Efficient Formation of Iron Nanoparticle Catalysts on Silicon Oxide by Hydroxylamine for Carbon Nanotube Synthesis and Electronics

Iron containing nanoparticles are found to spontaneously form on hydroxylated SiO2 substrates when immersed in a freshly mixed aqueous solution of FeCl3 and hydroxylamine. Upon calcination, a submonolayer of uniformly distributed iron oxide nanoparticles can be derived and used to catalyze the growth of single-walled carbon nanotubes by chemical vapor deposition. This simple method affords clean single-walled nanotube films on SiO2. The solution phase catalyst deposition approach allows for submicron scale catalyst patterning. Patterned growth of nanotubes with this catalyst retains high degrees of surface cleanliness and leads to arrays of nanotube electronic devices including field effect transistors. The population of hydroxyl groups on SiO2, reaction time, and pH of the solutions are found to be important to the deposition of nanoparticles through a surface-mediated hydroxylamine/FeCl3 chemistry. Introduction. Chemical vapor deposition (CVD) of singlewalled carbon nanotubes (SWNT) on substrates is a promising approach to organized molecular wire structures.1 Patterned growth assisted by self-assembly and electric-field directed assembly produces site-specific and oriented nanotube arrays,2-6 useful for device integration without postgrowth nanotube purification and assembly.7-10 Lying at the heart of nanotube CVD is catalytic nanoparticles. It is essential to develop novel methods for catalyst nanoparticle formation, and their delivery and patterning on substrates.1 Patterned growth of SWNTs at the level of a few microns was initially demonstrated with Fe catalyst supported on aluminum oxide powders.2 While clean SWNTs emanating from catalytic sites were readily obtained, the catalytic regions on the substrate were not clean. Mound-like patterned alumina particles could cause problems to characterizations by, for example, atomic force microscopy (AFM), as the imaging tip frequently picked up these particles. Catalysts supported on powders are also less desired for individual catalytic nanoparticle size control and patterning of nanoparticles at smaller scales (e.g., < 1 μm). Progress has been made recently in creating size-controlled discrete catalyst nanoparticles on flat substrates for SWNT growth.11-13 We have formed catalytic nanoparticles with appoferritin11 and PAMAM dendrimer13 hosts for growth of SWNTs with diameters in the range of 1-2 nm. Cheung and co-workers used catalyst nanoparticles derived in organic solvents for the synthesis of SWNTs with 3-12 nm diameters.12 Deposition and patterning of solution-derived discrete nanoparticles allows for patterned growth of nanotubes with clean substrate surfaces.14 Here, we report a simple approach to the formation of Fe containing catalyst nanoparticles (up to a monolayer) on SiO2 wafers for CVD synthesis of SWNTs. We find that simple soaking of SiO2 wafers in a freshly mixed aqueous solution of hydroxylamine and FeCl3 leads to spontaneous deposition of nanoparticles on the substrates. The nanoparticles are then fully converted to Fe2O3 and used to catalyze SWNT growth. The solution phase catalyst deposition approach allows for submicron scale patterned nanotube growth that retains a high degree of surface cleanliness, which facilitates characterization and device fabrication of, for example, nanotube field effect transistors (FETs). Hydroxylamine is known as a mild reducing agent and has been previously used to reduce AuCl4 ions on surface-supported Au nanoparticles for enlarging the particles.15 In our system, deposition of Fecontaining nanoparticles on SiO2 substrates from the hydroxylamine and FeCl3 solution appears to be mediated by the hydroxyl groups on SiO2. The reaction time and pH of the solution is found to determine the size and density of