ABSTRACT
The technique of chemical vapor deposition has been widely proved to be one of the most useful means to grow perpendicularly aligned CNTs on various substrates. Recently, Lu and coworkers reported an interesting work where controlled growth of CNTs with different alignments was achieved via a laser-assisted CVD process [30]. In this process, a Si wafer pre-patterned with ruthenium (Ru) electrodes was used as the substrate. In the subsequent CVD procedure, biasing voltages of different polarities were applied on the Ru electrodes to which tri-layer Al/Fe/Al catalysts were introduced. SEM observations showed that surface-bounded CNTs were obtained on the anodes, while perpendicularly aligned CNTs were produced on the cathodes. The growth alignment control was attributed to the movement of electrically charged catalyst nanoparticles under the influence of an external electric field. Besides the most commonly used Si or quartz substrate, the versatility of CVD technique makes it applicable for the CNT growth on various other substrates, such as single crystal magnesia [31], stainless steel [32], inconel [33], and even ceramic fibers [34]. A recent collaboration between Dai’s group and Wang’s group has exploited a low-pressure CVD process for growth of perpendicularly aligned MWCNT (PA-MWCNT) arrays that are dominated by a straight body segment but with a randomly entangled top layer [35]. After a small piece of such as-grown PA-MWCNT film (4 mm × 4 mm) was finger-pressed from the Si side onto a perpendicularly positioned glass slide, a 1480 g book was able to suspend from the glass surface, as shown in Fig. 3.1a. The calculated adhesion force parallel to the glass slide is 90.7 N⋅cm-2, almost 10 times that of a gecko foot, and much stronger than the normal adhesion force. That is to say, the array film is strongly bound along the shear direction and easily lifted in the normal direction. More important is that the array film could be repeatedly attached and detached from the glass surface with no decrease of the supported weight. Similar adhesion behaviors were also observed on other substrates such as poly(ethylene terephthalate) (PET) sheet, polytetrafluoroethylene
(PTFE) film, and rough sandpaper, demonstrating that the PAMWCNT arrays could be used for fabrication of gecko-foot-mimetic dry adhesives.
