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.     

Growth mechanism and field emission behavior of carbon nanotubes grown over 300 nm thick aluminium interlayer

The influence of thick aluminium (Al) ~ 300 nm interlayer on the growth and field emission (FE) properties of carbon nanotubes (CNTs) deposited on silicon coated with a 2 nm iron (Fe) catalyst was studied. The CNTs were grown over silicon substrate with and without Al-interlayer via CVD. It was observed that the presence of such high thickness of the interlayer on the substrate resulted in higher growth rate, narrower diameters and longer height of CNTs compared to CNTs grown on silicon (Si) substrate coated only with Fe. Al-interlayer hinders the diffusion of Fe into silicon, hence promotes the growth rate. Literature reports that a thick layer of Al causes Fe to diffuse into it, negatively affecting the growth. However, in our experiments, no evidence of depletion of Fe from the substrate was observed. Unique patterns of grown CNTs could be attributed to anisotropic Al-melting over the silicon substrate resulting in Al/Fe rich and deficient regions. The drastic improvement of current density from 0.41 mA/cm² to 20 mA/cm² at a field of 3.5 V/μm was found with Al-interlayer CNT grown samples. These mechanisms of improvements in field emission characteristics have been discussed in detail.     

Long-term stability of a horizontally-aligned carbon nanotube field emission cathode coated with a metallic glass thin film

A horizontally-aligned carbon nanotube (HACNT) field emission cathode was coated with a metallic glass thin film (MGTF) to improve the stability of the field emission properties. HACNT field emission cathodes have previously been fabricated on glass substrates using composite plating and crack-formation techniques. A carbon nanotubes/nickel (CNTs/Ni) composite film is deposited onto a glass substrate at 80 °C by the composite plating technique alone. Cracks are then formed in the CNT/Ni composite film during 30 min heating at 300 °C, and HACNTs are exposed in the cracks. The field emission properties of the HACNT field emission cathode show a low turn-on electric field E_on of about 2.3 V/μm, a low threshold electric field E_th of about 4.7 V/μm at an emission current density of 1 mA/cm², and a stability time of 78 h. The degradation of the HACNT field emission cathode is prevented by using a MGTF-coating technique and superior long-term stability (i.e. >125 h, with 5 nm MGTF; >270 h, with 10 nm MGTF) for the MGTF/HACNT field emission cathode is achieved.     

Fabrication and characterization of in-situ duplex plasma-treated nanocrystalline Ti/AlTiN coatings

Plasma nitriding and plasma-assisted PVD duplex treatment was adopted to improve the load-bearing capacity, fatigue resistance and adhesion of the AlTiN coating. Ion etch-cleaning was applied for better adhesion before plasma nitriding. After plasma nitriding Ti interlayer was in-situ deposited by high power impulse magnetron sputtering (HIPIMS), followed by the AlTiN coating through in-situ deposition by advanced plasma-assisted arc (APA-Arc). The microstructure and properties of the duplex-treated coating were carefully characterized and analyzed. The results show that the thicknesses of the nitriding zone, the γ′-Fe₄N compound layer, the Ti interlayer and the AlTiN top layer with nanocrystalline microstructures are about 60 μm, 2–3 μm, 100 nm and 6.1 μm, respectively. The nitriding rate is about 30 μm/h and the AlTiN coating deposition rate reaches 6.1 μm/h. The interfacial adhesion of the Ti/AlTiN coating is well enhanced by ion etch-cleaning and a Ti interlayer, and the load-bearing capacity is also improved by duplex treatment. In addition, the instinct hardness of the Ti/AlTiN coating reaches 3368HV_0.05 while the wear rate coefficient of 5.394×10⁻⁸ mm⁻³/Nm is sufficiently low. The Ti/AlTiN coating, which possesses a high corrosion potential (E_corr=−104.6 mV) and a low corrosion current density (i_corr=4.769 μA/cm²), shows highly protective efficiency to the substrate.     

Growth of carbon nanotube arrays using nanosphere lithography and their application in field emission devices

The present study investigates the patterned growth of carbon nanotubes (CNTs) by microwave plasma assisted chemical vapor deposition (MPCVD) and their field emission (FE) properties. The nanosphere monolayers were used as a mask for deposition of ultrathin (~ 3 nm) cobalt (Co) layer by DC sputtering. Periodic arrays of Co catalyst islands were obtained after the removal of spheres. Microscopic and Raman spectroscopic studies revealed the patterned growth of multiwall CNTs on catalyst islands. The CNTs length was around 10 µm and diameter was of 40–60 nm. The field emission properties were also compared with I–V characteristics of the un-patterned CNTs grown under the same conditions. The onset fields for un-patterned and patterned samples were nearly the same, 0.64 V/µm and 0.67 V/µm, respectively for a 10 µA current.     

Field emission characteristics of CNFB-CNT hybrid material grown by one-step MPCVD

A hybrid material consisting of carbon nanotubes (CNTs) and carbon nanoflake balls (CNFBs) was successfully synthesized by microwave-plasma-assisted chemical vapor deposition using a H₂/CH₄/N₂ ratio of 4:1:2 at 80 Torr for 30 min. The precursor used was a sol-gel solution containing ferric nitrate, tetrabutyl titanate, and n-propanol. The carbon hybrid material (CNFB-CNT) exhibited excellent field emission properties, with its turn-on field being 1.77 V/μm. It also showed two field enhancement factors (1536 and 7932) for different electric fields. The emission current density of the hybrid remained higher than 0.65 mA/cm² for more than 50 h and was 0.82 mA/cm² even after 50 h of continuous emission. Further, the field emission properties of the CNFB-CNT hybrid were better than those of other single-structured carbon nanomaterials (CNTs, CNFs, or CNFBs). Therefore, the CNFB-CNT hybrid material should be a promising candidate for use in high-performance field emitters.     

Electron field emission properties on UNCD coated Si-nanowires

The electron field emission (EFE) properties of Si-nanowires (SiNW) were improved by coating a UNCD films on the SiNWs. The SiNWs were synthesized by an electroless metal deposition (EMD) process, whereas the UNCD films were deposited directly on bare SiNW templates using Ar-plasma based microwave plasma enhanced chemical vapor deposition (MPE–CVD) process. The electron field emission properties of thus made nano-emitters increase with MPE–CVD time interval for coating the UNCD films, attaining small turn-on field (E₀ = 6.4 V/μm) and large emission current density (J_e = 6.0 mA/cm² at 12.6 V/μm). This is presumably owing to the higher UNCD granulation density and better UNCD-to-Si electrical contact on SiNWs. The electron field emission behavior of these UNCD nanowires emitters is significantly better than the bare SiNW ((E₀)_SiNWs = 8.6 V/μm and (J_e)_SiNWs < 0.01 mA/cm² at the same applied field) and is comparable to those for carbon nanotubes.     

Filtration and inactivation of aerosolized bacteriophage MS2 by a CNT air filter fabricated using electro-aerodynamic deposition

Carbon nanotubes (CNTs) were coated on a sample of glass fiber air filter medium at atmospheric pressure and room temperature using electro-aerodynamic deposition (EAD). In the EAD method, CNTs (diameter: 50 nm, length: 2–3 μm) were aerosolized, electrically charged, and injected through a nozzle. A voltage was applied externally between the ground nozzle and a planar electrode on which the sample was located. The charged CNTs were deposited on the sample in a vertically standing posture even at a low flow velocity. Before the deposition experiment, a calculation was performed to determine the applied voltage by simulating the electric field, flow field, and particle trajectory. Using CNT-coated filter samples, virus aerosol filtration and anti-viral tests were carried out using the aerosol number counting method and the plaque counting method, respectively. For this purpose, bacteriophage MS2 was aerosolized with an atomizer. The particle filtration efficiency was increased to 33.3% in the most penetration particle size zone (100 nm) and the antiviral efficiency of the CNT filter was 92% when the coating areal density was 1.5 × 10⁹ #/cm². The susceptibility constant of virus to CNTs was 0.2 cm²/μg.     

Carbon nanotubes growth in AlPO4-5 zeolites: Evidence for density dependent field emission characteristics

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO₄-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO₄-5 crystallites were impregnated, for a period of ∼ 10–60 min, in the Fe-catalyst solution and subjected to Electron Spectroscopy for Chemical Analysis (E.S.C.A.). The analysis revealed that the concentration of Fe-catalyst, C_Fe, was increased from ∼ 1.7% to ∼ 8.6%, respectively, with increase in impregnation time, I_T. The HRTEM results showed that Fe nano-clusters, with diameter ∼ 7–10 nm, were formed in the surface region of the crystallites. These crystallites were sprayed on the conducting substrates, under identical spraying conditions. SEM study revealed that the coverage of the crystallites on the substrates was ∼ 10³–10⁴ crystallites/cm². These substrates were subjected to direct current plasma enhanced chemical vapor deposition (dc-PECVD) process, to grow CNTs. The SEM micrographs were recorded for the CNT-grown substrates and the average areal density of CNTs, (σ_T)_av, on the crystallites (t/cm²) was estimated. The analysis indicated that (σ_T)_av increased from ∼ 6.24 ± 0.19 × 10¹⁰ to 2.04 ± 0.61 × 10¹¹ t/cm² with gradual increase in C_Fe. The field emission study of the samples revealed that the optimum values of the turn-on electric field, ∼ 3.69 V/μm and the field emission current density, ρ_d, ∼ 1.78 × 10³ μA/cm² were achieved for (σ_T)_av, ∼ 6.24 ± 0.19 × 10¹⁰ t/cm², at a concentration of Fe, C_Fe, ∼ 3.0%, encapsulated in the AlPO₄-5 crystallites.     

A layer-by-layer deposition mechanism for producing a pyrolytic carbon coating on carbon nanotubes

Pyrolytic carbon (PyC) was deposited on carbon nanotubes (CNTs) in order to modify them by introducing defects to their surface. The deposition of PyC was carried out at temperature between 800 and 1000 °C using propane as carbon source with or without a hydrogen carrier gas at low pressure of 20 kPa. The structure of PyC coatings was examined using transmission electron microscopy. The PyC coating could be distinguished from the original CNT walls due to the difference of the structure, with the coating showing a less orderly layer structure. When H₂ was introduced during deposition, PyC coating started to form at 900 °C, and the deposition rate increased rapidly with increasing temperature. Without H₂, PyC coating with a thickness of a few layers could be formed at temperatures between 800 and 900 °C in 10 min. The outmost layer of the PyC coating showed a structure of rough and curved carbon fragment. A layer-by-layer mechanism is proposed for the deposition consisting of alternating fragment formation (nucleation) and lateral growth to layer.     

Effect of Ti particle size on mechanical and tribological properties of TiCN coatings prepared by reactive plasma spraying

Titanium carbonitride (TiCN) coatings were successfully fabricated by reactive plasma spraying (RPS) from agglomerated Ti-graphite feedstock. The effect of Ti particle size on the microstructure and phase composition of plasma sprayed TiCN coatings was investigated. The Vickers microhardness of coatings was measured by a Microhardness Test and the corresponding Weibull distribution were also analyzed. In addition, a pin-on-disk tribometer was employed to determine the trobological properties of coatings. Results show that all the coatings consist of TiC_xN_1−x (0 ≤ x ≤1) and minor Ti₂O phases, and the amount of Ti₂O increases with the increase of Ti particle size. The Weibull distribution of Vickers microhardness of all the coatings shows apparent scattering, while the coating sprayed with Ti particle size of 28 µm exhibits a relatively even distribution. Compared with the coating sprayed with Ti particle size of 14 µm or 48 µm, the coating sprayed with Ti particle size of 28 µm exhibits improved mechanical and tribological properties, which are attributed to the high microhardness and strong bonding strength.     

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H₂O₂ of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ～ 2, 1 M H₂O₂, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H₂O₂. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm²) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.     

High-current field emission of point-type carbon nanotube emitters on Ni-coated metal wires

The fabrication and field emission characteristics are reported for point-type carbon nanotube (CNT) emitters formed by transferring a CNT film onto a Ni-coated Cu wire with a diameter of 1.24 mm. A Ni layer plays a role in enhancing the adhesion of CNTs to the substrate and improving their field emission characteristics. On firing at 400 °C, CNTs appear to directly bonded to a Ni layer. With a Ni layer introduced, a turn-on electric field of CNT emitters decreases from 1.73 to 0.81 V/μm by firing. The CNT film on the Ni-coated wire produces a high emission current density of 667 mA/cm² at quite a low electric field of 2.87 V/μm. This CNT film shows no degradation of emission current over 40 h for a current density of 60 mA/cm² at electric field of 6.7 V/μm. X-ray imaging of a printed circuit board with fine features is demonstrated by using our point-type CNT emitters.     

Electrophoretic deposition of MnO2-coated carbon nanotubes on a graphite sheet as a flexible electrode for supercapacitors

A flexible electrode was prepared by microwave heating deposition of manganese oxide (MnO₂) on carbon nanotubes (CNTs) followed by electrophoretic deposition of the MnO₂-coated CNTs on a flexible graphite sheet (FGS). The prepared MnO₂-coated CNTs were characterized by scanning and transmission electron microscopy, and X-ray diffraction. A uniformly thin nano-scale MnO₂ coating was formed on the surface of the CNTs. The MnO₂-coated CNTs–FGS electrode showed highly capacitive behaviour in the 0.5 M Na₂SO₄ aqueous solution, with a specific capacitance of 442.9 F/g based on MnO₂ at 2 mV/s. It exhibited an excellent cycling stability with no more than 1.1% capacitance loss after 1000 cycles at 50 mV/s.     

Preparation of antireflective SiO2 nanometric films

Antireflective nanometric SiO₂ films were formed on glass substrates by dip coating from a colloidal SiO₂ sol having an average particle size of 9 nm. Withdrawal speed of dip coating was varied between 100 and 200 mm/min with 25 mm increments, and baking temperature of the films was altered between 300 and 550 °C with 50 °C increments. Obtained SiO₂ films were in 80–200 nm thickness range. Film thickness was seen to increase with increasing withdrawal speed and to decrease with increasing baking temperature. A maximum light transmittance of 95% was obtained with 4.5% points increase, from the films which were withdrawn at 100 mm/min and baked at 450 or 500 °C. It was seen from SEM observations that the films exhibited full coverage on glass surface and contained no voids or cracks. Size of SiO₂ particles in the film was seen in the AFM analyses to increase with baking temperature. Sintering of SiO₂ particles appeared to accelerate at temperatures over 450 °C.     

Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.     

High performance electrochemical H2O2 sensor based on MWCNT thin films fabricated by novel electron beam evaporation

High quality multi-walled carbon nanotubes (MWCNT) were grown by electron beam evaporation (EBE) under a high vacuum of 10⁻⁶ mtorr. The influence of deposition thickness on the orientation, morphology and vibrational bands of MWCNT films fabricated on tantalum (Ta) substrate was discussed. XRD patterns of the film revealed the presence of (002) preferential plane of carbon. Raman spectral analysis show the G-band Raman feature corresponding to high frequency E₂g of first order mode, suggesting that CNTs were composed of crystalline carbon. SEM image of 200 nm thick MWCNT film shows well shaped homogenous fine nanotubes of length ~300 nm and diameter ~70 nm with high purity. The electrochemical performance of the MWCNTs/Ta electrodes was studied by cyclic voltammetry. The sensor prepared with optimum thickness can detect H₂O₂ in the wide range covering 5 µM to 0.025 mM, with the detection limit as low as 0.09 µM. The results demonstrate that the fabrication of MWCNTs/Ta electrode by EBE is a very interesting and useful approach, likely to be a focus of upcoming research efforts in electrochemical sensing.     

Template-assisted synthesis of PbTiO3 nanotubes

Nanotubes of ferroelectric lead titanate (PbTiO₃) have been made by a template-assisted method. An equimolar Pb–Ti sol was dropped onto porous alumina membranes and penetrated into the channels of the template. Single-phase PbTiO₃ perovskite nanotubes were obtained by annealing at 700 °C for 6 h. The nanotubes had diameters of 200–400 nm with a wall thickness of approximately 20 nm. Excess PbO or annealing in a Pb-containing atmosphere was not necessary in order to achieve single-phase PbTiO₃ nanotubes. The influence of the heating procedure and the sol concentration is discussed.     

Novel method for the synthesis of a β-tricalcium phosphate coating on a zirconia implant

A novel method for the synthesis of a thin β-tricalcium phosphate (β-TCP) coating on zirconia implants has been developed. The synthesis procedure involves two steps: (i) rapid wet-chemical deposition of a biomimetic CaP coating and (ii) subsequent post-deposition processing of the biomimetic CaP coating, which includes a heat treatment at 900 °C followed by a short sonication in a water bath. The obtained β-TCP coating showed a uniform and dense morphology with a thickness of ≈500 nm and displayed a roughness in the nanometre range (R_a = 28 nm). The β-TCP coating demonstrated an apatite-mineralization ability in a simulated body fluid and enhanced the adsorption of serum proteins on the zirconia. Moreover, the β-TCP coating adhered firmly to the zirconia substrate, developing a notable scratch resistance (L_c = 97 N) and tensile strength (52 MPa) and showed strong resistance towards mechanical forces present during implantation of the coated zirconia implant into the artificial bone.     

Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer

A robust solid state diffusion joining technique for SiC ceramics was designed with a thickness-controlled Ti interlayer formed by physical vapor deposition and joined by electric field-assisted sintering technology. The interface reaction and phase revolution process were investigated in terms of the equilibrium phase diagram and the concentration-dependent potential diagram of the Ti-Si-C ternary system. Interestingly, under the same joining conditions (fixed temperature and annealing duration), the thickness of the Ti interlayer determined the concentration and distribution of the Si and C reactants in the resulting joint layer, and the respective diffusion distance of Si and C into the Ti interlayer differentiated dramatically during the short joining process (only 5 min). In the case of a 100 nm Ti coating as an interlayer, the C concentration in the joint layer was saturated quickly, which benefited the formation of a TiC phase and subsequent Ti₃SiC₂ phase. The SiC ceramics were successfully joined at a low temperature of 1000 °C with a flexural strength of 168.2 MPa, which satisfies applications in corrosive environments. When the Ti thickness was increased to 1 μm, Si atoms diffused easily through the diluted Ti-C alloy (a dense TiC phase was not formed), and the Ti₅Si₃ brittle phase formed preferentially. These findings highlight the importance of the diffusion kinetics of the reactants on the final composition in the solid state reaction, particularly in the joining technique for covalent SiC ceramics.