**2.1. Different types of reactors for laser ablation experiments**

Previous laser ablation experiments have been focused on a chamber design commonly referred to as "front pumped counter flow pulse laser vaporization (PLV)". The design, presented in **Figure 3(a)** consists of an electric oven having a quartz tube inserted into it. The carbon target doped with small amount of metal catalyst (1) is located in the mid-position inside the quartz tube. The laser beam passes a quartz window (3) and is focused on the target. The ablation gas is fed from the front of reactor and a vacuum pump is located downstream of the cold finger (2) generating an even flow and pressure inside of the tube. The plume resulting from ablation of the target and evaporated species are transported by the inert gas toward the cold finger where they condense. Guo *et al.* reported for the first time SWCNTs production by laser ablation using a typically front pumped counter flow PLV [16].

Some other experiments have been carried out using a similar chamber design [18, 19].

Another chamber design includes a smaller quartz tube through which the carrier gas can be fed close to the target in order to confine the plume to a smaller region [17, 25].

In **Figure 3(b)**, the schematically design is represented. These simple designs proved to be effective.

bonding and structure by pressure, often introduced by hitting the material with a foreign

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

Pulsed laser vaporization (PLV) or laser ablation is a technique to produce high-quality carbon nanostructures. A laser beam is focused on a heated carbon target. The resulting plume and vapors are carried by an inert gas to a cooled cooper collector called cold finger (CF), where the raw material condenses. A sea urchin, also known as a dahlia structure, is a colloquial designation for the compact agglomeration of carbon nano-horns formed during the laser

PLV or PLD (pulsed laser deposition) is the technique that will be in great detail described

We will focused in this section on synthesis of SWCNTs and other carbon nanomaterials via laser ablation since this is the technique used in ours laboratories. The pulsed laser ablation process for production of SWCNTs has first been reported by Guo *et al.* [16]. Nanotubes produced by laser ablation are up to 90% pure [17]. The target consists of mainly graphite to

which an ablation gas is used to control the dynamics of the plume and carry the ablation products to a cooled collector. Among the gases investigated Argon has been the most studied [16, 18, 19], but we and others proved that other gases like nitrogen [20, 21], krypton [20, 22], neon [20, 23], and helium [24] are also suitable for SWCNTs production by laser ablation.

Previous laser ablation experiments have been focused on a chamber design commonly referred to as "front pumped counter flow pulse laser vaporization (PLV)". The design, presented in **Figure 3(a)** consists of an electric oven having a quartz tube inserted into it. The carbon target doped with small amount of metal catalyst (1) is located in the mid-position inside the quartz tube. The laser beam passes a quartz window (3) and is focused on the target. The ablation gas is fed from the front of reactor and a vacuum pump is located downstream of the cold finger (2) generating an even flow and pressure inside of the tube. The plume resulting from ablation of the target and evaporated species are transported by the inert gas toward the cold finger where they condense. Guo *et al.* reported for the first time SWCNTs

production by laser ablation using a typically front pumped counter flow PLV [16].

fed close to the target in order to confine the plume to a smaller region [17, 25].

Some other experiments have been carried out using a similar chamber design [18, 19].

Another chamber design includes a smaller quartz tube through which the carrier gas can be

C [16, 18, 19] through

below along the results of the synthesis obtained using this method.

The laser ablation takes place inside a quartz tube heated to 800–1200°

**2.1. Different types of reactors for laser ablation experiments**

which small amounts of metal catalysts are added.

object.

ablation process.

**2. Laser ablation**

**Figure 3.** (a) Design of front pumped counter flow PLV system [16, 18, 19]; (b) design of front pumped counter flow PLV system with inner tube [17, 25].

The short length of the oven is the main disadvantage of these designs. From here is resulting a small zone were the ablation medium could be maintained at a certain needed temperature. Short oven is equivalent with a high temperature gradient into the ablation chamber. In our new design this disadvantage is avoided.

**Figure 4.** (a) Design of "T"-shaped PLV chamber with laser entrance from the side [26]; (b) "X"-shaped PLV chamber [27].

For increasing the ablated surface Holloway *et al.* proposed A "T" shaped quartz tube where the laser enters the chamber from the side [26]. The design is schematically represented in **Figure 4(a)**. The design includes feeding the ablation gas through a narrow tube coiled around the transfer rod in order to obtain a higher surface, thus increasing the heat transfer. The narrow tube takes the hot gas close to the target so that the products are carried away towards the cold finger. By this method can be ablated a bigger surface of the target. Complexity of the system which implies a custom made quartz tube with a side viewport for laser is the main disadvantage of this design. Geometry of the oven providing heating and temperature into the chamber is changed by this design.

The geometry becoming cumbersome and harder to control, resulting in high temperature gradients and a turbulent flow of the gas.

A chamber in "X" shape similar to those used for thin film depositions was proposed by Yahya *et al.* [27]. In **Figure 4(b)** is shown this design. The target (1) is positioned at an angle (usually 45°) parallel with water cooled substrate (2) at different distances. A quartz tube is not needed for this design, only the laser entrance window resulting in a more robust and easy to fabricate system. The system is usefully since it can be used for other purposes such as film deposition. With this type of reactor only multi walled carbon nanotubes (MWCNTs) were obtained. The system is not heated into an oven making impossible for the plume to reach eutectic point. May be this is one of the reasons that SWCNTs have not been obtained using such a chamber design. Other disadvantage of such reactor is the inert gas is not directed to carry the products from target to substrate, but only provides an inert atmosphere inside.

## **2.2. Our new chamber design**

**Figure 5** presents a cut-away side view of the novel experimental set-up developed and used in our research for the laser ablation of the targets [28]. The laser ablation chamber consists of a quartz tube, 50 mm in diameter and 1260 mm long, inserted into an oven. The quartz tube is O-ring sealed and operates from 10−3 Torr up to atmospheric pressure and temperature is controlled from 30 to 1200°C.

**Figure 5.** New reactor – Patent pending.

Laser ablation starts by passing the laser beam through a UV transparent quartz window and enters into the quartz tube hitting the target, when the target material begins to be ablated. The target having 20 mm in diameter is mounted on a graphite rod. Target was rotated during ablation with constant speed to get uniform ablation. Then the inert gas, which enters from the left-up side of the reaction chamber, moves through the quartz tube to the heated area, e.g., at 1100°C, where the reaction takes place, transporting the ablation product toward a copper condenser called cold finger (CF), where it will be deposited as black soot. The cold finger was cooled to 12°C using water supplied by a chiller. The inert atmosphere and the transportation of the ablated material to the CF were maintained by using the carrier gas at a certain flow rate, e.g., 70 L/h. The pressure is maintained by pumping through a needle valve and measured by a vacuum gauge sitting just outside the chamber.

The KrF excimer laser used for the ablation is a Coherent COMPex Pro 205 equipment, having the wavelength of 248 nm and the pulse duration of 20 ns.

Our system has some special technical features:

vantage of this design. Geometry of the oven providing heating and temperature into the

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

The geometry becoming cumbersome and harder to control, resulting in high temperature

A chamber in "X" shape similar to those used for thin film depositions was proposed by Yahya *et al.* [27]. In **Figure 4(b)** is shown this design. The target (1) is positioned at an angle (usually 45°) parallel with water cooled substrate (2) at different distances. A quartz tube is not needed for this design, only the laser entrance window resulting in a more robust and easy to fabricate system. The system is usefully since it can be used for other purposes such as film deposition. With this type of reactor only multi walled carbon nanotubes (MWCNTs) were obtained. The system is not heated into an oven making impossible for the plume to reach eutectic point. May be this is one of the reasons that SWCNTs have not been obtained using such a chamber design. Other disadvantage of such reactor is the inert gas is not directed to carry the products

**Figure 5** presents a cut-away side view of the novel experimental set-up developed and used in our research for the laser ablation of the targets [28]. The laser ablation chamber consists of a quartz tube, 50 mm in diameter and 1260 mm long, inserted into an oven. The quartz tube is O-ring sealed and operates from 10−3 Torr up to atmospheric pressure and temperature is

Laser ablation starts by passing the laser beam through a UV transparent quartz window and enters into the quartz tube hitting the target, when the target material begins to be ablated. The

from target to substrate, but only provides an inert atmosphere inside.

chamber is changed by this design.

**2.2. Our new chamber design**

controlled from 30 to 1200°C.

**Figure 5.** New reactor – Patent pending.

gradients and a turbulent flow of the gas.

(i) The length of the oven was doubled compared with previous designs [16, 29], to ensure a more uniform temperature in the ablation reactor, allowing the product to travel longer time in the constant heated zone favoring the growth of the SWCNTs.

**Figure 6(a)** shows a schematic comparison between size accurate representations of our chamber design and other chamber designs [16, 18, 19]. Ratio of the length of the new chamber versus older designs is over 2:1. The temperature distribution expected as a result of increased oven length is represented by the red semicircles. Kataura *et al.* have shown that increasing the furnace temperature and the time spent by the ablation products at this elevated temperature increases the SWCNTs production yield [30].

**Figure 6.** Our design (above) [28] and previously used chamber designs (below) [16, 18, 19]: (a) schematic representation of the heated area inside the ablation chamber; (b). Schematic representation of temperature gradient given by the water cooled cold finger. Ratio of the length of the new cold finger versus older design is 4.5:1.

(ii) The cold finger is longer (260 mm) with a larger surface than the previous one [28, 31], increasing the temperature gradient over its length and improving the capture of the product. A comparison of them is displayed in **Figure 7**, from where we can see that ratio of the length of the new cold finger versus older designs is over 4.5:1.

**Figure 7.** Photograph of the new and previous designed cold fingers.

The longer cold finger provides a larger surface resulting in a better percentage collection of the ablation products. The temperature gradient induced by the collector outside the oven area is distributed on a larger length of the cold finger, as presented in **Figure 6(b)**. This provides the means to study the influence the cold finger temperature gradient has on the condensation of the ablation products, i.e., the final products.

(iii) The attached Alicat flow meter controller allows controlling accurately the flow rate of the carrier gas over a wide range, 0–10 normal liters per minute (NLPM), with a resolution of 0.01 NLPM.
