VO2薄膜表面氧缺陷的修复:F4TCNQ分子吸附反应

VO₂表面氧缺陷的存在对VO₂材料具有显著的电子掺杂效应, 极大地影响材料的本征电子结构和相变性质. 通过2, 3, 5, 6-四氟-7, 7', 8, 8'-四氰二甲基对苯醌(F₄TCNQ)分子表面吸附反应, 可以有效消除表面氧缺陷及其电子掺杂效应. 利用同步辐射光电子能谱和X射线吸收谱原位研究了修复过程中电子结构的变化以及界面的化学反应, 发现这种方式使得VO₂薄膜样品氩刻后得到的V³⁺失去电子成功地被氧化成原先的V⁴⁺, 同时F₄TCNQ分子吸附引起电子由衬底向分子层转移, 界面形成带负电荷的分子离子物种. 受电化学性质的制约, F₄TCNQ分子吸附反应修复氧缺陷较氧气氛退火更安全有效, 不会引起表面过度氧化形成V₂O₅.

Healing of oxygen defects on VO2 surface: F4TCNQ adsorption

Oxygen-defect vacancies that routinely exist in wet production of VO₂ material or on the surface of VO₂ single crystal after surface treatment have significant influence on the metal-insulator phase transition features mainly due to their enhanced effect of doping on V 3d electronic structure. The removal of the surface oxygen defects is highly desired for investigating the VO₂ intrinsic electronic properties. In this work, we propose a charge transfer doping method by using strong electric affinity molecule tetrafluorotetracyanoquinodimethane (F₄TCNQ) adsorption rather than the normal thermal annealing in oxygen atmosphere to heal the surface oxygen defects of VO₂ crystalline film. The healing effect is probed by the electronic structure evolution at the F₄TCNQ/VO₂ interface. The VO₂ crystalline film is grown by an oxygen plasma assisted molecular beam epitaxy method on an Al₂O₃(0001) substrate. Surface oxygen defects on VO₂ film are produced after a mild sputtering with an ionic energy of 1 keV and a thermal annealing in vacuum at 100 ℃. The influence of F₄TCNQ molecule adsorption on the electronic structure of the sputtered VO₂ film is studied by using in-situ synchrotron-based photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). XPS and XAS results demonstrate convincingly that V³⁺ species of sputtered VO₂ are oxidized into the V⁴⁺ and simultaneously negative molecular ions form at F₄TCNQ/VO₂ interface resulting from the electron transfer from VO₂ to the F₄TCNQ layer. The preferred adsorption on surface defects and the strong electron withdrawing function of F₄TCNQ molecules may account for the effective elimination of the electron doping effect of oxygen defects on VO₂ surface. This charge transfer effect at interface recovers the electronic properties of VO₂. Compared with thermal annealing in oxygen environment, the healing of oxygen defects by the molecular adsorption can prevent the surface from over oxidating VO₂ into V₂O₅, which opens a new route to surface defect healing.