A NIST facility for resonant soft x-ray scattering measuring nano-scale soft matter structure at NSLS-II

Resonant Soft X-ray Scattering for Organic Photovoltaics

Resonant Soft X-ray Scattering (RSoXS) has emerged as a powerful technique for probing the morphology in organic bulk heterojunction (BHJ) solar cells, frequently employed as a measurement of phase purity and compositional length scales. Here we use the National Institute of Standards and Technology RSoXS Simulation Suite to systematically examine how structural features common to BHJs would contribute to RSoXS patterns in the PM6:Y6 BHJ system. Starting from experimentally determined anisotropic optical constants, we simulate scattering from controlled morphological variations including compositional heterogeneity, interfacial sharpness, surface roughness, and molecular orientation. Our results demonstrate that noncompositional features can cause increases in scattering intensity exceeding those from compositional phase separation. Surface roughness of just a few nanometers produces substantial scattering due to the high contrast between organic materials and vacuum, and molecular orientation effects─whether random, interface-aligned, or independently correlated─can dramatically influence pattern intensity and shape. However, each structural feature exhibits a distinct energy-dependent scattering signature across the carbon K-edge, suggesting that careful analysis of the complete spectral response could enable deconvolution of multiple contributions. These findings provide a broader interpretation of the excellent correlations between RSoXS measurements and BHJ solar cell device performance, while highlighting the potential of forward simulation approaches to leverage the full information content of energy-dependent RSoXS measurements.

Resonant Soft X-ray Scattering Reveals the Distribution of Dopants in Semicrystalline Conjugated Polymers

The distribution of counterions and dopants within electrically doped semicrystalline conjugated polymers, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), plays a pivotal role in charge transport. The distribution of counterions in doped films of P3HT with controlled crystallinity was examined using polarized resonant soft X-ray scattering (P-RSoXS). The changes in scattering of doped P3HT films containing trifluoromethanesulfonimide (TFSI-) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ•-) as counterions to the charge carriers revealed distinct differences in their nanostructure. The scattering anisotropy of P-RSoXS from doped blends of P3HT was examined as a function of the soft X-ray absorption edge and found to vary systematically with the composition of crystalline and amorphous domains and by the identity of the counterion. A computational methodology was developed and used to simulate the soft X-ray scattering as a function of morphology and molecular orientation of the counterions. Modeling of the P-RSoXS at N and F K-edges was consistent with a structure where the conjugated plane of F4TCNQ•- aligns perpendicularly to that of the P3HT backbone in ordered domains. In contrast, TFSI- was distributed more uniformly between domains with no significant molecular alignment. The approach developed here demonstrates the capabilities of P-RSoXS in identifying orientation, structural, and compositional distributions within doped conjugated polymers using a computational workflow that is broadly extendable to other soft matter systems.

Self-Assembly of Hierarchical High-χ Fluorinated Block Copolymers with an Orthogonal Smectic-within-Lamellae 3 nm Sublattice and Vertical Surface Orientation

Hierarchical structure-within-structure assemblies offer a route toward increasingly complex and multifunctional materials while pushing the limits of block copolymer self-assembly. We present a detailed study of the self-assembly of a series of fluorinated high-χ block copolymers (BCPs) prepared via postmodification of a single poly(styrene)-block-poly(glycidyl methacrylate) (S-b-G) parent polymer with the fluorinated alkylthiol pendent groups containing 1, 6, or 8 fluorinated carbons (termed trifluoro-ethanethiol (TFET), perfluoro-octylthiol (PFOT), and perfluoro-decylthiol (PFDT), respectively). Bulk X-ray scattering of thermally annealed samples demonstrates hierarchical molecular assembly with phase separation between the two blocks and within the fluorinated block. The degree of ordering within the fluorinated block is highly sensitive to synthetic variation; a lamellar sublattice was formed for S-b-GPFOT and S-b-GPFDT. Thermal analyses of S-b-GPFOT reveal that the fluorinated block exhibits liquid crystal-like ordering. The complex thin-film self-assembly behavior of an S-b-GPFOT polymer was investigated using real-space (atomic force microscopy and scanning electron microscopy) and reciprocal-space (resonant soft X-ray scattering (RSoXS), grazing incidence small- and wide-angle scattering) measurements. After thermal annealing in nitrogen or vacuum, films thicker than 1.5 times the primary lattice spacing exhibit a 90-degree grain boundary, exposing a thin layer of vertical lamellae at the free interface, while exhibiting horizontal lamellae on the preferential (polystyrene brush) substrate. RSoXS measurements reveal the near-perfect orthogonality between the primary and sublattice orientations, demonstrating hierarchical patterning at the nanoscale.

Flatfielding of hybrid pixel detectors in tender x-ray scattering

The ability of the soft matter interfaces beamline at National Synchrotron Light Source II to access x-ray energy in the tender x-ray regime, i.e., from 2.1 to 5 keV, enables new resonant x-ray scattering studies at the sulfur K-edge and others. We present a new approach to correct data acquired in the tender x-ray regime with a Pilatus3 detector in order to improve the data quality and to correct the various artifacts inherent to hybrid pixel detectors, such as variations in modules' efficiency or noisy detector module junctions. This new flatfielding significantly enhances the data quality and enables detection of weak scattering signals.

Probing morphology and chemistry in complex soft materials within situresonant soft x-ray scattering

Small angle scattering methodologies have been evolving at fast pace over the past few decades due to the ever-increasing demands for more details on the complex nanostructures of multiphase and multicomponent soft materials like polymer assemblies and biomaterials. Currently, element-specific and contrast variation techniques such as resonant (elastic) soft/tender x-ray scattering, anomalous small angle x-ray scattering, and contrast-matching small angle neutron scattering, or combinations of above are routinely used to extract the chemical composition and spatial arrangement of constituent elements at multiple length scales and examine electronic ordering phenomena. Here we present some recent advances in selectively characterizing structural architectures of complex soft materials, which often contain multi-components with a wide range of length scales and multiple functionalities, where novel resonant scattering approaches have been demonstrated to decipher a higher level of structural complexity that correlates to functionality. With the advancement of machine learning and artificial intelligence assisted correlative analysis, high-throughput and autonomous experiments would open a new paradigm of material research. Further development of resonant x-ray scattering instrumentation with crossplatform sample environments will enable multimodalin situ/operando characterization of the system dynamics with much improved spatial and temporal resolution.