Selective area synthesis of 3-D carbon nanotube forest microsctructures
Carbon nanotubes (CNTs) are solid carbon allotropes that feature high strength and high transport capabilities. When large populations of CNTs are synthesized simultaneously, they self-assemble into vertically oriented CNT forests (also called CNT arrays or CNT turfs). In the current work, three-dimensional CNT forest microstructures are engineered by selectively activating CNT growth on specific regions of a patterned substrate. The process is realized using a combination of two distinct chemical vapor deposition (CVD) processes in concert with silicon substrates independently patterned with regions of alumina, iron, or alumina and iron. The CVD process of both techniques require a catalyst nanoparticle to facilitate CNT growth and a hydrocarbon gaseous precursor. Floating catalyst CVD, a process in which CNT catalyst is delivered to a substrate in the vapor state, is compatible with oxide-coated substrates and not compatible with bare silicon surfaces. Fixed catalyst CVD utilizes a static iron film catalyst layer to support CNT synthesis. Regions of alumina support selective CNT forest growth with floating catalyst CVD, while regions of layered Al2O3/Fe support both fixed and floating catalyst growth when the fixed catalyst is activated first. By selectively and sequentially activating specific regions of a lithographically-defined substrate using the various CVD processes, complex 3D microstructures are demonstrated. The resultant CNT forest microstructures are characterized using scanning electron microscopy (SEM) and nanoindentation. Atomic force microscopy is used to analyze the behavior of the fixed iron catalyst during the high temperatures required for CVD processing.