6 Quantum Dots as a Light Indicator for Emitting Diodes and Biological Coding Irati Ugarte 1 , Ivan Castelló 2 , Emilio Palomares 2 and Roberto Pacios 1 1 Ikerlan S. Coop 2 ICIQ Institut Català d'Investigació Química Spain 1. Introduction Quantum dots (QDs) are inorganic semiconductor particles that exhibit size and shape dependent optical and electronic properties (Alivisatos, 1996; Smith & Nie, 2010). Due to the typical dimension in the range of 1-100 nm, the surface-to-volume ratios of the materials become large and their electronic states become discrete. Moreover, due to the fact that the size of the semiconductor nanocrystal is smaller than the size of the exciton, charge carriers become spatially confined, which raises their energy (quantum confinement). Thus, the size and shape-dependent optoelectronic properties are attributed to the quantum confinement effect. Because of this effect, light emission from these particles can be tuned, throughout the ultraviolet, visible and near infrared spectral ranges. Quantum dots possess many advantages that make them interesting for several applications: - They show symmetrical and narrow emission spectra and broad absorption spectra, enabling that a single light source can be used to excite multicolour quantum dots simultaneously without signal overlap. - They have a brighter emission and a higher signal to noise ratio compared with organic dyes. - Their stability is due to its inorganic composition which reduces the effect of photobleaching compared to organic dyes. - The lifetime of the excited states that give rise to fluorescence in quantum dots is about 10 to 40ns, which is longer than the few nanoseconds observed for organic dyes. - The large Stokes shift (difference between peak absorption and peak emission wavelengths) reduces autofluorescence, which increases sensitivity to detect particular wavelengths. - QDs have high quantum yields in the visible range (0.65-0.85 for CdSe) as well as for the NIR (0.3-0.7 for PbS), while organic dyes are moderate in the NIR (0.05-0.25). Figure 1 shows an example of different size CdSe based nanoparticles narrow emission and broad absorption spectra. Both were excited at the same wavelength of 390 nm to record the emission.