Preparation and field emission properties of carbon nanotubes cold cathode using melting Ag nano-particles as binder

A new preparation process for carbon nanotubes (CNTs) cold cathode was studied through the replacement of traditional organic or inorganic binder with Ag nano-particles. This method has the advantages of low preparation temperature and fine electrical contact between CNTs paste and substrate. A mixture paste of CNTs, Ag nano-particles and other organic solvents was spreaded on Si substrate. By melting and connecting of Ag nano-particles after sintered 30 min at 250 °C, a flat CNTs films with good field emission properties was obtained. The measurements reveal that the turn on electric field and the threshold electric field of as-prepared CNTs cathode are 2.1 and 3.9 V/μm respectively and the field emission current density is up to 41 mA/cm² at an applied electric field of 4.7 V/μm.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Field emission properties of polymer-converted carbon films by heat treatment

Carbon films were prepared on single crystal silicon substrates by heat-treatment of a polymer-poly(phenylcarbyne) at 800 °C in Ar atmosphere. The heat-treatment caused the change of the polymer into carbon film, which exhibited good field emission properties. Low turn-on emission field of 4.3 V/μm (at 0.1 μA/cm²) and high emission current density of 250 μA/cm² (at 10 V/μm) were observed for the polymer-converted carbon films. This behavior was demonstrated to be mainly related to the microstructure of the carbon films, which consisted of fine carbon nanoparticles with high sp² bonding. The carbon films, which can be deposited simply with large areas, are promising for practical applications in field emission display.     

Effects of total CH4/Ar gas pressure on the structures and field electron emission properties of carbon nanomaterials grown by plasma-enhanced chemical vapor deposition

The effects of total CH₄/Ar gas pressure on the growth of carbon nanomaterials on Si (1 0 0) substrate covered with CoO nanoparticles, using plasma-enhanced chemical vapor deposition (PECVD), were investigated. The structures of obtained products were correlated with the total gas pressure and changed from pure carbon nanotubes (CNTs) through hybrid CNTs/graphene sheets (GSs), to pure GSs as the total gas pressure changed from 20 to 4 Torr. The total gas pressure influenced the density of hydrogen radicals and Ar ions in chamber, which in turn determined the degree of how CoO nanoparticles were deoxidized and ion bombardment energy that governed the final carbon nanomaterials. Moreover, the obtained hybrid CNTs/GSs exhibited a lower turn-on field (1.4 V/μm) emission, compared to either 2.7 V/μm for pure CNTs or 2.2 V/μm for pure GSs, at current density of 10 μA/cm².     

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I-V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm² at 2.5 V/μm.     

Electron field emission properties of conducting polymer coated multi walled carbon nanotubes

The present work describes the field emission characteristics of conducting polymer coated multi walled carbon nanotubes (MWNTs) field emitters fabricated over flexible graphitized carbon cloth. Nanocomposites involving the combination of MWNTs and conducting polymers polyaniline (PANI) and polypyrrole (PPy) have been prepared by in-situ polymerization method and have been characterized using scanning electron microscopy and transmission electron microscopy. Using spin coating method, field emitters based on PANI/MWNTs and PPy/MWNTs over flexible graphitized carbon cloth have been prepared. The field emission characteristics have been studied using an indigenously fabricated set up in a vacuum chamber with a base pressure of 2 × 10⁻⁵ Pa and the results are discussed. Our results display that the field emission performance of the emitters depends strongly on the work function of the emitting material. Low turn on emission field of 2.12 V/μm at 10 μA/cm² and high emission current density of 1 mA/cm² at 3.04 V/μm have been observed for PANI/MWNTs field emitter.     

Effect of deposition voltage on the field emission properties of electrodeposited diamond-like carbon films

Diamond-like carbon (DLC) films were deposited on Al substrates by electrodeposition technique under various voltages. The surface morphology and compositions of synthesized films were characterized by scanning electron microscopy and Raman spectroscopy. With the increase of deposition voltage, the sp² phase concentration decreased and the surface morphology changed dramatically. The influence of deposition voltage on the field electron emission (FEE) properties of DLC films was not monotonic due to two adverse effects of deposition voltage on the surface morphology and compositions. The DLC film deposited under 1200 V exhibited optimum FEE property, including a lowest threshold field of 13 V/μm and a largest emission current density of 904.8 μA/cm² at 23.5 V/μm.     

Application of printed nanocrystalline diamond film for electron emission cathode

The low-cost and large area screen-printed nano-diamond film (NDF) for electronic emission was fabricated. The edges and corners of nanocrystalline diamond are natural field-emitters. The nano-diamond paste for screen-printing was fabricated of mixing nano-graphite and other inorganic or organic vehicles. Through enough disperse in isopropyl alcohol by ultrasonic nano-diamond paste was screen-printed on the substrates to form NDF. SEM images showed that the surface morphology of NDF was improved, and the nano-diamond emitters were exposed from NDF through the special thermal-sintering technique and post-treatment process. The field emission characteristics of NDF were measured under all conditions with 10⁻⁶ Pa pressure. The results indicated that the field emission stability and emission uniformity of NDF were improved through hydrogen plasma post-treatment process. The turn-on field decreased from 1.60 V/μm to 1.25 V/μm. The screen-printed NDF can be applied to the displays electronic emission cathode for low-cost outdoor in large area.     

Fabrication of a three-dimensional complex carbon nanoneedle from carbon nanowalls

A three-dimensional complex carbon nanoneedle has been fabricated from carbon nanowalls by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibits novel three-dimensional complex nanostructure, the center of which is a carbon nanoneedle, and many 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 314 mA/cm² at the electric field of 2.5 V/μm.     

Flame synthesis of carbon nanotubes for panel field emission lamp

Multi-walled carbon nanotubes (CNTs) were synthesized on the surfaces of Ni-alloy plated Fe-wires with the diameter of 2 mm using a conventional laboratory ethanol (C₂H₅OH) flame method at 560 °C. SEM showed that the product had bush-shaped micron-structures with diameters from 100 to 450 nm and lengths of over 1.0 μm. TEM revealed that the micron-structures were composed of multi-walled nanotube bundles with the diameters of about 50 nm. The test on the diode configuration field emission of the Fe-wire arrays was performed. The onset electric field was 2.95 V/μm and the emission current can reach 50 mA/cm² at an electric field of 9 V/μm. The average fluctuation of the emission current density was less than 7%. The result suggests that the field emission was uniform and the present technique was feasible to fabricate Panel Field Emission Lamp (PFEL) with arrays of carbon nanotubes. PFEL has the advantages of high luminescence as well as stability, and thus, it can be used to replace ordinary lights.     

Field emission properties and synthesis of carbon nanotubes grown by rf plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to grow the Fe-catalyzed carbon nanotubes (CNTs). The grown CNTs with a uniform diameter in the range of about 10-20 nm and the typical lengths beyond 1 μm resulted in a very high aspect ratio. The Raman and TEM results showed that the grown CNTs contained a large amount of carbonaceous particles and crystal defects, such as pentagon-heptagon pair defects. XPS measurement indicated that the CNTs had CH covalent bonds. Field emission characteristics exhibited the low turn-on threshold field of 2.75 V/μm and the maximum emission current density of 7.75 mA/cm² at 6.5 V/μm. The growth mechanism of CNTs and the effects of hydrogen plasma on their structure were discussed.     

Synthesis of Cu-ZnO and C-ZnO nanoneedle arrays on zinc foil by low temperature oxidation route: Effect of buffer layers on growth, optical and field emission properties

Different densities of ZnO nanoneedle films have been prepared by pre-coated zinc foils with thin layer of copper and carbon followed by thermal oxidation at 400 °C in air. The X-ray diffraction patterns show well defined peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. The scanning electron microscope images clearly reveal formation of ZnO needles on the entire substrate surface. The X-ray photoelectron spectroscopy studies indicate that Cu and C ions are incorporated into the ZnO lattice. Photoluminescence studies evaluate different emission bands originated from different defect mechanism. From the field emission studies, the threshold field, required to draw emission current density of ∼100 μA/cm², is observed to be 2.25 V/μm and 1.57 V/μm for annealed zinc foil pre-coated with copper and carbon, respectively. The annealed film with copper layer exhibits good emission current stability at the pre-set value of ∼100 μA over a duration of 4 h. The results show that buffer layer is an important factor to control the growth rate, resulting in different density of ZnO needles, which leads to field emission properties. This method may have potential in fabrication of electron sources for high current density applications.     

Field emission characteristics of thin-metal-coated nano-sheet carbon films

Nano-sheet carbon films (NSCFs) coated with very thin (≈5-nm-thick) metal layers were fabricated on Si wafer chips by means of quartz-tube-type microwave-plasma chemical-vapour-deposition method with hydrogen-methane gas mixture and an electron beam evaporation method. Field emission (FE) current densities obtained at a macroscopic average electric field, E, of ≈10 V/μm changed from 13 mA/cm² for NSCF with no coated metal to 1.7, 0.7 and 30 mA/cm² for Ti-, Al- and Au-coated NSCFs, respectively, while the threshold E varied from 4.4 V/μm for the former one to 5.3, 5.4 and 2.0 V/μm for the corresponding latter ones, respectively. As the FE currents of Au-coated NSCFs tended to be saturated in a higher E region, compared to those of NSCFs with no coated metal, no simple Fowler-Nordheim (F-N) model is applicable. A modified F-N model considering statistic effects of the FE tip structures and a space-charge-limited-current effect is successfully applied to an explanation for the FE data observed in the low and high E regions.     

Characterization and structure study of the anodic oxide film on Zircaloy-4 synthesized using NaOH electrolytes at room temperature

Thick crystalline zirconium oxide films were synthesized on Zircaloy-4 substrates by anodic oxidation at room temperature in NaOH solution with a stable applied voltage (300 V). The film is approximately 4.7 μm in thickness. The XPS and SEM analysis shows that the film is a three-layer structure in water, hydroxide and oxide parts. The thickness of that order is ∼0.01 μm, ∼1 μm, ∼3.7 μm, respectively. The oxide layer is composed of tetragonal and monoclinic phases with the volume ratio about 0.2. Furthermore, the thick anodic film acts as a barrier to oxygen and zirconium migrations. It effectively protects zirconium alloys against the worse corrosion. An extremely low passive current density of ∼0.018 μA/cm² and a low oxidation weight gain of ∼0.411 mg/cm² were also observed in the films.     

Enhanced field emission from ZnO nanopencils by using pyramidal Si(1 0 0) substrates

Zinc oxide nanopencil arrays were synthesized on pyramidal Si(1 0 0) substrates via a simple thermal evaporation method. Their field emission properties have been investigated: the turn-on electric field (at the current density of 10 μA/cm²) was about 3.8 V/μm, and the threshold electric field (at the current density of 1 mA/cm²) was 5.8 V/μm. Compared with similar structures grown on flat Si substrates, which were made as references, the pyramidal Si-based ZnO nanopencil arrays appeared to be superior in field emission performance, thus the importance of the non-flat substrates has been accentuated. The pyramidal Si substrates could not only suppress the field screening effect but also improve the field enhancement effect during the field emission process. These findings indicated that using non-flat substrates is an efficient strategy to improve the field emission properties.     

Enhanced field emission and patterned emitter device fabrication of metal-tetracyanoquinodimethane nanowires array

Ag(TCNQ) and Cu(TCNQ) nanowires were synthesized via vapor-transport reaction method at a low temperature of 100 °C. Field emission properties of the as-obtained nanowires on ITO glass substrates were studied. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires were 9.7 and 7.6 V/μm (with emission current of 10 μA/cm²), respectively. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires decreased to 6 and 2.2 V/μm, and the emission current densities increased by two orders at a field of 8 V/μm with a homogeneous-like metal (e.g. Cu for Cu(TCNQ)) buffer layer to the substrate. The improved field emission is due to the better conduct in the nanowires/substrate interface and higher internal conductance of the nanowires. The patterned field emission cathode was then fabricated by localized growing M-TCNQ nanowires onto mask-deposited metal film buffer layer. The emission luminance was measured to be 810 cd/m² at a field of 8.5 V/μm.     

Emission properties of Ti-DLC films prepared by unbalanced magnetron sputtering

The field emission properties of Ti-DLC films in diode and coplanar device structures were studied. An emission current density of 1.14 A/cm² could be obtained at an applied field of 33 V/μm and the threshold field was 24 V/μm for the coplanar emission structure. The silicon substrate was found to limit the emission current in the diode structure because of its high resistivity.     

Field emission property of copper nitride thin film deposited by reactive magnetron sputtering

Copper nitride (Cu₃N) thin film was deposited on silicon (Si) substrate by reactive magnetron sputtering method. X-ray diffraction measurement showed that the film was composed of Cu₃N crystallites with anti-ReO₃ structure and exhibited preferential orientation of [1 0 0] direction. The field emission (FE) result showed that Cu₃N film had a turn-on electric field of about 3 V/μm at a current density of 1 μA/cm² and a current density of 700 μA/cm² was obtained at the electric field of 24 V/μm. The emission mechanism inferred by Fowler-Nordheim (FN) plot is shown as following: thermal electron emission at low field region and tunneling electron emission at high field region.     

Electron field emission from a patterned array of diamond-like carbon on anodic aluminum oxide template

A patterned array of diamond-like carbon (DLC) was grown on anodic aluminum oxide (AAO) template by filtered cathodic arc plasma (FCAP) technique at room temperature. The diameters of patterned array of DLC were ∼150 nm, and the patterned array density was estimated to ∼10⁹ cm⁻². A broad asymmetric band ranging from 1000 cm⁻¹ to 2000 cm⁻¹ was detected by Raman spectrum attributed to characteristic band of DLC. The fraction of sp³ bonded carbon atoms of the patterned array of DLC was measured by X-ray photoelectron spectrum (XPS) and the ratio was about 62.4%. Field emission properties of the patterned array of DLC were investigated. A low turn-on field of 3.4 V/μm at 10 μA/cm² with an emission area of 3.14 mm² was achieved. The results indicated that the electrons were emitted under both the effect of enhanced field because of the geometry and the work function of the DLC sample. Based on Fowler-Nordheim plot, the values of work function for the patterned array of DLC were estimated in range of 0.38 to 1.75 from a linearity plot.     

Carbon nano dots scale by focused ion beam system for MIS diode nano devices

Metal-insulator-semiconductor (MIS) structures with a nanocrystal carbon (nc-C) embedded in SiO₂ thin films were fabricated using a focused ion beam (FIB) system with a precursor of low-energy Ga⁺ ion and carbon source. The crystallinity of nc-C was investigated by Raman spectroscopy and atomic force microscopy (AFM). Raman spectra indicate the evidence of crystallization of nc-C after annealed at 600 °C by the sharp peak at 1565 cm⁻¹ in graphite (sp²), while no peak of diamond (sp³) could be seen at 1333 cm⁻¹. The AFM images showed the nc-C dots controlled with diameter of 100 nm, 200 nm and 300 nm, respectively. The above results revealed that the nc-C dots had sufficiently stuck onto SiO₂ films. The hysterisis loop in the capacitance-voltage characteristics appeared in the MIS device with SiO₂/nc-C/SiO₂ structure in which voltage shift is 0.32 V for radical oxidation and 0.14 V for dry oxidation, respectively.