George C. Schatz

Northwestern University, Evanston, Illinois, United States

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Publications (731)3603.11 Total impact

  • Michael B Ross, George C Schatz
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    ABSTRACT: We explore localized surface plasmon resonances in small (5–30 nm radius) aluminum and silver nanoparticles using classical electrodynamics simulations, focusing on radiative (far-field scattering) effects and the unique characteristics of aluminum as a plasmonic material. In Al spheres, higher-order plasmon resonances (e.g. quadrupoles) are significant at smaller sizes (>15 nm) than in Ag spheres. Additionally, although the plasmon width is minimized at a radius of about 15 nm for both materials, the Al plasmon linewidth (~1.4 eV) for the dipole mode is much larger than that observed in Ag (~0.3 eV). The radiative contribution to damping dominates over non-radiative effects for small (5–20 nm) Al spheres (>95%) whereas for similar size Ag spheres damping is almost entirely attributed to the bulk dielectric function (non-radiative). For Al nanorods the linewidths can be narrowed by increasing aspect ratio such that for an aspect ratio of 4.5, the overall Al (0.75 eV) linewidth is reasonably close to that of the same size Ag rod (0.35 eV). This narrowing arises from frequency dispersion in the real part of the Al dielectric function, and is associated with a 65% (1.5 to 0.5 eV) decrease in the radiative contribution to the linewidth for Al. Concurrently, an increase in the non-radiative width occurs as the aspect ratio increases and the plasmon tunes to the red. This demonstrates that anisotropy can be used as a parameter for controlling Al plasmon dephasing where the composition of the plasmon linewidth (radiative or non-radiative) can be tailored with aspect ratio. Overall, these data suggest that localized surface plasmon resonance dephasing mechanisms in Al nanostructures are inherently different from those in the noble metals, which could allow for new applications of plasmonic materials, tunable plasmon lifetimes, and new physics to be observed.
    Journal of Physics D Applied Physics 05/2015; 48(18). · 2.52 Impact Factor
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    ABSTRACT: Herein, we report the synthesis of structurally uniform gold circular disks as two-dimensional plasmonic nanostructures that complement the well-established one-dimensional rod and three-dimensional shell structures. We show that a Au conproportionation reaction can be used to etch a collection of nonuniform triangular prisms into a uniform circular disk product with thickness and diameter varying <10%. These new particles have broadly tunable plasmon resonances (650-1000 nm) with narrow bandwidths (0.23-0.28 eV) and can be described as "effectively two-dimensional" plasmonic structures, as they do not support a significant transverse mode.
    Nano letters. 01/2015;
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    ABSTRACT: Using on-wire lithography to synthesize well-defined nanorod dimers and trimers, we report a systematic study of the plasmon coupling properties of such materials. By comparing the dimer/trimer structures to discrete nanorods of the same overall length, we demonstrate many similarities between antibonding coupled modes in the dimers/trimers and higher-order resonances in the discrete nanorods. These conclusions are validated with a combination of discrete dipole approximation and finite-difference time-domain calculations and lead to the observation of antibonding modes in symmetric structures by measuring their solution-dispersed extinction spectra. Finally, we probe the effects of asymmetry and gap size on the occurrence of these modes and demonstrate that the delocalized nature of the antibonding modes lead to longer-range coupling compared to the stronger bonding modes.
    Nano Letters 11/2014; · 12.94 Impact Factor
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    ABSTRACT: We perform a detailed density functional theory assessment of the factors which determine shear interactions between carbon nanotubes (CNTs) within bundles, and in related CNT and graphene structures including yarns, providing an explanation for the shear force measured in recent experiments.\cite{Fi2012} The potential energy barriers separating AB stacked structures are found to be irrelevant to the shear analysis for bundles and yarns due to turbostratic stacking, and as a result, the tube-tube shear strength for pristine CNTs is estimated to be $< 0.24$ MPa, i.e., extremely small. Instead it is pinning due to the presence of defects and functional groups at the tube ends that primarily cause resistance to shear when bundles are fractured in weak vacuum ($\sim 10^{-5}$ Torr). Such defects and groups are estimated to involve 0.55 eV interaction energies on average, which is much larger than single-atom vacancy defects (approximately 0.039 eV). Furthermore since graphitic materials are stiff, they have large coherence lengths, and this means that push-pull effects result in force cancellation for vacancy and other defects that are internal to the CNTs. Another important factor is the softness of cantilever structures relative to the stiff CNTs in the experiments, as this contributes to elastic instability transitions which account for significant dissipation during shear that has been observed. The application of these results to the mechanical behavior of yarns is discussed, providing general guidelines for the manufacture of strong yarns composed of CNTs.
    Nano Letters 10/2014; · 12.94 Impact Factor
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    ABSTRACT: A novel, apertureless, cantilever-free pen array can be used for dual scanning photochemical and molecular printing. Serial writing with light is enabled by combining self-focusing pyramidal pens with an opaque backing between pens. The elastomeric pens also afford force-tuned illumination and simultaneous delivery of materials and optical energy. These attributes make the technique a promising candidate for maskless high-resolution photopatterning and combinatorial chemistry.
    Small 10/2014; · 7.51 Impact Factor
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    ABSTRACT: When used in nucleic acid duplexes, locked nucleic acid (LNA) and 2'-O-methyl RNA residues enhance the duplex stabilities, and this makes it possible to create much better RNA aptamers to target specific molecules in cells. Thus, LNA and 2'-O-methyl RNA residues are finding increasingly widespread use in RNA-based therapeutics. Herein, we utilize molecular dynamics (MD) simulations and UV melting experiments to investigate the structural and thermodynamic properties of 13 nucleic acid duplexes, including full DNA, RNA, LNA, and 2'-O-methyl RNA duplexes as well as hybrid systems such as LNA:RNA, 2'-O-methyl RNA:RNA, LNA/2'-O-methyl RNA:RNA, and RNA/2'-O-methyl RNA:RNA duplexes. The MD simulations are based on a version of the Amber force field revised specifically for RNA and LNA residues. Our results indicate that LNA and 2'-O-methyl RNA residues have two different hybridization mechanisms when included in hybrid duplexes with RNA wherein the former underwinds while the latter overwinds the duplexes. These computational predictions are supported by X-ray structures of LNA and 2'-O-methyl RNA duplexes that were recently presented by different groups, and there is also good agreement with the measured thermal stabilities of the duplexes. We find out that the "underwinding" phenomenon seen in LNA and LNA:RNA hybrid duplexes happens due to expansion of the major groove widths (Mgw) of the duplexes that is associated with decrease in the slide and twist values in base-pair steps. In contrast, 2'-O-methyl RNA residues in RNA duplexes slightly overwind the duplexes while the backbone is forced to stay in C3'-endo. Moreover, base-pair stacking in the LNA and LNA:RNA hybrid systems is gradually reduced with the inclusion of LNA residues in the duplexes while no such effect is seen in the 2'-O-methyl RNA systems. Our results show how competition between base stacking and structural rigidity in these RNA hybrid systems influences structures and stabilities. Even though both LNA and 2'-O-methyl RNA residues have C3'-endo sugar puckering, structurally LNA residues have a frozen sugar backbone which provides entropic enhancement of stabilities while the 2'-O-methyl RNA residues are more flexible and maintain base stacking that is almost untouched compared to RNA. Thus, enhancement of the structural stabilities of RNA duplexes by 2'-O-methyl RNA modifications is smaller than for the corresponding LNA modifications. Indeed, our experimental measurements show that on average each 2'-O-methyl RNA and LNA substitution in a RNA duplex enhances duplex stability by 0.2 and 1.4 kcal/mol, respectively. Our computational binding free energy predictions are qualitatively in line with these results. The only exception is for the full 2'-O-methyl RNA duplex, which is overstabilized, implying that further force field revisions are needed. Collectively, the results presented in this paper explain the atomistic details of the structural and thermodynamic roles of LNA and 2'-O-methyl RNA residues in RNA hybrid duplexes, shedding light on the mechanism behind targeting endogenous micro RNA (miRNA) in order to regulate mRNA activity and inhibit gene expression in the cell.
    The Journal of Physical Chemistry B 09/2014; · 3.38 Impact Factor
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    ABSTRACT: We show analytically and with rigorous computa-tional electrodynamics how inhomogeneous surface plasmon polaritons (ISPPs) can be generated by refraction of ordinary SPPs at metal−metal interfaces. ISPPs, in contrast with SPPs, propagate and decay in different directions and can therefore exhibit significantly different intensity patterns. Our analytical arguments are based on a complex generalization of Snell's law to describe how SPPs moving on one metal surface are refracted at an interface with a second, different metal surface. The refracted waveform on the second metal is an ISPP. Under suitable circumstances the decay of an ISPP can be almost perpendicular to the propagation direction, leading to significant confinement. It is also found that ISPPs on the second metal can retain information about the SPPs on the first metal, a phenomenon that we term "dispersion imprinting". The complex Snell's law predictions are validated with 3-D finite-difference time-domain simulations, and possible means of experimentally observing ISPPs are suggested. The idea of ISPPs and how they result from refraction may expand the potential for designing the propagation and dispersion features of surface waves in general, including surface phonon polaritons, surface magnons, and guided waves in metamaterials. S urface plasmon polaritons (SPPs) are surface waves created by coupling light into charge-density oscillations at a metal−dielectric interface that allow optical energy and information to be strongly confined to a two-dimensional surface. 1−7 Systems that permit the excitation of SPPs can exhibit interesting and unexpected optical properties, including extraordinary optical transmission 8 and superlensing. 9−11 Such properties are relevant to a wide range of applications including imaging and sensing 12−16 and optoelectronics, 17−20 so controlling and manipulating SPP propagation is a major goal of nanophotonics research. 21,22
    ACS Photonics. 08/2014;
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    ABSTRACT: This paper presents evidence that strongly adhered carbonaceous surface impurities, intrinsic impurities that accompany multiwall carbon nanotubes (MWCNTs) synthesized by arc-discharge, are a component that cannot be ignored in experiments involving single nanotubes and their interfaces with a second surface. At the interface that forms between a carbon nanotube and a graphitic surface, these impurities can significantly alter the adhesion properties of the underlying nanotube and can cause over 30% scatter in computed interaction energies, similar in magnitude to the scatter reported in experimental measurements involving individual CNTs. Also presented is high-resolution TEM evidence that commonly used purification techniques that are effective at removing larger impurity particles from as-produced arc-discharge MWCNT samples do not remove these strongly adhered carbonaceous surface impurities.
    Carbon 07/2014; 80:1–11. · 6.16 Impact Factor
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    ABSTRACT: High-performance solution-processed organic semiconductors maintain macroscopic functionality even in the presence of microscopic disorder. Here we show that the functional robustness of certain organic materials arises from the ability of molecules to create connected mesoscopic electrical networks, even in the absence of periodic order. The hierarchical network structures of two families of important organic photovoltaic acceptors, functionalized fullerenes and perylene diimides, are analyzed using a newly developed graph methodology. The results establish a connection between network robustness and molecular topology, and also demonstrate that solubilizing moieties play a large role in disrupting the molecular networks responsible for charge transport. A clear link is established between the success of mono and bis functionalized fullerene acceptors in organic photovoltaics and their ability to construct mesoscopically connected electrical networks over length scales of 10 nm.
    Proceedings of the National Academy of Sciences 06/2014; · 9.81 Impact Factor
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    Michael B Ross, Martin G Blaber, George C Schatz
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    ABSTRACT: The a priori ability to design electromagnetic wave propagation is crucial for the development of novel metamaterials. Incorporating plasmonic building blocks is of particular interest due to their ability to confine visible light. Here we explore the use of anisotropy in nanoscale and mesoscale plasmonic array architectures to produce noble metal-based metamaterials with unusual optical properties. We find that the combination of nanoscale and mesoscale anisotropy leads to rich opportunities for metamaterials throughout the visible and near-infrared. The low volume fraction (<5%) plasmonic metamaterials explored herein exhibit birefringence, a skin depth approaching that of pure metals for selected wavelengths, and directionally confined waves similar to those found in optical fibres. These data provide design principles with which the electromagnetic behaviour of plasmonic metamaterials can be tailored using high aspect ratio nanostructures that are accessible via a variety of synthesis and assembly methods.
    Nature Communications 06/2014; 5:4090. · 10.74 Impact Factor
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    ABSTRACT: We present theory and experiments which describe charge transfer from the [Formula: see text] and a(1)Δg states of molecular oxygen and atomic and molecular cations. Included in this work are new experimental results for O2(a(1)Δg) and the cations O(+), CO(+), Ar(+), and [Formula: see text], and new theory based on complete active space self-consistent field method calculations and an extended Langevin model to calculate rate constants for ground and excited O2 reacting with the atomic ions Ar(+), Kr(+), Xe(+), Cl(+), and Br(+). The T-shaped orientation of the (X - O2)(+) potential surface is used for the calculations, including all the low lying states up to the second singlet state of the oxygen molecule [Formula: see text]. The calculated rate constants for both [Formula: see text] and O2(a(1)Δg) show consistent trends with the experimental results, with a significant dependence of rate constant on charge transfer exothermicity that does not depend strongly on the nature of the cation. The comparisons with theory show that partners with exothermicities of about 1 eV have stronger interactions with O2, leading to larger Langevin radii, and also that more of the electronic states are attractive rather than repulsive, leading to larger rate constants. Rate constants for charge transfer involving O2(a(1)Δg) are similar to those for [Formula: see text] for a given exothermicity ignoring the electronic excitation of the O2(a(1)Δg) state. This means (and the electronic structure calculations support) that the ground and excited states of O2 have about the same attractive interactions with ions.
    The Journal of chemical physics. 06/2014; 140(21):214307.
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    ABSTRACT: Herein we utilize on-wire lithography (OWL) to synthesize a composite plasmonic–semiconductor material composed of Au nanorod dimers embedded within anatase TiO2 sheets. We demonstrate that, despite the harsh conditions necessary to synthesize crystalline TiO2, the gapped nanostructures remain intact. Additionally, we show that the optical properties of these structures can be tailored via the geometric control afforded by the OWL process to produce structures with various gap sizes exhibiting different electric field intensities near the surface of the metal particles and that those fields penetrate into the semiconductor material. Finally, we show that this composite amplifies the electric field of incident light on it by a factor of 103, which is more that 750 times greater than the isotropic materials typically used for these systems.
    Chemistry of Materials 06/2014; 26(12):3818–3824. · 8.54 Impact Factor
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    ABSTRACT: By introducing steric constraints into molecular compounds, it is possible to achieve atypical coordination geometries for the elements. Herein, we demonstrate that a titanium-oxo cluster [{Ti4(4-O)(2-O)2}(OPri)6(fdc)2], which possesses a unique edge-sharing Ti4O17 octahedron tetramer core, is stabilized by the constraints produced by two orthogonal 1,1′-ferrocenedicarboxylato (fdc) ligands. As a result, a square-planar tetracoordinate oxygen (ptO) can be generated. The bonding pattern of this unusual anti-van’t Hoff / Le Bel oxygen, which has been probed by theoretical calculations, can be described by two horizontally -bonded 2px and 2py orbitals along with one perpendicular non-bonded 2pz orbital. While the two ferrocene units are separated spatially by the ptO with an Fe•••Fe separation of 10.4 Å, electronic communication between them still takes place as revealed by the cluster’s two distinct one-electron electrochemical oxidation processes.
    Angewandte Chemie International Edition 06/2014; 2014(35):9193–9197. · 11.34 Impact Factor
  • Michael B. Ross, George C. Schatz
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    ABSTRACT: We use classical electrodynamics calculations to investigate the versatility and capability of aluminum and indium dimers with small gaps as ultraviolet plasmonic nanoantennas, focusing on the particle size and wavelength range that gives optimum near-field enhancement. We find that Al and In are highly capable plasmonic materials in the ultraviolet, even with the incorporation of Al2O3 shells on the Al spheres; however, Al is strongly influenced by quadrupole modes while In is not. Al is the optimal material in the deep-UV, while In is ideal in the near-UV and near-visible spectral regions. Unlike Au and Ag, Al and In are most effective with the lowest refractive index background media possible, with vacuum being ideal. Ag outperforms both Al and In red of 320 nm, but optimal surface-enhanced Raman spectroscopy enhancement factors are still substantial for Al and In, with peak |E|4 values (for dimers in vacuum with a 1 nm gap) determined to be: Al, 2.0 × 109 (at 204 nm); In, 1.2 × 109 (at 359 nm); Al/Al2O3, 1.2 × 107 (at 218 nm). For comparison, the optimal |E|4 for Au dimers is 2.8 × 1011 (at 723 nm) and for Ag is 1.3 × 1012 (at 794 nm), with background indices of 1.50 and 2.25, respectively. These data suggest that the continued exploration of Al and In as plasmonic materials could provide powerful opportunities in ultraviolet spectroscopic enhancement, fluorescence quenching, and cellular imaging.
    The Journal of Physical Chemistry C 05/2014; 118(23):12506–12514. · 4.84 Impact Factor
  • Tao Yu, One-Sun Lee, George C Schatz
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    ABSTRACT: We have employed molecular dynamics simulations and quantum chemistry methods to study the structures and electronic absorption properties of a novel type of photonic nanowire gel constructed by the self-assembly of peptide amphiphiles (PAs) and the chromophore-(PPIX)Zn molecules. Using MD simulations, structures of the self-assembled fiber were determined with atomistic detail, including the distribution of chromophores along the nanofiber and the relative distances and orientations of pairs of chromophores. In addition, quantum chemistry calculations were used to determine the electronic structure and absorption properties of the chromophores in the fiber, so as to assess the capabilities of the nanofiber for photonics applications. The calculations show that the PA nanofiber provides an effective scaffold for the chromophores in which the chromophores form several clusters in which nearest neighbor chromophores are separated by less than 20 Å. The calculations also indicate that the chromophores can be in both the hydrophilic shell and hydrophobic core portions of the fiber. There are only small spectral shifts to the B-band of the porphyrins arising from the inhomogeneous micro-electronic environment provided by the fiber. However there are much stronger electronic interactions between nearby pairs of chromophores, leading to a more significant red-shift of the B-band that is similar to what is found in the experiments, and to significant excitonic coupling that is seen in circular dichroism spectra. This electronic interaction between chromophores associated with the PA nanofiber structure is crucial to future applications of these fibers for light-harvesting applications.
    The Journal of Physical Chemistry A 04/2014; · 2.77 Impact Factor
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    ABSTRACT: We report the large-area assembly of anisotropic gold nanoparticles into lithographically defined templates with control over their angular position using a capillary force-based approach. We elucidate the role of the geometry of the templates in the assembly of anisotropic nanoparticles consisting of different shapes and sizes. These insights allow us to design templates that immobilize individual triangular nanoprisms and concave nanocubes in a shape-selective manner and filter undesired impurity particles from a mixture of triangular prisms and other polyhedra. Furthermore, by studying the assembly of two particles in the same template, we elucidate the importance of interparticle forces in this method. These advances allow for the construction of face-to-face and edge-to-edge nanocube dimers as well as triangular nanoprism bowtie antennas. As an example of the fundamental studies enabled by this assembly method, we investigate the surface-enhanced Raman scattering (SERS) of face-to-face concave cube dimers both experimentally and computationally and reveal a strong polarization dependence of the local field enhancement.
    Nano Letters 03/2014; · 13.03 Impact Factor
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    ABSTRACT: Trinucleotide and tetranucleotide repeat disorders are genetic inheritable diseases caused by mutations in DNA where the repeats in certain genes exceed the normal size. Once the repeats are transcribed, mRNA folds into a hairpin with repeating CXG (X = C, A, G, U) or CCUG motifs, which either attract cytoplasmic multiprotein complexes or translate into toxic polyQ proteins and cause the disease. These mRNA repeats have 1×1 or 2×2 internal loops, which make them ideal targets for pharmacologic development. Yet, the dynamic nature of RNA loops presents a significant challenge to obtaining reasonable predictions for targeting RNA repeats with small molecules. Two important results from our recent studies provide a point of entry into this challenging problem. First, we found that 1×1 AA internal loops in RNA CAG repeat expansions are dynamic and can form multiple different stable conformations; and second, we found that targeting 1×1 UU and 2×2 CU/UC internal loops with a small molecules produced complex structural changes in the RNA loop conformations with lowest free energy structures corresponding to one of the local minimum states predicted for 1×1 AA internal loops. These results suggest that RNA internal loops have multiple different free energy minimum states, which could be dominated with small molecules upon binding.
    ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, Dallas, Texas; 03/2014
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    ABSTRACT: Many naturally occurring peptides containing cationic and hydrophobic domains have evolved to interact with mammalian cell membranes and have been incorporated into materials for non-viral gene delivery, cancer therapy or treatment of microbial infections. Their electrostatic attraction to the negatively charged cell surface and hydrophobic interactions with the membrane lipids enable intracellular delivery or cell lysis. Although the effects of hydrophobicity and cationic charge of soluble molecules on the cell membrane are well known, the interactions between materials with these molecular features and cells remain poorly understood. Here we report that varying the cohesive forces within nanofibres of supramolecular materials with nearly identical cationic and hydrophobic structure instruct cell death or cell survival. Weak intermolecular bonds promote cell death through disruption of lipid membranes, while materials reinforced by hydrogen bonds support cell viability. These findings provide new strategies to design biomaterials that interact with the cell membrane.
    Nature Communications 02/2014; 5:3321. · 10.74 Impact Factor

Publication Stats

24k Citations
3,603.11 Total Impact Points

Institutions

  • 1977–2014
    • Northwestern University
      • • Department of Chemistry
      • • Department of Mechanical Engineering
      Evanston, Illinois, United States
  • 2011–2013
    • University of Maryland, College Park
      • Department of Chemistry and Biochemistry
      College Park, MD, United States
    • Chapman University
      Orange, California, United States
    • University of North Texas
      Denton, Texas, United States
    • Heriot-Watt University
      • School of Engineering and Physical Sciences
      Edinburgh, Scotland, United Kingdom
  • 1975–2011
    • California Institute of Technology
      • • Division of Chemistry and Chemical Engineering
      • • Arthur Amos Noyes Laboratory of Chemical Physics
      Pasadena, CA, United States
  • 2005–2010
    • Montana State University
      • Department of Chemistry & Biochemistry
      Bozeman, MT, United States
    • Chalmers University of Technology
      • Department of Applied Physics
      Göteborg, Vaestra Goetaland, Sweden
    • Université de Technologie de Troyes
      • Laboratoire de Nanotechnologie et d’Instrumentation Optique (LNIO)
      Troyes, Champagne-Ardenne, France
    • National Academy of Sciences of Ukraine
      Kievo, Kyiv City, Ukraine
  • 2009
    • University of Namur
      Namen, Walloon Region, Belgium
    • Cornell University
      Ithaca, New York, United States
    • University of Victoria
      • Department of Chemistry
      Victoria, British Columbia, Canada
  • 2008–2009
    • Delft University Of Technology
      • Department of Chemical Engineering
      Delft, South Holland, Netherlands
    • University of Illinois, Urbana-Champaign
      • Department of Biochemistry
      Urbana, IL, United States
    • Pennsylvania State University
      • Department of Chemistry
      University Park, MD, United States
    • Tulane University
      • Department of Chemistry
      New Orleans, LA, United States
    • University of Colorado Colorado Springs
      Colorado Springs, Colorado, United States
  • 2007–2008
    • Kangwon National University
      • Department of Chemistry
      Syunsen, Gangwon, South Korea
    • Max Planck Institute for Biophysical Chemistry
      Göttingen, Lower Saxony, Germany
  • 1992–2008
    • Northwest University
      Evanston, Illinois, United States
  • 1982–2008
    • Argonne National Laboratory
      Lemont, Illinois, United States
  • 1977–2008
    • Massachusetts Institute of Technology
      • Department of Chemistry
      Cambridge, Massachusetts, United States
  • 2004–2007
    • Pusan National University
      • • Department of Nanomaterials Engineering
      • • Department of Food Science and Technology
      Pusan, Busan, South Korea
    • Spectral Sciences Incorporated
      Burlington, Massachusetts, United States
  • 2005–2006
    • University of South Carolina
      • Chemistry and Biochemistry
      Columbia, SC, United States
    • Stanford University
      • Department of Chemistry
      Stanford, CA, United States
  • 2003
    • Institute for Molecular Science
      Okazaki, Aichi, Japan
  • 1986–1995
    • The University of Manchester
      • School of Chemistry
      Manchester, ENG, United Kingdom