Article pubs.acs.org/JPCC
Patterning of Forests of Carbon Nanotubes (CNTs) Using Copper Overlayers as Iron Catalyst Deactivators Reut Yemini, Merav Muallem, Tali Sharabani, Eti Teblum, Yossi Gofer, and Gilbert D. Nessim* Department of Chemistry, Bar Ilan Institute for Nanotechnology and Advanced materials (BINA), Bar Ilan University, Ramat Gan, 52900, Israel ABSTRACT: We show a simple technique to grow patterned carbon nanotube (CNT) forests by annealing the catalytic surface using copper patterns (as stencil or bridge) prior to, or during, CNT growth. We demonstrate that copper diffused into the iron catalyst and deactivated it, thus preventing CNT growth on the areas where the copper was present. This technique is a promising and simple way to pattern CNT forests since it does not require the usual lithography and lift-off of the catalyst. This catalyst deactivating overlayer principle can be extended to pattern other 1D nanostructures such as carbon nanofibers or nanowires and 2D nanostructures such as graphene or transition metal dichalcogenides using chemical vapor deposition (CVD).
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INTRODUCTION
EXPERIMENTAL SECTION Thermal Chemical Vapor Deposition Systems. To synthesize forests of VACNTs, we used a thermal chemical vapor deposition (CVD) system consisting of a three-zone furnace (Carbolite model HZS-E) and a fused silica (quartz) tube with 25-mm external diameter (22-mm internal diameter). The first two zones preheated the precursor gases to decompose the hydrocarbons and to form controlled ppm’s of water vapor from oxygen and hydrogen.12 The sample was introduced into the third zone of the furnace using the fast-heat technique to start the thermal process with annealing gases and temperatures at equilibrium.11 The gases used were Ar (99.9999%), Ar/O2 (a mixture with 99% Ar and 1% O2), C2H4 (99.999%), and H2 (99.9999%). Gas flows were controlled by electronic mass flow controllers (MKS P4B) with digital mass flows control unit (MKS model 247D). During all experiments, the two preheating zones were set at 770 °C and the growth zone was set at 755 °C. Sample Preparation and Experimental Procedures. The substrates were prepared using e-beam evaporation (5 rpm rotation, partial pressure of 10−6 Torr). We deposited 10 nm Al2O3 (underlayer) and 1.2 nm Fe (catalyst) on polished Si wafers (crystal orientation (100)). To pattern the CNT forests, we did a preannealing step in a reducing atmosphere of Ar (100 sccm) and H2 (400 sccm) with a patterned piece of copper foil positioned above the sample. The copper overlayer was either positioned in contact with the catalytic surface (stencil) or above it (bridge). We tested two different processes where we either (1) left the copper overlayer for the whole annealing and growth or (2) left the copper overlayer only during the annealing step and performed the subsequent growth step after removing the copper overlayer.
Since the discovery of carbon nanotubes and the fundamental understanding of many aspects of their complex growth mechanisms, significant research efforts have focused on the influence of different growth process parameters and a variety of material aspects for the growth of forests of vertically aligned carbon nanotubes (VACNTs). For instance, we now better understand the role of catalysts,1−5 underlayers,6−8 gases,9−12 and, most recently, thin film catalyst reservoirs positioned below the underlayer.13 Appropriate tuning of the abovementioned parameters allows to control the morphology of the CNTs grown (e.g., height, diameter), and to improve their microstructure (crystallinity) for superior mechanical, electrical, or thermal applications.14,15 Beyond the growth of forests of VACNTs, it is important to pattern the position of the CNTs or portions of the CNT forest for many applications and devices such as microelectromechanical systems (MEMS),16 membranes,17 scanning probes,18,19 sensors,20 field emitters,21 and in nanoelectronics.22 The established way to pattern CNT growth is to do lithography with lift-off of the catalyst layer.23−25 Although this method allows a good patterning precision, it requires lithography equipment, involves several fabrication steps, and exposes the thin catalyst layer to a solvent treatment. We show here a facile method where we pattern CNT growth using a patterned copper foil (overlayer) to interact with the iron catalyst layer during thermal treatment. The copper overlayer is either positioned above the catalytic surface (bridge) or in contact with it (stencil). We tested the role of the copper foil during CNT growth or only during a preannealing step, and always observed that the copper deactivated the iron catalyst below it. This technique is a simpler and quicker alternative to grow patterned CNT forests for applications that do not require lithographic precision. © 2016 American Chemical Society
Received: February 18, 2016 Revised: May 11, 2016 Published: May 11, 2016 12242
DOI: 10.1021/acs.jpcc.6b01676 J. Phys. Chem. C 2016, 120, 12242−12248
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Figure 1. Pictorial images of (a) copper strip directly positioned on the catalytic surface (stencil) and (b) copper strip suspended on small sections of wafers (bridge). Photo images of (c) anneal and growth with copper stencil; (d) anneal and growth with copper bridge; (e) anneal with copper stencil with subsequent CNT growth after removal of copper stencil; and (f) anneal with copper bridge with subsequent CNT growth after removal of copper bridge. The dashed red regions represent the areas without CNTs.
The steps in the first process were as follows: (1) purge with Ar and H2 (100 and 400 sccm) for 10 min at room temperature with the sample positioned outside the furnace; (2) introduce the sample into the growth zone and anneal for 5 or 15 min under the same Ar/H2 gas flows; (3) introduce C2H4 (200 sccm) and Ar/O2 (100 sccm) to grow CNTs (30 min); and (4) finally push the tube outside the furnace to cool under only Ar flow prior to removal. The steps in the second process were as follows: (1) preanneal the sample with the copper pattern under a flow of Ar/H2 for varying durations; (2) cool the sample and remove the copper pattern; and (3) process the sample a second time for CNT growth (purge, anneal, and growth as per the first method) without the copper overlayer. Characterization. We characterized the samples using an environmental scanning electron microscope (SEM, Quanta 2000, from FEI) operating at 3 keV, high-resolution SEM (HRSEM, Zeiss 982). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis HS spectrometer with monochromatized Al Kα X-ray radiation source (photon energy 1486.6 eV). All spectra were acquired at pass energy of 80 eV and source power of 150 W. Binding energies were recalibrated by setting the CC/CH component of the C 1s peak at 285 eV. Selected CNTs were removed and put on a transmission electron microscopy (TEM) grid and characterized using highresolution transmission electron microscope (HRTEM, JEM2100 from JEOL) operating at 200 Kev. HRTEM samples were prepared by dispersing a portion of the CNT carpet in 2propanol with gentle sonication for 1 h and then placing 1 drop of the solution on a 300-mesh Cu holey carbon grid (from SPI).
Figure 2. HRTEM showing crystalline CNTs.
this experiment, we infer that a copper overlayer positioned directly on the catalyst layer during the synthesis inhibits CNT growth. We repeated the same process but we suspended the copper strip above the catalytic surface at a distance of 525 μm using two small pieces of silicon wafer (of thickness 525 (±25 μm)) to support the copper strip as a bridge (Figure 1b). Interestingly, we again observed that there was growth on the entire sample except for the area under the copper bridge. From this experiment, we infer that a copper overlayer positioned in close proximity to the catalyst layer during the synthesis also inhibits CNT growth (Figure 1d). On the basis of the results obtained in the above-mentioned experiments, we wondered whether the CNT growth deactivation occurred because of the interaction of copper with the precursor gases26 or because of interaction between the iron catalyst and the copper overlayer (interdiffusion). We thus performed a third experiment in which we annealed the substrate (with argon and hydrogen; no hydrocarbon gas) with the copper foil positioned on the catalytic surface for 5 min. After annealing, we cooled the sample, removed the copper foil, and reinserted the sample into the CVD system for growing CNTs. We again observed that there was CNT growth on the entire sample except for the area that had been covered by the
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RESULTS AND DISCUSSION We first synthesized a CNT forest grown on a Fe/Al2O3 catalyst/underlayer system (deposited on a Si substrate) with a strip of copper foil positioned in the middle of the sample in contact with the catalytic surface to examine how the copper would affect CNT growth (Figure 1a). After growth, we visually observed that there was CNT growth on the entire sample except for the area under the copper strip (Figure 1c). From 12243
DOI: 10.1021/acs.jpcc.6b01676 J. Phys. Chem. C 2016, 120, 12242−12248
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Figure 3. HRSEM characterization of the catalytic surface after annealing for 5 min (above) and for 15 min (below) using a copper stencil during annealing. For each case, we distinguished three zones: (a) area before the copper bridge; (b) area under the copper bridge; (c) area after the copper bridge. Gases were flowing from left to right.
copper overlayer during the previous annealing step (Figure 1e). This result points to interdiffusion between the copper overlayer and the iron catalyst as the mechanism for catalyst deactivation. To test whether copper interacted with the precursor gases during CNT growth, we performed an annealing with the copper foil positioned as a bridge, 525 μm above the catalytic surface, followed by CNT growth after removing the copper bridge. If copper only killed the CNT catalysis by interaction with the growth gases (ethylene) during CNT growth, we would observe here full CNT growth, as we expect to observe before the copper bridge. However, we observed almost no CNT growth under the area where the bridge was positioned (Figure 1f), albeit slightly less “cleaned” compared to what we observed when using the copper stencil (Figure 1d). This further supports the hypothesis of interdiffusion between copper and iron catalyst with consequent catalyst deactivation during the CNT growth step as the dominant mechanism. HRTEM measurements of the dispersed CNTs (Figure 2) showed that the CNTs have an average diameter of 7 nm with 3−4 walls and exhibit a high degree of crystallinity. To examine what happened at the iron catalyst layer, we characterized the morphology of the catalytic surface using HRSEM after annealing the samples with a copper overlayer positioned directly above the catalyst layer (stencil) in the middle of the sample for two annealing durations: 5 and 15 min (Figure 3). We clearly observed larger catalyst dots on the surface where the copper foil was positioned during the annealing compared to the areas where there was no copper overlayer. We also noted that the density of the larger catalyst dots was almost four times higher for the sample annealed for 15 min compared to the sample annealed for only 5 min.
We then performed a CNT growth step for the abovementioned samples (Figure 3) in order to investigate CNT growth after annealing with a copper overlayer (Figure 4). On the sample that had been annealed for 15 min, we did not observe CNTs in the area that was positioned under the copper bridge while few very short filaments were visible on the sample that had been preannealed for only 5 min. These results show that the duration of the interaction between the copper overlayer and the iron catalyst is important. We observed that the CNT carpet was slightly taller for the 5-min anneal compared to the 15-min anneal. This is likely due to additional catalyst coarsening for increasing anneal in hydrogen, leading to CNTs of slightly larger diameter and shorter height.5 We also noticed that the CNTs are shorter after the copper stencil compared to the taller CNTs before the stencil (gas flow is from left to right). We hypothesize that this is due to some copper diffusion via gas phase, similar to autodiffusion, which we will discuss later. We chose copper since it is known to be a poor catalyst and an inhibitor for CNT growth. Although short carbon nanofiber growth using copper oxides has been reported, copper is known to be a poor catalyst27,28 because it does not lead to the formation of specific carbon−carbon bonds, and hence cannot be used as catalyst.29 Additionally, given the extremely low solubility of carbon in solid copper (e.g., only 0.0001 wt % C in Cu at 1100 °C),30,31 CNT nucleation is likely to be hindered by insufficient carbon diffusion into a copper catalyst nanoparticle. From the observation that no CNTs were present under the area where a copper stencil or bridge was positioned during the annealing step but then removed during CNT growth, we infer that the CNT growth inhibition is mainly caused by the interaction of copper with iron (interdiffusion) and not by the interaction of copper with the hydrocarbon gases. To understand whether copper diffuses into iron or vice versa, 12244
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Figure 4. Photo images (left) of CNT forests using preannealing durations of (a) 5 min and (b) 15 min. SEM images showing a magnification of different area in the samples: before the copper (left); where the copper was (center); after the copper (right). Gases were flowing from left to right.
we did an XPS measurement of a sample that was annealed for 15 min with copper foil on top of the catalyst layer (to check if Cu diffused into Fe) and of the side of the copper overlayer in contact with the sample (to check if Fe diffused into Cu). XPS results indicated the presence of trace amounts of copper on the sample but no iron was detected on the copper overlayer, indicating that copper diffused into the iron catalyst, thus poisoning it and inhibiting subsequent CNT growth (Figure 5). It is interesting to notice that a comparison of the bulk diffusivities of iron into copper and of copper into iron would have pointed to the opposite conclusion. From our calculations, using existing data on bulk diffusivity32,33 for T = 755 °C, we found that
D = (0.10 × 10−4) × exp[− 2.04 eV /(8.6 × 10−5 eV K−1 × 1028 K)] DFe−> Cu = 9.52 × 10−16 m 2 s−1
(b) the bulk diffusivity of copper in iron is D = (0.47 × 10−4) × exp[− 2.54 eV /(8.6 × 10−5 eV K−1 × 1028 K)]
DCu−> Fe = 1.56 × 10−17 m 2 s−1
The bulk diffusivity of iron in copper at 755 °C is almost 2 orders of magnitude larger than the bulk diffusivity of copper in iron at 755 °C. However, in our case we have bulk copper (foil)
(a) the bulk diffusivity of iron in copper is 12245
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Figure 5. XPS spectra of the sample after 15 min annealing with the copper strip above: full spectra (top left), copper peak (top right), iron peak (bottom left), and aluminum peak (bottom right).
diffusing into iron nanoparticles (iron catalyst dots) with radius of a few nanometers. Our XPS results indicate that the copper diffusivity into nanoscale iron particles is over 2 orders of magnitude faster compared to bulk diffusion. Considering that it was shown that many physical parameters such as melting
points dramatically diminish for nanoscale dots compared to bulk,34 we can understand the corresponding increase of diffusivity of copper into nanodots of iron that we observed. Looking carefully at the results obtained, we noticed that the area of no-growth under the copper overlayer was clearly 12246
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Finally, we tested our two-step technique (anneal with copper stencil and CNT growth after removing the stencil) with a simple pattern, with the synthesis results shown in Figure 8. To improve the contrast between the areas covered with CNTs and the areas where we expected only substrate, we performed a short oxygen etching (3 min) as described above. The pattern was well replicated although sharp corners ended up slightly rounded. Overall, the technique presented is a simple nonlithographic way to pattern CNT forests for applications that do not require lithographic precision.
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Figure 6. Patterned CNT growth using a two-step process: (1) anneal with copper overlayer and (2) subsequent CNT growth without copper foil. (a) Copper overlayer directly positioned above the catalytic surface during anneal and (b) copper bridge above the catalytic surface during anneal.
CONCLUSION In summary, we demonstrated a simple approach to grow patterned CNT forests by using a copper overlayer in contact with (stencil) or above (bridge) the catalyst layer to deactivate CNT growth. Our characterizations point to a mechanism of interdiffusion of copper into iron particles leading to catalyst deactivation. We showed that direct positioning of the copper directly above the iron layer and longer annealing times prior to CNT growth led to the best results. This study provides a novel and simple technique for patterning CNT forests and also sets the ground for future studies focused on the investigation of the effect of overlayers of different materials on CNTs or the effect of overlayers on the growth of other 1D and 2D nanostructures.
Figure 7. Short oxygen etching step to clean the copper-deactivated area.
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delineated when the copper overlayer was in contact with the catalytic surface of the sample. However, when the copper was positioned as a bridge, we noticed a “tail” of no growth after the bridge in the direction of the gas flow (Figure 6). A possible explanation is that an additional mechanism similar to autodiffusion, where two samples positioned in a reactor in close proximity interact with each other, is at play: some copper atoms are carried by the gas flow and deposited on the catalytic surface (note that the bridge is positioned 525 μm above the catalytic surface). When the copper overlayer is directly positioned on the catalytic surface, the copper atoms carried by the gas flow are so close to the catalytic surface that they almost immediately encounter the sample’s surface where they deposit, thus not forming a “tail” of no growth. In most samples, we noticed that the area with the copper stencil or bridge still had some small residual CNT growth. To increase the contrast between the areas with CNTs and the copper-deactivated areas, we performed short oxygen anneals (up to 3 min), which cleaned the area with few residual CNTs with not noticeable impact on the CNT forest (Figure 7). This post-process can also be integrated to the CNT growth step at the end of CNT growth, prior to cooling the sample.
AUTHOR INFORMATION
Corresponding Author
*
[email protected]; phone: +97237384540. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially funded by the Israel Science Foundation (ISF) through the Israel National Research Center for Electrochemical Propulsion (INREP) and I-CORE Program (2797/11). We are grateful to Dr. Michal Ejgenberg for her help in operating XPS instrument and to Itamar Padel for help in preparing copper stencils.
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Figure 8. Patterned CNT growth using a two-step process: (1) anneal with copper overlayer and (2) subsequent CNT growth without copper foil. (a) Sample before synthesis; (b) sample after synthesis of CNTs; (c) sample after “cleaning” by using Ar 500 sccm and Ar/O2 200 sccm for 3 min. 12247
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