Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

Abstract

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques (1, 2), and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization (3).

Statistical Analysis of Variation in Laboratory Growth of Carbon Nanotube Forests and Recommendations for Improved Consistency

“…millimeter repeatability with respect to the position of the furnace thermocouple • Calibration of all mass flow controllers, such as using a bubble flow meter, at intervals recommended by the manufacturer • Measurement of the deposited catalyst thickness using AFM, and calibration of thin-film deposition crystal gauges accordingly Complementary to these recommendations, a detailed video procedure for CNT forest growth from micropatterned catalysts has been published by Copic et al 49 This shows our methods for loading and unloading the reactor before and after the CNT growth process and provides a visual description of the catalyst preparation method and the CVD system.…”

Engineering Hierarchical Nanostructures by Elastocapillary Self‐Assembly

“…While capillary aggregation was initially described as being detrimental to the processing of high‐resolution polymer features made by lithography,15 advances in the controlled patterning of nanostructures have shown that capillary interactions can be used as a versatile and scalable fabrication step. For example, the wetting and drying of vertically aligned carbon nanotubes (CNTs) has been used to increase their packing density,17–22 to form periodic superstructures,23–25 to reorient thin films into horizontal sheets and multidirectional circuits,26–30 and to fabricate intricate 3D microarchitectures 28. 31–35 Similar principles have been used to manipulate nanofilaments made out of Si,36–39 Au,40 ZnO,41 Cu,38 hydrogels,1, 42 epoxy,1, 16 PMMA,12, 43, 44 PET,45 cyclic olefin copolymers,12 polyurethane,1 PDMS,46 various photoresists,15, 47, 48 and even biological fibers 49…”

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves

“…The aligned MWCNT array, as a core nanostructured material for thermopower waves, was synthesized by TCVD, [11][12][13] as shown in Figure 4A.…”