Field emission properties and ferromagnetic exchange interactions in γ-Fe2O3 and Fe3O4 nanoneedles-oleic acid-assisted growth

In various biomedical applications of iron oxide nanostructures, superparamagnetic nanoneedles offer benefits to access the interior of cells without damage. However, use of superparamagnetic nanoneedles should not be limited to biomedical applications. To explore and extend the field of application, this study is focused on detailed investigation of structural, magnetic, and field emission behavior of iron oxide. Iron oxide nanoneedles are prepared using template-free oleic acid-assisted sol-gel method by varying sols' molarity in the range 0.2-2.0 mM (interval 0.2 mM). Formation of nanoneedles with fine tips and diameter of 20, 23, and 25 nm and length of 700 nm, 1.0 mu m, and 1.2 mu m at 0.2, 1.0, and 2.0 mM sols are confirmed using Scanning Electron Microscopy. While for rest of the molarity range studied diameter of nanoneedles increases to similar to 50 nm. At 0.2 mM sol magnetite (Fe3O4) phase is observed and vacancy ordered and disordered maghemite (gamma-Fe2O3) phases are observed at 0.8-1.0 and 1.4-2.0 mM sols, respectively. Formation of these phases of iron oxide with variation in sols' molarity is also confirmed using FTIR spectra and Raman spectroscopy. Iron oxide nanoneedles show soft magnetic behavior with high saturation magnetization of 73.2, 43.43, and 75.18 emu/g at 0.2, 1.0, and 2.0 mM sols, respectively. These magnetic nanoneedles have potential applications in cell probing. Furthermore, nanoneedles are also tested for their field emission properties. It is observed that nanoneedles, with disordered maghemite phase, synthesized using 2.0 mM sol have high field emission properties along with low turn-on field of 3.77 V mu m(-1). Thus, this study reveals ordered and disordered phases of iron oxide using single route. Furthermore, these phases can be utilized for different applications, such as Fe3O4 (at 0.2 mM) phase for biomedical applications and gamma-Fe2O3 phase as field emitters.