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Samples to use on the STM

A project log for The PVC Scanning Tunneling Microscope

An easy-to-build scanning tunneling microscope.

Tom EkkensTom Ekkens 01/23/2018 at 00:560 Comments

Because the STM requires a current through the sample, anything we want to measure must be conducting.  This can be accomplished by either using only metal samples or by coating a non-conducting sample with metal.  Since the coating process is expensive, we will restrict ourselves to conducting samples that are easy to obtain.

DVD+R.

As mentioned in the operation section, a small piece of a DVD+R disk is the best sample to start with.  It is easy to prepare, cheap, and images well.

Preparation:  Cut into a DVD-R disk with a pair of scissors. Try to remove the clear plastic layer from the DVD, leaving the top of the disk with the label and the foil layer on it. Cut a section out just smaller than a #10 washer. Place it, foil side up, on the washer. Attach it down using conductive tape or glue. Make a good electrical connection between the foil and the washer.

Imaging:  Two images of the DVD are shown below.  The top image is the DVD as it appears when imaged using a light microscope.  This microscope has a magnification of 800 times or about 1.2 million pixels per meter.  Since the image is 675 pixels high, the spacing of the lines is determined to be 731 nm.  This matches the expected value of 740 nm from the DVD specification.  The bottom image is an image taken by the STM with a side length of about 2500 nm.  The line spacing is again about 740 nm.  The quality of the STM tip makes a large difference in the image.  A sharper tip would make the troughs in between the ridges more visible.  Several other parameters also make the image appear different.  The scan speed for this image was about 500 mS per line.  If we move too fast, the image blurs but if we don't move fast enough, external noise is worse.  The bias for this image was 600 mV.  

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