Kelvin Probe Force Microscopy Analysis of the Covalent

May 22, 2012 - Kelvin probe force microscopy (KPFM) was used to analyze the extent of plasma cleaning and how it affected the functionalization that ...
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Kelvin Probe Force Microscopy Analysis of the Covalent Functionalization and DNA Modification of Gallium Phosphide Nanorods David N. Richards,† Dmitry Y. Zemlyanov,‡ and Albena Ivanisevic*,§ †

Weldon School of Biomedical Engineering and ‡Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States § Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27606, United States ABSTRACT: The growth, covalent functionalization, and subsequent DNA modification of gallium phosphide (GaP) nanorods is presented. Analysis of the nanorods by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) revealed important information regarding their physical properties such as the presence of twinning defects. The nanorods were deposited onto glass substrates for further functionalization and biomolecule immobilization. Plasma cleaning was employed to remove the surfactant present on the nanorods’ surfaces. Kelvin probe force microscopy (KPFM) was used to analyze the extent of plasma cleaning and how it affected the functionalization that employed thiol chemistry. KFPM analysis of the subsequent modification of functionalized nanorods with single-stranded DNA (ssDNA) revealed that immobilization was dependent on the amount of plasma cleaning to which the nanorods had been exposed. Nanorods were then exposed to the cDNA strand and KPFM was again used to detect successful hybridization.

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INTRODUCTION The functionalization of semiconductor surfaces with organic molecules has become an increasingly popular topic because of its potential applications in the area of devices, especially biosensors. A large focus lies on silicon due to its wellcharacterized properties and variety of functionalization schemes.1,2 However, with the advancement of technology comes the advent of continuously smaller devices. At these smaller scales, silicon’s normally advantageous properties begin to break down.3 Furthermore, silicon’s band-gap energy is not suited for efficient biosensor devices.4 Therefore, the next generation of electronic devices will rely on a different variety of semiconductors, namely, III−V semiconductors. Gallium phosphide (GaP) is a III−V semiconductor with excellent inherent properties such as carrier mobility and has demonstrated favorable biocompatibility in the past.5 The main challenge with making III−V semiconductors, such as GaP, valuable for future biosensing devices is developing schemes for biomolecule immobilization. The covalent functionalization of a planar GaP surface with both thiol and terminal alkene organic linker molecules has been demonstrated by our group in the past.6 In another study, we used Kelvin probe force microscopy (KPFM), a derivative of atomic force microscopy that is capable of mapping the surface potential, to demonstrate the functionalization of planar GaP with a terminal alkene cross-linker using microcontact printing followed by the immobilization of single-stranded DNA and its complement.7 It was concluded that the concentration of the linker molecule solution that reacted with the GaP surface strongly affected the orientation of the DNA. © 2012 American Chemical Society

Nanorods are more advantageous to future biosensing devices compared to planar surfaces due to their advantageous electrical properties, their size, and their large surface area to volume (S/V) ratio, which allows for a maximum amount of biomolecule immobilization.8 Greater coverage of biomolecules on a semiconducting surface allows for greater sensitivity of a field-effect biosensing device, for example. Indeed, DNA immobilization on silicon nanorods has been successfully demonstrated by use of X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy.2 Biomolecule immobilization on III−V semiconductor nanorods is also feasible, and devices that incorporate them have demonstrated excellent sensitivity.9 Recent studies have demonstrated that KPFM on nanorods can reveal important information such as the location of electrical trapping centers10 and dopant distribution.11 In this study, we evaluate the progressive functionalization and immobilization of DNA on GaP nanorods via KPFM. The KPFM technique is once again appropriate for this study since thiol molecules have been shown to elicit a pronounced KPFM response.12 It also proved useful for investigating the effects of plasma cleaning on the removal of surfactant from the nanorods. We used 11-aminoundecanethiol as the cross-linker whose terminal amine group can provide for biomolecule immobilization. DNA was chosen as a representative biomolecule because it is a negatively charged molecule and therefore provides good KPFM contrast. In a recent study, Received: March 9, 2012 Revised: May 18, 2012 Published: May 22, 2012 12613

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Figure 1. Reaction scheme for the functionalization of GaP nanorods and subsequent modification with DNA.

surface potential mapping of DNA lying flat on a surface shows a distinct negative charge surrounded by the positively charged buffer salts.13 The thiol-functionalized GaP nanorods were exposed to solutions of single-stranded DNA (ssDNA) with and without buffer salts. Finally, select ssDNA-immobilized GaP nanorods were exposed to the cDNA strand. Figure 1 shows a schematic representation of the immobilization process.

was transferred via micropipet onto the surface of the patterned glass slide. A normal glass slide was placed over the patterned glass slide, which forced the solution to spread evenly. The glass slides were separated and the solution was allowed to air-dry. (Note: A patterned glass slide was utilized in order to locate the same nanorods after functionalization and modification steps.) The nanorod-coated glass slides were exposed to air plasma for 2.5−5 min by use of a PDC-32G plasma cleaner (Harrick Plasma, Ithaca, NY) to remove the nanorods’ surfactant layer as well as the residue that remained from the air-drying process. The nanorod samples were placed face-up in a 500 μM solution of 11-aminoundecanethiol (AUD) (Dojindo, Kumamoto, Japan) in ethanol. The samples remained in the cross-linker solution for 24 h. The sample was removed and immediately placed in a beaker of pure ethanol for 15 s and subsequently dried with N2. The functionalized nanorod samples were primed by placing them in a 1 mM solution of disuccinimidyl glutarate (DSG) (Thermo Scientific, Waltham, MA) in dimethyl sulfoxide (DMSO) for at least 20 h. Again, the samples were dipped in pure ethanol for 15 s and dried with N2. Cross-linker-functionalized nanorod samples that were selected for DNA immobilization with buffer salts were placed face-up in a phosphate-buffered saline (PBS) solution of 10 μM ssDNA with a 6-carbon amine spacer (5′-amine-C6TTAAGGTCTGGACTGGCCTG-3′) (IdtDNA, Coralville, IA). The samples were placed in ethanol for 15 s and dried with N2. Functionalized nanorod samples that were selected for DNA immobilization without buffer salts were placed face-up in a 10 μM ssDNA solution (same sequence as above) in 90%/ 10% dimethylformamide (DMF)/water. The sample was dipped in deionized (DI) water and dried with N2. In either case, each sample was incubated in the respective DNA solution for 24 h. The samples that were incubated in DNA without buffer salts were exposed to a 1 μM solution of the ssDNA complementary strand (5′-CAGGCCAGTCCAGACCTTAA-3′) along with 0.12 M NaCl and 0.025% sodium dodecyl sulfate (SDS) in

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MATERIALS AND METHODS All chemicals were purchased from Sigma Aldrich (St. Louis, MO) unless otherwise noted. Growth of Nanorods. GaP nanorods were grown by a solution−liquid−solid method developed by Liu et al.14OctaOctadecene (ODE; 10 mL of 90% solution) and 0.9 mmol of myristic acid were added to a 50 mL three-neck flask inside a glovebox. The flask was fitted with a cross-flow water condenser and placed on a basket heater. The O2 concentration within the glovebox was 1−2 ppm during nanorod growth. The contents of the flask were heated until boiling (∼310 °C). At this point, 2 mL of ODE, 0.9 mmol of triethylgallium (STREM Chemicals, Newburyport, MA), and 0.1 mmol of tris(trimethylsilyl)phosphine (Alfa Aesar, Ward Hill, MA) were mixed in a separate beaker. A glass syringe was used to transfer the contents of the beaker to the boiling solution in the three-neck flask. Upon addition of the beaker solution, the temperature within the flask dropped to ∼295 °C. The reaction proceeded for 5 min and the basket heater was removed. The flask was allowed to cool to room temperature until it was removed from the glovebox. The nanorod solution was subjected to multiple washing and centrifugation cycles with hexane. The nanorods were suspended in toluene via ultrasonication and stored in glass vials until further use. Nanorod Functionalization. A microetched patterned glass slide (Electron Microscopy Sciences, Hatfield, PA) was degreased via ultrasonication in methanol and ethanol, followed by drying with N2. An aliquot (20 μL) of the nanorods solution 12614

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Figure 2. (A) SEM image of a cluster of GaP nanorods (scale bar is 1 μm). (B) TEM image of a single GaP nanorod showing a layer of surfactant (scale bar is 5 nm). (C) TEM image of a single GaP nanorod showing twinning defects (scale bar is 20 nm). (D) XRD spectrum of a solid GaP nanorod sample.

GaP cluster dissolves into the gallium cluster and nanorod growth proceeds.14 Figure 2A depicts an SEM image of a cluster of as-grown GaP nanorods. Prior to any modification of the nanorods, the seed particles were removed with multiple cycles of washing in hexane followed by centrifugation. The as-grown GaP nanorods were also easily deposited onto a TEM grid for further analysis. TEM imaging revealed interesting details. First of all, it was determined that each rod is terminated in a relatively thick layer of surfactant that is most likely composed of myristic acid (see Figure 2B). Second, the nanorods possess a significant amount of twinning defects, which are easily identifiable in Figure 2C by the contrasting bands. Twinning defects are imperfections in the crystal structure whereby the crystallographic orientation abruptly changes. Other studies have noted the propensity for GaP nanorods to experience twinning15 and especially when they are grown by a solution−liquid−solid method similar to the one we report in this study.16 In fact, twinning in all zinc-blende structures is common due to the low energy required for twin formation.17 Finally, in order to identify that we indeed have GaP present in the rod structures, an XRD spectrum of a solid sample of the as-grown nanorods was acquired and is presented in Figure 2D. The positions of the peaks corresponding to the (111), (220), and (311) directions are indicative of GaP.18 The GaP nanorods can be dispersed evenly by micropipetting a drop of the nanorod solution on a surface, followed by placing a glass coverslip over the drop and allowing it to spread. This method allowed us to image individual nanorods by atomic force microscopy. In order to analyze a large sample

80%/20% DMF/water. The samples remained in the solution for 1 h, followed by rinsing with DI water and drying with N2. Surface Characterization. Kelvin probe force microscopy was performed with an Asylum Research Cypher atomic force microscope (Santa Barbara, CA). All KPFM images were collected at 3−6 V electric drive amplitude with a lift height of 2−5 nm using conducting tips (model ASYELEC, nominal resonant frequency 70 kHz, Ti/Ir coated) purchased from Asylum Research. All KPFM experiments were conducted under ambient conditions. Transmission electron microscopy (TEM) images were acquired on a Hitachi HF2000 capable of 0.2 nm resolution. Scanning electron microscopy (SEM) images were collected with a FEI NOVA nanoSEM at 5 kV accelerating voltage and a working distance of approximately 4 mm. The SEM is equipped with high-vacuum Everhardt−Thornley (ET) and through-thelens detectors (TLD). X-ray diffraction (XRD) was performed by use of a Bruker D8 Focus X-ray diffractometer with a Cu Kα source with a 3-circle goniometer and Lynseye 1D detector.

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RESULTS AND DISCUSSION

The as-grown GaP nanorods were deposited onto a silicon surface and subsequently imaged by SEM. This analysis revealed a heterogeneous mixture of nanorods with respect to diameter and length. Each rod was also capped with a seed particle that is composed solely of gallium. It is hypothesized that the gallium precursor (triethylgallium) decomposes into gallium clusters, while a side reaction between triethylgallium and the phosphorus precursor [tris(trimethylsilyl)phosphine] react to form an amorphous GaP cluster. Upon contact, the 12615

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Figure 3. Three-dimensional topographic image and corresponding phase image of an as-grown GaP nanorod on a glass substrate, (A) before and (B) after exposure to plasma cleaning. Scale bars are 250 nm.

size of nanorods and conduct this study in a timely manner, 20 μL of the nanorod solution was micropipetted onto the glass substrate surface, which left approximately five nanorods per 30 μm × 30 μm area. This process left a relatively thick layer of dried toluene residue on the surface of the glass. Any solutionbased method to remove this residue layer resulted in stripping of the nanorods from the surface as well as the residue. Plasma cleaning with air plasma proved to be the most effective method for removing the toluene residue as well as the surfactant from the nanorods’ surface. As mentioned earlier, the surfactant on the nanorods’ surface poses a problem for functionalization. Figure 3A depicts a 3D topographic image and phase image of an as-deposited (after seed particle removal) nanorod on a glass surface before any plasma cleaning. To capture this image, a single 2.5 μL drop of the nanorod solution was micropipetted onto the surface. The nanorod was found by chance in a break in the residue. The corresponding phase map demonstrates the contrast between the surfactant and the GaP nanorod. Figure 3B depicts the same nanorod after 1 min of exposure to plasma on the “high” power setting. The plasma cleaning provided the ability to remove the surfactant layer without providing any appreciable damage to the structure of the nanorod. In experimenting with the plasma cleaner, AFM topographic imaging determined that 2.5 min of exposure to the plasma on the “low” power setting was sufficient to remove the toluene residue enough to expose the underlying nanorods but left some visible surfactant. Exposure to plasma on the “high” power setting for 2.5 min removed more of the residue (and all visible surfactant), and exposure to plasma on high for 5 min removed the residue and visible surfactant completely. Functionalization and DNA modification proceeded on nanorods that had been exposed to each of these conditions in order to evaluate whether the presence (or absence) of surfactant is a factor.

The KPFM technique measures the difference in potential between the probe tip and the sample. When an AC voltage is applied to the tip, the tip vibrates due to electrostatic forces. A separate DC voltage is applied to the tip to nullify those forces and thus stop the tip vibrations. This applied DC voltage corresponds to the surface potential. Due to the nature of the imaging process, a topographic image is also obtained in addition to the surface potential map. After exposure to plasma cleaning, the nanorods were analyzed by KPFM. A 1 μm × 1 μm image was obtained for each nanorod. The surface potential of each nanorod was determined by subtracting the average potential of the surrounding glass substrate from the average potential of the nanorod itself. The surface potential of the plasma-cleaned nanorods was always higher compared to the glass substrate regardless of the plasma cleaning conditions. This result is to be expected since the glass substrate is composed mostly of silica (SiO2), a substance that has a naturally negative potential. Figure 4 is a histogram showing the average postplasma cleaning surface potential values for each plasma cleaning condition (error bars are the result of the analysis of multiple nanorods). For cases in which the plasma was set to high, the postplasma cleaning surface potentials are essentially the same (∼2−3 mV). However, for the nanorods exposed to plasma on the low setting, the surface potential is considerably higher, roughly 14 mV. These data indicated that the high-power plasma cleaning is indeed removing more of the myristic acid surfactant than the low-power plasma cleaning. Previous studies have demonstrated that the presence of one -CH2- can contribute between 9 mV19 and 14 mV12 of potential. However, the presence of residual surfactant was not expected to pose a problem for subsequent functionalization with AUD because thiols have demonstrated the ability to ligand-exchange with surfactant layers.20 The glass substrates were subsequently immersed in a 500 μM ethanolic solution of AUD for 24 h. The same nanorods were analyzed by KPFM within 24 h of being removed from the 12616

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After AUD functionalization, the surface potential increases in both cases in which the plasma was on high power. Therefore, these data suggest that there is at least some degree of covalent functionalization of the nanorod surface with AUD. However, the relatively low value of the surface potential (∼7 mV) indicates that the AUD is not forming a complete adlayer but rather lying more parallel to the nanorods’ surfaces. In the case of the low-power plasma cleaning, the surface potential did not change. Therefore, the likely scenario is that ligand exchange is occurring between the thiol and the remaining surfactant. Since the myristic acid surfactant and AUD are nearly identical in the amount of -CH2- groups present, we would expect to see little change. On the basis of findings presented later in this study, the scenario that the myristic acid surfactant layer was simply unaffected by the AUD is not plausible. After KPFM analysis, all of the nanorod-coated glass substrates were placed in a 1 mM solution of DSG to prime the amine terminus of the AUD. The substrates that were exposed to plasma on high power were immediately placed in a 10 μM PBS solution of amine-terminated ssDNA. A topographic analysis of the nanorods that were exposed to 2.5 min of plasma at high power after incubation in DNA revealed large deposits that were localized solely on the nanorods. Figure 5 depicts a 3D topographic progression of one nanorod along with the surface potential from KPFM after 2.5 min of high-

Figure 4. Histogram depicting the percent change in surface potential of individual GaP nanorods from plasma cleaning to AUD functionalization.

AUD solution. Figure 4 also summarizes the average values after AUD functionalization for each of the plasma cleaning conditions.

Figure 5. Three-dimensional topographic images with corresponding potential images of the same GaP nanorod on a glass substrate, (A) after 2.5 min of high-power plasma cleaning, (B) after AUD functionalization, and (C) after ssDNA modification. The line scans correspond to the red line in the surface potential images. Scale bars are 250 nm. 12617

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gallium and phosphorus oxides. The twinning defects seen in Figure 2 can affect the mechanical properties and are thus partly responsible for the observed degradation.21 Nanorods that had been exposed to 2.5 min of low-power plasma cleaning and primed with DSG were subjected to 10 μM of the same amine-terminated ssDNA in a solution of 90%/ 10% DMF/water.22 We chose to incubate these nanorods in this solution because it did not contain any salt that would screen the DNA’s negative charge. The results above indicated that lesser plasma cleaning provided better DNA modification. Therefore, we expected to see a decrease in the surface potential after DNA modification for nanorods subjected to 2.5 min low-power plasma cleaning that would contrast well with the relatively high post-AUD functionalization surface potential demonstrated by Figure 4. Indeed, for all nanorods that had been subjected to the DNA solution without salt, there was a considerable decrease in potential after DNA modification, and the large deposits that were visible in Figure 5 were lacking. Figure 7 is a histogram depicting the change in surface potential

power plasma cleaning (panel A), after AUD functionalization (panel B), and after DNA modification (panel C). The positive potential of the deposits in Figure 5C suggests that the deposits are salt that are screening the negative charge of the DNA backbone that lies underneath.13 The selectivity toward nanorods suggests that the DNA is covalently binding to the primed AUD cross-linker. Overall, KPFM analysis of all nanorods exposed to 2.5 min of plasma at high power indicated an average surface potential of 9.2 ± 6.1 mV. This small change from approximately 7 mV (from Figure 4) obtained for nanorods that were AUD-functionalized is expected since the negative charge from the DNA is neutralized by the presence of the salt counterions. These nanorods were not subjected to the complementary strand since the immobilized DNA was obviously unavailable for hybridization. A control experiment was conducted by exposing AUDfunctionalized nanorods (that had been cleaned with plasma at high power for 2.5 min) to the same buffer salt solution without DNA. These nanorods did not accumulate the large salt deposits seen in Figure 5. Furthermore, nanorods that were exposed to plasma at high power for 5 min showed no accumulation of salt deposits either. The extra amount of exposure to the plasma at high power (compared to the nanorods exposed for 2.5 min at high power) likely resulted in a detrimental amount of oxidation of the nanorods’ surfaces and inhibited a sufficient amount of AUD from covalently binding to gallium atoms. Furthermore, there were signs of disintegration after DNA modification, which was not visible with the nanorods exposed to DNA and plasma at high power for 2.5 min (Figure 6). Again, the disintegration is likely the result of insufficient binding of the AUD and DNA to the surfaces of the nanorods, leaving them susceptible to oxidation and subsequent dissolving of the

Figure 7. Histogram depicting the individual surface potential difference between ssDNA-modified nanorods and AUD-functionalized nanorods (blue bars). Red bars depict the surface potential difference between nanorods that were exposed to ssDNA and plasmacleaned nanorods without AUD functionalization as a control experiment.

for individual nanorods from post-AUD functionalization to post-DNA modification (blue bars). The relatively large variation in the magnitude of the surface potential change can be correlated with the amount of DNA binding to the surface and the orientation of the DNA. Interestingly, the nanorods with the highest post-AUD functionalization surface potential did not elicit the greatest decrease in potential after DNA modification. This was not expected, since a high postAUD functionalization was thought to correlate with high surface coverage. However, this phenomenon can be explained by the large variance in the structure of each nanorod. Indeed, some nanorods had rougher surfaces (and greater surface area) and more twinning defects than others and, when functionalized with AUD, produced a high surface potential but failed to provide open binding sites for the DNA. Figure 7 also depicts the results from a control experiment in which three different nonfunctionalized plasma-cleaned (2.5 min at low power) nanorods were exposed to a 10 μM solution of ssDNA in 90%/10% DMF/water (red bars). In each case,

Figure 6. Three-dimensional topographic images of a nanorod exposed to (A) plasma at high power for 5 min post-AUD functionalization and (B) post-DNA modification. Scale bars are 250 nm. 12618

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Figure 8. Three-dimensional topographic images with corresponding potential images of the same GaP nanorod on a glass substrate, (A) after 2.5 min of low-power plasma cleaning, (B) after AUD functionalization (B), (C) after ssDNA modification, and (D) after hybridization with the cDNA strand. Line scans correspond to the red line in the surface potential images. Scale bars are 250 nm.

cleaning conditions had a considerable effect on the nanorods’ surface potential and subsequent functionalization. High-power plasma cleaning resulted in an average nanorod surface potential of 3 mV (for both 5 and 2.5 min exposure times), while low-power plasma cleaning elicited a surface potential of approximately 13 mV. After functionalization with AUD, the surface potential increased to 7 mV for nanorods that were exposed to plasma at high power and remained at 13 mV for those exposed to plasma at low power. Subsequent modification with ssDNA in buffer salts of the nanorods that were exposed to 5 min of plasma at high power showed no signs of DNA immobilization while those exposed to 2.5 min of plasma at high power developed large salt deposits indicative of the presence of DNA. Nanorods that had been exposed to 2.5 min of plasma at low power, followed by AUD functionalization and DNA modification, all indicated that DNA immobilization had occurred to some extent via KPFM. Further modification of these surfaces with the cDNA sequence indicated successful hybridization. The results of this study provide useful guidance for biosensor development because we have described the creation, functionalization, and biomolecule immobilization of a III−V semiconductor nanorod surface in a relatively easy fashion. The nanorods were grown without the need for gas-phase techniques that require long periods of time and low pressures. The nanorods were subsequently functionalized under ambient conditions without the need for harsh chemicals or solvents. Future biosensor devices will likely benefit from the study presented here in order to increase feasibility for their mass production and reduce associated costs.

there was actually an increase in the surface potential, indicating that the DNA did not bind to the surfaces of the nanorods due to the lack of available binding sites. The observed increase for the control sample is likely caused by physisorption of DMF or redistribution of toluene residue. Figure 8 depicts a representative progression of a nanorod’s topography and surface potential from KPFM after 2.5 min of low-power plasma cleaning (panel A), after AUD functionalization (panel B), and after DNA modification (panel C). This finding suggests that the DNA is covalently binding selectively to the nanorod surface and exposing its negatively charged backbone. These samples were further subjected to the cDNA sequence and evaluated with KPFM. Figure 8D is the topography and the corresponding surface potential of the same nanorod after exposure to the cDNA sequence in a PBS buffer. For each nanorod that was exposed to the complementary strand, the potential of the nanorod surface increased. This increase in potential can be attributed to the presence of the salt counterions that are necessary for hybridization. Large salt deposits were not visible due to the short amount of time the substrates were incubated with the complementary sequence (1 h).

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CONCLUSIONS We have created GaP nanorods via a solution−liquid−solid method and characterized their physical properties by SEM, TEM, and XRD. TEM imaging revealed a significant layer of surfactant present on each nanorod. KPFM imaging indicated that air plasma cleaning could be used to remove the surfactant without eliciting damage to the GaP surface. However, plasma 12619

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(18) Gu, Z. J.; Paranthaman, M. P.; Pan, Z. W. Vapor-phase synthesis of gallium phosphide nanowires. Cryst. Growth Des. 2009, 9 (1), 525− 527. (19) Evans, S. D.; Ulman, A. Surface-potential studies of alkyl-thiol monolayers adsorbed on gold. Chem. Phys. Lett. 1990, 170 (5−6), 462−466. (20) Aldana, J.; Wang, Y. A.; Peng, X. G. Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols. J. Am. Chem. Soc. 2001, 123 (36), 8844−8850. (21) Meyers, M. A.; Mishra, A.; Benson, D. J. Mechanical properties of nanocrystalline materials. Prog. Mater. Sci. 2006, 51 (4), 427−556. (22) Demers, L. M.; Ginger, D. S.; Park, S. J.; Li, Z.; Chung, S. W.; Mirkin, C. A. Direct patterning of modified oligonucleotides on metals and insulators by dip-pen nanolithography. Science 2002, 296 (5574), 1836−1838.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF under CHE-1143525. We thank Debra Sherman for her help with acquiring SEM images and Roberto Garcia for his help in acquiring TEM images.

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REFERENCES

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