Long-term stability of a horizontally-aligned carbon nanotube field emission cathode coated with a metallic glass thin film

A horizontally-aligned carbon nanotube (HACNT) field emission cathode was coated with a metallic glass thin film (MGTF) to improve the stability of the field emission properties. HACNT field emission cathodes have previously been fabricated on glass substrates using composite plating and crack-formation techniques. A carbon nanotubes/nickel (CNTs/Ni) composite film is deposited onto a glass substrate at 80 °C by the composite plating technique alone. Cracks are then formed in the CNT/Ni composite film during 30 min heating at 300 °C, and HACNTs are exposed in the cracks. The field emission properties of the HACNT field emission cathode show a low turn-on electric field E_on of about 2.3 V/μm, a low threshold electric field E_th of about 4.7 V/μm at an emission current density of 1 mA/cm², and a stability time of 78 h. The degradation of the HACNT field emission cathode is prevented by using a MGTF-coating technique and superior long-term stability (i.e. >125 h, with 5 nm MGTF; >270 h, with 10 nm MGTF) for the MGTF/HACNT field emission cathode is achieved.     

The production of a flexible electroluminescent device on polyethylene terephthalate films using transparent conducting carbon nanotube electrode

An inorganic electroluminescent (EL) device on a flexible polyethylene terephthalate (PET) substrate and its properties were investigated. The transparent conducting film (TCF) made from carbon nanotubes (CNTs) (CNT-TCF) was employed in the flexible EL device. CNT-TCF was formed by filtration of CNT solution and was transferred to the PET film. It was found that the brightness of the inorganic EL device was strongly dependent on the quality of the CNT composite films. After a 3-aminopropyltriethoxysilane treatment of the PET substrate, CNTs uniformly were dispersed and showed a good adhesion to the substrate, and the resulting EL device showed better performance. The flexible EL device showed the brightness of 96.8 cd/m² at 28 kHz and 50 V.     

Field emission from nanodiamond grown with ‘ridge’ type geometrically enhanced features

Cold cathodes for vacuum field emission have many interesting applications including display devices. Chemical vapor deposited (CVD) diamond has long been considered such a candidate. A nanodiamond film with ridge features was grown on a highly doped (resistivity = 0.0035 Ω-cm) n-type silicon substrate by plasma enhanced CVD process using a H2/CH4/N2 gas mixture. The overall planar nanodiamond film was characterized for vacuum field emission in diode configuration. A low turn on field of ~ 2.3 V/µm at 1 µA was observed. The cathode was able to produce 150 µA at a field of 6.6 V/µm. This field emission behavior can be attributed to a combination of high sp2 content, higher electrical conductivity by nitrogen incorporation in the nanodiamond film and a high geometrical field enhancement factor because of the sharp ‘ridge’ like features on the surface.     

Nanodiamod lateral device field emission diode fabricated by electron beam lithography

Nitrogen incorporated nanodiamond film is known to aid in promoting enhanced electron emission via the induced graphitic behavior both in the bulk material and also the surface of the film. Since electron emission current is inversely proportional to the cathode to anode inter-electrode distance; it is necessary to implement electron beam lithography (EBL) to obtain a small emission gap. To achieve high resolution from EBL, a thinner nanodiamond film is required. In this work, we fabricated lateral field emitters on a 0.65 µm nanodiamond film. The nanodiamond film was deposited onto a silicon-on-insulator (SOI) substrate in CH₄/H₂/N₂ plasma ambient by microwave chemical vapor deposition. The SOI was prepared for diamond nucleation using mechanical abrasion and ultrasonication in nanodiamond powder. Electron beam lithography (EBL) was used to delineate a 10 emitter tipped diode with a 2 µm anode-to- anode emission gap.     

The stability of the CNT/Ni field emission cathode fabricated by the composite plating method

A novel composite plating method has been developed for the fabrication of carbon nanotube/Ni (CNT/Ni) field emission cathode. The field emission properties of the initial CNT/Ni field emitter show a low turn-on electric field E_on of about 1.1 V/μm with an emission current density of 1 μA/cm², and a low threshold electric field E_th of about 1.7 V/μm with an emission current density of 1 mA/cm². After performing a stability test with a high emission current density in high vacuum, the corresponding microstructure and the degree of graphitization of the CNT/Ni field emitter were measured by using scanning electron microscopy and Raman spectroscopy. We found that the degree of graphitization slowly decreases with the duration time t_FE of the stability test, the size of small rod-like CNT/Ni composite structures in the film increases with t_FE, and obvious cracks appear in the film as t_FE is larger than 60 h. The degradation of the field emission properties may be explained by the Joule heating effect on the CNT/Ni field emitter under high emission current density.     

Enhanced ferroelectric properties of 0.95Pb(Sc0.5Ta0.5)O3–0.05PbTiO3 thin films with Pb(Zr0.52,Ti0.48)O3 seed layer

0.95Pb(Sc_0.5Ta_0.5)O₃–0.05%PbTiO₃ (PSTT5) thin films with and without a Pb(Zr_0.52,Ti_0.48)O₃ (PZT52/48) seed layer were deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering. X-ray diffraction patterns indicate that the PSTT5 film with a PZT52/48 seed layer exhibited nearly pure perovskite crystalline phase with highly (4 0 0)-preferred orientation. Piezoresponse force microscopy observations reveal that a large out-of-plane spontaneous polarization exists in the highly (4 0 0)-oriented PSTT5 thin film. The PSTT5/PZT(52/48) possesses good ferroelectric properties with large remnant polarization P_r (12 μC/cm²) and low coercive field E_c (110 kV/cm). Moreover, The perfect butterfly-shaped capacitance–voltage characteristic curve and the relative dielectric constant as high as 733 is obtained in this PSTT5 thin film at 100 kHz.     

Effect of annealing temperature on ferroelectric electron emission of sol–gel PZT films

In this work, the influence of annealing temperature on the ferroelectric electron emission behaviors of 1.3-μm-thick sol–gel PbZr_0.52Ti_0.48O₃ (PZT) thin film emitters was investigated. The results revealed that the PZT films were crack-free in perovskite structure with columnar-like grains. Increasing annealing temperature led to the growth of the grains with improved ferroelectric and dielectric properties. The remnant polarization increased slightly from 35.3 to 39.6 μC/cm² and the coercive field decreased from the 56.4 to 54.6 kV/cm with increasing annealing temperature from 600 to 700 °C. The PZT film emitters exhibited remarkable ferroelectric electron emission behaviors at the threshold voltage above 95 V. The film annealed at 700 °C showed a relatively lower threshold voltage and higher emission current, which is related to the improved ferroelectric and dielectric properties at higher annealing temperature. The highest emission current achieved in this work was around 25 mA at the trigger voltage of 160 V.     

Growth of a three-dimensional complex carbon nanoneedle electron emitter for fabrication of field emission device

A three-dimensional complex carbon nanoneedle electron emitter film with high emission current density at low electric field has been developed by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibited novel film morphology. The scanning electron microscopy image taken from this film indicated a three-dimensional complex nanostructure emitter, the center of which was a carbon nanoneedle, and many small 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 251 mA/cm² at the electric field of 2.2 V/μm, and the field emission current was stable, making it possibly suitable for developing field emission device.     

Field emission devices for advanced electronics comprised of lateral nanodiamond or carbon nanotube emitters

Nanocarbon-derived electron emission devices, specifically, nanodiamond lateral field emission diodes and gated carbon nanotube triodes are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. Nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to utilize the material as an electron field emitter. Arrays of laterally arranged “finger-like” nanodiamond emitters constitute the cathode in a versatile diode configuration with small interelectrode separation. Nanodiamond lateral tip conditioning techniques are employed to improve emission and the subsequent device performance discussed. A low diode turn-on voltage of 7 V and a high emission current of 90 μA at an anode voltage of 70 V (electric field of ∼ 7 V/μm) is reported for the nanodiamond lateral device. Also, the development of a field emission triode amplifier based on aligned carbon nanotubes (CNTs) with low turn-on voltage and small gate leakage current, utilizing a dual-mask microfabrication process is reported. The2 × 20 μm CNT triode array displays a gate turn-on voltage of ∼ 44 V, and low gate currents less than 3% of the anode currents. The low gate leakage currents observed confirmed the effectiveness of the convex-shaped gated CNT emitter in alleviating the cathode-gate leakage problem that compromises the operation of a field emission triode.     

New carbon cone nanotip for use in a highly coherent cold field emission electron microscope

A new cathode for cold-field emission gun using a pyrolytic carbon-cone supported onto a carbon nanotube as the electron emitting tip has been developed. This tip was mounted in a TEM using a FIB based method, and the brightness measured under real operating conditions is five times better than obtained with a standard tungsten tip. Its use overcomes the many technical difficulties which have dogged the use of carbon nanotube-based tips as proposed replacements for tungsten tips. The resulting properties of the final CFEG exhibit a very good energy spread of 0.32 eV, a reduced brightness of 1.6 × 10⁹ A m^?2 sr^?1 V^?1 and a very good long-term stability with a current damping less than 16% per hour.     

Single-walled carbon nanotubes functionalized with polydiphenylamine as active materials for applications in the supercapacitors field

Composites based on polydiphenylamine (PDPA) doped with heteropolyanions of H₃PW₁₂O₄₀ and single-walled carbon nanotubes (SWNTs) were prepared by electrochemical polymerization of diphenylamine (DPA) on carbon nanotube films deposited onto Pt electrodes. HRTEM studies reveal that the electrochemical polymerization leads to the filling the spaces between tubes which compose the bundles, creating a monolithic film on the Pt electrode. The resulting composites were tested as active materials in supercapacitors. Resonant Raman scattering studies showed that the electropolymerization of DPA in the presence of H₃PW₁₂O₄₀ and SWNTs leads to the covalent functionalization of SWNTs with doped PDPA. The covalent functionalization of SWNTs with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions was revealed by FTIR spectroscopy, based on the changes in the vibrational features of PDPA and H₃PW₁₂O₄₀. These changes included i) a down-shift of the PDPA IR bands, which was attributed to the C–H bending vibrational mode of benzene (B), C_aromatic–N, C–C stretching (B) + C–H bending (B) and C–C stretching vibrations of the B ring, from 1174, 1321, 1495 and 1603 cm^− 1 to 1165, 1313, 1487 and 1599 cm^− 1, respectively; ii) a change in the peak positions of IR bands associated with the W = O and P-O-W vibration modes of H₃PW₁₂O₄₀; and iii) a down-shift of the IR band situated in the spectral range 650–725 cm^− 1, which was assigned to the inter-ring deformation vibration mode.The characterization of symmetric solid-state supercapacitors was performed for electrodes prepared from i) SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions, ii) SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions, and iii) PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. Preliminary results indicate high discharge capacitance values of up to 157.2 mF/cm² for SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. The discharge capacitance of this material is superior to those recorded for SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions (~ 18.2 mF/cm²) and PDPA doped with H₃PW₁₂O₄₀ heteropolyanions (~ 62.1 mF/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.     

Microstructure and dielectric properties of amorphous BaSm2Ti4O12 thin films for MIM capacitor

BaSm₂Ti₄O₁₂ (BST) film grown at room temperature was amorphous, while the film grown at 300 °C was also amorphous but contained a small amount of crystalline Sm₂Ti₂O₇ (ST). The crystalline BST phase was formed when the film was grown at 700 °C and subjected to rapid thermal annealing (RTA) at 900 °C. On the other hand, the ST phase was formed in the film grown at 300 °C and subjected to RTA at 900 °C. A high capacitance density of 2.12 fF/μm² and a low leakage current density of 1.15 fA/pF V were obtained from the 150 nm-thick BST film grown at 300 °C. Its capacitance density could conceivably be further increased by decreasing the thickness of the film. It had linear and quadratic coefficients of capacitance of −785 ppm/V and 5.8 ppm/V² at 100 kHz, respectively. Its temperature coefficient of capacitance was also low, being approximately 255 ppm/°C at 100 kHz.     

One-pot pyro-synthesis of a high energy density LiFePO4-Li3V2(PO4)3 nanocomposite cathode for lithium-ion battery applications

A highly crystalline carbon-coated 0.66LiFePO₄•0.33Li₃V₂(PO₄)₃ (LFP-LVP) nanocomposite was synthesized by a one-pot pyro-synthetic strategy using a polyol medium at low temperature. Prior to any additional heat treatment, electron microscopy confirmed the as-synthesized composite to consist of spherical particles with average diameters in the range of 30–60 nm. A crystal growth phenomenon and particle aggregation was observed upon heat treatment at 800 °C, thus resulting in an increase in the average particle size to 200–300 nm. When tested for a lithium-ion cell, the nanocomposite electrode demonstrated impressive electrochemical properties with higher operating potentials hence enhanced energy densities. Specifically, the composite cathode delivered a high reversible capacity of 156 mAh g⁻¹ at 0.1 C and exhibited a remarkable reversible capacity of 119 mAh g⁻¹, corresponding to an energy density of 46.88 Wh Kg⁻¹ at 6.4 C. When cycling was performed at 6.4 C, the electrode could recover up to 85% of the capacity observed at low current density of 0.1 C, which indicates the excellent rate capability of the nanocomposite electrode. The enhanced performance was attributed to the inclusion of the high potential LVP phase constituent in the present cathode by a simple one-pot polyol-assisted pyro strategy.     

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.     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.     

Study on the wet processable antimony tin oxide (ATO) transparent electrode for PLEDs

A polymer light emitting diodes (PLEDs) was fabricated using the wet processable antimony tin oxide (ATO) as the transparent electrode by spin coating method. PLED were fabricated with ATO (or ITO)/PEDOT:PSS/polymer/BaF₂/Ba/Al configurations. Electrical and optical properties of ATO transparent electrode were measured. Transmittance of ATO thin film was more than 90% in the visible region, sheet resistance was 30 Ω/□ and had a strong solvent resistance. The maximum brightness and maximum efficiency of PLED device using an ATO transparent electrode was 3637 cd/m² and 1.03 cd/A, respectively.     

High-current electron emission of thin diamond films deposited on molybdenum cathodes

The results of investigation of emission characteristics of cold cathodes employing diamond and related films are presented. The films were deposited in a new millimeter wave plasma-assisted CVD reactor using Ar–H₂–CH₄ and Ar–H₂–CH₄–N₂ gas mixtures. To study the emission properties of the high-current cathodes they were subjected to ~ 50 ns high-voltage pulses with amplitudes up to 100 kV. Experiments show that the emission properties strongly depend on methane and nitrogen concentration in gas mixture. The homogeneous emission with current density of 220 A/cm² has been obtained. The prepared cathodes were successfully tested in high-power rf pulse compressor employing electron beam triggering.     

Structural and electrical properties of Sr2NaNb4O13 thin film grown by electrophoretic method using nanosheets synthesized from K(Sr2Na)Nb4O13 compound

Sr₂NaNb₄O₁₃⁻ (SNNO⁻) nanosheets were exfoliated from the K(Sr₂Na)Nb₄O₁₃ compound that was synthesized at 1200 °C. The SNNO⁻ nanosheets were deposited on a Pt/Ti/SiO₂/Si substrate at room temperature by the electrophoretic method. Annealing was conducted at various temperatures to remove organic defects in the SNNO film. A crystalline SNNO phase without organic defects was formed in the film annealed at 500 °C. However, a SrNb₂O₆ secondary phase was formed in the films annealed above 600 °C, probably due to the evaporation of Na₂O. The SNNO thin film annealed at 500 °C showed a dielectric constant of 74 at 1.0 MHz with a dielectric loss of 2.2%. This film also exhibited a low leakage current density of 9.0 × 10⁻⁸ A/cm² at 0.6 MV/cm with a high breakdown electric field of 0.72 MV/cm.     

Towards the perfect graphene membrane? – Improvement and limits during formation of high quality graphene grown on Cu-foils

We investigated the structure and crystalline quality of monolayer graphene grown by hydrogen and methane chemical vapor deposition (CVD) on polycrystalline Cu foils. Our data show that the high temperature hydrogen pretreatment of the Cu foil has to be performed at a sufficiently high H₂ pressure in order to avoid graphene (g) formation already during the pretreatment, which limits the achievable domain size during subsequent growth in the CH₄/H₂ mixture. Methane–hydrogen CVD sustains g growth but induces the faceting of the Cu substrate. Characterization by low energy electron microscopy evidenced a staircase Cu substrate morphology of alternating (4 1 0) and (1 0 0) planes interrupted by (n 1 1) type facets. The g flakes cover the staircase shaped support as a coherent layer. The polycrystalline film mostly contains rotational domains that are preferentially, but not strictly, aligned with respect to the stepped support surface. The substrate induced corrugated morphology occurs also underneath large single crystalline flakes and is transferred to suspended membranes, produced by etching the Cu underneath the graphene. Thus, membranes manufactured from g-Cu are non flat. This explains their reported softened elastic response and the formation of so called nanorippled graphene after transfer from the Cu support which deteriorates its electrical conductivity.