Xu Meiling, Zhong Xin, Lv Jian, Cui Wenwen, Shi Jingming, Kanchana V, Vaitheeswaran G, Hao Jian, Wang Yanchao, Li Yinwei
Laboratory of Quantum Materials Design and Application, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China.
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, China.
J Chem Phys. 2019 Apr 21;150(15):154704. doi: 10.1063/1.5089697.
Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of TiO, TiO, and TiO. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO, 3.2 eV) to a narrow-gap semiconductor (TiO, 1.80 eV) and then to metals (TiO and TiO) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO) to ferromagnetic (TiO, TiO, and TiO) transformations. Notably, the TiO film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µ per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.
二氧化钛已广泛应用于现代工业领域，尤其是作为一种有效的光催化剂。最近，已合成出带隙显著减小至约1.8 eV的独立TiO薄膜，这表明尺寸对体带隙（>约3 eV）有相当大的影响，并扩大了可见光的吸附范围。氧化钛化合物具有多种化学计量比和多样的性质。因此，探索其他具有不同化学计量比的氧化钛薄膜的电子性质和功能非常必要。在这里，我们将结构搜索与第一性原理计算相结合，以探索具有不同化学计量比的候选Ti-O薄膜。除了实验合成的TiO薄膜外，结构搜索还确定了三种新的能量和动力学稳定的Ti-O薄膜，其化学计量比分别为TiO、TiO和TiO。计算表明，随着Ti含量的增加，Ti-O薄膜经历了几种有趣的电子转变，即从宽禁带半导体（TiO，3.2 eV）转变为窄禁带半导体（TiO，1.80 eV），然后由于未成对Ti_d电子的大量存在而转变为金属（TiO和TiO）。除了电子转变外，我们还观察到了从非磁性（TiO）到铁磁性（TiO、TiO和TiO）的转变。值得注意的是，TiO薄膜同时具有窄禁带半导体和铁磁性特性，每单位晶胞具有2.0 µ的大磁矩；因此，该薄膜在自旋电子器件等应用中具有很高的应用潜力。这些结果突出了过渡金属氧化物薄膜中金属含量诱导的电子和磁性转变，并为新型功能薄膜材料的设计提供了一条替代途径。
