Near-edge X-ray Fine Structure Research Articles

Anomalous lattice contraction and emerging topological phases in Bi-substituted Sm2Ir2O7

We demonstrate that substituting Bi for Sm in the pyrochlore Sm$_2$Ir$_2$O$_7$ induces an anomalous lattice contraction, with $\Delta a \sim -0.012$~\AA~observed at 10\% Bi substitution, where 'a' denotes the lattice constant. Beyond 10\% Bi substitution, the lattice expands according to Vegard's law. Within this anomalous substitution range, the resistivity shows a 1/T behavior up to 2\% Bi-substitution, while near 10\% substitution a -lnT dependence is observed. These resistivity behaviors suggest the possibility of a Weyl phase up to 2\% Bi substitution, which transforms to a semimetallic quadratic band touching (QBT) topological phase near 10\%. For the intermediate composition (Sm$_{0.95}$Bi$_{0.05}$)$_2$Ir$_2$O$_7$, the resistivity scales as $\rm 1/T^{1/4}$, possibly due to its proximity to a proposed quantum critical point at the Weyl-QBT phase boundary [Phys. Rev. X 4, 041027 (2014)]. The samples were characterized using synchrotron powder X-ray diffraction, X-ray near-edge fine structure (XANES), and Extended X-ray absorption fine structure (EXAFS) probes. Additionally, magnetic susceptibility and heat capacity measurements were conducted to provide further support.

Effect of bottom electrode on rectification performance in pyrene-terminated n-alkanethiolate

Rectification performance of pyrene-terminated alkanethiolate in large-area tunneling junction was explored on the effect of bottom-electrode identity (Ag, Au, Pt, Pd) as non-molecular factors. Rectification ratio (log|r–|) of pyrene-terminated junction significantly decreased in order of Ag (log|r–|= 2.2 ± 0.3) ≈ Pt (log|r–|= 2.2 ± 0.5) > Pd (log|r–|= 2.0 ± 0.5) > Au (log|r–|= 1.3 ± 0.6). Three hypotheses were considered for the origin of the bottom-electrode effect: (i) surface topography of bottom electrode, (ii) electronic structure, and (iii) supramolecular packing structure. Structural analyses on a molecular scale using atomic force microscopy, ultraviolet photoelectron spectroscopy, and near-edge X-ray fine structure spectroscopy revealed that the effect of bottom electrode on the supramolecular structure dominates the trend of rectification performance ruling out the other possibilities. These experimental results demonstrate that rectification performance in pyrene-based molecular diode is sensitive to the change in electrode's identity. Our work offers important insight about structure-rectification relationship in organic molecular diode

Calcite Nanotuned Chitinous Skeletons of Giant Ianthella basta Marine Demosponge.

Marine sponges were among the first multicellular organisms on our planet and have survived to this day thanks to their unique mechanisms of chemical defense and the specific design of their skeletons, which have been optimized over millions of years of evolution to effectively inhabit the aquatic environment. In this work, we carried out studies to elucidate the nature and nanostructural organization of three-dimensional skeletal microfibers of the giant marine demosponge Ianthella basta, the body of which is a micro-reticular, durable structure that determines the ideal filtration function of this organism. For the first time, using the battery of analytical tools including three-dimensional micro—X-ray Fluorescence (3D-µXRF), X-ray diffraction (XRD), infra-red (FTIR), Raman and Near Edge X-ray Fine Structure (NEXAFS) spectroscopy, we have shown that biomineral calcite is responsible for nano-tuning the skeletal fibers of this sponge species. This is the first report on the presence of a calcitic mineral phase in representatives of verongiid sponges which belong to the class Demospongiae. Our experimental data suggest a possible role for structural amino polysaccharide chitin as a template for calcification. Our study suggests further experiments to elucidate both the origin of calcium carbonate inside the skeleton of this sponge and the mechanisms of biomineralization in the surface layers of chitin microfibers saturated with bromotyrosines, which have effective antimicrobial properties and are responsible for the chemical defense of this organism. The discovery of the calcified phase in the chitinous template of I. basta skeleton is expected to broaden the knowledge in biomineralization science where the calcium carbonate is regarded as a valuable material for applications in biomedicine, environmental science, and even in civil engineering.

Optical properties and composition of viscous organic particles found in the Southern Great Plains

Abstract. Atmospheric high-viscosity organic particles (HVOPs) were observed in samples of ambient aerosols collected in April and May 2016 in the Southern Great Plains of the United States. These particles were apportioned as either airborne soil organic particles (ASOPs) or tar balls (TBs) from biomass burning based on spetro-microscopic imaging and assessments of meteorological records of smoke and precipitation data. Regardless of their apportionment, the number fractions of HVOPs were positively correlated (R2=0.85) with increased values of absorption Ångström exponent (AAE) measured in situ for ambient aerosol at the site. Extending this correlation to 100 % HVOPs yields an AAE of 2.6, similar to previous literature reports of the class of light-absorbing organic particles known as brown carbon (BrC). One out of the three samples investigated had a significant number of ASOPs, while the other two samples contained TBs. Although there are chemical similarities between ASOPs and TBs, they can be distinguished based on composition inferred from near-edge absorption X-ray fine structure (NEXAFS) spectroscopy. ASOPs were distinguished from TBs based on their average -COOH/C=C and -COOH/COH peak ratios, with ASOPs having lower ratios. NEXAFS spectra of filtered soil organic brine particles nebulized from field samples of standing water deposited after rain were consistent with ASOPs when laboratory particles were generated by bubble bursting at the air–organic brine interface. However, particles generated by nebulizing the bulk volume of soil organic brine had a particle composition different from ASOPs. These observations are consistent with the raindrop generation mechanism responsible for ASOP emissions in the area of study. In contrast, nebulized samples carry with them higher fractions of soil inorganics dissolved in the bulk volume of soil brine, which are not aerosolized by the raindrop mechanism. Our results support the bubble bursting mechanism of particle generation during rainfall resulting in the ejection of soil organics into the atmosphere. In addition, our results show that ASOPs may only be atmospherically relevant during times when suitable emission conditions are met.

Quantitative capabilities of STXM to measure spatially resolved organic volume fractions of mixed organic ∕ inorganic particles

Abstract. Scanning transmission X-ray microscopy coupled with near-edge X-ray absorption and fine structure (STXM-NEXAFS) spectroscopy can be used to characterize the morphology and composition of aerosol particles. Here, two inorganic ∕ organic systems are used to validate the calculation of organic volume fraction (OVF) and determine the level of associated error by using carbon K-edge STXM data at 278, 285.4, 288.6, and 320 eV. Using the mixture of sodium chloride and sucrose as one system and ammonium sulfate and sucrose as another, three solutions were made with 10:1, 1:1, and 1:10 mass ratios (inorganic to organic). The OVFs of the organic-rich aerosols of both systems deviated from the bulk OVF by less than 1%, while the inorganic-rich aerosols deviated by approximately 1 %. Aerosols from the equal mass mixture deviated more (about 4 %) due to thick inorganic regions exceeding the linear range of Beer's law. These calculations were performed after checking the data for poor image alignment, defocusing issues, and particles too thick to be analyzed. The potential for systematic error in the OVF calculation was also tested by assuming the incorrect composition. There is a small (about 0.5 %) OVF difference if the organic is erroneously assumed to be adipic acid rather than the known organic, sucrose. A much larger difference (up to 25 %) is seen if sodium chloride is assumed instead of ammonium sulfate. These results show that the OVF calculations are fairly insensitive to the organic while being much more sensitive to the choice of inorganic.

Soft X-ray Chlorine Photolysis on Chlorobenzene Ice: An Experimental and Theoretical Study.

An experimental and theoretical study of the photoinduced homolysis of the carbon-chlorine bond in an ice matrix of chlorobenzene is presented. A condensed chlorobenzene film has been grown in situ and near edge X-ray fine structure (NEXAFS) spectra were collected after exposing the condensed film to a monochromatic photon beam centered at the 2822 eV resonant excitation of chlorine and at 2850 eV. The photoabsorption to the Cl 1s → σ* and Cl 1s → π* states has been measured and the hypothesis of free radical coupling reactions was investigated via time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations. Also, potential energy pathways to the C-Cl cleavage have been obtained at the CASSCF level to the Cl 1s → σ*, 1s → π*, and 1s → ∞ states. A strong dissociative character was only found for the Cl 1s → σ* resonance.

Properties of NTCDA Thin Films on Ag(110): Scanning Tunneling Microscopy, Photoemission, Near-Edge X-ray Fine Structure, and Density Functional Theory Investigations

It is well proven that the properties of organic/metal interfaces play an utmost role in the performance of organic devices. Here we present a study on structural and electronic properties of high-quality 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) films grown on an Ag(110) surface. High-resolution scanning tunneling microscopy and low-energy electron diffraction show the presence of two molecular domains. Density functional theory calculations indicate that the most stable location of NTCDA corresponds to anhydride oxygen attached to the Ag atoms along the [110] direction. Photoemission results of the C 1s and O 1s core levels demonstrate a strong interfacial bonding, inducing a charge transfer from the Ag metal to the molecular monolayer. An angular-dependent study of the C K-edge near-edge X-ray fine structure spectra provides detailed information concerning the evolution of the NTCDA orientation with the film thickness.

Single-Shot near Edge X-ray Fine Structure (NEXAFS) Spectroscopy Using a Laboratory Laser-Plasma Light Source.

We present a proof of principle experiment on single-shot near edge soft X-ray fine structure (NEXAFS) spectroscopy with the use of a laboratory laser-plasma light source. The source is based on a plasma created as a result of the interaction of a nanosecond laser pulse with a double stream gas puff target. The laser-plasma source was optimized for efficient soft X-ray (SXR) emission from the krypton/helium target in the wavelength range from 2 nm to 5 nm. This emission was used to acquire simultaneously emission and absorption spectra of soft X-ray light from the source and from the investigated sample using a grazing incidence grating spectrometer. NEXAFS measurements in a transmission mode revealed the spectral features near the carbon K-α absorption edge of thin polyethylene terephthalate (PET) film and L-ascorbic acid in a single-shot. From these features, the composition of the PET sample was successfully obtained. The NEXAFS spectrum of the L-ascorbic acid obtained in a single-shot exposure was also compared to the spectrum obtained a multi-shot exposure and to numerical simulations showing good agreement. In the paper, the detailed information about the source, the spectroscopy system, the absorption spectra measurements and the results of the studies are presented and discussed.

2-D elemental mapping of an extreme ultraviolet-irradiated PET with a compact near edge X-ray fine structure spectromicroscopy

We present a near edge X-ray fine structure (NEXAFS) spectromicroscopy, that was developed based on a compact laboratory laser-plasma soft X-ray light source. The presented approach to spectromicroscopy is different from the usual synchrotron-based one when the series of full-field microscopy images are obtained at different energies. Herein, spatially localized NEXAFS spectra are obtained using a broad-band SXR emission, and the microscopic image is obtained by a raster scanning. The plasma is formed by the interaction of nanosecond laser pulses with a double stream gas puff target. The laser plasma source was optimized for efficient soft X-ray emission from a krypton plasma in the range ~2 nm to 5 nm wavelength radiation (energy of ~250 eV – 620 eV). This emission is used to acquire simultaneously emission and absorption spectra of soft X-ray light from the source and from the investigated sample. The spectra were acquired from a small area of PET polymer ~30 × 30 μm2 in size. A raster scanning was implemented to obtain 2-D elemental composition maps. A chemical (elemental) composition of EUV-irradiated PET was investigated. The data are in agreement with the reference measurements and the theoretical values. EUV irradiated PET spectra are showing hydrogen decrease in the PET structure due to the EUV irradiation. In the paper, a detailed information about the source, the system, the spectral measurements and the results are presented and discussed.

Compact system for near edge X-ray fine structure (NEXAFS) spectroscopy using a laser-plasma light source.

We present a compact laboratory system for near edge soft X-ray fine structure (NEXAFS) spectroscopy that was developed using a laser-plasma light source. The source is based on a double stream gas puff target. The plasma is formed by the interaction of a laser beam with the double stream gas puff target approach. The laser plasma source was optimized for efficient soft X-ray emission from a krypton/helium target in the range of 1.5 to 5 nm wavelength. This emission is used to acquire simultaneously the emission and absorption spectra of soft X-ray light from the source and from the investigated sample using a grazing incidence spectrometer. The measurements in the transmission mode reveal the features near the carbon K-α absorption edge of thin PET film. From those features, the composition of the sample was successfully obtained. The data are in agreement with synchrotron measurements. In the paper, the detailed information about the source, its optimization, the system, spectral measurements and the results are presented and discussed.

Systematic Investigation of π-π Interactions in Near-Edge X-ray Fine Structure (NEXAFS) Spectroscopy of Paracyclophanes.

NEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π-π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene-benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π-π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals.

Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate.

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C3S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C–S–H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C3S hydration was significantly extended, the degree of hydration of C3S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C3S in the C3S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C3S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C–S–H in the C3S-HVFA paste directly indicate that Al substitutes for Si in C–S–H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C–S–H in C3S-HVFA system and presented results consistent with previous literature.

Use of a DNA film on a self-assembled monolayer for investigating the physical process of DNA damage induced by core electron ionization

Purpose: A novel two-layer sample composed of a deoxyribonucleic acid (DNA) film and self-assembled monolayer (SAM) was prepared on an inorganic surface to mimic the processes in which DNA is damaged by soft X-ray irradiation.Materials and methods: A mercaptopropyltrimethoxysilane (MPTS) SAM was formed on a sapphire surface, then oligonucleotide (OGN) molecules were adsorbed on the MPTS-SAM. The thicknesses and chemical states of the layers were determined by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray fine structure (NEXAFS) around the phosphorus (P) and sulfur (S) K-edges. To induce the damage to the OGN molecules, the sample was irradiated with synchrotron soft X-rays. The chemical state of the OGN molecules before and after irradiation was examined by NEXAFS around the nitrogen (N) K-edge region.Results: The thickness of the MPTS-OGN layer was approximately 7.7 nm. The S atom of the OGN molecules was located at the bottom of the OGN layer. The peak shape of the N K-edge NEXAFS spectra of the MPTS-OGN layers clearly changed following irradiation.Conclusions: The MPTS-OGN layer formed on the sapphire surface. The chemical states and the structure of the interface were elucidated using synchrotron soft X-rays. The OGN molecules adsorbed on the MPTS films decomposed upon exposure to soft X-ray irradiation.

Benchmarking Metal and Metal Oxide Promoters for Oxygen Evolution Reaction in Li-O2 Cells

Despite high theoretical capacity, a Li-O2 cell has suffered from huge oxidation potential polarization on carbon-based positive electrode for charge (>4.2 V vs. Li/Li+), due to sluggish decomposition of non-conductive discharge product, lithium peroxide (Li2O2 ↔ 2Li+ + O2 + 2e-) [1]. Such high potential triggers side reactions such as degradation of electrolyte and carbonaceous electrode, which results in poor cycle-ability [1]. To mitigate this problem during oxygen evolution reaction (OER), solid-state metal or metal oxide nanoparticles (indicated as promoters), which have been widely employed as catalysts in aqueous media, were introduced to the electrode [2]. However, the specific role of promoters in the Li-O2 battery is little known due to complication from accompanying parasitic side reactions [3]. In addition, reasonable comparison of promoters’ activities is not feasible under different performance conditions when various reports were referred [2]. Therefore, to gain a reasonable assessment of their activities in the Li-O2 cell and an understanding of the promoters’ role, it is necessary to examine Li-O2 cells with these promoters under the same condition and analyze their reaction processes in detail. Here I present diagnosis of the true role of promoters, representative of platinum (Pt), gold (Au), palladium (Pd) and cobalt oxide (Co3O4), for OER in Li-O2 cells. After preparation of comparable size and mass loading of promoters on carbon nanotube (CNT) electrode, the Li-O2 cells containing these promoter/CNT combinations were examined using galvanostatic mode under the same operating conditions. The promoter/CNT electrodes show reasonably lower charge potentials than the promoter-free electrode for the 1st charge. Through in situ gas analysis of online electrochemical mass spectroscopy (OEMS) and ex situ chemical analysis of X-ray near-edge fine structure (XANES) spectroscopy, the evolved gas amount and remaining product after charge could be correlated, which accounted for the true reaction occurring for each promoter.

Fe L-Edge X-ray Absorption Spectra of Fe(II) Polypyridyl Spin Crossover Complexes from Time-Dependent Density Functional Theory

L-edge near-edge X-ray fine structure spectroscopy (NEXAFS) has become a powerful tool to study the electronic structure and dynamics of metallo-organic and biological compounds in solution. Here, we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time-dependent framework. Several key factors that control the L-edge excitations have been carefully examined using an Fe(II) polypyridyl complex [Fe(tren(py)3)](2+) (where tren(py)3 = tris(2-pyridylmethyliminoethyl)amine) as a model system. It is found that the electronic spectra of the low-spin (LS, singlet), intermediate-spin (IS, triplet), and high-spin (HS, quintet) states have distinct profiles. The relative energy positions, but not the spectral profiles, of different spin states are sensitive to the choice of the functionals. The inclusion of the vibronic coupling leads to almost no visible change in the resulting NEXAFS spectra because it is governed only by low-frequency modes of less than 500 cm(-1). With the help of the molecular dynamics sampling in acetonitrile at 300 K, our calculations reveal that the thermal motion can lead to a noticeable broadening of the spectra. The main peak position is strongly associated with the length of the Fe-N bond.

Investigation of phase segregation in sol–gel derived ZnMgO thin films

Highly c-axis-oriented Zn1-xMgxO multilayered thin films have been deposited on p-type Si substrates with different concentration of Mg (x = 0.00−0.40) using a sol–gel spin-coating technique. The x-ray diffraction (XRD) shows that single-phase wurtzite thin films start showing phase segregation for a Mg content of x = 0.25 for the sol–gel-derived ZnMgO thin films. The element specific near edge x-ray fine structure (NEXAFS) collected at O K-edge also clearly evidence the phase segregation at x = 0.25. These results also show that films are deposited with wurtzite structure as dominant phase even after phase segregation. The NEXAFS spectra collected at Zn L3-edge rule out the presence of any Zn-related defect due to Mg doping. The atomic force microscopy (AFM) depicts the spherical shape of nanosized grains, and grain size varies slightly with Mg content. The single-phase ZnMgO thin films show a band gap tuning from 3.38 to 3.84 eV, which is also consistent with blue shifting of near-band edge PL emission. The electrical resistivity of thin films increases with Mg content before phase segregation. However, the optical band gap, photoluminescence and electrical resistivity show anomalous behavior at phase segregation limit which has been discussed and correlated with each other.

Biological Change of Chemical States of Actinides and Lanthanides-effects of Organic Acids-

In this study, we have examined the chemical states change of lanthanides (Lns) of La, Ce, Pr, Eu, and actinides (Ans) of Th(IV), Pu(IV) in the complexes with organic acids of citric acids or desferrioxamine B (DFO) by the interaction with Pseudomonas fluorescens cells under the resting condition at pH 4 – 9. In the adsorption experiments the distribution coefficients (Kd) were determined. The oxidation states of Ce in the solution were determined by X-ray near edge fine structure (XAFS). In the absence of organic materials, Kd of Eu(III) increased gradually with pH up to 7; above 7, the Kd decreased with an increase in pH. In the presence of citric acid, we found that Kd was lower than that in the absence of organic materials, reflecting the formation of Eu(III)-citrate complexes competes with the Eu(III)-cell-surface complexes. For Ln-DFO complex, the higher Kds of La, Ce, and Pr were obtained at lower pHs on P. fluorescens cells. The Kd of Ce was lower than that of its neighboring REEs, La(III) and Pr(III). XAFS analysis showed that Ce exists as the Ce(IV)-DFO complex in higher pH than 6. Thus, the pH dependence of lower Kd of Ce is predominantly dependent on the stability of Ce(IV)-DFO complex. For the adsorption of Ans, the Kds of Th and Pu were larger than Eu, being inverse relationship to the stability constants of DFO complex. These results indicate that presence of organic acids affect the chemical states of Ln and An in the environments.

All-polymer solar cells utilizing low band gap polymers as donor and acceptor

All-polymer solar cells based on blends of the low band gap polymers poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]} (PTB7) and poly{[N,N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) are demonstrated. The use of the donor polymer PTB7 instead of poly(3-hexylthiophene) results in a higher open-circuit voltage and an overall spectral response better matched to the solar spectrum. A power conversion efficiency of 1.1% is reported with a peak external quantum efficiency of 18% at a wavelength of 680 nm. The microstructure of PTB7:P(NDI2OD-T2) blends is also investigated using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS), near-edge X-ray fine-structure (NEXAFS) spectroscopy, atomic force microscopy (AFM), and scanning transmission X-ray microscopy (STXM). GIWAXS measurements show that PTB7:P(NDI2OD-T2) blends contain P(NDI2OD-T2) crystallites with a (100) thickness of 9.5 nm dispersed in an amorphous PTB7 matrix. STXM measurements indicate a lack of mesoscale phase separation, with AFM and NEXAFS measurements revealing a P(NDI2OD-T2)-rich top surface with fibrillar morphology. These results indicate that the pairing of low band gap polymers as both donor and acceptor polymers in all-polymer solar cells may be an effective strategy for realizing high-efficiency all-polymer solar cells. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Controlling the Particle Size of ZrO2 Nanoparticles in Hydrothermally Stable ZrO2/MWCNT Composites

The composite of multiwalled carbon nanotubes (MWCNTs) decorated with ZrO(2) nanoparticles, synthesized by a grafting method followed by high-temperature annealing, was studied. The oxygen functionalized MWCNT surface uniformly disperses and stabilizes the oxide nanoparticles to an extent that is controlled by the metal oxide loading and thermal annealing temperature. This ZrO(2)/MWCNT also withstands decomposition in a hydrothermal environment providing potential applications in the catalysis of biomass conversion (e.g., aqueous phase reforming). The ZrO(2)/MWCNT have been characterized by (scanning) transmission electron microscopy ((S)TEM), X-ray diffraction (XRD), in situ small-angle X-ray scattering (SAXS), in situ wide-angle X-ray scattering (WAXS), and near edge X-ray fine structure (NEXAFS) for the purpose of a comprehensive analysis of the ZrO(2) particle size and particle size stability.

Theoretical Study of Core Excitations of Fullerene-Based Polymer Solar Cell Acceptors

First-principles simulations for the C K-edge X-ray photoelectron (XPS) and near-edge X-ray fine-structure (NEXAFS) spectra of representative polymer solar cell acceptor (PSCA) molecules have been carried out with special focuses on the [6,6]-phenyl-Cn+1-butyric acid methyl ester (PCnBM, n = 60, 70, 84). In the XPS spectra, evident red shift of core binding energies are observed in PCnBM as compared to the corresponding fullerene Cn, which is due to weak electronic charge transfer from the side chain to the fullerene backbone and consequently increased electron screening. Special emphasis is paid to a spectral peak at ca. 284.7 eV, which is caused by resonances of phenyl-ring carbons in the side chain and is important in characterizing the electronic structures of different PSCA molecules. Additionally, we propose a modified version of the building block (MBB) approach for quick estimation of the NEXAFS spectra, which is useful when computational resources are limited or massive systems are under study. T...