Template Guided Fluidic Assembly Process Mechanisms Characterized

Jaber-Ansari and colleagues characterize the role of surface properties for the assembly and control of SWNT architectures providing a potential basis for wafer-scale manufacturing of SWNTs for a range of emerging devices, systems, and applications.

Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network

• Jaber-Ansari, L, Hahm, MG, Somu, S, Sanz, YE, Busnaina, A, and Jung, YJ. 2009. Mechanism of Very Large Scale Assembly of SWNTs in Template Guided Fluidic Assembly Processes. J Am Chem Soc 131(2):804-808. DOI: 10.1021/ja8076523

Carbon nanotubes (CNTs) continue to be investigated for application in a range of devices due to their unique properties: high electron mobility, unique one-dimensional nanostructure, large current-carrying ability, and mechanical characteristics. In order to capitalize on these properties for defined devices and applications, a simple, reliable, and scalable  nanomanufacturing process is required to controllably assemble CNTs with precision placement and alignment over large areas.

Chemical vapor deposition with pre-patterned catalysts, patterned chemical surface functionalization, electrophoretic deposition, and dielectrophoresis all have been explored as methods for directed CNT assembly.While each of these techniques has merit, typical drawbacks include process compatibility, complex chemistries, and limited scalability. Recently, researchers from Ahmed Busnaina’s group at Northeastern University (Xiong et al. 2007) describe the liquid-phase fabrication of highly organized single wall nanotube (SWNT) networks. This method uses lithographically patterned templates and  a dip coating process to directly assemble SWNT onto exposed hydrophilic surfaces with varying geometries and feature sizes. While this method offers a scalable level of control to construct complex SWNT architectures for such applications as active elements in transistors, electrical interconnects or sensors, further understanding of the mechanisms surrounding the site-selective assembly for the SWNTs and related scaling issues is required.

A recent paper by Laila Jaber-Ansari and colleagues report a study characterizing the role of surface properties for the assembly of SWNT architectures and subsequent control towards fabricating wafer scale organized networks (Jaber-Ansari et al. 2008). The fluidic solution used in this study incorporated functionalized, negatively charged SWNTs, 2-3 µm in length at a concentration of 0.23 wt% in deionized water; the substrates were 3-inch silicon wafers with either a bare silicon or silicon dioxide surface. The authors pretreated the substrate using an inductively coupled plasma (ICP) with a gas flow mixture of O2 (20 sccm), SF6 (20 sccm), and Ar (5 sccm). They  exposed the substrates to plasma pretreatment for various times, after which the templates were formed by coating the wafers with photoresist and patterning. The authors then vertically placed the substrates into the SWNT solution using a dip-coater to control the procedure. Prior to assembling the SWNT architectures in the templates, the authors characterized the surfaces by several analytical techniques, including scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), in order to understand the physical and chemical properties of the surface as a function of the plasma treatment time, and relate those properties to the organized assembly of the SWNT networks.

The results indicate that the plasma pretreatment of the substrate and the subsequent dip-coating process effectively functionalized the silicon or silicon dioxide surface with hydrophilic chemical groups.  Additionally, the plasma pretreatment created nanometer-scale roughness on the surface that increased the density of hydroxyl functional complexes, further enhancing the hydrophilic properties. The authors conclude that the combination of these physical and chemical enhancements provide the means to control the fluidic assembly process, and build highly organized SWNT architectures at large scales.  By providing key insight into the underlying mechanisms for the SWNT fluidic assembly process, this study provides the potential basis for wafer-scale manufacturing of SWNTs for a range of emerging devices, systems, and applications.

Also cited

• Xiong X, Jaber-Ansari L, Hahm, MG, Busnaina A, Jung YJ. 2007. Building Highly Organized Single-Walled-Carbon-Nanotube Networks Using Template-Guided Fluidic Assembly. Small 3(12):2006-2010.

Image reproduced with permission from Jaber-Ansari, L, Hahm, MG, Somu, S, Sanz, YE, Busnaina, A, and Jung, YJ. 2009. Mechanism of Very Large Scale Assembly of SWNTs in Template Guided Fluidic Assembly Processes. J Am Chem Soc 131(2):804-808. Copyright 2009 American Chemical Society.