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Acta Phys. -Chim. Sin.  2019, Vol. 35 Issue (2): 139-144    DOI: 10.3866/PKU.WHXB201805111
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Characterization of α-Cu2Se Fine Structure by Spherical-Aberration-Corrected Scanning Transmission Electron Microscope
Lu CHEN,Jun LIU,Yong WANG*(),Ze ZHANG*()
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Abstract  

The structure of low-temperature α-Cu2Se, which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during the α-β phase 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 the α-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 $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis of α-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 $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis, while these details were not visible in a previous study along the $ <\bar{1}10{{>}_{\text{c}}} $ axis or in our results obtained along the ${{\left[ \bar{1}01 \right]}_{\text{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 an α-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 of α-Cu2Se are reasonable. This work provides new critical information for thoroughly understanding the structure of α-Cu2Se and the α-β phase transition of Cu2Se.



Key wordsα-Cu2Se      Focused ion beam      Electron diffraction      Cs-correction      STEM      Ordered fluctuation     
Received: 09 April 2018      Published: 11 May 2018
MSC2000:  O649  
Fund:  the National Natural Science Foundation of China(51390474);the National Natural Science Foundation of China(11327901)
Corresponding Authors: Yong WANG,Ze ZHANG     E-mail: yongwang@zju.edu.cn;zezhang@zju.edu.cn
Cite this article:

Lu CHEN, Jun LIU, Yong WANG, Ze ZHANG. Characterization of α-Cu2Se Fine Structure by Spherical-Aberration-Corrected Scanning Transmission Electron Microscope. Acta Phys. -Chim. Sin., 2019, 35(2): 139-144.

URL:

http://www.whxb.pku.edu.cn/10.3866/PKU.WHXB201805111     OR     http://www.whxb.pku.edu.cn/Y2019/V35/I2/139

Fig 1 Electron diffraction patterns of α-Cu2Se. (a) $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis; (b) ${{\left[ \bar{1}01 \right]}_{\text{c}}} $ axis; (c) [111] axis; (d) Enlarged view of the purple square marked area in (a).
Fig 2 HAADF images of α-Cu2Se. (a–d) ${{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}} $ axis; (e) ${{\left[ \bar{1}01 \right]}_{\text{c}}} $ axis. All scale bars: 2 nm.
Unit cell label a/nm b/nm c/nm α/(°) β/(°) γ/(°)
2L 0.7223 0.7206 1.4251 99.559 94.751 60.079
2R 1.4261 85.027 94.794
4L 2.8206 92.296 87.471
4R 2.8215 84.972 87.495
Table 1 Lattice parameters of the 4 kinds of unit cells for simulation.
Fig 3 Models and HAADF images of different variations. The lattice vectors a, b, c are expressed with red circles (⊙ and ⊗ mean outward and inward perpendicular to the paper, respectively), green and blue arrow, respectively. (a, d, f, h) Illustrations of lattice vectors of unit cells 2L, 2R, 4L and 4R, respectively. (b, e) Models and corresponding simulated HAADF images of different variations of unit cells 2L and 2R, respectively. The models from left to right in each row are 1 centrosymmetric variation and 2 non-centrosymmetric variations. (g, i) Models and corresponding STEM simulated images of unit cells 4L and 4R, respectively. (c, j) Experimental HAADF images of different variations of unit cell 2L and unit cell 4R.
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