Growth of tailored carbon nanotube arrays via block copolymer lithography

Abstract

Carbon nanotubes (CNTs) are of broad technological interest in electronics, photonics, energy devices, and other applications. However, establishing a straightforward process for mass production of uniform CNTs with desired structures and properties has been a long-standing challenge. In particular, it is highly desired to precisely control over the numbers of walls and diameter of CNTs, which are decisive parameters for the physical properties of CNTs. In this respect, the preparation of monodisperse catalyst array having a narrow size distribution is generally considered an effective pathway to produce well-defined CNTs, since the number of walls and diameter of the produced CNTs is closely related to the catalyst size. Catalytic chemical vapor deposition (CVD) has become a standard growth method for vertical or parallel CNT arrays. Plasma-enhanced CVD (PECVD) is particularly attractive, since the high-energy plasma environment enables low-temperature growth, high CNT alignment, and compatibility with conventional Si processes. Nevertheless, CVD methods suffer from low catalyst activity. Only a small fraction of the catalyst particles produce CNTs; the majority remain as undesired impurities due to contamination by amorphous carbon. For this reason oxygen-assisted CVD approaches have been developed. Oxygen-containing compounds, such as water ($H_2O$), alcohol ($CH_3OH$), and oxygen gas ($O_2$) has been included in the feed stream to remove undesired amorphous carbon contamination and promote highly efficient CNT growth. Ammonia ($NH_3$) has often been used as an environmental gas for PECVD growth of CNTs. The pretreatment of catalyst particles with $NH_3$ plasma results in a thin nitride layer on the catalyst surface, which suppresses catalyst deactivation by decomposing excessive carbon. As a constituent of the source gas, $NH_3$ is an effective source for the N-doping of CNTs. N-doping may greatly enhance the electrical conductivity of CNTs. When electro...