The confusion here is understandable. The two materials you describe are roughly the same size. That is, a nanodot or nanosphere may be the same size as a quantum dot. Furthermore, there is a little 'fuzzyness' in the term nano as it is a bit of a buzzword that people co-opt to make their research sound more interesting. This being said, the label 'quantum' refers to materials where quantum effects are perceptible. Allow me to explain this more explicitly:
Like a baseball reacts to gravitational potential energy, charge carriers (like electrons and holes) react to electrical potential energy. Different materials can have different electrical potential energies, and so one way to make quantum dots is to combine materials with different electrical properties. For example, you can grow GaAs on AlGaAs (Gallium Arsenide on Aluminum Gallium Arsenide... examples of
compound semiconductors). These materials have different
band gap energies and electrons moving in the conduction band and holes moving in the valence band will 'feel' different potential energies when traveling in GaAs than AlGaAs. If you make the GaAs layer small enough the potential well becomes thinner and classical statistics no longer describe the behavior of the charge carriers accurately and one must use quantum mechanics. This typically occurs when a size dimension reaches a few nanometers. Thus a quantum well may be very wide and long, but only a few nanometers thick (1D of confinement), a quantum wire will be long but not thick or wide (2Ds of confinement), and at last a quantum dot will not be very thick, wide, or long (3Ds of confinement).
Bottom line: It's a nanodot when it has nanoscale dimensions in three directions. It's a quantum dot when it exhibits quantum properties. A quantum dot can also be a nanodot, a nanodot is not necessarily a quantum dot. See attached files for representations of a quantum well, quantum rods, and quantum dots.
