Controlled partial embedding of carbon nanotubes within flexible transparent layers

Applications of carbon nanotubes (CNTs) like field emission displays, super-capacitors, and cell growth scaffolds can benefit from controllable embedding of the CNTs in a material such that the CNTs are anchored and protrude a desired length. We demonstrate a simple method for anchoring densely packed, vertically aligned arrays of CNTs into silicone layers using spin-coating, CNT insertion, curing, and growth substrate removal. CNT arrays of 51 and 120 µm in height are anchored into silicone layers of thickness 26 and 36 µm, respectively. Scanning electron microscopy (SEM) and optical microscopy are used to characterize the sample morphology, a 5.5 m s^-1 impinging water jet is used to apply shear stress, and a tensile test shows that the silicone layer detaches from the substrate before the CNTs are ripped from the layer. The CNTs are thus well anchored in the silicone layers. The spin-coating process gives control over layer thickness, and the method should have general applicability to various nanostructures and anchoring materials

Experimental Investigation on Patterning of Anchored and Unanchored Aligned Carbon Nanotube Mats by Fluid Immersion and Evaporation

Pattern formation by capillary forces in a nanoscale system was studied experimentally. Densely packed, vertically aligned mats of order 100 microns in height comprised of 20 nm diameter multi-walled carbon nanotubes were fabricated and treated with various liquids. The carbon nanotubes deflected and rearranged under the action of surface tension as the liquids evaporated, and remained fixed once dried. The size analysis of the resulting patterns in these experiments and in the literature showed they are distributed within one standard deviation from the mean, and there are, in general, many more small sizes than large ones within a pattern. Preexisting defects in the mats were found to play a significant role in the pattern formation process, both in this work and in the literature, whereas the properties of the specific liquid used and the height of the mats did not. A novel method for anchoring the aligned mats within another material using spin-coating was developed. An anchored mat made in this way was successfully held in place even under the application of a 5.5 m/s water jet. The anchoring method allowed the first known investigation of the role of boundary conditions in this pattern formation process. Under identical experimental conditions to cases where patterns are formed in the unanchored mats, it was found that no pattern formation occurs in the anchored mats. A population balance model based on conservation of area was applied to the pattern formation process, but sufficient details are lacking to make predictions. The anchoring method and its prevention of pattern formation is a very important finding, and is relevant to applications of the aligned mats, such as field emission displays, supercapacitors, tissue culture scaffolds, and friction drag reducing surfaces.</p

Capillography of Mats of Nanofibers

Capillography (from the Latin capillus, 'hair', and the Greek graphein, to write ) is a recently conceived technique for forming mats of nanofibers into useful patterns. The concept was inspired by experiments on carpetlike mats of multiwalled carbon nanotubes. Capillography may have the potential to be a less-expensive, less-time-consuming alternative to electron-beam lithography as a means of nanoscale patterning for the fabrication of small devices and instruments. In capillography, one exploits the lateral capillary forces exerted on small objects that pierce the surface of a liquid. If the small objects are identical, then the forces are always attractive. Two examples of the effects of such forces are the agglomeration of small particles floating on the surface of a pond and the drawing together of hairs of a wet paintbrush upon removal of the brush from water. Because nanoscale objects brought into contact remain stuck together indefinitely due to Van der Waals forces, patterns formed by capillography remain even upon removal of the liquid. For the experiments on the mats of carbon nanotubes, a surfactant solution capable of wetting carbon nanotubes (which are ultra-hydrophobic) was prepared. The mats were wetted with the solution, then dried. Once the mats were dry, it was found that the nanotubes had become ordered into various patterns, including nestlike indentations, trenches, and various combinations thereof. It may be possible to exploit such ordering effects through controlled wetting and drying of designated portions of mats of carbon nanotubes (and, perhaps, mats of nanofibers of other materials) to obtain patterns similar to those heretofore formed by use of electron-beam lithography. For making patterns that include nestlike indentations, it has been conjectured that it could be possible to control the nesting processes by use of electrostatic fields. Further research is needed to understand the physics of the patterning processes in order to develop capabilities to control patterns formed in capillography

Nanowicks

Nanowicks are dense mats of nanoscale fibers that are expected to enable the development of a variety of novel capillary pumps, filters, and fluidic control devices. Nanowicks make it possible obtain a variety of novel effects, including capillary pressures orders of magnitude greater than those afforded by microscale and conventional macroscale wicks. While wicking serves the key purpose of transporting fluid, the nanofiber geometry of a nanowick makes it possible to exploit additional effects -- most notably, efficient nanoscale mixing, fluidic effects for logic or control, and ultrafiltration (in which mats of nanofibers act as biomolecular sieves)

Inherent-opening-controlled pattern formation in carbon nanotube arrays

We have introduced inherent openings into densely packed carbon nanotube arrays to study self-organized pattern formation when the arrays undergo a wetting–dewetting treatment from nanotube tips. These inherent openings, made of circular or elongated hollows in nanotube mats, serve as dewetting centres, from where liquid recedes from. As the dewetting centres initiate dry zones and the dry zones expand, surrounding nanotubes are pulled away from the dewetting centres by liquid surface tension. Among short nanotubes, the self-organized patterns are consistent with the shape of the inherent openings, i.e. slender openings lead to elongated trench-like structures, and circular holes result in relatively round nest-like arrangements. Nanotubes in a relatively high mat are more connected, like in an elastic body, than those in a short mat. Small cracks often initialize themselves in a relatively high mat, along two or more adjacent round openings; each of the cracks evolves into a trench as liquid dries up. Self-organized pattern control with inherent openings needs to initiate the dewetting process above the nanotube tips. If there is no liquid on top, inherent openings barely enlarge themselves after the wetting–dewetting treatment