Combinatorial Evaluation for Field Emission Properties of Carbon

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J. Phys. Chem. C 2008, 112, 17974–17982

Combinatorial Evaluation for Field Emission Properties of Carbon Nanotubes Yosuke Shiratori, Hisashi Sugime, and Suguru Noda* Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: August 8, 2008; ReVised Manuscript ReceiVed: September 11, 2008

Field emission properties of carbon nanotubes (CNTs) were comparatively evaluated by using combinatorial CNT libraries. The libraries were prepared by combinatorial masked deposition of a Co catalytic layer on Al2O3/Si substrates and subsequent CNT growth by chemical vapor deposition from ethanol. Each library reproduced various types of single- and multiwalled carbon nanotubes with different morphologies and a variety of field emission properties. Combinatorial evaluations immediately identified the CNTs preferable as field emitters. The results obtained from individual field emission evaluations for samples with a constant nominal Co thickness agreed well with the results obtained from comparative evaluations for combinatorial CNT libraries. The results revealed that protrusive single-walled carbon nanotubes with a moderate interspacing showed the best field emission properties. 1. Introduction Carbon nanotubes (CNTs) are attractive as candidates for electron sources in field emission displays (FEDs) and lighting devices. Implementation of single- and multiwalled CNTs (SWCNTs and MWCNTs, respectively) on FED microcathodes1,2 and backlight units for liquid crystal displays3,4 are good examples of studies for application. Although MWCNTs have rather large tube diameters (typically 5-100 nm), they have a thermal stability suitable for field emitters.5 Recently, doublewalled CNTs (DWCNTs)6 or thin-walled CNTs (TWCNTs) are regarded as ideal emitters because they not only have a large tip curvature (diameter range of 2-3 nm) but also can withstand emission current up to 1.4 µA,7 as required for electron source applications. In this work, we focused on SWCNTs for application as coldcathodes. SWCNTs have the smallest diameters among all types of CNTs, and their current durability is believed to be smaller than that of DWCNTs.7 The large field-amplification effect due to their very small tip diameters is ideal for reducing the driving voltage of FE devices. In this context, we aimed at controlling the emitter morphologies in order to increase the number of emission sites (i.e., decrease the emission current per SWCNT emitter). For direct and selective implementation of SWCNT emitters on microcathodes, both a low-temperature or a shorttime chemical vapor deposition (CVD) process and protrusive emitter morphologies with a moderate interspacing are desired.8 Therefore, to identify preferable emitters and to understand the mechanisms of field emission itself, evaluations of various types of emitters prepared through a variety of processes are necessary.9 However, the morphologies of SWCNTs vary widely, depending on the catalyst and buffer layer material, thickness, process temperatures, pressures, and times, and therefore, comprehensive substrate-by-substrate evaluation of the emitting performance of such SWCNTs is not practical. The combinatorial masked deposition (CMD) method,10,11 in which a slit (CMD slit) placed above a substrate dilutes a sputtered metal flux, can reproduce various combinations of * Corresponding author. E-mail: [email protected]. Phone/ Fax: +81-3-5841-7332.

thicknesses of catalyst and buffer layers on a single substrate. CVD on such a catalyst library then provides a library of CNTs. In recent studies, a condition of rapid SWCNT growth was specified;12 mechanisms of CNT-growth were modeled based on the combinatorial investigation,13,14 and regions of a selective SWCNT growth in Co-Mo libraries were specified by the CMD method.15 Based on these studies, various types of CNTs and their morphologies can therefore appear in a single library depending on the nominal catalyst thickness, and thus various types of physical properties can be examined in a single library. Here, field emission (FE) evaluation was directly performed for CNT libraries, which were set as cathode substrates in parallel to an anode electrode. First, several CNT libraries on Si substrates with either flat or textured surfaces were prepared using the CVD method as reported in our previous work.9 Then, preferable field emitters were identified from combinatorial FE evaluation. Finally, the findings obtained from the combinatorial evaluation were compared with the evaluation results based on substrate-by-substrate measurements for individual uniform samples with a constant nominal catalyst thickness. 2. Experimental Section The n-type silicon wafers (resistivity: ca. 1 Ω · cm) were used as substrates. In addition to flat Si(100) substrates, textured Si substrates were also prepared by chemical etching in hydrazine hydrate at 80 °C. Si pyramids with (111) side faces were formed on the (100) surface because Si(111) surfaces are more resistant to etching by a strong base. After removal of an oxidized layer on the flat and textured substrates by hydrofluoric acid, first an Al2O3-buffer layer (catalyst supporting layer) and then a Co catalyst layer were deposited uniformly on the substrates by using an RF-magnetron sputtering method. In some samples, catalyst libraries with an exponential thickness profile of a Co layer were formed by the CMD method. After deposition of the Al2O3 and Co layers, the oxidized top surface was reduced at 785 °C for 10 min under H2(5 vol %)/Ar gas flow, and then CVD was performed (also at 785 °C) for short times (between 13-60 s) by introducing ethanol at 4 kPa in a hot-wall CVD chamber consisting of a quartz glass tube heated by a furnace. Tables 1 and 2 summarize the CVD conditions for combinatorial

10.1021/jp807078h CCC: $40.75  2008 American Chemical Society Published on Web 10/23/2008

Field Emission Properties of Carbon Nanotubes

J. Phys. Chem. C, Vol. 112, No. 46, 2008 17975

TABLE 1: CVD Conditions at 785°C on Flat and Textured Al2O3(10 nm)/Si Substrates with an Exponentially Profiled Co-Layer CNT Library

type of substrate

ethanol pressure (kPa)

growth time (s)

1 2 3 4

flat flat flat textured

4.0 6.7 2.7 4.0

30 30 13 30

CNT samples with a gradient nominal Co thickness (hereafter called combinatorial CNT libraries) and for individual CNT samples with a constant nominal Co thickness (hereafter called uniform CNT samples), respectively. Sample surfaces were observed by using a Hitachi S-4700 field emission scanning electron microscope (SEM). The quality of CNTs at the molecular level was evaluated using Raman scattering profiles recorded by a Chromex 501is spectrometer equipped with an Andor Technology DV401-FI CCD system under a backscattering configuration. Samples were excited with an Ar+ laser radiation (488.0 nm) focused on sample surfaces by a Seki Technotron optical unit and objectives with a magnification of 50 and a numerical aperture of 0.80. I-V characteristics under a diode configuration were measured at 1 × 10-5 Pa, where the bottom surface of the cathodes was grounded. To evaluate the cathode luminescence for both CNT libraries and uniform CNT samples, pulsed voltages (100 Hz, 1/2 duty) were applied to an anode electrode. Phosphor layers on indium-tin-oxide (ITO) glass substrates were prepared by screen-printing of binder-ZnO:Zn powder composites followed by a binder burn-out process in air at 515 °C for 10 min. An ITO glass was used as an anode, and 150 µm thick glass slides located above a CNT film were used as spacers. A Keithley 2410 high-voltage source meter was used as a voltage source and ammeter. I-V characterizations for uniform CNT samples were remote-controlled by LabTracer2 software and repeated eight times for each sample. Measurement areas were approximately 0.9 cm2, and I-V data were recorded every 30 ms in a voltage ramp of 200 V/s and were automatically loaded into a personal computer. 3. Results and Discussion 3.1. Characterization of CNT Libraries on Flat Substrates. Three CNT libraries were prepared on flat Si substrates. Table 1 summarizes the sample preparation conditions for these libraries. Figure 1 shows an image of Library 1 (single image at the top) with the Co thickness profile (white curve) where the horizontal axis is the distance x from the CMD slit, and the vertical axis is the nominal Co thickness (tCo). Also shown are SEM images (middle two rows of images) and Raman spectra (lower two rows) observed at representative x positions. At representative positions, one can see different types of CNTs. When we take a look at radial breathing modes (RBMs) and degree of splitting of the G band,16 the recorded Raman spectra indicate (a) the CNTs consisting of MWCNTs, (b) a mixture of MWCNTs and SWCNTs, and (c,d) larger content rates of SWCNTs. At x ) 3 mm, the diameters of CNTs are 20-30 nm (Figure 1a), and these CNTs do not grow from coarse particles (∼50 nm). To clarify the effect of packing density of CNTs on FE properties, Library 2 (Figure 2) was prepared with a higher source-gas pressure of 6.7 kPa. Compared with Library 1, Library 2 shows a wider dense-region, shorter protrusive MWCNTs (Figure 2a), a shorter SWCNT-grass adjacent to the

dense region (Figure 2d), and smaller intensity ratios of the G and D bands (IG/ID) in the obtained Raman spectra (Figure 2 bottom). Note that a higher source-gas pressure promotes growth of SWCNTs at around x ) 9 mm, at which the thickness of the SWCNT film (H) is 15.9 µm after the CVD for 30 s. To compare FE properties of MWCNTs and SWCNTs, Library 3 (Figure 3) was prepared. All CNTs on this library are grass-type because of the short growth time (13 s). The transition point from MWCNTs to SWCNTs occurs at 5 mm < x < 7 mm, corresponding to a nominal tCo of 2.9-1.4 nm. Growth mechanisms of CNTs in Libraries 1-3 can be explained by a model for the Co/SiO2 system as follows.13,14 As-deposited Co forms discontinuous and/or continuous films depending on tCo. During the reduction process, surface energy drives discontinuous and continuous layers to respectively form particles around 5 nm or smaller by surface diffusion of atoms and form particles around 10 nm or larger by deformation of continuous films. Consequently, SWCNTs and MWCNTs grow from specific particles whose sizes match the diameters of specific CNTs. The Co/Al2O3 system evaluated in our current study shows the same tendency of CNT structure on catalyst thickness as a Co/SiO2 system in this model. In the present study, Al2O3 was used as a buffer layer material, which increases the optimal tCo. A cause of this increment of an optimal tCo will be discussed elsewhere. Figure 4 shows Co particles formed on the Al2O3 layer by annealing at 785 °C for 10 min under H2 (5 vol%)/Ar gas flow. A Co film thicker than 6 nm (Figure 4a,b) has a mixture of coarse (>50 nm), medium (10-20 nm), and finer particles (difficult to specify their sizes because of restriction of the present microscopic resolution). As shown in Figures 1a, 2a, and 3a, particles larger than 50 nm do not catalyze CNT growth. As a result, sparse vertically aligned (VA)-MWCNTs are formed at x < 3 mm after CVD (Figures 1a, 2a, and 3a). At x ) 4 and 5 mm, which correspond to tCo of 4 and 3 nm, formation of particles with diameters of 10-40 nm is evident after the reduction process (Figure 4c,d). MWCNTs densely grow in the region x ) 4 to 5 mm. Particles with diameters of 10-20 nm are still evident at x ) 6 and 7 mm (Figure 4e,f, respectively); however, determining the particle sizes in these regions is difficult because of the resolution of the microscope used in this study. Raman spectra for Library 3 (Figure 3) reveal that a Co film less than 1.4 nm thick yields Co fine particles, which are catalysts of SWCNTs under the present experimental conditions (Figure 3c). When either the growth time or source gas pressure is increased, a thick, dense VA region is formed (Figures 1 and 2), resulting in a degradation in quality of SWCNTs (compare Figures 1b and 2c with Figure 3c). 3.2. Combinatorial FE Evaluation. Combinatorial FE evaluation was performed for Libraries 1-3. Figure 5 shows images of these libraries and their luminescence images from the rear surface of the anode at 4 and 5 V/µm (pulse mode). In the images, SW represents the region for SWCNT-type Raman profiles, and Roman numerals in the images at 5 V/µm indicate the FE-type (defined in section 3.4). An exponential curve on each graphic in Figure 5 indicates the tCo profile. Only specific x positions emit electrons, and bright spots correspond to protrusive emitter morphologies. In Library 1 (Figure 5a), even MWCNTs at x ) 2-3 mm exhibit a good FE property because of the lessening of the fieldscreening effect. As mentioned in section 3.1, protrusive MWCNTs at x < 3 mm (Figure 1a) originate from a mixture of self-organized coarse and fine catalyst particles (Figure 4a,b). Dense equipotential planes around emitters correspond to a field

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TABLE 2: CVD Conditions at 785 °C on Al2O3(10 nm)/Si Substrates with a Constant Co-Thickness (tCo) under an Ethanol Gas Pressure of 4.0 kPa and Turn-On Electric Fields to Extract an Emission Current of 1 µA/cm2 (Eto), Current Density at 4 V/µm (J4.0), and Field Enhancement Factor (β) Obtained for Samples A-F uniform CNT sample

type of Raman profile

morphology of CNTs

type of substrate

tCo (nm)a

A B C D E F

MWc MW SWc SW SW SW

VAd VA VA grass short,sparse grass

flat flat flat flat flat textured

15.0 4.9 1.2 1.2 0.3 1.4

a

growth time (s)

CNT film thickness H (µm)

G/Dratiob

Eto (V/µm)

J4.0 (µA/cm2)

17 30 60 60 45 30

2.3 3.3 3.0 7 mm) than for the MWCNT region (x < 5 mm). This result shows that SWCNT bundles at x > 7 mm with a larger tip curvature induce a larger field enhancement compared with results of MWCNT emitters at x < 5 mm. As revealed for Library 1, SWCNTs with a length of a few hundred nanometers do not act as field emitters under the present measurement configuration. 3.3. Effect of Substrate Texturing. To demonstrate the importance of moderate emitter spacing, Library 4 consisting of grass CNTs was prepared on a textured substrate. Figure 6 shows a photo of Library 4, SEM images, and Raman spectra observed at representative x positions. Because the sputtered Co flux is introduced from the CMD slit at the left, Co layers are formed on the left side-walls and tips of Si pyramids, and thus CNTs are mainly observed. CNTs at the tips protrude because of the macroscopic pyramid-type structures. Based on our previous work, which revealed a remarkable improvement in FE properties in comparison with CNTs on flat substrates,9 we believe that CNTs at the Si tips on a textured substrate can selectively be emission sites. Based on the Raman spectroscopy,

in which an incident laser is focused on a tip of a Si pyramid, the transition point from MWCNTs to SWCNTs occurs at 5 mm < x < 7 mm, which corresponded to tCo between 2.9 and 1.4 nm, respectively. This value of tCo for the transition point is the same as that for Library 3. Figure 7 shows the results of the combinatorial FE evaluation for Library 4 at 3 and 4 V/µm (note that the applied fields for Libraries 1-3 shown in Figure 5 are 4 and 5 V/µm). For Library 4, the SWCNT region (x > 7 mm) shows a much better FE property in comparison with the MWCNT region (x < 5 mm). The difference in brightness between the MWCNT and the SWCNT regions is clearer than that in Library 3, showing that SWCNT bundles with a moderate l and h induce an effective field enhancement. Similar to Libraries 1 and 3, Library 4 shows that h larger than 0.5 µm from a flat surface is necessary for field enhancement under the present measurement configuration. SWCNT bundles with a length of about 0.5 µm on Si tips (at x ) 11 mm in Library 4) strongly enhance an applied electric field, indicating that an effective field enhancement is induced by the presence of a macroscopic texturing. 3.4. Comparison of Combinatorial FE Evaluations with Results for Uniform CNT Samples. To demonstrate that the combinatorial FE evaluation is a quick and reproducible method to search for preferable CNT emitters, substrate-by-substrate FE evaluations were also done for uniform CNT samples. Table 2 summarizes the preparation conditions for the following six samples on individual substrates with uniform catalysts: (A) protrusive VA-MWCNTs, (B) VA-MWCNTs, (C) dense VA-


Shiratori et al.

Figure 3. Library 3 photo with the nominal Co thickness (tCo) profile (top), SEM images of the top surfaces and of the bases of CNTs (middle) and Raman spectra in the regions of RBM and D and G modes (bottom) for the representative positions (a-d) in the library. x is the distance from the CMD stlit. H is the thickness of the obtained CNT films.

Figure 4. SEM micrographs of Co particles formed on Al2O3(10 nm)/Si substrates during the reduction process at 785 °C under H2/Ar gas atmosphere. Distance from the CMD slit (x) and nominal Co thickness (tCo) are indicated above images.

SWCNTs, (D) SWCNT-grass, (E) short sparse SWCNTs, and (F) density-controlled SWCNTs. Figure 8 shows SEM images of the top surface and base of the obtained CNT emitters (A–F), Raman spectra, and luminescence images from the rear surface

of the anode substrate. Figure 9 shows the FE characteristics for these samples, and Table 2 lists the measured FE parameters. On the basis of the results shown in Figures 8 and 9, the FE properties can be categorized into three types. In samples B, C,



Figure 5. Results of combinatorial FE evaluations for Libraries 1-3 (a-c). Photographic images of luminescence from the rear surface of the anode at 4 and 5 V/µm (pulse mode) superimposed with photos of each library and an exponential profile of the nominal Co thickness (tCo) as a function of the distance from the CMD slit (x). SW indicates the region for SWCNT-type Raman profiles, and Roman numerals at 5 V/µm indicate FE types as defined in section 3.4.

and E, dark areas are dominant in the luminescence images. These three samples are therefore categorized as type I. In sample B, CNTs consist of MWCNTs without remarkable protrusions at the film surface. In samples C and E, SWCNTs with heights of approximately 0.5 µm are evident on the surfaces. On the basis of the combinatorial FE evaluation (section 3.2), the emitter morphologies in these three samples are not suitable for cold-cathode applications because of the field-screening effect. For samples B, C, and E, the achieved current densities at 4 V/µm (J4.0) are 26, 229, and 5 µA/cm2, respectively, and the field-enhancement factors (β) are 1.5 × 105, 1.0 × 105, and 1.6 × 105 cm-1 (Table 2), respectively. The J4.0 for sample C is the highest among these three samples and corresponds to the larger number of bright spots and higher brightness (Figure 8). In contrast, sample C has the smallest β, which is attributed to the large field-screening effect. The interspacing of CNTs in sample C is smaller than that of either

sample B or sample E. The increase in the number of emission sites in sample C in comparison with samples B and E might be due to edges of VA-SWCNTs at defects observed with the naked eye, inducing FE from ensembles of edge emitters. The x positions corresponding to the morphologies in samples B, C, and E are indicated as “I” in Libraries 1-3 in the right column of Figure 5. The luminescence images for these three samples (right row of images in Figure 5) are similar (i.e., relatively dark) to those obtained for the uniform samples B, C, and E. Compared with type I samples (B, C, and E), samples A and D show significantly more intense bright spots (right row of images in Figure 8). SEM images and Raman spectra (Figure 8) revealed that the film on sample A consists of sparse MWCNTs and that on sample D consists of a SWCNT-grass. We categorize these two samples into type II, in which protrusive bundles have h up to 1 µm and l of approximately 1


Shiratori et al.

Figure 6. Library 4 photo with the nominal Co thickness (tCo) profile (top), SEM images of the top surfaces and of the bases of CNTs (middle) and Raman spectra in the regions of RBM and D and G modes (bottom) for the representative positions (a-d) in the library. x is the distance from the CMD slit. H is the thickness of the obtained CNT films.

Figure 7. Results of combinatorial FE evaluations for Library 4. Photographic images of luminescence from the rear surface of the anode at 3 and 4 V/µm (pulse mode) superimposed with photos of each library and an exponential profile of the nominal Co thickness (tCo) as a function of the distance from the CMD slit (x). SW indicates the region for SWCNT-type Raman profiles, and Roman numerals at 5 V/µm indicate FE types as defined in section 3.4.

µm (sample A) and 0.5 µm (sample D). For sample A, J4.0 ) 145; for sample D, J4.0 ) 168 µA/cm2, and for both samples, β ) 0.9 × 105 cm-1 (Table 2). Although a CNT bundle in sample A (20-30 nm) is thicker than that in sample D (∼10 nm) as shown in the enlarged inset in the SEM pictures (Figure 8), the properties obtained for both samples are almost the same. The only notable differences between samples A and D are the smaller total area of bright spots and poorer luminescence homogeneity for sample D. Therefore, the total FE characteristics for sample A should be more stable than those for sample D. The Fowler-Nordheim (F-N) plots in repetitive measurement runs for sample D (Figure 9b, green) differ because of degradation in FE properties. Protrusive MWCNTs in sample A have uniform interspacing of about 1 µm, resulting in an

averaged β value as large as that for sample D, in which a considerable field-screening effect exists among SWCNT bundles (l ≈ h/2). Formation of MWCNT bundles is not likely to occur, and therefore tall MWCNTs with a l ≈ h morphology have a β comparable with that for protrusive bundles with a l ≈ h/2 morphology in SWCNT-grass. The x positions corresponding to the morphologies in samples A and D are indicated as “II” in the right column of Figure 5. The luminescence images in region II (bright spots) correspond with those obtained for samples A and D (Figure 8). Sample F (Figures 8 and 9) shows the best FE properties among all uniform CNT samples prepared in this work. SWCNT bundles with a moderate interspacing of 10-20 µm realized by Si tips (type III) induce an effective field enhancement. A



Figure 8. Cross-sectional SEM micrographs focused on top and base of CNT films, Raman spectra, and photos of luminescence from the rear surface of the anode at 5 V/µm obtained for samples A-F. Insets in SEM micrographs show enlarged images of CNT emitters.

corresponding region, which has the same morphology as that in sample F, is indicated as “III” in the right image of Figure 7. The results for the uniform CNT sample F agree well with the corresponding region in Library 4 (Figure 7). Note that the F-N plot for sample F (Figure 9b, brown) consists of two linear relationships: low-β and high-β regions at low electric field (3 V/µm). As discussed in our previous report,9 possible causes of nonlinear F-N plots include (1) adsorbate-enhanced field emission, (2) currentinduced evaporation (Joule heating), (3) stretching of entangled CNTs or “Y” shape splitting of a bundle during voltage sweep, (4) a local density of states (LDOS) at the tips of CNTs, (5) effect of space charge caused by ionization of residual gases, and (6) a large resistance between CNTs and a cathode electrode. Factors 2 and 3 are plausible mechanisms in transition of FE properties for “soft” SWCNTs. Considerable variations or inflections are evident in the F-N relationships for samples C (VA-SWCNTs) and D (SWCNT-grass; Figure 9b), but no recognizable inflection points are evident for samples A (protrusive VA-MWCNTs), B (VA-MWCNTs) and E (sparse

SWCNTs). Dynamic motions (stretching or “Y” shape splitting) of entangled SWCNT bundles induced by an applied electric field are considerable mechanisms for a high-β FE at a high applied electric field. Recently, Gupta et al.18 suggested (7) a multistep phenomenological FE model for nonlinear F-N plots. They explained the nonlinearity based on two successive processes: (I) electron tunneling from a metallic (semiconducting) region into a semiconducting (metallic) region and (II) tunneling from emitters into vacuum. This is also a plausible model to explain nonlinear F-N plots obtained for SWCNT emitters containing both metallic and semiconducting species. One of the most important issues for SWCNT emitters is their FE current durability. According to Saito et al.,7 sublimation of a SWCNT bundle occurs at an emission current of 1 µA. In contrast, a sustainable current of a single DWCNT (not a bundle) is 1.4 µA, and thus the current durability of SWCNTs is poorer than that of DWCNT. A possible design of a SWCNT cold cathode is one with a large number of emission sites. By assuming an ideal distribution of emission current into the SWCNT bundles on Si tips (number density of 4 × 105 cm-2)


Shiratori et al. exhibited good FE properties when the density of protrusive emitters was moderately controlled in a self-organized way. A key factor in controlling the self-organization of preferable CNT emitters is the optimal selection of materials and nominal thicknesses of catalyst layers and catalyst-support layers. In conclusion, the combinatorial FE evaluation simultaneously reveals not only desirable emitter morphologies but also optimal combinations of catalyst layers and buffer layers. This evaluation method will therefore accelerate the search for optimal conditions to prepare practical CNT emitters at the industrial level and can reveal insight into the self-organization of CNTs and their functions. Development of practical FE devices using SWCNT, DWCNT, or TWCNT emitters will require an increase in the number of emission sites via self-organization processes. A variety of CNTs can be prepared by controlling the growth alone. The concept of self-organization matches well with future device miniaturization and manufacturing.

Figure 9. (a) Current density (J)-electric field (E) curves and (b) F-N plots obtained from the repetitive measuring runs in the dc-voltage sweep mode for samples A-F. The F-N plot for sample F is composed of two linear relationships with slopes s1 (3 V/µm).

in sample F (Figure 8), an emission current per SWCNT bundle is about several nanoamperes. This value is more than two orders smaller than the sustainable current of a SWCNT bundle, resulting in a more stable field emission during several measurement runs.9 From the practical point of view, only selforganization can generate the formation of a high number density of SWCNT, DWCNT, or TWCNT emitters (>105 cm-2) on cathodes. Here, the combinatorial FE evaluation is a powerful tool to determine specific conditions to promote self-organization of CNT emitters with a moderate emitter spacing. Degradation of CNT emitters is triggered not only by Joule heating7 but also by reactions with reactive oxygen species generated by electroninduced radiolysis of contaminated water.19 If the measurement conditions (amount and species of contaminations and the outermost phosphor coatings) are controlled, then the combinatorial method is also a useful tool to simultaneously evaluate the lifetimes of various emitters. 4. Conclusions CNT libraries with a variety of CNTs on a single substrate were prepared by CVD after combinatorial masked deposition of a Co catalytic layer. Luminescence images and FE properties for uniform samples agree well with the findings obtained from combinatorial evaluations. The combinatorial FE evaluation is therefore a quick and reproducible method to search for optimal self-organized CNT emitters. The combinatorial FE evaluation identified preferable CNT emitters in a single sweep. Protrusive and screening-free SWCNT bundles with a moderate interspacing exhibited the best FE properties among the various types of carbon nanotubes prepared in this work. Even MWCNTs

Acknowledgment. We gratefully thank Dr. Yoshiko Tsuji, Mr. Kageyasu Kuroki, and Mr. Toshio Ohsawa for their technical support, and Professor Shigeo Maruyama for the use of Raman apparatus. This work is financially supported in part by Dainippon Screen Mfg, Co. and by Kakenhi (#1868602, #19054003) by MEXT, Japan. References and Notes (1) Jung, J. E.; Jin, Y. W.; Choi, J. H.; Park, Y. J.; Ko, T. Y.; Chung, D. S.; Kim, J. W.; Jang, J. E.; Cha, S. N.; Yi, W. K.; Cho, S. H.; Yoon, M. J.; Lee, C. G.; You, J. H.; Lee, N. S.; Yoo, J. B.; Kim, J. M. Physica B 2002, 323, 71. (2) Pirio, G.; Legagneux, P.; Pribat, D.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I. Nanotechnology 2002, 13, 1. (3) Kim, Y. C.; Yoo, E. H. Jpn. J. Appl. Phys. 2005, 44, L454. (4) Zang, Yu.; Deng, S. Z.; Xu, N. S.; Chen, J. J. Vac. Sci. Technol. B 2008, 26, 1033. (5) Jin, C.; Wang, J.; Wang, M.; Su, J.; Peng, L.-M. Carbon 2005, 43, 1026. (6) Machida, H.; Honda, S.; Ohkura, S.; Oura, K.; Inakura, H.; Katayama, M. Jpn. J. Appl. Phys. 2006, 45, 1044. (7) Saito, Y.; Seko, K.; Kinoshita, J. Diamond Relat. Mater. 2005, 14, 1843. (8) Bonard, J.-M.; Weiss, N.; Kind, H.; Sto¨ckli, T.; Forro´, L.; Kern, K.; Chaˆtelain, A. AdV. Mater. 2001, 13, 184. (9) Shiratori, Y.; Furuichi, K.; Noda, S.; Sugime, H.; Tsuji, Y.; Zhang, Z.; Maruyama, S.; Yamaguchi, Y. Jpn. J. Appl. Phys. 2008, 47, 4780. (10) Noda, S.; Kajikawa, Y.; Komiyama, H. Appl. Surf. Sci. 2004, 225, 372. (11) Noda, S.; Sugime, H.; Ohsawa, T.; Tsuji, Y.; Chiashi, S.; Murakami, Y.; Maruyama, S. Carbon 2006, 44, 1414. (12) Noda, S.; Hasegawa, K.; Sugime, H.; Kakehi, K.; Zhang, Z.; Maruyama, S.; Yamaguchi, Y. Jpn. J. Appl. Phys. 2007, 46, L399. (13) Kakehi, K.; Noda, S.; Maruyama, S.; Yamaguchi, Y. Jpn. J. Appl. Phys. 2008, 47, 1961. (14) Kakehi, K.; Noda, S.; Maruyama, S.; Yamaguchi, Y. Appl. Surf. Sci. 2008, 254, 6710. (15) Sugime, H.; Noda, S.; Maruyama, S.; Yamaguchi, Y. Carbon, accepted for publication. (16) Jorio, A.; Pimenta, M. A.; Souza Filho, A. G.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. New J. Phys. 2003, 5, 139.1. (17) Nilsson, L.; Groening, O.; Emmenegger, C.; Kuettel, O.; Schaller, E.; Schlapbach, L.; Kind, H.; Bonard, J.-M.; Kern, K. Appl. Phys. Lett. 2000, 76, 2071. (18) Gupta, S. J. Vac. Sci. Technol. B 2008, 26, 1006. (19) Fennimore, A. M.; Cheng, L. T.; Roach, G. A.; Reynolds, G. A. M.; Getty, R. R.; Krishnan, A. Appl. Phys. Lett. 2008, 92, 103104.

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Combinatorial Evaluation for Field Emission Properties of Carbon

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