Sol−Gel Processing of Semiconducting Metal Chalcogenide Xerogels:  Influence of Dimensionality on Quantum Confinement Effects in a Nanoparticle Network

Abstract

The synthesis and characterization of porous nanostructured inorganic polymers (gels and xerogels) of CdS, ZnS, PbS, and CdSe is described. Primary particles are synthesized via water-in-oil microemulsions, surface complexed with thiolate ligands, and dispersed in acetone. Oxidation of the thiolate ligands with H 2 O 2 leads to particle aggregation and the formation of wet gels, which are then dried under ambient conditions to produce xerogels. The xerogels exhibit optical band-edges that are intermediate between bulk precipitates and the more porous wet gels and aerogels, suggesting the extent of quantum confinement in such colloidal networks is intimately related to the pore structure and surface area (dimensionality). Heating of the xerogels in vacuo results in a continued decrease in the energy of absorption onset due to sintering of the nanoparticle network, and this is also reflected in a growth in crystallite size as probed by powder X-ray diffraction. The resulting xerogel networks demonstrate modest Brunauer-EmmettTeller (BET) surface areas of 29-65 m 2 /g with Barrett-Joyner-Halenda (BJH) average pore sizes of 3-15 nm, considerably smaller than values obtained for corresponding aerogels and consistent with increased density (decreased dimensionality).

Lead Selenide Nanostructured Aerogels and Xerogels

“…The optical data are summarized in Table 1. The observed energy values, when compared to the bulk value of 0.28 eV, are all blue‐shifted, suggestive of quantum confinement effects in the dried gels prepared from PbSe nanospheres 17–19. The xerogel is significantly red‐shifted from the aerogel, consistent with the effect of density on quantum confinement effects noted for other metal chalcogenide gel systems 18,41.…”

“…In 2004, we reported a method for preparation of aerogels of CdS by oxidation‐induced assembly of thiolate‐capped CdS particles to produce a gel, followed by supercritical drying. In 2005, we demonstrated the applicability of this method to PbS, ZnS, and CdSe as well and showed that the quantum confinement effects observed in both the aerogels and the more dense xerogels (ambient pressure dried gels) are due to the low dimensionality of the network, rather than from any barriers between the particles 17–19. Specifically, it was found that the extent of quantum confinement effects in gels correlates with the surface area and porosity of the gels.…”

Modern Inorganic Aerogels

“…[29,52,53] Furthermore they showed that the surface of the particles is still exposed and can be modified even after the assembly.Inalater study this feature was used to extend the possible aerogel materials to PbSe and CuSe through ac ation-exchange reaction in the pre-formed CdSe gels. [29,52,53] Furthermore they showed that the surface of the particles is still exposed and can be modified even after the assembly.Inalater study this feature was used to extend the possible aerogel materials to PbSe and CuSe through ac ation-exchange reaction in the pre-formed CdSe gels.…”

Moderne Anorganische Aerogele

“…[29,52,53] Des Weiteren zeigten sie,d ass die Partikeloberfläche noch immer zugänglich ist und sogar nach der Anordnung modifiziert werden kann. [29,52,53] Des Weiteren zeigten sie,d ass die Partikeloberfläche noch immer zugänglich ist und sogar nach der Anordnung modifiziert werden kann.…”