Highly Crystalline Single-Walled Carbon Nanotube Field Emitters

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Highly Crystalline Single-Walled Carbon Nanotube Field Emitters: Energy Loss-Free High Current Output and Long Durability with High Power Norihiro Shimoi, Yoshinori Sato, and Kazuyuki Tohji ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00008 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Electronic Materials

Highly Crystalline Single-Walled Carbon Nanotube Field Emitters: Energy Loss-Free High Current Output and Long Durability with High Power Norihiro Shimoi†*, Yoshinori Sato†,‡*, Kazuyuki Tohji1 †

Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-ku, Sendai

980-8579, Japan ‡

Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University,

Asahi 3-1-1, Matsumoto 390-8621, Japan

Corresponding author:

*

Norihiro Shimoi (e-mail address: [email protected]) Yoshinori Sato (e-mail address: [email protected])

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Abstract:

Power consumption reduction and energy savings for field-emission (FE) electron sources for the loading of high current output, long durability, and high power are considerably challenging in the field of materials. Here we show a new approach using a simple structure comprising highly crystalline single-walled carbon nanotubes (SWCNTs) in the cathode for meeting these goals of power consumption and energy efficiency. Efficacy and applicability were successfully demonstrated by revealing the ideal FE properties of SWCNTs via the control of their crystallinity. The FE fluctuation and emission lifetime of highly crystalline SWCNTs exhibited good stability for greater than 1300 h at 30 mA/cm2 and durability with an FE high current density of 10 A/cm2 via the application of a direct current constant voltage in the cathodic planar field emitter. Moreover, field emitters using highly crystalline SWCNTs permitted the use of new vacuum power switches for the loading power operation of greater than 27 kW, with a high loading current without energy loss and a cooling system. Highly crystalline SWCNTs as a flat plane-emission device can serve as a technological breakthrough for realizing energy savings and a low-carbon society in daily life.

Keywords single-walled carbon nanotubes, highly crystallinity, field-emission electron sources, high current output, long durability


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Introduction Compared with thermionic sources, field emitters exhibit excellent advantages of low power consumption, high current density, and rapid response speed, as well as feasibility of device miniaturization. Notably, planar field emitters with a high current density and power as well as long durability have been attracting considerable attention as electron sources for flat-panel displays,1 backlights in liquid-crystal displays,2 deep-ultraviolet planar light emission from AlGaN/AlN,3 X-ray sources in X-ray computed tomography, and microwave amplifiers. In field emitters, a high electric field allows electrons to penetrate through surface potential barrier and into vacuum via the quantum mechanical tunneling effect. To locally generate such a high field at a low voltage, an emitter is constructed with a pointed field-enhancing geometrical structure. Spindt-type field emitters contain metallic microcones with the cone tip on the order of tens of nanometers,4 which are produced by using “top-down” one-micrometer-scale lithography and some intricate geometrical processes. However, the micrometer-scale lithography scale-up of the metal-tip field emitters up to larger planar dimensions is difficult. In addition, metal tips are limited because they are sensitive to structural degradation under high electric fields. In this context, lightweight, structurally stable carbon nanotubes (CNTs) possess a high aspect ratio (a large curvature at the tip) and excellent electrical conductivity with a high carrier mobility, which have emerged as promising alternatives to traditional metal emitters. Since the discovery5–7 and purification8,9 of CNTs, several researchers in universities and industries (e.g., Motorola, Samsung, and Sony) have extensively investigated nanotube field emitters.10–15 CNT-based planar field emitters can be prepared by two methods: aligned CNTs directly synthesized on a substrate by chemical vapor deposition (CVD),16,17 and CNTs dispersed in a solvent covered on a substrate.18,19 The latter method can be developed in industry without limitations of the field emitter size. However, the emitters prepared by these methods are limited because of their stability and lifetime to high current emission with high power, which have not been resolved because defective CVD-prepared CNTs are used and a large number of defects are introduced into nanotubes during the fabrication of CNT dispersions by using sonication to disperse CNTs in the solvent. ACS Paragon Plus Environment

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Single-walled CNTs (SWCNTs) with a sharpened tip and large aspect ratio (diameter: 0.8–3.0 nm, length: > 1.0 µm) are ideal field emitters that exhibit high current emissions in a low electric field compared with multi-walled CNTs. However, the defects of SWCNTs lead to increased resistance for inducing the deterioration of field-emission (FE) properties, and SWCNTs become short to vaporize carbon atoms by Joule heat. If SWCNTs comprise an ideal carbon network (i.e., high crystallinity) and the fabrication can be achieved without the introduction of defects in the nanotubes, a field emitter with a high current density under a low electric field as well as a long lifetime at high power can be fabricated. In this work, we prepared a planar SWCNT field emitter to improve the planar emission homogeneity with an extremely high current output, extremely long durability, and high-energy consumption efficiency by using highly crystalline SWCNTs and positioning horizontally aligned nanotubes in the field emitter. In addition, the efficacy and applicability were successfully determined by revealing the ideal FE properties of SWCNTs via the control of their crystallinity. The FE fluctuation and emission lifetime of highly crystalline SWCNTs exhibited good stability for greater than 1300 h at 30 mA/cm2 and durability with a high FE current density of 10 A/cm2, under a direct current (DC) constant voltage in the planar field emitter. Moreover, highly crystalline SWCNT field emitters were confirmed to enable new vacuum power switches with a rapid response for the operation of a loading power of greater than 27 kW, with a high loading current without energy loss and a cooling system. These novel SWCNT field emitters developed herein could provide a powerful platform for field emitters that can be applied in flat-panel displays, backlights in liquid crystal displays, deep-ultraviolet light emission, X-ray sources, vacuum power switches, and microwave amplifiers, and serve as a technological breakthrough for realizing energy savings and a low-carbon society in daily life.

Results and Discussion Fabrication of planar SWCNT field emitters To demonstrate the indispensability of using highly crystalline SWCNTs in FE devices, the effect of increasing the crystallinity of SWCNTs on their FE properties was investigated by comparing highly ACS Paragon Plus Environment

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crystalline SWCNTs subjected to high-temperature annealing in a high vacuum and unannealed SWCNTs with crystal defects. For this purpose, some specific SWCNTs were prepared for the comparison of the electrical stability and reliability due to their crystallinity. The main highly crystalline SWCNTs were synthesized by a method reported previously by our group20–22: highly crystalline SWCNTs synthesized by the arc discharge method were purified by air oxidation and acid treatment, as well as annealing at 1200 °C (see Supporting Information). In addition, we modified some commercially available SWCNTs (ASP-100F, Hanwha Chemical Co., Ltd., Korea), as references for FE characteristics. Two planar field emitters were prepared (i.e., a buckypaper) by the vacuum filtration of ethanol-dispersed SWCNTs without additives (Figure 1a) and a scratched thin film including SWCNTs with a tin-doped indium oxide (ITO) matrix covered on a Si wafer, and SWCNTs protruding from grooved nicks on the ITO film emitted electrons (Figure 1b). Please check the detailed fabrication procedure in the Methods section. The field emitter fabricated by this method exhibited a low threshold and loading voltage, high planar FE homogeneity, and high brightness efficiency.23,24 Each sample for FE measurements was numbered from #1 to #7 (Table 1). Samples #1 to #5 were derived from commercial SWCNTs, and samples #6 and #7 were derived from the highly crystalline SWCNTs prepared by our group, where each number refers to the annealing treatment and the thin film fabrication conditions for SWCNTs. The crystallization of SWCNTs was controlled by the annealing temperature and vacuum pressure. Samples #1 to #5 were prepared for the comparison of any crystallinity effect. Sample #2 was a buckypaper using commercial SWCNTs, sample #3 used the SWCNTs annealed at 800 °C under a vacuum of 10−1 Pa, and samples #4 and #5 used the SWCNTs annealed at 1200 °C under a vacuum of 10−5 Pa. The SWCNTs were annealed at 1200 °C under 10−5 Pa using a turbomolecular pump or at 800 °C under a low vacuum < 0.1 Pa using an oil-sealed rotary pump to obtain highly or moderately crystalline SWCNTs. Nanotubes in a bundle of highly crystalline SWCNTs (#6 or #7) were clearly observed in the transmission electron microscopy (TEM) image of each wall of nanotube bundles, which were denoted as straight lines (Figures 1c and 1d). However, without annealing, commercial SWCNTs (#1) exhibited a wavy ACS Paragon Plus Environment

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defective lattice of the SWCNTs in the TEM image (Figure 1e). After the high-temperature annealing of the SWCNTs under high vacuum, their crystallinity was considerably improved. In particular, this result confirmed that the annealed SWCNTs exhibit a more highly crystalline carbon network. Bundled nanotubes were observed for the original SWCNTs and commercial SWCNTs, with diameters of bundled nanotubes ranging from 5 to 60 nm. The Raman frequencies in the radial breathing mode (RBM) band of the original SWCNTs (#6 and #7) and commercial SWCNTs (#1 and #2) were ~180 and 177 cm−1, respectively (Figure S1). In contrast, Raman shifts in the RBM of the highly crystalline commercial SWCNTs (#4 and #5) were clearly observed at 166 and 177 cm−1, indicative of the presence of the rearranged carbon skeleton of the SWCNTs by annealing. The SWCNT diameter can be approximated using the following equation:25 ω (cm1) = 217.8/d (nm) + 15.7. The diameter of the original SWCNTs was 1.32 nm; this value is less than those of commercial SWCNTs (#4 and #5; 1.35–1.45 nm). The D band at ~1350 cm−1 was observed, and the G band at 1595 cm−1 also appeared (Figure S1). The intensity ratio of the D band to that of the G line (ID/IG) increased (Table 1). The increased intensity of the D band has been reported because of the increase in the number of not only the vacancy

defects in CNTs26 but also the sp3-hybridized covalent bonds

functionalized to the surface of CNTs.27,28 However, all samples exhibited a marginal difference in the ID/IG ratio, indicative of the difficulty in evaluating the defects by Raman spectctroscopy20. Thermogravimetric analysis was carried out in the atmosphere, which is a method for obtaining information about the degree of defects (Figure 1f). The original SWCNTs (#6 and #7) exhibited a weight loss at the highest starting oxidation temperature of ~600 °C for all samples, indicating that the original SWCNTs comprise a stable structure with an incredibly small number of defects. Moderately crystalline SWCNTs (#3) exhibited a weight loss of ~9.0 wt% at 400 °C, corresponding to the atmospheric oxidation of amorphous carbon. Amorphous carbon was synthesized via the carbonization of back-streaming oils from a rotary pump at ~800 °C on the nanotube surface. Meanwhile, we estimated the percentages of metallic nanotubes to the total number of nanotubes by comparing the integral intensities of the M11 optical interband transition and the S22 optical interband transition in the Vis–Near infrared (NIR) spectrum of samples (Figure S2).29 The ACS Paragon Plus Environment

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percentages of metallic nanotubes in the original SWCNTs (#6 and #7) and highly crystalline commercial SWCNTs (#4 and #5) were estimated to be 11 and 13%, respectively. Both samples exhibited enriched semiconducting nanotubes with almost the same concentration. In addition, the bands (S11 and S22) in the vis–NIR spectrum of the original SWCNTs were blue-shifted. By using small-diameter SWCNTs, a high optical transition energy is achieved.30 Therefore, Vis–NIR analysis also revealed that the diameters of the original SWCNTs (#6 and #7) are less than those of highly crystalline commercial SWCNTs (#4 and #5).

Field emission properties Figure 2a shows the relationship between the FE current density and electric field, which was converted to a logarithmic formula (Fowler–Nordheim plot) in Figure 2b. Overall, the turn-on field for buckypaper at the initial FE property was less than 1.5 V/µm; otherwise, the turn-on field for a thin film with an ITO matrix at the initial FE property was ~3.0 V/µm at 0.1 mA/cm2. As shown in the planar lighting homogeneity in Figure 2c, the dispersion density of SWCNTs in the films was similar for the samples regardless of the SWCNT crystallization, indicative of homogeneous probability densities of SWCNTs that induce FE with respect to each sample. On the basis of the above FE properties, the top shapes of the SWCNTs as field emitters and their dispersion densities were similar for the respective SWCNTs with high crystallinity or defects used as field emitters (i.e., either the buckypaper or an ITO-matrix-coated film). However, differences in electrical properties between the buckypaper and coated thin films shown in Figures 1a and 1b can be explained by the conductivity on the field emitter surfaces. Therefore, the variation in the SWCNT crystallinity is thought to lead to differences in the conductance of the SWCNTs. Czerw et al. and Albrecht et al. found by scanning tunneling microscopy (STM) that crystal defects in SWCNTs originate in a local energy band that affects electrical conductivity.31,32 From the STM analysis, an electrical conduction model for SWCNTs was developed on the basis of an inelastic electron tunneling model in which electrons pass through the energy


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band, which is assumed to be an energy barrier, generated by the energy gap originating from the crystal defects of the SWCNTs. The mechanism of FE from SWCNTs has been reported to follow the F–N tunneling model.4,33 The density of the electric potential concentrated at the tips of CNTs is increased to a certain extract depending on their length and diameter, and the distance between neighboring CNTs. Electrons tunnel through the area with a thin quantum energy barrier localized near the tips of CNTs to the outside.34 Table 2 summarizes the electrical properties of each sample based on the FE initial properties and conductivity. The turn-on, threshold field (where current density reaches 10 mA/cm2), and FE properties (J-E relationship) in Table 2 were important for obtaining information about the crystallization and alignment homogeneity of SWCNTs activated for FE. By using the applied voltage V – FE current I characteristics obtained from FE measurements, the total FE emission site area α and the field enhancement factor β, which is the ratio of the intensity of the electric field concentrated at the tips of the SWCNTs to the actual electric field, from FE properties can be obtained (see in the Supporting Information about the estimation of the α and β), which were presumed to depend on the dispersion homogeneity of the SWCNTs.4 We suppose that the high field enhancement factor β of coated films can be attributed to the following two factors: first, the diameter of nanotube bundles was thin according to our observations of the nanotube dispersion method, and secondly the electric field was effectively applied to the tips of SWCNT bundles exposed by scratching the ITO film. The resistivity of a coated film was greater than that of the buckypaper because the contact resistance between SWCNTs and the ITO matrix was greater than the resistance in the SWCNTs.

Durability of FE devices Figure 3a shows the FE lifetime of up to 1300 h for each sample. These cathodes assembled in each simple diode structure with conductive anodes were set at an applied DC constant voltage, and an initial DC current of 10 mA/cm2 was obtained under a vacuum of less than 10−5 Pa. The FE currents for samples #6 and #7 were drastically activated, and each of the FE current densities for both emitters increased to greater than ACS Paragon Plus Environment

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35 and 16 mA/cm2, respectively, possibly related to the flushing and cleaning of the SWCNT surfaces. This flushing allows for the facile emission of electrons from the SWCNT tips. This result implied that the current density of highly crystalline SWCNTs is not attenuated for greater than 1000 h with a continuous loading power; otherwise, the field emission loading half time of commercial SWCNTs was presumed to depend on the annealing treatment condition or the fabrication condition of FE devices. In fact, the field emission loading half time was found to depend on the annealing treatment, reaching between 450 and 1200 h, and commercial SWCNTs (#1) with defects exhibited a shorter emission lifetime of ~200 h. The durability of the DC loading field emission current of each of the SWCNTs used as field emitters depends on their crystallinity. Highly crystalline SWCNTs exhibited superior FE properties at a high current density with a DC constant voltage supply. An acceleration test is an available route for supplying DC constant voltages to FE devices. With the application of a DC voltage, highly crystalline SWCNT devices will exhibit an additionally long lifetime. A long emission lifetime with a high loading current density by using highly crystalline SWCNTs was successfully achieved. The current #6 and #7 samples did not exhibit a field emission loading half time, and highly crystalline SWCNTs demonstrate greater potential compared with the results of hitherto known CNT FE devices and related FE materials shown in Figure 3b.35–47 In addition, electron emission devices with field emitters were fabricated by a wet coating process employing highly crystalline SWCNTs and SWCNTs with crystal defects under similar conditions, with similar dispersions, and structures to produce each electrode. Significant differences in the FE properties of each set of SWCNTs were observed, which depended on their crystallinity. The crystal defects in the carbon network in the SWCNTs led to the deterioration of the electrical properties.

Stability and switching of devices with a high current FE The suitability of SWCNTs for fabricating an effective electron emission source with stable FE current depends on the conductivity of the FE cathode. The original highly crystalline SWCNT buckypaper ACS Paragon Plus Environment

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(#7) was selected. Figure 4 shows the time-dependent FE current stability and thermal changes, as well as switching of the high-current drop under a high voltage for a power diode device with a size of 1.0 × 1.0 cm2 using the FE properties of the highly crystalline SWCNTs (Figure 4a). The FE current from the simple diode structure at a high DC current density of 10.08 A was stable without current attenuation (Figure 4b). Moreover, a supplied power consumption of 27.5 kW was stable for greater than 100 min. On the other hand, the current of the emissions from the commercial highly crystalline buckypaper (#5) gradually decreased. To obtain a stable and large FE current, it is typically imperative to construct a triode structure with vertically and uniformly aligned carbon nanotubes on a cathode electrode. Here, however, a stable and large FE current was obtained without the above-mentioned requirement for inducing FE. In addition, the energy loss from both electrodes due to calorific heating with a large FE current was measured by attaching thermocouples on the back surface of both electrodes (Figure 4a). The results indicated that the FE cathode with the highly crystalline SWCNTs had almost no energy loss. On the other hand, the anode exhibited a large energy loss with considerable heating. The anode phosphor lost 70 % of its induced energy via the irradiation of visible light, and no method of reducing the energy loss has been developed to the best of our knowledge. The increased crystallinity of the SWCNTs may prevent their degradation under a large FE current. As compared with the solid state power device, an FE electron source with the highly crystalline SWCNTs will enable new applications with low power consumption without energy loss and cooling systems. As described previously, a cold-cathode electron-emission-like field emission at room temperature is a phenomenon that can be used in vacuum power switches. According to our results, electron emitters of the original highly crystalline SWCNT buckypaper demonstrate promise for this application. This advanced configuration uses one of the unique properties of the original highly crystalline SWCNTs. High-voltage (E = 2.75 kV) switching experiments were expected to be performed under an external load of greater than 200 MV in a commercial power circuit. The FE current and supplied voltage herein were monitored using a highvoltage divider. When an emission current of greater than 10 A is obtained from our result, the expected voltage drop at the load by a simple calculation must be greater than 2.75 kV. Figure 4c shows the result ACS Paragon Plus Environment

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obtained from switching 2.75 kV at room temperature using the original electron emitter #7. When the loading current on the cathode immediately decreased from 10 A to 0 A, no residual FE current by the delayed effect from SWCNTs was termed as the on- and off-voltage of the vacuum switch. Such on-off switching of a current of 10 A was performed without any instability. On the basis of the above results, the high-crystallinity SWCNTs were shown to be to demonstrate promising for their stable use as field emitters with a high-power output.

Conclusion In this study, the original highly crystalline SWCNTs were applied as field emitters in a planar electron emission device. The crystallinity of the SWCNTs is an essential requirement for the fabrication of highly stable, reliable electronic devices. For the first time, a homogeneous planar electron source of highly crystalline SWCNTs was successfully realized. The original SWCNTs with high crystallinity are expected to significantly improve FE properties compared with those of devices using conventional carbon nanotubes. The fabrication of ITO thin films including the highly crystalline SWCNTs enabled us to obtain an FE source that radiates ballistic electrons and realize a field emission current with a long lifetime, a high loading efficiency, and loss-free energy with high current density, as well as feasibility for the fabrication of a largescale planar emission source. In addition, a recent achievement of 2.75-kV vacuum switches based on diode electron emitters using a highly crystalline SWCNT buckypaper as an electrical application was demonstrated. The 2.75-kV vacuum power switch using field emitters of a SWCNT buckypaper was expected to exhibit a breakthrough power transmission efficiency at ~3 kV during room-temperature operation. This observation was related to a high level of an electron emission source and the insulation provided by vacuum. Based on the principles validated by this result, vacuum power switches operating at a high-power potential with high efficiency will be possible in the near future.

Methods ACS Paragon Plus Environment

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Fabrication of buckypaper cathode. SWCNTs (5 mg) were put into a 100-mL conical flask containing an ethanol (100 mL) and sonicated for 5 min. The suspension was filtered using a membrane filter. After filtering the suspension, a buckypaper with a diameter of 16 mm removed from the membrane was dried up at 60 °C for over 12 h under vacuum. The buckypaper was cut for each field emission test and mounted on a Si wafer substrate by a conductive Ag paste adhesive. The fabricated cathode was well dried at 300 °C under vacuum (10−1 Pa) to remove any organic solvent. The activation of SWCNT bundles was fluffed up by tracing linearly the surface of the buckypaper with a needle.

Homogeneous dispersion of aggregated SWCNTs in solution and fabriacation of cathode for a thin film including SWCNTs. We fabricated a cathode containing SWCNTs by a wet-coating process and succeeded in obtaining a stable FE current at a low driving voltage. The key point in this study is to control the homogeneity of the dispersed SWCNTs in the solvent. Although the highly crystalline SWCNTs are expected to emit electrons stably with a low loading power supply, a dispersion process for obtaining homogeneous highly crystalline SWCNTs in a solvent has not been established. Generally, a dispersant is used to disperse carbon materials homogeneously in a solvent. However, highly crystalline SWCNTs cannot bind to the functional groups of a dispersant because they have few dangling bonds in their carbon network to bind with other functional groups. In this study, we used a tin-doped indium oxide (ITO; Kojundo-Kagaku Co., Ltd., Japan) precursor solution to physically separate the SWCNT aggregates into thin SWCNT bundles with an average diameter of approximately 20 nm that were aggregated with a small numbers of nanotubes. The SWCNT bundles in the mixture were dispersed by a jet mill machine (Sugino Machine, Star Burst Mini) using 99% butyl acetate, ethyl cellulose (EC) (Wako, abt. 49%-ethoxy 100 cP) as a dispersant to prevent the reaggregation of SWCNTs, and 95% sodium linear alkyl benzenesulfonate (DBS; Wako Co., Ltd., Japan). The initial mixture included the ITO precursor as a conductive matrix, DBS, and EC at a ratio of 1 : 600 : 1 : 4.


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The jet mill machine produced a mixture of dispersed highly crystalline SWCNTs in the form of thin SWCNT bundles. An FE electrode with a weight density of approximately 0.1 mg/cm2 was stacked on a Si wafer substrate heated at approximately 120 °C, and the stacked film was sintered at 630 °C under a pressure of 0.1 Pa to remove any organic components and to produce an ITO film including the well-dispersed highly crystalline SWCNTs. After forming the ITO film, it was activated for FE by physically carving the coated film to expose the SWCNTs while applying a low turn-on field; the width of each nick was 50 µm and the film was patterned with straight nicks separated by an interval of 100 µm. The illustration about the fabrication process for cathode film showed in the Supporting Information (Figure S3). Figure S4 shows SEM images of a patterned ITO film with the dispersed SWCNTs subjected to FE activation. We can observe dispersed SWCNT bundles protruding from both sides of the grooves in the ITO film. The white circles in Figure S4a indicate the exposed SWCNT bundles protruding from the ITO film. Although the highly crystalline SWCNTs homogeneously dispersed in the ITO film were aligned in random directions (Figures S4b and 4c), they are expected to exhibit high stability and provide a large FE current.

FE current density measurement. Once the substrate including the SWCNTs was set to an ITO-sputtered pattern on a glass, the FE characteristics of the cathode were analyzed. The coating thin film with SWCNTs and the buckypaper as the cathode were measured. Analyses included electron FE I–E curves and their light emission homogeneity. The anode plate used to measure I–E curves and the homogeneity of planar emission was assembled by placing a green phosphor (ZnS:Cu,Al, Nichia Chemicals Co., Ltd., Japan) having a thickness of approximately 20 µm on a sputtered ITO film. The distance between the cathode surface and the anode electrode was fixed at 0.85 mm, and the gap was maintained constant using ceramic spacers. The dimensions of each emitter were 0.7 × 0.7 cm2. The sample was placed in a vacuum chamber at a vacuum of less than 10−5 Pa and connected to an electrical circuit (Figure S5) to measure the FE properties. The FE measurement system in Figure S5 was ACS Paragon Plus Environment

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constructed from a power supply unit comprising an amplifier (Model 10/10B, Trek Holding Co., Ltd., USA), a function generator (DF1906, NF Corporation, Japan), an oscilloscope as an FE current monitor, and a PC to store the FE measurement data. The voltage applied to the sample for FE measurements was designed at the periodic triangle wave with a frequency of 60 Hz. The FE current was measured as voltage data then converted by a resistor of 100 kΩ to 1 MΩ to prevent any signal noise during the measurement from being detected.

Durability measurement. The measurement system used for the durability (lifetime of field emission currents) measurement was the same setup as that used for the I–E measurement. The cathode assembled in each simple diode structure with conductive anodes was set at an applied DC constant voltage, and an initial DC current of 10 mA/cm2 was obtained under a vacuum of less than 10−5 Pa.

Current fluctuation and high-voltage switching measurements. The cathode using a buckypaper with highly crystalline SWCNTs on a Si wafer was grounded, while a Ta plate as an anode had a DC constant voltage applied to evaluate the time-dependent FE stability and thermal changes of the diode device. The dimensions of the buckypaper were 1.0 × 1.0 cm2. Acceleration voltage for the electrons irradiated on the Ta plate was considered the distance between the buckypaper and the Ta plate was maintained constant at 0.45 mm and a voltage of 2.75 kV was applied to the devices under a pressure of 10−5 Pa. We obtained a high and stable current of 10 A for greater than 100 min, demonstrating for the durability of the highly crystalline SWCNTs. In addition, the energy loss from both the cathode and anode due to calorific heating with a large FE current was measured on both the cathode and anode by attaching thermocouples on the back surface of both electrodes (Figure 4a). A measurement system used for performing high-voltage switching measurements had the same setup as that used for current fluctuation. The pulse frequency of the signal input to the abovementioned diode was


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fixed at 60 Hz and duty cycle of 50% for the vacuum power switching measurement under a pressure of 10−5 Pa. The high voltage supplied to the diode was switched by triggering the ON/OFF output of a function generator.

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Acknowledgements ACS Paragon Plus Environment

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This study was conducted as a project consigned by the New Energy and Industrial Technology Development Organization and supported by JSPS KAKENHI Grant Number JP26220104. Y.S. was supported by JSPS KAKENHI Grant Number JP15H04131 and JP18H04145. We sincerely thank Marina Wayama (Hitachi High-Technologies Corporation, Japan) for help in the TEM observation and the guidance received.

Associated content Supplementary information 

Preparation of original SWCNTs (#6, #7).



Characterization of SWCNTs.



Estimation of the parameters α and β.



Raman scattering spectra of all the SWCNT samples.



Vis-NIR spectra of the commercial SWCNTs with highly crystallinity (#4, #5) and our original SWCNTs with highly crystallinity (#6, #7).



SEM photographs of ITO film with dispersant SWCNTs after sintering.



Schematic diagram of FE measurement system with a diode sample.

Author information Corresponding authors E-mail: [email protected] (Norihiro Shimoi) E-mail: [email protected] (Yoshinori Sato)

ORCID Yoshinori Sato: 0000-0002-2862-7708

Author contributions ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N.S. conceived and designed the preparation of devices and their FE measurements. Y.S. prepared our original highly crystalline SWCNYs (#6, #7) and performed the structural characterization of all the samples using Raman scattering spectroscopy, UV-Vis-NIR spectroscopy, and thermogravimetric analysis. K.T. characterized the structure of SWCNTs using transmission electron microscopy. All the authors contributed to discussion and manuscript preparation.

Notes The authors declare no competing financial interests.

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TOC Graphic 298x104mm (96 x 96 DPI)


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Figure 1. Flat field emitters using SWCNTs. (a) SEM images of a magnified view of a buckypaper with highly crystalline SWCNTs (#7). Inset shows the surface morphology of a buckypaper. (b) SEM image of ITO film with dispersed SWCNTs (#6) after scratching. White arrows indicate SWCNTs from the edge of the grooved ITO film. The inset shows the surface of scratched grooves. (c) Low-magnification TEM image of highly crystalline SWCNTs (#7). (d) Magnified image of highly crystalline bundled SWCNTs in an area surrounded by a red line of (c). (e) High-magnification TEM image of commercial SWCNTs (#1). (f) TG curves of all the SWCNTs in air. 190x254mm (96 x 96 DPI)



Figure 2. Field-emission properties. (a) J–E curves and (b) F–N plots of all devices. (c) Planar lighting homogeneity of all devices at a shade brightness of 3.5–3.7 mA/cm2 by using a neutral density filter. 176x149mm (96 x 96 DPI)


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Figure 3. Durability of devices. (a) Durability at FE current density up to 1300 h with a DC constant voltage. The field emission loading half time is defined against the half of the initial field emission current density. (b) Plots of the field emission loading half time of all devices as a function of the loading FE current density. The numbers (30–42) in the plot show the reference number. 167x231mm (96 x 96 DPI)



Figure 4. Stability and switching of devices. (a) Schematic of a diode structure for the measurement of a high emission current and temperature on each electrode. Buckypaper #7 was used as the cathode. (b) Current fluctuations and temperature change on each electrode under a DC load of 2.75 kV for greater than 100 min. The current fluctuation of buckypaper #5 was plotted as a reference. (c) 2.75 kV switching at room temperature using the buckypaper #7. Spike noises from the FE current were caused by the capacitance from the diode structure. 160x210mm (96 x 96 DPI)


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Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Sample number #1 #2 #3 #4 #5

Specification of SWCNT

Anneal condition (anneal temp. and vacuum)

Commercial SWCNTsa)

No treatment

Commercial SWCNTs

800 ºC at 10-1 Pa vac.

Commercial SWCNTs

1200 ºC at 10-5 Pa vac.

Our original SWCNTs

1200 ºC at 10-5 Pa vac.

I D/I G ratio

Film coated with ITO 0.0097 (±0.0006) Buckypaper Film coated with ITO

0.0114 (±0.0008)

Film coated with ITO 0.0117 (±0.0005) Buckypaper Film coated with ITO

#6 #7

Matrix in thin film

0.0092 (±0.0004) Buckypaper

a) Commercial SWCNTs used as a field emitter was obtained from Hanwha-Chemicals Co. Ltd. It is suitable for synthesizing highly crystalline CNTs.


ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Sample number

Turn-on field (V/μ Threshold field (V/μ m) m)

Page 28 of 28

Electron emission site [α]

Field enhancement factor [β]

Resistivity / Ω cm

Area density (mg/cm2)

#1

3.1

5.7

9.42 x 10-16

6.09 x 105

6.99 x 10-2

0.039

#2

1.5

2.6

6.70 x 10-16

1.51 x 106

7.24 x 10-5

2.5

-16

5

-2

#3

2.7

5.1

2.08 x 10

8.92 x 10

3.89 x 10

0.032

#4

2.4

5.4

1.36 x 10-15

1.40 x 106

1.79 x 10-2

0.039

#5

0.8

1.4

1.19 x 10-15

2.74 x 106

6.00 x 10-8

2.5

#6

2.9

5.2

7.98 x 10-14

6.99 x 105

2.28 x 10-2

0.026

1.5

-15

6

-8

#7

0.9

4.81 x 10


1.96 x 10

5.36 x 10

2.5

Highly Crystalline Single-Walled Carbon Nanotube Field Emitters

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