From Wikipedia: "A Scanning Tunneling Microscope (STM) can be considered a type of electron microscope, but it is really a type of scanning probe microscope and is non-optical. The STM employs principles of quantum mechanics to determine the height of a surface. An atomically sharp probe (the tip) is moved over the surface of the material under study, and a voltage is applied between probe and the surface. Depending on the voltage electrons will tunnel or jump from the tip to the surface (or vice-versa depending on the polarity), resulting in a weak electric current. The size of this current is exponentially dependent on the distance between probe and the surface."
So, technically, you aren't bouncing electrons off. You are tunneling them to and from the surface. Which means the nucleus doesn't have to be heavy.
But still, the nucleus IS heavy.