Plannar light source using a phosphor screen with single-walled carbon nanotubes as field emitters Sharon Bahena-Garrido, Norihiro Shimoi, Daisuke Abe, Toshimasa Hojo, Yasumitsu Tanaka, and Kazuyuki Tohji Citation: Review of Scientific Instruments 85, 104704 (2014); doi: 10.1063/1.4895913 View online: http://dx.doi.org/10.1063/1.4895913 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Self-formation of highly aligned metallic, semiconducting and single chiral single-walled carbon nanotubes assemblies via a crystal template method Appl. Phys. Lett. 105, 093102 (2014); 10.1063/1.4895103 Engineering the color rendering index of phosphor-free InGaN/(Al)GaN nanowire white light emitting diodes grown by molecular beam epitaxy J. Vac. Sci. Technol. B 32, 02C113 (2014); 10.1116/1.4865914 Hydration forces between surfaces of surfactant coated single-walled carbon nanotubes J. Chem. Phys. 138, 114701 (2013); 10.1063/1.4793763 Field-emission light sources utilizing carbon nanotubes and composite phosphor made of Si O 2 nanospheres covered with Y 2 O 3 : Eu J. Vac. Sci. Technol. B 27, 757 (2009); 10.1116/1.3070654 Study of high-brightness flat-panel lighting source using carbon-nanotube cathode J. Vac. Sci. Technol. B 26, 106 (2008); 10.1116/1.2825145

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 104704 (2014)

Plannar light source using a phosphor screen with single-walled carbon nanotubes as field emitters Sharon Bahena-Garrido,1 Norihiro Shimoi,1,a) Daisuke Abe,2 Toshimasa Hojo,3 Yasumitsu Tanaka,1 and Kazuyuki Tohji1 1 Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan 2 DOWA Holdings Co. Ltd., 14-1 Sotokanda 4-Chome, Chiyoda-ku, Tokyo 101-0021, Japan 3 Graduate School of Engineering, Tohoku University, 6-6-06 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

(Received 10 July 2014; accepted 4 September 2014; published online 14 October 2014) We developed and successfully fabricated a plannar light source device using a phosphor screen with single-walled carbon nanotubes (SWCNTs) as field emitters in a simple diode structure composed of the cathode containing the highly purified and crystalline SWCNTs dispersed into an organic In2 O3 – SnO2 precursor solution and a non-ionic surfactant. The cathode was activated by scratching process with sandpaper to obtain a large field emission current with low power consumption. The nicks by scratching were treated with Fourier analysis to determine the periodicity of the surface morphology and designed with controlling the count number of sandpapers. The anode, on the other hand, was made with phosphor deliberately optimized by coverage of ITO nanoparticles and assembled together with the cathode by the new stable assembling process resulting to stand-alone flat plane-emission panel. The device in a diode structure has a low driving voltage and good brightness homogeneity in that plane. Furthermore, field emission current fluctuation, which is an important factor in comparing luminance devices too, has a good stability in a simple diode panel. The flat plane-emission device employing the highly purified and crystalline SWCNTs has the potential to provide a new approach to lighting in our life style. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4895913] I. INTRODUCTION

Alternative technologies and approaches for energy saving through low power consumption in our day to day activities have been extensively studied. Our tremendous research effort significantly contributes to achieve our goal for energy efficiency and sustainable energy. For example, the developed solid state lighting devices represented by lightemitting diodes (LED) have become now widely used as a luminescence system in our living and working habitat. Many luminescence devices emitting visible light from focused positional or line light sources easily yield shadows in a room. Thus, flat plane lighting devices, for example organic light emitting device (OLED), have started to be developed recently to illuminate a room homogeneously.1–4 Instead of light-emitting diodes, we have focused on a cathode luminescence system as a cathode ray tube, and attempted to use carbon nanotubes (CNTs) for a field emitter having a nanoscale needle shape with chemical stability, thermal conductivity, and mechanical strength.5–8 Many studies have attempted to employ CNTs as field emitters to obtain a low power driving voltage and stable electron emission. For example, fabrication methods for electron emission cathodes using vertically aligned CNTs synthesized by chemical vapour deposition (CVD) or laser abrasion fabrication,9–11 as well as silk screen printing with high viscosity submicron scale metal-particle paste employed to fabricate patterned field emitters, have a) Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0034-6748/2014/85(10)/104704/6/$30.00

been proposed.12–15 However, these approaches only helped average out the non-homogeneous electron emitter plane with large field emission fluctuations; because they failed to form a uniform thin film with homogeneous dispersion of highly crystallized CNTs. To obtain a homogeneous lighting plane with a stable field emission in a flat plane device, it was thought to be important to disperse and align vertically and uniformly CNTs in a highly stable vacuum at over 10−4 Pa.16–19 Shimoi et al. have successfully obtained an effective cathode by utilization of CNTs—having a stable field emission and low FE current fluctuation that relies on the ability to disperse CNTs uniformly in liquid media.20 In particular, highly crystalline single-walled nanotubes (SWCNTs) synthesized by arcing can be expected to emit electrons stably with a low turn-on and driving voltage in a vacuum chamber, yet their homogeneous dispersion has just recently been reported. Furthermore, a low viscosity solvent containing highly crystalline SWNTs dispersed into an In2 O3 – SnO2 (tin-doped indium oxide; ITO) precursor solution was selected as the conductive matrix material. A non-ionic dispersant was added to facilitate dispersion of the highly crystalline SWNTs, and the mixture with the ITO solution, SWCNTs, and the dispersant was agitated by ceramic beads as the milling medium. In this paper, we report a newer approach of power and energy efficiency with the use of a simple diode structure employing the highly crystalized SWCNTs in the cathode and optimized phosphor condition in the anode and together assembled as a stand-alone flat plane-emission device in a cavity

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with glass panels. Fabrication and optimization of the cathode and the anode as well as its panel assembling process and the properties of the device has been discussed in the paper. This device in a diode structure has a low driving voltage, good brightness homogeneity, and stable current fluctuation with DC voltage addition in that plane. Furthermore, brightness efficiency, which is an important factor in comparing luminance devices, achieved over 60 lm/W employing a green phosphor in a simple diode panel. The flat plane-emission device has the potential to provide a new approach to lighting in our everyday life style. II. EXPERIMENTAL A. Cathode fabrication employing purified and highly crystalline single-walled carbon nanotubes

Purified and highly crystalline SWCNTs synthesized by arcing were employed as the source material for field emitters. Some SWCNTs twisted as a bundle and were difficult to separate to single nanotubes. Highly crystalline ArcSWCNTs (ASP-100F form Hanwha Chemicals Co. Ltd., Korea) were checked for high crystallization by Raman-shift measurement. The Raman spectrum measured by blue laser beam (Blue) from Cobalt at 473 nm wavelength and 10 mW output power is shown in Fig. 1. Raman peaks of 1589 cm−1 from the G+ band mode and 1562 cm−1 from the G- band mode appeared and were indicated as nanotube structures. A Raman peak intensity of 1352 cm−1 from the D band mode was dependent on a lack of any carbon network on a nanotube. The ratio between the G+ band at 1589 cm−1 and the D band at 1352 cm−1 was 0.0096; this confirms that the SWNT has a highly crystalline carbon network. Shimoi et al. attempted to disperse SWCNTs by adding a low viscosity ITO precursor solvent and non-ionic dispersant, and by shaking with ceramic beads.20, 21 Purified and highly crystallized SWCNTs were expected to serve as a source material for a field emitter though the homogeneous dispersion. This is because highly crystalline SWCNTs have almost no defects in the carbon network on their surface, nor can they combine with another functional group of dispersants. It was

FIG. 1. Raman spectrum with G and D band modes of SWCNTs.

FIG. 2. TEM photograph of dispersed SWCNTs.

difficult for the SWCNTs aggregated as a secondary particle to separate uniformly into single nanotubes. Furthermore, though a thin film homogeneously dispersing highly crystalline SWCNTs for a field emission cathode plate is necessary, no method for synthesizing a thin film with SWCNTs for a field emitter exhibiting a good homogeneity of plane-light emission has been found previously in vacuum chamber. Cathode fabrication employing the highly crystalline SWCNTs has been described in detail with slight modifications.20 The physical properties for producing a mixture including highly dispersant SWCNTs are as follows. Butyl acetate 99%, ethyl cellulose (EC) (abt. 49% ethoxy 100 cP), and sodium linear-alkyl-benzenesulfonate 95% (DBS) were obtained from Wako Co. Ltd., Japan. The initial mixture was prepared by mixing the SWCNTs powder with the ITO precursor from Kojundo-Kagaku Co. Ltd., Japan and the DBS and the EC in a proportion of about 1:600:1:4. The last component, EC, was tried in different proportions. The resulting solvent mixture including SWCNTs was shaken with ZrO2 beads to disperse the aggregates, and the mixing process was repeated for a range of ZrO2 bead diameters. We succeeded in obtaining a mixture of SWCNTs dispersed as thin bundles, aggregated with a few nanotubes. This mixture was sprayed onto a silicon wafer substrate and sintered around 900 K in vacuum to remove organic components, and to form bridges to make a conductive ITO film. The dispersed SWCNTs are shown by transmission electron microscopy (TEM) in Fig. 2; this shows SWCNTs in the mixture to be dispersed homogeneously as thin bundles aggregated with a few nanotubes through the dispersing process. Furthermore, the ITO electrode to ground the ITO coating film with SWCNTs was sputtered on the glass substrate. In this study, the SWCNTs coating weights per unit area were controlled by the numbers of spray cycles to verify both the field emission property and brightness efficiency of a flatplane light emission panel. The sintered film was fabricated using an activation process without any standing or aligning treatment of CNTs in order to emit electrons easily at low voltage. After the coated substrate with SWCNTs as a cathode plate was sintered, the cathode was activated by scratching with a sandpaper to obtain large field emission current with a low voltage as shown in Fig. 3. The cathode was ready

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Front

Rear Lighng area

15mm 15mm

FIG. 5. Overview of a stand-alone flat plane-emission diode panel.

FIG. 3. SEM photograph of coating surface scratched by a sandpaper.

to be analyzed. The analyses included the electron field emission and brightness efficiency at low voltage. B. Anode optimization and panel assembly

1. Phosphor coating for anode and its optimization

Polyvinyl alcohol (PVA) (from Nippon Gohsei) powder was dissolved in distilled water (8 wt. %) by water bath at 80 ◦ C. Five grams of green phosphor (ZnS:Cu, Al) from Nichia Chemicals Co. Ltd. was added into the dissolved PVA and stirred for 2 h. Five percent by weight of indium titanium oxide solution (ITO) (from DOWA Company) was later added and stirred for another 1 h. The mixture of PVA, phosphor, and ITO was used to coat the ITO patterned glass plate with a size of 15 × 15 mm2 by a bar coater and dried at 55 ◦ C for 15 min. Removal of PVA from the coated glass plate was done by oven-drying at 450 ◦ C for 2 h using a sintering machine. Phosphor layer with ITO solution proved effective in providing conductivity while preventing phosphor powders charging and dropping from the glass plate without aluminum coverage onto the coated phosphors. Figure 4 shows the surface of phosphors surrounded homogeneously by ITO nanomaterials. Moreover, the addition of ITO into the mixture made the property of the anode more stable and homogenous and contributed to the homogeneity and brightness of the whole diode device.

high bright light with high electron acceleration voltage and small field emission current with low power consumption at under 0.1 W. These assemblies were sintered to join at around 800 K for each panel using the glass paste Vaneetect II from Hitachi Chemicals Co. Ltd. Figure 5 shows an assembled flat plane panel employing cathode and anode electrodes after sintering. Specific sealant epoxy resin was being used for the binding of the glass plates of the cathode, the anode, and spacers to maintain the vacuum condition all throughout. The flat plane panel was evacuated by a vacuum pump without heating. However, electrical loading between the cathode and anode was maintained during vacuuming at a low pressure of 10−3 Pa—this consisted of a triangle wave of 4 kV voltage peak to peak for a few hours. This loading process made it possible to obtain a high vacuum of around 10−4 Pa in the panel due to the effective desorption of gas from the surface of the SWCNTs, the ITO matrix in the cathode and the phosphors in the anode. In this way, we could obtain a flat plane panel with high vacuum stability after chipping off the panel. The field emission current measurement system in Fig. 6 with field emission and lighting properties was constructed with a power supply unit having an amplifier, a function generator, an oscilloscope as a field emission current monitor, and a PC for data storage. The voltage for the sample was designed for a periodic voltage at 60 Hz of the triangle wave for field emission properties, the DC voltage, on the other hand, was added for measurement of the stability of field emission current fluctuation. The field emission current was converted to bias voltage data by a resistor at around 1 M for the purpose of preventing signal noise from any leaking current appearing in the measurement applications.

2. Panel assembly

A sintered cathode plate with SWCNTs and phosphor anode electrode were assembled with a 2.0 mm glass plate as a spacer. The gap of spacers was designed to obtain efficiently

FIG. 4. SEM photograph of surface of green phosphors. (a) Not coverage phosphors surface by ITO nanoparticles. (b) Phosphors surface covered by ITO nanoparticles.

FIG. 6. Schematic measurement circuit for field emission properties.

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FIG. 7. Overview of three-dimensional scratched surface morphology.

The brightness property was measured by a luminance meter (CS-2000) from KONIKA-MINOLTA Co. Ltd.

III. RESULTS AND DISCUSSION

After sintering a coated film with a mixture dispersing SWCNTs at 600 K, the sintered film was fabricated using sandpaper to scratch to emit electrons easily with a low voltage, as shown in Fig. 3 by scanning electron microscopy (SEM; from Hitachi Technologies Co. Ltd.). The SEM image shows the surface on coating layer, and the nicks shown in Fig. 3 play a role to make dispersant SWCNTs expose homogeneously in a planar. The SWCNTs were protruded from the groove face of the ITO film, and the distance between neighboring SWCNTs was controlled availably by the contents of SWCNTs in the mixture.20 Figure 7 shows an image of a three-dimensional nicked surface morphology by a probe type step profiler from Taylor-Hobson Co. Ltd. The average periodicity of scratched nicks was designed by a count number of a sandpaper. The surface roughness was analyzed quantitatively with the power spectrum obtained by the Fouriertransform method. Figure 8 shows the distribution spectrum of the wave number based on the spatial frequency of rough-

FIG. 9. Current density-electrical field characteristics of scratched cathode sample designed the scratched nicks by a Fourier-transform analysis and as coat sample.

ness obtained from scanning the surface roughness morphology of the scratched film in Fig. 3 and not scratched film for a comparison by the Fourier-transform method. The spectrum in Fig. 8(b) indicates that there are some periodical nicks of the surface morphology contained in the film. Some peaks in Fig. 8(b) indicated by green triangles express the scratched surface periodicity of the film. Figure 9 shows the current density-electrical field characteristics of the flat-plane diode panel employing the scratched cathode, as shown in Fig. 2. The turn-on field, which divided the bias by the separation between the emitters and the anode, was around 1.2 V/μm to get an emission current density of 0.1 μA/cm2 in Fig. 8. Figure 9 shows the distribution of electron emission site α 22–25 calculated from the current density-electrical field characteristics, which is an important factor in making the driving voltage steeper in current-voltage characteristics. The electron emission site α is analyzed by an F-N plot distribution transformed from field emission property in Fig. 8.26 According to the wave number from cathodes scratched by sandpapers having different count number

FIG. 8. Comparison of surface profile and the power spectrum against wave number by Fourier-transform method. (a) Only coated surface results without scratching. (b) Scratched surface results. The peaks indicated by green inverted triangles on the spectrum are revealed from periodical nicks by scratching.

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FIG. 12. Current fluctuation with 4 kVDC loading during 150 min.

FIG. 10. Dependent between electron emission site and wave number by Fourier analysis of the scratched surface morphology.

with surface roughness Ra, this shows that the homogeneity of plane light emission is improved as the value of the electron emission site and wave number is increased in Fig. 10. In this way, we could obtain good homogeneous plane lighting in that flat-plane panel employing an optimized cathode with the scratched surface morphology. The flat plane-emission from the simple diode panel generated a stable and homogeneous brightness of over 30 000 cd/m2 in the sampling area as shown in Fig. 11 without current fluctuation in the plane. The current fluctuation of emissions from the flat plane panel with an added voltage of 4 kV DC was stable during 150 min as shown in Fig. 12. This degree of electron emission stability is excellent for highly purified and crystallized SWCNTs in a diode panel. In particular, highly crystallized SWCNTs have no defects in their carbon network,27, 28 and the perfect crystallization prevents SWCNTs from breaking down with a large field emission current. Normally, it was necessary for a stable field emission current by CNTs with low power consumption to construct the triode structure, to synthesize carbon nanotube designed with vertical alignment and standing height uniformly on an electrode by CVD with high temperature and to obtain a high vacuum atmosphere around over 10−6 Pa in a panel.29 In this study, we could obtain a stable large field emission current in a simple diode panel for the first time in a world. We were able to achieve a high brightness efficiency of around 60 lm/W with a green phosphor employed in a flatplane emission panel at under 0.1 W. Lighting device with tri-

ode electrodes using carbon nanotubes as field emitters were already obtained the brightness efficiency results of around 60 lm/W;17, 30, 31 however, the plannar lighting device in this study had a simple diode structure with a brightness efficiency and stable emission current. We could establish an excellent device for a visible light emission with low power consumption and high drive stability. The control of SWCNTs dispersion, coating, vacuuming, and scratching processes when producing the flat-plane simple structured panel is important in order to obtain good lighting properties with low power consumption. IV. CONCLUSION

We developed a new coated CNTs cathode for a field emitter showing low emission fluctuation and high brightness efficiency. The mixture in our study was made using a low viscosity solvent with highly purified and crystallized SWCNTs dispersed in an organic ITO solution. The film was fabricated as a field emitter with low turn-on field using a simple scratching process. The scratching process is important to for a cathode device employing our SWCNTs to obtain a large field emission current with a low power consumption. The analysis of the nicks by a Fourier-transform method was available for designing analysis of the nicks periodicity by scratching. After assembling the developed cathode and anode electrodes into a cavity separated by glass plates, a stand-alone flat plane-emission panel could be fabricated with stable and high vacuum of under 10−4 Pa. The device in a diode structure has a low driving voltage and good brightness homogeneity in that plane. Furthermore, field emission current fluctuation, which is an important factor in comparing luminance devices too, has a good stability in a simple diode panel. The flat plane-emission device employing the highly purified and crystalline SWCNTs has the potential to provide a new approach to lighting in our life style. ACKNOWLEDGMENTS

This work was partially supported by DOWA Holdings Co. Ltd. and the authors appreciate the helpful discussion and advice of co-researchers from DOWA. 1 S.

FIG. 11. Plane-lighting homogeneity image of a stand-alone panel through a neutral density filter.

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Plannar light source using a phosphor screen with single-walled carbon nanotubes as field emitters.

We developed and successfully fabricated a plannar light source device using a phosphor screen with single-walled carbon nanotubes (SWCNTs) as field e...
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