more quantum physics on a budget.

In a recent article I wrote about a cloud chamber project built from a tupperware container and a couple Peltier devices.  Continuing in this vein of investigating materials at the particle level I ran across an open source scanning tunneling microscope (STM) from a research group at WWU Münster.

Their WWW page has a one page description of how these devices work.  To recap, they are based on electron tunneling, whereby electrons disobey the classical rules of physics and are able to trespass boundaries that they shouldn’t be able to.  Apparently at the quantum level electrons behave like my kids.  Grounding, either for the electrons or the kids, doesn’t seem to help.

The STM is equipped with a sharp electrical probe that is placed in very close proximity to the substance that is being examined.  The voltage ranging from a few mV to a few V is applied to the tip.  In quantum physics particles don’t have positions, they have probability distributions.  In the STM this means that there is a slight chance that an electron could be found outside its conductor.  The sample being studied also has a probability distribution for places that it can accept an electron.  When the tip is close enough to the sample that these probability fields overlap then a current will flow despite the fact that there is no continuous conductor.

An STM scans by moving the probe over the sample in fractions of a nanometer.  As the needle scans in the X and Y axis it is also raised and lowered in the Z axis in order to maintain a constant tunneling current.  The technique can easily discern subatomic features due to its sensitivity:  If the tip scans over a single step of gold atoms in a crystal the tunneling current changes by 1000, for a Z axis change of .3nm.

The fine scanning and Z axis motion is provided by piezo-electric crystals, which change shape when a voltage is applied.  The STM group stacked several crystals together in order to get a range of motion of about 500 nm with a few tens of volts.  Gross changes in Z axis are provided by micrometer screws.

They offer a couple suggestions for making the probe, which needs a very sharp tip.  One is to simply cut a platinum-iridium wire at a sharp angle with some diagonal cutters (albeit, you have to use expensive Swiss made cutters).  Another is to take a tungsten wire from a light bulb and etch it in a solution of sodium hydroxide.

Schematics for the power supply, bias circuit, control box, and preamplifier are all given in Eagle schematic form along with a board layout (2 layer board, so you can use the free version of Eagle).

You’re too late by a couple decades to win a Nobel Prize just for building an STM, but just think about your kid entering one in his school science fair.

Steve Ravet

EDN Gadgeteer