Michael F. Toney

University of California, Berkeley, Berkeley, California, United States

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Publications (433)2420.57 Total impact

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    ABSTRACT: The elevated level of atmospheric carbon dioxide (CO2) has caused serious concern of the progression of global warming. Geological sequestration is considered as one of the most promising techniques for mitigating the damaging effect of global climate change. Investigations over wide range of length-scales are important for systematic evaluation of the underground formations from prospective CO2 reservoir. Understanding the relationship between the micro morphology and the observed macro phenomena is even more crucial. Here we show Synchrotron based X-ray micro tomographic study of the morphological buildup of Sandstones. We present a numerical method to extract the pore sizes distribution of the porous structure directly, without approximation or complex calculation. We have also demonstrated its capability in predicting the capillary pressure curve in a mercury intrusion porosimetry (MIP) measurement. The method presented in this work can be directly applied to the morphological studies of heterogeneous systems in various research fields, ranging from Carbon Capture and Storage, and Enhanced Oil Recovery to environmental remediation in the vadose zone.
    Scientific Reports 06/2015; 5:10635. DOI:10.1038/srep10635 · 5.58 Impact Factor
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    Linda Y Lim, Shufen Fan, Huey Hoon Hng, Michael F Toney
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    ABSTRACT: Ge), and tin (Sn), are very attractive candidates for high capacity anodes for LIBs because they have low operating voltages, and possess high theoretical capacities (3850 mAh g −1 for Si, 1570 mAh g −1 for Ge, and 990 mAh g −1 for Sn). Their high capacities are attributed to their ability to alloy with as many as 4.4 Li atoms per Si, Ge, or Sn atom. [ 3 ] Even though Si is a potential candidate to replace graphite as anode, Ge is increasingly attracting attention for fast-charging and high-capacity LIBs applications, [ 4 ] due to its high lithium diffusivity (400 times faster than in Si, [ 5 ] and higher intrinsic electronic conductivity compared to Si. [ 6 ] In recent years, there have been a number of reports on the reaction mechanisms in Ge electrodes. [ 7–9 ] In situ and operando studies are more reliable than ex situ studies as the former allows us to watch the changes in the electrodes during battery operation, while the latter involves many postprocessing steps that modify the structure and chemistry of the electrodes. We previously investigated the reactions of crystalline Ge (c-Ge) electrodes cycled at C/15 using operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), and proposed a reaction mechanism. We showed that c-Ge lithiates inhomogeneously, fi rst forming an amorphous Li 9 Ge 4-like structure, followed by the conversion of the remaining c-Ge to amorphous Ge (aGe). Lithiation of this amorphous Ge forms ill-defi ned amorphous Li x Ge structures (a-Li x Ge), which are then further lithiated into crystalline Li 15 Ge 4 (c-Li 15 Ge 4). During delithiation, c-Li 15 Ge 4 transforms into a-Li x Ge and eventually amorphous Ge, although this transformation is incomplete, which partly explains the capacity fade. [ 8 ] In situ X-ray transmission microscopy imaging revealed a size dependent behavior of Ge particles during cycling at C/5, where smaller particles experience volume expansion before the larger ones and many small particles become inactive after one cycle. [ 9 ] A recent report used ex situ XRD, pair distribution function analyses, and in/ex situ high-resolution 7 Li solid-state nuclear magnetic resonance to investigate the fi rst lithiation cycle of c-Ge cycled at C/50. This reported a reaction sequence involving a succession of mostly disordered phases (similar to Li 7 Ge 3 and Li 7 Ge 2) ending with c-Li 15 Ge 4 and over-lithiated Li 15+ δ Ge 4 at the end of lithiation (although these were not quan-tifi ed). [ 10 ] The differences between this reaction sequence and Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies of Ge anodes are carried out to understand the effect of cycling rate on Ge phase transformation during charge/discharge process and to relate that effect to capacity. It is discovered that the formation of crystalline Li 15 Ge 4 (c-Li 15 Ge 4) during lithiation is suppressed beyond a certain cycling rate. A very stable and reversible high capacity of ≈1800 mAh g −1 can be attained up to 100 cycles at a slow Crate of C/21 when there is complete conversion of Ge anode into c-Li 15 Ge 4. When the Crate is increased to ≈C/10, the lithiation reaction is more heterogeneous and a relatively high capacity of ≈1000 mAh g −1 is achieved with poorer electrochemical reversibility. An increase in Crate to C/5 and higher reduces the capacity (≈500 mAh g −1) due to an impeded transformation from amorphous Li x Ge to c-Li 15 Ge 4 , and yet improves the electrochemical reversibility. A proposed mechanism is presented to explain the Crate dependent phase transformations and the relationship of these to capacity fading. The operando XRD and XAS results provide new insights into the relationship between structural changes in Ge and battery capacity, which are important for guiding better design of high-capacity anodes.
    Advanced Energy Materials 06/2015; DOI:10.1002/aenm.201500599 · 14.39 Impact Factor
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    ABSTRACT: A class of tunable, mesostructured self-assembled hydrogels has been developed based on a library of π–π stacking ABA-type telechelic triblock copolymers prepared by organocatalytic ring-opening polymerization (ROP). Poly(ethylene glycol) (PEG) served as a macroinitiator for ROP of methylene tricarbonate-benzyl ester (MTC-OBn), a benzyl-ester functionalized cyclic carbonate monomer, resulting in polymers with well-defined length and narrow polydispersity. Self-assembled hydrogels were formed by dissolving poly(MTC-OBn)-b-PEG-b-poly(MTC-OBn) copolymers in water. Physical cross-links in the hydrogels formed through hydrophobic and π–π stacking intermolecular interactions between poly(MTC-OBn) segments. The mesoscale material properties were probed using small angle X-ray scattering (SAXS) and dynamic mechanical measurements. SAXS spectra show a scattering peak, corresponding to a molecular domain spacing of 20–25 nm, which we attribute to the formation of electron-dense poly(MTC-OBn) assemblies. Results from oscillatory shear experiments were analyzed in conjunction with SAXS data in order to develop an understanding of the influence that nanoscale structure has on the mechanical properties of self-assembled gels. Modeling of scattering peaks using the Bragg spacing model demonstrated that the hydrophilic PEG network chains connecting the scattering moieties behaved as a two-dimensional self-avoiding walk. We compared the observed microstructural features with the mechanical properties of self-assembled gels and determined that the molecular weight of the polycarbonate and PEG segments controls the gel structure on both the mesoscale and macroscale. A central contribution of this work is a synthetic strategy that utilizes ROP to control polymer structure, which in turn controls both the structure and mechanical properties of these biodegradable self-assembled hydrogels.
    Polymer 05/2015; 65. DOI:10.1016/j.polymer.2015.03.029 · 3.77 Impact Factor
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    ABSTRACT: The solution shearing method has previously been used to tune the molecular packing and crystal thin film morphology of small molecular organic semiconductors (OSCs). Here, we study how the solution shearing method impacts the thin film morphology and causes structural rearrangements of two polymeric OSCs with interdigitated sidechain packing, namely P2TDC17FT4 and PBTTT-C16. The conjugated backbone tilt angle and the thin film morphology of the P2TDC17FT4 polymer were changed by the solution shearing conditions, and an accompanying change in the charge carrier mobility was observed. For PBTTT-C16, the out-of-plane lamellar spacing was increased by solution shearing, due to increased disorder of side chains. The ability to induce structural rearrangement of polymers through solution shearing allows for an easy and alternative method to modify OSC charge transport properties.
    Chemistry of Materials 04/2015; 27(7):2350-2359. DOI:10.1021/cm503780u · 8.54 Impact Factor
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    ABSTRACT: Vapor-deposited organic glasses can be produced with enhanced thermal stability and tunable molecular orientation by controlling the substrate temperature during deposition. Recent work has also shown improved charge carrier mobility associated with anisotropic molecular packing in organic electronics. Here grazing-incidence wide angle x-ray scattering (GIWAXS) is used to characterize the structural anisotropy in glasses of a hole transport material, N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine, commonly referred to as TPD. The TPD glasses were prepared by physical vapor deposition at substrate temperatures between 0.79 Tg and 0.98 Tg, where Tg is the glass transition temperature. A GIWAXS-derived orientation order parameter is used to quantify the anisotropy observed in the scattering patterns of the glasses. The GIWAXS-order parameter exhibits both positive and negative values as a function of substrate temperature, indicating either face-on or edge-on packing, and correlates well with a spectroscopic ellipsometry-derived order parameter that is sensitive to molecular orientation. We propose molecular packing arrangements based on the combination of the two order parameters and explore the relationship between kinetic stability and glass structure.
    Chemistry of Materials 04/2015; 27(9):150413092114002. DOI:10.1021/acs.chemmater.5b00583 · 8.54 Impact Factor
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    ABSTRACT: The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. Here, an open framework nanomaterial, copper hexacyanoferrate, in the Prussian Blue family is presented that allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb+, Pb2+, Al3+, and Y3+) in aqueous solution beyond that achieved in previous studies. Electrochemical measurements demonstrate the unprecedented kinetics of multivalent ion insertion associated with this material. Synchrotron X-ray diffraction experiments point toward a novel vacancy-mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results suggest a new approach to multi­valent ion insertion that may help to advance the understanding of this complex phenomenon.
    Advanced Energy Materials 04/2015; DOI:10.1002/aenm.201401869 · 14.39 Impact Factor
  • Elyse Coletta, Michael F. Toney, Curtis W. Frank
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    ABSTRACT: An understanding of the structure and properties of polymer electrolyte systems can be crucial to a variety of different applications. The current work performs a study of the composition, structure and properties of poly(ethylene glycol) (PEG)-aromatic polyimide systems incorporating ionic liquids that are relevant to several applications especially fuel cell membranes. Composition was varied through using different aromatic dianhydrides, aromatic diamines and in some cases synthesis solvent. Properties were characterized using Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle x-ray scattering, electrochemical impedance spectroscopy and cyclic voltammetry. By varying solvent, aromatic regularity and expected rigidity can be tuned, impacting average conductivity by 30%. Varying the aromatic diamine can influence the length scale and amount of aromatic regularity, which can ultimately affect the conductivity by a factor of four. The maximum conductivity reached was 83 mS/cm at 80 °C and 70 %RH. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015
    Journal of Polymer Science Part B Polymer Physics 04/2015; 53(7). DOI:10.1002/polb.23664 · 2.55 Impact Factor
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    ABSTRACT: There is high demand for efficient, robust and automated routines for tomographic data reduction, particularly for synchrotron data. Registration of the rotation axis in data processing is a critical step affecting the quality of the reconstruction and is not easily implemented with automation. Existing methods for calculating the center of rotation have been reviewed and an improved algorithm to register the rotation axis in tomographic data is presented. The performance of the proposed method is evaluated using synchrotron-based microtomography data on geological samples with and without artificial reduction of the signal-to-noise ratio. The proposed method improves the reconstruction quality by correcting both the tilting error and the translational offset of the rotation axis. The limitation of this promising method is also discussed.
    Journal of Synchrotron Radiation 03/2015; 22(2). DOI:10.1107/S160057751402726X · 3.02 Impact Factor
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    ABSTRACT: We observed a thermotropic phase transition in poly[3,4-dihexyl thiophene-2,2':5,6'-benzo[1,2-b:4,5-b']dithiophene] (PDHBDT) thin films accompanied by a transition from a random orientation to an ordered lamellar phase via a nearly-hexagonal lattice upon annealing. We demonstrate the effect of temperature-dependent molecular packing on charge carrier mobility (µ) in organic field-effect transistors (OFETs) and photovoltaic characteristics, such as exciton diffusion length (L<sub>) and power conversion efficiency (PCE), in organic solar cells (OSCs) using PDHBDT. The µ was continuously improved with increasing annealing temperature and PDHBDT films annealed at 270 oC resulted in a maximum µ up to 0.46 cm2/V∙s (µavg = 0.22 cm2/V∙s), which is attributed to the well-ordered lamellar structure with close a π-π stacking distance of 3.5 Å as shown by grazing incidence-angle X-ray diffraction (GIXD). On the other hand, PDHBDT films with a randomly orientated molecular orientation are more effective in photovoltaic devices than films with an ordered hexagonal or lamellar phase based on current-voltage characteristics of PDHBDT/C60 bilayer solar cells. This observation corresponds to an enhanced dark current density (JD) and a decreased LD upon annealing. This study provides insight into the dependence of charge transport and photovoltaic characteristics on molecular packing in polymer semiconductors, which is crucial for the management of charge and energy transport in a range of organic optoelectronic devices.
    Chemistry of Materials 02/2015; 27(4):150201103427005. DOI:10.1021/cm503773j · 8.54 Impact Factor
  • Johanna Nelson Weker, Michael F. Toney
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    ABSTRACT: Electrical vehicles (EVs) are an attractive option for moving towards a CO2 neutral transportation sector, but in order for widespread commercial use of EVs, the cost of electrical energy storage (i.e., batteries) must be reduced and the energy storage capacity must be increased. New, higher performing but Earth abundant electrodes are needed to accomplish this goal. To aid the development of these materials, in situ characterization to understand battery operation and failure is essential. Since electrodes are inherently heterogeneous, with a range of relevant length scales, imaging is a necessary component of the suite of characterization methods. In this Feature Article, the rapidly growing and developing field of X-ray based microscopy (XM) techniques is described and reviewed focusing on in situ and operando adaptations. Further, in situ transmission electron microscopy (TEM) is briefly discussed in this context and its complement to XM is emphasized. Finally, a perspective is given on some emerging X-ray based imaging approaches for energy storage materials.
    Advanced Functional Materials 02/2015; 25(11):n/a-n/a. DOI:10.1002/adfm.201403409 · 10.44 Impact Factor
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    ABSTRACT: Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.Keywords: organic photovoltaic; core-hole clock; resonant photoemission; charge-transfer state; electron dynamics; resonant Auger
    Journal of Physical Chemistry Letters 01/2015; 6(1):6-12. DOI:10.1021/jz502253r · 6.69 Impact Factor
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    ABSTRACT: Secondary phase segregation is hypothesized to have detrimental impacts on Cu2ZnSnS4 (CZTS) thin-film solar cells. In this study, we demonstrate the potential of using kinetic stabilization to inhibit phase decomposition in CZTS. By growing CZTS films at low temperature, we achieve a kinetically stabilized alloy with an expanded solid solution window in the pseudoternary CuS-ZnS-SnS phase diagram. Using X-ray absorption spectroscopy, we study the structural evolution and stability of this metastable alloy upon annealing. For near-stoichiometric samples, we observe a continuous emergence of short-range order toward crystalline CZTS that is nearly complete after a 1-min anneal at 450 °C. For Zn-rich samples, we detect precipitation of ZnS upon annealing, which suggests that the excess Zn exists as cation antisite defects in metastable CZTS.
    IEEE Journal of Photovoltaics 01/2015; 5(1):372-377. DOI:10.1109/JPHOTOV.2014.2360334 · 3.00 Impact Factor
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    ABSTRACT: Rapid thermal processing (RTP) is widely used for processing a variety of materials, including electronics and photovoltaics. Presently, optimization of RTP is done primarily based on ex-situ studies. As a consequence, the precise reaction pathways and phase progression during the RTP remain unclear. More awareness of the reaction pathways would better enable process optimization and foster increased adoption of RTP, which offers numerous advantages for synthesis of a broad range of materials systems. To achieve this, we have designed and developed a RTP instrument that enables real-time collection of X-ray diffraction data with intervals as short as 100 ms, while heating with ramp rates up to 100 °Cs(-1), and with a maximum operating temperature of 1200 °C. The system is portable and can be installed on a synchrotron beamline. The unique capabilities of this instrument are demonstrated with in-situ characterization of a Bi2O3-SiO2 glass frit obtained during heating with ramp rates 5 °C s(-1) and 100 °C s(-1), revealing numerous phase changes.
    Review of Scientific Instruments 01/2015; 86(1):013902. DOI:10.1063/1.4904848 · 1.58 Impact Factor
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    Chemistry of Materials 12/2014; DOI:10.1021/cm503828b · 8.54 Impact Factor
  • Chemistry of Materials 11/2014; 26(22):6531-6541. DOI:10.1021/cm5031987 · 8.54 Impact Factor
  • E. Coletta, M.F. Toney, C. W. Frank
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    ABSTRACT: Polymer electrolyte membrane (PEM) fuel cells can provide alternatively sourced energy, but current PEMs lose performance under certain conditions. The current work examines composition, structure and properties of poly (ethylene glycol)-aromatic polyimide-ionic liquid systems for PEM applications, as poly (ethylene glycol) (PEG) is an ion conductor and polyimides are stable. To evaluate polymer interactions, different PEG concentrations (0-50% by weight) and different PEG molecular weights (990-6000 g/mole) were examined. Characterization techniques included Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle x-ray scattering, electrochemical impedance spectroscopy and cyclic voltammetry. By increasing the PEG amount, PEG domains and polymer flexibility are increased, which increases conductivity by two to three orders of magnitude. By increasing PEG molecular weight, PEG segmental motion and PEG-polyimide interface quality are decreased, which decreases conductivity by a factor of two. The maximum conductivity was 64 mS/cm at 80 degrees C and 70 %RH.
    Polymer 11/2014; 55(26). DOI:10.1016/j.polymer.2014.10.075 · 3.77 Impact Factor
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    ABSTRACT: The crystallization and electrical characterization of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on a single layer graphene sheet is reported. Grazing incidence X-ray diffraction revealed that P3HT crystallizes with a mixture of face-on and edge-on lamellar orientations on graphene compared to mainly edge-on on a silicon substrate. Moreover, whereas ultrathin (10 nm) P3HT films form well oriented face-on and edge-on lamellae, thicker (50 nm) films form a mosaic of lamellae oriented at different angles from the graphene substrate. This mosaic of crystallites with π–π stacking oriented homogeneously at various angles inside the film favors the creation of a continuous pathway of interconnected crystallites, and results in a strong enhancement in vertical charge transport and charge carrier mobility in the thicker P3HT film. These results provide a better understanding of polythiophene crystallization on graphene, and should help the design of more efficient graphene based organic devices by control of the crystallinity of the semiconducting film.
    Advanced Functional Materials 11/2014; 25(5). DOI:10.1002/adfm.201403418 · 10.44 Impact Factor
  • Elyse Coletta, Michael F. Toney, Curtis W. Frank
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    ABSTRACT: Current fuel cell technology demands improvements for widespread use, and novel polymer materials may be able to achieve the necessary enhancements. This work inspects the composition, structure, and properties of poly(ethylene glycol) (PEG)–aromatic polyimide systems aimed at polymer electrolyte membrane applications, as PEG is a known ion conductor and aromatic polyimides are quite stable. Liquid electrolytes were incorporated into the polymers through soaking to achieve ionic conductivity. By varying polyimide and liquid electrolyte, the polymers were analyzed for their structure and conductivity. Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle X-ray scattering, electrochemical impedance spectroscopy, and cyclic voltammetry were used as characterization tools. Electrolyte identity impacts liquid uptake and conductivity. Polyimide identity can influence the size and variability of the doped polymer structure, which ultimately can change conductivity by up to 28%, with the maximum conductivity being 102 mS/cm at 80°C and 70% RH. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41675.
    Journal of Applied Polymer Science 11/2014; DOI:10.1002/app.41675 · 1.64 Impact Factor
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    ABSTRACT: ZnO nanotubes were prepared by selective dissolution of electrodeposited nanorods. The effect of solution pH, rod morphology and chloride ion concentration on the dissolution mechanism were studied. The selective etching was rationalized in terms of the surface energy of the different ZnO crystal faces and reactant diffusion. The nanorod diameter and chloride concentration are the most influential parameters on the dissolution mechanism because they control homogenous dissolution or selective etching of the (110) and (002) surfaces. Bulk solution pH only has an effect on the rate of dissolution. By accurate control of the dissolution process, the nanomorphology can be tailored and the formation of rods with a thin diameter (10-20 nm), cavity or ultra-thin-walled tubes (2-5 nm) can be achieved.
    Langmuir 10/2014; DOI:10.1021/la503765a · 4.38 Impact Factor
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    ABSTRACT: Alkyl substituents appended to the π-conjugated main chain account for the solution-processability and film-forming properties of most π-conjugated polymers for organic electronic device applications, including field-effect transistors (FETs) and bulk-heterojunction (BHJ) solar cells. Beyond film-forming properties, recent work has emphasized the determining role that side-chain substituents play on polymer self-assembly and thin-film nanostructural order, and, in turn, on device performance. However, the factors that determine polymer crystallite orientation in thin-films, implying preferential backbone orientation relative to the device substrate, are a matter of some debate, and these structural changes remain difficult to anticipate. In this report, we show how systematic changes in the side-chain pattern of poly(benzo[1,2-b:4,5-b']dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers can (i) influence the propensity of the polymer to order in the π-stacking direction, and (ii) direct the preferential orientation of the polymer crystallites in thin films (e.g., "face-on" vs. "edge-on"). Oriented crystallites, specifically crystallites that are well ordered in the π-stacking direction, are believed to be a key contributor to improved thin-film device performance in both FETs and BHJ solar cells.
    ACS Applied Materials & Interfaces 10/2014; DOI:10.1021/am505280a · 5.90 Impact Factor

Publication Stats

14k Citations
2,420.57 Total Impact Points


  • 2013
    • University of California, Berkeley
      Berkeley, California, United States
    • Diamond Light Source
      XPW, England, United Kingdom
    • Harvard University
      • Department of Chemistry and Chemical Biology
      Cambridge, Massachusetts, United States
  • 2012
    • King Abdullah University of Science and Technology
      Djidda, Makkah, Saudi Arabia
  • 2009–2012
    • National Institute of Standards and Technology
      • Polymers Division
      Maryland, United States
  • 2002–2012
    • Stanford University
      • • Department of Chemical Engineering
      • • Department of Materials Science and Engineering
      Palo Alto, California, United States
  • 2011
    • Carnegie Mellon University
      • Department of Materials Science and Engineering
      Pittsburgh, Pennsylvania, United States
  • 2007
    • Palo Alto Research Center
      Palo Alto, California, United States
    • University of Houston
      • Department of Chemical & Biomolecular Engineering
      Houston, Texas, United States
  • 2004–2007
    • University of Minnesota Duluth
      • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 2006
    • Pacific Northwest National Laboratory
      Ричленд, Washington, United States
  • 2005
    • New Jersey Institute of Technology
      • Department of Physics
      Newark, New Jersey, United States
  • 1999
    • Sogang University
      Sŏul, Seoul, South Korea
  • 1998
    • University of Wales
      Cardiff, Wales, United Kingdom
  • 1995
    • William Penn University
      Worcester, Massachusetts, United States
  • 1980–1994
    • University of Washington Seattle
      • Department of Physics
      Seattle, Washington, United States
  • 1989
    • Stanford Medicine
      Stanford, California, United States