Study of field emission characteristics of ultrathin film coated carbon nanotubes core shell structures 4

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 con

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]

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

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 5 mm × 5 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 3 min 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

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 min 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 3 min to 20 min 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

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 7 nm thick with

3 min 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

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 do not appear as thick hard rods but seem to be like free-standing spaghetti In addition, some CNTs at the edge do 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

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

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

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

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

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

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)

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 2 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