8466
Langmuir 2005, 21, 8466-8470
Constrained Iron Catalysts for Single-Walled Carbon Nanotube Growth Ryan M. Kramer, Laura A. Sowards, Mark J. Pender, Morley O. Stone, and Rajesh R. Naik* Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7702 Received March 11, 2005. In Final Form: June 7, 2005 The diameter of single walled carbon nanotubes (SWNTs) determines the electronic properties of the nanotube. The diameter of carbon nanotubes is dictated by the diameter of the catalyst particle. Here we describe the use of iron nanoparticles synthesized within the Dps protein cage as catalysts for the growth of single-walled carbon nanotubes. The discrete iron particles synthesized within the Dps protein cages when used as catalyst particles gives rise to single-walled carbon nanotubes with a limited diameter distribution.
Introduction Carbon nanotube synthesis has stimulated the material science community by offering many potential applications including high-strength polymer composites, battery electrode materials, nanoscale sensors, and for use in nanoelectronics.1-6 Structurally distinct single-walled carbon nanotubes (SWNTs) occur when the graphene sheet is cylindrically rolled along the (n,m) lattice vector in the graphene plane to form a single rolled sheet one atom thick. Nanotube chirality and diameters correspond directly to their unique wrapping vectors associated with the n and m lattice integers giving rise to SWNTs with distinct properties.7-9 Most SWNT production involves the decomposition of a volatile carbon species by a metal catalyst. Typical catalyst materials used for SWNT production are metal oxide nanoparticles. It is known that SWNT diameters and its related chiralities are dependent on the size and quality of the catalyst particle.10 Therefore, well-defined catalyst particles are crucial for the production of monodispersed SWNT populations. The controlled formation of metal nanoparticles with precise nanoscale dimensions has been an actively growing field. Polydispersed metal nanoparticles used in CNT synthesis results in the production of nanotubes with broad size distributions. Catalysts that give rise to narrow size * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (937) 255-4913. Tel: (937) 2553808. (1) Iijima, S. Nature 1991, 354, 56. (2) Cassell, A. M.; Franklin, N. R.; Tombler, T. W.; Chan, E. M.; Han, J.; Dai, H. Am. Chem. Soc. 1999, 121, 7975-7976. (3) Kong, J.; Soh, H. T.; Cassell, A. M.; Quate, C. F.; Dai, H. Nature 1998, 395, 878-881. (4) Franklin, N. R.; Li, Y.; Chen, R. J.; Javey, A.; Dai, H. App. Phys. Lett. 2001, 79. (5) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (6) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (7) Murakami, Y.; Yamakita, S.; Okubo, T.; Maruyama, S. Chem. Phys. Lett. 2003, 375, 393-398. (8) Cassell, A. M.; Verma, S.; Delzeit, L.; Meyyappan, M.; Han, J. Langmuir 2000, 17, 260-264. (9) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (10) Liu, J.; Fan, S.; Dai, H. Mater. Res. Soc. Bull. 2004, 29, 244249.
and chirality distributions in the nanotube production process are desired. Examples of catalyst synthesis techniques include preformed molybdenum-iron clusters, microporous aluminophosphate templates, and combinations of thin metal film evaporation and annealing techniques to produce localized metal clusters of similar sizes.11-15 Biologically derived metal nanoparticles can also serve as catalysts for the production of carbon nanotubes. For example, ferritin, a spherical protein complex containing an iron nanoparticle in the form of a hydrous ferric oxide, has also been employed as a catalyst for both multi- and single-walled carbon nanotube growth.13,16 The hollow cage of the ferritin complex comprises an ideal space for the constrained deposition of iron and can also be selectively loaded to control iron nanoparticle size. Additionally, the protein shell helps to prevent aggregation of iron particles in both sample preparation and SWNT growth stages.13 Dictating catalyst size in combination with the ability to deposit the protein shell in precise locations through micro-contact printing and other soft lithography techniques makes biological constructs a promising avenue in nanotube device construction. The mammalian ferritin complex used in previous studies is comprised of 24 identical or structurally similar subunits to form a spherical shell with a hollow core. The inner core of the protein complex is approximately 8 nm and is able to incorporate roughly 4500 iron atoms in the form of a paracrystalline iron hydroxide.17,18 When fully loaded and annealed, the complex gives rise to Fe particle that is capable of catalyzing growth of monodisperse multiwalled nanotube structures.16 Partial iron loading (11) Su, M.; Zheng, B.; Liu, J. Chem. Phys. Lett. 2000, 322, 321-326. (12) Satishkumar, B.; Govindaraj, A.; Sen, R.; Rao, C. Chem. Phys. Lett. 1998, 293, 47-52. (13) Li, Y.; Dim, W.; Zhang, Y.; Rolandi, M.; Wang, D.; Dai, H. J. Phys. Chem. B 2001, 105, 11424-11431. (14) Tang, Z. K.; Sun, H. D.; Wang, J.; Chen, J.; Li, G. App. Phys. Lett. 1998, 73(16), 2287-2289. (15) An, L.; Owens, J. M.; McNeil, L. E.; Liu, J. J. Am. Chem. Soc. 2002, 124, 13688-13689. (16) Bonard, J. M.; Chauvin, P.; Klinke, a. C. Nano Lett. 2002, 2, 665-667. (17) Lawson, D. M.; Artymiuk, P. J.; Yewdall, S. J.; Smith, J. M.; Livingstone, J. C.; Treffry, A.; Luzzago, A.; Levi, S.; Arosio, P.; Cesareni, G.; Thomas, C. D.; Shaw, W. V.; Harrison, P. M. Nature 349, 541-4. (18) Chasteen, N. D.; Harrison, P. M. J. Struct. Biol. 1999, 126, 182194.
10.1021/la0506729 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/05/2005
Constrained Iron Catalysts
of the ferritin core has been demonstrated by Dai and co-workers, resulting in the creation of iron catalyst with discrete particle size in the production of SWNTs with narrow size distributions.13 However, variations in the partial loading of the ferritin cage could possibly account for the variations found in the nanotube dimensions. Although partial loading of the ferritin complex has been found to be an important contribution to production of structurally similar SWNTs, the goal remains to produce identical SWNTs with the same diameter and chirality in a single production process. In this study, we use a protein cage termed Dps, which belongs to a superfamily of ferritin-like protein complexes, as a nanocontainer for growing discrete iron catalysts for use in the production of carbon nanotubes with a much narrower size distribution. Typically ferritin-like Dps complexes are comprised of only 12 identical subunits (compared to 24 for ferritin) that also self-assemble into a spherical structure, albeit with a reduced inner core diameter of ∼4 nm.19 When fully loaded, the dodecameric structure of Dps protein cages incorporates far less iron than ferritin. All ferritin-like particles including the Dps family of proteins show strong sequence, structural, and functional homology throughout prokaryotic species and incorporate anywhere between 250 and 400 iron atoms.20,21 The Dps protein from Bacillus subtilis has previously been shown to display DNA-binding properties, but the exact quaternary structure and inorganic iron templating nature of the Dps protein has not been elucidated.22 Here we show that the recombinantly expressed Dps protein cage from Bacillus subtilis is able to self-assemble into a spherical shell and can sequester iron into its central hollow cavity. Due to the nature of the constrained reaction environment, formation of the iron nanoparticle is limited to the confines of the hollow cavity. In contrast to the partial loading of the larger ferritin particle that gives rise to particle sizes with limited size distributions, loading the Dps protein under iron saturating conditions ensures discrete nanoparticles with a narrower size distribution. Investigation into other ferritin-like protein cages and their slight variations in iron incorporation, in conjunction with optimization of growth conditions, is the first step in the identification of a subset of bio-derived catalyst particles that give rise to SWNTs with limited size distributions. The Dps family of proteins has long been shown to be both structurally and functionally related to the ferritinlike family of proteins and has been theorized to be the evolutionary precursor to ferritins found in higher eukaryotes. The Dps complex, when fully assembled, is able to protect its host cell from free radical species in vivo through a bimodal action. Inside the cell, Dps is able to bind and condense DNA in oxidative stress conditions providing a physical barrier limiting access to the genetic material. The secondary protective effect of the Dps complex arises from its ability to actively sequester intracellular iron and deposit it within its core.22 Here we utilized the Dps complex as a size constrained protein cage in order to produce a homogeneous iron nanoparticle. To ensure that each catalyst particle contained precisely the same amount of iron atoms, we loaded the protein (19) Ilari, A.; Stefanini, S.; Chiancone, E.; Tsernoglou, D. Nat. Struct. Biol. 2000, 7, 38-43. (20) Ilari, A.; Pierpaolo, C.; Ferrari, D.; Rossi, G. L.; Chiancone, E. J. Bio. Chem. 2002, 277, 37619-37623. (21) Grant, R.; Filman, D. J.; Finkel, S. E.; Kolter, R.; Hogle, J. M. Nat. Struct. Biol. 1998, 5, 294-303. (22) Antelmann, H.; Engelmann, S.; Schmid, R.; Sorokin, A.; Lapidus, A.; Hecker, M. J. Bacterilo. 1997, 179, 7251-7256.
Langmuir, Vol. 21, No. 18, 2005 8467
Figure 1. Characterization of Dps protein cage. (A) Transmission electron micrograph of negatively stained empty Dps protein cages. (B) Prussian blue staining following iron loading of the DPS cage. A native-polyacrylamide gel with Ferritin as a positive control(lane 1), Apoferritin as a negative control (lane 2) iron loaded Dps (lane 3) and unloaded Dps (lane 4). (C) Elemental analysis of lyophilized Dps protein following iron loading and purification. SEM micrograph of the lyophilized Dps and the chlorine and iron EDS maps. 50 µm × 50 µm maps.
cage under iron saturating conditions. Following mineralization, the protein shell is removed by oxidization at elevated temperatures, thereby exposing the iron nanoparticle. When used as a catalyst for nanotube growth, SWNT populations with similarly sized diameters were obtained. Results and Discussion The Dps protein from Bacillus subtilis was previously shown to have amino acid sequence identity to bacterioferritins and other Dps proteins, though the exact structural nature and iron incorporation properties had not been explored.22 Initial purification of the protein complex used a combination of heat denaturation and ammonium-sulfate precipitation techniques. The protein stability under these highly denaturing conditions can be attributed to the fairly stable structure of the Dps complex. Further purification of the protein complex used a combination of ion exchange and gel-filtration chromatographies. Analysis of the relative sizes of the recombinantly expressed Dps protein were undertaken using dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS measurements estimate the outer diameter of the Dps protein cage to be 9.0 ( 1.1 nm (data not shown). The TEM micrographs revealed a selfassembled spherical protein cage with an outer diameter of ∼9 nm and an inner cavity diameter of 4 nm (Figure 1A). Based on the molecular weight and homology to other Dps proteins for which the crystal structure has been solved, the Dps complex studied in this report shares the self-assembling properties of the Dps family and is comprised of 12 subunits with outer and inner dimensions of approximately 9 and 4 nm respectively. Iron loading of the recombinant Dps protein cage was consequently performed as described in the Experimental Section and size exclusion chromotography separation using gel-filtration allowed for the removal of excess iron free in solution. UV-vis spectroscopy of the gel filtration eluant was then monitored to detect absorption of proteinaceous material (at 280 nm) and Fe oxide mineral (at 350 nm) (data not shown). The overlap of these two spectroscopic peaks during elution indicated that the coelution represented a composite protein-mineral fraction. Additionally, because the elution profile remained
8468
Langmuir, Vol. 21, No. 18, 2005
Figure 2. Dps-derived iron particles. (A) AFM topography image of discrete iron catalyst nanoparticles after heat treatment. (B) AFM image showing two carbon nanotubes grown Dps derived iron catalyst nanoparticles.
unchanged when compared to the unmineralized Dps cage, we expected any iron mineralization to occur within the protein cage. The iron loaded protein samples were analyzed using nondenaturing polyacrylamide gel electrophoresis (PAGE) and stained with Coomassie for protein and prussian blue for the presence of iron.26 Both iron loaded ferritin and iron loaded Dps stained darkly when exposed to the prussian blue stain (Figure 1B: lanes 1 and 3), whereas apoferritin (lacks iron) and unloaded Dps did not (Figure 1B: lanes 2 and 4). Iron incorporation was additionally confirmed using EDAX elemental analysis of the gel-purified and lyophilized iron loaded Dps protein. Figure 1C shows a scanning electron microscopy (SEM) image of the lyophilized bulk protein and elemental maps for chlorine (from NaCl contained in the buffer solution) and iron. The prussian blue staining in conjunction with the EDAX analysis confirms that the Dps protein was loaded with iron. Iron loaded Dps samples were repetitively dialyzed against water before use in SWNT catalysis on both SiO2/ Si and quartz substrates. Dps was spotted on these substrates and oxidized at 900 °C in air for 30 min to remove the proteinaceous shell that surrounded the iron nanoparticle. AFM measurements of the discrete particles left after the heat treatment revealed the average particle heights were 1.6 ( 0.1 nm as determined by measuring 50 individual nanoparticles (Figure 2A). Apparent topographic heights were used in measuring catalyst and nanotube dimensions due to AFM tip convolution in measuring apparent widths. Calculation of particle volumes based on height measurements, assuming a spherical nature, suggests that each catalyst is comprised of ∼182 iron atoms. Because the Dps particles were repetitively loaded under saturating conditions we can assume that each catalytic particle is representative of a fully loaded protein cage. The diameter distribution of the iron particles after heat treatment at 900 °C for 10 min in argon as determined by AFM analysis was found to be around 1.05 ( 0.11 nm (n ) 20). Nanotube growth using the Dps-derived iron catalyst was performed using chemical vapor deposition (CVD) with a 1:1 mixture of methane and hydrogen (250 sccm (23) Jorio, A.; Pimenta, M. A.; Souza Filho, A. G.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. N. J. Phys. 2003, 5, 139.1-139.17. (24) Su, M.; Li, Y.; Maynor, B.; Buldum, A.; Lu, J. P.; Liu, J. J. Phys. Chem. B 2000, 104, 6505-6508. (25) Tominaga, M.; Ohira, A.; Kubo, A. ChemComm 2004, 15181519. (26) Levi, S.; Santambrogio, P.; Cozzi, A.; Rovida, E.; Albertini, A.; Yewdall, S. J.; Harrison, P. M.; Arosio, P. Biochem. J. 1992, 288, 591596.
Kramer et al.
each) at 900 °C for 10 min to allow for nanotube growth. Both SEM and AFM analysis were used to confirm the growth of carbon nanotubes on the substrate. Carbon nanotubes could often be seen emanating from a Dpsderived iron catalyst as shown in Figure 2B. Our results show that the majority of catalysts failed to give rise to nanotubes and the catalyst efficiency was estimated to be ∼20% as determined by AFM analysis. Improvement of catalyst treatments using a combination of ozonation, ultraviolet irradiation, surfactants, and proteases could be used in conjunction with calcination to further increase the efficiency. The majority of SWNTs grown from the CVD process appeared as individual nanotubes with an approximate nanotube densitiy of 50 nanotubes/µm2 and with varied lengths from 20 nm to 1 mm as measured from composite SEM images. Almost all of the unusual curvatures and morphologies associated with SWNT growth were observed to include “crop circles”, “shepherd’s hooks”, and long curving nanotubes that arise due to their flexibility and high aspect ratios. Perhaps the greatest potential in using the iron loaded Dps protein cage as a source as a catalyst in SWNT growth arises from the monodisperse populations it can produce. We measured diameters of nanotubes grown from the Dpsderived iron catalyst using tapping mode AFM. To obtain quantitative data, we took averages of nanotube heights of long stretches (10-600 nm) of straight nanotubes so that slight variations in background roughness could be minimized (Figure 3A). The average diameter of carbon nanotubes found using this technique was 1.0 ( 0.1 nm. The size distribution of 50 individual nanotubes is shown in Figure 4A. Based on the AFM results, the majority of the nanotubes had a diameter of ∼1 nm. To elucidate the exact nature of the dispersity in our nanotube populations, we used Raman spectroscopy to characterize the radial breathing vibrational modes (RBM) associated with SWNTs grown on SiO2. Excitation wavelengths of 514, 633, and 725 nm were used in the micro-Raman studies using a 50× objective and a 1 µm spot size. Of all the RBM’s detected, the majority showed a single peak centered around 248 cm-1 which is indicative of a fairly monodisperse population with occasional peaks appearing at slightly lower and higher frequencies (Figure 4B). The Raman shift (ωRBM, in cm-1) of this vibrational mode can be used as a rough estimate of the nanotube diameter (d, in nm) using the relationship d ) 248 cm-1 nm/ωRBM.23 Based on the spectroscopic measurements, the nanotubes that gave rise to RBM peaks had a diameter of ∼1.0 nm. The tangential modes associated with the RBM peaks in Figure 4B typically showed a strong peak at 1590 cm-1 with smaller peaks centered at 1571 and 1551 cm-1. It is important to note that RBM peaks were only seen in a small percentage of the scans we performed despite the fact that they were areas rich in nanotubes with diameters of less than 1.4 nm as determined by AFM. Notably, the diameters represented through Raman experiments might only consist of a sampling of the possible diameters because only nanotubes whose electronic energy spacing between van Hove singularities matching the laser excitation energies used for our micro-Raman experiments can be resonant. Lattice oriented growth of carbon nanotubes has been demonstrated on Si (111), Si (100) surfaces (6-3) and Au (111) surfaces.24,25 Due to the flexibility and small diameter of the tubes, they align along the crystal grooves of the substrate. We also observed oriented nanotubes growth on quartz substrate. Of all nanotubes grown on the quartz surface using Dps using typical growth procedures, more than 60% show oriented growth in a single direction.
Constrained Iron Catalysts
Langmuir, Vol. 21, No. 18, 2005 8469
Figure 4. Dps-derived iron catalysts give rise to carbon nanotubes with limited size distribution. (A) Histogram showing the diameter distribution as determined from AFM images of 50 SWNTs. (B) Representative micro-Raman spectra of three independent nanotubes grown on a Si/SiO2 substrate using a 514 nm excitation wavelength. Peaks centered around 248 cm-1 comes from the radial breathing mode (RBM) of the nanotubes, whereas peaks centered at 300 cm-1 are from the Si/SiO2 substrate.
Figure 3. AFM measurements of nanotubes. (A) AFM images of six different carbon nanotubes grown using the Dps derived iron catalysts. Tube heights were averaged (boxed area in lower picture) along their lengths and background subtracted. (B) AFM image of two overlapping nanotubes.
Figure 5 shows AFM images of oriented SWNTs on the quartz substrate. Because there was only a weak correlation to tube growth in other directions, no direct determination could be drawn between the crystal surface of the quartz and overall orientation. Further investigation into this phenomenon is currently being explored. Conclusions In conclusion, we have demonstrated that the dodecameric Dps protein cage can be used to produce discrete iron nanoparticles, unlike selective loading of ferritin that gives rise to iron particles with a larger size distribution in catalyst populations. The nature of the decreased polydispersity results from the iron loading under saturating conditions ensuring the iron particles have little variation in iron content. Dps gives rise to monodisperse
Figure 5. Oriented growth of carbon nanotubes on a quartz surface. AFM image of SWNTs grown on quartz substrate using Dps-derived iron catalyst.
populations of SWNTs with limited size variation as supported by the data presented here. Table 1 summarizes protein derived catalysts and the populations of nanotubes they can produce. Currently the effects of varying growth parameters and its effects on size dimensions of Dpsderived SWNTs are being investigated. In addition, the Dps protein cage can also be loaded with cobalt, as an alternative catalyst source for SWNT synthesis. The development of a uniform catalyst particle that is inexpensive, durable, and functional represents a crucial step in eliminating variability in SWNT production.
8470
Langmuir, Vol. 21, No. 18, 2005
Kramer et al.
Table 1. Biologically Derived Iron Catalysts
loaded Dps outer diameter inner cavity diameter nanotube type catalyzed diameter distribution of nanotubes
9 nm 4 nm single-walled 1.0 ( 0.1 nm
Experimental Section Cloning. The Dps gene was PCR amplified from the Bacillus subtilis genome using primers (Integrated DNA Technologies) flanking the Dps open reading frame. The gene was subsequently cloned into the bacterial expression vector pET21b (Novagen, San Diego, CA). The sequence was confirmed using an automated DNA sequencer (Applied Biosystems 3100). Expression and Purification. BL21 (DE3) cells were transformed with the pET21b plasmid carrying the Dps gene. For expression, an overnight culture was diluted (1:50) into fresh LB medium containing ampicillin (100 µg/mL). Expression was induced by the addition of 1 mM IPTG when cells reached an optical density at 600 nm of ≈0.5. The cells were grown for approximately 4-5 h and harvested in 25 mL of buffer A (20 mM Tris-Cl pH 8.0, 100 mM NaCl) and frozen for storage. Protein was extracted by incubating the thawed cell suspension in 5 mg of lysozyme and ultrasonicated 5 times for 10 s. The lysed crude cell suspension was centrifuged at 5000 rpm for 30 min and the supernatant was transferred to a fresh tube and heat treated at 60 °C for 10 min. Again the sample was centrifuged at 10K rpm for 15 min to remove any denatured protein. The supernatant was subjected to ammonia sulfate precipitation and mixed to a final concentration of 2.0 M. Following a 10 min incubation the sample was centrifuged as above. The Dps protein was isolated by anion exchange chromatography (MonoQ Amersham Pharmacia) and eluted using a liner 0.1-0.6 NaCl gradient. The eluted fractions were concentrated for gel filtration chromatography (Superdex 200) using a column equilibrated with buffer B (20 mM phosphate buffer pH 7.0, 50 mM NaCl). Eluted fractions were analyzed using SDS-PAGE and purity was estimated to be greater than 90%. Prussian blue staining to confirm the presence of iron was done as previously described. Fe Loading into the Dps Protein Complex. A 10 mL solution of Dps (0.20 mg/mL) in buffer B was heated to 42 °C and stirred constantly. A 0.05 M ammonium iron (II) sulfate solution was added dropwise to the Dps solution followed by addition of H2O2 to ensure full oxidation of Fe(II) to Fe(III). This process was repeated several times to ensure complete mineralization of the Dps cavity (2000 Fe/Dps protein). Following the final
partially loaded ferritin
loaded ferritin
12 nm 8 nm single-walled 1.5 ( 0.4 nm 3.0 ( 0.9 nm
12 nm 8 nm multiwalled 5.2 ( 0.6 nm
addition the solution was centrifuged to remove bulk precipitate (10 min, 15K rpm). The solution was subsequently dialyzed overnight against a total of 6 L of water to remove ammonium iron (II) sulfate still in solution. Dps dodecameric self-assembly was confirmed using dynamic light scattering (Dyna-Pro -MS/X) and transmission electron microscopy (TEM) using a Phillips EM208 operating at 200 kV of negatively stained samples using uranyl acetate. Ferrocyanide staining of native PAGE gels was undertaken to confirm iron incorporation into the protein complex in addition to elemental analysis of lyophilized protein using XL30 Environmental Scanning Microscope field emmision gun by FEI Company equipped with EDAX Genesis elemental analysis software. Following dialysis, the sample was centrifuge (10 min, 15K rpm) and stored at 4 °C for further use. Synthesis of Single-Walled Carbon Nanotubes. Mineralized Dps was spotted onto a silicon (3 µm thick silicon oxide layer, P-type, Boron doped) or quartz (3′′ 500 µm thick, single side polished) wafers and placed into the center of Type F21100 Barnstead International tube furnace. The substrate was heated to 900 °C to burn off the protein shell and oxidize the particle. For nanotube production, the oxidized substrate was again heated in the tube furnace in Ar (150 sccm, 99.999%). Once the furnace reached 900 °C, methane (225 sccm, 99.999%) and hydrogen (225 sccm) were allowed to pass through the tube reactor for 10 min, followed by cooling to room temperature under Ar (150 sccm). SWNT production was confirmed using a combination of scanning electron microscopy (XL30 Environmental Scanning Microscope), atomic force microscopy (Digital Instruments MultiMode with a Nanoscope IIIa controller with NSC15 and HI′RES (DP14) µMasch probes), and Raman sprectroscopy (Renishaw 1000 microRaman system).
Acknowledgment. Funding for this work was provided by the Air Force Office of Scientific Research. The authors thank Dr. Song Tan from the Gene Regulation Center at the Pennsylvania State University for helpful discussion and use of equipment. LA0506729
