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[Yimeng Wu](https://orcid.org/0009-0001-9780-8203), Jie Tang, [Shuai Tang](https://orcid.org/0000-0003-0086-9104), You-Hu Chen, Ta-Wei Chiu, [Masaki Takeguchi](https://orcid.org/0000-0002-0282-6020), Lu-Chang Qin

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[Stable Field Emission from Single-Crystalline Zirconium Carbide Nanowires](https://mdr.nims.go.jp/datasets/1a0e7fbe-2b6a-4159-b188-aa40a5b8f9dd)

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Stable Field Emission from Single-Crystalline Zirconium Carbide NanowiresCitation: Wu, Y.; Tang, J.; Tang, S.;Chen, Y.-H.; Chiu, T.-W.; Takeguchi,M.; Qin, L.-C. Stable Field Emissionfrom Single-Crystalline ZirconiumCarbide Nanowires. Nanomaterials2024, 14, 1567. https://doi.org/10.3390/nano14191567Academic Editor: JakobBirkedal WagnerReceived: 28 August 2024Revised: 19 September 2024Accepted: 25 September 2024Published: 27 September 2024Copyright: © 2024 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).nanomaterialsArticleStable Field Emission from Single-Crystalline ZirconiumCarbide NanowiresYimeng Wu 1,2 , Jie Tang 1,2,*, Shuai Tang 3 , You-Hu Chen 1, Ta-Wei Chiu 1,2, Masaki Takeguchi 1and Lu-Chang Qin 4,*1 Research Center for Energy and Environmental Materials, National Institute for Materials Science,Tsukuba 305-0047, Ibaraki, Japan; wu.yimeng@nims.go.jp (Y.W.); chenyouhu@hotmail.com (Y.-H.C.);chiu.tawei@nims.go.jp (T.-W.C.); takeguchi.masaki@nims.go.jp (M.T.)2 Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8577, Ibaraki, Japan3 State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory ofDisplay Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University,Guangzhou 510275, China; tangsh58@mail.sysu.edu.cn4 Department of Physics and Astronomy, University of North Carolina at Chapel Hill,Chapel Hill, NC 27599-3255, USA* Correspondence: tang.jie@nims.go.jp (J.T.); lcqin@email.unc.edu (L.-C.Q.)Abstract: The <100> oriented single-crystalline Zirconium Carbide (ZrC) nanowires were controllablysynthesized on a graphite substrate by chemical vapor deposition (CVD) with optimized growth pa-rameters involving Zirconium tetrachloride (ZrCl4), flow of methane (CH4), and growth temperature.The length of nanowires is above 10 µm while the diameter is smaller than 100 nm. A single ZrCnanowire was picked up and fixed on a tungsten tip for field emission measurement. After surfacepretreatments, a sharpened and cleaned ZrC nanowire emitter showed a high emission currentdensity of 1.1 × 1010 A m−2 at a low turn-on voltage of 440 V. The field emission is stable for 150 minwith a fluctuation of 1.77%. This work provides an effective method for synthesizing and stabilizingsingle-crystalline ZrC nanowire emitters as an electron source for electron-beam applications.Keywords: zirconium carbide; nanowire; electric field emission; chemical vapor deposition1. IntroductionIn the past decades, field emission has garnered increasing attention as a method togenerate electron beams by applying a high electric field at room temperature, which ex-tracts electrons through the surface of a metal or semiconductor via quantum tunneling [1,2].Field emission electron sources have been developed and applied in microelectronic de-vices, electron microscopy, electron beam lithography, flat panel displays, and microwavedevices due to their low energy consumption, narrow energy spread of emitted electrons,and long operational lifespan [3–6]. Additionally, numerous studies have focused on en-hancing the electronic performance and controllability of the aforementioned devices atthe nanoscale [7,8]. Lopes et al. achieved tunable electronic properties for carbon-baseddiodes and rectifiers by controlling the current rectification ratio and direction in ensemblemolecular diodes (EMDs) through temperature regulation [9]. Shen et al. concentrated onmodifying organic–inorganic systems via molecular doping, demonstrating that organicmolecule doping significantly improves the performance of g-ZnO-based nano-electronicdevices and broadens the tunable range of their field emission capabilities [10]. Nanowireswith favorable geometry and controllable composition are considered promising nanos-tructures for cold field emission electron sources [11–14]. Various nanowires have beenextensively researched for their potential to serve as stable cold field emission electronsources [15,16]. For example, Laurent et al. achieved a controllable current density of1 mA cm−2 using a vertically aligned cobalt nanowire array [17]. Huang et al. demon-strated enhanced field emission characteristics through the fabrication of single In-dopedNanomaterials 2024, 14, 1567. https://doi.org/10.3390/nano14191567 https://www.mdpi.com/journal/nanomaterialshttps://doi.org/10.3390/nano14191567https://doi.org/10.3390/nano14191567https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.comhttps://orcid.org/0009-0001-9780-8203https://orcid.org/0000-0003-0086-9104https://orcid.org/0000-0002-0282-6020https://doi.org/10.3390/nano14191567https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.com/article/10.3390/nano14191567?type=check_update&version=2Nanomaterials 2024, 14, 1567 2 of 11ZnO nanowires [18]. Zhao et al. reviewed the synthesis methods of Si3N4 nanowires andtheir applications and prospects in optoelectronics [19].Our team obtained a stable and controllable nanostructure of a one-dimensional LaB6emitter with a low work function and achieved a high emission current density and lowflicker noise without decay [20–22]. We focused on further developing this nanostructureusing transition metal carbides, such as TiC, ZrC, HfC, and TaC, to achieve long-termstability at high current levels and enable its practical application in microelectronic de-vices [23–25]. Tang et al. demonstrated successfully the feasibility of using a single HfCnanowire as an electron source, owing to its low work function, high melting temperature,and outstanding chemical stability as a refractory ceramic material [24]. In addition to theaforementioned advantages, ZrC is also considered a potential material for future fieldemission applications due to its lower fabrication costs and safer production environment.Makie et al. obtained ZrC single crystals by arc floating zone refinement from sintered stock,which was then applied as a field emission emitter. Their work demonstrated the ability ofZrC emitters to operate under pressures far above those commonly found for field emissioncathodes in the 1980s [26]. Recently, Chiu et al. demonstrated the feasibility of a ZrC single-crystalline nanowire as a field emission emitter, which showed a high field enhancementfactor in their field emission measurements [23]. The nanowire tips with high crystallinity,clean surface, and low curvature can improve effectively their field emission performance.However, emission current stability and cathode reliability remained unsatisfactory whenZrC nanowires were utilized as field emitters [18]. In practical applications such as electronmicroscopy and electron beam lithography, stable field emission sources are essential forachieving precise imaging or etching [27,28]. An unstable electron beam current can leadto blurry images and poor etching precision, affecting directly the device’s efficiency andreducing its reliability and reproducibility [29,30]. Furthermore, the stability of the electronemission source is crucial for the long-term operation of the equipment. Significant currentfluctuations may shorten the lifespan of the device and increase maintenance costs [30–32].In this work, we focus on developing and utilizing optimized parameters of chemicalvapor deposition (CVD) to synthesize <100> oriented single-crystalline ZrC nanowires andstabilize them for the characterization of their field emission performance. We optimize thecrystallinity and morphology of the grown ZrC nanowires to improve their reliability as acathode by adjusting several reaction parameters, including temperature, CH4 flow rate,and amount of ZrCl4 [33]. After a ZrC nanowire was assembled as a field emitter, surfacepretreatment was applied to remove the irregular surface and oxidized layer from the ZrCnanowire tip. At the extraction voltage of 440 V, an emission current of density as highas 1.1 × 1010 A m−2 was achieved. Our single-crystalline ZrC nanowire emitter can emitstably for 2.5 h with a fluctuation of 1.77% in measurement. The results further confirm thepractical potential and feasibility of the ZrC nanowires in field emission applications.2. ExperimentalIn our synthesis experiment, ZrC structures were grown on a graphite substrate bysimultaneously introducing Zirconium tetrachloride (ZrCl4) and methane (CH4) in thepresence of nickel (Ni) nanoparticles as catalysts. The graphite substrate was designed as arectangular sheet with dimensions of 8 × 15 mm. Single-crystalline ZrC nanowires weresynthesized by using the CVD method. After the synthetic reactions were completed, ahigh density of ZrC nanowires was grown on the substrate. The chemical composition ofthe ZrC nanowires was characterized by X-ray diffraction (XRD; SmartLab, Rigaku, Tokyo,Japan) with Cu-Kα radiation. Scanning electron microscopy (SEM; JSM-6500F, JEOL, Tokyo,Japan) and transmission electron microscopy (TEM; JEM-2100F, JEOL, Tokyo, Japan) wereused to characterize the microstructure of the grown nanowires.To measure its field emission characteristics, a single ZrC nanowire with a smoothsurface and optimal aspect ratio was picked up and attached to a tungsten (W) needle tip,which featured a pre-prepared planar platform cut by focused ion beam milling (FIB; JFIB-2300, JEOL, Tokyo, Japan). Subsequently, carbon was deposited at the contact point betweenNanomaterials 2024, 14, 1567 3 of 11the ZrC nanowire and the W needle via electron beam-induced deposition, ensuringthe stability and reliability of the ZrC emitter. The field emission measurements andsurface pretreatments were applied in a high vacuum chamber with a pressure of less than1 × 10−7 Pa.3. Results and DiscussionThe CVD synthesis system of our ZrC nanowires comprises a reactant controller atthe front, a reaction zone, and a vacuum pump, as illustrated in Figure 1. In the synthesisexperiment, the reaction is conducted in a quartz tube under a vacuum of less than 10−1 Pa.As the temperature increased, ZrCl4 powders (99.95% purity) with a melting point of437 ◦C, placed in the low-temperature zone at the front end of the quartz tube, began toevaporate. The hydrogen (H2) gas flow carried the ZrCl4 vapors into the high-temperaturereaction zone located in the center of the quartz tube. Then, CH4 gas was introduced intothe reaction zone as a reactant. Subsequently, the two gases reacted for 20 min, and ZrCnanowires grew on a graphite substrate that was pre-coated with nickel (Ni) nanoparticlesas a catalyst with a diameter of a few tens of nanometers. Graphite was chosen to minimizepotential contamination at high temperatures due to the corrosive nature of HCl gas. Inthe preparation of Ni catalyst, 2 mg of Ni nanoparticles were added to 1 mL of alcohol,and 5 drops of the mixture were deposited onto the graphite substrate. Maintaining a hightemperature for 20 min was found to be sufficient for obtaining the best results. Extendingthe reaction time would increase the dimensions and length of the ZrC nanowires butwould not result in the higher density of nanowires [34].Figure 1. Schematic of the experimental setup for synthesizing ZrC nanowires.In order to synthesize ZrC nanowires with a high aspect ratio and smooth morphology,we conducted extensive preparation experiments to determine the optimal parameter range.Experimental temperatures ranged from 1220 to 1310 ◦C (in 30 ◦C intervals), ZrCl4 amountsfrom 0.6 to 1.0 g (in 0.1 g increments), and CH4 flow rates from 60 to 120 mL min−1 (in20 mL min−1 intervals). Each set of parameter combinations was repeated three times toensure reproducibility. Table 1 lists several sets of parameter combinations used in thecomparative experiments discussed below, discussing the effects of different parameters onthe ZrC nanowires.Figure 2 shows the results of the structural characterization, including X-ray diffraction(Figure 2a), scanning electron microscopy (Figure 2b,c), transmission electron microscopy(Figure 2e,f), and the statistical plots of diameter distribution (Figure 2d) of one ZrCnanowire sample synthesized at 1280 ◦C by using 0.7 g of ZrCl4 and a CH4 flow rateof 100 mL min−1. The ZrCl4 vapors were carried to the reaction zone by the hydrogen(H2) gas at a flow rate of 1 L min−1 during the entirety of 20-min reaction. The reactiontemperature of 1280 ◦C, slightly above the eutectic temperature of 1170 ◦C for the Ni-Zralloy, ensures the precipitation of ZrC crystals during the reaction.Nanomaterials 2024, 14, 1567 4 of 11Table 1. Summary of the experimental parameters used in each experiment.Case ZrCl4(g)CH4(mL min−1)Temperature(◦C) Description#10.6100 1280H2 gas with a flow rate of1 L min−1 carried ZrCl4 vaporsinto the reaction zone.0.71.0#2 0.7601280CH4 gas was introduced last intothe quartz tube, and the reactionwas conducted for 20 min.80120#3 0.7 1001220The heating rate of the reactionzone is 50 ◦C minute−1.12501310Figure 2. (a) XRD patterns display Bragg reflection originating from ZrC and graphite substrate.(b) The top-view and (c) tilt-view SEM images of ZrC nanowires show homogeneous growth. (d) Di-ameter distribution of ZrC nanowires. (e) TEM and (f) HRTEM image indicated that a single-crystalline ZrC nanowire grown in <100> direction.Figure 2a displays the X-ray diffractogram of the nanowires with graphite substrate.The XRD peaks are due to ZrC and graphite, corresponding to ICSD No. 01-089-2717 andICSD No. 00-056-0160. The high purity of ZrC nanowires ensured no evidence indicatingcontaminations or impurities in the obtained sample. Figure 2b shows the top-view SEMimages, revealing that the ZrC nanowires synthesized at 1280 ◦C exhibit a collimatedmorphology. The majority of nanowires are longer than 10 µm and have diameters of lessthan 100 nm. Figure 2d presents the diameter distribution of ZrC nanowires, based on SEMimages taken from ten different locations. Over 80% of the nanowires have diameters below100 nm, demonstrating the uniformity of the synthesis process. In the side-view imagedepicted in Figure 2c, more nanowires were observed to grow homogeneously and orientedat angles greater than 75 degrees relative to the graphite substrate at lower magnification.Further investigation of the nanowire structure and morphology using TEM is presented inFigure 2e,f. Figure 2e shows the typical morphology of a single nanowire with a diameterof 66 nm. A high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL,Tokyo, Japan) image of the same ZrC nanowire is displayed in Figure 2f. The surface of thenanowire is covered with a layer of thickness of 1–2 nm. The layer refers to graphite with alattice spacing of 0.34 nm. These graphite layers, along with the oxide layer, can affect thefield emission characteristics and will be removed during the pretreatment phase prior toNanomaterials 2024, 14, 1567 5 of 11testing. The lattice spacings measured inside from the HRTEM image are 0.271 nm and0.234 nm, corresponding to the interplanar spacing of the (111) and (200) lattice planes ofthe ZrC crystal, respectively. These results indicate that the ZrC nanowire is a single crystaloriented in the <100> direction, grown uniformly on the graphite substrate.To investigate the influence of ZrCl4 and CH4 as reactants on the growth of ZrCnanowires, a series of synthesis experiments were conducted by varying the amount ofZrCl4 and CH4 flow rate, respectively. Figure 3a shows the morphology of ZrC nanowiresgrown with 0.6 g ZrCl4 at 1280 ◦C. The ZrC nanowires synthesized on the substrate havediameters ranging from 50 to 150 nm. Additionally, most of the nanowires are alignedvertically and have smooth surfaces. Figure 3b,c show the SEM images of the ZrC nanowireswhen the amount of ZrCl4 was increased to 0.7 g and 1.0 g, respectively. The ZrC nanowiressynthesized with a slightly increased amount of ZrCl4 (0.7 g) did not show significantchanges, whereas those synthesized with a substantially increased amount of ZrCl4 (1.0 g)exhibited kinked morphologies. Figure 3d–f display the morphology of ZrC nanowiressynthesized at 1280 ◦C by using a CH4 flow rate of 60 mL min−1, 80 mL min−1, and120 mL min−1 while maintaining the amount of ZrCl4 and the H2 flow rate at 0.70 g and1 L min−1, respectively. The length and growth density of the ZrC nanowires increasedwith increased CH4 flow rate. Similar to the phenomena observed in the experiments withthe substantially increased amount of ZrCl4, higher flow rates of CH4 led to kink structuresas shown in Figure 3f. These observations suggest that a greater amount of ZrCl4 andCH4 flow rate would accelerate the reactions and facilitate the growth of ZrC nanowires,resulting in denser nanowires on the graphite substrate with a higher aspect ratio.Figure 3. SEM image of ZrC nanowires synthesized with (a) 0.6 g; (b) 0.7 g; (c) 1.0 g of ZrCl4 at1280 ◦C. SEM image of ZrC nanowires synthesized at 1280 ◦C with 0.7 g ZrCl4 and a CH4 flow rateof (d) 60 mL min−1; (e) 80 mL min−1; (f) 120 mL min−1.Additionally, to prepare suitable ZrC nanowires with smooth surfaces and a highaspect ratio for field emission applications, we also investigated the effect of temperature onthe morphology of ZrC nanowires synthesized by the CVD method. Figure 4a–c show theSEM images of ZrC nanowires synthesized at 1220 ◦C, 1250 ◦C, and 1310 ◦C, respectively,with the previously optimized ZrCl4 amount of 0.7 g and CH4 flow rate of 100 mL min−1.The ZrC nanowires synthesized at temperatures below 1280 ◦C exhibited much more kinkstructures, with a few even having multiple kinks. This phenomenon was also observedat a synthesis with the amount of ZrCl4 of 1.0 g and reduced temperatures in this set ofexperiments. Figure 4c shows that the ZrC nanowires synthesized at 1310 ◦C possessdiameters exceeding 200 nm and exhibit rough and angular surfaces.Nanomaterials 2024, 14, 1567 6 of 11Figure 4. SEM image of ZrC nanowires synthesized with 0.70 g ZrCl4 at (a) 1220 ◦C; (b) 1250 ◦C;(c) 1310 ◦C. (d) Schematic of Vapor–liquid–solid (VLS) mechanism applied for ZrC nanowire growth.Our synthesis process follows the vapor–liquid–solid (VLS) mechanism, originallyproposed by Wagner to account for whisker growth [35]. The VLS mechanism involves acatalytic liquid alloy phase capable of adsorbing rapidly vapors to supersaturation. Crystalgrowth proceeds from nucleated seeds at the liquid–solid interface. The synthesis relies onthe following chemical reaction [23]ZrCl4 + CH4 → ZrC + 4HCl (1)and two partial reactions are proposed to describe this processZrCl4 + CH4 = Zr + C + 4HCl (2)andZr + C = ZrC (3)Figure 4d illustrates the whole growth process of ZrC nanowires on substrate. Theintermediate product Zr formed alloys as droplets with Ni nanoparticles, which providedpreferential nucleation sites for the formation and growth of ZrC on the substrate. Thediameter of the ZrC nanowire is contingent upon the dimensions of the melting droplet.When the droplet reached the supersaturation, ZrC nuclei would start to form and growsequentially layer by layer. Before the catalyst droplet contacts the ZrC nanowire body, anequilibrium between the catalyst droplet and the surrounding gas phase should alreadybe established, as suggested by Schmidt et al. [36]. During the growth process, the surfacetension of the droplet and the solid nanowire, as well as the interfacial tension betweenthem, should remain in static equilibrium, ensuring effective synthesis of the nanowiresduring the CVD reactions.In our experiment, the higher amount of ZrCl4 or CH4 flow rate raised correspondinglythe partial pressure of Zr/C vapor in the reaction zone. This variation could alter thecomposition of the melting droplet, disrupting the static equilibrium and causing changesin the droplet dimension and interfacial tension. These changes, in turn, could create newwetting conditions that convert horizontal interfacial tension into vertical forces, resultingin the accumulation of ZrC on the new growth interface and causing kinked structures ofthe ZrC nanowires. On the other hand, Zr-Ni alloy droplets formed initially at 1280 ◦C.When these droplets became supersaturated, ZrC nanowires would grow via layer-by-layer stacking. Providing more reactants would accelerate the incorporation of Zr/C intothe Zr-Ni alloy droplets, thus speeding up the growth of ZrC nanowires. However, anexcessively fast growth rate can also destabilize the growth of the ZrC nanowires. Thecatalyst droplets are more likely to shift at the nanowire tip to reach a new equilibriumunder the new wetting conditions. Consequently, the nanowires would grow in a newNanomaterials 2024, 14, 1567 7 of 11direction, forming kinked structures. The oversupplied reactants would disrupt the staticequilibrium between the surface tension of the catalyst droplet and the solid nanowire,resulting in kink structures. Similar principles apply to the effect of lowering temperatures.At a lower temperature, the suppressed rate of molecular thermal motion would disruptthe interfacial equilibrium, causing the nanowire to bend and grow in new directions. Thegreater thermal motion at high temperatures would force more residual product gas toprecipitate at the solid–liquid interface, resulting in nanowires with lower aspect ratios.This epitaxial growth would also lead to the formation of jagged and rough surfaces.The assembly of a ZrC nanowire field emitter with high reliability is necessary to carryout field emission measurements. We used a nanomotor-driven probe to pick up one singleZrC nanowire and place it onto a W needle tip. All operations were monitored with anoptical microscope, ensuring the selection of a ZrC nanowire with a smooth surface andhigh aspect ratio. The selected ZrC nanowire was synthesized at 1280 ◦C with 0.7 g ZrCl4and CH4 flow rate of 100 mL min−1. Figure 5a shows a W needle was milled with FIB tofurnish a platform for holding the ZrC nanowire. Subsequently, the ZrC nanowire was fixedonto the W platform by using carbon deposition to ensure the stability of the entire emitter.The carbon deposition was induced by an electron beam in an SEM. The SEM image ofthe as-assembled ZrC field emitter is shown in Figure 5c. As illustrated schematically inFigure 5b, the measurement system comprises several components, including an extractor,field emitter, microchannel plate (MCP), and a pico-ammeter. During the operation of fieldemission, a negative voltage is applied to the emitter to induce the emission of electronsfrom the nanowire tip. The MCP is set to collect the emission current and image theemission pattern.Figure 5. (a) Schematic of as assembled ZrC nanowire emitter. (b) A simplified schematic of our fieldemission measurement system. (c) SEM image of the assembled ZrC nanowire emitter. (d) Tip of ZrCnanowire before field evaporation pretreatment. The inset is the divergent field emission pattern onthe MCP screen. (e) Tip of ZrC nanowire after field evaporation pretreatment. The inset displayedthat the field emission pattern was concentrated at a single point on the MCP screen.When the ZrC nanowire emitter with surface adsorbates and impurities is placed intothe high vacuum chamber (1 × 10−7 Pa) for the first time, the nanowire tip is usually asquare shape, as shown in Figure 5d, and its field emission pattern is divergent, as shownin the inset of Figure 5d. Pretreatments are therefore required to ensure the field emissioncurrent was from the single-crystalline ZrC nanowire tip surface instead of contaminationsand/or irregularities. A field evaporation was applied to an imaging gas with a positive andNanomaterials 2024, 14, 1567 8 of 11strong electric field, attracting the atoms to the tip. Since the electric field on the protrusionsites is strongest and the bonds between the adsorbates and the emitter are much weakerthan the bonds between the Zr-C atoms, the contamination and the corner atoms will beevaporated first. After a few hops, the polarized atoms would ionize eventually and movetoward the fluorescent screen. If the positive electric field is sufficient, atoms on the tipsurface will be ionized and extracted. Subsequently, the strong electric field will furnish aclean and sharpened nanowire tip after field evaporation, as shown in Figure 5e. A stablefield emission pattern from a rounded tip concentrated to a single point is shown in theinset of Figure 5e. Through a clean and sharpened nanowire tip, the electric field washighly concentrated at the center of the nanowire. Therefore, this field evaporation methodcan modify effectively the nanowire tip to improve its field emission characteristics.Figure 6a shows the correlation between the field-emission current (I) and extractionvoltage (V) from the recorded I–V data. The turn-on voltage is 440 V with an emissioncurrent of 1.2 nA and the emission current reached 100 nA at 535 V. The field emissioncharacteristics were evaluated by the Fowler–Nordheim (F-N) equation [37]J = 1.54× 106 F2ϕexp(−6.83× 109 ϕ32F)(4)where J is the field emission current density, ϕ is the work function of the emission surface,and F is the electric field at the emitter tip. By using J = IA with A being the field emissionarea and F = βV with β being the field enhancement factor, which is determined by thelocal geometry of the electron emitter, a linearized relationship (F-N plot) is obtainedln(IV2)= ln(1.54× 106 Aβ2ϕ)− 6.83× 109 ϕ32βV(5)andk = 6.83× 109 ϕ32β(6)is the intercept of the linear plot.Figure 6. (a) I–V curve of field emission characterization. (b) F-N plot of field emission characteriza-tion. (c) The 30 min field emission stability of ZrC nanowire emitter under emission current of 18 nA,49 nA, and 113 nA. (d) Long-term stability with a fluctuation of 1.77% in 2.5 h of measurement.Nanomaterials 2024, 14, 1567 9 of 11Figure 6b depicts the F-N plot of the ZrC nanowire emitter, exhibiting a highly linearrelationship with an R2-coefficient of 0.993, thereby reinforcing that the field emission fromthe ZrC nanowire emitter conforms closely to the conventional cold field emission modeldescribed by the F-N equation. In addition, if we substitute the work function of ZrC(100) into the slope of the F-N plot, we can calculate the field enhancement factor β ofthe ZrC nanowire emitter, which is 4.62 × 106 m−1, and a high emission current densityof 1.1 × 1010 A m−2 was obtained. This β value is significantly higher than that of thecommercial W needles, which is β = 1 × 106 m−1, and it aligns closely with the empiricalrelationship β = 1/5r, where r is equal to 43.2 nm, representing the radius of curvature ofthe ZrC nanowire tip. This value is consistent with our observations from the SEM imagesof the assembled emitter with a tip radius of approximately 40 nm. The results furtherconfirm the applicability of the empirical F-N equation at room temperature as reported inour earlier studies [20,22,23].The stability of emission current is of essential importance for applications as electronsources. The most commonly used commercial cold field electron source, the W (310)filament, operates under an extremely high vacuum (EHV, lower than 10−9 Pa) to extract ausable emission current due to its major shortcoming: unstable field emission current thatdeteriorates rapidly. Figure 6c shows the measurements of the 30 min emission stability ofour ZrC nanowire emitter at an emission current of 18 nA, 49 nA, and 113 nA, respectively,measured in a vacuum chamber operated in 1 × 10−7 Pa. The current stability, calculatedby ∑[(Ii − I)2]/[(n− 1)I]where Ii (i = 1, 2, . . ., n) are the recorded emission currents,and the formula represents the variance in emission current divided by the average valueof the current (I), was 0.88%, 1.88%, and 1.10%, respectively. On the other hand, afterthe field emission current exceeded 40 nA, which is a value of practical importance, theZrC nanowires maintained a stable emission current for over 2.5 h. Moreover, throughoutthe entire testing period, only six current values exhibited fluctuations exceeding 2%, asindicated by the light-colored region in Figure 6d.4. ConclusionsIn this work, single-crystalline ZrC nanowires with a high aspect ratio and smoothsurface were synthesized using the CVD method by adjusting temperature, amount ofZrCl4, and CH4 flow rate. Additionally, the influence of these three parameters on themorphology of the nanowires was studied. We observed that an oversupply of reactants andlowered temperatures result in kink structures. A lower CH4 flow rate reduces significantlythe growth density and length of the nanowires, while a higher reaction temperature resultsin nanowires with a smaller aspect ratio and rougher surfaces. Finally, the field emissioncharacteristics of the ZrC nanowire emitter follow the conventional cold field emissionmodel described by the Fowler–Nordheim equation. A high emission current density of1.1 × 1010 A m−2 and field enhancement factor β of 4.62 × 106 m−1 were obtained at alow extraction voltage of 440 V. The field emission current showed long-term stabilitywith a fluctuation of 1.77% in 2.5 h of measurement. The successful development ofZrC nanowire emitters could offer additional options and support for research in hybridmolecular systems utilizing methods like molecular doping. This development extendsthe potential applications of these systems, thereby enhancing the scope of the findingsand amplifying their relevance to the community of researchers and technologists. Theseresults further demonstrate the practical potential and feasibility of the ZrC nanowires inhigh-performance electron beam technology applications.Author Contributions: Conceptualization, Y.W., J.T., S.T. and L.-C.Q.; Data curation, Y.W. andS.T.; Formal analysis, Y.W., S.T., Y.-H.C. and M.T.; Funding acquisition, J.T.; Investigation, Y.W.;Methodology, Y.W., J.T., S.T., Y.-H.C., T.-W.C. and L.-C.Q.; Project administration, J.T.; Supervision,J.T.; Validation, Y.W. and S.T.; Writing—original draft, Y.W., J.T. and L.-C.Q.; Writing—review andediting, Y.W., J.T., S.T., Y.-H.C., T.-W.C., M.T. and L.-C.Q. All authors have read and agreed to thepublished version of the manuscript.Nanomaterials 2024, 14, 1567 10 of 11Funding: This work was funded by the National Institute for Materials Science (NIMS), ResearchCenter for Energy and Environmental Materials (GREEN). We also wish to thank the support foranalysis from the NIMS Transmission Electron Microscopy Unit and “Advanced Research Infras-tructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture,Sports, Science, and Technology (MEXT).Data Availability Statement: The original contributions presented in the study are included in thearticle; further inquiries can be directed to the corresponding authors.Conflicts of Interest: The funders had no role in the design of the study; in the collection, analyses,or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.References1. Mittal, G.; Lahiri, I. Recent progress in nanostructured next-generation field emission devices. J. Phys. D Appl. Phys. 2014, 47,323001. [CrossRef]2. Li, Y.; Sun, Y.; Yeow, J.T.W. Nanotube field electron emission: Principles, development, and applications. Nanotechnology 2015, 26,242001. [CrossRef] [PubMed]3. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1016/j.mtcomm.2020.101240https://doi.org/10.1007/s12274-020-2782-0https://doi.org/10.1039/D0NR04707Dhttps://www.ncbi.nlm.nih.gov/pubmed/32857075https://doi.org/10.1109/16.43775https://doi.org/10.1116/1.590052https://doi.org/10.1063/1.1289816https://doi.org/10.1088/0957-0233/11/12/703https://doi.org/10.1116/6.0001182https://doi.org/10.1088/0957-4484/20/32/325707https://doi.org/10.1021/nl072974whttps://doi.org/10.1088/0034-4885/73/11/114501https://doi.org/10.1063/1.1753975https://doi.org/10.1021/cr900141g Introduction  Experimental  Results and Discussion  Conclusions  References