As mentioned in Scanning Tunneling Microscopy, the scanning tunneling microscope (STM) can have a wide variety of working conditions. The tip is typically tungsten, but platinum-iridium alloy is used if the surface (or adsorbates) are too reactive. Some STMs can operate at room temperature and atmospheric pressure, but under those conditions most surfaces will get dirty very quickly. (That is, atoms and molecules will stick to the surface and get in the way.) So, for most work, STMs are operated at ultra-high vaccuum (about 10^-10 torr) and cryogenic temperatures (less than 100 kelvins).
General sequence for operating an STM:
- Seal STM assembly and chamber, containing sample. The chamber is usually made of thick, rolled stainless steel to prevent air from leaking through the chamber walls.
- Pump out the chamber until about 10^-8 torr is reached. A variety of vacuum pumps are used, including the ion pump and titanium pump.
- At 10^-8 torr, further pumping is futile, since there is usually a thin layer of water continually degassing from the walls of the chamber. So, the entire assembly is heated to about 150 Celsius, vaporizing the water and allowing it to be pumped out. After this, a vaccuum of about 10^-10 torr is achieved.
- Clean the sample. We want a surface that is pretty much atomically flat and clean. This is usually done with sputter and anneal cycles. High-energy argon or neon atoms bombard the surface, removing the first few layers of atoms. Heating the sample causes the surface to smooth from thermal diffusion.
- Measure surface with STM.
The surface can be probed in either constant-height or constant-current mode. In constant-height mode, the tip is held at one height and the current is measured as it passes over the surface. This is fast, but the tip could crash into an unexpected rise in the surface. In constant-current mode, the tip's height is controlled by a feedback signal so that the tunneling current is constant. This is slower, but you avoid tip-crashing. You can also do things like use the reactions between the tip atoms and surface (substrate or adsorbate) to move things on the surface. If there aren't enough things on the surface, you can drive a tungsten tip into the surface, digging up a lot of substrate atoms. It's quite popular to make things quantum corrals, logos, or quantum guns and to manipulate individual chemical reactons under situations where they normally would not occur.