Passivation surface states

A) minority carrier transfer catalysis and/or surface state passivation (B) electrostatic modification (C) catalysis of multi-electron photoprocesses (refer to text). 

Photoactivation of the as-prepared samples of QDs embedded in the above polymers increased their quantum yields, which was attributed to photoinduced surface state passivation, in addition to system relaxation. The resulting polymer-encapsulated QDs systems show an extended photochemical stabihty under continuous illumination in air without any observed blue shift, in contrast to the very short lifetime of the original QD solutions. While such fabricated QD-polymer composites in the initial work appear less sensitive toward hydrocarbon vapor exposures, many optimizations -for example, for the composite guest/host selection and the layer thickness -may be pursued to allow for the making of functional QD/polymer sensing materials based on the developed method. In addition, the encapsulation of QDs with selected monomers via in situ polymerization to make QD-based precursors is a useful technique with which to enhance QD hfe for, potentially, many other applications. 

A minority carrier transfer catalysis and or surface state passivation ... 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

The excellence of a properly formed Si02—Si interface and the difficulty of passivating other semiconductor surfaces has been one of the most important factors in the development of the worldwide market for siUcon-based semiconductors. MOSFETs are typically produced on (100) siUcon surfaces. Fewer surface states appear at this Si—Si02 interface, which has the fewest broken bonds. A widely used model for the thermal oxidation of sihcon has been developed (31). Nevertheless, despite many years of extensive research, the Si—Si02 interface is not yet fully understood. 

Myung, N., Bae, Y. and Bard, A. J. (2003) Enhancement of the photoluminescence of CdSe nanocrystals dispersed in CHCI3 by oxygen passivation of surface states. Nano Lett., 3, 747—749. 

First, I shall describe the hydrogenation method I used and then consider the passivation of surface states and that of bulk dangling bonds, including grain boundaries, dislocations and point defects. 

In silicon devices, which are operated at temperatures below 200°C, hydrogen annealing at > 450°C is used to passivate surface states at the oxide-semiconductor interface. Hydrogen forms electrically inactive complexes with the surface states. 

CD ZnSe has also been demonstrated to passivate surface states, 0.92 eV below the conduction band edge (measured by thermally stimulated exoelectron emission) on single crystal GaAs. This passivation resulted in bandgap luminescence from the originally non-luminescent GaAs [49a]. 

On most corroding metals, the above reactions occur at an oxidized surface and, depending on the peroperties of the surface layer, passivation may occur by which the kinetics of metal dissolution are substantially supressed either by ohmic, ionic, or electronic transport at a surface passivating film or by electrocatalytic hindrance. In passivation phenomena, a steady state with a balance between the formation and dissolution of the surface film takes place. As a result, the ionic flux of metal ions dissolving through the passivating film is highly reduced. 

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