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 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).     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.     

Stable Field Emitters for a Miniature X-ray Tube Using Carbon Nanotube Drop Drying on a Flat Metal Tip

Stable carbon nanotube (CNT) field emitters for a vacuum-sealed miniature X-ray tube have been fabricated. The field emitters with a uniform CNT coating are prepared by a simple drop drying of a CNT mixture solution that is composed of chemically modified multi-walled CNTs, silver nanoparticles, and isopropyl alcohol on flat tungsten tips. A highly thermal- and electrical-conductive silver layer strongly attaches CNTs to the tungsten tips. Consequently, the field emitters exhibit good electron emission stability: continuous electron emission of around 100 μA at 2.3 V/μm has stably lasted over 40 h even at non-high vacuum ambient (~10⁻³ Pa).     

Enhanced surface morphologies of screen-printed carbon nanotube films by heat treatment and their field-emission properties

A heating process for obtaining free-standing carbon nanotube emitters is presented with the aim of improving field-emission properties from the screen-printed multiwalled carbon nanotube (MWCNT) films. Using an atmosphere with an optimum combination of nitrogen and air for heat treatment of CNT films, the CNT emitters can be made to protrude from the surface. This allows for a high emission current and the formation of very uniform emission sites without special surface treatment. The morphological change of the CNT film by this technique has eliminated additional processing steps, such as surface treatment which may result in secondary contamination and damage to the film. Despite its simplicity the process provides a high reproducibility in emission current density which makes the films suitable for practical applications.     

High-performance field emission from a carbon nanotube carpet

We have created a field emitter composed of a carbon nanotube (CNT) yarn, which was prepared by direct spinning through chemical vapor deposition and then formed into a carpet structure by tying the yarn to a conductive substrate before cutting it. The structure of the carpet is arranged to induce the tips of the CNT yarn to protrude toward the anode for maximum electron emission. The turn-on field, threshold field, and field enhancement factor of the device are 0.33, 0.48 V/μm, and 19,141, respectively. Extremely low operating electric fields and a high field enhancement factor result from the high density of CNT emitters with high crystallinity, the electrically good contact between the emitters and the substrate, and the effects of the multistage structure. The emission is stable even at a high current density of 2.13 mA/cm², attributed to the strong adhesion between the emitters and the substrate. The emission performance is found to be customizable by adjusting the structure, for example, the CNT pile density. These results are relevant for practical applications, such as large-area flat-panel displays, large-area low-voltage lamps, and X-ray sources.     

Field emission characteristics of a single free standing carbon nanotube with gate electrode

In this paper, we have constructed and analyzed the field emission behavior of a single vertically aligned free standing carbon nanotube (CNT) with a gate electrode in order to verify the feasibility of using a single CNT as the low-voltage field emission electron source. The single vertically aligned CNT with gate electrode was fabricated by combining optical lithography, electron beam lithography (EBL) and inductively coupled plasma chemical vapor deposition (ICP-CVD) processes. A self-aligned process with a single mask was utilized to define the gated structure and the nano-size catalyst for CNT growth. A single vertically aligned CNT was then grown within the gate hole by ICP-CVD. The length-to-diameter ratio of CNT could be varied by adjusting the e-beam exposure time, and the CNT height was controlled to equal to the gate-to-cathode spacing (800) nm in one gated device and less than the spacing (530 nm) in another device. The field emission characteristics of the integrated gate electrode devices were then measured under a scanning electron microscopy (SEM) with a three-axis nano-positioning device. The turn-on field of the gated devices with 800 and 530 nm height CNT were 2.77 and 3.57 V/μm, respectively, with applying − 10 V gate voltage, and 0 V anode voltage.     

Stable and high current density electron emission using coniferous carbon nano-structured emitter

The electron emission properties of a coniferous carbon nano-structure (CCNS) based field emission electron gun have been measured. The CCNS was grown on a stainless steel substrate using chemical vapor deposition (CVD) and biased to about 50 kV. Stable, high-current-density electron emission (102 mA/cm²) was measured continuously for more than 1300 h. This result compares favorably to printed and directly grown carbon nanotube (CNT) based emitters. A wide range of potential applications are foreseen for CCNS based electron emitters, such as high intensity X-ray sources.     

Aligned carbon nanotubes sandwiched in epitaxial NbC film for enhanced superconductivity

Highly aligned carbon nanotube (CNT) ribbons were sandwiched in epitaxial superconducting NbC films by a chemical solution deposition method. The incorporation of aligned long CNTs into NbC film enhances the normal-state conductivity and improves the superconducting properties of the assembly.     

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.     

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.     

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.     

Highly uniform hole spacing micro brushes based on aligned carbon nanotube arrays

Highly uniform hole spacing micro brushes were fabricated based on aligned carbon nanotube (CNT) arrays synthesized by chemical vapor deposition method with the assistance of anodic aluminum oxide (AAO) template. Different micro brushes from CNT arrays were constructed on silicon, glass, and polyimide substrates, respectively. The micro brushes had highly uniform hole spacing originating from the regularly periodic pore structure of AAO template. The CNT arrays, serving as bristles, were firmly grafted on the substrates. The brushes can easily clean particles with scale of micrometer on the surface of silicon wafer and from the narrow spaces between the electrodes in a series of cleaning experiments. The results show the potential application of the CNT micro brushes as a cleaning tool in microelectronics manufacture field.     

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.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

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.     

Catalyst-free,self-assembly,and controllable synthesis of graphene flake–carbon nanotube composites for high-performance field emission

Catalyst-free and self-assembled growth of graphene flakes (GFs) on carbon nanotube (CNT) arrays have been realized by using microwave plasma enhanced chemical vapor deposition. The shape of GFs was highly manipulated by adjusting the growth time, C concentration, and microwave power. We qualitatively discussed the nucleation and growth mechanism of GFs based on the growth parameter–GF shape studies. The field emission (FE) properties of graphene flake–carbon nanotube (GF–CNT) composites for different GF shapes were measured and found to be strongly influenced by the GF distribution. The optimal shape of GFs for FE had small scales, sharp edges, and sparse distribution on CNTs. The best FE properties with the optimal shape were observed with a low turn-on electric field of 0.73 V/μm and excellent stability, which are superior to those of the as-grown CNT arrays and GF–CNT composites covered by densely distributed GFs. We consider that the large aspect ratio of CNTs and the unique FE stability of GFs play a synergetic effect on the improved FE properties.     

Thickness-, alignment- and defect-tunable growth of carbon nanotube arrays using designed mechanical loads

We demonstrate the thickness-, morphology-, and defect-tunable growth and simultaneous integration of aligned carbon nanotube (CNT) arrays using a novel microscale platform. This platform consists of a micromechanical spring of desired stiffness, which applies a precise vertical load to a vertically aligned CNT array during its growth by chemical vapor deposition (CVD). The micromechanical spring is strained by the extrusive growth force output from the aligned CNT array during its growth and, at the same time, exerts a mechanical restoring force against the buckling resistance of the CNTs. This application of a designed vertical load on the CNTs allows modulation of the thickness and degree of alignment of the CNT array, as well as the structural quality of the individual CNTs. Consequently, the electrical resistance between two opposing CNT arrays can be tuned by adjusting the vertical load. In addition, their sensing responsiveness toward chemical species can also be enhanced by applying larger vertical load on the CNTs. In contrast to conventional growth methods for producing aligned CNT arrays, our approach offers an efficient way for the growth engineering and on-chip integration of aligned CNT arrays in a single step of the CVD.