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projects:stm:log [2025/08/28 14:19] – [Lab Notes for the Scanning Tunneling Microscope] rahixprojects:stm:log [2025/09/30 22:27] (current) – Remove nolink rahix
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 ====== Lab Notes for the Scanning Tunneling Microscope ====== ====== Lab Notes for the Scanning Tunneling Microscope ======
  
 +==== Day 2025-09-20 (rahix) ====
 +Modelled more of the as-built of the current STM setup in mechanical CAD so we have a reference model:
 +
 +{{:projects:stm:cad-stm.png?900|}}
 +==== Day 2025-09-07 (rahix) ====
 +Took the video capture from yesterday, of the beam bouncing from the STM off a mirror 2.35m away and back to the camera on the STM and ran some analysis on the data.
 +
 +Analysis works like this:  For each frame, we find the beam center.  Then we look at the spectral content of the displacement in X and Y directions.  Check the full notebook for details: ''Laser-Wobble.ipynb'' from [[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Calculations|Calculations/]] ([[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Calculations/rendered/Laser-Wobble.md|Rendered Notebook]]).
 +
 +Here is the final figure:
 +
 +{{:projects:stm:2025-08-07-vibration-spectral-density.png?900|}}
 +
 +The most interesting result is the peak highlighted by the red line.  The red line marks the theoretical resonance frequency of the vibration isolation system that we had calculated in the past.  Seeing a peak right at this frequency sparks a lot of confidence!
 +
 +The results are limited up the 15 Hz because of the 30 fps video capture.  But towards the right of the figure, the data becomes pretty meaningless anyway as the displacements are too small for the camera to pick up already.
 +==== Day 2025-09-06 (rahix, q3k, hugo) ====
 +Rebuilt the entire experiment to clean up the space and install some shelving.  All connections are remade but the setup has not been reverified to tunneling.
 +
 +Set up the laser again to make some more measurements of the beam.  The following pictures show the beam at different distances. Left to right: 50mm, 100mm, 200mm, 400mm.  The 400mm measurement includes a mirror which is why the orientation is flipped.
 +
 +{{:projects:stm:2025-09-07-beam-50mm.png?400|}}
 +{{:projects:stm:2025-09-07-beam-100mm.png?400|}}
 +{{:projects:stm:2025-09-07-beam-200mm.png?400|}}
 +{{:projects:stm:2025-09-07-beam-400mm.png?400|}}
 +
 +The size change derived from the FWHM in these pictures would be about 0.6mrad, but this is most likely not at all a dependable result.
 +
 +Set up the laser to bounce off a mirror 2.35m away and back onto the STM vibration isolated mount.  Installed a ø2mm aperture in front of the beam to slightly tighten the profile.  This way we can watch the beam jump around on the camera.  Recorded 8 minutes of footage in hopes that we can recover the low-frequency content of the vibration isolation transmittance from this.
 +
 +Turned on the STM again and managed to get (very very unstable) tunneling.  So everything was set up correctly again!  The tip is still the broken one from the failed runs last time.  Adjusted the sample holding to avoid the collision that happened back then.  The STM is currently not covered in aluminum.
 +
 +We noticed that the vibration isolation is bouncing around a lot more after the rebuild. It seems this is caused by it now standing on a different part of the table that is not rigidly fastened to the table frame.  We should put some effort into stiffening the table structure here.
 +
 +Here is an overview of the current status of the experiment after it was rebuilt:
 +
 +{{:projects:stm:pxl_20250907_031437285.jpg?600|}}
 +
 +  * Oscilloscope
 +    * Yellow: Z displacement (control variable, this is the "output" signal)
 +    * Pink: Tunneling current scaled to 0-5V
 +    * Blue: X displacement output
 +    * Green: X displacement input
 +  * Multimeter (set to DC voltage
 +    * Setpoint current (''Sollstrom'') in 1V/nA (so 0.100V is 100pA)
 +  * Laptop
 +    * Connect white USB cable from the Raspberry Pi Pico
 +    * Temperature Measurement using ''serial-monitor-rust'' as detailed on [[projects:stm:log#day_2025-08-23_rahix_q3k_rob_hugo|2025-08-23]]
 +  * Red Pitaya (not currently running)
 +    * Can also measure the Z displacement (''IN1'')
 +    * And output the X displacement (''OUT1'')
 +==== Day 2025-09-05 (rahix) ====
 +Set up the beam camera for a test whether this whole plan of using the camera for interferometry will work.  Pointed the bare cheapo laser diode at it.
 +
 +{{:projects:stm:pxl_20250905_191140378.jpg?400|}}
 +
 +Configuring the camera is very odd — it seems you cannot directly set the exposure.  It seems that triggering the auto white balance once auto-fixes an adequate exposure.  Got to take a somewhat decent shot after then resetting all three white balance color channels to 50.
 +
 +{{:projects:stm:2025-09-05-210808-laser.jpg?400|}}
 +
 +Analyzing this image for beam quality, using ''Laserbeam.ipynb'' from [[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Calculations]].
 +
 +{{:projects:stm:2025-09-05-beam-quality.png?600|}}
 +
 +Please keep in mind that the sensor size is not precisely known as of right now so the absolute measurements in the image above are probably off by up to 20%.
 ==== Day 2025-08-28 (rahix) ==== ==== Day 2025-08-28 (rahix) ====
 Performed a study of the dimensions involved in our system based on some yet-to-be-confirmed assumptions.  This gives us some nice potential explanations for the behavior and lots of leads for probing the correctness of this model.  Keep in mind that all the results detailed below hinge on the assumptions detailed here. Performed a study of the dimensions involved in our system based on some yet-to-be-confirmed assumptions.  This gives us some nice potential explanations for the behavior and lots of leads for probing the correctness of this model.  Keep in mind that all the results detailed below hinge on the assumptions detailed here.
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   - Our piezo actuators have a maximum deflection of 0.3±0.1µm for the maximum voltage of +10V that we apply.  This was derived from looking at a similar piezo actuator's characteristics ([[https://content.kemet.com/datasheets/KEM_P0101_AE.pdf|KEMET AE0203D04DF]]).   - Our piezo actuators have a maximum deflection of 0.3±0.1µm for the maximum voltage of +10V that we apply.  This was derived from looking at a similar piezo actuator's characteristics ([[https://content.kemet.com/datasheets/KEM_P0101_AE.pdf|KEMET AE0203D04DF]]).
   - Our piezo actuators deflect by 31±10nm/V with a linear response between 0 and +10V.  We do not make assumptions about the behavior with negative bias as this is not characterized for the reference actuator either (they actually explicitly do not recommend negative bias).   - Our piezo actuators deflect by 31±10nm/V with a linear response between 0 and +10V.  We do not make assumptions about the behavior with negative bias as this is not characterized for the reference actuator either (they actually explicitly do not recommend negative bias).
 +  - Under the observed stable temperature conditions (drift by -0.04°C/min) we assume a worst-case thermal expansion of 18nm/min.  Real expansion is most likely below that as expansion of the different components will largely cancel out.
 +  - Tunneling currents require a distance of tip-to-sample below 1nm.
 +
 +== Derivations ==
 +Combining these assumptions with previous measurements lets us make some claims:
 +
 +  - The DVD sample we are looking at has a track spacing of 0.74±0.03µm (see [[https://ia801406.us.archive.org/32/items/dvd-iso-iec-specifications/c035641_ISO_IEC_16448_2002%28E%29.pdf|ISO/IEC 16448:2002]]).  We will not be able to resolve two tracks in one picture with the deflections of the piezos that we can achieve.  We may or may not be able to resolve a single pit which is about 320nm wide.
 +  - The measured control response from ''2025-08-27'' with a peak-to-peak amplitude of 2.2V corresponds to a distance variation of 90±22nm.  During these variations, the control system is able to keep the tip in range regardless.
 +  - In these 90±22nm variations, the change from adjusting tunneling current setpoint is vanishingly small.  This matches an expected change of a few Å (0.1nm) which correspond to a control signal change of 3±2mV.  Even a 1nm change would not be discernible at all.
 +  - Worst-case temperature drift would show up as a steady slope of 0.65±0.21V/min.  We are not seeing that so thermal expansion must have a much lower effect.
 ==== Day 2025-08-27 (mina, zdmx, rahix) ==== ==== Day 2025-08-27 (mina, zdmx, rahix) ====
 Tried setting everything up again.  The temperature sensor seems "stuck" at always reporting 25°C.  After a bit of investigation, it turns out that today it's too cold for the setup to work!  We accidentally designed the setup such that we can measure only down to exactly 25°C: Tried setting everything up again.  The temperature sensor seems "stuck" at always reporting 25°C.  After a bit of investigation, it turns out that today it's too cold for the setup to work!  We accidentally designed the setup such that we can measure only down to exactly 25°C:
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 Played a bit more with the [[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Calculations/Spring-Dimensioning.ipynb|calculations for the mass-spring-damper system]] to see where we are at, comparing to commercially available systems: Played a bit more with the [[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Calculations/Spring-Dimensioning.ipynb|calculations for the mass-spring-damper system]] to see where we are at, comparing to commercially available systems:
  
-{{:projects:stm:dampening-2025-06-08.png?nolink&400|}}+{{:projects:stm:dampening-2025-06-08.png?400|}}
  
  
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 Calculated the damping behavior of our setup: Calculated the damping behavior of our setup:
  
-{{:projects:stm:damping-ratio-2025-06-08.png?nolink&400|}}+{{:projects:stm:damping-ratio-2025-06-08.png?400|}}
  
 EMI Box: EMI Box:
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 Removed the formwork from the conrete block we poured on friday.  The block is so nice! Removed the formwork from the conrete block we poured on friday.  The block is so nice!
  
-{{:projects:stm:stm-concrete-weight.jpg?nolink&400|}}+{{:projects:stm:stm-concrete-weight.jpg?400|}}
  
   * Lessons learned:   * Lessons learned:
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 Poured a concrete weight for the vibration isolation system: Poured a concrete weight for the vibration isolation system:
  
-{{:projects:stm:concrete-pouring.jpg?nolink&400|}}+{{:projects:stm:concrete-pouring.jpg?400|}}
  
   * Lessons learned:   * Lessons learned: