Samples to use on the STM
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.
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. On the left 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. On the right 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.
Aluminum Foil - Shiny side.
The previous sample - a piece of DVD - is easy to image but only has good repeated features at the 740 nm level. Another sample is needed for additional size scales.
Preparation: A cheap option for the second sample is a piece of aluminum foil. It is mounted to a washer so that the magnet in the stage will hold it down. In the picture, I have made two additional electrical connections using silver paint but this really wasn't necessary. I have mounted it so the shiny side is up.
Imaging: Two images of the aluminum foil are shown below. On the left is the foil as it appears when imaged using a light microscope. This microscope has a magnification of 800 times so the field of view (top to bottom) is about 230 μm. Ridges from the manufacturing process are all in the same direction, but the ridge separation is not consistent. Some of the ridges are vary wide but some smaller ones are in the range of 10 to 30 μm. On the right is an image taken by a scanning electron micrscope with a field of view (top to bottom of about 3600 nm. This scan size matches the largest field of view that the STM has so the picture should be similar. At this magnification, there are smaller lines and bumps in the range of 100 to 300 nm.
Two pictures taken from the STM are shown below. Both pictures are taken at approximately the same location with the same field of view (2500 nm) and the same scan speed (400 mS per line). The image on the right was taken with a bias of 600 mV and is completely worthless. The image on the left was taken with a bias of 3000 mV and has excellent detail. A major ridge can been seen with additional small ridges and bumps in the range of 100 to 300 nm. Other bias settings of -600 mV and 2000 mV were tried but were not as good as the 3000 mV image.
Preparation: A cheap option for the third sample is a new US penny. A new penny is usually still shiny so it only needs to be cleaned with methanol to get the finger oil off it. It can be mounted to a washer but the simplest method is to just placed it on the STM sample holder and let gravity hold it down.
Imaging: Two images of the penny are shown below. In all cases, the flat area just above the date is where the images were taken. On the left is the penny as it appears when imaged using a light microscope. This microscope has a magnification of 800 times so the field of view (top to bottom) is about 210 μm. Ridges from the manufacturing process are all in the same direction, but the ridge separation is not consistent. At this size, the penny is quite similar to the aluminum foil sample. On the right is an image taken by an atomic force microscope with a field of view (top to bottom) of 15 μm. At this scan size, ridges and bumps are both visible. The ridges are spaced at about 4000 nm and the bumps are 700 nm and smaller.
Two pictures taken from the STM are shown below. Both pictures are taken at approximately the same location, with the same bias of +1 V, and with the same the same scan speed (300 mS per line). The image on the left has a scan size of 4200 nm and the image on the right has a scan size of 600 nm. Both images have their share of randomly-sized ridges but the smaller scan size shows a number of smaller bumps – some as small as 20 nm in diameter.