Defect-Selective Ferromagnetic Ordering and Optical\nTunability in C Ion-Implanted Microflowers Composed of TiO<sub>2</sub> Nanorods

Ion\nimplantation is one of the versatile techniques for controllable\ndoping of desired ions in solid materials. This work discusses the\ntunability of the structural, optical, and magnetic properties of\nhydrothermally synthesized rutile TiO<sub>2</sub> microflowers composed\nof nanorods due to C ion implantations. The C ions of 1.5 MeV are\nimplanted in the fluence range of 1 × 10<sup>15</sup> –\n2 × 10<sup>16</sup> ions/cm<sup>2</sup> in normal incidence.\nThe increase in the peak broadening of the X-ray diffraction (XRD)\nand Raman spectra in the ion-implanted TiO<sub>2</sub> compared to\nthe pristine TiO<sub>2</sub> indicates the degradation of crystallinity\ndue to the creation of lattice disorder and defects. The defect states\nare mainly attributed to the oxygen vacancy that assists in narrowing\nthe optical band gap and causes magnetism. As the fluence increases,\nmore defect states are created, which reduces the optical band gap\nfrom 3.04 to 2.98 eV and increases the Urbach energy from 144 to 193\nmeV. The ferromagnetic ordering with a tunable coercive field in the\nC-irradiated TiO<sub>2</sub> is further evidenced through density\nfunctional theory (DFT) calculations, which indicate an interaction between the 3d states of Ti and\n2p states of the O and C atoms. The spin-polarized total density of\nstates for pristine TiO<sub>2</sub> with Ti and O vacancies are calculated\nfor the net magnetic moments to match the experimental results. The\ndemonstration of C ion-implantation defect states plays an important\nrole in tuning the phononic, photonic, and magnetic properties of\nTiO<sub>2</sub> nanostructures, which are suitable for applications\nin versatile spintronic and optoelectronic devices based on nanorods.