22 research outputs found
Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers
An improved understanding of fundamental chemistry, electronic structure, morphology, and dynamics in polymers and soft materials requires advanced characterization techniques that are amenable to in situ and operando studies. Soft X-ray methods are especially useful in their ability to non-destructively provide information on specific materials or chemical moieties. Analysis of these experiments, which can be very dependent on X-ray energy and polarization, can quickly become complex. Complementary modeling and predictive capabilities are required to properly probe these critical features. Here, we present relevant background on this emerging suite of techniques. We focus on how the combination of theory and experiment has been applied and can be further developed to drive our understanding of how these methods probe relevant chemistry, structure, and dynamics in soft materials
Accounting for Nanometer-Thick Adventitious Carbon Contamination in X-ray Absorption Spectra of Carbon-Based Materials
From Mendelâs discovery on pea to todayâs plant genetics and breeding
In 2015, we celebrated the 150th anniversary of the presentation of the seminal work of Gregor Johann Mendel. While Darwinâs theory of evolution was based on differential survival and differential reproductive success, Mendelâs theory of heredity relies on equality and stability throughout all stages of the life cycle. Darwinâs concepts were continuous variation and âsoftâ heredity; Mendel espoused discontinuous variation and âhardâ heredity. Thus, the combination of Mendelian genetics with Darwinâs theory of natural selection was the process that resulted in the modern synthesis of evolutionary biology. Although biology, genetics, and genomics have been revolutionized in recent years, modern genetics will forever rely on simple principles founded on pea breeding using seven single gene characters. Purposeful use of mutants to study gene function is one of the essential tools of modern genetics. Today, over 100 plant species genomes have been sequenced. Mapping populations and their use in segregation of molecular markers and markerâtrait association to map and isolate genes, were developed on the basis of Mendel's work. Genome-wide or genomic selection is a recent approach for the development of improved breeding lines. The analysis of complex traits has been enhanced by high-throughput phenotyping and developments in statistical and modeling methods for the analysis of phenotypic data. Introgression of novel alleles from landraces and wild relatives widens genetic diversity and improves traits; transgenic methodologies allow for the introduction of novel genes from diverse sources, and gene editing approaches offer possibilities to manipulate gene in a precise manner
Competing Effects of Fluorination on the Orientation of Aromatic and Aliphatic Phosphonic Acid Monolayers on Indium Tin Oxide
Transparent conductive oxides such
as indium tin oxide (ITO) are
common substrates for optoelectronic devices, including organic light-emitting
diodes and organic solar cells. Tailoring the interface between the
oxide and the active layer by adjusting the work function or wettability
of the oxide can improve the performance of these devices in both
emissive and photovoltaic applications. The use of carefully designed
surface modifiers that form self-assembled monolayers (SAMs) can allow
the tuning of the surface of one oxide material to optimize its properties
for use with a variety of different organic semiconductors or for
different applications. Fluorinated phosphonic-acid-based SAMs can
affect the interface dipole and the work function of a metal oxide.
Fluorination may also affect the molecular packing and the orientation
of the SAM once bound to the surface. We utilize angle-dependent near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy to determine
the molecular orientations of octylphosphonic acid, phenylphosphonic
acid, and fluorinated derivatives on ITO and correlate the molecular
orientations derived from these studies with predictions from density
functional theory (DFT). We account quantitatively for the effect
of surface roughness on the measured orientations. We observe that
fluorination of the octylphosphonic acid SAM results in a more upright
orientation, an effect we attribute to intermolecular forces and increased
steric bulk. In contrast, fluorination of the phenylphosphonic acid
SAM leads to a less upright orientation that we associate with changes
in binding mode
Electron-Transfer Processes in Zinc PhthalocyanineâPhosphonic Acid Monolayers on ITO: Characterization of Orientation and Charge-Transfer Kinetics by Waveguide Spectroelectrochemistry
Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered
to indium
tin oxide (ITO) as a model for the donor/transparent conducting oxide
(TCO) interface in organic photovoltaics (OPVs), we demonstrate the
relationship between molecular orientation and charge-transfer rates
using spectroscopic, electrochemical, and spectroelectrochemical methods.
Both monomeric and aggregated forms of the phthalocyanine (Pc) are
observed in ZnPcPA monolayers. Potential-modulated attenuated total
reflectance (PM-ATR) measurements show that the monomeric subpopulation
undergoes oxidation/reduction with <i>k</i><sub>s,app</sub> = 2 Ă 10<sup>2</sup> s<sup>â1</sup>, independent of
Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent
charge-transfer rates are observed. For in-plane-oriented Pc aggregates, <i>k</i><sub>s,app</sub> = 2 Ă 10<sup>3</sup> s<sup>â1</sup>, whereas for upright Pc aggregates, <i>k</i><sub>s,app</sub> = 7 Ă 10<sup>2</sup> s<sup>â1</sup>. The rates for the
aggregates are comparable to those required for redox-active interlayer
films at the hole-collection contact in organic solar cells
Tuning hole charge collection efficiency in polymer photovoltaics by optimizing the work function of indium tin oxide electrodes with solution-processed LiF nanoparticles
Competing Effects of Fluorination on the Orientation of Aromatic and Aliphatic Phosphonic Acid Monolayers on Indium Tin Oxide
Intensity-Modulated Scanning Kelvin Probe Microscopy for Probing Recombination in Organic Photovoltaics
Orientation of Phenylphosphonic Acid Self-Assembled Monolayers on a Transparent Conductive Oxide: A Combined NEXAFS, PM-IRRAS, and DFT Study
Self-assembled monolayers (SAMs) of dipolar phosphonic
acids can
tailor the interface between organic semiconductors and transparent
conductive oxides. When used in optoelectronic devices such as organic
light emitting diodes and solar cells, these SAMs can increase current
density and photovoltaic performance. The molecular ordering and conformation
adopted by the SAMs determine properties such as work function and
wettability at these critical interfaces. We combine angle-dependent
near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and
polarization modulation infrared reflection absorption spectroscopy
(PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic
acid on indium zinc oxide, and correlate the resulting values with
density functional theory (DFT). We find that the SAMs are surprisingly
well-oriented, with the phenyl ring adopting a well-defined tilt angle
of 12â16° from the surface normal. We find quantitative
agreement between the two experimental techniques and density functional
theory calculations. These results not only provide a detailed picture
of the molecular structure of a technologically important class of
SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode
assignments for phosphonic acids on oxide surfaces, thus improving
the utility of PM-IRRAS for future studies