Quantum well structured CdSe nanoribbons with uniform and ultrathin thickness of 1.4 nm were synthesized from the low-temperature reaction of CdCl2 and ammonium selenocarbamate. The CdSe nanoribbons exhibited an extremely narrow photoluminescence band with fwhm of 70 meV.

Two-dimensional (2D) nanocrystals have attracted tremendous attention from many researchers in various disciplines because of their unique properties.[1] Since ways of making graphene were devised, [2] there have been significant research efforts to synthesize free-standing 2D nanocrystals of various materials, including metals, [3] oxides, [4] and chalcogenides.[5]Many of these 2D nanocrystals have been generated from exfoliation of materials with layered structures, and tiny amounts of products are generally produced.[6] CdSe nanocrystals are among the most intensively studied nanostructured materials, [7] owing to their many size-dependent optical and electrical characteristics [8] and resulting exciting applications.[9] Herein, we report on the large-scale synthesis of single-layered and lamellar-structured 2D CdSe nanocrystals with wurtzite crystal structure as thin as 1.4 nm by a soft colloidal template method. These free-standing 2D nanocrystals with insulating organic layers at the interface could find many interesting electronic and optoelectronic applications, including in quantum cascade lasers and resonant tunneling diodes utilizing their multiple quantum well structures. [10] Compared to materials with layered crystal structures such as graphite, the synthesis of free-standing 2D nanocrystals of nonlayered materials such as CdSe is extremely challenging, because selective growth along one specific facet among several with similar energies is required. For example, in CdSe with a hexagonal wurtzite crystal structure, a (0001 ) facet has significantly higher surface energy than other facets, which leads to the formation of many one-dimensional nanostructures.[11] Although there is a slight difference in the surface energies of AE (112 0) and AE (11 00) facets, [12] quantum-confined thin CdSe 2D nanocrystals could not be synthesized using a conventional colloidal chemical route that employs thermal decomposition of precursors at high temperature, because the small difference in the surface energies of these two facets is negated by the high reaction temperature. Consequently, there have been only a few reports on the successful chemical synthesis of 2D CdSe nanocrystals.[13] For example, CdSe inorganic-organic hybrid lamellar structures [13b,c] and CdSe nanoplatelets [13d] with zinc-blende structure were synthesized using colloidal chemical routes. However, their 2D growth mechanism has not been clearly elucidated. Furthermore, nanostructural control to form single-layered or multiple-layered nanosheets has not been demonstrated. In the current approach to creating 2D CdSe nanocrystals, we employed a soft template method, [14] and we were able to synthesize not only free-standing single-layered CdSe nanosheets but also lamellar-structured nanosheets by controlling the interaction between organic layers in 2D templates of cadmium chloride alkyl amine complexes. [15] It has been reported that the complex of cadmium halide and diamine can form a cadmium halide / diamine alternating layered structure thr...

Soldering semiconductor nanoparticles The optical and electronic properties of semiconductor nanoparticles can be tuned through changes in their size and composition. However, poor contact between interfaces can degrade nanoparticle performance in devices. Dolzhnikov et al. report the synthesis of a gel-like “solder” for metal chalcogenide nanoparticles, such as cadmium selenide and lead telluride, by cross-linking molecular wires of these materials. Science , this issue p. 425

We herein report on the large-scale synthesis of ultrathin Bi(2)Te(3) nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi(2)Te(3) nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ~1 nm, which corresponds to a single layer in Bi(2)Te(3) crystals. Bi(2)Te(3) nanostructured bulk materials were prepared by sintering of surfactant-removed Bi(2)Te(3) nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi(2)Te(3) nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.

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Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

We synthesized various hollow oxide nanoparticles from as-prepared MnO and iron oxide nanocrystals. Heating metal oxide nanocrystals dispersed in technical grade trioctylphosphine oxide (TOPO) at 300 degrees C for hours yielded hollow nanoparticles retaining the size and shape uniformity of the original nanocrystals. The method was highly reproducible and could be generalized to synthesize hollow oxide nanoparticles of various sizes, shapes, and compositions. Control experiments revealed that the impurities in technical grade TOPO, especially alkylphosphonic acid, were responsible for the etching of metal oxide nanocrystals to the hollow structures. Elemental mapping analysis revealed that the inward diffusion of phosphorus and the outward diffusion of metal took place in the intermediate stages during the etching process. The elemental analysis using XPS, EELS, and EDX showed that the hollow nanoparticles were amorphous metal oxides containing significant amount of phosphorus. The hollow nanoparticles synthesized from MnO and iron oxide nanocrystals were paramagnetic at room temperature and when dispersed in water showed spin relaxation enhancement effect for magnetic resonance imaging (MRI). Because of their morphology and magnetic property, the hollow nanoparticles would be utilized for multifunctional biomedical applications such as the drug delivery vehicles and the MRI contrast agents.