A general, one-pot, single-step method for producing colloidal silver chalcogenide (Ag(2)E; E = Se, S, Te) nanocrystals is presented, with an emphasis on Ag(2)Se. The method avoids exotic chemicals, high temperatures, and high pressures and requires only a few minutes of reaction time. While Ag(2)S and Ag(2)Te are formed in their low-temperature monoclinic phases, Ag(2)Se is obtained in a metastable tetragonal phase not observed in the bulk.

Facile Synthesis of Silver Chalcogenide (Ag 2 E; E: Se, S, Te) Semiconductor Nanocrystals. -Structurally well-defined and nearly monodisperse colloidal Ag2Se nanocrystals are synthesized from tri-n-octylphosphine solutions containing AgNO3, Se, oleic acid, 1-octadecylamine, and 1-octadecene (150°C, about 5 min). Ag2S and Ag2Te nanocrystals are prepared in a similar way. The samples are characterized by TEM, XPS, XRD, and optical absorption spectroscopy. While Ag 2 S and Ag 2 Te form in their low-temperature monoclinic phases, Ag2Se is obtained in a metastable tetragonal phase not observed in the bulk. The preparation method avoids exotic chemicals, high temperatures, high pressures, and requires only a few minutes of reaction time. The prepared materials have unique properties, and the nanocrystals should allow new phenomena and applications to be explored. -(SAHU, A.; QI, L.; KANG, M. S.; DENG, D.; NORRIS*, D. J.; J. Am. Chem. Soc. 133 (2011) 17, 6509-6512, http://dx.doi.org/10.1021/ja200012e ; Opt. Mater. Eng. Lab., ETH, CH-8092 Zuerich, Switz.; Eng.) -W. Pewestorf 32-204

We report the use of aerodynamic lenses to deposit micropatterns of colloidal semiconductor nanocrystals (or quantum dots). CdSe and CdSe/ZnS core/shell nanocrystals, with core diameters of 3.5-5 nm, were dispersed in hexane and then nebulized to generate agglomerates a few tens of nm in diameter, consisting of hundreds of nanocrystals. These agglomerates were then focused aerodynamically by a lens system. Microscale towers, lines, and patterns were deposited on thin sapphire plates and silicon wafers. The heights and widths of the deposits were adjustable by varying the experimental parameters. Line widths below 10 µm at fullwidth half-maximum are demonstrated. Upon exposure to near-UV illumination, these deposits exhibited robust fluorescence in the visible, with the color depending on the diameter of the individual nanocrystal cores. A red shift of the photoluminescence peaks from the nanocrystal dispersion to the glassy solid deposits was also observed, which confirmed that the deposits consist of closely packed nanocrystals embedded in the organic matrices. This approach, which is not restricted to semiconductor nanocrystals, provides an alternative means for the deposition of microscale patterns of colloidal nanoparticles.

We use aerosol techniques to investigate the cohesive and granular properties of solids composed of colloidal semiconductor nanocrystals (quantum dot solids). We form spherical agglomerates of nanocrystals with a nebulizer and direct them toward a carbon substrate at low (~0.01 m/s) or high (~100 m/s) velocities. We then study the morphology of the deposit (i.e., the "splat") after impact. By varying the size of the agglomerate and the spacing between the nanocrystals within it, we observe influences on the mechanical properties of the quantum dot solid. We observe a liquid-to-solid transition as the nanocrystals become more densely packed. Agglomerates with weakly interacting nanocrystals exhibit liquidlike splashing and coalescence of overlapping splats. More dense agglomerates exhibit arching and thickening effects, which is behavior typical of granular materials.

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