Part of my thesis work involved developing a room temperature prototype of a scanning tunneling microscope(STM).
Please refer to the full text of my thesis for a more elaborate write-up.
The STM was used to study the structure of Highly ordered pyrolytic graphite(HOPG) at room temperature, and monolayer steps were imaged. The thickness of each layer was estimated to be around 100 picometer. IV spectroscopy was also conducted on the surface of HOPG .
The final assembly.
Before scanning, the setup is covered and placed on a vibration isolation rig.
The mechanical assembly was machined at workshops in a nearby industrial area.
A tip made from tungsten wire is shown facing an HOPG sample.
The first scan obtained using this setup. A calibration grating from Asylum Research was imaged. This grating has a pitch of 10um and depth of 200nm. The entire surface is coated with platinum.
CAD render of the Grating. Extracted from the datasheet provided by Asylum Research
High resolution scan of one of the pits on the calibration grating
Flow diagram of the STM
Large area scan of an HOPG sample showing the layered structure
In Summary :
- Coarse Approach : Attocube nanopositioner with ANC300 controller.
- Scanning : Axially segmented piezo tube from Physik Instrumente
- Control Electronics : R9 from RHK. Software written in IHDL
- Vibration Isolation : Heavy platform suspended inside an empty instrument rack via bungee cords
- Tip Holder : Hypodermic needle.
- Matching piece (for Attocube Walker and Piezo tube) : Eventually settled on Delrin since Macor isn’t easy to procure, and the local machinists aren’t familiar with it.
- Guide rails : machined from SS rods.
- Electrical shielding : SS sheet metal construction.
Towards Developing a low temperature probe
With a liquid helium cryostat expected to arrive soon , I proceeded to design a probe compatible with its dimensions, and capable of housing the scanning probe.
The drawing was made using SolidWorks, and the scanning probe included an additional linear positioner that would allow moving to a cleaner region without disturbing the cryostat, vacuum etc.
The Cryostat was purchased from American magnetics, and features a 3-axis vector magnet. The vertical field can be set to a maximum of 6 Tesla, while vector fields are limited to 1 Tesla.
In order to ensure uninterrupted Liquid Helium supply , a helium plant with two 160 liter portable liquefiers was set up in an adjacent building. Helium transfer needs to carried out every 3-4 days, and is usually managed by the undergrads. I need to write a page dedicated to this cryostat, its vibration isolation pit, and RF shielding soon.