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Jones, Leighton O.

Publications and source records attributed to Jones, Leighton O..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Trifluoromethylation of 2D Transition Metal Dichalcogenides: A Mild Functionalization and Tunable p‐Type Doping Method

Chemical modification is a powerful strategy for tuning the electronic properties of 2D semiconductors. Here, in this study, we report the electrophilic trifluoromethylation of 2D WSe2 and MoS 2 under mild conditions using the reagent trifluoromethyl thianthrenium triflate (TTT). Chemical characterization and density functional theory calculations reveal that the trifluoromethyl groups bind covalently to surface chalcogen atoms as well as oxygen substitution sites. Trifluoromethylation induces p-type doping in the underlying 2D material, enabling the modulation of charge transport and optical emission properties in WSe 2 . This work introduces a versatile and efficient method for tailoring the optical and electronic properties of 2D transition metal dichalcogenides.

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

Utilizing Ultraviolet Photons to Generate Single-Photon Emitters in Semiconductor Monolayers

The understanding and controlled creation of atomic defects in semiconductor transition metal dichalcogenides (TMDs) are highly relevant to their applications in high-performance quantum optics and nanoelectronic devices. Here, we demonstrate a versatile approach in generating single-photon emitters in MoS 2 monolayers using widely attainable UV light. We discover that only defects engendered by UV photons in vacuum exhibit single-photon-emitter characteristics, whereas those created in air lack quantum emission attributes. In combination with theoretical calculations, we assign the defects generated in vacuum to unpassivated sulfur vacancies, whose highly localized midgap states give rise to single-photon emission. In contrast, UV irradiation of the MoS 2 monolayers in air results in oxygen-passivated sulfur vacancies, whose optical properties are likely governed by their pristine band-to-defect band optical transitions. Furthermore, these findings suggest that widely available light sources such as UV light can be utilized for creating quantum photon sources in TMDs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Mechanically interlocked pyrene-based photocatalysts

Triplet excited-state organic chromophores present countless opportunities for applications in photocatalysis. Here we describe an approach to the engineering of the triplet excited states of aromatic chromophores, which involves incorporating pyrene into pyridinium-containing mechanically interlocked molecules (MIMs). The π-extended nature of the pyrenes enforces [π···π] stacking, affording an efficient synthesis of tetrachromophoric octacationic homo[2]catenanes. These MIMs generate triplet populations and efficient intersystem crossing on account of the formation of a mixed charge-transfer/exciplex electronic state and a nanoconfinement effect, which leads to a high level of protection of the triplet state and extends the triplet lifetimes and yields. These compounds display excellent catalytic activity in photo-oxidation, as demonstrated by the aerobic oxidation of a sulfur-mustard simulant. Furthermore this research highlights the benefits of using the mechanical bond to fine-tune the triplet photophysics of existing aromatic chromophores, providing an avenue for the development of unexplored MIM-based photosensitizers and photocatalysts.

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

Atomic-Scale View of Redox Induced Changes for Monolayer MoO x on α-TiO 2 (110) with Chemical-State Sensitivity

Supported molybdenum oxide (MoO x ) plays an important role in catalytic transformations from alcohol dehydrogenation to transesterification. During these reactions, Mo and oxygen surface species undergo structural and chemical changes. A detailed, chemical-state specific, atomic-scale structural analysis of the catalyst under redox conditions is important for improving catalytic properties. In this study, monolayer of MoO x grown on α-TiO 2 (110) by atomic-layer deposition is analyzed by X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy (XPS). The chemical shifts for Mo 2p 3/2 and O 1s peaks are used to distinguish Mo 6+ from Mo 4+ and surface O from bulk O. Excitation of XPS by XSW allows pin-pointing the location of these surface species relative to the underlying substrate lattice. Finally, measured 3D composite atomic density maps for the oxidized and reduced interfaces compare well with our density functional theory models, and collectively create a unique view of the redoxdriven dynamics for this complex catalytic structure.

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

Atomic-Site-Specific Surface Valence-Band Structure from X-Ray Standing-Wave Excited Photoemission

X-ray standing-wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for ½ monolayer (ML) Pt grown on a SrTiO 3 (001) surface. The XSW induced modulations in the core level (CL), and VB photoemission from the surface and substrate atoms were monitored for three ℎ⁡kl substrate Bragg reflections. The XSW CL analysis shows the Pt to have a face-centered-cubic-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Altogether, this Letter represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Synthesis and Structure–Activity Characterization of a Single-Site MoO 2 Catalytic Center Anchored on Reduced Graphene Oxide

Molecularly derived single-site heterogeneous catalysts can bridge the understanding and performance gaps between conventional homogeneous and heterogeneous catalysis, guiding the rational design of next-generation catalysts. While impressive advances have been made with well-defined oxide supports, the structural complexity of other supports and the nature of the grafted surface species present an intriguing challenge. In this study, single-site Mo(=O) 2 species grafted onto reduced graphene oxide (rGO/MoO 2 ) are characterized by XPS, DRIFTS, powder XRD, N 2 physisorption, NH 3 -TPD, aqueous contact angle, active site poisoning assay, Mo EXAFS, model compound single-crystal XRD, DFT, and catalytic performance. NH 3 -TPD reveals that the anchored MoO 2 moiety is not strongly acidic, while Mo 3d 5/2 XPS assigns the oxidation state as Mo(VI), and XRD shows little rGO periodicity change on MoO 2 grafting. Contact angle analysis shows that MoO 2 grafting consumes rGO surface polar groups, yielding a more hydrophobic surface. The rGO/MoO 2 DRIFTS assigns features at 959 and 927 cm –1 to the symmetric and antisymmetric Mo=O stretching modes, respectively, of an isolated cis-(O=Mo=O) moiety, in agreement with DFT computation. Moreover, the Mo EXAFS rGO/MoO 2 structural data are consistent with isolated (C–O) 2– Mo(=O) 2 species having two Mo=O bonds and two Mo–O bonds at distances of 1.69(3) and 1.90(3) Å, respectively. rGO/MoO 2 is also more active than the previously reported AC/MoO 2 catalyst, with reductive carbonyl coupling TOFs approaching 1.81 × 10 3 h –1 . rGO/MoO 2 is environmentally robust and multiply recyclable with 69 ± 2% of the Mo sites catalytically significant. Altogether, rGO/MoO 2 is a structurally well-defined and versatile single-site Mo(VI) dioxo heterogeneous catalytic system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Regiospecific N -alkyl substitution tunes the molecular packing of high-performance non-fullerene acceptors

The rapid development of non-fullerene acceptors (NFAs) with strong near-infrared absorption has led to remarkably enhanced short-circuit current density (J sc ) values in organic solar cells (OSCs). NFAs based on the benzotriazole (Bz) fused-ring π-core have great potential in delivering both high J sc and decent open-circuit voltage values due to their strong intramolecular charge transfer with reasonably low energy loss. Here in this work, we have designed and synthesized a series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F and EHN6SEH-4F, via regiospecific N-alkyl engineering based on the high-performance NFA mBzS-4F that was reported previously. The molecular packing of mBzS-4F, AN6SBO-4F, and EHN6SEH-4F single crystals was analyzed using X-ray crystallography in order to provide a comprehensive understanding of the correlation between the molecular structure, the charge-transporting properties, and the solar cell performance. Compared with the typical honeycomb single-crystal structure of Y6 derivatives, these NFAs exhibit distinctly different molecular packing patterns. The strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm -2 .

36 MATERIALS SCIENCE↗