Yeah, some sort of photoluminescent plastic or some phosphorescent element mixed into the plastic. The trouble with GID stuff is (as far as I know) it has a short half-life and GID toys and such only retain their brightness after so much exposure to the direct light it needs to store the necessary energy for it to glow. So my guess is all these GID caps will slowly grow dimmer in time, but I'm no scientist. BC would know better on this.
I suspect CC made 'Murica and Senzubean with a radioluminescent element, allowing it to glow for much longer. These elements being radium, which has a half-life of 1600 years; tritium, which has a half-life of 12 years; and the man-made promethium which has a half-life of about 3 years. They are also slightly radioactive and expensive (which is reflected in the price point of 'Murica.)