Optical and tunneling spectroscopy of semiconductor nanocrystal quantum dots

Semiconductor nanocrystals are novel materials lying between the molecular and solid state regime with the unique feature of properties controlled by size [1–5]. Containing hundreds to thousands of atoms, 20–200 Å in diameter, nanocrystals maintain a crystalline core with periodicity of the bulk semiconductor. However, as the wavefunctions of electrons and holes are confined by the physical nanometric dimensions of the nanocrystals, the electronic level structure and the resultant optical and electrical properties are greatly modified. On reducing the size of direct gap semiconductors into the nanocrystal regime, a characteristic blue shift of the band gap appears, and discrete level structure develops as a result of the “quantum size effect” in these quantum dots (QDs) [6]. In addition, because of their small size, the charging energy associated with addition or removal of a single electron is very high, leading to pronounced single electron tunneling effects [7–9]. Owing to the unique optical and electrical properties, nanocrystals may play a key role in the emerging new field of nanotechnology in applications ranging from lasers [10,11] and other optoelectronic devices [12,13], to biological fluorescence marking [14–16].