Chapter Synthesis of Vertically-Aligned Carbon Nanotubes Chapter Synthesis of Vertically-Aligned Carbon Nanotubes This chapter focuses on the synthesis of carbon nanotubes (CNTs) used as substrate for the following thin film deposition The CNTs are required to be highly densed and vertically-aligned to be suitable for field emission (FE) application Therefore, plasma-enhanced chemical vapor deposition (PECVD) technique has been employed for CNT growth with Fe acting as catalyst SEM and TEM were utilized to characterize the as-grown CNTs The growth mechanism of the CNTs and the function of the metal catalyst will be discussed The growth rate of the CNTs will be investigated as well   4.1 Introduction Synthesis of CNTs has been extensively studied since their discovery [1-5] Arc discharge approach was used when Iijima first reported the synthesis of CNTs, which is similar with the method used for traditional fullerene preparation After that, several methods have been explored to produce CNTs, including CVD, laser ablation and some other approaches such as solar energy, electrolysis, underwater alternating current electric arc method and plasma torch method [6-8] 51    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes Essentially, all of the CNTs are formed in the same way but the process which causes the formation differs In both the arc discharge and laser ablation, a solid graphite carbon is heated to a very high temperature, leading to the separation of some carbon atoms from the solid These atoms then reassemble on the cathode in the case of arc discharge and on a cooled collector in the case of laser ablation During the reassembly process, carbon is arranged in the tubular formation on a nanoscale level In these fabrication methods, no catalyst is involved Alternatively, catalyst has been used in the growth of CNTs The catalytic CVD growth of CNTs is an entirely different process with the arc discharge and laser vaporization methods Instead of beginning with a solid carbon, the carbon atoms are extracted from a carbon monoxide or hydrocarbon gas, which dissociates either thermally or in the presence of plasma Subsequently, the dissociated carbon atoms once again self-assemble into highly ordered nanotubes However, in this case, the nanotubes form on a prepared substrate with small catalyst particles on it The nanosized catalyst particles act as seeds for nanotube growth Therefore, the size of the catalyst particles determines the size of the as-grown CNTs, as well as their locations [4] The advantages of the catalytic growth of CNTs are that long nanotubes can be achieved at relatively low temperatures [9] However, this kind of CNTs is rather thick with their diameters correlated to the sizes of the metal catalyst particles and the graphene layers of the catalytic produced CNTs usually contain defects The CNTs are also covered with amorphous carbon, which is the product of the thermal deposition of hydrocarbon [3] 52    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes In terms of other CNT growth approaches, solar energy method means that concentrated sunlight from a solar furnace is focused on a graphite sample in order to vaporize the carbon Afterward, the soot containing CNTs is condensed in a dark zone of a reactor, which is collected in a filter and water cooled Electrolysis method fabricates CNTs via passing an electric current in a molten ionic salt between graphite electrodes Underwater alternating current electric arc method combines the underwater growth with the application of an alternating current controlled power supply [2, 6-8] Some of these methods are innovative and effective in CNT growth However, the main problem of these methods is the incapability of producing vertically-aligned CNTs Although these CNTs are usually grouped into bundles, the bundles themselves are not generally aligned with each other The random orientation of CNTs has impeded their application in some areas such as microelectronic devices and field emission In order to obtain vertically-aligned CNTs, one method considered is the growth of CNTs by plasma-enhanced chemical vapor deposition (PECVD) technique There are some advantages of growing CNTs by PECVD technique: [4] (1) Dissociation of hydrocarbon gas and formation of CNTs take place at lower temperatures (typically 600 - 700 °C) because the energy in the plasma discharge replaces some of the heat energy; (2) Vertically-aligned CNTs can be obtained due to the existence of electric field during growth process; (3) This technique is simple, cheap, high yield, and capable of producing large 53    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes dimensional CNT substrates These advantages make PECVD a broadly used technique for CNTs preparation reported by considerate studies [10-13] In this research, we intended to investigate the FE properties of CNT-based materials Vertically-aligned one-dimensional nanostructures are one of the most favored geometries for FE application Therefore, PECVD technique was employed to grow CNTs 4.2 Experimental Details Before growing CNTs in PECVD system, a layer of Fe catalyst was deposited on the substrate A RF (13.65 MHz) magnetron sputtering system with the model of Denton Discovery-18 was applied to deposit the catalyst layers First, a clean N++ silicon (100) wafer was cut with diamond cutter into small specimens with dimension of around mm × mm These small specimens were then placed as a circle on the sample holder in the sputtering chamber, which was pumped down to ~10-3 Torr by a mechanical pump (rotary pump) and further down to ~10-6 Torr by a turbo bump When the vacuum pressure was ready, these specimens were deposited for with the RF power at constant 100 W In the sputtering process, the chamber pressure was set to be 0.01 Torr The sputtering rate was calibrated by a surface profiler to control the thickness of the catalyst layers After catalyst deposition, these substrates were transferred in air to the PECVD chamber for CNT growth Fig 4.1 shows the schematic setup of the PECVD system 54    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes utilized in this research The chamber was pumped down to ~10-5 Torr and the substrate holder was heated up to 700 °C Subsequently, H2 gas with a flow rate of 60 sccm (standard cubic centimeters per minute) was introduced into the chamber to start the H2 plasma and lasted for 10 in order to promote the formation of catalyst nanoparticles After that, C2H2 with the flow rate of 15 sccm was introduced into the chamber as the hydrocarbon gas Growth durations varied from to 20 in order to obtain the most suitable CNT length In the growth process, RF power was set to be 100 W and the chamber pressure was approximately kept at 1.2 Torr The substrate temperature was measured by attaching a thermocouple directly to the graphite substrate heater Fig 4.1 Schematic setup of the PECVD system used 55    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes 4.3 Results and Discussion 4.3.1 Characterization of the As-Grown Carbon Nanotubes Fig 4.2 shows the macrograph of the as-grown CNT samples, which appear to be dark and soot like The SEM images of the high density and vertically-aligned CNTs are shown in Fig 4.3 The Fe catalyst layer was approximately nm thick with sputtering duration measured by the surface profiler From the low magnification top view SEM image in Fig 4.3(a) it can be observed that the CNTs are highly densed on the substrate, confirming a high yield of CNTs by this growth method Under high magnification top view SEM image as shown in Fig 4.3(b), some small particles were observed at the top of the CNTs, which were speculated to be Fe particles Fig 4.3(c) shows that the CNTs are both uniform in length and diameter From this cross-sectional view, it is clear that the CNTs are well-aligned with   Fig 4.2 The macrograph of the as-grown CNT samples   56    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes   Fig 4.3 (a) Low magnification and (b) high magnification top view and (c) cross-section SEM images of CNT samples obtained with Fe catalyst   respect to the silicon substrate, and the nanotubes not appear as thick hard rods but seem to be like free-standing spaghetti In addition, some CNTs at the edge not appear to be vertically-aligned in this image The non-alignment is highly likely due to the external force used to break the sample in order to capture the cross-sectional SEM image of the nanotubes Besides, the slightly tilting ones are right at the edge and it is obvious that the majority of the CNTs beyond the edges are well-aligned Since the as-grown CNTs are high yield and vertically-aligned with respect to the silicon substrate, which exactly match our criteria for FE application, this growth recipe has been determined as the growth conditions for CNTs, and this kind of CNTs have been used as a standard substrate for the following coating process The growth duration to 57    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes obtain suitable CNT length will be examined in the subsequent portion TEM images of the as-grown CNTs are shown in Fig 4.4 It can be observed from the images that the CNT is multiwalled and consists of 35 layers of graphene sheets with spacing of around 0.31 nm Its outer diameter is approximately 30 nm and its inner diameter is about 10 nm Some defects can be observed in the hollow part of   Fig 4.4 TEM images of the as-grown CNTs obtained with Fe catalyst 58    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes the nanotube that a few layers of graphene sheets formed randomly in the hollow tube This kind of CNT structure is usually called bamboo-like structure [14-16].  Formation of these defects is probably due to highly dissolved carbon concentrations Provided that the hydrocarbon gas pressure in the CNT growth chamber is very high, bamboo-like structure would start from the nucleation at the junction between the outer carbon wall and the metal particle surface as it can be stabilized by the interaction with both the CNT wall and the catalyst Once an initial carbon layer forms on the catalyst particle, the carbon atoms on the catalyst surface can further lower the system energy through incorporation into carbon layers The elongation thereafter is due to the incorporation and attachment of carbon atoms to the interface between the initial carbon layer and the catalyst particle by surface diffusion of carbon atoms on the catalyst [17] Gradually, the entire bamboo-like structure forms as shown in the TEM images in Fig 4.4 The formation process of the bamboo-like structure is schematically elucidated in Fig 4.5   Fig 4.5 Schematic illustration of formation process of bamboo-like compartment structures in the center hollow part of CNTs (a) CNT growth; (b) nucleation of a partial bamboo-knot carbon layer at the carbon wall-catalyst junction; (c) subsequent growth of bamboo-like compartment in the hollow center of the CNT 59    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes 4.3.2 Catalytic Growth Mechanism of Carbon Nanotubes Basically, catalytic growth of CNTs is a vapor-solid synthesis process Its mechanism is divided into two types, tip growth and bottom growth mechanisms In this study, the catalyst particles were observed at the tips of CNTs as shown in Fig 4.6, thus their growth should have been controlled by the tip growth mechanism Fig 4.6 TEM image of the tip of the as-grown CNT with metal catalyst embedded in the nanotube A schematic description of the tip growth mechanism is shown in Fig 4.7 The process proceeds according to the following steps: First, the catalyst layer would shrink and form small islands due to the surface tension and the compressive stress resulted from the mismatch of the thermal expansion coefficients of silicon substrate and the transition metal [3] After being introduced into the PECVD chamber, the 60    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes hydrocarbon gas was decomposed on the surface of the catalyst particles in the present of hydrogen plasma In this step, the plasma may aid the decomposition by decreasing the mean free path of the gas molecules Meanwhile, the plasma would also provide a local surface heating that enabled the carbon fragments to diffuse across the catalyst particles [18] After supersaturation, carbon was precipitated out from the opposite side of the catalyst particles and reassembled to form nanotubes For catalyst particle sizes below a critical value, one nanotube grows per particle, with a diameter essentially equal to that of the particle Fig 4.7 Schematic illustration of catalytic tip growth mechanism of CNTs   In the other CNT growth mechanism, i.e., bottom growth process, the catalyst particles remain at the bottom of the nanotubes due to the strong adherence of the catalyst particles to the substrate surface such that the carbon precipitates from the top surface of the catalyst particles [19] The carbon fragments are feeded from the bottom of the catalyst particles instead from the tip surface, and the CNTs extend their 61    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes growth away from the catalyst, exhibiting a negative temperature gradient in the growth direction [20-22] In addition, the vertical alignment of the CNTs is probably due to the tip growth mechanism [23] The presence of the catalyst particles at the CNT tips may be essential for the alignment because in the electrical field, a stable negative feedback mechanism may be provided due to the interaction of the electrostatic force applied to the CNT tips with the catalyst particles On the contrary, the catalyst particles located at the bottom of the CNTs would result in a positive feedback mechanism that further misaligns the growth The crowding effect, i.e., adjacent CNTs supporting each other by van der Waals force was believed to contribute to the CNT alignment as well [24, 25] 4.3.3 Function of Metal Catalyst in CNT Growth The SEM images of the Fe catalyst particles are shown in Fig 4.8 It has been illustrated previously that at high temperatures, the metal catalyst layer would break into small islands due to the surface tension and the compressive stress resulted from the mismatch of the thermal expansion coefficients of silicon substrate and the transition metal Afterward, CNTs would form on these small metal catalyst seeds if they possess appropriate particle sizes In Fig 4.8(a), some big particles can be observed on the substrate Their sizes seem to be too large for the CNTs shown in Fig 4.3, and their density is very low However, the magnified SEM image of the selective 62    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes area shown in Fig 4.8(b) clearly demonstrates that the silicon substrate is fully covered with nanoparticles with sizes smaller than 50 nm, which is suitable to seed CNTs   Fig 4.8 (a) SEM image of the Fe catalyst particles resulted from thermal expansion; (b) magnified SEM image of the selective area within the white rectangular part from (a) 63    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes To explain the reason that CNTs choose to grow on certain sizes of metal particles whereas no CNTs form on large metal particles, the function of these metal particles in CNT growth process needs to be elucidated In this CNT growth process, the most time consuming step for the CNT formation on metal particles is the diffusion of the carbon species from the decomposition sites to the segregation sites If the metal catalyst particle is too large, the diffusion length for the carbon is too long whereas if the particle is too small, the strain energy of the nanotube with an appropriate diameter is too high Consequently, the suitable metal catalyst particle size should be ranged from to 100 nm [26, 27] Moreover, the diffusion process of carbon fragments in the metal catalyst particles is controlled by the activation energy gradient of the system There is a remarkable correlation between the activation energies for the CNT formation and for the carbon diffusion through the corresponding metal catalyst The hydrocarbon molecules were actually adsorbed on different parts of the metal particle surface, where possessed different catalytic activity due to various crystal orientations [14] When the active metal surface cracked the carbon-hydrogen bonds and diffused the carbon species through, the surface temperature of the catalytic active sites increased because of this highly exothermic reaction Given that the heat balances were maintained in this process, the CNT would grow continuously However, if perturbations took place in the chemistry process or excess carbon on the metal surface could not diffuse fast enough to build up the nanotube, the particle would become 64    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes completely encapsulated thus preventing further hydrocarbon decomposition and the CNT growth would stop Therefore, not all the metals are capable of acting as a catalyst for CNT growth Suitable metal catalyst for CNT growth implies all the above requirements should be satisfied Furthermore, the liquid catalyst particle surface is capable of reacting with the decomposed hydrocarbon gas molecules during CNT growth process In the 1930s and 1940s, researchers have studied the adsorption phenomenon on specific crystalline faces of highly purified metals After numerous experiments they found that exposed crystal faces with regular spacing were very active for some reactions and the activity was associated with the vacancies in the d-orbitals of metals [28, 29] The electron configuration of the transition metal Fe is [Ar]4s23d6 The d-orbital of Fe is incomplete and its occupation percentage is a measure of its affinity In the CNT growth process, the hydrocarbon molecules decomposed on the surface of Fe catalyst particles in the way that the carbon-hydrogen orbitals interacted with d-orbitals of Fe Through a charge transfer from hydrocarbon molecules to Fe, Fe reduced the energy required for the decomposition of hydrocarbon molecules [30] This energy reduction reflects its catalytic function as well Some other catalysts such as Ni and Co also play a similar role   65    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes 4.3.4 Growth Rate of Carbon Nanotubes with Fe Catalyst In order to investigate the growth rate of CNTs under the conditions mentioned in the preceding experiment portion, the lengths of CNTs obtained with different growth durations were measured In this investigation, all the other experiment conditions were kept the same except for the growth duration Fig 4.9 shows that the average CNT lengths were approximately 2.95, 3.71, 7.87 and 14.2 μm for CNTs grown for 3, 5, 10 and 20 respectively   Fig 4.9 Cross-sectional SEM images of CNTs grown for (a) min, (b) min, (c) 10 min, and (d) 20 min, showing the different CNT lengths     66    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes Fig 4.10 shows the length of CNTs as a function of the growth duration It can be observed that the CNT length exhibits a roughly linear relationship with the growth duration This observation agrees well with that obtained by Chhowalla et al [3] The growth rate for the CNTs with Fe catalyst was calculated to be approximately 0.80 μm min-1 Average CNT length (m) 14 12 10 2 10 12 14 16 18 20 22 Growth durations (min) Fig 4.10 The average CNT length as a function of the growth duration     The top view SEM images of the CNTs obtained with varying growth durations are shown in Fig 4.11 For CNTs grown for and min, it is clear that many larger particles are on the top surface of CNT forest For 10 CNTs, the top surface appears to be much cleaner These large particles are postulated to be amorphous carbon As the growth time was rather short, the function of H2 plasma may not be well completed in removing the excess amorphous carbon produced from the 67    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes decomposition of the feedgas, i.e., C2H2 gas[4] Therefore, the shorter the growth duration was, the larger the amorphous carbon particles can be observed CNTs with amorphous carbon on the surface was not suitable for the subsequent coating, hence the CNTs grown for longer than 10 are used as substrate for the following research   Fig 4.11 Top view SEM images of CNTs grown for (a) min, (b) min, and (c) 10   4.4 Summary High density vertically-aligned CNTs have been successfully synthesized by PECVD technique with Fe catalyst The catalytic growth mechanism for CNTs and the function of metal catalyst in CNT growth have been thoroughly discussed In addition, 68    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes the length of CNTs was found to be linearly increased as a function of growth duration However, with very short growth 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the PECVD system used 55    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes 4. 3 Results and Discussion 4. 3.1 Characterization of the As-Grown Carbon Nanotubes. .. covered with amorphous carbon, which is the product of the thermal deposition of hydrocarbon [3] 52    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes In terms of other CNT growth approaches,... the hollow part of   Fig 4. 4 TEM images of the as-grown CNTs obtained with Fe catalyst 58    Chapter Synthesis of Vertically-Aligned Carbon Nanotubes the nanotube that a few layers of graphene sheets