Semiconductor nanoparticles luminescence

The photoluminescence of these nanoparticles has very different causes, depending on the type of nanomaterial semiconductor QDs luminescence by recombination of excitons, rare-earth doped nanoparticles photoluminescence by atom orbital (AO) transitions within the rare-earth ions acting as luminescent centers, and metallic nanoparticles emit light by various mechanisms. Consequently, the optical properties of luminescent nanoparticles can be very different, depending on the material they consist of. 

The disadvantages of organic dyes (low photostability, insufficient brightness, short lifetimes, etc.) have resulted in competition from luminescent metal-ligand complexes, semiconductor nanoparticles (Quantum Dots), and conjugated polymers. These new materials show advanced performance in a variety of applications... 

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

Even larger probes of bent and kinked DNA are 40 A photoluminescent mineral colloidal particles of CdS [247-253]. These nanoparticles are approximately the size of proteins and can be made in a variety of sizes ( 20-100 A) and decorated with a variety of surface groups [267-279]. The emission spectrum of a nanoparticle solution depends on particle size and surface group synthetic procedures for CdS and other semiconductor nanoparticles have been developed so that the emission can be tuned throughout the visible spectrum and into the near infrared [267-279]. Moreover, the photoluminescence of CdS is sensitive to adsorbates [280-289], and thus these nanomaterials can function as luminescent chemical sensors. 

Absorption of and Emission fiom Nanoparticles, 541 What Is a Surface Plasmon 541 The Optical Extinction of Nanoparticles, 542 The Simple Drude Model Describes Metal Nanoparticles, 545 Semiconductor Nanoparticles (Quantum Dots), 549 Discrete Dipole Approximation (DDA), 550 Luminescence from Noble Metal Nanostructures, 550 Nonradiative Relaxation Dynamics of the Surface Plasmon Oscillation, 554 Nanoparticles Rule From Forster Energy Transfer to the Plasmon Ruler Equation, 558... 

Semiconductor nanoparticles and QDs are widely used in various fields such as luminescent biolabels [150-152] and have been demonstrated as components in regenerative solar cells [153-155], optical gain devices [24] and electroluminescent devices [23, 156, 157]. DMS have applications in spin-based electronics technologies, or spintronics [158-161]. Spintronic devices such as magnetic-optic switches, magnetic sensors, spin valve transistors and spin LEDs can be activated by implanting ferromagnetic Mn, Ni, Co and Cr in semiconductors [162-165]. Some of the applications of semiconductor nanoparticles or QDs have been explained in this section. 

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

CdTe semiconductor nanocrystaline luminescent markers together with magnetite nanoparticles were firstly included into polymer microcapsules to control capsules remotely via magnetic field. Embedding magnetic nanoparticles in microcapsule shells gives an opportunity to control capsule transfer and shell permeability. For example, the authors of. used goldcobalt nanoparticles to "open" capsules remotely. 

The bead probes to be described in next two sections will require additional fluores-cence/luminescence emission detector(s). The number of these detectors depends on the number of different colors or wavelength regions of emission to be detected. A typical maximum that has currently been feasible with organic dye emitters has been about six. This may be augmented due to the observed narrow bandwidths (typically, 30-50 nm, and as low as 26 nm, reproducibly produced) of emission from semiconductor nanoparticles, which have been recently shown to allow 10 colors. ... 

FRET (fluorescence resonance energy transfer) between luminescent semiconductor nanoparticles and other fluorescent molecnles such as amino acid residues in proteins, etc. that are attached to the particles has also been observed. Examples include... 

The tail of the spectra observed could refer to trapped carriers which play the main role in luminescence spectra (Fig. 10). Considering the luminescence phenomenon, the major difference arises between metal and semiconductor particles. The transition metals have an extremely high density of electronic states above the HOMO. The creation of excited states is followed in this case by ultrafast efectronic to vibrational radiationless transition. There are no reports of long-lived excited states of luminescence [12]. Semiconductor nanoparticles tyiMcally have Si - S energy gap of several electron volts, as has been shown [12] neglecting the presence of trap surface states. Direct Si - Sq internal conversion is a very high order process in lattice phonons (htu 199-300 cm )... 

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