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Figure 36. Optical image of the process of mechanical exfoliation.
3.2.2 Identifying the crystal orientation using optical contrast method
GeSe is structurally in-plane anisotropy. For these structural reasons, in-plane anisotropic optical and electrical properties are expected. In general, to check flake crystal orientation, we need to check the diffraction pattern or perform other complex analyses. However, we found the crystal orientation of flake by analyzing only the images obtained from optical microscopy and polarizers.
Figure 37. (a) Photograph image of microscopy & rotating stage. (b) The schematic of optical contrast method by using rotating stage.
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Figure 37 shows a rotating stage and an optical microscope image. After placing the sample in the center of the optical microscope, we used a polarizer to make a parallel polarization configuration. This configuration makes the incident light polarization direction parallel to the reflected light polarization direction. After that, we took the optical image of the target flake at every 10 degrees interval from 0 to 360 degrees.
Figure 38. The process of optical contrast analysis. (a) Split the image to red, green, blue channel respectively. (b) Select the green channel image. (c) Set the area and measure the green contrast of flake. (d) Measure the green contrast of background.
The optical images obtained while rotating 360 degrees are analyzed using a software called image J, which facilitates the analysis of optical microscope images. Figure 38 shows the process of analyzing an image from a certain angle among multiple images. First, we split the selected image into Red, Green, and Blue channels using Image J tools (Figures a, b). We then selected the green channel image. We measured the green contrast of the flake sample by setting a reference region. Bringing this reference region to the background substrate, we measured the green contrast. The formula of contrast is
(𝐼𝑠𝑎𝑚𝑝𝑙𝑒− 𝐼𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)/𝐼𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 .
By plotting the green contrast value for each angle, we could check the amount of contrast change. In the case of GeSe, the change in optical contrast showed most prominently in the green channel. In some cases, however, analysis may be performed in the red or blue channel.
44 3.2.3 GeSe nanoflake transfer to TEM grid
Figure 39. The process of transfer to TEM grid.
The target flake must be transferred to the TEM grid to confirm that the optical contrast value and the direction of the actual crystal orientation coincide. There are two ways to transfer, as shown in Figure 39.
One is the wet transfer method using IPA. After attaching a thin quant foil grid onto the desired target flake, dropped down the IPA solution. As the IPA solution slowly releases the grid and target flake, they were attached together. As the highly volatile IPA dries, flakes and grids stuck and fixed. When the grid detached from the substrate, transfer completed.
The other method was the dry transfer method using PDMS. First, GeSe nanoflake was transferred to the PDMS using mechanical exfoliation. After attaching the PDMS to the slide glass, inverted it, attached it to the silicon nitride grid and press it. When pressure was applied and detached, GeSe nanoflakes were transferred to the silicon nitride grid.
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3.2.4 Device fabrication by using parylene mask and electrode deposition
Device fabrication is required to measure the electrical properties of GeSe nanoflakes. There are two methods for device geometry design before electrode deposition. One is using a parylene mask as shadow mask and the other is using e-beam lithography.
Figure 40. Photograph image of manipulator and parylene mask.
Figure 41. (a) Schematic of Mask align and fabricating device process. (b) Optical image of each steps.
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The parylene mask was often used as a shadow mask because of its transparent property when you want to place an electrode on a small lateral material in micro units. The parylene mask was thin and vulnerable, making it difficult to handle by a single hand so it was handled with pink tape and manipulator (Figure 40).
The method of controlling the parylene mask using the manipulator is as follows (Figure 41). Punch a circular hole at the edge of the pink tape. Attach a patterned parylene mask to this hole. Attach the pink tape to the manipulator so that the parylene mask is clearly visible as the perforated part. Using a manipulator, align the target flake and parylene mask according to the pattern. After aligning, deposit metal with the mask attached. After deposition, check the GeSe device by detaching the parylene mask.
The disadvantage of using this method is that the electrode can be deposited only in the designed pattern of the shadow mask.
3.2.5 Device fabrication by using e-beam lithography and electrode deposition
Figure 42. Schematic of e-beam lithography process.
Another method uses e-beam lithography, which can design any desired device geometry using PMMA etching instead of aligning the shadow mask. The process can be seen in Figure 42.
First, we coated the substrate with the exfoliated target flake with PMMA at 4000 rpm for 1 minute.
After coating was finished, we strengthened the PMMA by heating it on a hot plate for 5 minutes at 180°C. After placing the coated sample in the e-beam lithography equipment, the e-beam was exposed in the desired design form, a process called Exposure. After removing the sample from e-beam lithography machine, we soaked it in the solution mixed with IPA and DI water at a ratio of 3:1 for 30 seconds and then removed it; this process is called Development and the solution is called Developer.
Developer can be used several solutions. After Development, we etched PMMA into the desired pattern
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shape, and then deposited the metal for device electrode. When deposition was complete, we soaked it in acetone for more than 2 hours to remove PMMA, a process called Lift-off. This resulted in the device we wanted because metal was deposited only on the part where PMMA was etched away.
Figure 43. Schematic of e-beam lithography details (align mark & device geometry pattern design).
Figure 43 shows the details of the process of drawing the desired pattern using e-beam lithography.
First, draw the align mark draw using the CAD program near the target flake. After exposure using e- beam lithography as an align mark, when developing was completed, an alignment marker was engraved near the flake. The image of the engraved align mark was obtained by optical microscopy.
The alignment marks in this optical microscope image and the alignment mark drawn with the CAD files were perfectly overlapped with each other. If e-beam lithography was performed without this process, the desired pattern may not match the target flake.
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