Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes

NANO LETTERS

Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition of the Carbon Feedstock

2009 Vol. 9, No. 10 3398-3405

Gilbert D. Nessim,† Matteo Seita,†,# Kevin P. O’Brien,§ A. John Hart,| Ryan K. Bonaparte,† Robert R. Mitchell,† and Carl V. Thompson*,† Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; Department of Materials, ETH Zürich, Wolfgang-Pauli-Str., CH-8093 Zürich, Switzerland; Components Research Department, Intel Corporation, Hillsboro, Oregon 97124; Department of Mechanical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 Received March 3, 2009; Revised Manuscript Received August 12, 2009

ABSTRACT Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 °C and a substrate temperature of 500 °C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 kΩ for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.

The CMOS industry has identified scaling of copper interconnects as one of the major obstacles to lithographic downscaling beyond the 22 nm integrated circuit node.1 The established copper dual damascene technology for interconnect vias may not be scalable because of copper failure by electromigration driven by high current densities and also due to high electrical resistances that result from surface scattering in small-diameter vias. Additionally, copper vias require a resistance liner to prevent copper diffusion into the surrounding dielectric, which further reduces the available conductive cross section and significantly complicates processing. Finally, the 3+ kilometers1 of copper interconnects in today’s microprocessors are becoming a major source of signal resistance-capacitance (RC) delay. Carbon nanotubes (CNTs) have been widely investigated as a promising new material for interconnect vias2 as they * To whom correspondence should be addressed. E-mail: [email protected]. † Massachusetts Institute of Technology. § Intel Corp. | University of Michigan. # Current address: Lab. for Nanometallurgy, Dept. of Materials, ETH Zurich, Wolfgang-Pauli-Str. 10, HCI E 522, CH-8093 Zu¨rich, Switzerland. 10.1021/nl900675d CCC: $40.75 Published on Web 08/31/2009

 2009 American Chemical Society

exhibit exceptional electrical,3 thermal4,5 and mechanical properties.6,7 Studies show that CNTs are stable for current densities up to 109 A/cm2, which is 2 orders of magnitude higher than copper,4 and that multiwall CNTs (MWCNTs) can exhibit multichannel ballistic conduction over distances of micrometers.8 Also, because of their higher chemical stability relative to copper, diffusion-barrier via liners are not needed for CNTs. Models described by Naeemi et al.9 indicate that MWCNTs longer than 100 µm can have conductivities higher than copper or single-wall CNT (SWCNT) bundles. Furthermore, CNTs can be grown in high aspect ratio vias10,11 and therefore allow greater separation of interconnect layers and inverse scaling of interconnect thicknesses, reducing overall RC losses and decreasing chiplevel energy dissipation.12 To match copper’s conductivity in vias, most groups have focused on packing dense bundles of SWCNTs rather than using individual MWCNTs. On the basis of electrical models, Naeemi et al.13 suggested a target density of SWCNTs of at least 3.3 × 1013 CNTs/cm2 and recommended using MWCNTs for long-range interconnects.9 The challenge is that the

maximum packing densities achieved experimentally are still about two orders of magnitude lower than desired. As MWCNTs are electrically conductive and exhibit multiple channels of conduction, the packing density required to approach copper conductivity may be lower compared to using SWCNTs. Moreover, individual MWCNTs can more readily be assembled into high-density interconnect networks than SWCNT bundles. Although the properties of CNTs suggest that they can replace copper in interconnect applications, important fabrication issues remain unresolved and therefore prevent their implementation. Specifically, it is still challenging to grow vertically aligned CNT (VACNT) “carpets” on metallic substrates at CMOS-compatible temperatures (400-450 °C). It is a further challenge to grow high-quality CNTs that make electrical contact with the substrate. An indicator of the difficulty of these tasks is that most CNT synthesis literature focuses on the easier task of growing carpets of crystalline CNTs on electrically insulating and nonmetallic substrates, such as alumina or silicon oxide, using thermal chemical vapor deposition (CVD)14-17 at temperatures between 700 and 900 °C. Replicating these high-temperature CVD processes with catalysts on metallic substrates has proven especially challenging due to phenomena such as alloying, catalyst coarsening, and segregation at the underlayer grain boundaries,18 whereas it is much simpler to stabilize metal nanoparticles on oxides. On the other hand, lowering the growth temperature has frequently proven to be ineffective because the structural quality of CNTs usually degrades as reaction temperatures are decreased, and because higher temperatures are often required to form the metal nanoparticles necessary for CNT growth. Our goal was to devise a straightforward approach using conventional physical vapor deposition to deposit the catalyst and the underlayer as thin films, and atmospheric pressure thermal CVD at CMOScompatible temperatures to grow MWCNTs on conductive layers. While no previous study has accomplished this goal, the existing literature suggests that low-temperature growth is enabled by thermal decomposition of the precursor gas and formation of catalyst nanoparticles at low temperatures. However, most of the effort to reduce growth temperature has focused on growing CNTs on insulating substrates. Initial approaches to reduce nanotube growth temperatures consisted of improving CVD recipes by varying the reactant gases, pretreating the catalyst, or using plasmas. Maruyama et al.19 synthesized floating SWCNTs at 550 °C using alcohol instead of hydrocarbons. Cantoro et al.20 showed growth of VACNTs at temperatures below 550 °C using a subnanometer catalyst iron layer on silicon oxide. Many groups have also used plasma to reduce growth temperatures and improve vertical alignment.21,22 However, plasmas also introduce defects in the nanotubes, which significantly increase their electrical resistance due to scattering.23 Hydrocarbon gases, such as ethylene or acetylene, decompose into a wide variety of compounds when heated at high temperature.24,25 This has previously been investigated through preheating of the source gases in relation to CNT Nano Lett., Vol. 9, No. 10, 2009

growth on insulating substrates. Meshot et al.26 showed that thermal rearrangement of C2H4/H2 created a population of hydrocarbons that affected the growth rate and the ultimate thickness of CNTs grown on insulating substrates. Jeong et al.27 synthesized VACNTs on silicon substrates at 550 °C using a hot-filament CVD (they also used a Pd plate positioned a few millimeters above the sample which improved CNT growth). Lee et al.28 used a two-stage heating technique to grow CNTs at 550 °C on silicon oxide by preheating the incoming gases at 850 °C. Kanzow et al.29 obtained growth of well graphitized MWCNTs by laser vaporization of nickel in a CVD flow of acetylene heated at 750 °C. The few and recent successful reports of CNT growth on conductive layers mostly focus on controlled formation of the catalyst. Talapatra et al.30 showed growth of MWCNTs on bulk alloy (Inconel) at 770 °C. A follow-up work by Bult et al.31 achieved growth on a conductive passivation layer (by oxidation in air at 850 °C) on the same alloy at 675 °C. Using a custom-designed particle generator (impactor) to deposit controlled-size cobalt nanoparticles on the substrate, Awano et al.32 showed growth of carpets of MWCNTs on TiN or Ti layers at 510 °C. The follow-up work by Yokoyama et al.33 achieved growth of CNT bundles at 390 °C (post-annealed at 400 °C) using microwave plasma to generate radicals at low pressure on the same engineered catalyst substrate. Although Cantoro et al.34 showed growth of SWCNTs below 450 °C by pretreating Al/Fe/Al thin metal layers in a NH3/H2 atmosphere, as the authors state, it is very likely that the Al layer was (at least) partly oxidized. The above examples show interesting progress on CNT growth on conductive layers but do not take advantage of thermal gas decomposition and often involve complex catalyst engineering techniques. In this work, we demonstrated that by appropriately preheating the incoming gases and by choosing the catalystunderlayer system, we were able to grow carpets of crystalline VACNTs on conductive substrates at temperatures as low as 500 °C. We also showed that the hydrocarbon gas decomposition is the limiting step to obtaining crystalline CNTs at low temperatures without using a plasma, which typically leads to structurally defective CNTs or carbon nanofibers. Our results were obtained using an Fe catalyst deposited by e-beam evaporation on a metallic Ta underlayer, which was chosen because of its high melting temperature.18 We verified electrical contact between the CNTs and the substrate, thus indicating that our decoupled approach can simultaneously stabilize the catalyst for vertically aligned CNT growth and maintain the CNT-underlayer contact needed for high-performance interconnects. Using the same approach, we have grown CNTs on other metallic underlayers including Pd, Cu, and W, demonstrating the versatility of our approach. The samples used in this study consisted of coupons with metallic square pads with sizes varying from 10 to 100 µm. The metals constituting the catalyst stack were e-beam evaporated using a CHA bell jar evaporator with a 4-pocket Temescal source. The base pressure was between 2.0 × 10-6 3399

Philips XL30) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30 FEG and Philips CM 30). Electrical characterization was performed using atomic force microscopy (AFM, Nanosurf) using a metal-coated (Pt-Ir) conductive tip. Electrical scans and I-V curves were obtained in contact mode.

Figure 1. Schematic of the fast-heat approach: the sample sits in a quartz tube outside the furnace (but still in an argon flow) while the temperature is ramped in the growth zone. It is then introduced into the growth zone when all temperatures are stable and the gas mixture is introduced.

and 2.0 × 10-7 Torr. We evaporated 5 nm of Ta (as an adhesion layer), 200 nm of Cu, 30 nm of Ta (underlayer) and 2 nm of Fe (catalyst) on silicon oxide without breaking vacuum. A standard lift-off process was used to generate the square pads. The samples were prepared on 2 in. square oxidized silicon substrates and were manually cleaved to form approximately 5 mm × 5 mm pieces that were used during the CNT growth experiments. Samples were cleaned with acetone and isopropyl alcohol prior to CNT growth. Growth was performed using a three-zone atmosphericpressure furnace (Lindberg Blue) in a fused-silica tube with an internal diameter of 22 mm. Flows of Ar (99.9995%, Airgas), C2H4 (99.5%, Airgas), and H2 (99.999%, Airgas) were maintained using electronic mass flow controllers (MKS 1179A). The samples were positioned in the quartz tube using a custom-made quartz fixture (Finkenbeiner Glass, Waltham, MA) and faced the flow at an inclination of 20°. All experiments were performed by using the “fast-heat” technique (Figure 1) in which the samples were sitting on the fixture inside the quartz tube and initially positioned outside the heated zone of the furnace. A fan was kept blowing from below to keep the exposed quartz tube walls at room temperature. An argon flow of 200 standard cubic centimeters per minute (sccm) was maintained while the three zones of the furnace were ramped to the desired temperature. Once the set temperatures were reached, 400 sccm of hydrogen and 150 sccm of ethylene were introduced, and the quartz tube was shifted, thus positioning the samples in the growth zone and starting the CNT growth process. The growth cycle lasted 15 min for each experiment. The preheating temperatures mentioned for the first two zones (which preheat the source gases before reaching the samples) are the ones indicated by the built-in furnace thermocouples. For the third zone, the growth zone where the samples were positioned, in addition to the furnace thermocouple reading, we included the reading from an additional thermocouple (Omega TJ36, accuracy 0.2 °C) inserted in the quartz tube (aligned with its axis) and positioned just behind the custom-made quartz fixture containing the samples. This thermocouple was connected to a digital temperature reader (Barnant 115). The CNTs were examined using high-resolution scanning electron microscopy (SEM, Zeiss/Leo Gemini 982 and 3400

To investigate the effects of gas preheating on CNT growth, we performed a series of thermal CVD experiments in which we varied the temperature of the first two furnace zones (the gas preheat zones) while setting the third zone at 475 °C (the growth zone where the samples were positioned). In most cases, we observed carpets of vertically aligned CNTs as shown in Figure 2. Increasing the gas preheat temperature (zones 1 and 2) slightly increased the sample temperature as recorded by the thermocouple in the growth zone. The CNT carpet height for increasing preheating temperatures is plotted in Figure 2. When the gas preheating temperature was below 650 °C, the CNT carpet height remained below 200 nm. However, above 710 °C we observed a higher density of CNTs, improved vertical alignment, and a rapid increase of the CNT carpet height for small increases of the gas preheating temperature. For instance, the average CNT carpet height for a gas preheating temperature of 710 °C was about 270 nm, while for 750 °C the carpet height reached almost 600 nm. We increased the gas preheating temperature up to 770 °C, as above that temperature soot formation may inhibit CNT growth.35,36 To confirm that the taller CNT carpets obtained were only dependent on the gas preheating and not the substrate temperature, we tested a growth process for increasing substrate temperatures without preheating the gases. Figure 3 shows that for recorded temperatures up to 571 °C carbon nanotubes were not vertically aligned, indicating low areal density, and their height did not exceed 200 nm. To confirm that gas preheating was the critical determinant of the CNT carpet height, we performed additional experiments where the preheating temperature was set at 750 °C and the substrate temperature was varied between 460 and 490 °C. The CNT carpet heights measured were very similar for these variations of the substrate temperature (not shown). We estimated the catalytic life to be above 45 min as we varied the growth duration from 8 min up to 60 min while fixing the gas preheat temperature at 750 °C and the growth zone temperature at 475 °C and observed a linear increase in the CNT carpet heights for the first 45 min, reaching a maximum height above 2 µm. This indicated that the growth rate could be considered constant for the 15 min CNT growth experiments described in this paper. SEM images and a plot of height versus time are provided as additional information in the supplemental online material. We examined the structure of the growth products of the samples grown when preheating the source gases at 770 °C using HRTEM and observed crystalline multiwall nanotubes (Figure 2) with a thin outer layer of amorphous carbon, which may be due to soot formation. Note that these crystalline CNTs were obtained for a substrate temperature of only 500 °C (measured by our additional thermocouple), a temperature Nano Lett., Vol. 9, No. 10, 2009

Figure 2. Plot showing evolution of CNT carpet height as a function of gas preheat temperature and selected SEM images of CNT carpets grown with the substrate zone set at 475 °C for different gas preheat temperatures. HRTEM image showing the crystalline structure obtained when gas preheating was set to 770 °C. For each experiment, the average CNT carpet height was calculated as an average of over 10 SEM images taken across the sample. Scale bar for SEM images is 200 nm for the first image (gas preheating off) and 500 nm for the remaining images.

Figure 3. SEM images of short CNTs grown at different substrate temperatures without gas preheating.

for which most often only nanofibers can be grown without use of a plasma (which introduces defects). We also lowered the growth zone temperature down to 390 °C while keeping the gas preheating above 750 °C to determine the lowest temperature at which we could still grow CNT carpets (Figure 4). Although growth was still possible, the CNTs obtained were short and not vertically aligned. We used an AFM to measure the electrical resistance of the CNTs grown on a sample with similar catalyst-underlayer stack but evaporated on a conductive (Pd) layer, and using a slightly different growth recipe to obtain a taller carpet of VACNTs. The catalyst-underlayer stack Fe(5 nm)/Ta(30 nm)/Cu(200 nm)/Ti(5 nm) was e-beam evaporated on top of a blanket Pd(200 nm) layer and a Ti(20 nm) adhesion Nano Lett., Vol. 9, No. 10, 2009

layer that were e-beam evaporated on Si/SiO2 coupons. We grew CNTs at 475 °C with the source gases preheated at 770 °C, and obtained a 2 µm-tall dense carpet of VACNTs. We first checked the internal resistance of our AFM with a metal-coated (Pt-Ir) conductive tip by measuring the resistance of the Pd layer and found values below 250 Ω. We then measured the electrical conductivity by approaching the AFM tip to the CNT carpet in increments of 20 nm until an I-V curve was observed. We measured resistances in the range of 35 kΩ, indicating electrical contact between the CNTs and the metallic substrate (Figure 5). Since the substrate temperature varied by only a few degrees when the gas preheating was above 710 °C and that without gas preheating we only obtained very short and not very aligned CNT carpets, even for substrate temperatures 3401

Figure 4. SEM images of CNTs obtained at growth at low temperature (390/410/440 °C) with gas preheating at 750 °C.

Figure 5. (a) SEM image showing a 2 µm-tall dense carpet of vertically aligned CNTs grown on a sample with Fe-Ta catalyst-underlayer over a Pd ground layer. (b) Image showing our AFM tip above a 100 µm × 1000 µm pads with CNTs; smaller CNT pads are also visible. (c) AFM-based I-V curve indicating a resistance of approximately 32 kΩ.

activated growth mechanisms, depending on whether the gases were preheated or not, so that k(T) ) A × exp(-Q/RTsub) with QTgas>710 °C > QTgas>650 °C

Figure 6. Arrhenius plot showing a distinct activation energy if gas preheating is above 650 °C compared to no gas preheat. With gas preheating off, the gas preheat temperature indicated is the temperature measured in zone 2 by the furnace thermocouple.

approaching 600 °C (Figure 3), we suspected that gas preheating was the key mechanism responsible for the results observed. In the Arrhenius plot in Figure 6, where the CNT growth rate is plotted as a function of the gas preheating temperature, we clearly observed two distinct thermally3402

where A is a weakly temperature-dependent parameter and R is the gas constant. These results clearly show a significant growth rate increase when the source gases were preheated above 710 °C, compared to no gas preheating. The activation energy Q measured from the slope on the Arrhenius plot in Figure 6 has a value of about 0.9 eV, which is very close to the activation energy for carbon diffusivity in BCC iron37 of 0.84 eV at the substrate temperatures we are considering (≈ 500 °C). This also indicates that the iron catalysts were reduced when the gases were preheated. When the gas preheating is turned off, the activation energy is much lower (Q ≈ 0.1 eV) indicating that gas preheating was the ratelimiting step for the growth of nanotubes, which is an important insight for synthesis of CNTs at low substrate temperatures. Although a mechanistic interpretation is difficult due to the interplay among gas dissociation, catalysis of various hydrocarbon compounds, evolution of catalyst morphology and interactions between the catalyst and the underlayer, we will suggest some specific insights regarding the mechanisms of nanotube growth. Sufficient gas preheating significantly enhanced the catalysis of CNT growth for a given substrate Nano Lett., Vol. 9, No. 10, 2009

temperature, thus allowing growth at lower temperatures compared to the case without gas preheating (compare the SEM images in Figures 2 and 3). When the incoming gases were sufficiently preheated, ethylene dissociated into multiple carbon compounds that were more easily catalyzed for CNT growth. Towell et al.24 showed that ethylene decomposed into multiple CxHy compounds when heated at high temperatures. More recently, Plata et al.25 analyzed the decomposition of ethylene as a function of temperature in a similar CNT growth process but using an electrically insulating substrate. They observed multiple volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) at the various temperatures. Their results showed that the concentrations of VOCs such as methane, propene, propyne, and pentane, and PAHs such as naphthalene, acenaphthylene, acenaphthene, phenanthrene, fluoranthene, and pyrene, increased with temperature. Based on molecular beam experiments, Eres et al.38 suggested a critical role for fluoranthene in the formation of a crystalline nanotube cap. The complex chemical characterization of the compounds into which ethylene decomposes at high temperature, which is beyond the scope of our study, indicates that many new compounds are generated by the thermal process and that they play a critical role in the formation and growth of CNTs on catalyzed particles. This is a crucial insight into CNT growth as most simulations usually consider only a single hydrocarbon gas (usually methane or ethylene or acetylene), when a whole series of carbon compounds most likely contribute to the CNT growth process. Given the high flow of hydrogen (400 sccm) in the gas mixture, we assumed that the catalyst surface reduction17 and restructuring did not depend on the additional compounds generated by the preheating. Considering that the substrate temperature measured with our additional thermocouple varied by only 7 °C for gas preheating temperatures between 730 and 770 °C and that we did not observe significant CNT carpet height changes when varying the substrate temperature by (15 °C for a preheat temperature of 750 °C, we also assumed that the evolution of the catalyst morphology was independent of the gas preheating at a given substrate temperature. To establish that electrical contact was made with the underlying metal film and to roughly characterize the electrical properties of the MWCNTs, we used an AFM with a conductive tip to measure the resistance of our MWCNT carpets grown on square pads with sizes of 10 to 100 µm, using Fe catalysts on Ta underlayers and a common Pd underlayer ground. The results obtained were always in the 35 kΩ range, indicating electrical contact with the substrate (Figure 5). Although the nominal radius of curvature of the AFM tip is only 20-30 nm and we approached the 2 µm-tall CNT carpet in 20 nm increments until an current flow was observed, it was difficult to estimate the number of CNTs in electrical contact with the AFM tip. Inspection of the conductive AFM tip before and after measurements on the CNT carpet revealed slight abrasive damage to the tip metallization, which conveniently provided a rough estimate Nano Lett., Vol. 9, No. 10, 2009

of the number of CNTs contacted. On the basis of the known geometry of the tip and the approximate density of the CNT carpet, we estimated that the AFM tip was in electrical contact with approximately 32 nanotubes. In principle, we expect that the resistance of a single MWCNT is equal to the number of CNTs contacted by the AFM tip multiplied by the measured resistance (35 kΩ). However, considering that the caps of our CNTs were most likely closed, the AFM tip could only make electrical contact with the most external wall of our CNTs that have 10-15 walls (see HRTEM image in Figure 2). Given that the conductivity of a MWCNT scales at least linearly with the number of walls (most models indicate that the conductivity increases faster9), we can consider that the inherent resistance of the MWCNTs in contact with the AFM tip is likely at least 10 to 15 times smaller than the resistance we measured. On the basis of the above speculative assumptions, we estimated the resistance a single MWCNT to be in the order of 75-100 kΩ () 35 kΩ × 32 ÷ 15 (or ÷ 10)). With this premise, we will compare our measurements with available theoretical and experimental measures of MWCNT resistance. The resistance of a CNT contacted at both ends is the sum of the quantum, scattering, and contact resistances. Using the formulas in McEuen et al.,39 the quantum resistance for a 2 µm-tall nanotube with 10 walls would be 1.3 kΩ [) 13 kΩ/10 walls)]. For an ideal MWCNT (i.e., no defects), the scattering resistance would be 2.6 kΩ [) 1.3kΩ × (2 µm/1 µm)]. On the basis of this theoretical data, if we assume that our CNTs are perfectly crystalline, the contact resistance would account for most of the resistance measured. However, although HRTEM shows the CNT walls (Figure 2), from our data it is not possible to quantify the density of defects and their influence on conductivity, thus we cannot separately determine the values of the scattering and of the contact resistances. One particular aspect of the contact resistance that may be relevant is the possibility that our Ta underlayer could be oxidized in situ during or after the growth process. A thermodynamic analysis based on an Ellingham diagram40 was used to estimate the oxygen concentration in our system. On the basis of this analysis, the oxygen partial pressure under our growth conditions may not be sufficient to oxidize the Ta layer. However, considering the presence of carbon species in the reaction, a carbide such as TaC could form. Gotoh et al.41 showed that the workfunction of TaC is 5.0 eV, which is very close to the theoretical estimates for the workfunction of MWCNTs (5.10 eV according to Shiraishi et al.42), leading, theoretically, to a good electrical contact. We also compared our measured values to estimated values based on electrical models and experiments made by others. Using the model developed by Naeemi et al.,9 which is based on equivalent electrical circuits, a 2 µm-tall MWCNT with 10 walls would exhibit a resistance of 3-16 kΩ depending on the assumptions made on the available conduction crosssection. Note that this model does not take into account the contact resistance or nanotube defects. Kajiura et al.43 attached bundles of 1-5 µm-tall MWCNTs with external 3403

diameter of 10-20 nm to a piezo-driven electrode using silver paste. They then dipped the most protruding MWCNT into Hg and measured resistances of R ) Rc + Fx, with the contact resistance Rc ) 79 ( 0.1 kΩ and F ) 9.7 ( 0.2 kΩ/µm. Using this formula, a 2 µm-tall MWCNT would exhibit a resistance R ≈ 92 kΩ. Awano et al.32 prepared a more elaborate structure specifically designed for electrical characterization of bundles of MWCNTs. They used thermal CVD at 510 °C to grow MWCNTs from Co nanoparticles deposited on a Ti layer at the bottoms of microwells (350 nm tall, 2 µm diameter) using a custom-built particle generator (impactor). When they contacted the top of the MWCNT bundle with a Ti film cap, they measured a resistance of 1MΩ, which they attributed to the oxidation of the bottom Ti layer prior to nanoparticle deposition. When they replaced the Ti layers with TiN, Awano’s team measured a resistance of 0.59 Ω. The HRTEM image shown in Figure 2 of Awano et al.32 indicated a MWCNT of ∼10 nm in external diameter similar to our CNTs (although their CNTs have ∼5 walls while ours have ∼10 walls). Given that their CNT density was stated as 1011 CNTs/cm2, we estimated that each of their holes contained ∼3141 MWCNTs [) π × (10-4)2 × 1011]. If we neglect the cross-CNT conduction, which is a reasonable assumption based on the large tunneling resistance of 2-140 MΩ indicated by Stahl et al.,44 and assuming that the 0.59 Ω resistance that Awano et al.32 measured is the parallel resistance of all the CNTs in the hole, we estimate that each of their CNTs had a resistance of ∼1.8 kΩ [) 0.59 Ω × 3141]. However, given that their CNTs were 350 nm-tall compared to our CNTs, which were 2000 nm-tall, we added a scattering resistance to the value calculated for the CNTs obtained by Awano’s group to make a fair comparison. This calculation leads to an estimated total CNT resistance of 6 kΩ [) 1.8 kΩ + ([13 kΩ/(5 walls)] × (2000-350 nm)/ 1000 nm))]. Despite the uncertainty as to the number of MWCNTs in contact with our AFM tip, which does not allow us to reach a conclusion on the resistance of a single MWCNT in our carpet, we can still infer from our measurements that we do observe electrical contact with the conductive substrate. In summary, we have shown the importance of preheating the incoming gases for growth of VACNTs on conductive substrates using thermal CVD at substrate temperatures as low as 400 °C. When the gases were preheated at 770 °C, we obtained crystalline CNTs for a substrate temperature of 500 °C. The ability to grow CNTs at low substrate temperatures has also reduced interactions between the catalyst and the metallic underlayer, such as alloying, which hinder CNT growth. By decoupling the gas preheating from CNT catalysis, we grew crystalline VACNTs on metallic substrates at low temperatures. Separate measurements of the temperature dependence of CNT growth as a function of the gas preheat temperature and the catalyst temperature indicated that gas decomposition is the ratelimiting process for CNT growth at low temperatures. AFMbased electrical measurements showed electrical contact with the substrate. Gas preheating as an enabler of low-temperature growth on conductive substrates is an important step toward 3404

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