1
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Gross N, Kuhs CT, Ostovar B, Chiang WY, Wilson KS, Volek TS, Faitz ZM, Carlin CC, Dionne JA, Zanni MT, Gruebele M, Roberts ST, Link S, Landes CF. Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2023; 127:14557-14586. [PMID: 37554548 PMCID: PMC10406104 DOI: 10.1021/acs.jpcc.3c02091] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/29/2023] [Revised: 06/24/2023] [Indexed: 08/10/2023]
Abstract
Ultrafast optical microscopy, generally employed by incorporating ultrafast laser pulses into microscopes, can provide spatially resolved mechanistic insight into scientific problems ranging from hot carrier dynamics to biological imaging. This Review discusses the progress in different ultrafast microscopy techniques, with a focus on transient absorption and two-dimensional microscopy. We review the underlying principles of these techniques and discuss their respective advantages and applicability to different scientific questions. We also examine in detail how instrument parameters such as sensitivity, laser power, and temporal and spatial resolution must be addressed. Finally, we comment on future developments and emerging opportunities in the field of ultrafast microscopy.
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Affiliation(s)
- Niklas Gross
- Department
of Chemistry, Rice University, Houston, Texas 77005, United States
| | - Christopher T. Kuhs
- Army
Research Laboratory-South, U.S. Army DEVCOM, Houston, Texas 77005, United States
| | - Behnaz Ostovar
- Department
of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
| | - Wei-Yi Chiang
- Department
of Chemistry, Rice University, Houston, Texas 77005, United States
| | - Kelly S. Wilson
- Department
of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
| | - Tanner S. Volek
- Department
of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
| | - Zachary M. Faitz
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
| | - Claire C. Carlin
- Department
of Materials Science and Engineering, Stanford
University, Stanford, California 94305, United States
| | - Jennifer A. Dionne
- Department
of Materials Science and Engineering, Stanford
University, Stanford, California 94305, United States
- Department
of Radiology, Molecular Imaging Program at Stanford (MIPS), Stanford University School of Medicine, Stanford, California 94305, United States
| | - Martin T. Zanni
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
| | - Martin Gruebele
- Department
of Chemistry, University of Illinois at
Urbana−Champaign, Urbana, Illinois 61801, United States
- Department
of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
- Center
for Biophysics and Quantitative Biology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
| | - Sean T. Roberts
- Department
of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
| | - Stephan Link
- Department
of Chemistry, Rice University, Houston, Texas 77005, United States
- Department
of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
| | - Christy F. Landes
- Department
of Chemistry, Rice University, Houston, Texas 77005, United States
- Department
of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
- Department
of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States
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2
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Hickey CL, Grumstrup EM. Direct Correlation of Charge Carrier Transport to Local Crystal Quality in Lead Halide Perovskites. NANO LETTERS 2020; 20:5050-5056. [PMID: 32520576 DOI: 10.1021/acs.nanolett.0c01244] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Although solution processing methods provide an attractive route toward development of low-cost functional materials, these accessible fabrication approaches can engender high concentrations of microscopic structural defects that are detrimental to performance. In lead halide perovskites, structural disorder derived from solution processing has been implicated as an important determiner of photophysical properties. However, a direct correlation between the functional properties of these materials and the local crystal structure in which non-equilibrium states evolve has remained elusive, in part because structural heterogeneities occur on length scales that defy conventional characterization techniques. To address this knowledge gap, we have combined ultrafast pump-probe microscopy and electron backscattering diffraction to directly correlate charge carrier transport with the local diffraction pattern contrast, an indicator of crystal quality. Spatial correlation of these measurements strongly suggests that even on individual single crystal CsPbBr3 domains, microscopic variability in the crystal quality profoundly impacts the efficiency of charge carrier transport.
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Affiliation(s)
- Casey L Hickey
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States
| | - Erik M Grumstrup
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States
- Montana Materials Science Program, Montana State University, Bozeman, Montana 59717, United States
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3
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Kwang SY, Frontiera RR. Spatially Offset Femtosecond Stimulated Raman Spectroscopy: Observing Exciton Transport through a Vibrational Lens. J Phys Chem Lett 2020; 11:4337-4344. [PMID: 32427490 DOI: 10.1021/acs.jpclett.0c01114] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
To design better molecular electronic devices, we need a strong understanding of how charges or excitons propagate, as many efficiency losses arise during transport. Exciton transport has been difficult to study because excitons tend to be short-lived, have short diffusion lengths, and can easily recombine. Here, we debut spatially offset femtosecond stimulated Raman spectroscopy (SO-FSRS), a three-pulse ultrafast microscopy technique. By offsetting the photoexcitation beam, we can monitor Raman spectral changes as a function of both time and position. We used SO-FSRS on 6,13-bis(triisopropylsilylethynyl) pentacene, a well-studied organic semiconductor used in photovoltaics and field-effect transistors. We demonstrated that the fast exciton and free charge carrier transport axes are identical and observed that exciton transport is less anisotropic by a factor of ∼3. SO-FSRS is the first technique that directly tracks molecular structural evolution during exciton transport, which can provide roadmaps for tailor-making molecules for specific electronic devices.
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Affiliation(s)
- Siu Yi Kwang
- Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
| | - Renee R Frontiera
- Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
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4
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Pandya R, Chen RYS, Gu Q, Gorman J, Auras F, Sung J, Friend R, Kukura P, Schnedermann C, Rao A. Femtosecond Transient Absorption Microscopy of Singlet Exciton Motion in Side-Chain Engineered Perylene-Diimide Thin Films. J Phys Chem A 2020; 124:2721-2730. [PMID: 32130861 PMCID: PMC7132576 DOI: 10.1021/acs.jpca.0c00346] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2020] [Revised: 03/04/2020] [Indexed: 12/21/2022]
Abstract
We present a statistical analysis of femtosecond transient absorption microscopy applied to four different organic semiconductor thin films based on perylene-diimide (PDI). By achieving a temporal resolution of 12 fs with simultaneous sub-10 nm spatial precision, we directly probe the underlying exciton transport characteristics within 3 ps after photoexcitation free of model assumptions. Our study reveals sub-picosecond coherent exciton transport (12-45 cm2 s-1) followed by a diffusive phase of exciton transport (3-17 cm2 s-1). A comparison between the different films suggests that the exciton transport in the studied materials is intricately linked to their nanoscale morphology, with PDI films that form large crystalline domains exhibiting the largest diffusion coefficients and transport lengths. Our study demonstrates the advantages of directly studying ultrafast transport properties at the nanometer length scale and highlights the need to examine nanoscale morphology when investigating exciton transport in organic as well as inorganic semiconductors.
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Affiliation(s)
- Raj Pandya
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Richard Y. S. Chen
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Qifei Gu
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Jeffrey Gorman
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Florian Auras
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Jooyoung Sung
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Richard Friend
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Philipp Kukura
- Physical
and Theoretical Chemistry Laboratory, Oxford
University, South Parks Road, Oxford OX1 3QZ, U.K.
| | - Christoph Schnedermann
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
| | - Akshay Rao
- Department
of Physics, Cavendish Laboratory, University
of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
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5
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Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors. Sci Rep 2020; 10:4344. [PMID: 32152385 PMCID: PMC7062911 DOI: 10.1038/s41598-020-61282-x] [Citation(s) in RCA: 28] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2019] [Accepted: 02/03/2020] [Indexed: 11/29/2022] Open
Abstract
In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grass-like curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general.
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6
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Jones AC, Kearns NM, Ho JJ, Flach JT, Zanni MT. Impact of non-equilibrium molecular packings on singlet fission in microcrystals observed using 2D white-light microscopy. Nat Chem 2019; 12:40-47. [PMID: 31792384 DOI: 10.1038/s41557-019-0368-9] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2018] [Accepted: 10/08/2019] [Indexed: 11/09/2022]
Abstract
Singlet fission, the process of splitting a singlet exciton into two triplet excitons, has been proposed as a mechanism for improving the efficiency of future photovoltaic devices. In organic semiconductors exhibiting singlet fission, the geometric relationship between molecules plays an important role by setting the intermolecular couplings that determine the system energetics. Here, we spatially image TIPS-pentacene microcrystals using ultrafast two-dimensional white-light microscopy and discover a low-energy singlet state sparsely distributed throughout the microcrystals, with higher concentrations at edges and morphological defects. The spectra of these singlet states are consistent with slip-stacked molecular geometries and increased charge-transfer couplings. The picosecond-timescale kinetics of these low-energy singlet states matches that of the correlated triplet-pair state, which we attribute to singlet/triplet-pair interconversion at these sites. Our observations support the conclusion that small populations of geometries with favourable energetics can play outsized roles in singlet fission processes.
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Affiliation(s)
- Andrew C Jones
- Department of Chemistry, University of Wisconsin, Madison, WI, USA
| | | | - Jia-Jung Ho
- Department of Chemistry, University of Wisconsin, Madison, WI, USA
| | - Jessica T Flach
- Department of Chemistry, University of Wisconsin, Madison, WI, USA
| | - Martin T Zanni
- Department of Chemistry, University of Wisconsin, Madison, WI, USA.
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7
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Jones AC, Kearns NM, Bohlmann Kunz M, Flach JT, Zanni MT. Multidimensional Spectroscopy on the Microscale: Development of a Multimodal Imaging System Incorporating 2D White-Light Spectroscopy, Broadband Transient Absorption, and Atomic Force Microscopy. J Phys Chem A 2019; 123:10824-10836. [DOI: 10.1021/acs.jpca.9b09099] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Affiliation(s)
- Andrew C. Jones
- Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
| | - Nicholas M. Kearns
- Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
| | - Miriam Bohlmann Kunz
- Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
| | - Jessica T. Flach
- Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
| | - Martin T. Zanni
- Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
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8
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Grumstrup EM. Spatiotemporal coupling of excited state dynamics in time-resolved microscopies. OPTICS EXPRESS 2019; 27:31385-31393. [PMID: 31684373 DOI: 10.1364/oe.27.031385] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2019] [Accepted: 10/01/2019] [Indexed: 06/10/2023]
Abstract
In the high-density excitation limit, as is often probed with ultrafast spectroscopies, spatial and temporal evolution of photogenerated excited states are strongly coupled, giving rise to artifacts that influence experimentally-determined material parameters. The interplay between spatial and temporal degrees of freedom is especially pronounced in pump-probe microscopy, where small laser spot sizes amplify the effects of spatiotemporal coupling on spectroscopic observables. To quantitatively model these effects, a continuum model is developed that accounts for laser spot size as well as nonlinear excited state decay and diffusion. It is shown that effective excitation densities cannot be used to determine quantitatively correct rate constants. Significant error is introduced unless experimental data is fit with a numerical model that accounts for spatial anisotropy in the excitation density. Furthermore, the quantitative determination of material diffusion coefficients is shown to be highly sensitive to experimental parameters.
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9
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Graefe CT, Punihaole D, Harris CM, Lynch MJ, Leighton R, Frontiera RR. Far-Field Super-Resolution Vibrational Spectroscopy. Anal Chem 2019; 91:8723-8731. [PMID: 31251563 DOI: 10.1021/acs.analchem.9b01731] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Potential label-free alternatives to super-resolution fluorescence techniques have been the focus of considerable research due to the challenges intrinsic in the reliance on fluorescent tags. In this Feature, we discuss efforts to develop super-resolution techniques based on vibrational spectroscopies and address possible sample applications as well as future potential resolution enhancements.
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Affiliation(s)
- Christian T Graefe
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
| | - David Punihaole
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
| | - Celina M Harris
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
| | - Michael J Lynch
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
| | - Ryan Leighton
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
| | - Renee R Frontiera
- Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States
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10
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Zhu T, Snaider JM, Yuan L, Huang L. Ultrafast Dynamic Microscopy of Carrier and Exciton Transport. Annu Rev Phys Chem 2019; 70:219-244. [PMID: 30883273 DOI: 10.1146/annurev-physchem-042018-052605] [Citation(s) in RCA: 50] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We highlight the recent progress in ultrafast dynamic microscopy that combines ultrafast optical spectroscopy with microscopy approaches, focusing on the application transient absorption microscopy (TAM) to directly image energy and charge transport in solar energy harvesting and conversion systems. We discuss the principles, instrumentation, and resolutions of TAM. The simultaneous spatial, temporal, and excited-state-specific resolutions of TAM unraveled exciton and charge transport mechanisms that were previously obscured in conventional ultrafast spectroscopy measurements for systems such as organic solar cells, hybrid perovskite thin films, and molecular aggregates. We also discuss future directions to improve resolutions and to develop other ultrafast imaging contrasts beyond transient absorption.
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Affiliation(s)
- Tong Zhu
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
- Laser/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
| | - Jordan M. Snaider
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
| | - Long Yuan
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
| | - Libai Huang
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
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11
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He Z, Zhang Z, Bi S. Nanoscale alignment of semiconductor crystals for high-fidelity organic electronics applications. APPLIED NANOSCIENCE 2019. [DOI: 10.1007/s13204-019-01068-4] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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12
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Wang M, Gong Y, Alzina F, Svoboda O, Ballesteros B, Sotomayor Torres CM, Xiao S, Zhang Z, He J. Raman antenna effect from exciton-phonon coupling in organic semiconducting nanobelts. NANOSCALE 2017; 9:19328-19336. [PMID: 29199314 DOI: 10.1039/c7nr07212k] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The highly anisotropic interactions in organic semiconductors together with the soft character of organic materials lead to strong coupling between nuclear vibrations and exciton dynamics, which potentially results in anomalous electrical, optical and optoelectrical properties. Here, we report on the Raman antenna effect from organic semiconducting nanobelts 6,13-dichloropentacene (DCP), resulting from the coupling of molecular excitons and intramolecular phonons. The highly ordered crystalline structure in DCP nanobelts enables the precise polarization-resolved spectroscopic measurement. The angle-dependent Raman spectroscopy under resonant excitation shows that all Raman modes from the skeletal vibrations of DCP molecule act like a nearly perfect dipole antenna IRaman ∝ cos4(θ - 90), with almost zero (maximum) Raman scattering parallel (perpendicular) to the nanobelt's long-axis. The Raman antenna effect in DCP nanobelt is originated from the coupling between molecular skeletal vibrations and intramolecular exciton and the confinement of intermolecular excitons. It dramatically enhances the Raman polarization ratio (ρ = I‖/I⊥ > 25) and amplifies the anisotropy of the angle-dependent Raman scattering (κRaman = Imax/Imin > 12) of DCP nanobelts. These findings have crucial implications for fundamental understanding on the exciton-phonon coupling and its effects on the optical properties of organic semiconductors.
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Affiliation(s)
- Mao Wang
- NTNU Nanomechanical Lab, Department of Structural Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, 7491, Norway.
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13
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Zhu T, Wan Y, Huang L. Direct Imaging of Frenkel Exciton Transport by Ultrafast Microscopy. Acc Chem Res 2017; 50:1725-1733. [PMID: 28678469 DOI: 10.1021/acs.accounts.7b00155] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
Long-range transport of Frenkel excitons is crucial for achieving efficient molecular-based solar energy harvesting. Understanding of exciton transport mechanisms is important for designing materials for solar energy applications. One major bottleneck in unraveling of exciton transport mechanisms is the lack of direct measurements to provide information in both spatial and temporal domains, imposed by the combination of fast energy transfer (typically ≤1 ps) and short exciton diffusion lengths (typically ≤100 nm). This challenge requires developing experimental tools to directly characterize excitation energy transport, and thus facilitate the elucidation of mechanisms. To address this challenge, we have employed ultrafast transient absorption microscopy (TAM) as a means to directly image exciton transport with ∼200 fs time resolution and ∼50 nm spatial precision. By mapping population in spatial and temporal domains, such approach has unraveled otherwise obscured information and provided important parameters for testing exciton transport models. In this Account, we discuss the recent progress in imaging Frenkel exciton migration in molecular crystals and aggregates by ultrafast microscopy. First, we establish the validity of the TAM methods by imaging singlet and triplet exciton transport in a series of polyacene single crystals that undergo singlet fission. A new singlet-mediated triplet transport pathway has been revealed by TAM, resulting from the equilibrium between triplet and singlet exciton populations. Such enhancement of triplet exciton transport enables triplet excitons to migrate as singlet excitons and leads to orders of magnitude faster apparent triplet exciton diffusion rate in the picosecond and nanosecond time scales, favorable for solar cell applications. Next we discuss how information obtained by ultrafast microscopy can evaluate coherent effects in exciton transport. We use tubular molecular aggregates that could support large exciton delocalization sizes as a model system. The initial experiments measure exciton diffusion constants of 3-6 cm2 s-1, 3-5 times higher than the incoherent limit predicted by theory, suggesting that coherent effects play a role. In summary, combining ultrafast spectroscopic methods with microscopic techniques provides a direct approach for obtaining important parameters to unravel the underlying exciton transport mechanisms in molecular solids. We discuss future directions to bridge the gap in understanding of fundamental energy transfer theories to include coherent and incoherent effects. We are still in the infancy of ultrafast microscopy, and the vast potential is not limited to the systems discussed in this Account.
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Affiliation(s)
- Tong Zhu
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Yan Wan
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
| | - Libai Huang
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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14
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Correlation of nanodomain morphology and ultrafast fluorescence dynamics in bisdecylsexithiophene-polyethylene oxide copolymer film. J Photochem Photobiol A Chem 2017. [DOI: 10.1016/j.jphotochem.2017.04.023] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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15
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Steiner F, Poelking C, Niedzialek D, Andrienko D, Nelson J. Influence of orientation mismatch on charge transport across grain boundaries in tri-isopropylsilylethynyl (TIPS) pentacene thin films. Phys Chem Chem Phys 2017; 19:10854-10862. [DOI: 10.1039/c6cp06436a] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We present a multi-scale model for charge transport across grain boundaries in molecular electronic materials that incorporates packing disorder, electrostatic and polarisation effects.
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Affiliation(s)
- Florian Steiner
- Department of Physics and Centre for Plastic Electronics
- Imperial College London
- London SW7 2AZ
- UK
| | - Carl Poelking
- Max Planck Institute for Polymer Research
- 55128 Mainz
- Germany
| | - Dorota Niedzialek
- Department of Physics and Centre for Plastic Electronics
- Imperial College London
- London SW7 2AZ
- UK
| | | | - Jenny Nelson
- Department of Physics and Centre for Plastic Electronics
- Imperial College London
- London SW7 2AZ
- UK
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16
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McDonough TJ, Zhang L, Roy SS, Kearns NM, Arnold MS, Zanni MT, Andrew TL. Triplet exciton dissociation and electron extraction in graphene-templated pentacene observed with ultrafast spectroscopy. Phys Chem Chem Phys 2017; 19:4809-4820. [DOI: 10.1039/c6cp06454j] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
Transient absorption measurements of pentacene, controlling molecular orientation (via graphene templating), fluence, and polarization, provide new evidence for charge generation.
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Affiliation(s)
| | - Lushuai Zhang
- Department of Materials Science and Engineering
- University of Wisconsin–Madison
- Madison
- USA
| | - Susmit Singha Roy
- Department of Materials Science and Engineering
- University of Wisconsin–Madison
- Madison
- USA
| | | | - Michael S. Arnold
- Department of Materials Science and Engineering
- University of Wisconsin–Madison
- Madison
- USA
| | - Martin T. Zanni
- Department of Chemistry
- University of Wisconsin–Madison
- Madison
- USA
| | - Trisha L. Andrew
- Department of Chemistry
- University of Wisconsin–Madison
- Madison
- USA
- Department of Materials Science and Engineering
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17
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Grieco C, Doucette GS, Pensack RD, Payne MM, Rimshaw A, Scholes GD, Anthony JE, Asbury JB. Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films. J Am Chem Soc 2016; 138:16069-16080. [DOI: 10.1021/jacs.6b10010] [Citation(s) in RCA: 67] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Christopher Grieco
- Department
of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
| | - Grayson S. Doucette
- Intercollege
Materials Science and Engineering Program, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
| | - Ryan D. Pensack
- Department
of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Marcia M. Payne
- Department
of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Adam Rimshaw
- Department
of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
| | - Gregory D. Scholes
- Department
of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - John E. Anthony
- Department
of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
| | - John B. Asbury
- Department
of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
- Intercollege
Materials Science and Engineering Program, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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18
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Schnedermann C, Lim JM, Wende T, Duarte AS, Ni L, Gu Q, Sadhanala A, Rao A, Kukura P. Sub-10 fs Time-Resolved Vibronic Optical Microscopy. J Phys Chem Lett 2016; 7:4854-4859. [PMID: 27934055 PMCID: PMC5684689 DOI: 10.1021/acs.jpclett.6b02387] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2016] [Accepted: 11/10/2016] [Indexed: 05/22/2023]
Abstract
We introduce femtosecond wide-field transient absorption microscopy combining sub-10 fs pump and probe pulses covering the complete visible (500-650 nm) and near-infrared (650-950 nm) spectrum with diffraction-limited optical resolution. We demonstrate the capabilities of our system by reporting the spatially- and spectrally-resolved transient electronic response of MAPbI3-xClx perovskite films and reveal significant quenching of the transient bleach signal at grain boundaries. The unprecedented temporal resolution enables us to directly observe the formation of band-gap renormalization, completed in 25 fs after photoexcitation. In addition, we acquire hyperspectral Raman maps of TIPS pentacene films with sub-400 nm spatial and sub-15 cm-1 spectral resolution covering the 100-2000 cm-1 window. Our approach opens up the possibility of studying ultrafast dynamics on nanometer length and femtosecond time scales in a variety of two-dimensional and nanoscopic systems.
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Affiliation(s)
- Christoph Schnedermann
- Physical
and Theoretical Chemistry Laboratory, University
of Oxford, South Parks
Road, Oxford OX1 3QZ, United Kingdom
| | - Jong Min Lim
- Physical
and Theoretical Chemistry Laboratory, University
of Oxford, South Parks
Road, Oxford OX1 3QZ, United Kingdom
| | - Torsten Wende
- Physical
and Theoretical Chemistry Laboratory, University
of Oxford, South Parks
Road, Oxford OX1 3QZ, United Kingdom
| | - Alex S. Duarte
- Physical
and Theoretical Chemistry Laboratory, University
of Oxford, South Parks
Road, Oxford OX1 3QZ, United Kingdom
| | - Limeng Ni
- Cavendish
Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
| | - Qifei Gu
- Cavendish
Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
| | - Aditya Sadhanala
- Cavendish
Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
| | - Akshay Rao
- Cavendish
Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
| | - Philipp Kukura
- Physical
and Theoretical Chemistry Laboratory, University
of Oxford, South Parks
Road, Oxford OX1 3QZ, United Kingdom
- E-mail:
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19
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Abstract
Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.
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Affiliation(s)
- Oksana Ostroverkhova
- Department of Physics, Oregon State University , Corvallis, Oregon 97331, United States
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20
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Zhu T, Wan Y, Guo Z, Johnson J, Huang L. Two Birds with One Stone: Tailoring Singlet Fission for Both Triplet Yield and Exciton Diffusion Length. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2016; 28:7539-7547. [PMID: 27348847 DOI: 10.1002/adma.201600968] [Citation(s) in RCA: 51] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/18/2016] [Revised: 05/04/2016] [Indexed: 06/06/2023]
Abstract
By direct imaging of singlet and triplet populations with ultrafast microscopy, it is shown that the triplet diffusion length and singlet fission yield can be simultaneously optimized for tetracene and its derivatives, making them ideal structures for application in bilayer solar cells.
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Affiliation(s)
- Tong Zhu
- Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA
| | - Yan Wan
- Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA
| | - Zhi Guo
- Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA
| | - Justin Johnson
- National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, CO, 80401, USA
| | - Libai Huang
- Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA.
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21
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Ostrander JS, Serrano AL, Ghosh A, Zanni MT. Spatially Resolved Two-Dimensional Infrared Spectroscopy via Wide-Field Microscopy. ACS PHOTONICS 2016; 3:1315-1323. [PMID: 27517058 PMCID: PMC4976945 DOI: 10.1021/acsphotonics.6b00297] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We report the first wide-field microscope for measuring two-dimensional infrared (2D IR) spectroscopic images. We concurrently collect more than 16 000 2D IR spectra, made possible by a new focal plane array detector and mid-IR pulse shaping, to generate hyperspectral images with multiple frequency dimensions and diffraction-limited spatial resolution. Both frequency axes of the spectra are collected in the time domain by scanning two pairs of femtosecond pulses using a dual acousto-optic modulator pulse shaper. The technique is demonstrated by imaging a mixture of metal carbonyl absorbed polystyrene beads. The differences in image formation between FTIR and 2D IR microscopy are also explored by imaging a patterned USAF test target. We find that our 2D IR microscope has diffraction-limited spatial resolution and enhanced contrast compared to FTIR microscopy because of the nonlinear scaling of the 2D IR signal to the absorptivity coefficient for the vibrational modes. Images generated using off-diagonal peaks, created from vibrational anharmonicities, improve the molecular discrimination and eliminate noise. Two-dimensional wide-field IR microscopy provides information on vibrational lifetimes, molecular couplings, transition dipole orientations, and many other quantities that can be used for creating image contrast to help disentangle and interpret complex and heterogeneous samples. Such experiments made possible could include the study of amyloid proteins in tissues, protein folding in heterogeneous environments, and structural dynamics in devices employing mid-IR materials.
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22
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Han S, Yang X, Zhuang X, Yu J, Li L. Tailoring the Dielectric Layer Structure for Enhanced Performance of Organic Field-Effect Transistors: The Use of a Sandwiched Polar Dielectric Layer. MATERIALS 2016; 9:ma9070545. [PMID: 28773667 PMCID: PMC5456942 DOI: 10.3390/ma9070545] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/15/2016] [Revised: 05/22/2016] [Accepted: 07/01/2016] [Indexed: 11/16/2022]
Abstract
To investigate the origins of hydroxyl groups in a polymeric dielectric and its applications in organic field-effect transistors (OFETs), a polar polymer layer was inserted between two polymethyl methacrylate (PMMA) dielectric layers, and its effect on the performance as an organic field-effect transistor (OFET) was studied. The OFETs with a sandwiched dielectric layer of poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) containing hydroxyl groups had shown enhanced characteristics compared to those with only PMMA layers. The field-effect mobility had been raised more than 10 times in n-type devices (three times in the p-type one), and the threshold voltage had been lowered almost eight times in p-type devices (two times in the n-type). The on-off ratio of two kinds of devices had been enhanced by almost two orders of magnitude. This was attributed to the orientation of hydroxyl groups from disordered to perpendicular to the substrate under gate-applied voltage bias, and additional charges would be induced by this polarization at the interface between the semiconductor and dielectrics, contributing to the accumulation of charge transfer.
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Affiliation(s)
- Shijiao Han
- State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China.
| | - Xin Yang
- Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China.
| | - Xinming Zhuang
- State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China.
| | - Junsheng Yu
- State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China.
- Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China.
| | - Lu Li
- Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China.
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23
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Wen J, Li W, Chen S, Ma J. Simulations of molecular self-assembled monolayers on surfaces: packing structures, formation processes and functions tuned by intermolecular and interfacial interactions. Phys Chem Chem Phys 2016; 18:22757-71. [DOI: 10.1039/c6cp01049k] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Simulations using QM and MM methods guide the rational design of functionalized SAMs on surfaces.
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Affiliation(s)
- Jin Wen
- Institute of Theoretical and Computational Chemistry
- Key Laboratory of Mesoscopic Chemistry of MOE
- School of Chemistry and Chemical Engineering
- Nanjing University
- Nanjing 210023
| | - Wei Li
- Institute of Theoretical and Computational Chemistry
- Key Laboratory of Mesoscopic Chemistry of MOE
- School of Chemistry and Chemical Engineering
- Nanjing University
- Nanjing 210023
| | - Shuang Chen
- Kuang Yaming Honors School
- Nanjing University
- Nanjing
- P. R. China
| | - Jing Ma
- Institute of Theoretical and Computational Chemistry
- Key Laboratory of Mesoscopic Chemistry of MOE
- School of Chemistry and Chemical Engineering
- Nanjing University
- Nanjing 210023
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24
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Arias DH, Ryerson JL, Cook JD, Damrauer NH, Johnson JC. Polymorphism influences singlet fission rates in tetracene thin films. Chem Sci 2015; 7:1185-1191. [PMID: 29910873 PMCID: PMC5975788 DOI: 10.1039/c5sc03535j] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2015] [Accepted: 11/05/2015] [Indexed: 12/21/2022] Open
Abstract
We report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials. SF has been comprehensively studied in one polymoprh (Tc I), but not in the other, less stable polymorph (Tc II). Using carefully controlled thermal evaporation deposition conditions and high sensitivity ultrafast transient absorption spectroscopy, we found that for large crystallite size samples, SF in nearly pure Tc II films is significantly faster than SF in Tc I films. We also discovered that crystallite size has a minimal impact on the SF rate in Tc II films, but a significant influence in Tc I films. Large crystallites exhibit SF times of 125 ps and 22 ps in Tc I and Tc II, respectively, whereas small crystallites have SF times of 31 ps and 33 ps. Our results demonstrate first, that attention must be paid to polymorphism in obtaining a self-consistent rate picture for SF in tetracene and second, that control of polymorphism can play a significant role towards achieving a mechanistic understanding of SF in polycrystalline systems. In this latter context we show that conventional theory based on non-covalent tetracene couplings is insufficient, thus highlighting the need for models that capture the delocalized and highly mobile nature of excited states in elucidating the full photophysical picture.
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Affiliation(s)
- Dylan H Arias
- National Renewable Energy Laboratory , Golden , Colorado 80401 , USA . .,Department of Chemistry and Biochemistry , University of Colorado , Boulder , Colorado 80309 , USA .
| | - Joseph L Ryerson
- National Renewable Energy Laboratory , Golden , Colorado 80401 , USA . .,Department of Chemistry and Biochemistry , University of Colorado , Boulder , Colorado 80309 , USA .
| | - Jasper D Cook
- Department of Chemistry and Biochemistry , University of Colorado , Boulder , Colorado 80309 , USA .
| | - Niels H Damrauer
- Department of Chemistry and Biochemistry , University of Colorado , Boulder , Colorado 80309 , USA .
| | - Justin C Johnson
- National Renewable Energy Laboratory , Golden , Colorado 80401 , USA .
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25
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Wan Y, Guo Z, Zhu T, Yan S, Johnson J, Huang L. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy. Nat Chem 2015; 7:785-92. [DOI: 10.1038/nchem.2348] [Citation(s) in RCA: 163] [Impact Index Per Article: 18.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2015] [Accepted: 09/14/2015] [Indexed: 12/22/2022]
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26
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Pan Z, Rawat N, Cour I, Manning L, Headrick RL, Furis M. Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films. Nat Commun 2015; 6:8201. [PMID: 26365682 PMCID: PMC4579592 DOI: 10.1038/ncomms9201] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2015] [Accepted: 07/24/2015] [Indexed: 12/03/2022] Open
Abstract
Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport. In-depth understanding of organic crystalline semiconductor thin films is critical for the development of many electronic and photonic devices. Here, the authors use combined linear dichroism and polarization-resolved photoluminescence scanning microscopy to unveil the nature of excited states in such systems.
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Affiliation(s)
- Z Pan
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
| | - N Rawat
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
| | - I Cour
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
| | - L Manning
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
| | - R L Headrick
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
| | - M Furis
- Department of Physics, Materials Science Program, University of Vermont, Burlington, Vermont 05405, USA
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27
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Devadas MS, Devkota T, Johns P, Li Z, Lo SS, Yu K, Huang L, Hartland GV. Imaging nano-objects by linear and nonlinear optical absorption microscopies. NANOTECHNOLOGY 2015; 26:354001. [PMID: 26266335 DOI: 10.1088/0957-4484/26/35/354001] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.
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Affiliation(s)
- Mary Sajini Devadas
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556-5670, USA
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28
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Penwell SB, Ginsberg LDS, Ginsberg NS. Bringing Far-Field Subdiffraction Optical Imaging to Electronically Coupled Optoelectronic Molecular Materials Using Their Endogenous Chromophores. J Phys Chem Lett 2015; 6:2767-2772. [PMID: 26266861 DOI: 10.1021/acs.jpclett.5b01200] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
We demonstrate that subdiffraction resolution can be achieved in fluorescence imaging of functional materials with densely packed, endogenous, electronically coupled chromophores by modifying stimulated emission depletion (STED) microscopy. This class of chromophores is not generally compatible with STED imaging due to strong two-photon absorption cross sections. Yet, we achieve 90 nm resolution and high contrast in images of clusters of conjugated polymer polyphenylenevinylene-derivative nanoparticles by modulating the excitation intensity in the material. This newfound capability has the potential to significantly broaden the range of fluorophores that can be employed in super-resolution fluorescence imaging. Moreover, solution-processed optoelectronics and photosynthetic or other naturally luminescent biomaterials exhibit complex energy and charge transport characteristics and luminescence variations in response to nanoscale heterogeneity in their complex, physical structures. Our discovery will furthermore transform the current understanding of these materials' structure-function relationships that have until now made them notoriously challenging to characterize on their native, subdiffraction scales.
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Affiliation(s)
| | | | - Naomi S Ginsberg
- ⊥Kavli Energy NanoSciences Institute, Berkeley, California 94720, United States
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29
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Abstract
Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we introduce a general model, based upon Förster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates, which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific hopping rates is broadened in a manner that results in a decrease in average exciton diffusivity relative to that in a perfectly ordered film. Second, since excitons prefer to make transitions that are downhill in energy, the steady state distribution of exciton energies is biased toward low-energy molecular subunits, those that exhibit reduced diffusivity relative to a perfectly ordered film. These effects combine to reduce the net diffusivity in a manner that is time dependent and grows more pronounced as disorder is increased. Notably, however, we demonstrate that the presence of energetic disorder can give rise to a population of molecular subunits with exciton transfer rates exceeding those of subunits in an energetically uniform material. Such enhancements may play an important role in processes that are sensitive to molecular-scale fluctuations in exciton density field.
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Affiliation(s)
- Elizabeth M Y Lee
- †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
| | - William A Tisdale
- †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
| | - Adam P Willard
- ‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
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