Light sources based on quantum dot technology 4551 Definition and background physics

Quantum physics describes particles having wave functions. This particularly applies to electrons, as the smallest stable particles with a rest mass. Should the wave function of the electron be increased until it reaches the geometric material realm, quantum mechanical effects can be anticipated, leading to interesting new properties.

This holds true for nanoparticles below about 20 nm in size (typically 15 nm). They are at the borderline between individual molecule and dimensional crystal. The movement of the electrons is constrained by the minuteness of the nanoparticles. Since the electrical and optical properties of solid bodies are determined by their electrons, a discretization of energetic states and a broadened band gap between valence band (completely filled with electrons) and conduction band (not completely filled so electrons are mobile) can be observed with decreasing material size (Haase & Kompe 2003). The resulting new properties can only be explained with the help of quantum physics. Such nanoparticles are therefore called quantum dots and are more or less zero-dimensional.

The electrons feel "squished" by the restricted particle boundaries and thus the distance (band gap) between original state and excited state increases with the decreasing size of the particles. If an electron shifts from the excited state into the original state, light is emitted; its wavelength and energy level are dependent on the band gap. It is therefore possible to generate wave lengths ranging from UV through the visible spectrum on up to the infrared (350-2300 nm) using quantum dots of the same composition (Evident Technologies 2003). The optical properties are determined by the size of the quantum dots. This is what makes the quantum dot so interesting for lighting technology. Earlier it was only possible to generate light of different wavelengths by combining different substances. Quantum dots even permit using substances for light generation that were previously unsuitable (Bertram & Weller 2002). Additionally, more than 50% of the atoms lie at the surface due to the small size of the quantum dots, thus permitting a precise tuning of light-emitting properties and suggesting that the emission of several colors from a single dot may be possible (SNL 2003). Quantum dot crystals used in combination with other crystals or phosphors can emit any desired color.

Quantum dots can be produced technically by means of epitaxial growth, for example in vacuum precipitation processes or by chemical methods from colloidal solutions (Rubahn 2002).

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