The track is a gold sample consisting of a perfect single crystal. But you have to clean the race track to be sure that there are only the nanocars and no other molecules from previous measurements.
Nanocar Race Part 6: The Environment
The STM is constructed in a UVH chamber. UHV stands for Ultra High Vacuum. The UHV chamber is pumped by primary pumps that are connected to a turbo pump. Additionaly there are ion getter pumps and Titanium sublimation pumps.
The ultra high vacuum is defined by the pressure of less than 10-7 mb. Here we have 10-11 mb in the chamber of the microscope. This is 1014 less than in our atmosphere. With such a low pressure there is almost nothing left in the vacuum chamber which can be adsorbed on the sample surface and which could disturb our nanocar.
At room temperature, which means around 20°C, the molecules would do vibrations and eventually move around on the surface. It would be impossible to measure a long time on a single molecule and to control its movement for the nanocar race. That’s why we cool down the microscope to 5 K.
0 K is the absolute temperature minimum. Nothing can be colder than 0 K. 300 K corresponds to 27°C. And our microscope is at 5 K = -268°C which means the temperature of the sample is close to the absolute temperature minimum. This ensures that all the molecules on the substrate freeze and don’t move any more. Except we want them to move for the race.
On top of the microscope there are two cryostats. An outter and an inner cryostat. A cryostat is like a very well isolated thermos bottle where you put something cold inside and it stays cold for a long time. The outter cryostat is filled with liquid Nitrogen, the inner one with liquid Helium. The temperature of 4.7 K are determined by the boiling point of liquid Helium. Liquid Nitrogen has a boiling point at 77 K.
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