**4. Nanomechanical properties of Bi2Te3 and Bi3Se2Te thin films**

Effects of helium ambient pressure (in PLD) on the nanomechanical properties of Bi2Te3 and Bi3Se2Te thin films have been investigated [9, 40]. The Bi2Te3 thin films were grown at *T*<sup>S</sup> of 250°C on c-plane sapphire substrates using excimer laser PLD with a power density of 5 J/ cm2 , at a repetition rate of 2 Hz. The helium pressures in PLD growth varied from 2 × 10−5 to 6.5 × 10−3 Torr. Similarly, Bi3Se2Te thin films were deposited on Al2O3 (0 0 0 1) substrates at a fixed substrate temperature of 250°C and *P*He ranging from 2 × 10−5 to 6.5 × 10−1 Torr through PLD. The light source of the PLD system was a KrF excimer laser with *λ* = 248 nm, pulse duration of 20 ns, fluence of 7.0 J/cm2 , and repetition rate of 2 Hz. The target-to-substrate distance, the number of laser pulses and deposition time were 40 mm, 3000, and 25 min, respectively. The grown films had the average thickness of 200 nm (the average growth rate was ~0.67 Å/pulse). Nanoindentation experiments were performed on a MTS Nano Indenter® XP system with a three-sided pyramidal Berkovich indenter tip using the continuous stiffness measurement (CSM) technique [41]. The hardness and Young's modulus of the Bi2Te3 and Bi3Se2Te thin films were determined from the load-displacement results through the analytical method proposed by Oliver and Pharr [42].

As shown in **Figure 9a**, the hardness monotonically increased from 2.92 ± 0.12 to 4.02 ± 0.14 GPa for Bi2Te3 films, and from 2.5 ± 0.2 to 3.0 ± 0.1 GPa for Bi3Se2Te films when *P*He was increased from 2.0 × 10−5 to 2.0 × 10−3 Torr. Similarly, the Young's modulus of Bi2Te3 thin films was 106.31 ± 0.63, 115.51 ± 1.92, and 127.46 ± 9.21 GPa for *P*He at 2.0 × 10−5, 2.0 × 10−4, and 2.0 × 10−3 Torr, respectively. For PHe > 2.0 × 10−3 Torr, the hardness (Young's modulus) of Bi3Se2Te films continues to increase with increasing *P*He, namely 3.2 ± 0.1 GPa (105.2 ± 10.2 GPa) at 2.0 × 10−1 Torr and 5.8 ± 0.2 GPa (188.5 ± 4.3 GPa) at 6.5 × 10−1 Torr (**Figure 9b**).

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 scatter-

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

Finally, **Figure 8d** shows the |*S*| vs. *σ* plot for the list in **Table 1**. The solid curves denote

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

Effects of helium ambient pressure (in PLD) on the nanomechanical properties of Bi2Te3 and Bi3Se2Te thin films have been investigated [9, 40]. The Bi2Te3 thin films were grown at *T*<sup>S</sup> of 250°C on c-plane sapphire substrates using excimer laser PLD with a power density of 5 J/

distance, the number of laser pulses and deposition time were 40 mm, 3000, and 25 min, respectively. The grown films had the average thickness of 200 nm (the average growth rate was ~0.67 Å/pulse). Nanoindentation experiments were performed on a MTS Nano Indenter® XP system with a three-sided pyramidal Berkovich indenter tip using the continuous stiffness measurement (CSM) technique [41]. The hardness and Young's modulus of the Bi2Te3 and Bi3Se2Te thin films were determined from the load-displacement results through the analytical

As shown in **Figure 9a**, the hardness monotonically increased from 2.92 ± 0.12 to 4.02 ± 0.14 GPa for Bi2Te3 films, and from 2.5 ± 0.2 to 3.0 ± 0.1 GPa for Bi3Se2Te films when *P*He was increased from 2.0 × 10−5 to 2.0 × 10−3 Torr. Similarly, the Young's modulus of Bi2Te3 thin films was 106.31 ± 0.63, 115.51 ± 1.92, and 127.46 ± 9.21 GPa for *P*He at 2.0 × 10−5, 2.0 × 10−4, and 2.0 × 10−3 Torr,

, at a repetition rate of 2 Hz. The helium pressures in PLD growth varied from 2 × 10−5 to 6.5 × 10−3 Torr. Similarly, Bi3Se2Te thin films were deposited on Al2O3 (0 0 0 1) substrates at a fixed substrate temperature of 250°C and *P*He ranging from 2 × 10−5 to 6.5 × 10−1 Torr through PLD. The light source of the PLD system was a KrF excimer laser with *λ* = 248 nm, pulse

**4. Nanomechanical properties of Bi2Te3 and Bi3Se2Te thin films**

*σ*). It can be found that TE nanomaterials usually possess low *σ*

, and repetition rate of 2 Hz. The target-to-substrate

ing at interfaces and grain boundaries.

duration of 20 ns, fluence of 7.0 J/cm2

method proposed by Oliver and Pharr [42].

employing nanostructured materials in the TE field.

different values of *PF*s (= *S*<sup>2</sup>

cm2

**Figure 9.** Material and nanomechanical properties of Bi2Te3 and Bi3Se2Te thin films grown on Al2O3 (0 0 0 1) substrates at *T*S of 250°C and various helium pressures (*P*He) between 2.0 × 10−5 and 6.5 × 10−1 Torr [9, 40]: (a and b) the hardness and Young's modulus, (c) grain size (*D*), (d) the Hall-Petch behavior observed on the Bi3Se2Te thin films, in which the hardness is observed to increase approximately with *D*−1/2 (*D* is grain size).

The crystallite sizes (*D*) of the films were estimated using the Scherrer equation *D* = 0.9*λ/ B*cos*θ*, where *λ*, *B*, and *θ* are the X-ray wavelength, full width at the half maximum of the Bi2Te3 (0 0 15) peak or Bi3Se2Te (0 0 5) peak, and Bragg diffraction angle, respectively. The *P*Hedependent *D* of the Bi2Te3 and Bi3Se2Te films is shown in **Figure 9c**. The grain size increases monotonically from 11.0 to 20.0 nm for Bi2Te3 and from 16.1 to 20.5 nm for Bi3Se2Te with increasing *P*He from 2.0 × 10−5 to 2.0 × 10−3 Torr. In the nanoscale, grain size can affect significantly the mechanical properties of materials. The dislocation activities can be drastically suppressed in a polycrystalline material when the grain size is decreased, and thus the grain boundary sliding and/or grain rotations become the dominant deformation behavior, which in turn would lead to the manifestations of the inverse Hall-Petch effect [43]. Softening caused by grain boundary sliding is mainly attributed to large amount of defects in grain boundaries, which allow rapid diffusion of atoms and vacancies under stress [44]. Consequently, the plastic deformation of Bi2Te3 films should be dominated by the grain boundary sliding and/or grain rotation rather than the dislocation activity because of *D* ≤ 20 nm [40], which is consistent with the results in Refs. [45–48].

In contrast, the 200-nm-thick Bi3Se2Te films with *D* of 16.1–25.1 nm grown at a larger *P*He range of 2.0 × 10−5 to 6.5 × 10−1 Torr exhibited the nanomechanical followed Hall-Petch relationship [44, 49]. The hardness and Young's modulus of the Bi3Se2Te thin films monotonically increased with increasing *P*He because of a corresponding decrease in grain sizes (**Figure 9a**–**c**). **Figure 9d** shows that hardness (*H*) increased linearly with *D−1/2* (where *D* is the grain size of the Bi3Se2Te films in the nanoscale regime) which is the typical Hall-Petch relationship [44, 49]. This is because the multiplication and mobility of dislocations are hindered by reducing the grain size [44]. It is reasonable for the observed phenomenon when the present grain sizes ranged between 25.1 and 16.1 nm which is larger than the typical critical *Dc* of 10 nm [44, 49]. It is demonstrated that the hardness and Young's modulus of the Bi2Te3 and Bi3Se2Te thin films can be enhanced by proper selection of the ambient pressure in PLD growths.
