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.
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.
- self-assembly nanostructures
- thermoelectric power factor
- pulsed laser deposition
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 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 = Π×I.
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 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)
and the coefficient of performance presents for the efficiency of air-conditioning and refrigeration :
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 . 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
The kinetic definition of
Where, the parameter
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
The electrical resistivity (
The electronic contribution to the thermal conductivity is proportional to the electrical conductivity (
Figure 3 shows the compromise of
In short, any attempt to increase
1.4 Nanostructuring thermoelectric materials
In classical physics, the coefficients α, κe 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 :
An approach for increasing α while keeping the values of σ and κe by looking for new materials with complex band structures, like heavy fermion compounds.
Controlling the disorder in materials (such as Skutterudites or Clathrates) to present a rattling effect which causes, (↑) σ and decreases (↓) κL (see for instance ref. ).
Developing nanostructured materials that could lead to (↑) α due to quantum confinement effects, while ↓κL due to the scattering of phonons at the interfaces. The latest improvements in the ZT of different materials has been achieved by this approach.
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, 17, 18]. Two ideas are dominant for the low-dimensional materials approach for improving ZT. Firstly, the presence of nanoscale constituents would introduce quantum confinement effects to enhance Seebeck coefficient and the power factor α2σ. 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 .
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 examples of different nanostructuring with different dimensionalities . 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 longer-wavelength phonons .
Figure 5 plots major milestones achieved for ZT over the past several decades as a function of both year and temperature . In the 1950s, Bi2Te3 was first investigated as a material of great thermoelectric with ZT∼0.6 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 . In recent year, great enhancements in ZT owning to low dimension and nanostructure materials have been reported [19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 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 .
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.
Gentherm designed and developed Automotive Climate Control Seat , which has TE heat pumps in the back and bottom cushions. The TE system makes conditioned flowing air through channels to the occupant for providing on-demand cooling or heating. As shown in the first panel in Figure 6, the seat has the heat pump consisting of a TE module (green box) and a fan (orange).
Thermal management of tiny laser diodes is used in fiber optic telecom, datacom backhaul networks. TEC can also be used for contact cooling of semiconductor lasers, infrared detectors, CCD- matrix, and miniconditioners for photomultipliers.
Localized cooling at hot spots of chips was created. For example, the Intel group is the first to demonstrate both concepts of applying the TE material only to a chip’s hottest spots (Figure 6) [33, 37]. On the substrate, the researchers grew a 100-μm-thick layered structure, called a superlattice, containing bismuth, tellurium, antimony, and selenium. The structure can pump 1300W/cm2 heat from the back side of the chip to the heat spreader. The superlattice induced an approximately 6°C temperature drop at the hot spot even before the device was powered up, because it conducts heat better than the grease that bonds the rest of the heat spreader to the chip. Yet, when a 3 A- current went through the thermoelectric cooler, the total temperature change was only of 15°C. Managing heat in electronics is a common issue, and TE 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) . 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.
2. Nanostructured Bi2Te3-based thermoelectric thin films grown using pulsed laser deposition
2.1 PLD growths of nanostructured Bi2Te3-based thin films
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 (
The enhancement of the PF of Bi2Te3-based thin films is challenging due to the coupling among TE material properties , and the difficulty in growing stoichiometric films . Indeed, stoichiometry is a key factor for obtaining better TE properties [5, 38, 43, 44, 45]. Yet, both tendency for re-evaporation of volatile elements (i.e., Te, Se) at elevated
2.2 Super-assembling of Bi2Te3 hierarchical nanostructured thin films
C.-H. Chen et al.  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 10−3 Torr. SEM images in Figure 10 clearly shows the morphological characteristics of the superassembling Bi2Te3 nanostructured thin films . At lower deposition temperatures (< 450°C), the films are mainly composed of vertically aligned nanoscaled flakes, but flakes are 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 . 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 out-of-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 .
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 , 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
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 .
Figure 12 presents the proposed growth model of the super-assemling nanostructured Bi2Te3 films prepared at various
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−1 for worm-like superassembly (450°C) to 160 S.cm−1 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/cmK2, primarily due to the low electrical conductivity of the films with voided morphologies.
|Material||Morphology||Method||Deposition conditions||μ (cm2/Vs)||σ (S/cm)||α (μV/K)||PF (μW/cmK2)||Ref.|
|Bi2Te3||Spindle-like super-assembly||PLD||350°C||0.13 Pa||4.0||12.4||79||−113||1.01|||
|Bi2Te3||Worm-like super-assembly||PLD||400°C||0.13 Pa||1.9||25.9||73||−119||1.03|||
|Bi2Te3||worm-like super-assembly||PLD||450°C||0.13 Pa||1.2||29.4||49||−138||0.93|||
|Bi2Te3||3D-layered super-assembly||PLD||600°C||0.13 Pa||5.1||20.3||160||−137||3.0|||
|Bi2Te3||Layered Structure||Sputtering||350°C||1.0 Pa||95||12.1||1840||−70||8.8|||
|Bi2Te3||Columnar Structure||Sputtering||350°C||1.0 Pa||246||7.5||2990||−46||6.4|||
|Bi2Se3||Layered HPs||PLD||300°C||40 Pa||7.4||81.4||963.8||−75.8||5.5|||
|Bi2Te3||Compact film||PLD||300°C||80 Pa||5||102||814.3||−172.8||24.3|||
|Bi2Te3||Layered-smooth film||PLD||250°C||10 Pa||10.1||90.6||1464||−186||50.6|||
|Bi3Se2Te||Nanocrystalline film||PLD||250°C||40 Pa||35.5||34.4||1747.5||−68.8||8.3|||
|Bi2Se3||Bulk||Melting and hot-pressing||─||─||251.9||−175||7.7|||
|Bi2Se0.3Te2.7||Bulk||Ball milling-hot pressing||─||─||892||−190||32.2|||
|Bi2Se1.8Te1.2||Nano-platelet bulk||Polyol method||─||─||199.6||−80.9||1.3|||
|Bi2Se2Te||Bulk||Ball milling- hot pressing||─||─||1613||−60||5.8|||
In PLD, tightly controlling substrate temperatures (
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
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, 63, 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 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].
|Sample, fabrication method||Avg. grain size||κ (W/m K)||σ (S/cm)||α (μV/K)||PF = σα2 (μW/cmK2)||ZT (300 K)||Ref.|
|Bi2Te2.7Se0.3 nanocrystalline thin film, flash evaporation||60 nm||0.8 (cross-plane)||540||−186.1 (in-plane)||18.7 (in-plane)||0.7|||
|Sintered bulk Bi2Te3-xSex material, hot-pressing||30 μm||1.6||930||−177.5||29.3||0.6|
|Nanocrystalline bismuth-telluride-based (Bi2Te3-xSex) thin film||10 nm||0.61||550||−84.0||3.9||0.19|||
|Nanocrystalline Bi-Sb-Te thin film, sputtering||26 nm||0.46||3.3||—||—||—|||
|Nanocrystalline BiSbTe (8:30:62) thin film, flash evaporation||150 nm||0.6||—||—||—||—|||
|Single crystal BiSbTe bulk alloys||—||0.75||—||—||—||—|||
|Bi2Te3/Sb2Te3 superlattices (period∼5 nm)||—||0.4||—||—||—||—|||
|Bi2Te3/Bi2(Te0.88Se0.12)3 superlattice film, MBE||80 nm||1.25||639||−204||27||0.60|||
|Bi2Te3 film, PLD||—||1.1||—||—||—||—|||
|Bi2Te3/Sb2Te3 superlattices film (layered thickness ∼ 6 nm), PLD.||—||0.11||—||—||—||—|
|Bi2Te3 films, laser ablation||—||0.2–0.3||—||—||—||—|||
|BixSb2-xTe3 nanolayer film, PLD||190 nm||1.16||2700||95||25||0.65|||
|BixSb2-xTe3 nanodisc film, PLD||100 nm||1.00||1100||132||20||0.60|
|BixSb2-xTe3 nanocolumn film, PLD||70 nm||0.93||280||207||12||0.39|
For PLD Bi2Te3-based films, Yamasaki et al.  measured thermal conductivity with an ac calorimetric method in the direction across the film, obtaining k∼1.1 W/m K for a Bi2Te3 film deposited by PLD in vacuum (Table 2). In addition, Walachova et al.  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.  reported the κ values of 0.93–1.16 W/mK for BixSb2-xTe3 films with the granular-layered morphologies (Figure 14B).
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
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.374, and the Ministry of Science and Technology, Taiwan under Contract Nos. MOST 109-2221-E-214-016 and MOST 110-2221-E-214-013.