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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Multichannel photodissociation dynamics in CS 2 studied by ultrafast electron diffraction

In this study, the structural dynamics of photoexcited gas-phase carbon disulfide (CS 2 ) molecules are investigated using ultrafast electron diffraction. The dynamics were triggered by excitation of the optically bright 1 B 2 ( 1 Σ u + ) state by an ultraviolet femtosecond laser pulse centred at 200 nm. In accordance with previous studies, rapid vibrational motion facilitates a combination of internal conversion and intersystem crossing to lower-lying electronic states. Photodissociation via these electronic manifolds results in the production of CS fragments in the electronic ground state and dissociated singlet and triplet sulphur atoms. The structural dynamics are extracted from the experiment using a trajectory-fitting filtering approach, revealing the main characteristics of the singlet and triplet dissociation pathways. Finally, the effect of the time-resolution on the experimental signal is considered and an outlook to future experiments provided.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Quantitative relationships between film morphology, charge carrier dynamics, and photovoltaic performance in bulk-heterojunction binary vs. ternary acceptor blends

Addressing pertinent and perplexing questions regarding why nonfullerene acceptors (NFAs) promote higher power conversion efficiencies (PCEs) than traditional fullerenes and how photoactive bulk heterojunction (BHJ) film morphology, charge photogeneration, and recombination dynamics dictate solar cell performance have stimulated many studies of polymer solar cells (PSCs), yet quantitative relationships remain limited. Better understanding in these areas offers the potential to advance materials design and device engineering, afford higher PCEs, and ultimate commercialization. Here we probe quantitative relationships between BHJ film morphology, charge carrier dynamics, and photovoltaic performance in model binary and ternary blend systems having a wide bandgap donor polymer, a fullerene, and a promising NFA. We show that optimal PC 71 BM incorporation in a PBDB-TF:ITIC-Th binary system matrix retains the original π-face-on orientation, ITIC-Th crystallinity and BHJ film crystallite dimensions, and reduces film upper surface ITIC-Th segregation. Such morphology changes together simultaneously increase hole (μ h ) and electron (μ e ) mobilities, facilitate light-activated ITIC-Th to PC 71 BM domain electron delocalization, reduce free charge carrier (FC) bimolecular recombination (BR) within PBDB-TF:ITIC-Th mixed regions, and increase FC extraction pathways viaPBDB-TF:PC 71 BM pairs. The interplay of these effects yields significantly enhanced inverted cell short-circuit current density (J SC ), fill factor (FF), and PCE. Unexpectedly, we also find that excessive PC 71 BM in the PBDB-TF:ITIC-Th binary system alters the PBDB-TF orientation to π-edge-on, increases large scale PC 71 BM-rich aggregations and BHJ upper surface PC 71 BM composition. Further, these morphology changes increase parasitic decay processes such as intersystem crossing from photoexcited PC 71 BM, compromising the J SC , FF, and PCE metrics. ITIC-Th X-ray diffraction reveals a unique sidechain-dominated molecular network with previously unknown sidechain-end group stacking, rationalizing the STEM and GIWAXS results, photophysics, and the high μ e . DFT computation reveals charge transfer networks within ITIC-Th crystallites, supporting excited-state electron delocalization from ITIC-Th to PC 71 BM. This structure–property understanding leads to a newly reported NFA blend with PCE near 17%.

14 SOLAR ENERGY↗
DOE OSTI2022

Coherent ring-current migration of Mg-phthalocyanine probed by time-resolved X-ray circular dichroism

The coherent ring current of Mg-phthalocyanine created by a broad band UV-visible pump pulse shows variation with time, where the ring currents at the corner benzene rings, around the Mg cation and on the outer ring oscillate with different time periods and the current density migrates among these regions. The 7 pairs of E u degenerate excited states populated upon photoexcitation, generate 21 distinct coherent ring currents. We further calculate the time-resolved X-ray circular dichroism (TRXCD) spectrum of the coherences contributing to the ring current obtained by an attosecond X-ray probe pulse resonant with the nitrogen K-edge. A frequency domain TRXCD signal obtained by a Fourier transform of the signal with respect to the pump-probe delay time clearly separates the currents induced by different state pairs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Tailoring electron transfer pathway for photocatalytic N 2 -to-NH 3 reduction in a CdS quantum dots-nitrogenase system

The combination of abiotic photosensitizers with purified enzymes in a biohybrid system offers a promising pathway to utilizing light to accomplish challenging chemical transformations and provides insights into the rational photocatalytic system design for efficient solar-to-chemical energy conversion. In this work, we demonstrate a hybrid photocatalytic system for ammonia production from N2 by combining cadmium sulfide quantum dots (CdS QDs) and Mo-nitrogenase from Azotobacter vinelandii, composed of the iron protein (FeP) and the molybdenum-iron protein (MoFeP). Photoexcited electrons from the CdS QD are delivered by an electron transfer mediator through the FeP to the catalytic MoFeP. The complete system was optimized for the ligand on the CdS QDs, mediators, and reaction conditions. The best results were achieved with β-mercaptoethanol as a QD ligand. The mediator test revealed that 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SPr)2V (-0.4 V vs. NHE) supports the reduction of protons and N 2 to H 2 and ammonia catalyzed by nitrogenase. However, in the presence of 1,1'-trimethylene-2,2'-bipyridinium TQ (-0.54 V vs. NHE) as a mediator, nitrogenase catalysis resulted in remarkably more products. The UV-vis and in situ potentiometric studies revealed that better performance with TQ is achieved due to the significantly more negative solution potential allowing for efficient reduction of FeP. As a result, the quantum yield for conversion of absorbed photons to ammonia attains 16%, far exceeding that of previously reported nitrogenase-based systems. This work reveals the importance of tuning the electron transfer pathways in photocatalytic systems and illustrates a potent strategy for efficient electronic coupling of a photosensitizer and an N 2 reduction catalyst.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Topological quantum devices: a review

The introduction of the concept of topology into condensed matter physics has greatly deepened our fundamental understanding of transport properties of electrons as well as all other forms of quasi particles in solid materials. It has also fostered a paradigm shift from conventional electronic/optoelectronic devices to novel quantum devices based on topology-enabled quantum device functionalities that transfer energy and information with unprecedented precision, robustness, and efficiency. In this article, the recent research progress in topological quantum devices is reviewed. We first outline the topological spintronic devices underlined by the spin-momentum locking property of topology. We then highlight the topological electronic devices based on quantized electron and dissipationless spin conductivity protected by topology. Finally, we discuss quantum optoelectronic devices with topology-redefined photoexcitation and emission. The field of topological quantum devices is only in its infancy, we envision many significant advances in the near future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy) 3 ] 2+ from ultrafast 2D electronic spectroscopy

Transition metal-based charge-transfer complexes represent a broad class of inorganic compounds with diverse photochemical applications. Charge-transfer complexes based on earth-abundant elements have been of increasing interest, particularly the canonical [Fe(bpy) 3 ] 2+ . Photoexcitation into the singlet metal–ligand charge transfer ( 1 MLCT) state is followed by relaxation first to the ligand-field manifold and then to the ground state. While these dynamics have been well-studied, processes within the MLCT manifold that facilitate and/or compete with relaxation have been more elusive. We applied ultrafast two-dimensional electronic spectroscopy (2DES) to disentangle the dynamics immediately following MLCT excitation of this compound. First, dynamics ascribed to relaxation out of the initially formed 1 MLCT state was found to correlate with the inertial response time of the solvent. Second, the additional dimension of the 2D spectra revealed a peak consistent with a ~20 fs 1 MLCT → 3 MLCT intersystem crossing process. These two observations indicate that the complex simultaneously undergoes intersystem crossing and direct conversion to ligand-field state(s). Resolution of these parallel pathways in this prototypical earth-abundant complex highlights the ability of 2DES to deconvolve the otherwise obscured excited-state dynamics of charge-transfer complexes.

2-dimensional electronic spectroscopy↗
DOE OSTI2024

A type-I van der Waals heterostructure formed by monolayer WS 2 and trilayer PdSe 2

Two-dimensional (2D) heterostructures, formed by stacking 2D semiconductors through the van der Waals force, have been extensively studied recently. However, the majority of the heterostructures discovered so far possess type-II interfaces that facilitate interlayer charge separation. Type-I interfaces, on the other hand, confine both electrons and holes in one layer, which is beneficial for optical applications that utilize electron–hole radiative recombination. So far, only a few type-I 2D heterostructures have been achieved, which has limited the construction of multilayer heterostructures with sophisticated band landscapes. Here, we report experimental evidence of a type-I interface between monolayer WS 2 and trilayer PdSe 2 . Two-dimensional PdSe 2 has emerged as a promising material for infrared optoelectronic and other applications. We fabricated the heterostructure by stacking an exfoliated monolayer WS 2 flake on top of a trilayer PdSe 2 film, synthesized by chemical vapor deposition. Photoluminescence spectroscopy measurements revealed that the WS 2 exciton peak is significantly quenched in the heterostructure, confirming efficient excitation transfer from WS 2 to PdSe 2 . Femtosecond transient absorption measurements with various pump/probe configurations showed that both electrons and holes photoexcited in the WS 2 layer of the heterostructure can efficiently transfer to PdSe 2 , while neither type of carriers excited in PdSe 2 can transfer to WS 2 . These experimental findings establish a type-I band alignment between monolayer WS 2 and trilayer PdSe 2 . Furthermore, our results further highlight PdSe 2 as an important 2D material for constructing van der Waals heterostructures with emergent electronic and optoelectronic properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

High-throughput and data-driven search for stable optoelectronic AMSe 3 materials

The rapid advancement in emerging optoelectronic technologies demands highly efficient, affordable, and ecofriendly materials. In this context, ternary chalcogenides, especially ternary selenides, show early promise as a material class due to their stability and remarkable electronic, optical, and transport properties. In this work, we integrate first-principles-based high-throughput computations with machine learning (ML) techniques to predict the thermodynamic stability and optoelectronic properties of 920 valency-satisfied selenide compounds. Through investigating polymorphism, our study reveals the edge-sharing orthorhombic Pnma phase (NH 4 CdCl 3 -type) as the most stable structure for most ternary selenides. High-fidelity supervised ML models are trained and tested to accelerate stability and band gap predictions. These data-driven models pin down the most influential features that dominantly control key material characteristics. The multistep high-throughput computations identify the ternary selenides with optimal direct band gaps, light carrier masses, and strong optical absorption edges. The extensive materials screening considering phase stability, toxicity, and defect tolerance, finally identifies the seven most suitable candidates for photovoltaic applications. Two of these final compounds, SrZrSe 3 and SrHfSe 3 , have already been synthesized in a single-phase form, with the latter showing an optically suitable band gap, aligning well with our findings. The non-adiabatic molecular dynamics reveal sufficiently long photoexcited charge carrier lifetimes (on the order of nanoseconds) in some of these selected selenide materials, indicating their exciting characteristics. Overall, our study suggests a robust in silico framework that can be extended to screen large datasets of various material classes for identifying promising photoactive candidates.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Excited-state structural conformations of Fe-amido photosensitizers revealed by picosecond X-ray absorption spectroscopy

Ultrafast X-ray spectroscopy and modelling were used to characterize distortions in the photo-generated quintet state of Fe sensitizers bearing hybrid N-heterocycle/amido ligands. Photoexcitation induces a 0.10–0.35 Å Fe–N elongation that is smaller for d(Fe–N amido ) compared to d(Fe-N heterocycle ), suggesting that the partial expansion of the Fe–N bonds involving the N-heterocyclic ligands is critical to the rapid population of ligand-field states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Influence of substitution pattern on the dynamics of internal conversion and intersystem crossing in thiopyridone isomers

We report a combined experimental and theoretical investigation of the ultrafast internal conversion (IC) and intersystem crossing (ISC) dynamics of two thiopyridone (TP) isomers in solution. Our study used ultrafast transient X-ray absorption spectroscopy (XAS) at the sulfur K-edge, in conjunction with electronic excited state surface hopping molecular dynamics and simulations of the excited state XAS, to investigate the impact of the functional group substitution pattern and solvent on the dynamics of IC and ISC. The combination of the localized X-ray probe and the simulation results enables, in part, the differentiation between ππ* and nπ* character excited states, as well as singlet and triplet states. Access to nπ* character excitations has particular value since they often prove challenging to assess with optical spectroscopy. For 2-TP, the photoexcited S 2 (ππ*) state rapidly undergoes IC to the S 1 (nπ*) state below the instrument response time, followed by ISC to the T 1 (ππ*) state on a timescale of 600 fs in acetonitrile. For 4-TP, the timescale of S 2 to S 1 IC increases to 330 fs and the timescale of ISC increases to more than 10 ps. The differences between isomers are rationalized by considering the key role of the, nπ* intermediates in mediating the intersystem crossing of these systems. Varying the substitution pattern of the molecule can stabilize or destabilize these intermediates leading to the increase in ISC rate in the ortho isomer as compared to the para isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.

Garratt, Douglas [SLAC National Accelerator Labora↗
DOE OSTI2025

Time-resolved momentum imaging of UV photodynamics in structural isomers of iodopropane probed by site-selective XUV ionization

The photodynamics of 1- and 2-iodopropane (1 and 2-IP) were studied in a time-resolved scheme incorporating ultraviolet (UV) excitation and extreme ultraviolet (XUV) probing, which initiates photoionization selectively from the I 4d core orbital. UV absorption in the A-band of both isomers leads to prompt C–I bond fission, with significant disposal of internal energy into the propyl radical product. Site-selective ionization enables a range of charge transfer (CT) processes between the nascent highly charged iodine ions and neutral propyl radicals, dependent on the interfragment distance at the instant of ionization. Subtle differences in the dynamics of these CT processes between the two isomers are observed. In 1-IP, the kinetic energies of iodine ions produced by UV photodissociation and subsequent XUV multiple ionization increased notably over the first few hundred femtoseconds, which could be understood in terms of differing gradients along the photodissociation coordinates of the neutral and polycationic states involved in the pump and probe steps, respectively. Led by a recent report of HI elimination in UV photoexcited 2-IP [Todt et al., Phys. Chem. Chem. Phys., 22(46), 27338 (2020)], we also model the most likely signatures of this process in the present experiment, and can identify signal in the 2-IP data (that is absent or significantly weaker in the data from the unbranched 1-IP isomer) that is consistent with such a process occurring on ultrafast timescales.

Allum, Felix [Deutsches Elektronen-Synchrotron (DE↗
DOE OSTI2025

Tunable and Robust Optical and Structural Properties of a Cooperative Squaraine-Dye Aggregate-DNA DX-DAE Tile System

Molecular excitons, which are excitations delocalized over multiple dyes in a wavelike manner, are of interest for a wide range of applications, including quantum information science. Numerous studies have templated a variety of synthetic dyes via a DNA scaffold to induce dye aggregation to create molecular excitons upon photoexcitation. Dye aggregate optical properties are critically dependent on relative dye geometry and local environment; therefore, an understanding of dye-dye and DNA-dye interactions is critical for advancing toward more complex DNA-dye systems. The extensively studied DNA Holliday junction (HJ) and less-studied double-crossover (DX) tile motif are fundamental test beds for designing complex and ultimately modular DNA-dye architectures. Here, we report the first study of single-linked squaraine dye aggregation and exciton delocalization on a larger and more stable (compared with the HJ) DX tile motif. We first highlight a few DNA-dye constructs that support single dyes and aggregates with distinct optical properties that are both tunable—through sample design, buffer conditions, and heat treatment—and robust to environment changes, including transfer to solid phase. Next, we assess several experimental and design considerations that demonstrate directed dye-driven assembly of a novel double-tile DNA configuration. Our results demonstrate that single-linked squaraine dyes templated to DX tiles provide a viable research path to design and evaluate dye aggregate networks that support exciton delocalization. We include herein the first report of exciton delocalization in the solid phase in a DNA-dye construct. Additionally, our findings indicate that dye aggregation impacts the assembly of the DNA-dye construct, and, in some cases, thereby cooperates with the DNA to determine a final robust system configuration. Finally, we show that a controlled annealing schedule can be employed to promote the homogeneous assembly of DNA-dye constructs. The findings in this study contribute to the understanding of DNA-dye systems and the relevant factors involved in their directed assembly to achieve specific constructs with desirable properties.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Phonon modulation of strongly coupled gold tetrahedral plasmonic nanoparticles and a carbocyanine J-aggregate

Coupling exciton and plasmon excitations to form polaritons are of great interest for manipulating energy transfer at the nanoscale via the formation of hybrid light–matter states. In this study, we successfully couple gold tetrahedral nanoparticles with the J-aggregate forming dye 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) to form a strongly coupled colloidal polariton system with a Rabi splitting energy of ∼206 meV. These gold tetrahedra exhibit coherent phonon modes upon photoexcitation, which produce transient oscillations of the LSPR energy for isolated tetrahedra. Transient absorption measurements of the polariton system were performed and show how these coherent phonon modes influence the polariton states’ extinction. We found that the oscillation period increases by 0.5 ± 0.14 ps upon surface deposition of TDBC dye, demonstrating LSPR sensitivity to the refractive index environment. Shifts in the plasmon resonance due to the coherent phonon modes transiently alters LSPR alignment with the J-aggregate exciton peak, resulting in shifts of the hybrid polariton states’ oscillator strength.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Simulation of ultrafast transient absorption spectra of a perylene-based light harvesting antenna

Atomistic simulations of photo-induced responses in artificial light-harvesting molecular systems help to reveal the mechanisms of ultrafast intramolecular energy transfer between individual chromophores. These light-induced processes mimic the primary events occurring in natural photosynthesis. Modeling studies contribute to the design of more efficient molecular architectures enabling performance optimization for applications in light harvesting, energy conversion, and optoelectronics. Within this context, the direct comparison between simulated and experimental transient absorption pump–probe (TA-PP) spectra are especially valuable for validating theoretical approaches and deepening mechanistic understanding. Herein, we investigate the photoinduced dynamics of an antenna system composed of two naphthalene monoimides donor units covalently linked to a perylene derived acceptor. Following photoexcitation, the exciton rapidly self-traps on one of the donor units. Thereafter, efficient ultrafast energy transfer to the acceptor unit takes place via two possible pathways: either through transient exciton localization on the second donor unit or by direct transfer to the acceptor. The simulated TA-PP spectra clearly capture these distinct energy transfer pathways and enable a detailed comparison of their relative efficiencies. This highlights the system's potential for tunable exciton dynamics towards advancing light-harvesting and optoelectronic molecular materials.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Highly-destabilized ligand field excited states of iron carbene complexes and their relation to charge transfer state lifetimes

Lifetimes of photoexcited charge transfer (CT) states in transition metal chromophores are influenced by low-lying ligand field (LF) excited states, especially for 3d metal complexes. To manipulate interactions between LF and CT states, it is important to be able to control LF excited state energies using tunable synthetic variables. In this report, we use Fe 2p3d L 3 -edge resonant inelastic X-ray scattering (RIXS) to measure LF excited state energies of three homoleptic iron chromophores coordinated by strong-field N-heterocyclic carbenes (NHCs). We investigate the effect of oxidation state and ligand scaffold on LF energies and covalency parameters. A cyclometalated bis(NHC) ligand affords both high LF excited state energies (and thus high 10 Dq) as well as high metal–ligand covalency compared to other iron complexes with very strong-field ligands. However, for the set of complexes investigated, we do not observe meaningful correlation between the LF excited state energies and the CT excited state lifetimes. These results illustrate that targeting long-lived CT excited states necessitates control of multiple molecular excited state properties, with destabilization of the LF excited state energies proving necessary, but insufficient, to control the CT excited state lifetime in Fe carbene complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Elucidating Norrish type I reactive pathways by ultrafast X-ray absorption spectroscopy

Norrish type I reactions selectively cleave carbon–carbon bonds directly adjacent to carbonyl groups. Despite their broad use in combination with aromatic carbonyls for additive manufacturing and dental UV curing applications, the nature of the photochemically active state and its population mechanism remain insufficiently understood. Detailed mechanistic insight requires mapping of the photoexcited population flow involving internal conversion and intersystem crossing. We present a time-domain study of gas phase acetophenone as a prototypical aromatic carbonyl combining soft X-ray time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge with ab initio multiple spawning (AIMS) simulations. Exploiting the specific sensitivity of TR-NEXAFS spectroscopy to states with nπ* character, we observe population transfer from the initially excited 1 ππ* state to the 1 nπ* state with a time constant of (0.13 ± 0.02) ps after an initial induction period of (0.12 ± 0.02) ps without population transfer, in quantitative agreement with the AIMS simulations. The population in the 1 nπ* state subsequently decays via intersystem crossing, likely mediated by a 3 ππ* state, within (3.17 ± 0.66) ps to a long-lived 3 nπ* state, which is presumed to be active towards Norrish type I chemistry.

Graßl, Martin [SLAC National Accelerator Laborator↗
DOE OSTI2020

Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy

Valence-to-core x-ray emission spectroscopy (VtC XES) combines the sample flexibility and element specificity of hard x-rays with the chemical environment sensitivity of valence spectroscopy. Here, we extend this technique to study geometric and electronic structural changes induced by photoexcitation in the femtosecond time domain via laser-pump, x-ray probe experiments using an x-ray free electron laser. The results of time-resolved VtC XES on a series of ferrous complexes [Fe(CN) 2n (2, 2'-bipyridine) 3-n ] -2n+2 , n = 1, 2, 3, are presented. Comparisons of spectra obtained from ground state density functional theory calculations reveal signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗