Semiconductor quantum dots luminescence

Sapsford, K.E., Pons, T., Medintz, I.L., and Mattoussi, H. (2006) Biosensing with luminescent semiconductor quantum dots. Sensors 6, 925-953. 

Abstract Silver clusters, composed of only a few silver atoms, have remarkable optical properties based on electronic transitions between quantized energy levels. They have large absorption coefficients and fluorescence quantum yields, in common with conventional fluorescent markers. But importantly, silver clusters have an attractive set of features, including subnanometer size, nontoxicity and photostability, which makes them competitive as fluorescent markers compared with organic dye molecules and semiconductor quantum dots. In this chapter, we review the synthesis and properties of fluorescent silver clusters, and their application as bio-labels and molecular sensors. Silver clusters may have a bright future as luminescent probes for labeling and sensing applications. 

Yoffe, A. D. Semiconductor quantum dots and related systems Electronic, optical, luminescence and related properties of low dimensional systems. Adv. Phys. 50, 1-208 (2001). 

More sophisticated designs involved semiconductor quantum dots with fluorescent protein receptors immobilized on the surface [146], The binding site of the protein receptor is occupied with an efficient fluorophore. On excitation a series of FRET (Forster resonant energy transfer) processes takes place excitation energy is transferred from the core of the quantum dot to the fluorescent protein and subsequently to the fluorophore. On substrate binding only one FRET step takes place and luminescence of the receptor is observed [146], In the simplest sensor architecture the protein contains bound quencher. Upon interaction with analyte the quencher is liberated and luminescence of the quantum dot is observed (Figure 16.25c). 

It was previously demonstrated theoretically [1] and experimentally [2] that semiconductor quantum dots (QDs) show strong dependence of optical properties on an electric field. Chemically synthesized semiconductor nanorods also exhibit the electric field effects. For example, quantum-confined Stark effect and luminescence quenching of single nanorods were previously demonstrated [3-5]. Unlike QDs, the nanorods exhibit quantum confinement only in two dimensions. It is reasonable to assume that the electric field applied along a nanorod may result in the strong polarization dependence of photoluminescence (PL). In the present paper, we investigate the influence of an external electric field onto luminescent properties of chemically synthesized CdSe/ZnS nanorods. 

Optoelectronic nanodevices that rely on electric field effects in optical absorption and emission provide the ability to be controlled conveniendy using integrated electronic platforms. Semiconductor quantum dots are theoretically expected as an excellent candidate for such optoelectronic nanomaterials to show optical properties strongly dependent on electric field [1]. In the general class of quantum dots, chemically synthesized semiconductor nanocrystals also exhibit electric field effects, for example, as demonstrated in their optical absorption (e.g. the quantum confined Stark effect [2,3]) and in their optical emission as the Stark shift and luminescence quenching [4,5]). 

Figure 6.2 Modification of semiconductor quantum dots (QDs) with functional encapsulating layersforwatersolubilization and preservation of luminescence properties and/or secondary covalent modification of the surface with biomolecules. Path A Exchange of the organic encapsulating layer with a water-soluble layer (a-d) thiolated or dithiolated functional...

Nanocrystalline semiconductor quantum dots have attracted attention primarily as sensitisers in solar cells, but have also been used as luminescent probes in biological systems, despite concerns over their toxicity (especially with quantum dots containing heavy metals such as Cd and Pb). The recombination of excitons post-excitation can lead to radiative emission with wavelengths shorter than that of the band gap of the bulk semiconductor due to perturbation of the exciton wave-function - so-called quantum confinement effects - at distances shorter than the exciton Bohr radius. For the popular semiconductor cadmium... 

In addition to properties associated with optical absorption, semiconductor nanocrystals exhibit interesting luminescent behavior. The luminescence is generally dependent on the size of the nanocrystal and the surface structure [83]. A photograph showing changes in the emission wavelength as a function of size of semiconductor quantum dots is shown in Fig. 1.18. The... 

Wannier-Frenkel hybrid exciton in organic-semiconductor quantum dot heterostructures. Journal of Luminescence, 125,196-200. 

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