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[Wataru Hayami](https://orcid.org/0000-0003-0497-8690), Shuai Tang, [Jie Tang](https://orcid.org/0000-0002-5871-5776), Lu-Chang Qin

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[Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters](https://mdr.nims.go.jp/datasets/4911259c-455a-4749-a6e0-821fb5cd68a1)

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Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emittersNanoscaleAdvancesPAPEROpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueEffects of low woaInternational Center for Materials NanMaterials Science, Tsukuba, Ibaraki 305-0nims.go.jpbCenter for Green Research on Energy and Enfor Materials Science, Tsukuba, Ibaraki 305cDepartment of Physics and Astronomy, TheHill, Chapel Hill, NC 27599-3255, USA† Present address: State Key LaboratoTechnologies, Guangdong Province KeyTechnology, School of Electronics andUniversity, Guangzhou 510275, China.Cite this: Nanoscale Adv., 2022, 4,4669Received 11th August 2022Accepted 26th September 2022DOI: 10.1039/d2na00536krsc.li/nanoscale-advances© 2022 The Author(s). Published byrk-function lanthanum oxides onstable electron field emissions from nanoscaleemittersWataru Hayami, *a Shuai Tang,†b Jie Tang b and Lu-Chang Qin cNanoscale electron field emitters are known to produce more stable electron emissions than conventionalemitters. This has been attributed to size effects; nanoscale emitters can operate with a small emissioncurrent and a low extraction voltage, which reduces the bombardment of residual gas ions on theemitter tip. However, our experiments discovered that nanoscale LaB6 emitters had extremely stableemissions, suggesting that chemical effects are present in addition to size effects. This suggests thatduring operations, a material other than LaB6 may be deposited on the surface of the tip to enhance thestability of emissions. Therefore, we searched for possible materials theoretically within the La–B–Oternary system and found that lanthanum oxides (LaO) and oxygen-deficient La2O3 (La2O3−x) had goodelectrical conductivity and a low work function comparable to that of LaB6. These lanthanum oxides arechemically less reactive to residual gases than LaB6. Thus, if they are present on the LaB6 surface, theycould stabilize electron emissions without diminishing the emission performance. These findings suggestthat lanthanum oxides could be used for electron field emitters.1. IntroductionElectron guns have been used for decades in various instru-ments, including cathode-ray tubes, electron microscopes, andelectron-beam lithography systems. They can be classied intothe following types of electron emissions: thermionic emissions(TE), Schottky emissions (SE), thermal eld emissions (TFE),and cold eld emissions (CFE). Electron source materials arechosen to correspond to each emission type, for instance,tungsten (W) for TE and CFE, LaB6 and CeB6 for TE, andtungsten coated with zirconium oxide (W/ZrO) for SE.1–3Currently, W is used for the emitter tip of CFE electron gunsbecause it can be processed to dimensions less than 100 nm,which is necessary to create a high electric eld (>109 V m−1)and produce a fairly constant emission current. Many studieshave been conducted to nd electron source materials withsuperior emission properties, for example, single-atom tips,4–6oarchitectonics, National Institute for044, Japan. E-mail: HAYAMI.Wataru@vironmental Materials, National Institute-0047, JapanUniversity of North Carolina at Chapelry of Optoelectronic Materials andLaboratory of Display Material andInformation Technology, Sun Yat-senthe Royal Society of Chemistrycarbon nanotubes,7–9 and etched sharp tips of borides,10–26carbides,27–39 and nitrides.40–46In particular, LaB6 has been studied repeatedly since the 1970sbecause its low work function (WF) (2.1–2.6 eV) and chemicalstability made it ideal as a TE emitter. Earlier studies of CFE froma LaB6 tip found that the emission current was not sufficientlystable and decayed aer several minutes.10 Since around the year2000, advanced technologies have been implemented to fabricatenanoscale LaB6 emitters.15–26 A LaB6 nanowire with a thickness ofless than 100 nm has been synthesized by chemical vapor depo-sition (CVD).21 When applied to an electron emitter, the LaB6nanowire produced a stable CFE.22–24 Notably, the emissioncurrent exhibited extreme stability without decay for over tens ofhours.24 Furthermore, a LaB6 nanoneedle created by focused ionbeam (FIB) milling demonstrated a higher degree of stability thannanowires and greatly exceeded the performance of the Wemitter.25 The LaB6 nanoneedle emitter was assembled in a TEMand was capable of long-term stable (<1%/100 h) atomic imaging,which has never been achieved by other CFE electron sources.26The reason for the high current stability is attributed to thefollowing:1 (i) to achieve the same probe current, the totalemission current for the nanoscale emitter is several orders ofmagnitude smaller than that of the conventional emitter, whichreduces the generation of residual gas ions and consequentlyreduces the ion bombardment on the emitter tip; (ii) the lowerextraction voltage for the nanoscale emitter is also advanta-geous for reducing residual gas ions and damage caused by theion bombardment. In addition, we proposed another possiblefactor in our previous study;25 (iii) the electric current density inNanoscale Adv., 2022, 4, 4669–4676 | 4669http://crossmark.crossref.org/dialog/?doi=10.1039/d2na00536k&domain=pdf&date_stamp=2022-10-22http://orcid.org/0000-0003-0497-8690http://orcid.org/0000-0002-5871-5776http://orcid.org/0000-0002-3424-0526http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536khttps://pubs.rsc.org/en/journals/journal/NAhttps://pubs.rsc.org/en/journals/journal/NA?issueid=NA004021Nanoscale Advances PaperOpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinethe apex region becomes so intense (�106 A cm−2) that elec-trons provide kinetic momentum to the adatoms on the surface,inducing an atomic ow toward the apex (electromigration) andeventually enhancing the desorption rate of the adatoms.The above ndings are size effects that apply to all nanoscaleemitters. In addition to size effects, it appeared in our experi-ments24,25 that chemical effects might also have been observedon the nanoscale LaB6 emitters, that is, a material other thanLaB6 was deposited on the surface, and it enhanced the currentstability. We conceived this idea aer noticing that the energy-dispersive X-ray spectroscopy (EDS) image of the nanoneedle tipappeared to show a reduction of boron atoms on the surfacelayers.25Although the EDS analysis suggested the presence of chem-ical effects, it was not decisive enough to reach this conclusion.Therefore, in this study, we theoretically investigated whetherchemical effects occur on the nanoscale LaB6 tip and attemptedto identify the materials covering the surface. The study wasconducted for the most part using rst-principles calculationsand molecular dynamics (MD) simulations. We extensivelysearched for materials in the phase diagram that satised therequired conditions for stabilizing the emission current.2. Calculation methodsThe calculations of the electronic structures and Car–ParrinelloMD simulations were conducted using the Quantum ESPRESSOcode,47,48 based on density functional theory with plane wavesand pseudopotentials. The ultraso pseudopotentials49 wereadopted from the library of Quantum ESPRESSO.50 The gener-alized gradient approximation functional of Perdew, Burke, andErnzerhof was employed.51 An energy cut-off of 80 Ry for planewaves and 560 Ry for electron density were sufficient to providethe convergence of the total energy. The DOS and WF werecalculated following the optimization of the lattice parametersand the atomic structures using Monkhorst–Pack k-pointsampling52 with an 8 � 8 � 8 mesh for the unit cell. Thecalculated lattice parameters were within an error of 1% fromthe corresponding experimental values.Slab models were used for the WF calculations and MDsimulations, which comprised several atomic layers separatedby a vacuum layer of 15 �A. The WF f was estimated using thefollowing formula:53f ¼ Vvac − Ef (1)where Vvac and Ef are the electrostatic potential of vacuum andthe Fermi energy, respectively. In the MD simulations, the timestep was taken as about 0.06 fs, and the temperature wascontrolled by rescaling the total kinetic energy of the atoms. Allcalculations were carried out using the Numerical MaterialsSimulator supercomputer at NIMS.Fig. 1 Calculated ternary phase diagram of the La–B–O system.Green and red circles denote stable and unstable (metastable)compounds, respectively. See the text for the yellow circle (La3BO6).3. Results and discussionAccording to the EDS analysis,25 the atomic elements detectedon the outermost surface of the LaB6 emitter were La, B, and O.4670 | Nanoscale Adv., 2022, 4, 4669–4676Therefore, we searched for materials consisting of these threeelements. Since hydrogen is not detected by EDS, the possibilityof the synthesis of hydroxides cannot be excluded. This is dis-cussed in Section 3.5. As there appear to be no experimentalphase diagrams available for the ternary La–B–O system, wecalculated the theoretical phase diagram using the OpenQuantum Materials Database (OQMD).54 The calculationmethod for judging the stability of compounds was based onDFT + U55 and the Quickhull algorithm.56The results are shown in Fig. 1, where stable and unstable(metastable) compounds are represented by green and redcircles, respectively. There are some other unstable compoundsomitted from the gure. La3BO6, denoted by the yellow circle,did not appear in this calculation although its existence hasbeen experimentally conrmed.57–59 Therefore, it is added toFig. 1. As the stable compounds are aligned along the lines of B–La, La–O, B–O, and B2O3–La2O3 binary systems, we searched forpossible materials covering the LaB6 tip along these binarylines.3.1. Lanthanum boridesFirst, we considered LaB4 in the B–La system as a potentialcovering material because of the following: (i) it is metallic;60 (ii)it has been observed on oxidized LaB6 surfaces;61–63 (iii) it isstructurally similar to LaB6.64 The covering materials must beconductive; otherwise, they will obstruct electron emissionsfrom the surfaces. LaB4 has a tetragonal crystal structure,including octahedral B6 clusters, like LaB6. By analogy withLaB6,65 we expected that LaB4 would have a lower WF when itssurface was terminated by a La layer, and we calculated the WFof the La-terminated LaB4 (001) surface using a slab model of (1� 1 � 6) unit cells separated by a 15�A space. The result showedthat the WF of the La-terminated surface was about 3.2 eV, andthat of the opposite surface, terminated by a B layer, was about4.5 eV. In contrast to our expectations, theWF of LaB4 wasmuchhigher than that of LaB6, meaning that LaB4 would diminish theperformance of the LaB6 emitter if present on the surface.Consequently, LaB4 could not be a candidate material.© 2022 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536kFig. 3 Snapshots of MD simulations of H2O adsorption on LaB6(001)(top left) and on LaO(001) (top right); O2 adsorption on LaB6(001)(bottom left) and on LaO(001) (bottom right). O atoms in the LaOcrystal and those from the adsorbents are denoted by red and orangespheres, respectively.Paper Nanoscale AdvancesOpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online3.2. Lanthanum monoxide LaOExcept for LaB4, no stable materials in the phase diagram(Fig. 1) are as conductive as LaB6. However, LaO (NaCl struc-ture), though judged to be unstable, has been reported to bemetallic.66–68 It was synthesized for the rst time under highpressure66 and recently as a thin lm on substrates.67,68 Thinlms of high-pressure phases sometimes appear because of thestress applied by the interface. Notably, LaO was also observedin the LaB6 oxidation experiment.69 For these reasons, weanticipated that LaO could be a covering material, and wecalculated the electronic density of states (DOS) and the WF.The DOS of LaO shown in Fig. 2 indicates that it is metallic,which is consistent with previous experimental and theoreticalresults.66–68 The states at the Fermi level originate mostly fromthe d-orbitals of the La atoms. The WF of the LaO(001) surfacewas calculated employing the slab model shown in the inset ofFig. 2. Themodel consisted of six layers of LaO separated by a 15�A space fully optimized before the WF calculation. The calcu-lated WF was approximately 2.3 eV, which is very close to that ofLaB6. Therefore, LaO does not greatly diminish electron emis-sions even if it covers the LaB6 surface. It was not previouslyknown that LaO has a WF as low as that of LaB6. LaO could beapplied to electron emitters if it generates stable electronemissions.If LaO is chemically stable and can cover the LaB6 emitter, itwould stabilize electron emissions. To investigate the chemicalstability of LaO, we conducted Car–Parrinello MD simulationsof gas molecule adsorption under the operating conditions ofthe emitter. For the (001) surfaces of LaB6 and LaO, slab modelsof (2� 2� 3) unit cells were employed (Fig. 3), on which the gasmolecules H2, O2, H2O, and CO were initially placed at a heightof 4�A from the surface. Although the operating temperature ofthe emitter tip is uncertain, it should be slightly higher thanroom temperature and was assumed to be 500 K in the simu-lations. The actual temperature would not exceed this becausethe energy dispersion of the emitted electron beam was small.25The duration of the simulations was 1.5 to 3.0 ps.In the case of H2O adsorption, the H2O molecule spontane-ously dissolved and was chemisorbed on LaB6(001) (Fig. 3, tople) with the H atom on a B atom and the OH on a La atom. InFig. 2 DOS of LaO with the Fermi level set to zero. The inset depictsa slab model for the WF calculation.© 2022 The Author(s). Published by the Royal Society of Chemistrycontrast, the H2O molecule did not immediately dissolve onLaO(001) (top right).With regard to O2 adsorption, the O2 molecule was dissolvedand chemisorbed on LaB6(001) (Fig. 3, bottom le) with Oatoms on B atoms when the WF was increased by 0.13 eV.Similarly, the O2 molecule was dissolved on LaO(001), but the Oatoms seemed to be absorbed into the bulk (bottom right). Thissuggests that LaO spontaneously transforms into the morestable La2O3 phase when plenty of O atoms are supplied. Sincethe adsorbed O atoms are far fewer than the bulk O atoms, LaOdoes not transform into La2O3 in the present case. The WF ofLaO(001) hardly increased upon O2 adsorption, probablybecause the dissolved O atoms did not remain on the surface,and the DOS of the bulk underwent little change with theadditional O atoms. As for H2 and CO adsorptions, the H2molecule was not adsorbed on LaB6(001) or LaO(001), and theCO molecule was adsorbed but not dissolved on these surfaces.The observed inactivity of H2 on the LaB6 surfaces was consis-tent with experimental results.70Although the duration time and the initial conditions werelimited, the results of the MD simulations suggest that theLaO(001) surface was chemically as stable as, or slightly morestable than, the LaB6(001) surface. The LaO(001) surface was notresistant to oxidation; however, it would appear to be resistantin practice because the WF was hardly inuenced by oxidation.Consequently, LaO is a candidate for the materials covering theLaB6 tip.Nanoscale Adv., 2022, 4, 4669–4676 | 4671http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536kFig. 4 Electronic DOS of La2O3 and La2O3−x. (a) no defects, (b)hexagonal with x ¼ 0.125, (c) monoclinic with x ¼ 0.167, (d) cubic (Ia�3)with x ¼ 0.125, and (e) cubic (Im�3m) with x ¼ 0.125. The insets depictthe surfaces on which the WF was calculated.Table 1 Calculated work functions (WFs) of La oxides, hydroxide, andboridesMaterial WF (eV) SurfaceLaO 2.3 (001)La2O3−x hexagonal 1.8 (11�20)La2O3−x monoclinic 2.0 (010)La2O3−x cubic (Ia�3) 1.9 (110)La2O3−x cubic (Im�3m) 2.1 (001)La(OH)3−x 1.9 (0001)LaB6 2.3 (001)Nanoscale Advances PaperOpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online3.3. Lanthanum oxides La2O3 and LaO2Along with LaO, a large amount of La2O3 was observed in theLaB6 oxidation experiment.69 This is to be expected becauseLa2O3 is the most stable phase in the La–O binary system(Fig. 1). La2O3 is a potential high-kmaterial that can be used forsemiconductor devices.71–74 When it is applied to gate insula-tors, the leak current becomes a problem in the presence ofoxygen vacancies, even at low temperatures. The vacancies aresupposed to work as donors and induce the electric current ofPoole–Frenkel types71,73,74 and space-charge limited types.73,74Having adequate electrical conductivity, La2O3 with Ovacancies (La2O3−x) would not obstruct emissions from the LaB6tip and would serve as LaO if theWF is sufficiently low. The DOSandWF calculations for La2O3−x require attention because of itspolymorphism. La2O3 of hexagonal (space group P�3m1),monoclinic (C2/m), and cubic (Ia�3) structures have beenobserved in the LaB6 oxidation experiment.69 With the inclusionof another reported cubic phase (LaO1.5, Im�3m),75 we investi-gated the electronic structures of La2O3−x in these four crystalstructures.In the cubic (Im�3m) phase, the oxygen sites are split andrandomized, as such a structure cannot be realized usinga single unit cell. An approximate structure model was con-structed employing a (2 � 2 � 2) cell with the oxygen positionsrandomized by MD simulations and annealing. There is anadditional hexagonal phase (P63/mmc), but it is merely a variantof the hexagonal (P�3m1) phase with split oxygen sites and,therefore, was excluded from the calculations. There are severaloxygen sites in hexagonal and monoclinic structures. To createvacancies, an O atom was removed from each structure to attainthe lowest total energy. For the hexagonal phase, an O atomcoordinated with six La atoms was removed, and for themonoclinic phase, an O atom coordinated with ve La atomswas removed.Fig. 4 shows the DOS of hexagonal La2O3 (no vacancies) andfour polymorphs of La2O3−x (with vacancies) mentioned above.The concentrations of O vacancies were adjusted to about 5% inthe calculations, depending on the number of atoms in the unitcell. In all the structures, it was observed that O vacanciesgenerated new states near the bottom of the conduction band,and the Fermi level laid at the top of the new states. The bandgaps from the new states to the conduction band were less than0.5 eV, so the materials practically became conductive by thethermal excitation of the electrons. The values of the band gapwere in good agreement with experimental values.73 Themobility of electrons is determined by the dispersion relation ofthe conduction band. The lower part of the conduction bandconsists of 5d and 6s orbitals of La atoms and the dispersionrelation is similar to that of LaB6, suggesting the same degree ofelectron mobility. As the vacancy concentrations increase, it islikely that the materials become more conductive and the DOSapproaches that of LaO (Fig. 2).It is laborious to calculate the WF for all the crystal surfaces.Empirically, in ionic crystals, neutral surfaces tend to appear onwhich the total charge of cations and anions is zero. Neutralsurfaces do not have electric dipoles perpendicular to the4672 | Nanoscale Adv., 2022, 4, 4669–4676surface, which implies that the work functions of neutralsurfaces are determined mainly by the electronic structure ofthe bulk rather than by the Miller indices. Based on this rule, wecalculated the WF on the neutral surfaces chosen, as illustratedin the insets of Fig. 4: the (11�20) surface for the hexagonalcrystal, (010) for the monoclinic, (110) for the cubic (Ia�3), and(001) for the cubic (Im�3m). The results summarized in Table 1show that the WFs of these surfaces were around 2.0 eV, close tothat of LaB6 and LaO. It should be noted that these values agreewell with the electron affinity of La2O3 (2 eV),72 which corre-sponds to the energy difference between the vacuum level andthe bottom of the conduction band. The good agreementbetween the calculated and experimental values affirms thevalidity of the choice of the neutral surfaces. La2O3−x, with sucha low WF, would not diminish the emission performance of theLaB6 tip.LaB4 3.2 (001)© 2022 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536kPaper Nanoscale AdvancesOpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineTo investigate the chemical stability of La2O3−x, as was donefor LaO (Fig. 3), MD simulations were conducted with H2O, O2,H2, and CO molecules on the (11�20) surface of hexagonalLa2O3−x (Fig. 4b inset). The conditions of the MD simulationswere the same as for LaO (500 K and 3 ps). As a result, thesemolecules did not dissolve on the surface within the simulationtime. Since the electronic states of other polymorphs of La2O3−xare very similar to the hexagonal one, the results indicate thatthe La2O3−x surfaces are more stable than the LaB6(001) andLaO(001) surfaces. Taking the low WF and high chemicalstability into consideration, La2O3−x can be a covering materialfor the LaB6 tip and may function as an electron emitter.According to the calculated phase diagram (Fig. 1), LaO2 wasclassied as being in a stable phase. However, there have beenno experimental reports on the synthesis of LaO2; there is onlya theoretical prediction of it.76 We investigated the electronicstructure of LaO2, based on the atomic structure provided byMarques et al.76,77 When the structure was optimized, itappeared to be locally stable. The calculated DOS showed thatLaO2 was an insulator with a band gap of approximately 3 eV.When an O vacancy was introduced, it remained an insulatorwithout any change in the value of the band gap. This is prob-ably because O atoms are redundant in LaO2 to complete theionic bonds with the La atoms. Since both La2O3 and LaO2 areinsulators, all compounds between them (LaOx, [1.5 # x# 2.0])are very likely to be insulators and would obstruct emissionsfrom the LaB6 tip.3.4. Lanthanum borates, metaborates, and boron oxidesBy analogy with La2O3, we considered that borates, metabo-rates, and boron oxides in the phase diagram (Fig. 1) mightpossibly have the same properties as La2O3, and we calculatedthe electronic structures of LaBO3, LaB3O6, B2O3, and B6O in thepresence of O vacancies. La3BO6, denoted by the yellow circle inFig. 1, was excluded from the considerations because its crystalstructure is unknown. Its properties will be inferred from theresults for LaBO3 and LaB3O6. Regarding LaB3O6, it has two Osites, one coordinated with a La atom and the other with two LaFig. 5 Electronic DOS of (a) LaBO3−x, (b) LaB3O6−x, (c) B2O3−x, and (d)B6O1−x. The blue arrows indicate the defect levels. The Fermi level isset to zero.© 2022 The Author(s). Published by the Royal Society of Chemistryatoms. An O atom was removed from the former site to reducethe total energy.Fig. 5 shows the calculated DOS of these materials with Ovacancies. For LaBO3−x (a), LaB3O6−x (b), and B2O3−x (c), thedefect levels (blue arrows) appear in the middle of the originalband gap. For B6O1−x (d), an unoccupied defect level is locatedat the bottom of the conduction band. In the DOS of B2O3−x (c),LaB3O6−x (b), and LaBO3−x (a), the defect level shis upward asthe composition approaches La2O3−x on the B2O3–La2O3 line(Fig. 1). This is consistent with the results for La2O3−x, wherethe defect levels were at the bottom of the conduction band(Fig. 4). Since La3BO6 is located at the midpoint between LaBO3and La2O3 in the phase diagram (Fig. 1), it is reasonable to inferthat La3BO6 with O vacancies still had a band gap. Thus, the Laborates, metaborates, and boron oxides cannot be conductiveeven in the presence of O vacancies and are unlikely to cover theLaB6 tip.3.5. Lanthanum hydroxide La(OH)3Except for the ternary compounds presented in Fig. 1, it isknown that La(OH)3 is produced from La2O3 by a simplehydration reaction.78 We therefore investigated the possibilitythat La(OH)3 might become conductive when defects areintroduced. The DOS of La(OH)3 was calculated with OHvacancies instead of O vacancies because O and H atoms arepaired in the structure. The DOS of La(OH)3−x (Fig. 6) is similarto that of La2O3−x (Fig. 4), where the defect levels appear at thebottom of the conduction band, which makes La(OH)3−x asconductive as La2O3−x. The WF of La(OH)3−x calculated on theneutral (0001) surface (Fig. 6 inset) was approximately 1.9 eV,which is comparable to that of La2O3−x and LaO (Table 1). Thus,La(OH)3−x could be a covering material for the LaB6 tip.However, La(OH)3 dehydrates at 330 �C and converts to La2O3,79so in this case, La(OH)3 does not need to be considered becausethe emitter tip was cleaned by ash heating at about 800 �C.253.6. Synthesis mechanism of LaO and La2O3Our theoretical studies found that LaO and La2O3−x couldenhance the chemical stability of the LaB6 emitter withoutFig. 6 Electronic DOS of La(OH)3−x with x ¼ 0.125. The Fermi level isset to zero. The inset depicts the (0001) surface on which the WF wascalculated.Nanoscale Adv., 2022, 4, 4669–4676 | 4673http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536kNanoscale Advances PaperOpen Access Article. Published on 11 October 2022. Downloaded on 10/26/2022 2:39:05 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinedegrading the emission performance. If these La oxides werepresent on the LaB6 surface, we can surmise their synthesismechanism from the various experiments we describe below.When a LaB6 crystal is exposed to an oxygen atmosphere, O2molecules are dissociatively adsorbed on the surface even atroom temperature.80–85 When the crystal is heated in an oxygenatmosphere to around 500–700 �C, the surface starts to beoxidized to yield LaO, La2O3, B2O3, LaBO3, LaB3O6, and La3BO6on the macroscopic scale.69,86 However, in our experimentalconditions,25 the LaB6 tip was heated for cleaning in an ultra-high vacuum, where the desorption processes are as signicantas the oxidation processes. The oxide layers would then belimited to several atomic layers. In the thermal desorptionexperiments,81,82 B oxides in the forms of BO and B2O2 began todesorb at temperatures around 1000 K (727 �C), and La oxides inthe form of LaO desorbed at higher temperatures of around1400 K (1127 �C). At about 1500 �C, the LaB6 surface becameclean.11 In our experiments,25 the LaB6 tip was heated ata moderate temperature of about 800 �C, which would inci-dentally have B oxides desorb and La oxides remain on thesurface.Regarding La borates and metaborates (LaBO3, LaB3O6, andLa3BO6) that have been synthesized in an oxygen atmo-sphere,69,86 it is uncertain whether they can be synthesizedunder oxygen-depleted conditions. Considering the fact thatthese materials are synthesized at temperatures (�900 �C)slightly higher than the temperature at which boron oxides BOand B2O2 are desorbed in a vacuum (727 �C),81,82 it seemsunlikely that La borates and metaborates are preferablysynthesized on the LaB6 surface in an ultrahigh vacuum.4. ConclusionsIn order to examine possible chemical effects on LaB6 nanoscaleemitters, we searched for materials that could cover the emittertip to stabilize electron emissions during long operating times.Extensive studies, based on the ternary phase diagram of theLa–B–O system, showed that LaO and La2O3−x could serve as thecovering material. Compared with LaB6, these materials havehigher chemical stability against typical residual gases (H2, O2,H2O, and CO) and have an equivalently low WF, which enablesthe emitter to maintain excellent emission performance. Itshould be noted that these chemical effects can coexist with sizeeffects. It is presumed that ash heating at a moderatetemperature of around 800 �C played a crucial role in the LaB6tip having LaO and/or La2O3−x layers, where B oxides areselectively desorbed and La oxides remain on the surface. It wasrecognized for the rst time that LaO and La2O3−x have sucha low WF that they could be used for electron emitters.Author contributionsW. H. conducted theoretical calculations and wrote the initialdra. S. T. performed the experiments and provided extensivecomments on the manuscript. J. T. and L.-C. Q. managed theentire research project. All authors contributed to thediscussion.4674 | Nanoscale Adv., 2022, 4, 4669–4676Conflicts of interestThere are no conicts to declare.AcknowledgementsPart of this work was supported by the Nanostructural Charac-terization Group of the NIMS Electron Microscopy AnalysisStation.References1 L. W. Swanson and G. A. Schwind, Adv. Imaging ElectronPhys., 2009, 159, 63–100.2 L. W. Swanson and G. A. Schwind, in Handbook of ChargedParticle Optics, ed. J. Orloff, CRC Press, New York, 2nd edn,2008, ch. 1, pp. 1–28.3 R. G. Forbes, in Handbook of Charged Particle Optics, ed. J.Orloff, CRC Press, New York, 2nd edn, 2008, ch. 3, pp. 87–128.4 N. D. Lang, A. 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Published by the Royal Society of Chemistryhttps://tddft.org/bmg/data.phphttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d2na00536k Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters