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43550-AC10
Development of a New Class of Porous Semiconductors with Photovoltaic, Sensing, and Catalytic Activity: Chalcogenide Aerogels
In the second year of the award, we have focused on (1) extending the chalcogenide gelation process to core-shell particles to optimize luminescence properties; (2) studying the effect of particle shape on gel morphology and mechanical strength; (3) evaluating the relationship between the extent of quantum confinement and the dimensionality of the gel; and (4) continuing to study the response of CdSe aerogels to triethylamine. Highlights from each of these areas are presented below.
(1) Improved luminescence behavior in CdSe porous networks (aerogels) was obtained by wrapping individual CdSe nanoparticles with a ZnS shell (preparing CdSe/ZnS core/shell nanoparticles) prior to gelation. The gels can be prepared as monoliths of different colors by varying the size of the CdSe cores. The photoluminescence of these materials is very intense and coincides with the band edge; there is no evidence for a broad feature to the red, which is characteristic of surface traps. The gels can be dried conventionally, resulting in significant loss of volume, or supercritically, in which case the volume is largely maintained. In contrast to analogous poorly emissive networks prepared from “naked” CdSe nanoparticles (see point 3 below), the extent of quantum confinement in the 3-D connected solid is a function of the precursor particle size and shows little dependence on the network density. This work has recently appeared as a communication in J. Am. Chem. Soc.
(2) Our original studies on CdSe gels and aerogels were conducted solely on gels prepared from spherical particles, which resulted in a colloidal “pearl necklace” type morphology. We reasoned that the shape of the building block (nanoparticle) would have a large impact on the connectivity of the network, and thus the mechanical strength and opto-electronic properties. In the past year, we have explored sol-gel chemistry with rod-shaped and branched CdSe nanoparticles. We find that the networks are polymeric in morphology and exhibit a higher degree of mechanical strength than dot-based materials. Consequently, the networks show a lower degree of compaction during ageing and a much larger surface area. These materials also exhibit enhanced luminescence intensity, which may make them more responsive to analytes than the present generation of chalcogenide aerogel sensors (see point 4 below). Additionally, the incorporation of linear structures (i.e. rods and branches) can be expected to augment the electrical conductivity, making these more promising for photovoltaic platforms.
(3) Quantum confinement effects in semiconductor nanomaterials have a strong dimensionality dependence, and this is well studied for particles, rods, wires, and wells. Metal chalcogenide gels are also low-dimensional materials, but the dimensionality is non-integer (fractal) in nature. Our prior data have shown that chalcogenide aerogels (with low density) exhibit a larger degree of quantum confinement than the xerogels (with higher density). We postulated that the dimensionality of the network can thus be tuned by density. In the past year, we have been performing systematic studies to quantify this effect. We have prepared a series of monolithic aerogels by changing the quantity of oxidant used to induce gelation. More dense monoliths are created when more oxidant is used. We see a small, but reproducible decrease in bandgap as the density (dimensionality) increases and the relationship appears to be linear. This is consistent with a decrease in quantum confinement with increasing density.
(4) The ability of CdSe nanostructured networks to act as luminescent optical sensors for small molecules is being tested for triethylamine (TEA). Powdered or monolithic CdSe mesoporous aerogels with high surface areas were placed in an optical cell and introduced to alternating streams of TEA saturated Ar and unadulterated Ar. An increase of intensity up to 30% is observed upon TEA introduction, saturating after just a few seconds. Switching to Ar results in a decrease in intensity, with a return to baseline occurring within several minutes. We find that the change in luminescence intensity depends on both the original surface treatment and the concentration of analyte. Thus, vacuum annealing or pyridine substitution of residual thiolate molecules on the surface of CdSe aerogels decreases the luminescence response to TEA. In the latter case, this is likely due to the fact that the Lewis basicity difference between TEA and pyridine is small relative to that between TEA and thiolates. In the former case, we speculate that we have oxidized the surface. Our data on as-prepared (thiolate-capped) CdSe suggests that there is a linear response over 0.005-0.04 atm partial pressure of TEA.
