Field Emission Applications Research Articles

Comprehensive analysis of charge carriers dynamics through the honeycomb structure of graphite thin films and polymer graphite with applications in cold field emission and scanning tunneling microscopy

Polymer graphite electron sources have performed satisfactorily as field emission emitters and scanning tunneling microscopy probes in the past few years. However, the emission process was characterized by limited total emission currents. This paper introduces the elemental, vibrational, electronic structure, and optical analysis of polymer graphite and glass-graphite composite field emission cathodes to study these limitations. Moreover, the field emission characteristics are studied including the changes in the potential energy barrier of the used materials and structures. Among the studied structures, the cathodes prepared from graphite thin films deposited on a micropointed glass substrate (film-GMF) showed superior performance as random field emission arrays. This includes obtaining much higher emission current values ≈ 20 times) and lower threshold voltages ≈ 1/2) compared to the results obtained from polymer graphite samples. The enhancement factor in such emitters is believed to be the three-dimensional honeycomb structure of graphite. Moreover, the study includes applying graphite coatings to tungsten nano-field emission cathodes and scanning tunneling microscopy probes, which improves the performance of such cathodes/probes in both microscopic techniques.

Comprehensive Analysis of E478 Single-Component Epoxy Resin and Tungsten-E478 Interface for Metallic-Polymer Composite Electron Source Applications.

This study provides comprehensive elemental, optical, and energy gap characteristics of the E478 single-component epoxy resin. This type of epoxy resin has imperative applications in medium voltage insulation and cold field emission of electrons. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and hydrogen nuclear magnetic resonance (1H-NMR) were used to study the elemental and structural analyses, ultraviolet photoelectron spectroscopy (UPS) was used to obtain the local work function and the ionization potential energies, and ultraviolet/visible light spectroscopy (UV/VIS) was used to report the optical and energy gap characteristics of the epoxy resin being studied. Moreover, the UPS and UV/VIS analyses were merged to obtain the electron affinity of the E478 epoxy resin and to study the epoxy's energy band diagram and the tungsten-epoxy interface band structure. The results showed that the E478 epoxy resin is considered an n-type semiconductor of energy gap ∼3.94 eV, local work function ∼3.42 eV, ionization potential ∼6.10 eV, electron affinity ∼2.16 eV, and tungsten-epoxy Schottky contact barrier height ∼2.50 eV.

Field emission from two-dimensional (2D) CdSSe flake flowers structure grown on gold coated silicon substrate: An efficient cold cathode.

Field emission finds a vital space in numerous scientific and technological applications, including high-resolution imaging at micro- and nano-scales, conducting high-energy physics experiments, molecule ionization in spectroscopy, and electronic uses. A continuous effort exists to develop new materials for enhanced field emission applications. In the present work, two-dimensional (2D) well-aligned CdSSe flake flowers (CdSSe-FFs) were successfully grown on gold-coated silicon substrate utilizing a simple and affordable chemical bath deposition approach at ambient temperature. The time-dependent growth mechanism from nanoparticles to FFs was observed at optimized parameters such as concentration of precursors, pH (~11), deposition time, and solution temperature. The crystalline nature of CdSSe-FFs is confirmed by high-resolution transmission electron microscopy (HRTEM) results, and selected area electron diffraction (SAED) observations reveal a hexagonal crystal structure. Additionally, the CdSSe-FFs thickness was confirmed by TEM analysis and found to be ~20-30 nm. The optical, photoelectric, and field emission (FE) characteristics are thoroughly explored which shows significant enhancement due to the formation of heterojunction between the gold-coated silicon substrate and CdSSe-FFs. The UV-visible absorption spectra of CdSSe-FFs show enhanced absorption at 700 nm, corresponding to the energy band gap (Eg) of 1.77 eV. The CdSSe-FFs exhibited field emission and photosensitive field emission (PSFE) characteristics. In FE study CdSSe-FFs shows an increase in current density of 387.2 μ A cm-2 in an applied field of 4.1 V m-1 which is 4.08 fold as compared to without light illumination (95.1 μ A cm-2). Furthermore, it shows excellent emission current stability at the preset value of 1.5 μA over 3 h with a deviation of the current density of less than 5% respectively. RESEARCH HIGHLIGHTS: Novel CdSSe flake flowers were grown on Au-coated Si substrate by a cost-effective chemical bath deposition route. The growth mechanism of CdSSe flake flowers is studied in detail. Field emission and Photoluminescence study of CdSSe flake flowers is characterized. CdSSe flake flowers with nanoflakes sharp edges exhibited enhanced field emission properties.

Analytical modeling of nucleation and growth of graphene layers on CNT array and its application in field emission of electrons

Carbon Nanotube (CNT) arrays and graphene have undergone several investigations to achieve efficient field emission (FE) owing to CNT’s remarkable large aspect ratio and graphene’s exceptional FE stability. However, when dense CNT arrays and planar graphene layers were used as field emitters, their field enhancement factor reduced dramatically. Therefore, in this paper, we numerically analyze the growth of a dense CNT array with planar graphene layers (PGLs) on top, resulting in a CNT-PGL hybrid and the associated field enhancement factor. The growth of the CNT array is investigated using Plasma Enhanced Chemical Vapor Deposition (PECVD) chamber in C2H2/NH3 environment with variable C2H2 flow, Ni catalyst film thickness, and substrate temperature followed by PGL precipitation on its top at an optimized cooling rate and Ni film thickness. The analytical model developed accounts for the number density of ions and neutrals, various surface elementary processes on catalyst film, CNT array growth, and PGLs precipitation. According to our investigation, the average growth rate of CNTs increases and then decreases with increasing C2H2 flow rate and catalyst film thickness. CNTs grow at a faster rate when the substrate temperature increases. Furthermore, as the chamber temperature is lowered from 750 °C to 250 °C in N2 environment and Ni film thickness grows, the number of the graphene layers increases. The field enhancement factors for the CNT array and hybrid are then calculated based on the optimal parameter values. The average height of the nanotubes, their spacing from one another, and the penetration of the electric field due to graphene coverage are considered while computing the field enhancement factor. It has been found that adding planar graphene layers to densely packed CNTs can raise its field enhancement factor. The results obtained match the current experimental observations quite well.

Modification of LaB6 with ZrO2-Al2O3-TiO2 for Improvement of Density and Mechanical and Electrical Properties

Pure lanthanum hexaboride (LaB6) ceramics were prepared using powders of different grain sizes. The ceramics could reach a relative density of 98.2% at high temperatures and pressures, but had a low flexural strength (136.9 MPa). LaB6 ceramics were synthesized using ZrO2-Al2O3-TiO2 (ZAT) as sintering additives. The ceramics demonstrate high density and excellent mechanical properties. The hot pressure sintering (HPS) method was utilized in the synthesis of the ceramics. Investigations were conducted on the effects of ZAT content, as well as the effects of the sintering temperature and pressure on the sintering behavior, microstructure, and mechanical and electrical properties of LaB6 ceramics. LaB6 ceramics fabricated with a ZAT addition of 6 wt.%, at a sintering temperature of 1700 °C, and under a pressure of 50 MPa, exhibited superior sintering and electrical properties, including a relative density of 97%, a conductivity of 7.2 MS/m, a flexural strength of 281.5 MPa, and a Vickers hardness of 21.2 GPa. The LaB6 ceramics synthesized in this research exhibit promising potential as electron-emitting cathodes for field emission applications.

Enhanced field emission characteristics of WS2 nano-films by diamond film and Mo film

WS2 is a type of transition metal chalcogenide materials (TMDCs) that exhibits exceptional photoelectric properties. Due to its atomically sharp edges, WS2 has the capability to enhance the electric field around its edges, thus presenting significant potential in field emission applications. In this study, the electron beam vapor deposition (EBVD) system and microwave plasma chemical vapor deposition (MPCVD) system were used to prepare a series of single-layer WS2 films，single-layer diamond films and diamond/WS2 nano-composite films on N-type highly doped Si substrate, and Mo/WS2 nano-composite films were prepared on Al2O3 ceramic substrate. Based on the characterization and analysis of the microstructure and chemical composition of each thin film layer in the above film devices, the field emission (FE) characteristics of each thin film device are compared and studied. The results demonstrate that the maximum FE current density of diamond/WS2 nano-composite film device is 912 μA/cm2, which is 2.7 times of the maximum FE current density of Mo/WS2 nano-composite film device 330 μA/cm2, and 3.4 times of the maximum FE current density of the single-layer diamond thin film device 267 μA/cm2. The single layer WS2 film did not exhibit measurable FE performance. The working principles of thin film field emission devices with various structures are introduced. It is considered that the diamond layer plays the role of electron accelerating layer, which not only effectively improves the FE characteristics of diamond/WS2 nanocomposite film, but also improves the repeatability and long-term stability of diamond/WS2 nanocomposite film FE devices.

Nitrogen admixture effects on growth characteristics and properties of carbon nanowalls

The plasma functionalization of carbon nanowalls (CNWs) is of great interest due to their potential applications in electron field emission and energy storage devices. In this study, the effects of nitrogen on the growth and properties of CNWs are thoroughly investigated by varying a wide range of the N2 concentrations in the CH4/H2/N2 discharge plasma. Nitrogen-doped carbon nanowalls (NCNWs) were synthesized in a single-step growth using the hydrogen radical injection plasma-enhanced chemical vapor deposition technique. The formation of dense NCNWs was found to be enhanced by nitrogen and was three times higher in the sample with a 150 sccm N2 flow rate than in the sample without N2 addition (N-0). As the N2 concentration increased from 0 to 200 sccm, the morphology of CNWs changed from relatively straight to curly-like nanowall structures. A high-resolution optical emission spectroscopic observation shows that as N2 flow increased, CN radicals became the dominant species, suppressing the formation of C2 and CH. Photoelectron spectroscopy analysis revealed the nitrogen doping effect on graphene layers at high N2 concentrations. According to Raman and Transmission Electron Microscopy analyses, the NCNWs had more branched and thicker nanowalls with higher defect concentrations than the N-0 sample. The wettability of NCNWs was improved, particularly for the sample with a N2 flow rate of 150 sccm.

Regulating the aspect ratio of bulk few-layer graphene to improve the field emission performance

Owing to its excellent electrical conductivity and local electric field enhancement capability from abundant electron tunneling edges, graphene is a promising material for field emission. However, graphene thin-film emitters mostly emit in planar structures, and due to the flexibility of the thin film, it is commonly challenging to achieve vertical structure emission with large aspect ratios. Therefore, the emission performance of current graphene field emitters is difficult to meet the requirements of high current applications. In the present investigation, bulk few-layer graphene cathodes are formed by cold pressing to achieve upright structural field emission, and the emission performance is enhanced by appropriately adjusting the aspect ratio. The obtained bulk few-layer graphene emitters exhibit a higher conductivity of 2498 S/cm and tensile strength of 6.02 MPa. The turn-on electric field (at 1 mA/cm2) and threshold electric field (at 10 mA/cm2) in order are determined to be 1.78 V/μm and 2.66 V/μm, with a maximum current density of 986 mA/cm2 (at 6.65 V/μm) and a maximum field enhancement factor of 14,891. Excellent emission stability is achieved over a continuous period of 110 h at 5 mA @ 526 mA/cm2. With the increase of emission current, the bulk few-layer graphene with large aspect ratios exhibits better emission stability. This work provides valuable insights into the application of graphene-based field emitters at high currents.

Highly enhanced and stable field emission performance of CNT – Dielectric (Si3N4) hybrids

In the present study, vertically aligned carbon nanotubes (CNTs) were grown onto Si substrates using a simple thermal chemical vapour deposition (TCVD) technique. Subsequently, the dielectric layer of Si3N4 was coated over the pristine CNTs using the Plasma Enhanced CVD (PECVD) method. Various characterizations were carried out on the as-deposited samples to study the structural properties, surface morphology, elemental composition, and electronic structure. The field emission measurements executed on CNT – Si3N4 samples showed an enhanced current density of 8.28 mA/cm2 compared to the 2.85 mA/cm2 current density of the pristine CNTs. A detailed study reveals that, apart from the basic conventional field emission parameters, the electronic structure of the CNTs, along with the presence of disordered carbon atoms and the presence of the dielectric material, are responsible for enhancing the current density and temporal stability of the CNT – Si3N4 hybrids. The study aids in exploring CNT-dielectric hybrids for better field emission properties with improved stability for field emission applications.

Surface modification of a biomass-derived self-supported carbon nano network as an emerging platform for advanced field emitter devices and supercapacitor applications.

Herein, a self-supported carbon network is designed through the sole pyrolysis of Carica papaya seeds (biomass) without any activation agent, demonstrating their field emission and supercapacitor applications. The pyrolysis of seeds in an argon atmosphere leads to the formation of interconnected, rod-like structures. Furthermore, the hydrofluoric acid treatment not only removed impurities, but also resulted in the formation of CaF2 nanocrystals with the addition of F-doping. From the field emission studies, the turn-on field values defined at an emission current density of ∼10 μA cm-2 were found to be ∼2.16 and 1.21 V μm-1 for the as-prepared carbon and F-doped carbon, respectively. Notably, F-doped carbon exhibits a high emission current density of ∼9.49 mA cm-2 and has been drawn at an applied electric field of ∼2.29 V μm-1. Supercapacitor studies were carried out to demonstrate the multi-functionality of the prepared materials. The F-doped carbon electrode material exhibits the highest specific capacitance of 234 F g-1 at 0.5 A g-1. To demonstrate the actual supercapacitor application, the HFC//HFC symmetric coin cell supercapacitor device was assembled. The overall multifunctional applicability of the fabricated hybrid structures provides a futuristic approach to field emission and energy storage applications.

Hierarchical growth of CdSe dendritic nanostructures for enhanced field emission application

Hierarchical growth of CdSe dendritic nanostructures for enhanced field emission application

Cold Cathodes with Two-Dimensional van der Waals Materials.

Two-dimensional van der Waals materials could be used as electron emitters alone or stacked in a heterostructure. Many significant phenomena of two-dimensional van der Waals field emitters have been observed and predicted since the landmark discovery of graphene. Due to the wide variety of heterostructures that integrate an atomic monolayer or multilayers with insulator nanofilms or metallic cathodes by van der Waals force, the diversity of van der Waals materials is large to be chosen from, which are appealing for further investigation. Until now, increasing the efficiency, stability, and uniformity in electron emission of cold cathodes with two-dimensional materials is still of interest in research. Some novel behaviors in electron emission, such as coherence and directionality, have been revealed by the theoretical study down to the atomic scale and could lead to innovative applications. Although intensive emission in the direction normal to two-dimensional emitters has been observed in experiments, the theoretical mechanism is still incomplete. In this paper, we will review some late progresses related to the cold cathodes with two-dimensional van der Waals materials, both in experiments and in the theoretical study, emphasizing the phenomena which are absent in the conventional cold cathodes. The review will cover the fabrication of several kinds of emitter structures for field emission applications, the state of the art of their field emission properties and the existing field emission model. In the end, some perspectives on their future research trend will also be given.

A two-dimensional hybrid of NiO nanowalls and reduced graphene oxide for superior field emission

The sharp and 2D nanostructure of nickel oxide (NiO) was synthesized hydrothermally on the nickel(Ni) foil and studied for field emission application. Graphene oxide (GO) and reduced graphene (rGO) is also good field emitter due to their unique electrical properties and geometry. The 2D_NiO nanostructured substrate was coated with GO solution, and it form a hetero-structure. The structure, morphology, and elemental details of as-synthesized materials, as well as hybrid samples, were determined by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). The field emission study of the samples was studied for NiO nanostructures samples showed field emission response. The turn-on field and maximum currents are 7.5 V/μm and 4.9 mA/cm2; for graphene oxide coated Ni foil, Ni_GO sample, 14 V/μm, and the maximum current is 2.6 mA/cm2. With a coating of GO on the NiO nanostructured surface, the field emission response enhanced abruptly. In the hybrid sample, NiO_GO, the field emission response is enhanced abruptly, the turn-on the field and the maximum current are 5.7 V/μm and 15.5 mA/ cm2, respectively. The field emission response is substantially increased than that of the individual 2D nanostructure, which has shown a low field emission. The field emission response increased further, introducing rGO on the Nanostructured substrate. In the NiO_rGO sample, the turn-on field and the maximum currents are 4.4 V/μm and 39.6 mA/ cm2, respectively.

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.

Low-Pressure CVD Synthesis of Tetragonal Borophene Single-Crystalline Sheets with High Ambient Stability

Being a typical graphene-like two-dimensional (2D) material, borophene has received a lot of attention due to its low density, high electron mobility, large Young’s modulus, and good chemical stability. Compared with the corresponding bulk counterparts, 2D borophene has larger specific surface area, higher conductivity, and smaller work function, finding potential applications in high-speed transistors, optoelectronic detection, field emission (FE), and mechanical enforcement areas. In this paper, 2D tetragonal borophene sheets with high ambient stability have been successfully fabricated on the surface of 1 cm2 copper foil using a developed low-pressure chemical vapor deposition (LPCVD) method. The vapor–solid mechanism was used to comprehend the formation of the tetragonal borophene sheets. The electrical conductivity of individual borophene sheets ranged from 4 to 5 × 10–4 S/cm, and the band gap was about 2.1 eV, suggesting their narrow semiconductive nature. Also, the maximum FE current of individual borophene sheets was close to 1 μA, and their emission properties enhanced with the decreasing sheet thickness, comparable to many other nanomaterials with excellent FE performances. More interestingly, individual borophene sheets were found to retain good enough electrical transport and FE behaviors even if they experienced several days’ atmospheric conservations. Our research results suggest that the LPCVD-grown borophene may be a promising building block for constructing high-performance nanodevices with good ambient stability.

Understanding substrate-driven growth mechanism of copper silicide nanoforms and their applications

The growth mechanism of various nanoforms of copper/copper silicide on Si substrates with different orientations is reported here. The triangular, square and rectangular copper/copper silicide nanostructures are deposited on silicon substrates with (111), (100) and (110) orientations respectively, by thermal evaporation of metallic copper at different substrate temperatures. Investigations confirm that the nanostructures consist of a pure copper layer on top of a copper silicide layer. The morphology and growth behaviour studies confirmed for the first time that the formation of well-defined structures and shapes are governed by the Si substrate orientation and the surface reconstruction of the Si planes. The copper silicide nanostructures are used in field emission applications and also serve as a template for the growth of carbon nano-fibre.

MoxWx–1S2 Nanotubes for Advanced Field Emission Application

AbstractTransition metal dichalcogenide (TMDC) nanotubes complement the field of low‐dimensional materials with their quasi‐1D morphology and a wide set of intriguing properties. By introducing different transition metals into the crystal structure, their properties can be tailored for specific purpose and applications. Herein, the characterization and a subsequent preparation of single‐nanotube field emission devices of MoxWx‐1S2 nanotubes prepared via the chemical vapor transport reaction is presented. Energy‐dispersive X‐ray spectroscopy, Raman spectroscopy, and X‐ray diffraction indicate that the molybdenum and tungsten atoms are randomly distributed within the crystal structure and that the material is highly crystalline. High resolution transmission electron microscopy and electron diffraction (ED) patterns further corroborate these findings. A detailed analysis of the ED patterns from an eight‐layer nanotube reveal that the nanotubes grow in the 2H structure, with each shell consists of one bilayer. The work function of the nanotubes is comparable to that of pure MoS2 and lower of pure WS2 NTs, making them ideal candidates for field emission applications. Two devices with different geometrical setup are prepared and tested as field emitters, showing promising results for single nanotube field emission applications.

Fullerene Transformed into a 3-D Structure of Nitrogen-Doped Few-Layer Graphene Sheets: Growth and Field Emission Properties

3-D-structured nitrogen-doped few-layer graphene sheets (3-D NFLGs) have been prepared using a fullerene C60 precursor combined with a microwave excited nitrogen plasma process. A deep understanding of the growth mechanism and regulating of morphology that can adjust the field emission properties is crucial for its widespread application. In this work, the detailed growth process and microstructure and the field emission properties of the as-synthesized vertical aligned 3-D NFLGs were studied. The growth of the 3-D NFLGs can be divided into three steps: evaporation of the C60, opening of the C60, and formation of the 3-D NFLGs. The growth process and morphology of the materials can be neatly regulated by changing the evaporating temperature of the C60, electron temperature of the plasma, and substrate temperature through our self-designed reaction equipment. Interestingly, the vertical 3-D NFLGs grew at a limited microwave power and nitrogen pressure and could be divided into three different morphological features, reflected in their distribution uniformity, interparietal distance, number of layers, and crystallinity, which exhibited different field emission properties. Additionally, the 3-D NFLGs were all in situ doped with ∼4% nitrogen atoms from nitrogen gas during the growth process, while different compositions of the nitrogen atoms were reflected in the graphitic N increasing with increasing nitrogen pressure. Furthermore, the field emission measurements show that the as-obtained vertical 3-D NFLGs exhibited the lowest turn-on electric field (1.30 V/μm@10 μA/cm2) and threshold field (2.1 V/μm@1 mA/cm2) at the M1 morphology and the highest stability at the M3 morphology. Remarkable field emission performance was obtained for the 3-D NFLGs with M2 morphology, showing their great potential for field emission applications.

Development of CoFe2O4 scaffolded reduced graphene oxide/carbon nanotube emitter for field emission application

The field emission (FE) characteristics of CoFe2O4/rGO/CNT (CGC) emitter is reported by comparing it with that of CoFe2O4/rGO (CG), CoFe2O4/CNT (CC), rGO/CNT (GC) and individual emitter. Among them, current density was high for CGC emitter i.e. 3.329 mA cm−2 for applied field of 5.4 V μm−1 as well as a good emission current stability. These superior FE characteristics of CGC emitter is due to the decreased bandgap as well as work function and, due to the sharp edges provided by CNTs and CoFe2O4 nanoparticles which results in easy electron liberation. A detailed mechanism for enhanced FE performance has been proposed based on obtained results.

High-Efficiency Ion Enrichment inside Ultra-Short Carbon Nanotubes.

The ion-enrichment inside carbon nanotubes (CNTs) offers the possibility of applications in water purification, ion batteries, memory devices, supercapacitors, field emission and functional hybrid nanostructures. However, the low filling capacity of CNTs in salt solutions due to end caps and blockages remains a barrier to the practical use of such applications. In this study, we fabricated ultra-short CNTs that were free from end caps and blockages using ball milling and acid pickling. We then compared their ion-enrichment capacity with that of long CNTs. The results showed that the ion-enrichment capacity of ultra-short CNTs was much higher than that of long CNTs. Furthermore, a broad range of ions could be enriched in the ultra-short CNTs including alkali-metal ions (e.g., K+), alkaline-earth-metal ions (e.g., Ca2+) and heavy-metal ions (e.g., Pb2+). The ultra-short CNTs were much more unobstructed than the raw long CNTs, which was due to the increased orifice number per unit mass of CNTs and the decreased difficulty in removing the blockages in the middle section inside the CNTs. Under the hydrated-cation–π interactions, the ultra-short CNTs with few end caps and blockages could highly efficiently enrich ions.