**1. Introduction**

Two-dimensional (2D) materials have intrigued physicists and material scientists for many decades due to an abundance of unusual physical phenomena that result from the confine‐ ment of charge, heat, and entropy flow to a plane [1]. For example, the ingenious harness of quantum mechanical phenomena, particular to lower dimensionality in graphene, has resulted

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in intriguing observations such as the quantum Hall effect at room temperatures, quantized optical transmittance, nonlocal hot carrier transport, and Klein tunneling [2, 3]. Despite such fundamental breakthroughs, the potential of 2D materials has not yet completely manifested into practical devices due to material limitations [1, 4]. For instance, the lack of a band gap resulted in serious limitations for using graphene in electronics [4]. Defects in material science and engineering are often perceived as performance limiters, but in the case of 2D materials, defect engineering could provide a way to overcome many roadblocks and forge new fron‐ tiers. In this regard, others and we have shown that defects in 2D materials (e.g., dopants, vacancies) can provide an excellent handle to control material properties [5–8]. Specifically, we have shown that defects such as vacancies and N dopants in graphene could be used to control the electron-electron and electron-phonon scattering pathways [8]. These results provided criticalbreakthroughsforimprovingthequantumcapacitanceofgrapheneanddopinggraphene without compromising its intrinsic characteristics [6]. Defects also play a vital role in improv‐ ing the properties of the so-called "beyond graphene" 2D materials. Previously, we used spark plasma sintering (SPS) to introduce charged grain boundaries (GB) in 2D Bi2Te3 for improving its thermoelectric (TE) figure of merit and compatibility factor [5]. Similarly, it has been demonstrated that defect engineering in 2D materials could improve many qualities ranging from electronic levels, conductivity, magnetism, and optics to structural mobility of disloca‐ tionsandcatalyticactivities[9,10].Asdiscussedinthischapter,defectengineeringin2Dmaterials leads to the discovery of potentially exotic properties, which can enable unprecedented technological applications. In particular, we present how dopants and defects in (i) graphene could be used for optical and electrochemical energy storage applications and (ii) 2D Bi2Te3 could be controlled for enhancing its thermoelectric efficiency.
