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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Probing exciton diffusion dynamics in photosynthetic supercomplexes via exciton–exciton annihilation

Photosynthesis converts solar energy into chemical energy through coordinated energy transfer between light-harvesting complexes and reaction centers (RCs). Understanding exciton motion, particularly the exciton diffusion length, is essential for optimizing energy efficiency in photosystems. In this work, we combine intensity-cycling transient absorption spectroscopy with kinetic Monte Carlo (kMC) simulation to investigate exciton motion in the C2S2 photosystem II supercomplex of spinach. Using exciton–exciton annihilation, revealed in the fifth-order response, we experimentally estimate an exciton diffusion length of 10.9 nm based on a 3D normal diffusion model, suggesting the ability of excitons to traverse the supercomplex. However, kMC simulations reveal that exciton motion is sub-diffusive because of spatial constraints and the strong RC traps. An anomalous diffusion model analysis of the experimental data yields a diffusion length of 9.7 nm, while the simulated diffusion length is 7.4 nm. The variable exciton residence time across subunits, partly influenced by their connectivity to the trap, indicates inhomogeneous annihilation probability and suggests how plants balance efficient light harvesting with photoprotection. We also explore the influence of specific assumptions in the annihilation simulation, which are challenging to access in more complex environments, such as the thylakoid membrane. Our study provides a framework for studying exciton dynamics using exciton–exciton annihilation, which can be extended to understand the light-harvesting efficiencies of larger, more complex photosynthetic assemblies.

Zhang, Kunyan (ORCID:000000026830409X)↗
DOE OSTI2025

Type-I and type-II interfaces in a MoSe 2 /WS 2 van der Waals heterostructure

We report experimental evidence that MoSe 2 and WS 2 allow the formation of type-I and type-II interfaces, according to the thickness of the former. Heterostructure samples are obtained by stacking a monolayer WS 2 flake on top of a MoSe 2 flake that contains regions of thickness from one to four layers. Photoluminescence spectroscopy and transient absorption measurements reveal a type-II interface in the regions of monolayer MoSe 2 in contact with monolayer WS 2 . In other regions of the heterostructure formed by multilayer MoSe 2 and monolayer WS 2 , features of type-I interface are observed, including the absence of charge transfer and dominance of intralayer excitons in MoSe 2 . Furthermore, the coexistence of type-I and type-II interfaces in a single heterostructure offers opportunities to design sophisticated two-dimensional materials with finely controlled photocarrier behaviors.

2D materials↗
DOE OSTI2025

Charge transfer in transition metal dichalcogenide alloy heterostructures

Two-dimensional (2D) transition metal dichalcogenides and their alloys provide a unique platform for exploring interlayer charge transfer in van der Waals heterostructures. These structures are crucial for advancing the next-generation electronic, optoelectronic, and quantum devices. In this study, interlayer charge transfer in heterostructures composed of MoSe 2 , MoS 2 , and their alloy, MoSSe, is investigated using transient absorption, Raman, and photoluminescence spectroscopy. The experimental results reveal that electron transfer in the alloy heterostructures, MoSSe/MoS 2 and MoSe 2 /MoSSe, is faster than in the pure MoSe 2 /MoS 2 heterostructure, despite the smaller conduction band offsets of the alloy systems. Raman spectroscopy confirms that alloy layers support phonon modes matching those of the pure layers, aligning with theoretical models of phonon-assisted interlayer charge transfer. Additionally, efficient hole transfer is observed in both alloy heterostructures. The findings suggest transition metal dichalcogenides alloys can be used for engineering heterostructures with desired charge transfer properties. By leveraging compositionally tunable band gaps and optical properties, alloy-based heterostructures offer opportunities for designing tailored materials suitable for diverse applications such as photodetectors, light-emitting devices, and flexible electronics. Furthermore, the ultrafast charge transfer observed in these systems provides insights into the fundamental mechanisms governing interlayer interactions in 2D materials.

2D material↗
DOE OSTI2023

Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria

In photosynthesis, absorbed light energy transfers through a network of antenna proteins with near-unity quantum efficiency to reach the reaction center, which initiates the downstream biochemical reactions. While the energy transfer dynamics within individual antenna proteins have been extensively studied over the past decades, the dynamics between the proteins are poorly understood due to the heterogeneous organization of the network. Previously reported timescales averaged over such heterogeneity, obscuring individual interprotein energy transfer steps. Here, we isolated and interrogated interprotein energy transfer by embedding two variants of the primary antenna protein from purple bacteria, light-harvesting complex 2 (LH2), together into a near-native membrane disc, known as a nanodisc. We integrated ultrafast transient absorption spectroscopy, quantum dynamics simulations, and cryogenic electron microscopy to determine interprotein energy transfer timescales. By varying the diameter of the nanodiscs, we replicated a range of distances between the proteins. The closest distance possible between neighboring LH2, which is the most common in native membranes, is 25 Å and resulted in a timescale of 5.7 ps. Larger distances of 28 to 31 Å resulted in timescales of 10 to 14 ps. Corresponding simulations showed that the fast energy transfer steps between closely spaced LH2 increase transport distances by ∼15%. Overall, our results introduce a framework for well-controlled studies of interprotein energy transfer dynamics and suggest that protein pairs serve as the primary pathway for the efficient transport of solar energy.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Harvesting electrons and holes from photodriven symmetry-breaking charge separation within a perylenediimide photosynthetic model dimer

Understanding how to utilize symmetry-breaking charge separation (SB-CS) offers a path toward increasingly efficient light-harvesting technologies. This process plays a central role in the first step of photosynthesis, in which the dimeric “special pair” of the photosynthetic reaction center enters a coherent SB-CS state after photoexcitation. Previous research on SB-CS in both biological and synthetic chromophore dimers has focused on increasing the efficiency of light-driven processes. In a chromophore dimer undergoing SB-CS, the energy of the radical ion pair product is nearly isoenergetic with that of the lowest excited singlet (S 1 ) state of the dimer. This means that very little energy is lost from the absorbed photon. In principle, the relatively high energy electron and hole generated by SB-CS within the chromophore dimer can each be transferred to adjacent charge acceptors to extend the lifetime of the electron–hole pair, which can increase the efficiency of solar energy conversion. To investigate this possibility, we have designed a bis-perylenediimide cyclophane (mPDI 2 ) covalently linked to a secondary electron donor, peri-xanthenoxanthene (PXX) and a secondary electron acceptor, partially fluorinated naphthalenediimide (FNDI). Upon selective photoexcitation of mPDI 2 , transient absorption spectroscopy shows that mPDI 2 undergoes SB-CS, followed by two secondary charge transfer reactions to generate a PXX •+ -mPDI 2 -FNDI •– radical ion pair having a nearly 3 µs lifetime. In conclusion, this strategy has the potential to increase the efficiency of molecular systems for artificial photosynthesis and photovoltaics.

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

Layer-engineered interlayer charge transfer in WSe 2 /WS 2 heterostructures

The layer thickness determines the electronic structure of two-dimensional (2D) materials, leading to different band alignments, which are crucial for the transition metal dichalcogenides heterostructures. Here, we investigated the heterostructure of WSe 2 /WS 2 with different layer thicknesses by steady-state and transient absorption spectroscopy. We observed different ultrafast charge transfer behaviors in 1L-WSe 2 /2L-WS 2 and 2L-WSe 2 /2L-WS 2 few-layer heterostructures. We demonstrate that the layer thickness determines the sequence of intralayer exciton relaxation and interlayer charge transfer. The valley transfer of the band edge induced by the layer thickness can effectively mediate the hot carrier transfer time and interlayer exciton lifetime. Furthermore, these provide us a deeper understanding of carrier dynamics in 2D indirect bandgap semiconductor heterostructures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Photocarrier dynamics at the interface between WS 2 and ozone-irradiated graphene

We investigate the photocarrier properties of heterostructures (HSs) formed by WS 2 and ozone-irradiated graphene (OI-graphene), with a focus on the impact of ozone irradiation. Photoluminescence and Raman spectroscopy reveal a pronounced doping effect induced by ozone irradiation on the graphene layer. Transient absorption measurements demonstrate a substantial suppression of charge transfer efficiency across the interface in the presence of ozone molecules. Spatially resolved pump-probe measurements further show a significant reduction in the diffusion coefficient of photocarriers in WS 2 on OI-graphene compared to that of individual WS 2 monolayers. The slower diffusion suggests enhanced spatial separation of electrons and holes in the WS 2 layer, induced by a built-in electric field at the interface. This effect arises from the combined influences of doping and dielectric property changes caused by ozone molecules. These findings highlight the critical influence of ozone irradiation on graphene and HS interfaces, offering valuable insights for the design of future graphene-based two-dimensional HSs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Attosecond XUV probing of vibronic quantum superpositions in Br 2 +

Ultrafast laser excitation can create coherent superpositions of electronic states in molecules and trigger ultrafast flow of electron density on a few femtosecond timescale. While recent attosecond experiments have addressed real-time observation of these primary photochemical processes, the underlying roles of simultaneous nuclear motions and how they modify and disturb the valence electronic motion remain uncertain. In this work, we investigate coherent electronic-vibrational dynamics induced among multiple vibronic levels of ionic bromine (Br 2 + ), including both spin-orbit (X 2 Π 3/2,g – X 2 Π 1/2,g ) and valence (A 2 Π 3/2,u – 4 Σ 3/2,u ) electronic superpositions, using attosecond transient absorption spectroscopy. Decay, revival, and apparent frequency shifts of electronic coherences are measured via characteristic quantum beats on the Br-3d core-level absorption signals. Quantum-mechanical simulations attribute the observed electronic decoherence to broadened phase distributions of nuclear wave packets on anharmonic potentials. Molecular vibronic structure is further revealed to be imprinted as discrete progressions in electronic beat frequencies. These results provide a future basis to interpret complex charge-migration dynamics in polyatomic systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic-level insight into photochemistry. Here, we performed a time-resolved study of ultrafast molecular symmetry breaking through geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft x-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry-broken cation, which were detected in the x-ray signal. These oscillations were damped within 58 ± 13 femtoseconds because vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens avenues for exploring complex systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water

Attosecond-pump/attosecond-probe experiments have long been sought as the most straightforward method for observing electron dynamics in real time. Although there has been much success with overlapped near-infrared femtosecond and extreme ultraviolet attosecond pulses combined with theory, true attosecond-pump/attosecond-probe experiments have been limited. Here we used a synchronized attosecond x-ray pulse pair from an x-ray free-electron laser to study the electronic response to valence ionization in liquid water through all x-ray attosecond transient absorption spectroscopy (AX-ATAS). Our analysis showed that the AX-ATAS response is confined to the subfemtosecond timescale, eliminating any hydrogen atom motion and demonstrating experimentally that the 1b 1 splitting in the x-ray emission spectrum is related to dynamics and is not evidence of two structural motifs in ambient liquid water.

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

Increased phase precision of spatial light modulators using irrational slopes: application to attosecond metrology

The ability of spatial light modulators (SLMs) to modify the amplitude and phase of light has proved them invaluable to the optics and photonics community. In many applications, the bit-depth of SLMs is a major limiting factor dictated by a digital processor. As a result, there is usually a compromise between refresh speed and bit-depth. Here, we present a method to increase the effective bit-depth of SLMs, which utilizes a linear slope, as is commonly applied to deal with the zeroth-order effect. This technique was tested using two interferometric transient absorption spectroscopy setups. Through the high harmonic generation in gases producing a train of attosecond pulses and harmonics from solids in the ultraviolet, two pulses are generated that interfere in the far field providing a measurement of the optical phase. An increase in the precision far beyond the limit dictated by the digital processor in the bit-depth was found.

Harrison, Geoffrey R. (ORCID:0000000152141866)↗
DOE OSTI2024

Exploring the impact of vibrational cavity coupling strength on ultrafast CN + c -C 6 H 12 reaction dynamics

Abstract Molecular polaritons, hybrid light-matter states resulting from strong cavity coupling of optical transitions, may provide a new route to guide chemical reactions. However, demonstrations of cavity-modified reactivity in clean benchmark systems are still needed to clarify the mechanisms and scope of polariton chemistry. Here, we use transient absorption to observe the ultrafast dynamics of CN radicals interacting with a cyclohexane ( c -C 6 H 12 ) and chloroform (CHCl 3 ) solvent mixture under vibrational strong coupling of a C–H stretching mode of c- C 6 H 12 . By modulating the c -C 6 H 12 :CHCl 3 ratio, we explore how solvent complexation and hydrogen (H)-abstraction processes proceed under collective cavity coupling strengths ranging from 55 to 85 cm −1 . Reaction rates remain unchanged for all extracavity, on-resonance, and off-resonance cavity coupling conditions, regardless of coupling strength. These results suggest that insufficient vibrational cavity coupling strength may not be the determining factor for the negligible cavity effects observed previously in H-abstraction reactions of CN with CHCl 3 .

Chen, Liying↗
DOE OSTI2021

Femtosecond Reflectance Spectroscopy for Energetic Material Diagnostics

Understanding the fundamental mechanisms underpinning shock initiation is critical to predicting energetic material (EM) safety and performance. Currently, the timescales and pathways by which shock-excited lattice modes transfer energy into specific chemical bonds remains an open question. Towards understanding these mechanisms, our group has previously measured the vibrational energy transfer (VET) pathways in several energetic thin films using broadband, femtosecond transient absorption spectroscopy. However, new technologies are needed to move beyond these thin film surrogates and measure broadband VET pathways in realistic EM morphologies. Herein, we describe a new broadband, femtosecond, attenuated total reflectance spectroscopy apparatus. Performance of the system is benchmarked against published data and the first VET results from a pressed EM pellet are presented. This technology enables fundamental studies of VET dynamics across sample configurations and environments (pressure, temperature, etc .) and supports the potential use of VET studies in the non-destructive surveillance of EM components.

42 ENGINEERING↗
DOE OSTI2021

Understanding Photocarrier and Gas Dynamics to Rationally Design Heterostructured Nanocatalysts for Solar CO 2 Conversion

Recent research in CO 2 photocatalysis has largely focused on exploring new catalysts; however, details of how these materials work often remain unclear. Knowledge of these processes will allow one to rationally design highly efficient catalysts for solar CO 2 conversion. This project aims to develop new techniques and establish new capabilities in SRNL to enable the study of photocatalysts and other materials in extreme detail. In FY21, we developed two new in situ techniques that are unique to SRNL, allowing the study of reaction intermediates and adsorbed gases during photocatalysis. We also established new in-house capabilities for catalyst synthesis and transient absorption (TA) spectroscopy, which enables the study of very short-lived excited states on photocatalysts. These capabilities will be used extensively for this project and in other current projects relevant to clean energy technologies such as photovoltaic cells, providing a good return on investment in the coming years.

14 SOLAR ENERGY↗
DOE OSTI2024

A Bottom-Up Approach to Rational Design of Crystalline Materials: Investigation of Vibronic Coherences Underlying Exciton Dynamics in Semiconductors

In this project we uncovered structure-function relationships of donor-acceptor co-crystals used to develop next-generation optoelectronic devices. Unraveling the photodynamics of molecular crystalline materials poses many challenges for spectroscopy due to broad, overlapping features representing numerous underlying dynamical processes. This leads researchers to make many assumptions about the dynamics of a system in choosing an appropriate kinetic fitting model. Computationally, electronic structure methods are either prohibitively expensive or underdeveloped for computing the excited state structure of molecular materials, especially states that exhibit charge transfer. Researchers must therefore perform calculations of excited electronic states using truncated models of molecular materials. Here we present a joint experimental-theoretical approach to bridging the gap between the photodynamics of a molecular material and its constituent molecules. We focus our efforts on quantifying the timescales and mechanisms of photoexcitation in donor-acceptor co-crystals and donor-acceptor dimers where the lowest-lying excited state is characterized by charge transfer from the donor to the acceptor. We employ ultrafast UV pump, UV-Vis probe transient absorption spectroscopy to unravel the time-resolved spectroscopic signatures of the photodynamics in both the crystalline material and donor-acceptor dimers in solution. We perform electronic structure and excited state dynamics calculations of the dimers to inform kinetic fitting models and assign the spectral features. The photodynamics of the crystal vs. dimer systems have many similarities, enabling unprecedented insights into the formation and evolution of charge transfer excitons in the crystalline systems.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Luminescent Zr Complexes with Long-lived Intraligand Charge Transfer Excited States

Understanding and establishing design principles to tune the excited states of earth-abundant sensitizers is crucial for identifying new photosensitizers for sustainable technological advancements. Here, we report a series of three bis Zr (IV) complexes of tridentate, dianionic ligands incorporating two phenoxide donors and aza arene acceptors of increasing electron affinity (pyridine < pyrimidine < pyrazine ) that are air and water-stable. These complexes emit via thermally assisted delayed fluorescence from an intraligand charge transfer excited state with varying metal contributions. The electronic structural changes from different acceptors vary the excited state character and the photophysical properties significantly between the complexes. The variation in photophysics that includes emission lifetime (76 μs to 265 ns), intersystem crossing (ISC) lifetime ( 390 – 290 ps), and the energy difference between the singlet and triplet excited states (∆EST. 170 -100 meV). We observe solvent-independent ISC rates when the excited state has metal contributions (acceptor = pyridine), and the ISC rates vary significantly with solvent polarity when the excited state is an intra-ligand charge transfer state (acceptor = pyrimidine and pyrazine). Transient absorption measurements provided the basis spectra and verified the metal contribution in the excited state. This work provides a basis for developing new sensitizers based on Zr (IV) by providing design principles that can be used to modulate the character of the excited state and its photophysics.

Nattikallungal, Thabassum A. [University of Southe↗
DOE OSTI2025

Excitation Energy Transfer Dynamics from Carotenoid to Bacteriochlorophyll a in the LH2 Complex of Rhodobacter sphaeroides: Insights from Reconstitution Experiments with Carotenoids and B800 Bacteriochlorophyll a

Carotenoids are crucial for photosynthesis, playing key roles in light harvesting and photoprotection. In this study, spheroidene and bacteriochlorophyll a (Bchl a) were reconstituted into the chromatophores of the carotenoidless mutant Rhodobacter sphaeroides R26.1, resulting in the preparation of high-quality LH2 complexes. Global and target analyses of transient absorption data revealed that incorporating B800 Bchl a significantly enhances excitation energy transfer (EET) efficiency from carotenoids to Bchl a. EET predominantly occurs from the carotenoid S2 state, with additional pathways from the S1 state observed in native LH2. Unique relaxation dynamics were identified, including the generation of the carotenoid S* state in reconstituted LH2 with both spheroidene and B800 Bchl a and the formation of the carotenoid T1 state in reconstituted LH2. These findings underscore the critical influence of pigment composition and spatial organization on energy transfer mechanisms. They provide valuable insights into the molecular interplay that governs excitation energy transfer in photosynthetic light-harvesting systems.

Biochemistry & Molecular Biology↗