Nanoscale structures need to be arranged into well-defined configurations in order to build integrated systems. Here we use a chemical-vapour deposition method with gas-phase catalyst delivery to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements. The preference of nanotubes to grow selectively on and normal to silica surfaces forces them to inherit the lithographically machined template topography of their substrates, allowing the sites of nucleation and the direction of growth to be controlled.

No abstract

We demonstrate a new, room-temperature approach to assemble two-dimensional and three-dimensional networks of gold nanowires by agitating nanoparticles in a toluene-aqueous mixture, without the use of templates. The nanowires have a uniform diameter of about 5 nm and consist of coalesced face-centered cubic nanocrystals. Toluene molecules passivate the gold surfaces during nanoparticle coalescence, rendering the nanowires hydrophobic and enabling their transfer into the toluene layer. Such templateless low-temperature assembly of mesostructures from nanoscale building blocks open up new possibilities for creating porous self-supporting nanocatalysts, nanowires for device interconnection, and low-density high-strength nanofillers for composites.

High-efficiency, solid-state thermoelectric energy conversion requires materials with a large thermoelectric figure of merit (ZT), defined as [1] rS 2 T/j, in which S is the Seebeck coefficient, r the electrical conductivity, j the thermal conductivity, and T the absolute temperature. The state of the art materials for thermoelectric cooling applications are alloys based on Bi 2 Te 3 /Sb 2 Te 3 and Bi 2 Te 3 /Bi 2 Se 3 with ZT ∼ 1, while a value of ZT ∼ 4 is necessary to surpass competing technologies.[2] Nanostructuring these thermoelectric materials has recently emerged as a successful strategy to gain factorial enhancements in ZT, [3][4][5][6] owing to quantum and classical size effects of the charge and heat carriers, respectively. [2,5] Quantum confinement of the charge carriers is believed to enhance the Seebeck coefficient S and electrical conductivity r owing to an increased density of states at the Fermi level. [7][8][9] . Moreover, intense boundary and interface scattering of heat carriers decreases thermal conductivity, [2,5,[10][11][12][13] exemplified in the Bi 2 Te 3 /Sb 2 Te 3 nanolayer superlattices [3] exhibiting ZT ∼ 2.3.Further increases in ZT are expected due to the presence of stronger quantum confinement and thermal conductivity reduction effects as the dimensionality of the nanostructures is decreased. Sub-nanometer-diameter nanorods of bismuth telluride are predicted to yield ZT values as high as 14, which is nearly threefold higher than the value predicted for two-dimensional (2D) quantum wells. [14][15][16] However, if the diameter is > 5 nm the electrical transport properties of the nanorods approach bulk properties. These studies have sparked a flurry of activity to produce nanorods and nanowires with small diameters from materials that have high ZT values in the bulk form. Most work to date on synthesizing nanorods or nanowires of bismuth telluride based materials has been by surfactantmediated solvothermal techniques [17][18][19] [27]Here, we describe a simple one-step, template-free, lowtemperature (100°C), aqueous-phase synthesis approach to obtain single-crystal bismuth telluride nanorods with diameters in the range 27-130 nm. These diameters are smaller than previously reported, and can be controlled by adjusting the reaction time and the molecular passivating agent, offering promise for realizing high-ZT thermoelectric nanostructures.In a typical synthesis procedure, either thioglycolic acid or L-cysteine was added to bismuth chloride solution, transforming the initially transparent solution to a yellow color due to thio-ligation of the bismuth ions. This solution was mixed with orthotelluric acid in water preheated to 80°C and then refluxed with excess hydrazine monohydrate at 100°C. The reaction was quenched with water at different time intervals to extract 5 mL of the refluxed mixture for microanalysis. Scanning electron microscopy (SEM) images in Figure 1 reveal the bismuth telluride nanorods obtained from a 5 h synthesis. The rods are straight with curved tips; there is no...