73 Search Results

Traditional heterogeneous catalysis is constrained by kinetic and thermodynamic limits, such as the Sabatier principle and reaction equilibrium. Dynamic and resonant catalysts hold promise to overcome these limitations by actively oscillating a catalyst’s physical or electronic structure at the time scale of the catalytic cycle, allowing programmable control over reaction pathways, and leading to improved rate and selectivity. External stimuli such as temperature swing, mechanical strain, electric charge, and light can perturb catalyst surfaces in different ways, altering adsorbate coverage, binding energies, and transition states beyond what steady-state catalysis allows. This work surveys the current state of dynamic catalysis, introducesmore » the concept of “stimulando” characterization for observing transient dynamics, and outlines key modeling, mechanistic, and benchmarking strategies to advance the field toward improved chemical transformation.« less

A set of modules for the informal learning of quantum science was developed. They include (1) Waves and Bottling Light in Quantum Dots, (2) Quantization of Energy Levels, (3) Particle-Wave Duality, (4) Magnetism and Electron Spin, and (5) Quantum Entanglement. Their teaching objective is to clarify concepts in quantum science, and they have been presented together as part of an hour-long show to ∼250 adults and school-age children. The learning outcomes of the modules were assessed by pre- and postevent quizzes as well as interactive clicker questions. The results suggest effective learning of all of the assessed concepts. These modulesmore » are detailed in a way that makes them deployable, together or in part, in other formal or informal settings to support the dissemination of information about quantum science to the general public.« less

Complex interactions between mobile ions and charge carriers in perovskite solar cells (PSCs) make it challenging to fully understand their dynamic interplay. Exposure to light further complicates these interactions, altering the system’s dynamics and inducing nonlinear effects that lead to changes in the J−V curve. Understanding these effects is crucial for improving the operational stability of PSCs. Impedance spectroscopy (IS) is a powerful technique for evaluating relaxation processes in the frequency domain; however, it is limited in capturing nonlinear contributions. Here, in this work, nonlinear impedance spectroscopy (NLIS) is employed to analyze the higher harmonic response to AC perturbation, bothmore » in the dark and after short-term light exposure. A shift in the low-frequency (LF) higher harmonic peak is observed after open-circuit light exposure, attributed to an altered electric field suggesting ion re-distribution, whereas closed-circuit exposure shows no LF shift, indicating minimal ion movement. Additionally, light exposure reduces higher-order admittance, more notably in open-circuit conditions, suggesting decreased recombination. Temperature-dependent analysis was conducted to characterize the activation energy of migrating species, identifying iodide as the dominant migrating ion.« less

Here, operando X-ray transient absorption measurements were successfully performed for the first time on the Co-Pi catalyst, confirming a photoreduction in Co-Pi under 400 nm blue light excitation. This provides key insights into spectroelectrochemical mechanistic studies of relevant oxygen evolution catalysts and guidance for photocatalysis using these metal oxides. Limiting blue-light exposure of these films should allow for better self-healing and higher turnover in photon-driven water oxidation. These results also demonstrate the importance of performing XTA measurements under operating conditions. Further development of this technique will allow for analysis of complex materials such as tethered molecules on surfaces and inmore » complex device architectures to yield beneficial data for applied materials.« less

The lack of intrinsic mirror symmetry in cavity mirrors poses a significant challenge for most organic chiral materials in generating circularly polarized (CP) lasers. However, nonreciprocal chiroptical materials, such as recently developed organic thin films exhibiting apparent circular dichroism (ACD), provide a promising approach to CP light generation. In this work, we integrate an ACD-based thin film into a free-space dye laser cavity, achieving direct CP laser emission with a degree of circular polarization (DOCP) up to 0.6, corresponding to a dissymmetry factor (g_lum) of 1.2, a new record for organic chiral lasers. The degree of polarization (DOP) is closemore » to 0.8, and the observed ellipticity in the emitted light originates from the ACD effect in the thin film, leading to asymmetric cavity losses for right- and left-circularly polarized light. This breakthrough demonstrates the potential of ACD-based materials to overcome the limitations of conventional chiral laser systems, marking a significant advancement in the field and paving the way for next-generation chiral photonic devices.« less

Radiation detectors are crucial in a wide variety of research and commercial applications, such as oil and gas exploration, medical imaging, nuclear nonproliferation, and homeland security. Neutron and gamma-ray detectors are fundamental components in portal monitors at ports and border crossings, bolstering national security against radiological threats. This study presents a dual-mode scintillator, undoped and Tl-doped ⁶Li-Cs₃Cu₂I₅, and demonstrates its potential as a promising material for simultaneous thermal neutron and gamma-ray detection. We explore the Bridgman growth of both undoped and thallium doped Li → Cu and Li → Cs substitutional systems with various Li doping levels and assess theirmore » impact on scintillation properties. Under 662 keV gamma-ray excitation, the undoped crystals had light yields up to 35,900 ph/MeV, with energy resolutions down to 4.5%. The Tl-doped crystals performed better than the undoped crystals with light yields peaking at 65,900 ph/MeV and energy resolutions as low as 3.5%. When exposed to a moderated ²⁵²Cf excitation source, our crystals had light yields between 102,900 and 167,200 photons per thermal neutron capture, with a full energy thermal neutron peak reaching 3 MeV in gamma equivalent energy. Pulse shape discrimination studies reveal well-separated gamma and neutron events, resulting in Figure-Of-Merit (FOM) as high as 3.7. Furthermore, these findings highlight the potential of Li-doped Cs₃Cu₂I₅ as a viable candidate for next-generation dual-mode scintillators.« less

The peculiarities of radiative and nonradiative processes associated with self-trapped intrinsic eXcitons in the excited β-Ga₂O₃ crystals are studied via time-resolved techniques of induced absorption, transient grating, and photoluminescence (PL) at room temperature. The excitation above the bandgap is produced by laser pulses with linear light polarization parallel and orthogonal in the (–201) and (001) planes. We elucidate that the nonradiative recombination rate occurring in the eXciton prevails over its radiative emission rate in a wide range of free carrier concentration composed of excited and equilibrium electrons. Hence, the nonradiative recombination has no effect on the strong anisotropy and themore » shape of the eXciton emission band. However, we find out that the conventional ABC model of electron effective lifetime is insufficient for explanation of the excitation dependences. Inclusion of two nonradiative Auger mechanisms in a modified ABC formula provides excellent agreement of these dependences. We conclude that the trap-assisted Auger process is in proportion to the free electron density with coefficient B = 1.1 × 10^–11 cm³/s and appears at low/intermediate excitation, while the triple-particle Auger process is in proportion to Δn² with coefficient C = 8 × 10^–30 cm⁶/s and appears at high excitation conditions. The transition between two Auger mechanisms is accompanied by a rise of the eXciton diffusivity in preferred crystallographic directions where the radiative PL intensity is maximal. The diffusion length LD in these directions can reach values ~300 nm, but, at high excitations, LD becomes limited by Auger lifetimes. These findings pave the way for the implementation of self-trapped eXcitons into specific optoelectronic devices.« less

Photocatalytic C–H activation is an emerging area of research. While cerium chloride photocatalysts have been extensively studied, the role of alcohol additives in these systems remains a subject of ongoing discussion. It was demonstrated that the photocatalyst [NEt₄]₂[Ce^IVCl₆] (1) produces •Cl and added alcohols exhibit zero-order kinetics. Prior studies by other researchers suggested that 1 and alcohols lead to cerium alkoxide [Ce–OR] and alkoxy radical intermediates. Here, to understand these seemingly divergent mechanistic proposals, an expanded investigation comparing cerium(IV) catalyst 1 and cerium(III) complex [NEt₄]₃[Ce^IIICl₆] (2), which exhibit markedly different reactivity and C–H selectivity, is disclosed. Our findings reveal thatmore » alcohol additives accelerate the conversion of cerium(III) to cerium(IV) catalysts, forming key intermediates such as [NEt₄]₂[Ce^IIICl₅(HOCH₃)] (5) and [NEt₄]₂[Ce^IVCl₅(OCH₃)] (6), driven by excited-state di-tert-butyl azodicarboxylate under blue light irradiation. The active complex 6 releases the •OCH₃ radical, in sharp contrast to •Cl radicals initiated by cerium(IV) photoredox catalyst 1. These different reactivity and selectivity profiles can be understood in the context of complex 5 generation and in situ formation of base to afford complex 6. Experimental validation shows enhanced selectivity toward C–H bonds with different reactivity with catalyst 1 and methanol upon the addition of base and decreased selectivity with catalyst 2 and methanol upon the addition of acid. These findings unify the previously contrasting observations of cerium halide/alkoxide photocatalytic systems and provide a comprehensive understanding on the essential role of base/acid and alcohol in selectivity and reactivity.« less

Direct detection of circularly polarized light (CPL) holds great promise for the development of various optical technologies. Chiral 2D organic–inorganic halide perovskites make it possible to fabricate CPL-sensitive photodetectors. However, selectively detecting left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light remains a significant challenge. Herein, we demonstrate a greatly enhanced distinguishability of photodiode-type CPL photodetectors based on chiral 2D perovskites with mixed chiral aryl (R)-(+),(S)-(–)-α-methylbenzylammonium (R,S-MBA) and achiral alkyl n-butylammonium (nBA) cations. The (R,S-MBA_0.5nBA_0.5)₂PbI₄ perovskites exhibit a 10-fold increase in circular dichroism signals compared to (R,S-MBA)₂PbI₄ perovskites. Here, the CPL photodetectors based on the mixed-cation perovskites exhibit self-poweredmore » capabilities with a specific detectivity of 2.45 × 10¹² Jones at a o V bias. Notably, these devices show high distinguishability (gres) factors of –0.58 and +0.54 based on (R,S-MBA_0.5nBA_0.5)₂PbI₄ perovskites, respectively, surpassing the performance of (R-MBA)₂PbI₄-based devices by over 3-fold and setting a record for CPL detectors based on chiral 2D n = 1 perovskites.« less

Metasurfaces have recently risen to prominence in optical research, providing unique functionalities that can be used for imaging, beam forming, holography, polarimetry, and many more, while keeping device dimensions small. Despite the fact that a vast range of basic metasurface designs has already been thoroughly studied in the literature, the number of metasurface-related papers is still growing at a rapid pace, as metasurface research is now spreading to adjacent fields, including computational imaging, augmented and virtual reality, automotive, display, biosensing, nonlinear, quantum and topological optics, optical computing, and more. At the same time, the ability of metasurfaces to perform opticalmore » functions in much more compact optical systems has triggered strong and constantly growing interest from various industries that greatly benefit from the availability of miniaturized, highly functional, and efficient optical components that can be integrated in optoelectronic systems at low cost. This creates a truly unique opportunity for the field of metasurfaces to make both a scientific and an industrial impact. Furthermore, the goal of this Roadmap is to mark this “golden age” of metasurface research and define future directions to encourage scientists and engineers to drive research and development in the field of metasurfaces toward both scientific excellence and broad industrial adoption.« less