**Abstract**

This book chapter reports recent advances in nanostructured Bi2Te3-based thermoelectric (TE) thin-films fabricated by pulsed laser deposition (PLD). By controlling the processing conditions in PLD growths, various fascinating Bi2Te3-based nanostructured films with promising or enhanced TE properties have been successfully fabricated, including super-assembling of Bi2Te3 hierarchical nanostructures, self-assembled Bi2Te3 films with well-aligned 0D to 3D nanoblocks, polycrystalline-nanostructured Bi2Se3 and Bi2Te3 thin-films, etc. In addition, a PLD-growth mechanism for fabricating the super-assembling Bi2Te3 thin-films is presented. This book chapter provides fundamental understanding the relationship amongst processing condition, structure-morphology, and TE property of PLD-growths Bi2Te3-based thin-films. It also presents an overview of TE materials and applications with the challenges and perspectives.

**Keywords:** Bi2Te3, thermoelectrics, self-assembly nanostructures, thermoelectric power factor, pulsed laser deposition

#### **1. Introduction**

Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating [1].

#### **1.1 Thermoelectric effects**

When an electric current flows through a pair of p-type and n-type semiconductors connected in series (**Figure 1(a)**, the holes in the p-type material and the electrons in the n-type material carry heat away from the top metal–semiconductor junctions, which leads to a cooling at the junctions called the Peltier effect. When current flows within the module, one side is cooled and the other heated. If the current is reversed, the hot and cold sides reverse also. For each material, the cooling effect is gauged by the Peltier coefficient Π that relates the heat carried by the charges to the electrical current through [1, 2, 4]: Q = ΠI.

In **Figure 1(b)**, when the two ends of the materials maintain a temperature difference, the higher thermal energy holes and electrons will diffuse from the hot

*<sup>η</sup>* <sup>¼</sup> *Th* � *Tc Th*

and the coefficient of performance presents for the efficiency of

*Th* � *Tc*

ZT is of the order of 1 at room temperature. This value gives a COP of about 1 (**Figure 2a**), which is still far lower than the COP = 2–4 of household refrigerators and air conditioners. Similar situation is true for power generation (**Figure 2b**) [2, 8]. Thermoelectric cooling and power generation generally still not competitive

*COP* <sup>¼</sup> *Tc*

applications, it is important to use high ZT thermoelectric materials.

properties of TE materials is to increase the power factor *α<sup>2</sup>*

tivity limit ZT to approximately 1 in large bulk materials [10].

air-conditioning and refrigeration [7]:

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

with the other energy conversion methods.

**1.3 Challenges in enhancing ZT**

quently, *α* decreases and thus α<sup>2</sup>

**Figure 2.**

**43**

*generation [2, 8].*

�

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

p

p

�

where *Th* and *Tc* are the hot-end and cold-end temperature of the thermoelectric materials, respectively, and *T* is the average temperature of *Th* and *Tc*. For practical

The best materials so far are alloys of Bi2Te3 with Sb2Te3 and Bi2Te3 with Bi2Se3.

A concept of "phonon-glass electron-crystal" (or PGEC in short) was proposed for designing efficient thermoelectric materials. This is a controversial concept from the aspect of materials science that the materials should have a high electrical conductivity as in a crystal and a low lattice thermal conductivity as in a glass [9]. However, the TE parameters are strongly interdependent, which makes the enhancement efforts of ZT very challenging. A normal approach for the enhanced

carrier concentration *n*, and/or to reduce the lattice thermal conductivity *κ<sup>L</sup>* by introducing the scattering centers. These parameters are the function of carrier effective mass *m\** and carrier mobility *μ*, scattering factor *r,* and their interconnec-

The kinetic definition of *α* is the energy difference between the average energy of mobile carriers and the Fermi energy [11]. When carrier concentration (*n*) is increased, both the Fermi energy and the average energy increase, but the Fermi energy increases more rapidly than the average energy as *n* is increased. Conse-

*Comparison of thermoelectric technology with other energy conversion methods for (a) cooling and (b) power*

*n* is dragged down rapidly. Therefore, the carrier

p

p

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ *ZT*

� <sup>1</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ *ZT*

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ *ZT*

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ *ZT*

<sup>þ</sup> *Tc Th*

> � *Th Tc*

þ 1

(2)

(3)

*σ* by optimizing the

#### **Figure 1.**

*Illustration of TE devices: (a) cooler (Peltier effect), (b) power generator (Seebeck effect). Redrawn after Ref. [2]. (c) Thermoelectric module showing the direction of charge flow on both cooling and power generation [3].*

side to the cold side, and consequently a potential difference is created. This is Seebeck effect and it is the principle for thermocouples. The power generation is measured by the Seebeck coefficient α, which relates the voltage generated to the temperature difference through ΔV = �αΔT. The Peltier and the Seebeck coefficients are related through the Kelvin relation [1, 2]: Π = αT.

Thermoelectric devices contain many thermoelectric couples (**Figure 1c**, bottom), which consist of p-type (containing free holes) and n-type (containing free electrons) thermoelectric elements connected electrically in series and thermally in parallel (**Figure 1c**, top). A thermoelectric generator uses heat flow across a temperature gradient to power an electric load through the external circuit.

#### **1.2 The thermoelectric figure of merit (ZT)**

The performance of the thermoelectric materials is often denoted as figure of merit Z whose unit is K�<sup>1</sup> , or ZT the dimensionless unit [5, 6].

$$ZT = \frac{\alpha^2 \sigma}{\kappa} T = \frac{\alpha^2 \sigma}{\kappa\_E + \kappa\_L} T \tag{1}$$

where *α*, *σ*,*T*, and *κ* are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. The total thermal conductivity can be split into electronic contribution (*κE*) and lattice contribution (*κL*). The thermoelectric power factor (PF) is calculated by the quantity of α<sup>2</sup> σ. The efficiency of a thermoelectric material is determined by its ZT. Meanwhile, the maximum efficiency (η) of a power generation is expressed by [3, 7]:

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

$$\eta = \frac{T\_h - T\_c}{T\_h} \cdot \frac{\sqrt{\mathbf{1} + Z\overline{T}} - \mathbf{1}}{\sqrt{\mathbf{1} + Z\overline{T}} + \frac{T\_c}{T\_h}} \tag{2}$$

and the coefficient of performance presents for the efficiency of air-conditioning and refrigeration [7]:

$$\text{COP} = \frac{T\_c}{T\_h - T\_c} \cdot \frac{\sqrt{\mathbf{1} + Z\overline{T}} - \frac{T\_h}{T\_c}}{\sqrt{\mathbf{1} + Z\overline{T}} + \mathbf{1}} \tag{3}$$

where *Th* and *Tc* are the hot-end and cold-end temperature of the thermoelectric materials, respectively, and *T* is the average temperature of *Th* and *Tc*. For practical applications, it is important to use high ZT thermoelectric materials.

The best materials so far are alloys of Bi2Te3 with Sb2Te3 and Bi2Te3 with Bi2Se3. ZT is of the order of 1 at room temperature. This value gives a COP of about 1 (**Figure 2a**), which is still far lower than the COP = 2–4 of household refrigerators and air conditioners. Similar situation is true for power generation (**Figure 2b**) [2, 8]. Thermoelectric cooling and power generation generally still not competitive with the other energy conversion methods.

#### **1.3 Challenges in enhancing ZT**

A concept of "phonon-glass electron-crystal" (or PGEC in short) was proposed for designing efficient thermoelectric materials. This is a controversial concept from the aspect of materials science that the materials should have a high electrical conductivity as in a crystal and a low lattice thermal conductivity as in a glass [9]. However, the TE parameters are strongly interdependent, which makes the enhancement efforts of ZT very challenging. A normal approach for the enhanced properties of TE materials is to increase the power factor *α<sup>2</sup> σ* by optimizing the carrier concentration *n*, and/or to reduce the lattice thermal conductivity *κ<sup>L</sup>* by introducing the scattering centers. These parameters are the function of carrier effective mass *m\** and carrier mobility *μ*, scattering factor *r,* and their interconnectivity limit ZT to approximately 1 in large bulk materials [10].

The kinetic definition of *α* is the energy difference between the average energy of mobile carriers and the Fermi energy [11]. When carrier concentration (*n*) is increased, both the Fermi energy and the average energy increase, but the Fermi energy increases more rapidly than the average energy as *n* is increased. Consequently, *α* decreases and thus α<sup>2</sup> *n* is dragged down rapidly. Therefore, the carrier

#### **Figure 2.**

*Comparison of thermoelectric technology with other energy conversion methods for (a) cooling and (b) power generation [2, 8].*

side to the cold side, and consequently a potential difference is created. This is Seebeck effect and it is the principle for thermocouples. The power generation is measured by the Seebeck coefficient α, which relates the voltage generated to the temperature difference through ΔV = �αΔT. The Peltier and the Seebeck

*Illustration of TE devices: (a) cooler (Peltier effect), (b) power generator (Seebeck effect). Redrawn after Ref. [2]. (c) Thermoelectric module showing the direction of charge flow on both cooling and power generation [3].*

Thermoelectric devices contain many thermoelectric couples (**Figure 1c**, bottom), which consist of p-type (containing free holes) and n-type (containing free electrons) thermoelectric elements connected electrically in series and thermally in parallel (**Figure 1c**, top). A thermoelectric generator uses heat flow across a

temperature gradient to power an electric load through the external circuit.

*ZT* <sup>¼</sup> *<sup>α</sup>*<sup>2</sup>*<sup>σ</sup> κ*

thermoelectric power factor (PF) is calculated by the quantity of α<sup>2</sup>

efficiency (η) of a power generation is expressed by [3, 7]:

The performance of the thermoelectric materials is often denoted as figure of

, or ZT the dimensionless unit [5, 6].

*<sup>T</sup>* <sup>¼</sup> *<sup>α</sup>*<sup>2</sup>*<sup>σ</sup> κ<sup>E</sup>* þ *κ<sup>L</sup>*

where *α*, *σ*,*T*, and *κ* are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. The total thermal conductivity can be split into electronic contribution (*κE*) and lattice contribution (*κL*). The

of a thermoelectric material is determined by its ZT. Meanwhile, the maximum

*T* (1)

σ. The efficiency

coefficients are related through the Kelvin relation [1, 2]: Π = αT.

**1.2 The thermoelectric figure of merit (ZT)**

merit Z whose unit is K�<sup>1</sup>

**42**

**Figure 1.**

*Materials at the Nanoscale*

concentration (*n*) increases electrical conductivity (σ) but reduces the Seebeck coefficient (α) for most of the homogeneous materials. For this reason, in metals and degenerate semiconductors with energy-independent scattering approximation, the Seebeck coefficient can be expressed as [3, 12]:

$$a = \frac{8\pi^2 k\_B^2}{3eh^2} m^\* T \left(\frac{\pi}{3n}\right)^{2/3} \tag{4}$$

Where, the parameter *m\** is density of states effective mass, and an increase of *m\** can raise the Seebeck coefficient according to the Eq. (4). However, most high *m\** materials have generally low *μ* which limits the α by a weighted mobility with a factor proportional to (*m\** ) 3/2*μ*. Moreover, there is no such thing as an optimal effective mass. There are high mobility low effective mass semiconductors (SiGe, GaAs) as well as low mobility high effective mass polaron conductors (oxides, chalcogenides) [3].

Noticeably, the defects scatter not only the phonons but also the electrons. When a thermoelectric material is designed for reducing lattice thermal conductivity, its carrier mobility is usually suppressed. Hence, the ratio of *μ/κ<sup>L</sup>* determines the improvement of ZT [5, 10]. The ratio is observed to increase experimentally through a more reduction in *κ<sup>L</sup>* rather than that in *μ*, but some fundamental issues in this mechanism are not understood well [10].

The electrical resistivity (*ρ*) and electrical conductivity (*σ*) are related to *n* through the carrier mobility *μ*:

$$\mathbf{1}/\rho = \sigma = n\epsilon\mu\tag{5}$$

reported to reduce *κL*. However, decreasing *κ<sup>L</sup>* with phonon scattering by adding defects results in decrease in *n* and *σ*. These are the major conflicts in the properties of bulk thermoelectric materials which have been addressed in the researches for

*Maximizing the efficiency (ZT) of a thermoelectric involves a compromise of thermal conductivity (κ; plotted on*

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

*) and Seebeck coefficient (α; 0–500 μVK*�*<sup>1</sup>*

*) with electrical*

In classical physics, the coefficients α, κ<sup>e</sup> and σ are interrelated in such a way that it is impossible to increase one without affecting the others. Therefore, a compromise has to be achieved to find the maximum ZT value. Three different strategies

a. An approach for increasing α while keeping the values of σ and κ<sup>e</sup> by looking for new materials with complex band structures, like heavy fermion compounds.

b. Controlling the disorder in materials (such as Skutterudites or Clathrates) to present a rattling effect which causes, (↑) σ and decreases (↓) κ<sup>L</sup> (see for

quantum confinement effects, while ↓κ<sup>L</sup> due to the scattering of phonons at the interfaces. The latest improvements in the ZT of different materials has

c. Developing nanostructured materials that could lead to (↑) α due to

In 1993, Hicks and Dresselhaus pioneered the concept of nanostructuring in design of thermoelectric materials (i.e. Bi2Te3). The addition of the dimensionality

more than a half century [10].

*conductivity (σ; 0–5000 Ω*�*<sup>1</sup> cm*�*<sup>1</sup>*

*the y-axis from 0 to a top value of 10 Wm*�*<sup>1</sup> K*�*<sup>1</sup>*

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

**Figure 3.**

**1.4 Nanostructuring thermoelectric materials**

*) [3].*

have appeared to improve the ZT [14]:

been achieved by this approach.

instance ref. [15]).

**45**

The electronic contribution to the thermal conductivity is proportional to the electrical conductivity (*σ*) of the materials according to Wiedemann–Franz Law [3], and the relationship is expressed as follows:

$$
\kappa \epsilon = L \sigma T = n \epsilon \mu LT \tag{6}
$$

where 'e' is electron charge, and *<sup>L</sup>* is Lorenz factor 2.48 � <sup>10</sup>�<sup>8</sup> <sup>J</sup> 2 /K<sup>2</sup> C<sup>2</sup> for free electrons and this can vary particularly with carrier concentration [3, 13].

**Figure 3** shows the compromise of *σ, κ* and *α* in thermoelectric materials that must be optimized to maximize the figure of merit ZT. Indeed, the lower carrier concentration will result in the lower *σ* and a decreasing ZT. Typically, the PF and ZT peaks occur at carrier concentrations of 1019–10<sup>21</sup> cm�<sup>3</sup> (depending on the material system), which falls in between common metals and heavily doped semiconductors [3]. High mobility carriers are most important for high value of electrical conductivity. Again from the Eq. (4), an increase of the carrier effective mass lead to increase the *α* but reduce the *μ* and hence the *σ* according to the Eq. (5). In case of the narrow semiconductor, the thermal excitation of carrier from valence band to conduction band creates holes and electrons. However, the concentration of the major carrier does not vary much. When two types of carriers are present, or bipolar effects takes place, and this is notorious to achieve effective thermoelectrics [4]. For example, the Seebeck coefficient for different carrier types is given by a weighted average of their electrical conductivity values (σ<sup>e</sup> and σp) [13].

$$\alpha \approx \frac{a\_{\epsilon}\sigma\_{\epsilon} + a\_{p}\sigma\_{p}}{\left(\sigma\_{\epsilon} + \sigma\_{p}\right)}\tag{7}$$

In short, any attempt to increase *σ*, will increase *κ<sup>e</sup>* which contributes to thermal conductivity (*κ*). In order to counter the increment of *κe*, various approaches are

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

**Figure 3.**

concentration (*n*) increases electrical conductivity (σ) but reduces the Seebeck coefficient (α) for most of the homogeneous materials. For this reason, in metals and degenerate semiconductors with energy-independent scattering approxima-

> 3*n* <sup>2</sup>*=*<sup>3</sup>

3/2*μ*. Moreover, there is no such thing as an optimal effective mass.

1*=ρ* ¼ *σ* ¼ *neμ* (5)

*κe* ¼ *LσT* ¼ *neμLT* (6)

(7)

2 /K<sup>2</sup>

C<sup>2</sup> for free

Where, the parameter *m\** is density of states effective mass, and an increase of *m\** can raise the Seebeck coefficient according to the Eq. (4). However, most high *m\** materials have generally low *μ* which limits the α by a weighted mobility with a factor

There are high mobility low effective mass semiconductors (SiGe, GaAs) as well as low mobility high effective mass polaron conductors (oxides, chalcogenides) [3]. Noticeably, the defects scatter not only the phonons but also the electrons. When a thermoelectric material is designed for reducing lattice thermal conductivity, its carrier mobility is usually suppressed. Hence, the ratio of *μ/κ<sup>L</sup>* determines the improvement of ZT [5, 10]. The ratio is observed to increase experimentally through a more reduction in *κ<sup>L</sup>* rather than that in *μ*, but some fundamental issues in

The electrical resistivity (*ρ*) and electrical conductivity (*σ*) are related to *n*

The electronic contribution to the thermal conductivity is proportional to the electrical conductivity (*σ*) of the materials according to Wiedemann–Franz Law [3],

**Figure 3** shows the compromise of *σ, κ* and *α* in thermoelectric materials that must be optimized to maximize the figure of merit ZT. Indeed, the lower carrier concentration will result in the lower *σ* and a decreasing ZT. Typically, the PF and ZT peaks occur at carrier concentrations of 1019–10<sup>21</sup> cm�<sup>3</sup> (depending on the material system), which falls in between common metals and heavily doped semiconductors [3]. High mobility carriers are most important for high value of electrical conductivity. Again from the Eq. (4), an increase of the carrier effective mass lead to increase the *α* but reduce the *μ* and hence the *σ* according to the Eq. (5). In case of the narrow semiconductor, the thermal excitation of carrier from valence band to conduction band creates holes and electrons. However, the concentration of the major carrier does not vary much. When two types of carriers are present, or bipolar effects takes place, and this is notorious to achieve effective thermoelectrics [4]. For example, the Seebeck coefficient for different carrier types is given by a

where 'e' is electron charge, and *<sup>L</sup>* is Lorenz factor 2.48 � <sup>10</sup>�<sup>8</sup> <sup>J</sup>

electrons and this can vary particularly with carrier concentration [3, 13].

weighted average of their electrical conductivity values (σ<sup>e</sup> and σp) [13].

*<sup>α</sup>*≈*αeσ<sup>e</sup>* <sup>þ</sup> *<sup>α</sup>pσ<sup>p</sup> σ<sup>e</sup>* þ *σ<sup>p</sup>*

In short, any attempt to increase *σ*, will increase *κ<sup>e</sup>* which contributes to thermal conductivity (*κ*). In order to counter the increment of *κe*, various approaches are

(4)

*<sup>α</sup>* <sup>¼</sup> <sup>8</sup>*π*2*k*<sup>2</sup> *B* <sup>3</sup>*eh*<sup>2</sup> *<sup>m</sup>*<sup>∗</sup> *<sup>T</sup> <sup>π</sup>*

tion, the Seebeck coefficient can be expressed as [3, 12]:

proportional to (*m\**

*Materials at the Nanoscale*

**44**

)

this mechanism are not understood well [10].

and the relationship is expressed as follows:

through the carrier mobility *μ*:

*Maximizing the efficiency (ZT) of a thermoelectric involves a compromise of thermal conductivity (κ; plotted on the y-axis from 0 to a top value of 10 Wm*�*<sup>1</sup> K*�*<sup>1</sup> ) and Seebeck coefficient (α; 0–500 μVK*�*<sup>1</sup> ) with electrical conductivity (σ; 0–5000 Ω*�*<sup>1</sup> cm*�*<sup>1</sup> ) [3].*

reported to reduce *κL*. However, decreasing *κ<sup>L</sup>* with phonon scattering by adding defects results in decrease in *n* and *σ*. These are the major conflicts in the properties of bulk thermoelectric materials which have been addressed in the researches for more than a half century [10].

#### **1.4 Nanostructuring thermoelectric materials**

In classical physics, the coefficients α, κ<sup>e</sup> and σ are interrelated in such a way that it is impossible to increase one without affecting the others. Therefore, a compromise has to be achieved to find the maximum ZT value. Three different strategies have appeared to improve the ZT [14]:


In 1993, Hicks and Dresselhaus pioneered the concept of nanostructuring in design of thermoelectric materials (i.e. Bi2Te3). The addition of the dimensionality and size of the system is added as a new parameter that affects the coupling of the electrical conductivity, Seebeck coefficient, and thermal conductivity, leading to substantially enhanced ZT [16–18]. Two ideas are dominant for the lowdimensional materials approach for improving ZT. Firstly, the presence of nanoscale constituents would introduce quantum confinement effects to enhance Seebeck coefficient and the power factor α<sup>2</sup> σ. Secondly, the numerous internal nanoinclusions and interfaces found in nanostructures would be designed so that the thermal conductivity would be reduced more than the electrical conductivity, based on differences in their respective scattering lengths [16].

examples of different nanostructuring with different dimensionalities [14]. A schematic diagram is shown in **Figure 4(e)** capturing these various phonon scattering mechanisms, along with the electrical transport within a thermoelectric material. For example, in material embedded nano-inclusions (nanoparticles), atomic defects

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

nanoparticles are required to scatter mid- and long-wavelength phonons effectively.

**Figure 5** plots major milestones achieved for ZT over the past several decades as a function of both year and temperature [20]. In the 1950s, Bi2Te3 was first investigated as a material of great thermoelectric with ZT0.6 near room temperature [5, 6]. It was quickly realized that alloying with Sb2Te3 and Bi2Se3 allowed for the fine tuning of the carrier concentration alongside a reduction in lattice thermal conductivity. These compounds have played a dominant role in the field of thermoelectrics through today. The alloys of Bi2Te3 with Sb2Te3 (such as Bi0.5Sb1.5Te3; p type) and of Bi2Te3 with Bi2Se3 (such as Bi2Te2.7Se0.3; n type), with a ZT 1 at room temperature are traditional cooling materials [6]. In recent year, great enhancements in ZT owning to low dimension and nanostructure materials have been reported [19–32] and achieved the highest ZT value of approximately 2.4.

The solid-state devices based on TE effect have the inherent advantages of reliability, silent and vibration-free operation (no moving fluids or moving parts), a very high power density, and the ability to maintain their efficiency in small scale

Commercial use has been made mostly from Peltier's thermoelectric cooling

*Thermoelectric figure-of-merit ZT as a function of temperature and year illustrating important milestones [20]. Although there have been several demonstrations of ZT > 1 in the past decade (2001–2010), no material has yet achieved the target goal of ZT* ≥ *3. The material systems that have achieved ZT > 1 have all been based on*

applications where only a moderate amount of power is needed [19].

(TEC) effect in applications, as demonstrated in **Figure 6** [35]:

are effective at scattering short wavelength phonons, but larger embedded

Grain boundaries can also play an effective role in scattering these longer-

wavelength phonons [20].

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

**Figure 5.**

**47**

*some form of nanostructuring.*

**1.5 Overview of thermoelectric applications**

As the dimensionality is decreased from 3D crystalline solids to 2D (quantum wells) to 1D (quantum wires) and finally to 0D (quantum dots), the spatial confinement are introduced that create the possibilities to tune the TE properties α, σ, and κ independently. When the system size decreases and approaches the scale comparable to the feature length of electron behavior (e.q. mean free path and wavelength) in any direction, the electronic density of states (D.O.S.) can split and become narrow as well as increase substantially (**Figure 4a**), resulting in the enhancement of α. Meanwhile, the thermal conductivity is also reduced because of the extensive phonon scattering at the surface, interfaces, and grain boundaries, as any dimension is less than the mean free path of phonons. **Figure 4(b)** illustrates

#### **Figure 4.**

*(a) Electronic density of states (D.O.S.) for a bulk 3D crystalline semiconductor, a 2D quantum well, a 1D nanowire or nanotube, and a 0D quantum dot [16]. (b) Examples of different nanostructuring with different dimensionalities [14]. (c) A spike in the density of states (solid line) above the bulk value (dashed line) occurs due to resonant states in Tl-doped PbTe [19]. (d) The measured ZT of Tl-PbTe and Na-PbTe samples for 300– 800 K indicates an improvement due to the addition of Tl [19]. (e) Schematic diagram illustrating various phonon scattering mechanisms within a thermoelectric material, along with electronic transport of hot and cold electrons [20].*

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

examples of different nanostructuring with different dimensionalities [14]. A schematic diagram is shown in **Figure 4(e)** capturing these various phonon scattering mechanisms, along with the electrical transport within a thermoelectric material. For example, in material embedded nano-inclusions (nanoparticles), atomic defects are effective at scattering short wavelength phonons, but larger embedded nanoparticles are required to scatter mid- and long-wavelength phonons effectively. Grain boundaries can also play an effective role in scattering these longerwavelength phonons [20].

**Figure 5** plots major milestones achieved for ZT over the past several decades as a function of both year and temperature [20]. In the 1950s, Bi2Te3 was first investigated as a material of great thermoelectric with ZT0.6 near room temperature [5, 6]. It was quickly realized that alloying with Sb2Te3 and Bi2Se3 allowed for the fine tuning of the carrier concentration alongside a reduction in lattice thermal conductivity. These compounds have played a dominant role in the field of thermoelectrics through today. The alloys of Bi2Te3 with Sb2Te3 (such as Bi0.5Sb1.5Te3; p type) and of Bi2Te3 with Bi2Se3 (such as Bi2Te2.7Se0.3; n type), with a ZT 1 at room temperature are traditional cooling materials [6]. In recent year, great enhancements in ZT owning to low dimension and nanostructure materials have been reported [19–32] and achieved the highest ZT value of approximately 2.4.

#### **1.5 Overview of thermoelectric applications**

The solid-state devices based on TE effect have the inherent advantages of reliability, silent and vibration-free operation (no moving fluids or moving parts), a very high power density, and the ability to maintain their efficiency in small scale applications where only a moderate amount of power is needed [19].

Commercial use has been made mostly from Peltier's thermoelectric cooling (TEC) effect in applications, as demonstrated in **Figure 6** [35]:

#### **Figure 5.**

and size of the system is added as a new parameter that affects the coupling of the electrical conductivity, Seebeck coefficient, and thermal conductivity, leading to

dimensional materials approach for improving ZT. Firstly, the presence of nanoscale constituents would introduce quantum confinement effects to enhance

nanoinclusions and interfaces found in nanostructures would be designed so that the thermal conductivity would be reduced more than the electrical conductivity,

As the dimensionality is decreased from 3D crystalline solids to 2D (quantum wells) to 1D (quantum wires) and finally to 0D (quantum dots), the spatial confinement are introduced that create the possibilities to tune the TE properties α, σ, and κ independently. When the system size decreases and approaches the scale comparable to the feature length of electron behavior (e.q. mean free path and wavelength) in any direction, the electronic density of states (D.O.S.) can split and become narrow as well as increase substantially (**Figure 4a**), resulting in the enhancement of α. Meanwhile, the thermal conductivity is also reduced because of the extensive phonon scattering at the surface, interfaces, and grain boundaries, as any dimension is less than the mean free path of phonons. **Figure 4(b)** illustrates

*(a) Electronic density of states (D.O.S.) for a bulk 3D crystalline semiconductor, a 2D quantum well, a 1D nanowire or nanotube, and a 0D quantum dot [16]. (b) Examples of different nanostructuring with different dimensionalities [14]. (c) A spike in the density of states (solid line) above the bulk value (dashed line) occurs due to resonant states in Tl-doped PbTe [19]. (d) The measured ZT of Tl-PbTe and Na-PbTe samples for 300– 800 K indicates an improvement due to the addition of Tl [19]. (e) Schematic diagram illustrating various phonon scattering mechanisms within a thermoelectric material, along with electronic transport of hot and cold*

σ. Secondly, the numerous internal

substantially enhanced ZT [16–18]. Two ideas are dominant for the low-

based on differences in their respective scattering lengths [16].

Seebeck coefficient and the power factor α<sup>2</sup>

*Materials at the Nanoscale*

**Figure 4.**

*electrons [20].*

**46**

*Thermoelectric figure-of-merit ZT as a function of temperature and year illustrating important milestones [20]. Although there have been several demonstrations of ZT > 1 in the past decade (2001–2010), no material has yet achieved the target goal of ZT* ≥ *3. The material systems that have achieved ZT > 1 have all been based on some form of nanostructuring.*

**Figure 6.**

*Overview of potential thermoelectric cooling (TEC) applications [33, 34].*

• Small refrigerator devices are used for camping and outdoor activities. For example, the cooler/warmer TE device (Engel Thermo 8) has volume 8 L and weighing just over 3 kg. Its features include cooling performance up to 22°C below ambient temperature and warming up to +65°C.

coolers can improve electronic systems in thermal performance, cost, noise,

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

**Figure 7** shows an overview of the present and potential applications of thermoelectric generators (TEGs) [34]. They include (1) heating systems and water boilers with TEG units which generate the electricity for the control units and pumping systems, (2) the long term perspective of waste heat recovery for

medium-scale industrial facilities, (3) waste heat recovery in automobiles and other combustion-engine-powered vehicles for enhanced efficiency and electric current supply of the electronic system, (4) miniaturized autarkic sensor systems powered by an integrated TEG with a wireless data transmitter, (5) ventilated wood stove powered by a thermoelectric generator with enhanced oxygen supply, improves

**2. Nanostructured Bi2Te3-based thermoelectric thin films grown using**

PLD is one of the most convenient thin film growth techniques that uses a high intensity pulsed laser beam as an external energy source to ablate a target, form a plume, and deposit thin films onto a substrate. In practice, a large number of variables affect the properties of the film, such as substrate temperature (*Ts*), background gas pressure (*P*) and laser fluence. **Figure 8** shows a PLD system for preparing thermoelectric thin films [38, 39]. The substrate was heated and

maintained at desired *Ts* using a thermocouple and a proportional-integral-derivative temperature controller. The thermocouple was buried inside a substrate holder which was heated by a tungsten lamp or electrical resistance heating. The pressure of ambient gas (He, Ar) could be fine-tuned by the needle valve. Laser source can

**2.1 PLD growths of nanostructured Bi2Te3-based thin films**

weight, size or efficiency.

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

*Overview of potential thermoelectric generator (TEG) applications [33, 34].*

**pulsed laser deposition**

burning process.

**49**

**Figure 7.**


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

**Figure 7.** *Overview of potential thermoelectric generator (TEG) applications [33, 34].*

coolers can improve electronic systems in thermal performance, cost, noise, weight, size or efficiency.

**Figure 7** shows an overview of the present and potential applications of thermoelectric generators (TEGs) [34]. They include (1) heating systems and water boilers with TEG units which generate the electricity for the control units and pumping systems, (2) the long term perspective of waste heat recovery for medium-scale industrial facilities, (3) waste heat recovery in automobiles and other combustion-engine-powered vehicles for enhanced efficiency and electric current supply of the electronic system, (4) miniaturized autarkic sensor systems powered by an integrated TEG with a wireless data transmitter, (5) ventilated wood stove powered by a thermoelectric generator with enhanced oxygen supply, improves burning process.
