Nanomaterials such as semiconductor quantum dots (SQDs) have electronic and optical properties between those of much larger macro and micro scale bulk semiconductor crystals and much smaller atoms and molecules. SQDs have been produced with peak optical emission wavelengths ranging from the ultraviolet through the visible and into the infrared.
Assembly
SQDs are currently assembled or synthesized using either “dry” or “wet” methods. Dry techniques are the most familiar to the traditional semiconductor micro fabrication community, while wet techniques are typically the domain of synthetic chemists. Though submicron nanostructures can also be formed through somewhat traditional paths using a top down approach involving high resolution ultraviolet or electron beam lithography and dry etching such as reactive ion etching, these techniques cannot be used to make true quantum dots of Bohr exciton scale.
Dry method
True dry Bohr-exciton scale quantum dots are formed using a bottom up approach, with the use of epitaxial techniques to produce SQDs from atomic constituents and/or molecular precursors. Such SQDs are assembled using strained layer growth in ultra-high vacuum, crystal growth systems, such as molecular beam epitaxy or metal organic chemical vapor deposition. SQD growth is a consequence of the lattice mismatch between the matrix or substrate material and the dot or epi-layer material.
Wet method
Recent work is in the area of bottom up synthesis of wet colloidal SQDs, grown in solution in a chemical reaction vessel. Wet SQDs form a major emerging component of the nanomaterial marketplace and SQDs are commercially available. Because of SQD particle size the peak luminescence emission wavelength can be controlled with great precision in wet colloidal synthesis. The surface chemistry of SQDs can be modified to have compatibility with a variety of solvents including water and so the first major commercial application of SQD is in luminescent tags for biomedical applications. Wet SQDs can be also assembled into quantum dot solids through various deposition techniques such as drop-casting, spin-casting, and Langmuir-Blodgett methods. Also, SQDs can in principle be epitaxially overgrown to form device hetero structures with novel nano structured active layers.