**Author details**

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

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

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

As exemplified by graphene and Bi2Te3, the presence of defects and dopants imparts the host material with new micro/quantum states or energy configurations that can strongly influence optical, electronic, and thermal properties. In addition to the above properties, S and F dopants are being explored to make graphene magnetic [10, 52–55], while N dopants are expected to provide much higher enhancements in quantum capacitance without compromising electrical conductivity [28]. Similar to Bi2Te3, defects in other layered systems such as SnSe and TaSe2 could be engineered to achieve better thermoelectric performance [56–62]. Although this chapter presented only some examples of defects in 2D materials, the same concepts also hold true for other 2D materials such as MoS2, WS2, and BN. Indeed, some properties (e.g., lumi‐ nescence and catalytic activity) of these materials can be tuned using defects [63–68]. As the design and development of new 2D materials are costly, complex, and limited due to the relatively poor air stability of many materials (e.g., silicene and phosphorene), the realization of desired properties and functionalities through control of defects (e.g., vacancies, dopants) in 2D materials is necessary. Though there is only one way for a given material to be defectfree, there are many possibilities for materials to be imperfect. The global scientific endeavors on understanding defects, such as the efforts presented in this chapter, provide a glimpse of the enormous potential of defects warranting further interdisciplinary research efforts.

temperature.

factor.

**5. Conclusions**

Sai Sunil Kumar Mallineni1 , Sriparna Bhattacharya1 , Fengjiao Liu1 , Pooja Puneet1 , Apparao Rao1,2, Anurag Srivastava3 and Ramakrishna Podila1,2,3,4\*

\*Address all correspondence to: rpodila@g.clemson.edu

1 Department of Physics and Astronomy, Clemson Nanomaterials Center, Clemson University, Clemson, SC, USA

2 Center for Optical Materials Science and Engineering Technologies, Clemson University, Clemson, SC, USA

3 ABV-Indian Institute of Information Technology and Management, Gwalior, MP, India

4 Laboratory of Nano-bio Physics, Clemson University, Clemson, SC, USA
