Q-Dots

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

A novel, electrochemically assisted method of obtaining semiconductor quantum dots supported on a surface has been introduced by Penner and his group [123], It comprised a hybrid electrochemical/chemical (E/C) process consisting of electrochemical deposition followed by chemical modification and it was described as a general, rapid, and low-cost solution-phase method for synthesizing supported Q-dots of metal salts. 

The primary goal of the researchers has been to produce Q-dots possessing all of the attributes of the Q-dots prepared using liquid-phase synthetic methods (that is adjustability of the nanocrystal identity and diameter and size monodispersity) and also the technological utility of Q-dots prepared by MBE (specifically, the deposition of nanocrystals with a defined orientation and an electrical output contact). It was shown that the E/C-synthesized 5-CuI and CdS Q-dots were indeed epitaxial with narrow size distribution and strong photoluminescence tunable by the particle size. Qne of the advantages of the E/C method is that it can be made size selective. The key point is that the size as well as the size dispersion of product nanoparticles are directed actually by the corresponding properties of the metal nanoparticles therefore the first deposition step assumes special importance. 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

Fig. 5.19 (a) Linking CdSe quantum dots to Ti02 particle with bifunctional surface modifier (b) light harvesting assembly composed of T102 film functionahzed with CdSe Q-dots on optically transparent electrode (OTE). [Adapted (in gray scale) from [348]]... 

Fig. 6.8 Q dependence of the two eigenvalues Ai(Q) solid line) and A2(Q) dotted line) predicted by a two-component dynamic RPA approach for the case of an hA-dB labelled diblock copolymer melt. Calculations were performed with/=0.5, Rg =Rg =40 A, Na=Ny=200, Ku=0, Ai(Q) describes the collective mode of the diblock copolymer chains. The Rouse rates were taken from PE and PEE at 473 K (see Table 6.2). (Reprinted with permission from [44]. Copyright 1999 American Institute of Physics)...

Modeled as eta-Catnot etA-tJiaiAal Taax T n -Faax P in aax K-dot Fonei in Povec out nec-po et back-vork-ratio woEk-tatio Q-dot in 0-dot out -net Q-dflt ... 

Poueii in Povec ou nen-pouet baok-voch-cetio vocK-tdCiio Q-dot in C-dot out net Q-dot... 

Tniio -Pmox Palo p ax-a doc Fouet Id p Foxki ouc uet-powet back voEk xatio = Hork-xatio Q-dot iji p... 

Vadeled as eta- CaEnot -eta-theEnal -l ax l in Paax -Fain > aaM-a-dot PoveE in -PoacE out net-povai m back Work-tat10 ttotk-tatio Q-dot in O-dot out net Q -dot ... 

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