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[Yimeng Wu](https://orcid.org/0009-0001-9780-8203), [Jie Tang](https://orcid.org/0000-0002-5871-5776), Shuai Tang, You-Hu Chen, Ta-Wei Chiu, [Masaki Takeguchi](https://orcid.org/0000-0002-0282-6020), [Ayako Hashimoto](https://orcid.org/0000-0002-1985-7667), Lu-Chang Qin

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[Stable Field Emissions from Zirconium Carbide Nanoneedle Electron Source](https://mdr.nims.go.jp/datasets/cb6b9ca9-88eb-405f-a6d9-eafe9a393f47)

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Stable Field Emissions from Zirconium Carbide Nanoneedle Electron SourceAcademic Editor: Paolo M. ScriminReceived: 2 December 2024Revised: 25 December 2024Accepted: 7 January 2025Published: 9 January 2025Citation: Wu, Y.; Tang, J.; Tang, S.;Chen, Y.-H.; Chiu, T.-W.; Takeguchi,M.; Hashimoto, A.; Qin, L.-C. StableField Emissions from ZirconiumCarbide Nanoneedle Electron Source.Nanomaterials 2025, 15, 93. https://doi.org/10.3390/nano15020093Copyright: © 2025 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/).ArticleStable Field Emissions from Zirconium Carbide NanoneedleElectron SourceYimeng Wu 1,2 , Jie Tang 1,2,*, Shuai Tang 3 , You-Hu Chen 1, Ta-Wei Chiu 1,2, Masaki Takeguchi 1 ,Ayako Hashimoto 1,2 and 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.); hashimoto.ayako@nims.go.jp (A.H.)2 Graduate School of Science and Technology, 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: In this study, a single zirconium carbide (ZrC) nanoneedle structure oriented inthe <100> direction was fabricated by a dual-beam focused ion beam (FIB-SEM) system, andits field emission characteristics and emission current stability were evaluated. Benefitingfrom controlled fabrication with real-time observation, the ZrC nanoneedle has a smoothsurface and a tip with a radius of curvature smaller than 20 nm and a length greater than2 µm. Due to its low work function and well-controlled morphology, the ZrC nanoneedleemitter, positioned in a high-vacuum chamber, was able to generate a single and collimatedelectron beam with a current of 1.2 nA at a turn-on voltage of 210 V, and the currentincreased to 100 nA when the applied voltage reached 325 V. After the treatment of thenanoneedle tip, the field emission exhibited a stable emission for 150 min with a fluctuationof 1.4% and an emission current density as high as 1.4 × 1010 A m−2. This work presentsan efficient and controllable method for fabricating nanostructures, and this method isapplicable to the transition metal compound ZrC as a field emission emitter, demonstratingits potential as an electron source for electron-beam devices.Keywords: zirconium carbide; nanoneedle; electron source; stable field emission1. IntroductionCold field emissions, also known as room-temperature field emissions, refer to theprocess of electron emissions from a cold cathode under the influence of a strong electricfield. This quantum phenomenon has been the focus of extensive scientific investigationsover the past ninety years [1–4]. As early as the late 19th century, it was first noted thatstrong electric fields could induce electron emissions from surfaces and that the currentbetween two metals in a vacuum exceeds theoretical predictions [5]. Fowler and Nordheimlater developed the quantum mechanical theory to describe the field emission process inbulk metals, and a set of equations, commonly referred to as the Fowler–Nordheim (F-N)equations, have been frequently used as a good approximation to describe field emissionsfrom crystalline materials like metals and semiconductors [6]. As the research progressed,field emission electron sources, initially employed in rudimentary applications such asradar and microwave amplifiers [7–9], have evolved to meet the demands of applicationsNanomaterials 2025, 15, 93 https://doi.org/10.3390/nano15020093https://doi.org/10.3390/nano15020093https://doi.org/10.3390/nano15020093https://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://orcid.org/0000-0002-1985-7667https://doi.org/10.3390/nano15020093https://www.mdpi.com/article/10.3390/nano15020093?type=check_update&version=2Nanomaterials 2025, 15, 93 2 of 10such as electron microscopy, electron beam lithography, X-ray tubes, and displays [10–13].These developments have been applied to meet the growing demands of miniaturized andhighly efficient electron sources in modern technologies.In contrast to thermionic emissions, where filaments are heated to high temperatures(1000–3000 ◦C) to supply electrons with sufficient energy to overcome the material’s workfunction [14], cold field emissions apply a high electric field to the surface of the mate-rial, effectively narrowing the potential barrier to a few nanometers. This allows freeelectrons at the Fermi level (following the Dirac–Einstein distribution) to tunnel throughthe energy barrier via quantum tunneling and be emitted into the vacuum state at roomtemperature [5,15–18]. This mechanism provides field emissions with several advantages:a high response speed, narrow energy spread, and low power consumption. To achievethe extremely high local electric field required for inducing field emission, often reaching afew V/nm, reducing the diameter of the emitter tip can significantly increase the electronflux density near the tip [19], effectively reducing the macroscopic extraction voltage by anorder of magnitude from several kilovolts [20,21]. On the other hand, cathode materialswith low work functions exhibit lower energy spread, resulting in electrons that are moreconducive to tunneling under smaller electrostatic forces [22,23]. The ongoing developmentof electron emitters with a low work function and structures with a high aspect ratio hasbeen a key focus in the development of field emission electron sources.In recent decades, significant efforts have been directed toward the systematic in-vestigation of materials with a low work function and high melting point using the one-dimensional nanostructure for application as field emission electron sources. Due to theirspecific geometry and controllable composition and structure, one-dimensional nanos-tructures have been considered promising for field emission electron sources [24–27].One-dimensional boride, nitride, and carbide nanostructures have been extensively studiedas stable cold field emission electron sources [28–32]. Zhao et al. reviewed the synthesismethods of Si3N4 nanowires and their applications and prospects in optoelectronics [32].Tang et al. successfully produced HfC nanowire field emitters with long-term stable emis-sions, and their further research and development led to the realization of long-term stableemissions from CeB6 and LaB6 nanoneedle field emitters [29–31]. Zirconium carbide (ZrC)is a well-known refractory ceramic material with a NaCl-type crystal structure and hasbeen highly valued for its potential in field emission applications due to its low work func-tion (3.6 eV), high melting temperature (3532 ◦C), and excellent chemical stability [33–36].Makie et al. demonstrated the field emission capability of ZrC single crystals, which wereobtained via arc floating zone refinement from sintered stock, showing that ZrC emitterscould be operated under pressures far exceeding those typically used for field emissioncathodes as early as the late 1980s [37]. Recently, our team has successfully utilized a singleZrC nanowire as a field emission electron source, which was synthesized via chemicalvapor deposition (CVD) and assembled into an electron emitter using a nano-manipulationsystem under an optical microscope [38], demonstrating the great potential of the ZrCnanowire as a field emission filament. However, nanowire structures must face significantchallenges in industrial applications. The complexities of the synthesis and assembly pro-cesses considerably impact the manufacturing efficiency of nanowire emitters. Additionally,the points of contact with the platform could result in inadequate structural robustness.The vibrations of the emitter tip during emission also hinder their ability to provide a stableemission current under practical operating conditions.In this study, we report the successful fabrication and development of a single ZrCnanoneedle as a field emission point electron source and detailed characterizations of itsemission properties and emission current stability. The ZrC nanoneedle emitter showedan extraction voltage of 210 V and achieved an emission current density as high asNanomaterials 2025, 15, 93 3 of 101.4 × 1010 A m−2, which can be attributed to its optimized morphology and surface struc-ture with a low work function.2. Experimental SectionFollowing the successful stabilization of the field emission current from ZrC nanowireemitters, we developed a two-step fabrication process employing ion milling to produceZrC emitters with improved structural robustness [30,31]. Figure 1a presents a schematicof our controlled fabrication method for producing ZrC nanoneedle field emission electronsources utilizing a dual-beam focused ion beam (FIB-SEM, Helios 650, FEI, Hillsboro, OR,USA) system.Nanomaterials 2025, 15, x FOR PEER REVIEW 3 of 10   an extraction voltage of 210 V and achieved an emission current density as high as 1.4 × 1010 A m−2, which can be attributed to its optimized morphology and surface structure with a low work function. 2. Experimental Section Following the successful stabilization of the field emission current from ZrC nan-owire emitters, we developed a two-step fabrication process employing ion milling to pro-duce ZrC emitters with improved structural robustness [30,31]. Figure 1a presents a sche-matic of our controlled fabrication method for producing ZrC nanoneedle field emission electron sources utilizing a dual-beam focused ion beam (FIB-SEM, Helios 650, FEI, Hills-boro, OR, USA) system.  Figure 1. Schematic of (a) the fabrication process of ZrC nanoneedles using the FIB-SEM system. (b) The experimental setup for the field emission test. In the first step, a (100)-oriented ZrC bulk crystal was sectioned into a lamella with a dimension of 3 × 10 µm and transferred using an Omni probe. It should be noted that the rocking curve measurement of the ZrC crystal was performed on the ZrC (200) peak (2θ = 38.44°). In this context, omega (ω) refers to the rotation angle of the sample relative to the incident X-ray beam, and the theoretical omega was calculated as half of 2θ with θ being the Bragg angle and satisfying Bragg’s law. The experimental omega, obtained from the rocking curve scan, indicated an “off-angle along the incident beam direction”, which was estimated to be less than ±0.06°, suggesting the near-perfect alignment of the sample along the vertical axis. The lamella was subsequently mounted onto a tungsten (W) needle tip, which has a pre-prepared planar platform created by FIB milling. Platinum (Pt) was deposited at the contact point between the ZrC lamella and the W needle using electron beam-induced deposition, ensuring the stability and reliability of the ZrC emitter. Finally, Ga-ion milling was employed to sharpen the ZrC lamella into a nanoneedle as an emitter. The crystallographic orientation of the ZrC crystal was measured by X-ray diffraction (XRD; SmartLab, Rigaku, Tokyo, Japan). Scanning electron microscopy (SEM; JSM-6500F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; JEM-ARM200F, JEOL, Tokyo, Japan) were used to characterize the microstructure of the ZrC nanoneedle. This fabrication method for electron sources offers the advantages of high efficiency and con-trollability, significantly reducing the preparation time to 50 min for a single filament. Additionally, progress can be monitored in real-time and regulated by adjusting the mill-ing current. The field emission characteristics of the ZrC nanoneedle were evaluated in a high-vacuum chamber (1 × 10−7 Pa), as shown in Figure 1b, designed for both field emissions and thermal flashing procedures. Prior to measurement, thermal flashing pretreatment was applied to remove the adsorbates and contaminants on the ZrC nanoneedle surface. A negative field was applied to the ZrC nanoneedle emitter to extract electron emissions Figure 1. Schematic of (a) the fabrication process of ZrC nanoneedles using the FIB-SEM system.(b) The experimental setup for the field emission test.In the first step, a (100)-oriented ZrC bulk crystal was sectioned into a lamella witha dimension of 3 × 10 µm and transferred using an Omni probe. It should be noted thatthe rocking curve measurement of the ZrC crystal was performed on the ZrC (200) peak(2θ = 38.44◦). In this context, omega (ω) refers to the rotation angle of the sample relativeto the incident X-ray beam, and the theoretical omega was calculated as half of 2θ with θbeing the Bragg angle and satisfying Bragg’s law. The experimental omega, obtained fromthe rocking curve scan, indicated an “off-angle along the incident beam direction”, whichwas estimated to be less than ±0.06◦, suggesting the near-perfect alignment of the samplealong the vertical axis. The lamella was subsequently mounted onto a tungsten (W) needletip, which has a pre-prepared planar platform created by FIB milling. Platinum (Pt) wasdeposited at the contact point between the ZrC lamella and the W needle using electronbeam-induced deposition, ensuring the stability and reliability of the ZrC emitter. Finally,Ga-ion milling was employed to sharpen the ZrC lamella into a nanoneedle as an emitter.The crystallographic orientation of the ZrC crystal was measured by X-ray diffraction (XRD;SmartLab, Rigaku, Tokyo, Japan). Scanning electron microscopy (SEM; JSM-6500F, JEOL,Tokyo, Japan) and transmission electron microscopy (TEM; JEM-ARM200F, JEOL, Tokyo,Japan) were used to characterize the microstructure of the ZrC nanoneedle. This fabricationmethod for electron sources offers the advantages of high efficiency and controllability,significantly reducing the preparation time to 50 min for a single filament. Additionally,progress can be monitored in real-time and regulated by adjusting the milling current.The field emission characteristics of the ZrC nanoneedle were evaluated in a high-vacuum chamber (1 × 10−7 Pa), as shown in Figure 1b, designed for both field emissionsand thermal flashing procedures. Prior to measurement, thermal flashing pretreatmentwas applied to remove the adsorbates and contaminants on the ZrC nanoneedle surface. Anegative field was applied to the ZrC nanoneedle emitter to extract electron emissions andNanomaterials 2025, 15, 93 4 of 10record different current values at the picometer. A grounded microchannel plate (MCP)was placed 5 cm from the emitter to record the emission current and observe the fieldemission pattern.3. Results and DiscussionA field emission electron source composed of a sharpened tip supported with a hairpinfilament is illustrated in Figure 2a. Tungsten is typically used as the cathode material ofelectron sources, and the cathode filament begins emitting electrons when an electric fieldis created. The anode, maintained at a positive potential relative to the filament, generatesa strong electric field that attracts and accelerates the electrons toward it. Some electronspass through the anode and continue traveling to the column toward the detector. In thisstudy, we replaced the W tip with ZrC to act as the electron emitter. Figure 2b,c presentthe SEM images of the emitter tip during and after the fabrication process, respectively.The images reveal that Ga-ion milling effectively sharpened the emitter tip. The conicalshape of the electron emitter tip, in combination with the hairpin structure, ensured thestructural robustness of the single nanoneedle without severe vibration affecting the fieldemissions. In both images, three regions of distinct contrast correspond to the ZrC tip, Ptdeposition, and W needle. These regions are highlighted and distinguished using differentcolors for clarity. The fabricated nanoneedle possesses a total length exceeding 10 µmwith a tip of a radius of curvature less than 20 nm. Following real-time observationsusing a controlled milling current, the high-magnification SEM image of the nanoneedletip exhibited a sharpened tip apex with a radius of curvature of approximately 20 nmand a smooth surface, as shown in the inset of Figure 2c. The morphology and curvaturediscussed above were also confirmed in the TEM image of the emitter’s tip, as shown inFigure 2d. Additionally, due to the extended exposure of the nanoneedle sample to theair during transfer to the TEM grid and the impact of Ga-ion milling on the surface, anoxidized amorphous layer with a thickness of approximately 5 nm was observed on the tipsurface. The formation of the oxidized amorphous layer was initially due to the amorphouslayer generated by Ga ion bombardment during the fabrication process, followed by theoxidation layer formed during surface exposure to oxygen in the transfer process. Typically,this oxidized amorphous layer can be removed via thermal flashing and high-current fieldemissions during surface pretreatment. The inset in Figure 2d shows an electron diffractionpattern from the tip region, corresponding to the <100> zone axis of the ZrC crystal. Theuppermost part of the ZrC nanoneedle retained the single crystallinity of the <100> orientedsingle crystal. In addition, the high-resolution TEM image near its surface region (Figure 2e)shows a crystal lattice spacing of 0.23 nm in two directions, corresponding to the {200}lattice spacing of the ZrC crystal.The prepared ZrC nanoneedle emitter was placed in a high-vacuum chamber(1 × 10−7 Pa), where a gradually increasing electric field was applied to collect the emissioncurrent from the emitter tip for the evaluation of its field emission properties. Figure 3ashows the relationship between the field emission current (I) and extraction voltage (V).The ZrC nanoneedle emitter exhibited a field emission current of 1.2 nA at a turn-on voltageof 210 V, and the current increased to 100 nA as the applied voltage reached 325 V. The fieldemission characteristics were analyzed by the Fowler–Nordheim (F-N) equation [5].I =Ac1F2ϕexp(−c2ϕ32F)(1)Here, I is the field emission current, A is the emission area, ϕ is the work function of theemitter surface, c1 = 1.54 × 106 AeV V−2 and c2 = 6.83 × 109 eV−3/2 V m−1 are constants,and F is the electric field applied on emitter tip, which can be expressed as follows:Nanomaterials 2025, 15, 93 5 of 10F = βV (2)Here, β is the field enhancement factor, which is a proportionality factor between theextraction voltage V and the electric field F, and is determined by the local geometry ofthe electron emitter. The linearized relationship (F-N plot) between ln(I/V2) and 1/V iscalculated as follows:ln(IV2)=SV+ b (3)This produced the following slope:S = −6.83 × 109ϕ3/2/β (4)The F-N plot shown in Figure 3b demonstrates excellent linearity with an R2 coefficientof 0.995, indicating that the field emission behavior of the ZrC nanoneedle emitter closelyfollows the traditional cold field emission model described by the F-N equation. Further-more, by substituting the slope S = −1979 V and the work function of ZrC(100) ϕ = 3.6 eVinto Equation (4), we calculated the local field enhancement factor β = 2.18 × 107 m−1. Thisallowed us to evaluate the emitter tip’s electric field at each extraction voltage duringthe field emission. The ZrC nanoneedle emitter, produced by using our efficient and con-trollable fabrication process, exhibited a field enhancement factor β that was an order ofmagnitude higher than the W(310) filament with β = 1 × 106 m−1.Nanomaterials 2025, 15, x FOR PEER REVIEW 5 of 10    Figure 2. (a) Schematic of the ZrC nanoneedle field emission electron source with hairpin structure. (b) SEM image of ZrC nanoneedle during the process of Ga-ion milling. (c) SEM image of ZrC nanoneedle after fabrication was completed. (d) TEM image and electron diffraction pattern (inset) of the sharpened ZrC nanoneedle tip. (e) High-resolution TEM image near the surface region.  Figure 3. Field emission characteristics of the ZrC nanoneedle emitter. (a) I-V curve of field emis-sions and (b) its corresponding F-N plot. (c) FEM pattern of the ZrC nanoneedle with a single emis-sion spot in the axial direction. (d) Field emission intensity following a Gaussian distribution with FWHM of 7.1 mm. Figure 2. (a) Schematic of the ZrC nanoneedle field emission electron source with hairpin structure.(b) SEM image of ZrC nanoneedle during the process of Ga-ion milling. (c) SEM image of ZrCnanoneedle after fabrication was completed. (d) TEM image and electron diffraction pattern (inset) ofthe sharpened ZrC nanoneedle tip. (e) High-resolution TEM image near the surface region.The emitter gave off a single electron beam during testing, which can be observed inthe field emission microscope (FEM) pattern shown in Figure 3c. The center of the imageclearly shows a single emission point on the microchannel plate (MCP). Located at the upperright of this emission point is the MCP’s central aperture, which is positioned to collectprobe current and exhibit fluorescence due to its positive potential. The intensity of the fieldNanomaterials 2025, 15, 93 6 of 10emission beam in the FEM pattern follows a Gaussian distribution (Figure 3d), exhibitinga diameter of 7.1 mm at its full width at half maximum (FWHM) and a correspondingsemi-angle of divergence of 71 mrad. The Gaussian distribution curve (red) is fitted to theintensity profile of the FEM pattern (black) for evaluations. Based on the parameters ofthe FEM, we determined that the electrons were emitted from a region of approximately3.7 nm2 at the ZrC nanoneedle tip. This accounts for the fact that when the field emissioncurrent reached 50 nA at 300 V, a local electric field of 6.5 V nm−1 at the nanoneedle tipwas created. The reduced brightness of the ZrC emitter was calculated using the followingformula [39,40]:Br =1.44Jπd(5)where J is the areal density of the emission current and the variable is given as follows:d = 9.76 × 10−11 Fϕ1/2t(y)(6)This equation represents the transverse energy, while the function t(y), relatedto the image potential in F-N theory, can be approximated as t(y) = 1 + 0.1107y1.33.y = 3.79 × 10−5F0.5ϕ−1. By substituting a work function of 3.6 eV, a local electric field of6.5 V nm−1, and an emission area of 3.7 nm² into the above equations, we obtained thetransverse energy d = 0.39 eV. When the field emission current reached 50 nA at 300 V, thecurrent density was J = 1.4 × 1010 A m2, and the reduced brightness of this nanoneedleemission current was 1.4 × 1010 A m2 sr−1 V−1. The significantly higher brightness andcurrent density of the ZrC nanoneedle emitter were actually obtained in a relatively lowervacuum (10−7 Pa) compared to the commercial W(310) filament. Such performance high-lights the ZrC nanoneedle’s potential as an effective field emission source, particularly inenvironments where W filaments struggle to maintain stable emissions.Benefiting from its optimized morphology and low work function of the tip material,the ZrC nanoneedle emitter showed an extraction voltage of 210 V, a field enhancementfactor β of 2.18 × 107, and an emission current density as high as 1.4 × 1010 A m−2.Compared to recently developed InSb nanowire arrays and WS2 nanotube arrays [41,42],both arrays had turn-on voltages in the range of tens of volts due to the short distance fromthe anode (500 nm). The ZrC nanoneedle emitter not only benefits from the exceptionallyhigh field enhancement inherent to single-point field emissions but also achieves a currentdensity that is more than an order of magnitude higher, ensuring a high brightness for coldfield emission sources. When compared with conventional W and LaB6 single emitters,the ZrC emitter significantly outperforms the W needle, which has a turn-on voltageexceeding 2000 V and a field enhancement factor β on the order of 1 × 106. Although LaB6nanoneedles exhibit comparable current density and a lower local electric field, their lowerwork function enables field emissions at an even lower voltage of 165 V. The stability underhigh currents has always been a challenge that LaB6 needs to address [30,43].After confirming the practicality of the ZrC nanoneedle emitter’s field emission char-acteristics, the stability of the field emission current becomes another crucial parameter toconsider for practical use in microelectronic devices. The most commonly used commercialfield emitter, the W(310) filament, requires operation in an extremely high vacuum (EHV,10−9 Pa) due to its instabilities and significant decay in field emission currents. This strin-gent requirement of the vacuum level substantially increases both the cost and efforts ofmaintenance of the devices [44,45].Figure 4a–c display three comparisons of before (red line) and after (black line) stabi-lizing the ZrC nanoneedle emitter under field emission currents of 3 nA, 10 nA, and 50 nAat a vacuum of 1 × 10−7 Pa, respectively. During this process, thermal flashing and highNanomaterials 2025, 15, 93 7 of 10current treatment were applied to clean the adsorbates and contaminants on the emitter’stip surface, effectively stabilizing its surface structure. As a result, the initially stepped andfluctuating field emission current (red line) gradually diminished, eventually showing asmooth and steady current (black line). The current stabilities, calculated by the formula∑[(Ii − I)2]/[(n − 1)I], where Ii (i = 1,2, . . .,n) represents the recorded emission currentsand the expression denotes the variance of emission current divided by the average valueof the current (I), were 0.30%, 0.31%, and 0.60%, respectively. It should be noted that thestepped instability in the current was only observed during the low current (Figure 4a,b)stabilization process, while the 50 nA stabilization process only exhibited current fluctua-tions. This can be attributed to the prolonged field emission and multiple thermal flashingtreatments in the low current stages that had already removed most of the gas moleculesexistent at the tip. The duration of the stepped instability in the current also significantlydecreased with the increase in the field emission current. The current value of up to 50 nA,combined with the application of high current treatments (sustained field emissions at over100 nA for approximately 10 s) at the onset of this stage, readily facilitated energy transferto the adsorbates and contaminants on the emitter surface under one single emission beamand a smaller emission area, assisting in their desorption from the surface and allowingfor the gradual stabilization of the surface structure over the remaining time. Upon thecompletion of all stabilization processes, we observed stability at 50 nA with a fluctuationof 1.41%, and this was maintained for 2.5 h, as presented in Figure 4d.Nanomaterials 2025, 15, x FOR PEER REVIEW 5 of 10    Figure 2. (a) Schematic of the ZrC nanoneedle field emission electron source with hairpin structure. (b) SEM image of ZrC nanoneedle during the process of Ga-ion milling. (c) SEM image of ZrC nanoneedle after fabrication was completed. (d) TEM image and electron diffraction pattern (inset) of the sharpened ZrC nanoneedle tip. (e) High-resolution TEM image near the surface region.  Figure 3. Field emission characteristics of the ZrC nanoneedle emitter. (a) I-V curve of field emis-sions and (b) its corresponding F-N plot. (c) FEM pattern of the ZrC nanoneedle with a single emis-sion spot in the axial direction. (d) Field emission intensity following a Gaussian distribution with FWHM of 7.1 mm. Figure 3. Field emission characteristics of the ZrC nanoneedle emitter. (a) I-V curve of field emissionsand (b) its corresponding F-N plot. (c) FEM pattern of the ZrC nanoneedle with a single emissionspot in the axial direction. (d) Field emission intensity following a Gaussian distribution with FWHMof 7.1 mm.Nanomaterials 2025, 15, 93 8 of 10Nanomaterials 2025, 15, x FOR PEER REVIEW 8 of 10   nA with a fluctuation of 1.41%, and this was maintained for 2.5 h, as presented in Figure 4d.  Figure 4. The 30 min field emission stability before (red line) and after (black line) the ZrC nanon-eedle emitter stabilized under emission currents of (a) 3 nA, (b) 10 nA, and (c) 50 nA with fluctua-tions of 0.30%, 0.31%, and 0.60%, respectively. (d) Long-term stability with a fluctuation of 1.41% after 2.5 h of measurement. The field emission stability of the ZrC nanoneedle shows clear advantages over the commercial W(310) filament in terms of emission current decay and fluctuation. For the W(310) emitters, even under an extremely high vacuum (EHV, 10−9 Pa), the emission cur-rent drops by 5% in 2.5 h, with current fluctuations increasing from 1.0% to 3.2% over time. In contrast, the ZrC nanoneedle exhibited no observable current decay at a vacuum of 10−7 Pa, which maintained a steady current fluctuation of around 1.5% during its pro-longed emission. This performance is expected to improve further in higher vacuum con-ditions. 4. Conclusions A ZrC nanoneedle field emitter was fabricated using an efficient and controllable SEM-FIB process, achieving a length exceeding 2 µm and a tip with a radius of curvature less than 20 nm. Under high-vacuum conditions of 10−7 Pa, the ZrC nanoneedle emitter exhibited a turn-on field of 6.5 V nm−1 with an emission current of 50 nA and achieved an emission current density as high as 1.4 × 1010 A m−2. The FEM pattern showed a single bright and well-focused emission electron beam. Additionally, the ZrC nanoneedle exhib-ited outstanding emission stability, fluctuating by only 1.41% after 150 min of continuous emission with a current of 50 nA in a 10⁻⁷ Pa vacuum. The stable emission is dependent on the interplay of a low work function, ideal nanoneedle morphology, and appropriate surface treatment. These results satisfy the requisite application standards for practical deployment in electron-beam devices. Author Contributions: Conceptualization, Y.W., J.T., S.T., and L.-C.Q.; Data curation, Y.W. and S.T.; Formal analysis, Y.W., S.T., Y.-H.C., and M.T.; Funding acquisition, J.T., A.H.; Investigation, Y.W.; Methodology, Y.W., J.T., S.T., Y.-H.C., T.-W.C., A.H., and L.-C.Q.; Project administration, J.T.; Re-sources, J.T.; Supervision, J.T.; Validation, Y.W. and S.T.; Writing—original draft, Y.W., J.T., and L.-C.Q.; Writing—review and editing, Y.W., J.T., S.T., Y.-H.C., T.-W.C., M.T., A.H., and L.-C.Q. All authors have read and agreed to the published version of the manuscript. Figure 4. The 30 min field emission stability before (red line) and after (black line) the ZrC nanoneedleemitter stabilized under emission currents of (a) 3 nA, (b) 10 nA, and (c) 50 nA with fluctuations of0.30%, 0.31%, and 0.60%, respectively. (d) Long-term stability with a fluctuation of 1.41% after 2.5 hof measurement.The field emission stability of the ZrC nanoneedle shows clear advantages over thecommercial W(310) filament in terms of emission current decay and fluctuation. For theW(310) emitters, even under an extremely high vacuum (EHV, 10−9 Pa), the emissioncurrent drops by 5% in 2.5 h, with current fluctuations increasing from 1.0% to 3.2% overtime. In contrast, the ZrC nanoneedle exhibited no observable current decay at a vacuum of10−7 Pa, which maintained a steady current fluctuation of around 1.5% during its prolongedemission. This performance is expected to improve further in higher vacuum conditions.4. ConclusionsA ZrC nanoneedle field emitter was fabricated using an efficient and controllable SEM-FIB process, achieving a length exceeding 2 µm and a tip with a radius of curvature less than20 nm. Under high-vacuum conditions of 10−7 Pa, the ZrC nanoneedle emitter exhibited aturn-on field of 6.5 V nm−1 with an emission current of 50 nA and achieved an emissioncurrent density as high as 1.4 × 1010 A m−2. The FEM pattern showed a single brightand well-focused emission electron beam. Additionally, the ZrC nanoneedle exhibitedoutstanding emission stability, fluctuating by only 1.41% after 150 min of continuousemission with a current of 50 nA in a 10−7 Pa vacuum. The stable emission is dependenton the interplay of a low work function, ideal nanoneedle morphology, and appropriatesurface treatment. These results satisfy the requisite application standards for practicaldeployment in electron-beam devices.Author Contributions: Conceptualization, Y.W., J.T., S.T. and L.-C.Q.; Data curation, Y.W. and S.T.;Formal analysis, Y.W., S.T., Y.-H.C. and M.T.; Funding acquisition, J.T. and A.H.; Investigation,Y.W.; Methodology, Y.W., J.T., S.T., Y.-H.C., T.-W.C., A.H. and L.-C.Q.; Project administration, J.T.;Resources, J.T.; Supervision, J.T.; Validation, Y.W. and S.T.; Writing—original draft, Y.W., J.T. andL.-C.Q.; Writing—review and editing, Y.W., J.T., S.T., Y.-H.C., T.-W.C., M.T., A.H. and L.-C.Q. Allauthors have read and agreed to the published version of the manuscript.Nanomaterials 2025, 15, 93 9 of 10Funding: This work was funded by the National Institute for Materials Science (NIMS) and theResearch Center for Energy and Environmental Materials (GREEN). We also wish to thank theTransmission Electron Microscopy Unit, Surface and Bulk Analysis Unit in NIMS, and “AdvancedResearch Infrastructure for Materials and Nanotechnology in Japan (ARIM: JPMXP1224NM5231)” ofthe Ministry of Education, Culture, Sports, Science and Technology (MEXT) for their support.Data Availability Statement: The original contributions presented in the study are included in thearticle, and further inquiries can be directed to the corresponding author/s.Conflicts of Interest: The funders had no role in the design of the study; in the collection, analysis, orinterpretation of the data; in the writing of the manuscript; or in the decision to publish the results.References1. Dyke, W.P.; Dolan, W.W. Field emission. Adv. Electron. Electron Phys. 1956, 8, 89–185.2. Stratton, R. Theory of field emission from semiconductors. Phys. Rev. 1962, 125, 67–82. [CrossRef]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/10.1002/smll.201301293https://doi.org/10.1038/am.2013.25https://doi.org/10.1039/D0NR03406Ahttps://www.ncbi.nlm.nih.gov/pubmed/32608436https://doi.org/10.1039/D1NA00167Ahttps://www.ncbi.nlm.nih.gov/pubmed/36134182https://doi.org/10.1039/D1NR04907Khttps://www.ncbi.nlm.nih.gov/pubmed/34636392https://doi.org/10.1016/j.ceramint.2021.02.139https://doi.org/10.1111/j.1151-2916.1965.tb14728.xhttps://doi.org/10.1116/1.573700https://doi.org/10.1016/j.matchemphys.2009.02.012https://doi.org/10.1116/1.587380https://doi.org/10.1109/16.43775https://doi.org/10.3390/nano14191567https://www.ncbi.nlm.nih.gov/pubmed/39404294https://doi.org/10.1103/PhysRev.113.110https://doi.org/10.1116/1.2907780https://doi.org/10.1002/smll.202002880https://www.ncbi.nlm.nih.gov/pubmed/32761781https://doi.org/10.1002/aelm.202000402https://doi.org/10.1116/1.3488988https://doi.org/10.1116/6.0001182https://doi.org/10.1088/0957-4484/20/32/325707https://www.ncbi.nlm.nih.gov/pubmed/19620758 Introduction  Experimental Section  Results and Discussion  Conclusions  References