Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

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.     

Field emission of ribonucleic acid–carbon nanotube films prepared by electrophoretic deposition

Carbon nanotube (CNT) films are fabricated on indium tin oxide (ITO) glass substrates by combining electrophoresis with photolithography using ribonucleic acid (RNA)–CNT hybrids as functionalized CNTs and their emission properties are investigated. The CNTs are well-dispersed by wrapping them with RNA and well-defined RNA–CNT patterns are obtained on the ITO glass substrate. The RNA–CNT films show good field emission properties, such as high current densities, low turn-on fields, and uniform emission images. The RNA–CNT hybrids compare favorably to other functionalized CNTs for use in the electrophoretic deposition.     

Influence of length distributions of carbon nanotubes on their field emission uniformity in the paste-printed dot arrays

Arrays of carbon nanotube (CNT) emitter dots with a diode structure were fabricated by screen printing and subsequent photolithographic patterning of a CNT paste. Two different types of raw CNTs, which were of narrow and wide distributions of lengths, were used in preparing the photosensitive CNT pastes. Field emission uniformity of the arrays of CNT emitter dots was characterized by statistically analyzing brightness data of individual dots. It seemed that the narrow length distribution of raw CNTs resulted in uniform heights of standing CNT emitters in dots, consequently giving rise to high emission uniformity over an area.     

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N₂. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.     

New approach to enhance adhesions between carbon nanotube emitters and substrate by the combination of electrophoresis and successive electroplating

Complementary electroplating combined with electrophoresis enhanced the field emission characteristics of emitters by improving the adhesions between CNT emitters and substrate. The emitting current of the CNT emitters prepared by our combined method increased nine times higher than that of CNT emitters prepared by electrophoresis only, since electroplating improved the adhesion of CNT emitters. During the life-time measurement for 10 h, the emitting current of CNT emitters fabricated by electrophoresis only was drastically decreased to 13% of the initial current, while that prepared by the combination of electrophoresis and successive electroplating decreased to 64% of the initial current. We suggest that our method is a promising approach for the efficient fabrication of reliable CNT emitters.     

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.     

Synthesis of patterned carbon nanotube arrays for field emission using a two layer Sn/Ni catalyst in an ethanol flame

Patterned carbon nanotube (CNT) arrays on Si substrate have been fabricated by using a two layer Sn/Ni catalyst in a diffusion ethanol flame. Vertically well-aligned CNT arrays were achieved on a Si substrate without any catalyst pretreatment. The Sn underlayer activated the substrates for CNT growth with Ni as catalyst, and provided a good contact between CNTs and the substrate, which is useful for field emission. Since the adhesion of Sn/Ni nanoparticles to the substrate is very strong, the growth of the CNTs follows a base-growth mode. The thickness of the Sn underlayer largely determines the diameter and diameter distribution of the as-grown CNTs. The morphologies and field electron emission properties of CNT arrays grown on Si substrates with different thicknesses of Sn and growth times have been investigated. The variation of emission current density was less than 5% during a 4 h test under a field of 1.77 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.     

Enhanced field emission properties of a reduced graphene oxide/carbon nanotube hybrid film

A facile vacuum filtration method for the preparation of hybrid films to achieve superior field emission properties from carbon nanotubes (CNTs) using reduced graphene oxide (rGO) as a bi-functional filler has been proposed. In the hybrid films, CNTs serve as electron emitters, while rGO helps to control the density of the CNT-emitters and reduce electrical resistance of the films. Via controlling volumes of CNTs and rGO dispersions, electron field emission properties of the hybrid films can be easily tailored. Higher weight ratio of rGO:CNT results in better electrical properties and the best field emission property is achieved when a rGO:CNT weight ratio of 1:3 is employed. The hybrid film reveals a significant improvement in field emission properties, as compared with the CNT film without adding rGO. Decreases in sheet resistance, turn-on field, and threshold field are attributed to the formation of extended conjugated network between CNTs and rGO in association with the reduction of screening effect through the optimization of density of CNT-emitters. The concept that rGO can be employed to control the density of CNT emitters will be of special interest for field emission enhancement.     

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).     

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.     

Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays

Strong, conducting, transparent carbon nanotube sheets were prepared by solid-state draw from well-ordered, aligned multiwalled carbon nanotube (MWCNT) forests [Zhang et al., 2005] [1]. Study of electron field emission from such transparent MWCNT sheets shows threshold fields of less than 0.5 V/μm with current densities high enough for display applications. Step-like field emission current increase and hysteresis behavior in I-V curves has been observed. The origin of such behavior is discussed in terms of mechanical rearrangement of the nanotube network in high electric field. Studied MWCNT transparent sheet field emission cathodes have several advantages when used as multi-functional electrodes. They are high current, high stability, transparent, and flexible field emission sources and can be used in an inverted geometry, with cathode being in front of the light emitting plate. At the same time transparent CNT sheets may serve as a transparent conducting electrode for electrical connection and pixel addressing in field emission displays (FEDs). Also, these sheets can be used as an optical polarizer in FEDs.     

Enrichment of metallic carbon nanotubes by electric field-assisted chemical vapor deposition

We report herein a rational approach to increase the proportion of metallic carbon nanotubes (CNTs) in horizontally aligned ultralong CNT arrays by electric field-assisted chemical vapor deposition. In a gas flow-directed growth mode, the buoyancy caused by temperature differences near the substrate can lift catalyst particles or CNTs from the substrate into the laminar flow so that ultralong CNT arrays with mixed metallic (m-) and semiconducting (s-) CNTs can be obtained. It was verified that the percentage of m-CNTs was about 47% for pristine CNTs. When an electric field was introduced during CNT growth, the grown CNTs were polarized and the generated electric field force assisted them into the laminar flow. The greater polarizability of m-CNTs compared to s-CNTs resulted in more m-CNTs lifted and an increased m- to s-CNT ratio in the array. Measurements of CNT electrical properties showed that the percentage of m-CNTs could reach 80% when the electric field intensity was set at 200 V/cm.     

Improvement of electron field emission from carbon nanotubes by Ar neutral beam treatment

To improve the field emission properties of screen printed carbon nanotube (CNT) films, an Ar neutral beam was used as one of the surface treatment techniques and the CNT field emission characteristics after the treatment were compared with those after Ar ion beam treatment. The Ar neutral beam treatment enhanced the field emission properties of the CNTs and by decreasing the turn-on field and by increasing emission sites. When the field emission properties were measured after the treatment for 10 s with an energy of 100 eV, the turn-on field decreased from 1.7 to 0.9 V/μm while that after the ion beam treatment increased from 1.7 to 2.8 V/μm showing damage of exposed CNTs due to the intensive bombardment by the positive ions in the beam. The neutral beam treatment appeared to expose more CNT field emitters from the CNT paste without cutting or severely damaging the already exposed long CNT emitters because there were no charged particles in the beam.     

Highly stable carbon nanotube field emitters on small metal tips against electrical arcing

Carbon nanotube (CNT) field emitters that exhibit extremely high stability against high-voltage arcing have been demonstrated. The CNT emitters were fabricated on a sharp copper tip substrate that produces a high electric field. A metal mixture composed of silver, copper, and indium micro- and nanoparticles was used as a binder to attach CNTs to the substrate. Due to the strong adhesion of the metal mixture, CNTs were not detached from the substrate even after many intense arcing events. Through electrical conditioning of the as-prepared CNT emitters, vertically standing CNTs with almost the same heights were formed on the substrate surface and most of loosely bound impurities were removed from the substrate. Consequently, no arcing was observed during the normal operation of the CNT emitters and the emission current remained constant even after intentionally inducing arcing at current densities up to 70 mA/cm².     

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.     

Electrophoretic deposition of carbon nanotubes

Electrophoretic deposition (EPD) has been gaining increasing interest as an economical and versatile processing technique for the production of novel coatings or films of carbon nanotubes (CNTs) on conductive substrates. The purpose of the paper is to present an up-to-date comprehensive overview of current research progress in the field of EPD of CNTs. The paper specifically reviews the preparation and characterisation of stable CNT suspensions, and the mechanism of the EPD process; it includes discussion of pure CNT coatings and CNT/nanoparticle composite films. A complete discussion of the EPD parameters is presented, including electrode materials, deposition time, electrode separation, deposition voltage and resultant electric field. The paper highlights potential applications of the resulting CNT and CNT/composite structures, in areas such as field emission devices, fuel cells, and supercapacitors.     

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).     

Optimization for field emission from carbon nanotubes array in hexagon

In order to investigate the influence of the arrangement of the carbon nanotubes (CNTs) array on the field emission, the more practical model in hexagon was proposed. From the calculation with the image floated sphere model, the results showed that the arrangement has little influence on the field emission and the intertube distance R of CNTs array critically affects the field emission from the CNTs array, which accords with the results from the numerical simulations and experiments. When R is less than the height of tube h, the enhancement factor decreases rapidly with R. Considering the field emission current density, the field emission can be optimized when R is comparable with h, which accorded with the results from experiments. Furthermore, the influence of the anode–cathode distance d on the field emission from CNTs array was also discussed, which proved that d has little effect on the field emission from CNTs array. For the fixed field strength E for the certain materials in filed emission, we can reduce the threshold voltage to some extent by decreasing d in the case of R > 2h.