**2.2 Super-assembling of Bi2Te3 hierarchical nanostructured thin films**

C.-H. Chen et al. [41] reported the PLD growths of super-assembling of Bi2Te3 hierarchical nanostructured thin films on the SiO2/Si substrates and their thermoelectric properties. Interesting Bi2Te3 super-assemblies were successfully grown using PLD with controlling the substrate temperatures from 350–600°C and at a fixed Ar ambient pressure of approximately 10<sup>3</sup> Torr. SEM images in **Figure 10** clearly shows the morphological characteristics of the superassembling Bi2Te3 nanostructured thin films [41]. At lower deposition temperatures (< 450°C), the films are mainly composed of vertically aligned nanoscaled flakes, but flakes are

*Nanostructuring Bi2Te3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition DOI: http://dx.doi.org/10.5772/intechopen.99469*

#### **Figure 9.**

*(a) Vapor pressures of Bi, Sb,Te, Se, Bi2Se3, and Bi2Te3 as a function of temperature [46]. (b) The variation of sticking coefficient Ks (Bi,Te) as a function of substrate temperature Ts at fixed flux ratio FR = 4.5 [49].*

#### **Figure 10.**

be KrF excimer laser beam (λ = 248 nm) and Q-switched Nd:YAG laser (λ = 355 nm)

The enhancement of the PF of Bi2Te3-based thin films is challenging due to the coupling among TE material properties [3], and the difficulty in growing stoichiometric films [38]. Indeed, stoichiometry is a key factor for obtaining better TE properties [5, 38, 43–45]. Yet, both tendency for re-evaporation of volatile elements (i.e., Te, Se) at elevated *Ts* [45–48] and the low sticking coefficient Te (< 0.6 for Bi2Te3) at *Ts* beyond 300°C [49, 50] constrain to grow stoichiometric Bi2Te3-based

C.-H. Chen et al. [41] reported the PLD growths of super-assembling of Bi2Te3 hierarchical nanostructured thin films on the SiO2/Si substrates and their thermoelectric properties. Interesting Bi2Te3 super-assemblies were successfully grown using PLD with controlling the substrate temperatures from 350–600°C and at a fixed Ar ambient pressure of approximately 10<sup>3</sup> Torr. SEM images in **Figure 10** clearly shows the morphological characteristics of the superassembling Bi2Te3 nanostructured thin films [41]. At lower deposition temperatures (< 450°C), the films are mainly composed of vertically aligned nanoscaled flakes, but flakes are

5–20 ns, repetition rate of 5–10 Hz [38, 40–42]. The laser beam was guided by several UV mirrors and focused on a stoichiometric polycrystalline target (e.g., Bi2Se3, Bi2Te3, Bi0.5Sb1.5Te3, etc.) inside the vacuum chamber by the UV lens. The deposition chamber was evacuated to a base pressure of <sup>10</sup><sup>6</sup> Torr, and highpurity ambient gas (He or Ar) was then introduced until obtaining a target pressure

**2.2 Super-assembling of Bi2Te3 hierarchical nanostructured thin films**

) pulsed duration of

with properly selected laser fluence (e.g., 3.8, 6.2, or 8.3 J/cm<sup>2</sup>

(e.g., usually 10<sup>5</sup> – <sup>3</sup><sup>10</sup><sup>1</sup> Torr).

*A schematic illustration of a PLD system.*

*Materials at the Nanoscale*

films (**Figure 9a** and **b**).

**50**

**Figure 8.**

*(a)–(d) The cross-sectional and (e)–(l) the corresponding top-view SEM images of the Bi2Te3 superassemblies deposited at 350°C, 400°C, 450°C, and 600°C, respectively [41].*

horizontally stacked for 600°C-film (**Figure 10a–d**). Moreover, the bottom of each of the deposited super-assemblies has a relatively continuous and dense layer, and this layer thickness increases with increasing substrate temperature from 350–450° C (**Figure 10a–d**). The top-view SEM images confirm for the high uniformity and presents the unique super-assembling features of the repetitively and regularly assembled nano-flakes (**Figure 10e–h**). These four films are uniformly composed of spindle-like (**Figure 10e**), worm-like (**Figure 10f** and **g**) and island-like (**Figure 10h**) hierarchical nanostructures. Magnified top-view SEM images (**Figure 10i–k**) further show that the nanoflakes are composed of oriented and regular assemblies of numerous rice-like and elongated primitive nanoparticles [41]. At a higher substrate temperature, thin- and large-size nanoflakes are formed from packing of dense rice-like nanoparticles, driving by the relatively sufficient thermal energy for diffusion. In addition, the out-of- plane superassembly structure (600°C) has a limited column width, which is not always consistent along the outof-plane direction (**Figure 10d**). Also, the parallel nano-flakes (at 600°C) are

evidently formed by flake stacking along c-axis orientation or epitaxial-like growth. The special three-dimensional mesh-like structure of 600°C-film would also be an effective design for scattering phonons, and it's extremely smooth top surface is certainly beneficial for subsequent analyses and applications [41].

**Figure 11(a)** shows the crystal structure of Bi2Te3, which is usually described by a hexagonal cell that consists of 15 layers of atoms stacking along the c-axis with a sequence [5], namely Te(1)–Bi–Te(2)–Bi–Te(1) Te(1)–Bi–Te(2)–Bi–Te(1)Te(1)–Bi– Te(2)–Bi–Te(1) . The superscripts refer to two different types of bonding for Te atoms. The 5-atomic-layer thick lamellae of–(Te(1)–Bi–Te(2)–Bi–Te(1))– is called quintuple layers, QLs. The Te(1)… Te(1) refers Van der Waals force between Te atoms, whereas the Te(1)–Bi and Bi–Te(2) are ionic-covalent bonds. This weak binding between the Te(1)… Te(1) accounts for the anisotropic thermal and electrical transport properties of Bi2Te3. For example, the thermal conductivity along the *c*-axis direction (0.7 Wm<sup>1</sup> K <sup>1</sup> ) is approximately a haft of the value along the plane perpendicular to the *<sup>c</sup>*-axis (1.5 Wm<sup>1</sup> K <sup>1</sup> ) [5, 6, 13]. The weak binding of Te(1)… Te(1) also make the ease of cleavage along the plane perpendicular to the *c*-axis.

**Figure 11(b)** shows XRD patterns of the Bi2Te3 super-assemblies deposited at various substrate temperatures from 350–600°C. Clearly, all the films exhibited rhombohedral Bi2Te3 (JCPDS no. 89–4302) without traceable impurities or oxides. When substrate temperature increases, the (00 l) preferential orientation gradually becomes stronger, the 600°C- film is highly (00 l)-preferred orientation, which is consistent with the SEM observation (**Figure 10d**). The gradually enhanced (00 l) peaks from 350–450°C mainly originate from the increased thickness of the bottom layer (**Figure 10a–d**), which has similar lamellar morphology with (00 l)-preferential orientation of 600°C-film [41].

**Figure 12** presents the proposed growth model of the super-assemling nanostructured Bi2Te3 films prepared at various *TS*. The growth mechanisms are layerthen-fake for *TS* = 350–450°C and layer-by-layer for higher *TS* of 600°C. We can only find a monotone morphology and single preferential orientation of (00 l) for 600°C-film, which lead to a fully lamellar morphology with the (00 l) preferential

orientation. Meanwhile, drastic changes in morphologies from layer to flake and orientations from (00 l) to (015) are observed at lower temperatures (350–450°C). The (00 l)-preferred orientation should be attributed to the thin bottom layer of the films prepared at 350–450°C. The thickness of this layer increases with increasing *TS*. Since the bottom layer at 350°C is extremely thin, the required *Ts* for obtaining layer growth should be just below 350°C. The drastic change in the morphology and orientation at *TS* of 350–450°C, namely, the layer- then-flake growth can be

*Schematic illustration of the layer-then-flake and layer-by-layer growth models and the resulting Bi2Te3 inplane (350–450°C) and out- of-plane (600°C) super-assemblies. Inset is the optical image of the prepared*

*Nanostructuring Bi2Te3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition*

temperature at top surface of the as-deposited film should be slightly lower than

**Table 1** summarizes the detailed properties of the super-assembling nanostructured Bi2Te3 thin-films. Due to such the voided structures, the films exhibited low electrical conductivity from 49 S.cm<sup>1</sup> for worm-like superassembly (450°C) to 160 S.cm<sup>1</sup> for 3D-layered super-assembly (600°C). Seebeck coefficient of the films was in range of 113–138 μV/K. As a result, the power factor (PF) is relatively low in

In PLD, tightly controlling substrate temperatures (*Ts*) and ambient pressures (*P*) enables the morphologies and compositions of films to be manipulated extensively, which offers a new method for enhancing the TE properties of films [38, 43, 51, 58, 59]. For example, self-assembled Bi2Te3 films featuring well-aligned zero- to three-dimensional nanoblocks have been fabricated (**Figure 13a**–**d**), but the room-

A. Li Bassi et al. [43] obtained several microstructured Bi2Te3 films (**Figure 13e**–**h**)

, primarily due to the low electrical conductivity of

) [51]. By contrast,

, **Figure 13f,f1**) at room-temperature;

, **Figure 13e,**

induced by a temperature gradient along the growth direction that the

temperature PFs of these films remain low (≤ 1.9 μWcm<sup>1</sup> K<sup>2</sup>

with high PFs for morphologies: layered-smooth (50.6 μWcm<sup>1</sup> K<sup>2</sup>

at the substrate [41].

**53**

**Figure 12.**

range of 0.93 to 3.0 μW/cmK<sup>2</sup>

the films with voided morphologies.

*super-assembled films with a size of 1.5 1.5 cm2 [41].*

*DOI: http://dx.doi.org/10.5772/intechopen.99469*

**e1**), and compact-smooth (21.2 μWcm<sup>1</sup> K<sup>2</sup>

#### **Figure 11.**

*(a) Crystal structures of Bi2Te3. (b) X-ray diffraction (XRD) patterns of of the Bi2Te3 super-assemblies deposited at various deposition temperatures from 350–600°C and at an Ar ambient pressure <sup>10</sup><sup>3</sup> Torr [41].*

*Nanostructuring Bi2Te3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition DOI: http://dx.doi.org/10.5772/intechopen.99469*

**Figure 12.**

evidently formed by flake stacking along c-axis orientation or epitaxial-like growth. The special three-dimensional mesh-like structure of 600°C-film would also be an effective design for scattering phonons, and it's extremely smooth top surface is

**Figure 11(a)** shows the crystal structure of Bi2Te3, which is usually described by a

**Figure 11(b)** shows XRD patterns of the Bi2Te3 super-assemblies deposited at various substrate temperatures from 350–600°C. Clearly, all the films exhibited rhombohedral Bi2Te3 (JCPDS no. 89–4302) without traceable impurities or oxides. When substrate temperature increases, the (00 l) preferential orientation gradually becomes stronger, the 600°C- film is highly (00 l)-preferred orientation, which is consistent with the SEM observation (**Figure 10d**). The gradually enhanced (00 l) peaks from 350–450°C mainly originate from the increased thickness of the bottom layer (**Figure 10a–d**), which has similar lamellar morphology with (00 l)-preferen-

**Figure 12** presents the proposed growth model of the super-assemling nanostructured Bi2Te3 films prepared at various *TS*. The growth mechanisms are layerthen-fake for *TS* = 350–450°C and layer-by-layer for higher *TS* of 600°C. We can only find a monotone morphology and single preferential orientation of (00 l) for 600°C-film, which lead to a fully lamellar morphology with the (00 l) preferential

*(a) Crystal structures of Bi2Te3. (b) X-ray diffraction (XRD) patterns of of the Bi2Te3 super-assemblies deposited at various deposition temperatures from 350–600°C and at an Ar ambient pressure*

) is approximately a haft of the value along the plane perpendicular

) [5, 6, 13]. The weak binding of Te(1)… Te(1) also make

hexagonal cell that consists of 15 layers of atoms stacking along the c-axis with a sequence [5], namely Te(1)–Bi–Te(2)–Bi–Te(1) Te(1)–Bi–Te(2)–Bi–Te(1)Te(1)–Bi– Te(2)–Bi–Te(1) . The superscripts refer to two different types of bonding for Te atoms. The 5-atomic-layer thick lamellae of–(Te(1)–Bi–Te(2)–Bi–Te(1))– is called quintuple layers, QLs. The Te(1)… Te(1) refers Van der Waals force between Te atoms, whereas the Te(1)–Bi and Bi–Te(2) are ionic-covalent bonds. This weak binding between the Te(1)… Te(1) accounts for the anisotropic thermal and electrical transport properties of Bi2Te3. For example, the thermal conductivity along the *c*-axis direction

certainly beneficial for subsequent analyses and applications [41].

K <sup>1</sup>

the ease of cleavage along the plane perpendicular to the *c*-axis.

(0.7 Wm<sup>1</sup>

**Figure 11.**

**52**

*<sup>10</sup><sup>3</sup> Torr [41].*

K <sup>1</sup>

tial orientation of 600°C-film [41].

to the *<sup>c</sup>*-axis (1.5 Wm<sup>1</sup>

*Materials at the Nanoscale*

*Schematic illustration of the layer-then-flake and layer-by-layer growth models and the resulting Bi2Te3 inplane (350–450°C) and out- of-plane (600°C) super-assemblies. Inset is the optical image of the prepared super-assembled films with a size of 1.5 1.5 cm2 [41].*

orientation. Meanwhile, drastic changes in morphologies from layer to flake and orientations from (00 l) to (015) are observed at lower temperatures (350–450°C). The (00 l)-preferred orientation should be attributed to the thin bottom layer of the films prepared at 350–450°C. The thickness of this layer increases with increasing *TS*. Since the bottom layer at 350°C is extremely thin, the required *Ts* for obtaining layer growth should be just below 350°C. The drastic change in the morphology and orientation at *TS* of 350–450°C, namely, the layer- then-flake growth can be induced by a temperature gradient along the growth direction that the temperature at top surface of the as-deposited film should be slightly lower than at the substrate [41].

**Table 1** summarizes the detailed properties of the super-assembling nanostructured Bi2Te3 thin-films. Due to such the voided structures, the films exhibited low electrical conductivity from 49 S.cm<sup>1</sup> for worm-like superassembly (450°C) to 160 S.cm<sup>1</sup> for 3D-layered super-assembly (600°C). Seebeck coefficient of the films was in range of 113–138 μV/K. As a result, the power factor (PF) is relatively low in range of 0.93 to 3.0 μW/cmK<sup>2</sup> , primarily due to the low electrical conductivity of the films with voided morphologies.

In PLD, tightly controlling substrate temperatures (*Ts*) and ambient pressures (*P*) enables the morphologies and compositions of films to be manipulated extensively, which offers a new method for enhancing the TE properties of films [38, 43, 51, 58, 59]. For example, self-assembled Bi2Te3 films featuring well-aligned zero- to three-dimensional nanoblocks have been fabricated (**Figure 13a**–**d**), but the roomtemperature PFs of these films remain low (≤ 1.9 μWcm<sup>1</sup> K<sup>2</sup> ) [51]. By contrast, A. Li Bassi et al. [43] obtained several microstructured Bi2Te3 films (**Figure 13e**–**h**) with high PFs for morphologies: layered-smooth (50.6 μWcm<sup>1</sup> K<sup>2</sup> , **Figure 13e, e1**), and compact-smooth (21.2 μWcm<sup>1</sup> K<sup>2</sup> , **Figure 13f,f1**) at room-temperature;


**Table 1.**

*Material, type, method, processing conditions, carrier concentration (n), mobility (μ), electrical conductivity (σ), Seebeck coefficient (α), power factor (PF = α2σ) of the Bi2Te3 films deposited by PLD and RF sputtering, as compared to properties of Bi2Se3, Bi2Se3, Bi2SexTe1-x bulk and films reported in the literature. All the selected values were recorded at room temperature.*

whereas the PFs remained low values of 8.8

Usually, TE nanomaterials possess low

some other films [52]. Unfortunately, the

μWcm � 1 K �

**Table 1** summaries the morphology and properties of Bi

*μWcm* � *1 K* � *2*

structure-morphology, but bulk and thin films have superior

3Se

works to calculate ZT of the films. Thermal conductivity (

**κ of Bi**

**Table 2** summarizes thermal transport properties (at room

2Te

**2Se**

of thermoelectric films. The detail of this technique can be found in refs. [62

deposited by PLD, sputtering, in comparison with the properties of TE bulks.

*PLD at various substrate temperatures and ambient pressures, reported by (i) Chang and Chen [51] and (ii)*

nanostructured thermoelectrics is expected to achieve low values thanks to the extensive phonons scattering at interfaces, surfaces and grain boundaries.

*κ* values have been noted for the monocrystalline Bi

**<sup>3</sup> and Bi**

crystalline–nanostructured Bi2Te3-based thin films and bulk materials in the literature. Generally, the thermal conductivity κ value for polycrystalline films is expected to be smaller than that of bulk alloys because of the extensive phonons scattering at interfaces, surfaces and grain boundaries [5, 60, 66]. Moreover, the

**2Te**

ω technique is usually employed in measuring thermal conductivity

3-based films will further decrease when the grain size of

*κ* = 0.8 W/m K for an average grain size of 60 nm) [60], and for Bi-Sb-Te

2Te achieved

(**Figure 13g,g1**) and 0.08

*The morphology and power factor (unit*

compact-polycrystalline Bi

**2.3 Thermal conductivity**

A transient 3

of nanocrystalline Bi

Indeed, reduced

films [61, 62].

films (

**55**

**Figure 13.**

*Li Bassi et al. [43].*

*Nanostructuring Bi*

*2Te*

*DOI: http://dx.doi.org/10.5772/intechopen.99469*

μWcm � 1 K �

*) of nano/micro-structured Bi*

*3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition*

<sup>2</sup> for 3D crystallite shapes

*2Te*

3-based thin-films

*<sup>3</sup> thin-films grown by*

*σ*. For example, the

*κ*) of nanocrystalline and

–temperature) of nano-

2Se0.3Te2.7

–64].

κ

<sup>2</sup> for 3D-voided platelets (**Figure 13h,h1**).

*σ* values due to the separating or voided

2Te

*σ* = 1747.5 S/cm [53] or even higher for

**<sup>3</sup> and Bi-Te-Se compounds**

*κ* of films are missed in many published

*Nanostructuring Bi2Te3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition DOI: http://dx.doi.org/10.5772/intechopen.99469*

#### **Figure 13.**

*The morphology and power factor (unit μWcm*�*<sup>1</sup> K*�*<sup>2</sup> ) of nano/micro-structured Bi2Te3 thin-films grown by PLD at various substrate temperatures and ambient pressures, reported by (i) Chang and Chen [51] and (ii) Li Bassi et al. [43].*

whereas the PFs remained low values of 8.8 μWcm�<sup>1</sup> K�<sup>2</sup> for 3D crystallite shapes (**Figure 13g,g1**) and 0.08 μWcm�<sup>1</sup> K�<sup>2</sup> for 3D-voided platelets (**Figure 13h,h1**).

**Table 1** summaries the morphology and properties of Bi2Te3-based thin-films deposited by PLD, sputtering, in comparison with the properties of TE bulks. Usually, TE nanomaterials possess low *σ* values due to the separating or voided structure-morphology, but bulk and thin films have superior *σ*. For example, the compact-polycrystalline Bi3Se2Te achieved *σ* = 1747.5 S/cm [53] or even higher for some other films [52]. Unfortunately, the *κ* of films are missed in many published works to calculate ZT of the films. Thermal conductivity (*κ*) of nanocrystalline and nanostructured thermoelectrics is expected to achieve low values thanks to the extensive phonons scattering at interfaces, surfaces and grain boundaries. Indeed, reduced *κ* values have been noted for the monocrystalline Bi2Se0.3Te2.7 films (*κ* = 0.8 W/m K for an average grain size of 60 nm) [60], and for Bi-Sb-Te films [61, 62].
