Semiconductor Nanostructures
Semiconducting nanocrystals, otherwise known as Quantum Dots (QD’s), form a specific class of gas sensing materials. As a rule, QD’s consist of elements of II-VI groups. Present chapter gives information about QDs and gas sensors, usually optical ones, de
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Semiconductor Nanostructures
5.1 5.1.1
Quantum Dots General Consideration
Semiconducting nanocrystals, otherwise known as quantum dots (QDs), were first discovered in the early 1980s (Ekimov et al. 1985). Since then, interest in QDs as alternatives to traditional organic dyes has increased dramatically (Costa-Fernandez 2006; Jorge et al. 2007; Callan et al. 2007; Smith and Nie 2010). Typically, QDs are colloidal nanocrystalline particles, roughly spherical, with particle diameters typically ranging from 1 to 12 nm. At such small sizes (close to or smaller than the dimensions of the exciton Bohr radius within the corresponding bulk material), these nanostructured materials behave differently from bulk solids because of quantum-confinement effects (Alivisatos 1996). As a rule, QDs consist of elements of Groups II and VI (Gaponik et al. 2002; Smith and Nie 2010). Much of the research in quantum dots has been concentrated on cadmium selenide quantum dots due to their intense light emission properties. However, other II–VI compounds, such as CdS, CdTe, PbS, PbSe, or ZnS, and III–V compounds, such as InP, InAs, or InGaAs, are used as well. A major drawback of the cadmium selenide dots is their toxicity. Zinc sulfide quantum dots could be much less toxic than cadmium selenide. However, ZnS does not possess the level of light emission intensity of cadmium selenide. Recent research has shown that this shortcoming can be overcome partially by doping the zinc sulfide quantum dots with an appropriate metal ion such as manganese and lanthanides (Mohanta et al. 2003). At present there are a variety of methods which can be used for synthesis of nanomaterials with specific size and shape, including QDs. However, Guo and Wang (2011) believe that diverse wetchemical and electrochemical approaches have advantages due to their simplicity and rapidity during preparing high-quality nanomaterials with required morphologies. A general scheme shown in Fig. 5.1 demonstrates how to use simple wet-chemical and electrochemical methods to make nanomaterials. It was established that quantum dots, in contrast to organic dyes, have broad absorption spectra, higher quantum yields, better chemical and photoluminescence stability, reduced photobleaching, and narrow emission spectra without red tailing (Jaiswal and Simon 2004). Moreover, QDs have the sizedependent nature of the emission wavelength (see Fig. 5.2), which is related to the three-dimensional quantum confinement of their charge carriers. The smaller the dot, the greater the blue shift observed relative to the typical Eg of the bulk semiconductor. This means that by controlling the growth of the nanocrystal, the emission wavelength can be tailored. In fact, QDs can be made to emit luminescence from the ultraviolet to the near-infrared spectral region. In particular, depending on the particles size, the emission of CdSe quantum dots can be continuously tuned from 465 to 640 nm, corresponding to a size ranging from 1.9 to 6.7 nm (diameter), respectively. For CdTe the emission is observed i
