Publications

Click on the title to reveal the abstract and manuscript links. Group theses are found here.

C. G. Bischak, J. G. Raybin, J. W. Kruppe, N. S. Ginsberg, "Charging-driven coarsening and melting of a colloidal nanoparticle monolayer at an ionic liquid-vacuum interface," Soft Matter, Advance Article (2020).

We induce and investigate the coarsening and melting dynamics of an initially static nanoparticle colloidal monolayer at an ionic liquid–vacuum interface, driven by a focused, scanning electron beam. Coarsening occurs through grain interface migration and larger-scale motions such as grain rotations, often facilitated by sliding dislocations. The progressive decrease in area fraction that drives melting of the monolayer is explained using an electrowetting model whereby particles at the interface are solvated once their accumulating charge recruits sufficient counterions to subsume the particle. Subject to stochastic particle removal from the monolayer, melting is recapitulated in simulations with a Lennard-Jones potential. This new driving mechanism for colloidal systems, whose dynamical timescales we show can be controlled with the accelerating voltage, opens the possibility to manipulate particle interactions dynamically without need to vary particle intrinsic properties or surface treatments. Furthermore, the decrease in particle size availed by electron imaging presents opportunities to observe force and time scales in a lesser-explored regime intermediate between typical colloidal and molecular systems. doi link PDF
C. G. Bischak*, M. Lai*, Z. Fan*, D. Lu, P. David, D. Dong, H. Chen, A. S. Etman, T. Lei, J. Sun, M. Grünwald, D. T. Limmer, P. Yang, N. S. Ginsberg, "Liquid-Like Interfaces Mediate Structural Phase Transitions in Lead Halide Perovskites," Matter, 3: 534-545 (2020). *equal contribution

Microscopic pathways of structural phase transitions in metal halide perovskites are difficult to probe because they occur over disparate time and length scales and because electron-based microscopies typically used to directly probe nanoscale dynamics of phase transitions often damage metal halide perovskite materials. Using in situ nanoscale cathodoluminescence microscopy with low electron beam exposure, we visualize nucleation and growth in the thermally driven transition to the perovskite phase in hundreds of non-perovskite phase nanowires. In combination with molecular dynamics simulations, we reveal that the transformation does not follow a simple martensitic mechanism, but proceeds despite a substantial energy barrier via ion diffusion through a liquid-like interface between the two structures. While cations are disordered in this liquid-like region, the halide ions retain substantial spatial correlations. This detailed picture not only reveals how phase transitions between disparate structures can proceed, but also opens the possibility to control such processes. doi link PDF
D. T. Limmer and N. S. Ginsberg, "Photoinduced phase separation in the lead halides is a polaronic effect," J. Chem. Phys., 152:230901 (2020). (Cover article)

We present a perspective on recent observations of the photoinduced phase separation of halides in multi-component lead-halide perovskites. The spontaneous phase separation of an initial homogeneous solid solution under steady-state illumination conditions is found experimentally to be reversible, stochastic, weakly dependent on morphology, yet strongly dependent on composition and thermodynamic state. Regions enriched in a specific halide species that form upon phase separation are self-limiting in size, pinned to specific compositions, and grow in number in proportion to the steady-state carrier concentration until saturation. These empirical observations of robustness rule out explanations based on specific defect structures and point to the local modulation of an existing miscibility phase transition in the presence of excess charge carriers. A model for rationalizing existing observations based on the coupling between composition, strain, and charge density fluctuations through the formation of polarons is reviewed. doi link PDF Cover article
T. D. Roberts*, R. Yuan*, L. Xiang, R. Pokhrel, K. Yang, P. Trefonas, K. Xu, N. S. Ginsberg, "Direct correlation of single-particle motion to amorphous microstructural components of semicrystalline polyethylene oxide electrolytic films," J Phys Chem Lett, 11:4849–4858 (2020). *equal contribution

Semicrystalline polymers constitute some of the most widely used materials in the world, and their functional properties are intimately connected to their structure on a range of length scales. Many of these properties depend on the micro- and nanoscale heterogeneous distribution of crystalline and amorphous phases, but this renders the interpretation of ensemble averaged measurements challenging. We use superlocalized widefield single-particle tracking in conjunction with AFM phase imaging to correlate the crystalline morphology of lithium-triflate-doped poly(ethylene oxide) thin films to the motion of individual fluorescent probes at the nanoscale. The results demonstrate that probe motion is intrinsically isotropic in amorphous regions and that, without altering this intrinsic diffusivity, closely spaced, often parallel, crystallite fibers anisotropically constrain probe motion along intercalating amorphous channels. This constraint is emphasized by the agreement between crystallite and anisotropic probe trajectory orientations. This constraint is also emphasized by the extent of the trajectory confinement correlated to the width of the measured gaps between adjacent crystallites. This study illustrates with direct nanoscale correlations how controlled and periodic arrangement of crystalline domains is a promising design principle for mass transport in semicrystalline polymer materials without compromising their mechanical stability. doi link PDF
M. Delor, A. H. Slavney, N. R. Wolf, M. R. Filip, J. B. Neaton, H. I. Karunadasa, N. S. Ginsberg, "Carrier diffusion lengths exceeding 1 micron despite trap-limited transport in halide double perovskites," ACS Energy Letters, 5:1337-1345 (2020).

We image charge carrier transport over nanometers−micrometers and picoseconds−microseconds in halide double perovskites Cs2AgBiBr6 and Cs2AgTlBr6 single crystals using stroboscopic scattering microscopy. Both materials exhibit long, microsecond carrier lifetimes because of their indirect or symmetry-forbidden direct bandgaps. We extract free-charge and trap-limited mobilities near the surface and in the crystal bulk. The free-charge mobilities for both materials (∼10−50 cm2/(V s)) can reach those reported for archetypal lead halide perovskites. We measure trap densities exceeding 1017 cm-3 within ∼20 nm of the crystal surface. Measurements on freshly cleaved or thermally annealed crystals suggest the traps are primarily halide vacancies that likely form through surface bromine degassing. Although these traps considerably slow charge transport, they are energetically shallow, enabling thermally induced detrapping and mobile carriers over microseconds at room temperature. This defect tolerance yields carrier diffusion lengths exceeding 1 μm even in the presence of large trap densities and under solar excitation conditions where traps are not saturated. These results suggest that halide double perovskites could rival the best lead-based perovskites for photovoltaic and optoelectronic applications. doi link PDF
B. Guzelturk, J. K. Utterback, I. Coropceanu, V. Kamysbayev, E. Janke, M. Zajac, N. Yazdani, B. L. Cotts, S.-J. Park, A. Sood, M.-F. Lin, A. H. Reid, M. E. Kozina, X. Shen, S. P Weathersby, V. Wood, A. Salleo, X. Wang, D. V. Talapin, N. S. Ginsberg, A. M. Lindenberg," Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy," ACS Nano, 14:4792-4804 (2020).

Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron−phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron−phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications. doi link PDF
R. B. Wai, N. Ramesh, C. D. Aiello, J. G. Raybin, S. E. Zeltmann, C. G. Bischak, E. Barnard, S. Aloni, D. F. Ogletree, A. M. Minor, and N. S. Ginsberg, "Resolving enhanced Mn2+ luminescence at the surface of CsPbCl3 with time-resolved cathodoluminescence imaging," J. Phys. Chem. Letters, 11:2624-2629 (2020).

Mn2+ doping of lead halide perovskites has garnered recent interest because it produces stable orange luminescence in tandem with perovskite emission. Here, we observe enhanced Mn2+ luminescence at the edges of Mn2+-doped CsPbCl3 perovskite microplates and suggest an explanation for its origin using the high spatiotemporal resolution of timeresolved cathodoluminescence (TRCL) imaging. We reveal two luminescent decay components that we attribute to two different Mn2+ populations. While each component appears to be present both near the surface and in the bulk, the origin of the intensity variation stems from a higher proportion of the longer lifetime component near the perovskite surface. We suggest that this higher emission is caused by an increased probability of electron−hole recombination on Mn2+ near the perovskite surface due to an increased trap concentration there. This observation suggests that such surface features have yet untapped potential to enhance emissive properties via control of surface-to-volume ratio. doi link PDF
B. D. Folie, J. A. Tan, J. Huang, P. C. Sercel, M. Delor,M. Lai, J. L. Lyons, N. Bernstein, Al. Efros, P. Yang, N. S. Ginsberg, "Effect of Anisotropic Confinement on Electronic Structure and Dynamics of Band Edge Excitons in Inorganic Perovskite Nanowires," J. Phys. Chem. A, 124:1867-1876 (2020).

Inorganic lead halide perovskite nanostructures show promise as the active layers in photovoltaics, light emitting diodes, and other optoelectronic devices. They are robust in the presence of oxygen and water, and the electronic structure and dynamics of these nanostructures can be tuned through quantum confinement. Here we create aligned bundles of CsPbBr3 nanowires with widths resulting in quantum confinement of the electronic wave functions and subject them to ultrafast microscopy. We directly image rapid one-dimensional exciton diffusion along the nanowires, and we measure an exciton trap density of roughly one per nanowire. Using transient absorption microscopy, we observe a polarization-dependent splitting of the band edge exciton line, and from the polarized fluorescence of nanowires in solution, we determine that the exciton transition dipole moments are anisotropic in strength. Our observations are consistent with a model in which splitting is driven by shape anisotropy in conjunction with long-range exchange. doi link PDF
N. S. Ginsberg and W. A. Tisdale, "Spatially Resolved Exciton and Charge Transport in Emerging Semiconductors," Annual Review of Physical Chemistry, 71:1-30 (2020) [First published as a Review in Advance on November 22, 2019]

We review recent advances in the characterization of electronic forms of energy transport in emerging semiconductors. The approaches described all temporally and spatially resolve the evolution of initially localized populations of photogenerated excitons or charge carriers. We first provide a comprehensive background for describing the physical origin and nature of electronic energy transport both microscopically and from the perspective of the observer. We introduce the new family of far-field, time-resolved optical microscopies developed to directly resolve not only the extent of this transport but also its potentially temporally and spatially dependent rate. We review a representation of examples from the recent literature, including investigation of energy flow in colloidal quantum dot solids, organic semiconductors, organic-inorganic metal halide perovskites, and 2D transition metal dichalcogenides. These examples illustrate how traditional parameters like diffusivity are applicable only within limited spatiotemporal ranges and how the techniques at the core of this review, especially when taken together, are revealing a more complete picture of the spatiotemporal evolution of energy transport in complex semiconductors, even as a function of their structural heterogeneities. doi link PDF
M. D. Delor, H. L. Weaver, Q. Yu, N. S. Ginsberg, "Imaging material functionality through three-dimensional nanoscale tracking of energy flow," Nature Materials, 19, 56-62 (2020). [Published online 7 October 2019]

The ability of energy carriers to move between atoms and molecules underlies biochemical and material function. Understanding and controlling energy flow, however, requires observing it on ultrasmall and ultrafast spatio-temporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. Here, we developed a non-invasive optical scheme that leverages non-resonant interferometric scattering to track tiny changes in material polarizability created by energy carriers. We thus map evolving energy carrier distributions in four dimensions of spacetime with few-nanometre lateral precision and directly correlate them with material morphology. We visualize exciton, charge and heat transport in polyacene, silicon and perovskite semiconductors and elucidate how disorder affects energy flow in three dimensions. For example, we show that morphological boundaries in polycrystalline metal halide perovskites possess lateral- and depth-dependent resistivities, blocking lateral transport for surface but not bulk carriers. We also reveal strategies for interpreting energy transport in disordered environments that will direct the design of defect-tolerant materials for the semiconductor industry of tomorrow. doi link PDF
H. Stern, R. Wang, Y. Fan, R. Mizuta, J.C. Stewart, L.-M. Needham, T.D. Roberts, R. Wai, N.S. Ginsberg, D. Klenerman, S. Hofmann, S.F. Lee, " Spectrally Resolved Photodynamics of Individual Emitters in Large-Area Monolayers of Hexagonal Boron Nitride, " ACS Nano, 13, 4538-4547 (2019).

Hexagonal boron nitride (h-BN) is a 2D, wide band gap semiconductor that has recently been shown to display bright room-temperature emission in the visible region, sparking immense interest in the material for use in quantum applications. In this work, we study highly crystalline, single atomic layers of chemical vapor deposition grown h-BN and find predominantly one type of emissive state. Using a multidimensional super-resolution fluorescence microscopy technique we simultaneously measure spatial position, intensity, and spectral properties of the emitters, as they are exposed to continuous wave illumination over minutes. As well as low emitter heterogeneity, we observe inhomogeneous broadening of emitter line-widths and power law dependency in fluorescence intermittency; this is strikingly similar to previous work on quantum dots. These results show that high control over h-BN growth and treatment can produce a narrow distribution of emitter type and that surface interactions heavily influence the photodynamics. Furthermore, we highlight the utility of spectrally resolved wide-field microscopy in the study of optically active excitations in atomically thin two-dimensional materials. doi link PDF
M. Lai, A. Obliger, D. Lu, C.S. Kley, C.G. Bischak, Q. Kong, T. Lei, L. Dou, N.S. Ginsberg, D.T. Limmer, P. Yang, " Intrinsic anion diffusivity in lead halide perovskites is facilitated by a soft lattice, " Proc. Natl. Acad. Sci., 115, 11929-11934 (2018).

Facile ionic transport in lead halide perovskites plays a critical role in device performance. Understanding the microscopic origins of high ionic conductivities has been complicated by indirect measurements and sample microstructural heterogeneities. Here, we report the direct visualization of halide anion interdiffusion in CsPbCl3–CsPbBr3 single crystalline perovskite nanowire heterojunctions using wide-field and confocal photoluminescence measurements. The combination of nanoscale imaging techniques with these single crystalline materials allows us to measure intrinsic anionic lattice diffusivities, free from complications of microscale inhomogeneity. Halide diffusivities were found to be between 10−13 and ~10−12 cm2/second at about 100 °C, which are several orders of magnitudes lower than those reported in polycrystalline thin films. Spatially resolved photoluminescence lifetimes and surface potential measurements provide evidence of the central role of halide vacancies in facilitating ionic diffusion. Vacancy formation free energies computed from molecular simulation are small due to the easily deformable perovskite lattice, accounting for the high equilibrium vacancy concentration. Furthermore, molecular simulations suggest that ionic motion is facilitated by low-frequency lattice modes, resulting in lowactivation barriers for vacancy-mediated transport. This work elucidates the intrinsic solid-state ion diffusion mechanisms in this class of semisoft materials and offers guidelines for engineering materials with long-term stability in functional devices. doi link PDF
Q. Kong, W. Lee, M. Lai, C.G. Bischak, G. Gao, A.B. Wong, T. Lei, Y. Yu, L.-W. Wang, N.S. Ginsberg, P. Yang, " Phase-transition–induced p-n junction in single halide perovskite nanowire, " Proc. Natl. Acad. Sci., 115, 8889-8894 (2018).

Semiconductor p-n junctions are fundamental building blocks for modern optical and electronic devices. The p- and n-type regions are typically created by chemical doping process. Here we show that in the new class of halide perovskite semiconductors, the p-n junctions can be readily induced through a localized thermal-driven phase transition. We demonstrate this p-n junction formation in a single-crystalline halide perovskite CsSnI3 nanowire (NW). This material undergoes a phase transition from a double-chain yellow (Y) phase to an orthorhombic black (B) phase. The formation energies of the cation and anion vacancies in these two phases are significantly different, which leads to n- and p- type electrical characteristics for Y and B phases, respectively. Interface formation between these two phases and directional interface propagation within a single NW are directly observed under cathodoluminescence (CL) microscopy. Current rectification is demonstrated for the p-n junction formed with this localized thermal-driven phase transition. doi link PDF
C.G. Bischak, A.B. Wong, E. Lin, D.T. Limmer, P. Yang, N. S. Ginsberg, "Tunable polaron distortions control the extent of halide demixing in lead halide perovskites," J. Phys. Chem. Lett., 9, 3998-4005 (2018).

Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I–-rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br-/I- methylammonium (MA+) lead halide perovskite (MAPb(BrxI1-x)3) nanoplates, eliminating effects from extended structural defects. Even in these nanoplates, we find that phase separation occurs, resulting in I–-rich clusters that are nucleated stochastically and stabilized by polarons. Upon lowering the electron–phonon coupling strength by partially exchanging MA+ for Cs+, a phase-separated steady state is not reached, nevertheless transient I- clustering still occurs. Our results, supported by multiscale modeling, demonstrate that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics of which depends on the electron–phonon coupling strength. doi link PDF
M. Delor, J. Dai, T. D. Roberts, J. R. Rogers, S. M. Hamed, J. B. Neaton, P. L. Geissler, M. B. Francis, N. S. Ginsberg, "Exploiting chromophore–protein interactions through linker engineering to tune photoinduced dynamics in a biomimetic light-harvesting platform," J. Am. Chem. Soc., 140, 6278 - 6287 (2018).

Creating artificial systems that mimic and surpass those found in nature is one of the great challenges of modern science. In the context of photosynthetic light harvesting, the difficulty lies in attaining utmost control over the energetics, positions and relative orientations of chromophores in densely packed arrays to transfer electronic excitation energy to desired locations with high efficiency. Toward achieving this goal, we use a highly versatile biomimetic protein scaffold from the tobacco mosaic virus coat protein on which chromophores can be attached at precise locations via linkers of differing lengths and rigidities. We show that minor linker modifications, including switching chiral configurations and alkyl chain shortening, lead to significant lengthening of the ultrafast excited state dynamics of the system as the linkers are shortened and rigidified. Molecular dynamics simulations provide molecular-level detail over how the chromophore attachment orientations, positions, and distances from the protein surface lead to the observed trends in system dynamics. In particular, we find that short and rigid linkers are able to sandwich water molecules between chromophore and protein, leading to chromophore–water–protein supracomplexes with intricately coupled dynamics that are highly dependent on their local protein environment. In addition, cyclohexyl-based linkers are identified as ideal candidates to retain rotational correlations over several nanoseconds and thus lock relative chromophore orientations throughout the lifetime of an exciton. Combining linker engineering with judicious placement of chromophores on the hydrated protein scaffold to exploit different chromophore–bath couplings provides a clear and effective path to producing highly controllable artificial light-harvesting systems that can increasingly mimic their natural counterparts, thus aiding to elucidate natural photosynthetic mechanisms. doi link PDF
B. D. Folie, J.B. Haber, S. Refaely-Abramson, J.B. Neaton, N. S. Ginsberg, "Long-Lived Correlated Triplet Pairs in a Π-Stacked Crystalline Pentacene Derivative," J. Am. Chem. Soc., 140, 2326–2335 (2018).

Singlet fission is the spin-conserving process by which a singlet exciton splits into two triplet excitons. Singlet fission occurs via a correlated triplet pair intermediate, but direct evidence of this state has been scant, and in films of TIPS-pentacene, a small molecule organic semiconductor, even the rate of fission has been unclear. We use polarization-resolved transient absorption microscopy on individual crystalline domains of TIPS-pentacene to establish the fission rate and demonstrate that the initially created triplets remain bound for a surprisingly long time, hundreds of picoseconds, before separating. Furthermore, using a broadband probe, we show that it is possible to determine absorbance spectra of individual excited species in a crystalline solid. We find that triplet interactions perturb the absorbance, and provide evidence that triplet interaction and binding could be caused by the π-stacked geometry. Elucidating the relationship between the lattice structure and the electronic structure and dynamics has important implications for the creation of photovoltaic devices that aim to boost efficiency via singlet fission. doi link PDF
C. G. Bischak, R. B. Wai, C. Cherqui, J. A. Busche, S. C. Quillin, C. L. Hetherington, Z. Wang, C. D. Aiello, D. G. Schlom, S. Aloni, D. F. Ogletree, D. J. Masiello, N. S. Ginsberg, "Non-invasive cathodoluminescence-activated nano-imaging of dynamic processes in liquids," ACS Nano, 11, 10583-10590 (2017).

In situ electron microscopy provides remarkably high spatial resolution, yet electron beam irradiation often damages soft materials and perturbs dynamic processes, requiring samples to be very robust. Here, we instead noninvasively image the dynamics of metal and polymer nanoparticles in a liquid environment with subdiffraction resolution using cathodoluminescence-activated imaging by resonant energy transfer (CLAIRE). In CLAIRE, a free-standing scintillator film serves as a nanoscale optical excitation source when excited by a low energy, focused electron beam. We capture the nanoscale dynamics of these particles translating along and desorbing from the scintillator surface and demonstrate 50 ms frame acquisition and a range of imaging of at least 20 nm from the scintillator surface. Furthermore, in contrast with in situ electron microscopy, CLAIRE provides spectral selectivity instead of relying on scattering alone. We also demonstrate through quantitative modeling that the CLAIRE signal from metal nanoparticles is impacted by multiplasmonic mode interferences. Our findings demonstrate that CLAIRE is a promising, noninvasive approach for super-resolution imaging for soft and fluid materials with high spatial and temporal resolution. doi link PDF
S. B. Penwell, L. D. S. Ginsberg, R. Noriega, N. S. Ginsberg, "Resolving ultrafast exciton migration in organic solids at the nanoscale," Nature Materials, 16, 1136-1141 (2017).

Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis. doi link PDF
M. Delor, D. G. McCarthy, B. L. Cotts, T. Roberts, R. Noriega, D. D. Devore, S. Mukhopadhyay, T. S. De Vries, N. S. Ginsberg, “Resolving and controlling photoinduced ultrafast solvation in the solid state,” J. Phys Chem Lett., 8, 4183-4190 (2017).

 

 

Solid-state solvation (SSS) is a solid-state analogue of solvent–solute interactions in the liquid state. Although it could enable exceptionally fine control over the energetic properties of solid-state devices, its molecular mechanisms have remained largely unexplored. We use ultrafast transient absorption and optical Kerr effect spectroscopies to independently track and correlate both the excited-state dynamics of an organic emitter and the polarization anisotropy relaxation of a small polar dopant embedded in an amorphous polystyrene matrix. The results demonstrate that the dopants are able to rotationally reorient on ultrafast time scales following light-induced changes in the electronic configuration of the emitter, minimizing the system energy. The solid-state dopant–emitter dynamics are intrinsically analogous to liquid-state solvent–solute interactions. In addition, tuning the dopant/polymer pore ratio offers control over solvation dynamics by exploiting molecular-scale confinement of the dopants by the polymer matrix. Our findings will enable refined strategies for tuning optoelectronic material properties using SSS and offer new strategies to investigate mobility and disorder in heterogeneous solid and glassy materials. doi link PDF
L. Dou, M. Lai, C. S. Kley, Y. Yang, C. G. Bischak, D. Zhang, S. W. Eaton, N. S. Ginsberg, P. Yang, "Spatially Resolved Multi-Color CsPbX3 Nanowire Heterojunctions via Anion Exchange," Proc. Natl. Acad. Sci., 114, 7216-7221 (2017).

 

 

Semiconductor heterojunction is a key building block in modern electronics and optoelectronics. The accurate control over the composition, band gap, energy level (band bending), and doping level is the foundation of ideal functional heterojunctions. We demonstrate highly spatially resolved heterojunctions in a type of semiconductor, halide perovskites, which show great potential in photovoltaic and solid-state lighting applications. We accomplish this through the combination of facile anion-exchange chemistry with nanofabrication techniques. The halide perovskite nanowire heterojunction provides an ideal platform for fundamental studies and technological applications. For example, multicolor lasers or LEDs could be made using such localized heterojunctions; quantitative interdiffusion and ion migration dynamics could be examined at elevated temperatures, etc. doi link PDF
B. L. Cotts, D. G. McCarthy, R. Noriega, S. B. Penwell, M. Delor, D. D. Devore, S. Mukhopadhyay, T. S. De Vries, N. S. Ginsberg, "Tuning thermally activated delayed fluorescence emitter photophysics through solvation in the solid state," ACS Energy Letters,  2, 1526–1533 (2017).

 

 

Solid-state solvation (SSS) is analogous to liquid-phase solvation but occurs within glassy matrices. Organic solutes with singlet charge transfer (1CT) excited states are especially susceptible to solvatochromism. Their 1CT states and photon emission energies decrease when surrounding molecules with sterically unhindered polar moieties reorient to stabilize them. Thermally activated delayed fluorescence (TADF) organic light-emitting diodes feature such solutes as emitters in the solid state, employing efficient reverse intersystem crossing to harvest the majority of electrogenerated triplets. Here we explore the potential of SSS to manipulate not only these emitters’ 1CT states but also, concurrently, their singlet–triplet energy gaps (ΔEST) that control TADF. By solvating the TADF emitter 2PXZ-OXD with progressively increasing concentrations of camphoric anhydride (CA) in a polystyrene film, we find that it is possible to finely tune the emitter’s photophysics. We observe a maximum increase in prompt lifetime and corresponding decrease in delayed lifetime of ~60%. By contrast, the photoluminescence quantum yield peaks at an intermediate CA concentration, reflecting competition between increasing reverse intersystem crossing yield and decreasing singlet oscillator strength. Our findings demonstrate technologically relevant fine control of emitter photophysical properties, as varying the extent of SSS reveals the convolved evolution of different kinetic rates as a function of the 1CT state energy and ΔEST. doi link PDF
C. G. Bischak, C. L. Hetherington, H. Wu, S. Aloni, D. F. Ogletree, D. T. Limmer, N. S. Ginsberg "Origin of photoinduced phase separation in hybrid perovskites," Nano Letters, 17, 1028-1033 (2017).

 

 

The distinct physical properties of hybrid organic–inorganic materials can lead to unexpected nonequilibrium phenomena that are difficult to characterize due to the broad range of length and time scales involved. For instance, mixed halide hybrid perovskites are promising materials for optoelectronics, yet bulk measurements suggest the halides reversibly phase separate upon photoexcitation. By combining nanoscale imaging and multiscale modeling, we find that the nature of halide demixing in these materials is distinct from macroscopic phase separation. We propose that the localized strain induced by a single photoexcited charge interacting with the soft, ionic lattice is sufficient to promote halide phase separation and nucleate a light-stabilized, low-bandgap, ∼8 nm iodide-rich cluster. The limited extent of this polaron is essential to promote demixing because by contrast bulk strain would simply be relaxed. Photoinduced phase separation is therefore a consequence of the unique electromechanical properties of this hybrid class of materials. Exploiting photoinduced phase separation and other nonequilibrium phenomena in hybrid materials more generally could expand applications in sensing, switching, memory, and energy storage. doi link PDF
M. Lai, Q. Kong, C. G. Bischak, Y. Yu, L. Dou, S. W. Eaton, N. S. Ginsberg, P. Yang, "Structural, optical, and electrical study of phase controlled cesium lead iodide nanowires," Nano Research, 1-8 (2017).

 

 

Cesium lead iodide (CsPbI3), in its black perovskite phase, has a suitable bandgap and high quantum efficiency for photovoltaic applications. However, CsPbI3 tends to crystalize into a yellow non-perovskite phase, which has poor optoelectronic properties, at room temperature. Therefore, controlling the phase transition in CsPbI3 is critical for practical application of this material. Here we report a systematic study of the phase transition of one-dimensional CsPbI3 nanowires and their corresponding structural, optical, and electrical properties. We show the formation of perovskite black phase CsPbI3 nanowires from the non-perovskite yellow phase through rapid thermal quenching. Post-transformed black phase CsPbI3 nanowires exhibit increased photoluminescence emission intensity with a shrinking of the bandgap from 2.78 to 1.76 eV. The perovskite nanowires were photoconductive and showed a fast photoresponse and excellent stability at room temperature. These promising optical and electrical properties make the perovskite CsPbI3 nanowires attractive for a variety of nanoscale optoelectronic devices. doi link PDF
R. Noriega, E. S. Barnard, B. Ursprung, B. L. Cotts, S. B. Penwell, P. J. Schuck, N. S. Ginsberg "Uncovering single-molecule photophysical heterogeneity of bright, thermally-activated delayed fluorescence emitters dispersed in glassy hosts," J. Am. Chem. Soc., 138, 13551–13560 (2016).

 

 

Recently developed all-organic emitters used in display applications achieve high brightness by harvesting triplet populations via thermally activated delayed fluorescence. The photophysical properties of these emitters therefore involve new inherent complexities and are strongly affected by interactions with their host material in the solid state. Ensemble measurements occlude the molecular details of how host–guest interactions determine fundamental properties such as the essential balance of singlet oscillator strength and triplet harvesting. Therefore, using time-resolved fluorescence spectroscopy, we interrogate these emitters at the single-molecule level and compare their properties in two distinct glassy polymer hosts. We find that nonbonding interactions with aromatic moieties in the host appear to mediate the molecular configurations of the emitters, but also promote nonradiative quenching pathways. We also find substantial heterogeneity in the time-resolved photoluminescence of these emitters, which is dominated by static disorder in the polymer. Finally, since singlet–triplet cycling underpins the mechanism for increased brightness, we present the first room-temperature measurement of singlet–triplet equilibration dynamics in this family of emitters. Our observations present a molecular-scale interrogation of host–guest interactions in a disordered film, with implications for highly efficient organic light-emitting devices. Combining a single-molecule experimental technique with an emitter that is sensitive to triplet dynamics, yet read out via fluorescence, should also provide a complementary approach to performing fundamental studies of glassy materials over a large dynamic range of time scales. doi link PDF
L. Dou, A. B. Wong, Y. Yu, M. Lai, N. Kornienko, S. W. Eaton, A. Fu, C. G. Bischak, J. Ma, T. Ding, N. S. Ginsberg, L. W. Wang, A. P. Alivisatos. P. Yang, "Atomically thin two-dimensional organic-inorganic hybrid perovskites," Science, 349, 1518-1521 (2015).

 

 

Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials. doi link PDF
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, 6, 3155-3162 (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

 

Theses:

Namrata Ramesh, Honors B.S. May 2020. Investigating Self Assembly of Gold Nanocrystal Superlattices. PDF

Brendan D. Folie, Ph.D. December 2018. Extremeley Small and Incredibly Fast: Combining Spectroscopy and Microscopy to Reveal Local Excited State Dynamics in Disordered Semiconductors. PDF

Connor G. Bischak, Ph.D. August 2017. Elucidating heterogeneities and dynamic processes at the nanoscale with cathodoluminescence and cathodoluminescence-activated microscopies. PDF

Dannielle McCarthy, Honors B.S. January 2017. Tuning Thermally Activated Delayed Fluorescence through the Solid State Solvation Effect. PDF

Benjamin L. Cotts, Ph.D. December 2016. Time-resolved optical spectroscopy of organic electronics as a function of local environment. PDF

Samuel B. Penwell, Ph.D. August 2016. Spatially Resolving Dynamics and Nanoscale Migration of Excitons in Organic Semiconductors Using Transient Absorption Imaging and STED Microscopy. PDF