α-Cu_2Se精细结构的球差校正扫描透射电镜表征

Characterization of α-Cu_2Se Fine Structure by Spherical-Aberration-Corrected Scanning Transmission Electron Microscope

目前学界对Cu_2Se低温α相的结构仍未认识清楚,而解决这一问题对理解Cu_2Se在相变过程中热电性能提升等特性具有重要意义。本文首次报道了由球差校正扫描透射电镜(STEM)拍摄到的沿■带轴的原子级分辨率高角环形暗场(HAADF)像,揭示了由Se原子以多种形式有序起伏产生的复杂结构。结合电子衍射图谱,分析了包含不同层数、通过相互组合构成α-Cu_2Se晶体的多种结构变体。使用QSTEM软件对构建的结构变体进行高分辨图像模拟,得到了与实验对应的HAADF像。该工作为更全面地理解α-Cu_2Se的结构提供了新的重要信息。

The structure of low-temperatureα-Cu2Se,which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during theα-βphase transition of Cu2Se,has still not been fully solved.Because it is restricted by the quality of polycrystal and powder specimens and the accuracy of characterization methods such as the conventional transmission electron microscopy(TEM)and X-ray diffraction(XRD),direct observation with atomic-scale resolution to reveal the structural details has not been realized,although electron diffraction and high-resolution transmission electron microscopy(HRTEM)studies have indicated the complexity of theα-Cu2Se layered structure.Owing to developments in the focused ion beam(FIB)milling preparation method,high-quality single crystalline specimens with specific crystallographic orientations can be prepared to ensure that atomic-resolution images along a specific orientation can be acquired.Furthermore,the developments in aberration correction technology in TEM and scanning transmission electron microscopy(STEM)allow us to observe the subtle details of structural variation and evolution.Herein,we report,for the first time,the atomic-resolution high-angle annular dark field(HAADF)images acquired along the[1_1_2]c axis ofα-Cu2Se using spherical-aberration(Cs)-corrected STEM from FIB-prepared single crystalline specimens.The observations revealed that the complex structure is generated by ordered fluctuations of Se atoms with various forms,including that some of the Se atoms on the two sides of the Cu deficiency layer get closer to each other than the others and the neighboring Cu deficiency layers have different forms of ordered Se fluctuations.These characteristics can only be observed along the[1_1_2]c axis,while these details were not visible in a previous study along the<1_10 >c axis or in our results obtained along the[1_01]c axis.By combining the electron diffraction patterns,several models of the unit cell variants were established,including the two-layer and four-layer cells(both have two different shapes)and the two-layer variants with and without central symmetry.These variants can also transform into each other,and anα-Cu2Se crystal can be formed through the random assembly of these variants.Using the program QSTEM,the corresponding HAADF images of these variants were simulated.The simulation results were similar to the experimental HAADF images and reflected most of the observed details,including the different forms of the ordered fluctuations of Se atoms and the dispersion of Cu atoms,which indicates that our structure models ofα-Cu2Se are reasonable.This work provides new critical information for thoroughly understanding the structure ofα-Cu2Se and theα-βphase transition of Cu2Se.