CVD Synthesis of Hierarchical 3D MWCNT/Carbon-Fiber Nanostructures

Multiwalled carbon nanotubes (MWCNTs) were synthesized by CVD on industrially manufactured highly crystalline vapor-grown carbon fibers (VGCFs). Two catalyst metals (Ni and Fe) and carbon precursor gases (C2H2 and CO) were studied. The catalysts were deposited on the fibers by sputtering and experiments carried out in two different reactors. Samples were characterized by electron microscopy (SEM and TEM). Iron was completely inactive as catalyst with both C2H2 and CO for reasons discussed in the paper. The combination of Ni and C2H2 was very active for secondary CNT synthesis, without any pretreatment of the fibers. The optimal temperature for CNT synthesis was 750 ∘C, with total gas flow of 650 cm exp 3 min exp -1  of C2H2, H2, and Ar in 1.0:6.7:30 ratio.Peer reviewe

High-Yield of Memory Elements from Carbon Nanotube Field-Effect Transistors with Atomic Layer Deposited Gate Dielectric

Carbon nanotube field-effect transistors (CNT FETs) have been proposed as possible building blocks for future nano-electronics. But a challenge with CNT FETs is that they appear to randomly display varying amounts of hysteresis in their transfer characteristics. The hysteresis is often attributed to charge trapping in the dielectric layer between the nanotube and the gate. This study includes 94 CNT FET samples, providing an unprecedented basis for statistics on the hysteresis seen in five different CNT-gate configurations. We find that the memory effect can be controlled by carefully designing the gate dielectric in nm-thin layers. By using atomic layer depositions (ALD) of HfO$_{2}$ and TiO$_{2}$ in a triple-layer configuration, we achieve the first CNT FETs with consistent and narrowly distributed memory effects in their transfer characteristics.Comment: 6 pages, 3 figures; added one reference, text reformatted with smaller addition

Robust Bessel-function-based method for determination of the (n, m) indices of single-walled carbon nanotubes by electron diffraction

We report a calibration-free method for the determination of chiral indices (n,m) of single-walled carbon nanotubes from their electron diffraction patterns based on Bessel function analysis of the diffracted layer lines. An approach has been developed for confident identification of the orders of the Bessel functions from the intensity modulations of the diffraction layer lines, to which (n,m) are correlated. In particular, we critically evaluate the effect of nanotube inclination on the validity of the method and show that the layer lines governed by high-order Bessel functions tolerate higher tilt angles than those of low-order Bessel functions and thus are favored for (n,m) evaluation. The method is of particular significance in that it considerably enhances the precision of chiral indexing and makes possible the analysis of high-order Bessel functions, especially when EDPs are of relatively low pixel resolution. The technique can be extended to structural analysis of double-walled carbon nanotubes.Peer reviewe

Determination of helicities in unidirectional assemblies of graphitic or graphiticlike tubular structures

Here we propose a universal method for the determination of all helicities present in unidirectional assemblies of hexagon-based graphitic or graphiticlike tubular structures, e.g., multiwalled or bundled carbon nanotubes(CNTs) or boron-nitride nanotubes and their structural analogs. A critical dimension characteristic of a fundamental structural property, i.e., the atomic bond length, is discerned from electron diffraction patterns by which all helicities present in the assemblies are identified. Using this method, we determine the helicity population in a single-walled CNT sample produced by laser ablation technique.Peer reviewe

Influence of the diameter of single-walled carbon nanotube bundles on the optoelectronic performance of dry-deposited thin films

The optoelectronic performance of thin films of single-walled carbon nanotubes (SWCNTs) was studied with respect to the properties of both individual nanotubes and their bundles. The SWCNTs were synthesized in a hot wire generator aerosol reactor, collected by gas filtration and dry-transferred onto various substrates. By thus completely avoiding liquid dispersion steps, we were able to avoid any artifacts from residual surfactants or sonication. We found that bundle lengths determined the thin-film performance, as would be expected for highly resistive bundle–bundle junctions. However, we found no evidence that contact resistances were affected by the bundle diameters, although they did play a secondary role by simply affecting the absorption. The individual SWCNT diameters and their graphitization level as gauged by the Raman D band intensity did not show any clear correlation with the overall performance.Peer reviewe

Effect of Carbon Nanotube Aqueous Dispersion Quality on Mechanical Properties of Cement Composite

An effect of the quality of carbon nanotube (CNT) dispersions added to cement on paste mechanical properties has been studied. High-quality dispersions of few-walled CNT (FWCNTs) were produced in two steps. First, FWCNTs were functionalized in a mixture of nitric and sulfuric acids (70 wt.% and 96 wt.%, resp.) at 80 ∘C . Second, functionalized FWCNTs were washed out by acetone to remove carboxylated carbonaceous fragments (CCFs) formed during CNT oxidation. Mechanical test results showed 2-fold increase in the compressive strength of the cement paste prepared from the dispersion of acetone-washed functionalized FWCNTs, which is believed to occur due to the chemical interaction between cement matrix and functional groups (–COOH and –OH). Utilisation of unwashed FWCNTs led to a marginal improvement of mechanical properties of the cement pastes, whereas surfactant-treated functionalized FWCNT dispersions only worsened the mechanical properties.Peer reviewe

Influence of nanotube length and density on the plasmonic terahertz response of single-walled carbon nanotubes

We measure the conductivity spectra of thin films comprising bundled single-walled carbon nanotubes (CNTs) of different average lengths in the frequency range 0.3-1000 THz and temperature interval 10-530 K. The observed temperature-induced changes in the terahertz conductivity spectra are shown to depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which we obtain from Drude fits of the measured conductivity in the range 0.3-2 THz for 10 $\mu$m length CNTs. This increasing scattering rate with temperature results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS$_2$ nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak.Comment: Submitted to Journal of Physics D. Main manuscript: 9 pages, 8 figures. Supplementary material: 5 pages, 6 figure

Mechanistic investigation of ZnO nanowire growth

ZnO nanowire (NW) growth mechanism was investigated in a nonvapor and noncatalytic approach for the controlled NW synthesis in a second time scale. The experimental results showed what ZnO NW growth was determined by migration of zincinterstitials and vacancies in a ZnO layer, which should be also considered in other synthesis techniques and mechanisms. The mechanism of the ZnO NW growth was explained as due to the advantageous diffusion through grain boundaries in ZnO layer and crystal defects in NWs. Additionally, on the basis of photoluminescence measurements, a feasible application of as-produced wires for optoelectronic devices was demonstrated.Peer reviewe