**1. Introduction**

The (re)discovery [1, 2] of graphene—a single layer of carbon atoms arranged in a honeycomb lattice—in 2004 by Novoselov et al. has triggered a new aspect of research in two-dimensional (2D) materials [3, 4]. Although the existence of materials with their properties governed by their 2D units was well known for quite some time [5, 6], it is the experimental realization of a single layer graphene has showed that it is possible to exfoliate stable 2D materials from the 3D solids exhibiting various fascinating properties.

A huge number of crystalline solid-state materials having different mechanical, electronic, and transport properties exist from which stable 2D materials can be created due to the presence of weak interaction between the layers [7]. 2D allotropes (e.g., silicene, graphyne, germa‐ nene), compounds (e.g., graphane, hexagonal boron nitride, transition metal di-chalcoge‐ nides) are the few examples of 2D materials. These 2D materials have the potential for a wide range of applications due to the interesting electronic and structural properties [2, 8–12].

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To exploit these various properties, the samples have to be made in a scalable way. Chemi‐ cal vapor deposition (CVD) has become a very common method for large-scale fabrication. Nonetheless, the CVD samples inevitably contain defects, for example, edges, hetero struc‐ tures, grain boundaries, vacancies, and interstitial impurities [13–15]. These defects can be seen very easily in transmission electron microscopy (TEM) experiments [16] or scanning tunnel‐ ling microscopy (STM) experiments [17]. **Figure 1a**, **b** shows experimental STM and TEM images of an isolated single vacancy in graphene. In the STM image, the single vacancy can be seen as a blob because of increased local density of states. These states appear due to the presence of dangling bonds around the single vacancy.

**Figure 1.** (a) Experimental STM image of single isolated vacancy in graphene. Reprinted with permission from Ugeda et al. [17], copyright (2010) by the American Physical Society. (b) Experimental TEM image of reconstructed single vacancy with atomic configurations. Reprinted (adapted) with permission from Meyer et al. [16], copyright (2008) by the American Chemical Society.

In general, these defects manipulate the properties of the materials and hence their avoid‐ ance or deliberate engineering requires a thorough understanding. In one hand, defects can be detrimental to device properties [13], but on the other hand, especially at the nanoscale, defects can bring new functionalities which could be utilized for applications [18, 19].

In this book chapter, we address a few cases of defects in 2D materials such as graphene and its derivatives. We show how one can tune the various properties of the pristine materials with the control insertion of defects in these systems and use them in various applications.
