**2. ZnO nanostructures from solvothermal method**

The process of the growth of the ZnO NWs is similar to Law et al. [2]. Arrays of ZnO NWs were synthesized on fluorine-doped tin oxide (FTO) substrates that were first cleaned thoroughly by sonication with acetone/ethanol and then coated with a thin film of ZnO QDs, 3–4 nm in diameter, by dip-coating in a 0.005 M Zn(OAc)2 concentrated ethanol solution. The arrays of ZnO NWs were synthesized on seeded FTO substrates by immersing the seeded substrates in aqueous solutions containing 0.06 M zinc nitrate hydrate, 0.06 M hexamethylenetetramine (HMTA), and 7.5 mM polyethylenimine (PEI) at 95°C for 2.5–7.5 hours. **Figure 1** shows the tilted-view SEM images of the ZnO NWs on FTO layers and alumina-doped ZnO layers (AZO, 50~200 Ω per square), respectively. It seems that the aspect ratio (length against diameter) of nanowires can be increased when using PEI. The length and diameter of the PEI-absent ZnO NWs can be controlled in the range of about 300–500 nm and 300–400 nm, respectively. But, the length and diameter of the PEI-present ZnO NWs can be adjusted in the range of about 500–700 nm and 150–250 nm, respectively. The lengths of ZnO NWs are also prolonged while increasing the growth times. For example, the length and diameter of the ZnO NWs were in the range of about 2–3 μm and 250–350 nm, respectively, while the growth time is of 7.5 hours. Furthermore, the ZnO nanostructures with different morphology can be observed while using various substrates. The density and diameter of ZnO NWs seem to be influenced while using AZO substrates. The length and diameter of the ZnO NWs were in the range about 0.2–1.2 μm and 200–500 nm, respectively. By sputtering technique, it is reasonable to presume that the grain size of thick AZO films (200–300 nm) is bigger than dip-coating method, which caused the increase in the NW diameter and the good orientation as well. However, the fluctuation of diameter for ZnO NWs is huge while using AZO substrates, which could be related to the uniformity of AZO under layers.

Beyond the ZnO NWs, the impressive branched ZnO NWs have been fabricated successfully from solvothermal method [4]. First, the arrays of ZnO NWs were synthesized on seeded FTO substrates by immersing the seeded substrates in aqueous solutions containing 0.08 M zinc nitrate hydrate, 0.08 M HTMA, and 12 mM PEI at 95°C for 10 hours. Second, the ZnO NW substrate obtained from the first step were re-coated with seed layers of ZnO NPs by dip-coating in a 0.005 M Zn(OAc)2 in ethanol. Then the branched NWs were grown by immersing the seeded ZnO NWs for a 5-hour duration in an aqueous solution containing 0.02 M zinc nitrate hydrate, 0.02 M HMTA, and 3 mM PEI at 95°C ambient. The final products were immediately rinsed with deionized water and baked in air at 450°C for 30 minutes to

**39**

density of about 7 × 108

**Figure 1.**

cm<sup>−</sup><sup>2</sup>

*grown 7.5 hours. (d) AZO substrate, with PEI and grown 7.5 hours.*

the branched ZnO NWs can successfully be fabricated.

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

remove any residual organics. The evolution of the ZnO NWs to the branched ZnO NWs is illustrated in **Figure 2a** and its corresponding FESEM images **Figure 2b**–**d**. By using a solvothermal method, the bare ZnO NWs with slight vertical off-alignment were grown perpendicularly on the FTO substrate, as shown in **Figure 2b**. Through the pre-coating process on the ZnO NWs, little ZnO crystallites with diameter 10–20 nm were formed on the backbone NWs, as shown in **Figure 2c**. After the second growth step, radial secondary ZnO branches emanated from the seeds, as shown in **Figure 2d**. The entire substrate in which the backbone nanowire has a

*The SEM images of the ZnO nanowires and nanorods with different growth conditions (a) FTO substrate, without PEI, grown 2.5 hours. (b) FTO substrate, with PEI, grown 2.5 hours. (c) FTO substrate, with PEI,* 

backbone NWs have length and diameter in the range of 7–8 μm and 150–250 nm, respectively, whereas the secondary branches have length and diameter ranging from 100 to 300 nm and 20 to 50 nm, respectively. By the infiltration of moderately concentrated Zn(OAc)2 solution into interstitial voids between backbone ZnO NWs,

Further structural characterizations of the branched ZnO NWs were performed by TEM. **Figure 3a** reveals the secondary ZnO branches which were grown on the side walls of nanowire backbone with different radial angles. The evidence confirmed the secondary ZnO branches were not the derivatives of ZnO NWs but definitely originated from the small crystallite ZnO seeds via the pre-coating process. Different from the other reports of comblike ZnO nanostructures which demonstrated the monolithically single-crystalline relationship between the branches and backbone nanowires [5, 6], in the present mechanism, the secondary ZnO branches were derived on the ZnO seeds in spite of the coordinate crystal relationship. The magnified intersection area of ZnO

was covered with branched ZnO nanostructures. The

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

**Figure 1.**

*Nanostructures*

device performance. As a wide-bandgap semiconductor, ZnO has been reported as an alternative for dye-sensitized solar cells (DSCs) because ZnO offers a large direct

for the high-quality thin film [1]. ZnO provides a promising alternative for improving the performance of the photoelectrode in DSCs because ZnO can be tailored to various nanostructures. In the present proposal, first, I will present the ZnO nanowire structures via a low-pressure vapor-phase deposition and a simple solvothermal method. The one-dimensional ZnO NWs could simultaneously afford a direct conduction pathway to significantly enhance the overall efficiency of the DSCs [2, 3]. Next, I also will demonstrate how to employ the hierarchical structure of the ZnO NPs, fabricated from sol-gel method, which would promote light scattering through the presence of secondary colloidal spheres, thus, enhancing photon absorption to improve the short-circuit current density and the overall light conversion efficiency.

The process of the growth of the ZnO NWs is similar to Law et al. [2]. Arrays of ZnO NWs were synthesized on fluorine-doped tin oxide (FTO) substrates that were first cleaned thoroughly by sonication with acetone/ethanol and then coated with a thin film of ZnO QDs, 3–4 nm in diameter, by dip-coating in a 0.005 M Zn(OAc)2 concentrated ethanol solution. The arrays of ZnO NWs were synthesized on seeded FTO substrates by immersing the seeded substrates in aqueous solutions containing 0.06 M zinc nitrate hydrate, 0.06 M hexamethylenetetramine (HMTA), and 7.5 mM polyethylenimine (PEI) at 95°C for 2.5–7.5 hours. **Figure 1** shows the tilted-view SEM images of the ZnO NWs on FTO layers and alumina-doped ZnO layers (AZO, 50~200 Ω per square), respectively. It seems that the aspect ratio (length against diameter) of nanowires can be increased when using PEI. The length and diameter of the PEI-absent ZnO NWs can be controlled in the range of about 300–500 nm and 300–400 nm, respectively. But, the length and diameter of the PEI-present ZnO NWs can be adjusted in the range of about 500–700 nm and 150–250 nm, respectively. The lengths of ZnO NWs are also prolonged while increasing the growth times. For example, the length and diameter of the ZnO NWs were in the range of about 2–3 μm and 250–350 nm, respectively, while the growth time is of 7.5 hours. Furthermore, the ZnO nanostructures with different morphology can be observed while using various substrates. The density and diameter of ZnO NWs seem to be influenced while using AZO substrates. The length and diameter of the ZnO NWs were in the range about 0.2–1.2 μm and 200–500 nm, respectively. By sputtering technique, it is reasonable to presume that the grain size of thick AZO films (200–300 nm) is bigger than dip-coating method, which caused the increase in the NW diameter and the good orientation as well. However, the fluctuation of diameter for ZnO NWs is huge while using AZO substrates, which could be related

Beyond the ZnO NWs, the impressive branched ZnO NWs have been fabricated successfully from solvothermal method [4]. First, the arrays of ZnO NWs were synthesized on seeded FTO substrates by immersing the seeded substrates in aqueous solutions containing 0.08 M zinc nitrate hydrate, 0.08 M HTMA, and 12 mM PEI at 95°C for 10 hours. Second, the ZnO NW substrate obtained from the first step were re-coated with seed layers of ZnO NPs by dip-coating in a 0.005 M Zn(OAc)2 in ethanol. Then the branched NWs were grown by immersing the seeded ZnO NWs for a 5-hour duration in an aqueous solution containing 0.02 M zinc nitrate hydrate, 0.02 M HMTA, and 3 mM PEI at 95°C ambient. The final products were immediately rinsed with deionized water and baked in air at 450°C for 30 minutes to

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> )

bandgap which is close to TiO2 and even higher electron mobility (155 cm2

**2. ZnO nanostructures from solvothermal method**

to the uniformity of AZO under layers.

**38**

*The SEM images of the ZnO nanowires and nanorods with different growth conditions (a) FTO substrate, without PEI, grown 2.5 hours. (b) FTO substrate, with PEI, grown 2.5 hours. (c) FTO substrate, with PEI, grown 7.5 hours. (d) AZO substrate, with PEI and grown 7.5 hours.*

remove any residual organics. The evolution of the ZnO NWs to the branched ZnO NWs is illustrated in **Figure 2a** and its corresponding FESEM images **Figure 2b**–**d**. By using a solvothermal method, the bare ZnO NWs with slight vertical off-alignment were grown perpendicularly on the FTO substrate, as shown in **Figure 2b**. Through the pre-coating process on the ZnO NWs, little ZnO crystallites with diameter 10–20 nm were formed on the backbone NWs, as shown in **Figure 2c**. After the second growth step, radial secondary ZnO branches emanated from the seeds, as shown in **Figure 2d**. The entire substrate in which the backbone nanowire has a density of about 7 × 108 cm<sup>−</sup><sup>2</sup> was covered with branched ZnO nanostructures. The backbone NWs have length and diameter in the range of 7–8 μm and 150–250 nm, respectively, whereas the secondary branches have length and diameter ranging from 100 to 300 nm and 20 to 50 nm, respectively. By the infiltration of moderately concentrated Zn(OAc)2 solution into interstitial voids between backbone ZnO NWs, the branched ZnO NWs can successfully be fabricated.

Further structural characterizations of the branched ZnO NWs were performed by TEM. **Figure 3a** reveals the secondary ZnO branches which were grown on the side walls of nanowire backbone with different radial angles. The evidence confirmed the secondary ZnO branches were not the derivatives of ZnO NWs but definitely originated from the small crystallite ZnO seeds via the pre-coating process. Different from the other reports of comblike ZnO nanostructures which demonstrated the monolithically single-crystalline relationship between the branches and backbone nanowires [5, 6], in the present mechanism, the secondary ZnO branches were derived on the ZnO seeds in spite of the coordinate crystal relationship. The magnified intersection area of ZnO

#### **Figure 2.**

*(a) The schematic growth procedure from the original ZnO nanowires to the branched ZnO nanowires. (b) Before and (c) after re-coating a seed layer of the original ZnO nanowires obtained from a solvothermal method. (d) The branched ZnO nanowires after second growth. Scale bar, 1 μm [4].*

#### **Figure 3.**

*(a) TEM image of a single-branched ZnO nanowire. (b) The magnified intersection area of ZnO branch and nanowire. (c and d) The corresponding nano-beam diffraction (NBD) and selected area electron diffraction (SAED) for the secondary ZnO branch and the ZnO nanowire backbone, respectively [4].*

branch and nanowire was shown in **Figure 3b**. **Figure 3c**, **d** shows the corresponding nano-beam diffraction (NBD) of the secondary ZnO branch and selected area electron diffraction (SAED) of the ZnO nanowire backbone, respectively. The diffraction patterns confirmed each ZnO nanostructure was single-crystal wurtzite and preferentially oriented in the *c*-axis direction even though the two components of ZnO nanostructures were not fabricated simultaneously. The *θ*-2*θ* X-ray diffraction patterns of ZnO nanostructures, which corresponds to the hexagonal wurtzite crystallites with cell constants of *a* = 3.251 Å and *c* = 5.208 Å, were shown in **Figure 4a**. The strong {0001} diffraction family of ZnO once indicates that the nanowires are moderately oriented in the *c*-axis direction. It is presumed that all the precursors have been completely decomposed as no excess peaks can be detected. A Raman spectrum of the branched ZnO NWs, as shown in **Figure 4b**, which was taken from a 5 μm2 spot size excited by a frequency-doubled

**41**

**Figure 4.**

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

Yb:YAG laser (λ = 515 nm), obviously indicates the remarkable *E2* (low) and *E2* (high)

utable to the second-order Raman scattering caused by the zone-boundary phonons

superposition of *A1*(LO) and *E1*(LO). The substrate signal did not appear due to the penetrating limitation of the 515 nm laser. The good crystalline quality of ZnO nanostructures confirmed above ensures that the photoelectrode can provide good electronic

Secondary NPs herein were synthesized via sol-gel method [7]. The detailed synthetic process is similar to that described by Seelig et al. [8]. The structural evolution of products synthesized in various aging time using 10 ml of primary supernatant was shown in a series of SEM photographs as shown in **Figure 5**. The ZnO NPs were accumulated by white seeds from the beginning shown in **Figure 5a**.

*(a) θ-2θ XRD profiles of A (the branched ZnO nanowires) and B (FTO substrate only). (b) Raman spectra of* 

*the branched ZnO nanowires, using a frequency-doubled Yb:YAG laser (*λ*= 515 nm) [4].*

2-*E2*(M) of ZnO. The weak and almost invisible signal near 581 cm<sup>−</sup><sup>1</sup>

conductivity without the defect trapping within the structures.

, respectively. The peak at 332 cm<sup>−</sup><sup>1</sup>

is attrib-

contributes to the

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

modes of ZnO located at 98 and 438 cm<sup>−</sup><sup>1</sup>

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

Yb:YAG laser (λ = 515 nm), obviously indicates the remarkable *E2* (low) and *E2* (high) modes of ZnO located at 98 and 438 cm<sup>−</sup><sup>1</sup> , respectively. The peak at 332 cm<sup>−</sup><sup>1</sup> is attributable to the second-order Raman scattering caused by the zone-boundary phonons 2-*E2*(M) of ZnO. The weak and almost invisible signal near 581 cm<sup>−</sup><sup>1</sup> contributes to the superposition of *A1*(LO) and *E1*(LO). The substrate signal did not appear due to the penetrating limitation of the 515 nm laser. The good crystalline quality of ZnO nanostructures confirmed above ensures that the photoelectrode can provide good electronic conductivity without the defect trapping within the structures.

Secondary NPs herein were synthesized via sol-gel method [7]. The detailed synthetic process is similar to that described by Seelig et al. [8]. The structural evolution of products synthesized in various aging time using 10 ml of primary supernatant was shown in a series of SEM photographs as shown in **Figure 5**. The ZnO NPs were accumulated by white seeds from the beginning shown in **Figure 5a**.

**Figure 4.**

*(a) θ-2θ XRD profiles of A (the branched ZnO nanowires) and B (FTO substrate only). (b) Raman spectra of the branched ZnO nanowires, using a frequency-doubled Yb:YAG laser (*λ*= 515 nm) [4].*

*Nanostructures*

**Figure 2.**

**Figure 3.**

**40**

in **Figure 4b**, which was taken from a 5 μm2

branch and nanowire was shown in **Figure 3b**. **Figure 3c**, **d** shows the corresponding nano-beam diffraction (NBD) of the secondary ZnO branch and selected area electron diffraction (SAED) of the ZnO nanowire backbone, respectively. The diffraction patterns confirmed each ZnO nanostructure was single-crystal wurtzite and preferentially oriented in the *c*-axis direction even though the two components of ZnO nanostructures were not fabricated simultaneously. The *θ*-2*θ* X-ray diffraction patterns of ZnO nanostructures, which corresponds to the hexagonal wurtzite crystallites with cell constants of *a* = 3.251 Å and *c* = 5.208 Å, were shown in **Figure 4a**. The strong {0001} diffraction family of ZnO once indicates that the nanowires are moderately oriented in the *c*-axis direction. It is presumed that all the precursors have been completely decomposed as no excess peaks can be detected. A Raman spectrum of the branched ZnO NWs, as shown

*(SAED) for the secondary ZnO branch and the ZnO nanowire backbone, respectively [4].*

*(a) TEM image of a single-branched ZnO nanowire. (b) The magnified intersection area of ZnO branch and nanowire. (c and d) The corresponding nano-beam diffraction (NBD) and selected area electron diffraction* 

*(a) The schematic growth procedure from the original ZnO nanowires to the branched ZnO nanowires. (b) Before and (c) after re-coating a seed layer of the original ZnO nanowires obtained from a solvothermal* 

*method. (d) The branched ZnO nanowires after second growth. Scale bar, 1 μm [4].*

spot size excited by a frequency-doubled

The zinc complexes were initially connected as a network (see **Figure 5b**) and condensed isotropic, eventually forming a hierarchical packing of colloidal particle, as shown in **Figure 5c**. The unidirectional aggregate phenomenon and formation mechanism in other metal oxide colloidal systems were presented by Serna et al. [9]. In at least 1-hour aging time, the monodispersed spherical ZnO NPs with an average particle size of ca. 185 nm could successfully be synthesized. The EDS spectra of the products with different aging time, as shown in **Figure 6**, reveal that they contain Zn, O, and C. As the aging time increases, the carbon ratio decreases, which means that the product requires complete aging time to remove the acetate ions.

Typical TEM micrographs of the ZnO NPs are shown in **Figure 7a**–**d**. A hierarchical packing of secondary ZnO NPs is formed in the condensation reaction of the sol-gel process, and the spherical shape of the ZnO NPs is recognized by the aggregation of many primary single crystals (also called subcrystals) ranging from 6 to 12 nm. It should be particularly noted here that when SAED is performed on several secondary ZnO NPs, the pattern exhibits a polycrystalline wurtzite structure of ZnO, as shown in the inset of **Figure 7a**. On the contrary, the pattern reveals the single-crystal-like diffraction, as shown in the inset of **Figure 7b**, while restricting the SAED area within only one ZnO NP. Obviously, the secondary ZnO NPs are polycrystals consisting of much smaller subcrystals of the same crystal orientation. More evidence can be demonstrated in the high-resolution TEM (HRTEM) image from both the center and the edge of the ZnO NPs in **Figure 7b**, **c**, respectively. In most cases, van der Waals interacts as a driving force for selfassembly between surface molecules of nanocrystallites, which can then assemble colloidal nanocrystals to form solids. If the size distribution of the nanocrystals is sufficiently small, an ordered array (also known as a superlattice), a quantum dot, or an artificial solid is formed by self-assembly [11, 12]. Therefore, the growth of the above secondary ZnO nanoparticles could be carried out in substantially the same way, with some discontinuities between the subunits, and each subcrystal is a subunit of the secondary ZnO NPs. Sugimoto et al. also reported similar selfassembly structures in α-Fe2O3 particles [13, 14]. The reason for the discontinuity of the internal structure is explained by the strong adsorption of ions used in the

#### **Figure 5.**

*Large and local scale of scanning electron micrographs of various aging time products synthesized using 10 ml of primary supernatant. The aging times are (a) 15 minutes, (b) 30 minutes, and (c) 60 minutes, respectively [7].*

**43**

**Figure 6.**

of ZnO, which belongs to the space group *C*<sup>6</sup><sup>υ</sup>

*(a) 15 minutes, (b) 30 minutes, and (c) 60 minutes [7].*

(λ = 515 nm). The remarkable feature at 520 cm<sup>−</sup><sup>1</sup>

from the Si substrate, while the peak at 437 cm<sup>−</sup><sup>1</sup>

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

reaction to prevent the fusion between the surface grains [15–17]. Although there is no existence of ions throughout the whole process in this present work. It is suspected that due to the blockage of DEG, the solvent appears as a microemulsion system, resulting in the separate growth of the ZnO subcrystals alone and eventually assembled under the driving force of the van der Waals interaction to form secondary NPs. As shown in **Figure 7d**, the subcrystal is a perfect crystal and exhibits a facet for which the evidence is specifically described in the HRTEM image of the edge of ZnO NPs. Thus, the subcrystals slowly self-assemble by sintering and belonging to the same defined orientation as the adjacent subcrystals. It is interesting that the evolutions of morphology of ZnO NPs show the subcrystals significantly fused with the neighbor crystals during the heating process. **Figure 8a–c** displays the SEM images of as-grown ZnO NPs and the samples after post-annealing at 350 and 500°C in air ambient for 1 hour, respectively. The grain growth was also investigated from the XRD profiles (not shown here) and Scherrer formula, as the crystalline sizes were estimated to be approximately 9, 14, and 20 nm for as-grown, 350°C-annealed, and 500°C-annealed samples, respectively. Raman spectroscopy was performed to investigate the vibrational properties of the secondary ZnO NPs before and after being heat treated. For the wurtzite structure

*Composition-variation analysis by energy dispersive x-ray spectra (EDS) of different aging time products as* 

two formula units, with all of the atoms occupying 2*b* sites of symmetry *C*<sup>3</sup>ν. **Figure 9** shows a normal Raman spectra by a frequency-doubled Yb:YAG laser

4 (*P*63mc), one primitive cell includes

corresponds to *E2*(high) of

is due to the TO phonon mode

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

#### **Figure 6.**

*Nanostructures*

ions.

The zinc complexes were initially connected as a network (see **Figure 5b**) and condensed isotropic, eventually forming a hierarchical packing of colloidal particle, as shown in **Figure 5c**. The unidirectional aggregate phenomenon and formation mechanism in other metal oxide colloidal systems were presented by Serna et al. [9]. In at least 1-hour aging time, the monodispersed spherical ZnO NPs with an average particle size of ca. 185 nm could successfully be synthesized. The EDS spectra of the products with different aging time, as shown in **Figure 6**, reveal that they contain Zn, O, and C. As the aging time increases, the carbon ratio decreases, which means that the product requires complete aging time to remove the acetate

Typical TEM micrographs of the ZnO NPs are shown in **Figure 7a**–**d**. A hierarchical packing of secondary ZnO NPs is formed in the condensation reaction of the sol-gel process, and the spherical shape of the ZnO NPs is recognized by the aggregation of many primary single crystals (also called subcrystals) ranging from 6 to 12 nm. It should be particularly noted here that when SAED is performed on several secondary ZnO NPs, the pattern exhibits a polycrystalline wurtzite structure of ZnO, as shown in the inset of **Figure 7a**. On the contrary, the pattern reveals the single-crystal-like diffraction, as shown in the inset of **Figure 7b**, while restricting the SAED area within only one ZnO NP. Obviously, the secondary ZnO NPs are polycrystals consisting of much smaller subcrystals of the same crystal orientation. More evidence can be demonstrated in the high-resolution TEM (HRTEM) image from both the center and the edge of the ZnO NPs in **Figure 7b**, **c**, respectively. In most cases, van der Waals interacts as a driving force for selfassembly between surface molecules of nanocrystallites, which can then assemble colloidal nanocrystals to form solids. If the size distribution of the nanocrystals is sufficiently small, an ordered array (also known as a superlattice), a quantum dot, or an artificial solid is formed by self-assembly [11, 12]. Therefore, the growth of the above secondary ZnO nanoparticles could be carried out in substantially the same way, with some discontinuities between the subunits, and each subcrystal is a subunit of the secondary ZnO NPs. Sugimoto et al. also reported similar selfassembly structures in α-Fe2O3 particles [13, 14]. The reason for the discontinuity of the internal structure is explained by the strong adsorption of ions used in the

*Large and local scale of scanning electron micrographs of various aging time products synthesized using 10 ml of primary supernatant. The aging times are (a) 15 minutes, (b) 30 minutes, and (c) 60 minutes,* 

**42**

**Figure 5.**

*respectively [7].*

*Composition-variation analysis by energy dispersive x-ray spectra (EDS) of different aging time products as (a) 15 minutes, (b) 30 minutes, and (c) 60 minutes [7].*

reaction to prevent the fusion between the surface grains [15–17]. Although there is no existence of ions throughout the whole process in this present work. It is suspected that due to the blockage of DEG, the solvent appears as a microemulsion system, resulting in the separate growth of the ZnO subcrystals alone and eventually assembled under the driving force of the van der Waals interaction to form secondary NPs. As shown in **Figure 7d**, the subcrystal is a perfect crystal and exhibits a facet for which the evidence is specifically described in the HRTEM image of the edge of ZnO NPs. Thus, the subcrystals slowly self-assemble by sintering and belonging to the same defined orientation as the adjacent subcrystals.

It is interesting that the evolutions of morphology of ZnO NPs show the subcrystals significantly fused with the neighbor crystals during the heating process. **Figure 8a–c** displays the SEM images of as-grown ZnO NPs and the samples after post-annealing at 350 and 500°C in air ambient for 1 hour, respectively. The grain growth was also investigated from the XRD profiles (not shown here) and Scherrer formula, as the crystalline sizes were estimated to be approximately 9, 14, and 20 nm for as-grown, 350°C-annealed, and 500°C-annealed samples, respectively. Raman spectroscopy was performed to investigate the vibrational properties of the secondary ZnO NPs before and after being heat treated. For the wurtzite structure of ZnO, which belongs to the space group *C*<sup>6</sup><sup>υ</sup> 4 (*P*63mc), one primitive cell includes two formula units, with all of the atoms occupying 2*b* sites of symmetry *C*<sup>3</sup>ν. **Figure 9** shows a normal Raman spectra by a frequency-doubled Yb:YAG laser (λ = 515 nm). The remarkable feature at 520 cm<sup>−</sup><sup>1</sup> is due to the TO phonon mode from the Si substrate, while the peak at 437 cm<sup>−</sup><sup>1</sup> corresponds to *E2*(high) of

#### **Figure 7.**

*TEM images of secondary ZnO NPs recognized of crystalline subcrystals. (a) A typical low-magnification TEM image and SAED pattern of several uniform ZnO NPs. (b) High-magnification TEM image of one individual ZnO NP and its corresponding single crystal-like SAED spots. (c and d) High-resolution TEM images of central area and boundary part of one individual ZnO NP, respectively. Inset of (c) corresponding fast Fourier transform image [10].*

#### **Figure 8.**

*SEM micrographs of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing for 1 hour secondary ZnO NPs, respectively [10].*

**45**

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

can be assigned to the second-order Raman scat-

, is almost imperceptible. Multiphonon scat-

) as the crystal

is

tering arising from zone-boundary phonons 2-*E2* (M) of ZnO. No significant change of Raman spectra and intense *E2* (high) peak for the 350°C-annealed and 500°C-annealed samples means good crystallinity. The full width at half-maximum

*Normal Raman spectra of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing for 1 hour* 

size increases as anneal temperature from 350˚C to 500˚C, which is consistent with the XRD results. Another imperceptible broadened peak around 580 cm<sup>−</sup><sup>1</sup>

contributed to the superposition of *A1*(LO) and *E1* (LO). The lattice behavior of *A1* (LO) and *E1* (LO) modes is associated with the existence of some nonstoichiometric defects while heat treatment, such as oxygen vacancy, interstitial zinc, or their complexes [18–20] those are produced due to the unfavorable process environment. In principle the electron-phonon interaction could be investigated by utilizing resonant Raman scattering (RRS) experiments. The excitation photon energy resonates with the transition energy above the electron bands of wurtzite ZnO, so that a He-Cd laser (= 325 nm) was used as the excitation source for RRS. The *A1* (LO) and *E1* (LO) modes would dominate as their polar symmetric and exhibit different frequencies from the TO modes as well. As shown in **Figure 10**, intense multiphonon scatterings of the secondary ZnO NPs before and after heat treatment were observed, where the major peaks were observed as a result from the polar symmetry modes *A1* (LO) and *E1*(LO) and their overtones. For the superposition of LO phonon mode, the Zn atoms and the O atoms have the same vibration direction as the adjacent lattice, respectively [21], while the weak peak, which is contributed to

tering processes also have been previously reported for single-crystalline bulk ZnO [22], ZnO films [23], ZnO-opal structures [24], and ZnO NWs [25, 26] but rarely

It is worth noting that the intensity of the first-order Raman mode and its overtone are enhanced in the grown ZnO NP compared to the annealed sample. The reason can be explained by the total Raman cross section for an *n*-phonon process

(FWHM) of Raman E2(high) peak decreases (from 14 to 11 cm<sup>−</sup><sup>1</sup>

*secondary ZnO NPs, using a frequency-doubled Yb:YAG laser (=515 nm) [10].*

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

ZnO. The peak at 331 cm<sup>−</sup><sup>1</sup>

**Figure 9.**

the *E2*(high) mode around 437 cm<sup>−</sup><sup>1</sup>

mentioned for ZnO NPs.

written as [27, 28]

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

#### **Figure 9.**

*Nanostructures*

**44**

**Figure 8.**

**Figure 7.**

*secondary ZnO NPs, respectively [10].*

*SEM micrographs of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing for 1 hour* 

*TEM images of secondary ZnO NPs recognized of crystalline subcrystals. (a) A typical low-magnification TEM image and SAED pattern of several uniform ZnO NPs. (b) High-magnification TEM image of one individual ZnO NP and its corresponding single crystal-like SAED spots. (c and d) High-resolution TEM images of central area and boundary part of one individual ZnO NP, respectively. Inset of (c) corresponding fast Fourier transform image [10].*

*Normal Raman spectra of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing for 1 hour secondary ZnO NPs, using a frequency-doubled Yb:YAG laser (=515 nm) [10].*

ZnO. The peak at 331 cm<sup>−</sup><sup>1</sup> can be assigned to the second-order Raman scattering arising from zone-boundary phonons 2-*E2* (M) of ZnO. No significant change of Raman spectra and intense *E2* (high) peak for the 350°C-annealed and 500°C-annealed samples means good crystallinity. The full width at half-maximum (FWHM) of Raman E2(high) peak decreases (from 14 to 11 cm<sup>−</sup><sup>1</sup> ) as the crystal size increases as anneal temperature from 350˚C to 500˚C, which is consistent with the XRD results. Another imperceptible broadened peak around 580 cm<sup>−</sup><sup>1</sup> is contributed to the superposition of *A1*(LO) and *E1* (LO). The lattice behavior of *A1* (LO) and *E1* (LO) modes is associated with the existence of some nonstoichiometric defects while heat treatment, such as oxygen vacancy, interstitial zinc, or their complexes [18–20] those are produced due to the unfavorable process environment.

In principle the electron-phonon interaction could be investigated by utilizing resonant Raman scattering (RRS) experiments. The excitation photon energy resonates with the transition energy above the electron bands of wurtzite ZnO, so that a He-Cd laser (= 325 nm) was used as the excitation source for RRS. The *A1* (LO) and *E1* (LO) modes would dominate as their polar symmetric and exhibit different frequencies from the TO modes as well. As shown in **Figure 10**, intense multiphonon scatterings of the secondary ZnO NPs before and after heat treatment were observed, where the major peaks were observed as a result from the polar symmetry modes *A1* (LO) and *E1*(LO) and their overtones. For the superposition of LO phonon mode, the Zn atoms and the O atoms have the same vibration direction as the adjacent lattice, respectively [21], while the weak peak, which is contributed to the *E2*(high) mode around 437 cm<sup>−</sup><sup>1</sup> , is almost imperceptible. Multiphonon scattering processes also have been previously reported for single-crystalline bulk ZnO [22], ZnO films [23], ZnO-opal structures [24], and ZnO NWs [25, 26] but rarely mentioned for ZnO NPs.

It is worth noting that the intensity of the first-order Raman mode and its overtone are enhanced in the grown ZnO NP compared to the annealed sample. The reason can be explained by the total Raman cross section for an *n*-phonon process written as [27, 28]

**Figure 10.**

*Resonant Raman scatterings (RRS) of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing for 1 hour secondary ZnO NPs, using a He-Cd laser (= 325 nm) [10].*

$$\boldsymbol{\sigma}\_{n} = \int \boldsymbol{\sigma}\_{n}^{R}(\boldsymbol{\alpha}) f(\boldsymbol{R}) d\boldsymbol{R}\_{n} \tag{1}$$

**47**

*a*

*b*

**Table 1.**

[37] and ZnO NWs [38].

1LO (cm<sup>−</sup><sup>1</sup> )

(FWHM)

2LO (cm<sup>−</sup><sup>1</sup> )

(FWHM)

*This content*

*are also listed as a Ref. [10].*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

The phonon scattering is not limited to the center of the Brillouin region. In order to observe the displacement, broadening, and asymmetry of the first-order optical phonon, the phonon dispersion near the center of the region must also be considered. Alim et al. [30, 31] have shown that the large red-shift in the resonant Raman spectrum from 20 nm ZnO NPs is most likely due to local heating by UV laser excitation. In this study, since the as-grown secondary ZnO NPs contain more air gaps than the annealed NPs, an unfavorable heat dissipation may be the other possibility of causing higher temperatures and greater phonon red-shift. For the clarity, detailed numerical analysis of the ZnO NPs of this experiment is clearly listed in **Table 1**. It was found that the ratio between the second-order and first-order Raman scattering cross sections increased from 0.38 to 2.05, while the ZnO crystallite size increased from nanoparticle to bulk. In the Franck-Condon approximation, the coupling strength of the [32, 33] exciton transition to the LO phonon can be expressed by the Huang-Rhys parameter *S*. The cross section of the RRS depends on the particle size, temperature, and excitation wavelength. Scamarico et al. [34] proposed that due to the strong energy dependence of the Raman scattering cross section, it is necessary to maintain resonance conditions in order to make meaningful comparisons with spectra of nanocrystals of different sizes to maintain different electronic transitions. It is important to purposefully use the same experimental conditions, such as laser power, wavelength, spot size, etc. for each sample. In this study, I emphasize that the tendency here is the increasing electron-phonon interaction with increasing nanocrystal size. It is generally accepted that the electron-phonon coupling is determined by the deformation potential and the Fröhlich potential. TO Raman scattering cross section is mainly determined by the deformation potential that involves the short-range interaction between the lattice displacement and the electrons [35, 36]. On the other hand, the LO Raman scattering cross section includes contributions not only the Fröhlich potential that involves the long-range interaction generated by the macroscopic electric field associated with the LO phonons but also the deformation potential. The intensity of TO phonons in ZnO NPs was found to be almost insensitive, while the intensity of LO phonons was greatly enhanced under resonance conditions. This study shows that electron-LO-phonon coupling is related to the Fröhlich interaction as the size of the nanocrystals decreases. Although the complex origin is not clear, the results of this study are very similar to those of other low-dimensional ZnO nanostructures, such as ZnO-based quantum wells

**As-growna 350o**

Grain size (nm) 9 14 20 >1000

I2 LO/I1LO 0.38 0.59 1.07 2.05

*Wave number, broadening, and the ratio of n-LO phonons found in RRS spectra. The assignments of bulk ZnO* 

578 (27.2)

1149 (54.7)

*From Ref. [22] (used the same He-Cd laser,* λ*= 325 nm, as the excitation source for RRS)*

**Ca 500o**

584 (23.9)

1158 (47.3)

582 (24.2)

1154 (49.3) **Ca Bulkb**

585 (N/A)

1165 (N/A)

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

$$\sigma\_n^R(\alpha) = \mu^4 \left| \sum\_{m=0}^{\infty} \frac{\langle n|m\rangle \langle m|0\rangle}{E\_0 + n\hbar \alpha\_{LO} - \hbar \alpha + i\hbar \Gamma} \right|^2 \times \exp\left(-\frac{i\hbar \alpha\_{LO}}{k\_B T}\right),\tag{2}$$

where *μ* is the electronic dipole transition moment; *E*0 is the size-dependent energy of the electronic transition; ℏω and ℏω*LO* are the energies of the excitation photon and the LO phonon, respectively; *m* denotes the intermediate vibrational level in the excited state; Γ is the homogeneous linewidth; *kB* is Boltzmann's constant; *T* is the temperature; and the bracket indicates the overlap integral between the ground and excited state wave functions. Consequently, the RRS intensity can be enhanced as the denominator in Raman scattering cross section tending to zero, while the electronic state in the material is close to the incident or scattered photons. Similar results have been previously reported for CdS and ZnO, using various laser wavelengths [29]. Due to the quantum confinement effect of the subcrystal size relative to the exciton radius, the bandgap of the as-grown ZnO NPs would tend to approach the excitation laser energy. Evidence of quantum confinement can be found by the intensive tail of the blue-shifted photoluminescence (PL) signal of the as-grown ZnO NPs, or it can be found in the RRS spectrum rather than in the normal RS spectrum due to their red-shifted, broadening and asymmetry.

Due to the infinite correlation length, the phonon eigenstate in an ideal crystal is a plane wave; therefore, the *K* = 0 momentum selection rule of the first-order Raman spectrum can be satisfied. When the crystalline is reduced to nanometer scale, the momentum selection rule will be relaxed. This allows the phonon with wave vector |*k*| = |*k*'| ± 2*π*/*L* to participate in the first-order Raman scattering, where *k*' is the wave vector of the incident light and *L* is the size of the crystal.

### *Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

The phonon scattering is not limited to the center of the Brillouin region. In order to observe the displacement, broadening, and asymmetry of the first-order optical phonon, the phonon dispersion near the center of the region must also be considered. Alim et al. [30, 31] have shown that the large red-shift in the resonant Raman spectrum from 20 nm ZnO NPs is most likely due to local heating by UV laser excitation. In this study, since the as-grown secondary ZnO NPs contain more air gaps than the annealed NPs, an unfavorable heat dissipation may be the other possibility of causing higher temperatures and greater phonon red-shift. For the clarity, detailed numerical analysis of the ZnO NPs of this experiment is clearly listed in **Table 1**. It was found that the ratio between the second-order and first-order Raman scattering cross sections increased from 0.38 to 2.05, while the ZnO crystallite size increased from nanoparticle to bulk. In the Franck-Condon approximation, the coupling strength of the [32, 33] exciton transition to the LO phonon can be expressed by the Huang-Rhys parameter *S*. The cross section of the RRS depends on the particle size, temperature, and excitation wavelength. Scamarico et al. [34] proposed that due to the strong energy dependence of the Raman scattering cross section, it is necessary to maintain resonance conditions in order to make meaningful comparisons with spectra of nanocrystals of different sizes to maintain different electronic transitions. It is important to purposefully use the same experimental conditions, such as laser power, wavelength, spot size, etc. for each sample. In this study, I emphasize that the tendency here is the increasing electron-phonon interaction with increasing nanocrystal size. It is generally accepted that the electron-phonon coupling is determined by the deformation potential and the Fröhlich potential. TO Raman scattering cross section is mainly determined by the deformation potential that involves the short-range interaction between the lattice displacement and the electrons [35, 36]. On the other hand, the LO Raman scattering cross section includes contributions not only the Fröhlich potential that involves the long-range interaction generated by the macroscopic electric field associated with the LO phonons but also the deformation potential. The intensity of TO phonons in ZnO NPs was found to be almost insensitive, while the intensity of LO phonons was greatly enhanced under resonance conditions. This study shows that electron-LO-phonon coupling is related to the Fröhlich interaction as the size of the nanocrystals decreases. Although the complex origin is not clear, the results of this study are very similar to those of other low-dimensional ZnO nanostructures, such as ZnO-based quantum wells [37] and ZnO NWs [38].


#### **Table 1.**

*Nanostructures*

σ*<sup>n</sup>* = ∫σ*<sup>n</sup>*

*for 1 hour secondary ZnO NPs, using a He-Cd laser (= 325 nm) [10].*


*R* (ω) = *μ*<sup>4</sup>

σ*<sup>n</sup>*

**Figure 10.**

*R*

*Resonant Raman scatterings (RRS) of (a) as-grown, (b) 350°C annealing for 1 hour, and (c) 500°C annealing* 

<sup>∞</sup> 〈*n*|*m*〉〈*m*|0〉 \_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>E</sup>*<sup>0</sup> <sup>+</sup> *<sup>n</sup>*ℏω*LO* <sup>−</sup> ℏω <sup>+</sup> *<sup>i</sup>*ℏΓ<sup>|</sup>

where *μ* is the electronic dipole transition moment; *E*0 is the size-dependent energy of the electronic transition; ℏω and ℏω*LO* are the energies of the excitation photon and the LO phonon, respectively; *m* denotes the intermediate vibrational level in the excited state; Γ is the homogeneous linewidth; *kB* is Boltzmann's constant; *T* is the temperature; and the bracket indicates the overlap integral between the ground and excited state wave functions. Consequently, the RRS intensity can be enhanced as the denominator in Raman scattering cross section tending to zero, while the electronic state in the material is close to the incident or scattered photons. Similar results have been previously reported for CdS and ZnO, using various laser wavelengths [29]. Due to the quantum confinement effect of the subcrystal size relative to the exciton radius, the bandgap of the as-grown ZnO NPs would tend to approach the excitation laser energy. Evidence of quantum confinement can be found by the intensive tail of the blue-shifted photoluminescence (PL) signal of the as-grown ZnO NPs, or it can be found in the RRS spectrum rather than in the normal RS spectrum due to their red-shifted, broadening and asymmetry.

Due to the infinite correlation length, the phonon eigenstate in an ideal crystal is a plane wave; therefore, the *K* = 0 momentum selection rule of the first-order Raman spectrum can be satisfied. When the crystalline is reduced to nanometer scale, the momentum selection rule will be relaxed. This allows the phonon with wave vector |*k*| = |*k*'| ± 2*π*/*L* to participate in the first-order Raman scattering, where *k*' is the wave vector of the incident light and *L* is the size of the crystal.

(ω)*f*(*R*)*dR***,** (1)

<sup>×</sup> exp(−\_\_\_\_\_ *i*ℏω*LO*

*kBT* )**,** (2)

2

**46**

*Wave number, broadening, and the ratio of n-LO phonons found in RRS spectra. The assignments of bulk ZnO are also listed as a Ref. [10].*
