Other Projects – Homebrew STM

Applications

This page is still under construction. Since my STM is not producing images yet, I cannot include any images or other ‘application’ results of my own. I have included suggestions for preparing STM tips here, most of which were generously provided by readers of these web pages.

Tip preparation

Mechanical Sharpening

PtIr cutting (adapted from 1998 NanoSurf instructions)

This is what I tried so far, following Nanosurf’s suggestions for users of their EasyScan STM. The procedure seemed straightforward, but I can’t comment on the quality of the tips so far (since I can’t judge the image resolution I obtain yet).

I used PtIr wire with 0.25 mm diameter.
Required tools: A wire cutter (edge cutter), and tweezers or flat nose pliers to hold the wire.
Clean tools with ethanol.
Cut a piece of 5 to 8 mm length of PtIr wire.
Holding this piece with the tweezers or pliers, grab the wire end with the wire cutter at a pointed angle. The angle between the cutter’s blade and the wire should be approx. 20�.
Cut off the wire end, while simultaneously pulling the wire lightly with the tweezers. The idea is to ‘tear’, rather than cut, the final part of the wire, to obtain a sharply pointed tip.
Immediately install the tip in the STM, continuing to hold it with the tweezers, only. (Don’t touch the tip end in the process…)

PtIr grinding and cutting (courtesy of Michael Vogelgesang)

A clear case of alchemy – every lab has its own method. One easy approach is to grind PtIr wire with 1000 grain abrasive paper (no problem with a bit of practice). An alternative is to clip off PtIr wire with a wire cutter (edge cutter), while pulling at the wire at the same time. Such tips have disadvantages for spectroscopy use, but at room temperature and ambient air, that is excluded anyway.

Field Emission

Field emission cleaning in the STM (courtesy of Michael Vogelgesang)

After retracting the tip just out of the tunneling region, one can apply a high DC voltage (200 - 600V) between tip and sample, to obtain a field emission that cleans the tip. Attention, these voltages can be deadly!! Current should not exceed 50µA; a current limiting series resistor of 10 MOhm or more is absolutely required. When performing this procedure in air, there might be problems with arcing; no warranties!

Electrochemical Etching

Tungsten tips etched in NaOH (courtesy of Janik Zikovski)

Fill a small container with a 2-mol solution of NaOH
Suspend a bit of W wire above the solution, with a system to slowly lower it while holding it in place.
Apply 30 Volts to the wire (I might have been -30 V, I’m not sure).
The tungsten electrochemically etches away. The place where etching is fastest is right at the meniscus - so that is where the tip will be formed. Eventually, the W at the meniscus is etched away completely and the rest just falls to the bottom.
Your 30V source should have some sort of current sensor and a threshold current below which it cuts off automatically - that is, at the exact moment the bit of wire falls off, you have a nearly atomic point. If you continue etching, you will dull the tip. So the circuit shuts off immediately when the current is very low (4 mA or so if I recall correctly).
Take the wire out soon, rinse it and check under a microscope.

You usually get 5-10 bad tips before you get a good one.

Tungsten tips etched in NaOH (courtesy of J. F. Gomez)

I use a tungsten wire, 0.25 mm diameter, and etch it electrochemically in a NaOH/2M solution film that is formed by surface tension on a Au (or Pt) ring of ~1cm diameter.
I use an AC voltage source between the wire (future tip) and the Au ring, around 6-8 VAC.
Put the wire perpendicular to the center of the circular film and control the desired length.
After some 5-10 min the tip is etched, and ready to use.
Renew the NaOH when it collapses.
Hint: If you use a length of wire 2x the desired one and etch it at its middle, you will have 2 tips at once !!!…

For PtIr tips the process is analogous, but the solution is something like NaCN (I’m not sure).