(Re-)building a **Scanning Tunneling Microscope** (STM).

We are currently in the process of getting an STM back up and running.  The hardware was inherited from a past project by one of our members, where it successfully imaged sample surfaces.  Now we are trying to improve on those past results.

{{:projects:pxl_20250607_233415336.jpg?1200|}}

[Current working state of the experiment]

==== Links ====

  * [[https://git.fa-fo.de/fafo/k8ik-stm|git repository with technical documentation]]
  * [[projects:stm:log|Lab Notes]]


==== STM key information ====

=== STM Scanning Head ===
The scanning head of our STM uses 3 piezo actuators oriented along the 3 cartesian axes.  We currently do not have data on their actuation range.

The tip is made from tungsten wire and mounted to the scanning head using a screw terminal.

To be documented.

=== STM Mount ===
The STM mount consists of two aluminum plates which seat against each other precisely using a [[http://www.g2-engineering.com/technology-kinematics|kinematic mount]] made from 3 micrometer screws.

The tip/probe is mounted slightly in front of two of the screws with the last one sitting very far back.  This makes a fulcrum with the back screw having even more fine adjust (factor 20).

Thread pitch of micrometer screws (according to lab book 16.3.2012): 
  * Front 0.25mm per revolution
  * Back is hence 0.0125mm per revolution or 12.5µm 

=== Electronics ===
The control loop for running the STM in constant current mode is currently an entirely analog design.  There is a transimpedance amplifier which preamplifies the tunneling current with a gain of 10⁹ V/A.  This nicely maps pA to mV (and measured tunneling currents are in the hundreds of pA range).

Learn more about the electronics here: [[https://git.fa-fo.de/fafo/k8ik-stm/src/branch/main/Electronics]]

=== Vibration Isolation ===

We have built ourselves a simple passive vibration isolation using tension springs and a large mass.  The intended damping element (an eddy current brake) did not end up working, but also wasn't really needed after all.

{{:projects:stm-frame-cad.png?600|}}

We have not measured performance, but the real world resonance frequency matches the calculations quite well. Here is the calculated transmissibility (blue curve):

{{:projects:stm:dampening-2025-06-08.png?400|}}
==== Other Projects ====
There are quite a few other DIY/open STM builds out there.  Here are a few links for further reading:

  * [[https://www.sciencedirect.com/science/article/pii/S2468067223001116|Open STM — Weilin Ma (2023)]]
  * [[https://github.com/MechRedPanda/red-panda-stm|3D-Printed STM — MechPanda (2023)]]
  * [[https://hackaday.io/project/29332-the-pvc-scanning-tunneling-microscope|The PVC Scanning Tunneling Microscope — Tom Ekkens (2018)]]
  * [[http://www.joniemeyer.com/jo_niemeyer/STM.html|Project  „20 steps“  into the microcosmos — Jo Niemeyer (2009)]] ([[https://web.archive.org/web/20210518134908/http://www.joniemeyer.com/jo_niemeyer/STM.html|archive.org]])
  * [[https://www.e-basteln.de/other/stm/overview/#start|Homebrew STM — Jürgen Müller (2006)]]
  * [[https://www.e-basteln.de/other/stm/references/|STM References Overview — Jürgen Müller (2006)]]