C. Y. Wong, B. D. Folie, B. L. Cotts, N. S. Ginsberg, "Discerning Variable Extents of Inter-Domain Orientational and Structural Heterogeneity in Solution-Cast Polycrystalline Organic Semiconductor Thin Films," Journal of Physical Chemistry Letters, ASAP (2015).
By spatially resolving the polarized ultrafast optical transient absorption within several tens of individual domains in solution-processed polycrystalline small-molecule organic semiconducting films, we infer the domains’ extents of structural and orientational heterogeneity. As metrics, we observe variations in the time scales of ultrafast excited state dynamics and in the relative strength of competing resonant probe transitions. We find that films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) exhibit a much higher degree of both structural and orientational heterogeneity among their domains than do films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn), despite the apparent structural similarity between these two small molecules. Since both molecules feature prominently in solution-processed organic transistors, correlating the extent of heterogeneity to bulk transport using our approach will be highly valuable toward determining the underlying design principles for creating high-performing devices. Furthermore, our ability to characterize such variation in heterogeneity will enable fundamental studies of the interplay between molecular dynamics and driving forces in controlling emergent unequilibrated structures.
doi link PDF
S. B. Penwell, L. D. S. Ginsberg, N. S. Ginsberg, "Bringing Far-Field Sub-Diffraction Optical Resolution to Electronically-Coupled Optoelectronic Molecular Materials using their Endogenous Chromophores," Journal of Physical Chemistry Letters, 6, 2767-2772 (2015).
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.
doi link PDF
C. G. Bischak, E. M. Sanehira, J. T. Precht, J. M. Luther, N. S. Ginsberg, "Heterogeneous charge carrier dynamics in organic-inorganic hybrid materials: nanoscale lateral and depth-dependent variation of recombination rates in methylammonium lead halide perovskite thin films," Nano Letters, 15, 4799-4807 (2015).
We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using high-resolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depth-dependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic–inorganic hybrid materials.
doi link PDF
R. Noriega, D. T. Finley, J. Haberstroh, P. L. Geissler, M. B. Francis, N. S. Ginsberg. "Manipulating excited state dynamics of light harvesting chromophores through restricted motions in a hydrated nanoscale protein cavity," Journal of Physical Chemistry B, 119, 6963-6973 (2015).
Manipulating the photophysical properties of light-absorbing units is a crucial element in the design of biomimetic light-harvesting systems. Using a highly tunable synthetic platform combined with transient absorption and time-resolved fluorescence measurements and molecular dynamics simulations, we interrogate isolated chromophores covalently linked to different positions in the interior of the hydrated nanoscale cavity of a supramolecular protein assembly. We find that, following photoexcitation, the time scales over which these chromophores are solvated, undergo conformational rearrangements, and return to the ground state are highly sensitive to their position within this cavity and are significantly slower than in a bulk aqueous solution. Molecular dynamics simulations reveal the hindered translations and rotations of water molecules within the protein cavity with spatial specificity. The results presented herein show that fully hydrated nanoscale protein cavities are a promising way to mimic the tight protein pockets found in natural light-harvesting complexes. We also show that the interplay between protein, solvent, and chromophores can be used to substantially tune the relaxation processes within artificial light-harvesting assemblies in order to significantly improve the yield of interchromophore energy transfer and extend the range of excitation transport. Our observations have implications for other important, similarly sized bioinspired materials, such as nanoreactors and biocompatible targeted delivery agents.
doi link PDF
C. G. Bischak, C. L. Hetherington, Z. Wang, J. T. Precht, D. M. Kaz, D. G. Schlom, N. S. Ginsberg. "Cathodoluminescence-activated nano-imaging: Non-invasive near-field scanning optical microscopy in an electron microscope," Nano Letters, 15, 3383-3390 (2015).
We demonstrate a new nanoimaging platform in which optical excitations generated by a low-energy electron beam in an ultrathin scintillator are used as a noninvasive, near-field optical scanning probe of an underlying sample. We obtain optical images of Al nanostructures with 46 nm resolution and validate the noninvasiveness of this approach by imaging a conjugated polymer film otherwise incompatible with electron microscopy due to electron-induced damage. The high resolution, speed, and noninvasiveness of this “cathodoluminescence-activated” platform also show promise for super-resolution bioimaging.
doi link PDF
C. Y. Wong, B. L. Cotts, H. Wu, N. S. Ginsberg. “Exciton dynamics reveal aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films,” Nature Communications, 6, 5946 (2015).
Large-scale organic electronics manufacturing requires solution processing. For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. Here we use transient absorption microscopy to isolate a unique signature of a hidden interface in a TIPS-pentacene thin film, exposing its exciton dynamics and intermolecular structure. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystallites interleaved by a web of interfaces that compound decreases in charge mobility. Our novel approach provides critical missing information on interface morphology necessary to correlate solution-processing methods to optimal device performance. doi link PDF
S. Sharifzadeh, C. Y. Wong, H. Wu, B. L. Cotts, N. S. Ginsberg, J. B. Neaton. "Relating the physical structure and optoelectronic function of crystalline TIPS-pentacene," Advanced Functional Materials (2014).
Theory and experiment are combined to investigate the nature of low-energy excitons within ordered domains of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) polycrystalline thin films. First-principles density functional theory and many-body perturbation theory calculations, along with polarization-dependent optical absorption spectro-microscopy on ordered domains, show multiple low-energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the first absorption peak is composed of a single excitonic transition and retains the polarization-dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure-dependence of polarization-dependent absorption reveals the exact inter-grain orientation within the TIPS-PEN film. Additionally, the degree of exciton delocalization can be significantly tuned by modest changes in the solid-state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These findings pave the way for tailoring the singlet fission efficiency of organic crystals by solid-state structure. doi link PDF
D. M. Kaz, C. G. Bischak, C. L. Hetherington, H. H. Howard, X. Marti, J. D. Clarkson, C. Adamo, D. G. Schlom, R. Ramesh, S. Aloni, D. F. Ogletree, N. S. Ginsberg. “Bright Cathodoluminescent Thin Films for Scanning Nano-Optical Excitation and Imaging,” ACS Nano, 7, 10397-10404 (2013).
Demand for visualizing nanoscale dynamics in biological and advanced materials continues to drive the development of subdiffraction optical probes. While many strategies employ scanning tips for this purpose, we instead exploit a focused electron beam to create scannable nanoscale optical excitations in an epitaxially grown thin-film of cerium-doped yttrium aluminum perovskite, whose cathodoluminescence response is bright, robust, and spatially resolved to 18 nm. We also demonstrate lithographic patterning of the film’s luminescence at the nanoscale. We anticipate that converting these films into free-standing membranes will yield a powerful near-field optical microscopy without the complication of mechanical scanning. doi link PDF
C. Y. Wong, S. B. Penwell, B. L. Cotts, R. Noriega, H. Wu, N. S. Ginsberg. “Revealing Exciton Dynamics in a Small-Molecule Organic Semiconducting Film with Subdomain Transient Absorption Microscopy.” Journal of Physical Chemistry C, 117, 22111-22122 (2013).
The ultrafast spectroscopy of single domains of polycrystalline films of TIPS-pentacene, a small-molecule organic semiconductor of interest in electronic and photovoltaic applications, is investigated using transient absorption microscopy. Individual domains are distinguished by their different polarization-dependent linear and nonlinear optical responses. As compared to bulk measurements, we show that the nonlinear response within a given domain can be tied more concretely to specific physical processes that transfer exciton populations between specified electronic states. By use of this approach and a simple kinetic model, the signatures of singlet fission as well as vibrational relaxation of the initially excited singlet state are identified. As such, observing exciton dynamics within and comparing exciton dynamics between different TIPS-pentacene domains reveal the relationship between photophysics and film morphology needed to improve device performance. doi link PDF
Professor Ginsberg's Previous Work:
G. S. Schlau-Cohen, A. Ishizaki, T. R. Calhoun, N. S. Ginsberg, M. Ballottari, R. Bassi, and G. R. Fleming. “Elucidation of the timescales and origins of quantum electronic coherence in LHCII,” Nature Chemistry, 4, 389 (2012). doi link
N. S. Ginsberg, J. D. Davis, M. Ballottari, Y.-C. Cheng, R. Bassi, and G. R. Fleming. “Solving structure in the CP29 light harvesting complex with polarization-phased 2D electronic spectroscopy,” Proceedings of the National Academy of Science, 108, 3848-3853 (2011). doi link
G. S. Schlau-Cohen, T. R. Calhoun, N. S. Ginsberg, M. Ballottari, R. Bassi, G. R. Fleming. “Spectroscopic Elucidation of Uncoupled Transition Energies in the Major Photosynthetic Light Harvesting Complex, LHCII,” Proceedings of the National Academy of Science, 107, 13276 (2010). doi link
T. R. Calhoun, N. S. Ginsberg, G. S. Schlau-Cohen, Y-C. Cheng, M. Ballottari, R. Bassi, and G. R. Fleming. “Quantum Coherence Enabled Determination of the Energy Landscape in Light Harvesting Complex II,” Journal of Physical Chemistry B, 113, 16291 (2009). (cover article). doi link
G. S. Schlau-Cohen, T. R. Calhoun, N. S. Ginsberg, E. L. Read, M. Ballottari, R. Bassi, G. R. Fleming. “Mapping Pathways of Energy Flow in LHCII with Two-Dimensional Electronic Spectroscopy,” Journal of Physical Chemistry B, 113, 15352 (2009). doi link
N. S. Ginsberg, Y.-C. Cheng, and G. R. Fleming. “Two-dimensional electronic spectroscopy of molecular aggregates,” Accounts of Chemical Research 42, 1352 (2009). doi link
N. S. Ginsberg, S. R. Garner, L. V. Hau. “Coherent control of optical information with matter wave dynamics,” Nature 445, 623 (2007). doi link,
- cover article; featured in New York Times, National Public Radio, Nature video stream and podcast
C. Slowe, N. S. Ginsberg, T. Ristroph, A. Goodsell, L. V. Hau. “Ultraslow Light & Bose-Einstein Condensates: Two-way Control with Coherent Light & Atom Fields,” Optics & Photonics News 16, 30 (2005). doi link
N. S. Ginsberg, J. Brand, L. V. Hau. “Observation of Hybrid Soliton Vortex-Ring Structures in Bose-Einstein Condensates,” Physical Review Letters 94, 040403 (2005). doi link
- highlighted in American Institute of Physics’ “Physics News Update,” Physics Today’s “Physics Update,” and selected as one of 44 articles from 2005 to be highlighted in APS News, February 2006
Z. Dutton, N. S. Ginsberg, C. Slowe, L. V. Hau. “The Art of Taming Light: Ultra-slow and Stopped Light,” Europhysics News 35, 33 (2004). doi link
D. I. Hoult, N. S. Ginsberg. “The Quantum Origins of the Free Induction Decay Signal and Spin Noise,” Journal of Magnetic Resonance 148, 182 (2001). doi link