Mid-IR Colloidal Nanocrystals

Colloidal quantum dots presenting optoelectronic properties into the mid-infrared are reviewed with an emphasis on HgTe. Interband transitions with narrow band gap material and intraband transitions with wide band gap semiconductors can both address the infrared range of wavelengths. Semimetals are particularly promising since, by controlling the particle size, a gap can in principle be opened and tuned through the full infrared spectrum. HgTe has been quite successful recently, and colloidal synthesis has allowed the tuning of the absorption edge from 1 and 5 mu m at room temperature. The cutoff wavelength and absorption coefficient of these materials have been discussed. Electrochemistry experiments have demonstrated that both n and p nanoparticles are stable. Films of the HgTe colloidal quantum dots have shown room temperature photoconduction when excited in the mid-IR. With novel processing leading to improved detectivity, these materials are becoming of significant interest as an alternative to the much more expensive epitaxial detector technologies.