Yttrium-Doped Hydroxyapatite Membranes with High Proton

Apr 27, 2012 - texture of HAP seeds and Y-HAP dense film was evaluated by pole figures of the ... from zero degrees (normal to the substrate) to 85 de...
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Yttrium-Doped Hydroxyapatite Membranes with High Proton Conductivity Xue Wei† and Matthew Z. Yates*,† †

Department of Chemical Engineering and Laboratory for Laser Energetics, 206 Gavett Hall, University of Rochester, Rochester, New York 14627, United States ABSTRACT: Fully dense yttrium-doped hydroxyapatite membranes were synthesized having aligned crystal domains that span the membrane thickness. The membranes were grown by multistage hydrothermal crystallization onto a palladium substrate electrochemically seeded with hydroxyapatite nanocrystals. Synthesis conditions were chosen to promote the alignment of the crystallographic c-axis normal to the palladium substrate to promote proton transport through the membrane. The palladium substrate was used as an electrode for electrochemical characterization of the membrane, with platinum as the counter electrode. The measured proton conductivity of the novel membranes approaches 10−2 S/cm at 700 °C, a value higher by a factor of >100 than previously recorded for any apatite ceramic at similar conditions. The enhancement in proton conductivity is attributed to the combined effects of yttrium doping and the novel membrane microstructure that eliminates grain boundary resistance to proton transport. With a hydrogen atmosphere at the palladium electrode and dry air at the counter electrode, open circuit voltages of 0.9−1.0 V were measured. The high open circuit voltage indicates that there is minimal electronic conductivity through the membrane and that the membrane is a barrier maintaining separation of hydrogen and air. The high proton conductivity and good barrier properties suggest that the novel membranes may be effective in fuel cells and other electrochemical devices. KEYWORDS: hydroxyapatite, hydrothermal synthesis, proton conductors, membranes, coatings

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affects the number of hydroxyl groups in the material, as OH− is converted to O2‑ for electrical charge compensation. As the yttrium doping level increases, the proton conductivity is increased, and the activation energy for proton transport is reduced. The proton conductivity of bulk Y-HAP was shown to increase with yttrium doping until reaching a maximum value of ∼10−4 S/cm at 800 °C for yttrium-doped hydroxyapatite in which ∼7% of the calcium atoms were substituted with yttrium.7 Replacing greater than ∼7% of the calcium atoms with yttrium results in a decrease in overall conductivity and a transition from pure proton conduction to mixed oxygen/ proton conduction. A proof of concept fuel cell device was tested using Y-HAP as the proton conducting membrane.7 However, the maximum reported conductivity of Y-HAP is still too low to expect good performance in electrochemical devices unless the membranes can be made ultrathin to reduce the area specific resistance to an acceptable level.8 A novel alternative approach to improve proton transport of hydroxyapatite membranes is to engineer the membrane microstructure so that the c-axis of crystal domains passes through the membrane to minimize the transport path length

INTRODUCTION Hydroxyapatite (Ca10(PO4)6(OH)2) is most widely used as a bioceramic due to its compositional similarity to naturally occurring apatites in teeth and bones. At elevated temperatures, hydroxyapatite is a pure proton conductor with conductivities of 10−9−10−7 S/cm from 500 to 800 °C.1−3 The hexagonal hydroxyapatite crystals display highly anisotropic proton conductivity, with much faster transport along the crystallographic c-axis due to proton migration along hydroxyl groups that line that axis.4,5 Electrical polarization/depolarization measurements indicate an onset of proton mobility along the c-axis of hydroxyapatite crystals at temperatures as low as 200− 250 °C.5 However, grain boundary resistance limits the proton conductivity of bulk hydroxyapatite.1 As a result, the proton conductivity of traditional bulk sintered hydroxyapatite ceramics is too low for the material to be considered a candidate for use in electrochemical devices such as fuel cells or sensors that require rapid transport of ions. One approach to improve the conductivity of hydroxyapatite is to dope the crystals with ions such as fluorine or yttrium.4,6 Yttrium-doped hydroxyapatite (Y-HAP), with the chemical formula Ca10‑XYX(PO4)6(OH)2‑XOX, has been shown to have much higher proton conductivity than undoped hydroxyapatite.6 Yttrium ions (Y3+) substitute for calcium ions (Ca2+) in the hydroxyapatite crystal framework. Doping with yttrium © 2012 American Chemical Society

Received: November 10, 2011 Revised: April 24, 2012 Published: April 27, 2012 1738

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Hydroxyapatite seed crystals were deposited onto a clean Pd plate (12.5 × 12.5 × 0.1 mm) by electrochemical deposition. The Pd plate was used as the cathode, and the anode was a platinum plate (25 × 25 × 0.127 mm). The electrolyte solution consisted of 138 mM NaCl, 50 mM tris(hydroxymethyl)aminomethane, 1.3 mM CaCl2, and 0.84 mM K2HPO4 in deionized water. The solution was buffered to pH 7.2 using 37% hydrochloric acid. The electrochemical reaction was carried out at ∼95 °C for 5 min with constant current density of 9.3 mA/cm2. After the reaction, the Pd plate seeded with HAP crystals was taken out of the electrolyte solution, rinsed with deionized water several times, and dried in air. For hydrothermal crystallization, Na2EDTA was first completely dissolved into 30 mL of deionized water. Ca(NO3)2, Y(NO3)3, and (NH4)2HPO4 were successively added, and the solution was stirred for 30 min. The concentrations used for hydrothermal synthesis were 0.3 M Ca(NO3)2, 0.015 M Y(NO3)3, 0.3225 M Na2EDTA, and 0.09 M (NH4)2HPO4. The solution was adjusted to pH = 10.0 with ammonium hydroxide and then transferred to a Teflon-lined vessel (Parr Instruments model 4744) to immerse the seeded Pd substrate. The Pd substrate was positioned with the seed layer facing down and tilted up at ∼45° relative to the bottom of the vessel. The Teflon-lined vessel was placed into a convective oven for hydrothermal synthesis at 200 °C. Hydrothermal crystallization was repeated four times, and each crystallization step was carried out for 15 h at 200 °C. Between each crystallization step, the vessel was allowed to cool to room temperature. Then the sample was taken out, rinsed with deionized water, and placed back into the vessel with fresh synthesis solution. After the final reaction, the vessel was cooled to room temperature in air, and the sample was taken out, rinsed with deionized water several times, and dried in air. Morphology and elemental analysis of the products were examined using a field emission scanning electron microscope (FESEM, ZeissLeo DSM982) equipped with an energy dispersive X-ray spectrometer (EDX Phoenix). The crystal structure was determined by X-ray diffraction (XRD) (Philips PW3020) with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 20° to 40°. The crystallographic texture of HAP seeds and Y-HAP dense film was evaluated by pole figures of the (002) and (112) planes using a Philips Hi-Resolution X’PERT PRO X-ray diffractometer (PANalytical, Netherlands). The peak intensity was measured at every 5 degrees of tilt angle, starting from zero degrees (normal to the substrate) to 85 degrees. At each tilt angle, data were collected as the sample was rotated 360 degrees. In each XRD plot the intensity was normalized to the most intense peak. The proton conductivity of the membranes was measured by twopoint probe alternating current impedance spectroscopy (EIS 300, Gamry Instruments) over the frequency range from 300k Hz to 1 Hz. The Pd substrate served as one electrode. The counter electrode was a platinum layer (∼100 nm thickness) sputter coated on the upper surface of the Y-HAP membrane. Platinum leads were attached to both electrodes using platinum paint (SPI Supplies/Structure Probe, Inc.). The membrane was attached to the end of a ceramic tube using ceramic adhesive (Ceramabond 569, Aremco Products, Inc.), with the Y-HAP film facing outward. The sample was placed in dry air inside a tube furnace. To avoid palladium embrittlement, nitrogen was fed to the inside of the tube as it was heated. The gas flow was switched to hydrogen when the temperature reached 300 °C. The membrane was maintained at constant temperature for at least one hour prior to each measurement.

and to eliminate grain-boundary resistance. We reported the first synthesis of hydroxyapatite membranes with the desired alignment of crystal domains using a multistage crystallization onto seeded surfaces.9 The method used for microstructural engineering of hydroxyapatite is similar to that employed to align pores and improve mass transport properties of crystalline nanoporous zeolite and molecular sieve membranes.10 The synthesis method is practical for coating hydroxyapatite over large surface areas, onto substrates of irregular geometry, and onto a wide range of substrate types. The membranes containing crystal domains aligned with the c-axis normal to the surface displayed proton conductivity several orders of magnitude higher than traditional sintered hydroxyapatite membranes.9 However, the maximum measured hydroxyapatite conductivity of ∼10−4 S/cm at 700 °C is still less than state of the art proton conducting ceramics used as membranes in fuel cells and other electrochemical devices. More recently, an analogous microstructural engineering approach produced remarkable enhancement in proton conductivity of yttriumdoped barium zirconate membranes by eliminating grain boundaries.11 The crystal alignment in yttrium-doped barium zirconate membranes was only achieved by epitaxial growth on a specific crystal facet with good lattice matching. However, these recent results clearly show that microstructural engineering is worthy of greater investigation as a promising new approach for improving the performance of ion conducting ceramic membranes. We recently reported on the microstructural engineering of Y-HAP membranes grown on titanium substrates following methods similar to that developed for hydroxyapatite.12 It was found that yttrium can promote the formation of dense, wellcrystallized Y-HAP films on titanium with near perfect alignment of the crystal c-axis normal to the substrate. In the present study, we investigate the synthesis and characterization of dense Y-HAP membranes grown on palladium in order to characterize the proton conductivity of these novel Y-HAP films. Palladium was chosen as the supporting substrate because of its hydrogen permeation properties, catalytic activity, and its ability to act as an electrode during electrochemical characterization of the membrane.13,14 Hydrogen permeates through palladium by forming palladium hydride. In operating electrochemical devices, the palladium hydride can be converted to palladium metal, giving off protons and electrons in the process.14 The proton flux from the palladium electrode is high enough to give good performance in “hydrogen membrane fuel cells”, in which the palladium acts as the anode.14 For fuel cell operation, the palladium is coated with a proton conducting ceramic membrane and an appropriate cathode layer.13 Growth of Y-HAP membranes with good proton conductivity onto palladium is the first step in developing an apatite-based hydrogen membrane fuel cell.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Electrochemical deposition was used to coat the Pd substrate with hydroxyapatite seed crystals. The electrochemical crystallization technique was originally developed for coating titanium and stainless steel to improve biocompatibility of orthopedic implants.15,16 Growth of hydroxyapatite onto a metal substrate acting as a cathode is achieved by passing a direct current through an aqueous solution containing calcium and phosphate ions. Electrolysis of water causes a local increase

Palladium (Pd) foil (0.1 mm thick), platinum (Pt) foil (0.127 mm thick), and yttrium nitrate hexahydrate (99.9% purity) were obtained from Alfa Aesar. (NH4)2HPO4 (>99.0% purity) was purchased from EMD. K2HPO4 (99.99% purity), CaCl2·2H2O (99+% purity), 37% hydrochloric acid, and 28.0−30.0% ammonium hydroxide were purchased from Mallinckrodt Chemicals. NaCl (≥99.0% purity), tris(hydroxymethyl)aminomethane (99.8+% purity), Ca(NO3)2·4H2O (99.0% purity), and disodium ethylenediaminetetraacetate dihydrate (Na2EDTA·2H2O) (99.0−101.0% purity) were all obtained from Sigma-Aldrich. 1739

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microscope. For the hydroxyapatite seed layer, the Ca/P ratio was measured to be 1.56, compared to the Ca/P ratio in the synthesis solution of 1.67. Calcium deficiency in the deposited seed crystals is consistent with other studies of hydrothermal synthesis of hydroxyapatite.18,19 The Ca/Y ratio in the Y-HAP membrane was measured to be 14.5, which is very close to the value of 14.4 previously reported to give rise to maximum proton conductivity in bulk Y-HAP.7 The concentration of yttrium in the film is enhanced relative to the starting synthesis solution that had a Ca/Y ratio of 20. As recently reported, the Ca/Y ratio in the Y-HAP film is not directly correlated to the Ca/Y ratio used for hydrothermal synthesis.12 The crystal structure and orientation were probed with powder X-ray diffraction (XRD), as shown in Figure 2. The

in pH at the surface of the cathode and an enhanced concentration of calcium ions by electrostatic attraction. As a result, the solution becomes locally supersaturated with respect to hydroxyapatite, causing crystals to nucleate and grow selectively on the cathode surface. Electrolysis of water also causes hydrogen gas evolution at the cathode surface. With palladium as the cathode, the deposition time and electric current density must be kept low to avoid palladium embrittlement due to exposure to hydrogen.17 Figure 1(a) is

Figure 1. Scanning electron micrograph of (a) hydroxyapatite seed layer and (b) fully dense yttrium-doped hydroxyapatite on a palladium substrate.

a scanning electron microscopy (SEM) image showing the topview of hydroxyapatite crystals electrochemically grown on palladium at 95 °C using a DC current density of 9.3 mA/cm2 for 5 min. The rod-shaped crystals are ∼150 nm in width and ∼1 μm in length. The morphology of the hydroxyapatite seed crystals on palladium is similar to that obtained by electrochemical deposition on titanium and stainless steel, where the long axis of the hydroxyapatite rods is associated with the crystallographic c-axis.15,16 To form a dense Y-HAP membrane onto the seeded surface, we used a hydrothermal crystal growth method mediated by ethylenediaminetetraacetic acid disodium salt (EDTA). The EDTA was added to chelate calcium and yttrium ions in solution to control supersaturation of Y-HAP in order to promote surface crystal growth by limiting homogeneous crystal nucleation from solution.9 As the solution is heated, it slowly becomes supersaturated with respect to Y-HAP due to the release of free ions of calcium and yttrium as the EDTA complexes break down. The Y-HAP crystals grow onto the seeded palladium surface as well as homogeneously nucleate from solution. It was found that crystal growth on the seeded substrate is initially rapid but slows and eventually stops after ∼10−15 h at 200 °C due to depletion of reagents.12 Therefore, hydrothermal growth steps were repeated to obtain fully dense coatings. Figure 1(b) shows the fully dense Y-HAP coating on palladium after four repeated hydrothermal growth steps of 15 h (60 total hours of crystallization at 200 °C). After each hydrothermal reaction, the sample was allowed to cool to room temperature and rinsed with water, and fresh reagents were added to continue crystal growth. The image in Figure 1(b) shows a side-view of a broken section of the Y-HAP membrane on the palladium surface. The crystal domains are intergrown to form a continuous membrane, approximately 30 μm thick, which coats the entire surface. On the upper surface, some hexagonal facets of individual crystal domains are visible, indicating c-axis alignment normal to the substrate. The elemental composition of the hydroxyapatite seeds and Y-HAP membranes was probed with an energy dispersive X-ray spectrometer (Phoenix EDX) attached to the electron

Figure 2. Powder XRD patterns of (a) hydroxyapatite seeds and (b) fully dense Y-HAP membrane on palladium. The XRD peaks are identified from that of the standard hydroxyapatite powder pattern (JCPDS card 09-0432).

diffraction pattern of the seed layer is consistent with hydroxyapatite, with no evidence of secondary crystal phases. The diffraction pattern for the seed layer shows the shoulder of a very broad peak near 23 degrees that indicates some amorphous material is present. For the hydrothermally crystallized Y-HAP membrane, there is no evidence of amorphous material remaining. The most notable feature of the Y-HAP membrane diffraction pattern is the remarkable enhancement of the (002) peak. Enhancement of the (002) peak is consistent with preferred orientation of the c-axis normal to the substrate. The degree of preferred orientation can be probed by calculating the texture coefficient of the (002) peak, Tc(002), defined as20,21 Tc(002) =

I(002)/Ir(002) 1 [∑ I(hkl)/Ir(hkl)] n

where I(hkl) are the peak intensities measured from films, Ir(hkl) are the intensities from the XRD reference peaks (JCPDS card 09-0432) of randomly oriented hydroxyapatite powders, and n is the number of diffraction peaks considered. Therefore, a randomly oriented powder would have Tc(002) = 1, and preferential orientation of the c-axis normal to the substrate would result in Tc(002) > 1. The value of Tc(002) was calculated from the X-ray diffraction data relative to five other diffraction peaks corresponding to (102), (211), (112), (300), and (202) planes. Since a total of 6 peaks was considered (n = 6), the maximum possible value of Tc(002) = 6 would indicate perfect 1740

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alignment of the c-axis normal to the substrate. For the hydroxyapatite seed layer, Tc(002) = 1.6, which indicates some preferred crystal orientation of the c-axis normal to the substrate. The additional crystallization during hydrothermal growth greatly enhanced the preferred crystal orientation. For the fully dense Y-HAP membrane, Tc(002) = 5.9, indicating near perfect alignment of the crystal domains since the texture coefficient is near the maximum value. The texture coefficient analysis is consistent with the SEM observations in Figure 1 that show rod-shaped seed crystals preferentially oriented normal to the substrate and show hexagonal facets on the topview image of the Y-HAP membrane. Figure 3 shows three-dimensional XRD pole scans of the (002) and (112) peaks of the hydroxyapatite seed layer and Y-

seed crystals show preferential orientation of the (112) planes normal to the substrate, as the pole figure shows maximum (112) peak intensity at zero tilt angle. For the fully dense YHAP film, in contrast, the (112) peak intensity contour plot shows a ring of maximum intensity at 35° tilt angle. These observations verify the preferential orientation of the c-axis normal to the substrate.22 It is interesting to note that the misaligned seed crystals do not significantly contribute to the X-ray diffraction from the Y-HAP membranes, indicating the volume fraction of misaligned crystals is very low in the portion of the final membrane sampled by X-ray diffraction. It is possible that some of the seed crystals were consumed through a dissolution/reprecipitation process during the long hydrothermal reaction or that growing crystals were sterically selfaligned during film growth as individual crystals became larger. The proton conductivity of the Y-HAP membrane was measured at temperatures from 300 to 700 °C. The measurement was carried out with the membrane exposed to dry air. The palladium foil was sealed at the end of a ceramic tube to which hydrogen was fed. The bulk resistance of the membrane was determined from real values in Nyquist plots by extrapolation of impedance arcs,11,23 as shown in Figure 4(a). The proton conductivity was calculated from the equation σ = l/(R*A), where σ is the proton conductivity, l is the thickness of the membrane, A is the contact area of the membrane with Pt electrode, and R is the bulk resistance from the Nyquist plot.

Figure 3. Pole figures showing relative intensity of X-ray diffraction from the (002) and (112) planes of hydroxyapatite seed crystal layer (left) compared to the Y-HAP membrane (right), both on a palladium substrate. The inset shows the polar coordinate system of the measurement that was converted to Cartesian coordinates in the x−y plane of the plots.

HAP membrane. The peak intensity was measured at every 5 degrees of tilt angle, starting from zero degrees (normal to the substrate) to 85 degrees. At each tilt angle, data were collected as the sample was rotated 360 degrees azimuthally. The x−y plane in Figure 3 is in Cartesian coordinates obtained by converting the polar coordinates of tilt angle and rotation angle in degrees. For perfectly aligned crystal domains with the c-axis normal to the substrate, the (002) pole figure contour would appear as a point of maximum intensity at zero degree tilt angle, and falling to zero intensity as the sample is tilted. The seed layer has significant reflections from (002) planes at all tilt angles measured. This indicates that some fraction of the seed crystals are oriented with the c-axis normal to all tilt angles measured. For the Y-HAP membrane, the (002) peak intensity is maximum at zero degrees and falls markedly as the sample is tilted, indicating strong alignment of the c-axes normal to the substrate. The results show that nearly all of the crystal domains in the fully dense Y-HAP film are oriented within 15 degrees of normal to the substrate, since there is little measurable intensity from the (002) peak at tilt angles of 15 degrees or higher. The

Figure 4. (a) Nyquist plots and (b) proton conductivity of Y-HAP membrane on palladium surface. 1741

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K).28 The difference of 2.8 μm/(m K) between thermal expansion coefficients of hydroxyapatite and palladium is therefore larger than the difference of 2.0 μm/(m K) between the most common solid oxide fuel cell membrane and its anode support. In addition, palladium will further expand upon exposure to hydrogen due to hydride formation. As a result, thermal cycling will lead to cracks in the hydroxyapatite coating. Thermal stress may possibly be reduced by growing Y-HAP on the concave side of a curved palladium surface. In that way, the expansion of palladium would result in some compressive stress on the Y-HAP rather than pure tensile stress. In addition, it is possible that the Y-HAP membrane synthesis could be extended to ceramic anode substrates that have much lower mismatch in thermal expansion.

During impedance testing, the measured open circuit voltage (OCV) values were between 0.9 and 1.0 V, while the theoretical Nernst potential values at 400 and 600 °C are 1.09 and 1.04 V respectively.24 The high OCV indicates that there is no significant contribution from electronic conduction and proton conductivity is predominant. In addition, the high OCV indicates the Y-HAP membrane was acting as a barrier to hydrogen permeation. At 300 °C, the measured conductivity increased from ∼10−6 to ∼10−5 S/cm when switching nitrogen to hydrogen flow into the tube. Earlier studies of proton conductivity of apatites using palladium electrodes noted a similar enhancement in measured conductivity in hydrogen atmosphere.4 The enhancement in conductivity is attributed to the increased concentration of protons due to catalytic oxidation of hydrogen by palladium. The proton conductivity increased steadily with increasing temperature above 300 °C, approaching 10−2 S/cm at 700 °C. The proton conductivity exceeds the highest recorded value for an apatite material by a factor of >100, and it compares favorably with many proton conducting ceramics used in fuel cells and other electrochemical devices.25 The linear curve fit to the data as shown in Figure 4(b) was used to calculate an activation energy for proton conduction of 0.73 eV. The activation energy is significantly lower than the value of 1.0 eV measured for proton conductivity of bulk Y-HAP having nearly the same composition as our membrane.7 We speculate that the enhancement in conductivity and the decrease in activation energy are attributed to the reduction in grain boundaries. Polarization/depolarization studies have indicated that hydroxyapatite grain boundaries act as traps for protons.1,5 Previous studies of hydroxyapatite proton conductivity had significantly higher contribution of grain boundaries to the impedance measurements. The novel apatite membrane structure reported here is near ideal for promoting proton transport and eliminating grain boundary resistance. It should be noted that hydroxyapatite proton conductivity is time-dependent in dry air due to dehydroxylation of the samples at high temperature. Dehydroxylation causes vacancies of the OH− groups lining the c-axis of the crystal domains. The vacancies initially enhance proton conductivity, but increasing fraction of vacancies significantly suppresses conduction. The changes in conductivity in dry air were observed in a prior study to occur over tens to hundreds of hours at 700−800 °C.1 Conductivity measurements on Y-HAP were not carried out long enough to examine time dependent changes due to dehydroxylation. The dehydroxylation was shown to be reversed by the introduction of steam, and the original conductivity restored.1 Future experiments are planned to investigate the conductivity of the Y-HAP membranes as a function of partial pressure of steam in air. Steam may allow the fraction of hydroxyl vacancies to be controlled in order to further enhance conductivity. Practical applications of the Y-HAP membranes supported on palladium are hindered to some extent by the mismatch in thermal expansion coefficients. The thermal expansion coefficient of Y-HAP is not known but can be expected to be similar to undoped hydroxyapatite. At 600 °C, the thermal expansion coefficient of palladium is 15 μm/(m K), while it is 12.2 μm/(m K) for hydroxyapatite along the a-axis.26,27 In comparison, yttria-stabilized zirconia, the most commonly used membrane in solid oxide fuel cells, has a thermal expansion coefficient of 10.5 μm/(m K) and is typically used with an anode having a thermal expansion coefficient of 12.5 μm/(m

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CONCLUSIONS The high proton conductivity achieved by optimizing Y-HAP membrane microstructure suggests new applications of hydroxyapatite in a variety of electrochemical devices such as fuel cells, hydrogen pumps, and electrochemical sensors. Excellent performance has been demonstrated in the literature for fuel cells using Pd as the anode and proton conducting ceramics as the membrane.13,14,29 Growth of the Y-HAP film directly onto Pd is the first step in using the Y-HAP membranes in hydrogen fuel cells with a Pd anode. The high proton conductivity measured for Y-HAP is directly related to the near perfect alignment of the crystallographic c-axis of Y-HAP normal to the Pd surface. The Y-HAP membrane microstructure promotes proton transport through the membrane by minimizing the proton transport path length and eliminating grain boundary resistance. In addition, the microstructure is ideal for electric-field induced polarization along the c-axis to produce hydroxyapatite electrets that have been shown to improve osteoconductive properties in biomedical implants.30 The microstructural engineering technique can potentially be used to enhance the ionic conductivity of other apatites, including those recently discovered to have fast oxygen ion conduction.31

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-585-273-2335. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge NSF (CMMI-0856128), the DOE through the Laboratory for Laser Energetics (DE-FC03-92SF19460), and the University of Rochester for supporting this research.

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REFERENCES

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