LabAdviser/314/Microscopy 314-307/Technique/Holo/Off-axis ATEM: Difference between revisions
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'''Feedback to this page''': '''[mailto:labadviser@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/LabAdviser/314/Technique/Holo/Off-axis_ATEM click here]''' | '''Feedback to this page''': '''[mailto:labadviser@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/LabAdviser/314/Microscopy_314-307/Technique/Holo/Off-axis_ATEM click here]''' | ||
(''content by Ganapathi Prabhu Sai Balasubramanian, April 2020'') | (''content by Ganapathi Prabhu Sai Balasubramanian @DTU Nanolab, April 2020'') | ||
__FORCETOC__ | __FORCETOC__ | ||
[[Category:314]] | [[index.php?title=Category:314]] | ||
[[Category:314-Microscopy]] | [[index.php?title=Category:314-Microscopy]] | ||
= Procedure for Performing Electron Holography in the ATEM = | = Procedure for Performing Electron Holography in the ATEM = | ||
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# Fill Liquid nitrogen in the dewar twice or thrice; once at the beginning of the session prior to inserting the TEM holder, once after 30 minutes into the session, and the once around half time of the full-day session. <br/><br/> | # Fill Liquid nitrogen in the dewar twice or thrice; once at the beginning of the session prior to inserting the TEM holder, once after 30 minutes into the session, and the once around half time of the full-day session. <br/><br/> | ||
# Switch on the power supply at the backside of the room | # Switch on the power supply at the backside of the room | ||
#:[[file:Off-axis-Holo_1.png|thumb| | #:[[file:Off-axis-Holo_1.png|thumb|left|200px|power supply biprism]] <br clear=all> <br/> | ||
# Switch on the control box (under the microscope PC screen) | # Switch on the control box (under the microscope PC screen) | ||
#:[[file:Off-axis-Holo_2.png|thumb| | #:[[file:Off-axis-Holo_2.png|thumb|left|200px|biprism controller]] <br clear=all> <br/> | ||
# Switch cables on SAD from red to grey <br/><br/> | # Switch cables on SAD from red to grey <br/><br/> | ||
# Load the appropriate TEM alignment, e.g. 120 keV TEM. This is done by going to Alignment tab → Alignment → Flapout → load the alignment file for the correct keV, e.g. Main_120kV. Select one particular alignment and the press Ctrl+A for selecting all and then move them from “Available” to “Selected” by clicking on the Apply button. <br/><br/> | # Load the appropriate TEM alignment, e.g. 120 keV TEM. This is done by going to Alignment tab → Alignment → Flapout → load the alignment file for the correct keV, e.g. Main_120kV. Select one particular alignment and the press Ctrl+A for selecting all and then move them from “Available” to “Selected” by clicking on the Apply button. <br/><br/> | ||
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#: If the sample is slightly out of focus then don’t try to fix it. Also, if the FFT of the image shows signs of astigmatism, then don’t try to fix it. <br/><br/> | #: If the sample is slightly out of focus then don’t try to fix it. Also, if the FFT of the image shows signs of astigmatism, then don’t try to fix it. <br/><br/> | ||
# Set the biprism. Insert the SAD aperture (lever from right to left). Position of the biprism is SAD aperture #1. Find the biprism and center it. If you can’t see it, excite the biprism to ~100 V with the volume knob on the controls for the biprism. | # Set the biprism. Insert the SAD aperture (lever from right to left). Position of the biprism is SAD aperture #1. Find the biprism and center it. If you can’t see it, excite the biprism to ~100 V with the volume knob on the controls for the biprism. | ||
#:[[file:Off-axis-Holo_3.png|thumb| | #:[[file:Off-axis-Holo_3.png|thumb|left|200px|biprism controller close-up]] <br clear=all> <br/> | ||
#: Align the long axis of the biprism wire in such a way that it is parallel to the long axis of the feature of interest in case of nanowire in plan view OR align the long axis of the biprism wire along the edge of the lamella parallel to the grid, in case of dealing with specimens in the form of a lamella. For rotating the biprism wire, turn the black cylinder attached to the area where the controls for the SAD aperture exist in the microscope. Next, play with condenser stigmatism controls for getting the beam elliptical. When the beam is condensed it should be elliptical with the long axis of the ellipse perpendicular to the biprism wire. Upon turning the intensity knob clockwise, when the beam expands, the long axis of the ellipse should be parallel to the biprism wire. If what you see is opposite of this then you should try swapping the MFx and MFy values.<br/><br/> | #: Align the long axis of the biprism wire in such a way that it is parallel to the long axis of the feature of interest in case of nanowire in plan view OR align the long axis of the biprism wire along the edge of the lamella parallel to the grid, in case of dealing with specimens in the form of a lamella. For rotating the biprism wire, turn the black cylinder attached to the area where the controls for the SAD aperture exist in the microscope. Next, play with condenser stigmatism controls for getting the beam elliptical. When the beam is condensed it should be elliptical with the long axis of the ellipse perpendicular to the biprism wire. Upon turning the intensity knob clockwise, when the beam expands, the long axis of the ellipse should be parallel to the biprism wire. If what you see is opposite of this then you should try swapping the MFx and MFy values.<br/><br/> | ||
#:[[file:Off-axis-Holo_4.png|thumb|left|150px]] | #:[[file:Off-axis-Holo_4.png|thumb|left|150px]] | ||
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# Correct for objective lens astigmatism. For every magnification change you should recheck objective lens astigmatism and the focus (with the focus knob). You do the rotation centering at the beginning and not when you play with the magnification-control. <br/><br/> | # Correct for objective lens astigmatism. For every magnification change you should recheck objective lens astigmatism and the focus (with the focus knob). You do the rotation centering at the beginning and not when you play with the magnification-control. <br/><br/> | ||
# Acquire a reference hologram with acquisition time of 10 s and if it is noisy reacquire the hologram with 10 s or 5 s. Then draw a spectrum line across a few fringes and see how many pixels are there per fringe. We should keep 4 pixels per fringe. To increase the number of fringes per unit are, increase the biprism voltage. A biprism voltage of 120-130V is recommended. | # Acquire a reference hologram with acquisition time of 10 s and if it is noisy reacquire the hologram with 10 s or 5 s. Then draw a spectrum line across a few fringes and see how many pixels are there per fringe. We should keep 4 pixels per fringe. To increase the number of fringes per unit are, increase the biprism voltage. A biprism voltage of 120-130V is recommended. | ||
<br clear=all> | |||
== Finishing-up the holography part of the measurement == | |||
# Stop the camera acquisition <br/><br/> | |||
# Put the viewing screen down (with R1 button) <br/><br/> | |||
# Come out of the Lorentz and Holography modes. To do this go to Holography tab → Click on Holography button again to unselect it. Then go to Lorentz → Click on Lorentz button again to unselect it. Then switch to the TEM mode (Gun → BeamSettings → Click on TEM button). <br/><br/> | |||
# Turn down the volume knob of the biprism. Then turn off this box. <br/><br/> | |||
# Turn off the power supply box in the back of the room. <br/><br/> | |||
# Swap the cables at the SAD apertures (from grey to red) <br/><br/> | |||
# Switch to aperture #3. Remove the SAD apertures from the column by moving the lever from left to right. | |||
== Acquiring CBED patterns == | |||
# Decrease the magnification to SA 7200X or something in the SA range. <br/><br/> | |||
# Start the camera acquisition. Lift the viewing screen (press R1 button) <br/><br/> | |||
# Go to Mono tab → Monochromator tune → Click on focus button and play with the intensity knob by turning it clockwise and decrease the intensity drastically, say to the value of 100 or 175 or 200, the higher the better as there will be less damage to the sample. We want to have a beam that gives a CBED pattern that is not saturated under acquisition times of 0.001-0.01 s. This way you will get a beam that doesn’t burn the featyre of interest. Go to a magnification around SA 49000X. <br/><br/> | |||
# Go to Gun tab → Click on the TEM button <br/><br/> | |||
# Acquire CBED pattern after tilting the sample appropriately. Use an acquisition time of 0.001 sec or 0.01 sec. Use an acquisition time as low as possible otherwise you will burn the camera. | |||
== Closing down the session == | |||
# Stop the camera acquisition. <br/><br/> | |||
# Put the screen down (press R1 button) <br/><br/> | |||
# Reset the holder <br/><br/> | |||
# Make sure that all apertures (SAD and objective) are retracted. <br/><br/> | |||
# Close the column isolation valve. Go to Set up tab → Vacuum → Click on column valve closed button (the button’s color will change from grey to yellow) <br/><br/> | |||
# Remove the holder <br/><br/> | |||
# Fill out the process log <br/><br/> | |||
# Check on the lab manager whether there is a next user | |||
== For the next user: Changing the beam kV and settings for the alignment and FEG register == | |||
# Go to Set up → High Tension → Decrease/Increase the HT in steps from 300 kV/120 kV to the needed setting <br/><br/> | |||
# Go to Alignments tab → Alignments → Flapout → select the required setting (e.g. Main_120kV) and then press Ctrl+A for selecting all and then move them from “Available” to “Selected” by clicking on the Apply button. <br/><br/> | |||
# Go to Gun → FEG Register → Load the FEG register for the electron energy of interest to the next user <br/><br/> | |||
# Go to Setup tab → Vacuum → Flapout → Click on the cryo cycle button and it will turn yellow <br/><br/> | |||
# Stop your session in Lab Manager <br/><br/> | |||
# Log out from lab manager and the support PC. | |||
== Notes/Tips == | |||
* For obtaining electron holograms from the region of interest, tilt the sample appropriately so that the diffraction contrast within the region of interest i.e. black areas are at a minimum <br/><br/> | |||
* Avoid Fresnel fringe in the camera’s field of view, so center the biprism wire accordingly <br/><br/> | |||
* In case of a core-shell structure, go to a high enough biprism voltage so that you can see the edges of the core sharp. <br/><br/> | |||
* While obtaining electron holograms from the sample you must be away from the membrane edges; you should have only the feature of interest in the holographic field of view <br/><br/> | |||
* The fringes in the object and reference holograms must be sharp, if they aren’t sharp reacquire them <br/><br/> | |||
* In obtaining reference holograms be sure to keep away from all parts of the sample; you should have only vacuum in the reference holograms. <br/><br/> | |||
* In the object holograms, put the sample at focus using the focus knob; the object edges will be sharp when it is at focus. If there is a hole near the object of interest then it will be sharp when in focus. You can use the edges of the hole for the correction of objective lens astigmatism. The fringe contrast around the holde should be homogeneous. <br/><br/> | |||
* The width of the feature of interest should be smaller than the fringe width of the biprism. <br/><br/> | |||
* The holographic FOV should be at the center of the image field-of-view <br/><br/> | |||
* Obtain many, say, 5-10 sets of the reference and object holograms for each condition <br/><br/> | |||
* Do the rotation centering before adjusting objective focus <br/><br/> | |||
* Remember that in Lorentz mode we can’t get a diffraction pattern <br/><br/> | |||
* You should aim to have 4-5 steps per fringe in the reference and object holograms. <br/><br/> | |||
* For analyzing the holograms while the experiment is running, in Digital Micrograph, go to | |||
*: Holo3 → Reconstruct → Reconstruct with ref, OR | |||
*: Holo3 → Reconstruct → Reconstruc. (reconstruct using just one image) OR | |||
*: Holo3 → Reconstruct → Reconstruct with ++, press space bar and adjust the mask size with the up and down arrow keys, press space bar and this will give you the phase image <br/><br/> | |||
* Go to a high enough biprism voltage to have fine enough fringes that will lead to non-overlapping central and side bands in the FFT <br/><br/> | |||
* Make sure that the fringes contrast across the imaging FOV is uniform. <br/><br/> | |||
* If in the imaging FOV of the reference fringes, you see some black areas then try spreading the beam. If you see some disturbance (e.g. curving) in the fringes of the reference it could be due to either nearness of the sample or some charged contaminants on the birism. So to get rid of it, move the sample away and/or move to a different part of the biprism my moving along its length. <br/><br/> | |||
* When obtaining CBED patterns ensure that you can clearly see features in the central disk <br/><br/> | |||
* If during your experiment involving electron holography, you see that the visibility of fringes has gone low, then for increasing the visibility of the fringes you can decrease the exposure time, decrease the biprism voltage, reduce the magnification, play with track ball and the intensity knob to get the beam to uniformly illuminate the region of interest. | |||