The study of quantum dots began in the 1980s. Quantum dots are very small amounts of semiconductor material (nanoparticles) whose size affects the allowed energy levels of the material. The electrons of the material usually reside in the lowest band of energy levels called the valence band. When the electron absorbs energy it is excited to a higher band of energy levels, levels called the conduction band, leaving behind an empty spot known as a hole. When the electron returns to the lower valence energy level it emits energy. How far apart the valence band and conduction band are depends on the size of the particle. The size of the particle controls what is known as the confinement energy, Figure 1. This means that the size of the particle can be used to control the different types of light the particles absorb and emit. Quantum dots have been created that absorb ultraviolet light and emit all of the colors of the rainbow depending on their size, rather than just what it is made of.