted by the presence of strong
surface corrugations of the p-6P template layer (more details
see Supporting Information). As analogous observations are
also reported for 6T/p-6P,15,20 it can be concluded that carbonbased, rod-like molecules seem to provide a strong tendency to
adsorb within the corrugation of the p-6P template layer. This
certainly underlines the high potential of organic−organic
relative to the p-6P template for both, the azimuthal orientation and
the molecular tilt angle out of the contact plane, as depicted by a real
space model in Figure 6b.
On the basis of the XRD-PF analysis (a detailed analysis is
presented in Supporting Information), a top view onto the NNN/p6P* interface is deduced and depicted in Figure 7a. The real space
model visualizes a parallel alignment of both molecular species, which
perfectly correlates with polarization-dependent PL analysis presented
in Figure 3c,d. The surface unit cells and vectors are also indicated,
yielding an ≈3.5° tilt of [11̅0]NNN relative to [11̅0]p‑6P. As both
crystallographic directions characterize the crystals’ LNA the obtained
molecular packing is consistent with the observed needle-like
morphology, presented in Figure 2. Red- (NNN) and blue- (p-6P)
crosses further indicate the heads of the LMA which point in the upper
hemisphere.
Figure 7b finally sketches a side view onto the NNN/p-6P*
interface, and it can be shown that the obtained crystal configurations
are also characterized by a nearly perfect adoption of the herringbone
stacking sequence (see also Supporting Information). Moreover, the
obtained crystal geometry results in an adoption of the NNN
molecular (10.09 Å) stacking period relative to the p-6P template
(9.82 Å), yielding a mismatch of ≈2.7%.
■
DISCUSSION
On the basis of the presented optical, morphological and
structural analysis, a clear molecular picture emerges on how p6P templates NNN versus using plain muscovite mica (001). In
particular, the following conclusions can be drawn:
(I) Morphological and structural analysis revealed that the
nucleation of (001) oriented NNN crystallites25 can be
prohibited by introducing a p-6P template layer. Consequently,
the formation of an island-shaped surface morphology, due to
the nucleation of NNN in a standing molecular configuration,
can be suppressed and only lying molecular configurations are
formed. These crystal types coincide with a needle shaped
morphology. The latter observation is further consistent with
other reports on p-6P/6T organic−organic heterostructures,4,15
and the results presented herein are relevant to the design of
optoelectronic devices as the emission efficiency is directly
related to the alignment of the optical transition dipole.36,37
Most likely, the observed behavior can be attributed to the
geometrical alignment of the p-6P low energy facets relative to
the substrate surface. In that sense, the nucleation of standing
molecular configurations of 6T4 and NNN25 is traced back to
ledge-directed epitaxy at the relatively strongly tilted side facets
of needle -shaped crystallites. In contrast, the formation of a
nearly rectangular cross-sectional needle shape in the case of p5725
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is characterized by a clear minimum at φx = 10° (labeled as B).
For a better visualization, real space models of the selected
adsorption geometries (A−B) are presented in Figure 8b.
Whereas edge-on NNN molecules (φx = 90°) represent the
least favorable configuration (Ead = −1.63 eV), force-field
simulations yield the best adsorption geometry for an
approximately flat-on molecule (φx = 10°, Ead = −2.59 eV).
Again, the obtained result can be understood by a maximization
of the molecular contact area.
It should be stated that calculations for a single NNN
molecule do not take into account molecule−molecule
interactions, which certainly play a major role when increasing
the molecular surface coverage. In that sense, the observed
discrepancy to the expected stacking sequence, visualized in
Figure 7b, can be explained. In order to overcome that
drawback, force-field simulations were preformed for a closed
monolayer (ML) of NNN (details see Supporting Information), and the energetically most favorable adsorption geometry
is presented in Figure 8c. Strikingly, the deduced real space
model now reveals a nearly perfect adoption of the NNN’s
herringbone stacking sequence to the p-6P template stack.
In summary, the experimentally observed adopted alignment
of NNN relative to p-6P can be explained by an energetically
preferred adsorption geometry. Certainly, analogous considerations also hold true for other carbon-based rod-like
molecules, and a similar behavior can be expected for a broad
spectrum of organic compounds, which would allow a precise
tuning of the obtained material properties by a proper
molecular selection. Consequently, the observed behavior
underlines the potential of organic−organic heteroepitaxy
based on p-6P fibers for the fabrication of organic
optoelectronic devices. Moreover, it should be stated that the
obtained results implyin contrast to inorganic heteroepitaxy38,39a minor role of lattice match during the formation of
the organic−organic interface but a dominant role of the
molecular adsorption geometry.
(III) NNN molecules tend to crystallize with a different
contact plane on p-6P nanofibers. Whereas a (110) orientation
is found on top of p-6P template crystals, a (111) contact plane
was verified for NNN/muscovite mica (001).25 This observeration is closely connected to the previously discussed geometrical aligment. As indicated in Figure 7 the formation of a
(110) contact plane not only enables a nearly perfect adoption
of the molecular tilt angles relative to the template, but also
provides a nearly parallel molecular stacking sequence along the
LNA. The latter observations imply that NNN molecules do
not have to align across step edges, formed along the LNA of
the p-6P stack. In contrast, a (111) oriented NNN crystallite
cannot fulfill all these criteria simultanously, which correlates
with a disadvantageous adsorption geometry (see Supporting
Information for more details).
(IV) XRD-PF analysis has revealed that not only p-6P
template crystallites lack of a 2-fold rotational symmetry but
also NNN crystals which nucleated on top of the template
fibers. The latter observation allows two valuable conclusions:
(1) Concerning the nucleation of organic crystallites the
atomic configuration of the muscovite mica (001) surfaces must
play a dominant role. Previous investigations11 already
demonstrated that the presence of surface corrugations on
the muscovite mica substrate is sufficient to break a quasi
hexagonal symmetry40 to avoid the formation of multidirectional oriented p-6P nanofibers. Strikingly, experimental
investigation presented herein show that the distortion of the
Figure 8. (a) Color-coded adsorption energy Ead (φx, φz) of a single
NNN molecule on a p-6P (111)̅ template layer. For each data point
the energetically most favorable molecular tilt angle φy has been
chosen. Labels A−B mark selected adsorption geometries which are
sketched as real space models in panel (b). The color-coded
representation indicates a sharp minimum at φz = 105° independent
of the herringbone angle φx. The latter observation is further
underlined by plotting the most favorable Ead versus φz in the bottom
panel. The graph in the right panel presents the adsorption energy Ead
versus herringbone angle φx at the optimized azimuthal orientation
(φz = 105°). The best adsorption geometry is found for an almost flat
lying NNN molecule (φx = 10°) (B). (b) Real space models for the
molecular adsorption of a single NNN (red) for selected herringbone
angles (φx = 90°, 10°). For all representations φz = 105° and φy = 4°
are found to be the energetically most favorable geometries. The
obtained value of φz = 105° coincides with a parallel alignment of the
probed NNN molecule relative to the p-6P molecules within the
template layer. (c) Obtained geometries for a closed monolayer of two
NNN (red) molecules per surface unit cell on a p-6P (111̅) template
stack (blue). A nearly perfect adoption of the NNN’s herringbone
stacking sequence to p-6P is visualized.
heterostructures to gain control of the azimuthal alignment of a
broad spectrum of organic molecules.
(2) An adoption of the molecular tilt out of the contact plane
XRD-PF analysis revealed the presence of NNN/p-6P crystal
pairs where both molecular species are characterized by the
same molecular tilt out of the contact plane. Force-field
simulations, which investigate the adsorption energy Ead versus
φz and φy (see Supporting Information) underline the
experimental findings. In particular, simulations reveal an
energetic minimum for Ead, when NNN molecules are tilted
≈4° to adopt the out of plane tilt of the p-6P template layer.
The observed behavior can be explained by the molecular
planarity and a desirable optimized atomic distance to reach the
global minimum of the Lennard−Jones potential.
(3) The herringbone stacking angles
In order to discuss the influence of the herringbone stacking
angles of the p-6P template on the NNN molecular geometry,
the right panel of Figure 8a presents Ead at the energetically
preferable azimuth (φz = 105°) versus φx. The obtained curve
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muscovite mica (001) surface, which reduces its symmetry to
cm,14 is also sufficient to force a different adsorption energy of
180° azimuthally rotated p-6P crystallites. At this point it
should be mentioned that the observed imbalance of both
crystal types cannot be explained with models implying the
existence of 2-fold rotational symmetry or do not take into
account the atomic configuration within the surface unit cell of
the muscovite mica substrate, e.g., pure lattice match.41,42
(2) As the discussed imbalance is also reflected by NNN
crystallites, which have nucleated on top of p-6P template
fibers, it can be further concluded that also the generated p1
geometry at the p-6P/NNN interface, caused by (a) the
herringbone stacking angles and (b) the tilt of ≈5° of p-6P
molecules out of the contact plane plays a major role during the
crystal nucleation at the organic−organic interface.
Article
ASSOCIATED CONTENT
S Supporting Information
*
A detailed description of the performed force field simulations,
structural investigations, and X-ray pole figure analysis. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work has been financially supported by the Austrian
Science Fund (FWF): P25154 and by the Federal Government
of Upper Austria (Project “Organische Nanostrukturen”).
SUMMARY AND CONCLUSION
By a combined experimental and theoretical approach, NNN/
p-6P organic−organic bilayers were investigated. We successfully demonstrated that all methods yield a consistent picture
which underlines the potential of organic−organic heteroepitaxy to gain control on the azimuthal molecular order of rodlike molecules. In particular we demonstrated that NNN tends
to adopt the geometrical alignment of the p-6P molecules in
the organic template layer. We showed by experiments and
simulations that (1) an azimuthal parallel molecular alignment,
(2) an adoption of the molecular tilt out of the contact plane ,
and (3) an adoption of the herringbone angles is correlated
with an energetically preferable adsorption geometry. In that
sense, force-field simulations underline the major role of strong
surface corrugations from the p-6P template surface, which
should act as highly attractive adsorption sites not only for
NNN. As an azimuthal parallel molecular orientation correlates
with high anisotropy and molecular order, the obtained results
highlight the potential of the chosen strategy for the fabrication
of optoelectronic devices.
Besides a detailed discussion on the formation of the
organic−organic interface, a comparison of the prepared
NNN/p-6P heterostructures with NNN deposited on plain
mica is presented. We showed that the insertion of a p-6P
template layer not only leads to (1) an increased molecular and
optical anisotropy, but (2) the formation of a different crystal
contact plane and (3) the suppression of island shaped crystal
morphologies. In particular, it is shown that only flat lying
molecular configurations of NNN nucleate on top of the p-6P
fibers, which is relevant for the fabrication of optoelectronic
devices.
Finally, X-ray pole figure analysis reveals that both p-6P and
NNN crystallites do not follow 2-fold rotational symmetry.
Consequently, experimental investigations show that the
distortion of the muscovite mica (001) surface is sufficient to
force a different adsorption energy for 180° rotated crystals.
The observed imbalance of both crystal types questions models,
which postulate 2-fold rotational symmetry, e.g., pure lattice
matching.
As the obtained results are perfectly consistent and
consequently complement reports on thiophene-based 6T/p6P heterostructures, we conclude that the concept of organic−
organic heteroepitaxy can be applied to a broader range of
chain-like carbon-based molecules. In doing so, epitaxially
overgrown p-6P template fibers can be seen as a general
concept for the fabrication of organic−organic heterojunctions
and optoelectronic device structures.
■
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