Microwave-Assisted Fabrication of Carbon Nanotube AFM Tips

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SCHEME 1. Experimental Setup a 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. 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 intoa5mMethanoic 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 © 2010 American Chemical Society 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 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. © 2010 American Chemical Society 4011 DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009-–4012
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Microwave-Assisted Fabrication of Carbon Nanotube AFM Tips

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