Nanocar racing competition and the race tracks used
While I can't find specific justification for the surface, the first competition involved both gold and silver "racetracks."


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The competition involves propulsion and imaging using STM so the substrate must be conductive. Gold (and silver to a lesser degree) is particularly useful, since it won't easily oxidize and is easy to purify.

I've seen several comments about having 'tracks' in the gold surface, which suggests a well-defined crystal face with reconstruction, e.g. Au(100):



(Image from Wikipedia)