Molecular Packing in Organic Solar Cell Materials: Insights from the

Aug 18, 2014 - Binglin Sui , Mykhailo V. Bondar , Dane Anderson , Hector J. Rivera-Jacquez , Artëm E. ... Artëm E. Masunov , Dane Anderson , Alexand...
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Molecular Packing in Organic Solar Cell Materials: Insights from the Emission Line Shapes of P3HT/PCBM Polymer Blend Nanoparticles Angela M. Crotty,†,‡ Alicia N. Gizzi,‡,% Hector J. Rivera-Jacquez,‡,§ Artem ̈ E. Masunov,*,‡,§,∥,⊥ ‡,§ ‡,& ,‡,§,# Zhongjian Hu, Jeff A. Geldmeier, and Andre J. Gesquiere* †

Burnett Honors College, ‡NanoScience Technology Center, §Department of Chemistry, ∥Department of Physics, ⊥Florida Solar Energy Center, and #CREOL, School of Optics and Photonics, University of Central Florida, Orlando, Florida 32826, United States & Department of Materials Science and Engineering, Virginia Tech, Blacksburg, Virginia 24060, United States % Department of Chemistry, Central Connecticut State University, 1615 Stanley Street, New Britain, Connecticut 06053, United States ABSTRACT: Semiconducting polymer devices have seen tremendous progress in development of material and device designs, while device efficiencies have made substantial gains. Still, the effect of material morphology on the optoelectronic properties of semiconducting polymers is not completely understood even though these materials make up the active device layer. In this study we use computational methods to simulate different poly(3-hexylthiophene) (P3HT) morphologies, predict their emission spectra, and compare them to experimentally observed emission spectra for P3HT nanoparticles. We use published X-ray diffraction data on P3HT polymorphs to build the molecular models of nanodomains that differ in the side-chain packing. The atomic and electronic structures of both nanodomains are studied with the force field, Hartree−Fock, CIS, and density functional theory methods. The results confirm the coexistence of type I and II nanodomains, where the shift of the backbones in the same stack is determined by the differences in side-chain packing. Upon excitation, the polymer chains in type II domain are free to slide to their optimal arrangement in the stack, whereas in type I domain this sliding is hindered by the steric repulsion of the side chains and the chains are essentially constrained to keep the ground state geometry. These nanodomains, therefore, differ in their emission spectra: type I emission has a single 0−0 vibronic band, while type II demonstrates pronounced vibronic progression. In agreement with Frenkel exciton theory, splitting of the excited state depends on the longitudinal shift of the π-systems. However, we find that due to the constraints arising from P3HT being confined in nanosized particles, the type I nanodomain increasingly appears as an additional emitter that exhibits J-aggregate character. As a result, a pronounced vibronic structure appears as PCBM blending ratios increase, as opposed to the changes in emission profile due to a different degree of disorder present in weakly coupled H-aggregates. These findings are distinct from those made for bulk P3HT materials.

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INTRODUCTION Since the first reports on organic optoelectronic devices, conjugated polymers have been at the forefront of research in this field. They have given great promise in a number of exciting device technologies such as organic photovoltaics (OPV), organic light-emitting diodes (OLED), and organic field effect transistors (OFET).1−9 The rapid and steady progress of the conjugated polymer materials has been fueled by some immense advantages intrinsic to conjugated polymers such as low-cost fabrication of materials, ease of preparation of large area flexible plastic films, and exceptional optoelectronic properties such as high extinction coefficients, tunability of absorption spectra, and high charge mobilities.10−12 In devices such as OPVs and OFET, where the conjugated polymer forms the active layer, the resulting morphology of the conjugated polymer active layer after processing and fabrication consists of a mixture of disordered and ordered structures. These amorphous and highly ordered (semicrystalline) regions in © XXXX American Chemical Society

the active layer occur both at the nano- to macroscale. They are a key factor in determining device properties, function, and performance.13,14 Different approaches such as thermal annealing, solvent annealing, or change of composition through doping have been employed to achieve a favorable nanoscale morphology.15−17 It is well established that optoelectronic properties of conjugated polymer materials are affected by the physical conformation of individual polymer chains and the interactions between polymer chains in the material.18,19 A conjugated polymer chain is basically an ensemble of quasi-independent and localized conjugated segments (chromophores)20 that govern the optoelectronic properties of these conjugated polymer molecules. In addition, interactions between conReceived: April 26, 2014 Revised: May 13, 2014

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and 0−1 transitions in P3HT emission spectra. This observation is due to the sensitivity of the 0−0 transition to the variations in P3HT material morphology as well as the extent of long-range order, i.e., variations in the degree of crystallinity of the P3HT material as represented by weakly coupled H-aggregates.37,47,48 The 0−0 transition will increase or decrease in intensity with decreasing or increasing degree of crystallinity of the P3HT material, respectively, as it is partially forbidden for the ordered H-aggregate state. From this model it is thus shown that polythiophene spectra (e.g., P3OT, rrP3HT) for bulk materials (i) consist of a single emitting species, (ii) that exhibits variations in relative peak intensities of the 0− 0 and 0−1 transitions from the same excited state, (iii) due to changes in the intensity of the 0−0 transition caused by variations in the degree of crystallinity of the P3HT material at the nanoscale, and (iv) for which the material morphology is modeled by weakly coupled H-aggregates. In this contribution, we address the bimodal distribution of emission maxima observed in experimentally measured emission spectra from single nanoparticles of the P3HT polymer using representative atomistic models and first principle theoretical methods. These methods are able to provide higher accuracy, necessary when molecular packing is determined by rather weak intermolecular interactions.49,50 Previously, density functional theory was found helpful in interpretation of photochemical processes,51−53 crystal packing of conjugated polymers,54 emission behavior of organic molecules,55 their aggregates,56,57 and nanoparticles.58 Here we optimize the atomic structures of P3HT nanodomains, predict their emission spectra, and compare our predictions to experimental results. We confirm the earlier interpretation59 that links this bimodality to the coexistence of two polymorphic types of crystalline nanodomains, rather than to a different degree of disorder present in weakly coupled H-aggregates. From the computational work reported herein it can be concluded that the confinement of P3HT into nanoscale materials leads to the introduction of the second emitter that appears to have J-aggregate character (red-shift and predominance of 0−0 transition). Our results show that the model for P3HT spectra reported by Spano et al., while appropriate for bulk P3HT materials, needs to be revised for nanoscale confined P3HT materials such as P3HT nanoparticles and P3HT nanowires.

jugated polymer chains lead to interchain chromophores that may dominate conjugated polymer material properties and function due to energy migration processes.18 These properties, however, lead to complications in material and device design and development due to the highly inhomogeneous nature of the material morphology and associated optoelectronic properties. Specifically, nanoscale structural variations in polymer chain morphology and interchain interactions consistently lead to the formation of a variety of interchain species that are spatially and inhomogeneously distributed in the films, which make conjugated polymer material properties challenging to understand. Polymer chain morphology and interchain interactions in materials are very difficult to control during material and device fabrication,18 although, some reports have shown potential in guiding the nature and occurrence of these interchain electronic species by choice of solvent, temperature, concentration of the solution from which films are cast, and spin-coating speed.21−27 Thus, understanding the electronic interactions between polymer chains in conjugated polymer materials as a function of polymer chain morphology at the molecular and nanoscale level is of great importance for the continued development of conjugated polymer materials and optoelectronic devices. Poly(3-hexylthiophene) (P3HT) is a prototypical conjugated polymer that has been successfully applied in efficient bulk heterojunction−organic photovoltaic devices (BHJ-OPV) and OFET.15,28−31 P3HT is a highly ordered conjugated polymer material that shows intimate contact between adjacent conjugated polymer backbones.32 The absorption spectra of P3HT films reveal several specific optical signatures that are related to this morphology of P3HT materials.33 A faint shoulder at 560 nm and a pronounced shoulder at 610 nm that appear in P3HT solid state materials are due to planarization of the polymer chains driven by intermolecular interactions, particularly van der Waals interactions and intermolecular πstacking.34 This arrangement has also been identified as being responsible for the high charge mobilities that have been observed for P3HT.35,36 While under these conditions P3HT materials can be processed to retain a microscale crystalline order, the application of P3HT in bulk heterojunction−organic photovoltaics results in a contradictory situation where highly ordered domains are needed to transport charge, but disorder is desired as well to achieve efficient exciton dissociation upon the light absorption. An interesting consequence of the disorder introduced in P3HT materials exhibiting long-range order is the observation that the optical properties are highly sensitive to morphological variations. Researchers have reported extensively on changes in the optical properties of polythiophenes as a function of material morphology.37−44 Although several computational investigations on polymer electronic structure were published in the past,45,46 they did not address the emission spectra. Recently, Spano et al. reported a significant advancement in understanding the effect of aggregation and disorder on the optical properties of P3HT in the bulk. Specifically, the role of weakly coupled Haggregates (including excitonic intermolecular coupling and exciton−phonon coupling) and their effect on P3HT spectroscopic properties in view of the introduction of disorder into the material system have been considered in their theoretical works.37,47,48 Under these circumstances the excitonic emission is (partially) forbidden for crystalline P3HT. The introduction of disorder in these structures leads to the observation of changes in relative intensities of the 0−0

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EXPERIMENTAL AND COMPUTATIONAL DETAILS P3HT and composite P3HT/PCBM nanoparticles were fabricated by the reprecipitation method. In the reprecipitation method, the materials of interest are dissolved in a good solvent that is miscible in a bad solvent for these materials. The solution is then rapidly injected in the bad solvent, which leads to aggregation of the materials of interest into nanoparticles. In the studies discussed herein, tetrahydrofuran was used as the good solvent and water as the bad solvent. Preparation and characterization details have been reported previously.59,60 All calculations were performed using the Gaussian 2009 software package. The emission line shape predictions were performed using Frequency = (FC, Emission) keyword in Gaussian 2009. Excitations of up to 5 quanta on each normal mode as well as their combinations were included. The overall profile was obtained as the sum of all transitions with line width of 10 cm−1. Since CIS theory level is known to overestimate the electronic excitation energies, single point energy evaluations were performed using M05qx density functional with 35% HF B

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Figure 1. (a, b) Basic characterization of nanoparticles by TEM and AFM. The inset in panel b shows a line scan for one of the particles. (c) Laser confocal fluorescence microscopy image of single particle sample; each dot represents the location of an individual nanoparticle. Data shown in (a− c) are for 50 wt % PCBM-doped P3HT nanoparticles. Reproduced with permission from ref 64. Copyright 2009 Elsevier B.V.

nanoparticle composition (0%, i.e., pure P3HT, 5, 50, 75 wt % PCBM blended P3HT/PCBM) the single particle emission spectra were averaged and are presented here as single particle ensemble spectra in Figure 2 (insets). These data show (i) that

exchange, which was optimized to reproduce the experimental excitation energies.61 The 0−0 transition energy was calculated as the difference between the total excited state energy at the TD-M05qx/6-31G* theory level evaluated at the CIS/STO-3G optimized geometry and the total ground state energy at the M05qx/6-31G* theory level evaluated at the HF/STO-3G optimized geometry (denoted in the following as TD-M05qx/ 6-31G*//CIS/STO-3G and M05qx/6-31G*//HF/STO-3G, respectively).

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RESULTS AND DISCUSSION Experimental Background. P3HT and composite P3HT/ PCBM nanoparticles were fabricated by the reprecipitation method. The P3HT/PCBM blending ratio for composite nanoparticles was varied between 5, 50, and 75 wt % PCBM The resulting nanoparticles have been extensively characterized by Transmission Electron Microscopy, Atomic Force Microscopy, steady-state absorption and fluorescence spectroscopy, and single particle spectroscopy (Figure 1).59,60 Our assignment of nanoparticle morphology and function based on these studies was corroborated by time-resolved studies reported recently.62 The nanoparticles have a size of 40−60 nm and contain domains of PCBM trapped by ordered domains of P3HT. The extent of P3HT ordering and PCBM domain formation depends on the P3HT/PCBM blending ratio. These morphological changes were studied one particle at a time with single particle spectroscopy. Individual nanoparticles (dispersed in a poly vinyl alcohol matrix and isolated several micrometer from each other) were first located by fluorescence imaging using a home-built sample-scanning confocal microscope described elsewhere.63 The highly sensitive setup has single photon counting capability and was able to easily detect the highly quenched blended P3HT/PCBM nanoparticles (Figure 1c). Even though the charge transfer process and resulting quenching are efficient, this is a dynamic quenching process, and thus the detection of P3HT emission is still probable.65 Emission spectra for individual nanoparticles were collected by placing a nanoparticle in the focal spot of the objective lens with the sample scanner at which point photoexcitation was achieved by laser. Several hundred emission spectra for each nanoparticle composition were collected by sending the fluorescence signal to a monochromator coupled to a charge coupled device (CCD) as described previously.63 For each

Figure 2. Single particle ensemble emission spectra made by averaging single particle spectra (insets in the left column) and corresponding subensemble spectra (red and blue lines) constructed based on the corresponding peak energy histograms (right column, emission maxima at 1.88 eV (blue) and 1.73 eV (red)). Reproduced with permission from ref 59. Copyright 2009 Elsevier B.V.

two emission features exist at 1.88 and 1.73 eV and (ii) that the relative intensity of the peaks at 1.88 and 1.73 eV changes with nanoparticle blending ratio. The power of single particle spectroscopy lies in the fact that besides the ensemble average, the distribution of spectral properties can also be considered. Figure 2 shows histograms of peak energy distribution for each of the nanoparticle compositions. Two key observations can be made: (i) the peak energy distribution histograms reveal a bimodal distribution, and (ii) the occurrence of nanoparticles with emission maximum at 1.73 eV increases as the PCBM content in the composite nanoparticles increases. To evaluate these observations further, the nanoparticles for each composition C

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Figure 3. Type I lamellar (a) and type II interdigitated (b) arrangement of the side chains in nanodomains of P3HT.

conjugated stacks. This structure provides ideal π-stacking of thiophene units and low energy emission.66 In the type II packing structure the alkyl chains are interdigitated and only moderately tilted. In this arrangement the π-stacking distance increases to 4.5 Å, which leads to higher energy emission.40 Two factors are thus at play to cause the spectral changes observed for P3HT nanoparticles as the blending ratio with PCBM is increased: (i) addition of PCBM leads to an increased abundance of type I packing structures, and (ii) as shown later (vide inf ra) computational data reveal that the type I packing structure has an additional 1/4 backbone shifted, i.e., by the length of half a monomer unit (1/4 of the translation vector of the P3HT chain)in order for the side chains to sterically fit between the side chains of the underlying backbone in the same stackcompared to the type II packing structure. This additional shift introduces J-aggregate-like emission properties (red-shift and predominance of 0−0 transition). These Jaggregate-like emission properties are not observed in bulk P3HT and P3HT/PCBM blended films and are attributed to the confinement of P3HT chains in the nanoparticle materials.

were sorted into two populations according to the distribution observed in the peak energy histograms, with 1.80 eV used as the sorting criterion to place nanoparticles in the blue or red subensembles. Subensemble spectra were constructed by averaging spectral data accordingly and are shown in Figure 2. These data reveal the emission spectra of two emitting species: one with a higher peak emission energy and one with a lower peak emission energy exist in the P3HT and composite P3HT/PCBM nanoparticles. The relative intensity of the emitters with lower peak emission energy increases with increasing PCBM blending level, which indicates an increase in abundance of these emitters with increasing PCBM blending level. This observation is in agreement with observations made from the peak emission energy distribution histograms. The emitting species can be assigned to type I and type II packing structures based on the reported data and polythiophene packing structures reported in the literature.40,41 The lamellar type I packing structure consists of π-conjugated stacks of thiophene rings separated by 3.8 Å spaced by alkyl chains that are tilted with respect to the propagation direction of the πD

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Figure 4. QM models of P3HT structure for type I lamellar (a) and type II interdigitated (b) packing modes.

keyword IOp(3/124=1). The local minimum was verified with the normal-mode analysis. CIS/STO-3G theory level with empirical dispersion correction was used for optimization and normal-mode analysis of the lowest excited state with large oscillator strength. The structures of the nanodomains optimized with molecular mechanics are shown in Figure 5a,b. Apart from some edge irregularities, the type II nanodomain with interdigitated side chain packing (Figure5b) retains crystallographic long-distance order. In contrast, the type I packing demonstrates a helical twist, making the backbones in the stack slightly nonparallel (ca. 5° dihedral angle). The stacks are also nonparallel with

Nanoscale confinement effects on P3HT morphology have recently been reported.67,68 The increased occurrence of type I packing structures is likely due to the fact that PCBM does not fit in between the alkyl side chains of P3HT and thereby interrupts the interdigitation found in type II structures. The inability of PCBM to fit within P3HT alkyl chains has been discussed extensively in the literature.69,70 Geometry Optimization. The initial geometries of the nanodomains were constructed as shown in Figure 3, according to the X-ray data on P3HT polymorphs of type I41 and type II40 by Prosa et al. Both polymorphs are formed by π-stacking of polythiophene main chains but differ by the packing of the hexane side chains and π-stacking distance of the thiophene units. The lamellar type I packing structure consists of πconjugated stacks of thiophene rings separated by 3.8 Å spaced by alkyl chains that are tilted with respect to the propagation direction of the π-conjugated stacks. In the type II packing structure the alkyl chains are interdigitated and only moderately tilted. In this arrangement the π-stacking distance increases to 4.5 Å.40 We used thiophene octamers to represent each main chain. The lamellar structure of type I nanodomain was built of two π-stacks containing eight main chains each; side chains were twisted 60° off the molecular plane (Figure 3a). The side chains in type II were kept in the molecular plain and interdigitated with the side chains of the neighboring main chains placed in the same plane in antiparallel orientation. One layer was represented by four main chains, and nanodomains were built by stacking four of these layers (Figure 3b). Both domains were optimized at the molecular mechanics (MM) theory level using the universal force field (UFF). As nanodomains were too large for the quantum mechanical (QM) treatment, smaller representative fragments were selected (Figure 4) such that geometry optimization did not alter their structural organization significantly. The main chains in these fragments were represented by four thiophene units. This size seems to be sufficient, as experimental studies of oligothiophenes indicate that the exciton is localized on two thiopene units,71 while polarons (at least the positively charged ones) are localized on four thiophene units.72 Type I QM fragment was represented by one stack of two backbones (Figure 4a), and type II QM fragment contained two such stacks, with all the side chains pointing outside removed (Figure 4b). To ensure stability of the QM models, they were optimized for both polymorphs at the HF/STO-3G theory level with empirical dispersion correction added by the

Figure 5. P3HT structure after MM energy optimization for type I lamellar (a) and type II interdigitated (b) nanodomains. E

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Figure 6. Essential orbitals corresponding to the lowest excited state in the geometry of the ground state for type I QM model. This state is bright (oscillator strength of 1.453) and consists of nearly equal mix of two electron transitions (HOMO to LUMO and HOMO−1 to LUMO+1). Each row above illustrates one of these transitions: occupied orbital is on the left, and virtual orbital is on the right. Each orbital, involved in these transitions, is formed by an equal mix of HOMO and LUMO orbitals of the monomeric PT backbone.

Figure 7. Essential orbitals corresponding to the lowest excited state in the geometry of the ground state for type II QM model. This state is dark (oscillator strength of 0.002) and consists of nearly equal mix of four electron transitions. Each row above illustrates one of these transitions: occupied orbital is on the left, and virtual orbital is on the right. Each orbital, involved in these transitions, is formed by an equal mix of HOMO and LUMO orbitals of the monomeric PT backbone. One can see that the electron transitions are largely confined within the same stack of PT backbones, with the exception of small interstack charge transfer in the top two rows. The overall excitation, however, is delocalized between the two PT stacks.

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Figure 8. Essential orbitals corresponding to the lowest excited state in its optimized geometry for type I QM model (top) and type II QM model (bottom). Both states are bright and consist predominantly of the HOMO to LUMO transition. Both excitations are localized on a single PT backbone, with a small fraction of charge transfer within the stack in type II case.

Figure 9. (a) Franck−Condon prediction of the vibronic line shape for the emission band from QM model type I: vibronic progression is well formed, and the 0−1 transition is the dominant peak. (b) Franck−Condon prediction of the vibronic line shape for the emission band from QM model type II: the maximum corresponds to the 0−0 transition, and the remaining peaks are much less pronounced.

each other, with the dihedral angle between the backbones of different stacks being close to 12°. Clearly, type I is more disordered. The QM models retain the chain twist in type I and parallel order in type II. More importantly, they clearly demonstrate the difference between the backbone stacking in type I and type II. The backbones in the type I model are shifted by the length of one monomer unit (1/2 of the translation vector of the P3HT chain) in order for the side chains to sterically fit between the side chains of the underlying backbone in the same stack. On the other hand, the shift of the backbone in the type II model is close to the one-half length of the monomer unit (1/4 of the translation vector along P3HT chain). This time the close packing of the side chains takes place between the backbones of the neighboring stacks, and the next backbone in the stack has to fit in the valley formed by the side chains of the alternating stacks. Electronic Structure Calculations. The electronic structures of the excited states obtained for QM model systems are reported in Figures 6−8. In the case of type I model the ground state optimization followed by single point prediction of the singlet excited states at the TD-M05qx/6-31G*//HF/STO-3G theory level reveals that the lowest excited state has a low oscillator strength (“dark” state), and light absorption will initially populate the second excited state with oscillator strength close to 1.5 (“bright” state, which is 0.25 eV higher

in energy than the dark state). This situation is characteristic for H-aggregates.57,73 The HOMO to LUMO transition makes a largest contribution to this dark state, followed closely by (HOMO−1) to (LUMO+1) transition. As geometry optimization of the dark state progresses, it gradually gains the oscillator strength at the expense of the second excited state. By the time optimization is completed, the oscillator strengths reaches 1.450 (corresponding to the short radiative lifetime), the state almost exclusively consists of HOMO to LUMO transition, and both of these orbitals are localized on the same backbones in stack (Figure 8, top). The electronic structure is similar in the type II QM model. Here the lowest excited state is also dark in the ground state geometry but becomes bright in the optimized excited state geometry. It consists of nearly equal contributions from four electron transitions. Each orbital, involved in these transitions, is formed by an equal mix of HOMO and LUMO orbitals of the monomeric PT backbone. One can see from Figure 7 that the electron transitions are largely confined within the same stack of PT backbones, with the exception of small interstack charge transfer for the two of them. The excitation as a whole is, however, delocalized between the two PT stacks. In the geometry of excited state the excitation is again localized on one PT backbone with a small fraction of the charge transfer between the stacked backbones. G

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Prediction of Emission Line Shapes. The emission line shapes predicted for type I and type II models and comparison with experiment are reported in Figure 9a,b. One can see that both spectra have 0−0 transition maximum ca. 2.1 eV, in good agreement with the experimental value of 1.9 eV. However, the 0−1 transition dominates in type I, while the 0−0 transition corresponds to the maximum in type II. The reason for these differences can be rationalized as follows. The π-stacks in type I nanodomains are composed of polymer chains that are free to slide to an optimum geometric arrangement upon electronic excitation. Therefore, the Dushinsky shift is large and results in pronounced vibronic progression on emission spectrum and thus large Huang−Rhys factor. On the other hand, backbone chains in π-stacks of type II are locked in their arrangement by the interdigitated side chains, the excitation does not alter this arrangement much, and 0−0 transition dominates in the emission spectrum. Using these predictions, one can interpret the experimental emission as follows. The chromophores in pristine P3HT nanoparticles are represented in majority by type II nanodomains. Upon increasing mass percent of PCBM in the blend the fraction of type I nanodomains is increasing, and at 75% doping ratio a large fraction (nearly half) of the chromophores are represented by type I. This mixture results in a second, red-shifted peak and broadens the original peak somewhat, as the type II 0−0 transition overlaps with the type I one.

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CONCLUSIONS In summary, our QM/MM calculations confirm the possibility of two types of nanodomains in regioregular semicrystalline P3HT, which differ by side-chain packing. Both of these nanodomains are formed by stacking of the backbone chains, but differ by longitudinal shift of the backbones. The relative presence of two crystalline structures in P3HT is altered by blending with PCBM and confinement of P3HT chains in nanoparticle materials. The increasing abundance of type I emitters under these conditions leads to emission spectra with vibronic signature of J-aggregate character rather than being due to a different degree of disorder present in weakly coupled H-aggregates. This finding is distinct from those made for bulk P3HT materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.E.M.). *E-mail [email protected] (A.J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (CCF-0740344 and CHE-0832622). Research was performed using the Stokes HPCC facility at the UCF Institute for Simulation and Training (IST), and supercomputers at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility at Lawrence Berkeley National Laboratory. A.M.C. acknowledges support by UCF RAMP scholarship. Z.H., J.G., and A.J.G. thank the National Science Foundation (NSF) for financial support of this work through a CAREER award (CBET-0746210), through award CMMI-1335295, and through REU site EEC1247324. H

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