紫外光辐照对TiO2纳米线电输运性能的影响及磁阻效应研究

采用溶胶凝胶法以及静电纺丝法, 利用热处理工艺, 成功制备出了多晶锐钛矿型TiO₂纳米线, 通过两线法在室温下测试单根TiO₂纳米线的V-I曲线来研究其电输运性能及磁阻效应. 结果表明: 在无光照环境下其V-I曲线为不过零点的直线, 零场电阻较大, 在磁场作用下电阻下降, 表现出负磁阻效应; 紫外光辐照环境下TiO₂纳米线载流子浓度增加使得电阻变小, 然而在磁场作用下电阻增大, 表现为正磁阻效应. 紫外光辐照导致的载流子浓度变化, 使得负磁阻转变为正磁阻, 我们将磁阻变化归结为d电子局域导致的负磁阻与能带劈裂导致的正磁阻两种机理相互竞争的结果.

Electronic transportation properties and magnetoresistance effects on single TiO2 nanowire under ultraviolet irradiation

The polycrystalline anatase TiO₂ nanowires with a diameter of about 300 nm are successfully prepared by the sol-gel method together with electrospinning method under a heat treatment at 500℃. The effect of illumination on electronic transport property and magnetoresistance (MR) effect are studied via voltage-current (V-I) curves measured at room temperature in the cases of the dark and the ultraviolet irradiation. The results show that the V-I plots are straight lines without passing through zero point and the resistance of the nanowire is as high as 7.5×10¹¹ Ω in the dark. The resistance decreases gradually with the magnetic field increasing and after reaching a minimum 4.7×10¹¹ Ω at B=0.7 T it turns to increase rapidly, but is still smaller than the resistance without magnetic field, indicating a negative MR effect. With the increase of the magnetic field, the negative MR effect increases and then decreases, and the negative MR achieves a maximum value of -37.5% under B=0.7 T. Interestingly, the resistance of nanowires in the ultraviolet irradiation is reduced by about 10 times compared with that in the dark without applying a magnetic field. As the magnetic field increases, the resistance increases monotonically, presenting a positive MR effect. The MR increases rapidly with the increase of magnetic field, and reaches the maximum positive MR effect 620% under B=1.0 T. At room temperature only a few carriers are generated by the thermal excitation in the TiO₂ nanowires, which leads to a large resistance in the dark situation. In the ultraviolet irradiation case, the carrier concentration of the nanowires increases because of the generation of a large number of electron-hole pairs, resulting in huge decrease of resistance compared with in the dark. We attribute the change of the MR to the competition betwen two MR mechanisms: negative MR effect due to the localization of d electron and positive MR effect due to spin splitting of the conduction band. In the dark, due to the low carrier concentration, the negative MR mechanism caused by the localization of d electron is dominant under the magnetic field. However, in the ultraviolet irradiation, because carrier concentration increases hugely due to the irradiation, the positive MR mechanism caused by spin splitting of the conduction band is dominant. The fact that the V-I curves does not pass through zero point implies that the contact between TiO₂ nanowire and Pt metal is Schottky contact due to the difference in work function. In the dark, the initial voltage first increases with the increase of magnetic field, and then remains steady. In the ultraviolet irradiation the initial voltage is smaller than in the dark and increases monotonically with the magnetic field increasing. In this paper, the physical mechanism of the electrical transport property and MR effect of TiO₂ nanowire are discussed, which may provide a meaningful exploration for developing the new electronic device based on the oxide nanowires.