Colloidal Nanocrystal-Based Gels and Aerogels: Material Aspects and

Perspective pubs.acs.org/JPCL

Colloidal Nanocrystal-Based Gels and Aerogels: Material Aspects and Application Perspectives Nikolai Gaponik, Anne-Kristin Herrmann, and Alexander Eychmüller* Physical Chemistry, TU Dresden, Bergstrasse 66b, 01062 Dresden, Germany ABSTRACT: Gels and aerogels manufactured from a variety of metal and semiconductor nanoparticles available in colloidal solutions have recently proven to provide an opportunity to marry the nanoscale world with that of materials of macro dimensions that can be easily manipulated and processed while maintaining the nanoscale properties. The aerogel materials may be further processed in order to achieve improvements in their properties relevant to applications in optical sensing, photovoltaics, LEDs, nonlinear optics, thermoelectrics, and catalysis. This Perspective reviews the young field, lines out some of the synthetical challenges, and touches on application-related aspects.

M

atter in its nanoparticulate form has gained considerable attention in recent decades due to the fascinating properties exhibited as materials evolve from the atomic scale, through the molecular, and approach the extended solid. These “size-tunable” properties have the potential to have an impact on many interesting areas of development and applications. However, the marriage of the nanoscale world with that of materials of macro dimensions that can be easily manipulated and processed while maintaining the nanoscale properties is still a challenge to be surmounted. Gels and aerogels built from a variety of metal and semiconductor nanoparticles available in colloidal solutions have recently been demonstrated to provide such an opportunity. The evolving aerogels are extremely light, highly porous solids and have shown to keep the important properties of the nanosized objects that they consist of instead of simply those of the respective bulk solids. The resulting aerogel materials have been characterized with respect to their morphology and composition, and their resulting properties have been examined in light of the inherent electronic nature of the nanosized constituents. The aerogel materials may be further processed in order to achieve improvements in their properties relevant to applications in optical sensing, photovoltaics, LEDs, nonlinear optics, thermoelectrics, and catalysis. Aerogels as fine inorganic superstructures bridging, in their properties, the macro and nano dimensions are known to be exceptional highly porous materials with a variety of applications.1 The chemistry of the aerogel synthesis originated from the pioneering work2 from the early 1930s and was further developed starting from the late 1960s.1,3 Attractive catalytic, thermoresistant, piezoelectric, antiseptic, and many other properties of the aerogels have originated from the unique combination of the specific properties of nanomaterials magnified by their macroscale self-assembly.1,3,4 Until very recently, the most investigated materials forming fine aerogel superstructures were silica and other metal oxides together with © 2011 American Chemical Society

The marriage of the nanoscale world with that of materials of macro dimensions that can be easily manipulated and processed while maintaining the nanoscale properties is still a challenge to be surmounted.

their mixtures. If the functionality of the oxides was not enough to achieve the property demanded, additional impregnation of the aerogel with metals, polymers, and molecules could be performed, leaving for the initial aerogel itself the role of a passive porous matrix. Recently, attention was attracted by the possibility to create oxide-free functional aerogels, based mainly on metal chalcogenides, 5 which may open enormous opportunities for semiconductor technology, catalysis and photocatalysis, optoelectronics and photonics, sorbents, and filters. According to the literature,5 there are three different approaches to the formation of metal chalcogenide aerogels, (i) thiolysis reactions of molecular metal precursors,6 (ii) condensation reactions between small clusters and metal ions,7 and (iii) condensation reactions of discrete colloidal nanocrystals.8,9 In this Perspective, we attempt to summarize Received: October 10, 2011 Accepted: December 2, 2011 Published: December 2, 2011 8

dx.doi.org/10.1021/jz201357r | J. Phys. Chem. Lett. 2012, 3, 8−17

The Journal of Physical Chemistry Letters

Perspective

proposed that are based on an additional modification of the ligand shells instead of their exchange. Among others, modifications with polyethylene glycols,20 phospholipid micelles,21 and amphiphilic polymers22 are reported so far. However, these additional shells are not considered to be optimal for the intended gelation as they will increase the amount of organics in the gel structures and will decrease the accessibility of the nanocrystal surfaces. Moreover, an increased complexity of the ligand shell may cause the formation of multiple byproducts of the oxidative gelation, possibly resulting in unpredictable changes of the final properties of the aerogel. A very interesting alternative approach based on the ligand exchange with molecular metal chalcogenides has been introduced recently by Talapin and co-workers.23,24 Although these functional all-inorganic nanoparticles may be considered as very attractive building blocks for the formation of various superstructures including aerogels, their controllable gelation has not been reported so far. In general, the phase transfer for each particular nanocrystal system should be developed and optimized by individually taking into account all different parameters like the original capping agent, the solvents and concentrations, the material properties of the core nanocrystals, the presence of inorganic shells, the demanded functionality of the new ligand, and so forth. As an example of such a kind of optimization heading for an improved colloidal stability and better light-emission properties of InP-based core−shell nanocrystals, we refer to a recently published work of Reiss and co-workers.25 It is mentioned that occasionally a phase transfer might be avoided, as recently shown on the example of trioctylphophine oxide (TOPO)-capped CdTe nanocrystals synthesized in organics and subjected to oxidative gelation in toluene/methanol mixtures.26 However, more work has to be done to transfer this approach to other kinds of nanocrystals. A phase transfer can be also avoided by synthesizing then nanoparticles in polar media right away. Indeed, our group has developed the controllable photochemical gelation of thiolcapped CdTe nanocrystals directly synthesized in water.27 The formation of highly porous (pore surface of ∼210 m2/g and monolith density of less than 1/500th of the corresponding bulk material) strongly emitting aerogels as well as the hybridization of the wet gels with processable polymers has successfully been demonstrated (Figure 1). As reported in ref 12 and as is seen from Figure 1 (cf. also Figure 2, bottom), in such semiconductor nanocrystal gel and aerogel networks, the photoluminescence of the individual nanoparticles is generally preserved. However, due to the close proximity of the emitting species in the network, their efficient coupling is possible, resulting in electron or energy transfer between neighboring nanocrystals. As a result, the aerogels show a slightly red-shifted intensity reduced photoluminescence (PL) compared to that of the colloidal solution (Figure 2). The explanations of the mechanisms of the PL changes and discussions of related phenomena of charge and energy transfer in semiconductor nanocrystal assemblies may be found in the corresponding literature.28−31 These changes of the optical properties may be reduced if the distance between the individual nanoparticles is increased, for example, by using matrixes or core−dielectric shell nanoparticle building blocks. As an example of matrix-supported structures, CdTe nanocrystals can be assembled within 3D networks of Cd-thiolates.32 In these gels and aerogels, the optical properties of the individual nanoparticles are better prevented from deterioration; the nanoparticles are separated by the thiolate

the recent success in the field of condensation (or selfassembly) of colloidal nanocrystals, stressing on the wide applicability of the presently developed methods to a variety of materials, for example, metals, metal chalcogenides, and their combinations. Special attention is paid to desired architectures of the aerogels determined by their application perspectives. The general approach leading to the fabrication of colloidal nanocrystal aerogels is based on their controllable destabilization followed by supercritical solvent removal. Destabilization may be achieved by different methods including chemical or photochemical oxidation of surface ligands, depleting the shell of ligands by removal through washing with suitable solvents, reducing of electrostatic stabilization by adjusting the ionic strength of the solution, and so forth. The technique of supercritical drying is based on the exchange of the interstitial solvent with liquid carbon dioxide, bringing the CO2 to its critical state, and removal by reducing the pressure while maintaining the temperature above the critical point (around 31 °C). This method preserves 3D structures of very delicate species from the destructive influence of surface tension and turbulences appearing during drying under ambient conditions.10 Gel samples dried supercritically are typically named aerogels, while those dried under ambient conditions are called xerogels.1 It is noted that the drying is also possible without a solvent exchange to carbon dioxide, for example, it may be performed from methanol, acetone, or similar solvents originally used as media for the gelation. However, the advantage of this approach, namely, the reduced processing time, is compensated for by the necessity to apply higher temperatures (around 240 °C and higher) and the related risk of explosion. Moreover, higher temperatures may cause undesirable structural rearrangements in the gel structure.1 The first nanocrystal aerogels reported by Brock et al. composed of CdS and CdSe showed a certain blue shift in the absorption typical for quantum-confined nanocrystal building blocks but appeared to be barely emitting.8,9,11 Later, the same group managed to overcome the initial problem of quenched nanocrystal emission, and strongly emitting samples of CdSe/ ZnS nanocrystal aerogels were produced.12 These CdSe/ZnS aerogels exhibited surface areas ranging from 188 to 234 m2/g and extremely low monolith densities (around 0.08 g/cm3). Subsequently, the initial success and perspectives of the emerging field of colloidal nanocrystal-based aerogels were analyzed in early reviews.13,14 At that early stage, the main attention was paid to extending the material range of the nanocrystal-based aerogels (e.g., PbS,8 ZnS,8 GeSx,6 Ag2Se,15 CdS−Au,16 and CdS−Ag17). Relatively fewer attempts were reported to achieve a special design (e.g., thin films, processable composites, etc.) or to demonstrate application perspectives. However, already, these pioneering papers explicitly stress on the necessity of “preparing these materials in thin-film form and evaluating their potential for photovoltaic and sensing applications”.8 The attractivity of light-emitting aerogels for LED and optical gain applications was also mentioned.12 In all cases discussed above, the building blocks were nanocrystals prepared in nonpolar organic media. For a controllable gelation, they had to be transferred to polar organic media or to water. Such a phase transfer itself is a critical step in the fabrication of functional aerogels. Indeed, the phase transfer via a ligand exchange may result in a deterioration of the original optical properties and in a reduced colloidal stability of the semiconductor nanocrystals.18,19 To avoid this deterioration, a number of approaches have been 9



Perspective

Figure 1. (a) TEM image of a CdTe NC aerogel structure; (b) strongly emitting CdTe NC hydrogel; (c) self-supporting emitting CdTe aerogel (size is ∼0.5 cm3); (d,e) optically transparent and processable aerogel−PMMA composite. Images in frames (b), (c), and (e) are acquired under 365 nm UV lamp excitation. Reproduced with permission from ref 27. Copyright 2008, Wiley VCH Verlag GmbH & Co. KGaA.

network, and quenching of the emission due to energy transfer between nanocrystals is less probable. The aerogels of this type show very strong emission and possess “saturated” pure emission colors. By this, they may find applications, for example, in color conversion layers,33,34 although, to the best of our knowledge, this kind of application is not yet explored with aerogel materials. At the same time, the application of matrixsupported NC aerogels in LEDs or energy-harvesting systems is not favorable because of the dielectric barriers preventing efficient interparticle charge flows. Sensor and catalytic applications of the matrix-supported aerogels may be less favorable due to their relatively lower porosity and surface area, which is determined not by the size of the individual nanoparticles but by the thiolate matrix. To date, in the case of nanoparticle-based aerogels, the understanding of the fundamental phenomena related to the mechanisms of their formation, to the interactions between single nanoparticles in the chains, and to the collective effects responsible for the optical and electric properties of the aerogels is still in its infancy and will definitely attract growing attention in the near future. The recently suggested mechanism for the gelation of CdSe nanocrystals is based on the formation of diselenide bridges and may be described as follows.35 During the gelation, the oxidation of the thiolate ligand leads to the formation of disulfides or sulfonates, producing decomplexed Cd2+ ions on the nanocrystal surface. The Cd2+ ions can then be solvated (e.g., by the carboxylate species), leaving a seleniderich NC surface. In the presence of excess oxidizing agent, surface selenide groups can oxidize and form diselenide (or polyselenide) species, linking the particles together. The formation of polyselenide bonds between the nanocrystals was confirmed by Raman and X-ray photoelectron spectroscopies.35

Figure 2. (Top) Optical absorption (gel data converted from reflectance, black) and photoluminescence (gray) spectra of CdSe and CdSe/ZnS samples, as reproduced from ref 12. (Bottom) Typical absorption (left) and PL (right) spectra of an aqueous solution of CdTe NCs and of the resulting aerogel. The bottom panel is reproduced with permission from ref 27. Copyright 2008, Wiley VCH Verlag GmbH & Co. KGaA.

It is also expected that similar phenomena are responsible for the oxidative gelation of CdTe aerogels.26 A very recent study revealed the option to gelate CdTe nanoparticles regardless of their thiol capping, that is, an oxidative gelation is performed both with thiolated CdTe nanocrystals in polar media and with trioctylphosphine-oxide-capped particles dispersed in toluene/ methanol mixtures. This fact and the related optical and morphological studies support the idea of bi- or polytelluride linkage as the main mechanism responsible for gelation of CdTe NCs synthesized in organic media.26 It is noted that CdTe nanocrystals synthesized directly in water in the presence of short-chain thiols (e.g., thioglycolic acid) display a sulfur-rich surface.36 The gelation and interparticle bonding in this case are likely to proceed through the formation of bi- or polysulfide species. This peculiarity may explain some deviations in the gelation processes observed between aqueous and organically synthesized CdTe NCs.26,27 Among several other examples relevant to formation mechanisms, we mention the use of analytical ultracentrifuge measurements as a tool allowing for monitoring of the CdTe gel network growth kinetics27 and the utilization of energy dispersive spectroscopy to confirm the loss of thiolate functionalities during CdSe/ZnS gel formation.12 The pore 10



Perspective

structure and interconnectivity in CdS aerogels were probed by a combination of conventional techniques, with hyperpolarized xenon NMR revealing the important role of residual organics and drying procedures on the accessibility of pores.37 Optical spectroscopy allowed the establishing of a correlation of the density of the CdSe aerogel network and its optical band gap,38 as well as the correlations between the nanocrystal morphologies and the porosities and emission intensities of the CdSe aerogels.39,40 For example, it was found that an aerogel made of CdSe nanorods exhibits a unique morphology, which is similar to an acid-catalyzed silica aerogel with a polymer-type framework (Figure 3a,b).39 These rod aerogels

Figure 4. TEM images of (a) spherical PbTe nanoparticles (nps) and (b) cubic PbTe nanoparticles, along with images of the resultant aerogels and xerogels. The insets show HRTEM images of the individual nanoparticles. Reproduced with permission from ref 45. Copyright 2011, Royal Society of Chemistry.

decreasing the thermal transport and potentially leading to enhanced thermoelectric performance.43,44 Very recently, PbTe nanocrystal-based aerogels with controlled structure and porosity were fabricated (Figure 4) as promising nanostructured thermoelectric materials.45 However, thermoelectric properties of these aerogels are still not reported. Another material of choice, Bi2Te3 in its nanoparticulate form, has also attracted recent attention as promising for thermoelectrics.43,44 Scheele et al. have investigated the thermoelectric properties (electric conductivity, thermopower, and thermal conductivity) of bismuth telluride nanoparticles pressed using spark plasma sintering and found that after ligand removal and compression, the samples yielded an electrical conductivity that is virtually identical to typical n-type bulk samples.46,47 The total thermal conductivity of these nanoparticle pellets was as much as 1 order of magnitude smaller than that of the bulk material. Dirmyer et al. investigated thermoelectric properties of annealed and pressed Bi2Te3 nanoparticles stabilized with long-chain thiols and found that annealing (removing of thiol barriers between the nanoparticles) resulted in an improvement of the electrical conductivity while maintaining a low thermal conductivity, which is important for high thermoelectric performance.48 Brock et al. recently reported the thermoelectric properties of the first Bi2Te3 nanoparticle aerogels ever prepared,49 and the results were very encouraging because the thermoelectric performance of the Bi2Te3 aerogels was similar to that published by Dirmyer et al. for pellets and may be improved by the optimization of the aerogel structure. Quite a number of different approaches have focused on modifying oxide-based aerogels with metal nanoparticles (e.g., Pt, Au) in order to carry the catalytic properties from the

Figure 3. TEM images of CdSe colloidal aerogels (A) and polymeric aerogels (B) and their building blocks (dots and rods, inset). (C) Graph of room-temperature emission spectra (λex = 440 nm) of CdSe dot and rod aerogel monoliths (pictured in the insets). Reproduced from ref 39.

exhibit more than twice of the surface area and cumulative pore volume of the dot aerogels as well as a larger number of accessible pores. Moreover, the CdSe rod aerogel monolith shows much stronger fluorescence than a dot aerogel monolith (Figure 3c), despite the fact that the primary particles (rods and dots) have comparable emission intensities.39 Very relevant studies were also performed on the pore wall thickness versus energy gap dependence in hexagonal mesoporous germanium.41 Several years ago, Lin and Dresselhaus reported on a theoretical model for the electronic structure and transport properties of superlattices of PbS, PbSe, and PbTe.42 The lead chalcogenides were chosen for these studies because they are conventionally good thermoelectric materials with well-established transport properties. Impressive thermoelectric performances were predicted for 5 nm diameter PbSe/PbS and PbTe/PbSe superlattices (ZT values higher than 4 and 6 at 77 K, respectively). These ZT values indicate that nanowire superlattices are promising systems for thermoelectric applications. Moreover, the presence of pores and interfaces on the nanoscale (as in the case of nanowire bundles forming typical nanoparticle-based aerogels) is expected to result in phonon scattering, thereby 11



Perspective

metal50,51 into the porous structures of the aerogels.1,52−54 Our group has contributed to the formation of fine mesoporous assemblies of catalytically active metal nanoparticles on artificial opals55 and on fungi56 as templates. Among other superstructural metallic nanomaterials reported so far are mesoporous platinum−carbon composites,57 nanoporous gold,58 Au nanoparticles interlinked with dithiols,59 necklace nanochains of hybrid Pd−lipid nanospheres,60 electrocatalytically active nanoporous Pt aggregates61 and Pd nanosheets,62 foams,63,64 ordered 2D and 3D supercrystals,65−68 and highly porous assemblies from metallic and bimetallic “supraspheres” of several hundred nanometers in diameter cross-linked with dithiols.69,70 The first report on solely metallic nanoparticle based aerogels originates from our group.71,72 These monometallic (Au, Ag, Pt) and bimetallic (Au/Ag and Pt/Ag) gels and aerogels exhibit an average density 2 orders of magnitude lower than that of metallic foams,73 and their primary structural units match the size of the original colloidal nanoparticles (5−20 nm), which is an order of magnitude smaller than that of the self-assembled supraspheres.70 No chemical cross-linkers were involved in the self-assembly process. The formation of such noble metal nanoparticle based mesoporous monometallic and bimetallic aerogels is an important step toward self-supported monoliths with enormously high catalytically active surfaces. Considering that metal nanoparticles possess very specific optical properties due to their pronounced surface plasmon resonances, aerogels from metal nanoparticles may also find future applications in nanophotonics, for example, as advanced optical sensors and ultrasensitive detectors.74 Typical structures of exemplarily chosen Pt/Ag bimetallic gels and aerogels are shown in Figure 5. A similar approach based on the replacement of the capping agent was further utilized by Liu et al. for the fabrication of monometallic (Au, Pd, Pt, Ru, and Rh) and trimetallic (Au/Pd/Pt) nanoporous films.75 Three-dimensional porous nanowire networks fabricated from noble metal salts in aqueous solution were dried under ambient conditions to form high-surface-area noble metal (Au, Ag, Pt, and Pd) nanosponges showing broad-band nonlinear optical response.76 As mentioned above, for the preparation of an aerogel, an efficient destabilization is usually necessary in order to gelate the colloids. This might be performed by adding chemicals such as hydrogen peroxide, by changing the ionic strength of the media, or by a photochemical treatment. However, such treatments may have disadvantages like higher costs, nonfacile control, and activity degradation of the nanocrystals in the networks compared to the original nanocrystals. Therefore, the development of alternative methods to gelate the colloids is of great interest. The recently introduced approach to the stabilization of the colloidal nanocrystals with tetrazole derivatives77−79 opens up an alternative opportunity for the formation of gel networks. This method is based on the ability of tetrazole derivatives to build strong complexing bridges with several metal ions,80,81 which offers a facile and highly reproducible interconnection of nanoparticles, leading to a multibranched network (Figure 6a,b). 82 Moreover, the technique developed does not involve any sophisticated processing like chemical destabilization of the initial colloid or its photochemical treatment. The approach can easily be scaled up and provides the option of producing large amounts of gels/aerogels. The 3D structures obtained are optically transparent and maintain the emission properties of the original nanocrystals. This gelation is very quick (on the time scale of

Figure 5. (A,B) SEM images of platinum/silver aerogels at different magnifications and (C,D) TEM micrographs of a platinum/silver hydrogel (C) and aerogel (D). (E) HR-TEM image of the platinum/ silver nanochains, showing individual silver and platinum nanodots. Reproduced with permission from ref 71. Copyright 2009, Wiley VCH Verlag GmbH & Co. KGaA.

seconds to minutes) and is also shown to be reversible if stronger complexants such as EDTA are added to the network. This reversibility may be easily followed by visual observations and also spectroscopically as the gel is characterized by pronounced scattering in the UV−vis absorbance and by a red-shifted photoluminescence with relatively lower intensity (Figure 6c−e).

The development of alternative methods to gelate the colloids is of great interest. It is noted that this tetrazole-based gelation approach is versatile and may be applied to a variety of nanocrystal materials and, even more importantly, to mixtures of different materials. It is obvious that establishing optimal joint gelation conditions for nanocrystals of different nature (e.g., metals and semiconductors or metals and oxides) is a challenging task. Indeed, the colloidal destabilization of each kind of combinations demands “individual” conditions. Moreover, even if the conditions are similar, segregation of the gel fragments may occur due to different affinities of the nanocrystal surfaces to each other. For complexation of tetrazole-stabilized nanocrystals, the nature of the core material plays no role in the gelation process. As seen from Figure 7, the complexation of tetrazolestabilized CdTe and Au nanocrystals allows formation of networks with uniform distribution and a controllable ratio of the constituents.83 Moreover, in this kind of multifunctional 12



Perspective

Figure 6. (a) Mechanism proposed for the gelation of CdTe/5-HSCH2Tz nanocrystals; (b) TEM and HRTEM images of a CdTe nanocrystal hydrogel fragment dried on a TEM grid. (Inset) A typical true-color image of a CdTe hydrogel under daylight (left) and under a UV lamp (right); (c−e) absorbance (top panels) and emission (bottom panels) spectra of a CdTe−tetrazole nanocrystal colloid measured upon a stepwise addition of Cd2+ ions into the initial colloid (c) and into the colloid regenerated by EDTA (e). (d) Nanocrystal solution regeneration after step (c) by the addition of EDTA. The arrows indicate increased Cd2+ or EDTA concentration. Adapted from ref 82.

Figure 7. (Top) TEM images of fragments of hybrid CdTe/Au aerogels of different compositions. The numbers shown are the CdTe/Au weight ratios in the initial colloidal mixture. (Bottom) PL (λex = 450 nm) (a), absorbance (b), and PL lifetime (λex = 403 nm) (c) of pure green-emitting CdTe, Au (absorbance only), and hybrid CdTe/Au aerogels of the same compositions as those in the top panel. Adapted from ref 83.

nanostructures, an efficient coupling of the optical properties of the nanocrystals and plasmon-enhanced emission were observed. Detailed analysis of similar effects in semiconductor and metal nanocrystal assemblies may be found in the corresponding recent literature.84−88 The combination of a pronounced surface-sensitive optical signal with the extremely high surface area (around 130 m2/g) makes these multifunctional materials promising candidates for optical sensing applications. The successful application of gels and aerogels is very much dependent on efficient addressing (e.g., electric contacting, coupling to waveguides, etc.) and on their manipulation (e.g., insertion into and removal out of reaction mixtures as in the case of catalysis and hybridizing with other functional layers and structures as in case of LED and solar cell applications).

One of the important steps toward this is the fabrication of aerogels in thin-film form on various substrates. Indeed, conductive patterned substrates will allow contact and any kind of electric-transport measurements, transparent substrates are optimal for coupling with optical systems, and conducting and transparent supports are optimal for optoelectronic and photovoltaic applications. Aerogels fixed on surfaces are easy to handle as heterogeneous catalysts, and they may be combined with microfluidic reactors as catalytic or sensoric units. Thinfilm xerogel or aerogel networks formed from metal nanoparticles may find applications as transparent conducting substrates. Not without reason, the importance of the fabrication of thin-film aerogels was mentioned by Brock et al. already in their early work.8 One of the possible fabrication methods presently developed in our group may be described as 13



Perspective

Although the colloidal nanocrystal gel and aerogel research is only in its infant state, this emerging multidisciplinary field attracts more and more interest. The main research streams include new materials, a fundamental understanding of the assembly processes and interactions, and finally application-related studies. With the huge variety of existing potential building blocks92,93 including II−VI, III−V, and IV−VI semiconductor nanocrystals and their ternary compounds, core−shell geometries, type-I and type-II structures, metal nanoparticles, and various metal oxides, researchers possess already a vast set of possible constituents from which to form aerogels. Additionally, the material options increase substantially when considering the sizes of the NCs, which (depending on the compounds) may vary from 1.5 to 300 nm, which is a further parameter influencing the chemical and physical properties of the gels. Last but not least, we are also in the position to alter the shapes of the nanoentities from spherical to rodlike and to tetrapoidal in some cases. With this set alone, one would have several hundreds of different possible building blocks from which gels and aerogels could be generated. Because we believe especially in the potential of combinations of these “bricks” into mixed and hybrid structures, the number of options is countless. In many cases, owing to the unique combination of nano and macro dimensions in one structure, gels and aerogels will have physicochemical properties different from their building blocks and from the corresponding bulk materials. Studying these properties includes appropriate modeling of transport phenomena and interparticle coupling as well as thorough structural, electrical, and optical characterization. In some cases (e.g., freestanding, low mass, and fragile samples), existing measurement protocols and setups have to be specially adjusted to enable appropriate characterization. Understanding of the formation mechanisms is especially important for extending the material range and making gelation approaches facile, versatile, and reproducible, which is a main demand of application-related studies.

follows. The surface of a substrate (glass, quartz, silicon wafer, etc.) is premodified by polyelectrolyte molecules in order to improve its adhesive properties toward nanocrystals and gel fragments.89 This premodified substrate can be further immersed into a reaction mixture, where gelation of the same type of nanocrystals proceeds, resulting in the fabrication of thin-film gel networks electrostatically bound to the substrate. Figure 8a shows an example of such a thin film of a Au

Figure 8. (a) SEM image of a thin Au network formed on a glass substrate with the aid of poly(diallyldimethylammonium chloride); (b) SEM image of a thin-film Au/Ag xerogel nanostructure formed on a vertical glass substrate by slow evaporation of the solvent from the gel solution. Insets show photographs of the corresponding samples.

nanoparticle network (too thin to be named gel) formed on a glass substrate by electrostatic assembly of hydrogel fragments forming in the solution. This approach shows potential for controlling the thickness and particle density by the immersion time, the concentration of the colloidal solutions, and the gelation conditions. Alternatively, substrates may be vertically immersed into a hydrogel solution, and the deposition of the film is performed by slow evaporation of the solvent. In this case, only relatively thick films were formed on the glass surface (Figure 8b). The specific resistance of the Au/Ag xerogel film shown in Figure 8b is ∼3.6 Ω mm2/m, and the layer resistance is around 2 Ω, which is comparable with typical characteristics of conventional ITO. It is obvious that full control of the thickness, conductivity, porosity, and so forth of the films demanded by each specific application needs more research. We note that critical point drying (and forming aerogels) seems to be necessary for forming catalyst substrates with very large surface areas. However, for the fabrication of conducting transparent substrates, drying under ambient conditions (and forming thin-film xerogels) may already be sufficient. Another important approach allowing better handling of the originally fine and fragile nanocrystal aerogels is their hybridization with polymers (see, e.g. Figure 1d,e). Cross-linking with polymers has previously been demonstrated to allow for the improvement of mechanical properties of SiO2 aerogels.90,91 In the case of colloidal nanocrystal-based aerogels, mechanical improvement may be even more important, taking into account the delicate nature of the interparticle connections and the reduced robustness of the semiconductor nanocrystals in comparison to silica. Besides the mechanical strength, polymers may provide chemical stability, elasticity, and shape-forming ability to the aerogel. At the same time, the aerogel network will still be percolated through the polymer matrix, allowing for energy and/or charge transfer. The optical properties of the aerogel may also be preserved in the transparent polymer matrix. Such hybrids may be less attractive for catalytic applications because of their reduced surface area and low porosity but possess a potential for optoelectronics and photovoltaics.

The mechanical strength of colloidal nanocrystal-based aerogels is an important issue taking into account the delicate nature of the interparticle connections. The list of promising application fields of nanocrystal-based aerogels is impressive: heterogeneous and electrocatalysis, LEDs, photodetectors and energy-harvesting systems, optical sensors, surface-enhanced Raman scattering (SERS) substrates, broadband optical limiters, color conversion layers, thermoelectrics, and conducting transparent substrates. For some of the mentioned applications, preliminary results are obtained, but none of them are presently finalized, which opens enormous research and development opportunities for the related fields.

■

AUTHOR INFORMATION

Biographies Nikolai Gaponik received his Ph.D. from the Belarussian State University in Minsk. He was a visiting scientist at the LMU in Munich 14



Perspective

(17) Gill, S. K.; Hope-Weeks, L. J. Monolithic Aerogels of Silver Modified Cadmium Sulfide Colloids. Chem. Commun. 2009, 4384− 4386. (18) Aldana, J.; Wang, Y. A.; Peng, X. Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols. J. Am. Chem. Soc. 2001, 123, 8844−8850. (19) Breus, V. V.; Heyes, C. D.; Nienhaus, G. U. Quenching of CdSe−ZnS Core−Shell Quantum Dot Luminescence by WaterSoluble Thiolated Ligands. J. Phys. Chem. C 2007, 111, 18589−18594. (20) Susumu, K.; Uyeda, H. T.; Medintz, I. L.; Pons, T.; Delehanty, J. B.; Mattoussi, H. Enhancing the Stability and Biological Functionalities of Quantum Dots via Compact Multifunctional Ligands. J. Am. Chem. Soc. 2007, 129, 13987−13996. (21) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. In Vivo Imaging of Quantum Dots Encapsulated in Phospholipid Micelles. Science 2002, 298, 1759−1762. (22) Pellegrino, T.; Manna, L.; Kudera, S.; Liedl, T.; Koktysh, D.; Rogach, A. L.; Keller, S.; Raedler, J.; Natile, G.; Parak, W. J. Hydrophobic Nanocrystals Coated with an Amphiphilic Polymer Shell: A General Route to Water Soluble Nanocrystals. Nano Lett. 2004, 4, 703−707. (23) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417−1420. (24) Kovalenko, M. V.; Bodnarchuk, M. I.; Zaumseil, J.; Lee, J.-S.; Talapin, D. V. Expanding the Chemical Versatility of Colloidal Nanocrystals Capped with Molecular Metal Chalcogenide Ligands. J. Am. Chem. Soc. 2010, 132, 10085−10092. (25) Tamang, S.; Beaune, G.; Texier, I.; Reiss, P. Aqueous Phase Transfer of InP/ZnS Nanocrystals Conserving Fluorescence and High Colloidal Stability. ACS Nano 2011, DOI: 10.1021/nn203598c. (26) Yao, Q.; Brock, S. L. Porous CdTe Nanocrystal Assemblies: Ligation Effects on the Gelation Process and the Properties of Resultant Aerogels. Inorg. Chem. 2011, 50, 9985−9992. (27) Gaponik, N.; Wolf, A.; Marx, R.; Lesnyak, V.; Schilling, K.; Eychmüller, A. Three-Dimensional Self-Assembly of Thiol-Capped CdTe Nanocrystals: Gels and Aerogels as Building Blocks for Nanotechnology. Adv. Mater. 2008, 20, 4257−4262. (28) Kagan, C. R.; Murray, C. B.; Nirmal, M.; Bawendi, M. G. Electronic Energy Transfer in CdSe Quantum Dot Solids. Phys. Rev. Lett. 1996, 76, 1517−1520. (29) Rogach, A. L.; Klar, T. A.; Lupton, J. M.; Meijerink, A.; Feldmann, J. Energy Transfer with Semiconductor Nanocrystals. J. Mater. Chem. 2009, 19, 1208−1221. (30) Lunz, M.; Bradley, A. L.; Chen, W.-Y.; Gerard, V. A.; Byrne, S. J.; Gun’ko, Y. K.; Lesnyak, V.; Gaponik, N. Influence of Quantum Dot Concentration on Forster Resonant Energy Transfer in Monodispersed Nanocrystal Quantum Dot Monolayers. Phys. Rev. B 2010, 81, 205316. (31) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Device Topologies. J. Phys. Chem. Lett. 2011, 2, 1879−1887. (32) Chen, H.; Lesnyak, V.; Bigall, N. C.; Gaponik, N.; Eychmüller, A. Self-Assembly of TGA-Capped CdTe Nanocrystals into ThreeDimensional Luminescent Nanostructures. Chem. Mater. 2010, 22, 2309−2314. (33) Jang, E.; Jun, S.; Jang, H.; Lim, J.; Kim, B.; Kim, Y. White-LightEmitting Diodes with Quantum Dot Color Converters for Display Backlights. Adv. Mater. 2010, 22, 3076−3080. (34) Nizamoglu, S.; Demir, H. V. Foerster Resonance Energy Transfer Enhanced Color-Conversion Using Colloidal Semiconductor Quantum Dots for Solid State Lighting. Appl. Phys. Lett. 2009, 95, 151111. (35) Pala, I. R.; Arachchige, I. U.; Georgiev, D. G.; Brock, S. L. Reversible Gelation of II−VI Nanocrystals: The Nature of Interparticle Bonding and the Origin of Nanocrystal Photochemical Instability. Angew. Chem., Int. Ed. 2010, 49, 3661−3665.

with J. Feldmann, a DAAD-fellow, and a postdoc with H. Weller at the University of Hamburg. Since 2005, he has been a senior staff scientist at the TU Dresden. Anne-Kristin Herrmann studied Chemistry at the Technical University of Dresden and received her Master of Science in 2010. Presently, she works as a Ph.D. student in the group of Professor Eychmü ller, and her research is focused on metallic porous nanostructures. The academic career of Alexander Eychmüller started in Göttingen (Ph.D., MPI for Biophysical Chemistry, K.H. Grellmann) and continued at UCLA (postdoc with M. A. El-Sayed), Berlin (HMI with A. Henglein), and the University of Hamburg (with H. Weller). Since 2005, he has held a chair in Physical Chemistry/Electrochemistry at the TU Dresden. http://www.chm.tu-dresden.de/pc2/

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by the German Science Foundation (DFG) under Projects EY16/10-1 and EY16/10-2. A.-K.H. is grateful to the DFG under GRK 1401 for financial support. N.G. acknowledges an IKERBASQUE Fellowship from The Basque Foundation of Science, Spain.

(1) Hüsing, N.; Schubert, U. Aerogels  Airy Materials: Chemistry, Structure, and Properties. Angew. Chem., Int. Ed. 1998, 37, 22−45. (2) Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741. (3) Gesser, H. D.; Goswami, P. C. Aerogels and Related Porous Materials. Chem. Rev. 1989, 89, 765−788. (4) Eychmüller, A. Aerogels from Semiconductor Nanomaterials. Angew. Chem., Int. Ed. 2005, 44, 4839−4841. (5) Brock, S. L.; Yu, H., Chalcogenide Aerogels. In Aerogels Handbook, 1st ed.; Aegerter, M. A., Leventis, N., Koebel, M. M., Eds. Springer: New York, 2011. (6) Kalebaila, K. K.; Georgiev, D. G.; Brock, S. L. Synthesis and Characterization of Germanium Sulfide Aerogels. J. Non-Cryst. Solids 2006, 352, 232−240. (7) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.; Kanatzidis, M. G. Porous Semiconducting Gels and Aerogels from Chalcogenide Clusters. Science 2007, 317, 490−493. (8) Mohanan, J. L.; Arachchige, I. U.; Brock, S. L. Porous Semiconductor Chalcogenide Aerogels. Science 2005, 307, 397−400. (9) Mohanan, J. L.; Brock, S. L. A New Addition to the Aerogel Community: Unsupported CdS Aerogels with Tunable Optical Properties. J. Non-Cryst. Solids 2004, 350, 1−8. (10) Tewari, P. H.; Hunt, A. J.; Lofftus, K. D. Ambient-Temperature Supercritical Drying of Transparent Silica Aerogels. Mater. Lett. 1985, 3, 363−367. (11) Arachchige, I. U.; Brock, S. L. Sol−Gel Assembly of CdSe Nanoparticles to Form Porous Aerogel Networks. J. Am. Chem. Soc. 2006, 128, 7964−7971. (12) Arachchige, I. U.; Brock, S. L. Highly Luminescent QuantumDot Monoliths. J. Am. Chem. Soc. 2007, 129, 1840−1841. (13) Brock, S. L.; Arachchige, I. U.; Kalebaila, K. K. Metal Chalcogenide Gels, Xerogels and Aerogels. Comments Inorg. Chem. 2006, 27, 103−126. (14) Arachchige, I. U.; Brock, S. L. Sol−Gel Methods for the Assembly of Metal Chalcogenide Quantum Dots. Acc. Chem. Res. 2007, 40, 801−809. (15) Yao, Q.; Arachchige, I. U.; Brock, S. L. Expanding the Repertoire of Chalcogenide Nanocrystal Networks: Ag2Se Gels and Aerogels by Cation Exchange Reactions. J. Am. Chem. Soc. 2009, 131, 2800−2801. (16) Gill, S.; Brown, P.; Hope-Weeks, L. Gold Modified Cadmium Sulfide Aerogels. J. Sol−Gel Sci. Technol. 2011, 57, 68−75. 15



Perspective

(36) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmüller, A.; Rakovich, Y. P.; Donegan, J. F. Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals: State-of-theArt. J. Phys. Chem. C 2007, 111, 14628−14637. (37) Pawsey, S.; Kalebaila, K. K.; Moudrakovski, I.; Ripmeester, J. A.; Brock, S. L. Pore Structure and Interconnectivity of CdS Aerogels and Xerogels by Hyperpolarized Xenon NMR. J. Phys. Chem. C 2010, 114, 13187−13195. (38) Yu, H.; Liu, Y.; Brock, S. L. Tuning the Optical Band Gap of Quantum Dot Assemblies by Varying Network Density. ACS Nano 2009, 3, 2000−2006. (39) Yu, H.; Bellair, R.; Kannan, R. M.; Brock, S. L. Engineering Strength, Porosity, and Emission Intensity of Nanostructured CdSe Networks by Altering the Building-Block Shape. J. Am. Chem. Soc. 2008, 130, 5054−5055. (40) Yu, H.; Brock, S. L. Effects of Nanoparticle Shape on the Morphology and Properties of Porous CdSe Assemblies (Aerogels). ACS Nano 2008, 2, 1563−1570. (41) Armatas, G. S.; Kanatzidis, M. G. Size Dependence in Hexagonal Mesoporous Germanium: Pore Wall Thickness Versus Energy Gap and Photoluminescence. Nano Lett. 2010, 10, 3330−3336. (42) Lin, Y. M.; Dresselhaus, M. S. Thermoelectric Properties of Superlattice Nanowires. Phys. Rev. B 2003, 68, 075304. (43) Kanatzidis, M. G. Nanostructured Thermoelectrics. The New Paradigm? Chem. Mater. 2010, 22, 648−659. (44) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features. Adv. Mater. 2010, 22, 3970−3980. (45) Ganguly, S.; Brock, S. L. Toward Nanostructured Thermoelectrics: Synthesis and Characterization of Lead Telluride Gels and Aerogels. J. Mater. Chem. 2011, 21, 8800−8806. (46) Scheele, M.; Oeschler, N.; Meier, K.; Kornowski, A.; Klinke, C.; Weller, H. Synthesis and Thermoelectric Characterization of Bi2Te3 Nanoparticles. Adv. Funct. Mater. 2009, 19, 3476−3483. (47) Scheele, M.; Oeschler, N.; Veremchuk, I.; Reinsberg, K.-G.; Kreuziger, A.-M.; Kornowski, A.; Broekaert, J.; Klinke, C.; Weller, H. ZT Enhancement in Solution-Grown Sb(2−x)BixTe3 Nanoplatelets. ACS Nano 2010, 4, 4283−4291. (48) Dirmyer, M. R.; Martin, J.; Nolas, G. S.; Sen, A.; Badding, J. V. Thermal and Electrical Conductivity of Size-Tuned Bismuth Telluride Nanoparticles. Small 2009, 5, 933−937. (49) Dong, W.; Rhine, W.; Caggiano, G.; Evans, O. R.; Gould, G.; White, J.; Sharp, J.; Gilbert, P.; Ganguly, S.; Brock, S. L. Characterization of Bismuth Telluride Aerogels for Thermoelectric Applications. MRS Online Proc. Libr. 2011, 1306, mrsf10-1306-bb12-02. (50) Campbell, C. T. Physics: The Active Site in Nanoparticle Gold Catalysis. Science 2004, 306, 234−235. (51) Jiang, Y.; Gao, Q. Heterogeneous Hydrogenation Catalysis over Recyclable Pd(0) Nanoparticle Catalysts Stabilized by PAMAM-SBA15 Organic-Inorganic Hybrid Composites. J. Am. Chem. Soc. 2006, 128, 716−717. (52) Pajonk, G. M. Catalytic Aerogels. Catal. Today 1997, 35, 319− 337. (53) Vallribera, A.; Molins, E. Aerogel Supported Nanoparticles in Catalysis. Nanopart. Catal. 2008, 161−194. (54) Anderson, K.; Cortinas Fernandez, S.; Hardacre, C.; Marr, P. C. Preparation of Nanoparticulate Metal Catalysts in Porous Supports Using an Ionic Liquid Route; Hydrogenation and C−C Coupling. Inorg. Chem. Commun. 2003, 7, 73−76. (55) Lu, L.; Capek, R.; Kornowski, A.; Gaponik, N.; Eychmüller, A. Selective Fabrication of Ordered Bimetallic Nanostructures with Hierarchical Porosity. Angew. Chem., Int. Ed. 2005, 44, 5997− 6001. (56) Bigall, N. C.; Reitzig, M.; Naumann, W.; Simon, P.; van Pee, K.-H.; Eychmüller, A. Fungal Templates for Noble-Metal Nanoparticles and Their Application in Catalysis. Angew. Chem., Int. Ed. 2008, 47, 7876−7879. (57) Warren, S. C.; Messina, L. C.; Slaughter, L. S.; Kamperman, M.; Zhou, Q.; Gruner, S. M.; DiSalvo, F. J.; Wiesner, U. Ordered

Mesoporous Materials from Metal Nanoparticle−Block Copolymer Self-Assembly. Science 2008, 320, 1748−1752. (58) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Bäumer, M. Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature. Science 2010, 327, 319−322. (59) Joseph, Y.; Guse, B.; Nelles, G. Aging of 1,ω-Alkyldithiol Interlinked Au Nanoparticle Networks. Chem. Mater. 2009, 21, 1670− 1676. (60) Zhou, Y.; Kogiso, M.; Shimizu, T. Necklace-Like Chains of Hybrid Nanospheres Consisting of Pd Nanocrystals and Peptidic Lipids. J. Am. Chem. Soc. 2009, 131, 2456−2457. (61) Viswanath, B.; Patra, S.; Munichandraiah, N.; Ravishankar, N. Nanoporous Pt with High Surface Area by Reaction-Limited Aggregation of Nanoparticles. Langmuir 2009, 25, 3115−3121. (62) Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.; Zheng, N. Freestanding Palladium Nanosheets with Plasmonic and Catalytic Properties. Nat. Nano 2011, 6, 28−32. (63) Banhart, J. Manufacture, Characterization and Application of Cellular Metals and Metal Foams. Prog. Mater. Sci. 2001, 46, 559−632. (64) Tappan, B. C.; Steiner, S. A. III; Luther, E. P. Nanoporous Metal Foams. Angew. Chem., Int. Ed. 2010, 49, 4544−4565. (65) Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Colloidal Synthesis and Self-Assembly of CoPt3 Nanocrystals. J. Am. Chem. Soc. 2002, 124, 11480−11485. (66) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Electrostatic Self-Assembly of Binary Nanoparticle Crystals with a Diamond-Like Lattice. Science 2006, 312, 420−424. (67) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Structural Diversity in Binary Nanoparticle Superlattices. Nature 2006, 439, 55−59. (68) Fan, H.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lopez, G. P.; Brinker, C. J. Self-Assembly of Ordered, Robust, Three-Dimensional Gold Nanocrystal/Silica Arrays. Science 2004, 304, 567−571. (69) Klajn, R.; Bishop, K. J. M.; Fialkowski, M.; Paszewski, M.; Campbell, C. J.; Gray, T. P.; Grzybowski, B. A. Plastic and Moldable Metals by Self-Assembly of Sticky Nanoparticle Aggregates. Science 2007, 316, 261−264. (70) Klajn, R.; Gray, T. P.; Wesson, P. J.; Myers, B. D.; Dravid, V. P.; Smoukov, S. K.; Grzybowski, B. A. Bulk Synthesis and Surface Patterning of Nanoporous Metals and Alloys from Supra Spherical Nanoparticle Aggregates. Adv. Funct. Mater. 2008, 18, 2763−2769. (71) Bigall, N. C.; Herrmann, A.-K.; Vogel, M.; Rose, M.; Simon, P.; Carrillo-Cabrera, W.; Dorfs, D.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Hydrogels and Aerogels from Noble Metal Nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 9731−9734. (72) Bigall, N. C.; Eychmüller, A. Synthesis of Noble Metal Nanoparticles and Their Non-Ordered Superstructures. Philos. Trans. R. Soc. London, Ser. A 2010, 368, 1385−1404. (73) Schroers, J.; Veazey, C.; Johnson, W. L. Amorphous Metallic Foam. Appl. Phys. Lett. 2003, 82, 370−372. (74) Cao, Y. W. C.; Jin, R.; Mirkin, C. A. Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection. Science 2002, 297, 1536−1540. (75) Liu, R.; Liu, J.-f.; Yu, S.-j.; Liu, Q.; Jiang, G.-b. Capping Agent Replacement Induced Self-Organization of Ultrathin Nanowires: A New and General Approach for Fabricating Noble Metal Nanoporous Films with Small Ligament Sizes. Chem. Commun. 2011, 47, 1613− 1615. (76) Krishna, K. S.; Sandeep, C. S. S.; Philip, R.; Eswaramoorthy, M. Mixing Does the Magic: A Rapid Synthesis of High Surface Area Noble Metal Nanosponges Showing Broadband Nonlinear Optical Response. ACS Nano 2010, 4, 2681−2688. (77) Voitekhovich, S. V.; Talapin, D. V.; Klinke, C.; Kornowski, A.; Weller, H. CdS Nanoparticles Capped with 1-Substituted 5Thiotetrazoles: Synthesis, Characterization, and Thermolysis of the Surfactant. Chem. Mater. 2008, 20, 4545−4547. 16



Perspective

(78) Nichick, M. N.; Voitekhovich, S. V.; Lesnyak, V.; Matulis, V. E.; Zheldakova, R. A.; Lesnikovich, A. I.; Ivashkevich, O. A. 1-Substituted Tetrazole-5-Thiol-Capped Noble Metal Nanoparticles. J. Phys. Chem. C 2011, 115, 16928−16933. (79) Nichick, M. N.; Voitekhovich, S. V.; Shavel, A.; Lesnikovich, A. I.; Ivashkevich, O. A. 1-Substituted 5-Thiotetrazoles as Novel Capping Agents for Stabilization of Gold Nanoparticles. Polyhedron 2009, 28, 3138−3142. (80) Gaponik, P. N.; Voitekhovich, S. V.; Ivashkevich, O. A. Metal Derivatives of Tetrazoles. Russ. Chem. Rev. 2006, 75, 507−539. (81) Andrews, P. C.; Junk, P. C.; Massi, M.; Silberstein, M. Gelation of La(III) Cations Promoted by 5-(2-Pyridyl)Tetrazolate and Water. Chem. Commun. 2006, 3317−3319. (82) Lesnyak, V.; Voitekhovich, S. V.; Gaponik, P. N.; Gaponik, N.; Eychmüller, A. CdTe Nanocrystals Capped with a Tetrazolyl Analogue of Thioglycolic Acid: Aqueous Synthesis, Characterization, and MetalAssisted Assembly. ACS Nano 2010, 4, 4090−4096. (83) Lesnyak, V.; Wolf, A.; Dubavik, A.; Borchardt, L.; Voitekhovich, S. V.; Gaponik, N.; Kaskel, S.; Eychmüller, A. 3D Assembly of Semiconductor and Metal Nanocrystals: Hybrid CdTe/Au Structures with Controlled Content. J. Am. Chem. Soc. 2011, 133, 13413−13420. (84) Ma, X.; Fletcher, K.; Kipp, T.; Grzelczak, M. P.; Wang, Z.; Guerrero-Martínez, A. s.; Pastoriza-Santos, I.; Kornowski, A.; LizMarzán, L. M.; Mews, A. Photoluminescence of Individual Au/CdSe Nanocrystal Complexes with Variable Interparticle Distances. J. Phys. Chem. Lett. 2011, 2, 2466−2471. (85) Lunz, M.; Gerard, V. A.; Gun’ko, Y. K.; Lesnyak, V.; Gaponik, N.; Susha, A. S.; Rogach, A. L.; Bradley, A. L. Surface Plasmon Enhanced Energy Transfer between Donor and Acceptor CdTe Nanocrystal Quantum Dot Monolayers. Nano Lett. 2011, 11, 3341− 3345. (86) Ozel, T.; Nizamoglu, S.; Sefunc, M. A.; Samarskaya, O.; Ozel, I. O.; Mutlugun, E.; Lesnyak, V.; Gaponik, N.; Eychmüller, A.; Gaponenko, S. V.; Demir, H. V. Anisotropic Emission from Multilayered Plasmon Resonator Nanocomposites of Isotropic Semiconductor Quantum Dots. ACS Nano 2011, 5, 1328−1334. (87) Matsumoto, Y.; Kanemoto, R.; Itoh, T.; Nakanishi, S.; Ishikawa, M.; Biju, V. Photoluminescence Quenching and Intensity Fluctuations of CdSe−ZnS Quantum Dots on an Ag Nanoparticle Film. J. Phys. Chem. C 2008, 112, 1345−1350. (88) Achermann, M. Exciton−Plasmon Interactions in Metal− Semiconductor Nanostructures. J. Phys. Chem. Lett. 2010, 1, 2837− 2843. (89) Gaponik, N. Assemblies of Thiol-Capped Nanocrystals as Building Blocks for Use in Nanotechnology. J. Mater. Chem. 2010, 20, 5174−5181. (90) Leventis, N.; Sotiriou-Leventis, C.; Zhang, G.; Rawashdeh, A.-M. M. Nanoengineering Strong Silica Aerogels. Nano Lett. 2002, 2, 957−960. (91) Leventis, N. Three-Dimensional Core−Shell Superstructures: Mechanically Strong Aerogels. Acc. Chem. Res. 2007, 40, 874−884. (92) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (93) Gaponik, N.; Hickey, S. G.; Dorfs, D.; Rogach, A. L.; Eychmü ller, A. Progress in the Light Emission of Colloidal Semiconductor Nanocrystals. Small 2010, 6, 1364−1378.

17


Colloidal Nanocrystal-Based Gels and Aerogels: Material Aspects and

Recommend Documents