4.1 Characterization of carbon nanotubes density through Ni nanoparticle formation using hydrogen plasma treatment on the TiN buffer layer and nanoindentation
4.1.1 Introduction
Carbon nanotube (CNT) is the most important material [41], exhibit unique electronic and extraordinary mechanical properties. The control of such structure has technical significance in the sense that the structural diversity leads to different electronic, mechanical and other physical properties. These useful properties of CNTs make themselves good candidates for various application fields, such as a transistor [42, 43], sensor [44], field emission display [45-46], nanoscale interconnects [47]. The growth is in these cases usually achieved by means of catalytical chemical vapor deposition (CVD) with high purity, high yield CNTs for practical application. Thus, it was found that the CNTs would only be deposited under specific pretreatment conditions before CNTs growth [48~53].
Therefore, in order to avoid an undesired metal silicide formation at high temperature, the buffer layer was proposed in annealing process [54~56].
In the recent, some materials such as thin film [57~61], nanocomposite [62], CNTs [63], and nanoparticle [64] were measured using indenter technology with loaded in axial compression. It should be emphasized that special care is required when measuring the mechanical response of particle aggregates or thin films deposited on substrates, the tension, bending, vibration, and compression experiments can strongly contribute to the overall mechanical response of the film/substrate system.
When dealing with metal nanoparticles from the pretreatment behaviors of the agglomerates mechanism to the substrate becomes an aim of issue for nanoindentation.
In this cause, the resolution to measure directly the interparticle forces in the nanometer regime is lacked. Nevertheless, the relationship between both of the interparticle forces on metal particles and the effect of the grown CNTs is not yet being exposed in detail. In this chapter, we synthesized CNTs with a microwave plasma chemical vapor deposition (MPCVD) system on Ni/TiN/Si substrate. The effect of pretreatment of Ni catalyst layer on growing characteristic and properties of CNTs according to the pretreatment of period process was investigated. It is suggested that the surface performances of catalyst can be modified by hydrogen plasma [57]. Therefore, small agglomerates of catalyst nanoparticles were formed on TiN buffer substrates. The aid of focused is find out the relatively large particles mechanical property of pretreated nanoparticles, in the nanometer range. The
deformation behavior of the agglomerates under loads control-mode of nanoindentation technique was studied.
4.1.2 Results and discussion 4.1.2.1 Catalysts pretreatment
Layers of Ni were not only etched, but also conglomerated, by hydrogen plasma treatment at 3 minutes from SEM observation (Fig. 4-1 (a)). Above 5 minutes, the conglomeration effect dominates so the heating and plasma power transform the film of Ni catalysts film into large Ni particles (Fig. 4-1 (b)). The size and distribution of Ni nanoparticles varied with the pretreatment time. The average diameter of the Ni nanoparticles declined at 10 minutes, because small Ni nanoparticles were removed and large particles were shrunk by long etching (Fig. 4-1 (c)). Ni nanoparticles tend to melt away and conglomerate with each other over 15 minutes (Fig. 4-1 (d)).
Fig. 4-2 presents the AFM analysis on surface etching associated with pretreatment.
The results were shown that the flat nickel layer is arrived at island-like of catalysts by pretreatment (3, 5, 10, and 15 minutes). The intermediate TiN layer was to be used as a diffusion barrier to prevent reaction between Ni and Si. In fact, nucleation also induces the formation of islands on the buffer layer in the presence of hydrogen plasma. Tab. 4-1 summarized that the RMS surface roughness of the catalytic-layer changed from 6.1 to
11.9 nm, as the pretreatment time increased from 3 to 15 minutes. Therefore, the nucleation and etching by heating and hydrogen plasma treatment are obvious. This treatment contributes to the formation of particles, which act as seeds for nucleation in the growth of CNTs.
Fig. 4-3 presents HRTEM images of the metal particles from 3 to 15 minutes. In
the pretreatment, the metal film was being melting due to heated by thermal and plasma.
It is show that the flat Ni layers arrive at island-like of catalysts and become a droplet upon buffer layer (Fig. 4-3 (a)). Because of continue treatment with 5 minutes, a droplet is defined by decreasing size between island-like of catalyst in thermal equilibrium on a horizontal surface (Fig. 4-3 (b)). At 10 minutes pretreatment, the droplet became smallest due to the suitable interfacial tension force on horizontal surface. The Ni nanoparticles are seemed corresponds to perfect forming from a film (Fig. 4-3 (c)). Over 15 minutes, large nanoparticles were occurred by unequal interfacial tension force (Fig.
4-3 (d)). It is indicates that increasing the period of pretreatment increased the change in
volume of the metal clusters, and conic particles were formed by pretreatment.
4.1.2.2 Indentation of catalyst from pretreatment
Fig. 4-4 illustrated the pretreatment Ni/TiN/Si substrate to endure the Berkovich indenter tip (tip radius ~50 nm) (a) before and (b) after. The loadings and unloadings
were performed with an approximately constant rate of 0.0166 mNs-1. Fig. 4-5(a) shows that before the indentations, the SEM images were performed us with images of the agglomerates. Thus, compare with after indentation (see in Fig. 4-5(b)), it is obvious that the Ni nanoparticles on the surface were identified by the Berkovich indenter tip within the indents part in low load force (0.25 mN). Especially, from the inset figure, the area of tip is almost 1 micrometer. These images were actually convolutions of the true surface and the shape of the tip, and therefore, were of limited resolution. However, the imprints of the tip are clearly visible. In contrast, the particles on the indented surface became enlarged, such that Ni particles are formation also occurred on the TiN surface.
Fig. 4-6 reveals the plots of load-displacement curves for the (a) Ni/TiN/Si thin
film and (b) Ni particles/TiN/Si. From the Fig. 4-6(a), Ni/TiN was indented with deformed material around the indents at low force. Based on the Fig. 4-6(b), the particles were observed only inside the indented parts, thus lead the decreased in modulus (from 238.9±8.4 to 176.2±6.1GPa) and hardness (from 17.2±1.6 to 11±0.8 GPa GPa) as compared in Tab. 4-2.
Fig. 4-7 shows the SEM image within the after Ni-coated catalyst were pretreated
for 3, 5, 10 and 15 minutes, hydrogen and methane (9:1) flowed into the chamber at 550°C for 10 minutes. The relationship among these conditions is presented in this
figure. CNTs were found variation of length, diameter, and morphology. Based on the pretreatment, the size of nanoparticle was depending on the increased pretreatment time (correspond in Fig. 4-1). Therefore, based on the constant grow time (10 minutes), it is induced that the CNTs have well aligned via 10 minutes pretreatment condition. It is believed that diffusion, nucleation and etching would occur during hydrogen plasma pretreatment and heating. The combined effects contribute to formation of nanoparticles with small spacing. These can act as the nucleation sites for growth of CNTs.
The crystal quality and structure of CNTs have been measured by Raman spectra according to the pretreatment condition. Fig. 4-8 shows Raman spectra of CNTs that were grown on the Ni/TiN systems. The Raman spectra of all samples show D-band peak and G-band peak around 1300 cm-1, 1550 cm-1 respectively. The intensity increases according to increasing pretreatment time. The D and G-band correspond to sp2 and sp3 carbon stretching modes, relatively and their intensity ratio is a measure of the amount of disorder in the CNTs. The D-band has known to be attributed to the carbonaceous particles, defects in the curved graphitic sheet and tube ends. The intensity of D-band peak is higher than that of the G-band at all samples, and it indicates that there were many carbonaceous particles or clusters in the CNTs. The ratio of the intensities of the D-band and G-bands (ID and IG) was summarized in Fig. 4-9. The ID to IG ratio decrease indicating the effect of the sp3 to sp2 bonded carbon configuration. In case of
pretreatment, the ratio decrease according to pretreatment time, the ID/IG ratio is 0.90, 0.89, 0.86, and 0.96 respectively. This result shows that the increase of pretreatment time decrease the ID/IG ratio and the disorder of CNTs (correspond in Tab. 4-1), so it reduces amorphous carbon and carbonaceous particles in CNTs and this result is consistent with the SEM result.
Fig. 4-10 displays typical TEM image of the CNTs synthesized after hydrogen
plasma pretreatment. TEM investigations reveal that the CNTs are not very straight at their root, their walls being corrugated. Particles at the CNTs root exhibit a faceted morphology. The CNTs associated to these particles exhibit outer diameter in the 50–80 nm range and inner diameter less than 30 nm. Particle size is considerably larger than the CNTs inner diameter and is often slightly larger than the CNTs outer diameter. Also, the critical value is related to the hydrogen plasma pretreatment, the CNTs quality also changes with the pretreatment and gas increases.
4.1.3 Conclusion
We have described the effect of pretreatment on the increasing time. From the SEM image of pretreated Ni catalyst layer, the increase time decrease the diameter of Ni catalyst grain size. The microstructure and properties of the pretreated Ni nanoparticles was characterized by in situ HR-TEM. The nanoindentation technique was used to study
the behavior of Ni nanoparticles. By the low load force testing, it is observed that the decreased in modulus (from 238.9±8.4 to 176.2±6.1GPa) and hardness (from 17.2±1.6 to 11±0.8 GPa) compared with the Ni film. Also, the density and vertical alignment of CNTs could be controlled by adjusting the density of nickel nanoparticles. From the Raman characterization indicates that the increase of pretreatment time decrease the ID/IG ratio and the disorder of CNTs, therefore, it indicates reducing amorphous carbon and carbonaceous particles in CNTs.
4.2 Effects of hydrogen plasma pretreatment on the TaN buffer layer for growth of carbon nanotubes
4.2.1 Introduction
The CNT undoubtedly occupies a unique position among advanced materials because of its novel electrical, mechanical and chemical characterizations [65,68].
These useful properties of CNTs make themselves good candidates for various application fields, such as the field-effect transistor [68], sensor [69], field emission display [45,70] and nanoscale interconnects [71]. CNTs can be synthesized by a variety of techniques, such as arc discharge, laser ablation, plasma-enhanced and thermal chemical vapor depositions (CVDs) [72~75]. While the former two techniques are suitable for large-scale production of CNTs, they cannot be used for self-assembly on material surfaces. CNTs synthesized by CVD are known to be longer than those obtained by other processes. It is possible to grow dense arrays of aligned CNTs by CVD [76] as well. Therefore, CVD is one of the prominent methods to synthesize high purity, high yield CNTs for practical applications.
Meanwhile, the control of CNT structure has technical advantage in the
sense that the structural diversity leads to different electronic and mechanical characterizations. There have been several attempts to control the structure of CNTs by various methods, including the pretreatment of the metal films on which CNTs are grown [77] and the direct control of structure by varying synthesis parameters [78]. In particular, the plasma etching can be used to transform a catalytic layer into catalytic nanoparticles, which may be applied to the density control of CNTs. In addition, however, in order to avoid the formation of metal silicide at a high temperature, a buffer layer was adopted in the annealing process [54].
Herein, the effects of H2 plasma pretreatment flow rate on the synthesis of CNTs on a Ni/TaN/Si substrate by means of microwave plasma chemical vapor deposition (MPCVD) system are investigated. The structure and composition of Ni catalyst nanoparticles are investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Raman spectroscopy equipped with a change coupled device detector is used to study the effect of the flow rate on the intensity ratio of G and D bands (ID/IG), which in turn measures the amounts of the amorphous carbon and carbonaceous particles in the CNT.
4.2.2 Results and discussion
The synthesis of CNTs by CVD often involves three mainly steps: first, decomposition of hydrocarbon gas at the surface of the catalyst nanoparticles; then, diffusion of resultant carbon atom in the nanoparticles to form the nucleation seed; and finally, precipitation of carbon atoms at the nanoparticles interface to form CNTs. It is well known and often proposed that the size and chemical composition of metal nanoparticles determine the diameter and structural perfection of the CNT [79].
Ni catalyst metal layers transformed into nanoparticles after various H2 plasma pretreatment flow rates are illustrated in Fig.4-11. From this figure, it is clearly observed that higher H2 plasma pretreatment flow rates leads to denser Ni catalyst nanoparticles. With the treatment of etching by H2 plasma, it can be found that the Ni catalyst metal layers breaks into small islands. From SEM observations, it is confirmed that the H2 plasma pretreatment plays an important role in promoting the uniform formation of Ni nanoparticles. The particle sizes of Ni catalyst metal layers treated by H2 plasma etching are approximately 20-30 nm, which are displayed in the cross-sectional TEM images in Fig.4-12. It is interesting to note that the geometries of the Ni catalyst particles were obviously affected by the H2 plasma pretreatment flow rate. As shown in Fig.4-11, at the flow rates of 100 sccm and 200 sccm the Ni catalyst particles have broad-based shapes, while at the flow rate of 300 sccm the Ni catalyst
particle has a semicircle-like one. Such a morphology difference is not surprising to be noticed because at a higher flow rate, the atoms in the catalyst particle can move around more easily via H2 plasma etching than at a lower flow rate. In the plasma environment, the H2 plasma plays a role in reducing Ni nanoparticles like the results proposed in Ref.
80. These observations also indicate that the geometry of a large catalyst particle can be
reshaped more easily at a higher flow rate for the CNT nucleation and growth.
Fig.4-13 shows the cross-sectional SEM images of the CNTs grown at the synthesis flow rates 100, 200 and 300 sccm, respectively. It can be seen that amorphous carbon and carbonaceous particles were decreased and denser vertically-aligned CNTs were displayed at a higher flow rate, as shown in Fig.4-13. In addition, the CNTs shown in Fig.4-13(c) were 30-40 nm in diameter and several micrometers in length. From this observation, the ability of Ni catalyst particles to change their shape can also explain why in the present experiment the highest density of CNTs was synthesized at the flow rate of 300 sccm.
The structure of CNTs, which is obtained from the Ni catalyst particles treated by H2 plasma at the flow rate of 300 sccm, is displayed in Fig.4-14. An embryonic Ni catalyst particle is formed in the course of H2 plasma pretreatment because of the difference of the interfacial energies between Ni catalyst particle/substrate and Ni catalyst particle/gas, with its catalytic decomposition of CH4 to liberate carbon atoms.
The change of elastic energy and surface energy of carbon layer caused the radius of curvature of Ni catalyst particle to become small. Then the rising gradient of the surface energy enhanced the surface diffusion of carbon atoms from bottom to top of the Ni catalyst particles. Therefore, significantly, a spindle-shaped Ni catalyst particle exists within the CNTs. The details of CNTs growth mechanisms can be found elsewhere [81].
In addition, the TEM image reveals that there are well-graphitized layer and the direction of graphite basal planes is parallel to the tube axis, as illustrated in Fig.4-14.
Raman spectroscopy (Raman spectroscopy equipped, CCMS in NTU) has been used to investigate the vibrational characterizations of the carbon samples. Raman spectra of CNTs obtained at H2 plasma pretreatment flow rates of 100, 200 and 300 sccm, respectively, are illustrated in Fig.4-15. All of Raman spectra display two broad bands at 1330 cm-1 (D-band) and 1580 cm-1 (G-band), respectively. The D-band is associated with the vibrations of carbon atoms with dangling bonds in plane terminations of “disordered graphite” or glassy carbons. The G-band corresponds to the E2g mode of graphite and, is related to the vibration of sp2-bonded carbon atoms in the two dimensional hexagonal lattice of the graphite layer. In addition, G-band indicates the degree of crystallinity in the graphite structure, while the intensity of D-band represents the impurities, defects or lattice distortions in CNTs.
In 2000, Ferrari et al. [82] proposed that the intensity ratio of G and D bands (ID/IG)
is related to the sp2 carbon cluster sizes in the graphene sheet and is nearly proportional to the defect density. The ID/IG ratio is 0.96, 0.92 and 0.84 of H2 plasma pretreatment flow rates of 100, 200 and 300 sccm, respectively, which is shown in Fig.4-16. This shows that a lower degree of structural disorder exists after the CNTs were H2 plasma pretreated. Results indicated that ID/IG ratio is decreased with increasing the flow rate.
Further from the analysis of Raman spectra, we observed that the higher flow rate induces the amorphization of lattice and formation of defects in CNTs, indicating that the decrease of the degree of disorder in CNTs and this result is consistent with the SEM observations.
4.2.3 Conclusions
To summarize, we have combined SEM, Raman and TEM techniques to investigate the effects of H2 plasma pretreatment flow rate on the synthesis of CNTs.
We synthesized CNTs by using MPCVD system on Ni/TaN/Si substrates. From SEM observations, higher flow rates leads to denser Ni catalyst nanoparticles.
Furthermore, the results of Raman spectra and TEM indicate that the morphologies of CNTs transform from amorphous carbon to crystalline graphite structure or finite sized graphite structure, depending on the H2 plasma pretreatment flow rate. A decrease in the number of defects and optimized morphologies therefore is believed to play a
significant role in the improvement in the field emission characterizations observed in the future.
4.3 Effects of fluorocarbon/oxygen plasma post-treatment on the surface performance of multiwalled carbon nanotubes
4.3.1 Introduction
CNTs have attracted tremendous interest from fundamental and technological aspects this last decade since their discovery in 1991 because of their special physical and chemical characteristics [83]. It is well-known that the multiwalled carbon nanotubes (MWCNTs) have mechanical characterizations quite different from those of single-waled carbon nanotubes (SWCNTs) [84~86]. These differences are reflected in either their macroscopic material characterizations or their processing and handling behaviors or both. Also, MWCNTs have been investigated widely on their electrical transport characterizations as the most promising material for nanoelectronics [87~88].
The electrical transport characterizations of MWCNTs are exported to depend on their structures such as multiplicity, chirality, and so on.
Among various methods (such as arc discharge, laser ablation and chemical vapor deposition) developed to grow CNTs, plasma-enhanced chemical vapor deposition (PECVD) methods are attracting great interest because of their specificities, such as
synthesis temperature [89]. To go further on the control of CNTs structure, an understanding of the growth mechanisms of CNTs is needed but it has not been clearly provided yet. The roles of various gases as precursors for applying the synthesis of CNTs have been presented [79, 90], but we have not fully understood the effects of each gas on the post-treatment of CNTs. In our previously study [91], the effects of ammonia plasma treatment on the surface characteristics of carbon fibers was analyzed during the plasma treatment procedure, but never, at the best of our knowledge, the role of fluorocarbon/oxygen plasma treatment in the characteristics of CNTs by PECVD.
The aim of this chapter is to present the results obtained with the CNTs grown by
The aim of this chapter is to present the results obtained with the CNTs grown by