Diameter-controlled and nitrogen-doped vertically aligned single-walled carbon nanotubes

Diameter controlled and vertically aligned single-walled carbon nanotubes were synthesized from pure and mixed ethanol/acetonitrile feedstock. With increasing acetonitrile concentration in the feedstock, nitrogen incorporation into the sp² carbon network increased until saturating at approximately one atomic percent. The incorporation of nitrogen correlates with a significant diameter reduction from a mean diameter of 2.1 nm down to 0.7 nm. Heteroatom-mediated diameter control is independent of catalyst preparation and represents a versatile tool for the direct synthesis of tailored single-walled carbon nanotubes.     

Synthesis of vertically aligned carbon nanotubes on submicron-sized dot-catalyst array using plasma CVD method

Vertically aligned carbon nanotubes were synthesized on submicron-sized dot-catalyst array which have a dot size of 400 nm and dot intervals varying from 0.3 μm to 10 μm, using direct current plasma enhanced chemical vapour deposition(PECVD) method. The dot-catalyst array was fabricated by lift-off process using electron beam lithography. Field emission characteristics were measured and compared for different dot intervals. Experimental results of field emission characteristics and the Fowler–Nordheim plots suggest that spatial control of dotted-catalyst is very important to improve the electric field emission characteristics. The results show that the highest field enhancement factor β was obtained when the dot interval was two times longer than the length of CNTs.     

Freestanding vertically oriented single-walled carbon nanotubes synthesized using microwave plasma-enhanced CVD

Freestanding single-walled carbon nanotubes (SWCNTs) have been synthesized in a vertical direction, perpendicular to the growth substrate, using applied DC substrate bias in a microwave plasma-enhanced chemical vapor deposition (PECVD) synthesis process. The degree of alignment and spatial density of SWCNTs demonstrate a strong dependence on the magnitude of applied bias, with increased alignment and decreased density with increased bias. The unique synthesis environment created by the application of a negative substrate bias in PECVD aligns SWCNTs along electric field lines and decreases SWCNT density due to bombardment by positively charged hydrogen ions. Multi-excitation wavelength Raman spectroscopy reveals shifts in dominant RBM peaks with the application of dc bias. Use of this technique to orient SWCNTs in the vertical direction may allow for three-dimensional SWCNT-based device architectures.     

Semi-quantitative study on the fabrication of densely packed and vertically aligned single-walled carbon nanotubes

This paper presents, for the first time, a semi-quantitative study on the production of densely packed and vertically aligned (DPVA) single-walled carbon nanotubes (SWNTs) from ultra-thin catalytic films. An up-to-date highest volume density (60-70 kg m⁻³) and the corresponding high surface density on the order of 10¹⁶ m⁻² of DPVA-SWNTs have been achieved by point-arc microwave plasma chemical vapor deposition. The precise thickness control of the sandwich-like catalytic nanostructure of 0.5 nm Al₂O₃/0.5 nm Fe/>5 nm Al₂O₃, developed by the authors, and a short-time (5 min) heat pretreatment of substrates at a temperature as low as 600 °C play the very key role in the process of fabricating DPVA-SWNTs.     

Growth dynamics of vertically aligned single-walled carbon nanotubes from in situ measurements

An in situ optical absorbance measurement was used to study the growth dynamics of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) synthesized by chemical vapor deposition of ethanol. The growth rate of the VA-SWCNT film was found to decay exponentially from an initial maximum, resulting in an effective growth time of approximately 15 min. Investigation of various growth conditions revealed an optimum pressure at which growth is maximized, and this pressure depends on the growth temperature. Below this optimum pressure the synthesis reaction is first-order, and the rate-limiting step is the arrival of ethanol at the catalyst. We also present a novel method for determining the burning temperature of low-mass materials, which combines the in situ absorbance measurement with controlled oxidation.     

Synthesis of vertically aligned single-walled carbon nanotubes with metallic chirality through facet control of catalysts

For nanotube synthesis, iron platinum (FePt) alloy particles have been prepared on a single crystalline magnesium oxide (MgO) substrate by alternate sputter deposition of FePt and MgO. Partially facetted {1 1 1}-nano particles of FePt have been epitaxially formed on the substrate and periodically exposed on the surface. The particles of FePt were half-buried between deposited MgO showed superior thermal stability and microparticulations were also achieved by optimization of film layer thickness. By using the substrates for growth of carbon nanotubes, vertically aligned single-walled carbon nanotubes (forest) have been successfully grown on the substrate containing the faceted FePt nanoparticles. Raman spectra of the forest have revealed prominent features of metallic nanotubes in the radial breathing-mode region.     

The production of horizontally aligned single-walled carbon nanotubes

The current progress on the production of aligned single-walled carbon nanotubes (SWCNTs), particularly the horizontally aligned ones, is reviewed. There are two main categories for the alignment of SWCNTs: the post synthesis assembly and the in situ growth approaches. The post synthesis assembly approach mainly involves dispersing SWCNTs in solutions and aligning SWCNTs using spin-coating, Langmuir–Blodgett assembly, mechanical shearing, or blown bubble film techniques. The in situ growth approach produces aligned SWCNTs directly during their growth using controlled chemical vapor deposition and arc discharge techniques. The latter approach has the advantage of avoiding the defects generated during the post treatment methods, and may also be combined with other growth controls such as structure selectivity of SWCNTs and direct device patterning for scale up applications.     

A simple combinatorial method to discover Co-Mo binary catalysts that grow vertically aligned single-walled carbon nanotubes

A simple yet versatile combinatorial method to discover binary metal nanoparticle catalysts was developed. In this method, the nominal thickness of component metals can be independently screened for a wide range by simply setting a mask with a slit above a substrate during sputter-deposition. Using this method, we prepared a catalyst library with Mo (0.2-4 nm) and Co (0.2-8 nm) thickness profiles on a SiO₂/Si wafer and discovered active catalysts that grow vertically aligned single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition.     

Covering vertically aligned carbon nanotubes with a multiferroic compound

This work highlights the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO₃ (BFO) films were fabricated in-situ deposited on the surface of VA-MWCNTs by RF (radio frequency) magnetron sputtering. For in situ deposition temperature of 400 °C and deposition time up to 2 h, BFO films cover the MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by piezo force microscopy. G type antiferromagnetic ordering with weak ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe.     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Fabrication and field emission property studies of vertically aligned multiwalled carbon nanotubes grown by double plasma chemical vapour deposition technique

The field emissive carbon nanotubes (CNTs) with nickel tips were grown by a novel technique, double plasma hot filament chemical vapor deposition technique. Surface morphologies and structure of the sample were studied by a field emission scanning electron microscope, high resolution transmission electron microscope, Fourier transform infrared and Raman spectroscopy. Field emission (FE) of the sample was observed with a threshold field of ~ 3.4 V/μm. The high enhancement factor β derived from the slope of Fowler-Nordheim is attributed to the isolated nature of CNTs and nickel present at the CNT tip.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Temperature gradient chemical vapor deposition of vertically aligned carbon nanotubes

We present temperature gradient chemical vapor deposition (TG CVD) for producing vertically aligned (VA-) carbon nanotubes (CNTs). Independent heaters on the gas inlet and catalyst substrate sides of a cold-wall, vertical CVD reactor can modulate the gas temperature gradient to lead to controlled thermal histories of acetylene precursor. Our growth results reveal that such a precursor thermal history can play a significant role in the growth and structural features of the resultant VA-CNTs. We find several gas thermal zones particularly important to the VA-CNT growth by evaluating the precursor dwell time in different zones. Thermal treatment of the acetylene precursor at 600–700 °C is found crucial for the synthesis of VA-CNTs. When this thermal zone is conjoined in particular with a zone >700 °C, efficient growths of single-walled and double-walled VA-CNTs can be achieved. These gas thermal zones can contribute to VA-CNT growths by mixing various secondary hydrocarbons with acetylene, corroborated by the results of our reacting flow simulation. Our findings emphasize the influence of gas-phase reactions on the VA-CNT growth and suggest that our TG CVD approach can be practically utilized to modulate complex gas-phase phenomena for the controlled growth of VA-CNTs.     

Oil sorption and recovery by using vertically aligned carbon nanotubes

We used carbon nanotubes as oil adsorbents and evaluated recycling performance by squeezing method. The sorption capacity of 3 mm long vertically aligned carbon nanotubes is almost 6.9 times higher than that of agglomerated carbon nanotubes due to the existence of large-sized macropores. Compared with exfoliated graphite (41 g/g), aligned carbon nanotubes exhibit higher sorption capacity (69 g/g) and better recycling performance due to their unique mechanical strength and excellent rebound resilience properties at high strains.     

High surface area diamond-like carbon electrodes grown on vertically aligned carbon nanotubes

Electrochemically active diamond-like carbon (DLC) electrodes featuring high specific surface area have been prepared by plasma-enhanced chemical vapour deposition (CVD) onto densely packed forests of vertically aligned multiwall carbon nanotubes (VACNTs). The DLC:VACNT composite film exhibits a complex topography with web like features and ridges generated by partial coalescence of the DLC over the CNT arrays. DLC:VACNT electrodes exhibit low background responses over a large potential window, low uncompensated resistance, as well as low charge-transfer impedance in the presence of ferrocyanide as a redox probe. The interfacial capacitance associated with the DLC:VACNT electrode is in the range of 0.6 mF cm⁻², a value two orders of magnitude larger than in conventional flat carbon electrodes. DLC films grown onto single-crystal Si(1 0 0) under identical conditions resulted in essentially insulating layers. Conducting-atomic force microscopy studies reveal that the film electro-activity does not arise from specific topographic features in the highly corrugated film. The ensemble of experimental results suggests that the enhanced electrochemical responses are not connected to areas in which the CNT support is exposed to the electrolyte solution. This is remarkable behaviour considering that no dopants have been included during the DLC film growth.     

Improvement in growth yield of single-walled carbon nanotubes with narrow chirality distribution by pulse plasma CVD

A pulse plasma chemical vapor deposition (CVD) technique was developed for improving the growth yield of single-walled carbon nanotubes (SWNTs) with a narrow chirality distribution. The growth yield of the SWNTs could be improved by repetitive short duration pulse plasma CVD, while maintaining the initial narrow chirality distribution. Detailed growth dynamics is discussed based on a systematic investigation by changing the pulse parameters. The growth of SWNTs with a narrow chirality distribution could be controlled by the difference in the nucleation time required using catalysts comprising relatively small or large particles as the key factor. The nucleation can be controlled by adjusting the pulse on/ofF time ratio and the total processing time.