Direct Synthesis of Single Crystalline $\alpha$ -Fe Nanoparticles With High Saturation Magnetization by Mixed Surfactant

Magnetic nanoparticles (NPs) have attracted attention for their potential use in electronic devices and nano-bioengineering applications. Fe NPs made by thermal decomposition from an Fe(CO)x-Oleylamine (OlAm) reacted precursor show high saturation magnetization (M_s) (~ 140 emu/g_net at 300 K). However, the M_s never reaches to the bulk value (218 emu/g at 300 K) of iron due to the unique crystalline structure of the Fe NPs. Fe NPs coated with OlAm (OlAm-Fe NP) have an expanded α (b.c.c.) structure and ultra-fine grains. In this paper, we focus on the adsorption ability of the surfactant to improve M_s via control of phase and grain size. Fe NPs with high saturation magnetization were synthesized by thermal decomposition of Fe(CO)₅ with some surfactants that had weak absorption ability, such as tribenzylamine, trioctylamine, and their mixture. Slow decomposition rate of Fe(CO)₅ and large grain size were obtained by applying surfactants with weak adsorption ability. Especially, Fe NPs that were synthesized by mixed surfactants (mixed surfactant-Fe NPs) have polygonal shape with 6.7 nm in diameter. HRTEM and XRD results suggest that mixed surfactant-Fe NPs are single crystallines of α -Fe. M_s of mixed surfactant-Fe NPs is 194 and 183 emu/g_net at 5 K and 300 K, respectively. These results strongly indicate that single crystallization of α-Fe enhances the M_s of Fe NPs and also suggest that optimization of the surfactant adsorption ability is indispensable to promote the single crystallization of Fe NPs.