Sb

Fig. 9. Directed growth of suspended SWNT. (a) A nanotube power-line structure. (b) A square of nanotubes. (c) An extensive network of suspended SWNTs the tops of pillars pre-fabricated on a silicon substrate. The stamped substrate is calcined and then used in CVD growth.

Remarkably, the SWNTs grown from the pillar tops tend to direct from pillar to pillar. Well-directed SWNT bridges are obtained in an area of the substrate containing isolated rows of pillars as shown in Fig. 9a, where suspended tubes forming a power-line like structure can be seen. In an area containing towers in a square configuration, a square of suspended nanotube bridges is obtained (Fig. 9b). Suspended SWNTs networks extending a large area of the substrate are also formed (Fig. 9c). Clearly, the directions of the SWNTs are determined by the pattern of the elevated structures on the substrate. Very few nanotubes are seen to extend from pillars and rest on the bottom surface of the substrate.

The self-directed growth can be understood by considering the SWNT growth process on the designed substrates [25]. Nanotubes are nucleated only on the tower-tops since the catalytic stamping method does not place any catalyst materials on the substrate below. As the SWNTs lengthen, the methane flow keeps the nanotubes floating and waving in the 'wind' since the flow velocity near the bottom surface is substantially lower than that at the level of the tower-tops. This prevents the SWNTs from being caught by the bottom surface. The nearby towers on the other hand provide fixation points for the growing tubes. If the waving SWNTs contact adjacent towers, the tube-tower van der Waals interactions will catch the nanotubes and hold them aloft.

Growing defect-free nanotubes continuously to macroscopic lengths has been a challenging task. In general, continuous nanotube growth requires the catalytic sites for SWNTs remaining active indefinitely. Also, the carbon feedstock must be able to reach and feed into the catalytic sites in a continuous fashion. For CVD growth approaches, this means that catalyst materials with high surface areas and open pore structures can facilitate the growth of long nanotubes, as discussed earlier.

We have found that within the methane CVD approach, small concentrations (~0.03%) of benzene generated in-situ in the CVD system can lead to an appreciable increase in the yield of long nanotubes. Individual SWNTs with length up to 0.15 mm (150 ^.m) can be obtained [25]. An SEM image of an approximately 100|j.m long tube is shown in Fig. 10. The SWNTs appear continuous and strung between many pillars. In-situ generation of benzene is accomplished by catalytic conversion of methane in the CVD system using bulk amounts of alumina supported Fe/Mo catalyst [25]. This leads to enhanced SWNT growth, and a likely explanation is that at high temperatures, benzene molecules are highly reactive compared to methane, therefore enhancing the efficiency of carbon-feedstock in nanotube growth. However, we find that when high concentrations of benzene are introduced into the CVD growth system, exceedingly low yields of SWNTs result with virtually no suspended tubes grown on the sample. This is because high concentrations of benzene undergo extensive pyrolysis under the high temperature CVD condition, which causes severe catalyst poisoning as amorphous carbon is deposited on the catalytic sites therefore preventing SWNT growth.

Fig. 10. SEM image of a CVD grown approximately 100|i.m long SWNT strung between silicon pillars

Fig. 10. SEM image of a CVD grown approximately 100|i.m long SWNT strung between silicon pillars

2.3 Growth of Isolated Nanotubes on Specific Surface Sites

A CVD growth strategy has been developed collaboratively by our group and Quate to grow individual single-walled nanotubes at specific sites on flat silicon oxide substrates [20]. The approach involves methane CVD on substrates containing catalyst islands patterned by electron beam lithography. 'Nanotube chips' with isolated SWNTs grown from the islands are obtained. Atomic Force Microscopy (AFM) images of SWNTs on such nanotube-chips are shown in Fig. 10, where the synthesized nanotubes extending from the catalyst islands are clearly observed. This growth approach readily leads to SWNTs originating from well controlled surface sites, and have enabled us to develop a controlled method to integrate nanotubes into addressable structures for the purpose of elucidating their fundamental properties and building devices with interesting electrical, electromechanical and chemical functions [23,38,39,40,41,42,43,44].

2.4 From Growth to Molecular-Wire Devices

Integrating individual nanotubes into addressable structures is important to the characterization of nanotubes. It is necessary to investigate individual tubes because the properties of nanotubes are highly sensitive to their structural parameters, including chirality and diameter. Currently, all of the growth methods yield inhomogeneous materials containing nanotubes with various chiralities. Therefore, measurements of ensembles of nanotubes can only reveal their bulk averaged properties.

2.4.1 Electrical Properties of Individual Nanotubes

Previous approaches to individual SWNT electrical devices include randomly depositing SWNTs from liquid suspensions onto pre-defined electrodes [4,45], or onto a flat substrate followed by locating nanotubes and patterning electrodes [46,47,48,49]. We have demonstrated that controlled nanotube growth strategies open up new routes to individually addressable nanotubes. The SWNTs grown from specific sites on substrates can be reliably contacted by electrodes (Figs. 11c, 12a) and characterized [38]. Metal electrodes are placed onto the two ends of a nanotube via lithography patterning and electron beam evaporation. Detailed procedure for contacting a SWNT can be found in [38].

The formation of low ohmic contacts with SWNTs is critical to elucidating their intrinsic electrical properties [40] and building devices with advanced characteristics. This is accomplished by our controlled approach of growing and contacting nanotubes. We found that titanium metal contacts give rise to the lowest contact resistance compared to other metals. Metallic SWNTs that are several microns long typically exhibit two-terminal resistance on the order of tens to hundreds of kilo-ohms. The lowest single-tube resistance measured with our individual metallic SWNT is ~ 12 k ^ (Fig. 12b). The low contact resistance achieved in our system can be attributed to several factors. The first is that our method allows the two metal electrodes to contact the two ends of a nanotube. Broken translational symmetry at the nanotube ends could be responsible for the strong electrical coupling between the tube and metal [50]. Secondly, titanium-carbon (carbide) bond formation at the metal-tube interface may have occurred during the electron-beam evaporation process. This is based on our result that titanium yields lower contact resistance than other metals, including aluminum and gold, and the fact that

Fig. 11. Growth of single-walled nanotubes on controlled surface sites and device integration. (a) An atomic force microscopy image of SWNTs grown from patterned catalyst dots (while spots). (b) AFM image of a SWNT grown between two catalyst sites (white corners). (c) AFM image of a SWNT grown from a catalyst island and contacted by metal electrodes

Fig. 11. Growth of single-walled nanotubes on controlled surface sites and device integration. (a) An atomic force microscopy image of SWNTs grown from patterned catalyst dots (while spots). (b) AFM image of a SWNT grown between two catalyst sites (white corners). (c) AFM image of a SWNT grown from a catalyst island and contacted by metal electrodes aluminum and gold do not form strong bonding with carbon and stable carbide compounds.

For individual semiconducting SWNTs grown on surfaces, relatively low resistance devices on the order of hundreds of kilo-ohms can be made by our approach [40]. These nanotubes exhibit p-type transistor behavior at room temperature as their conductance can be dramatically changed by gate voltages (Fig. 12c). This property is consistent with the initial observation made with high resistance samples (several mega-ohms) by Dekker [51] and by Avouris' group [52]. The transconductance (ratio of current change over gate-voltage change) of our semiconducting tube samples can be up to ~200nA/V [40] which is two orders of magnitude higher than that measured with earlier samples. The high transconductance is a direct result of the relatively low resistance of our semiconducting SWNT samples, as high currents can be transported through the system at relatively low bias voltages. This result should not be underestimated, given the importance of high transcon-ductance and voltage gain to transistors. Nevertheless, future work is needed in order to create semiconducting SWNT devices with transconductance and voltage gains that match existing silicon devices.

Fig. 12. Electrical properties of individual single-walled carbon nanotubes. (a) AFM image of an individual single-walled nano-tube contacted by two metal electrode. The length of the nanotube between electrodes is ~ 3|i.m.

(b) Temperature dependent resistance of a metallic SWNT.

(c) Current (I) vs. voltage (V) characteristics of a semiconducting nanotube under various gatevoltages (Vg)

Fig. 12. Electrical properties of individual single-walled carbon nanotubes. (a) AFM image of an individual single-walled nano-tube contacted by two metal electrode. The length of the nanotube between electrodes is ~ 3|i.m.

(b) Temperature dependent resistance of a metallic SWNT.

(c) Current (I) vs. voltage (V) characteristics of a semiconducting nanotube under various gatevoltages (Vg)

2.4.2 Nanotube Electromechanical Properties and Devices

Controlled growth of nanotubes on surfaces combined with microfabrication approaches allows the construction of novel nanotube devices for new types of studies. For instance, the question of how mechanical deformation affects the electrical properties of carbon nanotubes has been intriguing [53,54,55,56,57], and is important to potential applications of nanotubes as building blocks for nanoscale electro-mechanical devices. To address this question experimentally, it will be desired to obtains suspended nanotubes that can be manipulated mechanically and at the same time addressed electrically. To this end, we have grown individual SWNTs from patterned catalyst sites across pre-fabricated trenches on SiO2/Si substrates [43]. This led to an individual SWNTs partially suspended over the trenches (Fig. 13a). The nanotube is then contacted at the two ends by metal electrodes. The suspended part of the nanotube can be manipulated with an AFM tip, while the resistance of the sample is being monitored. For a metallic SWNT with ~ 610 nm suspended length, an AFM tip is used to repeatedly push down the suspended nanotube and then retract (Fig. 13b). We observe that the nanotube conductance decreases each time the AFM tip pushes the nanotube down, but recovers as the tip retracts. The repeated pushing and retracting cause oscillations in the cantilever/nanotube deflection and sample conductance, with equal periodicity in the two oscillations (Fig. 13c). The full reversibility of the nanotube electrical conductance upon tip retraction suggests that the metal-tube contacts are not affected each time the tip deflects the suspended part of the nanotube. The observed change in sample conductance is entirely due to the mechanical deformation of the SWNT caused by the pushing tip. The reversibility also suggests that the suspended part of the SWNT is firmly fixed on the substrate next to the trench. The length of the SWNT resting on the SiO2 surface (> 1.5|im on both sides of the trench) does not stretch or slide on the substrate when the suspended part is pushed. Anchoring of the nanotube on the SiO2 substrate is due to strong tube-substrate van der Waals interactions.

The conductance is found to decrease by a factor of two at ~ 5° bending angle (strain ~ 0.3%), but decrease more dramatically by two orders of magnitude at a bending angle ~ 14° (strain ~ 3%, Fig. 13d) [44]. To understand these electromechanical characteristics, Wu and coworkers at the University of Louisville have carried out order-W non-orthogonal tight-binding molecular-dynamics simulations of a tip deflecting a metallic (5,5) SWNT, with the tip modeled by a short and stiff (5,5) SWNT cap [44,58]. At relatively small bending angles, the nanotube is found to retain sp2 bonding throughout its structure, but exhibits significant bond distortion for the atoms in the region near the tip. As tip-pushing and bending proceed, the nanotube structure progressively evolves and larger structural-changes occur in the nanotube region in the vicinity of the tip. At a 15° bending angle, the average number of bonds per atom in this region is found to increase to ~ 3.6, suggesting the appearance of sp3-bonded atoms (marked in red in Fig. 13e). This causes a significant decrease in the local ^-electron density as revealed by electronic structure calculations. Since the ^-electrons are delocalized and responsible for electrical conduction, a drastic reduction in the ^-electron density is responsible for the significant decrease in conductance. Simulations find that the large local sp3 deformation is highly energetic, and its appearance is entirely due to the forcing tip. The structure is found to fully reverse to sp2 upon moving the tip away in the simulation.

Using the Landau-Buttiker formula, Wu and coworkers have calculated the conductance of the (5,5) tube vs. bending angle [58]. It is found that

Fig. 13. Electromechanical characteristics of suspended nanotubes. (a) AFM image of a SWNT with a ~ 605 nm long suspension over a trench. The bright spots around the suspended tube part are caused by tube touching and sticking to the side of the pyramidal scanning tip. (b) A schematic view of the electromechanical measurement setup. (c) Cantilever deflection (A Zc, upper graph) and nanotube electrical conductance (G, lower graph) evolution during repeated cycles of pushing the suspended SWNT and retracting. (d) Electrical conductance (G) of the SWNT sample vs. bending angle (ff). (e) Simulated atomic configurations of a (5,5) SWNT pushed to a 15 degrees bending angle by an AFM tip

Fig. 13. Electromechanical characteristics of suspended nanotubes. (a) AFM image of a SWNT with a ~ 605 nm long suspension over a trench. The bright spots around the suspended tube part are caused by tube touching and sticking to the side of the pyramidal scanning tip. (b) A schematic view of the electromechanical measurement setup. (c) Cantilever deflection (A Zc, upper graph) and nanotube electrical conductance (G, lower graph) evolution during repeated cycles of pushing the suspended SWNT and retracting. (d) Electrical conductance (G) of the SWNT sample vs. bending angle (ff). (e) Simulated atomic configurations of a (5,5) SWNT pushed to a 15 degrees bending angle by an AFM tip at relatively small bending angles, the conductance of the SWNT exhibits appreciable decrease, but the decrease is relatively gradual. The conductance decrease is caused by the relatively large C-C bond distortions in the nanotube region near the tip. The decrease is gradual because the overall nanotube structure remains in the sp2 state. At large bending angles, the nanotube conductance decreases dramatically, as sp3 bonding sets into the nanotube structure. These results agree qualitatively with experimental data and thus provide a detailed rationale to the observed nanotube electromechanical characteristics. The combined experimental and theoretical study leads to an in-depth understanding of nanotube electromechanical properties, and suggests that SWNTs could serve as reversible electro-mechanical transducers that are potentially useful for nano-electro-mechanical devices.

The electro-mechanical characteristics are elucidated when the nanotube deformation is caused by a manipulating local probe. This should be distinguished from previous theoretical considerations of 'smoothly' bent nano-tubes. Nanotube bending in most of the earlier investigations is modeled by holding the ends of a nanotube such that the nanotube is at a certain bending configuration [54,55,56]. In most of the cases studied, the nanotube remains in the sp2 state with only small bond distortions throughout the structure. Therefore, the nanotube conductance has been found to be little changed (< 10 fold) under bending angles up to 20°. Nevertheless, Rochefort et al. did find that at larger bending angles (e.g. 0=45°), the electrical conductance of a metallic (6,6) SWNT is lowered up to 10-fold [55,56]. The physics studied in our case should be applicable to SWNTs containing large local deformations caused by other forces. For instance, if a highly kinked SWNT stabilized by van der Waals forces on a substrate develops sp3 type of bonding characteristics at the kink, the electrical conductance should be significantly reduced compared to a straight tube.

0 0

Post a comment