Effect of Oxygen Plasma Alumina Treatment on Growth of Carbon

Article pubs.acs.org/JPCC

Effect of Oxygen Plasma Alumina Treatment on Growth of Carbon Nanotube Forests Junwei Yang,† Santiago Esconjauregui,*,† Rongsie Xie,† Hisashi Sugime,† Taron Makaryan,‡ Lorenzo D’Arsié,† David Leonardo Gonzalez Arellano,§ Sunil Bhardwaj,∥ Cinzia Cepek,∥ and John Robertson† †

Department of Engineering, University of Cambridge, 9 J. J. Thomson Avenue, Cambridge CB3 0FA, United Kingdom Radiophysics Department, Yerevan State University, 1 A. Manoogian Street, Yerevan 0005, Armenia § Polymer Science and Engineering, Conte Research Center, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States ∥ Istituto Officina dei Materiali-CNR, Laboratorio TASC, Trieste I-34149, Italy ‡

ABSTRACT: We use oxygen plasma to increase the height of forests from ∼0.2 to >2 mm. The effectiveness of treating alumina by oxygen plasma, prior to iron nanoparticle formation, is studied using cycles of nanotube growth, nanotube burning, and regrowth. This demonstrates that plasma-treated alumina is more resistant to iron bulk diffusion than an untreated one. Secondary ion mass spectroscopy shows there is negligible iron diffusion into the bulk of treated alumina. Plasma treatment of catalyst supports is potentially useful for growth of ultra-high-density nanotube forests for applications such as interconnects in integrated circuits and heat sinks.

■

INTRODUCTION Carbon nanotube (CNT) forests possess a unique set of properties which make them attractive for industrial applications.1−19 Forests are typically synthesized by chemical vapor deposition (CVD) using Fe-coated Al2O3 support. Compared to other synthesis techniques, CVD is the only method that ensures surface growth of forests with control of nanotube purity, diameter, density, and length.20−28 However, the height of forests tends to maximize when tube length approaches the millimeter range.29−34 The reasons for growth termination remain unclear, so that millimeter-range forests can only be obtained by empirically optimizing the growth conditions. Briefly, a thin film of Fe (∼0.3−1.5 nm) is deposited on an Al2O3-coated support layer and then transformed into catalytically active nanoparticles by annealing in a reducing atmosphere. This is followed by supply of a carbon feedstock (e.g., acetylene) until forest growth stops. Only a selected set of processing conditions, optimized to each type of catalyst, will lead to the tallest forests. For catalyst systems where the support is a metal or metal compound, growth of forests is more challenging.35−39 It can only be achieved by engineering the catalyst−support interaction using unconventional catalyst preparation (e.g., plasma pretreatment).40,41 These forests, however, tend to be shorter than those grown on Al2O3 support. Forest growth in the Al2O3−Fe system arises from a particular interaction between Fe and Al2O3.42 A reaction at the Al2O3−Fe interface promotes formation of Fe2+ and Fe3+ interface states which restrict Fe surface mobility. This induces a narrow Fe nanoparticle size distribution and the nucleation of © 2014 American Chemical Society

CNTs vertically aligned to the Al2O3 support. Despite the reduced mobility, Fe can still subsurface diffuse into the Al2O3, and this can be detrimental to nanotube growth. Surface diffusion followed by nanoparticle sintering as well as Brownian movement of Fe particles also detriment nanotube growth. Other reports have suggested that growth termination is related to catalyst poisoning.20 This model supports the idea that production of amorphous carbon during CNT CVD gradually covers the particles, thus reducing the active area where the carbon source is decomposed, until becoming completely inactive. Deactivation is thus thermally dependent and may be minimized by adding a small amount of vapor water throughout the growth. It is believed the water etches amorphous and other graphitic carbon impurities. As a result, not only the life of the catalyst is extended but the nanotube material is purer than most other forms. It has also been postulated that water addition inhibits the Ostwald ripening process, due to the ability of oxygen and hydroxyl species to reduce diffusion rates of Fe atoms on Al2O3.30 It is likely that sudden growth termination of forests is a combination of both catalyst deactivation (due to byproducts of the reaction) and uncontrolled catalyst−support interaction (surface or bulk diffusion of the catalytic particles). Herein we aim to further control the growth of CNT forests and focus on nanoparticle diffusion into the bulk of Al2O3. Understanding and controlling catalyst diffusion is a prerequisite to nucleate ultra-high-density Received: March 4, 2014 Revised: July 21, 2014 Published: July 21, 2014 18683

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

Article

Figure 1. Cartoon of cyclic growth and burning of nanotube forests. (a) Fe catalyst particles are prepared on Al2O3 support, then exposed to CNT CVD conditions, and then heated in open air for tube burning. (b) Schematic of all the steps of a cycle. Cycles can be repeated several times while tube growth is verified. Conditions for pretreatment are 700 °C, 5 min in 1 bar Ar:H2 (1000:500 sccm), while for growth conditions are 700 °C, 10 min, Ar:H2:C2H2 (1000:500:10 sccm).

CNT forests for applications as interconnects,3,14−16,19 supercapacitors,43,44 stretchable conductors,45 supertough fibers,46 thermal management surfaces, or heat sinkers.7 To mitigate Fe diffusion into the bulk of Al2O3, Zhong et al.22 treated Al2O3 layers with oxygen plasma. This allowed them to create a denser, diffusion-resistant Al2O3 support and grow forests with densities of the order of 1013 CNTs cm−2. In this paper we further investigate the effect of oxygen plasma when treating Al2O3. Using a cyclic approach consisting of CNT growth, burning of the tubes, and regrowth (as schematically depicted in Figure 1a and detailed in the Experimental Section), we demonstrate that plasma-treated Al2O3 is more resistant to Fe diffusion than untreated Al2O3. This approach of cyclic growth allows us to quantitatively determine the effectiveness of the plasma sealing, i.e., nanoparticle resistance to diffusion into the bulk of the Al2O3 support. Our results show that while untreated Al2O3 layers show practically no regrowth of CNTs after the first tube burning (mainly due to catalyst diffusion in to the bulk of Al2O3), plasma-sealed Al2O3 allows up to 5 cycles of growth, burn, and regrowth. Systematic secondary ion mass spectroscopy (SIMS) shows a strong correlation between Fe diffusion and oxygen plasma treatment. For comparison, we evaluate identical cyclic growth and catalyst characterization on Fe− SiO2 systems, as SiO2 naturally inhibits Fe diffusion into its bulk.47,48 Altogether, these results show that for growing taller forests the catalyst particles must be fully immobilized. This allows us to increase 1 order of magnitude the height of the forests, from ∼0.2 mm to >2 mm using untreated or treated Al2O3.

100 W RF power oxygen plasma for 15 min at room temperature (RT). For catalyst, we thermally evaporate 1 nm high-purity Fe on untreated and plasma-treated Al2O3 samples and, for comparison, on bare SiO2. All samples are exposed to air after each processing step. CNT Growth, Forest Burning, and Film Characterization. For CNT growth, samples are pretreated in a hot wall system (quartz furnace tube) at 700 °C in 1 bar Ar:H2 (1000:500 sccm) for 5 min. Samples are introduced at RT and heated at a nominal heating rate of 50 °C min−1. Immediately after reaching 700 °C, growth is carried out for 10 min by adding C2H2 (10 sccm) to the Ar:H2 flow. After growth, samples are cooled with a 1000 sccm Ar flow until reaching RT. For cyclic growth, as schematically depicted in Figure 1b, samples (after due characterization) are replaced in the quartz furnace tube at 650 °C in open air to burn away the as-grown forests. Each cycle finishes with nanotube growth. This allows evaluating the effectiveness of plasma treatment. CNT burning re-exposes the nanoparticles, where after reduction they are again active for nucleation and growth of CNTs (using same previous conditions). The growth process and successive burning are repeated until no growth is verified. In the ideal case of particles being fully immobilized, the cycles can be repeated an infinite number of times. However, if nanoparticles diffuse into the bulk of Al2O3 or undergo sintering, the growth degrades until eventually all particles are inactive for the initial growth conditions. In all cases assuming no evaporation of the nanoparticles, the total amount of deposited catalyst remains in the sample (either on the surface or on both surface and in the bulk of the sample). The morphology of the samples is characterized by atomic force microscopy (AFM). CNTs are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The density and size of the nanoparticles are estimated from the AFM images using Gwyddion software.

■

EXPERIMENTAL SECTION Support Layer and Catalyst Preparation. We use as substrate Si(100) coated with 200 nm thermal SiO2 on which we dc magnetron sputter nominally 10 nm Al2O3. After sputtering, one-half of the samples are subject to a 150 mbar, 18684

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

Article

Figure 2. (a) Evolution of nanotube forest height for treated and untreated Al2O3 samples. (b, c, and d) Side-view SEM analysis of forests grown on treated Al2O3 for cycles 1, 3, and 5, respectively. (e and f) SEM images for the same cycles but on untreated Al2O3.

■

Surface Characterization. The chemical state of the sample surfaces is analyzed by X-ray photoemission spectroscopy (XPS). All photoemission spectra are acquired at room temperature in normal emission geometry using a conventional Mg X-ray source (hν = 1253.6 eV) and a 120 mm hemispherical electron energy analyzer with an overall instrumental energy resolution of 0.7 eV. For data analysis, the photoelectron binding energy (BE) is calibrated by fixing the Si 2p signal coming from the bulk to 99.3 eV. Spectra are normalized to the incident photon flux and analyzed by performing a nonlinear mean square fit of the data in the energy range of the studied photoemission peaks following the Levenberg−Marquardt algorithm. We use a Shirley background and reproduce the photoemission intensity using, as fit function, asymmetric Doniach−Sunjic line shapes for the 2p levels of Al. SIMS Characterization. We performed time-of-flight (TOF) SIMS analysis using TOF.SIMS5 by ION-TOF equipped with a Bi liquid metal gun and a Cs ion gun. Bi ions are filtered for Bi1+ ions, and Cs ions are filtered for Cs+ ions. The sputtering raster is 250 μm × 250 μm using Cs+ at 500 eV (current 41 nA), and surface spectra are taken from an area of 50 μm × 50 μm using Bi1+ at 25 keV (target current 0.87 pA). The guns are operated in high-current bunched mode, the extractor in positive mode, and the analyzer optimized for high mass resolution, acquiring over a range from 0.5 to 740 Da. An electron flood gun is used between pulses to compensate for the charging effect due to insulation. During analysis, metal−Cs clusters are followed and labeled as M+ for the respective metal ions. The depth of the SIMS crater is measured using a Wyko NT1100 optical profiling system; a constant sputtering rate is assumed throughout.

RESULTS Figure 2 summarizes the growth results after each cycle of nanotube growth and burning for both untreated and plasmatreated Al2O3 samples. For a direct comparison, all samples are subject to the same pretreatment and growth conditions. We find that during the first cycle, both supports lead to the nucleation and growth of nanotube forests of comparable heights, Figure 2a. The height of the forests for the plasmatreated Al2O3 samples averages a millimeter, while for the untreated ones it is slightly shorter of about 0.90 mm. Striking differences appear after the first nanotube burning and regrowth. Untreated samples show poor nanotube growth. The height of the forest for cycle 2 is less than 10% of that obtained from the first cycle. This suggests a rapid catalyst deactivation. Further cycles (3−5) yield no forests but sparse unaligned tubes. In contrast, the plasma-treated samples renucleate forests for up to five cycles. The forest height decreases only by ∼12% after each cycle. After cycle 5, the forest height still averages >500 μm. Such results indicate a much greater lifespan of the catalytic activity for plasma-treated samples. We attribute the variation in nanotube height with the number of cycles to the growth conditions which are fixed and optimized for the initial as-formed nanoparticles. However, it is noted that the final height for both cases (treated and untreated) is similar. This suggests that while plasma treatment extends the lifespan of the catalyst, a similar growth termination mechanism is at play. Yamada et al. proposed in 2008 that catalyst deactivation occurs due to carbon coating.49 Figure 2b, 2c, and 2d shows side-view SEM images of the forests grown on plasma-pretreated samples, cycles 1, 3, and 5 respectively. The surface density of all these forests is on the 18685

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

Article

Figure 3. Evolution of nanoparticle number density after annealing and successive 5 cycles of growth and burning. Below is AFM analysis of 1 nm Fe deposited onto treated and untreated Al2O3 supports after deposition and after cycles 2 and 4 (conditions as in Figure 1).

Figure 4. SIMS depth profile of as-deposited and after cycle 1 samples for treated and untreated Al2O3. Fe+ diffusion into the bulk of Al2O3 is observed only on untreated Al2O3.

order of 1011 CNTs cm−2 and appears to be very homogeneous across the samples. Closer inspection shows that neighboring tip tubes collapse together by van der Waals forces, forming voids at the topmost surfaces, typical of forests with these area densities. Nanotube collapse by van der Waals forces is more evident as the cycle number progresses. HRTEM reveals no

nanoparticles at the nanotube tips, suggesting root growth in all cases. The CNTs are multiwalled type of, typically, 3−5 walls; tube diameter averages 5.1 ± 0.3 nm for cycle 1 and increases cycle by cycle up to 7.8 ± 0.5 nm for cycle 5. For the untreated case, SEM shows forests only for the two first cycles (Figure 2e shows those for cycle 1). For the rest of the cycles, the surface 18686

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

■

appears covered by sparse, unaligned tubes lying on the Al2O3 surface, Figure 2f and 2g. No aligned nanotubes but only defective structures nucleate and grow after cycle 3. The area density of these structures is extremely low (2 mm in height) while the untreated samples yield forests whose height is 1 order of magnitude shorter (Figure 8). This set of experiments is systematically repeated to ensure reproducibility. In all cases, growth proves to be similar. The results thus allow us to conclude the need of fully immobilizing the catalyst (regardless of the employed substrates) to control growth of forests on a surface. In surfacegrowth nanotube CVD, each tube is seeded by a catalytic particle so that nanoparticle immobilization is one of the key steps to achieve controlled forest growth. This is a common issue on supported catalysts since the catalytic activity of particles is strongly related to the interactions between particles’ atoms and support. If the metal atoms are highly mobile, the nanoparticles tend to easily lose their catalytic activity by sintering into larger particles and/or diffusing into the bulk of the support. It is for this reason that CNT CVD requires strategies to minimize the mobility of metal atoms (and hence nanoparticles) during both catalyst formation and nanotube growth. It remains, however, of fundamental value to fully understand forest growth termination. Only this knowledge will pave the way toward man-tailored forest synthesis and countless applications of nanotubes.

of the catalyst show small and dense nanoparticles with a narrow size distribution for the first cycle. As the cycle number progresses, surface diffusion (and resulting nanoparticle sintering, Ostwald ripening, and Brownian motion) becomes more evident. The nanoparticles then coarsen and the size distribution widens to the extent that they become catalytically inactive after the third cycle of growth and burning, as observed on treated Al2O3 samples after five cycles. Oxygen Plasma Treatment for Growth Termination. We now comment on how plasma treatment also contributes to enlarge the catalyst lifetime, avoiding poisoning by amorphous carbon. In Hata’s forest growth,20 addition of water is believed to etch the amorphous carbon that deactivates the particles while simultaneously minimizing the coarsening effect. In our case of plasma-treated samples, the surface and bulk diffusion of the particles is minimized, so that the catalyst ensemble tends to conserve its size and number density throughout the growth process. Keeping these two parameters constant and using optimized growth conditions, production of amorphous carbon (or other byproducts of the reaction) is minimized, hence extending the lifetime of catalytic nanoparticles. To prove this, we performed a series of growth experiments using untreated and treated Al2O3. All samples are coated with 1 nm Fe and subject to exactly the same pretreatment and growth conditions (as detailed in the Experimental Section), except for the temperature and time. We evaluate forest growth at 450, 600, and 750 °C for 1 h (Figure 7). This growth time ensures the

■

CONCLUSIONS Our approach of cyclic growth and burning of CNT forests allows evaluating the catalytic activity of nanoparticles while assuring no removal from the Al2O3 substrate. In using these cycles, we demonstrate that oxygen plasma-treated Al2O3 support is more resistant to bulk diffusion of Fe, which is directly related to blockage of Fe bulk diffusion (as shown by SIMS characterization). In contrast, untreated Al2O3 allows just two cycles of growth and burning of tubes; afterward, the catalyst particles become inactive or disappear underneath. SiO2 support shows similar results to treated alumina, but nanoparticles surface diffuse during the processing and sinter until eventually becoming inactive. Finally, we demonstrate that treated Al2O3 supports, coated with an ultrathin Fe film, allow

Figure 7. SEM images of nanotube growth untreated and treated Al2O3 support at (a and b) 450, (c and d) 600, and (e and f) 750 °C. Conditions for pretreatment are as in Figure 1. Growth time is 1 h.

forests have reached their plateau. We observe that at low temperatures (450 °C), growth is poor (i.e., a nucleation density 2 mm in height (treated samples), while on untreated Al2O3 the height of forests is ∼200 μm. Pretreatment is performed at 750 °C, 5 min in 1 bar Ar:H2 (1000:500 sccm), while growth is performed at 750 °C in 1 bar Ar:H2:C2H2 (1000:500:5 sccm) for 1 h. capacitor Electrodes Operable at 4 V with High Power and Energy Density. Adv. Energy Mater. 2010, 22, E235−E241. (6) Xu, M.; Futaba, D. N.; Yumura, M.; Hata, K. Tailoring Temperature Invariant Viscoelasticity of Carbon Nanotube Material. Nano Lett. 2011, 11, 3279−3284. (7) Akoshima, M.; Hata, K.; Futaba, D. N.; Mizuno, K.; Baba, T.; Yumura, M. Thermal Diffusivity of Single-Walled Carbon Nanotube Forest Measured by Laser Flash Method. Jpn. J. Appl. Phys. 2009, 48, 05EC07. (8) Wang, X.; Liu, Y.; Hu, P.; Yu, G.; Xiao, K.; Zhu, D. Controllable Fabrication of Aligned Carbon Nanotubes: Selective Position and Different Lengths. Adv. Mater. 2002, 14, 1557−1560. (9) Hayamizu, Y.; Yamada, T.; Mizuno, K.; Davis, R. C.; Futaba, D. N.; Yumura, M.; Hata, K. Integrated Tree-Dimensional Microelectromechanical Devices from Processable Carbon Nanotube Wafers. Nat. Nanotechnol. 2008, 3, 289−294. (10) Acquaviva, D.; Arun, A.; Esconjauregui, S.; Bouvet, D.; Robertson, J.; Smajda, R.; Magrez, A.; Forro, L.; Ionescu, A. M. Capacitive Nanoelectromechanical Switch Based on Suspended Carbon Nanotube Array. Appl. Phys. Lett. 2010, 97, 233508. (11) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Carbon Nanotube Arrays with Strong Shear Binding-On and Easy Normal Lifting-Off. Science 2008, 322, 238−242. (12) Ge, L.; Sethi, S.; Ci, L.; Ajayan, P. M.; Dhinojwala, A. Carbon Nanotube-Based Synthetic Gecko Tapes. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10792−10795. (13) Chen, B.; Goldberg Oppenheimer, P.; Shean, T. A. V.; Wirth, C. T.; Hofmann, S.; Robertson, J. Adhesive Properties of Gecko-Inspired Mimetic via Micropatterned Carbon Nanotube Forests. J. Phys. Chem. C 2012, 116, 20047−20053. (14) Robertson, J.; Zhong, G. F.; Esconjauregui, C. S.; Bayer, B. C.; Zhang, C.; Fouquet, M.; Hofmann, S. Applications of Carbon Nanotubes Grown by Chemical Vapor Deposition. Jpn. J. Appl. Phys. 2012, 51, 01AH01. (15) Robertson, J.; Zhong, G.; Hofmann, S.; Bayer, B. C.; Esconjauregui, C. S.; Telg, H.; Thomsen, C. Use of carbon nanotubes for VLSI interconnects. Diamond Relat. Mater. 2009, 18, 957−962. (16) Nihei, M.; Horibe, M.; Kawabata, A.; Awano, Y. Simultaneous Formation of Multiwall Carbon Nanotubes and Their End-Bonded

forest growth up to 1 order of magnitude higher during the same growth time. These results highlight the importance of catalyst immobilization for synthesizing inf inite long forest films.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge financial support from the European project GRAFOL. L.D. acknowledges funding from EPSRC, UK. C.C. acknowledges ABNANOTECH supported by MIUR (Progetto Premiale 2012) and In-Kind (PIK) EX-PRO-REL for the financial support. S.B. acknowledges ICTP funding for Training and Research in Italian Laboratory (TRIL) fellowship.

■

REFERENCES

(1) Avouris, P.; Chen, J. Nanotube Electronics and Optoelectronics. Mater. Today 2006, 9, 46−54. (2) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Properties. Science 1999, 283, 512−514. (3) Awano, Y.; Sato, S.; Nihei, M.; Sakai, T.; Ohno, Y.; Mizutani, T. Carbon Nanotubes for VLSI: Interconnect and Transistor Applications. Proc. IEEE 2010, 98, 2015−2031. (4) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Shapeengineerable and Highly Densely Packed Single-Walled Carbon Nanotubes and Their Application as Super-Capacitor Electrodes. Nat. Mater. 2006, 5, 987−994. (5) Najafabadi, A. I.; Yasuda, S.; Kobashi, K.; Yamada, T.; Futaba, D. N.; Hatori, H.; Yumura, M.; Iijima, S.; Hata, K. Extracting the Full Potential of Single-Walled Carbon Nanotubes as Durable Super18690

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

Article

Ohmic Contacts to Ti Electrodes for Future ULSI Interconnects. Jpn. J. Appl. Phys. 2004, 43, 1856−1859. (17) Robertson, J.; Zhong, G.; Telg, H.; Thomsen, C.; Warner, J. H.; Briggs, G. A. D.; Dettlaff-Weglikowska, U.; Roth, S. Growth and Characterization of High-Density Mats of Single-Walled Carbon Nanotubes for Interconnects. Appl. Phys. Lett. 2008, 93, 163111. (18) Katagiri, M.; Sakuma, N.; Suzuki, M.; Sakai, T.; Sato, S.; Hyakushima, T.; Nihei, M.; Awano, Y. Carbon Nanotube Vias Fabricated by Remote Plasma-Enhanced Chemical Vapor Deposition. Jpn. J. Appl. Phys. 2008, 47, 2024−2027. (19) Chiodarelli, N.; Li, Y.; Cott, D. J.; Mertens, S.; Peys, N.; Heyns, M.; De Gendt, S.; Groeseneken, G.; Vereecken, P. M. Integration and Electrical Characterization of Carbon Nanotube Via Interconnects. Microelectron. Eng. 2011, 88, 837−843. (20) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306, 1362−1364. (21) Li, X.; Cao, A.; Jung, Y. J.; Vajtai, R.; Ajayan, P. M. Bottom-Up Growth of Carbon Nanotube Multilayers: Unprecedented Growth. Nano Lett. 2005, 5, 1997−2000. (22) Zhong, G.; Warner, J. H.; Fouquet, M.; Robertson, A. W.; Chen, B.; Robertson, J. Growth of Ultrahigh Density Single-Walled Carbon Nanotube Forests by Improved Catalyst Design. ACS Nano 2012, 6, 2893−2903. (23) Esconjauregui, S.; Fouquet, M.; Bayer, B. C.; Ducati, C.; Smajda, R.; Hofmann, S.; Robertson, J. Growth of Ultra-High Density Vertically-Aligned Carbon Nanotube Forests for Interconnects. ACS Nano 2010, 4, 7431−7436. (24) Cantoro, M.; Hofmann, S.; Pisana, S.; Scardaci, V.; Parvez, A.; Ducati, C.; Ferrari, A. C.; Blackburn, A. M.; Wang, K.-Y.; Robertson, J. Catalytic Chemical Vapor Deposition of Single-Wall Carbon Nanotubes at Low Temperatures. Nano Lett. 2006, 6, 1107−1112. (25) Harutyunyan, A. R.; Chen, G.; Paronyan, T. M.; Pigos, E. M.; Kuznetsov, O. A.; Hewaparakrama, K.; Kim, S. M.; Zakharov, D.; Stach, E. A.; Sumanasekera, G. U. Preferential Growth of SingleWalled Carbon Nanotubes with Metallic Conductivity. Science 2009, 326, 116−120. (26) Li, Y.-L.; Kinloch, I. A.; Windle, A. H. Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis. Science 2004, 304, 276−278. (27) Yamada, T.; Maigne, A.; Yudasaka, M.; Mizuno, K.; Futaba, D. N.; Yumura, M.; Iijima, S.; Hata, K. Revealing the Secret of WaterAssisted Carbon Nanotube Synthesis by Microscopic Observation of the Interaction of Water on the Catalysts. Nano Lett. 2008, 8, 4288− 4292. (28) Yasuda, S.; Hiraoka, T.; Futaba, D. N.; Yamada, T.; Yumura, M.; Hata, K. Existence and Kinetics of Graphitic Carbonaceous Impurities in Carbon Nanotube Forests to Assess the Absolute Purity. Nano Lett. 2009, 9, 769−773. (29) Amama, P. B.; Pint, C. L.; Kim, S. M.; Mc Jilton, L.; Eyink, K. G.; Stach, E. A.; Hauge, R. H.; Maruyama, B. Influence of Alumina Type on the Evolution and Activity of Alumina-Supported Fe Catalysts in Single-Walled Carbon Nanotube Carpet Growth. ACS Nano 2010, 4, 895−904. (30) Amama, P. B.; Pint, C. L.; McJilton, L.; Kim, S. M.; Stach, E. A.; Murray, P. T.; Hauge, R. H.; Maruyama, B. Role of Water in Super Growth of Single-Walled Carbon Nanotube Carpets. Nano Lett. 2009, 9, 44−49. (31) Kim, S. M.; Pint, C. L.; Amama, P. B.; Zakharov, D. N.; Hauge, R. H.; Maruyama, B.; Stach, E. A. Evolution in Catalyst Morphology Leads to Carbon Nanotube Growth Termination. J. Phys. Chem. Lett. 2010, 1, 918−922. (32) Amama, P. B.; Pint, C. L.; Mirri, F.; Pasquali, M.; Hauge, R. H.; Maruyama, B. Catalyst−Support Interactions and Their Influence in Water-Assisted Carbon Nanotube Carpet Growth. Carbon 2012, 50, 2396−2406. (33) Kim, S. M.; Pint, C. L.; Amama, P. B.; Hauge, R. H.; Maruyama, B.; Stach, E. A. Catalyst and Catalyst Support Morphology Evolution

in Single-Walled Carbon Nanotube Supergrowth: Growth Deceleration and Termination. J. Mater. Res. 2010, 25, 1875−1885. (34) Sakurai, S.; Nishino, H.; Futaba, D. N.; Yasuda, S.; Yamada, T.; Maigne, A.; Matsuo, Y.; Nakamura, E.; Yumura, M.; Hata, K. Role of Subsurface Diffusion and Ostwald Ripening in Catalyst Formation for Single-Walled Carbon Nanotube Forest Growth. J. Am. Chem. Soc. 2012, 134, 2148−2153. (35) de los Arcos, T.; Vonau, F.; Garnier, M. G.; Thommen, V.; Boyen, H.-G.; Oelhafen, P.; Düggelin, M.; Mathis, D.; Guggenheim, R. Influence of Iron−Silicon Interaction on the Growth of Carbon Nanotubes Produced by Chemical Vapor Deposition. Appl. Phys. Lett. 2002, 80, 2383. (36) García-Céspedes, J.; Thomasson, S.; Teo, K. B. K.; Kinloch, I. A.; Milne, W. I.; Pascual, E.; Bertran, E. Efficient Diffusion Barrier Layers for the Catalytic Growth of Carbon Nanotubes on Copper Substrates. Carbon 2009, 47, 613−621. (37) Rao, A. M.; Jacques, D.; Haddon, R. C.; Zhu, W.; Bower, C.; Jin, S. In Situ-Grown Carbon Nanotube Array with Excellent Field Emission Characteristics. Appl. Phys. Lett. 2000, 76, 3813. (38) Wang, Y.; Luo, Z.; Li, B.; Ho, P. S.; Yao, Z.; Shi, L.; Bryan, E. N.; Nemanich, R. J. Comparison Study of Catalyst Nanoparticle Formation and Carbon Nanotube Growth: Support Effect. J. Appl. Phys. 2007, 101, 124310−124318. (39) Yokoyama, D.; Iwasaki, T.; Yoshida, T.; Kawarada, H.; Sato, S.; Hyakushima, T.; Nihei, M.; Awano, Y. Low Temperature Grown Carbon Nanotube Interconnects Using Inner Shells by Chemical Mechanical Polishing. Appl. Phys. Lett. 2007, 91, 263101. (40) Esconjauregui, S.; Bayer, B. C.; Fouquet, M.; Wirth, C. T.; Yan, F.; Xie, R.; Ducati, C.; Baehtz, C.; Castellarin-Cudia, C.; Bhardwaj, S.; et al. Use of Plasma Treatment to Grow Carbon Nanotube Forests on TiN Substrate. J. Appl. Phys. 2011, 109, 114312−114320. (41) Esconjauregui, S.; Cepek, C.; Fouquet, M.; Bayer, B. C.; Gamalski, A. D.; Chen, B.; Xie, R.; Bhardwaj, S.; Ducati, C.; Hofmann, S.; Robertson, J. Plasma Stabilisation of Metallic Nanoparticles on Silicon for the Growth of Carbon Nanotubes. J. Appl. Phys. 2012, 112, 034303−034311. (42) Mattevi, C.; Wirth, C. T.; Hofmann, S.; Blume, R.; Cantoro, M.; Ducati, C.; Cepek, C.; Knop-Gericke, A.; Milne, S.; Castellarin-Cudia, C.; et al. In-situ X-ray Photoelectron Spectroscopy Study of Catalyst− Support Interactions and Growth of Carbon Nanotube Forests. J. Phys. Chem. C 2008, 112, 12207−12213. (43) Kim, Y. S.; Kumar, K.; Fisher, F. T.; Yang, E. H. Out-of-Plane Growth of CNTs on Graphene for Supercapacitor Applications. Nanotechnology 2012, 23, 015301−015308. (44) Izadi-Najafabadi, A.; Yasuda, S.; Kobashi, K.; Yamada, T.; Futaba, D. N.; Hatori, H.; Yumura, M.; Iijima, S.; Hata, K. Extracting the Full Potential of Single-Walled Carbon Nanotubes as Durable Supercapacitor Electrodes Operable at 4 V with High Power and Energy Density. Adv. Mater. 2010, 22, E235−E241. (45) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Stretchable Active-Matrix Organic Light-Emitting Diode Display using Printable Elastic Conductors. Nat. Mater. 2009, 8, 494−499. (46) Dalton, A. B.; Collins, S.; Muñoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. SuperTough Carbon-Nanotube Fibres. Nature 2003, 423, 703. (47) Winterbottom, W. L. Equilibrium Shape of a Small Particle in Contact with a Foreign Substrate. Acta Metall. Mater. 1967, 15, 303− 310. (48) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. The Surface Energy of Metals. Surf. Sci. 1998, 411, 186−202. (49) Yamada, T.; Maigne, A.; Yudasaka, M.; Mizuno, K.; Futaba, D. N.; Yumura, M.; Iijima, S.; Hata, H. Revealing the Secret of WaterAssisted Carbon nanotube Synthesis by Microscopic Observation of the Interaction of Water on the Catalysts. Nano Lett. 2008, 8, 4288− 4292. (50) Bennett, C. E. G.; McKinnon, N. A.; Williams, L. S. Sintering in Gas Discharges. Nature 1968, 217, 1287−1288. 18691

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692

The Journal of Physical Chemistry C

Article

(51) Grigorov, G. I.; Grigorov, K. G.; Sporken, R.; Caudano, R. IonInduced Densification of PVD Filmsa Choice of the Optimum Density of Ion Bombardment. Appl. Phys. A: Mater. 1996, 63, 399− 401. (52) Seitz, F.; Koehler, J. S. Displacement of Atoms during Irradiation. Solid State Phys. 1956, 2, 305−448. (53) Aisenberg, S.; Chabot, R. W. Physics of Ion Plating and Ion Beam Deposition. J. Vac. Sci. Technol. 1973, 101, 104−107. (54) Martin, P. J. Ion-Assisted Thin Film Deposition and Applications. Vacuum 1986, 36, 585−590. (55) Fu, Q.; Wagner, T. Interaction of Nanostructured Metal Overlayers with Oxide Surfaces. Surf. Sci. Rep. 2007, 62, 431−498.

18692

dx.doi.org/10.1021/jp5022196 | J. Phys. Chem. C 2014, 118, 18683−18692