Multi-walled Structure Research Articles

Synthesis and emission properties of carbon nanotubes grown by sandwich catalyst stacks

We report in this article a method to grow carbon nanotubes (CNTs), which are well aligned and vertically oriented. Furthermore, these CNTs have a uniform length and diameter. A sandwiched catalyst structure is utilized to form multiwalled carbon nanotubes. It is feasible to grow CNTs between two terminals directly and grow straight vertical carbon nanotube bundles. The transmission electron microscope image of the as-grown CNTs shows a typical multiwalled CNT lattice structure with a few defects. After treating the as-grown CNTs in an ammonia radio-frequency plasma, a highly stable field-emission current density of more than 6A∕cm2 at an electric field of 7.7V∕μm with a total field-emission current of >1.2mA was obtained. Our experiments indicate a fabrication route for largely improving the field-emission characteristics of CNT-based field emitters.

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

The methane conversion and carbon yield of the chemical vapor deposition (CVD) reaction suggests that the optimum reaction conditions of the formation of multi-wall carbon nanotubes (MWCNTs) can be obtained by using a 50 mg of nano-MgNi alloy under pyrolysis of the pure CH 4 gas with the flow rate about 100–120 cm 3/min at 650 °C for 30 min. Raman results indicate the CNTs are in multi-wall structure, since no single-wall characteristic features appearing in the 200–400 cm −1 region. This is consistent with those of the XRD and TGA findings. Under selected condition, the carbon yield and the CNTs purity can reach up to 1231% and 92% in the presence of hydrogen. It is presumable that the presence of hydrogen in the pyrolysis of CH 4 prevents the deactivation of catalysts and enhances the graphitization degree of CNTs. In addition, the presence of Mg metal in the alloy can prevent the aggregation of the Ni metal and forms the active Mg 2Ni phase to enhance the CH 4 pyrolysis to form CNTs. After the purification procedures with both air oxidation at 550 °C and HCl treatments, the final purified yield and purity of CNT reach to 73.2% and (98.04 ± 0.2)% respectively.

Structural characteristics of carbon nanofibers for on-chip interconnect applications

In this letter, we compare the structures of plasma-enhanced chemical vapor deposition of Ni-catalyzed and Pd-catalyzed carbon nanofibers (CNFs) synthesized for on-chip interconnect applications with scanning transmission electron microscopy (STEM). The Ni-catalyzed CNF has a conventional fiberlike structure and many graphitic layers that are almost parallel to the substrate at the CNF base. In contrast, the Pd-catalyzed CNF has a multiwall nanotubelike structure on the sidewall spanning the entire CNF. The microstructure observed in the Pd-catalyzed fibers at the CNF-metal interface has the potential to lower contact resistance significantly, as our electrical measurements using current-sensing atomic force microscopy indicate. A structural model is presented based on STEM image analysis.

Synthesis of titania nanotubes by microwave irradiation

A novel method for synthesis of titanium oxide (TiO 2) nanotubes by microwave irradiation was investigated. Under certain microwave power, TiO 2 nanotubes were gained via the reaction of TiO 2 crystals of either anatase or rutile or mixed phase and NaOH aqueous solution. The products were characterized by—TEM, HRTEM, XRD, BET and ICP. The results showed that the titania nanotubes had the central hollow, open ended and multi-wall structure. The diameters of the nanotubes were 8–12 nm, and the lengths were up to 200–1000 nm. The compositions were titanium and oxygen and the atomic ratio of O/Ti was about 2/1. XRD showed that the titanium oxide nanotubes could be anatase phase.

In Situ Formation of BN Nanotubes during Nitriding Reactions

High-yield multiwalled boron nitride (BN) nanotubes have been produced using a ball milling-annealing method. The BN nanotubes with a diameter less than 10 nm and a well-crystallized multiwalled structure were formed via an in situ nitriding reaction. The systematic investigation of the formation process at different annealing temperatures and for different times suggested that the formation of the unique multiwalled structure was attributed by a two-dimensional growth of the BN phase and a nonmetal catalytic growth.

Characteristics of carbonaceous materials with nanotubes grown by hot-filament plasma-enhanced chemical vapor deposition method

We have grown carbonaceous materials with nanotubes (CNTs) using a HFPECVD (hot-filament plasma-enhanced chemical vapor deposition) method for the purpose of finding their growth characteristics according to the growth conditions and angles between substrate and cathode. C 2H 2 gas was used as the carbon precursor and NH 3 gas used as the dilution gas. SEM images of Ni-catalyst layers pretreated with NH 3 plasma show that nano-size grains were formed, and the diameter as well as the density of CNTs depended on the pretreatment of the catalytic layer. The characteristics of CNTs were supposed to have some relationship with the angle ( θ) between substrate and cathode. Raman spectra showed both D-band and G-band peaks around 1360 cm − 1 and 1598 cm − 1, respectively, and the position of these two peaks was not changed with both plasma bias and temperature. However, I G/ I D relative intensity ratio decreased with the substrate angle ( θ), which indicates that the amounts of amorphous carbon and defective structure in CNTs increased. TEM image showed that CNTs synthesized in this work had multiwall structure and hollow in their body.

Synthesis of well-aligned carbon nanotubes with open tips

Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10 μm and diameters ranging from 92 to 229 nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.

열 CVD법에 의한 CNTs/Metal/Al2O3나노복합분말의 합성 및 특성

An optimum route to synthesize composite powders with homogeneous dispersion of carbon nanotubes (CNTs) was investigated. nanocomposite powders were fabricated by thermal chemical vapor deposition of gas over nanocomposite catalyst prepared by selective reduction of metal powders. The FT-Raman spectroscopy analysis revealed that the CNTs have single- and multi-walled structure. The CNTs with the diameter of 25-43 nm were homogeneously distributed in the powders, and their characteristics were strongly affected by a kind of metal catalyst and catalyst size. The experimental results show that the composite powder with required size and dispersion of CNTs can be realized by control of synthesis condition.

Synthesis of multiwalled carbon nanotubes using C14H10O7W catalyst

High-purity carbon nanotubes were synthesized using a novel catalyst: tungsten-containing complex (C14H10O7W). The carbon nanotubes have a multiwalled structure that is hollow on the inside and a clean surface without carbonaceous particles. The carbon nanotubes have average diameters in the range of 20–60nm and lengths of 30μm. Transmission electron microscopy analysis reveals that the inner graphite sheets of these nanotubes are highly crystallized, but the outer graphite sheets are defective. The microstructures of the carbon nanotubes produced from C14H10O7W, tungsten hexacarbonyl [W(CO)6], and iron pentacarbonyl [Fe(CO)5] were analyzed using high-resolution transmission electron microscopy. We demonstrate that the tungsten-based catalyst can effectively be used to produce the carbon nanotubes.

Formation of nanotubes TiO 2 from layered titanate particles by a soft chemical process

Nanotubes TiO 2 were synthesized from layered titanate Na 2Ti 3O 7 particles by a soft chemical process. These nanotubes have a hollow structure with an opening end. The wall of nanotubes is very well crystallized multi-wall structure. The inner diameters of nanotubes are ca. 30–40 nm and the length up to several hundreds nanometers. XRD results show that TiO 2 phase was formed during the procedure of exfoliating Na 2Ti 3O 7 particles into nanosheets. Based on our experimental results, an exfoliating-rolling model was proposed for formation of nanotube structure.

Improved growth of aligned carbon nanotubes by mechanical activation

Straight aligned carbon nanotubes with multiwalled cylindrical structure have been produced by pyrolysis of iron phthalocyanine (FePc) after ball milling treatment. The pre-ball milling treatment prevented the formation of curved nanotubes with bamboo or conelike structures. X-ray diffraction analysis revealed that the milled FePc has an activated and disordered structure, which contributes a lower vaporization temperature determined by thermal gravimetric analysis. The low formation temperature and an increased nanotube growth rate are favorable to the formation of cylindrical structure than bamboo tubes.

Synthesis of Stepped Carbon Nanotubes in Anodic Aluminum Oxide Templates

Anodic aluminum oxide (AAO) with pores of various diameter, density, and thickness values was obtained through control of the anodization parameters including voltage, temperature, pore widening time, anodization time, etc. The pore diameter was controlled by a pore widening in an etchant, and alumina templates having stepped nano-channels were fabricated by repetition of anodization and pore widening processes. Stepped carbon nanotubes (CNTs) were then grown on the stepped AAO templates by pyrolysis of acetylene without using the catalyst. High-resolution transmission electron microscopy images revealed that CNTs have a multi-wall structure made of graphite flakes of several nm sizes. The current-voltage characteristic of the sloped and linear CNTs were also examined.

Electric transport measurement of a multi-walled carbon nanotube in scanning transmission electron microscope

We used a carbon nanotube (CNT) manipulator in scanning transmission electron microscope (STEM). The manipulator was composed of two tungsten heater stages facing each other. One was fixed and the other stage could be moved. Titanium nickel (TiNi) particles whose average size was about 100 nm were put on the tungsten heaters. Finally, we constructed multi-walled CNT bridge structure by heating TiNi in STEM. Furthermore, we measured the electric transport of the CNT bridge in STEM. The current–voltage characteristics of the CNT bridge showed that the maximum current was 157 μA at 2.6 V and the current density was estimated to be 2×10 11 A/m 2 . The resistance showed a constant of 13 kΩ . This quite coincided with the quantum conductance of G 0=2 e 2/ h. This means that the CNT bridge using TiNi particles can reproducibly function as a ballistic conductor.

Synthesis of CNTs/Metal/Al<sub>2</sub>O<sub>3</sub> Nanocomposite Powders by Chemical Vapor Deposition

CNTs/metal/Al2O3 nanocomposite powders were fabricated by thermal chemical vapor deposition (CVD) of C2H2 gas over metal/Al2O3 nanocomposite catalysts prepared by the selective reduction of oxide/Al2O3 powders. The yield and diameter of CNTs significantly depended on the kind of metal catalyst and catalyst size. X-ray diffraction (XRD) was used for phase identification. The morphology of CNTs was determined using field emission scanning electron microscopy (FESEM). FT-Raman spectroscopy revealed that the CNTs have single- and multi-walled carbon nanotubes structure. The relationship between the CNT yield/diameter and the characteristics of the composite catalyst was systematically investigated.

The emission effect of metal-tip removal in carbon nanotubes by bias-assisted ICPHFCVD

This paper presents a technique for the preparation of vertically grown carbon nanotubes (CNTs) by bias-assisted inductively coupled plasma hot-filament chemical vapor deposition. Purification of the CNTs using r.f. plasma in a one-step process, based on the different etching property of the metal tip is also discussed. The Ni at the tip of the CNTs was effectively removed by using r.f. plasma based on the different etching property. After purification CNTs show the multi-walled and hollow-type structure. The measured critical current density on CNTs with a Ni tip was 3.52×10−7 A cm−2 at 2.47 V μm−1 turn-on field and 6.6×10−4 A cm−2 at 4.8 V μm−1 of the critical field. On the other hand, the critical current density on purified CNTs after Ni removal by an r.f. source was 1.36×10−7 A cm−2 at 2.1 V μm−1 turn-on field and 1.5×10−3 A cm−2 at 6 V μm−1 of the critical field, respectively.

Stepped carbon nanotubes synthesized in anodic aluminum oxide templates

Anodic aluminum oxide templates with stepped nano channels were fabricated by repeated anodization and pore widening processes, and carbon nanotubes were grown on the templates without catalytic metals by pyrolysis of acetylene. High-resolution transmission electron microscopy revealed that the wall of the CNTs has a multi-wall structure made of stacked graphite flakes of several nm sizes. For the carbon nanotube formation on alumina, we propose a pyrolytic CVD of carbon on alumina surface instead of a synthesis via catalytic action of the alumina. The electrical properties of the stepped and linear CNTs did not show remarkable difference because of the highly defective structure of the flaked tubes.

Stepped carbon nanotubes synthesized in anodic aluminum oxide templates

Anodic aluminum oxide templates with stepped nano channels were fabricated by repeated anodization and pore widening processes, and carbon nanotubes were grown on the templates without catalytic metals by pyrolysis of acetylene. High-resolution transmission electron microscopy revealed that the wall of the CNTs has a multi-wall structure made of stacked graphite flakes of several nm sizes. For the carbon nanotube formation on alumina, we propose a pyrolytic CVD of carbon on alumina surface instead of a synthesis via catalytic action of the alumina. The electrical properties of the stepped and linear CNTs did not show remarkable difference because of the highly defective structure of the flaked tubes.

Synthesis of Carbon Nanotubes from Catalytic Decomposition of C2H2through Pd/Al2O3Catalysts

CNTs have been synthesized by catalytic <TEX>$C_2H_2$</TEX> decomposition through <TEX>$Pd/Al_2O_3$</TEX> at low temperature. The CNTs were grown to a length of about 10 <TEX>${\mu}$</TEX>m and diameter 150-200 nm with multiwalled structure. Pd catalysts have two major roles; one is the active catalyst for <TEX>$C_2H_2$</TEX> decomposition, the other is a nucleation site of CNT's growth.

Room temperature preparation of novelCu2−xSe nanotubes in organic solvent

A compound with a non-layered structure composed ofCu2−xSe nanotubes was prepared in a room temperature redox reaction in ethylene diamine solution.The product was characterized by x-ray diffractometry, transmission electron microscopy, highresolution transmission electron microscopy, and x-ray photoelectron spectroscopy. Analysisshows that the nanotubes have a multiwall structure with (002) planes of face-centred cubicCu2−xSe and openends. The Cu2−xSe nanotubes have length 500 nm, with an inner diameter of 20 nm and an outer diameterof 30 nm on average. Ethylene diamine’s ability to achieve strong coordinationand the presence of the reducing agent hydrazine hydrate aid the formation ofCu2−xSe nanotubes with mixed valence.

플라즈마 화학 기상 증착법을 이용한 탄소나노튜브의 성장 분석 및 전계방출 특성

Carbon nanotubes(CNTs) are grown by using Co catalyst metal. CNTs fabricated by PECVD(plasma enhanced chemical vapor deposition) method are studied in terms of surface reaction and surface structure by TEM and Raman analysing method and ate analysed in its electrical field emission characteristics with variation of space between anode and cathode. Acetylene(C<TEX>$_2$</TEX>H<TEX>$_2$</TEX>) gas is used as the carbon source, while ammonia and hydrogen gas are used as catalyst and dilution gas. The CNTs grown by hydrogen(H<TEX>$_2$</TEX>) gas plasma indicates better vortical alignment, lower temperature process, and longer tip, compared to that grown by ammonia(NH<TEX>$_3$</TEX>) gas plasma. The CNTs fabricated with Co(cobalt) catalyst metal and PECVD method show the multiwall structure in mid-circle type in tip-end and the inner vacancy of 10nm. Emission properties of CNTs indicate the turn-on field to be 2.6 V/<TEX>${\mu}{\textrm}{m}$</TEX> We suggest that CNTs can be possibly applied to the emitter tip of FEDs and high brightness flat lamp because of low temperature CNTs growth, low turn-on field.