Effect of nickel impurity on structural, magnetic and electronic properties of WX₂MLs (X = S, Se): An ab-initio study

Abstract

We have studied the spin dependent structural, electronic and magnetic properties of pristine and Ni doped WX₂ (X = S, Se) 2D monolayers by using density functional theory (DFT). The doping process improves and control the electronic and magnetic properties of TMDCs which is essential for application in electronics, optoelectronics and spintronics. WX₂MLs with Ni impurity shows ferromagnetic behaviour. Spin resolved energy band gap have been observed. The Ni doped WSe₂ exhibits the characteristics of half metal with 0.88 (D) and 0.02 (D) eV energy band gap for up and down spin respectively. The WS₂ML with Nickel impurity shows 1.03 (I) and 0.17 (D) eV energy band gap for up and down spin respectively. The total magnetic moment of both Ni doped WX₂MLs is found to be approximately 4µ_B which is due to local effect of the dopant. The exclusive combination of magnetic response and narrow bandgap which expedite the application of spintronics and yields magnetic semiconductors.

Introduction

The discovery of graphene drew the attention of scientists towards 2D materials [1]. This leads to excitement in the world of material science and researchers started discovering a new class of materials called Transition Metal Dichalcogenides (TMDCs). These TMDCs have the power to completely revolutionize industries like electronics and energy storage. TMDCs belongs to a novel class of two-dimensional materials that have garnered significant attention in recent years. These materials possess unique electronic, optical and mechanical properties that have the potential to transform the industries, from electronics and energy storage to biomedicine [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. TMDCs are also explored as a possible replacement for conventional semiconductors due to their wide energy bandgap. TMDC monolayers are MX₂-type atomically thin semiconductors, where M is a transition metal and X is a chalcogen atom (S, Se, Te). The WX₂MLs are one important category of TMDCs in which a W-atom is sandwiched between two layers of X-atoms (X = S, Se). The monolayers of WS₂ and WSe₂ exhibit metallic and semiconducting properties and have direct band gaps of 1.8 eV and 1.566 eV, respectively [14]. To enhance the significant potential of 2D materials for use in optoelectronics, energy storage devices, and catalysis impurity defects, one important technique is to modify the bandgap of 2D materials. There are several approaches for this purpose [15], [16], [17]. Generally, substitutional and doping methods are used to tune the energy bandgap of 2D materials. Theoretically, predictions proved the transition elements prefer to substitute at the M−site instead of the X-site. Several experimental and theoretical studies on the effect of defects and impurities on transition metal dichalcogenide monolayers (TMDMLs) have been conducted in recent years. It has been demonstrated that the electrical and magnetic characteristics of WX₂ monolayers are significantly affected by the doping of magnetic impurities [16], [17], [18], [19]. Local magnetic moments created by magnetic impurities may interact with the magnetic moments of nearby WX₂ atoms. This interaction can result in changes to the electronic structure of the WX₂ monolayer, including modifications to the bandgap and magnetic ordering. According to studies, magnetic impurities can significantly change the magnetic characteristics of WX2 monolayers, causing the formation of novel magnetic phases and improving magnetic ordering [17]. Furthermore, it has been demonstrated that the creation of magnetic skyrmions, which are topologically protected spin structures, results from the doping of WX2 monolayers with magnetic impurities. This type of material shows possible applications in the fields of electronics and spintronics [18]. Recently, TMDC's 2D monolayer (ML) of WS₂ doped with Fe has become paramagnetic in nature [19]. WSe₂MLs are doped with Cr, Mn, Fe, and Co impurities [20] and Fe doped system a robust and stable half metallic materials due to the presence of three connected states peak spanning to the fermi level. Kyungnam Kang investigated the effect of Fe-doping in MoS₂ and WS₂ monolayers, concluding that Fe-doped MoS₂ is ferromagnetic while Fe-doped WS₂ monolayer is paramagnetic [21]. As researchers continue to unravel the mysteries of TMDCs, it is becoming increasingly clear that these materials have the potential to play a major role in shaping our future landscape of technology. In the present paper, we have investigated the density functional theory (DFT)-based structural, electronic, and magnetic properties of 2D TMDMLs of WX₂ (X = S, Se) doped with Ni impurity and observed the effect of impurity on band structure, which leads to an induced magnetic moment and is helpful in spintronics.