Vacancy Induced Magnetism In Low-dimensional Semiconductor Nanomaterials

With the development of new technologies,multifunctional materials that are convenient,environmentally friendly,and safe have become the focus of research.Diluted magnetic semiconductor materials are one of the hot spots.Dilute magnetic semiconductor materials can simultaneously utilize the charge properties and spin properties of electrons,greatly improving the processing speed and storage density of information,and exist broad application prospects in fields such as magnetic sensors,optical isolators,semiconductor integrated circuits,semiconductor lasers,and spin quantum computers.At present,dilute magnetic semiconductor materials have not been widely used due to their lower Curie temperatures and lower saturation magnetization.Therefore,improving the Curie temperature and saturation magnetic moment of dilute magnetic semiconductors and designing new dilute magnetic semiconductor materials have become a research hotspot in this field.In recent years,it has been found that some semiconductor materials,particularly metal oxide semiconductor materials,have ferromagnetism and may have a high Curie temperature without introducing magnetic impurities.Fellow Coey of the Royal Academy of Sciences named the ferromagnetism of these materials as d⁰ferromagnetic^[1].However,the magnetic origin of d⁰ ferromagnetism has alwaysbeencontroversial,constrainingtheefficientuseand commercialization of such semiconductor materials.Regarding the d⁰ magnetic source of semiconducting metal oxides,a large number of theoretical and experimental studies have been conducted,but the results are not the same.Most experimental results show that the magnetic properties of dilute magnetic semiconductors are caused by oxygen vacancies,while the most theoretical studies have concluded that the local magnetic properties of materials are caused by cation vacancies.In order to more reasonably explain the experimental phenomena,we propose a new research idea that considers the effect of chemical adsorption of oxygen molecules in the air on the magnetic properties of the defect metal oxide d⁰.In addition,we also designed new types of binary and ternary metal oxide ultrathin nanotubes,and studied the d⁰ magnetic properties induced by their intrinsic vacancies.The main conclusions of the above study are as follows:?1?Based on the density functional theory,we investigated the electronic structures and magnetic properties of the oxygen adsorption on the defective cubic HfO₂?110?surface.The adsorption capacities of the perfect and defective surfaces follow in the sequence as:oxygen-deficient model>perfect surface>hafnium-deficient model.When the oxygen molecules adsorb on the nonmagnetic perfect HfO₂ surface,the system has a local magnetic moment and the antiferromagnetic coupling is more stable.For the defective cubic HfO₂?110?surface,it is found that the oxygen vacancy is more easy to form than the hafnium vacancy by comparing the vacancy formation energy.And the hafnium vacancy could induce a large magnetic moment while the oxygen vacancy alone could not.After adsorption of the oxygen molecule,the nonmagnetic oxygen deficient HfO₂ model is transformed into the magnetic surface.On the contrary,the introduction of adsorbed oxygen molecule causes the magnetic moment of hafnium-deficient HfO₂ surface to reduce.Additionally,iftheoxygenmoleculesadsorbonthedefective HfO₂?110?surface,the ferromagnetic coupling is energetically favorable.?2?The structural,electronic and magnetic properties of intrinsic defects in anatase-type ultrathin TiO₂ nanotube have been investigated systematically by first-principles calculations.The neutral Ti vacancies could induce a large magnetic moments and long-range ferromagnetic coupling,nevertheless,it is difficult to create a significant amount of Ti vacancies in nanotube due to its higher formation energy.The neutral oxygen vacancies are more easily to form than vacancies,and the type of their magnetic coupling is related not only to the kinds of oxygen vacancies but also to the distance of vacancies,and their antiferromagnetic couplings are stable in most cases.Thus,the monovalence positively-and negatively-charged vacancy defects in nanotube are predicted.By comparing the formation energies of these neutral and charged vacancies,the positively-charged O vacancies are more easily formed in nanotube under O-poor condition,and their ferromagnetic couplings are energetically favorable.The negatively-charged Ti vacancies are more stable under O-rich condition,and they prefer to couple in a antiferromagnetic way.Our work offers a possible route toward high Curie temperature ferromagnetism in TiO₂materials.?3?Based on density functional theory,we predict the electronic structures,stabilities,and magnetism of ultrathin SrTiO₃ nanotubes?u-STONTs?with different terminals and curvatures,which are constructed by rolling up the rectangular?001?STO monolayer.By comparing the total energies and cohesive energies of STO monolayer and u-STONTs,it is found that the STO monolayer is more stable than TiO₂-out terminal NTs,but not as stable as SrO-out terminal NTs,where?18,0?SrO-out NT is most stable among various tubular structures.In STO monolayer,the stable ferromagnetic coupling is closely related to Ti vacancies instead of O vacancies,although Ti vacancy is more difficult to generate in any conditions.On the contrary,Ti vacancies are more likely to emerge in SrO-out NTs under O-rich condition,and it can induce a large magnetic moment and ferromagnetic coupling.Under O-poor condition,O vacancies are much easier to form in NTs,and the magnetic coupling induced by O vacancies depends upon the kinds of vacancies.And the absence of twofold-coordinated O atoms in the axial direction can lead to a long-rang ferromagnetic coupling in SrO-out NT.