**2.3 Thermal conductivity κ of Bi2Se3 and Bi2Te3 and Bi-Te-Se compounds**

A transient 3ω technique is usually employed in measuring thermal conductivity of thermoelectric films. The detail of this technique can be found in refs. [62–64]. **Table 2** summarizes thermal transport properties (at room–temperature) of nanocrystalline–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 κ of nanocrystalline Bi2Te3-based films will further decrease when the grain size of

**Material**

**54**

Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Se3 Bi2Te3 Bi2Te3 Bi3Se2Te

Bi2Se3 Bi2Se0.3Te2.7

Bi2Se1.5Te1.5 Bi2Se1.8Te1.2

Bi2Se2Te

**Table 1.** *Material, type, method, processing conditions,*

*PLD and RF sputtering,*

 *as compared to properties of Bi2Se3, Bi2Se3, Bi2SexTe1-x bulk and films reported in the literature.*

 *carrier* 

*concentration*

 *(n), mobility (μ), electrical conductivity*

 *(σ), Seebeck coefficient (α), power factor (PF = α2*

 *All the selected values were recorded at room* 

Nano-platelet

 bulk

> Bulk

Ball milling- hot pressing

Polyol method

Nanocrystalline

Bulk Bulk Bulk

Zone melting

Ball milling-hot

 pressing

Melting and

hot-pressing

 film

Layered HPs

Compact film

Layered-smooth

 film

Columnar Structure

Sputtering

PLD PLD PLD PLD

250°C

 40 Pa

 35.5

─

─

1.2

─

─

 ─

1613

�60 *σ) of the Bi2Te3 films deposited by*

*temperature.*

5.8

 [55]

 ─

199.6

�80.9

1.3

 [57]

230

 441.6

�193

16.5

 [56]

 ─

892

�190

32.2

 [55]

 ─

251.9

�175

7.7

 [54]

34.4

 1747.5

�68.8

8.3

 [53]

250°C

 10 Pa

 10.1

90.6

 1464

�186

50.6

 [43]

300°C

 80 Pa

5

102

 814.3

�172.8

 24.3

 [40]

300°C

 40 Pa

350°C

 1.0 Pa

 246 7.4

81.4

 963.8

�75.8

5.5

 [38]

7.5

 2990

�46

6.4

 [52]

Layered Structure

Nanorods

3D-layered

Nanoparticles Nanoparticles

super-assembly

PLD PLD PLD Sputtering

350°C

250°C

 0.9 Pa

 9.1

2.0

 29

�81

0.19

 [51]

 1.0 Pa

300°C

 1.0 Pa

 105

95

12.1

 1840

�70

8.8

 [52]

8.3

 1390

�60

5.0

 [52]

300°C

 20 Pa

600°C

 0.13 Pa

 5.1 9.7

14.8

 230

�91

1.90

 [51]

20.3

 160

�137

3.0

 [41]

worm-like

super-assembly

Worm-like

super-assembly

PLD PLD

450°C

 0.13 Pa

 1.2

29.4

 49

�138

0.93

 [41]

400°C

 0.13 Pa

 1.9

25.9

 73

�119

1.03

 [41]

*Materials at the Nanoscale*

Spindle-like

super-assembly

PLD

350°C

 0.13 Pa

 4.0

12.4

 79

�113

1.01

 [41]

**Morphology**

**Method**

**Deposition**

 **conditions** *n* **(1019 cm**�**3**

**) μ**

**(cm2/Vs) σ (S/cm) α**

**(μV/K)**

 **PF** 

**(μW/cmK2)**

 **Ref.**


#### **Table 2.**

*Room–temperature thermal transport properties of nanocrystalline–nanostructured Bi2Te3-based thin films and bulk materials in the literature, included: sample and fabrication method, average grain size, thermal conductivity κ, electrical conductivity σ, Seebeck coefficient α, power factor PF (= α<sup>2</sup> σ), and ZT (at 300 K).*

decreases (κ ≤ 0.81 W/mK, **Figure 14A**) [62, 65]. For Bi2Te3/Sb2Te3 superlattice films, the coherent backscattering of phonon waves at the superlattice interfaces is outlined for the reduction of lattice thermal conductivity, resulting in the low κ ≤ 0.4 W/mK [67, 69].

For PLD Bi2Te3-based films, Yamasaki et al. [69] measured thermal conductivity with an ac calorimetric method in the direction across the film, obtaining k1.1 W/m K for a Bi2Te3 film deposited by PLD in vacuum (**Table 2**). In addition, Walachova et al. [70] used direct ZT measurement with the Harman method to estimate the κ value, and it is about 0.2–0.3 W/mK for the Bi2Te3 films. Recently, Chang et al. [71] reported the κ values of 0.93–1.16 W/mK for BixSb2-xTe3 films with the granular-layered morphologies (**Figure 14B**).

**3. Conclusions**

*Bi2Te3-xSex films [65], (B) the BixSb2-xTe3 films [71].*

**Figure 14.**

**57**

In this book chapter, we present an overview of thermoelectric materials and applications, challenging of enhancing TE properties, and the nanostructuring approach in development TE materials. Various interesting nanostructured Bi2Te3 based thin films have been grown successfully by PLD with properly controlled substrate temperatures ambient gas pressures. For example, super-assembling of

*The morphology and thermal conductivity of Bi2Te3-based films with different grain sizes: (A) nanocrystalline*

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

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

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

**Figure 14.**

decreases (κ ≤ 0.81 W/mK, **Figure 14A**) [62, 65]. For Bi2Te3/Sb2Te3 superlattice films, the coherent backscattering of phonon waves at the superlattice interfaces is outlined for the reduction of lattice thermal conductivity, resulting in the low

*Room–temperature thermal transport properties of nanocrystalline–nanostructured Bi2Te3-based thin films and bulk materials in the literature, included: sample and fabrication method, average grain size, thermal*

with an ac calorimetric method in the direction across the film, obtaining

*conductivity κ, electrical conductivity σ, Seebeck coefficient α, power factor PF (= α<sup>2</sup>*

the granular-layered morphologies (**Figure 14B**).

For PLD Bi2Te3-based films, Yamasaki et al. [69] measured thermal conductivity

k1.1 W/m K for a Bi2Te3 film deposited by PLD in vacuum (**Table 2**). In addition, Walachova et al. [70] used direct ZT measurement with the Harman method to estimate the κ value, and it is about 0.2–0.3 W/mK for the Bi2Te3 films. Recently, Chang et al. [71] reported the κ values of 0.93–1.16 W/mK for BixSb2-xTe3 films with

κ ≤ 0.4 W/mK [67, 69].

**Sample, fabrication method Avg.**

Bi2Te2.7Se0.3 nanocrystalline thin film, flash evaporation

Sintered bulk Bi2Te3-xSex material, hot-pressing

*Materials at the Nanoscale*

Nanocrystalline bismuthtelluride-based (Bi2Te3-xSex) thin

Nanocrystalline Bi-Sb-Te thin

Nanocrystalline BiSbTe (8:30:62) thin film, flash evaporation

Bi2Te3/Sb2Te3 superlattices

Bi2Te3/Bi2(Te0.88Se0.12)3 superlattice film, MBE

Bi2Te3/Sb2Te3 superlattices film (layered thickness 6 nm), PLD.

BixSb2-xTe3 nanocolumn film,

PLD

**Table 2.**

**56**

film, sputtering

(period5 nm)

film

**grain size**

60 nm 0.8

**κ (W/ m K)**

(crossplane)

Single crystal BiSbTe bulk alloys — 0.75 —— — — [5]

Bi2Te3+0.63 bulk — 2.2 1000 240 58 0.87 [5] Bi2(Te0.95Se0.05)3 bulk — 1.59 901 223 45 0.85 [5]

Bi2Te3 film, PLD — 1.1 —— — — [69]

Bi2Te3 films, laser ablation — 0.2–0.3 —— — — [70] BixSb2-xTe3 nanolayer film, PLD 190 nm 1.16 2700 95 25 0.65 [71] BixSb2-xTe3 nanodisc film, PLD 100 nm 1.00 1100 132 20 0.60

**σ (S/ cm)**

**α (μV/ K)**

540 186.1 (inplane)

30 μm 1.6 930 177.5 29.3 0.6

45 nm 0.65 6.7 ——— 84 nm 0.81 33.3 ———

10 nm 0.61 550 84.0 3.9 0.19 [65] 27 nm 0.68 540 138.1 10.3 0.46 60 nm 0.80 540 186.1 18.7 0.70

26 nm 0.46 3.3 ——— [62]

150 nm 0.6 —— — — [66]

— 0.4 —— — — [67]

80 nm 1.25 639 204 27 0.60 [68]

— 0.11 —— — —

70 nm 0.93 280 207 12 0.39

**PF = σα<sup>2</sup> (μW/ cmK<sup>2</sup> )**

18.7 (inplane)

**ZT (300 K)** **Ref.**

0.7 [60]

*σ), and ZT (at 300 K).*

*The morphology and thermal conductivity of Bi2Te3-based films with different grain sizes: (A) nanocrystalline Bi2Te3-xSex films [65], (B) the BixSb2-xTe3 films [71].*

## **3. Conclusions**

In this book chapter, we present an overview of thermoelectric materials and applications, challenging of enhancing TE properties, and the nanostructuring approach in development TE materials. Various interesting nanostructured Bi2Te3 based thin films have been grown successfully by PLD with properly controlled substrate temperatures ambient gas pressures. For example, super-assembling of

Bi2Te3 hierarchical nanostructures were grown at *TS* from 350 to 600°C, and the films possessed relative high Seebeck coefficient of 113–138 μV/K, but exhibited low electrical conductivities of 49–160 S.cm�<sup>1</sup> , and thus they had relatively low PF in range of 0.93 to 3.0 μW/cmK<sup>2</sup> . Our intensive literature review on Bi2Te3-based TE materials can make general conclusion that TE nanomaterials possess low *σ* values when their structure-morphology are separating or voided, meanwhile, bulk and compact-smooth thin films can achieve high *σ* values. The PF values of Bi2Te3-based thermoelectrics varied in a wide range, i.e. below 5.0 μW/cmK<sup>2</sup> for voided structure-morphology, and reaching intermediate-high PF values of 5.0–50.6 μW/cmK<sup>2</sup> for compact-smooth or compact-layered structures. An advantage of nanocrystalline and nanostructuring thermoelectrics is the reduced thermal conductivity (possibly below 1 W.m�<sup>1</sup> K�<sup>1</sup> ). This book chapter provides fundamental understanding the relationship amongst processing condition in PLD growths, structure-morphology, and TE properties of Bi2Te3-based thin films.

**References**

science.1158899

asiamat.2010.138

York: Springer; 2001.

[1] Bell LE. Cooling, heating , generating power, and recovering waste heat with thermoelectric systems. Science. 2008;

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

merit from bulk to nano-thermoelectric materials. Nano Energy. 2013; **2**: 190– 212. DOI: 10.1016/j.nanoen.2012.10.005

[12] Cutler M, Leavy JF, Fitzpatrick RL. Electronic transport in semimetallic cerium sulfid. Phys. Rev. 1964; **133:**

[11] Pichanusakorn P, Bandaru P. Nanostructured thermoelectrics. Mater. Sci. Eng. R. 2010; **67**:19–63. DOI: 10.1016/j.mser.2009.10.001

[13] Tritt TM. Thermoelectric Phenomena, Materials, and

matsci-062910-100453.

Calero O, Díaz-Chao P.

j.rser.2013.03.008.

Applications. Annu. Rev. Mater. Res. 2011; **41:** 433. DOI: 10.1146/annurev-

[14] Martín-González M, Caballero-

Nanoengineering thermoelectrics for 21st century: Energy harvesting and other trends in the field. *Renew. Sustain. Energy Rev.* 2013; **24**: 288. DOI: 10.1016/

[15] Eilertsen J, Li J, Rouvimov S, Subramanian MA. Thermoelectric properties of indium-filled InxRh4Sb12 skutterudites. J. Alloys Compd. 2011;

[16] Dresselhaus MS, Chen G, Tang MY, Yang RG, Lee H, Wang DZ, Ren ZF, Fleurial J-P, Gogna P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007; **19**: 1043. DOI: 10.1002/adma.200600527.

[17] Hicks LD, Dresselhaus MS. Effect of

Thermoelectric figure of merit of a onedimensional conductor. Phys. Rev. B

quantum-well structures on the thermoelectric figure of merit. Phys. Rev. B 1993; **47:** 12727. DOI: 10.1103/

[18] Hicks LD, Dresselhaus MS.

PhysRevB.47.12727

**509**: 6289. DOI: 10.1016/j. jallcom.2011.03.057.

A1143.

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

[2] Chen G., Shakouri A. Heat Transfer in Nanostructures for Solid-State Energy Conversion. J. Heat Transfer. 2002; **124**: 242. DOI: 10.1115/1.1448331.

[3] Snyder GJ, Toberer ES. Complex thermoelectric materials. Nat. Mater. 2008; 7:105–114. DOI: 10.1038/nmat2090

[5] Rowe DM. Thermoelectrics

handbook: macro to nano. D.M. Rowe Ed. Boca Raton: CRC Press; 2006. 1014 p. DOI: 10.1201/9781420038903

[6] Nolas GS, Sharp J, Goldsmid HJ, Thermoelectrics: Basic Principles and New Materials Developments. New

[7] Lan Y, Minnich AJ, Chen G, Ren Z. Enhancement of thermoelectric figureof-merit by a bulk nanostructuring approach. Adv. Funct. Mater. 2010; **20**: 357–376. DOI: 10.1002/adfm.200901512

[8] http://www.electronics-cooling.com/ 2005/11/advances-in-high-performa

nce-cooling- for-electronics/

matsci.29.1.89.

**59**

[9] Nolas GS, Morelli DT, Tritt TM. KUTTERUDITES: A Phonon-Glass-Electron Crystal Approach to Advanced Thermoelectric Energy Conversion Applications. Annu. Rev. Mater. Sci. 1999; **29**: 89. DOI: 10.1146/annurev.

[10] Alam H and Ramakrishna S. A review on the enhancement of figure of

[4] Li J-F, Liu W-S, Zhao L-D, Zhou M. High-performance nanostructured thermoelectric materials. NPG Asia Mater. 2010; **2**: 152. DOI: 10.1038/

**321**: 1457–1461. DOI: 10.1126/
