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Fig. 1. Electron microscope images of (A) CdS aerogels and (B) CdSe aerogels, illustrating the colloidal nature of the aerogel aggregates. The presence of mesopores (2 to 50 nm) can be clearly seen. (C) A high-magnification image of a CdS aerogel heated in vacuo at 100°C, illustrating the crystalline nature of the nanoparticle architecture. The lattice fringe separation is 2.5 A, corresponding to the (102) reflection of hexagonal CdS. (D) A photograph provides a comparison of a wet CdS gel (center) with an unwashed CdS xerogel (left) and a monolithic CdS aerogel (right). In contrast to xerogel formation, minimal volume loss is observed in the conversion of wet gels to aerogels. The scale is in millimeters.

features of the nanoparticle building blocks. As illustrated in Fig. 2, all of the aerogel materials examined exhibit a sharp absorption onset at an energy far greater than the bandgap for bulk solids (Fig. 2, tabular inset), consistent with retention of quantum-confinement effects in the solids (19). This strongly suggests that the nanoparticle chro-mophore remains effectively isolated, which can be attributed to the fractal connectivity of the network or to the presence of grain interfaces. These chromophores can be ripened by heating, resulting in a systematic red shift of the absorption onset that is nearly linear with temperature (Fig. 3). This also correlates with a growth in the average crystallite size, as probed by x-ray powder diffraction (Fig. 3). Thus, the sharp absorption onset in these materials can effectively be tuned. In addition to the growth of particles, there is a systematic transformation from the cubic crystal structure (as observed for as-prepared aerogels produced from room-temperature reverse-micelle nanopar-ticles) to the more thermodynamically stable hexagonal modification (dominant phase at 500°C) (fig. S3). Even at conditions where the x-ray data suggest a dominant cubic structure (100°C), nanocrystallites with hexagonal features can clearly be seen with transmission electron microscopy (TEM), and indexing of lattice fringes and electron diffraction patterns are consistent with some degree of cubic-to-hexagonal transformation (Fig. 1 and fig. S1).

Photoluminescence studies on as-prepared CdS aerogels reveal only broad, trap-state emission near 600 nm (fig. S4). Upon heating at 100°C, some of the band-edge emission apparent in the nanoparticle precursors is

Pbs Diffused Reflectance Spectra

Fig. 2. Diffuse reflectance data for as-prepared aerogels of PbS, CdSe, CdS, and ZnS and comparative values for bulk crystalline samples. Data were acquired on a Shimadzu (Columbia, MD) model UV-3101PC double-beam, double-monochromator spectrophotometer equipped with an integrating sphere, using BaSO4 as a 100% reflectance standard. The bandgaps of the samples were estimated from the low-energy onset in the absorbance data (converted from reflectance).

Fig. 2. Diffuse reflectance data for as-prepared aerogels of PbS, CdSe, CdS, and ZnS and comparative values for bulk crystalline samples. Data were acquired on a Shimadzu (Columbia, MD) model UV-3101PC double-beam, double-monochromator spectrophotometer equipped with an integrating sphere, using BaSO4 as a 100% reflectance standard. The bandgaps of the samples were estimated from the low-energy onset in the absorbance data (converted from reflectance).

recovered. This is coincident with the beginning of the transformation from cubic to hexagonal and an increase in crystallinity as evidenced by x-ray powder diffraction. It is well documented that the highly crystalline hexagonal nanoparticles produced from high-temperature (~200°C to 300°C) routes exhibit greater photoluminescence quantum yields than the cubic nanoparticles produced at room temperature with reverse-micellar strategies (27). Thus, we sought to test whether nano-particles produced from high-temperature routes would give rise to as-prepared aerogels demonstrating strong band-edge photoluminescence.

CdSe nanoparticles were prepared in trioctylphosphine oxide at elevated temperatures and capped with mercaptoundecanoic acid (MUA) to form hexagonal CdSe nanoparticles of ~5.1 nm in diameter, as estimated from the absorbance onset according to the mass approximation model (20, 28, 29). The CdSe nanoparticles exhibit a sharp band-edge emission near 540 nm, along with a broad shoulder centered near 610 nm, attributable to trap states near the surface (Fig. 4). These nanoparticles transform into opaque orange gels upon standard treatment

Fig. 3. Influence of temperature on crystallite size of the primary particle components in CdS aerogels (open circles, x-ray powder diffraction data) and on absorption onset (filled triangles, diffuse reflectance data).

with tetranitromethane. However, gels were also observed to form in alcohol solutions after standing in ambient light over several days, likely as a result of photooxidation of the MUA capping groups from the surface, representing an alternate method of condensation (20). Aerogels formed from supercritical drying of these gels demonstrate slightly lower surface areas than those prepared from reverse-micellar strategies (30). However, the sharp band-edge emission characteristic of the quantum-confined nanoparticles is retained, with no further processing required (19). The appreciable change in the chromophore environment is evidenced by the shift in the broad trap-state emission peak to near 650 nm (Fig. 4), which suggests very different surface characteristics after condensation and drying than is present in the pristine CdSe nanoparticles in solution.

Together, these data suggest that the controlled removal of surface groups under

Incised Diffraction Device

Wavelength (nm)

Fig. 4. Photoluminescence data for MUA-capped CdSe nanoparticles (open triangles) and an as-prepared aerogel (filled squares), acquired by using an excitation wavelength of 480 nm. Data were collected at 77 K on a SPEX Fluorolog model spectrometer (Jobin Yvon Horiba, Edison, NJ) with 1681 Spex 0.22 m excitation module and 0.34 m emission module. Powdered aerogel samples were placed in evacuated quartz tubes for measurement.

Wavelength (nm)

Fig. 4. Photoluminescence data for MUA-capped CdSe nanoparticles (open triangles) and an as-prepared aerogel (filled squares), acquired by using an excitation wavelength of 480 nm. Data were collected at 77 K on a SPEX Fluorolog model spectrometer (Jobin Yvon Horiba, Edison, NJ) with 1681 Spex 0.22 m excitation module and 0.34 m emission module. Powdered aerogel samples were placed in evacuated quartz tubes for measurement.

conditions that lead to the formation of lacunar aggregates and eventually gelation is quite general for metal chalcogenide nanoparticles. This is consistent with a previously reported study by Kotov in which CdSe nanoparticle samples stripped of excess thioglycolic acid spontaneously assembled first into chainlike aggregates and subsequently into wires (31). Likewise, Peng found that photooxidation of surface thiolate ligands from CdSe nanoparticles resulted in aggregate formation, and this was reversible upon introduction of more capping agent (20). We find that by adjusting the conditions under which capping groups are removed, robust gels can be produced, and the characteristic pore structure of these colloidal gels can be maintained by supercritical drying. The generality of this method should lend itself to a number of new aerogel materials if the surface chemistry can be appropriately tailored. Because the resulting aerogels retain the photophysical properties of the quantum-confined building blocks, gelation and aerogel formation represents an excellent strategy for the assembly of nanoparticles from solution into solid state devices where exploitation of size-dependent properties is desired.

The aerogels presented here based on PbS, CdSe, CdS, and ZnS cover the optical spectrum from the infrared through the ultraviolet, and the optical features of each material can be effectively "tuned" over a substantial range by adjusting the heating profile employed. The aerogel structure effectively preserves the integrity of the quantum dots by locking them into a network while providing a pore structure through which chemical species can be introduced, either as analytes or as secondary components for composite formation. Current efforts are devoted to preparing these materials in thin-film form and evaluating their potential for photovoltaic and sensing applications.

References and Notes

1. H. Li, A. Laine, M. O'Keeffe, O. M. Yahgi, Science 283, 1145 (1999).

2. N. Zheng, X. Bu, P. Feng, Nature 426, 428 (2003).

3. N. Zheng, X. Bu, B. Wang, P. Feng, Science 298, 2366 (2002).

4. W. Park, J. S. King, C. W. Neff, C. Liddell, C. J. Summers, Phys. Status Solidi B 229, 949 (2002).

5. P. V. Braun, P. Wiltzius, Nature 402, 603 (1999).

6. Y. A. Vlasov, N. Yao, D. J. Norris, Adv. Mater. 11, 165 (1999).

7. P. V. Braun, P. Osenar, V. Tohver, S. B. Kennedy, S. I. Stupp, J. Am. Chem. Soc. 121, 7302 (1999).

8. B. M. Rabatic, M. U. Pralle, G. N. Tew, S. I. Stupp, Chem. Mater. 15, 1249 (2003).

9. P. N. Trikalitis, K. K. Rangan, T. Bakas, M. G. Kanatzidis, Nature 410, 671 (2001).

10. P. N. Trikalitis, K. K. Rangan, M. G. Kanatzidis, J. Am. Chem. Soc. 124, 2604 (2002).

12. A. E. Riley, S. H. Tolbert, J. Am. Chem. Soc. 125, 4551 (2003).

13. N. Husing, U. Schubert, Angew. Chem. Int. Ed. Engl. 37, 22 (1998).

Table 1. Surface area and porosity data for metal chalcogenide xerogels and aerogels. All samples were treated with H2O2 to induce gelation, with the exception of CdSe, which was treated with tetranitromethane. Data were acquired on a Micromeritics (Norcross, GA) ASAP 2010 Surface Area Analyzer. Samples were degassed for a minimum of 24 hours at 100°C before analysis. In an effort to extract the full pore volume of the aerogels, long equilibrium intervals were used and granular specimens were evaluated in lieu of monoliths (32). Surface areas were computed with the Brunauer, Emmett, and Teller (BET) multimolecular layer adsorption model, and average pore size and cumulative pore volumes were computed with the Barrett, Joyner, and Halenda (BJH) model. The range of values reflects data from two or more samples prepared and measured under similar conditions.

Table 1. Surface area and porosity data for metal chalcogenide xerogels and aerogels. All samples were treated with H2O2 to induce gelation, with the exception of CdSe, which was treated with tetranitromethane. Data were acquired on a Micromeritics (Norcross, GA) ASAP 2010 Surface Area Analyzer. Samples were degassed for a minimum of 24 hours at 100°C before analysis. In an effort to extract the full pore volume of the aerogels, long equilibrium intervals were used and granular specimens were evaluated in lieu of monoliths (32). Surface areas were computed with the Brunauer, Emmett, and Teller (BET) multimolecular layer adsorption model, and average pore size and cumulative pore volumes were computed with the Barrett, Joyner, and Halenda (BJH) model. The range of values reflects data from two or more samples prepared and measured under similar conditions.

Metal chalcogenide xerogel/aerogel

BET surface area (m2/g)

Mean BET surface area (m2/g)

BJH adsorption average pore diameter (nm)

BJH adsorption cumulative pore volume (cm3/g)

CdS xerogel

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