**4.2. Impact of charged grain boundaries in few-layered Bi2Te3**

defect concentration results in poor electrical conductivity leading to a drop in Cmeas suggesting

Thus, as evidenced by our data in graphene, the presence of defects does not necessarily deteriorate the material performance. Though there is only one way for a given material to be defect-free, there are many possibilities for it to be imperfect. While defect configuration is important in determining the mobility through carrier scattering rate, controlling defect concentration is critical for electrochemical applications. Accordingly, future efforts must be focused on finding new approaches to identify and control the right defect configurations (e.g., N in graphitic configuration to increase carrier concentration without compromising carrier scattering rates or mobility) and concentrations, which could improve material properties

**4. Defects in 2D bulk materials for thermoelectric power generation**

Thermoelectric (TE) materials have the potential to reduce global energy crisis and global warming effects by converting waste heat to electricity. As of 2005, the world energy usage was ~15 terawatts of energy, of which ~90 % was first converted to heat and the remainder ~10 % of energy was utilized [24]. In general, power plants and the transportation industry are the two main sources of heat energy losses that contribute to global warming. In recent years, prototype car models developed by automobile industries BMW and Ford have successfully transformed the waste heat from car exhausts to electricity using thermoelectric power

A basic thermoelectric energy conversion module consists of *n* and *p*-type semiconducting materials, connected electrically in series and thermally in parallel [35]. The maximum thermoelectric efficiency is a product of the Carnot Efficiency and a term consisting of *ZT* or the thermoelectric figure of merit, which embodies interrelated material parameters, as given

and *k*(= *kE* + *kL*) is the total thermal conductivity comprised of electronic (*kE*) and lattice contributions (*kL*), respectively. The main challenge of improving the energy conversion efficiency and consequently the *ZT* of thermoelectric materials is the inherent *coupling* between the electrical conductivity and the Seebeck coefficient. In recent years, there has been signifi‐ cant scientific progress in the field of thermoelectrics with the use of nanostructured materials (e.g., superlattices, nanowires, and nanocomposites) that have simultaneously increased the power factor (the numerator of *ZT*) and reduced the thermal conductivity to achieve a high

In the early 1990s, Hicks et al. [37] predicted intriguing changes in transport properties upon lowering the dimensionality of existing bulk materials (e.g., from 3D to 2D) that were not observed in the corresponding bulk materials. A dramatic increase in the density of states (DOS) of low-dimensional materials was predicted that could increase the Seebeck coefficient

*σT*/*k*, where *α* is Seebeck coefficient or thermopower, *σ* is the electrical conductivity,

the importance of defect concentration in determining 2D material properties.

92 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

instead of dismissing all defects as detrimental for carrier mobility.

generators, thus improving the fuel efficiency [24, 34].

**4.1. Quantum confinement effects in 2D thermoelectric materials**

by *ZT* = *α<sup>2</sup>*

*ZT* > 1 [36].

Discovered in the early 1950s by Goldsmid [42], Bi2Te3 is one of the most used and commer‐ cialized TE materials for room-temperature power generation and refrigeration applications [43]. The first TE refrigerator was designed using the *p*-type Bi2Te3, which was estimated to have a figure of merit ~ 0.48. In 2001, the thin-film superlattices were reported to exhibit enhanced *ZT* ~ 2.4 and ~1.4 at 300 K in the *p*-type Bi2Te3/Sb2Te3 and the *n*-type Bi2Te3/ Bi2Te2.83Se0.17, respectively [44]. While a high *ZT* ~ 1.5 was realized in the bulk *p*-type Bi2Te3 via nanostructuring techniques such as ball-milling [45], melt-spinning [46, 47], and hydrothermal synthesis [48], it was found that these methods were ineffective for the *n*-type Bi2Te3. Like graphite, *n*-type Bi2Te3 is easily cleavable, and these traditional nanostructuring methods were too harsh and resulted in the deterioration of its basal plane (or *in-plane*) properties (**Figure 7a**).

Recently, Puneet et al. [5] utilized a novel technique of chemical exfoliation followed by spark plasma sintering (CE-SPS) in the *n*-type Bi2Te2.7Se0.3 that significantly improved the TE compatibility factor and stabilized the *ZT* peak at higher temperatures (300–450 K) as com‐ pared to the commercial ingot. Based on the studies of emerging two-dimensional (2D) materials (e.g., graphene), it may be expected that the electronic, thermal, and optical proper‐ ties of chemically exfoliated *n*-type Bi2Te3 should exhibit properties that are different from the bulk. In the following paragraphs, we discuss the effect of the chemical exfoliation and spark plasma sintering of *n*-type Bi2Te3 as compared to the bulk commercial ingot.

The bulk bismuth telluride (Bi2Te3) exhibits a rhombohedral crystal structure belonging to the space group R 3 ¯ m(D5), which is more commonly represented by a hexagonal crystal structure as shown in **Figure 7a**. The hexagonal unit cell of Bi2Te3 is composed of three quintuples with lattice constants *a* = 0.4384 Å and *c* = 3.036 Å, respectively. As shown in the figure, each quintuple consists of five atoms stacked in the order Te1 -Bi-Te2 -Bi-Te1 along the *c*-axis, which are bonded by ionic-covalent bonds. The Te1 -Te1 layers between the two quintuples are held together by the weak van der Waals forces that make the Bi2Te3 easily cleavable. The *n*-type Bi2Te3 was obtained by partially doping Se at the Te1 or Te2 sites, which represent the Te atoms with two types of bonding.

**Figure 7.** (a) A schematic representation showing Bi2Te3 rhombohedral crystal structure belonging to the space group R 3 ¯ m(D5). Upon chemical exfoliation, the crystal breaks at the van der Waals gap. (b) A scanning electron micrograph showing the layered structure of bulk Bi2Te3 ingot. (c) A transmission electron micrograph showing chemically exfoliat‐ ed quasi-2D Bi2Te3, which is repacked using spark plasma sintering (d) to achieve better thermoelectric performance through charged grain boundaries.

The nanostructuring of the commercial *n*-type Bi2Te3 ingot (see **Figure 7b**, purchased from the Marlow Industries, USA) was achieved by the chemical exfoliation method, using N-methyl-2 pyrrolidone (NMP) solution which was ultrasonicated using a tip sonicator (Branson 250) for 3, 5, 8, or 13 h, respectively. After centrifuging the solution and vacuum filtering the superna‐ tant through a 0.45 mm nylon filter paper, the filtered powder was washed several times using deionized water to remove any residual NMP. The exfoliated nanolayers (see **Figure 7c**) were ~50 nm in thickness, which were then compacted using the spark plasma sintering (SPS, Dr. Sinter LabH-515S) technique at a holding temperature of 500 °C and an applied pressure of 30 MPa for 5 min under a dynamic vacuum. For the SPS process, the samples were loaded into graphite dies, and the pressure was applied using graphite rods. The resulting SPS-processed pellets were 12.5 mm in diameter and 2–3 mm in thickness, yielding samples with 98–99 % of the theoretical density (see **Figure 7c**).

bulk. In the following paragraphs, we discuss the effect of the chemical exfoliation and spark

The bulk bismuth telluride (Bi2Te3) exhibits a rhombohedral crystal structure belonging to the

as shown in **Figure 7a**. The hexagonal unit cell of Bi2Te3 is composed of three quintuples with lattice constants *a* = 0.4384 Å and *c* = 3.036 Å, respectively. As shown in the figure, each

together by the weak van der Waals forces that make the Bi2Te3 easily cleavable. The *n*-type

**Figure 7.** (a) A schematic representation showing Bi2Te3 rhombohedral crystal structure belonging to the space group R

¯ m(D5). Upon chemical exfoliation, the crystal breaks at the van der Waals gap. (b) A scanning electron micrograph showing the layered structure of bulk Bi2Te3 ingot. (c) A transmission electron micrograph showing chemically exfoliat‐ ed quasi-2D Bi2Te3, which is repacked using spark plasma sintering (d) to achieve better thermoelectric performance

The nanostructuring of the commercial *n*-type Bi2Te3 ingot (see **Figure 7b**, purchased from the Marlow Industries, USA) was achieved by the chemical exfoliation method, using N-methyl-2 pyrrolidone (NMP) solution which was ultrasonicated using a tip sonicator (Branson 250) for 3, 5, 8, or 13 h, respectively. After centrifuging the solution and vacuum filtering the superna‐ tant through a 0.45 mm nylon filter paper, the filtered powder was washed several times using

¯ m(D5), which is more commonly represented by a hexagonal crystal structure

or Te2




along the *c*-axis, which

sites, which represent the Te atoms

plasma sintering of *n*-type Bi2Te3 as compared to the bulk commercial ingot.

quintuple consists of five atoms stacked in the order Te1

94 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Bi2Te3 was obtained by partially doping Se at the Te1

are bonded by ionic-covalent bonds. The Te1

space group R 3

3

through charged grain boundaries.

with two types of bonding.

The rapid densification technique by SPS has distinct advantages over other types of sintering techniques, such as the hot pressing. The SPS process is capable of sintering material powders within a very short time, in the order of minutes [49, 50]. As a result, it is possible to retain the metastable micro-/nanostructures of materials by limiting their grain growth and excessive diffusion during the sintering process. Furthermore, unlike other sintering techniques like hot pressing which uses furnace heating, only the graphite cylinder, rods, and sample are heated by the joule heating produced by pulsed dc in the SPS, which leads to even shorter processing times.

**Figure 8.** (a) Charged defects near the grain boundaries (GB) can induce excess majority carriers (b) and preferentially scatter holes over electrons and low kinetic energy carriers (red arrow in panels a and c) due to engineered charged GB potential barriers shown in (c).

Contrary to the general understanding that defects in a crystal lattice are detrimental to the transport properties of materials, the defects in 2D materials are extremely useful and could be manipulated to generate controlled defects for novel and innovative applications. As observed in the few-layered bulk Bi2Te3, the localized positive charges in the grain boundaries introduced extra electrons in the material, thereby increasing the carrier concentration *n* (see **Figure 3a** in Ref. [5]). This increase in *n* was attributed to the injection of donor-like defects (**Figure 8a** and **b**), arising from positively charged anti-sites/Te vacancies on the exfoliated grain boundaries, due to chemical/mechanical exfoliation [51]. Moreover, these positively charged or interfacial charged *defects* acted as a potential barrier (see **Figure 8c**) to selectively filter out low-energy holes (or minority carriers) as shown schematically by Puneet et al. (see **Figure 3** in Ref. [5]). These charged *defects* in the few-layered *n*-type Bi2Te3 thus shifted the onset of the bipolar (or two carrier) effects and consequently the maximum *ZT* value to higher temperatures, thus optimizing the *ZT* over a broader range of temperature. In addition, the thermoelectric compatibility factor of the few-layered *n*-type Bi2Te3 was significantly im‐ proved, which determined the ability of these materials to be segmented to other thermoelec‐ tric materials such as PbTe at higher temperatures, for operation over a broader range of temperature.

In summary, the CE-SPS processing of 2D Bi2Te3 leads to preferential scattering of electrons at charged grain boundaries and optimizes the band filling, thereby increasing the electrical conductivity despite the presence of numerous grain boundaries, and mitigates the bipolar effect via band occupancy optimization leading to an upshift in *ZT* peak (by ~100 K) and stabilization of the *ZT* peak over a broad temperature range of ~150 K. These changes in electrical and thermal transport led in turn to a more device-design friendly compatibility factor.
