1558-1748 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2018.2884971, IEEE Sensors Journal > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—Defects are often symbolised as deformity in material that deteriorate its performance. However in nanoscale regime, defects lead to generate a useful and novel material for device applications. In the present report, the vacancy defects i.e. single vacancy and double vacancy defects (with different symmetry) on graphene sheet have been analysed to understand the electronic as well as transport properties using density functional theory and NEGF approach. Conductance, current-voltage and sensitivity analysis, these defected graphene sheet have been examined for its suitability for gas sensing application particularly for ammonia gas. The study observes that a single vacancy defected graphene is a good candidate for ammonia molecule sensing, in comparison to double vacancy. Index Terms - DFT, Graphene, Defects, Sensitivity, Conductance, Ammonia I. INTRODUCTION raphene has been the subject of extensive research, since its discovery[1] due to its outstanding electrical, mechanical and thermal properties[2] and applications in numerous technological fields such as ultra-high frequency electronic devices[3], photonics[4], nanopores for protein detection[5], energy storage devices and battery[6], nano electromechanical systems[7], flexible nonvolatile memory devices[8] and highly efficient gas sensors [9]. Graphene is made up of six carbon atom ring, in which each carbon atom is sp 2 hybridized with a two-dimensional periodic lattice structure. This single atom carbon sheet in honeycomb pattern provides a unique band structure with nonzero curvature. Graphene contains a huge surface area of (2630 m 2 /g) with the high electron mobility of 40000 cm 2 V −1 s −1 [10] at room temperature. The occurrence of charge impurities [11], the corrugation of sheet [12] and short-range disorder [13] are the possible aspects that need to be addressed. The disorder such as defects usually modifies the structure and the properties of the pristine graphene sheet [14-15]. Point defects or vacancies in nature are usually fundamental defects which arise during graphene growth and are also produced by energetic electrons bombarding the sample during the process of SEM/TEM [14] investigation. Recently, our group has analyzed various defects in graphene for enhancing the energy density of S. Dandeliya and A. Srivastava are with the Computational Nanoscience and Technology Laboratory, Advanced Materials Research Group, Atal Bihari Vajpayee–Indian Institute of Information Technology and Management, Gwalior 474010, India (e-mail: sushmitadandeliya@gmail.com; profanurag@gmail.com). * corresponding author supercapacitor electrode [16-17]. Detecting toxic gas molecules has significant applications in agriculture, environment and industrial pollution monitoring. Toxic ammonia is a colorless, irritant, and a hazardous gas in atmosphere. High exposure to NH 3 may cause damage of lungs in living beings, and fatalities in aquatic animals as it can dissolve easily in water. Near farming areas, the concentration level can be more than the allowed limit. This affects the health of farmers as well as animals within the close proximity of high concentration area. Pure ammonia is used in chemical industry, refrigeration systems and fertilizers factories. A leak in the system may cause serious and life- threatening situations, as exposure of ammonia at high concentration is a severe health threat [18]. A good number of attempts have been made on the fabrication and analysis of graphene based gas and chemical sensors. Mechanically exfoliated pyrolytic graphite was experimented to sense the NO 2 [19]. Doped graphene (with gold nanoparticles) [20] or foam like graphene model[21] have been experimented for enhanced ammonia sensing. The outcome of device based on graphene can be improved by functionalizing its surface. Investigations have suggested that the defects at graphene surface leads to higher reactivity for adsorption. An earlier report show that the adsorption of H 2 CO on surface of defected grahene, and interaction of H 2 CO molecule is present due to the C atoms of vacancy area [22] while no interaction in case of pristine graphene. Water molecule also adsorbs more likely on the vacancy site with high stability [23]. Similarly, NO 2 [24] CO and CO 2 molecule [25] are sensed on defective graphene sheet with high adsorption energy and charge transfer as compare to its pristine counterpart. These evidences imply that defects can play significant role in graphene based sensor for NH 3 . To exploit the properties of defected graphene sheet, a first principle analysis of vacancy defected graphene has been performed, including single vacancy as well as double vacancy defects. We also explored how the symmetry of defect modifies the electronic and structural properties of sheet. As a consequence of vacancy defects, variations in electrical and transport properties have been investigated. Furthermore, the proposed defected graphene structures have been tested for ammonia (NH 3 ) gas sensing to confirm which configuration is most suitable for ammonia sensing. II. COMPUTATIONAL METHODOLOGY The present analysis has been performed using density functional theory [DFT][26] and non-equilibrium Green’s Defected graphene as ammonia sensor: Theoretical Insight Sushmita Dandeliya, Anurag Srivastava * , Member, IEEE G