【作者】 王丽平；

【导师】 许并社；

【作者基本信息】 太原理工大学 ， 材料科学与工程， 2013， 博士

【摘要】 氟化镁(MgF2)具有良好的光学性能,是一种传统的光学薄膜材料。从真空紫外波段到红外波段,具有透过率高、吸收率低、折射率低且热稳定性好等优异性能,使其在光学领域的应用越来越广泛。MgF2/金属多层膜结构、金属插层的MgF2光子晶体、金属-MgF2纳米金属复合陶瓷膜,是近年来新出现的研究热点,其中Ag/MgF2多层膜、Ag-MgF2纳米金属复合陶瓷膜,由于量子尺寸效应引起的独特光电特性而备受关注。这些结构中,存在大量的表面、界面及原子间相互作用,会对材料的微观结构及相关性能产生重要影响,尤其Ag/MgF2体系的界面结构、互扩散行为及对光学性能的研究还尚待进一步深入,鉴于实验研究的难度和局限性,原子层次的模拟计算就显得尤为重要。近年来,原子层次上探索实验机理、材料性能预测的研究,正在成为材料科学研究的重要手段。本论文基于第一性原理研究了Ag/MgF2纳米薄膜界面的相关问题,主要研究成果如下：(1)系统分析了MgF2块体表面、MgF2内米薄膜的结构稳定性、电子特性及光学性能。(001)、(100)、(110)表面的稳定性依次增强,其中最密排面(110)为MgF2块体最稳定存在的表面；对比分析了以(001)、(100)、(110)为表面纳米薄膜的表面能,表明(100)的表面能低于(001)、(110)的表面能,(100)是MgF2纳米薄膜易于稳定存在的表面之一；块体MgF2的折射率的实验值与理论计算值非常吻合,(100)、(001)、(110)面构成纳米薄膜的折射率较块体材料小；研究发现(001)面纳米薄膜原子层数达25层,即膜厚为4nm时的MgF2薄膜结构较为稳定,并有随薄膜厚度的增加薄膜的折射率及消光系数减小的趋势。(2)分析了Ag(111)表面、Ag(111)面构成纳米薄膜的结构稳定性、电子特性及光学性能。不同原子层厚度的Ag(111)面纳米薄膜表面能研究发现,原子层厚度增加到13层时,即膜厚约为2.8nm时,纳米薄膜表面能趋于稳定,为薄膜稳定存在的临界厚度；光学特性来看,在可见光及红外波段,Ag(111)面构成的纳米薄膜的折射率明显高于块体材料而消光系数略高于块体材料,在所研究纳米尺度范围内有随着薄膜厚度的增加,折射率降低、消光系数增加的趋势。(3)系统研究了Ag在MgF2(100)、(001)、(110)表面吸附、扩散结构模型,以及吸附、扩散对MgF2电子结构及光学性能的影响规律。结果表明,Ag在MgF2(100)、(001)、(110)面的吸附为化学吸附,稳定吸附位分别为穴位、短桥位、长桥位；(100)、(110)、(001)面吸附Ag体系的稳定性依次增强,Ag、Mg间以离子键形式结合和Ag、F间以共价键形式作用；Ag在MgF2表面的稳定性较强,不易由表面扩散至次表面；MgF2表面含Ag吸附、扩散体系的折射率在可见光波段,较块体MgF2减小,吸收系数较块体MgF2增加。(4)探讨了MgF2/Ag纳米薄膜理想界面、互扩散界面的结构稳定性、扩散行为及光学性能。获得了Ag(111)/MgF2(001)理想界面结构界面能最小的稳定结构；Mg、Ag在Ag(111)/MgF2(001)界面结构体系中的扩散行为来看,界面处Mg比Ag更容易扩散入Ag中,而Ag较难扩散入MgF2。Ag扩散引起体系折射率减小,尤其在可见光波段,折射率减小幅度更大。更多还原

【Abstract】 As one of the most important optical film materials currently available, magnesium fluoride (MgF2) possesses a low refractive index, low absorption, a high thermal stability and good transparency over a wide range of wavelengths. Recently, a lot of new hotspots have attracted researchers’attention, such as MgF2/metal multilayers, metal-intercalated MgF2photonic crystals, metal-MgF2nanoparticle cermet films, among which Ag/MgF2composite films is now an important concern because of their novel photoelectric properties. However, there are a lot of surfaces, interfaces and atomic interaction in Ag/MgF2composite films, which will have a significant effect on the microstructure and performances. So far, the research of Ag/MgF2about interface structure, interdiffusion behavior, and their effect on optical properties of remain to be further investigated. Due to the difficulties and limitations of experimental research, it is very important to simulate from atomic level. In recent years, exploration of the experimental mechanism and prediction of material’s properties from atomic level is becoming important means of materials science research. In this paper, interface of Ag/MgF2nanofilms were studied based on the first-principles. The main research results are as follows: (1) The structural stability, electronic properties, and optical properties of MgF2surfaces and nanofilms were analyzed. The structural stability of MgF2surfaces increases in the following order:(001),(100), and (110), which reveals the most close-packed surface, that is (110), is most stable. The surface energy of MgF2(100) nanofilms is smaller than that of (100) and (110) nanofilms and MgF2(100) nanofilms can exist stably. Further, MgF2(001) nanofilms are energetically stabilized when the layer number is increased to25, corresponding to a thickness of4nm. Refractive index of MgF2nanofilms is decreased compared with the bulk. With increasing thin film thickness, refractive index and their extinction coefficient of MgF2(001) nanofilms are decreased.(2) The structural stability, electronic and optical properties of Ag (111) surface and Ag (111) nanofilms were analyzed. Ag (111) nanofilms are energetically stabilized when the layer number is increased to13, corresponding to a thickness of2.8nm. Analysis on the optical properties of nanofilms suggests that refractive index is significantly increased and extinction coefficient is slightly decreased compared with the bulk in the visible light and infrared wavelengths. With increasing thin film thickness, the refractive index decreased slightly and the extinction coefficient increase.(3) The adsorption and diffusion of Ag on MgF2(100),(001) and (110) surfaces were studied. The results show that the adsorption is chemical and the energetically favorable adsorption sites of (100),(001), and (110) are hollow site, short bridge site and long bridge site, respectively. The structural stability of Ag-adsorbed surfaces increases in the following order:(100),(110) and (001). The interaction between Ag and Mg is ionic, while covalent between Ag and F. The diffusion of Ag at MgF2(001) surface is not easy. In the visible wavelength region, the refractive index of MgF2surface with Ag adsorption is slightly decreased compared with that of the bulk, while the optical absorption is slightly increased.(4) The interface characteristics of Ag/MgF2nanofilms were discussed. The most stable ideal interface structure of Ag (111)/MgF2(001) is obtained, which has the lowest interface energy. The analysis on the diffusion behavior shows that Mg diffuses into Ag film more easily, while Ag diffuses into MgF2film difficultly. Further, Ag diffusion results in a decrease in refractive index, especially in the visible light wavelength region.更多还原