Showing papers by "Peidong Yang published in 2000"

Microemulsion Templating of Siliceous Mesostructured Cellular Foams with Well-Defined Ultralarge Mesopores

Abstract: Siliceous mesostructured cellular foams (MCFs) with well-defined ultralarge mesopores and hydrothermally robust frameworks are described. The MCFs are templated by oil-in-water microemulsions and are characterized by small-angle X-ray scattering, nitrogen sorption, transmission electron microscopy, scanning electron microscopy, thermogravimetry, and differential thermal analysis. The MCFs consist of uniform spherical cells measuring 24−42 nm in diameter, possess BET surface areas up to 1000 m2/g and porosities of 80−84%, and give, because of their pores with small size distributions, higher-order scattering peaks even in the absence of long-range order. Windows with diameters of 9−22 nm and narrow size distribution interconnect the cells. The pore size can be controlled by adjusting the amount of the organic swelling agent that is added and by varying the aging temperature. Adding ammonium fluoride selectively enlarges the windows by 50−80%. In addition, the windows can be enlarged by postsynthesis treatm...

Mirrorless lasing from mesostructured waveguides patterned by soft lithography

Abstract: Mesostructured silica waveguide arrays were fabricated with a combination of acidic sol-gel block copolymer templating chemistry and soft lithography. Waveguiding was enabled by the use of a low-refractive index (1.15) mesoporous silica thin film support. When the mesostructure was doped with the laser dye rhodamine 6G, amplified spontaneous emission was observed with a low pumping threshold of 10 kilowatts per square centimeter, attributed to the mesostructure's ability to prevent aggregation of the dye molecules even at relatively high loadings within the organized high-surface area mesochannels of the waveguides. These highly processible, self-assembling mesostructured host media and claddings may have potential for the fabrication of integrated optical circuits.

Germanium Nanowire Growth via Simple Vapor Transport

Abstract: Germanium nanowires are synthesized in bulk quantities and high purity using a simple vapor transport process. X-ray diffraction, scanning electron microscopy and transmission electron microscopy studies show that these wires are single crystalline with [111] growth direction; they have diameters ranging from 5 to 300 nm, and lengths up to hundreds micrometer. With brief vacuum thermal treatment, the average diameters of the wires can be further reduced to 16 nm.

Hexagonal to Mesocellular Foam Phase Transition in Polymer-Templated Mesoporous Silicas

Abstract: We have investigated the phase transition between two distinct mesoporous silicas: SBA-15, comprising a hexagonally packed arrangement of cylindrical pores (6−12 nm in diameter), and mesocellular silica foams (MCF), consisting of spherical voids (22−42 nm in diameter) interconnected by “windows” of ∼10 nm. Both SBA-15 and MCF are formed using an amphiphilic triblock copolymer (Pluronic P123) as a template. The synthesis conditions for the two materials are identical, except substantial trimethylbenzene is added to form MCF. We find that the phase transition occurs at an oil−polymer mass ratio of 0.2−0.3. Although the pore structures and pore sizes change dramatically, the mean surface curvature of the system remains essentially the same throughout the transition.

Microchannel Networks for Nanowire Patterning

Abstract: One-dimensional (1D) nanostructures, such as nanowires, nanotubes, and molecular wires are currently being investigated in great detail for their unique electronic and mechanical properties and their potential implementation as devices.1-4 Integration of the nanotubes and nanowires into useful devices requires placing them in specific positions with desired configurations reproducibly.1,4 This, however, remains to be a major challenge in the field. We describe a strategy for aligning and patterning conductive [Mo3Se3]∞ nanowires on substrates using microchannel networks.5 This strategy relies on the solvent evaporation induced self-assembly of [Mo3Se3]∞ infinite chains within the microchannels. Aligned [Mo3Se3]∞ wires6,7 as thin as several nanometers can be readily patterned using micrometer-sized channels. Multilevel cross-bar junction configurations and nano-to-macroscale connections were also demonstrated. Optical microscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) studies, and electrical measurements show that these wires are highly oriented, crystalline, and conductive. The current process in constructing and measuring nanotube or other nanowire devices has been mainly involved the deposition on prefabricated electrodes in hope of getting tubes/wires at the right place and configuration.1 Atomic force microscope (AFM) has also been used to push or deposit nanotubes into desired configurations.8,9 Unfortunately, these device fabrication methods have the intrinsic limitations of being highly serendipitous or timeconsuming. We wish to develop simple and parallel methods for this purpose using soft lithography. Soft lithography has been extensively used to create microstructures with lateral dimensions of 30 nm to 500 μm.5 The patterned materials include metals, polymers, colloids, proteins,10 and ceramics.5,11 Generally, the patterned feature size reflects the actual dimension of the surface features on the micromolds. Here we show [Mo3Se3]∞ molecular wires can be patterned on a nanometer scale using micromolds having micrometer-sized channel network. [Mo3Se3]∞ molecular wires, 6 A in diameter, are infinite chains of polycondensed Mo6Se8 polyhedra.7 They have been experimentally determined to be highly conductive with a resistivity of 10-2-10-4 Ω‚cm at room temperature.6,12 Individual [Mo3Se3]∞ molecular wires can be obtained as a solution (10-4-10-6 M) by dissolving single crystals of LiMo3Se3 in the polar solvent dimethyl sulfoxide or N-methylformamide.7 Microchannel networks were formed between a poly(dimethylsiloxane) (PDMS) micromold and a silicon/glass substrate. The microchannels have variable height of 1-4 μm, width of 1-10 μm, and length of 5-10 mm. A droplet (∼0.1-10 μL) of the wire solution was placed at the open end of the microchannels, and the channels were filled within minutes via capillary action. The solution-filled microchannel network was placed in a vacuum to evaporate the solvent. We found that molecular wires tend to form nanowire bundles and align along the corners of the microchannels upon solvent evaporation and PDMS micromold removal, leaving an otherwise clean contact area. Cross-polarized optical microscopy studies indicate the wires are continuous up to several millimeters and are well aligned (Figure 1a). FESEM studies (Figure 1b-d) show the width of nanowires varies from 10 to 200 nm, which is largely determined by the molecular wire solution concentration and the microchannel volume. This feature size is significantly smaller than the microchannel size. Each nanowire generally consists of several thinner wires with diameter of ∼10 nm or less (Figure 1c). Double helix formation between these nanowires was (1) Fuhrer, M.; Nygard, J.; Shih, L.; Forero, M.; Yoon, Y.; Mazzoni, M. S. C.; Choi, H. J.; Ihm, J.; Louie, S. G.; Zettl, A, McEuen, P. L. Science 2000, 288, 494-496. (2) Collier, C. P.; Wong, E. W.; Behloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391394. (3) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435446. (4) Rueches, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94-97. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (6) (a) Venkataraman, L.; Lieber, C. M. Phys. ReV. Lett. 1999, 83, 53345336. (b) Venkataraman, L. Thesis, Harvard University, 1999. (7) Tarason, J. M.; DiSalvo, F. J.; Cheve, C. H.; Carroll, J.; Walsh, M.; Rupp, L. J. Solid State Chem. 1985, 58, 290-300. (8) Lefebvre, J.; Lymch, J. F.; Llaguno, M.; Radosavljevic, M.; Johnson, A. T. Appl. Phys. Lett. 1999, 75, 3014-3016. (9) Cheung, C. L.; Hafner, J. H.; Odom, T. W.; Kim, K.; Lieber, C. M. Appl. Phys. Lett. 2000, 76, 3136-3138. (10) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H., R.; Delamarche, E. AdV. Mater. 2000, 12, 1067-1070. (11) (a) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. AdV. Mater. 1999, 11, 1433-1437. (b) Aizenberg, J.; Braun, P. V.; Wiltzus, P. Phys. ReV. Lett. 2000, 84, 2997-3000. (c) Aizenberg, J.; Black, A. J. J.; Whitesides, G. M. Nature 1999, 398, 495-498. (12) Golden, J. H.; DiSalvo, F. J.; Frechet, J. M. J.; Silcox, J.; Thomas, M.; Elman, J. Science 1996, 273, 782-785. Figure 1. (a) Cross-polarized optical image of nanowire patterns on glass substrate. (b-d) FESEM images of wire arrays on silicon substrate. Arrow in c indicates the thinner wires are pulling together to form a thicker bundle. 10232 J. Am. Chem. Soc. 2000, 122, 10232-10233

Germanium/carbon core–sheath nanostructures

Abstract: Germanium/carbon core–sheath nanostructures and junctions are produced when Ge nanowires are subject to a thermal treatment in an organic vapor doped vacuum. The organic molecules pyrolyze on the surface of the Ge nanowires and form a continuous graphitic coating. The carbon-sheathed Ge nanowires undergo melting and evaporation at high temperature, which results in the formation of germanium/carbon junctions. These core–sheath nanostructures and junctions generally have diameters of 5–100 nm, sheath thickness of 1–5 nm, and lengths up to several micrometers. This process may prove to be an effective approach to prevent the nanowire surface oxidation and create nanowires with chemically inert surface.

Surfactant‐Induced Mesoscopic Assemblies of Inorganic Molecular Chains

Abstract: CdCl2 or ZnCl2. The jars were sealed and kept static at room temperature in the dark for up to 7 days. Then the precipitates were filtered and washed with absolute alcohol and distilled water, then dried in vacuum at 20 C. After extracting the organic phase, post thermal treatments were performed by heating the water phase in a water bath at 60 C for an hour with magnetic stirring. X-ray powder diffraction (XRD) measurements of the as-prepared sample were carried on a Rigaku D/max-cA X-ray diffractometer with Cu Ka radiation (k = 1.54178 Š). The TEM images and ED pattern were taken on a Hitachi H800 transmission electron microscope with an accelerating voltage of 200 kV. HRTEM images were taken on a JEOL-2010 transmission electron microscope.

Germanium Nanowire Growth via Simple Vapor Transport.

Abstract: Germanium nanowires are synthesized in bulk quantities and high purity using a simple vapor transport process. X-ray diffraction, scanning electron microscopy and transmission electron microscopy studies show that these wires are single crystalline with [111] growth direction; they have diameters ranging from 5 to 300 nm, and lengths up to hundreds micrometer. With brief vacuum thermal treatment, the average diameters of the wires can be further reduced to 16 nm.

Ag Nanowire Formation within Mesoporous Silica.

Abstract: Uniform Ag nanowires have been synthesized within nanoscale channels of mesoporous silica SBA-15 by a simple chemical approach, which involves AgNO3 impregnation, followed by thermal decomposition.