Operando Study Insights into Lithiation/Delithiation Processes in a Poly(ethylene oxide) Electrolyte of All-Solid-State Lithium Batteries by Grazing-Incidence X-ray Scattering.

Poly(ethylene oxide) (PEO)-based composite electrolytes (PCEs) are considered as promising candidates for next-generation lithium-metal batteries (LMBs) due to their high safety, easy fabrication, and good electrochemical stability. Here, we utilize operando grazing-incidence small-angle and wide-angle X-ray scattering to probe the correlation of electrochemically induced changes and the buried morphology and crystalline structure of the PCE. Results show that the two irreversible reactions, PEO-Li+ reduction and TFSI- decomposition, cause changes in the crystalline structure, array orientation, and morphology of the PCE. In addition, the reversible Li plating/stripping process alters the inner morphology, especially the PEO-LiTFSI domain radius and distance between PEO-LiTFSI domains, rather than causing crystalline structure and orientation changes. This work provides a new path to monitor a working battery in real time and to a detailed understanding of the Li+ diffusion mechanism, which is essential for developing highly transferable and interface-stable PCE-based LMBs.

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting.

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV-visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

Diamond-lattice photonic crystals assembled from DNA origami.

Colloidal self-assembly allows rational design of structures on the micrometer and submicrometer scale. One architecture that can generate complete three-dimensional photonic bandgaps is the diamond cubic lattice, which has remained difficult to realize at length scales comparable with the wavelength of visible or ultraviolet light. In this work, we demonstrate three-dimensional photonic crystals self-assembled from DNA origami that act as precisely programmable patchy colloids. Our DNA-based nanoscale tetrapods crystallize into a rod-connected diamond cubic lattice with a periodicity of 170 nanometers. This structure serves as a scaffold for atomic-layer deposition of high-refractive index materials such as titanium dioxide, yielding a tunable photonic bandgap in the near-ultraviolet.

Microstrain and Crystal Orientation Variation within Naked Triple-Cation Mixed Halide Perovskites under Heat, UV, and Visible Light Exposure.

The instability of perovskite absorbers under various environmental stressors is the most significant obstacle to widespread commercialization of perovskite solar cells. Herein, we study the evolution of crystal structure and microstrain present in naked triple-cation mixed CsMAFA-based perovskite films under heat, UV, and visible light (1 Sun) conditions by grazing-incidence wide-angle X-ray scattering (GIWAXS). We find that the microstrain is gradient distributed along the surface normal of the films, decreasing from the upper surface to regions deeper within the film. Moreover, heat, UV, and visible light treatments do not interfere with the crystalline orientations within annealed polycrystalline films. However, when subjected to heat, the naked perovskite films exhibit a rapid component decomposition, induced by phase separation and ion migration. Conversely, under exposure to UV and 1 Sun light soaking, the naked perovskite films undergo a self-optimization structure evolution during degradation and develop into smoother films with reduced surface potential fluctuations.

Nanostructured Black Silicon as a Stable and Surface-Sensitive Platform for Time-Resolved In Situ Electrochemical Infrared Absorption Spectroscopy.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial chemical processes. However, SEIRAS-active nanostructured metallic thin films for the in situ analysis of electrochemical phenomena are often unstable under biased aqueous conditions. In this work, we present a surface-enhancing structure based on etched black Si internal reflection elements with Au-coatings for in situ electrochemical ATR-SEIRAS. Using electrochemical potential-dependent adsorption and desorption of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates offer advantages over commonly used structures, such as electroless-deposited Au on Si and electrodeposited Au on ITO-coated Si, due to the combination of high stability, sensitivity, and conductivity. These characteristics are especially valuable for time-resolved measurements where stable substrates are required over extended times. Furthermore, the low sheet resistance of Au layers on black Si reduces the RC time constant of the electrochemical cell, enabling a significantly higher time resolution compared to that of traditional substrates. Thus, we employ black Si-based substrates in conjunction with rapid- and step-scan Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption and desorption kinetics of 4-methoxypyridine during in situ electrochemical potential steps. Adsorption is shown to be diffusion-limited, which allows for the determination of the mean molecular area in a fully established monolayer. Moreover, no significant changes in the peak ratios of vibrational modes with different orientations relative to the molecular axis are observed, suggesting a single adsorption mode and no alteration of the average molecular orientation during the adsorption process. Overall, this study highlights the enhanced performance of black Si-based substrates for both steady-state and time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful platform for kinetic and mechanistic investigations of electrochemical interfaces.

Tuning Carbon Dioxide Reduction Reaction Selectivity of Bi Single-Atom Electrocatalysts with Controlled Coordination Environments.

Control over product selectivity of the electrocatalytic CO2 reduction reaction (CO2RR) is a crucial challenge for the sustainable production of carbon-based chemical feedstocks. In this regard, single-atom catalysts (SACs) are promising materials due to their tunable coordination environments, which could enable tailored catalytic activities and selectivities, as well as new insights into structure-activity relationships. However, direct evidence for selectivity control via systematic tuning of the SAC coordination environment is scarce. In this work, we have synthesized two differently coordinated Bi SACs anchored to the same host material (carbon black) and characterized their CO2RR activities and selectivities. We find that oxophilic, oxygen-coordinated Bi atoms produce HCOOH, while nitrogen-coordinated Bi atoms generate CO. Importantly, use of the same support material assured that alternation of the coordination environment is the dominant factor for controlling the CO2RR product selectivity. Overall, this work demonstrates the structure-activity relationship of Bi SACs, which can be utilized to establish control over CO2RR product distributions, and highlights the promise for engineering atomic coordination environments of SACs to tune reaction pathways.

Bright circularly polarized photoluminescence in chiral layered hybrid lead-halide perovskites.

Hybrid perovskite semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while softness of their crystal lattice accommodates lattice strain to maintain high crystal quality with low defect densities, necessary for high luminescence yields. We report photoluminescence quantum efficiencies as high as 39% and degrees of circularly polarized photoluminescence of up to 52%, at room temperature, in the chiral layered hybrid lead-halide perovskites (R/S/Rac)-3BrMBA2PbI4 [3BrMBA = 1-(3-bromphenyl)-ethylamine]. Using transient chiroptical spectroscopy, we explain the excellent photoluminescence yields from suppression of nonradiative loss channels and high rates of radiative recombination. We further find that photoexcitations show polarization lifetimes that exceed the time scales of radiative decays, which rationalize the high degrees of polarized luminescence. Our findings pave the way toward high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis.

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO2 reduction, but its impact on the performance of CO2 electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO2 electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO2 electrolysis performance, resulting in variations of C2+ product Faradaic efficiency (FE) by >3×, with C2+ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO2 reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO2/H2O concentration ratio and promoting high CO surface coverage to facilitate C2+ production in long-term CO2 electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO2 reduction systems.

Solution-Processed NiPS3 Thin Films from Liquid Exfoliated Inks with Long-Lived Spin-Entangled Excitons.

Antiferromagnets are promising materials for future opto-spintronic applications since they show spin dynamics in the THz range and no net magnetization. Recently, layered van der Waals (vdW) antiferromagnets have been reported, which combine low-dimensional excitonic properties with complex spin-structure. While various methods for the fabrication of vdW 2D crystals exist, formation of large area and continuous thin films is challenging because of either limited scalability, synthetic complexity, or low opto-spintronic quality of the final material. Here, we fabricate centimeter-scale thin films of the van der Waals 2D antiferromagnetic material NiPS3, which we prepare using a crystal ink made from liquid phase exfoliation (LPE). We perform statistical atomic force microscopy (AFM) and scanning electron microscopy (SEM) to characterize and control the lateral size and number of layers through this ink-based fabrication. Using ultrafast optical spectroscopy at cryogenic temperatures, we resolve the dynamics of photoexcited excitons. We find antiferromagnetic spin arrangement and spin-entangled Zhang-Rice multiplet excitons with lifetimes in the nanosecond range, as well as ultranarrow emission line widths, despite the disordered nature of our films. Thus, our findings demonstrate scalable thin-film fabrication of high-quality NiPS3, which is crucial for translating this 2D antiferromagnetic material into spintronic and nanoscale memory devices and further exploring its complex spin-light coupled states.

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction.

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO2. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cucub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cucub/C, wherein the CoPc and Cucub share an interface, and CoPc-C/Cucub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cucub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C2 products almost doubles for the CoPc-Cucub/C, whereas it decreases by half for the CoPc-C/Cucub, compared to the Cucub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

Using Metal-Organic Frameworks to Confine Liquid Samples for Nanoscale NV-NMR.

Atomic-scale magnetic field sensors based on nitrogen vacancy (NV) defects in diamonds are an exciting platform for nanoscale nuclear magnetic resonance (NMR) spectroscopy. The detection of NMR signals from a few zeptoliters to single molecules or even single nuclear spins has been demonstrated using NV centers close to the diamond surface. However, fast molecular diffusion of sample molecules in and out of the nanoscale detection volumes impedes their detection and limits current experiments to solid-state or highly viscous samples. Here, we show that restricting diffusion by confinement enables nanoscale NMR spectroscopy of liquid samples. Our approach uses metal-organic frameworks (MOF) with angstrom-sized pores on a diamond chip to trap sample molecules near the NV centers. This enables the detection of NMR signals from a liquid sample, which would not be detectable without confinement. These results set the route for nanoscale liquid-phase NMR with high spectral resolution.

Tandem electrocatalytic CO2 reduction with Fe-porphyrins and Cu nanocubes enhances ethylene production.

Copper-based tandem schemes have emerged as promising strategies to promote the formation of multi-carbon products in the electrocatalytic CO2 reduction reaction. In such approaches, the CO-generating component of the tandem catalyst increases the local concentration of CO and thereby enhances the intrinsic carbon-carbon (C-C) coupling on copper. However, the optimal characteristics of the CO-generating catalyst for maximizing the C2 production are currently unknown. In this work, we developed tunable tandem catalysts comprising iron porphyrin (Fe-Por), as the CO-generating component, and Cu nanocubes (Cucub) to understand how the turnover frequency for CO (TOFCO) of the molecular catalysts impacts the C-C coupling on the Cu surface. First, we tuned the TOFCO of the Fe-Por by varying the number of orbitals involved in the π-system. Then, we coupled these molecular catalysts with the Cucub and assessed the current densities and faradaic efficiencies. We discovered that all of the designed Fe-Por boost ethylene production. The most efficient Cucub/Fe-Por tandem catalyst was the one including the Fe-Por with the highest TOFCO and exhibited a nearly 22-fold increase in the ethylene selectivity and 100 mV positive shift of the onset potential with respect to the pristine Cucub. These results reveal that coupling the TOFCO tunability of molecular catalysts with copper nanocatalysts opens up new possibilities towards the development of Cu-based catalysts with enhanced selectivity for multi-carbon product generation at low overpotential.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

Catalytic Metasurfaces Empowered by Bound States in the Continuum.

Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO2-x) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO2-x BIC metasurface, thus providing a general framework for maximizing light-matter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.

Optically Induced Long-Lived Chirality Memory in the Color-Tunable Chiral Lead-Free Semiconductor (R)/(S)-CHEA4Bi2BrxI10-x (x = 0-10).

Hybrid organic-inorganic networks that incorporate chiral molecules have attracted great attention due to their potential in semiconductor lighting applications and optical communication. Here, we introduce a chiral organic molecule (R)/(S)-1-cyclohexylethylamine (CHEA) into bismuth-based lead-free structures with an edge-sharing octahedral motif, to synthesize chiral lead-free (R)/(S)-CHEA4Bi2BrxI10-x crystals and thin films. Using single-crystal X-ray diffraction measurements and density functional theory calculations, we identify crystal and electronic band structures. We investigate the materials' optical properties and find circular dichroism, which we tune by the bromide-iodide ratio over a wide wavelength range, from 300 to 500 nm. We further employ transient absorption spectroscopy and time-correlated single photon counting to investigate charge carrier dynamics, which show long-lived excitations with optically induced chirality memory up to tens of nanosecond timescales. Our demonstration of chirality memory in a color-tunable chiral lead-free semiconductor opens a new avenue for the discovery of high-performance, lead-free spintronic materials with chiroptical functionalities.

Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality.

We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO4) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO4 films are smooth and compact, possessing remarkably high structural quality across complete 2'' wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO4, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO4, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.

The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells.

Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.

Surface NMR using quantum sensors in diamond.

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method's capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid-liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon.

Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.

Metastable Ta2N3 with highly tunable electrical conductivity via oxygen incorporation.

The binary Ta-N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach - by which the carrier concentration is reduced by the oxygen impurity - offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.