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contrast value was maximum, the ZZ direction of the crystal and the polarized light direction were parallel, and when the value was minimum, the ZZ direction of crystal and the polarized light direction were perpendicular to each other. (Figure 44b). In the case of Figure 44a samples, the direction of the edge which was set the reference is parallel to the crystal ZZ direction.
Figure 45. TEM measurement of GeSe nanoflake. (a) Low magnification TEM image of GeSe edge as shown in Figure 42(a) very next to black arrow, (b) TEM diffraction image at the same location and (c) High resolution TEM image at the same position. (d) High-angle annular dark- field STEM cross section image of GeSe FET device.
To confirm the reliability of optical contrast method, the GeSe flake on the Si/SiO2 substrate was picked up and transferred to a silicon nitride TEM grid. The method of finding the crystal orientation using the optical contrast was consistent with transmittance electron microscopy (TEM) measurements (Figure 45). Figure 45a shows the low magnification TEM image and Figure 45b shows the selected area electron diffraction patterns (SAED) image at the edge which is very next to the black arrow in figure 1(a). It was confirmed that the red arrow direction (ZZ direction) and the black arrow direction in Figure 44a are parallel. Figure 45c was a high resolution (HR) image magnifying the edge of GeSe in Figure 44a. From the inset image in Figure 45c, atomic structure model image and the real atomic
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position image were in good agreement. And to confirm the cross section an atomic structure of GeSe TEM image, finding the AC direction of GeSe crystal through optical contrast method and then, focused ion beam (FIB) process was performed. The real atomic position image (Figure 43d) and atomic structure model image (inset of Figure 43d) were in good agreement.
Figure 46. Angle dependent of Raman measurement. (a) Raman measurement at θ=1°, (b) θ=45°, and (c) θ=89°. (d) Angle dependent of B3g Raman peak intensity and (e) Ag2 Raman peak intensity.
We also performed polarized Raman with flakes that confirmed the ZZ direction of GeSe crystals using optical contrast method. Among the GeSe Raman peaks, B3g peak and Ag2 peak are intensively studied. Since we confirmed that the edge of the flake set as a reference in Figure 44, 45 was the ZZ direction, that θ was zero means that the ZZ direction of the crystal and the polarized Raman laser were parallel to each other.
As can be seen from Figure 46a, d, e, when the ZZ direction of the crystal and the polarized laser direction were parallel or perpendicular to each other, the B3g Raman peak intensity was minimum. As can be seen from Figure 46b, d, that the B3g Raman peak intensity was maximum when the ZZ direction of the crystal and the polarized laser was 45 degrees to each other. When the ZZ direction of the crystal and the polarized light direction are perpendicular to each other, Ag2 peak Raman intensity was maximum and ZZ direction of the crystal and the polarized light direction were parallel to each other, Ag2 peak Raman intensity was minimum. These results are well known through other studies.35,37
By performing optical contrast, TEM, and Raman studies, we could obtain consistent results. Also, through this optical contrast method, we found easy way to identify the crystal direction of nanoflake with only optical microscope without complex procedures such as Raman spectroscopy or TEM.
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3.3.2 Surface changing with air ambient conditions (degradation)
Figure 47. (a) Optical image of GeSe nanoflake right after exfoliation and (b) after 14 days of exposure in air. (c) AFM image of GeSe nanoflake and (d) 3D AFM image after 14 days of exposure in air.
In the case of black phosphorus, degradation issue is known to be severe when exposed to air for a long time.137-139 Degradation means that when exposed to air, the BP surface is oxidized and bubbles are formed, and theses bubbles degrade the electrical properties of intrinsic BP.
We have also confirmed that this degradation issue appears in GeSe. First, GeSe nanoflakes were transferred to a Si/SiO2 substrate by mechanical exfoliation. After sample was exposed to the air for 14 days, the state change was observed. Figure 47a shows the state immediately after exfoliation, and Figure 47b shows the optical image after exposure to air for 14 days. When comparing the images of Figure 47a, b it was confirmed that many black bubbles were appeared. To observe these surface changes in more details, an image was obtained using atomic force microscopy. Figure 47c was a typical AFM image and Figure 47d was a 3D AFM image. As a result of the AFM image analysis, the flake thickness was 150nm and the bubble was small and high with an average of 100nm and a diameter of 500nm. It is not yet known exactly what the bubble is and for what reason. We will study more about
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this degradation in the future. Although the study of this phenomenon has not been conducted accurately, when preparing all GeSe samples, it was conducted in a glove box or as much as possible to avoid air exposure.