LabAdviser/314/Microscopy 314-307/Technique/Holo/Off-axis ETEM: Difference between revisions
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In order to remove effects from the TEM or other instabilities, reference holograms are usually acquired right after the object holograms. They are later used for reconstruction. | In order to remove effects from the TEM or other instabilities, reference holograms are usually acquired right after the object holograms. They are later used for reconstruction. | ||
[[file:Schematic_holo.jpg|thumb|left|200px|Figure 1: Schematic of the set-up for off-axis electron holography. U0 is the biprism voltage, s is the fringe spacing, β is the angle that the object and reference wave is superimposed onto one another, and w is the width of the interference pattern.]] <br clear=all | :[[file:Schematic_holo.jpg|thumb|left|200px|Figure 1: Schematic of the set-up for off-axis electron holography. U0 is the biprism voltage, s is the fringe spacing, β is the angle that the object and reference wave is superimposed onto one another, and w is the width of the interference pattern.]] <br clear=all> | ||
The detailed procedure for obtaining electron holograms is described below. The procedure has been used for obtaining holograms of in liquid phase transmission electron microscopy or of single features on a C-grid. Some tips are also shown at the end of the document. | The detailed procedure for obtaining electron holograms is described below. The procedure has been used for obtaining holograms of in liquid phase transmission electron microscopy or of single features on a C-grid. Some tips are also shown at the end of the document. | ||
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# Align the biprism to the feature of interest. Usually the biprism should be at an angle to the feature, e.g. a straight line. When aligning the biprism, one has to consider where the object and reference wave are passed through. If the biprism is perpendicular to the feature, the reference and object wave both passes through that feature and would terminate each other. Therefore, the reference wave is usually passed through vacuum/substrate and the object wave over that feature. Rotate the biprism with the black knob on the SAD aperture (outermost knob). You may want to zoom out to not lose the biprism from the FOV | # Align the biprism to the feature of interest. Usually the biprism should be at an angle to the feature, e.g. a straight line. When aligning the biprism, one has to consider where the object and reference wave are passed through. If the biprism is perpendicular to the feature, the reference and object wave both passes through that feature and would terminate each other. Therefore, the reference wave is usually passed through vacuum/substrate and the object wave over that feature. Rotate the biprism with the black knob on the SAD aperture (outermost knob). You may want to zoom out to not lose the biprism from the FOV | ||
# Align the beam to the biprism. The beam should be elliptical and perpendicular to the biprism to achieve high spatial coherence. Aligning the beam is done by stigmating the beam through the condenser lens. For this: Press stigmator → Condenser Stigmation → Rotate MFx or MFy. Note, either MFx or MFy has to be at their maximum, e.g. MFx = +1 or -1. When that is fixed, rotate the other multifunction knob until the beam is perpendicular to the biprism. In order to see if the beam is aligned to the biprism, check for the “step” that is produced when condensing the beam (Intensity knob). The idea is illustrated in the figure below. | # Align the beam to the biprism. The beam should be elliptical and perpendicular to the biprism to achieve high spatial coherence. Aligning the beam is done by stigmating the beam through the condenser lens. For this: Press stigmator → Condenser Stigmation → Rotate MFx or MFy. Note, either MFx or MFy has to be at their maximum, e.g. MFx = +1 or -1. When that is fixed, rotate the other multifunction knob until the beam is perpendicular to the biprism. In order to see if the beam is aligned to the biprism, check for the “step” that is produced when condensing the beam (Intensity knob). The idea is illustrated in the figure below. | ||
#:[[file:Holo-beam-alignment.png|thumb|left|200px|Figure 6: Aligning the beam (schematic)]] <br clear=all | #:[[file:Holo-beam-alignment.png|thumb|left|200px|Figure 6: Aligning the beam (schematic)]] <br clear=all> | ||
# Recheck the alignments: Rotation Center, PPx/PPy, Beam shift and object astigmatism. Most crucial is rotation center and object astigmatism! | # Recheck the alignments: Rotation Center, PPx/PPy, Beam shift and object astigmatism. Most crucial is rotation center and object astigmatism! | ||
# Tune the biprism voltage: Usually around 150-180V is useful, but higher voltages can be used for better resolution. Note that the fringes should be separated enough to see the individual fringes + it is recommended that 4 pixels per fringe is the minimum of fringe width. | # Tune the biprism voltage: Usually around 150-180V is useful, but higher voltages can be used for better resolution. Note that the fringes should be separated enough to see the individual fringes + it is recommended that 4 pixels per fringe is the minimum of fringe width. | ||
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# If you see a blister on the biprism, it means the biprism is contaminated at this region. Move the biprism by adjusting the knobs on the SAD aperture until the blister is not there | # If you see a blister on the biprism, it means the biprism is contaminated at this region. Move the biprism by adjusting the knobs on the SAD aperture until the blister is not there | ||
#* You don’t want a curved interference pattern, the lines should be straight | #* You don’t want a curved interference pattern, the lines should be straight | ||
# Do not jump around or cause vibrations | # Do not jump around or cause vibrations → Move the chair and yourself away from the desk if necessary | ||
# If the biprism is not stable, even after waiting, tab either of the knobs of the SAD ever so slightly. It is possible the SAD aperture is moving, tabbing will help it fall into a stable position. | # If the biprism is not stable, even after waiting, tab either of the knobs of the SAD ever so slightly. It is possible the SAD aperture is moving, tabbing will help it fall into a stable position. | ||
# Name the reference and object holograms appropriately. I usually have a string for the file along the lines: Sample_Magnification_BiprismVoltage_ExposureTime_Obj_Number for object holograms, and “Obj” is “Ref” for reference holograms. | # Name the reference and object holograms appropriately. I usually have a string for the file along the lines: Sample_Magnification_BiprismVoltage_ExposureTime_Obj_Number for object holograms, and “Obj” is “Ref” for reference holograms. | ||
#* You need to know the biprism voltage and exposure time for later documentation, since these values determine the width and carrier frequency and are always reported in literature. | #* You need to know the biprism voltage and exposure time for later documentation, since these values determine the width and carrier frequency and are always reported in literature. | ||