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- W2143455005 abstract "The door to a new class of materials has been opened through the combination of aerogels and semiconductor nanocrystals. Semiconductor nanoparticles are used as building blocks to give dry, mesoporous aerogels with large internal surface areas (see picture, center: wet CdS gel; left: unwashed CdS aerogel; right: monolithic CdS aerogel; scale in mm). These materials have many potential uses including in photovoltaic devices and sensors. In a recently published paper in Science1 a remarkable wedding has been announced which unites two very active fields of current materials research: semiconductor nanocrystals and aerogels. The Wayne State University team of Mohanan, Arachchige, and Brock employs colloidally prepared II–VI (CdS, ZnS, CdSe) or IV–VI (PbS) semiconductor nanoparticles as building blocks for dry, mesoporous, high surface area, and light weight aerogels which form after solvent removal under supercritical conditions from the gelated material. Semiconductor nanocrystals2 have been the focus of research for about 20 years and have encountered interest over the years from such diverse disciplines as solid-state physics, inorganic chemistry, physical chemistry, colloid chemistry, materials science, and recently even biological sciences, medical sciences, and engineering. Narrow and intensive emission spectra, continuous absorption bands, high chemical and photobleaching stability, processability, and surface functionality are among the most attractive properties of these materials. The breath-taking development of “nanochemistry” is reflected in an immense number of publications on the synthesis of semiconductor nanoparticles. Nearly all II–VI semiconductors and many of the IV–VI compounds have been prepared in colloidal form, some of them in a variety of different approaches. A great number of these developments have been summarized in surveys, in special issues of journals, and in books.3 The ability to achieve desired particle sizes over the largest possible range, narrow size distributions, good crystallinity, desired surface properties and—should the occasion arise—high luminescence quantum yields as well as adjustable electronic properties, are all considered to be characteristics of a “good preparation”. The properties of the semiconductor nanocrystals also known as quantum dots depend strongly on the type of nanocrystal, the size (quantum-confinement) and the capping agent, thus they can be tuned by a proper choice of the synthetic conditions. There is a wide range of very efficient light-emitting semiconductor nanocrystals which can be synthesized both in organic media or as aqueous solutions. Worthy of mention are CdS, CdSe, and ZnSe4 (UV spectral region) CdSe, InP, CdTe5 (visible light region), PbSe, HgTe, and InAs6 (near-infrared and infrared region). Furthermore, phase transition pressures, melting points, optical and optoelectronic, catalytic, magnetic, and electrical properties of nanomaterials differ from those of the bulk as well as from those of the molecular species they consist of, a situation which is appealing both scientifically and for applications. Assemblies of nanoparticles have been formed by coupling the particles by linker molecules. The formation of bioconjugates has been performed by connecting the nanoparticles to biological material, for example, for the use as fluorescent labels. The build-up of complex nanoparticle superstructures has been demonstrated with a very elaborate concept using highly specific interactions between bound biological functional groups such as complementary DNA strands.7 The other field of research touched by the work of Brock et al. is that of aerogels8 which was opened by Kistler as early as in 1931.9 Even in his first Nature article on this class of materials he reported the successful preparation of aerogels from “silica, nickel tartarate, stannic oxide, tungstic oxide, gelatine, agar, nitrocellulose, cellulose, and egg albumin” and he saw “no reason why this list may not be extended indefinitely”! A small constraint to this optimistic statement was added by Kistler in a follow-up publication in 1932:10 “Rubber offered difficulties not yet surmounted, but the way has been indicated.” In the early 1940s he completed a license agreement with industry for the production of silica aerogel and the commercialization of his ideas began. In the 1960s scientists started to consider them as a medium for storing liquid rocket fuel. Silica aerogels are nonflammable, nontoxic, lightweight, transparent, and thermally stable to about 650 °C. Organic aerogels have been made for their thermal capabilities and they are stiffer and stronger than silica aerogels. For aerogels containing a conducting skeletal material such as carbon, electronic conductivity is introduced into the material. Aerogels have continuous porosity, nanosized pores, and an extremely high surface area of up to 1000 m2 per gram of material. Very recently, Leventis et al. reported11 that the strength of silica aerogel monoliths has been improved by a factor of over 100 through cross-linking the nanoparticle building blocks of preformed silica hydrogels with a polymer (Figure 1). Additionally they showed that composite monoliths are much less hygroscopic than native silica aerogels, and that they do not collapse when in contact with liquids. Strong silica aerogel resulting from cross-linking of the skeleton with a polymer.11 Today, aerogels have broad applications in the commercial and military sectors, for example, as nanoporous thermal insulators, providing 3–10 times more thermal performance at a given thickness when compared to existing materials. Other reports stress the encapsulation capabilities of aerogels for bioactive molecules,12 proteins, and even whole cells. Very promising is also the application of aerogels in the field of microphotonics.13 At this point it may have become more obvious why the paper from Brock and co-workers may be of such importance: by combining the properties of aerogels with those of semiconductor nanocrystals a new class of materials and new applications become accessible with possibly far reaching consequences. The monoliths prepared from CdS and ZnS have densities of between 0.07 and 0.35 g cm−3 compared to the respective bulk values of 4.83 and 4.04 g cm−3 and they contained mesopores of 2–50 nm in diameter. Most of the aerogels exhibit sharp absorption onsets at energies far above the respective bulk band gaps, thus, retaining their nanoparticle energetic features, namely the size quantization. This implies that the quantum dots remain electronically isolated which is attributed to the “fractal connectivity” of the network. This characteristic might be altered by post-preparative annealing of the structures which permits the absorption energy to be tuned to the red spectral region as the average crystallite size grows. The emission properties of the structures, however, are in need of considerable improvement: so far only weak and mainly trapped emission has been observed. In future efforts also pure band-to-band emission may be obtainable by using nanoparticles prepared by more elaborate synthetic routes such as, core–shell14 or core–shell–shell15 structures or nanocrystals with intrinsically stronger emission characteristics (Figure 2).5d Additionally, narrower distributions of nanocrystal sizes may be possible, which would lead to even sharper emission bands and consequently to purer colors. Absorption and emission spectra (λex=400 nm) of thiol-capped CdTe nanocrystals; PL=photoluminescence.5d As optimistic as Kistler in his 1931 paper, Brock et al. state “the generality of this method should lend itself to a number of new aerogel materials if the surface chemistry is appropriately tailored”. The latter can be agreed with completely and is largely based on the observations of Kotov, Peng, and others16 that loss of particle-stabilizing molecules and/or photooxidation of surface ligands leads to aggregate formation in solution which is the basis of the production of robust gels. Current efforts of the Wayne State team “are devoted to preparing these materials in thin-film form and evaluating their potential for photovoltaic and sensing applications”. We are looking forward to many more papers on and possibly even applications of this door-opening synthetic route to materials combining two very topical areas of current materials research." @default.
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- W2143455005 date "2005-07-29" @default.
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- W2143455005 title "Aerogels from Semiconductor Nanomaterials" @default.
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- W2143455005 doi "https://doi.org/10.1002/anie.200501052" @default.
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