Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional structures of helical or zigzagged GaN, ZnGa2O4, and Zn2SnO4 nanowires. The GaN nanowires adopt a helical structure that consists of six equivalent <011> growth directions with the axial [0001] direction. We also confirmed that the ZnGa2O4 nanosprings have four equivalent <011> growth directions with the [001] axial direction. The zigzagged Zn2SnO4 nanowires consisted of linked rhombohedrons having the side edges matched to the <110> direction and the [111] axial direction.

Vertically aligned Mn (10%)-doped Fe 3 O 4 (Fe 2.7 Mn 0.3 O 4 ) nanowire arrays were produced by the reduction/ substitution of pregrown Fe 2 O 3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn 2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe 3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules.

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Straight and zigzagged Ga1- x Mn x N (0 ≤ x ≤ 0.05) nanowires were selectively synthesized by the vapor transport method, using different growth temperature. They consisted of single-crystalline wurtzite GaN nanocrystals grown along the [101̄0] and [0001] directions for the straight and zigzagged morphologies, respectively. The lattice constant, c, decreases initially with increasing amount of Mn doping (x), and then increases as x increases above 0.03. X-ray photoelectron spectroscopy revealed that as the Mn content increases to x = 0.02, the binding energy of Ga 2p shifts to a higher energy, suggesting the possibility of hybridization between the Mn2+ ions and host defects. X-ray absorption spectroscopy and X-ray magnetic circular dichroism confirmed that the Mn2+ ions substitute into the tetrahedrally coordinated sites. The magnetization measurement revealed that all of these nanowires exhibited room-temperature ferromagnetic behaviors, most significantly for the straight nanowires grown with the [101̄0] direction, having x = 0.02−0.03.

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