Self-organized multi-layered graphene-boron-doped diamond hybrid nanowalls for high-performance electron emission devices

Tailoring Defects in B, N-Codoped Carbon Nanowalls for Direct Electrochemical Oxidation of Glyphosate and its Metabolites

Tailoring the defects in graphene and its related carbon allotropes has great potential to exploit their enhanced electrochemical properties for energy applications, environmental remediation, and sensing. Vertical graphene, also known as carbon nanowalls (CNWs), exhibits a large surface area, enhanced charge transfer capability, and high defect density, making it suitable for a wide range of emerging applications. However, precise control and tuning of the defect size, position, and density remain challenging; moreover, due to their characteristic labyrinthine morphology, conventional characterization techniques and widely accepted quality indicators fail or need to be reformulated. This study primarily focuses on examining the impact of boron heterodoping and argon plasma treatment on CNW structures, uncovering complex interplays between specific defect-induced three-dimensional nanostructures and electrochemical performance. Moreover, the study introduces the use of defect-rich CNWs as a label-free electrode for directly oxidizing glyphosate (GLY), a common herbicide, and its metabolites (sarcosine and aminomethylphosphonic acid) for the first time. Crucially, we discovered that the presence of specific boron bonds (BC and BN), coupled with the absence of Lewis-base functional groups such as pyridinic-N, is essential for the oxidation of these analytes. Notably, the D+D* second-order combinational Raman modes at ≈2570 cm-1 emerged as a reliable indicator of the analytes' affinity. Contrary to expectations, the electrochemically active surface area and the presence of oxygen-containing functional groups played a secondary role. Argon-plasma post-treatment was found to adversely affect both the morphology and surface chemistry of CNWs, leading to an increase in sp3-hybridized carbon, the introduction of oxygen, and alterations in the types of nitrogen functional groups. Simulations support that certain defects are functional for GLY rather than AMPA. Sarcosine oxidation is the least affected by defect type.

A Study on the Field Emission Characteristics of High-Quality Wrinkled Multilayer Graphene Cathodes

Field emission (FE) necessitates cathode materials with low work function and high thermal and electrical conductivity and stability. To meet these requirements, we developed FE cathodes based on high-quality wrinkled multilayer graphene (MLG) prepared using the bubble-assisted chemical vapor deposition (B-CVD) method and investigated their emission characteristics. The result showed that MLG cathodes prepared using the spin-coating method exhibited a high field emission current density (~7.9 mA/cm2), indicating the excellent intrinsic emission performance of the MLG. However, the weak adhesion between the MLG and the substrate led to the poor stability of the cathode. Screen printing was employed to prepare the cathode to improve stability, and the influence of a silver buffer layer was explored on the cathode's performance. The results demonstrated that these cathodes exhibited better emission stability, and the silver buffer layer further enhanced the comprehensive field emission performance. The optimized cathode possesses low turn-on field strength (~1.5 V/μm), low threshold field strength (~2.65 V/μm), high current density (~10.5 mA/cm2), and good emission uniformity. Moreover, the cathode also exhibits excellent emission stability, with a current fluctuation of only 6.28% during a 4-h test at 1530 V.

Tuneable effects of pyrrolic N and pyridinic N on the enhanced field emission properties of nitrogen-doped graphene

Graphene is one of the most potential field emission cathode materials and a lot of work has been carried out to demonstrate the effectiveness of nitrogen doping (N doping) for the enhancement of field emission properties of graphene. However, the effect of N doping on graphene field emission is lacking systematic and thorough understanding. In this study, undoped graphene and N-doped graphene were prepared and characterized for measurements, and the field emission property dependence of the doping content was investigated and the tuneable effect was discussed. For the undoped graphene, the turn-on field was 7.95 V μm-1 and the current density was 7.3 μA cm-2, and for the 10 mg, 20 mg, and 30 mg N-doped graphene samples, the turn-on fields declined to 7.50 V μm-1, 6.38 V μm-1, and 7.28 V μm-1, and current densities increased to 21.0 μA cm-2, 42.6 μA cm-2, and 13.2 μA cm-2, respectively. Density functional theory (DFT) calculations revealed that N doping could bring about additional charge and then cause charge aggregation around the N atom. At the same time, it also lowered the work function, which further enhanced the field emission. The doping effect was determined by the content of the pyrrolic-type N and pyridinic-type N. Pyridinic-type N is more favourable for field emission because of its smaller work function, which is in good agreement with the experimental results. This study would be of great benefit to the understanding of N doping modulation for superior field emission properties.

Improvement of field emission performances by DMSO and PEDOT:PSS treated freestanding CNT clusters

In this paper, we present two methods to improve the field emission (FE) performance of vertically aligned carbon nanotube emitters that are treated with dimethyl sulfoxide (DMSO) solution and poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) solution, respectively, and compared their performances. Both treatments force CNTs to be bundled into clusters, but the formed structures are very different. After treatment, both methods reduced the threshold electric field due to the reduction in screening effects, whereas the PEDOT:PSS treatment significantly reduced the threshold field to far lower than that of DMSO treatment. In addition, the FE efficiency and lifetime of treated CNT emitters are significantly improved. For both treated emitters, there is only slight degradation of the emission current after 80 hours of continuous FE at around 50 μA. In addition, the uniformity of both treated CNT emitters is improved which enables more CNTs to contribute to the overall current emission. This, in turn, lowers the current emitted by individual CNTs, and thereby increases the lifetime of the emitters. Therefore, this study demonstrates that these simple treatment methods of bundling CNTs into unique cluster-structures significantly improve the lifetime of FE and make them excellent candidates for large currents and long-term FE.

Covalent organic frameworks-derived hierarchically porous N-doped carbon for 2,4-dichlorophenol degradation by activated persulfate: The dual role of graphitic N

A series of hierarchically porous carbon catalysts with high N content and large surface area were prepared via self-templated carbonization of covalent organic frameworks (COFs). The catalyst was used to activate persulfate (PS) for degrading 2,4-dichlorophenol (2,4-DCP). Experimental results demonstrated that the prepared catalyst treated at 700 °C (PNC-700) showed both strong adsorption ability and enhanced PS activity for 2,4-DCP degradation. A variety of characterization techniques were used to investigate the properties of prepared catalysts. We found that the graphitic N functional groups acted as both activity sites and electron transfer access. The activity of the catalyst was also closely related to the hierarchical pore structure and good electrical conductivity. The influencing factors of PNC-700/PS system in 2,4-DCP degradation were discussed. In addition, PNC-700 displayed excellent recyclability. The activation process especially non-radical pathway was promoted by increasing graphitic N contents. The possible reaction mechanism and degradation pathways were also proposed.

Bottom-Up Evolution of Diamond-Graphite Hybrid Two-Dimensional Nanostructure: Underlying Picture and Electrochemical Activity

The diamond-graphite hybrid thin film with low-dimensional nanostructure (e.g., nitrogen-included ultrananocrystalline diamond (N-UNCD) or the alike), has been employed in many impactful breakthrough applications. However, the detailed picture behind the bottom-up evolution of such intriguing carbon nanostructure is far from clarified yet. Here, the authors clarify it, through the concerted efforts of microscopic, physical, and electrochemical analyses for a series of samples synthesized by hot-filament chemical vapor deposition using methane-hydrogen precursor gas, based on the hydrogen-dependent surface reconstruction of nanodiamond and on the substrate-temperature-dependent variation of the growth species (atomic hydrogen and methyl radical) concentration near substrate. The clarified picture provides insights for a drastic enhancement in the electrochemical activities of the hybrid thin film, concerning the detection of important biomolecule, that is, ascorbic acid, uric acid, and dopamine: their limits of detections are 490, 35, and 25 nm, respectively, which are among the best of the all-carbon thin film electrodes in the literature. This work also enables a simple and effective way of strongly enhancing AA detection.

Enhanced Field Emission from Ultrananocrystalline Diamond-Decorated Carbon Nanowalls Prepared by a Self-Assembly Seeding Technique

Vertically aligned nanographite structures, the so-called carbon nanowalls (CNWs), are decorated with ultrananocrystalline diamond particles by an electrostatic self-assembly seeding technique, followed by short-term growth in plasma chemical vapor deposition, to enhance field emission efficiency and stability. A nanodiamond suspension diluted with a dispersion medium with high wettability on CNWs enables seeding of diamond nanograins consisting of nanoparticles of 3-5 nm in diameter on CNWs with high uniformity and minimal aggregation and control of their number density. The field emission turn-on field depends upon the density of diamond nanograins and decreases from 3.0 V μm^-1 for bare CNWs to 1.8 V μm^-1 for diamond-decorated CNWs together with about an order of magnitude increase in current density. Finite element modeling indicates that only a part of decorating diamond located at the top of nanowalls actually contributes to field amplification and emission. The diamond-decorated CNWs show also higher emission stability with much larger time constants of current degradation than the bare CNWs for long-term duration. The enhanced emission efficiency is explained by larger field amplification rather than lowering of the tunneling barrier, while the enhanced emission stability is attributed to the higher robustness of diamond.

Influence of B/N co-doping on electrical and photoluminescence properties of CVD grown homoepitaxial diamond films

Boron doped diamond (BDD) has great potential in electrical, and electrochemical sensing applications. The growth parameters, substrates, and synthesis method play a vital role in the preparation of semiconducting BDD to metallic BDD. Doping of other elements along with boron (B) into diamond demonstrated improved efficacy of B doping and exceptional properties. In the present study, B and nitrogen (N) co-doped diamond has been synthesized on single crystalline diamond (SCD) IIa and SCD Ib substrates in a microwave plasma-assisted chemical vapor deposition process. The B/N co-doping into CVD diamond has been conducted at constant N flow of N/C ∼ 0.02 with three different B/C doping concentrations of B/C ∼ 2500 ppm, 5000 ppm, 7500 ppm. Atomic force microscopy topography depicted the flat and smooth surface with low surface roughness for low B doping, whereas surface features like hillock structures and un-epitaxial diamond crystals with high surface roughness were observed for high B doping concentrations. KPFM measurements revealed that the work function (4.74-4.94 eV) has not varied significantly for CVD diamond synthesized with different B/C concentrations. Raman spectroscopy measurements described the growth of high-quality diamond and photoluminescence studies revealed the formation of high-density nitrogen-vacancy centers in CVD diamond layers. X-ray photoelectron spectroscopy results confirmed the successful B doping and the increase in N doping with B doping concentration. The room temperature electrical resistance measurements of CVD diamond layers (B/C ∼ 7500 ppm) have shown the low resistance value ∼9.29 Ω for CVD diamond/SCD IIa, and the resistance value ∼16.55 Ω for CVD diamond/SCD Ib samples.

Control of electrostatic self-assembly seeding of diamond nanoparticles on carbon nanowalls

Seeding of diamond nanoparticles on vertically-aligned multi-layer graphene, the so-called carbon nanowalls (CNWs), is studied by using deionized water, ethylene glycol, ethanol, and formamide as dispersion mediums. Detonation nanodiamond particles show the smallest mean size and size distribution with a high positive zeta potential when dispersed in ethanol. The contact angle of ethanol on CNWs is almost zero degree, confirming highly wetting behaviour. The diamond nanoparticles dispersed in ethanol are distributed the most uniformly with minimal aggregation on CNWs as opposed to those dispersed in other liquids. The resulting diamond nanoparticle-seeded CNWs, followed by short-term growth in microwave plasma chemical vapor deposition, show a marked decrease in field emission turn-on field down to 1.3 Vμm^-1together with a large increase in current density, compared to bare CNWs without diamond seeding. The results provide a way to control the density, size, and uniformity (spacing) of diamond nanoparticles on CNWs and should be applied to fabricate hybrid materials and devices using nanodiamond and nanocarbons.

Nitrogen-Incorporated Boron-Doped Nanocrystalline Diamond Nanowires for Microplasma Illumination

The origin of nitrogen-incorporated boron-doped nanocrystalline diamond (NB-NCD) nanowires as a function of substrate temperature (T_s) in H₂/CH₄/B₂H₆/N₂ reactant gases is systematically addressed. Because of T_s, there is a drastic modification in the dimensional structure and microstructure and hence in the several properties of the NB-NCD films. The NB-NCD films grown at low T_s (400 °C) contain faceted diamond grains. The morphology changes to nanosized diamond grains for NB-NCD films grown at 550 °C (or 700 °C). Interestingly, the NB-NCD films grown at 850 °C possess one-dimensional nanowire-like morphological grains. These nanowire-like NB-NCD films possess the co-existence of the sp³-diamond phase and the sp²-graphitic phase, where diamond nanowires are surrounded by sp²-graphitic phases at grain boundaries. The optical emission spectroscopy studies stated that the CN, BH, and C₂ species in the plasma are the main factors for the origin of nanowire-like conducting diamond grains and the materialization of graphitic phases at the grain boundaries. Moreover, conductive atomic force microscopy studies reveal that the NB-NCD films grown at 850 °C show a large number of emission sites from the grains and the grain boundaries. While boron doping improved the electrical conductivity of the NCD grains, the nitrogen incorporation eased the generation of graphitic phases at the grain boundaries that afford conducting channels for the electrons, thus achieving a high electrical conductivity for the NB-NCD films grown at 850 °C. The microplasma devices using these nanowire-like NB-NCD films as cathodes display superior plasma illumination properties with a threshold field of 3300 V/μm and plasma current density of 1.04 mA/cm² with a supplied voltage of 520 V and a lifetime stability of 520 min. The outstanding plasma illumination characteristics of these conducting nanowire-like NB-NCD films make them appropriate as cathodes and pave the way for the utilization of these materials in various microplasma device applications.

Mixed sp2-sp3 Nanocarbon Materials: A Status Quo Review

Carbon nanomaterials with a different character of the chemical bond-graphene (sp2) and nanodiamond (sp3)-are the building bricks for a new class of all-carbon hybrid nanomaterials, where the two different carbon networks with sp3 and sp2 hybridization coexist, interacting and even transforming into one another. The extraordinary physiochemical properties defined by the unique electronic band structure of the two border nanoallotropes ensure the immense application potential and versatility of these all-carbon nanomaterials. The review summarizes the status quo of sp2 - sp3 nanomaterials, including graphene/graphene-oxide-nanodiamond composites and hybrids, graphene/graphene-oxide-diamond heterojunctions, and other sp2-sp3 nanocarbon hybrids for sensing, electronic, and other emergent applications. Novel sp2-sp3 transitional nanocarbon phases and architectures are also discussed. Furthermore, the two-way sp2 (graphene) to sp3 (diamond surface and nanodiamond) transformations at the nanoscale, essential for innovative fabrication, and stability and chemical reactivity assessment are discussed based on extensive theoretical, computational and experimental studies.

The Central Role of Nitrogen Atoms in a Zeolitic Imidazolate Framework-Derived Catalyst for Cathodic Hydrogen Evolution

Platinum usually offers the most effective active center for hydrogen evolution reaction (HER), because of the optimal trade-off between the adsorption and desorption of hydrogeN atoms (H*) on Pt atoms. Herein, we report an unusual result regarding the active center of a HER catalyst, which was synthesized by electrodepositing traces of Pt nanoparticles (NPs) into a porous nitrogen-rich dodecahedron matrix derived from zeolitic imidazolate framework ZIF-8. With an ultra-low Pt loading of 2.76 μg cm^-2 , the N-Pt-bonded catalyst can produce a current density of 117 mA cm^-2 for the HER in 1.0 m H₂ SO₄ at an overpotential of 50 mV, whereas the commercial Pt/C (300 μg cm^-2 Pt) can only reach 50 mA cm^-2 under the same conditions. Cyclic voltammetry demonstrates that both the H* adsorption and the Pt oxidation are not allowed to occur on this catalyst, due to a full surface coverage of the trace Pt NPs by imidazole. The results from the specially designed experiments indicate that the imidazole N atoms may act as proton anchor-sites for the HER due to their electron donor nature. Density functional theory calculations also support a catalytic HER mechanism centered at the Pt-supported N active center, which needs a Gibbs free energy of H* absorption (ΔG_H* ) significantly smaller than the absolute value of ΔG_H* on the Pt(111) surface. We hope that the results of this study will encourage the research on novel N-centered catalysts for the HER.

Electrochemical determination of nitroaromatic explosives at boron-doped diamond/graphene nanowall electrodes: 2,4,6-trinitrotoluene and 2,4,6-trinitroanisole in liquid effluents

The study is devoted to the electrochemical detection of trace explosives on boron-doped diamond/graphene nanowall electrodes (B:DGNW). The electrodes were fabricated in a one-step growth process using chemical vapour deposition without any additional modifications. The electrochemical investigations were focused on the determination of the important nitroaromatic explosive compounds, 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitroanisole (TNA). The distinct reduction peaks of both studied compounds were observed regardless of the pH value of the solution. The reduction peak currents were linearly related to the concentration of TNT and TNA in the range from 0.05-15 ppm. Nevertheless, two various linear trends were observed, attributed respectively to the adsorption processes at low concentrations up to the diffusional character of detection for larger contamination levels. The limit of detection of TNT and TNA is equal to 73 ppb and 270 ppb, respectively. Moreover, the proposed detection strategy has been applied under real conditions with a significant concentration of interfering compounds - in landfill leachates. The proposed bare B:DGNW electrodes were revealed to have a high electroactive area towards the voltammetric determination of various nitroaromatic compounds with a high rate of repeatability, thus appearing to be an attractive nanocarbon surface for further applications.

In Situ Construction of Hierarchical Diamond Supported on Carbon Nanowalls/Diamond for Enhanced Electron Field Emission

The integration of sp²-/sp³-bonded carbon has aroused increasing attention on attaining a great electron field emission (EFE) performance. Herein, a novel hierarchical diamond@carbon nanowalls/diamond (D@C/D) architecture is facilely prepared through the growth of the hybrid carbon nanowalls/diamond (C/D) film followed by the in situ hydrogen plasma treatment using microwave plasma chemical vapor deposition. The hierarchical D@C/D architecture is composed of thin diamond nanoplatelets sandwiched into carbon nanowalls (CNWs) as the bottom layer and the thickened nanoplatelets constituted by diamond nanograins as the upper layer. The hydrogen plasma plays an effective role in the transformation of sacrificial sp²-bonded CNWs to sp³-bonded diamond, eventually leading to the template thickening of diamond nanoplatelets in the upper layer. Impressively, the D@C/D-90 film demonstrates much better EFE behaviors of low turn-on potential (E_o = 4.3 V μm^-1), high current density (J_e@8 V μm^-1 = 20.81 mA cm^-1), and superior long-term stability, in comparison with the pristine C/D film (E_o = 6 V μm^-1, J_e@8 V μm^-1 = 0.33 mA cm^-1). The enhanced EFE performance of the hierarchical D@C/D film is ascribed to the well-established graphite pathway for electrons transported from the bottom to the top and the increased diamond emitting sites with negative electron-affinity and robust nature at the top. This work will promote the development of the high-performance cathode EFE material based on hybrid sp²/sp³-bonded carbon, and the method proposed here also provides an effective strategy to construct a diamond nanostructure for various applications.

Enhanced charge storage properties of ultrananocrystalline diamond films by contact electrification-induced hydrogenation

We report the enhanced charge storage characteristics of ultrananocrystalline diamond (UNCD) by contact electrification-induced hydrogenation. The non-catalytic hydrogenation of UNCD films was achieved by using platinum as an electron donor and sulfuric acid as a hydrogen proton donor, confirmed by Raman spectroscopy and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Chemical treatment with only a H₂SO₄ solution is responsible for the surface oxidation. The oxidation of UNCD resulted in an increase in the quantity and duration of the tribocharges. After non-catalytic hydrogenation, the generation of friction-induced tribocharges was enhanced and remained for three hours and more. We show that the hydrogen incorporation on grain boundaries is responsible for the improvement of charge storage capability, because the doped hydrogen acts as a trap site for the tribocharges. This lab-scale and succinct method can be utilized to control charge trap capability in nanoscale memory electronics.

The enhanced charge storage characteristics of ultrananocrystalline diamond caused by contact electrification-induced hydrogenation was demonstrated by using atomic force microscopy.

Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source

Electron emission signifies an important mechanism facilitating the enlargement of devices that have modernized large parts of science and technology. Today, the search for innovative electron emission devices for imaging, sensing, electronics, and high-energy physics continues. Integrating two materials with dissimilar electronic properties into a hybrid material is an extremely sought-after synergistic approach, envisioning a superior field electron emission (FEE) material. An innovation is described regarding the fabrication of a nanostructured carbon hybrid, resulting from the one-step growth of boron-doped nanocrystalline diamond (BNCD) and carbon nanospikes (CNSs) by a microwave plasma-enhanced chemical vapor deposition technique. Spectroscopic and microscopic tools are used to investigate the morphological, bonding, and microstructural characteristics related to the growth mechanism of these hybrids. Utilizing the benefits of both the sharp edges of the CNSs and the high stability of BNCD, promising FEE performance with a lower turn-on field of 1.3 V/μm, a higher field enhancement factor of 6780, and a stable FEE current stability lasting for 780 min is obtained. The microplasma devices utilizing these hybrids as a cathode illustrate a superior plasma illumination behavior. Such hybrid carbon nanostructures, with superb electron emission characteristics, can encourage the enlargement of several electron emission device technologies.

Origin of Conductive Nanocrystalline Diamond Nanoneedles for Optoelectronic Applications

Microstructural evolution of nanocrystalline diamond (NCD) nanoneedles owing to the addition of methane and nitrogen in the reactant gases is systematically addressed. It has been determined that varying the concentration of CH₄ in the CH₄/H₂/N₂ plasma is significant to tailor the morphology and microstructure of NCD films. While NCD films grown with 1% CH₄ in a CH₄/H₂/N₂ (3%) plasma contain large diamond grains, the microstructure changed considerably for NCD films grown using 5% (or 10%) CH₄, ensuing in nanosized diamond grains. For 15% CH₄-grown NCD films, a well-defined nanoneedle structure evolves. These NCD nanoneedle films contain sp³ phase diamond, sheathed with sp²-bonded graphitic phases, achieving a low resistivity of 90 Ω cm and enhanced field electron emission (FEE) properties, namely, a low turn-on field of 4.3 V/μm with a high FEE current density of 3.3 mA/cm² (at an applied field of 8.6 V/μm) and a significant field enhancement factor of 3865. Furthermore, a microplasma device utilizing NCD nanoneedle films as cathodes can trigger a gas breakdown at a low threshold field of 3600 V/cm attaining a high plasma illumination current density of 1.14 mA/cm² at an applied voltage of 500 V, and a high plasma lifetime stability of 881 min is evidenced. The optical emission spectroscopy studies suggest that the C₂, CN, and CH species in the growing plasma are the major causes for the observed microstructural evolution in the NCD films. However, the increase in substrate temperature to ∼780 °C due to the incorporation of 15% CH₄ in the CH₄/H₂/N₂ plasma is the key driver resulting in the origin of nanoneedles in NCD films. The outstanding optoelectronic characteristics of these nanoneedle films make them suitable as cathodes in high-brightness display panels.

Nanoscale investigation of improved triboelectric properties of UV-irradiated ultrananocrystalline diamond films

We report improved the triboelectric properties of ultraviolet (UV)-irradiated ultrananocrystalline diamond (UNCD) films that were measured using atomic force microscopy (AFM). Fabricated using the chemical vapor deposition (CVD) method, UNCD is an artificial diamond film with mechanical properties similar to single-crystal diamond. Surface modification by means of UV irradiation is a simple method to modify the surface properties of carbon-based and oxide materials. While the physical properties (e.g., roughness, adhesion, and friction) of these UNCD films did not exhibit any significant change following the UV treatment, we found that the UV-irradiated UNCD surface was oxidized and became graphitic, as confirmed using X-ray photoelectron spectroscopy, work function measurements using Kelvin probe force microscopy, and ultraviolet photoelectron spectroscopy. The work function of the samples increased with increasing UV exposure time, which is associated with the reduction of carbon atoms on the surface and oxygen-rich surfaces. Tribocharges were generated by scratching the surface of the UNCD films with a diamond-coated AFM tip. The duration of the tribocharges increased because of reactive radicals and the insulating property resulting from the UV/ozone treatment. The radicals were responsible for trapping charges; the UV-irradiated UNCD films preserved the charges for more than 5 h, which is five times longer than that on bare UNCD. This study demonstrated that UNCD is a promising material for generating triboelectricity and that UNCD can be used as a charge-trapping layer in charge-trap flash memory devices.

Tailoring Electro/Optical Properties of Transparent Boron-Doped Carbon Nanowalls Grown on Quartz

Carbon nanowalls (CNWs) have attracted much attention for numerous applications in electrical devices because of their peculiar structural characteristics. However, it is possible to set synthesis parameters to vary the electrical and optical properties of such CNWs. In this paper, we demonstrate the direct growth of highly transparent boron-doped nanowalls (B-CNWs) on optical grade fused quartz. The effect of growth temperature and boron doping on the behavior of boron-doped carbon nanowalls grown on quartz was studied in particular. Temperature and boron inclusion doping level allow for direct tuning of CNW morphology. It is possible to operate with both parameters to obtain a transparent and conductive film; however, boron doping is a preferred factor to maintain the transparency in the visible region, while a higher growth temperature is more effective to improve conductance. Light transmittance and electrical conductivity are mainly influenced by growth temperature and then by boron doping. Tailoring B-CNWs has important implications for potential applications of such electrically conductive transparent electrodes designed for energy conversion and storage devices.

Oriented Carbon Nanostructures by Plasma Processing: Recent Advances and Future Challenges

Carbon, one of the most abundant materials, is very attractive for many applications because it exists in a variety of forms based on dimensions, such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and-three dimensional (3D). Carbon nanowall (CNW) is a vertically-oriented 2D form of a graphene-like structure with open boundaries, sharp edges, nonstacking morphology, large interlayer spacing, and a huge surface area. Plasma-enhanced chemical vapor deposition (PECVD) is widely used for the large-scale synthesis and functionalization of carbon nanowalls (CNWs) with different types of plasma activation. Plasma-enhanced techniques open up possibilities to improve the structure and morphology of CNWs by controlling the plasma discharge parameters. Plasma-assisted surface treatment on CNWs improves their stability against structural degradation and surface chemistry with enhanced electrical and chemical properties. These advantages broaden the applications of CNWs in electrochemical energy storage devices, catalysis, and electronic devices and sensing devices to extremely thin black body coatings. However, the controlled growth of CNWs for specific applications remains a challenge. In these aspects, this review discusses the growth of CNWs using different plasma activation, the influence of various plasma-discharge parameters, and plasma-assisted surface treatment techniques for tailoring the properties of CNWs. The challenges and possibilities of CNW-related research are also discussed.

Low Temperature Synthesis of Lithium-Doped Nanocrystalline Diamond Films with Enhanced Field Electron Emission Properties

Low temperature (350 °C) grown conductive nanocrystalline diamond (NCD) films were realized by lithium diffusion from Cr-coated lithium niobate substrates (Cr/LNO). The NCD/Cr/LNO films showed a low resistivity of 0.01 Ω·cm and excellent field electron emission characteristics, viz. a low turn-on field of 2.3 V/µm, a high-current density of 11.0 mA/cm² (at 4.9 V/m), a large field enhancement factor of 1670, and a life-time stability of 445 min (at 3.0 mA/cm²). The low temperature deposition process combined with the excellent electrical characteristics offers a new prospective for applications based on temperature sensitive materials.

Improving Field Electron Emission Properties of ZnO Nanosheets with Ag Nanoparticles Adsorbed by Photochemical Method

Zinc oxide is a low cost and practical II-VI chemical material, which is utilized to absorb silver (Ag) nanoparticles (NPs) on zinc oxide nanosheets (ZnO NSs). Using the Ag NP-decorated ZnO NSs can improve the electrical characteristics of zinc oxide. Field electron emission characteristics of ZnO NSs and Ag-ZnO NSs indicate the turn-on fields were 5.3 and 3.2 V/μm in the dark, whereas the turn-on field were 4.3 and 2 V/μm under UV light, respectively. In addition, the field electron emission characteristics of ZnO NSs and Ag-ZnO NSs indicate the enhanced field enhancement factors were 3002 and 3420 in the dark and 3276 and 4815 under UV light, respectively.