Improved Field Emission Properties Research Articles

Field Emission from Carbon Nanotubes on Titanium Nitride-Coated Planar and 3D-Printed Substrates.

Carbon nanotubes (CNTs) are well known for their outstanding field emission (FE) performance, facilitated by their unique combination of electrical, mechanical, and thermal properties. However, if the substrate of choice is a poor conductor, the electron supply towards the CNTs can be limited, restricting the FE current. Furthermore, ineffective heat dissipation can lead to emitter-substrate bond degradation, shortening the field emitters' lifetime. Herein, temperature-stable titanium nitride (TiN) was deposited by plasma-enhanced atomic layer deposition (PEALD) on different substrate types prior to the CNT growth. A turn-on field reduction of up to 59% was found for the emitters that were generated on TiN-coated bulk substrates instead of on pristine ones. This observation was attributed exclusively to the TiN layer as no significant change in the emitter morphology could be identified. The fabrication route and, consequently, improved FE properties were transferred from bulk substrates to free-standing, electrically insulating nanomembranes. Moreover, 3D-printed, polymeric microstructures were overcoated by atomic layer deposition (ALD) employing its high conformality. The results of our approach by combining ALD with CNT growth could assist the future fabrication of highly efficient field emitters on 3D scaffold structures regardless of the substrate material.

Improved Field Emission Properties of Carbon Nanotube Paste Emitters Using an Electrically Conductive Graphite Binder

Improved Field Emission Properties of Carbon Nanotube Paste Emitters Using an Electrically Conductive Graphite Binder

The Influence of Morphological Characteristics of Graphene on Its Field Emission Properties

Graphene is one of the most potential field emission cathode materials due to its excellent electrical, thermal, and mechanical properties, as well as rich edge structures. In this paper, we studied the growth parameters of graphene prepared by chemical vapor deposition, and prepared three kinds of morphologies of graphene: single-layer graphene, graphene islands, and graphene with buffer layers, and then we explored the influence of the morphological characteristics of graphene on its field emission properties, and analyzed the mechanism of the morphological characteristics of graphene on its field emission properties through COMSOL. Compared to single-layer graphene, the turn-on field of graphene islands, and graphene with buffer layers was 5.55 V/μm and 5.85 V/μm, respectively. The current density has also been increased to 40.3 μA/cm<sup>2</sup> and 26.4 μA/cm<sup>2</sup>. On the other hand, the field emission currents of single-layer graphene and graphene with buffer layers are more stable. In the 5-hour test, the current density only decreased by 2% and 4%, respectively. COMSOL simulation shows that the morphological characteristics of graphene have a significant impact on the electric field distribution characteristics and heat dissipation capacity. Graphene islands and graphene with buffer layers have exposed edges, leading to local electric field concentration and thus improving field emission properties. Graphene islands are distributed discretely on the substrate, without forming a continuous graphene film and lacking transverse heat dissipation channels, so the accumulation of heat will cause damage to the graphene emitter, and affect the stability of its field emission current. This study would be of great benefit to the understanding of the influence of the morphological characteristics of graphene on its field emission properties, and improving the field emission properties of graphene materials.

CH4/(Ar–H2) plasma post-treatments produce nano-diamond aggregation and improvement in field emission properties of ultrananocrystalline diamond films

CH4/(Ar–H2) plasma post-treatments produce nano-diamond aggregation and improvement in field emission properties of ultrananocrystalline diamond films

Growth mechanism and field emission of B doped AlN films

This paper highlights the effect of boron dopant on the field emission and growth mechanism of AlN films. The films have been grown by reactive RF magnetron sputtering method. For undoped films, columnar AlN structure was obtained. In the case of boron doped AlN films, the structure growth proceeded gradually through four layers of different thicknesses and structures. It was found that boron favors the formation of an amorphous layer within which oxygen and boron based precipitates develops, retarding the growth of columnar morphology. Depth profiling X-ray photoelectron spectroscopy analyses confirm the presence of B-O, B-N, Al-B and Al-O groups.The study on the field emission (FE) properties of undoped and B doped AlN films on Si substrates indicates that the addition of boron allow to improve FE properties with low turn on field of 2.6 V/μm and low threshold filed of 3.07 V/μm. Such good field emission properties can be attributed to the formation of densely nanograined surface and the appearance of boron based groups.

Improved field emission properties of reduced graphene oxide decorated with indium nanoparticles

Low work function metals decoration of graphene is a effective method for the improvement of electron field emission properties. Indium, a relatively lower work function metal, is used to decorate reduced graphene oxide (RGO) films for the first time by simple and efficient solution process combined with magnetron sputtering method and the field emission properties are investigated. At continue voltage mode, the indium decorated RGO films display lower turn-on and threshold field as well as better emission stability than those of pristine RGO films. Moreover, the field emission performance of indium decorated RGO films is affected by the spacing between the cathode and anode. At pulse voltage mode, the indium decorated RGO films show approximately 10 times higher maximum emission current density compared to that at continue voltage mode. The calculation results by density functional theory indicate that the indium decoration of RGO significantly increases the density of states (DOS) near Fermi level and reduces the work function.

Field emission characteristics of MWCNT- bimetal (Cu and In) hybrids: Correlation with morphology, microstructure and electronic structure

In the current study, vertically aligned multiwalled carbon nanotubes (MWCNTs) and bi-metal (Cu and In) decorated CNTs were synthesized over Si substrates using Thermal Chemical Vapour Deposition technique and the thermal evaporation method. Different characterization techniques studied the surface morphology, structural properties, and elemental composition of the prepared samples. Field emission measurements were executed on the as-prepared samples. In-Cu-MWCNTs exhibit an enhanced current density of 7.46 mA/cm2 as compared to 2.83 mA/cm2 of pristine CNTs. A meticulous study of the electronic structure along with the basic field emission parameters reveals that the graphitic content of carbon (C) as well as the density of state and electronic structure variation are responsible for the improved field emission properties of the CNT-bimetal hybrids. The study opens a new area of research in CNT-bi-metal decorated hybrids for various field emission applications.

Field Emission of Multi-Walled Carbon Nanotubes from Pt-Assisted Chemical Vapor Deposition

Multi-walled carbon nanotubes (MWNTs) were grown directly on a metal substrate with the assistance of Pt using a chemical vapor deposition method. In addition, the growth mechanism of Pt-assisted catalytic CNT was discussed. MWNTs were characterized by SEM, TEM, AFM, Raman, and EDS, and the field emission (FE) properties were investigated, comparing with the direct grown MWNTs. The results showed that CNTs could not been synthesized by Pt particles alone under the experimental condition, but Pt may accelerate the decomposition of the carbon source gas, i.e., assisting MWNT growth with other catalysts. The Pt-assisted MWNTs were longer with larger diameters of around 80 nm and possessed better structural qualities with very few catalyst particles inside. Improved field emission properties were demonstrated for the Pt-assisted MWNTs with lower turn-on fields (for 0.01 mA·cm−2 current density) of 2.0 V·μm−1 and threshold field (for 10 mA·cm−2 current density) of 3.5 V·μm−1, as well as better stability under a long-term test of 80 h (started at 3.0 mA for the Pt-assisted emitter and 3.25 mA for the direct grown emitter). This work demonstrated a promising approach to develop high performance CNT field emitters for device applications.

Field emission enhancement from directly grown N-doped carbon nanotubes on stainless steel substrates

N-doped carbon nanotubes (NCNT) are grown directly on the stainless steel (SS) substrates by chemical vapor deposition at atmospheric pressure, and the field emission (FE) enhancement mechanism is investigated. The turn on and threshold fields of the NCNT emitters are 1.61 and 2.05 V μm−1, respectively, lower than those from the CNT emitters. The improved FE properties are mainly attributed to the reductions of the work function and the surface resistance. First-principles calculations suggest that the work function of NCNT depends highly on the N doping concentration. In addition, the N-doping in CNT leads to the charge aggregation around N, benefiting the field emission enhancement. More importantly, for the CNTs, the emission current dropped by 28.6% after ∼7h continuous emission at the pressure of 5.38 × 10−4 Pa, while the current reduced only 5.8％after 7 h test at the pressure of 8.35 × 10−4 Pa for the NCNTs, suggesting that the NCNTs exhibit better resistance to the ion bombardment in high pressure. The investigation demonstrates that the facilely grown NCNTs are promising to develop practical FE cathodes.

Enhanced field emission properties of rGO wrapped Ga2O3 micro/nanobricks: Experimental investigation with theoretical validation

Aiming for efficient cold cathode applications in low dimension, pure and rGO wrapped gallium oxide micro/nanobricks were synthesized via cost-effective solid-state and hydrothermal routes. The synthesized samples were characterized using X-ray diffractometry, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy for phase, morphology, composition, and structure-related investigations. In addition, all samples were thoroughly investigated, and as a result, experimental modifications were adapted accordingly for improving the field emission properties. Remarkable enhancement of field-emission performances with a high emission current-density of 1.08 mA/cm2 and enhancement factor of 7400 has been observed as an effect of rGO wrapping. The results have been correlated with the increase in the availability of emission sites and proper charge carrier transport between the components in the hybrid structure. Furthermore, the probability of charge carrier transport across Ga2O3 /rGO junction was validated using theoretical analysis via DFT calculations. Moreover, the improvement of field emission properties due to rGO wrapping was also predicted from ANSYS simulations. Hence, 3.5 times increment of current density and 2.5 times lowering of turn-on field, the Ga2O3/rGO hybrid system emerged as one of the most functional future cold cathodes. This work opens up new applications for Ga2O3-based composites beyond the sensing and catalysis sectors.

Improve field emission properties of SiC nanowires by doping rare earth cerium under different methane ventilation rate

Cerium (Ce)-doped SiC nanowires (NWs) were synthesized by chemical vapor reaction (CVR) method under diverse methane (CH4) ventilation rates, with milled Si–SiO2 mixed powders being used as silicon source. Afterwards, the products were characterized to be β-SiC. In addition, FT-IR analysis reveals that the vibration intensity of Si–O group has been significantly reduced after doping with Ce, and the contraction vibration peak of Ce–O group appears at 882 cm−1. After Ce doping, the energy gap shows a decreasing trend from 1.37 to 0.9 eV, as determined by Material Studion 4.2. At the CH4 ventilation rate of 0.12 sccm, the content of Ce displays a relatively high value of 0.93 at%, the coating layer of SiO2 is about 12 nm, and the turn-on field achieves a minimum value of about 2.47 V μm−1. A synergetic mechanism was proposed for explaining the influence of the morphology and Ce concentration on the field emission (FE) properties of Ce-doped SiC NWs. Findings in the present study may provide certain foundation for future studies examining the FE properties of additional one-dimensional nano-material.

Enhanced electron emission from ternary solid solution-MWCNT hybrid with theoretical validation

Coalescence of unique properties of semiconductor and carbon nanotubes (CNTs) produces new advanced materials with exciting properties, which unwraps several new fields. Here, finite element modeling is employed to optimize the diameter of semiconducting zinc cadmium sulfide (Zn0.8Cd0.2S) microspheres on the wall of metallic CNTs to get the improved field emission property. Then, Zn0.8Cd0.2S microspheres are attached on COOH functionalized multiwalled CNTs (MWCNTs) by a simple one step solvothermal method for experimental corroboration. Field emission experiment shows a considerable enhancement of current density with respect to bare microspheres and functionalized CNTs. The turn-on field is found to be as low as 3V/μm at a current density of 10 μΑ/cm2 for the hybrid system. Density functional theory (DFT) calculation is carried out for the model structures of Zn0.8Cd0.2S and CNT-Zn0.8Cd0.2S to calculate the work functions. This improvement in field emission property of the hybrid system is demonstrated in terms of band bending, geometry, high field enhancement factor, low work function and electron affinity. Thus, non-field emitting microspheres with less toxicity have been transformed into a decent field emitter by the contribution of few carbon nanotubes.

Hydrogen Plasma Treatment of Aligned Multi-Walled Carbon Nanotube Arrays for Improvement of Field Emission Properties.

Vertically aligned carbon nanotube (CNT) arrays show potential for the development of planar low-voltage emission cathodes. The characteristics of cathodes can be improved by modifying their surface, e.g., by hydrogen plasma treatment, as was performed in this work. The surface of multi-walled CNT arrays grown on silicon substrates from toluene and ferrocene using catalytic chemical vapor deposition was treated in a high-pressure (~104 Pa) microwave reactor. The structure, composition, and current-voltage characteristics of the arrays were studied before and after hydrogen plasma treatment at various power values and durations. CNT tips were destroyed and catalytic iron was released from the CNT channels. The etching rate was influenced by iron particles that formed on the array surface. The lower emission threshold in the plasma-treated arrays than in the initial sample is explained by the amplification factor of the local electric field increasing due to graphene structures of unfolded nanotube layers that formed at the CNT tips.

Improved Field Emission Properties of Carbon Nanostructures by Laser Surface Engineering.

We herein present an alternative geometry of nanostructured carbon cathode capable of obtaining a low turn-on field, and both stable and high current densities. This cathode geometry consisted of a micro-hollow array on planar carbon nanostructures engineered by femtosecond laser. The micro-hollow geometry provides a larger edge area for achieving a lower turn-on field of 0.70 V/µm, a sustainable current of approximately 2 mA (about 112 mA/cm2) at an applied field of less than 2 V/µm. The electric field in the vicinity of the hollow array (rim edge) is enhanced due to the edge effect, that is key to improving field emission performance. The edge effect of the micro-hollow cathode is confirmed by numerical calculation. This new type of nanostructured carbon cathode geometry can be promisingly applied for high intensity and compact electron sources.

Improved analytical framework for quantifying field emission from nanostructures

Field emission properties of well characterized thin films, prepared using hydrothermal method, have been analysed to propose a new set of parameters for quantification of electron emission quality. Hydrothermally grown Nanopetals and nanorods of NiO and TiO2 respectively show good field emission properties of course different from each other due to variation in microstructures. The proposed analytical method enables one to understand the field emission properties in better sense as compared to the traditionally used framework, namely the Fowler-Nordhiem approach. The proposed model improves the analysis by introducing a boost-factor to take care of the field emission data in totality unlike the traditional method where only currents for higher electric fields are considered. The proposed model is found to be more appropriate as it addresses the ambiguity present in the previously used method. A quantum mechanical approach has been adopted for explaining the improved field emission properties from these nanostructures using the concept of tunnelling probability.

Improving field emission properties of vertically aligned carbon nanotube arrays through a structure modification

Vertically aligned carbon nanotube (VACNT) emitters were synthesized directly on stainless steel substrate using DC plasma-enhanced chemical vapor deposition. Remarkable field emission (FE) properties, such as low turn-on electric field (ETO = 1.40 V/μm) and low threshold electric field (ETH = 2.31 V/μm), were observed from VACNT arrays with long length and moderate density. The FE performance was significantly enhanced by a uniquely bundled structure of VACNTs formed through a simple water treatment process. The FE properties of VACNTs were further improved by coating the exterior of CNTs with a uniform layer of crystalline SnO2 nanoparticles; the ETO and ETH were reduced to 1.18 and 2.01 V/μm, respectively. The enhancement of FE properties by SnO2 coating can be attributed to the morphological change of VACNTs caused by the solution phase coating process. The coated samples also exhibited an improved FE stability which is attributed to the enhancement of the mechanical strength and chemical stability of the VACNTs after the SnO2 coating. The VACNT emitters with characteristic features such as a conductive substrate, low contact resistance between the VACNTs and the substrate, uniform coating, and bundled morphology can be ideal candidates for FE devices.

Study of film thickness effect on carbon nanotube based field emission devices

Carbon nanotube (CNT) has been considered as an ideal cathode material for field emission application for a long time. To improve field emission properties, coating CNT films to form a composite cathode is an effective approach. However, few reports have studied and revealed the effect of coated layer thickness. Hence, in this study, we employed an Au layer to coat CNT cathodes with capability to precisely control the coated film thickness and systematically investigated the film thickness effect on field emission properties of the CNT composite field-emission devices. A nonmonotonic relationship between field emission performances and cathode film thickness was observed, and a high performance with a rather low turn-on field of 0.60 V/μm and an ultrahigh field enhancement factor near to 20000 was optimally demonstrated for the CNT composite cathode with an Au layer thickness of 3 nm. Based on experimental data and analysis, the nonmonotonic tendency was found mainly due to effective field emitter density, and the cathode conductivity works only for ultrathin films. Besides, the CNT composite cathode had good stability and uniformity for low heat contact resistant. The superior performance of the CNT cathode composited with a proper Au layer demonstrated potential applications in vacuum electronic devices and an optimized thickness of the coated layer was important for field emission enhancement of composite cathodes.

Large Scale Synthesis of Nanopyramidal-Like VO₂ Films by an Oxygen-Assisted Etching Growth Method with Significantly Enhanced Field Emission Properties.

The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO2 thin films into pyramidal-like VO2 nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO2 crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO2 structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO2 films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO2 films showed potential benefits for application such as temperature−regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO2-based device applications.

Controllable Synthesis of Special Reed-Leaf-Like Carbon Nanostructures Using Copper Containing Catalytic Pyrolysis for High-Performance Field Emission

Special reed-leaf-like carbon nanostructures have been realized by using chemical vapor deposition (CVD) under the combined action of copper containing catalytic pyrolysis and ammonia (NH3) gas. The nucleation and growth mechanisms of CNLs based on growth parameters are discussed. The Raman spectra of carbon nanotubes (CNTs), CNLs and CNT-CNL composites were measured and found to be strongly influenced by the type of gas. Field emission (FE) properties of CNL-CNT composites were observed with a lower turn-on electric field of 0.73 V/µm, and a higher current density of 18.0 mA/cm2 at an electric field of 2.65 V/µm, which are superior to those of CNTs and flower-like CNLs. This is because there are more field emitters in CNLs inter-planted in CNTs. We consider that the unique FE stability of CNTs and defects in CNLs play a synergetic role on the improved FE properties.

Investigation on field-emission properties of graphdiyne–ZnO composite

Graphdiyne–ZnO composite is constructed to investigate field-emission. We hope the tip effect of graphdiyne can be strengthened by doping ZnO. We find that the effective movement of the Fermi level and the dwindling of band gap have contributed to the modification of the electronic structures of this composite significantly with increase in the electric field. The high binding energy indicates that the composite model is very stable. In addition, the ionization energies and work functions decrease linearly with the increasing electric field, which represents an improvement of field-emission properties. It implies that graphdiyne–ZnO composite may become a promising material for field-emission.