Excitons nanostructured material

Measurements of the optical properties in this range of wavelengths can probe the fundamental electronic transitions in these nanostructures. Some of the aforementioned effects have in fact been experimentally revealed in this series of experiments (90). As mentioned above, the IF nanoparticles in this study were prepared by a careful sulfidization of oxide nanoparticles. Briefly, the reaction starts on the surface of the oxide nanoparticle and proceeds inward, and hence the number of closed (fullerene-like) sulfide layers can be controlled quite accurately during the reaction. Also, the deeper the sulfide layer in the nanoparticle, the smaller is its radius and the larger is the strain in the nanostructure. Once available in sufficient quantities, the absorption spectra of thin films of the fullerene-like particles and nanotubes were measured at various temperatures (4-300 K). The excitonic nature of the absorption of the nanoparticles was established, which is a manifestation of the semiconducting nature of the material. Furthermore, a clear red shift in the ex-citon energy, which increased with the number of sulfide layers of the nanoparticles, was also observed (see Fig. 21). The temperature dependence of the exciton... 

Optically excited semiconductor nanostructures show effects of QC if at least one spatial dimension of the material becomes comparable to, or smaller than the characteristic length scale (the classical Bohr radius) of an e-h pair. Different regimes of QC have been defined which depend on the semiconductor nanocrystallite size R relative to that of the Bohr radius of the exciton an, the electron ae or the hole ah ... 

Fig. 2 illustrates (he room-temperature photoluminescence (PL) spectra recorded from the as-prepared ZnO colloidal solution and the ZnO nanostructure formed after deposition of the colloid on the silicon substrate. An UV band at 385 nm was detected from all ZnO products. In addition, a broad orange-red photoluminescence band centered at around 620 nm could be also observed in some materials. The UV photoluminescence peak at 385 nm is well known to be related to the exciton emission, ihe mechanism of visible emission is suggested mainly due to the present of various point defects, either extrinsic or intrinsic, which can easily form recombination centers. Photoluminescence measurements show that the deposited ZnO nanostructures have the stronger UV emission than the ZnO nanoparticles in the colloidal solutions. The better UV emission characteristic of deposited ZnO is suggested to be due to the lower defect density and oxygen vacancies in ZnO nanocrystals in the first case. Similar results have also been reported previously [8]. In addition, the aqueous surrounding can change the surface states of ZnO nanocrystals. It is well known that surface states may... 

A quantum dot is made from a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. A quantum dot contains a small finite number (of the order of 1 to 100) of conduction band electrons, valence band holes, or excitons, that is, a finite number of elementary electric charges (Scheme 16.2). The reason for the confinement is either the presence of an interface between different semiconductor materials (e.g. in coie-sheU nanocrystal systems) or the existence of the semiconductor surface (e.g. semiconductor nanocrystal). Therefore, one quantum dot or numerous quantum dots of exactly the same size and shape have a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but they always extend over many periods of the crystal lattice (5). 

Nanostructured semiconducting block copolymers containing triphenylamine as hole transport moiety and perylene bisimide as dye and electron transport, have been investigated in view of applications in photovoltaic devices. The polymers show nanowire like structure which formation is driven by the crystallization of perylene bisimides via n- n stacking and since this self-assembly gives rise to domains size comparable to the exciton diffusion length, these materials offer perspectives for the implementation of organic solar cells [357]. 

Nowadays, more than fifty years later, a continuously improved control on matter at small length-scales has been made possible by the advancement of lithographic, etching, assembly and surface functionalization technologies. The new properties observed in matter at the nanoscale include many quantum effects due to the confinement of electrons or of excitons within nanostructures, together with a great variety of phenomena related to the enormously increased surface-to-volume ratios of nanostructures compared to bulk materials. The control of scientists over atoms and molecules extends... 

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