**3.3. Thermoelectric properties of polycrystalline Bi2Te3, Bi2Se3, and Bi3Se2Te thin films with controlled structure morphology**

Some typical HRTEM images of Bi2Te3, Bi2Se3, and Bi3Se2Te grown using PLD are shown in **Figure 7** [14–16]. HRTEM images performed on a high *μ* Bi2Te3 film with nanodisk-like morphology grown at 220°C are shown in **Figure 7a**. Clearly, the lower inset in **Figure 7a** shows the film with uniform thickness of approximately 295 nm and a SiO2 layer with a thickness of 300 nm. It shows that projected period of 0.508 nm along the *c*-axis corresponds to the lattice spacing of the (0 0 6) planes. The highly (0 0 1)-orientated and crystallized structures of the film should facilitate the transport of charge carriers. The *c*-axis lattice constant of the Bi2Te3 film is 30.48 Å, which agrees closely with the value (30.44 Å) presented in JCPDS 82-0358. The other Bi2Te3 films grown at *T*S ≥ 220°C also display similar HRTEM results.

For a Bi2Se3 film deposited at 300°C and 40 Pa, an HRTEM image taken at the boundary of three platelets (P1, P2, and P3) revealed the granular-polycrystalline structure of the films (**Figure 7b**). Moreover, P1 and P2 partly overlapped and the corresponding fast Fourier transform (FFT) of this overlapping region indexed by 003 patterns of [0 1 0] zone axis was performed from the dashed-square area (**Figure 7b**, inset). The projected period along the *c*axes of both P1 and P2 was 9.60 Å, corresponding to (0 0 3) planes, which was close to the reported value of 9.55 Å in Ref. [28].

HRTEM images of a Bi3Se2Te film deposited at 250°C and 40 Pa are shown in **Figure 7c** and **d**. Nanocrystallites with sizes of 10–20 nm are clearly observed in **Figure 7c**, confirming the nanocrystalline type of the Bi3Se2Te films. The interplanar spacing of the Bi3Se2Te (0 0 5) planes in the nanocrystallites is approximately 0.464 nm. Therefore, the *c*-axis lattice constant is determined to be 23.2 Å, closely agreeing with the value of 23.25 Å for Bi3Se2Te bulk (JCPDS 00-053-1190). In addition, the white lines in **Figure 7c** indicate the orientations of the (0 0 5) planes. It is seen that the overall orientation of the crystallites is disorganized. Intriguingly, near the interface of the film and substrate, the film has some nanoinclusions with sizes of 12– 17 nm, as shown in **Figure 7d**. The EDS analysis shows that these are Bi semimetal nanoprecipitates (the inset in **Figure 7d**). The lattice spacing of Bi nanoinclusions is ~0.32 nm, corresponding to the Bi (0 1 2) planes. It has been found that Bi nanoinclusions can lead to the enhanced Seebeck coefficient and reduced lattice thermal conductivity owing to the lowenergy electron filtering and phonon scattering at the nanoinclusions, respectively [29–31].

**Figure 6.** Overview of the potential applications of thermoelectric generators [27].

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

other Bi2Te3 films grown at *T*S ≥ 220°C also display similar HRTEM results.

**controlled structure morphology**

reported value of 9.55 Å in Ref. [28].

**3.3. Thermoelectric properties of polycrystalline Bi2Te3, Bi2Se3, and Bi3Se2Te thin films with**

Some typical HRTEM images of Bi2Te3, Bi2Se3, and Bi3Se2Te grown using PLD are shown in **Figure 7** [14–16]. HRTEM images performed on a high *μ* Bi2Te3 film with nanodisk-like morphology grown at 220°C are shown in **Figure 7a**. Clearly, the lower inset in **Figure 7a** shows the film with uniform thickness of approximately 295 nm and a SiO2 layer with a thickness of 300 nm. It shows that projected period of 0.508 nm along the *c*-axis corresponds to the lattice spacing of the (0 0 6) planes. The highly (0 0 1)-orientated and crystallized structures of the film should facilitate the transport of charge carriers. The *c*-axis lattice constant of the Bi2Te3 film is 30.48 Å, which agrees closely with the value (30.44 Å) presented in JCPDS 82-0358. The

For a Bi2Se3 film deposited at 300°C and 40 Pa, an HRTEM image taken at the boundary of three platelets (P1, P2, and P3) revealed the granular-polycrystalline structure of the films (**Figure 7b**). Moreover, P1 and P2 partly overlapped and the corresponding fast Fourier transform (FFT) of this overlapping region indexed by 003 patterns of [0 1 0] zone axis was performed from the dashed-square area (**Figure 7b**, inset). The projected period along the *c*axes of both P1 and P2 was 9.60 Å, corresponding to (0 0 3) planes, which was close to the

**Figure 7.** (a) HRTEM images of a high carrier mobility (*μ*) Bi2Te3 film with nanodisk-like morphology grown at 220°C and *PAr* of 80 Pa. (b) HRTEM image of an optimized Bi2Se3 film deposited at 300°C and *P*He of 40 Pa. The inset shows the FFT patterns of the dashed-square area in the HRTEM image. (c and d) HRTEM images of the Bi3Se2Te film grown at 250°C and *P*He of 40 Pa. The white lines in (c) indicate the (0 0 5)-orientation of nanograins. Inset in (d): FFT patterns and EDS spectra performed at film and Bi nanoinclusion positions.

**Figure 8a** shows the *T*S-dependent *α*, *σ*, and *PF* (= *α*<sup>2</sup> *σ*) of some nanostructured Bi2Te3 films [15]. The *σ* value gradually increased from 34.5 ± 0.1 to 814.3 ± 1.5 S/cm when *T*S was increased from 30 to 300°C, and then sharply decreased to 647.3 ± 0.4 S/cm at 340°C and 414.0 ± 1.2 S/cm at 380°C. The enhanced *σ* (= 647.3 – 814.3 S/cm) of the films grown at 220–340°C originated from the substantially enhanced *μ* because the *n* exhibited a slight decrease [15]. Although the coupled relationship between *σ* (= *neμ*) and |*α*| (~*n*−2/3) generally constrains the concurrent enhancement of *σ* and |*α*|, a reduction in *n* and a substantial increase in *μ* in the same optimal range of *T*<sup>S</sup> (= 220–340°C) could lead to high values of both *σ* and |*α*|. Consequently, the *PF* of the stoichiometric Bi2Te3 films grown in the range of 220–340°C reached remarkably high values, ranging between 18.2 ± 0.25 and 24.3 ± 0.44 μW cm−1 K−2, whereas the *PF* was low (≤0.44 μW cm−1 K−2) in the case of nonstoichiometric films deposited at *T*S ≤ 120 or 380°C (**Figure 8a**).

**Figure 8.** (a) Substrate temperature (*T*S) dependence of room temperature Seebeck coefficient *α* (red circles), electrical conductivity *σ* (blue triangulars), and power factor (*PF* = *α*<sup>2</sup> *σ*, black squares) of the Bi2Te3 and Bi4Te5 (for "PH" point) films. The morphology abbreviations: CNP, columnar nanoparticle; CNF, columnar nanoflower; ND, nanodisk; CP, compact polycrystalline; LTP, layered triangular platelet; PH, polyhedral. (b) Contour plot of the Bi2Se3 film's *PF* as a function of *P*He and *T*S. The morphology abbreviations: SC, smooth and compact; RG, rice grain; TP, triangular polygonal; S-LFs, super-layered flakes; L-HPs, layered hexagonal platelets. (c) Contour plot of the film's *PF* as a function of *T*<sup>S</sup> from 200 to 350°C and *P*He from 0.027 to 86.7 Pa. (d) |*S*| vs. *σ* of the films in this study and the relevant novel TE materials in the literature, listed in **Table 1**. Solid curves denote different *PF*s from 1 to 50 μW cm−1 K−2.

In order to check the evolution of the *PF* (= α2 *σ*) as a function of *P*He and *T*S, the contour plot is illustrated **Figure 8b**. The *PF* of Bi2Se3 films increased with increasing *T*<sup>S</sup> from 200 to 300°C because *σ* became considerably larger but the Seebeck coefficient diminished only slightly. However, for films deposited at 350°C, *PF* was lowered primarily because of the reduction in *S* and not the increase in *σ*. At intermediate pressures (40–93 Pa), the Bi2Se3 films remained stoichiometric or slightly Se-rich compositions, which in turn led to the reduced carrier concentrations and significantly enhanced the *α* values [14]. Thus, the *PF* of Bi2Se3 films grown at intermediate pressure was typically higher than the *PF* of films grown at a low or high pressure. The optimal value of *PF* was 5.54 ± 0.34 μW cm−1 K−2 for the layered hexagonal platelet Bi2Se3 films deposited at 300°C and 40 Pa [14].

**Figure 8a** shows the *T*S-dependent *α*, *σ*, and *PF* (= *α*<sup>2</sup>

conductivity *σ* (blue triangulars), and power factor (*PF* = *α*<sup>2</sup>

In order to check the evolution of the *PF* (= α2

The *σ* value gradually increased from 34.5 ± 0.1 to 814.3 ± 1.5 S/cm when *T*S was increased from 30 to 300°C, and then sharply decreased to 647.3 ± 0.4 S/cm at 340°C and 414.0 ± 1.2 S/cm at 380°C. The enhanced *σ* (= 647.3 – 814.3 S/cm) of the films grown at 220–340°C originated from the substantially enhanced *μ* because the *n* exhibited a slight decrease [15]. Although the coupled relationship between *σ* (= *neμ*) and |*α*| (~*n*−2/3) generally constrains the concurrent enhancement of *σ* and |*α*|, a reduction in *n* and a substantial increase in *μ* in the same optimal range of *T*<sup>S</sup> (= 220–340°C) could lead to high values of both *σ* and |*α*|. Consequently, the *PF* of the stoichiometric Bi2Te3 films grown in the range of 220–340°C reached remarkably high values, ranging between 18.2 ± 0.25 and 24.3 ± 0.44 μW cm−1 K−2, whereas the *PF* was low (≤0.44 μW cm−1 K−2) in the case of nonstoichiometric films deposited at *T*S ≤ 120 or 380°C (**Figure 8a**).

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

**Figure 8.** (a) Substrate temperature (*T*S) dependence of room temperature Seebeck coefficient *α* (red circles), electrical

films. The morphology abbreviations: CNP, columnar nanoparticle; CNF, columnar nanoflower; ND, nanodisk; CP, compact polycrystalline; LTP, layered triangular platelet; PH, polyhedral. (b) Contour plot of the Bi2Se3 film's *PF* as a function of *P*He and *T*S. The morphology abbreviations: SC, smooth and compact; RG, rice grain; TP, triangular polygonal; S-LFs, super-layered flakes; L-HPs, layered hexagonal platelets. (c) Contour plot of the film's *PF* as a function of *T*<sup>S</sup> from 200 to 350°C and *P*He from 0.027 to 86.7 Pa. (d) |*S*| vs. *σ* of the films in this study and the relevant novel TE mate-

illustrated **Figure 8b**. The *PF* of Bi2Se3 films increased with increasing *T*<sup>S</sup> from 200 to 300°C because *σ* became considerably larger but the Seebeck coefficient diminished only slightly. However, for films deposited at 350°C, *PF* was lowered primarily because of the reduction in

rials in the literature, listed in **Table 1**. Solid curves denote different *PF*s from 1 to 50 μW cm−1 K−2.

*σ*) of some nanostructured Bi2Te3 films [15].

*σ*, black squares) of the Bi2Te3 and Bi4Te5 (for "PH" point)

*σ*) as a function of *P*He and *T*S, the contour plot is


**Table 1.** Material, type, method, carrier concentration (*n*), mobility (*μ*), electrical conductivity (*σ*), Seebeck coefficient (*S*), power factor (*PF* = *S*<sup>2</sup> *σ*) of the optimal bismuth chalcogenide films in this study as compared to properties of Bi2Te3, Bi2Se3, Bi2Se*x*Te1−*x* bulk and films reported in the literature. All the selected values were recorded at room temperature.

The *T*S*-* and *P*He*-*dependent *PF* of nanocrystalline Bi3Se2Te films is further shown in **Figure 8c**. The films grown at 200°C only have *PF*s of 1.0–2.8 μW cm−1 K−2. The *PF*s of the films grown at higher *T*S are significantly enhanced because of their high *σ* values. Around *T*S = 250–350°C and *P*He = 40 Pa, a window for high *PF* is clearly observed. An optimal *PF* of 8.3 μW cm−1 K−2 is achieved for a Bi3Se2Te film deposited at 250°C and 40 Pa.

**Table 1** summarizes the transport and room-temperature TE properties of bismuth chalcogenides in the literature [14, 15, 32–39]. For PLD growths, the highly (0 0 1)-oriented layered Bi2Te3 films achieved a *PF* of 50.6 μW cm−1 K−2 [39], and the layered compact polycrystalline film possessed a *PF* value of 24.3 μW cm−1 K−2 [15]. The Bi2Se3 films generally have lower TE properties than those of Bi2Te3 films. For example, the optimal *PF* of the Bi2Se3 films grown by PLD was 5.5 μW cm−1 K−2 [14], which was slightly lower than the *PF* of Bi2Se3 bulk (*PF* ≈ 7.7 μW cm−1 K−2 ) [32]. The nanocrystalline Bi3Se2Te films had an optimal *PF* of 8.3 μW cm−1 K−2 [16]. Further, PLD growth allows fabrication of nanostructured TE films with different morphologies of nanoparticle Bi2Te3 film (*PF* = 1.9 μW cm−1 K−2) [34] and super-assembled Bi2Te3 film (*PF* = 1.0 μW cm−1 K−2) [33]. The Bi2Te3 film deposited by the sputtering technique had *PF* of 8.8 μW cm−1 K−2 [35]. There are some reports of TE properties for bulk materials of bismuth chalcogenides, such as Bi2Se1.8Te1.2 nanoplatelet (*PF* ≈ 1.3 μW cm−1 K−2 ) [38], Bi2Se2Te (*PF* ≈ 5.8 μW cm−1 K−2 ), Bi2Se1.5Te1.5 (*PF* ≈ 16.5 μW cm−1 K−2 ) [37], and Bi2Se0.3Te2.7 (*PF* ≈ 32.2 μW cm−1 K−2 ) [36]. Unfortunately, the thermal conductivity *κ* of the films is missed in the reports to fully evaluate the TE performance of the films. Nevertheless, the *κ* of polycrystalline films with small grain sizes should be reduced thanks to the extensive phonon scattering at interfaces and grain boundaries.

Finally, **Figure 8d** shows the |*S*| vs. *σ* plot for the list in **Table 1**. The solid curves denote different values of *PF*s (= *S*<sup>2</sup> *σ*). It can be found that TE nanomaterials usually possess low *σ* values due to the separating or voided structure morphology, but bulk and thin films have superior *σ*. Note, the significant reduction in thermal conductivity *κ* is the key factor for employing nanostructured materials in the TE field.
