A quantum dot including a core and a shell disposed on an outer surface of the core. The core includes a first semiconductor nanocrystal including a Group II-VI compound. The shell includes a second semiconductor nanocrystal. An effective mass of the second semiconductor nanocrystal is about 0.5 times to about 2.0 times an effective mass of the first semiconductor nanocrystal and the quantum dot does not include cadmium, lead, mercury, or a combination thereof.