Improved field emission properties of carbon nanotubes decorated with Ta layer

The field emission (FE) properties of vertically aligned carbon nanotube (CNT) arrays having a surface decorated with Ta layer were investigated. The CNTs with 6 nm thickness of Ta decoration showed improved FE properties with a low turn-on field of 0.64 V/μm at 10 μA/cm², a threshold field of 1.06 V/μm at 1 mA/cm² and a maximum current density of 7.61 mA/cm² at 1.6 V/μm. After Ta decoration, the increased emission centres and/or defect sites on the surface of CNTs improved the field enhancement factor. The work function of CNTs with Ta decoration measured with ultraviolet photoelectron spectroscopy decreased from 4.74 to 4.15 eV with increasing Ta thickness of 0–6 nm. The decreased work function and increased field enhancement factor were responsible for the improved FE properties of the vertically aligned CNTs. Moreover, a significant hysteresis in the cycle-testing of the current density with rising and falling electric field process was observed and attributed to the adsorption/desorption effect, as confirmed by the photoelectron spectrum.     

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

The field emission properties of double-walled carbon nanotubes (DWCNTs) were remarkably improved by decorating their surface with ruthenium (Ru) metal nanoparticles. The Ru nanoparticles were attached effectively on the surface of DWCNTs via a chemical procedure. The Ru-decorated DWCNTs showed lower turn-on voltage, higher emission current density, and improved emission uniformity compared with pristine DWCNTs. The effect of Ru nanoparticles on the work function and density of states was evaluated by the first-principles calculation. The enhanced field emission properties of Ru-DWCNTs were mainly attributed to the Ru nanoparticles which increased the field enhancement factor and the density of emission sites. Our results indicate that the Ru-decorated DWCNTs can be used as an effective field emitter for various field emission devices.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

Noncovalently silylated carbon nanotubes decorated with quantum dots

A nanoassembly of single-walled carbon nanotubes coated by a thin layer of silica followed by quantum dots was prepared. That the quantum dots retained their photoluminescent properties after deposition onto the silylated carbon nanotubes suggests that the thin layer of silica prevented the quenching of the fluorescence by the nanotubes. This fluorescent nanoassembly represents an excellent building block for photoelectric and optical devices and biological nanoprobes.     

Improved field emission from amorphous carbon nanotubes by surface functionalization with stearic acid

Amorphous carbon nanotubes (a-CNTs) were synthesized by a simple process using ammonium chloride and ferrocene at a low temperature (∼250 °C) in open atmosphere. The as-prepared samples were oxidized by a simple acid treatment and further functionalized with stearic acid. For the confirmation of oxidation and functionalization, the pristine and the modified a-CNTs were characterized by Fourier transformed infrared spectroscopy, transmission and scanning electron microscopy. The as-prepared samples showed good field emission (FE) and the FE characteristics were significantly improved for the stearic acid functionalized samples compared to the pristine sample. The effects of inter-electrode distance on the FE properties of the functionalized samples were also studied.     

Electromechanical self-oscillations of carbon nanotube field emitter

We report the observation of the electromechanical self-oscillations excited in a vacuum diode with a field emission cathode made of single-wall carbon nanotubes. The mechanical oscillations of the cathode accompanied by the periodic variations of the emission current intensity have been observed under the action of a constant voltage applied between the cathode and the anode of the diode. An empirical model of the phenomenon is proposed. It provides a qualitative agreement of the experimental results and the theoretical predictions. The modeling assuming a common behavior of the electromechanical systems with nano-sized mechanically flexible field emitters has been performed. It has demonstrated a dependence of the oscillation frequency value on the geometrical size of flexible emitters and on the value of voltage applied. This phenomenon opens a way for generation of the high frequency electromagnetic waves by using the electromechanical systems with emitters of nanometer size.     

Vacuum annealing of carbon nanotubes produced from amorphous carbon

In the vacuum annealing of carbon nanotubes formed by the mechanical activation of amorphous carbon (from sphagnum moss), the yield of purified nanotubes is 19–75 wt %. The potential of materials derived from sphagnum moss as enteric sorbents is considered.     

Effect of UV irradiation on magnetic behavior of reduced graphene oxide decorated with nickel nanostructure

Reduced graphene oxide (RGO) decorated with nickel has been synthesized via an in-situ reduction of graphene oxide (GO) and nickel nitrate using NaOH and hydrazine. The starting materials Ni (NO₃)₂ and GO were taken in two different ratios and the products formed were designated as RGNi₂ and RGNi₁. The formation of the composite was confirmed by the appearance of X-ray diffraction peaks at 44.5°, 51.9°, 76.5° corresponding to Ni and at 24.8°and 43.2° for RGO. The RGNi₂ was irradiated with UV light (λ=254 nm) for different durations (2, 6, 12, 24 and 48 h). Intensity ratio of d and g-bands (I_d/I_g) of Raman spectra increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). The magnetization measurements using the vibrating sample magnetometer (VSM) of these samples reveal their ferromagnetic behavior. The calculated saturation magnetization (M_S) value of Ni, RGNi₁and RGNi₂ is 47.86, 30.56 and 8.25 emu/g respectively and the corresponding coercivity (H_C) value is found to be 181, 227 and 296 Oe. The M_S of RGNi₂ is found to increase to 10.65 emu/g after 48 h of irradiation. This enhancement in the M_S(~23%) with irradiation may be due to defect formation by the UV light.     

Carbon nanotubes grown on cobalt-containing amorphous carbon composite films

Carbon nanotubes (CNTs) have been produced on silicon wafer by filtered cathodic vacuum arc technique using cobalt-containing graphite targets followed by thermal chemical vapor deposition. The Co-containing amorphous carbon (a-C:Co) composite films have various contents of Co as a catalyst for CNTs growth. It is found that dense and random CNTs were grown on the a-C:Co composite film deposited using a 2 at.% Co-containing graphite target and nanoforest CNTs on the composite films using 5, 10 and 15 at.% Co-containing targets. The nanoforest CNTs using a 15 at.% Co-containing target have very good field emission properties with a low threshold field of 1.6 V/μm and a high and stable current density of 2.1 mA/cm² at 3 V/μm, which may result from the smaller diameter of CNTs. It is found that the field emission properties of the CNTs are significantly affected by the diameter of CNTs rather than its orientation.     

H2S oxidation by multi-wall carbon nanotubes decorated with tungsten sulfide

Tungsten sulfide catalysts decorated on single and multiwall carbon nanotubes (SWNTs & MWNTs) and activated carbon were synthesized, and XRD, ICP, SEM, TEM and ASAP analyses were employed to acquire the characteristics of each catalyst. Afterwards a gas flow containing 5,000 ppm of H₂S was passed over the catalyst in gas hour space velocity (GHSV) of 5,000 h^?1, temperature of 65 °C, steam volume percent of 20 and O₂/H₂S ratio equal to 2. The results revealed that the catalyst supported on MWNTs exhibited higher conversion amongst its counterparts. Then effects of GHSV, steam volume percent in the feed, catalyst loading and temperature were investigated on conversion of hydrogen sulfide to elemental sulfur for tungsten sulfide catalyst decorated on MWNTs.     

Improvement of thermo-electrical properties of polymer composites filled with multiwall carbon nanotubes decorated carbon fillers

To enhance the thermo-electrical properties of liquid silicone rubber (LSR) in applications, the carbon fibres (CFs) modified by multiwall carbon nanotubes (MWCNT) on the surfaces were used as the fillers. The MWCNT-modified CFs (MPCFs) were analysed by Fourier transform infrared spectra, thermogravimetric analysis, scanning electron micrograph and energy dispersive X-ray spectroscopy. It was found that MWCNT were successfully adsorbed onto the surface of CFs. The MPCFs functioned as conductive fillers in LSR for thermal and electrical conductivity application and exhibited significant enhancement. The effects of MPCFs loading on thermal conductivity and volume resistivity of LSR composites were investigated in detail. Results of this work revealed that the MPCFs/LSR composites possessed a thermal conductivity of 0.73?W?m^?1?K^?1 with 14?vol.-% filler loading, approximately 3.48-fold higher than that of pure LSR substrate. And with the increase of MPCFs loading, the least volume resistivity of MPCFs/LSR composites is 10?Ω?cm. Besides, compared with that of neat LSR, the tensile strength of MPCFs/LSR composites increased 0.913?MPa.     

Epoxy functionalized multi-walled carbon nanotubes for improved adhesives

Three different types of epoxy-functionalized multi-walled carbon nanotubes (EpCNTs) were prepared by multiple covalent functionalization methods. The EpCNTs were characterized by thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), and Raman spectroscopy to confirm covalent functionalization. The effect of the different chemistries on the adhesive properties was compared to a neat commercial epoxy (Hexion formulation 4007) using functionalized and unfunctionalized multi-walled carbon nanotubes (MWCNT) at 0.5, 1, 2, 3, 5, and 10 wt%. It was found that an EpCNT at 1 wt% increased the lap shear strength, tested using the American Society for Testing and Materials standard test D1002, by 36% over the unfilled epoxy formulation and by 27% over a 1 wt% unmodified MWCNT control sample. SEM images revealed a fracture surface morphology change with the incorporation of EpCNT and a deflection of the crack fronts at the site of embedded CNTs, as the mechanism accounting for increased adhesive strength. Rheological studies showed non-linear viscosity and DSC cure studies showed an alteration of cure kinetics with increased CNT concentration, and these effects were more pronounced for EpCNT.     

Fullerene-functionalized carbon nanotubes as improved optical limiting devices

We report the synthesis, characterization and optical limiting behavior of a nanohybrid built by grafting C₆₀-fullerenes on carbon nanotubes (CNTs). The nonlinear optical limiting properties of the CNT-C₆₀ complex were investigated at wavelengths where C₆₀ does not absorb. We found that the nanohybrid had superior performances to those of CNTs and fullerenes, either taken individually or as a mixture. This enhanced optical limitation of the nanohybrid suggests not only cooperative but also synergistic effects between the two carbon forms. A mechanism involving higher excitonic states of the CNTs formed by Auger recombination of low energy excitons is proposed.     

Focused-ion-beam assisted fabrication of individual multiwall carbon nanotube field emitter

We report the fabrication of an individual carbon nanotube (CNT) electron field emitter using a focused-ion-beam (FIB) technique. The monolithic multiwall CNT with a graphitic shield is synthesized using chemical vapor deposition technique. The FIB technique is applied to attach the monolithic multiwall CNT on an etched tungsten tip. Field emission measurements are carried out in a vacuum of 10⁻⁷ Torr. Threshold voltage as low as 120 V has been obtained.     

Bamboo-like amorphous carbon nanotubes clad in ultrathin nickel oxide nanosheets for lithium-ion battery electrodes with long cycle life

In this work, we report the synthesis of one-dimensional (1-D) hierarchical NiO nanosheets covering bamboo-like amorphous CNT composites (NiO@CNT) via a facile and a low-cost solution route based on sulfonated polymeric nanotubes (PNTs) used simultaneously as both, a template, and a source of nano-structured carbon derived by a low-temperature thermal carbonization treatment. The electrochemical performance of the NiO@CNT composite electrode indicate that this novel hybrid nanostructure is potentially capable of delivering excellent reversible capacity when used as an anode material in a lithium-ion battery (LIB). A large discharge capacity of 1034 mAh g⁻¹ is delivered by the NiO@CNT composite even after 300 cycles at a relatively high current density of 800 mA g⁻¹, with an average coulombic efficiency of 98.1%. A significant achievement in the reversible capacity of the NiO@CNT composite is attributed to the outstanding nanostructure resulting in synergistic effects of the hollow amorphous CNT backbone and ultrathin NiO nanosheets. Furthermore, the generic solution method to fabricate 1-D metal oxides@amorphous CNT nanostructures, developed in this work, is expected to have a wide range of applications in improving the properties of transition metal oxides.     

Water‐based amorphous carbon nanotubes filled polymer nanocomposites

A novel method of making water‐based amorphous carbon nanotubes (ACNTs) for advanced polymer nanocomposites is presented. In this approach, sodium dodecyl sulfate (SDS) is introduced onto the amorphous carbon nanotubes to improve the solubility in water and the dispersion in polyvinyl alcohol [PVA] matrix. As a result, the addition of 0.6 wt % ACNTs in the polymer resulted in the significant improvement (167.5 and 175.8%) in the tensile strength and modulus of the polymer, respectively. The improved mechanical property could be ascribed to the load transfer to the nanotubes in the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.