Auslender Avi, Halabi Mahdi, Levi George, Diéguez Oswaldo, Kohn Amit
Department of Materials Science and Engineering, The Iby and Aladar Fleischman Faculty of Engineering, Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel.
Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel.
Ultramicroscopy. 2019 Mar;198:18-25. doi: 10.1016/j.ultramic.2018.12.017. Epub 2018 Dec 30.
The mean inner potential (MIP) of a single crystal α-AlO sapphire was measured using off-axis electron holography. To measure the MIP, we use mechanically polished wedge specimens for transmission electron microscopy (TEM). This approach also enabled us to measure the plasmon mean free path for inelastic scattering (IMFP). The wedge specimen, chosen here at an angle of approximately 45°, allows to determine the MIP by measuring the gradient of phase variations of the reconstructed electron wave over extended regions across the sample. The angle of the wedge was measured to an accuracy of better than 1° by two methods: first, perpendicular sectioning in a focused ion beam for direct measurement by TEM and second, by a non-destructive approach of confocal optical microscopy. The validity of this methodology was examined on a single crystal Si(001) sample showing that the mechanically polished wedge approach can be applied to a wide range of materials. Our measurements concluded that the MIP of sapphire is V = 16.90 ± 0.36 V. Furthermore, the IMFP of sapphire was measured at 136 ± 2 nm for 197 keV electrons with a collection angle of 18mrad. The measured MIP of sapphire reflects its degree of ionicity, which lies between theoretical calculations based on electron scattering factors of charged and neutral isolated atoms obtained by Dirac-Fock calculations. Our MIP measurements tend to the expected value for this predominantly ionic material. To account for chemical bonding and the role of the crystallographic plane at the surface of the sample, we compared the experimental measurements to density-functional-theory calculations of the MIP. Calculations of α-AlO slabs cut along (0001) and (1-100) planes obtained MIP values of 15.7 V and 16.7 V, respectively.
使用离轴电子全息术测量了单晶α - AlO蓝宝石的平均内势（MIP）。为了测量MIP，我们使用用于透射电子显微镜（TEM）的机械抛光楔形样品。这种方法还使我们能够测量非弹性散射的等离子体平均自由程（IMFP）。这里选择的楔形样品角度约为45°，通过测量重建电子波在样品上扩展区域的相位变化梯度来确定MIP。通过两种方法将楔形角度测量到优于1°的精度：第一，在聚焦离子束中垂直切片以便通过TEM直接测量；第二，通过共焦光学显微镜的非破坏性方法。在单晶Si(001)样品上检验了该方法的有效性，结果表明机械抛光楔形方法可应用于多种材料。我们的测量得出蓝宝石的MIP为V = 16.90±0.36 V。此外，对于收集角为18mrad的197 keV电子，测量得到蓝宝石的IMFP为136±2 nm。测量得到的蓝宝石MIP反映了其离子性程度，该程度介于基于狄拉克 - 福克计算获得的带电和中性孤立原子的电子散射因子的理论计算值之间。我们对MIP的测量结果趋向于这种主要为离子性材料的预期值。为了考虑化学键合以及样品表面晶体学平面的作用，我们将实验测量结果与MIP的密度泛函理论计算进行了比较。沿(0001)和(1 - 100)平面切割的α - AlO平板的计算分别得到MIP值为15.7 V和16.7 V。
