P.S. Alegaonkar

Alignment and wall control of ultra long carbon nanotubes in water assisted chemical vapour deposition

This study examined the correlation between the thickness of the iron catalyst film deposited by e-beam evaporation and carbon nanotube (CNT) growth by water-assisted chemical vapour deposition. The number of walls and the alignment of the CNTs in the CNT forest could be controlled by the iron thickness. High resolution transmission electron microscopy was used to examine the catalyst size distribution and individual CNT structure. Atomic force microscopy was used to analyse the surface roughness. The correlation between the surface roughness, catalyst agglomeration, number of walls in the CNT and the alignment of the CNTs in the CNT forest was established. The CNTs grown on an iron catalyst ranging in thickness from 0.1 to 0.5 nm were well aligned with 2–4 walls. On the other hand, the CNTs grown on an iron catalyst ranging in thickness from 0.6 to 1 nm were less well aligned with 4–10 walls. The CNTs grown on an iron catalyst >1 nm in thickness were misaligned with 8–16 walls. The significance of these experimental trends is discussed within the framework of the clusters aggregation and Van der Waals interaction.

Water-assisted synthesis of carbon nanotubes: Acetylene partial pressure and height control

The effect of the partial pressure of acetylene on the height of carbon nanotube films synthesized by the water-assisted CVD (WA-CVD) was studied. Initially, Fe (2 nm)/Al2O3(15 nm) bilayers were deposited onto a silicon substrate and subjected to the WA-CVD to grow carbon nanotubes. The growth was carried out at 800 °C for a period of ~10 min using a mixture of acetylene/argon gases with a water bubbling system. The partial pressure of acetylene was varied from ~5 to 95% while keeping the total pressure of the gas mixture constant. The height of the carbon nanotube film was measured using scanning electron microscopy. The analysis showed that the height of the film gradually increased from 800±15 nm to 1.4±0.002 mm as the partial pressure of the acetylene feedstock was sequentially increased from 5 to 40%. Thereafter, the height decreased gradually to 0.77±0.002 mm as the partial pressure of acetylene was further increased to 95%. These samples were subjected to X-ray photoelectron spectroscopy after removing the nanotube film from the surface. The analysis revealed that at a lower acetylene content surface oxidation is predominant, whereas in the higher acetylene content regime carbonization influenced the height of the carbon nanotube film. Details of the analysis are presented.

Growth of carbon nanotubes: effect of Fe diffusion and oxidation

The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO2 and Al2O3/SiO2 have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO2/Si and Fe/Al2O3/SiO2/Si samples were subjected to thermal chemical vapour deposition (CVD) at ∼650°C for ∼30 min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al2O3/SiO2/Si samples was relatively high (∼9.5–11 µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ∼650°C for ∼30 min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x Fe, and magnitude of the diffusivity, D Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al2O3/SiO2/Si samples, the magnitudes of x Fe and D Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al2O3 layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al2O3/SiO2/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples.

Fabrication of Carbon Nanotube Embedded Nylon Nanofiber Bundles by Electrospinning

The electrospinning is a novel and efficient tool for fabrication of carbon nanotube (CNTs) -polymer composites. We have fabricated polymer/CNTs composite by doping multi-walled carbon nanotubes (MWNTs) in nylon fibers using electrospinning technique. The solution, containing MWNTs/nylon, was ejected from the spinneret to form fibers. Spun fibers were collected on the water surface in the water bath and transfer to the winding drum. We observed that, the unwoven fibers were transformed into aligned bundles. The fiber alignment is discussed. The electrical properties of the aligned fibers were analyzed.

Fabrication of Porous Al2O3 and TiO2 Thin Film Hybrid Composite Using Atomic Layer Deposition and Properties Study

Atomic layer deposition (ALD) has been used in advanced applications where thin layers of materials with precise thickness down to the nanometer scale are needed. Using anodic oxidation, we prepared the porous alumina. Anodic oxidation was carried out in 5C 0.3M oxalic acid with anodizing voltages (~ 40 V) and two step anodization method. SEM shows that, these porous anodic oxides are well aligned and organized into high-density uniform arrays. Afterward, titanium dioxide thin films were coated by ALD on the porous anodic aluminum oxide. ALD films were influenced by the deposited interface morphology between Al2O3 and TiO2 and narrow channel of ~ 10 nm was obtained by controlling ALD cycle.

Investigation of Disorder in Mixed Phase, sp2–sp3 Bonded Graphene-Like Nanocarbon

Disorder in a mixed phase, sp2-sp3 bonded graphene-like nanocarbon (GNC) lattice has been extensively studied for its electronic and field emission properties. Morphological investigations are performed using scanning electron microscopy (SEM) which depicts microstructures comprising of atomically flat terraces (c-planes) with an abundance of edges (ab planes which are orthogonal to c-planes). Scanning tunneling microscopy (STM) is used to observe the atomic structure of basal planes whereas field emission microscopy (FEM) is found to be suitable for resolving nanotopography of edges. STM images revealed the hexagonal and non-hexagonal atomic arrangements in addition to a variety of defect structures. Scanning tunneling spectroscopy is carried out to study the effect of this short-range disorder on the local density of states. Current versus voltage (I-V) characteristics have been recorded at different defect sites and are compared with respect to the extent of the defect. As sharp edges of GNC are expected to be excellent field emitters, because of low work function and high electric field, enhancement in current is observed particularly when applied electric field is along basal planes. Therefore, it is worthwhile to investigate field emission from these samples. The FEM images show a cluster of bright spots at low voltages which later transformed into an array resembling ledges of ab-planes with increasing voltage. Reproducible I-V curves yield linear Fowler-Nordheim plots supporting field emission as the dominant mechanism of electron emission. Turn on field for 10 μA current is estimated to be ~3 V/μm.

Graphene composite using easy soluble expanded graphite: Synthesis and emission parameters

Since the carbon nanotubes (CNTs) have received a considerable attention because of their interesting properties such as high aspect ratio, electrical and thermal conductivity, and chemical and mechanical stability1. Various low-dimensional carbon materials have been extensively studied to explore their applications as the electron source2,3. Recently, graphene have attracted enormous scientific attention on account of its extraordinary electronic and mechanical properties resulting from one-atom-thick layers and hexagonally arrayed sp2-hybridized carbon atom structure. Field emission studies on graphitic sheets have become important issue from the point of view of technological applications as well as fundamental sciences4. So far, a few papers has been paid to field emission studies of graphene sheet (thickness 1-2 nm)5, synthesized via composite approach which is scalable for vacuum microelectronics and other applications, because the difficulty of graphene dispersion in solvent as well as matrix.

Variation of Conductivity of Fullerite Structures Under Different Types of Pressure

The crystalline state of fullerene C/sub 60/, called fullerite, is a semiconductor with a wide band gap. The type of bond between C/sub 60/ molecules in fullerite crystals means that the resistance of fullerite changes dramatically as result of applied pressure. In principal, fullerene can be used as a functional material for electronic devices or sensors. The technical realization of fullerite sensors is not easy, but the measured properties depend, to some extent, on the structure of the fullerite used. The paper describes an investigation of the pressure dependence of fullerite conductivity for differently assembled C/sub 60/ structures. It was found that fullerite powder, tubules or thin films can be used. as sensitive material to probe unidirectional mechanical stress and isotropically applied pressure by changes in resistance. As good pressure sensitivity was found from pre-vacuum to at least atmospheric pressure, fullerite-based pressure-sensors can also be applied as vacuum manometers.