The amazing optical properties of semiconductor nanocrystals known as quantum dots have led to an explosion of research activity into their potential uses in medical nanotechnology, solar cells, quantum dot lasers, sensors and other technologies. This brief quantum dot tutorial cannot possibly describe all the exciting nanoparticle research currently underway but will cover a basic quantum dot definition, a little theory and the mechanism behind quantum dot fluorescence.
Quantum Dot Definition
Quantum dots are semiconductor nanocrystals that have bright, highly tunable fluorescence emission that depends sensitively on their size, which typically ranges from 1-15 nanometers (nm). The most widely researched types are cadmium selenide (CdSe), cadmium sulfide (CdS), zinc sulfide (ZnS) and core-shell structures such as CdSe coated with a thin layer of ZnS. Quantum dots represent an exciting and important area of nanotechnology research.
How Semiconductors Work
A semiconductor is a material that can be made to conduct electricity with the input of a small amount of energy. This is in contrast with conductors such as metals, which freely conduct electricity with no input of energy, and insulators such as diamond, which require a large input of energy to conduct electricity. In chemical terms, a semiconductor's energy levels are grouped into a valence band of tightly bound electrons and a conduction band of more loosely bound electrons. (Physicists use different terms to describe semiconductor electron structure). The conduction band can be thought of as a set of empty orbitals into which the electrons can be promoted with the input of a modest amount of energy. Once promoted into the conduction band, the electrons are available to conduct electricity. The gap in energy between the valence and conduction bands is known as the band gap. The smaller the band gap, the less energy is needed to make the material conduct electricity. When an electron returns from the higher-energy conduction band (excited state) back down to the lower-energy valence band (ground state), it may emit a photon of light with an energy that matches the energy of the band gap. This emission is known as fluorescence. The "color" of the fluorescence depends on the size of the band gap. A larger band gap may result in a "bluer" (i.e., higher-energy) light, whereas a smaller band gap would produce a "redder" (i.e., lower-energy) light. How Stuff Works has a great explanation of How Semiconductors Work.
Quantum Dot Theory
A quantum dot is a semiconducting nanocrystal, meaning that it can be made to conduct electricity fairly easily and it has one or more physical dimensions between 1-100 nanometers. When the electrons of a quantum dot return back to the ground state the resulting fluorescence is bright enough to be easily detected with the naked eye. What makes quantum dot fluorescence so interesting is three-fold: 1) the color emitted by the quantum dot depends on its size; 2) the fluorescence is not easily quenched or dampened by the surrounding medium, as is true for fluorescent dyes made from organic molecules; and 3) the quantum dots do not degrade easily, as do many fluorescent dyes. Thus quantum dots are a potential replacement in many technologies where fluorescence emission is used as an analytical tool.
The size-dependent fluorescence color results from a phenomenon known as "quantum confinement" where the size of the particle limits the space in which an electron wave can travel throughout the substance. As the semiconducting nanocrystal gets smaller, the spacing between energy levels increases resulting in a blue-shift to higher-energy quantum dot fluorescence very much analogous to the canonical "particle-in-a-box" model in quantum theory. (In chemistry terms, it has a larger band gap.) The size of commonly studied quantum dots can be tuned to emit throughout the visible region of the electromagnetic spectrum, allowing their emission to be traced optically. In the figure you can see solutions of quantum dots exposed to ultraviolet light, which stimulates emission. The size of the quantum dots increases from left to right across the image. I.e., the blue solution contains the smallest quantum dots and the red one contains the largest. Size-dependent color changes are an example of what happens to matter when it is made at the nanoscale. Thus, quantum dots are a quintessential nanotechnology!