9 research outputs found
Altering the Polymorphic Accessibility of Polycyclic Aromatic Hydrocarbons with Fluorination
Substituting hydrogen
with fluorine is an extensively employed
strategy to improve the macroscopic properties of compounds for use
in fields as diverse as pharmaceutics and optoelectronics. The role
fluorine substitution plays on polymorphismî—¸the ability of
a compound to adopt more than one crystal structureî—¸has not
been previously studied. Yet, this understanding is important as different
polymorphs of the same compound can result in drastically different
bulk properties (e.g., solubility, absorptivity, and conductivity).
Strategies to either promote or suppress the crystallization of particular
polymorphs are thus desired. Here, we show that substituting hydrogen
with fluorine affects the polymorphic behavior of contorted hexabenzocoronene
(cHBC). A polycyclic aromatic hydrocarbon and molecular semiconductor,
cHBC exhibits two polymorphs (i.e., <i>P</i>2<sub>1</sub>/<i>c</i> crystal structure which we refer to as polymorph
I and a triclinic crystal structure which we refer to as polymorph
II) that are accessible through postdeposition processing of amorphous
films. While the same two polymorphs remain accessible in fluorinated
derivatives of cHBC, fluorination appears to favor the formation of
polymorph I, with progressively smaller energy barrier for transformation
from polymorph II to polymorph I with fluorination
Altering the Polymorphic Accessibility of Polycyclic Aromatic Hydrocarbons with Fluorination
Substituting hydrogen
with fluorine is an extensively employed
strategy to improve the macroscopic properties of compounds for use
in fields as diverse as pharmaceutics and optoelectronics. The role
fluorine substitution plays on polymorphismî—¸the ability of
a compound to adopt more than one crystal structureî—¸has not
been previously studied. Yet, this understanding is important as different
polymorphs of the same compound can result in drastically different
bulk properties (e.g., solubility, absorptivity, and conductivity).
Strategies to either promote or suppress the crystallization of particular
polymorphs are thus desired. Here, we show that substituting hydrogen
with fluorine affects the polymorphic behavior of contorted hexabenzocoronene
(cHBC). A polycyclic aromatic hydrocarbon and molecular semiconductor,
cHBC exhibits two polymorphs (i.e., <i>P</i>2<sub>1</sub>/<i>c</i> crystal structure which we refer to as polymorph
I and a triclinic crystal structure which we refer to as polymorph
II) that are accessible through postdeposition processing of amorphous
films. While the same two polymorphs remain accessible in fluorinated
derivatives of cHBC, fluorination appears to favor the formation of
polymorph I, with progressively smaller energy barrier for transformation
from polymorph II to polymorph I with fluorination
Post-deposition Processing Methods To Induce Preferential Orientation in Contorted Hexabenzocoronene Thin Films
The structuring in organic electrically active thin films critically influences the performance of devices comprising them. Controlling film structure, however, remains challenging and generally requires stringent deposition conditions or modification of the substrate. To this end, we have developed post-deposition processing methods that are decoupled from the initial deposition conditions to induce different out-of-plane molecular orientations in contorted hexabenzocoronene (HBC) thin films. As-deposited HBC thin films lack any long-range order; subjecting them to post-deposition processing, such as hexanes-vapor annealing, thermal annealing, and physical contact with elastomeric poly(dimethyl siloxane), induces crystallization with increasing extents of preferential edge-on orientation, corresponding to greater degrees of in-plane π-stacking. Accordingly, transistors comprising HBC thin films that have been processed under these conditions exhibit field-effect mobilities that increase by as much as 2 orders of magnitude with increasing extents of molecular orientation. The ability to decouple HBC deposition from its subsequent structuring through post-deposition processing affords us the unique opportunity to tune competing molecule–molecule and molecule–solvent interactions, which ultimately leads to control over the structure and electrical function of HBC films
Tuning Polymorphism and Orientation in Organic Semiconductor Thin Films via Post-deposition Processing
Though both the crystal
structure and molecular orientation of
organic semiconductors are known to impact charge transport in thin-film
devices, separately accessing different polymorphs and varying the
out-of-plane molecular orientation is challenging, typically requiring
stringent control over film deposition conditions, film thickness,
and substrate chemistry. Here we demonstrate independent tuning of
the crystalline polymorph and molecular orientation in thin films
of contorted hexabenzocoronene, c-HBC, during post-deposition processing
without the need to adjust deposition conditions. Three polymorphs
are observed, two of which have not been previously reported. Using
our ability to independently tune the crystal structure and out-of-plane
molecular orientation in thin films of c-HBC, we have decoupled and
evaluated the effects that molecular packing and orientation have
on device performance in thin-film transistors (TFTs). In the case
of TFTs comprising c-HBC, polymorphism and molecular orientation are
equally important; independently changing either one affects the field-effect
mobility by an order of magnitude
Understanding Heterogeneous Nucleation in Binary, Solution-Processed, Organic Semiconductor Thin Films
Heterogeneous nucleation is often the precursor to crystallization
in solution-processed organic semiconductor thin films. Here, we study
the efficacy of a series of nine small-molecule organic semiconductor
additives in seeding the crystallization of solution-processable triethylsilylethynyl
anthradithiophene (TES ADT). By systematically varying the concentrations
of the additives in TES ADT thin films, we found the tendency of the
additives to crystallize, their solubility in the casting solvent,
and their similarity in chemical structure to TES ADT, to determine
the nucleation and resulting density of nuclei. Tracking the crystallization
process further yields information about the mechanism of nucleation.
While pure TES ADT nucleates instantaneously at the onset of crystallization,
nucleation transitions to a distributed process occurring throughout
crystallization with the incorporation of increasing amounts of additives
Macroscopic Molecular Ordering and Exciton Delocalization in Crystalline Phthalocyanine Thin Films
We present spatially-, temporally-
and polarization-resolved dual
photoluminescence/linear dichroism microscopy experiments that investigate
the correlation between long-range order and the nature of exciton
states in solution-processed phthalocyanine thin films. The influence
of grain boundaries and disorder is absent in these films because
typical grain sizes are 3 orders of magnitude larger than focused
excitation beam diameters. These experiments reveal the existence
of a delocalized singlet exciton, polarized along the high mobility
axis in this quasi-1D electronic system. The strong delocalized π
orbitals overlap, controlled by the molecular stacking along the high
mobility axis, is responsible for breaking the radiative recombination
selection rules. Using our linear dichroism scanning microscopy setup,
we further established that a rotation of molecules (i.e., a structural
phase transition) that occurs above 100 K prevents the observation
of this exciton at room temperature
High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs
van der Waals Epitaxial Growth of Graphene on Sapphire by Chemical Vapor Deposition without a Metal Catalyst
van der Waals epitaxial growth of graphene on <i>c</i>-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH<sub>4</sub> partial pressure, growth temperature, and H<sub>2</sub>/CH<sub>4</sub> ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm<sup>2</sup>/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10–10] || sapphire [11–20]) for about 80–90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ∼24 μm<sup>–2</sup>, and a lateral growth rate of ∼82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation
High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs