|Techniques to align CNT networks in-plane on a substrate have been investigated as a means to enable CNT-based electronic devices including transistors ad interconnects. One approach—transferring CNT thin films from one surface to another via a soft lithography—suffers from limited ability to achieve good adhesion of the CNT films to the transfer substrate and imprecise alignment of the CNT patterns. Ultimately, a transfer technique that can be scaled to large area with high throughput processing at low temperature is required to achieve flexible substrates for many emerging display, lighting, and solar PV applications. Pint et. al. report a scalable means to create aligned CNT thin film patterns on both rigid and flexible substrates.|
Reviewed by Jeff Morse, PhD., National Nanomanufacturing Network
Pint CL, Xu YQ, Moghazy S, Cherukuri T, Alvarez NT, Haroz EH, Mahzooni S, Doorn SK, Kono J, Pasquali M, and Hague RH. 2010. Dry Contact Transfer Printing of Aligned Carbon Nanotube Patterns and Characterization of Their Optical Properties for Diameter Distribution and Alignment. ACS Nano 4(2): 1131-1145. DOI: 10.1021/nn9013356
Integration of nanomaterials with electronics has gained significant traction as a result of enhanced properties such as electronic conduction for hybrid ink dispersions at low to moderate loadings. As one leading example, carbon nanotubes (CNTs) have exhibited competitive performance as electrodes for applications ranging from energy storage, where the large surface area enhances performance, to photovoltaics (PV) and displays, where high optical transparency is a desirable feature. More recently, techniques to align CNT networks in-plane on a substrate have been investigated as a means to enable CNT-based electronic devices including transistors ad interconnects. In part, these techniques have utilized wet processes to either form densely packed films of CNTs via dielectrophoresis-directed assembly methods or transfer CNT thin films from one surface to another via a soft lithography approach. The later technique suffers from limited ability to achieve good adhesion of the CNT films to the transfer substrate and imprecise alignment of the CNT patterns. Ultimately, a technique is required that can be scaled to large area withhigh throughput processing and isnominally compatible with low temperature, flexible substrates for many emerging display, lighting, and solar PV applications.
Recently, Pint et. al. investigated a dry transfer process for patterned, aligned single wall carbon nanotubes (SWNTs) based on the principle that patterned SWNT films experience greater van der Waals interactions for the film in contact with the transfer substrate than the growth substrate. This process is facilitated by first etching the bond between the vertically grown SWNTs and the catalyst on the growth substrate. The transfer process is completed using a physical “scraping” mechanism to effectively push the patterned SWNTs over horizontally onto the transfer substrate where the total surface area of the film in contact with the transfer substrate is significantly greater.
Vertically aligned SWNT patterns were formed by lithographic patterning of wafer-scale traces of Al2O3 (10 nm thick) followed by 0.5 nm of Fe catalyst. Growth is conducted by water-assisted chemical vapor deposition (CVD) in which the height and diameter of the SWNTs are determined by the pattern width and reaction exposure time. This is followed by a post-growth H2/H2O vapor etch which effectively releases the chemical bond between the catalyst layer and the SWNTs, therefore the interface is controlled by the weaker van der Waals interactions. Transfer is achieved by toppling the SWNT patterns over via a process in which the transfer substrate is drawn across the surface of the growth substrate, essentially toppling the patterned arrays. As a result of the pattern line edge, the toppled SWNT film has large van der Waals interactions with the transfer substrate, producing anefficient transfer of the vertically grown SWNT patterns into horizontally aligned thin film patterns.
A key advantage of the transfer process is the removal of catalyst from the as-grown nanotubes during the vapor etch process, thereby eliminating possible contaminants from subsequent process steps, as well as allowing reuse of the original patterned catalyst template. The authors further demonstrated the ability to from multilayer pattern transfers in which a SWNT grid pattern was formed by overlaying two SWNT lined patterns offset by 90 degrees, thereby forming aligned patterns of SWNT thin film traces at wafer-scale integration levels. While further exploration of optimal nanotube growth and pattern conditions is necessary to tailor the properties of the SWNT thin films, the dry transfer technique investigated by the authors offers a scalable means to create aligned CNT thin film patterns on both rigid and flexible substrates.
Image reproduced with permission from Pint CL, et. al. 2010. ACS Nano 4(2): 1131-1145. DOI: 10.1021/nn9013356. Copyright 2010 American Chemical Society.