Two methods, coupling self-assembled monolayer and chemical bath deposition ͑CBD͒, were utilized to assemble cadmium sulfide ͑CdS͒ quantum dots ͑QDs͒ onto mesoporous TiO 2 films for dye-sensitized solar cell ͑DSSC͒ applications. Colloidal CdS QDs were first self-assembled on the TiO 2 surface. CBD was then introduced to replenish the incorporated amount and increase the coverage ratio of CdS QDs on the TiO 2 surface. The preassembled CdS QDs act as nucleation sites in the CBD process, forming a CdS nanofilm with an interfacial structure capable of inhibiting the recombination of injected electrons. An efficiency as high as 1.35% for the QD-sensitized DSSC was achieved using the present strategy.

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Colloidal cadmium sulfide (CdS) quantum dots (QDs) were prepared and surface modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-CdS). The MSA-CdS QDs were then assembled onto bare TiO(2) mesoporous films using the carboxylic groups/TiO(2) interaction. The TiO(2) film was also surface modified by 3-mercaptopropyl trimethoxysilane (MPTMS) or 3-aminopropyl-methyl diethoxysilane (APMDS) to prepare, respectively, a thiol (-SH) or amino (-NH(2)) terminated surface for binding with the CdS QDs. The experimental results showed that the MPTMS-modified film has the highest adsorption rate and adsorption amount to the CdS QDs, attributable to the strong thiol/CdS interaction. In contrast, the adsorption rate and incorporated amount of the QDs on the bare TiO(2) film are much lower than for the silane-modified films. The incident photon-to-current conversion efficiency (IPCE) obtained for the CdS-sensitized TiO(2) electrode was about 20% (at 400 nm) for the bare TiO(2), 13% for the MPTMS-TiO(2), and 6% for APMDS-TiO(2). The current-voltage measurement under dark conditions reveals a higher dark current on the MPTMS- and APMDS-modified electrodes, indicating a lower coverage ratio of CdS on these TiO(2) films. This result is attributed to the fast adsorption rate of CdS QDs on the bottleneck of a mesopore which inhibits the transport of the QDs deep into the inner region of a pore. For the bare TiO(2) film, the lower incorporated amount of CdS but higher energy conversion efficiency indicates the formation of a better-covered CdS QDs monolayer. The moderate adsorption rate of MSA-CdS QDs using the carboxylic acid/TiO(2) interaction is responsible for the efficient assembly of QDs onto the mesoporous TiO(2) films.

We have examined the critical nucleus through density functional calculations of crystalline fluctuations in a Lennard-Jones fluid. The free energy functional we employ has a square-gradient form, with the parameters for a Lennard-Jones interaction potential determined by a modified weighted density approximation applied locally through the liquid–solid interface. We have found that the interface and the center of the critical nucleus behave differently at large undercoolings and at large superheatings. At large undercoolings, the interface of the critical nucleus sharpens, in contrast to the critical nucleus near the spinodal (at large superheatings) that is wide in extent and small in amplitude. We compare the radius and work of formation from classical nucleation theory to that from density functional calculations.

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