Exciton–Exciton Annihilation in Copper-phthalocyanine Single-Crystal

Article pubs.acs.org/JPCC

Exciton−Exciton Annihilation in Copper-phthalocyanine SingleCrystal Nanowires Ying-Zhong Ma,*,† Kai Xiao,‡ and Robert W. Shaw† †

Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6142, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6030, United States

‡

ABSTRACT: Femtosecond one-color pump−probe spectroscopy was applied to study exciton dynamics in single-crystal α-phase copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a biexponential decay function with lifetimes of 22 and 204 ps, but the corresponding relative amplitudes depend on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton−exciton annihilation process. Detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, the exciton−exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusionlimited, which possibly involves intrachain exciton diffusion along 1D molecular stacks.

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INTRODUCTION The remarkable chemical and thermal stability, tunable electronic properties through variation of the central metal atom, and the propensity for self-assembly into stacks through a π−π supramolecular interaction create a wide variety of novel metallophthalocyanine (MPc) applications. Several demonstrations have been reported for their use in organic field effect transistors,1 sensors,2 light-emitting devices,3 photovoltaic cells,4 and nonlinear optical devices.5,6 These unique properties have also stimulated extensive research toward a fundamental understanding of their linear and nonlinear optical properties, electronic structures, and excited-state dynamics. Ultrafast optical spectroscopy has been applied to study exciton relaxation processes in colloidal particles and thin films of various MPc materials. It has been shown that the exciton lifetimes depend critically on the central metal atoms7−9 and the molecular arrangements corresponding to different morphologies (amorphous versus α-, β-, Y-, and M- crystalline forms).10−12 A strong dependence of the exciton relaxation dynamics on the excitation intensity has been commonly observed and attributed to nonlinear exciton−exciton annihilation.7,13,14 On the basis of the need for a time-dependent annihilation rate to satisfactorily describe the transient absorption decay profiles measured under high excitation intensity, it has been concluded that the annihilation is a 1D diffusion-limited process.7,13,14 Alternatively, exciton−exciton annihilation involving static excitons was suggested by Kobayashi and co-workers in interpreting their transient absorption data collected from various vanadyl phthalocyanines (VOPcs) that exhibit distinct molecular arrangements.12 © 2012 American Chemical Society

The expectation that well-defined and ordered crystals of organic semiconductors should provide significantly enhanced properties such as high carrier mobilities has motivated efforts to synthesize 1D organic nanostructures. Recently, synthesis of crystalline CuPc nanowires with either the metastable α-phase or the stable β-phase was reported.15 The molecular arrangement in these two phases differs only in the tilt angle of the stacked molecules relative to the b-axis of the crystal, and the αphase can be converted to the β-phase by simply annealing at 250 °C. The diameter of these nanowires ranges from ∼50 to ∼800 nm, and the corresponding length varies from ∼15 to ∼24 μm, which leads to an aspect ratio as large as 300. Even for the narrowest wires the diameter is presumably still an order of magnitude greater than the size of the exciton, i.e., the exciton Bohr radius. Given the similarity in the exciton binding energies and dielectric constants between CuPc materials16−18 and semiconducting single-walled carbon nanotubes (SWNTs),19−21 it is reasonable to assume that the Q-band excitons of CuPc materials should have similar Bohr radii, i.e., on the order of a few nanometers.22 Consequently, the excitons created in these 1D CuPc nanowires can, in principle, move not only along the longitudinal direction of the wires but in all possible directions. However, ab initio density functional calculations of α- and β-phase CuPc crystals by Lozzi et al.23 and Giovannetti et al.17 show that the interchain interactions between the molecules in different stacks are negligible, Received: June 12, 2012 Revised: August 28, 2012 Published: September 14, 2012 21588

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whereas the intrachain interactions between the molecules within a given stack are somewhat stronger. These theoretical results suggest that exciton motion along the stacking direction is more favorable than the motion from one stack to another. The question we would like to address experimentally is how the excitons move in these structurally quasi-1D single-crystal nanowires: do they retain 1D diffusion along the stacking direction as found in other disordered crystalline polymorphs such as short-length-scale colloidal particles and thin films, or do they move not only along the stacking direction (intrachain) but also between different stacks (interchains)? A more general question is whether the exciton dynamics in these well-defined and ordered single-crystal nanowires differs from that found previously in other crystalline polymorphs. Here, we report our results obtained through one-color femtosecond pump−probe measurements on selected single-crystal α-CuPc nanowires at different pump intensities.

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MATERIALS AND METHODS

Single-crystal α-CuPc nanowires were synthesized on a 14 × 75 mm2 rectangular Si wafer substrate by a physical vapor transport method. Control of the nanowire diameter ranging from ∼50 to ∼800 nm was achieved by creating an approximately linear temperature variation along the long edge of the substrate in the furnace. The corresponding wire length varied from ∼15 to ∼24 μm. Details of the synthesis and size control can be found in ref 15. We studied the exciton dynamics in α-CuPc nanowires using one-color femtosecond pump−probe spectroscopy. The light source was a commercially available regenerative Ti:sapphire amplifier with a repetition rate of 250 kHz, which generates 150 fs (full width at half-maximum, fwhm) pulses centered at 800 nm. The output was used to pump an optical parametric amplifier (OPA) to produce pulses centered at 594 nm to resonantly excite the Q-band of the spatially selected nanowires with a ∼83 nm diameter and ∼18 μm length (see Figure 1 for a SEM image and the linear absorption spectrum of these αCuPc nanowires, as well as the laser pulse spectrum). A dual prism compressor consisting of two SF10 prisms was employed to compensate for group velocity dispersion, producing nearly transform-limited 120 fs (fwhm) pulses. After compression, the output beam was split into a strong pump and a weak probe (∼20:1 intensity ratio), and their relative delay was controlled using a DC-motor driven optical delay stage (UTS150CC, Newport) with 0.3 fs resolution. The pump and probe beams were focused to spots with a ∼75 μm diameter using a 12 cm focal length lens. Because the Si substrate is opaque at the chosen wavelength, the pump−probe measurements were performed in a reflection configuration. The reflected probe beam was directed to a Si photodiode after passing through a pair of irises to minimize scattered pump light and then detected using a lock-in amplifier (SR 850, Stanford Research Systems). The polarization of the pump beam was set to the magic angle (54.7°) with respect to the probe beam. A combination of a polarizer and an air-spaced achromatic halfwaveplate (CVI, ACWP-400-700-06-2) was used to control the pump intensity. All experiments were performed at room temperature (∼24 °C).

Figure 1. SEM image and linear absorption spectrum of CuPc nanowires grown at 186 °C on a Si and a quartz substrate, respectively. The diameter and length of these wires are very similar to those employed for the pump−probe experiments described in this paper. The dashed line shows the spectrum of the 120 fs (fwhm) laser pulse.

different pump intensities are shown in Figure 2a. For an increase of pump intensity from 42 to 190 μJ/cm 2 , corresponding to a change of peak power from 0.33 to 1.49 GW/cm2, the observed decay of the transient reflectance signal accelerated significantly. The accelerated decays can be seen more clearly in Figure 2b, where the two data sets measured at the highest and lowest pump intensities have been scaled to an equal amplitude at 100 ps delay time. Further, the corresponding initial peak amplitudes of the measured transient reflectance signals are found to scale linearly with pump intensity (see Figure 3). Note that the fitted line does not go through the origin even after the reflected portion of the pump power (measured using a laser power meter) is subtracted (open squares in Figure 3). This is most probably caused by overestimation of the pump intensity, as the scattered pump light cannot be measured and subtracted from the pump intensity data used in Figure 3. The results shown in Figure 2a and 2b are clearly distinct from those reported previously for some MPc materials, including ClAlPc colloidal particles13 and a thin film of MgPc,14 in terms of the intensity dependence of the initial peak amplitudes of the transient absorption signals. The nonlinear dependence found in these earlier studies is clearly in contrast to the linear dependence observed in this work. We believe that differences in the laser pulse lengths and pump intensities used for the previous and current experiments as well as the time scales associated with nonlinear exciton−exciton annihilation processes in the corresponding systems are responsible for the distinct differences in the observed intensity dependence. While the laser pulses employed by Mi et al. in their measurements on the MgPc film are very similar to ours, the highest pump intensity is more than an order of magnitude greater than what we used in our experiment. Under such a high pump intensity,

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RESULTS AND DISCUSSION Representative pump−probe data collected for the α-CuPc nanowires with ∼83 nm diameter and ∼18 μm length at 21589

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impulsive annihilation process. These authors employed 30 and 200 ps laser pulses to measure the amplitudes of an induced absorption signal at 500 nm under different excitation intensities. As in the same study an intensity-dependent, rapid initial relaxation with a typical ∼1 ps time scale was found through a separate experiment with much shorter ∼200 fs laser pulses, nonlinear exciton−exciton annihilation must occur during the temporal overlap of the pump and probe pulses in the experiments employing 30 ps laser pulses. Consequently, a nonlinear dependence of the peak amplitudes on pump intensity is expected.24 In contrast, the low pump intensities employed in our work in combination with the much slower exciton relaxation relative to our pulse duration eliminate the possibility of either absorption saturation or impulsive annihilation even at the highest pump intensity used and therefore gave rise to the linear dependence of the initial peak amplitudes on pump intensity (Figure 3). Furthermore, our results are also distinct from the more recent study of Gadalla et al. on CoPc films.25 Although a similar linear dependence of peak transient absorption signal on pump intensity was also observed, no intensity dependence could be identified for the transient absorption decays obtained at different pump intensities. To gain a quantitative measure of the exciton relaxation time scales in the CuPc nanowires, we employed a global lifetime analysis algorithm to analyze the transient reflectance kinetics. In this analysis, the data collected at different pump pulse intensities were fitted to a chosen model function simultaneously. The lifetimes were treated as global parameters. A single set of their values that could best describe all the data was sought during the fitting, whereas all the amplitudes were taken as independent, local variables. We found that all the data can be satisfactorily fitted to a biexponential model function with lifetimes of 22 and 204 ps. While the slower decay component dominates all the measured kinetics, the relative amplitude associated with the faster decay component changes from 5% to 26% with increasing pump pulse intensity. This increasing contribution of the faster decay component consequently leads to an accelerated decay behavior at high pump intensity as shown in Figure 2. While we have assigned the observed transient reflectance signals to the CuPc nanowires in the foregoing discussion, unambiguous confirmation of this assignment is necessary as the Si substrate absorbs at the wavelength of our pump−probe measurements. To this end, we further performed successive measurements on both the CuPc nanowires and the same Si substrate used to grow these nanowires under otherwise identical experimental conditions. We chose a very edge portion of the substrate where no nanowires have been grown for the Si measurements. As shown in Figure 4, the transient reflectance signal detected from the Si substrate is more than 100 times smaller than that observed from the nanowires. Also, its negative-signed signal, which indicates a dominant contribution from induced absorption, is in contrast to the positive, increased transient reflectance signal seen from the nanowires. Because of this very weak Si signal, over 40 test measurements were carried out through incremental translation of the sample along the propagation direction of the pump and probe beams. This allowed us to find the optimal sample position with the largest pump−probe signal. In view of this very weak signal from the Si substrate, which should be even smaller in the presence of the nanowires owing to the strong attenuation of both pump and probe pulses, we can safely

Figure 2. (A) One-color pump−probe data collected at 594 nm at different pump intensities (shown in μJ/cm2). (B) Comparison of the pump−probe data measured at the highest and lowest pump intensities after normalizing the signal amplitudes at 100 ps.

Figure 3. Plot of the peak amplitude of the pump−probe signals as a function of the pump fluence. The filled circles and open squares show the same peak amplitude data but with a different abscissa, corresponding to the pump fluence before and after subtracting the reflected portion of the pump power, respectively. The solid line depicts a linear fit to the corrected data.

it is generally not surprising to observe a nonlinear intensity dependence of the peak amplitude owing to saturation of the related electronic transition. When the time scale associated with exciton−exciton annihilation is shorter than or comparable to the laser pulse duration, an impulsive annihilation occurring during the temporal overlap of the pump and probe pulses will also contribute to the observed nonlinearity. In comparison, the pump intensities employed by Gulbinas et al. in their study on the ClAlPc colloidal particles are similar to ours. The observed square-root dependence must be instead a result of the 21590

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amplitude but would not alter this proportionality. This relation also holds during an exciton−exciton annihilation process. When two excitons approach each other to a distance smaller than the so-called reaction radius of the annihilation process, they will annihilate immediately.30 On the basis of the experimental verification for several molecular organic crystals (see ref 31 as an example), this nonlinear annihilation process is commonly considered to induce rapid relaxation of one exciton by releasing its energy to the second exciton and consequently promoting the latter to a higher-energy excited state, which will subsequently relax back to the S1 state.26,32 It should be emphasized that the annihilation process described by eq 1 or its solutions, eqs 2 and 3, is diffusion-limited. Previously, both eqs 2 and 3 as well as their variants have been successfully used to analyze ultrafast exciton relaxation dynamics obtained for various systems, including J-aggregates,33 photosynthetic lightharvesting complexes,30,34 semiconducting single-walled carbon nanotubes,35 and MPc materials.7,12 Detailed analyses of the transient reflectance kinetics measured over the full delay scan range (∼150 ps) were performed separately using eqs 2 and 3. In either case, the data collected at different pump intensities were analyzed globally, where k and γ0 were taken as global fitting parameters, and a single set of their values that could best describe all kinetics were sought during the fitting. Furthermore, n(0) was treated as a local parameter, and a separate value giving the best fit to each decay was determined. Because the transient reflectance kinetics obtained at all pump intensities exhibit much slower decays than the pulse duration, no deconvolution procedure was applied in our global analyses. Representative results of our global data fitting based on eq 3 are shown in Figure 5a. While

Figure 4. Comparison of the pump−probe data collected from the CuPc nanowires (gray line) and the Si substrate (black line) used to grow these nanowires. The inset shows the same Si substrate data but magnified for clarity. Note that all the data in this figure were measured using 65 fs laser pulses and a parallel polarization for the pump and probe beams.

ignore any contribution from the Si substrate to the measured transient reflectance signals from the CuPc nanowires. The dependence of the relaxation dynamics on pump intensity shown in Figure 2a indicates the occurrence of exciton−exciton annihilation. Such a nonlinear dynamical process has been observed in several MPc materials previously.7,12−14 In the case of our one-color pump−probe experiment with resonant Q-band excitation (S0 → S 1 transition), the exciton relaxation can be described by the following rate equation26 dn(t ) 1 = −kn(t ) − γ(t )n2(t ) dt 2

(1)

where n(t) is the population of the excitons; k is the rate of linear relaxation to the ground state; and γ(t) is the exciton− exciton annihilation rate. For diffusion-limited annihilation in an extended system whose size is comparable to or larger than the exciton diffusion radius, the annihilation rate becomes timedependent for systems with dimensionality (d) less than two. This rate can be expressed by γ(t) = γ0/tα, where γ0 is the timeindependent annihilation rate and α = 1 − d/2 (d < 2).27 However, the annihilation rate becomes time-independent for systems with d ≥ 2 because α should be equal to zero in this case.27,28 Analytical solutions of eq 1 for both d = 1 and d ≥ 2 cases can be readily derived n(0)e−kt

n(t ) = 1+ n(t ) =

γ0n(0) πk

(d = 1)

erf( kt )

n(0)e−kt 1 + n(0)γ0k−1(1 − e−kt )

Figure 5. Results of global analysis for the pump−probe data collected at 89.5, 125.0, and 138.2 μJ/cm2, respectively. The gray lines in (A) and (B) are the fits obtained using eqs 3 and 2, respectively; the top panels show the residuals of the corresponding fits to the data measured at 138.2 μJ/cm2.

(2)

(d ≥ 2) (3)

where n(0) is the initial population of the excitons, and erf() is the error function. These formulas can be directly applied to analyze pump−probe kinetics probed at high pump intensities because the signal amplitude at a given delay time t is proportional to [σESA(λ) − σSE(λ) − σ0(λ)]n(t), where σESA(λ) and σSE(λ) are the cross sections of excited state absorption and stimulated emission of the S1 state at probe wavelength λ and σ0(λ) denotes the corresponding cross-section of ground-state absorption.29 It is clear that the presence of excited-state absorption at the probe wavelength will change the signal

the transient reflectance kinetics can be reasonably well described by eq 3, some deviation is clearly noticeable, especially in the initial portion of the delay time, which can be even more clearly seen from the residuals shown in the top panels of Figure 5. The parameters determined from the analysis are k = 0.0043 ps−1 and γ0n(0) = 0.016, 0.014, and 0.010 ps−1, respectively. Note that only the γ0n(0) values are given here because both γ0 and n(0) obtained from this analysis are relative numbers. To obtain absolute values for γ0 and n(0), one needs to independently determine the mean number of the 21591

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(5) Tajalli, H.; Jiang, J. P.; Murray, J. T.; Armstrong, N. R.; Schmidt, A.; Chandross, M.; Mazumdar, S.; Peyghambarian, N. App. Phys. Lett. 1995, 67, 1639−1641. (6) Casstevens, M.; Samoc, M.; Pfleger, J.; Prasad, P. N. J. Chem. Phys. 1990, 92, 2019−2024. (7) Gulbinas, V.; Chachisvilis, M.; Valkunas, L.; Sundström, V. J. Phys. Chem. 1996, 100, 2213−2219. (8) Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. J. Phys. Chem. A 1999, 103, 7587−7596. (9) Nikolaitchik, A. V.; Rodgers, M. A. J. J. Phys. Chem. A 1999, 103, 7597−7605. (10) Gulbinas, V. Chem. Phys. 2000, 261, 469−479. (11) Zhu, R.-Y.; Chen, Y.; Zhou, J.; Li, B.; Liu, W.-M.; Qian, S.-X.; Hanack, M.; Araki, Y.; Ito, O. Chem. Phys. Lett. 2004, 398, 308−312. (12) Terasaki, A.; Hosoda, M.; Wada, T.; Tada, H.; Koma, A.; Yamada, A.; Sasabe, H.; Garito, A. F.; Kobayashi, T. J. Phys. Chem. 1992, 96, 10534−10542. (13) Gulbinas, V.; Chachisvilis, M.; Persson, A.; Svanberg, S.; Sundström, V. J. Phys. Chem. 1994, 98, 8118−8123. (14) Mi, J.; Guo, L.; Liu, Y.; Liu, W.; You, G.; Qian, S.-X. Phys. Lett. A 2003, 310, 486−492. (15) Xiao, K.; Li, R.; Tao, J.; Payzant, E. A.; Ivanov, I. N.; Puretzky, A. A.; Hu, W.; Geohegan, D. B. Adv. Funct. Mater. 2009, 19, 3776− 3780. (16) Hill, I. G.; Kahn, A.; Soos, Z. G.; Pascal, J., R. A. Chem. Phys. Lett. 2000, 327, 181−188. (17) Giovannetti, G.; Brocks, G.; van den Brink, J. Phys. Rev. B 2008, 77, 035133. (18) Knupfer, M.; Schwieger, T.; Peisert, H.; Fink, J. Phys. Rev. B 2004, 69, 165210. (19) Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. Science 2005, 308, 838−841. (20) Maultzsch, J.; Pomraenke, R.; Reich, S.; Chang, E.; Prezzi, D.; Ruini, A.; Molinari, E.; Strano, M. S.; Thomsen, C.; Lienau, C. Phys. Rev. B 2005, 72, 241402(R). (21) Ma, Y.-Z.; Valkunas, L.; Bachilo, S. M.; Fleming, G. R. J. Phys. Chem. B 2005, 109, 15671−15674. (22) Spataru, C. D.; Ismail-Beigi, S.; Benedict, L. X.; Louie, S. G. Phys. Rev. Lett. 2004, 92, 077402. (23) Lozzi, L.; Santucci, S.; La Rosa, S.; Delley, B.; Picozzi, S. J. Chem. Phys. 2004, 121, 1883−1889. (24) Ma, Y.-Z.; Valkunas, L.; Dexheimer, S. L.; Bachilo, S. M.; Fleming, G. R. Phys. Rev. Lett. 2005, 94, 157402. (25) Gadalla, A.; Crégut, O.; Gallart, M.; Hönerlage, B.; Beaufrand, J.-B.; Bowen, M.; Boukari, S.; Beaurepaire, E.; Gilliot, P. J. Phys. Chem. C 2010, 114, 4086−4092. (26) van Amerongen, H.; Valkunas, L.; van Grondelle, R. Photosynthetic Excitons; World Scientific: Singapore, New Jersey, London, Hongkong, 2000. (27) Bunde, A.; Havlin, S., Eds. Fractals and disordered systems; Springer Verlag: Berlin, 1991. (28) Ovchinnikov, A. A.; Timashev, S. F.; Belyĭ, A. A. Kinetics of diffusion controlled chemical processes; Nova Science Publishers: New York, 1989. (29) Valkunas, L.; Ma, Y.-Z.; Fleming, G. R. Phys. Rev. B 2006, 73, 115432. (30) Barzda, V.; Gulbinas, V.; Kananavicius, R.; Cervinskas, V.; van Amerongen, H.; van Grondelle, R.; Valkunas, L. Biophys. J. 2001, 80, 2409−2421. (31) Avakian, P.; Caris, J. C.; Kepler, R. G.; Abramson, E. J. Chem. Phys. 1963, 39, 1127−1128. (32) Valkunas, L.; Trinkunas, G.; Liuolia, V. Exciton annihilation in molecular aggregates. In Resonance energy transfer; Andrews, D. L., Demidov, A. A., Eds.; Wiley and Sons: Chichester, 1999; pp 244−307. (33) Scheblykin, I. G.; Varnavsky, O. P.; Bataiev, M. M.; Sliusarenko, O.; Van der Auweraer, M.; Vitukhnovsky, A. G. Chem. Phys. Lett. 1998, 298, 341−350. (34) Valkunas, L.; Akesson, E.; Pullerits, T.; Sundström, V. Biophys. J. 1996, 70, 2373−2379.

excitons created in the nanowires at different excitation intensities. This determination requires an accurate absorption cross-section per nanowire and the sample optical density at the probed spot, but both these parameters are currently unavailable. Improved fits for the same data set were obtained using eq 2 as shown in Figure 5b. The parameters obtained in this case are k = 0.0077 ps−1 and γ0n(0) = 0.14, 0.12, and 0.09 ps−1/2, respectively. On the basis of the quality of the data fitting shown in Figures 5a and 5b, and similar results obtained with other data sets (not shown), it is reasonable to consider that the exciton−exciton annihilation in the CuPc nanowires is diffusion-limited and involves intrachain excitons that are within 1D molecular stacks. Such a 1D diffusion-limited annihilation process has been found previously in other crystalline polymorphs including colloidal particles and thin films of various MPc materials.7,13 In conclusion, our femtosecond one-color pump−probe measurements on single-crystal α-CuPc nanowires with a typical diameter/length of ∼83 nm/18 μm show a remarkable intensity-dependent decay behavior for the transient reflectance kinetics measured at different pump intensities. The initial peak amplitudes of the transient reflectance signals were further found to scale linearly with the intensities of the pump pulses, in striking contrast to the saturating or square root dependence found previously. Global analysis of the data collected at different pump intensities reveals that the exciton−exciton annihilation is 1D diffusion-limited, which presumably occurs along the molecular stacking direction. Such a 1D diffusionlimited annihilation process was also found previously in other MPc materials such as colloidal particles and thin films. The directional 1D exciton diffusion makes CuPc nanowires attractive for photovoltaic applications, which should enable the excitons to move a relatively longer distance during their lifetimes and in turn help to achieve high power conversion efficiency.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 865-574-7213. Fax: 865-574-8363. E-mail: may1@ ornl.gov. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research sponsored by the Laboratory Directed Research and Development Program of ORNL, managed by UT-Battelle, LLC, for the U.S. Department of Energy (YZM, FY2010, 2011), and the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (YZM and RWS, FY2012). A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy (CNMS2010-057).

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REFERENCES

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