A theoretical study on the influences of vacancies and impurities on the electronic and magnetic properties of two-dimensional materials

Abstract

Graphene, the thinnest material with a stable two-dimensional (2D) structure, was considered to be a potential alternative for silicon based electronics due to its high surface area, flexibility and mobility at room temperature. However, zero band gap of graphene causes a poor on-off ratio of current thus making it unsuitable for logic operations. To resolve this issue researcher then focused on tuning graphene band gap through several methods including functionalization, doping and creating vacancy etc. However, modifying graphene band gap without sacrificing its major properties is still a great challenge. Then the search of new graphene like materials was become hot topic in order to replace it with other suitable materials for appropriate applications. Recently, 2D layered materials emerged as a fascinating research area due to their unique physical and chemical properties, which differ from their bulks counterparts. This dissertation is divided into two different parts. In the first part, the efforts were made to modify the electronic and magnetic properties of two-dimensional layered materials such as SnS2 and SnS using first principle calculations. In the second part, a new approach was investigated to achieve the polarization switching in graphene through embedding Mn@SV and Mn@DV. Firstly, we performed first-principles calculations to investigate the formation and migration of vacancies and doping with M (M=Ti, V, Co, Mo,W, Re) at the Sn sites and X (X=O, Se, Te) at the S sites in monolayer SnS2. It was found that the formation energies for S vacancy under both Sn- and S-rich environments are lower than those for Sn vacancy, indicating that the vacancies at the S sites are likely to be formed during the synthesis. It was observed that both the vacancies at the Sn and S sites remain robust due to high migration barrier. Additionally, SnS2 with the vacancies at the Sn sites induces magnetic ground states with a magnetic moment of 4.00 μB with a half-metallic band gap of 1.64 eV while SnS2 with the vacancies at the S sites preserves the semiconducting nature of pristine SnS2 with a narrow band gap of 0.30 eV. Furthermore, we found that the dopants Ti, V, Mo, W, Re can be easily incorporated at the Sn sites in monolayer SnS2 due to the low formation energies under the S-rich environment. However, Co may not be easily incorporated into SnS2. The doping with M at the Sn sites induces magnetic ground states in non-magnetic SnS2. Additionally, a long-range magnetic ordering was observed in SnS2 doped with V, Co, and Mo. On the other hand, easy incorporation of O, Se, and Te at the S sites under the Sn-rich environment has been observed while the semiconducting nature of SnS2 preserves with small band gaps. Then, we investigated the structural, electronic, and magnetic properties of defects in monolayer SnS. We studied the formation and migration of vacancies at both Sn- and Ssites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at Sn-site requires lesser energy, indicating that vacancy at Sn-site is more likely to be formed in experiments with energetic particle irradiation. For Sn-rich (S-rich) environment, vacancy at S-site (Sn-site) is more likely to be found than vacancy at Sn-site (S-site). Reducing the formation of vacancy clusters, S vacancy remain at the position where it was created because of high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also studied the transition metal (TM=Mn, Fe, Co) doping at Sn-site, and found a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM leads nonmagnetic monolayer SnS to magnetic while keeping it semiconducting. Additionally, long- range ferromagnetic behaviour was observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising way to realize two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices. Thirdly, we investigated the optoelectronic and magnetic properties of In- and Sb- doped monolayer SnS. The computed binding energies exhibiting energetically favorable nature of the doped systems. The doping of In at Sn sites preserves the semiconducting nature of monolayer SnS. Whereas, a metallic behaviour is observed with Sb doping in SnS. Moreover, ferromagnetism is observed for the In- doped monolayer SnS even at a higher distance. Furthermore, a high magnetic anisotropy energy (MAE) of 40.91 meV is computed for In-doped system. Further experimental investigations are highly desirable to investigate the efficient use of these systems in the field of photo-electronics and magnetic semiconductor devices. Finally, we investigates the generation of polarization switching in graphene using density functional theory calculations. It was found that by embedding Mn in single or double vacancy (Mn@SV and Mn@DV) in a graphene induces a dipole moment perpendicular to the sheet, which can be switched from up to down by Mn penetration through the graphene. We estimated the energy barriers for dipole switching, which were found to be 2.60 eV and 0.28 eV for Mn@SV and Mn@DV, respectively. However, by applying biaxial tensile strain, the energy barrier was reduced to 0.16 eV for Mn@SV. Additionally, both Mn@SV and Mn@DV induces high magnetic moment of 3.00 μB, suggestion its potential application for spintronics and nanoscale electro-mechanical or memory devices.