STM
In this experiment you will learn how a Scanning Tunneling Microscope works and will take “pictures” of the surface of graphite and gold. You should be able to see atomic resolution in the case of graphite and determine the lattice constant directly.
A scanning tunneling microscope works by bringing the tip of a metallic wire very close to the surface of a conducting material. The wire is made “atomically sharp” by luck: when prepared properly it may have an atom or two “sticking out”. When this tip is brought close to a conducting surface, and a small positive voltage is applied, it will draw a small tunneling current even before it touches the surface. This current, which is an extremely sensitive function of the distance between tip and surface, can be measured and amplified and used to control the location of the tip above the surface. The device you will use is a commercial one from nanoSurf. The tip is scanned in all three directions using a piezoelectric motor which can move the surface in steps smaller than a atomic dimensions. In the normal operation of the microscope, the z-distance of the tip above the surface is adjusted to maintain a constant current value and x and y are scanned lateral to the surface. The z value plotted versus x and y thus gives a map of the topography of the surface.
This apparatus is VERY DELICATE. PLEASE DO NOT DESTROY IT. If in doubt , ask before you do something. The easiest way to destroy it is to break off the tip mount while mounting the wire tip. The next easiest way is to smash the sample holder into the piezoelectric motor. You could also drop the entire apparatus onto the floor, or put your fingers all over the surfaces you are measuring. Please try to avoid all of these. All manipulations of the sample holder (the small chrome cylinder) or the tip should be done with extreme care. There are high power magnifiers provided to help you see what you are doing while moving the sample. Use them. Please keep the tools clean when you prepare the tip. Never touch any clean surface with your fingers.
The operation of the STM is described in the easyScan manual. You should read pp 1-5 in this manual before you start. It is also a good idea to do a simulated scan as described on p. 12. You may then proceed to p. 13 and prepare a tip. This is the most sensitive part of the whole microscope. If you have any doubt that you can do this without damaging the apparatus, please ask for help from your instructor. PLEASE DO NOT DESTROY THE TIP MOUNT. Do not clean the sample unless your instructor suggests it. Of course, do not touch the sample surface or any other working part of the STM. Start with the graphite sample and follow the instructions on p. 17. Be very careful not to crash the tip into the surface. You can usually see the reflection of the tip in the surface of the graphite, using the built-in magnifier. Stop advancing the surface toward the tip well short of touching, and do the rest using the piezoelectric motor. Follow the instructions given on pp.19-28 of the manual.
You should try to do at least the following experiments:
1) Get a good scan of graphite. Try to get an atomic resolution scan , enough to get the lattice constant for graphite. See p. 33 for interpretation of the pictures. Be sure you record and print out your pictures using the snapshot. Export as a bitmap.
2) Look at the plot of I versus V for a couple of spots on the graphite scan. Use the spectroscopy panel.
3) Scan the gold sample and interpret your results.
Tips: Clean the tools and carriage and maybe the graphite
surface (with tape) before starting. Start the scan at “full”, but with a
scan range of 50 nm. Zoom one step at a time, always looking for dull parts
of the surface and decreasing the Z range. Display Z position, not current.