1536-125X (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/TNANO.2018.2884171, IEEE Transactions on Nanotechnology 1 1536-125X © 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. Abstract— The target of this paper is to theoretically investigate the probability of gas (both oxidizing and reducing) adsorption on the van der Waals (vdW) heterojunction formed between p- type rGO and n-type 2D-ZnO, using density functional theory (DFT) based first principle calculation employing Virtual Nanolab (VNL) Atomistix Toolkit (ATK) (v2016.4). Two types of heterostructures are considered viz. heterostructure type-1 where hydroxyl group (sp 2 ) is at the edge of the reduced graphene oxide (rGO) and heterostructure type-2 where hydroxyl group (sp 3 ) is perpendicular to the plane of rGO. Adsorption energy, charge transfer and the distance of the nearest atom from the adsorbent are calculated for oxidizing (NO 2 as the test case) and reducing (NH 3 as the test case) species and compared with that of Oxygen on rGO/2D-ZnO heterostructures as well as on its individual constituent (rGO nanoflakes and 2D-ZnO, separately). Like 2D- ZnO, heterostructure type-1 was also found to be selective towards NO 2 with almost three times and five times higher adsorption energy than that of 2D-ZnO and rGO, respectively. On the other hand, it was found that charge distribution in the underlying 2D-ZnO of heterostructure type-2 remained almost unaltered even after gas adsorption and therefore lead to insignificant improvement compared to its 2D-ZnO counterpart. Index Terms— van der Waals heterostructure, rGO-ZnO hybrid system, Density functional theory, charge transfer, gas adsorption probability. I. INTRODUCTION UDICIOUS fabrication of localized heterojunctions between graphene or its derivatives (rGO) and metal oxide semiconductors (like ZnO, TiO 2 ) has been proved to be an efficient route for improving gas sensor performance where Manuscript received 22 nd May, 2018. This publication is an outcome of the R&D work undertaken in the project under the Visvesvaraya PhD Scheme of Ministry of Electronics & Information Technology, Government of India, being implemented by Digital India Corporation (formerly Media Lab Asia). Kasturi Ghosh would like to thank MCIT, India, for supporting to carry the research work through SMDP C2SD project. Kasturi Ghosh, Niladri S. Mahapatra and Hafizur Rahaman are with the School of VLSI Technology, IIEST Shibpur, India. Partha Bhattacharyya (Corresponding author) is with the Nano thin films and solid state gas sensor devices laboratory, Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology (IIEST), Shibpur, India (e-mail: pb_etc_besu@yahoo.com ). synergistic hybridization enhances the sensing properties of individual components [1]- [11]. In this regard, Zinc oxide is an important member in the family of frequently used metal oxide semiconductors for gas sensing application due to its chemical and thermal stability, low-cost and good oxidation resistant nature [1], [3], [7]-[11]. For the purpose of gas sensing, different nano-forms of ZnO, like nanoflowers, nanoparticles, nanofibers, nanorods and quantum dots have so far been investigated to form heterojunction with rGO, a popular two dimensional (2D) material whose oxygen functional groups act as active adsorption sites [2], [7]-[13]. Like graphene and its two dimensional derivatives such as graphene oxides, other 2D materials and their heterostructures have recently gained attention for possible use in designing gas sensors due to large specific surface area and strong surface activities [12] –[19]. Hu et al. investigated the sensing behavior of monolayer tin sulfide (SnS) for four target gas molecules (NH 3 , NO 2 , CO, and H 2 O) by the first-principle calculation based on density-functional theory [17]. The adsorption of N 2 , CO 2 , O 2 , and NO 2 molecules on the GeS monolayer from atomic scale to device scale by the first principles and molecular dynamics method were investigated by Wang et al. [18]. 2D monolayer of ZnO with planar honeycomb structure known as graphene like ZnO (g-ZnO) or 2D-ZnO has also been investigated thoroughly from experimental and theoretical perspective [20]- [23]. Potentiality of 2D-ZnO nano-walls as hydrogen sensors was reported by Chen et al. [21]. Zhang et al. investigated physisorption probability of carbon mono-oxide (CO) on 2D-ZnO by first principle analysis [23]. Two-dimensional net-like SnO 2 /ZnO heteronanostructures for high-performance H 2 S gas sensor was developed by Fu et al. [24]. Due to layered structure of these 2D nanomaterials, their heterostructures can be formed either in stacked vertical architecture (known as van der Waals (vdW) heterostructures) or laterally (in-plane) [14], [16]. However, very few attempts were made towards the successful fabrication of lateral heterostructure, as the main bottlenecks of fabricating such lateral heterojuction are strain release between two materials (which hinders formation of defect-free lateral heterojunction) and possibility of alloy formation instead of formation of junction [25]. On the other hand, Prediction of Adsorption Probability of Oxidizing and Reducing Species on 2D Hybrid Junction of rGO-ZnO from First Principle Analysis Kasturi Ghosh, Member, IEEE, Niladri S. Mahapatra, Hafizur Rahaman, Senior Member, IEEE and Partha Bhattacharyya, Senior Member, IEEE J