Recently, Zheng Xiaohong, a researcher of the Computational Physics and Quantum Materials Research Department of the Institute of Solid State Physics, Hefei Institute of Material Science, Chinese Academy of Sciences, has made progress in the research of two-dimensional tin dioxide (SnO2) materials. The researchers predicted that the two-dimensional single-layer delta phase (P-4m2) of SnO2 can exist stably based on the first-principles calculation method, and found that the two-dimensional SnO2 material of the delta phase has a negative Poisson ratio in the plane. In addition, through hole carrier doping, the system can be induced to transform from a non-magnetic state to a ferromagnetic sequence, and achieve a completely spin-polarized semi-metallicity. Related research results were published in Physical Review B with the topic of Computational Prediction of a Two-dimensional Semiconductor SnO2 with Negative Poisson's Ratio and Tunable Magnetism by Doping.
SnO2 bulk is an n-type wide band gap semiconductor material, and the common structure is rutile. Due to its good chemical stability, high light transmittance, low resistivity and acid and alkali resistance, SnO2 has application prospects in the fields of electrode materials, chemical sensor elements, and solar cells. Studies have shown that the inclusion of 3d transition metal impurities in the SnO2 bulk can achieve room temperature ferromagnetism. When oxygen vacancies are introduced into the system, the ferromagnetic order can be observed in the thin film samples, but not in the bulk samples. Although SnO2 bulk materials have been extensively studied experimentally and theoretically, there are still few studies on the corresponding two-dimensional monolayer phase. To further explore the existence of SnO2 two-dimensional phase and the possibility of doping to induce room temperature ferromagnetism is of great significance to the field of two-dimensional semiconductors and spintronics.
Based on density functional theory calculations, the researchers found that the two-dimensional SnO2 (δ-SnO2) of the δ phase can exist stably, and it is a single-layer material with the property of negative Poisson's ratio (ν=-0.11) in the plane. The formation of Poisson's ratio is mainly caused by the synergy between the symmetry of the lattice structure and the SnO4 tetrahedron under the limitation of low-dimensional effects. At the same time, the single-layer SnO2 is an indirect band gap semiconductor with a band gap of 3.7 eV, and the electron mobility can be as high as 103 cm2V-1s-1. SnO2 exhibits a double Mexican hat-like energy band in the valence band near the Fermi level. Researchers can induce a ferromagnetic phase transition in the system through hole carrier doping, which can exhibit semi-metallicity in a wide range of doping concentration. The Stoner mechanism can explain this magnetic phase transition. In addition, the transition between the two-dimensional XY magnet and the Ising magnet can be achieved by changing the hole carrier doping concentration, and with a proper doping concentration, the ferromagnetic transition temperature can be higher than room temperature. Studies have shown that the predicted SnO2 phase is another important example of p-type magnetic systems. It also shows that the predicted SnO2 two-dimensional phase has application prospects in nanomechanics and low-dimensional spintronics.
The research work was funded by the National Natural Science Foundation of China and the China Scholarship Council, and related calculations were completed in the Hefei Branch of the Supercomputing Center of the Chinese Academy of Sciences.
Figure 1. (a) Top view and side view of the crystal structure of a two-dimensional monolayer δ-SnO2; (b) The relationship between the strain in the x direction and the tensile strain in the y direction of the system
Figure 2. (a) The relationship between the magnetic energy difference of different magnetic configurations (non-magnetic, zigzag antiferromagnetic and striped antiferromagnetic) and ferromagnetic configuration with the change of hole carrier doping concentration; ( b) The relationship between the magnetic moment of the system and the doping concentration of hole carriers
Figure 3. (a) The magnetic anisotropy of hole-doped two-dimensional δ-SnO2 system and (b) the relationship between ferromagnetic transition temperature and hole carrier doping concentration
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