A Simple Combinatorial Method Aiding Research on ... - IOPscience

Japanese Journal of Applied Physics 49 (2010) 02BA02

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A Simple Combinatorial Method Aiding Research on Single-Walled Carbon Nanotube Growth on Substrates Suguru Noda, Hisashi Sugime, Kei Hasegawa, Kazunori Kakehi, and Yosuke Shiratori Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan Received July 24, 2009; accepted September 14, 2009; published online February 22, 2010 Establishing fabrication methods of carbon nanotubes (CNTs) is essential to realize many applications expected for CNTs. Catalytic growth of CNTs on substrates by chemical vapor deposition (CVD) is promising for direct fabrication of CNT devices, and catalyst nanoparticles play a crucial role in such growth. We have developed a simple method called ‘‘combinatorial masked deposition (CMD)’’, in which catalyst particles of a given series of sizes and compositions are formed on a single substrate by annealing gradient catalyst layers formed by sputtering through a mask. CMD enables preparation of hundreds of catalysts on a wafer, growth of single-walled CNTs (SWCNTs), and evaluation of SWCNT diameter distributions by automated Raman mapping in a single day. CMD helps determinations of the CVD and catalyst windows realizing millimeter-tall SWCNT forest growth in 10 min, and of growth curves for a series of catalysts in a single measurement when combined with realtime monitoring. A catalyst library prepared using CMD yields various CNTs, ranging from individuals, networks, spikes, and to forests of both SWCNTs and multi-walled CNTs, and thus can be used to efficiently evaluate self-organized CNT field emitters, for example. The CMD method is simple yet effective for research of CNT growth methods. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.02BA02

1.

Introduction

Carbon nanotubes (CNTs), especially single-walled CNTs (SWCNTs), have been attracting much attention due to their unique one-dimensional structure and excellent electrical, mechanical, thermal, optical, and chemical properties, and thus many applications have been proposed and extensively studied.1) Figure 1 shows some examples out of many potential applications. Performance of integrated circuits might be drastically improved with semiconducting SWCNT transistors2) and metallic SWCNT wirings3) (I in Fig. 1). High specific surface area with excellent conductivity makes CNTs attractive candidates for battery/capacitor electrodes4) and high specific surface area with excellent tensile strength makes CNTs attractive candidates for fillers in composite materials5) (II in Fig. 1). Field emission displays with low energy consumption might be realized by using spiky CNTs as field electron emitters,6) and flexible printable transparent electrodes7) used in both displays/lightings and solar cells might be realized by using CNT networks (III in Fig. 1). Despite CNTs have such a huge potential realizing various applications without using multiple chemical elements, however, their practical application is limited due to the performance of current production methods. To realize integrated circuits with SWCNT transistors and wirings (I in Fig. 1), for example, the obstacle is not the current price of 1;000 USD/g-SWCNTs, but the breakthrough needed in the precise control of the chiralities and position of individual SWCNTs. In contrast, to realize capacitor/battery electrodes and composites (II in Fig. 1), the obstacle is not their structural control, but the breakthrough needed in the cost and scale of CNT production to meet such bulk use. In the case of planar electronic devices such as field emitters and transparent electrodes (III in Fig. 1), structural control is needed not for individual CNTs (such as chiralities and positioning) but for ensembles of CNTs (such as spikes and networks), and production scale and cost is important not for the mass of CNTs but for the device area (note that a 1-mm-thick CNT layer roughly corresponds to 1 g-CNT/m2 ). Figure 1 classifies CNT 

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Fig. 1. (Color online) Map of CNT potential applications from practical viewpoints of allowable production cost (determined by added value of the products) and required accuracy of structural control.

applications from the practical viewpoints of allowable production cost (determined by added value of the products) and required accuracy of structural control. The range of potential applications of CNTs varies widely, from highperformance devices using individual CNTs, general-purpose materials using CNTs in bulk, and to planar (electronic) devices using CNT ensembles two-dimensionally. A ‘‘universal’’ production method is not feasible, and therefore developing production methods customized for each application is the key to fully realize practical applications. In this paper, first we explain our simple combinatorial catalyst preparation method called the CMD method (combinatorial masked deposition) to aid research of SWCNT synthesis. Then, we present our current and previously reported experimental results that demonstrate achievements in diameter control of SWCNTs toward precise control of individual SWCNTs (I in Fig. 1), in rapid SWCNT growth toward mass production of SWCNTs (II in Fig. 1), and in morphological control of SWCNT ensembles

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toward planar electronic devices (III in Fig. 1). Among the three categories of CNT applications shown in Fig. 1, we consider planar devices that use CNT ensembles (III) the closest to achieving practical application. Self-organization process of CNTs into ensembles is the key for the highthroughput device fabrication, and as one example, we report our achievement of such fabrication for field emitters. 2.

Combinatorial Masked Deposition: CMD

Synthesis of SWCNTs using either physical vapor deposition (PVD) or chemical vapor deposition (CVD) methods has been extensively researched. Catalytic CVD methods, in which SWCNT production is enhanced by metal nanoparticle catalysts either suspended in the gas-phase8) or supported on either substrates9) or particles,10) are now widely used to synthesize SWCNTs. The preparation method of catalyst nanoparticles is critical in the growth of SWCNTs directly on device substrates. There are mainly two approaches to this catalyst preparation method; (1) deposition of pre-synthesized catalyst nanoparticles on substrates [Fig. 2(a)], and (2) conversion of pre-deposited catalyst precursors into nanoparticles over the substrates [Fig. 2(b)]. CVD temperatures (typically as high as 1000 K) sometimes cause coarsening of catalyst particles due to the poorer stability of smaller particles, and this coarsening phenomenon sometimes makes the former approach of catalyst preparation [Fig. 2(a)] difficult. In contrast, typical catalyst metals (Fe, Co, Ni) form particles wet partially on a typical support layer (SiO2 , Al2 O3 , or MgO) under equilibrium due to the balance among their surface/interfacial energies (Fig. 3),11) and this equilibrium structure makes the latter approach of catalyst preparation [Fig. 2(b)] effective. Extensive research has been made to develop preparation methods/conditions of the catalyst precursors by time-consuming trial-and-error approaches. Aiming at efficient screening of catalyst preparation conditions, several groups have developed combinatorial methods12–18) using either dry-12,16,18) or wet-13–15,17) processes. These methods enabled rapid screening for catalyst conditions of large parameter-spaces. Let us extract the key parameter in the approach of Fig. 2(b). Annealing at high CVD temperatures makes catalyst metals to approach equilibrium within the characteristic area of surface diffusion (i.e., the square of the diffusion length), and therefore, v (m3 ) metals existing over the area of surface diffusion s (m2 ) spontaneously yield single metal nanoparticles of v (m3 ) in volume at a number density of s1

Fig. 2. (Color online) Two major approaches for preparation of catalyst nanoparticles on substrates. (a) Deposition of pre-synthesized nanoparticles on substrates. Nanoparticles of suitable sizes for CNT growth (left) are sometimes coarsened (right) at CVD temperatures. (b) Conversion of pre-deposited catalyst precursors into nanoparticles over substrates. Catalyst particles of suitable sizes for CNT growth (right) can grow up from catalyst precursors (left) at CVD temperatures.

Fig. 3. (Color online) A map showing the growth modes of sub-monolayer metals on rutile TiO2 (110) surfaces.11) Solid circles and open triangles represent experimentally observed three-dimensional (3D) island and two-dimensional (2D) layer growth modes, respectively, and small crosses represent metals whose growth modes have not been determined. x -Axis is heat of oxidation of metals, which is correlated with the interfacial energy between metals and oxide underlayer. y-Axis is heat of sublimation of metals, which is correlated with the surface energy of metals. The line x ¼ y separates 3D island and 2D layer growth modes. Fe, Co, and Ni are the representative catalysts for CNT growth, and are plotted on or near the x ¼ y line, showing that these metals wet partially on oxide surfaces and their wettability decreases in this order.

Fig. 4. (Color online) Schematic explaining our combinatorial mask deposition (CMD) method. (a) Gradient thickness profiles of catalysts are prepared by setting a mask as a physical filter above a substrate during sputter-deposition. (b) Catalyst layers of different thicknesses yield a series of particles of different sizes. (c) Introduction of carbon feedstock gases induces growth of CNTs dependent on the size of catalyst particles.

(m2 ). Because the area for surface diffusion is currently difficult to estimate for most cases, nominal thickness tn ¼ v=s (m) becomes the key parameter. We therefore applied our CMD method19) to screen the nominal thickness tn of the catalyst for SWCNT growth. Figure 4 schematically shows our CMD method for SWCNT growth on substrates. A mask (acting as a physical filter) either with holes19,20) or a slit21) set above a substrate during sputter-deposition yields gradient thickness profiles in deposited catalysts according to the distance from the hole/slit [Fig. 4(a)]. Because the optimal nominal thickness of catalyst for SWCNT growth is typically as thin as 0.1– 1 nm, direct measurement of the catalyst layer thickness is

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Fig. 5. Typical TEM images of (a) as-grown SWCNTs transferred to TEM grids20) and (b) catalyst Co particles on SiO2 /Si substrates ionmilled after ACCVD. CVD conditions were 1.3 kPa C2 H5 OH, 1073 K, and 10 min and nominal Co thicknesses were 0.10 nm (a) and 0.03 nm (b).

Fig. 6. (Color online) Schematics showing side (a) and top (b) views of our standard CMD housing that can hold four 15  15 mm2 substrates for two catalyst components or twenty 3  15 mm2 substrates for one catalyst component.

difficult. The incubation time for deposition is usually negligible for sputter-deposition of metals at ambient temperature22) so that the thickness profiles of catalyst layers are estimated by deposition rate profiles and deposition times (typically 1 min). Deposition rate profiles are obtained by depositing thicker layers (typically 5 – 500 nm) by sputtering for long times (typically 1 h) and measuring their thickness profiles by a surface profiler. Annealing at the CVD temperatures converts the catalyst layers into particles with diameters depending on the initial layer thickness [Fig. 4(b)]. By flowing carbon source gases, CNTs start growing depending on the catalyst particle sizes [Fig. 4(c)]. We previously applied this CMD method first to the alcohol catalytic CVD (ACCVD) method23,24) and clarified that the optimum Co thickness was as thin as 0:1 nm (less than a monolayer) on SiO2 for the growth of SWCNTs.20) Figure 5 shows typical transmission electron microscopy (TEM) images of as-grown SWCNTs transferred to TEM grids and catalyst Co particles on SiO2 /Si substrates ionmilled after ACCVD. (Sub)monolayer Co20,25) and Ni,26) (0.1– 0.2 nm) which form discontinuous layer as deposited, typically form nanometer-sized particles (around 2 – 5 nm in diameter) on SiO2 surfaces at a number density 1  1012 cm2 by H2 annealing at 1000 K, and these particles efficiently grow SWCNTs. Thicker Co (1 nm), which forms continuous layer as deposited, typically forms larger particles (diameter  10 nm) at lower density (1010 – 1011 cm2 ) by H2 annealing at 1000 K, and these particles grow multi-walled CNTs (MWCNTs).25)

binary catalyst libraries covering wide ranges of both thickness and composition can be prepared on a single substrate.23,29) Figures 6(a) and 6(b) show top- and side-view schematics of our standard CMD housing, which can hold four 15  15 mm2 substrates suitable for two catalyst components or twenty 3  15 mm2 substrates suitable for one catalyst component, respectively. Figure 7 shows a typical CMD result for ACCVD with Co–Mo catalysts.29) A library of orthogonal thickness gradients of Co and Mo [Fig. 7(a)] enables conditions for active catalysts for growing CNTs to be identified easily by the naked eye [Fig. 7(b)]. As previously reported,29) such conditions are largely affected by the reaction conditions such as C2 H5 OH pressure and growth temperature. Therefore, simultaneous optimization of reaction and catalyst conditions were crucially important although those conditions are separately optimized in most works. The catalyst conditions of interest can be studied in detail by preparing those catalyst layers uniformly on separate substrates. Figure 7(c) shows scanning electron microscopy (SEM) images of the cross-sections of such CNT samples, showing forest morphologies of vertically-aligned (VA-) CNTs. Figure 7(d) shows TEM images of these CNTs transferred to TEM grids by simply scratching the CNT film surface with the grids. CNTs were SWCNTs with diameters around 2 – 3 nm. Evaluation by TEM is time consuming, and thus some other method to screen numerous catalyst conditions needs to be developed. Figure 7(e) shows Raman scattering spectra for the radial breathing mode (RBM) of SWCNTs obtained using automated mapping over the library at 169 points (13  13) at a 1 mm interval. Because the resonant wavelength for excitation differs among different SWCNTs, three lasers (i.e., 488, 515, and 633 nm) were used for the RBM mapping. The SWCNT diameters were widely distributed from 0.9 to 3 nm (note that detectable diameter range is 3 nm and smaller in this measurement). And we can grasp the tendency that both the diameters and yields of SWCNTs increased with increasing Co/Mo ratio. Mo possibly suppressed the surface diffusion of Co, resulting in reduced coarsening of Co catalyst particles by Ostwald ripening processes.29,30) SWCNT diameters under 169

3.

Binary Catalyst Libraries with Automated Raman Mapping for Diameter Control of SWCNTs

Binary catalysts such as Co–Mo sometimes show improved catalytic properties. CoMoCAT is a representative method yielding SWCNTs from CO by using Co–Mo catalyst supported on mesoporous SiO2 .27) ACCVD is the first method realized SWCNT forests by using Co–Mo catalyst supported on SiO2 substrates.28) By depositing the first catalyst component on a substrate through a slit-mask, rotating the substrate by 90 , and then depositing the second catalyst component through a slit-mask on the first layer,

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Fig. 8. Rapid growth of VA-SWCNT forests.32) (a) Photo of VA-CNT forests grown on a catalyst library of 0.2 – 3-nm Fe on SiO2 (top) and Al2 Ox (bottom) underlayers. (b) TEM image of SWCNTs grown by 0.5nm-thick Fe catalyst layer. (c) Raman spectra taken from the top of VASWCNT forests grown by 0.5-nm-thick Fe catalyst layer. (d) Height profiles of VA-CNT forests grown with different H2 O addition. Standard growth conditions were 7.9 vol % C2 H4 /26 vol % H2 /50 –100 ppmv H2 O/ Ar balance at ambient pressure, 1093 K with a residence time of 1– 5 s.

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Fig. 7. (Color online) CMD result of SWCNT growth by ACCVD. CVD conditions were 4.0 kPa C2 H5 OH, 1123 K, and 10 min.29) (a) Thickness profiles of Co–Mo catalyst layers. (b) Photo of CMD library after ACCVD. (c) SEM images of SWCNT forests prepared on different substrates with uniform catalyst layers. Catalyst conditions were (A) Co 0.11 nm and Mo 0.28 nm, (B) Co 0.22 nm and Mo 0.14 nm, (C) Co 0.07 and Mo 0.05 nm, and correspond to points indicated in (b). (d) TEM images and histograms of SWCNTs shown in (c). (e) Raman spectra for RBM modes of SWCNTs obtained by automated mapping at blue circles indicated in (b). Nominal Co thickness varied from 0.04 nm at the bottom to 0.82 nm at the top.

catalyst conditions (or more, if we take data at smaller intervals) can now be studied in only 1 day from catalyst preparation, SWCNT growth, and to diameter evaluation by automated Raman mapping. Such rapid preparation, growth, and evaluation will thus contribute to development of more precise control of SWCNT growth.

Catalyst Libraries with Real-Time Monitoring for Rapid SWCNT Growth

Extensive progress has been made recently in growth techniques of SWCNTs on substrates; sub-micrometer-thick films of randomly oriented SWCNTs until 2003,24) micronthick films of VA-SWCNT forests in 2003,28) and millimeter-thick films of VA-SWCNT forests in 2004.31) So called ‘‘SuperGrowth’’ by water-assisted CVD31) realized amazing growth rates, 2.5 mm in 10 min. Such rapid growth is possible in a rather wide CVD and catalyst windows for MWCNTs but in narrow windows for SWCNTs. When sputtered catalyst layers are used, thinner layers yield smaller catalysts, which in turn grow thinner CNTs. Based on this concept, we applied our CMD method and quickly reproduced rapid SWCNT growth and clarified its CVD and catalyst windows.32,33) Figure 8(a) shows a typical photo of such CNT forests grown on an Fe library on both Al2 Ox and SiO2 underlayers, clearly showing that the Al2 Ox underlayer is essential for Fe to rapidly grow VA-CNTs. The photo also reveals the height profiles of CNTs as a function of Fe catalyst layer thickness and reveals the existence of a threshold Fe thickness for VA-CNT growth. Above the threshold, CNT height gradually decreased with increasing Fe thickness, which is not influenced by the gas-flow direction. Figure 8(b) is a typical TEM image showing SWCNTs with somewhat large diameters around 4 nm. Figure 8(c) shows Raman spectra taken from the top of the SWCNT films. RBM peaks indicate that these SWCNTs have diameters of 1– 2 nm. The discrepancy between the TEM and RBM results for the SWNT diameters originates from the diameter increase in SWCNTs during growth.34)

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fits well the growth curves for certain periods, but then suddenly deviates due to the sudden termination of CNT growth although its mechanism is unclear. Repeatability of CNT growth is sometimes poor, and this phenomenon of sudden termination might be one cause. Real-time monitoring coupled with CMD (CMD-RTM) method provides growth curves for a series of catalysts with minimized uncertainty due to run-by-run deviation. Rapid SWCNT growth with clarified CVD and catalyst windows provides new routes for large-scale SWCNT production using supported catalysts. The CMD-RTM method provides information on catalyst activity and lifetime, both of which are crucial in designing practical production processes. 5. Fig. 9. (Color online) Real-time monitoring of VA-CNT growth on a combinatorial catalyst library.34) (a) Schematic showing CVD apparatus for real-time monitoring. (b) Snapshots of VA-CNT forests growing on a gradient Fe catalyst layer on Al2 Ox underlayer from 7.9 vol % C2 H4 / 26 vol % H2 /50 –100 ppmv H2 O/Ar balance at ambient pressure and 1093 K. CNTs are bright red due to elevated temperature. (c) Growth curves obtained for different Fe thickness obtained in a single experimental run. Dots are measured data and curves are of fitting using eq. (2).

Figure 8(d) shows height profiles of VA-SWCNT forests with different H2 O addition. H2 O addition actually widens the catalyst window for rapid CNT growth but also suppresses the growth of thin SWCNTs as can be seen in the weakened RBM peaks and the G-band shape [Fig. 8(c)]. Note that SWCNTs can actually be grown without H2 O addition, although the catalyst window is narrow (0.4 – 1.0 nm). Height increase in millimeter-thick VA-CNT forests during CVD can be easily detected even by the naked eye. Through a glass window at one end of the reaction tube [Fig. 9(a)], VA-CNT growth was monitored in real-time by using a digital camera.34) Figure 9(b) shows side-view snapshots of the VA-CNT forests growing on a combinatorial catalyst library. CNTs are bright red due to the blackbody radiation at the elevated temperature. The photos reveal that the VA-CNT films stopped growing after 6 min. From the photos in Fig. 9(b), growth curves (i.e., height change with growth time) can be obtained for a series of catalyst conditions. Figure 9(c) shows such growth curves, clearly revealing that the height of VA-CNTs initially increased at relatively constant rates but suddenly stopped increasing after 3 – 6 min. Such sudden termination has been reported by Meshot and Hart.35) In previous works,36,37) gradual termination of CNT growth was reported and the following equations were proposed:   t rðtÞ ¼ r0 exp  ; ð1Þ     t ; ð2Þ LðtÞ ¼ r0  1  exp   where rðtÞ is the growth rate of VA-CNTs at time t, r0 is the initial growth rate,  is the time constant of the growth rate decay, and LðtÞ is the height of VA-CNTs at t. Equation (2)

SWCNT Libraries for High-Throughput Preparation and Evaluation of Field Emitters

CNTs grown on substrates vary widely not only in their individual structure25) but also in the morphology of their ensembles.38) Figure 10 shows SEM images of CNTs grown under the same conditions except for layer thickness of Co catalyst (0.05 – 0.63 nm) and for the reaction time (1– 3 min).38) The 0.05-nm-thick Co grew SWCNT at low density, resulting in individuals after 1 min of growth but networks after 3 min. The 0.14-nm-thick Co grew SWCNTs at a higher density, resulting in networks after 1 min of growth but spikes after 3 min. The 0.21-nm-thick Co grew SWCNTs at the highest density, resulting in forests after 1– 3 min of growth. In contrast, thicker Co mainly grew MWCNTs and formed networks after 1 of growth and forests after 3 min of growth. In summary, by changing only the catalyst layer thickness, CNTs grew at different densities and yielded various morphologies, and by simply increasing the growth time, CNT morphologies evolved from individuals, networks, spikes, and to forests. CNTs form ensembles due to bundling among themselves. SWCNTs are flexible and have strong van der Waals interactions, and thus easily form bundles and yield various morphologies. Figure 11 schematically summarizes such morphologies spontaneously evolving in SWCNTs. The extent of bundle formation correlates with the coverage C of SWCNTs over the substrate surface. C can be expressed as

Fig. 10. Top-view (a,c) and side-view (b,d) SEM images of CNT films of various structures and morphologies grown from 1.3 kPa C2 H5 OH at 1003 K by combinatorial Co catalyst layer on SiO2 substrates.38) SWCNTs mainly grew when the Co thickness was 0.05, 0.14, and 0.21 nm, whereas MWCNTs mainly grew when the Co thickness was 0.38 and 0.63 nm.

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Fig. 11. (Color online) Schematic showing various morphologies spontaneously evolving in SWCNTs during growth on substrates.

C ¼ dlN

ð3Þ

where d (m), l (m), N (m2 ) are diameter, length, and number density of the SWCNTs, respectively. This coverage C should be an effective index for classifying morphologies; however, even if C is the same, there should be differences in fine structures. For example, C  1 should yield network morphologies, but bundling should be more enhanced for larger l with smaller N. For SWCNT networks with a mixed chirality, bundling causes percolation of metallic SWCNTs, leading the network films to have metallic properties. SWCNT networks with metallic/semiconducting properties will be obtained by increasing/decreasing l and decreasing/ increasing N while keeping C  1. Various morphologies of practical interests, in addition to networks, evolve in SWCNTs spontaneously. Next, we consider the application of SWCNTs to planar (electronic) devices (III in Fig. 1). For such applications, their device layer thickness is typically less than 1 mm. Rapid SWCNT growth shown in Figs. 8 and 9 is as fast as 4 mm/s. This means that SWCNT device elements of self-organized morphologies can be mounted whole over the substrates within 1 s or less. This ‘‘instant’’ mounting of SWCNT device elements can overcome the time/cost-consuming mounting process that is one of the barriers in the practical applications of CNTs. Next, we show the field emitters as an example of using SWCNT libraries fabricated with the CMD method for highthroughput evaluation of CNT devices. Figure 12 shows CNTs of various structure and morphology prepared using the CMD method and shows their field emission (FE) performance.39) On flat Si(100) substrates, MWCNTs grown by 6.6-nm-thick Co (f) showed the best FE performance among the various CNTs including VA-MWCNT forests by 1.4-nm-thick Co (e), SWCNT spikes by 0.8-nm-thick Co (d), and SWCNT networks by 0.5-nm-thick Co (c). This is because MWCNTs by 6.6-nm-thick Co (f) had protrusive ones with both height and interspacing of a few micrometers, which are characteristics suitable for increasing the emitter density while suppressing the screening effect of electric field among themselves. The results for CNTs grown on (111)-textured Si(100) substrates, prepared by anisotropic wet-etching by hydrazine hydrate, differed significantly. Protrusive MWCNTs by 6.6-nm-thick Co (l) had similar FE performance as on the flat Si(100) substrates, whereas CNTs by thinner Co (i–k) had significantly improved FE performance than on the flat (100) substrates. FE performance improved as CNTs changes from sparse MWCNTs by 2.9nm-thick Co (k), disordered SWCNT forests by 0.8-nmthick Co (j), to sparse SWCNTs by 0.5-nm-thick Co (i). In conclusion, textures significantly enhance the FE performance, especially for SWCNTs, by increasing the interspacing

Fig. 12. (Color online) Self-organized CNT field emitters formed on (a–f) flat and (g–l) (111)-textured Si(100) substrates.39) Growth conditions were 4.0 kPa C2 H5 OH, 1058 K, and 30 s with combinatorial Co catalyst layers on Al2 O3 underlayer. (a,g) Cathode luminescence (CL) tests of the CNT emitter libraries using ZnO:Zn phosphors on ITO/glass substrates under an electric field of 4.0 V/mm. (b,h) Top-view photos of CNT emitter libraries. Horizontal positions are the same as the CL images (a,g). Symbols c–f and i–l indicate the positions of the SEM images (c–f) and (i–l). (c–f) SEM images of CNTs grown on flat Si(100) substrates taken at same magnifications. (i–l) SEM images of CNTs grown on (111)-textured Si substrates taken at same magnifications. Note that the sputtered Co flux introduced from the CMD slit at the left yielded Co layers and thus CNTs on the left side of the textures. Thicknesses in nanometers in (c–f, i–l) show the nominal thickness of Co catalyst layers.

between the protrusive SWCNTs. This effect is easily reproduced on uniform CNT samples grown on larger device area by uniform catalysts.40) Extensive research and development of CNT field emitters has focused on the individual structure of CNTs, such as single-, double-, and thin-walled CNTs. Figure 12 clearly shows, however, the importance of morphologies of their ensembles. There is a wide variety in the CNT morphology as well as in the hierarchical structure such as textures in the substrates. The CMD method enables identification of suitable CNT field emitters that can be prepared easily on large scale via self-organization. 6.

Conclusions

In this work, we explained our simple combinatorial method, CMD, by reviewing our achievements in developing CNT growth methods on substrates using this method. CNTs vary widely in their individual structure as well as in the morphology of their ensembles. Although this variety widens their potential applications, it is a major barrier toward practical applications because CNTs act differently depending on their structure and morphology. As a first step

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for the precise control of individual structures of SWCNTs, we explained the binary catalyst library yielding SWCNTs of various diameters, which can then be evaluated rapidly by automated Raman mapping. Toward mass-production of SWCNTs, we explained the CMD-RTM method to determine both the activity and lifetime of a series of catalysts in a single experimental run for rapidly growing millimeterthick VA-SWCNT forests. For targeting practical applications, we explained our libraries of CNT field emitters of various morphologies with hierarchical structures, from which CNT field emitters with high FE performance can easily be determined and then be prepared easily at a large scale by self-organization. A universal fabrication method of SWCNTs is not feasible, and thus developing fabrication methods customized for each application should be important to move toward the widespread application of CNTs. Our simple combinatorial method, CMD, will contribute to develop customized production methods of SWCNTs. Acknowledgements

The authors are grateful to Professor S. Maruyama for introducing us to CNT research, to Mr. T. Osawa and Dr. Y. Tsuji for supports in experiments, to Mr. S. Nakamura for TEM observation of catalyst nanoparticles, and to Mr. T. Ito, Mr. H. Tsunakawa, and Mr. K. Ibe for assistance in TEM observations of CNTs. This work was financially supported in part by Grants-in-Aid for Young Scientists A (18686062 and 21686074) and Grants-in-Aid for Specific Area Research (19054003) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, by Nanotech Challenge (07005623-0) from New Eergy and Industrial Technology Development Organization, Japan, and by DAINIPPON SCREEN MFG. Co., Ltd., Japan. H.S. was supported by the Global COE Program for Chemistry Innovation and by a JSPS fellowship. K.H. was supported by the Global COE Programs for Chemistry Innovation and Mechanical System Innovation.

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