Block copolymer-templated iron oxide nanoparticles for bimodal growth of multi-walled carbon nanotubes

Abstract

Since their discovery carbon nanotubes (CNTs) have sparked great interest due to their exceptional mechanical, electrical, and thermal properties. These properties make carbon nanotubes desirable for numerous applications including: nanoelectronics, high-strength composites, energy storage, superhydrophobic surfaces, sensors, and biomaterial interfaces. Bulk synthesis of carbon nanotubes with controlled physical features, i.e. length, diameter, multiwalled vs. single walled, carbon nanotube chirality, etc. is necessary to make full use of carbon nanotubes exceptional properties in commercial aspects. Typical carbon nanotube synthesis processes use chemical vapor deposition (CVD), arc-discharge, and laser ablation. Synthesizing carbon nanotubes via CVD typically involves depositing a thin metal film on a silicon substrate, and heating the substrate so that the thin metal film dewets and forms metallic nanoparticles. A hydrocarbon gas is then flowed over the nanoparticles to initiate carbon nanotube growth. Though these thin metal film catalysts are easy to prepare, they offer poor control over nanoparticle diameters and areal density. It has been shown that physical properties of carbon nanotubes, such as diameter and uniformity of growth, are directly related to the diameter of the catalyst nanoparticle, and that chirality of the carbon nanotube is inversely related to the catalyst nanoparticle diameter. Therefore, fully exploiting the unique properties of carbon nanotubes requires an understanding of how to control catalyst nanoparticle diameters, and thereby carbon nanotube physical characteristics. Bennett et al demonstrated that controllability of nanoparticle diameters is possible using a simple poly(styrene-b-acrylic acid) (PS-b-PAA) amphiphilic block copolymer. The amphiphilic PS-b-PAA block copolymer forms micelles, when dissolved in toluene, with anionic carboxylic acid groups available from the PAA.

(cont.) The anionic PAA carboxylic acid groups can be used to sequester metal cations, so that metal is effectively loaded into the micelles. The size of nanoparticles can be controlled by the size of the PAA portion of the block copolymer. When spin cast onto a substrate, the metal-loaded PS-b-PAA micelles form a quasi-ordered block copolymer thin film. Maximizing the amount of metal-loaded micelles in solution can maximize the resulting areal density of nanoparticles, thereby forming a monodisperse, quasi-hexagonal nanoparticle array. The deposited micellular thin film and substrate can then be etched with oxygen plasma, removing the organic polymer so that only the nanoparticle array is left, and the substrate is ready for carbon nanotube growth.