39 research outputs found
Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates
The
resolution of inkjet printing technology is determined by wetting
and evaporation processes after the jet drop contacts the substrate.
Here, the wetting of different picoliter solubilized polymer droplets
jetting onto one-end-closed porous alumina was investigated. The selected
polymers are commonly used in inkjet ink. The synergistic effects
of the hierarchical structure and substrate surface modification were
used to control the behavior of polymer-based ink drops. A model that
invokes the effect of surface tension was applied to calculate the
amount of polymer solution penetrating into the pores. The calculation
corroborates experimental observations and shows that the volume of
polymer solution in the pores increases with an increase in pore radius
and depth, resulting in less solution remaining on the substrate surface.
The structure of the porous substrate coupled with intrinsic polymer
properties and surface modifications all contribute to the resolution
that can be achieved via inkjet printing
Creating and Optimizing Interfaces for Electric-Field and Photon-Induced Charge Transfer
We create and optimize a structurally well-defined electron donorâacceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by <i>in situ</i> thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an <i>in situ</i> test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed
Regioregularity and Intrachain Ordering: Impact on the Nanostructure and Charge Transport in Two-Dimensional Assemblies of Poly(3-hexylthiophene)
The properties of supramolecular assemblies in conjugated
macromolecular
systems are strongly dependent on single chain effects. We report
that differences in regioregularity (RR) of side chain attachment
in polyÂ(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient
to induce dramatic changes in the electronic and morphological properties
of the material. Casting the electronic absorption spectra in the
framework of Spanoâs model reveals that the conjugation length
is surprisingly sensitive to RR, with differences in free exciton
bandwidth between the two P3HT samples approaching 73 meV. The enhanced
main chain planarization motivates a concomitant increase in nanofibril
width as well as crystallinity observed in thin films of the higher
RR variant. This observation correlates well with the field effect
mobilities that are attenuated by 1 to 2 orders of magnitude in the
lower RR polymer film. We suggest that the increased intrachain order
coupled with a reduced fraction of grain boundaries in the higher
RR film is responsible for the reported differences
Memory and Photovoltaic Elements in Organic Field Effect Transistors with Donor/Acceptor Planar-Hetero Junction Interfaces
Interfacial charge transfer at organic/organic planar-hetero
junctions
allows access to device structures that create new opportunities for
flexible electronic devices. Fundamental characteristics of a pentacene/[6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) interface are explored via a comprehensive
study of charge transfer between these two materials using the field-effect
transistor (FET) geometry both in the dark and under illumination.
Organic memory elements in a field effect transistor are demonstrated
for a device fabricated with a pentacene/PCBM interface. Electric
field induced charge transfer at the interface, in the dark, induced
a nonvolatile memory effect with a large hysteresis characterized
by a memory window of 43 V in the transfer characteristics. A photoinduced
threshold voltage shift induced by exciton dissociation at the interface,
in the absence of a gate electric field, is consistent with the formation
of the photoinduced conducting channel in pentacene
Automated Analysis of Orientational Order in Images of Fibrillar Materials
Nanofibers
are a ubiquitous structural motif in a variety of functional
materials. In the field of organic electronics, ĎâĎ-stacking
of conjugated polymers leads to fibrillar morphologies with a wide
array of fiber packing behavior. Fiber orientation and alignment are
known to influence the charge transport properties of devices such
as organic field effect transistors. The solution processing methods
used to create these devices give rise to large variations in these
structural parametersî¸however, they are only observable with
imaging techniques such as atomic force microscopy (AFM). To bring
more rigorous quantification of orientation and alignment to these
materials, a comprehensive image analysis tool is introduced to quantify
the two-dimensional orientation and alignment of nanofibers from AFM images. It has been developed in
MATLAB and packaged as a stand-alone application, so that researchers
with no computational expertise can produce publication-ready figures
directly from their images. AFM frequently yields images with low
contrast and moderate noise, making quantitative feature extraction
a significant challenge. In this protocol, each image is analyzed
in the context of an Orientation Map, in which nanofibers are thinned
to single-pixel width and an orientation is extracted for each of
these pixels. The Orientation Map is obtained through a five-step
process: fiber smoothing by anisotropic diffusion filtering, contrast
enhancement by top hat filtering, binarization by adaptive thresholding,
skeletonization, and recovery of orientations from the result of diffusion
filtering. Each step involves parameters that can be set using physical
heuristics, which are examined in detail. This Orientation Map yields
an orientation distribution and a plot of <i>S</i><sub>2D</sub>, an orientational order parameter, as a function of frame size.
The image analysis procedure is used to quantify differences in P3HT
nanofiber morphology induced by various solution processing recipes,
as well as the effect of spin-coating when used to deposit solutions
of nanofibers. All examples presented in this protocol can be reproduced
from beginning to end using the included software, with visualizations
produced at each stage of processing
Anisotropic Assembly of Conjugated Polymer Nanocrystallites for Enhanced Charge Transport
The anisotropic assembly of P3HT
nanocrystallites into longer nanofibrillar
structures was demonstrated via sequential UV irradiation after ultrasonication
to the pristine polymer solutions. The morphology of resultant films
was studied by atomic force microscopy (AFM), and quantitative analysis
of intra- and intermolecular ordering of polymer chains was performed
by means of static absorption spectroscopy and quantitative modeling.
Consequently, the approach to treat the precursor solution enhanced
intra- and intermolecular ordering and reduced the incidence of grain
boundaries within P3HT films, which contributed to the excellent charge
carrier transport characteristics of the corresponding films (Îź
â 12.0 Ă 10<sup>â2</sup> cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup> for 96% RR P3HT)
Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide
Liquid crystals can organize dispersed
particles into useful and
exotic structures. In the case of lyotropic cholesteric polypeptide
liquid crystals, polypeptide-coated particles are appealing because
the surface chemistry matches that of the polymeric mesogen, which
permits a tighter focus on factors such as extended particle shape.
The colloidal particles developed here consist of a magnetic and fluorescent
cylindrically symmetric silica core with one rounded, almost hemispherical
end. Functionalized with helical polyÂ(Îł-stearyl-l-glutamate)
(PSLG), the particles were dispersed at different concentrations in
cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran
(THF). Defects introduced by the particles to the director field of
the bulk PSLG/THF host led to a variety of phases. In fresh mixtures,
the cholesteric mesophase of the PSLG matrix was distorted, as reflected
in the absence of the characteristic fingerprint pattern. Over time,
the fingerprint pattern returned, more quickly when the concentration
of the PSLG-coated particles was low. At low particle concentration
the particles were âguidedâ by the PSLG liquid crystal
to organize into patterns similar to that of the re-formed bulk chiral
nematic phase. When their concentration increased, the well-dispersed
PSLG-coated particles seemed to map onto the distortions in the bulk
hostâs local director field. The particles located near the
glass vialâChLC interfaces were stacked lengthwise into architectures
with apparent two-dimensional hexagonal symmetry. The size of these
âcrystallineâ structures increased with particle concentration.
They displayed remarkable stability toward an external magnetic field;
hydrophobic interactions between the PSLG polymers in the shell and
those in the bulk LC matrix may be responsible. The results show that
bio-inspired LCs can assemble suitable colloidal particles into soft
crystalline structures
Ultrasound-Induced Ordering in Poly(3-hexylthiophene): Role of Molecular and Process Parameters on Morphology and Charge Transport
Facile
methods for controlling the microstructure of polymeric semiconductors
are critical to the success of large area flexible electronics. Here
we explore ultrasonic irradiation of solutions of polyÂ(3-hexylthiophene)
(P3HT) as a simple route to creating ordered molecular aggregates
that result in a one to two order of magnitude improvement in field
effect mobility. A detailed investigation of the ultrasound induced
phenomenon, including the role of solvent, polymer regioregularity
(RR) and film deposition method, is conducted. Absorption spectroscopy
reveals that the development of low energy vibronic features is dependent
on both the regioregularity as well as the solvent, with the latter
especially influential on the intensity and shape of the band. Use
of either higher regioregular polymer or ultrasonic irradiation of
lower regioregular polymer solutions results in high field effect
mobilities that are nearly independent of the dynamics of the film
formation process. Surprisingly, no distinct correlation between thin-film
morphology and macroscopic charge transport could be ascertained.
The relationships between molecular and process parameters are very
subtle: modulation of one effects changes in the others, which in
turn impact charge transport on the macroscale. Our results provide
insight into the degree of control that is required for the development
of reproducible, robust materials and processes for advanced flexible
electronics based on polymeric materials
Imparting Chemical Stability in Nanoparticulate Silver via a Conjugated Polymer Casing Approach
Only limited information is available on the design and
synthesis
of functional materials for preventing corrosion of metal nanostructures.
In the nanometer regime, even noble metals are subject to chemical
attack. Here, the corrosion behavior of noble metal nanoparticles
coated with a conjugated polymer nanolayer was explored for the first
time. Specifically, electrochemical corrosion and sulfur tarnishing
behaviors were examined for Ag-polypyrrole (PPy) coreâshell
nanoparticles using potentiodynamic polarization and spectrophotometric
analysis, respectively. First, the Ag-PPy nanoparticles exhibited
enhanced resistance to electrochemically induced corrosion compared
to their exposed silver counterparts. Briefly, a neutral PPy shell
provided the highest protection efficiency (75.5%), followed by sulfate
ion- (61.3%) and dodecylbenzenesulfonate ion- (53.6%) doped PPy shells.
However, the doping of the PPy shell with chloride ion induced an
adverse effect (protection efficiency, â120%). Second, upon
exposure to sulfide ions, the Ag-PPy nanoparticles preserved their
morphology and colloidal stability while the bare silver analog underwent
significant structural deformation. To further understand the function
of the PPy shell as a protection layer for the silver core, the catalytic
activity of the nanostructures was also evaluated. Using the reduction
of 4-nitrophenol as a representative example of a catalytic reaction,
the rate constant for that reduction using the PPy encased Ag nanoparticles
was found to be 1.1 Ă 10<sup>â3</sup> s<sup>â1</sup>, which is approximately 33% less than that determined for the parent
silver. These results demonstrate that PPy can serve as both an electrical
and chemical barrier for mitigating undesirable chemical degradation
in corrosive environments, as well as provide a simple physical barrier
to corrosive substances under appropriate conditions