Indika U. Arachchige

Porous Semiconductor Chalcogenide Aerogels

Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepared from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercritical solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepared as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chemical or photochemical oxidation, and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.

Expanding the Repertoire of Chalcogenide Nanocrystal Networks: Ag2Se Gels and Aerogels by Cation Exchange Reactions

Cation exchange is shown to be a simple and efficient method to prepare nanostructured Ag(2)Se gels and aerogels from CdSe gel precursors. Treatment of CdSe wet gels with AgNO(3) yields, for the first time, Ag(2)Se gels, and these are transformed to high surface area, nanostructured aerogels by supercritical fluid drying. The striking similarity in crystallite size, morphology, and surface area characteristics between CdSe and the corresponding Ag(2)Se aerogels supports a mechanism in which the bonding within the gel network remains globally unchanged, even as the structural attributes of the primary particle components are undergoing a dramatic transformation (hexagonal CdSe to cubic Ag(2)Se). Intriguingly, the rapid exchange also enables exquisite control of composition on the macroscale; reduced concentrations of Ag(+) lead to two-component (i.e., CdSe inside, Ag(2)Se outside) heterogeneous structures of mm-cm dimensions. Overall, this methodology offers a simple approach for the generation of porous nanocrystalline metal chalcogenide networks of known or new compositions.

Reversible Gelation of II–VI Nanocrystals: The Nature of Interparticle Bonding and the Origin of Nanocrystal Photochemical Instability

Semiconducting nanocrystals (NCs) with dimensions smaller than the bulk exciton Bohr radius exhibit unique, size-tunable opto-electronic properties due to quantum confinement effects. Accordingly, there has been tremendous interest in the synthesis and characterization of colloidal semiconductor NCs, and they have been investigated for a variety of applications ranging from biological labeling and diagnostics to photovoltaics, photodetectors, sensors, and catalysts. However, the stability of colloidal NCs is a major issue in many of these applications. The most common method of stabilization is by chemically attaching ligands to the surface atoms of the NCs. Consequently, semiconducting NCs are generally synthesized in the presence of coordinating surfactant ligands, for example, trioctylphosphine oxide, which confers dispersibility in non-polar media. For biological applications, dispersibility in polar media (i.e. water) is needed and this is often achieved by substitution of nonpolar surfactant ligands with carboxylate-terminated thiolates by treatment with mercaptoundecanoic acid, mercaptoacetic acid, dihydrolipoic acid, etc., in base. However, the stability of thiolate coated II–VI semiconductor NCs, such as CdSe, is typically poor, often leading to precipitation. In 2001, Peng and co-workers reported a detailed study of the photochemical instability of thiolate-capped CdSe NCs. The photooxidation of the thiolate ligands on the NCs can be catalyzed by the CdSe NCs in the presence of light and O2, producing disulfides and thus, effectively decomplexing the particle en route to aggregation. However, if free thiols are present in the solution, they can replace the thiolates lost as disulfide, dispersing aggregates as they form and prolonging the stability of the sol. We and others have exploited oxidative removal of the thiolates as a means to link particles together into three-dimensional architectures (gels), in which metal chalcogenide NCs are assembled into porous network structures. Importantly, analyses of oxidized CdS sols have shown that the thiols and their oxidized products (disulfides and sulfonates) can be completely removed from the gel network, and are therefore not participating in interparticle bonding, which leads to the conclusion that CdS NCs are physically connected to each other without any organic linkers. The extent of particle interaction in the network (and the related extent of quantum confinement) is found to be a direct function of the dimensionality of the network, itself controlled by the density. Such architectures are of interest for applications requiring maximal transport of charge (through the gel network) and small molecules (through the interconnected pore network), such as sensing and photocatalysis. Here we show that, analogous to the work of Peng et al., thiolates can be employed to break up the gel network into its constituent NCs. Additionally, for the first time, we probe the properties of the interparticle bonding in CdSe gels and aggregates. Dispersion studies were performed principally with CdSe gels, aerogels and xerogels prepared from oxidation of high temperature prepared NCs that were capped with 11-mercaptoundecanoic acid (MUA) in the presence of base (tetramethylammonium hydroxide (TMAH)). Treatment of CdSe wet gels with fresh methanolic solutions of MUA and TMAH (pH 12.0) results in formation of a sol that is visually identical to the precursor NCs (Figure 1) in 2–3 min. Like-

Oxidation-Induced Self-Assembly of Ag Nanoshells into Transparent and Opaque Ag Hydrogels and Aerogels

The synthesis of hollow Ag nanoshells (NSs) with tunable plasmon bands in the visible spectrum and their oxidative-assembly into high-surface-area, mesoporous, transparent, and opaque Ag gel frameworks is reported. Thiolate-coated Ag NSs with varying size and shell thickness were prepared by fast chemical reduction of preformed Ag2O nanoparticles (NPs). These NSs were assembled into monolithic Ag hydrogels via oxidative removal of the surface thiolates, followed by CO2 supercritical drying to produce metallic Ag aerogels. The gelation kinetics have been controlled by tuning the oxidant/thiolate molar ratio (X) that governs the rate of NP condensation, which in turn determines the morphology, optical transparency, opacity, surface area, and porosity of the resultant gel frameworks. The monolithic Ag hydrogels prepared using high concentration of oxidant (X > 7.7) leads to oxidative etching of precursor colloids into significantly smaller NPs (3.2-7.6 nm), which appeared to eliminate the visible light scattering yielding transparent gel materials. In contrast, the opaque Ag aerogels composed entirely of hollow NSs exhibit enormously high surface areas (45-160 m(2)/g), interconnected meso-to-macro-pore network that can be tuned by varying the inner cavity of Ag colloids, and accessibility of chemical species to both inner and outer surface of the hollows, offering perspectives for a number of new technologies. An advantage of current synthesis is the ability to transform Ag NSs into monolithic hydrogels within 4-12 h, which otherwise is reported to require weeks to months for the oxidation-induced metallic gel synthesis reported to date.

Anomalous Band Gap Evolution from Band Inversion in Pb1−xSnxTe Nanocrystals

We report the synthesis of a series of narrowly disperse Pb(1-x)Sn(x)Te nanocrystals by employing a colloidal synthetic strategy. As synthesized nanocrystals are solid solutions with cubic NaCl-type structure and exhibit band energy gaps in the mid-IR region. We show that these ternary nanocrystals display qualitatively the same anomalous trend in band gaps as a function of x that is attributed to the band inversion phenomenon of the corresponding bulk materials; however unlike the bulk the band gap does not vanish at any Sn concentration but achieves a minimum of 0.28 eV for x = 0.67.

METAL CHALCOGENIDE GELS, XEROGELS AND AEROGELS

Metal chalcogenide gels represent an intriguing class of nanostructured solids that are largely unexplored, in contrast to oxides. This Comment reviews the two synthetic approaches applied for gelation of metal chalcogenides—thiolysis and nanoparticle condensation—and presents a survey of the materials prepared by these strategies. Drying strategies to produce dense xerogels (ambient pressure) and highly porous aerogels (supercritical fluid extraction) are described and the effect of density on the extent of quantum confinement in semiconducting metal chalcogenide gel structures is discussed.

Nanoscale Stabilization of New Phases in the PbTe–Sb2Te3 System: PbmSb2nTem+3n Nanocrystals

A series of novel rock-salt-type Pb(m)Sb(2n)Te(m+3n) nanocrystals (m = 2, 3, 4, 6, 8, and 10; n = 1 and 2) were successfully prepared using a colloidal synthesis route. These materials are stable only on the nanoscale and have no bulk analogues. Elemental compositions were determined using scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The nanocrystals exhibit well-defined band energies in the mid-IR region that are nearly independent of their atomic compositions. Pb(m)Sb(2n)Te(m+3n) nanocrystals behave as metastable homogeneous solid solutions at room temperature and tend to phase separate into the respective binary PbTe + Sb(2)Te(3) at 300 °C. Furthermore, pair distribution function (PDF) analysis suggests that the local structure of these Pb(m)Sb(2n)Te(m+3n) nanocrystals is distorted with respect to the rock-salt structure.

Metal–Semiconductor Hybrid Aerogels: Evolution of Optoelectronic Properties in a Low-Dimensional CdSe/Ag Nanoparticle Assembly

Hybrid nanomaterials composed of metal-semiconductor components exhibit unique properties in comparison to their individual counterparts, making them of great interest for optoelectronic applications. Theoretical and experimental studies suggest that interfacial interactions of individual components are of paramount importance to produce hybrid electronic states. The direct cross-linking of nanoparticles (NPs) via controlled removal of the surfactant ligands provides a route to tune interfacial interactions in a manner that has not been thoroughly investigated. Herein, we report the synthesis of CdSe/Ag heteronanostructures (aerogels) via oxidation induced self-assembly of thiol-coated NPs and the evolution of optical properties as a function of composition. Three hybrid systems were investigated, where the first and second excitonic energies of CdSe were matched with plasmonic energy of Au or Ag NPs and Ag hollow NPs. Physical characterization of the aerogels suggests the presence of an interconnected network of hexagonal CdSe and cubic Ag NPs. The optical properties of hybrids were systematically examined through UV-vis, photoluminescence (PL), and time-resolved (TR) PL spectroscopic studies that indicate the generation of alternate radiative decay pathways. A new emission (640 nm) from CdSe/Ag aerogels emerged at Ag loading as low as 0.27%, whereas absorption band tailing and PL quenching effects were observed at higher Ag and Au loading, respectively. The TRPL decay time of the new emission (∼600 ns) is markedly different from those of the band-edge (1.83 ± 0.03 ns) and trap-state (1190 ± 120 ns) emission maxima of phase pure CdSe, supporting the existence of alternate radiative relaxation pathways in sol-gel derived CdSe/Ag hybrids.

Nanoparticles Change the Ordering Pattern of n-Carboxylic Acids into Nanorods on HOPG

This paper describes the formation of organic nanorods induced by monolayer-protected inorganic nanoparticles. Alkanes and alkane derivatives, such as n-carboxylic acids, self-assemble on highly oriented pyrolytic graphite (HOPG) into a persistent molecular packing structure that is dictated by the epitaxial interaction between the carbon chain plane and the HOPG basal plane. Carboxylic acids form 2-D crystalline layers consisting of nanostripe domains whose periodicity is one or two times the molecular chain length. However, when the molecular ordering occurs in the vicinity of a nanoparticle, this persistent HOPG-dominated nanostripe pattern is disrupted, and nanorods attached to the nanoparticles become the dominant structure. In order to understand the underlying mechanism of the nanoparticle-mediated nanorod formation, the effects of film-forming conditions, carboxylic acid chain length, nanoparticle size, and chemical composition of the nanoparticle are examined. It is determined that carboxylic acid nanorods can be induced by nanoparticles of different core materials including CdSe, CdS, and Au, as long as the protecting monolayer allows sufficient dispersion and colloidal stability of the nanoparticles in solution. A carboxylic chain length range amenable to the nanorod formation is identified, as is the relationship between the nanoparticle size and the number of nanorods per nanoparticle. This study contributes to the understanding of seed-mediated crystallization and molecular ordering. Moreover, it defines the parameters governing solution-based formation of hybrid nanostructures and nanopatterns incorporating dual functionality as defined by the inorganic nanoparticle and organic nanorod, respectively.

Ultra-small Ge1−xSnx quantum dots with visible photoluminescence

Ge1-xSnx alloy quantum dots (QDs) were synthesized with sizes ranging from 1-3 nm exhibiting visible orange-red photoluminescence. Composition dependent optical properties were characterized and supported by theoretical calculations. Structural analysis suggests the QDs are diamond cubic phase, characteristic of Ge1-xSnx thin films and nanocrystals (NCs) reported to date.