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-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.     

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.     

Simple approach for the fabrication of carbon nanotube field emitter using conducting paste

A conducting layer using carbon nanotube (CNT) paste was prepared by mixing multi-walled CNT (MWNT), organic vehicles and spin on glass (SOG). The effect of SOG on the properties of the CNT paste was evaluated and compared to that of CNT paste with a glass frit. CNT powders were coated on the conducting CNT film either by sprinkling CNT powders onto the overall conducting layer area or by dropping a solution containing well dispersed CNTs. CNTs were strongly fixed by the formation of silica after heat treatment. The samples showed good field emission characteristics with turn-on electric fields of approximately 1.6 ∼ 2.2 V/μm. SOG was found to be an efficient inorganic binder for CNTs in the CNT paste.     

In situ fabrication of HfC-decorated carbon nanotube yarns and their field-emission properties

HfC-decorated carbon nanotube (CNT) yarns have been fabricated by pre-coating pure CNT yarns with Hf and in situ current heating in vacuum. HfC nanocrystals were formed at about 1600 K through reaction of the CNTs with Hf. The fabricated HfC-CNT yarns had a work function of 3.9 eV, lower than that of pure CNT yarns. For field-emission applications, HfC-CNT sharp tips made up of HfC nanorods were obtained by Joule-heating-induced electrical breakdown above 2100 K. Their emission current could reach 320 μA with a calculated density of 800 A/cm² at an extraction voltage of 400 V. The emitters could operate under a rough vacuum of 10⁻² Pa without obvious degradation. These excellent field-emission properties are attributed to the HfC nanorods, which have a low work function and are resistant to ion bombardment.     

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.     

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).     

Simple fabrication process of a screen-printed triode-CNT field emitter array

We introduced a simple fabrication process for field emission devices based on carbon nanotubes (CNTs) emitters. Instead of using the ITO material as a transparent electrode, a metal (Au) with thickness of 5–20 nm was used. Moreover, the ITO patterning process was eliminated by depositing metal layer, before the CNT printing process. In addition, the thin metal layer on a photoresist (PR) layer was used as UV block. We fabricated the CNT field emission arrays of triode structure with a simple process. And I–V characteristics of field emission arrays were measured. The maximum current density of 254 μA/cm² was achieved when the gate and the anode voltages were kept 150 and 3000 V, respectively. The distance between anode and cathode was kept constant.     

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.     

Fabrication and properties of under-gated triode with CNT emitter for flat lamp

We investigated under-gate type carbon nanotube field emitter arrays (FEAs) for back light unit (BLU) in liquid crystal display (LCD). Gate oxide was formed by wet etching of ITO coated glass substrate instead of depositing SiO₂ on the glass substrate. Wet etching is easier and simpler than depositing and etching thick gate oxide to isolate the gate metal from cathode electrode in triode. To optimize the triode, we simulated the electric field distribution and electron trajectory in triode structures by the SIMION simulator. CNT emitters were formed using screen printing of photosensitive CNT paste. Field-emission characteristics of triode structure were measured. The maximum current density of 92.5 μA/cm² was when the gate and anode voltage was 95 and 2500 V, respectively, at the anode–cathode spacing of 1500 μm.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

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.     

Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution

Nitrogen-doped carbon nanotubes (CNTs) are known to be better field emitters than the pristine ones. But the field emissions (FE) property closely depends on what kinds of nitrogen moieties are substituted in the CNT matrix. While graphitic N-substituents (gN) give rise to additional sub-levels in the unoccupied states near the Fermi level, pyridinic N-substituents (pN) destroy the existing sub-levels. Here, we show that the FE property of N-doped CNTs can be tailored by controlling the gN/pN ratio therein. Vertically aligned N-doped CNTs were grown by chemical vapor deposition of camphor in the presence of ferrocene catalyst, using dimethylformamide (DMF) as a nitrogen source. A step-wise increase of DMF concentration (0-45 wt.%) in camphor caused a systematic rise of N-doping (0.8-5.2 at.%) in the resulting CNTs. The gN- and pN-dopings were identified by XPS analysis, and gN/pN ratio was found to increase from 1.7 to 3.5 in a regular manner. The FE measurements of as-grown N-doped CNTs exhibited a corresponding decrease of turn-on field from 1.8 to 0.6 V/μm and threshold field from 4.2 to 2.0 V/μm. This study thus presents the first experimental demonstration of FE enhancement by controlling the gN/pN ratio.     

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.     

Enhanced field emission of open-ended, thin-walled carbon nanotubes filled with ferromagnetic nanowires

A general strategy for in situ synthesis of open-ended, thin-walled carbon nanotubes (CNTs) filled with long ferromagnetic (FeNi, FeCo, FeCoNi, etc.) nanowires is proposed. The key feature of this strategy is the introduction of Cl-contained benzene (e.g. trichlorobenzene) as carbon precursor. Size-dependent etching effect of Cl radicals on the sidewalls of CNTs is proved by theoretical calculations. As-prepared thin-walled FeNi-filled CNTs (FeNi-CNTs) show much lower turn-on field of 0.30 V/μm (at 10 μA/cm²) and lower threshold field of 0.65 V/μm (at 1.0 mA/cm²) than those of thick-walled counterparts. Moreover, as-prepared thin-walled FeNi-CNTs show good field-emission stability at a low vacuum level (10⁻⁶ Torr). The enhanced field-emission performance can be attributed to a combined contribution of open-end tips, thinner sidewalls and metal nanowire fillings.     

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).     

Field emission from the film of the finely dispersed arc discharge black core material

We have studied, for the first time, the field emission from the film, prepared by a spray method, of the finely dispersed black core material, including multi-walled carbon nanotubes (MWNTs), fabricated by arc discharge. We dispersed the black core material by using an ultrasonic processor and found that the dispersed ones were much finer than those observed when treated with a ball mill and normal ultra-sonic bath. By SEM, HRTEM and Raman analyses, the MWNTs were almost not deformed and damaged during ultra-sonication. The field emission current density measured from the film of the dispersed black core material was about 15 mA/cm² at an applied field of 8 V/μm, which was about 23 times higher than that found by a ball mill. A current density of 1 mA/cm², which is required basically for flat panel display, has been obtained at 5.3 V/μm. The lifetime test of the dispersed black core material showed that the current density was almost unchanged while the field was applied. Therefore, it is concluded that a black core material fabricated by arc discharge could be used to flat panel displays as field emitters by dispersing with an ultrasonic processor, without further treatment like extraction or purification.     

Long lifetime emission from screen printing carbon nanotubes over 45,000 h at 1.27 mA/cm2 with 10% duty ratio

A long time operation of field electron emission over 45,000 h was achieved using the screen printing carbon nanotube (CNT) emitter fabricated on an ITO coated glass substrate. The measurement was carried out in an ultra-high vacuum chamber at around 3 × 10^− 7 Pa in pressure with continuously applying voltage of rectangular pulses at 743 V or 730 V peak, 60 Hz frequency and 10% duty ratio. The emission current corresponding to the voltage pulse at 1.27 mA/cm² density was repeatedly and reproducibly observed for more than 5 years. Because the operated condition was 10 times higher at the current density than that of the conventionally developed field emission lamps (FELs), the expected lifetime of the presented emitter will be 450,000 h though the vacuum level of the measurement was extremely higher than that of the actual operation of FELs. By investigating the morphology of the emitter after terminating the operation, it was found that sparse bundles of agglomerated CNTs were standing in the long lifetime CNT emitter.     

Field emission property of arrayed nanocrystalline diamond

Arrays of nanocrystalline diamond (NCD) stripes were fabricated by plasma etching of a NCD film. Electron field emission (EFE) of NCD arrays with 100-μm-wide stripes separated by different spacings was analyzed. The NCD arrays had higher EFE efficacy than the non-patterned blanket NCD film. The turn-on electric field (E_on) decreased from 5.4 V/μm^-1 for the blanket NCD film to 4.2, 4.4 and 4.7 V/μm^− 1 for the NCD arrays with 100, 500 and 1000 μm of spacing, respectively. Both the effective emitting area and the field enhancement factor for the NCD emitters were increased by patterning. The enhanced EFE from arrayed NCD stripes was possibly attributed to the edge effect and reduction of electrostatic screening.     

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).