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Introduction

Hi, this is a DIY STM(Scanning Tunneling Microscope) project!

In June 2022: the distance-tunneling current curve, and bias voltage-tunneling current curve of HOPG were measured.

In May 2023, the image of HOPG in atom resolution was obtained.

This project has been published on HardWareX, all the details can be found in the article. DOI: https://doi.org/10.1016/j.ohx.2023.e00504

Branches

Currently, the repository has two branches: main and Ref-Document.

The open-source file was included in the main, and the references(datasheet, paper, etc.) documents were in the Ref-Documents branches. Since the documents file occupies a large amount of space, I have to separate the documents file into another branch.

File Structure

• 3DModels

  Included design file of structure and STEP file for CNC

• Docs

  Included project introduction and build tutorial

• Hardware

  MCU source code(by ESP-IDF and Platform IO) and firmware

• PCB

  PCB file(Easy EDA), browsing online: OSHWHUB

• PythonScript

  Control software code

Document

The document has been published on Arxiv: https://arxiv.org/abs/2310.05413, and you can also find it in the doc folder.

Version Release Naming Rules

As of October 2023, three different mechanical structures for the STM (Scanning Tunneling Microscope) have been published, with version 3.0.0 capable of basic STM functionality.

This project will follow the software release approach, where after each update in the STM design, a release will be made. The version numbering for each released STM design will follow the following format:

The version number will be in the format of A.B.C. When there is a significant reconstruction of the mechanical structure, A will change. When there are substantial changes to the circuitry, software, or mechanical structure of the design, B will change. When there are minor modifications to the design, C will change.

For example, 1.0.0 represents the first-generation mechanical structure design of the STM.

Releases

Release OpenSTM v1.0.0

This is the first-generation STM (Scanning Tunneling Microscope) design, constructed using two aluminum plates. You can view it here: https://www.youtube.com/watch?v=NZt7yUdhfqY

This design is relatively simple and did not yield experimental results suitable for analysis. However, subsequent designs are built upon this first-generation design. This version of the design is provided for reference only and does not currently include detailed documentation.

The released design files include:

• 3D model files (SolidWorks).
• Arduino programs: These programs control the STM and include the ESP32 microcontroller control program (using LVGL for interaction) and a vibration detection program based on the MPU9250. Both were developed using Arduino and Platform IO.
• PCB and schematic files.
• Python scripts for measuring interference fringes.
• LTSpice simulation files for the power supply chip.

Release OpenSTM v2.0.0

This version of the design represents the second-generation microscope structure. You can view it here: https://www.youtube.com/watch?v=2gPIJrTNqIA

The structure of this design allows for the measurement of:

• Tunneling distance-current curves.
• Scanning Tunneling Spectroscopy (STS).

The files included in v2.0.0.zip are:

• 3DModel: 3D model files drawn in SolidWorks and STEP files required for CNC machining.
• PCB: Schematics and PCB files drawn using the professional version of Jialichuang EDA.
• Software: In the Arduino folder, it includes control programs for the ESP32 microcontroller and the ATMEGA 328P microcontroller. In the Python folder, it contains the upper-level control software and image conversion programs.

Release OpenSTM v3.0.0

This is the third-generation scanning tunneling microscope, a version with basic functionality that can perform curve measurements and HOPG atomic imaging. Video: https://www.youtube.com/watch?v=noWLMcfyqbQ Replication paper manuscript: https://arxiv.org/abs/2310.05413 For detailed information, please refer to the manuscript.

Results

Distance-tunneling current curve

Bias voltage-tunneling current curve of HOPG

Atom image of HOPG

Contact me

If you would like to build an STM microscope yourself or have any suggestions for me, you can submit an issue on this page.

Development Record

• November 2021

  Unstable tunneling.

• January 11, 2022

  Submission of the open-source page.

• January 18, 2022

  Added tennis balls under the vibration isolation table, significantly improving vibration isolation.

• February 05, 2022

  1. Changed the power supply for the system simulation part to 9V battery while continuing to use a switch power supply for the digital part.
  2. ADP5070 stopped working. Frustrating!
  3. Discovered that tunneling current should be between 100pA - 10nA, which suggests signals within 1V for the existing op-amps, realizing previous mistakes.s
  4. OPA627 has an open-loop voltage gain of 120dB and input bias current of 1pA, making it reasonable to use a 100MΩ feedback resistor?

• February 16, 2022

  Tested the input-output characteristics of the op-amp, confirming the feasibility of the front-end tunnel current amplification circuit.

• March 14, 2022

  1. Completed CNC machining for the new structural design.
  2. Abandoned LVGL and display as the control system.

• March 21, 2022

  Conducted tunnel current approach tests using the new structural system, circuit, and control system:

  1. Tunnel current is initially stable and can be maintained for about ten seconds.
  2. Found that the tunnel current-piezoelectric ceramic deformation curve, sampled at eight points, generally follows exponential characteristics.
  3. Confirmed that a complex vibration isolation system is not required.
  4. Essentially confirmed that the previous output jumping phenomenon was due to thermal expansion mismatch.

• April 12, 2022

  1. Tunnel current has become very stable after adjusting the heat isolation of the stepping motor in the approach mechanical structure. It can now be maintained for at least 30 minutes.
  2. Analyzed the new tunneling curve and suspected that the current-piezoelectric ceramic deformation relationship is not purely exponential. It might be due to the formation of a capacitor between the tip and the sample.
  3. Improved the fine approach control algorithm, allowing you to engage in polar battles after clicking "start approach."
  4. Changed the power supply for the simulation part from a 9V battery to a 3S lithium-polymer battery.

• April 20, 2022

  Completed STM image scanning in constant height mode and verified it through repeated experiments, but could not reliably determine the image scale and content.

• May 01, 2022

  Started writing the graduation thesis.

• May 04, 2022

  70% of the graduation thesis writing is complete, began writing the constant current scanning algorithm.

• June 16, 2022

  Released open-source materials for the second-generation microscope.

• October 21, 2022

  Started designing stick-slip piezoelectric motors.

• October 26, 2022

  Improved technical documentation.

• January 4, 2023

  Completed the reconstruction of the third-generation circuit and mechanical structure, designed the stick-slip piezoelectric motor, and entered the debugging phase (not yet released, pending validation).

  (1) Regarding circuit power supply: The reconstructed circuit uses the ADP5070 with a low-noise LDO scheme to provide multiple power rails (dual ±12V, 5V). The ADP5070 is powered by a 35W dual-C port power adapter with extremely low ripple (peaking at around 13mV, resembling a sawtooth wave).

  (2) Regarding PCB layer design: The new generation circuit board is divided into three parts: power board, MCU board, and control board. Coaxial signal lines are used for connections between the boards, while IDC ribbon cables are used for data lines.

  (3) Regarding circuit improvements: The MCU board continues to use ESP32 as the controller, but the module model is updated to ESP32-S3, with a slot reserved for a WIFI antenna for future upgrades. The control board largely maintains the design of the second generation, continuing to use the AD5761+OPA2227 scheme to control the scanning head. However, due to the introduction of the piezoelectric slide stage, the control board adds an AD8761 as a DAC to apply bias voltage to the sample, repurposing the originally used DAC for biasing to control the piezoelectric slide stage.

  (4) Regarding mechanical structure design: The new generation mechanical structure has reduced overall dimensions and introduced a stick-slip piezoelectric slide stage for coarse approach (referencing the article "Open-source XYZ nanopositioner for high-precision analytical applications"). Additionally, the front-end amplifier has been further shielded with full metal wrapping to further reduce noise coupling.

• January 18, 2023

  The MCU module was changed to ESP32-WROOM-32E, and some issues arose with the S3 module that currently cannot be resolved due to limited documentation. To improve MCU efficiency, the development framework was changed from Arduino to ESP-IDF (simple GPIO code flipping revealed that Platform IO + Arduino runs at 800KHz, Arduino IDE at 1.2MHz, and Platform IO + ESP-IDF at 1.44MHz).

• February 12, 2023

  Completed modifications and validation of the third-generation circuit, mechanical structure, and testing are ongoing. Currently, PID control for probe approach has been implemented, resulting in more stable current compared to the previous generation, and a significant improvement in thermal drift issues.

• March 31, 2023

  1. Encountered some issues during the assembly of the piezoelectric slide stage: Although the assembly difficulty of the piezoelectric slide stage is not high, the slide stage needs to be kept somewhat parallel to the magnet applying pressure during installation; otherwise, the piezoelectric slide stage will not work over long distances.
  2. Planning to design two piezoelectric slide stage structures to adapt to different shapes of piezoelectric ceramics.
  3. Software completed D-I curve testing and bias testing functionality.

• May 02, 2023

  1. Completed scanning of HOPG, and observed the blurred outline of carbon atoms.
  2. Discovered an interesting phenomenon: during the detection of HOPG samples, the tunneling current will fluctuate as the probe approaches (caused by environmental vibrations). If the sharpness of the probe reaches atomic resolution at this time, the fluctuation curve of the tunneling current will be coupled into a sinusoidal-like curve, and this coupling will disappear after a collision with the sample (confirmed not to be interference from the power grid, with a fluctuation period of about 1.4ms and disappearing after a probe collision).

  I believe this is because after entering the tunneling distance, the probe's X/Y axis moves with the vibration caused by environmental vibrations, creating a "scanning" effect, and the undulations on the carbon atom surface lead to periodic variations in current.

• May 03, 2023

  Observed clear outlines of carbon atoms on HOPG.

• October 10, 2023

  The HardwareX manuscript for the third-generation hardware has been published on Arxiv: https://arxiv.org/abs/2310.05413.

Acknowledgement

Wuyi University and Teachers

JLC PCB and Easy EDA Team

Jürgen Müller

Daniel Berard's Project'

John D. Alexander's Project

Institude of Optics and Electronics, Chinese Academy of Sciences

All friends from Bilibili

Reference Project

[1] John Alexander: STM Project, http://web.archive.org/web/20121107205242/http://www.geocities.com/spm_stm/Project.html

[2] Dan Berard: Home-Built STM, https://dberard.com/home-built-stm/

[3] Jürgen Müller: Homebrew STM, http://www.e-basteln.de/other/stm/overview/

[4] NanoSurf: NaioSTM, https://www.nanosurf.com/en/products/naiostm-stm-for-nanoeducation

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