**3. Pulsed laser deposition of ITO nanostructures.**

The growth of ITO nanostructures (nanowires, nanorods, nanowhiskers, and nanocrystals) is desirable because of the distinct nanoscale effects in addition to the advantages in large contact areas offered by nanomaterials. The nanoeffects of ITO nanostructures have been demonstrated in various aspects based on the UV light-emitting properties [34], terahertz and far-infrared transmitting characteristics [35], and field emission properties [36]. PLD, as a flexible and versatile tool for materials deposition, was first shown to be capable for the growth of nanowires by using excimer laser ablation in 2006 [37]. ITO nanowires were grown on catalyst-free oxidized silicon substrates at 500°C in nitrogen atmosphere. The growth of nanostructures was not observed by others although the growth involving non-oxygen gases such as N2 [19], rare gas Ne, Ar, and Xe [22] (**Tables 2** and **3**) has been reported. In this report, ITO films with lower resistivity and higher transmittance were obtained when deposited in O2 . However, in the report by Savu and Joanni [37], when higher substrate temperate and Si substrate were used, nanostructural growth of ITO was observed in N2 . The morphology of the nanostructures was found to be highly dependent on the N2 gas pressure as shown in

**Figure 6.** Surface images of nanostructured films deposited at 0.1 mbar (a, b), 0.5 mbar (c, d), 1 mbar (e, f), and 2 mbar (g, h) [37].

**Figure 5.** Effects of substrate temperature on the crystallinity of the ITO films deposited by (a) 248 nm laser [12] and (b)

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

355 nm laser [26].

**Figure 6**. The growth mechanisms were proposed to be related to vapor-liquid-solid growth as a large amount of liquid was formed, especially at low pressure, thus promoting the growth of thin, branched nanowires. As the deposition pressure increases, the amount of liquid phase decreases, resulting in formation of nanowires having fewer branches. At a pressure of 1 mbar, the wires were almost perpendicular to the substrate and free of branches. At 2 mbar, columnar dense film is formed with large pyramidal and triangular structures.

Based on a standard PLD setup as shown in **Figure 2**, we studied the effects of different background gases Ar, He, N2 , and O2 in details [38–40]. Glass substrates were used, and the growth was performed at a lower substrate temperature of 250°C. ITO nanostructures were formed, and the morphology of the nanostructures formed in different gases is shown in **Figure 7** [40]. The ITO film was uniform in size when deposited in O2 , while ITO deposited in Ar consisted of ultrafine nano-grains with a size of < 50 nm. For ITO deposited in N2 , the nanostructure consisted of porous network of nanorods of about 30 nm in diameter and 300 nm in length. Larger structures were obtained when deposited in He. ITO nanostructures formed in Ar, He, and O2 were highly crystalline and possess higher transmittance than those obtained in N2 . The resistivity for ITO nanostructures deposited in N2 was also higher than those deposited in Ar, He, and O2 . The results show that nanostructures can be obtained under specific conditions, which is not limited to higher temperature range and Si substrates in the first report by Savu and Joanni [37].

**Figure 7.** Effects of the background gas on the ITO microstructures. (a) O2 , (b) Ar, (c) N2 , and (d) He [40].

Further studies were performed for the growth in He and Ar at different background pressures based on the same setup [39]. The pressure range was chosen by considering the molecular

**Figure 6**. The growth mechanisms were proposed to be related to vapor-liquid-solid growth as a large amount of liquid was formed, especially at low pressure, thus promoting the growth of thin, branched nanowires. As the deposition pressure increases, the amount of liquid phase decreases, resulting in formation of nanowires having fewer branches. At a pressure of 1 mbar, the wires were almost perpendicular to the substrate and free of branches. At 2 mbar, columnar dense film is formed with large pyramidal and triangular structures. Based on a standard PLD setup as shown in **Figure 2**, we studied the effects of different back-

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

was performed at a lower substrate temperature of 250°C. ITO nanostructures were formed, and the morphology of the nanostructures formed in different gases is shown in **Figure 7** [40].

sisted of porous network of nanorods of about 30 nm in diameter and 300 nm in length. Larger structures were obtained when deposited in He. ITO nanostructures formed in Ar, He, and O2

. The results show that nanostructures can be obtained under specific conditions, which is not limited to higher temperature range and Si substrates in the first report by Savu and Joanni [37].

in details [38–40]. Glass substrates were used, and the growth

was also higher than those deposited in Ar, He, and

, (b) Ar, (c) N2

, and (d) He [40].

, while ITO deposited in Ar consisted

, the nanostructure con-

. The resistiv-

ground gases Ar, He, N2

O2

, and O2

The ITO film was uniform in size when deposited in O2

**Figure 7.** Effects of the background gas on the ITO microstructures. (a) O2

ity for ITO nanostructures deposited in N2

of ultrafine nano-grains with a size of < 50 nm. For ITO deposited in N2

were highly crystalline and possess higher transmittance than those obtained in N2

**Figure 8.** XRD and SEM of ITO samples grown in Ar ambient at (a) 20 mTorr, (b) 30 mTorr, and (c) 40 mTorr and grown in He at (d) 250 mTorr, (e) 1 Torr, and (f) 2 Torr [39].


**Table 4.** Resistivity, carrier density, and Hall mobility of commercial ITO and ITO samples grown in Ar and He ambient.

weight of the gas that will affect the collision with ablated species. The growth was performed by PLD using a 355 nm laser at a substrate temperature of 250°C. The results show that nanostructures growth was dependent critically on the background pressure (**Figure 8**), similar to those reported in N2 [37]. ITO nanowires were obtained in both gases. For ITO deposited in Ar, XRD diffraction patterns corresponding to (222), (400), (440), and (622) orientations of cubic bixbyite structure of ITO were detected. As the pressure of Ar increased, the (400) diffraction peak became relatively stronger, indicating the increase in crystalline orientation.

**Figure 9.** Optical transmittance of commercial ITO and ITO samples grown in Ar and He background gases [39].

At 30 mTorr, ITO nanowires were formed, and some spherical particles were observed on their tips, which suggest that they are formed by the vapor-liquid-solid (VLS) mechanism. At higher pressure of 40 mTorr, nanowires with smaller diameters were observed. No spherical particles were observed, and the tips were sharp, unlike those obtained at lower pressure. When deposited in He, nanowires were obtained at 250 mTorr, and spherical tips were formed on the tips. As the pressure increased, larger pyramid shape crystals were obtained, and the crystals orientation are aligned to (400). Both nanostructures grown in Ar and He exhibited good resistivity in the range of 10−4 Ω cm, and nanowires grown in Ar exhibited even higher carrier mobility than those measured from a commercial ITO sample. The values are shown in **Table 4**. **Figure 9** shows the optical transmittance of ITO nanostructures grown in Ar and He as compared to a commercial ITO sample. In addition, ITO nanowires grown by PLD are tested as TCO layer for a standard OLED device. ITO nanowires with larger contact areas and higher charge injection have led to higher emission current density for the devices [39].
