Defect-Selective Ferromagnetic Ordering and Optical Tunability in C Ion-Implanted Microflowers Composed of TiO₂ Nanorods

Ion implantation is one of the versatile techniques for controllable doping of desired ions in solid materials. This work discusses the tunability of the structural, optical, and magnetic properties of hydrothermally synthesized rutile TiO2 microflowers composed of nanorods due to C ion implantations. The C ions of 1.5 MeV are implanted in the fluence range of 1 × 1015 – 2 × 1016 ions/cm2 in normal incidence. The increase in the peak broadening of the X-ray diffraction (XRD) and Raman spectra in the ion-implanted TiO2 compared to the pristine TiO2 indicates the degradation of crystallinity due to the creation of lattice disorder and defects. The defect states are mainly attributed to the oxygen vacancy that assists in narrowing the optical band gap and causes magnetism. As the fluence increases, more defect states are created, which reduces the optical band gap from 3.04 to 2.98 eV and increases the Urbach energy from 144 to 193 meV. The ferromagnetic ordering with a tunable coercive field in the C-irradiated TiO2 is further evidenced through density functional theory (DFT) calculations, which indicate an interaction between the 3d states of Ti and 2p states of the O and C atoms. The spin-polarized total density of states for pristine TiO2 with Ti and O vacancies are calculated for the net magnetic moments to match the experimental results. The demonstration of C ion-implantation defect states plays an important role in tuning the phononic, photonic, and magnetic properties of TiO2 nanostructures, which are suitable for applications in versatile spintronic and optoelectronic devices based on nanorods.