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[Yimeng Wu](https://orcid.org/0009-0001-9780-8203), Jie Tang, Shuai Tang, You-Hu Chen, Ta-Wei Chiu, Ankit Singh, [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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[Geometric influence on field emission characteristics of zirconium carbide nanoneedles](https://mdr.nims.go.jp/datasets/0f30ded3-7471-4323-86f1-810c9f4cb86e)

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Geometric influence on field emission characteristics of zirconium carbide nanoneedlesJapanese Journal ofApplied Physics      LETTER • OPEN ACCESSGeometric influence on field emissioncharacteristics of zirconium carbide nanoneedlesTo cite this article: Yimeng Wu et al 2025 Jpn. J. Appl. Phys. 64 040904 View the article online for updates and enhancements.You may also likeComparison of ensemble forecasting ofsolar irradiance with different numbers ofensemble membersPerawut Chinnavornrungsee, NuwongChollacoop, Sasiwimon Songtrai et al.-Cryogenic etching of SiOxFy and SiO2 inSF6/H2 plasmaR. Dussart, T. Tillocher, L. Becerra et al.-Photomask-driven selective oxidation andoptical imaging of ultrathin gold oxide filmson antireflective substratesSatoshi Tanigaki, Daiki Murata, MasatoshiKitamura et al.-This content was downloaded from IP address 144.213.253.16 on 16/05/2025 at 05:12https://doi.org/10.35848/1347-4065/adcbf9/article/10.35848/1347-4065/adc937/article/10.35848/1347-4065/adc937/article/10.35848/1347-4065/adc937/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcbb3/article/10.35848/1347-4065/adcd1a/article/10.35848/1347-4065/adcd1a/article/10.35848/1347-4065/adcd1ahttps://pagead2.googlesyndication.com/pcs/click?xai=AKAOjstyMAIF7edtmVXSJnRaz6nsC68VUPZSkoB3uepqLBYrXpmB0IqPR0Dn8gCwBaOlvz8hPLfVJYXyVmAXS87CI2w7r08R-5rD4g2_idBwZIc4K58Aap5fjEQ3tDE_QVA3-jLKJ5FYpP6wGAr0nagn2kP7BmVLfLD0sgqqKbOS-2dPFIBSI2Xe2UCWiceEzp_rICOZxma8eBQeOTiG8FxnWgxT-ve2ormjwvFzFA1N0i41TFYk4EzWM28A4eN2z6IkUSZSkPZtaaV6u1GvjYMub8vjA5hJXodk7Zw7I1E-oPgqBalC-9MeAizR7WH5KL8b59RPupYLYG6pVwEE6bW1RZMLd5BjC91lmLIcWJ7UoajnJNVM&sig=Cg0ArKJSzN4dFKd6it22&fbs_aeid=%5Bgw_fbsaeid%5D&adurl=https://www.electrochem.org/247/registration%3Futm_source%3DIOP%26utm_medium%3Dbanner%26utm_campaign%3DIOP_247_regular_registration%26utm_id%3DIOP%2B247%2Bregular%2BregistrationGeometric influence on field emission characteristics of zirconium carbidenanoneedlesYimeng Wu1,2 , Jie Tang1,2*, Shuai Tang3, You-Hu Chen1, Ta-Wei Chiu1,2, Ankit Singh1†, Masaki Takeguchi1 ,Ayako Hashimoto1,2 , and Lu-Chang Qin4*1Research Center for Energy and Environmental Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan2Graduate School of Science and Technology, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan3State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School ofElectronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, People’s Republic of China4Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, United States of America*E-mail: tang.jie@nims.go.jp; lcqin@email.unc.edu†Current address: Department of Metallurgical and Materials Engineering at the Central University of Jharkhand, Ranchi, Cheri-Manatu, Ranchi, 835222,India.Received March 5, 2025; revised March 26, 2025; accepted April 10, 2025; published online April 28, 2025〈100〉 oriented single-crystalline ZrC nanoneedles were successfully fabricated using a dual-beam FIB-SEM system. Atomic characterization ofthe pristine ZrC crystal confirmed the consistency and uniformity of nanoneedles’ crystallographic orientation, ensuring tip stability and precision.Nanoneedles with 10–100 nm tip radii were evaluated as field emitters, yielding field enhancement factors of 1.78 × 107–4.85 × 106 m−1 and tipemission areas of 0.93–97.3 nm2. This work underscores the importance of geometric optimization in improving field emission performance anddemonstrates a scalable and efficient method for developing high-performance electron sources for advanced electron-beam applications.© 2025 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing LtdF ield emission electron sources are known for theirunique working mechanism, where electrons areemitted via quantum tunneling under high electricfields at room temperature.1,2) The advantages including highresponse speed, low energy spread, and low power con-sumption, have driven research into next-generation emittersto overcome the limitations of current commercial W(310)filaments, such as high vacuum requirements, high workfunction, and emission instability.1,3–9) Achieving the highlocal electric fields required for field emission involvesreducing emitter tip dimensions to enhance electron fluxdensity and lower extraction voltage.3,10,11) Additionally,low-work-function materials like LaB₆, and HfC havedemonstrated promising field emission performance due totheir combined low work function and excellent thermalstability.12–16) Tang et al. optimized HfC nanowire emittertips using FIB milling and extended this approach tofabricating CeB₆ and LaB₆ nanoneedle tips, achievingexcellent current stability and a well-converged single emis-sion beam.17–20) ZrC, another refractory transition metalcarbide with a low work function (3.6 eV), high meltingpoint (3532 °C), and remarkable chemical stability, has beenstudied since the 1980s for its ability to function under higherpressures than typical field emission cathodes.21–24) Recentstudies on 1D ZrC nanostructured emitters highlight theirhigh crystallinity, pristine surface, and small tip curvatureradii for stable, high-brightness electron beams.25–27) Ourteam fabricated single ZrC nanoneedles as field emissionsources using a dual-beam system,28) achieving controllablefabrication and robust tip morphology. In this study, weevaluated the consistency of ZrC nanoneedles fabricated viaFIB-SEM and reported four sets of field emission sourceswith tip curvature radii of 10, 25, 50, and 100 nm toinvestigate the relationship between curvature radius andfiled emission characteristics.A dual-beam focused ion beam (FIB-SEM, Helios 650)fabrication process using ion milling was developed toproduce ZrC nanoneedle emitters, achieving both structuralrobustness and controllable fabrication outcomes.17,28)Figure 1(a) illustrates the schematic overview of the con-trolled fabrication method using FIB-SEM to produce a ZrCfield emission nanoneedle as electron source from a singlecrystal. The 〈100〉-oriented ZrC crystal (Crystal Base Co.,Ltd.) was rough milled into a lamella and picked up by anOmni probe [Fig. 1(b), scanning ion microscopy (SIM)image]. It was then precisely mounted onto a tungsten (W)needle with a prefabricated platform using Pt deposition. Gaion milling was applied to modify the lamella, first removingthe protective carbon coating and then shaping it into a ZrCnanoneedle emitter. Scanning electron microscopy (SEM,JSM-6500F) and transmission electron microscopy (TEM,JEM-ARM200F, JEM-2100F) were used for microstructuralcharacterization. Figure 1(c) shows the ZrC nanoneedle afterFIB milling, with a length over 10 μm and a high aspect ratio.The tip region displays three contrast levels: ZrC (lightest), Ptdeposition (darkest), and the W needle (moderate), asindicated on the right. Overall, this fabrication processinvolves replacing the tip of the W needle with a ZrC singlecrystal to serve as the field emission emitter. The fieldemission characteristics were tested in a high vacuumchamber (1× 10⁻7 Pa), as shown in Fig. 1(d), designed forfield emission and thermal flashing. Before measurement,thermal flashing was performed to remove surface adsorbatesand contaminants. A negative field was applied to extractelectron emission, and the current was recorded at thepicoampere level. A grounded microchannel plate (MCP)placed 5 cm from the emitter recorded the emission currentand field emission microscopy (FEM) pattern.TEM characterization was performed on both the pristineZrC bulk crystal and the fabricated nanoneedle to confirmContent from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of thiswork must maintain attribution to the author(s) and the title of the work, journal citation and DOI.040904-1© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJapanese Journal of Applied Physics 64, 040904 (2025) LETTERhttps://doi.org/10.35848/1347-4065/adcbf9https://crossmark.crossref.org/dialog/?doi=10.35848/1347-4065/adcbf9&domain=pdf&date_stamp=2025-04-28https://orcid.org/0009-0001-9780-8203https://orcid.org/0009-0001-9780-8203https://orcid.org/0000-0002-0282-6020https://orcid.org/0000-0002-0282-6020https://orcid.org/0000-0002-1985-7667https://orcid.org/0000-0002-1985-7667mailto:tang.jie@nims.go.jpmailto:lcqin@email.unc.eduhttps://creativecommons.org/licenses/by/4.0/https://doi.org/10.35848/1347-4065/adcbf9their single crystallinity. The TEM image of the extractedZrC lamella [Fig. 2(a)] shows the ZrC crystal with a 600 nmcarbon coating applied to prevent Ga ion damage duringrough milling. Figure 2(b) presents a high-resolution TEM(HRTEM) image and fast Fourier transform patterns, re-vealing rotational lattice distortions and a partially crystallinestructure located at a depth of 300 nm due to surfaceoxidation from prolonged air exposure. Fine milling of atleast 1 μm was necessary for the ZrC nanoneedle tip to avoidsuch surface distortions. Further TEM analysis at depths of1–2 μm [Figs. 2(c) and 2(d)] confirmed the absence of latticedistortions, with lattice spacings of 0.23 and 0.16 nmcorresponding to the (200) and (220) planes of the ZrCcrystal. Figure 2(e) shows the ZrC nanoneedle after FIB-SEM fabrication, with its tip exhibiting an oxidized amor-phous layer from air exposure and Ga ion milling. The insetdiffraction pattern aligns with the [001] zone axis of the ZrCcrystal, confirming the preservation of single crystallinity atthe nanoneedle tip. The HRTEM image of the tip-formingregion [Fig. 2(f)] shows a lattice spacing of 0.23 nm,corresponding to the (200) lattice plane. They confirmedthe uniformity and single crystallinity of the ZrC bulk and thenanoneedle tip at depths of 1–2 μm. We used energy-dispersive X-ray spectroscopy (EDS) mapping to analyzethe chemical composition of the nanoneedle tip. Figure 2(g)illustrates that oxygen is primarily distributed within theamorphous surface layer of the nanoneedle. Figure 2(h)presents the EDS line profile along the axial direction,revealing a gradual decrease in Zr and C concentrations asthe diameter decreases, while the O concentration peaks at adepth of 5 nm. This indicates the presence of a 5 nm thickoxidized amorphous layer at the tip, consistent with theobservations in Fig. 2(e). This oxidized layer can beminimized and removed during surface pre-treatment in avacuum chamber. The characterization establishes the stabi-lity and consistency of the ZrC nanoneedle tip-formingregion, the material properties remain consistent regardlessof bulk depth.The emitter tip curvature radius and the internal crystallinestructure influence the field emission properties. During FIB-SEM milling fabrication, the depth of the bulk materialutilized varies with the curvature radius, with larger radiicorresponding to shallower depths and smaller radii requiringdeeper sections of the bulk. Based on the characterizationsabove, it can be concluded that the curvature radius is theprimary factor influencing the field emission characteristicsof emitters fabricated using this method.To examine the impact of tip curvature radius on fieldemission, we selected ZrC nanoneedles with 10 nm (red),25 nm (yellow), 50 nm (blue), and 100 nm (green) radii fromFig. 1. (a) Schematic illustration of the fabrication process for the ZrC nanoneedle structure. (b) SIM image of the ZrC lamella picked up by an Omni probefrom the ZrC bulk material. (c) SEM image of the finished ZrC nanoneedle after fabrication. (d) Experimental setup for field emission measurement.040904-2© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJpn. J. Appl. Phys. 64, 040904 (2025) Y. Wu et al.a controlled fabrication process under SEM observation[Fig. 3(a)]. Their field emission properties were then char-acterized. The tip curvature radii are highlighted with coloroverlays, consistently used as the legend throughout Fig. 3.All nanoneedles exhibit well-defined geometry, with in-creasing bluntness as the curvature radius increases.The ZrC nanoneedles with varying tip curvature radii weretested in a high-vacuum environment (1× 10−7 Pa), wherean extraction voltage was applied to induce field emission.The field emission currents were recorded by internalcircuitry. Figure 3(b) displays the emission current (I) versusextraction voltage (V ) curves. The data demonstrate thatlarger tip curvature radii increase the turn-on voltage from205 to 490 V, while sharper tips yield higher emissioncurrents at the same voltage, emphasizing the role of tipsharpness in field emission. The field emission characteristicsFig. 2. Structural characterization of ZrC bulk crystal and ZrC nanoneedle. (a) TEM image of the ZrC lamella picked up from the ZrC bulk crystal. (b)HRTEM image of the upper region of the ZrC lamella, corresponding to section below the surface of the ZrC bulk crystal. (c) HRTEM image of the regionlocated 1 μm beneath the surface. (d) HRTEM image of the region located 2 μm beneath the surface. (e) TEM image of the ZrC nanoneedle after completingthe FIB milling process. (f) HRTEM image of the tip-forming region of the ZrC nanoneedle. (g) Elemental mapping image corresponding to the overlappingdistribution of C (green), O (red), and Zr (blue). (h) Line profiling with the concentrations of C, O, Zr, and Ga shown along the axial direction.Fig. 3. (a) SEM images of the fabricated ZrC nanoneedles tips with curvature radii of 10 nm (red), 25 nm (yellow), 50 nm (blue), and 100 nm (green). (b)I–V curves of ZrC nanoneedles with different tip curvature radii. (c) F–N plots corresponding to the I–V curves in (b). (d) Field emission intensity obtainedfrom FEM pattern normalized by Gaussian distribution.040904-3© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJpn. J. Appl. Phys. 64, 040904 (2025) Y. Wu et al.were evaluated using the Fowler–Nordheim (F–N)equation:29)⎜ ⎟⎛⎝⎞⎠( )= ´f- ´ fIAFF1.54 10 exp6.83 10, 162 9 32where I is the field emission current, A is the emission area, fis the work function of the emitter surface. By using b=F Vwith β being the field enhancement factor which is deter-mined by the local geometry of the electron emitter, alinearized relationship (F–N plot) is obtained⎜ ⎟⎛⎝⎞⎠⎛⎝⎞⎠( )bb=- ´f+ ´fIV VAln 6.83 10 log 1.54 10 , 22932 62and( )b= ´fk 6.83 10 3932is the slope of the linear plot. The F–N plots corresponding tothe four sets of I–V curves are shown in Fig. 3(c). All fittedtrend lines demonstrated a high degree of linearity withR2-coefficients confirming that the field emission behavioraligns with the cold field emission model. By incorporating thework function of ZrC (f= 3.6 eV) and the slopes of the fourlinear functions obtained from the F–N plots, we calculated thecorresponding field enhancement factors (β) for the four ZrCnanoneedles. The results indicate that a decrease in β withincreasing tip curvature radius: 1.78× 107 m−1, 1.15×107 m−1, 8.67× 106 m−1, and 4.85× 106 m−1, respectively.The MCP at the anode recorded FEM patterns of singleelectron beams emitted from four ZrC nanoneedles undervarying voltages. The spot intensity follows a Gaussiandistribution, changing with the applied voltage. Figure 3(d)shows the normalized Gaussian distribution curves from FEMpatterns, enabling direct comparison of emission beam dis-tributions across different tip diameters. Measurement of thefull width at half maximum showed the spot diameters of5.3 mm, 8.0 mm, 11.5 mm, and 16.5mm, with correspondingsemi-angles of 53mrad, 80mrad, 115 mrad, and 165 mradwhen the curvature radius increases. FEM parameters indicateelectron emission from regions of ∼0.93 nm2, 5.78 nm2,23.3 nm2, and 97.3 nm2 at 50 nA. These emission areas alignwith previously calculated field emission characteristics, con-firming that smaller curvature radii enhance the local electricfields, reducing the emission area at the ZrC nanoneedle tip.The scattering angle depends on the emission area and tipcurvature radius, as variations in the electric field distributioncause smaller curvatures to concentrate the field and reduceemission area, while larger curvatures distribute it moreuniformly, leading to a power-law rather than linear relation-ship.A balance between the thermal effects and emission noiseis achieved in the ZrC nanoneedle emitter through their highthermal stability, mechanical hardness, and overall structuralrobustness. Generally, reducing the emission area whilemaintaining a high current leads to emission current instabil-ities and a shortened emitter lifetime owing to two primaryfactors: Joule heating more likely occurs at the emitter apex,and the emission current becomes increasingly sensitive tosurface roughness. As the emission area decreases, currentdensity rises, causing localized heating that acceleratesmaterial evaporation and thermal noise. In addition, the fieldemission current is highly dependent on the local electricfield and the work function of the tip surface. Surfaceevaporation, atomic adsorption, and contamination at thetip can all degrade the current stability and increase theemission noise. Nevertheless, the ZrC nanoneedle emitterexhibited high emission stability and a prolonged lifetime.28)Our study reveals that optimizing the tip curvature radiusbalances thermal effects and emission noise. By fabricatingZrC nanoneedles with a curvature radius as small as 10 nmusing the FIB-SEM system, we achieved a clean, low-work-function surface and a structurally stable emitter. Afterremoving the 5 nm thick oxidized layer during the pretreat-ment, the exposed ZrC single crystal enabled field emissionat over 200 V. The intrinsic physical properties of thematerial including high melting point and hardness, com-bined with the emitter’s structural robustness effectivelybalanced current instabilities and noise caused by tip mor-phology. The ZrC nanoneedle emitter exhibited a 100-foldincrease in the emission area with a tenfold increase in the tipcurvature radius while maintaining a low turn-on voltage andhigh current density. This balance of emission stability,brightness, and energy efficiency underscores ZrC’s potentialas a next-generation electron source, particularly for applica-tions such as low-voltage SEM imaging and chemicalanalysis.We fabricated 〈100〉-oriented single-crystalline ZrC nano-needles with tip curvature radii of 10, 25, 50, and 100 nmusing a dual-beam FIB-SEM system. Structural characteriza-tion confirmed the positional stability and geometric preci-sion of the tip. Field emission measurement in a vacuumchamber showed high F–N plot linearity, confirming coldfield emission. The field enhancement factors are from1.78× 107 to 4.85× 106 m−1, with emission areas from0.93 to 97.3 nm2, indicating that smaller curvature radiienhance local electric fields and emission performance. TheZrC nanoneedle emitter balances structural robustness andsurface properties, enabling high-density emission at lowturn-on voltage. This makes it promising for low-voltageSEM imaging, chemical analysis, and future electron beamapplications.Acknowledgments This work was funded by National Institute forMaterials Science (NIMS), Research Center for Energy and EnvironmentalMaterials (GREEN). We also wish to thank the support for the TransmissionElectron Microscopy Unit, Surface and Bulk Analysis Unit in NIMS, and“Advanced Research Infrastructure for Materials and Nanotechnology in Japan(ARIM: JPMXP1224NM5231)” of the Ministry of Education, Culture, Sports,Science and Technology (MEXT).ORCID iDs Yimeng Wu https://orcid.org/0009-0001-9780-8203 Masaki Takeguchi https://orcid.org/0000-0002-0282-6020 Ayako Hashimoto https://orcid.org/0000-0002-1985-76671) W. P. Dyke and W. W. Dolan, “Field emission,” Adv. Electron. ElectronPhys. 8, 89 (1956).2) R. Stratton, “Theory of field emission from semiconductors,” Phys. Rev.125, 67 (1962).040904-4© 2025 The Author(s). 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Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJpn. J. Appl. Phys. 64, 040904 (2025) Y. 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