15 research outputs found
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
Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells
The
effects of interface roughness between donor and acceptor in a bilayer
heterojunction solar cell were investigated on a polymer–polymer
system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-<i>alt</i>-benzothiadiazole) (F8BT). Both polymers are known to
reorganize into semicrystalline structures when heated above their
glass-transition temperature. Here, the bilayers were thermally annealed
below glass transition of the bulk polymers (≈140 °C)
at temperatures of 90, 100, and 110 °C for time periods from
2 min up to 250 min. No change of crystallinity could be observed
at those temperatures. However, X-ray reflectivity and device characteristics
reveal a coherent trend upon heat treatment. In X-ray reflectivity
investigations, an increasing interface roughness between the two
polymers is observed as a function of temperature and annealing time,
up to a value of 1 nm. Simultaneously, according bilayer devices show
an up to 80% increase of power conversion efficiency (PCE) for short
annealing periods at any of the mentioned temperatures. Together,
this is in agreement with the expectations for enlargement of the
interfacial area. However, for longer annealing times, a decrease
of PCE is observed, despite the ongoing increase of interface roughness.
The onset of decreasing PCE shifts to shorter durations the higher
the annealing temperature. Both, X-ray reflectivity and device characteristics
display a significant change at temperatures below the glass transition
temperatures of P3HT and F8BT
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 {4̅11} 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 {4̅11} and {100} contact planes unveiled that
they share a common crystallographic direction which is explained
by ledge directed epitaxy
Complex Behavior of Caffeine Crystallites on Muscovite Mica Surfaces
Defined fabrication of organic thin
films is highly desired in
technological, as well as pharmaceutical, applications since morphology
and crystal structure are directly linked to physical, electrical,
and optical properties. Within this work, the directed growth of caffeine
deposited by hot wall epitaxy (HWE) on muscovite mica is studied.
Optical and atomic force microscopy measurements reveal the presence
of caffeine needles exhibiting a preferable alignment in the azimuthal
directions with respect to the orientation of the defined mica surface.
Specular X-ray diffraction and X-ray diffraction pole figure measurements
give evidence that the β-polymorphic form of caffeine forms
on the mica surface. All results consent that caffeine molecules have
an edge-on conformation i.e. minimizing their interaction area with
the surface. Furthermore, the azimuthal alignment of the long caffeine
needle axis takes place along the [11̅0], [100], and [110] real
space directions of mica; needles are observed every 60° azimuthally.
While mica has a complex surface structure with mirror planes and
lowered oxygen rows, the slightly disturbed 3-fold symmetry dictates
the crystal alignment. This is different to previous findings for
solution cast caffeine growth on mica. For HWE the needles align solely
along the mica main directions whereby solution cast needles show
an additional needle splitting due to a different alignment of caffeine
with respect to the surface
Accessing Phase-Pure and Stable Acetaminophen Polymorphs by Thermal Gradient Crystallization
We employ a simple
and reproducible methodology based on thermal
gradient crystallization to access forms <b>I</b> and <b>II</b> of acetaminophen. This methodology provides insight into
the role of heat flux in the control over polymorphism and phase transitions.
We report the crystallization of different polymorphs of acetaminophen
as a function of the thermal gradient parameters (magnitude of the
gradient, sample velocity) in a thin film geometry. The thin-film
samples were displaced at well-defined velocities (1 ≤ <i>v</i> ≤ 75 μm/s) to control both the direction
and the rate of crystal growth. We carried out a detailed structural
analysis combining polarized optical microscopy and X-ray diffraction
(specular and grazing-incidence) to characterize the crystalline forms
isolated by the thermal gradient technique. The resulting polymorphic
forms have been found to exhibit high phase purity and remarkable
stability over time
Icephobic Gradient Polymer Coatings Deposited via iCVD: A Novel Approach for Icing Control and Mitigation
Materials against ice formation and accretion are highly
desirable
for different industrial applications and daily activities affected
by icing. Although several concepts have been proposed, no material
has so far shown wide-ranging icephobic features, enabling durability
and manufacturing on large scales. Herein, we present gradient polymers
made of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane
(V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate
(PFDA) deposited in one step via initiated chemical vapor deposition
(iCVD) as an effective coating to mitigate ice accretion and reduce
ice adhesion. The gradient structures easily overcome adhesion, stability,
and durability issues of traditional fluorinated coatings. The coatings
show promising icephobic performance by reducing ice adhesion, depressing
the freezing point, delaying drop freezing, and inhibiting ice nucleation
and frost propagation. Icephobicity correlates with surface energy
discontinuities at the surface plane resulting from the random orientation
of the fluorinated groups of PFDA, as confirmed by grazing-incidence
X-ray diffraction measurements. The icephobicity could be further
improved by tuning the surface crystallinity rather than surface wetting,
as samples with random crystal orientation show the lowest ice adhesion
despite high contact angle hysteresis. The iCVD-manufactured coatings
show promising results, indicating the potential for ice control on
larger scales and various applications
Initial Steps of Rubicene Film Growth on Silicon Dioxide
The film growth of the conjugated organic molecule rubicene
on silicon dioxide was studied in detail. Since no structural data
of the condensed material were available, we first produced high quality
single crystals from solution and determined the crystal structure.
This high purity material was used to prepare ultrathin films under
ultrahigh vacuum conditions, by physical vapor deposition. Thermal
desorption spectroscopy (TDS) was applied to delineate the adsorption
and desorption kinetics. It could be shown that the initial sticking
coefficient is only 0.2 ± 0.05, but the sticking coefficient
increases with increasing coverage. TDS further revealed that first
a closed, weakly bound bilayer develops (wetting layer), which dewets
after further deposition of rubicene, leading to an island-like layer.
These islands are crystalline and exhibit the same structure as the
solution grown crystals. The orientation of the crystallites is with
the (001) plane parallel to the substrate. A dewetting of the closed
bilayer was also observed when the film was exposed to air. Furthermore,
Ostwald ripening of the island-like film takes place under ambient
conditions, leading to films composed of few, large crystallites.
From TDS, we determined the heat of evaporation from the multilayer
islands to be 1.47 eV, whereas the desorption energy from the first
layer is only 1.25 eV
One Polymorph and Various Morphologies of Phenytoin at a Silica Surface Due to Preparation Kinetics
The preparation of solid crystalline
films at surfaces is of great
interest in a variety of fields. Within this work the preparation
of pharmaceutically relevant thin films containing the active pharmaceutical
ingredient phenytoin is demonstrated. The preparation techniques applied
include drop casting, spin coating, and vacuum deposition. For the
solution processed samples a decisive impact of the solution concentration
and the applied film fabrication technique is observed; particular
films form for all samples but with their extensions along different
crystallographic directions strongly altered. Vacuum deposition of
phenytoin reveals amorphous films, which over time crystallize into
needle-like or particular-type structures whereby a nominal thickness
of 50 nm is required to achieve a fully closed layer. Independent
of all preparation techniques, the resulting polymorph is the same
for each sample as confirmed by specular X-ray diffraction scans.
Thus, morphologies observed via optical and atomic force microscope
techniques are therefore a result of the preparation technique. This
shows that the different time scales for which crystallization is
obtained is the driving force for the various morphologies in phenytoin
thin films rather than the presence of another polymorph forming
Stimulated Emission Properties of Sterically Modified Distyrylbenzene-Based H‑Aggregate Single Crystals
J-aggregation has
been shown to be beneficial for light amplification
in single crystals of π-conjugated organic molecules. In the
case of H-aggregation, the criteria for such processes are still under
debate. It has also been shown that H-aggregate arrangements with
considerable π–π overlap are detrimental for light
amplification. We show here that a proper alignment of the molecules
in the crystal lattice, which minimizes π–π overlap
between adjacent molecules, gives rise to (random) stimulated emission
from cofacial arrangements similar to that of the herringbone aggregates
Substrate-Induced Phase of a [1]Benzothieno[3,2‑<i>b</i>]benzothiophene Derivative and Phase Evolution by Aging and Solvent Vapor Annealing
Substrate-induced
phases (SIPs) are polymorphic phases that are
found in thin films of a material and are different from the single
crystal or “bulk” structure of a material. In this work,
we investigate the presence of a SIP in the family of [1]benzothieno[3,2-<i>b</i>]benzothiophene (BTBT) organic semiconductors and the effect
of aging and solvent vapor annealing on the film structure. Through
extensive X-ray structural investigations of spin coated films, we
find a SIP with a significantly different structure to that found
in single crystals of the same material forms; the SIP has a herringbone
motif while single crystals display layered π–π
stacking. Over time, the structure of the film is found to slowly
convert to the single crystal structure. Solvent vapor annealing initiates
the same structural evolution process but at a greatly increased rate,
and near complete conversion can be achieved in a short period of
time. As properties such as charge transport capability are determined
by the molecular structure, this work highlights the importance of
understanding and controlling the structure of organic semiconductor
films and presents a simple method to control the film structure by
solvent vapor annealing