**3.4. SEM-Z contrast-STEM-EDX combined experiments**

An advanced surface science system, capable of SEM and scanning transmission electron microscopy (STEM) investigations, performed at the same location of a specimen, was involved for the synthesized SWCNTs.

The special features of the synthesized SWCNTs are presented in **Figure 11** based on the capabilities of the special equipment presented above: *SWCNTs images in SEM, Z contrast (atomic mass contrast), STEM and energy dispersive X-ray spectroscopy (EDX) modes at the same sample location*. The images allow to a clear identification of catalysts grains and amorphous carbon (EDX mapping sustained also by Z contrast image) and in **Figure 11(d)** we observe that carbon is wide-spread. In SEM and STEM mode we can see the nanotubes shape and observe the opposite contrast of the images (a) and (c).

**Figure 11.** SWCNTs images taken at the same sample location in (a) SEM mode; (b) Z-contrast mode; (c) STEM mode; (d–f) EDX mapping mode for C, Co, Ni, respectively.

### **3.5. HR-STEM experiments**

In order to get direct information on the morphology of the synthesized SWCNTs, we performed HR-STEM investigations.

We measured the diameters of several SWCNTs using HR-STEM images as in **Figure 12**. The measured diameters distribution and its histogram are plotted in **Figure 13(a)** and **(b)**, respectively. The histogram of diameters distribution was in the range of 1.0–1.7 nm, and the center between 1.25 and 1.35 nm [23]. This result is in great agreement with the result obtained from micro-Raman measurements (average *d*A = 1.35 nm).

**Figure 12.** The measurement of SWCNTs diameter by HR-STEM.

**Figure 13.** (a) Measured diameters distribution of SWCNTs. (b) Histograms of diameter distribution of the SWCNTs.

### **3.6. TGA experiments**

**3.4. SEM-Z contrast-STEM-EDX combined experiments**

298 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

the opposite contrast of the images (a) and (c).

(d–f) EDX mapping mode for C, Co, Ni, respectively.

**3.5. HR-STEM experiments**

formed HR-STEM investigations.

for the synthesized SWCNTs.

An advanced surface science system, capable of SEM and scanning transmission electron microscopy (STEM) investigations, performed at the same location of a specimen, was involved

The special features of the synthesized SWCNTs are presented in **Figure 11** based on the capabilities of the special equipment presented above: *SWCNTs images in SEM, Z contrast (atomic mass contrast), STEM and energy dispersive X-ray spectroscopy (EDX) modes at the same sample location*. The images allow to a clear identification of catalysts grains and amorphous carbon (EDX mapping sustained also by Z contrast image) and in **Figure 11(d)** we observe that carbon is wide-spread. In SEM and STEM mode we can see the nanotubes shape and observe

**Figure 11.** SWCNTs images taken at the same sample location in (a) SEM mode; (b) Z-contrast mode; (c) STEM mode;

In order to get direct information on the morphology of the synthesized SWCNTs, we per-

We measured the diameters of several SWCNTs using HR-STEM images as in **Figure 12**. The measured diameters distribution and its histogram are plotted in **Figure 13(a)** and **(b)**, reThermo-gravimetric analysis (TGA) was involved for the synthesized SWCNTs characterization as well as for the target composition used for obtaining the SWCNTs.

We analyzed by TGA, graphite powder, and materials used for our target. They burn in the same zone denoting that our target is graphitized. On the other hand collected material from cold finger (SWCNTs, green curve) burn in totally different place confirming it is a different material. What we also depicted in **Figure 14** is that ablation take place with respecting the target stoichiometry since the target materials and SWCNTs contain same percentage of remaining catalyst metals. From TGA curves we estimate a 70% yield of SWCNTs in raw product.

**Figure 14.** TGA curves for graphite powder (black), target material (red), SWCNTs (green).

### **3.7. Influence of carrier gas over SWCNTs synthesis**

### *3.7.1. Experimental description*

In an effort to compare our work with the work of Nishide *et al.* [20] which used Nd-YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) laser, we study the SWCNTs synthesis versus different inert carrier gas used during the ablation with KrF excimer laser. For this study we varied the ablation gas types while the other parameters: laser type, laser energy, pulse repetition rate, oven temperature, gas pressure, gas flow rate, and ablation time remained constant (**Table 2**). The laser ablation experiments were carried out in four different inert gases: argon, nitrogen, neon, and helium.


**Table 2.** Parameters used for SWCNTs synthesis in different gas carrier environment.

### *3.7.2. Ablated mass and collected mass*

We measured the mass of the target before and after ablation and also the raw product collected mass, i.e., SWCNTs. We observed linear decrease of ablated mass with the increase of carrier gas molecular mass. Also the collected mass follows the same trend, so we can increase and optimize the SWCNTs production by changing carrier gas. Fitting lines are shown in **Figure 15**. High-Quality Carbon Nanomaterials Synthesized by Excimer Laser Ablation http://dx.doi.org/10.5772/65309 301

**Figure 15.** Ablated mass and collected mass versus gas molecular mass.

### *3.7.3. Micro-Raman experiments*

**Figure 14.** TGA curves for graphite powder (black), target material (red), SWCNTs (green).

300 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

In an effort to compare our work with the work of Nishide *et al.* [20] which used Nd-YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) laser, we study the SWCNTs synthesis versus different inert carrier gas used during the ablation with KrF excimer laser. For this study we varied the ablation gas types while the other parameters: laser type, laser energy, pulse repetition rate, oven temperature, gas pressure, gas flow rate, and ablation time remained constant (**Table 2**). The laser ablation experiments were carried out in four different

We measured the mass of the target before and after ablation and also the raw product collected mass, i.e., SWCNTs. We observed linear decrease of ablated mass with the increase of carrier gas molecular mass. Also the collected mass follows the same trend, so we can increase and optimize the SWCNTs production by changing carrier gas. Fitting lines are shown in **Figure 15**.

**3.7. Influence of carrier gas over SWCNTs synthesis**

inert gases: argon, nitrogen, neon, and helium.

**Parameter Value**

Laser energy 600 mJ Repetition rate 30 Hz Pulse period 20 ns Oven temperature 1100°C Gas pressure 500 Torr Ablation time 60 min

*3.7.2. Ablated mass and collected mass*

Laser type Excimer KrF, 248 nm

**Table 2.** Parameters used for SWCNTs synthesis in different gas carrier environment.

*3.7.1. Experimental description*

Typical micro-Raman spectra for SWCNTs obtained in different inert gases are shown in **Figure 16(a)**. The excitation laser was green, 532nm.

**Figure 16.** (a) Micro-Raman spectra for SWCNTs obtained using different ablation gases; (b) Radial breathing mode (RBM) part of the spectra; (c) G band part of the spectra.

In the zone of radial breathing mode (RBM) **Figure 16(b)** we found two peaks whose frequencies are strongly SWCNTs diameter dependent. The diameters were calculated in accordance with Eq. (1). For all gases two diameters distribution of 1.31 and 1.47 nm were calculated see **Figure 16(b)** (black lines) in great agreement with the result obtained from statistically HR-STEM measurements 1.25–1.35 nm (**Figure 13**).

### *3.7.4. TGA experiments*

As can be seen in **Figure 17** we found different yields of SWCNTs versus the gas carrier. In **Table 3** using the yields and collected mass we calculate the mass of SWCNTs contained in the soot and the highest value was obtained in helium, almost 10 times more than in argon.

**Figure 17.** TGA curves of the ablation product, i.e., SWCNTs, obtained in different inert gases (Gas: air, *T* = 100–850°C).


**Table 3.** SWCNTs mass calculated from the TGA curves.
