| With the astonishing development of science,technology and social economy,people are increasingly demanding electronic products such as mobile phones and computers.Electronic products,with large capacity,convenient reading,and fast storage are becoming more and more popular.Diluted magnetic semiconductors?DMSs?having spin properties and charge properties are widely applied to semiconductor lasers,spin quantum computers,semiconductor integrated circuits,and magnetic sensors,which because they can greatly improve information reading and storage speed.However,DMSs have not been widely used so far,and there are mainly two problems:?1?the Curie temperature is lower than room temperature,and it cannot be used normally at room temperature;?2?the saturation magnetization is low.Therefore,what attracted the attention of researchers is how to improve the Curie temperature and saturation magnetic moment of DMSs.The research found that there are three main ways to generate magnetism in non-magnetic semiconductors: 1.Doping magnetic elements in non-magnetic semiconductors.2.Adsorbing gas on the surface of materials.3.Intrinsic vacancy inducing magnetism in non-magnetic materials.However,since doping forms magnetic clusters and secondary phases,attention has been paid more to the magnetic properties of intrinsic vacancy-inducing materials.And this fascinating magnetic behavior was named by Coey as “d0 magnetism”,but there is no uniform understanding of the origin of d0 magnetism.Most experimental results reported that the anion vacancies can induce magnetism in undoped dilutemagnetic semiconductors.However,theoretical calculations showed that cation vacancies induce magnetic properties in the material.Vacuum annealing and aie tempering are often used in experiments to explore the source of magnetic material.The semiconductor metal oxide is magnetically enhanced after vacuum annealing,and the magnetism are weakened after air tempering.Based on the experimental phenomenon,the researchers believe that the vacuum annealing leads to an increase in oxygen vacancies,and after air tempering,the oxygen vacancies in the materials decrease,and the magnetic first increases and then decreases.The proportion of oxygen in the air is 21%,and the chemical nature of oxygen is very active.Therefore,the samples prepared during the experiment will inevitably come into contact with oxygen.It is believed that it is of great necessity to investigate the effect of oxygen adsorption on the d0 magnetism in Zn O.In addition,most previous ferromagnetic studies have focused on Ti O2 thin films and bulk crystals,whereas few have investigated the magnetic properties of Ti O2 nanotubes?NTs?.Due to the rapid development of experimental techniques,Ti O2 nanotubes have been successfully fabricated in a variety of methods,and have been extensively studied in the fields of photocatalysis,electrochemistry,adsorption,and structure.Recent experimental results indicated that the magnetic properties of undoped Ti O2 nanotubes are very sensitive to its tubular structure.Therefore,it is of great significance to explore the d0 magnetism of guided by the intrinsic vacancies of Ti O2 NTs.Based on the research done by us,we give some conclusions as followed:It is difficult for oxygen molecules to form chemisorption on defect-free wZn O?100?surface.The calculation of vacancy formation energy shows that oxygen vacancies are more stable than zinc vacancies in most cases,and surface vacancies are more likely to occur than the subsurface vacancies.Oxygen molecules have higher adsorption capacity on the oxygen-deficient Zn O?100?surface than the zinc-deficient surface.In addition,the zinc vacancies presence ofsurface can inhibit the adsorption of more oxygen molecules.Magnetic calculations exhibit that the perfect and oxygen-deficient w-Zn O?100?surface are non-magnetic,but zinc vacancies can induce local magnetic moments on the surface.For the oxygen-deficient Zn O?100?surface,the local magnetic moment can become larger as the number of adsorbed oxygen molecules is further increased.Conversely,the introduction of adsorbed oxygen molecules reduces the magnetic moment on the surface of the zinc-deficient Zn O?100?.Zn O nanocrystalline powders prepared by sol-gel method is annealed at 600? for 1 h to exhibit room temperature ferromagnetism,which can be enhanced by a subsequent vacuum annealing at 600? for 30 min.The further treatment in 0.1 Mpa O2 atmosphere at 25? for 16 hours increases the RTFM again,indicating that the O2 adsorption has a positive effect on the enhancement of the magnetism.We have performed detailed DFT studies of the adsorption of O2 molecule on the Zn O?100?surface,and the chemisorption of O2 is unfavorable at all adsorption sites in the perfect Zn O surface.For the oxygen-deficient surface,O2 molecule can be chemisorbed exothermically and fill the vacancy spontaneously.And the spin polarization of its O 2p orbitals at Fermi level can introduce the magnetic moment.It can be deermined that the O2 adsorption provides a good choice for adjusting the electronic property and spin polarization.The curvature energy calculations showed that?m,0?u-Ti O2 NTs follow the classical elasticity theory.Compared with NTs curled by a single atomic layer,more energy is required to form a Ti O2 NT from a Ti O2 nanosheet due to its larger wall thickness.As the diameter of the NTs increases,the wall thickness becomes thinner,and the binding energy and band gap grow larger until they reach saturation.We considered a representative?10,0?u-Ti O2 NT as an example to analyze the vacancy formation energy and magnetic properties.For?10,0?u-Ti O2 NT,VTi0 can induce a large magnetic moment and long-range ferromagneticcoupling,whereas VO0 vacancies cannot.The vacancy formation energy calculations showed that the VTi-and VO+ vacancies are more stable than the VTi0 and VO0 vacancies.In addition,the introduction of VTi-vacancies leads to unstable ferromagnetic coupling in?10,0?u-Ti O2 NT,whereas theVO+ vacancies favor stable ferromagnetic coupling.Moreover,the same types of oxygen vacancies in NT attract each other and the different types of oxygen vacancies are mutually exclusive. |
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