UNCD electron emitters using Si nanostructure as a template

The electron field emission (EFE) properties of silicon nanostructures (SiNSs) coated with ultra-nanocrystalline diamond (UNCD) were characterized. The SiNS, comprising cauliflower-like grainy structure and nanorods, was generated by reaction of a Si substrate with an Au film at 1000 °C, and used as templates to grow UNCD. The UNCD films were deposited by microwave plasma-enhanced chemical vapour deposition (MPECVD) using methane and argon as reaction gases. The UNCD films can be grown on the SiNS with or without ultrasonication pretreatment with diamond particles. The EFE properties of the SiNS were improved by adding an UNCD film. The turn-on field (E₀) decreased from 17.6 V/μm for the SiNS to 15.2 V/μm for the UNCD/SiNS, and the emission current density increased from 0.095 to 3.8 mA/cm² at an electric field of 40 V/μm. Ultrasonication pretreatments of SiNS with diamond particles varied the structure and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degraded the field emission properties of the UNCD/SiNS in this study.     

Electron field emission properties of carbon nanoflakes prepared by RF sputtering

Carbon nanoflakes (CNFs) with corrugated geometry were synthesized using RF sputtering process with Ar/CH₄ gas mixture. Transmission electron microscopic examination reveals that the introduction of H₂ in sputtering chamber leads to the preferential etching of amorphous carbons, while maintaining integrity for the nano-crystalline phases. The proportion of nano-sized crystalline clusters is thus increased, which improved the electron field emission (EFE) properties of the materials, viz. with turn-on field of E ₀ = 6.22 V/μm and FEE current density of J _e = 90.1 μA/cm² at 11.0 V/μm. The cathodes made of screen printing of CNFs-Ag paste exhibit even better EFE properties than the as-deposited CNFs. The EFE of the CNFs cathodes can be turned on at E ₀ = 5.71 V/μm, achieving J ₀ = 340.1 μA/cm² at 11.0 V/μm applied field. These results showed that the CNFs are inheritantly more robust in device fabrication process than the other carbon materials and thus possess better potential for electron field emitter applications.     

Influence of Ti coatings on field emission characteristics of nano-sheet carbon films

Nano-sheet carbon films (NSCFs) coated with a 2-nm Ti layer were fabricated on n-type Si (110) by means of a quartz-tube-type microwave-plasma chemical-vapour-deposition (MWPCVD) method with hydrogen-methane gas mixture and an electron beam (EB) evaporation method. The field emission (FE) properties of the NSCF were changed by depositing a thin Ti film on its surface. The threshold field was decreased from 3.7 V/μm to 2.5 V/μm and the FE current density at a macroscopic electric field (E) of 10 V/μm was decreased from 41.7 mA/cm² to 26.3 mA/cm² for Ti-coated NSCFs. Moreover, the saturation tendency of the emission current density was not improved for Ti-coated NSCFs. A three-region E model considering statistical size effects of FE tip structures in the low E region and space-charge-limited-current (SCLC) effects in the high E region was proposed and the FE data in the low, middle and high E regions were reasonably interpreted.     

Characterization and enhanced field emission properties of carbon nanotube bundle arrays coated with N-doped nanocrystalline anatase TiO2

Anatase TiO₂ nanocrystals (NCs) were deposited onto patterned carbon nanotube (CNT) bundle arrays to form a TiO₂/CNT composite using metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (Ti(OC₃H₇)₄) as a source reagent. The N-doped TiO₂/CNT composite was then fabricated using nitrogen plasma treatment. The structural and spectroscopic properties of TiO₂/CNT composites were characterized by field-emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The combined geometrical structure and low electron affinity effects of N-doped TiO₂ led to a low turn-on field of 1.0 V μm⁻¹ at a current density of 10 μA cm⁻², a low threshold field of 1.9 V μm⁻¹ at a current density of 1 mA cm⁻², a high field enhancement factor of 3.0 × 10³, and long-term stability for the N-doped TiO₂/CNT composite. The results revealed that the N-doped TiO₂/CNT composite can be a potential candidate for field emission devices.     

Growth and optical and field emission properties of flower-like ZnO nanostructures with hexagonal crown

Flower-like zinc oxide (ZnO) nanostructures with hexagonal crown were synthesized on a Si substrate by direct thermal evaporation of zinc power at a low temperature of 600 °C and atmospheric pressure. Field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy and photoluminescence were applied to study the structural characteristics and optical properties of the product. The result indicated that the flower-like product with many slender branches and hexagonal crowns at the ends were single-crystalline wurtzite structures and were preferentially oriented in the <001> direction. The photoluminescence spectrum demonstrated a strong UV emission band at about 386 nm and a green emission band at 516 nm. The field emission of the product showed a turn-on field of 3.0 V/µm at a current density of 0.1 μA/cm², while the emission current density reached about 1 mA/cm² at an applied field of 5.9 V/μm.     

Room temperature mass production of carbon nanotube field emission micro-cathode arrays using electroplating of a CNT/Ni composite followed by micro-machining

A room temperature fabrication method for the mass production of carbon nanotube (CNT) field emission micro-cathode arrays is reported. The technique combines electroplating of a CNT/Ni composite and micro-machining. This method combines the advantages of direct growth and screen printing conventionally used to fabricate such structures and avoids their disadvantages. Due to its integration and room temperature processing, the technique is proven to be advantageous in mass production and low cost. Results of field emission testing show that the CNT micro-cathodes have excellent field-emission properties, such as high current density (15.7 mA/cm²), field enhancement factor (2.4 × 10⁶/cm), and good stability (109 h for 10% degradation of current density from 400 μA/cm²).     

Electron Field Emission Enhancement of Vertically Aligned Ultrananocrystalline Diamond‐Coated ZnO Core–Shell Heterostructured Nanorods

Enhanced electron field emission (EFE) behavior of a core–shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR‐UNCDN core–shell heterostructures show a high emission current density of 5.5 mA cm^?2 at an applied field of 4.25 V μm^?1, and a low turn‐on field of 2.08 V μm^?1 compared to the 1.67 mA cm^?2 emission current density (at an applied field of 28.7 V μm^?1) and 16.6 V μm^?1 turn‐on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR‐UNCDN core–shell heterostructures in the enhancement of electron emission.     

Synthesis and improved electron field emission characteristics of Au-coated nano-carbon needle films

Nano-carbon needle films (NCNFs) coated with a 5-nm Au layer were prepared on p-type Si (100) substrates by means of quartz-tube type microwave plasma chemical vapor deposition (MWPCVD) at different total gas pressures and an electron beam (EB) method. The NCNF deposited at the total gas pressure of 60 Torr had better field emission (FE) characteristics due to the dense structure of carbon sheets, good direction and high density of carbon needles. The FE properties were obviously improved due to depositing Au thin layer on NCNFs. The FE current density at a macroscopic electric field, E, of 10 V/μm was increased from 68.2 mA/cm² to 154.6 mA/cm² and the threshold field was decreased from 2.4 V/μm to 2.1 V/μm for the Au-coated NCNF deposited at the total gas pressure of 60 Torr. The three-region E model was employed to reasonably explain the FE data.     

Synthesis and improved electron field emission characteristics of Au-coated nano-carbon needle films

Nano-carbon needle films (NCNFs) coated with a 5-nm Au layer were prepared on p-type Si (100) substrates by means of quartz-tube type microwave plasma chemical vapor deposition (MWPCVD) at different total gas pressures and an electron beam (EB) method. The NCNF deposited at the total gas pressure of 60 Torr had better field emission (FE) characteristics due to the dense structure of carbon sheets, good direction and high density of carbon needles. The FE properties were obviously improved due to depositing Au thin layer on NCNFs. The FE current density at a macroscopic electric field, E, of 10 V/μm was increased from 68.2 mA/cm² to 154.6 mA/cm² and the threshold field was decreased from 2.4 V/μm to 2.1 V/μm for the Au-coated NCNF deposited at the total gas pressure of 60 Torr. The three-region E model was employed to reasonably explain the FE data.     

Fabrication of carbon nanoneedle and carbon nanowall mixture film with two-step growth method

A novel nano-carbon electron emitter film has been developed on a stainless steel substrate by a direct current plasma chemical vapor deposition system. Samples grown at temperatures of 900 °C and 1100 °C showed different surface morphologies. It is found that a two-step growth process established by combining these two temperature growths together is suitable for deposition of a high density emitter array film. The as-grown nano-carbon film indicates a carbon nanoneedle and carbon nanowall mixture film, where the needle array density is about 3 × 10⁷/cm². The I-V characteristic shows an emission current density of 228 mA/cm² at 2.5 V/μm, and the field emission current is stable, making it possibly suitable for developing field emission devices.     

Field emission from oriented tin oxide rods

Tin oxide (SnO₂) films were grown on silicon substrates by a wet chemical route. It was found from scanning electron microscopy investigations that oriented SnO₂ rods normal to the substrates were obtained. Field emission studies were carried out in diode configuration in an all metal ultra high vacuum chamber at a base pressure ∼ 1.33 × 10^− 8 mbar. The ‘onset’ field required to draw 0.1 μA/cm² current density from the emitter cathode was found to be ∼ 3.4 V/μm for SnO₂ rods. The field emission current and applied field follows the Folwer-Nordheim relationship in low field regime. The observed results indicate that the field emission characteristics of chemically grown SnO₂ structures are comparable to the vapor grown nanostructures.     

Oxygen plasma assisted end-opening and field emission enhancement in vertically aligned multiwall carbon nanotubes

This paper highlights the changes in micro-structural and field emission properties of vertically aligned carbon nanotubes (VACNTs) via oxygen plasma treatment. We find that exposure of very low power oxygen plasma (6 W) at 13.56 MHz for 15–20 min, opens the tip of vertically aligned CNTs. Scanning electron microscopy and transmission electron microscopy images were used to identify the quality and micro-structural changes of the nanotube morphology and surfaces. Raman spectra showed that the numbers of defects were increased throughout the oxygen plasma treatment process. In addition, the hydrophobic nature of the VACNTs is altered significantly and the contact angle decreases drastically from 110° to 40°. It was observed that the electron field emission (EFE) characteristics are significantly enhanced. The turn-on electric field (ETOE) of CNTs decreased from ∼0.80 V μm⁻¹ (untreated) to ∼0.60 V μm⁻¹ (oxygen treated). We believe that the open ended VACNTs would be immensely valuable for applications such as micro/nanofluidic based filtering elements and display devices.     

Preparation and pseudo-capacitance of birnessite-type MnO2 nanostructures via microwave-assisted emulsion method

A microwave-assisted emulsion process has been developed to synthesize birnessite-type MnO₂ one-dimensional (1D) nanostructures. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). TEM images confirmed that the particles were composed of nanowires and nanobelts. As a consequence of the small size, such MnO₂ nanostructures exhibit a high specific capacitance of 277 F g⁻¹ at the current density of 0.2 mA cm⁻². Furthermore, the simple synthetic approach may provide a convenient route for the preparation of birnessite-type MnO₂ nanowires and other 1D nanostructured materials on a large scale.     

Fabrication of carbon nanoflakes by RF sputtering for field emission applications

In this paper, an ultra thin sheet-like carbon nanostructure, carbon nanoflake (CNF), has been effectively fabricated by RF sputtering on Si substrate without any catalyst or special substrate pre-treatment. The CNFs were chosen to be the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The effect of deposition parameters such as substrate temperature, gas flow ratio and RF power on the field emission properties is discussed in detail. The sheet-like structures with thickness of about 10 nm or less stand on edge on the substrate and have a defective graphite structure. The field emission properties of the sample deposited at the optimum deposition conditions are turn-on field of 5.5 V/μm and current density of 1.4 mA/cm² at 11 V/μm. Considering the inexpensive manufacturing cost, lower synthesis temperature and ease of large-area preparation, the CNFs with low turn-on field deposited by RF sputtering might have a potential application in field emission devices.     

Transformation of c-oriented nanowall network to a flat morphology in GaN films on c-plane sapphire

The work significantly optimizes growth parameters for nanostructured and flat GaN film in the 480–830 °C temperature range. The growth of ordered, high quality GaN nanowall hexagonal honeycomb like network on c-plane sapphire under nitrogen rich (N/Ga ratio of 100) conditions at temperatures below 700 °C is demonstrated. The walls are c-oriented wurtzite structures 200 nm wide at base and taper to 10 nm at apex, manifesting electron confinement effects to tune optoelectronic properties. For substrate temperatures above 700 °C the nanowalls thicken to a flat morphology with a dislocation density of 10¹⁰/cm². The role of misfit dislocations in the GaN overlayer evolution is discussed in terms of growth kinetics being influenced by adatom diffusion, interactions and bonding at different temperatures. The GaN films are characterized by reflection high energy electron diffraction (RHEED), field emission scanning electron (FESEM), high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL).     

Field emission properties of electrodeposited cobalt nanowire arrays grown in anodic aluminum oxide

The excellent vertically aligned cobalt nanowire arrays were electrodeposited into anodic aluminum oxide (AAO) templates. Each nanowire has the same length with 20 μm and the diameter with 60 nm. The field emission characteristics of the nanowires were firstly studied based on current-voltage measurements and the Fowler-Nordheim equation. The electron field emission measurements on the samples showed a turn-on field (1 mA/cm²) of 1.66 V/μm, a field enhancement factor of β = 3054 and a current density of 600 mA/cm² at a relatively low voltage of 4.3 V/μm. The nanowire arrays could be an ideal alternative to carbon nanotubes and ZnO nanowires for the fabrication of flat panel displays.     

Enhanced field emission from the ZnO nanowires by hydrogen gas exposure

ZnO nanowires (ZNWs) were synthesized on Co-coated Si wafer via a carbon thermal reduction vapor transport method. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy investigations show that these ZNWs present a high-quality single-crystalline hexagonal structure. Field emission (FE) characteristics of the ZNWs film were measured. A low turn-on voltage for driving a current density of 0.1 μA/cm² is about 3.9 V/μm. The field enhancement factor was determined to be ∼ 1180 for ZNWs film. Exposure of H₂ during FE causes a permanent increase in the FE current and a decrease in the turn-on field. Also, the field enhancement factor γ was finally increased from 1180 ± 20 to 1510 ± 20 after FE saturation.     

A novel kind of porous carbon nitride using H-magadiite as the template

A novel kind of porous carbon nitride was prepared by pyrolysis of polymerized ethylenediamine using H-magadiite as the template. This material was characterized by XRD, FT-IR, XPS, and nitrogen sorption at 77 K. The BET surface area, pore volume and the median pore width are 436 m²/g, 0.69 cm³/g and 0.84 nm, respectively, based on the N₂ sorption. Using glass carbon electrodes modified by this material, an excellent linear determination range (0.05-0.99 μM) with a low detected concentration limit of 6.9 × 10^− 3 μM could be obtained on the determination of differential pulse voltammetries of dopamine in the presence of 400-fold excess of ascorbic acid.     

Growth and characterization of carbon nanofibers by a technique of polymer doped catalyst and hot-filament chemical vapor deposition

Carbon nanostructures have been prepared from the catalytic conversion of polyethylene glycol using a rapid immersion in hot-filament system fed with ethanol, hydrogen and argon. Fiber structures of external diameter about 30 nm have been observed by field emission scanning electron microscopy (FESEM). Raman measurements indicate high degree of C-C sp² ordering which suggests that the samples correspond to CNTs of good tube crystallinity. The samples presented remarkable field emission properties. Lowest threshold field achieved for electron emission was 1.0 V/μm.     

Enhancing electron field emission of carbon nanoflakes by hydrogen post-annealing process

In this study, the carbon nanoflakes (CNFs) fabricated by sputtering were chosen as the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The as-deposited CNFs were annealed in the furnace under hydrogen atmosphere. The results showed that the optimum field emission properties with smaller turn-on field and larger current density were obtained at annealing temperature of 600 °C for 10 min. The hydrogen thermal annealing has chemical etching on the surface of the CNFs and produces appropriate emission site density to increase the emission current density. The turn-on field was reduced from 6.7 to 5.8 V/μm and electric current density was increased from 22 to 187 μA/cm² under 8 V/μm after hydrogen thermal annealing.