Bending actuation and charge distribution behavior of polyurethane/carbon nanotube electroactive nanocomposites

The electroactive nanocomposite films were fabricated using polyurethane (PU) with modified carbon nanotubes (CNTs) as the filler. The CNTs were modified using microwave‐induced polymerization route and they were found to be highly dispersed in polar solvents such as dimethylformamide. The modified CNTs were characterized using transmission electron microscopy, field emission scanning electron microscopy, thermogravimetric analysis, and X‐ray photoelectron spectroscopy. To evaluate these films we mainly focused on electrical properties such as actuation behavior, resistivity, impedance analysis, and space charge measurements. We found that the PU/CNT films bent toward the cathode when an electric field was applied and they reverted to its original position when the electric field was removed. Upon the incorporation of CNTs as the filler for the polymer, the electrical properties of the films improved significantly. Asymmetric charge accumulation was observed from space charge measurements in some of the films and this explains the bending deformation and the actuation behavior. POLYM. COMPOS., 262–269, 2016. © 2014 Society of Plastics Engineers     

Novel plasma chemical vapor deposition method of carbon nanotubes at low temperature for field emission display application

We developed a novel growth method of aligned carbon nanotubes. A high density plasma chemical vapor deposition (PECVD) has been employed to grow high-quality carbon nanotubes (CNTs) at low temperatures. High-density, aligned CNTs can be grown on Si and glass substrates. The CNTs were selectively-deposited on the patterned Ni catalyst layer, which was sputtered on Si. The CNTs exhibited a turn-on field of 0.9 V/μm and an emission current of 480 μA/cm² at a field of 3 V/μm.     

Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I–V measurements show that, with a thin ta-C film coating, the threshold electric field (E_thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E_thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm² at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.     

Field emission from metal-containing amorphous carbon composite films

Metal-containing (Co, Al and Ti) amorphous carbon composite films (a-C:Me) have been prepared by the filtered cathodic arc technique using metal-containing graphite targets at room temperature. Field emission properties of the heat-treated a-C:Me films were improved and were found to be dependent on the metal content and variety of metals. After heat-treatment at 550°C in a mixture of acetylene and nitrogen gases, the field emission properties of a-C:Co films were significantly improved, in which Co acted as catalysts to enhance graphitization as well as formation of carbon nanotubes during heat-treatment. A threshold electric field of less than 2 V/μm was obtained from the heat-treated a-C:Co composite films without conditioning. The heat-treated a-C:Al and a-C:Ti films, though the conditioning step could be avoided and relatively low threshold fields could be obtained, exhibited relatively low emission site densities, however. The a-C:Me films, which can be deposited with a high rate at room temperature and require a relatively low temperature, heat-treatment process to enhance electron emission, are promising for practical applications in field emission display.     

Enhanced field emission properties of films consisting of Fe-core carbon nanotubes prepared under magnetic field

Films consisting of carbon nanotubes containing iron (CNTs-Fe) were prepared on tungsten substrates for electron field emission. The films were prepared by drying the mixture of the CNTs-Fe with poly [3-octyl-thiophene] (P3OT) or toluene under a magnetic field using a permanent magnet of surface magnetic flux 340 mT. The field emission properties of the films were measured in comparison with the films prepared without the magnetic field. The films prepared under the magnetic field showed better field emission characteristics than those without the magnetic field, and the electric field to generate the required field emission current was significantly decreased by applying the magnetic field in the film preparation process. Microscopic analyses indicated that the magnetic field had the effect of forcing the CNTs-Fe to stand perpendicular to the substrate, which causes the enhanced field emission properties.     

Field emission properties of Cu/multiwalled carbon nanotube composite films fabricated by an electrodeposition technique

Composite films of Cu and multiwalled carbon nanotubes (MWCNTs) were fabricated by an electrodeposition technique, and their field emission properties were examined. Commercially available MWCNTs with various diameters (60–150 nm) were used. The microstructure of the composite films was analyzed by scanning electron microscopy and the field emission properties were measured using a diode-type system. Cu/MWCNT composite films with homogeneous dispersion of MWCNTs were fabricated using each type of MWCNT. Bare MWCNTs were present on the surface of the composite films and the ends of the protruding tips were fixed by the deposited copper matrix. The composite films produced clear emission currents and the corresponding Fowler–Nordheim (F–N) plots showed that these were field emission currents. The turn-on electric field tended to decrease with decreasing MWCNT diameter. A light-emitting device incorporating the Cu/MWCNT composite film as a field emitter was fabricated, and its light-emitting properties were investigated. Light emission with a brightness of around 100 cd m^?2 was observed for approximately 100 h.     

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.     

Field emission from carbon nanotubes on a graphitized carbon fabric

Multi-walled carbon nanotubes (MWCNTs) have been directly grown over a flexible graphitized carbon fabric by water assisted chemical vapor deposition. Field emission properties are compared with randomly oriented multi-walled and single walled carbon nanotube field emitters obtained by spin coating on to carbon fabric. The MWCNTs and single walled carbon nanotubes (SWCNTs) used in spin coating were characterized by X-ray diffraction (XRD) and Raman spectroscopy. High resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) were used to characterize the field emitters. The use of graphitized carbon fabric as substrate has brought in flexibility in the fabrication of carbon nanotube field emitters. The samples show good field emission properties with a fairly stable emission current. Analysis of field emission based on the Fowler-Nordheim theory reveals current saturation effects at high applied fields for all the samples.     

CVD growth and field emission characteristics of nano-structured films composed of vertically standing and mutually intersecting nano-carbon sheets

Nano-structured films composed of rather flat nano-carbon sheets which roughly stood vertically on Si wafers and which intersected each other at large angles were grown by means of a high-power microwave-plasma chemical-vapour-deposition (MWPCVD) method. The structure of the fabricated films was investigated using scanning electron microscopes and Raman spectroscopy. Field emission (FE) currents obtained from these films reached 50 mA/cm² at a macroscopic electric field of 3.6 V/μm. The observed FE characteristics were analyzed using a modified Fowler–Nordheim (F–N) equation where the field enhancement factor and effective emission area were treated as field-dependent parameters. It was found from the analysis that the vertically standing nano-carbon flat sheets had larger field enhancement factors and less FE areas, compared with those obtained for wrinkled nano-carbon sheets previously reported by the authors. It is suggested that the observed variations in the FE current without saturation behavior as a function of the macroscopic electric field can be explained in terms of an effective surface geometry phenomenon of the concerned films.     

Carbon nanotube growth at 420 °C using nickel/carbon composite thin films as catalyst supports

Nickel/carbon composite (Ni/C) thin films were used as catalyst supports for the growth of vertically aligned multiwalled carbon nanotubes (MWCNTs) at temperature as low as 420 °C. Nickel nanoparticles embedded within the carbon matrix of Ni/C films have served as catalysts for the synthesis of nanotubes by PECVD using acetylene/ammonia plasma. Two different nickel contents (40 at.% and 60 at.%) in the films were used. Analysis indicated a diffusion of nickel atoms in the form of nanoparticles to the film surface upon annealing. This diffusion depends on both annealing temperature and nickel concentration in the films and affects the MWCNT growth at low temperature. The MWCNT synthesis was tested at growth temperature ranging between 335 and 520 °C. The growth of MWCNTs at 420 °C was only achieved by using Ni/C films with a high nickel content (60 at.%). These MWCNTs did not present considerable loss in their growth rate and structural quality compared to MWCNTs grown on classical substrates (Ni catalysts deposited on TiN), at higher temperature (520–600 °C). The results suggest that carbon saturation at the surface and subsurface of nickel catalysts of the Ni/C films is responsible for the improvement of MWCNT growth at low temperature.     

Effect of Purity and Substrate on Field Emission Properties of Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapour decomposition (CVD) of acetylene over Rare Earth (RE) based AB₂ (DyNi₂) alloy hydride catalyst. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Thermo Gravimetric Analysis and Raman Spectroscopy. Fully carbon based field emitters have been fabricated by spin coating a solutions of both as-grown and purified MWNT and dichloro ethane (DCE) over carbon paper with and without graphitized layer. The use of graphitized carbon paper as substrate opens several new possibilities for carbon nanotube (CNT) field emitters, as the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon paper and reduces contact resistance. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. CNT field emitter prepared by spin coating of the purified MWNT–DCE solution over graphitized carbon paper shows excellent emission properties with a fairly stable emission current over a period of 4 h. Analysis of the field emission characteristics based on the Fowler–Nordheim (FN) theory reveals current saturation effects at high applied fields for all the samples.     

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.     

Improved field emission characteristics of nano-structured carbon films deposited on polycrystalline CVD diamond

Nano-structured carbon films were deposited on polycrystalline diamond films grown on Si wafers by means of high-power microwave-plasma chemical-vapour-deposition (MWPCVD) method. Scanning electron microscope images show that the deposited carbon films were composed of wrinkled graphitic nano-sheets with considerably disordered structures and carbon needles on the CVD diamond grains. Field emission (FE) characteristics obtained from such films yielded very high FE currents, being larger than 100 mA/cm² at a macroscopic electric field of 9.5 V/μm. A possible mechanism of the observed strong FEs is discussed in relation to a modified Fowler–Nordheim (F–N) equation considering field-dependent parameters. Rugged CVD diamond grains played an important role in enhancing the FE current density. These experimental results suggest that some field-dependent effect should be taken into account as well as the surface geometry effect to quantitatively explain the increases in the FE current density observed in the region where no saturation behaviour of the FE current occurred.     

Growth and high current field emission of carbon nanofiber films with electroplated Ni catalyst

We report high current-density field emission from carbon nanofiber (CNF) films synthesized using electroplated Ni catalysts on gold-buffer layers via hot-filament chemical vapor deposition. High-density thick CNFs which had a solid structure without hollow cores and many protrusions on the outside of CNF body were formed. The protrusions consisted of buckled small graphitic sheets, and some protrusions had very small tip radius to which we attribute good field emission from CNF films. The maximum emission current of 3.67 mA was measured from the area of 4.9 × 10^{− 3} cm², corresponding to the current density of 750 mA/cm², at the electric field of 12.5 V/μm. There was a distinctive hysteresis in emission–current curves measured while ramping up and down the bias-voltage. The deviations between up- and down-sweep emission currents, and the slope change in Fowler–Nordheim curves were most prominent in medium-voltage and -current regime. Moreover, the emission–current hysteresis showed dependence on the pressure during measurement and the voltage-sweep speed. We propose that adsorbate-enhanced field emission and adsorbate desorption during field-emission measurement were responsible for the observed emission behavior.     

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.     

The stability of the CNT/Ni field emission cathode fabricated by the composite plating method

A novel composite plating method has been developed for the fabrication of carbon nanotube/Ni (CNT/Ni) field emission cathode. The field emission properties of the initial CNT/Ni field emitter show a low turn-on electric field E_on of about 1.1 V/μm with an emission current density of 1 μA/cm², and a low threshold electric field E_th of about 1.7 V/μm with an emission current density of 1 mA/cm². After performing a stability test with a high emission current density in high vacuum, the corresponding microstructure and the degree of graphitization of the CNT/Ni field emitter were measured by using scanning electron microscopy and Raman spectroscopy. We found that the degree of graphitization slowly decreases with the duration time t_FE of the stability test, the size of small rod-like CNT/Ni composite structures in the film increases with t_FE, and obvious cracks appear in the film as t_FE is larger than 60 h. The degradation of the field emission properties may be explained by the Joule heating effect on the CNT/Ni field emitter under high emission current density.     

Optical and electrical properties of a-C:H deposited by magnetic confinement of r.f. PECVD plasma

Amorphous hydrogenated carbon (a-C:H) thin films have been deposited in an r.f. PECVD chamber using a magnetic multipole system to confine the plasma. The influence of magnetic field on both the plasma parameters and the film properties has been studied. The results are compared with those obtained under similar conditions using a standard PECVD system. Optical emission spectroscopy (OES) shows an increase in the intensity of the hydrogen and C–H lines in the plasma. EELS, optical, electrical and electron field emission measurements have been used to characterize the deposited films. The sp³/sp² ratio was increased using the magnetic field and the optical gap was also increased as compared to films grown using a standard process. The electron field emission was found to be improved (higher current density and smaller barrier height) for samples deposited in the presence of the magnetic field.     

Excellent field emission characteristics from few-layer graphene–carbon nanotube hybrids synthesized using radio frequency hydrogen plasma sputtering deposition

The growth of few-layer graphene (FLG) on carbon nanotubes (CNTs) was realized by using radio frequency hydrogen plasma sputtering deposition. A defect nucleation mechanism and a two dimensional growth model of the FLG were proposed, and field emission characteristics of these FLG–CNT hybrids were studied. They show excellent field emission properties, with a low turn-on electric field (0.98 V/μm) and threshold field (1.51 V/μm), large field enhancement factor (～3980) and good stability behavior, which are much better than those of the as-grown CNT arrays. The sharp edges and the low work function of the hybrids are believed to be responsible for the improved field emission properties.     

Effects of Heat Treatment on Structural,Optical and Magnetic Properties of Electro Deposited Fe–Ni–P Thin Films

Nanocrystalline Fe–Ni–P thin films have been prepared by electrodeposition method for different temperature. The structural, morphological, mechanical and magnetic properties are analyzed by different measurements. The structural and surface morphology of thin film were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD results showed all the electro deposited Fe–Ni–P films exhibits nanocrystalline FCC structure. The SEM pictures of Fe–Ni–P thin films show that the deposits of thin films are crack free, uniform and bright surface. The mechanical properties of Fe–Ni–P thin films have been analyzed by VHT. The electroplated Fe–Ni–P thin films were strongly adherent to the copper substrate. The VHT result of Fe–Ni–P thin films shows that thin film coated at high bath temperature has highest hardness value. Also VSM result shows that thin film coated at high bath temperature has higher magnetization value. This shows that the soft magnetic properties of Fe–Ni–P thin films are greatly enhanced by various temperatures.     

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.