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.     

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been directly grown on Inconel 600 substrates by microwave plasma enhanced chemical vapor deposition without using any external catalyst. Grown CNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and field emission measurements. Characterization results show that field emission current density increases from 200 μA/cm² at ∼5.5 V/μm to 14.5 mA/cm² at ∼1.6 V/μA when substrate is heat-treated and incident microwave is attenuated before reaching it. Detailed characterization reveals that heat-treatment results in migration of Cr and Fe oxides towards the top surface which completely changes substrate morphology also. Microwave attenuation reduces reflection of microwaves from the substrate and increases residence time of the precursor over the substrate promoting high density growth of CNTs. The combination of these two process parameters resulted in growth of long, dense CNTs with bamboo-like defects that contributes to enhanced current density at lower applied field.     

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.     

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.     

The use of camphor-grown carbon nanotube array as an efficient field emitter

Three-dimensional growth of well-aligned high-purity multiwall carbon nanotubes (CNTs) is achieved on silicon, nickel-coated silicon and cobalt-coated silicon substrates by thermal decomposition of a botanical carbon source, camphor, with different catalyst concentrations. Field emission study of as-grown nanotubes in a parallel-plate diode configuration suggests them to be an efficient emitter with a turn-on field of ∼1 V/μm (for 10 μA/cm²) and a threshold field of ∼4 V/μm (for 10 mA/cm²). Maximum current density lies in a range of 20-30 mA/cm² at 5.6 V/μm with significant reversibility. Prolonged stability test of camphor-grown CNT emitters suggests a life time of ∼5 months under continuous operation. A new feature, metal-assisted electron emission from CNTs, has been addressed. Isolated nanotubes used as a cold cathode in a field emission microscope reveal the pentagonal emission sites and hence the atomic structure of the nanotube tips.     

High performance field emission of carbon nanotube film emitters with a triangular shape

We report novel two-dimensional (2D) shaped carbon nanotube (CNT) field emitters using triangular-shaped CNT films and their field emission properties. Using the 2D shaped CNT field emitters, we achieved remarkable field emission performance with a high emission current of 22 mA (equivalent to an emission current density >10⁵ A/cm²) and long-term emission stability at 1 mA for 20 h. We also discuss the field emission behavior of the 2D shaped CNT field emitter in detail.     

Fabrication of Ni-matrix carbon nanotube field emitters using composite electroplating and micromachining

Ni-matrix carbon nanotube (CNT) field emitters have been fabricated by composite electroplating and micromachining (CEMM) at room temperature. Pretreated multi-walled CNT and Ni are deposited onto a Cr/Cu conducting layer by composite electroplating and protruding tips of CNTs are obtained as emitters by etching away a layer of Ni, followed by emitter pixels which are formed by micromachining. Through the process of CEMM, CNTs are vertically embedded in the flat Ni substrate. No further treatment is needed to initiate or augment field emission and the field emitters exhibit good field-emission properties such as high current density (13 mA cm⁻² at an applied electric field of 3.4 V μm⁻¹), low turn-on field (0.53 V μm⁻¹), and good stability (110 h for 10% degradation of current density from 400 μA cm⁻²).     

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.     

Horizontally aligned carbon nanotube field emitters fabricated on ITO glass substrates

Horizontally aligned carbon nanotube (CNT) field emitters, in which electrons are emitted from the side of CNTs, are fabricated on indium tin oxide (ITO) glass substrates by electrophoretic deposition and fissure formation techniques. A thin film of CNTs is deposited onto an ITO glass plate using an aqueous mixture of CNTs and the cationic detergent cetyltrimethylammonium bromide by applying a negative voltage to the ITO glass plate. Then, an additional layer of sodium dodecyl sulfate (SDS), an anionic detergent, is deposited on the CNT film. This is done using an aqueous solution of SDS by applying a positive voltage. Through the process of firing, CNTs with a clean surface are exposed in the fissures produced. No further treatment is needed to initiate or augment field emission. The CNT field emitters show relatively good field-emission properties such as high current density (11 mA/cm² at an applied electric field of 4.3 V/μm), low turn-on field (2.2 V/μm), and good stability (98 h for 10% degradation of current density from 400 μA/cm²).     

Low-temperature CVD growth of carbon nanotubes for field emission application

For field emission application, carbon nanotube emitters were synthesized on catalyst-mixed thick-film electrode lower than 500 °C by chemical vapor deposition (CVD) method. Mixtures of Ni/tetraethyl silicate (TEOS) and conducting Ag powders were applied to fabricate the electrode by screen-printing processes. These processes are simple, low cost and easy to scale up for large sized panels. The field emission properties performed uniform emission image and high brightness (no less than 500 nits). The turn-on electric field was about 3.85 V/μm with an emission current destiny of 10 μA/cm², and the achieved current density was 1 mA/cm² driven by 5 V/μm.     

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.     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

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.     

Effect of Electrodeposition Parameters on the Microstructure and Corrosion Behavior of ‎DCPD Coatings on Biodegradable Mg–Ca–Zn Alloy

In this study, DCPD (Brushite, CaHPO₄.2H₂O) coatings were prepared on the surface of a Mg–Ca–Zn alloy using different current density (0.15–1.2 mA/cm²) and deposition time (5–90 min). The results revealed that DCPD with needle‐like morphology was observed for the current density between 0.15 and 0.4 mA/cm²?, whereas ?plate‐like morphology was obtained at current density above 0.8 mA/cm². The results showed that surface roughness increased with increasing current density. The lowest corrosion rate of 0.14 mm/year was obtained for the dense and uniform DCPD coating ?at 0.4 mA/cm², while further increase has deleterious effect on the corrosion resistance.     

Correlation between carbon–oxygen atomic ratio and field emission performance of few-layer reduced graphite oxide

We report the effect of carbon–oxygen atomic ratio (C/O ratio) on the field emission properties of the chemically reduced few-layer graphite oxide (GO). The field emission properties are found to be a non-monotonic function of the C/O ratio in a wide range of 2.06–14.80. Samples with C/O ratio of 6.98 show the lowest turn-on (1.80 MV/m), threshold fields (5.15 MV/m) and much higher current density (44.08 mA/cm² at 9.00 MV/m). Long-time (10 h) current stability test of the GO at a high current density (～13 mA/cm²) resulted in the reduction of the GO. The samples with field emission induced reduction show the same non-monotonic effect of the C/O ratio on the field emission properties as that of the chemically reduced GOs. The average current fluctuation of the GO is higher than that of the reduced GO, which is due to the oxygen desorption during the electron emission. The effect of the carbon–oxygen bonds on the surface potential barrier of the reduced GO edges is proposed in detail for interpreting the experimental observations.     

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.     

Self-heating Schottky emission from a ballasted carbon nanotube array

A ballast resistor is utilized in a low density vertically aligned carbon nanotube array. Based on the nature of the ballast resistor, the uniformity of the emission improves remarkably. A highly stable field emission current is obtained under a constant voltage and a current density of 300 mA/cm² is achieved. Joule heat generated by this field emission current increases the temperature of the CNT array significantly. The high temperature changes the emission to Schottky emission regime. The Schottky emission achieves 900 mA/cm², which is three times the field emission current density. Simulation result shows the corresponding temperature is about 1700 K. A color change of the emission area is observed after the experiment. When compared to the conventional Schottky cathode, the emitter is self-heating and no extra heater is needed. This is the first report of a successful utilization of a ballast resistor in a CNT based emission array and the first observation of Schottky emission from a vertically aligned CNT array used as an electron emitter.     

Growth of a three-dimensional complex carbon nanoneedle electron emitter for fabrication of field emission device

A three-dimensional complex carbon nanoneedle electron emitter film with high emission current density at low electric field has been developed by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibited novel film morphology. The scanning electron microscopy image taken from this film indicated a three-dimensional complex nanostructure emitter, the center of which was a carbon nanoneedle, and many small carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I–V characteristic addressed an emission current density of 251 mA/cm² at the electric field of 2.2 V/μm, and the field emission current was stable, making it possibly suitable for developing field emission device.     

The evaluation of nickel deposit obtained via Watts electrolyte at ambient temperature

A series of electroplating works were conducted to investigate the best conditions for the electrodeposition of nickel on a mild steel substrate. The electrodeposition was done at ambient electrolyte temperature with mild agitation and under current density ranging from 10 to 50 mA/cm². X-ray diffraction analysis (2θ for first three peaks = 44.6, 51.9, and 76.8) and Energy Dispersive Spectrometer verified the presence of a pure nickel coating. Under field emission scanning electron microscopy analysis, the coating shows a typical nodular surface morphology, while cross-sectional microstructures show a compact nickel layer. Vickers hardness testing shows that the coating hardness gave the highest value of 293 HV at 30 mA/cm² current density.     

Enhanced electron emission from functionalized carbon nanotubes with a barium strontium oxide coating produced by magnetron sputtering

Carbon nanotubes (CNTs) were functionalized with a surface coating using magnetron sputter deposition. The CNT samples used were prepared by plasma enhanced chemical vapor deposition and were vertically aligned to the surface of the tungsten substrate. A thin layer of barium strontium oxide approximately 100 nm in thickness was deposited on their surface using magnetron sputtering. The oxide coating was uniform, covering the whole surface of the CNTs and significantly lowered the work function while preserving the geometry. The resulting oxide coated CNTs had a work function of 1.9 eV and a field enhancement factor of 467, which led to a significant improvement in both field and thermionic emission. Compared to uncoated CNTs, the field emission was increased by a factor of two, while the thermionic emission increased by more than four orders of magnitude. At 4.4 V/μm, a field emission current of 23.6 μA was obtained from an emitting surface of 0.012 cm². Similarly, at 1.1 V/μm and 1221 K, a thermionic emission current of 14.6 mA was obtained.