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.     

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.     

Growth and electron field emission properties of ultrananocrystalline diamond on silicon nanostructures

The electron field emission (EFE) properties of Si nanostructures (SiNS), such as Si nanorods (SiNR) and Si nanowire (SiNW) bundles were investigated. Additionally, ultrananocrystalline diamond (UNCD) growth on SiNS was carried out to improve the EFE properties of SiNS via forming a combined UNCD/SiNS structure. The EFE properties of SiNS were improved after the deposition of UNCD at specific growth conditions. The EFE performance of SiNR (turn-on field, E₀ = 5.3 V/μm and current density, J_e = 0.53 mA/cm² at an applied field of 15 V/μm) was better than SiNW bundles (turn-on field, E₀ = 10.9 V/μm and current density, J_e < 0.01 mA/cm² at an applied field of 15 V/μm). The improved EFE properties with turn-on field, E₀ = 4.7 V/μm, current density, J_e = 1.1 mA/cm² at an applied field of 15 V/μm was achieved for UNCD coated (UNCD grown for 60 min at 1200 W) SiNR. The EFE property of SiNW bundles was improved to a turn-on field, E₀ = 8.0 V/μm, and current density, J_e = 0.12 mA/cm² at an applied field of 15 V/μm (UNCD grown for 30 min at 1200 W).     

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.     

Carbon nanotube loop arrays for low-operational power,high uniformity field emission with long-term stability

We present a rational field emitter array architecture composed of thin multi-walled carbon nanotube “loops” which simultaneously satisfies the important requirements for practical applications. We achieved low turn-on voltage (1.27 V/μm for 10 μA/cm² emission), high enhancement factor (2400), uniformity, and long-term emission stability exceeding 10,000 h at 1 mA/cm², where each of the values approaches or exceeds the highest reported values to date for field emission arrays.     

Field emission properties of nano-structured carbon films with carbon needles deposited on Si using high-power-density microwave-plasma CVD method

Nano-structured carbon films (NCFs) were fabricated on Si wafer chips with hydrogen–methane gas mixture by creating high-power-density microwave (MW) plasma in a quartz tube with a low-output MW power source. Carbon sheets with many nano-protuberances were complicatedly twisted each other in the center region of the NCF while carbon needles were additionally formed in the fringe region of the NCF. These NCFs yielded field emission (FE) characteristics with the FE current density of 68 mA/cm² at a macroscopic electric field, E, of 10 V/μm. Since the FE currents tended to be saturated even in a medium E region, no simple Fowler–Nordheim (F–N) model was applicable. A model considering the F–N mechanism, statistic effects of FE tip structures and a space-charge-limited-current effect has been applied successfully to explain all the FE data observed for E < 10 V/μm. It turned out that E-dependent parameters such as the effective total emission area and the effective field enhancement factor should be employed to obtain sufficient quantitative agreements between the model and the experiments.     

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.     

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.     

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.     

Fabrication of double-sided field-emission light source using a mixture of carbon nanotubes and phosphor sandwiched between two electrode layers

A double-sided surface light source based on field emission (FE) using an alternating current power source is demonstrated. Carbon-nanotube (CNT) emitters and ZnS phosphor are mixed and screen-printed onto two pieces of indium tin oxide glass that were assembled together with the coated surfaces facing each other to make a parallel-plate, diode-structure FE device. The device has a double-sided luminance distribution with a turn-on field of 2 V/μm, a good uniformity, and a stable luminance of 4000 cd/m². The results show that CNTs not only act as good field emitters but also as an electrically conductive network around the isolated phosphors. The network prevents electric arcing and thus extends the lifetime of the device.     

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.     

Packing density control of carbon nanotube emitters in an anodic aluminum oxide nano-template on a Si wafer

Carbon nanotubes (CNTs) emitters in the AAO templates on a silicon wafer were fabricated. The packing density of CNTs was changed in the order of 1 × 10⁷ − 7 × 10⁷ tips cm^− 2 depending upon the concentration of hydrogen in the reactant gas mixture. The emission current density was strongly dependent upon the packing density of the CNT emitters. Low turn-on fields of 1.6–2.1 V/μm and field enhancement factor of 1900–4970 were observed.     

Super growth of vertically-aligned carbon nanofibers and their field emission properties

We demonstrate a very efficient synthesis of vertically-aligned ultra-long carbon nanofibers (CNFs) with sharp tip ends using thermal chemical vapor deposition. Millimeter-scale CNFs with a diameter of less than 50 nm are readily grown on palladium thin film deposited Al₂O₃ substrate, which activate the conical stacking of graphitic platelets. The field emission performance of the as-grown CNFs is better than that of previous CNFs due to their extremely high aspect ratio and sharp tip angle. The CNF array gives the turn-on electric field of 0.9 V/μm, the maximum emission current density of 6.3 mA/cm² at 2 V/μm, and the field enhancement factor of 2585.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

Enhancement of electron field emission of nitrogenated carbon nanotubes on chlorination

Multi-wall nitrogenated carbon nanotubes (N-CNTs) are modified by the chlorine–plasma treatment (N-CNTs:Cl) and hence studied their field emission characteristics. It is observed that the turn-on voltage of N-CNTs is decreased from ~ 1.0 V/µm to ~ 0.875 V/µm on chlorination. The current density is enhanced from 1.3 mA/cm² (N-CNTs) to ~ 15 mA/cm² (N-CNTs:Cl) at an electric field of 1.9 V/µm. The X-ray absorption near edge structure spectra revels, the formation of different bonding of chlorine with carbon and nitrogen presence in the N-CNTs during the process of chlorine–plasma treatment by the charge transfer (or else) that increase the density of free charge carriers and hence enhanced the field emission characteristics of N-CNTs:Cl.     

High current field emission from individual non-linear resistor ballasted carbon nanotube cluster array

Field emission (FE) electron sources based on carbon nanotubes (CNTs) have the potential to serve as cold cathodes for various vacuum microelectronic and nanoelectronic devices. Emission currents are extremely sensitive to variation in emitter geometry and local surface states, both of which are difficult to synthesize uniformly when fabricating a CNT field emission array (FEA). Such non-uniformities cause unstable emission, limiting the current output. Here, we propose a method for simulating and fabricating a high performance CNT-FEA with emission units that are individually connected to a single crystalline silicon pillar (SP), which acts as an non-linear ballast resistor. Results showed that the driving field for this CNT-FEA was greatly reduced relative to CNT-FEAs on a flat silicon substrate. This improvement was due to the high aspect ratio of the CNT clusters combined with SPs. The FE behavior demonstrated that the emission current was limited by the non-linear resistors (NLRs). Emitted currents density over 1.65 A/cm² at a low extraction field of 5.8 V/μm were produced by a 1 mm² emmiting area. The proposed technology may be used to fabricate cathodes capable of reliable, uniform, and high current emission.     

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.