ultra-robust graphene oxide-silk fibroin nanocomposite membranes

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Kesong Hu , Maneesh K. Gupta , Dhaval D. Kulkarni , and Vladimir V. Tsukruk *

Ultra-Robust Graphene Oxide-Silk Fibroin NanocompositeMembranes

TION
1. Introduction
Nanocomposite materials in forms of membranes, fi lms, and coatings are gaining surging interests in structural and func-tional applications, because they are more effi cient in loading transfer than conventional composites and can substantially eliminate catastrophic failure caused by poor loading transfer between components. To enhance the mechanical properties of polymeric nanocomposites, carbon nanotubes, intercalated clay, graphene, and graphene oxide are added as high-performance reinforcing nanofi llers. For example, ultrahigh toughness was reported for polyvinyl alcohol nanocomposite fi lms fi lled with single-walled carbon nanotubes; [ 1 ] and ultrahigh modulus was reported for crosslinked nanoclay containing nanocompos-ites. [ 2 ] However, improving toughness is usually achieved by increasing the ultimate strain and compromising the strength, which is not desired for high-performance applications. [ 3 ]

Among popular fi llers, graphene oxide (GO) exhibits huge potential due to its outstanding mechanical properties, high binding potential, high aspect ratio, high fl exibility, and supe-rior processibility. [ 4–6 ] Graphene oxide microscopic sheets can be easily produced from graphite fl akes by thermal oxidation as suggested by Hummers. [ 7 ] This procedure introduces a high density of epoxide and hydroxyl groups on both sides and car-boxyl groups around the edges, which is critically important for nanocomposites performance ( Scheme 1 a,b). [ 8 ] Moreover, graphene oxide remains negatively charged in acidic condi-tions (down to pH around 2) [ 6 ] and can be incorporated into nanocomposite polyelectrolyte materials via aqueous solution processing routines. In spite of structural defects their elastic modulus is still very high at 0.25 TPa even in comparison with traditional graphenes with elastic modulus of 1 TPa. [ 9 , 10 ] Graphene oxide-polyelectrolyte nanomembranes have demon-strated high toughness of 1.9 MJ m − 3 and high elastic modulus of about 20 GPa, [ 11 ] but the low saturation limit prohibited fur-ther advances in traditional polymer composites.

Natural biomaterials have drawn attention as a choice for nanocomposite matrices because of their inspiring morpholo-gies, biocompatibility, biodegradability, and superior perfor-mances such as those demonstrated by silk materials. [ 12–14 ] Thanks to its hierarchical multidomain morphology, silk mate-rials and fi bers are one of the strongest elastomeric natural

© 2013 WILEY-VCH Verlag G

K. Hu, M. K. Gupta, D. D. Kulkarni, Prof. V. V. TsukrukSchool of Materials Science and EngineeringGeorgia Institute of TechnologyAtlanta, Georgia 30332-0245, USA E-mail: [email protected]

DOI: 10.1002/adma.201300179

Adv. Mater. 2013, DOI: 10.1002/adma.201300179

biomaterials. [ 15 , 16 ] An advanced secondary structure with antiparallel β -sheet nanocrystals can dramatically improve the macroscopic mechanical properties of silk materials. [ 17 , 18 ] Due to the diffi culties in extracting spider silk fi broin from webs or glands, reconstituted silk fi broins from silkworm cocoons are commonly used in research and production. [ 17 , 19–28 ] To date, researchers have made them into threads, [ 16 ] fi lms, [ 17 ] hydrogels, [ 25 ] scaffolds, [ 21 ] capsules, [ 15 ] microcapsules, [ 29 ] and coatings [ 20 ] by different techniques. Very good mechanical properties of various silk-based nanocomposites have been demonstrated, but the ultimate values are still well below those of the high performance nanocomposites and thus different reinforcing strategies are deliberated. [ 13 , 30 ] However, traditional reinforcing components, such as nanoparticles or clays, pos-sess homogeneous surface compositions with preferred polar or hydrophobic interactions, and thus cannot be very effi cient because of the heterogeneous, multiblock nature of silk back-bones with alternating hydrophilic and hydrophobic nano-scale domains facilitating a combination of hydrogen bonding, polar-polar, and hydrophobic-hydrophobic interactions at the interfaces. [ 14 , 31 ]

In this communication, we report novel ultrathin, robust nanocomposite membranes by incorporating graphene oxide sheets into silk fi broin matrix through heterogeneous sur-face interactions in an organized layer-by-layer (LbL) manner (Scheme 1 c). The outstanding values of the mechanical prop-erties achieved here are manyfold higher than those reported to date, which include a tensile modulus of 145 GPa, an ulti-mate stress of more than 300 MPa, and a toughness of above

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Scheme 1 . Molecular models of silk fi broin hydrophilic segments with polar interactions (a, upper part) and GO (a, bottom part: side view; b: top view). Elements in the ball-and-stick model are grey-scale coded: H: white, C: light grey, O: dark grey, N: black. c) The layered structure of nanocomposite silk fi broin-graphene oxide membrane.

http://doi.wiley.com/10.1002/adma.201300179


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Figure 1 . AFM morphology of the GO-SF nanocomposite membrane: a) topography ( z -scale: 60 nm, inset: optical image of the membrane suspending on a 300 μ m copper aperture) and b) phase (z-scale: 45 ° ) images of the same area. c) Silk fi broin adsorption on graphene oxide ( z -scale: 7.5 nm). The height profi le of the cross section shows the GO thickness is ≈ 1.8 nm due to silk fi broin beneath the fl ake. inset: 500 nm × 500 nm area on GO, the height of the silk fi broin molecules is around 2 nm. d) The thickness of the nanomembranes increases linearly with the number of the bilayers assembled. e,f) AFM (e) and optical microscopy (f) images of buckled membranes for compressive tests.

2.2 MJ m − 3 for silk nanocomposites with a silk content around 80%. We suggest that this outstanding performance that well exceeds the theoretical values predicted by conven-tional mechanical models is facilitated by the effective 2D graphene oxide fi ller which max-imize all hydrogen bonding, polar-polar, and hydrophobic-hydrophobic interactions of the defective graphene oxide sheets with the silk fi broin matrix composed of polar random silk domains and the hydrophobic β -sheet nano-crystals. We suggest that the dense network of weak interactions between the silk fi broin domains and graphene oxide sheets, which are in intimate contact within a 5 nm-thick bilayer, facilitates the formation of molecular interphase zones, thus effectively increasing the reinforcing effect and allowing a record high mechanical strength and toughness, unheard of for biopolymer based nanocom-posite fi lms, to be acheived.

2. Results and Discussion

We applied the spin assisted layer-by-layer (SA-LbL) technique to assemble alternative layers of graphene oxide and silk fi broin according to a well-established procedure (Scheme 1 ) (see Experimental Section). [ 32–35 ] We tuned the volume concentrations of graphene oxide from 3 to 6, 9, 11.5, and 23.5 vol% or the total thickness of the nanomembranes from 5 to 55 nm by increasing the number of graphene oxide layers ( Figure 1 ).

As AFM imaging demonstrated, the topo-graphy of silk layers covered with graphene oxide sheets is uniform with a root-mean square (RMS) roughness of 4.3 ± 1.9 nm within 1 μ m × 1 μ m surface areas (Figure 1 a). The thickness of the graphene oxide sheets (0.95 nm) agrees with other experimental results and indicates predominantly single/double-layered fl akes. [ 11 , 36 ] The uniformity and overall integrity of the nanocomposite membrane was also confi rmed by optical microscopy, which shows fl at and homoge-neous semi-transparent membranes trans-ferred to and freely suspended on a 300 μ m aperture (Figure 1 a, inset). The AFM phase images provide contrast between graphene

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oxide sheets and silk fi broin matrix, and show that 69 ± 9% sur-face coverage of graphene oxide can be achieved for specimensprepared from either graphene oxide methanol or aqueoussuspensions without folding and wrinkling of these fl akes(Figure 1 b).

The thickness of a typical LbL membrane increases linearlywith increasing number of layers with the average bilayerthickness of around 5.4 nm (Figure 1 d). This increment value

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suggests around 5 nm-thick silk fi broin layers that are covered with 0.95 nm-thick graphene oxide sheets with 69% coverage density. The silk layer thickness of around 5 nm is common for silk fi broin adsorbed on modestly hydrophilic surfaces and corresponds to the monolayer state with some layering of the secondary structure. [ 37 ]

High-resolution AFM image of silk fi broin macromolecules adsorbed on a graphene oxide sheet shows the characteristic

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multidomain morphology of silk fi broin macromolecules. The silk macromolecules are uniformly distributed on graphene oxide surface with nanofi brils mostly formed along the edges of the graphene oxide fl akes (Figure 1 c). The surface density of adsorbed silk fi broin macromolecules is signifi cantly higher on graphene oxide sheets than on a silicon surface, indicating strong interactions between those two components after effi -cient adsorption from silk solution during fast solvent removal. On the other hand, the vast majority of fi brillar structures are individual nanofi brils or limited multidomains, rather than bundles as usually observed for long-time adsorption on hydrophilic substrates. [ 31 ] The absence of signifi cant aggrega-tion and self-folding is critically important for maximizing interfacial interactions among different silk domains and het-erogeneous substrate surfaces. [ 38 ]

Buckling and bulging micromechanical tests were carried out to measure the compressive and tensile moduli, ultimate stress, ultimate strain, and toughness of the nanomembranes in accordance with common procedures widely adapted in engineering mechanical studies of ultrathin fi lms and coat-ings. [ 39–41 ] The stress–strain curves were derived from defl ec-tion–pressure measurements with optical interferometry to calculate the tensile elastic modulus. On the other hand, the periodic buckling patterns of the compressed nanocomposite fi lms were used to evaluate the compressive elastic modulus (see Supporting Information for details).

These techniques are especially applicable and widely uti-lized for nanocomposite ultrathin fi lms where conventional tensile tests are not applicable. [ 42–45 ] These micromechanical testing techniques are applied to study mechanical properties of metal, semiconductor, and polymeric fi lms over recent dec-ades by using fundamental equations of membrane deforma-tions derived in classical work by Timoshenko and others. [ 46–52 ] The results from the bulging tests match well with those made by nanoindentation, microtensile, and point membrane defl ec-tion tests.

Figure 2 a shows the values of the elastic moduli for graphene oxide-silk fi broin nanocomposites after methanol treatment to induce β -sheet formation obtained from both bulging and buck-ling tests for specimens with various graphene oxide concentra-tions. The pristine silk fi broin nanomembranes after methanol treatment showed the Young’s moduli of about 10 GPa, which is slightly higher as compared to the reported values for LbL silk fi broin fi lms, and that can be related to extensive methanol treatment in this study. [ 17 , 26 ] In fact, the Young’s modulus of β -sheet crystals was measured to be around 22 GPa [ 53 ] and the fraction of β -sheet crystal in intensely methanol-treated silk fi broin is around 45%. [ 28 ] With the 4–5 GPa of Young’s modulus of the random silk fi broin known, [ 17 ] the Halpin–Tsai model for randomly oriented nanoparticles [ 11 , 54 ] gives a Young’s modulus of 9.5 GPa, which fi ts well to our experimental results.

After the addition of graphene oxide, the Young’s moduli of the nanomembranes subjected to tensile stress (bulging tests) increased linearly with graphene oxide concentration, eventually reaching the highest value of 145 ± 4 GPa at 23.5 vol% of graphene oxide concentration. The compressive elastic mod-ulus also increases linearly but is slightly lower than the ten-sile modulus for the same graphene oxide content (Figure 2 a). The outstanding value of tensile modulus is by far the highest

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, DOI: 10.1002/adma.201300179

modulus recorded for nanocomposite membranes without the expense of a very high fi ller concentration [ 1 , 2 , 55 ] Moreover, it is even more surprising to fi nd that the experimental values are systematically and signifi cantly higher than the theoretical values predicted by the Halpin–Tsai laminated model for perfect orientation of 2D reinforcing sheets, as well as other mechan-ical models (Figure 2 a and Supporting Information). [ 38 , 56 , 57 ]

To elucidate this discrepancy, we consider the possibility of rare but known interphase reinforcement mechanism in nano-composites based upon the formation of extended interphase zones between the two components with enhanced interme-diate properties. [ 58–62 ] It is worth noting that, in the case of graphene oxide sheets, the interphase region might play a sig-nifi cant role, since the fi ller is below 1 nm thick and the overall thickness of the complete bilayer unit is only 5.4 nm.

In order to determine the thickness of the interphase and the extent of the corresponding reinforcing phenomenon, we developed a mechanistic model of interphase reinforcement (Figure 2 b). In this model, we assumed that the elastic modulus varies monotonically along the normal direction within the inter-phase zone and the rate of the decay depends on the local mod-ulus and the total percentage of decay, similar to those suggested for layered nanocomposites in nanoindentation experiments. [ 63 ] By rearranging the analytical equations from Kovalev et al., [ 63 ] we derived an expression for the theoretical sigmoid variation of the composite elastic modulus at the interphase region (see Supporting Information for detail). [ 64 , 65 ] This model suggests the equation for the variation of the composite modulus:

E ∗(t) = �E

1 + exp[η

(tτ

− 1)] + ESF

(1)

where E ∗ ( t ) is the current composite modulus at a distance t from the graphene oxide-silk fi broin interface; Δ E = E GO − E SF , where E GO and E SF are the Young’s moduli of graphene oxide and silk fi broin, respectively; η is the shape factor, which is pro-portional to the relative interfacial strength; τ is the effective thickness at which the modulus decays by 50%. In this model, η remains a characteristic constant if the nature of interactions should remain unchanged.

By assuming a continuous modulus profi le across the inter-phase zone the fi tting of the compositional variation of the elastic modulus gives values of η = 6.00 for both tensile and compressive tests, thus indicating similar nature of interfacial interactions (see Supporting Information). The fi tting of the experimental results gives the effective thickness τ for compres-sive and tensile conditions of 0.27 nm and 0.48 nm, respectively. These values correspond to the total interphase thickness of about 0.6 nm and 1.0 nm (Figure 2 b). This interphase thickness is comparable to cross-sections of silk backbones and individual domains as measured by AFM (0.6 to 2 nm). Moreover, con-sidering the thickness of the graphene oxide sheets (0.95 nm) and silk phase (about 5 nm), such dimensions create a unique situation for layered nanocomposites with the overall fraction of the reinforcing region (graphene oxide and the interphase zone) reaching about 40% of the total nanocomposite volume, even for a low fi ller content (10–20%). Overall, this extended interphase volume results in the doubling of the effective high-modulus fi ller volume concentration, thus dramatically

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Figure 2 . Micromechanical properties of the nanocomposite membranes: a) Young’s moduli of the nanomembranes with different graphene oxide concentrations. The sigmoid interphase decay model-fi tted data for bulging and buckling results are shown by the dashed and dotted curves, respec-tively. The value of the parallel-platelets Halpin–Tsai model is represented by the solid line. b) Sigmoid decay curves for buckling and bulging tests at the interphase region (between the shaded areas). GO and SF regions are partly shown. c–e) GO concentration dependence of ultimate stress (c), ultimate strain (d) and toughness (e). f) Representative stress–strain data derived from bulging tests. Some data are shown for pristine and methanol-treated membranes.

increasing the reinforcing effect well beyond the nominal phys-ical content of graphene oxide component.

The modulus value for different graphene oxide concentra-tions calculated according to the interphase zone model fi t well to the experimental data in contrast to traditional mechanical models with sharp interfaces (Figure 2 a). It is important to note that the interphase model proposed is reduced to standard rule-of-mixture model for composites in the limit of zero effective thickness of the interphase region. To verify if the dramatic reinforcement of the mechanical properties is not related to merely scale effect arising from nanoscale thickness of the fi lms (around 50 nm), we conducted additional bulging experi-ments for samples with the same composition and bilayer structure but different thicknesses (up to 1.34 um) by varying the number of bilayers and demonstrated that extremely high

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elastic modulus does not degrade for fi lms with much higher thickness (Supporting Information, Figure S3). In addition, the consistent lower compressive modulus than tensile modulus is probably caused by progressive delamination of the layered nanomembranes due to local wrinkling of fl exible graphene oxide sheets in buckling experiments. [ 17 , 27 , 66 ]

The ultimate stress of the graphene oxide-silk fi broin nano-composites reaches very high value of above 300 MPa (up to 330 MPa in some cases) with the ultimate strain staying within 1.0 ± 0.4% (Figure 2 c,d,f). Retaining virtually constant compli-ance without signifi cant stiffening despite the tripled ultimate strength is another unique feature of these interphase nanocom-posites. This behavior can be related to recoverable hydrogen bonding network between components and low fl exural stiff-ness of graphene oxide sheets. Finally, the average toughness of

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Figure 3 . Comparison of mechanical properties among GO- and SF-related fi lm materials. All of the data for the nanocomposites were taken from the respective compositions that provide the highest moduli. The data points are greyscale coded for ultimate mechanical strength. The dashed borders separate regions with improving combined properties, and the arrow shows the overall desired direction for nancomposite rein-forcement. ( ∗ Nomenclature: Filler-matrix (crosslinker). ∗ ∗ Abbreviations: PVA: poly(vinyl alcohol); PU: polyurethane; PCL: poly( ε -caprolactone); PAH: poly(allylamine hydrochloride); PSS: poly(sodium 4-styrene sul-fonate); NFC: native nanofi brillated cellulose; HFBI: hydrophobin; GA: glutaraldehyde; AgNPL: silver nanoplatelets; POSS: polyhedral oligo-meric silsesquioxane; MMT: montmorillonite. ∗ ∗ ∗ Reference: GO-PU, [ 67 ] GO-PCL, [ 69 ] Grapehene-NFC(HFBI), [ 70 ] GO-PAH/PSS, [ 11 ] MMT-SF(GA), [ 26 ] POSS-SF, [ 26 ] graphene paper, [ 72 ] GO-PVA, [ 68 ] SF(GA), [ 26 ] AgNPL-SF, [ 27 ] GO paper, [ 71 ] SF, [ 17 ] GO(borate). [ 73 ] )

methanol treated nanocomposites reaches high values of 2.2–2.4 MJ m − 3 with the highest toughness of 3.4 MJ m − 3 recorded for some specimens (Figure 2 e).

Finally, a comparison of the nanocomposites containing silk fi broin with different fi llers and different polymeric materials with graphene oxide fi llers provides a clear perspective on the performance of the silk fi broin-graphene oxides nanocompos-ites fabricated in this work ( Figure 3 ). [ 11 , 17 , 26 , 27 , 67–73 ] In particular, the modulus, ultimate stress, and toughness of the graphene oxide-silk fi broin nanocomposite membranes are manyfold (2–20) times higher than those of the nanocomposite mem-branes made of traditional reinforced silks and cross-linked silk materials. On the other hand, the major mechanical parameters are 1.5–6 times higher than those for synthetic nanocomposites from conventional homopolymers with graphene oxide fi llers (Figure 3 ). In addition, no other nanocomposites possess a comprehensive combination of outstanding mechanical proper-ties as the graphene oxide-silk fi broin nanocomposites do. For example, even if graphene oxide-polyurethane nanocomposites show a very high toughness of 16 MJ m − 3 , their ultimate stress and elastic modulus are very low due to weak polyurethane elastomeric matrix. [ 67 ] Another example is that graphene oxide fi lm crosslinked by borate possesses high modulus of 127 GPa,


but the ultimate stress is modest at 185 MPa and toughness is very low at 0.12 MJ m − 3 due to the high brittleness of these nanocomposites. [ 73 ] Moreover, popular graphene oxide and graphene papers with a high content of fl exible sheets show modest mechanical properties (Figure 3 ).

To sum up, we suggest that the synergistic enhancement from both components with complementary heterogeneous nature of surface functionalities is facilitated by the satura-tion of all hydrogen bonding, polar, and hydrophobic interac-tions participating in the interfacial bonding of the ultrathin graphene oxide-silk fi broin nanocomposites. The 145 GPa ten-sile modulus is by far the highest reported value for synthetic and bioderived nanocomposite fi lms with predominant con-tent of a soft component (around 80%), and is comparable to that of ultrathin stainless-steel fi lms. [ 74 ] The dense network of weak interactions between the modestly aggregated silk fi broin and graphene oxide sheets being in intimate contact within a bilayer of about 5 nm thickness facilitates the formation of strong molecular interphase zones of confi ned silk material thus dramatically increasing the reinforcing effect. The model of interphase reinforcement shows an excellent match with the experimental data for the thickness of the interphase zone of about 1 nm, which is close to the backbone diameter of spread silk macromolecules “arrested” by the heterogeneous surface of graphene oxides during a fast (tens of milliseconds) assembly routine. It is worth noting that the micromechanical character-istics of these nanocomposites remains high over the course of long shelf-storage time.

With the outstanding micromechanical properties, fast fab-rication, potential enhanced electrical and thermal conduc-tivity, controlled permeability, and inherent biocompatibility, the ultrathin graphene oxide-silk fi broin nanocomposite mem-branes in their supported and freely standing states can be valuable for potential applications in bio-nanosensing devices, protective molecular coatings, cell protection and support, per-meable membranes for separation and delivery, energy har-vesting and ion separation, nanoporous biological and chemical fi lters, interfacial thermal management, and electromagnetic interference shielding coatings.

3. Experimental Section Materials Preparation : We prepared silk fi broin aqueous solution

from Bombyx mori silkworm cocoons by a conventional procedure, including splitting, degumming, dissolving, and dialysis. [ 19 ] The silk fi broin solution was collected after the dialysis and purifi ed twice by centrifugation (9000 rpm, 20 min, 5 ° C). The concentration of the resulting solution was 4.2 ± 0.5 wt%. The solution was diluted to the desired concentrations immediately and stored in a refrigerator (2 ° C). Graphene oxide suspensions were made following the Hummers method [ 7 , 11 ] (see Supporting Information). The condensed graphene oxide was redispersed (0.04 wt%) in water or methanol for spin-casting. All of the water used in this work was Nanopure water (18.2 M Ω cm, Barnstead).

All of the nanomembranes were fabricated by the spin-assisted layer-by-layer (SA-LbL) technique. [ 17 , 75 ] Initially, we deposited a sacrifi cial layer (100 nm) of polystyrene (PS) on a silicon wafer (14 mm × 25 mm). Then the silk fi broin aqueous solution (0.2 wt%) and graphene oxide suspension (0.04 wt%) were alternatively spun on the substrate (3000 rpm, 25 s) and the routine was repeated until the desired


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thickness (50 ± 10 nm) was reached. After each deposition of graphene oxide, the samples were rinsed with water to remove excessive material. The graphene oxide concentration in the nanomembrane was calculated by the equation:

φGO =

n · a · tGO

tf (2)

where Φ GO is the concentration of graphene oxide in the nanomembrane; n is the number of graphene oxide sheets, and α is the graphene oxide coverage percentage on each layer; t GO and t f are the thickness of single graphene oxide and the whole nanomembrane, respectively. To visualize silk fi broin adsorption on graphene oxide sheets, we mixed graphene oxide aqueous suspension (0.1 wt%) and silk fi broin solution (0.2 wt%) at a ratio of 5:1, then spin-cast the mixture on silicon.

Characterization : The thickness of the resulting nanomembrane was measured using an M2000 ellipsometer (Woollam) and an Icon AFM (Veeco) instrument. After the deposition, we fl oated the nanomembranes in water and freed them by dissolving the PS layer in toluene according to usual procedure. [ 75 ] All of the samples were dried overnight in a clean room at ambient temperature. ScanAsyst mode and soft tapping mode of the AFM were used to determine the topography, RMS roughness and the graphene oxide coverage of the nanomembranes. [ 76 ] Buckling and bulging tests were conducted according to the established experimental setups in our lab (see Supporting Information). [ 11 , 17 , 26 , 27 , 41 , 75 , 77 ]

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We thank the fi nancial support provided by the Air Force Offi ce for Scientifi c Research FA9550-11-1-0233 Grant.

Received: January 14, 2013Published online:

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