Etching Atomically Sharp Tips for ST Microscopy

Doing science at home is a challenge, because research instruments and lab equipment tend to be very expensive. They’re niche products marketed towards commercial labs, universities, and research hospitals that have huge budgets, so affordability is rarely a priority. However, you might get lucky and find used equipment at a price that is accessible to citizen scientists. But what about consumables? If scanning tunneling microscopy is your goal, then check out Baird Bankovic’s DIY etcher that is capable of producing atomically sharp tips.

A scanning tunneling microscope (STM) can image individual atoms, which is far beyond the capability of an optical microscope. STM works by moving a conducting probe across the surface of the material in a vacuum. By creating a voltage bias between the material and the probe, electrons tunnel through the vacuum from the material to the probe. Measuring the current provides data on the physical structure of the atoms in the material. That data can then be analyzed to create an image. A good STM can “see” features smaller than 0.1 nanometers and can detect depth with a resolution of 0.01 nanometers.

But all of that requires a probe with an extremely sharp tip that has a point on the same scale as the measured atoms. That tip also needs to be conductive. Machining such tips by traditional mechanical means is out of the question — such fabrication cannot produce a point fine enough. Instead, STM tips are etched by dipping a wire “blank” into an etchant and passing a current through. When controlled properly, this process causes the wire to dissolve until it snaps off, leaving an atomically sharp tip.

Bankovic build a machine to control this process. An Arduino Mega board handles all of the fabrication according to the input parameters. The user just needs to trim the tungsten wire to length and place the container of etchant. The Arduino, which mounts onto a custom PCB with a few keys and an LCD, controls the rotation of a stepper motor and monitors the current passing through the wire. The stepper mounts to a 3D-printed frame and drives a lead screw that lowers and raises the wire feeder, which rides on a linear rail.

To avoid blunting the tip, the wire must be removed from the etchant as soon as it snaps off. To lift the wire at exactly that moment, the Arduino looks at the electrical current and acts when it sees a dip. It uses a similar technique to locate the surface of the etchant so that it submerges the tip to the correct depth.

If you have an STM and need some tips, Bankovic uploaded all of the files and code for this DIY etcher to GitHub.

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