nanomaterials

Article A Cylindrical Ultrahigh Vacuum Ionization Gauge with a Carbon Nanotube

Jian Zhang 1 , Jianping Wei 2, Detian Li 2, Huzhong Zhang 2 , Yongjun Wang 2,* and Xiaobing Zhang 1,*

1 School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China; [email protected] 2 Science and Technology on Vacuum Technology and Physics Laboratory, Lanzhou Institute of Physics, Lanzhou 730000, China; [email protected] (J.W.); [email protected] (D.L.); [email protected] (H.Z.) * Correspondence: [email protected] (Y.W.); [email protected] (X.Z.)

Abstract: In this study, a cylindrical triode ultrahigh vacuum ionization gauge with a screen-printed carbon nanotube (CNT) source was developed, and its metrological performance in different gases was systematically investigated using an ultrahigh vacuum system. The resulting ionization gauge with a CNT cathode responded linearly to nitrogen, argon, and air pressures in the range from ~4.0 ± 1.0 × 10−7 to 6 × 10−4 Pa, which is the first reported CNT emitter-based ionization gauge whose lower limit of pressure measurement is lower than its hot cathode counterpart. In addition, the sensitivities of this novel gauge were ~0.05 Pa−1 for nitrogen, ~0.06 Pa−1 for argon, and ~0.04 Pa−1 for air, respectively. The trend of sensitivity with anode voltage, obtained by the   experimental method, was roughly consistent with that gained through theoretical simulation. The advantages of the present sensor (including low power consumption for electron emissions, invisible Citation: Zhang, J.; Wei, J.; Li, D.; to infrared light radiation and thermal radiation, high stability, etc.) mean that it has potential Zhang, H.; Wang, Y.; Zhang, X. A applications in space exploration. Cylindrical Triode Ultrahigh Vacuum Ionization Gauge with a Carbon Nanotube Cathode. Nanomaterials Keywords: ionization gauge; CNT; field emission; pressure measurement; lower limit 2021, 11, 1636. https://doi.org/ 10.3390/nano11071636

Academic Editors: 1. Introduction Maurizio Passacantando and In recent years, ultrahigh and extremely high vacuum pressure measurements have Nikos Tagmatarchis become widely demanded in many science and engineering fields [1–3], including space exploration, surface science, particle accelerator, storage ring, etc. To date, hot cathode Received: 30 May 2021 and ionization gauges are the only practically available electronic vacuum Accepted: 18 June 2021 sensors in this pressure region [4,5]. However, conventional ionization gauges have long- Published: 22 June 2021 standing intractable problems when measuring extremely low pressure, i.e., X-ray effects, electron-stimulated desorption effects, and outgassing effects [1,6,7]. Fortunately, recent Publisher’s Note: MDPI stays neutral studies [8–19] have shown that ionization gauges with carbon nanotube (CNT) with regard to jurisdictional claims in have unique advantages, such as low power consumption for electron emissions, fast re- published maps and institutional affil- sponses, free from visible to infrared light radiation and thermal radiation, etc., in extremely iations. low pressure measurements, which are largely due to the application of the novel CNT field emission cathode. For instance, Murakami et al. [9] reported the application of a CNT field emission cathode in a Bayard–Alpert gauge (BAG) for the first time in 2001, and the lower limit of pressure measurement by this novel sensor was approximately 1 × 10−4 Pa. Copyright: © 2021 by the authors. Dong et al. [8] reported on the design and investigation of a commercial extractor gauge Licensee MDPI, Basel, Switzerland. (Leybold, IE 514) with a CNT cathode, which showed excellent measurement linearity This article is an open access article from 10−10 to 10−6 Torr in nitrogen. Sheng et al. [10] described a high-sensitivity saddle distributed under the terms and field gauge with a CNT cathode, and a linear pressure response was achieved from 10−5 to conditions of the Creative Commons 10−2 Pa. In theory, the CNT cathode ionization gauges have several advantages; in reality, Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ however, none of the reported ionization gauges with CNT cathodes have a lower limit of 4.0/). pressure measurement than that of corresponding hot cathodes with the same structural

Nanomaterials 2021, 11, 1636. https://doi.org/10.3390/nano11071636 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1636 2 of 11

types. For example, the lower limit of a commercial extractor gauge (Leybold, IE 514) is as low as ~10−10 Pa, but the lower limit achieved by the extractor gauge with a CNT cathode in Ref. [8] was only 4 × 10−8 Pa. Likewise, in Ref. [18], the lower limit achieved by BAG with a CNT cathode was approximately 1 × 10−4 Pa; the value for the hot BAG cathode was about 1 × 10−9 Pa. In this sense, the advantages of the application of such novel CNT field emission cathodes in extremely low pressure measurements have not yet been fully demonstrated. In this work, a high-quality CNT electron source was produced by a screen-printing method, and a cylindrical triode ultrahigh vacuum ionization gauge with a screen-printed CNT cathode was further developed by replacing the original hot filament with the result- ing CNT electron source in order to demonstrate the advantage of an ionization gauge equipped with a CNT cathode in suppressing X-ray effects; the sensing characteristics of this novel gauge in nitrogen, argon, and air were evaluated systematically. The effects of anode voltage on gauge sensitivity were also studied by using the theoretical simu- lation method. Here, a lower limit of ~3.0 × 10−7 Pa was achieved in nitrogen using the present sensor under optimized conditions, which is the first reported CNT emitter- based ionization gauge whose lower limit of pressure measurement is lower than its hot cathode counterpart.

2. Materials and Methods 2.1. CNT Electron Source and Its Field Emission Characteristics In this study, a novel field emission cathode was fabricated by screen-printing CNT paste on one end of a 5 mm diameter stainless steel rod. Here, the CNT paste, roughly 100 ± 20 µm in thickness, was composed of multi-walled CNTs of 10~15 nm in diameter, with inorganic fillers of alloy, and organic powder of ethyl cellulose and terpineol; the latter was used as a solvent. The CNT paste was formulated by using only ball-milling apparatus. The inner and outer diameters of the utilized CNT were ~7.0 and ~12.8 nm, respectively, and the number of walls was ~8. More detailed descriptions about the CNT cathode preparation processes are presented in Refs. [20,21]. The microstructure and morphology of the CNT cathodes were characterized by a LabRAM HR800 micro-Raman spectrometer (HORIBA Jobin Yvon, Edison, NJ, USA) operating with a 532 nm Ar+ laser as the excitation and a field emission scanning (FE-SEM, Quanta FEI 200, Eindhoven, The Netherlands). The Raman spectra were recorded from ~75 to 3000 cm−1, and the scanning time for each sample was 30 s. Finally, the Raman spectra were fitted based on two Gaussian shape curves with a curve-fitting software to identify the peak positions and calculate the intensity ratio of D and G peaks. The field emission electron source was constructed with a gate and a CNT cathode, and they were 200 µm away from each other. The gate electrode, used to extract and accelerate the electron from the CNT emitters, was made of sheet (15 mm in diameter, 50 µm in thickness), and the center of the molybdenum sheet, ~8 mm in diameter, was a mesh structure with ~80% physical transparency. The gate and the CNT cathode were electrically separated by insulating ceramics. The field emission properties of the resulting CNT electron source, including I-V characteristics and stability, were investigated in an ionization gauge, which is described in the next section in detail. In order to realize a reliable pressure measurement, the CNT electron source was firstly fired at 25 ◦C for 2 h in a muffle furnace in air in order to evaporate the organic residual slurry to reduce the outgassing from paste during field emission processes; then, it was conditioned in a vacuum of ~10−8 Pa at a current density of ~1 mA/cm2 for 10 h before it was used in the ionization gauge.

2.2. Cylindrical Triode Ionization Gauge with a CNT Electron Source 2.2.1. Experimental Study In this work, a commercialized hot cathode ionization gauge with a triode structure (ZJ-27, Guoguang Electric Co. Ltd., Chengdu, China) was modified by replacing the hot Nanomaterials 2021, 11, 1636 3 of 11

2.2. Cylindrical Triode Ionization Gauge with a CNT Electron Source 2.2.1. Experimental Study Nanomaterials 2021, 11, 1636 3 of 11 In this work, a commercialized hot cathode ionization gauge with a triode structure (ZJ-27, Guoguang Electric Co. Ltd., Chengdu, China) was modified by replacing the hot filamentfilament with with a a screen-printed CNT electron source. The modified modified ionization gauge was mainlymainly composed composed of of a a screen-printed screen-printed CNT CNT electron electron source, source, a a cylindrical collector, collector, and a helix-shapedhelix-shaped anode anode grid. grid. The The gate gate electrode electrode was was 2 2 mm away from the anode apex. The helix-shapedhelix-shaped anode, ~8 mm in diameter,diameter, andand thethe cylindrical cylindrical collector, collector, ~25 ~25 mm mm in in diameter, diame- ter,were were coaxial, coaxial, and and situated situated ~8.5 ~8.5 mm mm away away from from each each other. other. The The measurement measurement range range of the of theoriginal original hot hot cathode cathode ionization ionization gauge gauge (ZJ-27) (ZJ-27) was was from from 1.0 1.0× ×10 10−−55 to 1.0 1.0 ×× 10101 1Pa,Pa, with with a sensitivitya sensitivity of of0.15 0.15 Pa Pa−1− [22].1 [22 ].The The gauge gauge performances performances were were evaluated evaluated in in an an ultrahigh vacuumvacuum system system with with an an ultimate ultimate pressure pressure of of 1 1× 10× −108 Pa,−8 whichPa, which was wasmonitored monitored by a bycali- a bratedcalibrated extractor extractor gauge gauge (Leybold, (Leybold, IE IE514). 514). The The experimental experimental gases gases (N (N2,2 ,Ar Ar and and air) air) were were introducedintroduced into into the the system system via via a a needle needle inlet inlet valve valve to to obtain the de desiredsired pressures (given byby the the nitrogen equivalent value). The The voltages voltages applied applied to to the gate and the helix-shaped anodeanode were were provided provided by two Keithley 2290-5 source meters, and the cylindrical collector asas well as the CNTCNT cathodecathode waswas grounded.grounded. TheThe smallsmall ionion collectorcollector current,current,I collectorIcollector,, was measuredmeasured with a picoam-meter (Keithley 6487),6487), andand thethe CNTCNT cathodecathode current,current,I cathodeIcathode,, and and thethe helix-shapedhelix-shaped anodeanode current, current,Ianode Ianode, were, were detected detected using using two high-precisiontwo high-precision digital digital multi- multi-metersmeters (FLUKE,17B). (FLUKE,17B). Finally, Finally, in order in to order determine to determine the linear the pressure linear measurementpressure measure- range mentof the range present of ionizationthe present gauge ionization with a gauge CNT cathode, with a CNT the background cathode, the signal background obtained signal under obtainedthe ultimate under pressure the ultimate condition pressure was processed condition withwas processed the same procedurewith the same as described procedure in asRefs. described [14,23]. Thein Refs. original [14,23]. cylindrical The original triode ionizationcylindrical gauge triode and ionization the schematic gauge diagram and the of schematicthe modified diagram ionization of the gauge modified with aionization CNT electron gauge source with useda CNT in thiselectron work source are illustrated used in thisin Figure work1 are. illustrated in Figure 1.

Figure 1. TheThe originaloriginal cylindricalcylindrical triode triode ionization ionization gauge gauge (a ),(a the), the schematic schematic diagram diagram of the of modifiedthe modified ionization ionization gauge gauge with witha CNT a CNT cathode cathode (b), and (b), theand inset the inset in (b in) is (b the) is schematic the schematic illustration illustration of the of screen-printedthe screen-printed CNT CNT electron electron source source used used in this in this work. Here, ① hot filament/CNT electron source; ② cylindrical collector; and ③ helix-shaped anode. In addition, work. Here, 1 hot filament/CNT electron source; 2 cylindrical collector; and 3 helix-shaped anode. In addition, the the electrode voltages used in the pressure sensing experiment are also given (blue numbers). The Chinese characters in electrode voltages used in the pressure sensing experiment are also given (blue numbers). The Chinese characters in (a) are (a) are patent number 200420060808.0. patent number 200420060808.0. 2.2.2. Theoretical Simulation 2.2.2. Theoretical Simulation In order to investigate the effect of anode voltage on gauge sensitivity, we built the In order to investigate the effect of anode voltage on gauge sensitivity, we built gaugethe gauge model model using using SolidWorks SolidWorks 3D Modeling 3D Modeling Software Software according according to the to structure the structure pa- rametersparameters of ofthe the real real ionization ionization gauge gauge with with a aCNT CNT cathode, cathode, as as shown shown in in Figure Figure 11b,b, andand Ions Optic Software Simion 8.1 was used to determine the effective electron path length and the mean electron energy along its trajectory as a function of anode voltage; the ionization cross-section for the studied gas was further calculated by using an analytical formula

obtained from the so-called binary encounter Bethe (BEB) model [24,25]. In this model, the Nanomaterials 2021, 11, 1636 4 of 11

Ions Optic Software Simion 8.1 was used to determine the effective electron path length and the mean electron energy along its trajectory as a function of anode voltage; the ion- Nanomaterials 2021, 11, 1636 ization cross-section for the studied gas was further calculated by using an analytical4 of 11 formula obtained from the so-called binary encounter Bethe (BEB) model [24,25]. In this model, the ionization cross-section is calculated by each molecular orbital and then ionization cross-section is calculated by each molecular orbital and then summed over all summed over all orbitals, which is finally given as: σ = 1 − +1− − K h ln t  1  1 ln t i orbitals, which is finally given as: σ = 1 − + 1 − − ; here, t = T/B, BEB t+u+1 2 t2 t t+1 ; here, t=T/B, u=U/B, K=4πaNR /B ，𝑎 = 0.5292Å, R=13.61 eV. T is the incident u = U/B, K = 4πa2NR2/B2, a = 0.5292A˚ , R = 13.61 eV. T is the incident electron energy electron energy (eV),0 B and U 0are the binding energy (eV) and kinetic energy (eV) of an (eV), B and U are the binding energy (eV) and kinetic energy (eV) of an electron on a given electron on a given molecular orbital, respectively, and N is the number of on molecular orbital, respectively, and N is the number of electrons on the orbital [24]. By the orbital [24]. By using the above parameters, the gauge sensitivity was calculated as using the above parameters, the gauge sensitivity was calculated as follows: follows: S =S=( (LL··σσ)/k)/kT (1)(1)

Here,Here, SS isis the gaugegauge sensitivity,sensitivity,L Lis is the the effective effective electron electron path path length, length, which which is defined is de- finedas the as electronthe electron trajectory trajectory length length in in the the effective effective ionization ionization space, space, kk isis the Boltzmann Boltzmann constant,constant, TT isis the the absolute absolute temperature temperature of of molecules molecules in in the the ionization ionization region, region, and and σσ isis the the electron-impactelectron-impact ionization ionization cross-section cross-section of of gas gas molecules molecules [26]. [26]. InIn the the simulation simulation procedure, procedure, in in order order to to improve improve the the calculation calculation accuracy accuracy and and the the reliabilityreliability of of the simulationsimulation results,results, a a three-dimensional three-dimensional structural structural model model was was adopted. adopted. The Thegrids grids of the of calculationthe calculation model model for the for studied the studied ionization ionization gauge gauge were all were hexahedral, all hexahedral, and the andtotal the grid total number grid number was about was 10 8about. The more108. The detailed more calculation detailed calculation methods were methods adequately were adequatelydescribed indescribed our previous in our papers previous [12,27 ].papers The gate [12,27]. voltage, The thegate collector voltage, voltage, the collector and the voltage,CNT cathode and the voltage CNT cathode were assumed voltage towere be 300,assumed 0, and to 0 be V, 300, respectively, 0, and 0 V, and respectively, the studied andanode the voltagestudied increasedanode voltage from increased 50 to 400 from V. A total50 to of400 600 V. electronsA total of with 600 initialelectrons energies with initialof 0.1 energies eV were of randomly 0.1 eV were arranged randomly on the arranged CNT tips. on Thethe CNT effect tips of anode. The effect voltage of anode on the voltageionization on the cross-section, ionization thecross-section, effective electron the effective path length, electron and path the length, mean electronand the energymean electronalong electron energy trajectories along electron were stimulated,trajectories andwere the stimulated, corresponding and the results corresponding are presented re- in sultsFigure are2 .presented Here, the ionizationin Figure 2. cross-section Here, the io andnization the effective cross-section electron and path the lengtheffective will elec- be tronused path to calculate length will the be gauge used sensitivity. to calculate the gauge sensitivity.

Figure 2. The stimulated electron-impact ionization cross-section, the mean electron path length, and the mean electron energy along electron trajectories as a function of anode voltage. The error bars are the standard deviation obtained by three calculations for the corresponding physical values.

Nanomaterials 2021, 11, 1636 5 of 11

Figure 2. The stimulated electron-impact ionization cross-section, the mean electron path length, and the mean electron energy along electron trajectories as a function of anode voltage. The error bars are the standard deviation obtained by three calculations for the corresponding physical values. Nanomaterials 2021, 11, 1636 5 of 11 3. Results and Discussion 3.1. Characterization of the Screen-Printed CNT Cathode 3. Results and Discussion 3.1. CharacterizationThe morphology of theand Screen-Printed crystallinity CNT of the Cathode CNT cathode were characterized by FE-SEM and Raman techniques, respectively, and the corresponding results are pre- The morphology and crystallinity of the CNT cathode were characterized by FE- sented in Figure 3. The present CNT cathode principally consisted of highly tangled, SEM and Raman techniques, respectively, and the corresponding results are presented randomly oriented, dense CNTs together with a certain number of impurities, as shown in Figure3 . The present CNT cathode principally consisted of highly tangled, randomly inoriented, Figure dense3a. It is CNTs alsotogether obvious with that anumerous certain number CNT ofbodies impurities, protruded as shown from intheFigure binder3 a. matrix,It is also and obvious the defects that on numerous CNT bodies CNT in bodies addition protruded to the CNT from tips the could binder become matrix, the andac- tivethe defectselectron on emission CNT bodies sites induring addition field to em theission CNT tipsprocesses could [28,29]. become In the addition, active electron small amountsemission of sites Bi blocks during could field be emission found on processes the cathode [28,29 surface,]. In addition, which was small used amounts to enhance of Bi theblocks connection could be between found on the the CNTs cathode and surface, the substr whichate [21]. was usedIn order to enhance to gain themore connection insights intobetween the structural the CNTs characteristics and the substrate of the [21 ].CNT In order cathode, to gain we more performed insights Raman into the analysis structural in additioncharacteristics to FE-SEM of the for CNT the cathode, as-received we performed cathode. RamanFigure analysis3b shows in additionthe typical to FE-SEMRaman spectrumfor the as-received of the present cathode. CNT Figurecathode.3b There shows are the two typical strong Raman peaks spectrum and two weak of the peaks present in theCNT wavenumber cathode. There range are of two 75strong to 3000 peaks cm−1 and. The two weak weak peaks, peaks present in the wavenumberat around 100 range to 300 of cm75− to1, result 3000 cm from−1. Thescattering weak peaks,by the present radial br ateathing around 100modes to 300 (RBMs), cm−1, namely, result from coherent scattering vi- brationby the radialin phases breathing of the modes carbon (RBMs), atoms namely,in the radial coherent direction. vibration The in appearance phases of the of carbon RBM peaksatoms in in the radialRaman direction. spectrum The indicated appearance that ofthere RBM was peaks the in definite the Raman existence spectrum of few-walledindicated that CNTs there in wasthe cathode the definite [13]. existence The peak of located few-walled at around CNTs 2688 in the cm cathode−1 was assigned [13]. The topeak the located2D peak, at aroundwhich originated 2688 cm−1 fromwas assigneda double toresonance the 2D peak, process which involving originated two from pho- a nonsdouble of opposite resonance wave process vectors. involving The strong two phononspeaks at of~1348.0 opposite cm−1 wave and ~1585.6 vectors. cm The−1 are strong the so-calledpeaks at ~1348.0D and G cm peaks,−1 and respectively. ~1585.6 cm− 1Theare D the peak so-called is related D and to G the peaks, breathing respectively. modes The of six-foldD peak iscarbon related rings to the in breathing carbonaceous modes mate of six-foldrials, and carbon the rings G peak in carbonaceous is generated materials, by the stretchingand the G mode peak isof generated all pairs of by sp the2 atoms stretching in both mode rings of alland pairs chains of sp of2 graphiticatoms in bothmaterials rings [30,31].and chains The ofintensity graphitic ratio materials of the [G30 ,peak31]. Theto D intensity peak, IG ratio/ID, is of commonly the G peak used to D peak,to estimateIG/ID , theis commonly structural perfection used to estimate of CNTs, the which structural was perfectionas high as 3.2 of CNTs,in this whichcase, indicating was as high that as the 3.2 usedin this CNTs case, had indicating a high thatcrystalline the used perfection CNTs had [13,32]. a high It crystallineis reasonable perfection to believe [13 ,that32]. Itthe is goodreasonable crystallization to believe of that the the present good crystallization CNTs endows of thethe present cathode CNTs with endows exceptional the cathode field emissionwith exceptional performances field emission [32,33]. performances [32,33].

FigureFigure 3. 3. MicrostructureMicrostructure characterization characterization of of the the resulting resulting CNT CNT cathode. cathode. (a (a) )FE FE–SEM­SEM image, and ( b) Raman analysis. The field electron emission properties of the integrated CNT electron source were evaluatedThe field in theelectron modified emission cylindrical properties triode of ionization the integrated gauge CNT prior electron to vacuum source pressure were evaluatedsensing application. in the modified In this cylin process,drical the triode CNT cathodeionization and gauge the helix-shaped prior to vacuum anode pressure voltages were set to be 0 and 350 V, respectively, and the gate voltage gradually increased from 207 to 267 V; the corresponding results are given in Figure4. It is observable from Figure4a that the present CNT electron source exhibited typical field emission characteristics, because the emission current exponentially increased with increasing the gate voltage [34]. The Nanomaterials 2021, 11, 1636 6 of 11

sensing application. In this process, the CNT cathode and the helix-shaped anode volt- ages were set to be 0 and 350 V, respectively, and the gate voltage gradually increased from 207 to 267 V; the corresponding results are given in Figure 4. It is observable from

Nanomaterials 2021, 11, 1636 Figure 4a that the present CNT electron source exhibited typical field emission charac-6 of 11 teristics, because the emission current exponentially increased with increasing the gate voltage [34]. The maximal field emission current achieved at a gate voltage of 267 V is onlymaximal 80 μ fieldA, because emission the current gate voltage achieved does at anot gate increase voltage further of 267 Vin is order only 80to µpreventA, because the CNTthe gate from voltage being does degraded. not increase The electron further in transmittance order to prevent over the the CNT gate from mesh being is about degraded. 70% andThe electronis not strongly transmittance dependent over on the the gate gate mesh voltage is about in the 70% range and isfrom not 207 strongly to 267 dependent V. In ad- dition,on the in gate this voltage case, the in anode the range current from reached 207 to up 267 to V. 20 In μ addition,A when the in gate this voltage case, the was anode 250 V.current reached up to 20 µA when the gate voltage was 250 V.

Figure 4. TheThe field field electron emission characteristics of the present CNT electron source. (a) I-V curve, and (b)) stability.stability.

The field field emission stability of a CNT electronelectron source is of practicalpractical importance in vacuum electronic device applications. The sh short-termort-term emission stability of the present CNT electron source source was was estimated estimated by by applyi applyingng a aconstant constant gate gate voltage voltage of of 270 270 V Vfor for a perioda period of of20 20h. h.The The initial initial emission emission current current was wasset to set be to ~90 be μ ~90A, whichµA, which is a more is a morestrin- gentstringent value value than than the theone one used used in inthe the next next pr pressureessure sensing sensing experiment. experiment. It It is is observable that, as as demonstrated demonstrated in in Figure Figure 4b,4b, the the emission emission current current was was almost almost constant constant over over 20 20 h hof continuousof continuous operation operation only only with with a minor a minor emission emission fluctuation fluctuation of 2.3% of 2.3% (the (theemission emission cur- 4 × rentcurrent fluctuation fluctuation is defined is defined as: f = as: △If/I=ave × 100%.I/Iave Here,100%. f is the Here, emissionf is the current emission fluctuation, current I Ifluctuation,ave is the averagedave is the field averaged emission field current emission during current the duringwhole thetest wholeperiod, test and period, △I is andthe standard4I is the standarddeviation deviation[32], indicating [32], indicating an excelle annt excellentfield emission field emissionstability. stability.In fact, the In fact,out- gassingthe outgassing from screen-printed from screen-printed CNT electron CNT electronsource is source one of isthe one main of thereasons main of reasons emission of instabilityemission instability for such electron for such sources. electron In sources. this work, In this the work, field emission the field emissionstability of stability the CNT of the CNT electron source was estimated after a series of experimental treatments, including electron source was estimated◦ after a series of experimental treatments, including high-temperature (250 °C)C) treatment, aging treatment, and an I-V test. Consequently, the heat induced during these treatment processes can accelerate the evaporation of residual organic slurry, and thus a stable emission characteristic of the present CNT electron source organic slurry, and thus a stable emission characteristic of the present CNT electron is obtained. It is universally recognized that the electron emission stability of a CNT source is obtained. It is universally recognized that the electron emission stability of a electron source is one of the key factors for determining its potential application in devices; CNT electron source is one of the key factors for determining its potential application in the good emission stability of an integrated CNT electron source is definitely favorable for devices; the good emission stability of an integrated CNT electron source is definitely the development of high-performance electric vacuum devices. favorable for the development of high-performance electric vacuum devices. 3.2. Metrology Behaviors of the Novel Ionization Gauge 3.2. Metrology Behaviors of the Novel Ionization Gauge For ionization gaugse, a low anode current (i.e., 20 µA) would greatly reduce the adverseFor impactionization of X-ray gaugse, effects a [low12,18 anode,35] in current extremely (i.e., low 20 pressure μA) would measurements, greatly reduce even ifthe it wouldadverse cause impact difficulties of X-ray due effects to weak [12,18,35] collector in extremely current detection low pressure [7,8]. Thus, measurements, the metrological even ifbehaviors it wouldof cause themodified difficulties ionization due to weak gauge collector with a CNTcurrent electron detection source [7,8]. were Thus, investigated the met- rologicalunder a low behaviors anode currentof the modified together withionization constant gauge electrode with a voltages, CNT electron as demonstrated source were in Figure1b. In this situation, the anode current was only about 20 µA, which is high enough for extremely low pressure measurement, according to our previous experience [11,14,17]. The sensing principle of an ionization gauge is described as follows: I /(p × I ) = S, ion anode where Iion and Ianode are the collector and the anode currents, respectively, p is the pressure, and S is the gauge sensitivity [5]. The pressure-sensing characteristics of the modified ionization gauge with a CNT electron source were investigated, and the corresponding Nanomaterials 2021, 11, 1636 7 of 11

results are presented in Figure5. The normalized ion currents ( Iion/Ianode) for three testing gases (i.e., nitrogen, argon and air) are linear relative to the pressure in the range from ~4.0 ± 1.0 × 10−7 to ~6.0 × 10−4 Pa, indicating good characteristics for the pressure sensor. Here, it is worth noting that the lower limit of the studied ionization gauge is two orders of magnitude lower than its hot cathode counterpart [22]. To the best of our knowledge, this is the first reported CNT emitter-based ionization gauge whose lower limit of pressure measurement is lower than its hot cathode counterpart [5,36]. It is well-known that in cylindrical triode ionization gauges, X-ray effects are one of the most dominant factors to restrain the extension of the lower limit of pressure measurements of such a gauge [1,6]. This is mainly due to the fact that the X-rays mainly produced by electron-impact on the anode generate a photoelectron current from the ion collector, and thus produce a pressure- independent background current. This background current, in reality, is comparable to or even larger than the ion current originating from ionizing gas molecules in extremely high even ultrahigh vacuum pressure regions; consequently, a nonlinear characteristic in normalized ion current–pressure plots is generally observed in hot cathode ionization, as widely reported in previous studies [18,37,38]. In order to suppress the adverse impact of X-ray effects in very low pressure measurements, some measures (i.e., reducing the collector area, locating the collector out of the line of sight of the gauge grid and the filament, adding a suppresser electrode, and utilizing a current modulation technique) were employed to extend the lower limit of pressure measurements of the ionization gauge [6,35,39]; however, to date, the results have not been satisfactory. In the present work, a field emission CNT electron source was used, which has several advantages in extending the lower limit of pressure measurements of the modified ionization gauge. On the one hand, the novel CNT electron source successfully avoids any radiation (thermal, light and ultraviolet) effects which exist in hot cathodes, and thus the photoelectron current emitting from the collector induced by strong ultraviolet radiation does not exist in the present gauge, which is conducive to extending the lower limit of pressure measurements of the present gauge [40,41]. On the other hand, the present ionization gauge had a very low power consumption, of about 9.5 mW, compared with ~3.45 W of the cylindrical triode ionization gauge with a hot cathode [22]; as a result, outgassing of the gauge materials induced by thermal radiation or thermal conductivity would be extremely low, which is favorable for achieving extremely low pressure measurements [7,36,42]. Additionally, the X-ray photocurrent was proportional to the anode current [43], which was as high as 1 mA in a cylindrical triode ionization gauge with a hot cathode [22]. In this work, as mentioned above, the anode current was about 20 µA, only one-fiftieth times lower than that for the cylindrical triode hot cathode ionization gauge, which is conducive to greatly reducing the X-ray photocurrent, and is thus beneficial for obtaining lower limits of pressure measurements for the present gauge. The sensitivity values of the ionization gauge, derived from the slopes of the linear regions in Figure5, were ~0.05 Pa −1 for nitrogen, ~0.06 Pa−1 for argon, and ~0.04 Pa−1 for air, which are even slightly higher than that of the most prevalent CNT cathode extractor gauge [8,11,12,17] and BAG [16], and are acceptable values for extremely low pressure measurements [7]. The detailed variation of sensitivity of this ionization gauge will be further discussed below. Nanomaterials 2021, 11, 1636 8 of 11 Nanomaterials 2021, 11, 1636 8 of 11

FigureFigure 5. 5.The The normalized normalized ion ion current current of of the the modified modified ionization ionization gauge gauge with with a CNT a CNT electron electron source source as aas function a function of testing of testing pressure pressure in different in different gases. gases.

TheThe gaugegauge sensitivitysensitivity isis aa keykey parameterparameter forfor pressurepressure sensing,sensing, whichwhich isis relatedrelated toto severalseveral factors,factors, including the the gauge gauge physical physical structure, structure, the the electrode electrode voltage, voltage, and and the the gas gasspecies species [5,10,14]. [5,10,14 In]. general, In general, a high a highsensitivit sensitivityy is necessary is necessary for an for ionization an ionization gauge gauge with a withlow anode a low anodecurrent current to monitor to monitor the extremely the extremely low pressure low pressure [44]. In [this44]. work, In this the work, effect the of effectanode of voltage anode on voltage gauge on sensitivity gauge sensitivity was investig wasated investigated by both experimental by both experimental and theoretical and theoreticalmethods. In methods. the experimental In the experimental study, the vari study,ation the of variation gauge sensitivity of gauge sensitivityas a function as aof −5 functionanode voltage of anode was voltage investigated was investigated under a vacuum under a pressure vacuum pressureof ~6.3 × of10 ~6.3−5 Pa,× the10 meas-Pa, theurement measurement was repeated was repeated four times four to times confirm to confirm the repeatability the repeatability of the ofexperimental the experimental result, result,and the and corresponding the corresponding results resultsare presented are presented in Figure in Figure6. Here,6. in Here, order in to order ensure to ensurethat the thatanode the current anode currentwas consistent was consistent with the with value the in value Figure in 5, Figure the gate5, the voltage gate voltageis set to isbe set 300 to V bein 300this V case. in this It case.is apparent It is apparent that under that under a constant a constant gate gatevoltage, voltage, the thegauge gauge sensitivity sensitivity in- increasescreases as as the the anode anode voltage voltage increases increases from ~50 to ~100 V, andand thenthen graduallygradually decreasesdecreases asas thethe anodeanode voltagevoltage increasesincreases further.further. InIn aa hothot cathodecathode ionizationionization gauge,gauge, thethe highesthighest sensitivitysensitivity is is usually usually obtained obtained with with an an anode anode voltage voltage of of ~100 ~100 to to 150 150 V V (the (the corresponding corresponding electronelectron energyenergy isis inin thethe rangerange fromfrom 100100 toto 150150 eV)eV) forfor mostmost pervasivepervasive gasgas moleculesmolecules [[13].13]. However,However, in in the the ionization ionization gauge gauge with with a a CNT CNT cathode, cathode, the the situation situation is is more more complicated complicated becausebecause thethe emitting electron generated generated from from CNT CNT emitters emitters is isnot not only only accelerated accelerated by bythe thegate gate voltage, voltage, but butis also is alsoimpacted impacted by the by anode the anode voltage. voltage. In our Incase, our the case, highest the highestsensitiv- sensitivity, ~0.15 Pa−1, was achieved at an anode voltage of ~100 V, which is comparable to ity, ~0.15 Pa−1, was achieved at an anode voltage of ~100 V, which is comparable to the the sensitivity of ZJ-27 commercial hot cathode gauges (~0.15 Pa−1)[22]. Here, it should be sensitivity of ZJ-27 commercial hot cathode gauges (~0.15 Pa−1) [22]. Here, it should be pointed out that the sensitivity obtained from Figure5 is lower than that of the value given pointed out that the sensitivity obtained from Figure 5 is lower than that of the value in Figure6 , which is mainly attributed to the fact that the sensitivity in Figure5 is derived given in Figure 6, which is mainly attributed to the fact that the sensitivity in Figure 5 is from the slope of normalized ion current–pressure plot. This is a relatively averaged value derived from the slope of normalized ion current–pressure plot. This is a relatively av- corresponding to the full pressure region from ~4.0 × 10−7 to ~6.0 × 10−4 Pa; however, eraged value corresponding to the full pressure region from ~4.0 × 10−7 to ~6.0 × 10−4 Pa; the sensitivity in Figure6 is obtained by single point calculation, namely, it is achieved however, the sensitivity in Figure 6 is obtained by single point calculation, namely, it is at a pressure of ~6.3 × 10−5 Pa. It can be seen from Figure5 that the slope in the high- achieved at a pressure of ~6.3 × 10−5 Pa. It can be seen from Figure 5 that the slope in the pressure region (p > 3 × 10−5 Pa) is clearly higher than that of the low-pressure region high-pressure region (p > 3 × 10−5 Pa) is clearly higher than that of the low-pressure region (p < 3 × 10−5 Pa), and hence the sensitivity achieved from single point calculation at a (p < 3 × 10−5 Pa), and hence the sensitivity achieved from single point calculation at a pressure of ~6.31 × 10−5 Pa is higher relative to the averaged value obtained from the full −5 pressurepressure region.of ~6.31 At × 10 constant Pa is gatehigher voltages relative (250 to the and aver 300aged V), the value trend obtained of sensitivity from the with full anodepressure voltage, region. obtained At constant by the gate experimental voltages (250 method, and is300 consistent V), the trend with theof sensitivity one gained with by theoreticalanode voltage, simulation, obtained as demonstratedby the experimental in Figure method,6. It is generallyis consistent believed with thatthe theone gaugegained sensitivityby theoretical is determined simulation, by as the demonstrated ionization cross-section in Figure 6. ofIt is gas generally molecules believed as well that as the the gauge sensitivity is determined by the ionization cross-section of gas molecules as well as

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effectivethe effective electron electron path path length, length, i.e., thei.e., gauge the gauge sensitivity sensitivity is the is integral the integral over over the path the path of the of electronsthe electrons of the of probability the probability of ionization of ionization of a gasof a molecule gas molecule by an by electron an electron [45]. [45]. Thus, Thus, it can it explaincan explain the variation the variation of the of experimental the experimental sensitivity sensitivity with anode with voltage.anode voltage. As is clearly As is shownclearly inshown Figure in2 ,Figure both the 2, electron-impactboth the electron-impac ionizationt ionization cross-section cross-section and the effective and the electron effective pathelectron length path reach length their reach maximum their maximum values at va anlues anode at an voltage anode ofvoltage ~100 of V. ~100 Consequently, V. Conse- thequently, highest the sensitivity highest sensitivity is naturally is naturally obtained obtained according according to Equation to Equation (1). In addition,(1). In addi- it shouldtion, it beshould noted be that noted under that some under anode some voltages anode (e.g.,voltages 250 V),(e.g., the 250 sensitivity V), the sensitivity derived from de- experimentalrived from experimental testing was highertesting than was thehigher one th obtainedan the one by theoreticalobtained by simulation, theoretical which simula- is mainlytion, which because is mainly in the theoreticalbecause in simulation,the theoretical only simulation, the contribution only the of gascontribution phase ions of was gas considered.phase ions However,was considered. in the experiment, However, thein contributionsthe experiment, from the ions contributions generated by from electron- ions stimulatedgenerated by desorption electron-stimulated and weak X-raydesorption effects and were weak also X-ray included, effects except were foralso gas included, phase ions;except hence for gas a higher phase sensitivity ions; hence was a higher achieved. sens Ititivity is reasonable was achieved. that the It highest is reasonable sensitivity that shouldthe highest be used sensitivity in pressure should sensing be used in our in pressure work; however, sensing whenin our the work; anode however, voltage when was 100the V,anode the anode voltage current was 100 was V, very the low,anode ~5.4 currentµA. As was a result,very low, the collector~5.4 μA. currentAs a result, for thethe pressurecollectormeasurement current for the of pressure 3.0 × 10 −measurement7 Pa was extremely of 3.0 × low, 10−7 which Pa was poses extremely a huge low, challenge which forposes detecting a huge thechallenge weak currentfor detecting signal the using weak the current picoam-meter, signal using and the is alsopicoam-meter, the reason and for theis also unstable the reason sensitivity for the in unstable the low anodesensitivit voltagey in the region, low anode especially voltage at 50 region, V, as shownespecially in Figureat 50 V,6 . as In shown order to in keep Figure the 6. ion In current order to signal keep intensity the ion atcurrent a sufficient signal level, intensity a compromise at a suffi- iscient needed level, between a compromise the sensitivity is needed and between the ion current;the sensitivity therefore, and an the anode ion current; voltage therefore, of 350 V wasan anode used involtage this study. of 350 V was used in this study.

FigureFigure 6.6.The The experimentalexperimental andand simulatedsimulated sensitivitiessensitivities ofof thethe studiedstudied ionizationionization gaugegauge withwith aa CNT CNT cathodecathode asas aa functionfunction of of anode anode voltage. voltage.

4.4. ConclusionsConclusions InIn summary,summary, an an ionization ionization gauge gauge with with an an integrated integrated CNT CNT electron electron source source was was devel- de- opedveloped in this in work.this work. The fieldThe emissionfield emission performances performances of the integratedof the integrated CNT electron CNT electron source, assource, well as as the well metrological as the metrological behaviors behaviors of this novel of ionizationthis novel gauge,ionization were gauge, investigated. were inves- The electrontigated. sourceThe electron exhibited source good exhibited field emission good field characteristics emission characteristics with excellent with stability, excellent and thestability, lower limitand the of pressure lower limit measurement of pressure of themeasurement ionization gaugeof the withionization a CNT gauge cathode with was a −7 asCNT low cathode as ~3.0 ×was10 as lowPa in as nitrogen; ~3.0 × 10 this−7 Pa is in the nitrogen; first CNT this emitter-based is the first CNT ionization emitter-based gauge withionization a lower gauge limit ofwith pressure a lower measurements limit of pressure than its measurements hot cathode counterpart. than its hot The cathode gauge −1 sensitivitycounterpart. is anodeThe gauge voltage-dependent, sensitivity is anode and a voltage-dependent, highest value of ~0.15 and Pa a highestwas achieved value of at~0.15 an anodePa−1 was voltage achieved of ~100 at an V. anode In addition, voltage the of ~100 resulting V. Inionization addition, the gauge resulting with aioniza- CNT electrontion gauge source with had a anCNT extremely electron low source power had consumption an extremely of aboutlow power 9.5 mW, consumption which makes of it advantageous over hot cathode ionization gauges in space exploration.

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Author Contributions: Conceptualization, Y.W. and X.Z.; methodology, J.Z., J.W., H.Z. and D.L.; investigation, J.Z., J.W. and H.Z.; writing—original draft preparation, Y.W. and J.Z.; writing—review and editing, Y.W. and J.Z.; funding acquisition, Y.W., J.Z., D.L. and X.Z.; supervision, D.L. and X.Z. All authors have read and agreed to the published version of the manuscript. Funding: The work was funded by the National Natural Science Foundation of China (Grant No. 61671226, 61971133, 61627850), the National Key Research and Development Program of China (Grant No. 2018YFE0125500), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0095). Institutional Review Board Statement: The study did not involve humans or animals. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available from Y.W. upon reasonable request. Acknowledgments: The authors thank the National Natural Science Foundation of China (Grant No. 61671226, 61971133, 61627850), the National Key Research and Development Program of China (Grant No. 2018YFE0125500), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0095) for their financial support. Conflicts of Interest: The authors declare no conflict of interest.

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