Papers by Author: Sean Li

Abstract: MXenes is a new group of two-dimensional materials via etching of the ‘A’ element from MAX phases. Depending on the functional group, MXenes can be semiconductors or metals. In this paper, first-principles calculations have been performed to investigate the effects of single vacancy defects on a semiconducting MXene, Sc₂CF₂ monolayer. The theoretical results show that V-Sc can induce magnetism in the host monolayer, while V-C and V-F result in n-type conductivity. For V-Sc doped Sc₂CF₂, tensile strains enhance the total magnetic moment which remains constant with applied compressive strains. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of Sc₂CF₂ can be tuned without the need of chemical doping.

Abstract: In this work, the structural and magnetic properties of polycrystalline Zn1-xCoxO (x = 0, 0.02, 0.05, 0.0625, 0.10 and 0.15) oxides were studied in detail. Rietveld refinement of x-ray diffraction spectra indicates that a single-phase wurtzite structure was formed in Zn1-xCoxO samples for x up to 0.10. The magnetization for x = 0.02 can be fitted to a model with a paramagnetic Curie term and a diamagnetic constant which could arise from spins of isolated free Co ions and a diamagnetic background, respectively. For x > 0.02, however, an additional antiferromagnetic Curie-Weiss term needs to employ for fitting. This is due to an additional contribution from clustered Co ions that are in nearest neighbor positions through oxygen ions. Results show that the substitution of Co at the Zn site does not occur in a completely random manner but Co ions appear to have a tendency for clustering. In addition, the homogenous ZnO:Co thin film prepared by Pulsed Laser Deposition on SiO2/Si substrate shows ferromagnetic behavior at room temperature.

Abstract: Polycrystalline stoichiometric Co-substituted ZnO oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in air. The precise nature of magnetic coupling is determined by studying carefully structural and magnetic properties. The magnetization as a function of temperature for Zn1-xCoxO (x = 0.02, 0.05, 0.0625 and 0.10) can be fitted well to a model with a paramagnetic Curie term, an antiferromagnetic Curie-Weiss term and a diamagnetic constant, which could arise from spins of isolated Co ions, grouped Co ions that are in nearest neighbor positions and a diamagnetic background, respectively. The substitution of Co at the Zn sites does not occur in a completely random manner but the Co ions appear to have a tendency for grouping. It is interesting to note in Raman measurements that host lattice defects with 2 distinct impurity modes may be induced by isolated and grouped Co spins.