Resonant Soft X-ray Scattering Research Articles

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO3 Thin Film.

SrRuO3 (SRO) thin films and their heterostructure have attracted much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft X-ray scattering (RSXS) at the oxygen K edge to investigate the spin configuration of a four-unit-cell SRO film that was grown epitaxially on a single-crystal SrTiO3. The RSXS signal under a magnetic field (∼0.4 tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.

Mixed-flow design for microfluidic printing of two-component polymer semiconductor systems

The rational creation of two-component conjugated polymer systems with high levels of phase purity in each component is challenging but crucial for realizing printed soft-matter electronics. Here, we report a mixed-flow microfluidic printing (MFMP) approach for two-component π-polymer systems that significantly elevates phase purity in bulk-heterojunction solar cells and thin-film transistors. MFMP integrates laminar and extensional flows using a specially microstructured shear blade, designed with fluid flow simulation tools to tune the flow patterns and induce shear, stretch, and pushout effects. This optimizes polymer conformation and semiconducting blend order as assessed by atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (R-SoXS), photovoltaic response, and field effect mobility. For printed all-polymer (poly[(5,6-difluoro-2-octyl-2H-benzotriazole-4,7-diyl)-2,5-thiophenediyl[4,8-bis[5-(2-hexyldecyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-2,5-thiophenediyl]) [J51]:(poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}) [N2200]) solar cells, this approach enhances short-circuit currents and fill factors, with power conversion efficiency increasing from 5.20% for conventional blade coating to 7.80% for MFMP. Moreover, the performance of mixed polymer ambipolar [poly(3-hexylthiophene-2,5-diyl) (P3HT):N2200] and semiconducting:insulating polymer unipolar (N2200:polystyrene) transistors is similarly enhanced, underscoring versatility for two-component π-polymer systems. Mixed-flow designs offer modalities for achieving high-performance organic optoelectronics via innovative printing methodologies.

Molecular Packing in Double Gyroid Cubic Phases Revealed via Resonant Soft X-Ray Scattering.

The bicontinuous double gyroid phase is one of the nature's most symmetric and complex structures, the electron density map of which was established long ago. By utilizing small-angle x-ray scattering, resonant soft x-ray scattering at the carbon K edge and model-dependent tensor-based scattering theory, we have not only elucidated morphology but also identified molecular packing in the double gyroid phases formed by molecules with different shapes, i.e., rodlike vs taper shaped, thus validating some of the hypothetical packing models and disproving others. The spatial variation of molecular orientation through the channel junctions in the double gyroid phase can be either continuous in the case of anisotropic channels or discontinuous in the case of isotropic channels depending on the molecular structure and shape.

Analysis of Chemical Contents Spatial Distribution in EUV Resist Using Resonant Soft X-ray Scattering Method

Analysis of Chemical Contents Spatial Distribution in EUV Resist Using Resonant Soft X-ray Scattering Method

Resonant Soft X-Ray Scattering from Stripe-Ordered La2−xBaxCuO4 Detected by a Transition-Edge Sensor Array Detector

Resonant soft x-ray scattering (RSXS) is a leading probe of valence band order in materials best known for detecting charge density wave order in the copper-oxide superconductors. One of the biggest limitations on the RSXS technique is the presence of a severe fluorescence background which, like the RSXS cross section itself, is enhanced under resonant conditions. This background prevents the study of weak signals such as diffuse scattering from glassy or fluctuating order that is spread widely over momentum space. Recent advances in superconducting transition edge sensor (TES) detectors have led to major improvements in energy resolution and detection efficiency in the soft x-ray range. Here, we perform a RSXS study of stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ at the Cu $L_{3/2}$ edge (932.2 eV) using a TES detector with 1.5 eV resolution, to evaluate its utility for mitigating the fluorescence background problem. We find that, for suitable degree of detuning from the resonance, the TES rejects the fluorescence background, leading to a 5 to 10 times improvement in the statistical quality of the data compared to an equivalent, energy-integrated measurement. We conclude that a TES presents a promising approach to reducing background in RSXS studies and may lead to new discoveries in materials exhibiting valence band order that is fluctuating or glassy.

Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T‐2DT/Nonfullerene Organic Solar Cells

The temperature dependent aggregation behavior of PffBT4T polymers used in organic solar cells plays a critical role in the formation of a favorable morphology in fullerene-based devices. However, there has been little investigation into the impact of donor/acceptor ratio on morphology tuning, especially for non-fullerene acceptors (NFAs). Herein, the influence of composition on morphology is reported for blends of PffBT4T-2DT with two NFAs, O-IDTBR and O-IDFBR. The monotectic phase behavior inferred from differential scanning calorimetry provides qualitative insight into the interplay between solid-liquid and liquid-liquid demixing. Transient absorption spectroscopy suggests that geminate recombination dominates charge decay and that the decay rate is insensitive to composition, corroborated by negligible changes in open-circuit voltage. Exciton lifetimes are also insensitive to composition, which is attributed to the signal being dominated by acceptor excitons which are formed and decay in domains of similar size and purity irrespective of composition. A hierarchical morphology is observed, where the composition dependence of size scales and scattering intensity from resonant soft X-ray scattering (R-SoXS) is dominated by variations in volume fractions of polymer/polymer rich domains. Results suggest an optimal morphology where polymer crystallite size and connectivity are balanced, ensuring a high probability of hole extraction via such domains.

Fine-Tuning Semiconducting Polymer Self-Aggregation and Crystallinity Enables Optimal Morphology and High-Performance Printed All-Polymer Solar Cells.

Polymer aggregation and crystallization behavior play a crucial role in the performance of all-polymer solar cells (all-PSCs). Gaining control over polymer self-assembly via molecular design to influence bulk-heterojunction active-layer morphology, however, remains challenging. Herein, we show a simple yet effective way to modulate the self-aggregation of the commonly used naphthalene diimide (NDI)-based acceptor polymer (N2200), by systematically replacing a certain amount of alkyl side-chains with compact bulky side-chains (CBS). Specifically, we have synthesized a series of random copolymer (PNDI-CBSx) with different molar fractions (x = 0-1) of the CBS units and have found that both solution-phase aggregation and solid-state crystallinity of these acceptor polymers are progressively suppressed with increasing x as evidenced by UV-vis absorption, photoluminescence (PL) spectroscopies, thermal analysis, and grazing incidence X-ray scattering (GIWAXS) techniques. Importantly, as compared to the highly self-aggregating N2200, photovoltaic results show that blending of more amorphous acceptor polymers with donor polymer (PBDB-T) can enable all-PSCs with significantly increased PCE (up to 8.5%). The higher short-circuit current density (Jsc) results from the smaller polymer phase-separation domain sizes as evidenced by PL quenching and resonant soft X-ray scattering (R-SoXS) analyses. Additionally, we show that the lower crystallinity of the active layer is less sensitive to the film deposition methods. Thus, the transition from spin-coating to solution coating can be easily achieved with no performance losses. On the other hand, decreasing aggregation and crystallinity of the acceptor polymer too much reduces the photovoltaic performance as the donor phase-separation domain sizes increases. The highly amorphous acceptor polymers appear to induce formation of larger donor polymer crystallites. These results highlight the importance of a balanced aggregation strength between the donor and acceptor polymers to achieve high-performance all-PSCs with optimal active layer film morphology.

Connecting soft x-ray anisotropy with local order in conjugated polymers

Abstract

Multimodal Resonant x-Ray Scattering: Elucidating Electrochemical Membranes

The development of functionally complex mesoscale (nm - µm) materials requires comparable evolution in the analytical instruments and techniques in order to understand the physical and chemical structure-property relationships underlying their performance. Resonant X-Ray Scattering (ReXS) is a powerful technique due to its ability to statistically-significant sub-nm morphological and chemical sensitivity over broad lengthscales, but it has so far been underutilized in many of the scientific communities whose progress demands spatiochemically-sensitive in-situ or operando characterization techniques. In this presentation, we will show how our group has developed and applied a versatile in-situ cell to harness the potential of ReXS while maintaining compatibility with a variety of synchrotron x-ray techniques and using an environment that is also amenable to other advanced lab-scale techniques such as transmission electron microscopy (TEM). First, we will go over the basic physics of why ReXS measurements can be especially beneficial for in-situ characterization. Then, we will demonstrate the adaptability and multimodality of our cell by spotlighting both spectroscopic and scattering experiments conducted across different “soft” (i.e. 200 eV – 1.5 keV) and “tender” (i.e. 1.5 keV – 5 keV) x-ray systems at different synchrotrons. In particular, we will highlight the cell’s fuel-cell like capabilities (i.e. heating, electrochemical, and gas/liquid flow) by focusing on recent studies conducted on a variety of proton exchange membranes (PEM) materials spanning a wide range of thicknesses, and which have proven to be difficult to characterize with other techniques. Advanced characterization of PEMs is critical, since they are are still poorly understood cost and performance-limiting components that are used across fuel cells[1], electrolyzers,[2] and redox-flow batteries.[3] Perfluorinated sulfonic acid (PFSA) ionomers are the dominant class of PEMs used across these aforementioned applications, where an electrically neutral chemically inert polytetrafluoroethylene (PTFE) backbone is tethered with SO3 - terminated side-chains that rearrange into a phase-separated morphology where their hydrophilic sulfonate nano-domains endow PEMs with their crucial proton conductivity. Therefore, results from resonant soft x-ray scattering (RSoXS) and absorption spectroscopy conducted at the oxygen and fluorine K-edges will be used to show the response of nanometer-thick PFSA films (such as those relevant for water desalination, fuel cell catalysts, and more) to hydration and interactions with an organic solvent. Finally, the ability of tender resonant x-ray scattering (T-ReXS) near the sulfur K-edge to improve scattering contrast for nanostructural studies of PFSAs spanning a wide range of industrially relevant thicknesses (i.e., 2-50 micrometers). In addition, the unique ability to measure the hydrophilic domains of a variety of cation exchanged/contaminated (i.e. Co, Ce, Na, etc) samples and impact beam damage on PFSA membranes and their resonant response will be discussed with the audience in attendance.

Resonant Inelastic Soft X-ray Scattering Study of Co-Doped Maghemite Nanoparticles.

Cobalt ferrite nanoparticles have been attracting considerable interest in the recent years because of the large number of potential applications, including magnetic storage, magnetic fluid hyperthermia and as contrast agents for magnetic resonance imaging. Physical properties of this class of materials depend critically on a number of parameters, including crystallinity, stoichiometry and cation distribution. In this work we have performed a Resonant Inelastic soft X-ray Scattering (RIXS) study on a series of 5 nm cobalt-doped maghemite nanoparticles to obtain direct quantitative information on cation distribution as a function of cobalt doping. We found that the distribution of divalent cobalt is stable in the investigated doping range and slightly different from that of bulk, stoichiometric cobalt ferrite. These results confirm that cobalt doping can be used to finely tune the magnetic properties of nanostructured ferrites without modifying their structural integrity.

Resonant Soft X-ray Scattering for the Stochastic Origin Analysis in EUV Resist

Resonant Soft X-ray Scattering for the Stochastic Origin Analysis in EUV Resist

Intertwined Spin and Orbital Density Waves in MnP Uncovered by Resonant Soft X-Ray Scattering

X-ray scattering experiments on the superconductor manganese phosphide reveal two types of helical orbital arrangements that appear intertwined with the double helix of spins, suggesting that orbital physics may underlie its superconductivity.

Reconstructing the three-dimensional latent image of extreme ultraviolet resists with resonant soft x-ray scattering

Extreme ultraviolet (EUV) lithography is one of the most promising printing techniques for high-volume semiconductor manufacturing at the 14-nm half-pitch device node and beyond. However, key challenges around EUV photoresist materials, such as the exposure-dose sensitivity or the line-width roughness, continue to impede its full adoption into industrial nanofab facilities. Metrology tools are required to address these challenges by helping to assess the impact of the EUV materials’ properties and processing conditions along different steps of the nanofabrication process. We apply the resonant soft x-ray scattering (RSoXS) technique to gain insights into the structure of patterned EUV resists before the development step takes place. By using energies around the carbon K-edge to take advantage of small differences in chemistry, the electronic density contrast between the exposed and unexposed regions of the resists could be enhanced in order to image the patterns with subnanometer precision. Critical-dimension grazing-incidence small-angle x-ray scattering is then performed at energies where the contrast is maximized, enabling the reconstruction of the three-dimensional shape of the latent image. We demonstrate the potential of RSoXS to provide a high-resolution height-sensitive profile of patterned EUV resists, which will help in quantifying the evolution of critical features, such as the line-edge roughness, at a key step of the nanofabrication process.

(Invited) Role of Domain Purity in Non-Fullerene Acceptor Based Organic Solar Cells

The field of non-fullerene organic solar cells (OSC) has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors with device efficiencies approaching 15%. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure–property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates a large pool of possible donor–acceptor combinations, which poses a daunting challenge for rationally screening material combinations. Resonant soft X-ray scattering (RSoXS) has proven to be a powerful technique to measure domain purity in OSC active layer blends. In polymer-fullerene systems, RSoXS has shown that higher average domain purity is generally correlated to device performance. A simple interpretation of RSoXS purity measurements is complicated by the consideration that real morphologies may be composed of three or more phases, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Whilst charges are understood to migrate to the aggregated domains due to a favorable electronic landscape, these charges might be effectively trapped in aggregates if there is insufficient number of percolating pathways in the amorphous mixed regions. Conversely, if the mixed regions have a composition much beyond the percolation threshold or if the volume faction of the mixed regions is large, then recombination is enhanced. Therefore, understanding the role of domain purity over different length scales and controlling the mixed amorphous phase for maximum charge creation and minimum recombination remains a principal challenge for non-fullerene OSCs. We will summarize several of our RSoXS studies that compare non-fullerene acceptor based organic solar cells having different primary chemical structures of the constituents and with significant processing variations. Our analysis suggests that mixed regions are often detrimental to good performance in these systems, typically having a pronounced negative impact on FF. Optimizing performance continues to be a delicate balance of possibly competing factors and the volume fraction of the ideal morphology remains to be determined. However, the miscibility of blend components, which might be probed using near-edge X-ray absorption spectroscopy as a screening tool, can guide expectations of achievable phase purity and ultimately the achievable performance of the active layer.

Polarized Soft X-ray Scattering Reveals Chain Orientation within Nanoscale Polymer Domains

Fully conjugated donor–acceptor block copolymers can serve as the active layer in organic photovoltaics and other organic electronic devices. Self-assembly into periodic domains and crystallization of the constituent blocks are crucial to enable control of the multiscale structure and consequently electronic properties. Resonant soft X-ray scattering (RSoXS) is an invaluable tool to characterize such materials, where tuning the X-ray energy and polarization reveals molecular orientation and domain spacing. Here, anisotropic soft X-ray scattering data reveal the type and degree of orientation within conjugated block copolymers composed of poly(3-hexylthiophene-2,5-diyl) and various push–pull copolymers. The maximum anisotropy is observed at the scattering vector corresponding to the spacing between block copolymer domains, which scales with the end-to-end distance of the blocks, as expected. Furthermore, the anisotropy in RSoXS data reveals that the crystalline P3HT blocks orient, on average, parallel or n...

Aluminum oxide free-standing thin films to enable nitrogen edge soft x-ray scattering

Abstract

Electrical transport properties and fractional dynamics of twist-bend nematic liquid crystal phase

The recent experimental observation of twist-bend nematic phases has kindled interest on this new class of materials. The natural twist-bend deformations of the director results in a nanoscale pitch (pseudo-layers) probed by freeze-fracture transmission electron microscopy (FFTEM) and very recently it was verified by resonant soft x-ray scattering (RSoXS) technique. It is pivotal that other mechanisms arise to certify the recent experiments and corroborate the theoretical models. Impedance spectroscopy is a method commonly used to investigate the dynamics of bound or mobile charge in the bulk or interfacial regions of materials in the vast field of condensed matter. Here we use impedance spectroscopy technique to investigate the electrical transport properties of twist-bend nematic phase in a dimeric liquid crystal mixture. The experimental data is analyzed in the framework of an anomalous diffusion equation coupled with Poisson’s equation where bulk and surface contributions are taken into account. Our fitting results agree well with the impedance measurements in the nematic (N) and the twist-bend (Ntb) phase and indicate that strong subdiffusive regimes take place in the system. We also find a large diffusion anisotropy depending on the imposed orientation, which is associated with motions along or through the pseudo-layers, showing that the impedance technique can be used to explore the molecular arrangement of Ntb phases.

Resonant Soft X-Ray Scattering Provides Protein Structure with Chemical Specificity

We introduce resonant soft X-ray scattering (RSoXS) as an approach to study the structure of proteins and other biological molecules in solution. Scattering contrast calculations suggest that RSoXS has comparable or even higher sensitivity than hard X-ray scattering because of contrast generated at the absorption edges of constituent elements, such as carbon and oxygen. Here, we demonstrate that working near the carbon edge reveals the envelope function of bovine serum albumin, using scattering volumes of 10-5 μL that are multiple orders of magnitude lower than traditional scattering experiments. Furthermore, tuning the X-ray energy within the carbon absorption edge provides different signatures of the size and shape of the protein by revealing the density of different types of bonding motifs within the protein. The combination of chemical specificity, smaller sample size, and enhanced X-ray contrast will propel RSoXS as a complementary tool to existing techniques for the study of biomolecular structure.

High-efficiency quaternary polymer solar cells enabled with binary fullerene additives to reduce nonfullerene acceptor optical band gap and improve carriers transport

The polymer/small-molecule electron donor and nonfullerene organic electron acceptor are of structural similarity with both donor and acceptor molecules consisting of polycyclic fused-ring backbone and being decorated with alkyl-chains. In this study, we report that the introduction of binary fullerenes (C60-/C70-PCBM and C60-/C70-ICBA) into a nonfullerene binary system PBDB-T:ITIC reduces the polymer-nonfullerene acceptor intermixing, obtaining higher crystallinity with (100) crystal coherence length from 28 to 29–33 nm for the ITIC, and from 14 to 20–24 nm for the PBDB-T, and improved electron and hole mobilities both. Unprecedentedly, such a protocol reduces the ITIC optical band gap from 1.59 to 1.55 eV. As consequences, higher short-circuit current-density (17.8–18.4 vs. 15.8 mA/cm2), open-circuit voltage (0.92 vs. 0.90 V) and fill-factor (0.72–0.73 vs. 0.68) are simultaneously obtained, which ultimately afford higher efficient quaternary polymer solar cells with power conversion efficiencies (PCEs) up to 12.0%–12.8% comparing to the host binary device with 9.9% efficiency. For the polymer, ITIC, and ICBA/PCBM ternary blends, 11% PCEs were recorded. The use of PCBM leads to larger red-shifting in thin film absorption and external quantum efficiency (EQE) response. Such effect is more pronounced when ICBA:PCBM mixture is used. These results indicate the size and shape of C60 and C70 as well as the substituent position of the second indene unit on C60-/C70-ICBA affect not only the blend morphology but also the electronic coupling in BHJ mixtures: the quaternary device performance increased in sequences of C70-PCBM:C70-ICBA ® C70-PCBM:C60-ICBA ® C60-PCBM:C70-ICBA ® C60-PCBM:C60-ICBA. The resonant soft X-ray scattering (RSoXS) data indicated the most refined phase separation in the C60-PCBM:C60-ICBA based blend, corresponding to its best device function among the quaternary devices. These results indicate that the using of binary fullerenes as the acceptor additives allows for tuning nonfullerene blended film’s optical properties and film-morphologies, shedding light on the designing high-performance multi-acceptor polymer solar cells.

Resonant soft X-ray scattering reveals cellulose microfibril spacing in plant primary cell walls

Cellulose microfibrils are crucial for many of the remarkable mechanical properties of primary cell walls. Nevertheless, many structural features of cellulose microfibril organization in cell walls are not yet fully described. Microscopy techniques provide direct visualization of cell wall organization, and quantification of some aspects of wall microstructure is possible through image processing. Complementary to microscopy techniques, scattering yields structural information in reciprocal space over large sample areas. Using the onion epidermal wall as a model system, we introduce resonant soft X-ray scattering (RSoXS) to directly quantify the average interfibril spacing. Tuning the X-ray energy to the calcium L-edge enhances the contrast between cellulose and pectin due to the localization of calcium ions to homogalacturonan in the pectin matrix. As a consequence, RSoXS profiles reveal an average center-to-center distance between cellulose microfibrils or microfibril bundles of about 20 nm.