Stephen Hill

National High Magnetic Field Laboratory, Tallahassee, Florida, United States

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Publications (88)298.32 Total impact

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    ABSTRACT: We report a room temperature study on the electrical response of field-effect transistors (FETs) based on few-layered MoSe2, grown by a chemical vapor transport technique, mechanically exfoliated onto SiO2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current on to off ratios up to 106 for both hole and electron channels when applying a small excitation voltage. A rather small hysteresis is observed when sweeping the back-gate voltage between positive and negative values, indicating the near absence of charge “puddles”. For both channels the Hall-effect indicates Hall mobilities μ_H ≈ 250 cm^2/Vs which are comparable to the corresponding field-effect mobilities, i.e. μ_FE 150 to 200 cm2/Vs evaluated through the conventional two-terminal field-effect configuration. Therefore, our results suggest that MoSe2 could be a good candidate for p-n junctions composed of a single atomic layer and for low-power, complimentary logic applications.
    ACS Nano 07/2014; · 12.03 Impact Factor
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    ABSTRACT: The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below TN=13 K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.
    Physical Review B 06/2014; 89:214403. · 3.66 Impact Factor
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    ABSTRACT: Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic coordination complexes. These complexes, [M(C2O4)3](3-) (M = Ru, Cr, Fe) and [M(CN)6](3-) (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (S = (1)/2, (3)/2, (5)/2) and spin-orbit coupling (ζ = 464, 880, 3100 cm(-1)) on quantum decoherence. Coherence times (T2) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin-orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru(C2O4)3](3-), was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, T2, and the presence of Rabi oscillations. The complex [Ru(C2O4)3](3-) exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru(C2O4)3](3-) as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence.
    Journal of the American Chemical Society 05/2014; · 10.68 Impact Factor
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    ABSTRACT: Low-energy excitations in the rare earth kagome spin system Pr3Ga5SiO14 (PGS) have been investigated using high-field electron spin resonance (ESR) techniques. Previous work has shown that PGS does not exhibit long-range magnetic order at temperatures down to 30 mK. The present low-temperature (1.3–20 K) field-scan measurements, made on a single crystal sample in applied fields up to 30 T and for several microwave frequencies, give a series of temperature-dependent absorption peaks that are very different to conventional ESR spectra. The observed response is interpreted in terms of discrete spin-wave excitations in antiferromagnetically correlated spin clusters. We present a theoretical model which, in the limit of small magnetic frustration effects, qualitatively describes the microwave-induced transitions.
    Physical Review B 09/2013; 88(9). · 3.66 Impact Factor
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    ABSTRACT: The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.
    Angewandte Chemie International Edition 09/2013; · 11.34 Impact Factor
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    ABSTRACT: Microwave-assisted synthesis has been used to obtain the family of dodecanuclear Ni(II) complexes [Ni12 (NO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ][ClO4 ]2 (1), [Ni12 (NO3 )(MeO)12 (BrC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ][ClO4 ]2 (2), [Ni12 (CO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)10 (H2 O)2 ]2 [SO4 ] (3) and [Ni12 (NO3 )(MeO)12 (MeC6 H4 CO2 )9 (MeOH)8 (H2 O)7 ][NO3 ]2 (4). They contain three {Ni4 O4 } cubane units which template around a central μ6 anion, either NO3 (-) or CO3 (2-) . Their magnetic properties have been studied by superconducting quantum interference device (SQUID) magnetometry and high-field EPR measurements. The nanostructuration of the Ni12 species on mica surfaces is studied by AFM and grazing-incidence X-ray diffraction, which reveal the formation of polycrystalline thin layers.
    Chemistry 05/2013; · 5.93 Impact Factor
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    ABSTRACT: We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T1e(-1) of other free radicals used in DNP at the same concentration, trityl OX063 has slower T1e(-1) than BDPA and 4-oxo-TEMPO. At T > 20 K, the T1e(-1)vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd(3+) doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T1e(-1) especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: gx = gy = 2.00319(3) and gz = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T1e(-1) is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.
    Physical Chemistry Chemical Physics 05/2013; · 3.83 Impact Factor
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    ABSTRACT: In this work we investigated the magnetic anisotropy of a Mn (IV) monomer via axial and rhombic zero field splitting terms D, E. The d^3 ion sits in an octahedral environment in a P 21/c space group. The complex is studied via single crystal and powder HFEPR over a wide range of frequencies 49GHz to 416GHz and temperatures 2 to 60K. The angle dependence at low temperature and frequency (˜88GHz) reveals a minimum of the resonance field, when the long axis of the crystal is along the magnetic field. The same behavior is observed at higher frequency (˜240GHz). Furthermore, pulse EPR experiments in high frequency quasi-optical spectrometer at low temperature (˜1.487K) a spin echo could be observed and we were able to observe the variation of the T2 times as a function of the magnetic field orientation, and as a function of the temperature.
    03/2013;
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    ABSTRACT: The Cavity Perturbation Technique (CPT) is a contact-free technique that measures the change of the characteristics of a cavity resonator upon the introduction of the sample. In this experiment, we study the effect of crystal size with regards to the CPT transmission spectra for a single crystal of the Fe8 single-molecule magnets. It is interesting to study the interaction between these two resonance systems, i. e. a cavity and a crystal of Fe8. We want to know whether it is a quantum mechanical or a classical interaction. The frequency shift and suppression of the cavity Q value increase linearly with increasing sample size. These observations are in agreement with the theoretical expectation for a classical coupling between the Fe8 crystal and the cavity. From cavity perturbation theory, these phenomena may be explained by the following classical formula: δφ/φ =-βχ, where φ is the complex frequency, β is the filling factor that depends on the sample volume and the resonant mode of the cavity, and χ is the complex susceptibility.
    03/2013;
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    ABSTRACT: Collective spin excitation spectra in frustrated antiferromagnets have been detected using high field electron magnetic resonance (EMR). At low temperatures the langasite kagomé systems R3Ga5SiO14(R = Pr and Nd) exhibit short range spin correlation effects. Neutron scattering has shown that these systems do not exhibit long-range magnetic order at temperatures down to 30 mK. Field-sweep EMR measurements made on single crystals of Pr3Ga5SiO14 and Nd3Ga5SiO14 in the temperature range 1.3 - 20 K, and in fields up to 22 T, give a series of absorption peaks which are quite different to conventional EMR spectra. The resonances are interpreted using a model which involves spin-wave excitations in short range antiferromagnetically correlated spin loops or clusters.
    03/2013;
  • Junjie Liu, Enrique del Barco, Stephen Hill
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    ABSTRACT: This chapter takes a microscopic view of quantum tunneling of magnetization (QTM) in single-molecule magnets (SMMs), focusing on the interplay between exchange and anisotropy. Careful consideration is given to the relationship between molecular symmetry and the symmetry of the spin Hamiltonian that dictates QTM selection rules. Higher order interactions that can modify the usual selection rules are shown to be very sensitive to the exchange strength. In the strong coupling limit, the spin Hamiltonian possess rigorous $D_{2h}$ symmetry (or $C_{\infty}$ in high-symmetry cases). In the case of weaker exchange, additional symmetries may emerge through mixing of excited spin states into the ground state. Group theoretic arguments are introduced to support these ideas, as are extensive results of magnetization hysteresis and electron paramagnetic resonance measurements.
    02/2013;
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    ABSTRACT: This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, the [Ni(Me6tren)Cl](ClO4) 1 and [Ni(Me6tren)Br](Br) 2. High Field, High frequency Electron Paramagnetic Resonance spectroscopy performed on a single crystal of the [Ni(Me6tren)Cl](ClO4) 1 shows a giant uniaxial magnetic anisotropy with an experimental Dexp value (energy difference between the Ms = ±1 and Ms = 0 components of the ground spin state S = 1) estimated to be between -120 cm-1 and -180 cm-1. The theoretical study shows that for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first order spin-orbit coupling that results in a splitting of the Ms = ±1 and Ms = 0 components of ca. -600 cm-1. Despite, the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of first order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical Dth between -100 cm-1 and -200 cm-1) is obtained.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: Dedicated to Professor George Christou on the occasion of his 60th birthday. One hundred years ago Alfred Werner won the Nobel Prize in chemistry for his pioneering work with inorganic coordination compounds, which was mainly attributed to his work on mononuclear cobalt complexes. [1] Although this chemistry has been well-developed, in recent years it continues to reveal new and interesting magnetic properties derived from the molecular geometry of these complexes. [2] Indeed since the discovery of magnet-like behavior in a mononuclear transi-tion-metal complex, [3] a sudden re-emergence of interest occurred in 3d molecules acting as molecular magnets.This is mainly due to the fact that these model mononuclear complexes can unravel the origin of magnetic anisotropy due to their unquenched first-order orbital angular momen-tum. [4, 5] When large uniaxial anisotropy (D) is coupled with the intrinsic spin (S) of a molecule, an anisotropic barrier (U = S 2 j D j) for the reversal of the magnetization can be seen. [6] Such molecules are termed single-molecule magnets (SMMs) or, for mononuclear complexes, single-ion magnets (SIMs). [4, 7, 8] To compensate for low-spin values in 3d ions, highly anisotropic metal ions, such as Fe II or Co II , are used. [4, 6c] Moreover, 3d complexes can exhibit spin crossover (SCO) behavior. [9] For 3d 4 –3d 7 metal ions, high-spin (HS)–low-spin (LS) crossover can occur if the ligand field is tuned such that a balance between strong and weak field ligands is ach-ieved. [10] Strong-field terpyridine (terpy) ligands can be ideal for isolating such systems. With this in mind we have carefully studied three related compounds based on the Co II -terpy system where we fine-tune the ligand field by controlling the number of coordinated terpy ligands as well as the remaining terminal ligands, leading to unique magnetic properties of SIM and SCO behavior. Herein we unravel the inherent physical properties of [Co(terpy)Cl 2 ] (1), [Co(terpy)(NCS) 2 ] (2), and [Co(terpy) 2 ](NCS) 2 ·1.5 H 2 O (3) through structural, spectroscopic, computational, and magnetic studies. The synthetic procedure and crystallographic data for the complexes are described in the Supporting Information (Tables S1–S3). In 1, the Co II ion adopts C s symmetry and is elevated in respect to the plane formed by three N atoms of the terpy ligand (N1–N3), yielding a distorted square-based pyramid (Figure 1). [11] The packing diagrams along the a-, b-and c-axes show antiparallel packing of Co II units (Supporting Information, Figures S1–S3) with the closest intermolecular distance between terpy centroids of 3.79 Š, leading to p–p stacking. In complex 2, [12] the Co II ion shifts towards trigonal bipyramidal and coordinates within the plane of the terpy ligand. The packing diagrams (Supporting Information, Figures S4–S6) indicate parallel alignment of the Co II units with a distance of 3.66 Š between the centroids of the terpy ligands. These solid-state interactions are strong enough to induce a change in the Co II geometry, which adopts C 2v as opposed to C s symmetry. In 3, the six-coordinate bis(terpy) complex adopts a distorted octahedral geometry (Figure 1 c) and packs in a similar fashion to 1 (Supporting Information, Figures S7, S8), with an intermolecular terpy centroids distance of 3.70 Š. Unlike the weak-field chloride ligands in complex 1, nitrogen-containing ligands of 2 and 3 are known to have stronger fields, which will have significant implica-tions on the electronic distribution and in turn the magnetic properties of these complexes. To probe further the electronic properties of the com-plexes, density functional theory (DFT) calculations were performed (see the Supporting Information). Geometry optimization of 1 resulted in a structure with C s symmetry and a HS state of S = 3/2, in close agreement with the X-ray data (Figure 1), while for 2, a C s symmetric structure (Supporting Information, Scheme S1) was obtained. There-Dateiname: Z303005E Pagina:
    Angewandte Chemie International Edition in English 01/2013; · 13.45 Impact Factor
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    ABSTRACT: The heterobimetallic complex [Cu(II)Mn(III)(L) 2 (py) 4 ](ClO 4)·EtOH (1) built using the pro-ligand 2,2'-biphe-nol (LH 2), contains a rare example of a Jahn–Teller compressed Mn(III) centre. Dc magnetic susceptibility measurements on 1 reveal a strong antiferromagnetic exchange between the Cu(II) and Mn(III) ions mediated through the phenolate O-atoms (J = −33.4 cm −1), with magnetisation measurements at low temperatures and high fields suggesting significant anisotropy. Simulations of high-field and high fre-quency powder EPR data suggest a single-ion anisotropy D Mn(III) = +4.45 cm −1 . DFT calculations also yield an antiferromagnetic exchange for 1, though the magnitude is overestimated (J DFT = −71 cm −1). Calcu-lations reveal that the antiferromagnetic interaction essentially stems from the Mn(d x 2 −y 2)–Cu(d x 2 −y 2) interaction. The computed single-ion anisotropy and cluster anisotropy also correlates well with exper-iment. A larger cluster anisotropy for the S = 3/2 state compared to the single-ion anisotropy of Mn(III) is rationalised on the basis of orbital mixing and various contributions that arise due to the spin–orbit interaction.
    Dalton Transactions 01/2013; 42:207. · 3.81 Impact Factor
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    Angewandte Chemie International Edition 01/2013; · 11.34 Impact Factor
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    ABSTRACT: High-frequency Electron Paramagnetic Resonance (HFEPR) measurements have been performed on both a single-crystal and powder samples of a weakly coupled antiferromagnetic dinuclear [Mn(III)](2) molecular magnet, [Mn(III)(2)L(2)(py)(4)], where L is the trianion of 3-(3-oxo-3-phenylpropionyl)-5-methylsalicylic acid, and py is pyridine. The experimental results were analyzed on the basis of a multispin Hamiltonian using both a perturbative approach and numerical simulations. It is found that the single-crystal HFEPR results provide a direct and simple means of determining both the axial anisotropy of the individual Mn(III) ions and the isotropic exchange coupling between them. Previously unpublished low-temperature magnetization data are then simulated using the same model Hamiltonian, yielding excellent agreement. This work highlights the limitations of widely used protocols for analyzing magnetic and powder EPR data obtained for multinuclear molecular magnets in which the exchange and single-ion anisotropies are comparable, thus emphasizing the value of single-crystal, multifrequency EPR measurements.
    Inorganic Chemistry 12/2012; · 4.59 Impact Factor
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    ABSTRACT: The synthesis and properties are reported of a rare example of a Mn(12) single-molecule magnet (SMM) in truly axial symmetry (tetragonal, I4̅). [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(MeOH)(4)]·MeOH (3·MeOH) was synthesized by carboxylate substitution on [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)]·2MeCO(2)H·4H(2)O (1). The complex was found to possess an S = 10 ground state, as is typical for the Mn(12) family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on 3·MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for 1. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for 1, suggesting that the crystals of 3 are homogeneous and do not contain multiple Mn(12) environments. In the single-crystal (55)Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn(4+) peak was detectable, suggesting a possible decrease in the local symmetry of the Mn(4+) site. Spin-lattice (T(1)) relaxation studies were performed on single crystals of 3·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn(12) complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as 1.
    Inorganic Chemistry 12/2012; · 4.59 Impact Factor
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    ABSTRACT: Continuous-wave, multi-frequency electron paramagnetic resonance (EPR) studies are reported for a series of single-crystal and powder samples containing different dilutions of a recently discovered mononuclear Ho(III) (4f(10)) single-molecule magnet (SMM) encapsulated in a highly symmetric polyoxometalate (POM) cage. The encapsulation offers the potential for applications in molecular spintronics devices, as it preserves the intrinsic properties of the nanomagnet outside of the crystal. A significant magnetic anisotropy arises due to a splitting of the Hund's coupled total angular momentum (J = L + S = 8) ground state in the POM ligand field. Thus, high-frequency (50.4 GHz) EPR studies reveal a highly anisotropic eight line spectrum corresponding to transitions within the lowest m(J) = ±4 doublet, split by a strong hyperfine interaction with the I = 7/2 Ho nucleus (100% natural abundance). X-band EPR studies reveal the presence of an appreciable tunneling gap between the m(J) = ±4 doublet states having the same nuclear spin projection, leading to a highly non-linear field-dependence of the spectrum at low-frequencies.
    Dalton Transactions 09/2012; 41(44):13697-704. · 3.81 Impact Factor
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    ABSTRACT: The dimeric complex [Mn(III)(2)(Naphth-sao)(2)(Naphth-saoH)(2)(MeOH)(2)]·4MeOH (1·4MeOH), acts as a simple model complex with which to examine the magneto-structural relationship in polymetallic, oxime-bridged Mn(III) complexes. Dc magnetic susceptibility studies reveal that ferromagnetic exchange is mediated through the heavily twisted Mn-O-N-Mn moiety (J = +1.24 cm(-1)) with magnetisation measurements at low temperatures and high fields suggesting significant anisotropy. Simulations of high field, high frequency EPR data reveal a single ion anisotropy, D((Mn(III))) = -3.94 cm(-1). Theoretical studies on simplified model complexes of 1 reveal that calculated values of the exchange coupling and the anisotropy are in excellent agreement with experiment, with the weak ferromagnetism resulting from an accidental orthogonality between the Mn-N-O plane of the first Mn(III) ion and the Jahn-Teller axis of the second Mn(III) ion.
    Dalton Transactions 05/2012; 41(27):8340-7. · 3.81 Impact Factor

Publication Stats

324 Citations
298.32 Total Impact Points

Institutions

  • 2010–2014
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2013
    • University of Ottawa
      • Department of Chemistry
      Ottawa, Ontario, Canada
  • 2009–2013
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 2009–2012
    • The University of Edinburgh
      • School of Chemistry
      Edinburgh, SCT, United Kingdom
  • 2003–2012
    • University of Florida
      • • Department of Chemistry
      • • Department of Physics
      Gainesville, FL, United States
  • 2005–2011
    • University of California, San Diego
      • Department of Chemistry and Biochemistry
      San Diego, CA, United States