Unravelling photoisomerization dynamics in a metastable-state photoacid.

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Machine Intelligence-Centered System for Automated Characterization of Functional Materials and Interfaces.

Classic design of experiment relies on a time-intensive workflow that requires planning, data interpretation, and hypothesis building by experienced researchers. Here, we describe an integrated, machine-intelligent experimental system which enables simultaneous dynamic tests of electrical, optical, gravimetric, and viscoelastic properties of materials under a programmable dynamic environment. Specially designed software controls the experiment and performs on-the-fly extensive data analysis and dynamic modeling, real-time iterative feedback for dynamic control of experimental conditions, and rapid visualization of experimental results. The system operates with minimal human intervention and enables time-efficient characterization of complex dynamic multifunctional environmental responses of materials with simultaneous data processing and analytics. The system provides a viable platform for artificial intelligence (AI)-centered material characterization, which, when coupled with an AI-controlled synthesis system, could lead to accelerated discovery of multifunctional materials.

Magnetic and Optical Properties of Au-Co Solid Solution and Phase-Separated Thin Films and Nanoparticles.

The chemical composition and morphology of AuxCo1-x thin films and nanoparticles are controlled via a combination of cosputtering, pulsed laser-induced dewetting (PLiD), and annealing, leading to tunable magnetic and optical properties. Regardless of chemical composition, the as-deposited thin films and as-PLiD nanoparticles are found to possess a face-centered cubic (FCC) AuxCo1-x solid-solution crystal structure. Annealing results in large phase-separated grains of Au and Co in both the thin films and nanostructures for all chemical compositions. The magnetic and optical properties are characterized via vibrating sample magnetometry (VSM), ellipsometry, optical transmission spectroscopy, and electron energy loss spectroscopy (EELS). Despite the exceptionally high magnetic anisotropy inherent to Co, the presence of sufficient Au (72 atom %) in the AuxCo1-x solid solution results in superparamagnetic thin films. Among the as-PLiD nanoparticle samples, an increased Co composition leads to a departure from traditional ferromagnetism in favor of wasp-waisted hysteresis caused by magnetic vortices. Phase separation resulting from annealing leads to ferromagnetism for all compositions in both the thin films and nanoparticles. The optical properties of AuxCo1-x nanostructures are also largely influenced by the chemical morphology, where the AuxCo1-x intermixed solid solution has significantly damped plasmonic performance relative to pure Au and comparable to pure Co. Phase separation greatly enhances the quality factor, optical absorption, and electron energy loss spectroscopy (EELS) signatures. The enhancement of the localized surface plasmon resonances (LSPRs) scales with the reduction in Co composition, despite EELS evidence that excitation of the Co portions of a nanoparticle can provide a similar, and in some instances enhanced, LSPR resonance compared to Au. This behavior, however, is seemingly limited to the LSPR dipole mode, while higher-order modes are greatly damped by a Co aloof position. This observed magneto-plasmonic functionality and tunability could be applicable in biomedicine, namely, cancer therapeutics.

Quantifying fish otolith mineralogy for trace-element chemistry studies.

Otoliths are frequently used to infer environmental conditions or fish life history events based on trace-element concentrations. However, otoliths can be comprised of any one or combination of the three most common polymorphs of calcium carbonate-aragonite, calcite, and vaterite-which can affect the ecological interpretation of otolith trace-element results. Previous studies have reported heterogeneous calcium carbonate compositions between left and right otoliths but did not provide quantitative assessments of polymorph abundances. In this study, neutron diffraction and Raman spectroscopy were used to identify and quantify mineralogical compositions of Chinook salmon Oncorhynchus tshawytscha otolith pairs. We found mineralogical compositions frequently differed between otoliths in a pair and accurate calcium carbonate polymorph identification was rarely possible by visual inspection alone. The prevalence of multiple polymorphs in otoliths is not well-understood, and future research should focus on identifying otolith compositions and investigate how variations in mineralogy affect trace-element incorporation and potentially bias environmental interpretations.

Analysis of trypsin activity at ß-casein layers formed on hydrophobic surfaces using a multiharmonic acoustic method.

Proteolysis of milk proteins, such as caseins, caused by milk proteases, can change the organoleptic and nutritional characteristics of milk, and therefore it is essential to monitor this enzymatic activity. We used trypsin as a model protease because of its role as a biomarker for pancreatitis. The aim of this work was to demonstrate the detection of proteolysis of ß-casein by trypsin using a multiharmonic quartz crystal microbalance (QCM) biosensor. The ß-casein layer was deposited from a 0.1 mg mL-1 solution on a hydrophobic surface consisting of a self-assembled monolayer of 1-dodecanethiol on the gold electrode of the QCM. The addition of an increasing concentration of trypsin leads to the removal of the casein layer due to proteolysis, and correlates with an increase in the resonant frequency of the QCM. We investigated the effect of trypsin concentrations (0.3-20 nM) on the kinetics of the proteolysis of ß-casein and demonstrated that the frequency increase is proportional to the protease concentration. Consequently, an inverse Michaelis-Menten model was used to estimate the Michaelis-Menten constant (KM = 0.38 ± 0.02 nM) and the limit of detection (LOD = 0.16 ± 0.02 nM). The thickness, mass and viscoelastic properties of the protein adlayer after its formation and following the proteolytic cleavage were evaluated by means of multi-harmonic analysis. We found that ß-casein is preferably adsorbed on the hydrophobic surfaces as an asymmetrical double layer, of which the innermost layer was found to be denser and thinner (about 2.37 nm) and the outermost layer was found to be less tightly bound and thicker (about 3.5 nm).

Optically Induced Static Magnetization in Metal Halide Perovskite for Spin-Related Optoelectronics.

Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin-related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth-sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect. It is demonstrated that a circularly polarized photoexcitation induces a stable magnetization signal within the depth up to 7.5 nm into the surface of high-quality perovskite (MAPbBr3) film underneath a ferromagnetic cobalt layer at room temperature. In contrast, a linearly polarized light does not induce any detectable magnetization in the MAPbBr3. The observation reveals that photoexcited orbital magnetic dipoles at the surface of perovskite are coupled with the spins of the ferromagnetic atoms at the interface, leading to an optically induced magnetization within the perovskite's surface. The finding demonstrates that perovskite semiconductor can be bridged with magnetism through optically controllable method at room temperature in this heterojunction design. This provides the new concept of utilizing spin and orbital degrees of freedom in new-generation spin-related optoelectronic devices.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy.

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI3 )0.85 (MAPbBr3 )0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

SMART transfer method to directly compare the mechanical response of water-supported and free-standing ultrathin polymeric films.

Intrinsic mechanical properties of sub-100 nm thin films are markedly difficult to obtain, yet an ever-growing necessity for emerging fields such as soft organic electronics. To complicate matters, the interfacial contribution plays a major role in such thin films and is often unexplored despite supporting substrates being a main component in current metrologies. Here we present the shear motion assisted robust transfer technique for fabricating free-standing sub-100 nm films and measuring their inherent structural-mechanical properties. We compare these results to water-supported measurements, exploring two phenomena: 1) The influence of confinement on mechanics and 2) the role of water on the mechanical properties of hydrophobic films. Upon confinement, polystyrene films exhibit increased strain at failure, and reduced yield stress, while modulus is reduced only for the thinnest 19 nm film. Water measurements demonstrate subtle differences in mechanics which we elucidate using quartz crystal microbalance and neutron reflectometry.

Detection of Chymotrypsin by Optical and Acoustic Methods.

Chymotrypsin is an important proteolytic enzyme in the human digestive system that cleaves milk proteins through the hydrolysis reaction, making it an interesting subject to study the activity of milk proteases. In this work, we compared detection of chymotrypsin by spectrophotometric dynamic light scattering (DLS) and quartz crystal microbalance (QCM) methods and determined the limit of chymotrypsin detection (LOD), 0.15 ± 0.01 nM for spectrophotometric, 0.67 ± 0.05 nM for DLS and 1.40 ± 0.30 nM for QCM methods, respectively. The sensors are relatively cheap and are able to detect chymotrypsin in 3035 min. While the optical detection methods are simple to implement, the QCM method is more robust for sample preparation, and allows detection of chymotrypsin in non-transparent samples. We give an overview on methods and instruments for detection of chymotrypsin and other milk proteases.

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers.

We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.

Scalable synthesis of nanoporous atomically thin graphene membranes for dialysis and molecular separations via facile isopropanol-assisted hot lamination.

Scalable graphene synthesis and facile large-area membrane fabrication are imperative to advance nanoporous atomically thin membranes (NATMs) for molecular separations. Although chemical vapor deposition (CVD) allows for roll-to-roll high-quality monolayer graphene synthesis, facile transfer with atomically clean interfaces to porous supports for large-area NATM fabrication remains extremely challenging. Sacrificial polymer scaffolds commonly used for graphene transfer typically leave polymer residues detrimental to membrane performance and transfers without polymer scaffolds suffer from low yield resulting in high non-selective leakage through NATMs. Here, we systematically study the factors influencing graphene NATM fabrication and report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination (IHL) process that enables scalable, facile and clean transfer of CVD graphene on to polycarbonate track etched (PCTE) supports with coverage ≥99.2%, while preserving support integrity/porosity. We demonstrate fully functional centimeter-scale graphene NATMs that show record high permeances (∼2-3 orders of magnitude higher) and better selectivity than commercially available state-of-the-art polymeric dialysis membranes, specifically in the 0-1000 Da range. Our work highlights a scalable approach to fabricate graphene NATMs for practical applications and is fully compatible with roll-to-roll manufacturing processes.

New Insights on Plasmin Long Term Stability and the Mechanism of Its Activity Inhibition Analyzed by Quartz Crystal Microbalance.

We used the research quartz crystal microbalance (RQCM) to monitor regulatory effects of plasmin and trypsin in the presence of their inhibitor α2-antiplasmin. The gold surface of quartz crystals was modified with a ß-casein layer that served as a substrate for protease digestion. The addition of plasmin or trypsin as well as their mixtures with α2-antiplasmin resulted in an increase of resonant frequency, f, and in a decrease of motional resistance, Rm, depending on the molar ratio of protease: antiplasmin. At equimolar concentrations of protease and α2-antiplasmin (5 nM:5 nM) full inhibition of protease activity took place. Monitoring of plasmin activity on an hourly and daily basis revealed a prominent effect of autolysis and decrease of plasmin activity in freshly activated samples. The degree of inhibition as well as plasmin half-life (t1/2 = 2.48 ± 0.28 days) connected with its degradation was determined.

Non-Equilibrium Synthesis of Highly Active Nanostructured, Oxygen-Incorporated Amorphous Molybdenum Sulfide HER Electrocatalyst.

Molybdenum sulfide emerged as promising hydrogen evolution reaction (HER) electrocatalyst thanks to its high intrinsic activity, however its limited active sites exposure and low conductivity hamper its performance. To address these drawbacks, the non-equilibrium nature of pulsed laser deposition (PLD) is exploited to synthesize self-supported hierarchical nanoarchitectures by gas phase nucleation and sequential attachment of defective molybdenum sulfide clusters. The physics of the process are studied by in situ diagnostics and correlated to the properties of the resulting electrocatalyst. The as-synthesized architectures have a disordered nanocrystalline structure, with nanodomains of bent, defective S-Mo-S layers embedded in an amorphous matrix, with excess sulfur and segregated molybdenum particles. Oxygen incorporation in this structure fosters the creation of amorphous oxide/oxysulfide nanophases with high electrical conductivity, enabling fast electron transfer to the active sites. The combined effect of the nanocrystalline pristine structure and the surface oxidation enhances the performance leading to small overpotentials, very fast kinetics (35.1 mV dec-1 Tafel slope) and remarkable long-term stability for continuous operation up to -1 A cm-2. This work shows possible new avenues in catalytic design arising from a non-equilibrium technique as PLD and the importance of structural and chemical control to improve the HER performance of MoS-based catalysts.

Optical and Magnetic Properties of Ag-Ni Bimetallic Nanoparticles Assembled via Pulsed Laser-Induced Dewetting.

Pulsed laser-induced dewetting (PLiD) of Ag0.5Ni0.5 thin films results in phase-separated bimetallic nanoparticles with size distributions that depend on the initial thin film thickness. Co-sputtering of Ag and Ni is used to generate the as-deposited (AD) nanogranular supersaturated thin films. The magnetic and optical properties of the AD thin films and PLiD nanoparticles are characterized using a vibrating sample magnetometer, optical absorption spectroscopy, and electron energy loss spectroscopy (EELS). Magnetic measurements demonstrate that Ag0.5Ni0.5 nanoparticles are ferromagnetic at room temperature when the nanoparticle diameters are >20 nm and superparamagnetic <20 nm. Optical measurements show that all nanoparticle size distributions possess a local surface plasmon resonance (LSPR) peak that red-shifts with increasing diameter. Following PLiD, a Janus nanoparticle morphology is observed in scanning transmission electron microscopy, and low-loss EELS reveals size-dependent Ag and Ni LSPR dipole modes, while higher order modes appear only in the Ag hemisphere. PLiD of Ag-Ni thin films is shown to be a viable technique to generate bimetallic nanoparticles with both magnetic and plasmonic functionality.

Correlation of Spatiotemporal Dynamics of Polarization and Charge Transport in Blended Hybrid Organic-Inorganic Perovskites on Macro- and Nanoscales.

Progress in flexible organic electronics necessitates a full understanding of how local inhomogeneities impact electronic and ionic conduction pathways and underlie macroscopic device characteristics. We used frequency- and time-resolved macro- and nanoprobe measurements to study spatiotemporal characteristics of multiscale charge transport dynamics in a series of ternary-blended hybrid organic inorganic perovskites (HOIPs) (MA0.95-xFAxCs0.05PbI3). We show that A-site cation composition defines charge transport mechanisms across broad temporal (102-10-6 s) and spatial (millimeters-picometers) scales. Ab initio molecular dynamic simulations suggest that insertion of FA results in a dynamic lattice, improved ion transport, and dipole screening. We demonstrate that correlations between macro- and nanoscale measurements provide a pathway for accessing distribution of relaxation in nanoscale polarization and charge transport dynamics of ionically conductive functional perovskites.

Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures.

Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to determine the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300 °C) temperatures and bottom layer replacement for complete conversion to WSe2. Atomic-resolution electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Molecular dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable composition.

High-Resolution Laser-Induced Graphene. Flexible Electronics beyond the Visible Limit.

Laser-induced graphene (LIG) is a multifunctional graphene foam that is commonly direct-written with an infrared laser into a carbon-based precursor material. Here, a visible 405 nm laser is used to directly convert polyimide into LIG. This enabled the formation of LIG with a spatial resolution of ∼12 µm and a thickness of <5 µm. The spatial resolution enabled by the relatively smaller focused spot size of the 405 nm laser represents a >60% reduction in LIG feature sizes reported in prior publications. This process occurs in situ in an SEM chamber, thus allowing direct observation of LIG formation. The reduced size of the LIG features enables the direct-write formation of flexible electronics that are not visible to the unaided eye. A humidity sensor is demonstrated which could detect human breath with a response time of 250 ms. With the growing interest in LIG for flexible electronics and sensors, finer features can greatly expand its utility.

Hierarchical TiO2:Cu2O Nanostructures for Gas/Vapor Sensing and CO2 Sequestration.

We investigate the effect of high-surface-area self-assembled TiO2:Cu2O nanostructures for CO2 and relative humidity gravimetric detection using polyethylenimine (PEI), 1-ethyl-3-methylimidazolium (EMIM), and polyacrylamide (PAAm). Introduction of hierarchical TiO2:Cu2O nanostructures on the surface of quartz crystal microbalance sensors is found to significantly improve sensitivity to CO2 and to H2O vapor. The response of EMIM to CO2 increases fivefold for 100 nm-thick TiO2:Cu2O as compared to gold. At ambient CO2 concentrations, the hierarchical assembly operates as a sensor with excellent reversibility, while at higher pressures, the CO2 desorption rate decreases, suggesting possible application for CO2 sequestration under these conditions. The gravimetric response of PEI to CO2 increases by a factor of 3 upon introduction of a 50 nm TiO2:Cu2O layer. The PAAm gravimetric response to water vapor also increases by a factor of 3 and displays improved reversibility with the addition of 50 nm TiO2:Cu2O structures. We found that TiO2:Cu2O can be used to lower the detection limits for CO2 sensing with EMIM and PEI and lower the detection limits for H2O sensing with PAAm by over a factor of 2. Coarse-grained and all-atom molecular dynamics simulations indicate the dissociative character of ionic liquid assembly on TiO2:Cu2O interfaces and different distributions of CO2 and H2O molecules on bare and ionic liquid-coated surfaces, confirming experimental observations. Overall, our results show high potential of hierarchical assemblies of TiO2:Cu2O/room temperature ionic liquid and polymer films for sensors and CO2 sequestration.

Two-Photon Up-Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi-2D Perovskite Films.

A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA)2 (MA)4 Pb5 Br16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two-photon up-conversion PL signal. This confirms that the gap states are indeed responsible for generating the two-photon up-conversion PL in quasi-2D perovskites. Furthermore, mechanical scratching indicates that the different-n-value nanoplates are essentially uniformly formed in the quasi-2D perovskite films toward generating multi-photon up-conversion light emission. More importantly, the two-photon up-conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi-photon excitation. Polarization-dependent up-conversion PL studies reveal that the gap states experience the orbit-orbit interaction through Coulomb polarization to form spatially extended states toward developing multi-photon up-conversion light emission in quasi-2D perovskites.

Synthesis of zinc-gallate phosphors by biomineralization and their emission properties.

Reduction of target species by microorganisms and their subsequent precipitation into sparingly soluble mineral phase nanoparticles have been referred to as microbially mediated nanomaterial synthesis. Here, we describe the microbially mediated production of nano-dimensioned spinel structured zinc-gallate (ZnGa2O4) phosphors exhibiting different emission performance with varying substituted elements. Interestingly, in the microbially mediated phosphor production described herein, there were no reducible metal- and non-metal species composing the target minerals. By varying substituted elements, zinc-gallate phosphors present typical red, green, and blue (RGB) emission. An apparent whitish emission was accomplished by blending phosphors. A promising potential for white light produced by biosynthesized mixtures of Cr-, Mn-, and Co- substituted zinc-gallates representing RGB emissions was evidenced. Microbial activity supplied a reducing driving force and provided appropriate near neutral pH and reduced Eh conditions to thermodynamically precipitate spinel structured nanomaterials from supersaturated divalent and trivalent cations. This result complemented conventional biomineralization concepts and expanded the realm of biomanufacturing nanomaterials for further applications. STATEMENT OF SIGNIFICANCE: This study substantiated that circumstances of a suitable pH/Eh derived from bacterial activity, divalent/trivalent ion supply, buffering capacity, and supersaturation could precipitate spinel structure nanoparticles. Even though live or dead cells with membrane could enhance the nuclei generation, the spinel structured phases were produced regardless of existence of live or dead cells and reducible metal or non-metal species incorporating into the produced solid phases. This finding led to production of a series of metal-substituted zinc-gallates with specific RGB emission that can result in whitish light using simple blending. We believe our findings could expand the realm of nanomaterial synthesis using low cost, highly scalable bio-nanotechnology.