pubs.acs.org/NanoLett
Microwave-Assisted Fabrication of Carbon Nanotube AFM Tips Tamara S. Druzhinina,† Stephanie Hoeppener*,†,‡,§ and Ulrich S. Schubert*,†,‡,§ †
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands, ‡ Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands, and § Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University Jena, Humboldtstrasse 10, D-07743 Jena, Germany ABSTRACT A new, fast, alternative approach for the fabrication of carbon nanotube (CNT) atomic force microscopy (AFM) tips is reported. Thereby, the tube material is grown on the apex of an AFM tip by utilizing microwave irradiation and selective heating of the catalyst. Reaction times as short as three minutes allowed the fabrication of CNT AFM tips in a highly efficient process. This method represents a promising approach toward a cheaper, faster, and straightforward synthesis of CNT AFM tips. KEYWORDS Carbon nanotubes, microwave, AFM tip, carbon nanotube synthesis, catalyst
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ment of the CNT on the AFM tip is usually performed by using scanning electron microscope (SEM) manipulators, where individual tubes are picked and stabilized on the AFM tip with locally deposited carbon. This process is timeconsuming and requires a rather expensive experimental infrastructure and is also difficult to be used for a scale-up of the manufacturing process. Alternatively, the direct growth of CNTs onto AFM tips can be used. For this purpose different methods can be utilized, e.g., surface or pore growth. In particular the chemical vapor deposition (CVD) is frequently used and yields thin CNTs grown directly on the tip apex. Besides a relatively fast production time, still dedicated equipment as well as rather harsh reaction conditions are required using these conventional CVD approaches. Due to the fact that all these methods are time-consuming and costly, there is a demand for alternative methods for the formation of AFM CNT-tips, which makes them affordable and allows their use not only for very specialized applications. Here we introduce an alternative approach that allows the fabrication of carbon nanotube AFM probes utilizing the microwave-assisted growth of CNTs directly on the apex of a commercially available AFM tip. This approach benefits from the selective heating of tip mounted catalyst particles due to a preferential absorption of the microwave irradiation, which results in a strong, local increase of the temperature that is sufficient to grow multiwall carbon nanotubes in the presence of ethanol vapor within very short time scales of a few minutes only. Different aspects of the fabrication process are discussed, and an optimized procedure to fabricate CNT tips is presented. The microwave-assisted synthesis of CNTs on AFM tips was performed according to a previously reported method.30 For the microwave irradiation in a synthetic laboratory single mode microwave (Emrys Liberator, Biotage) was used.
canning force microscopy has developed into a standard tool in material research and represents a frequently used technique in nearly all fields of science, including, e.g., chemistry, physics, biology, and others.1-4 The resolution of this technique is, however, strongly related to the quality of the available tip material, which limits not only the lateral resolution but also implies limitations with respect to the investigation of, e.g., steep edges.5,6 While commercially available atomic force microscopy (AFM) tips are fabricated by using silicon microfabrication techniques reaching a typical resolution of approximately 10 nm, tailormade tip layouts have been proposed to improve the tip performance. The tip quality depends mainly on the dimensions and shape of the probe, the durability of the tip apex, and the nature of the interaction between sample and probe. In this respect, in particular, AFM tips functionalized with a carbon nanotube (CNT) have attracted considerable attention. Due to the high Young’s modulus of the CNTs and their excellent aspect ratio,7 attempts have been made to use them as probes for AFM experiments. Not only their unique mechanical but also their chemical and electronic properties8-12 open attractive possibilities that might result in improving imaging performance13,14 or in measuring the properties of CNTs.15 Due to the high resolution of CNT AFM tips, they can be used to image very fine structures, such as biological and molecular materials. Several studies have been performed where CNT AFM tips were used to image biological materials, such as, DNA or proteins.16-18 Different methods have been developed either to directly grow CNTs on AFM tips19-23 or to place CNTs on tips.24-27 The place-
* Corresponding authors. E-mail:
[email protected] (S.H.),
[email protected] (U.S.S.). Telephone: +49 (0)3641 948261 (S.H.), +49 (0)3641 948202 (U.S.S.). Received for review: 06/1/2010 Published on Web: 09/24/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009–4012
SCHEME 1.
Experimental Setupa
a Quartz glass pressure vial with mounted AFM cantilevers, support, and liquid ethanol reservoir. Microwave cavity of the single mode synthetic microwave. The pressure vial is transferred to the cavity, and microwave irradiation is applied.
Typical irradiation times of 5 min were applied.31 The reactions were performed in capped pressure vials which were loaded with 5 mL of ethanol. The samples were placed onto a quartz glass support above the liquid level of the ethanol (Scheme 1). The setup permits the use of a liquid ethanol reservoir as the carbon source in the bottom of the vial and a glass support that affords the placement of the substrate above the liquid level. This approach was demonstrated to allow the synthesis of carbon nanotubes under relatively mild synthetic conditions in rather short time scales of less than five minutes. To use this approach also to grow CNTs directly on an AFM tip, a few critical issues had to be addressed. In particular the question if the conditions to initiate the growth of CNTs onto the small area of the AFM tip can be matched had to be investigated. In the previously reported experiments it was essential for the successful growth of CNTs onto substrates that a sufficiently high pressure was generated in the vial, due to the evaporation of the carbon source, i.e., ethanol. To obtain the required synthesis conditions the total power of the microwave was limited to 200 W in the course of the experiments. Due to the restriction of the maximum temperature (250 °C) and pressure (21 bar) that can be generated within the pressure vials, the microwave irradiation usually stops automatically due to a safety shut down of the microwave. As a consequence the experimental conditions had to be adjusted to these limitations. Moreover, heat dissipation effects had to be taken into consideration. Therefore, commercially available AFM tips were mounted onto small pieces (0.5 × 1 cm) of silicon wafer by means of a conducting silver paste which was used to glue the chip, to which the cantilevers and the tips are connected, onto the substrate. This allowed the control of the heat dissipation from the relatively small area of the tip material itself and, moreover, permited the convenient handling of the tips. © 2010 American Chemical Society
FIGURE 1. SEM micrographs of the AFM tip before (a) and after microwave irradiation (b). The whole AFM tip is covered with nickel acetate catalyst (a) and CNTs after microwave irradiation (b).
In a first experiment, commercially available AFM tips were just immersed into a 5 mM ethanoic solution of nickel acetate (Sigma Aldrich) dissolved in ethanol. Subsequently, the solvent was allowed to dry, and the AFM tips were mounted onto the silicon support. Scanning electron microscopy (SEM) investigations (Quanta 3D FEG, FEI, The Netherlands) prior to the microwave irradiation revealed that the catalyst material was homogenously deposited on the whole tip area (Figure 1a) as indicated by the flake-like structures on the AFM tip. This nickel acetate covering was transformed into nickel catalyst particles in the course of the microwave irradiation by thermal activation, and the individually formed particles were used as catalyst particles for the growth of CNTs. Figure 1b depicts a SEM image that was recorded after the microwave irradiation process, and it is observed that a homogeneous coating of the tip with CNTs was obtained. Thus, it could be confirmed that the chosen irradiation conditions were sufficient to obtain the required temperature and pressure conditions to form CNTs also on the AFM tip. In the next step the optimization of the catalyst deposition process was addressed to ultimately be able to grow only individual CNTs on an AFM tip. For this purpose, different approaches were tested to limit the amount of catalyst deposition. It was found that this can be achieved best by 4010
DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009-–4012
FIGURE 2. SEM micrographs of an AFM-tip before (a) and after microwave irradiation (b).
simple scanning of the tip over a dried solution of nickel acetate drop casted onto a silicon substrate at higher contact forces (NTegra Aura AFM, NT-MDT, Russia). In this case a lower amount of nickel acetate is deposited onto the tip, which can provide the catalyst seed for the CNT growth. Figure 2a depicts the deposited material present on the AFM tip after scanning a small area on the nickel acetate loaded substrate. The presence of material is very visible at the slope of the tip, indicating that small amounts of the nickel acetate were attached to the tip. The prepared tips were subsequently mounted in the microwave vials, and the CNT growth was pursued. Figure 2b shows the conversion of the nickel acetate material into the catalyst particles, which are observed both on the tip apex as well as on the slope of the tip itself. It could be observed that few CNTs are grown from the tip, and in particular, one CNT protrudes vertically away from the tip. This CNT has a length of approximately 600 nm and a diameter of 20 nm. Due to the length, a bending of the CNT is observed.28,29 This result demonstrates that it is possible to obtain reaction conditions within the pressure vial that permit the growth of individual CNTs due to an effective compensation of the heat dissipation effects. In previous experiments, performed to grow individual CNTs onto solid substrates, it was observed that this represents a challenging task, which was up to now not sufficiently implemented into the surface-based synthesis approach. However, still the amount of catalyst material attached to the tip is not yet well controlled due to the fact that the AFM tip collects considerable amounts of nickel acetate during the scanning process of the dried layer. AFM force spectroscopy was conducted to further demonstrate the successful functionalization of the AFM tips with carbon nanotubes. Therefore, a set of measurements was performed that included first the recording of an approach and retraction curve with a CNT-modified AFM tip in the static AFM mode. Figure 3a displays a representative curve that clearly demonstrates the bending of the cantilever away from the surface, when the CNTs are in contact with the surface and start to slide away or buckle. Relatively large adhesion forces suggest in this case that several CNTs are attached to the cantilever, which have a length of approximately 60 nm, as estimated from the z-displacement position of the onset of the bending curve until the typical proportional deflection of the cantilever is observed. These curves are reproducible © 2010 American Chemical Society
FIGURE 3. Force spectroscopy of CNT functionalized AFM tips. (a) Representative deflection vs distance plot of a CNT modified tip. (b) Measurement with the same tip after the CNT material was removed by appling higher forces. (c) I-V curve of a CNT-modified tip gently approached onto the substrate. 4011
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indicating the stability of the CNTs onto the tip. After this significantly higher forces were applied onto the tip to remove the CNT material on purpose. The force spectroscopic measurements after this process (Figure 3b) indicate the characteristic deviation of the deflection vs distance curves, and a significant decrease of the adhesion forces was observed. Moreover, the characteristic snap-in points are clearly visible, without any indication for a bending of the cantilever prior to the contact of the AFM tip. Additionally, the current-voltage characteristic (Figure 3c) was measured on a CNT-modified metal-coated AFM that was gently brought into contact with a graphite substrate. In this case a small significantly reduced conductivity could be measured compared to nonmodified metal-coated tips. In conclusion, a powerful process was developed that allows the direct fabrication of CNT AFM tips utilizing efficient synthetic conditions generated in a single mode microwave reactor. It could be demonstrated that the growth of individual CNTs can be achieved, and the optimization of the preparation conditions resulted in a promising approach that enabled the fabrication of CNT AFM tips utilizing relatively mild synthesis conditions. In particular the relatively low experimental affords as well as the fast fabrication times are a general advantage of the introduced method and provide a promising, cheap technique to fabricate CNT AFM tips. The deposition of the catalyst material could be further improved by utilizing particle picking approaches, e.g., by force vs distance curve recording, to further increase the controllability of the presented approach.
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Acknowledgment. The authors are grateful for the financial support of the Dutch Council of Scientific Research (NWO) by a VICI grant awarded to U.S.S. This research has been carried out with the support of the Materials and Interface Chemistry Research Unit (SEM) and the Department of Chemical Engineering and Chemistry, Eindhoven University of Technology. Dr. Alexander Alexeev is kindly acknowledged for help with the conductivity measurements and for fruitful discussions. We thank the Dutch Polymer Institute (DPI, technology area HTE) for funding.
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DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009-–4012
