A First Principles Study of Electronic Transport & Magnetocrystalline Anisotropy in one and two-dimensional Materials

Abstract

The peculiar properties of two-dimensional (2D) materials are the hot topic of research among the researchers in the last two decades because of their peculiar properties in a wide range of applications. But, the most important among these 2D family is graphene, phosphorene, MoS2, hexagonal boron nitride, and many other transition metal dichalcogenides, which are vastly studied. Therefore, in this thesis, using the first principles calculations is used to study the 2D and some of the one-dimensional (1D) nanoribbons for different attractive properties such as physical, electronic, magnetic and electronic transport are investigated. This thesis is basically consist of three parts; where the first section is about the magnetic and magnetocrystalline anisotropy of the recently reported 2D intrinsic ferromagnetic materials called CrI3 and VI3 will be instigated, while the next part is consist of the study about the tilted phosphorene nanoribbons passivated with hydrogen and oxygen atoms will be reported, here in this section we also investigated the effect of defects in the hydrogenated tilted phosphorene nanoribbons over the electronic and electronic transport properties. In the last section, the gas sensing effect is investigated for the separation of different toxic and non-toxic gases over their adsorption on the g-C4N3 sheet using the electronic transport method. As it is recently reported that chromium-tri iodide (CrI3) is a newly exfoliated 2D intrinsic ferromagnetic (FM) material where its magnetic ground state is strongly sensitive to the combination of the even-odd layers. Therefore, in the first section, we study the bilayer CrI3 and investigated the magnetic ground state of this material. In the present study, we focus on the magnetic ground state of the bilayer CrI3 by using van der Waals and GGA+U. Interestingly, for the first time considering the natural stacking in the bilayer, we found an antiferromagnetic (AFM) ground state. In this work, we consider two types of stackings called AB (natural bilayer) and AA (where the upper layer is moved up to 4Å). Further, by calculating the magnetocrystalline anisotropy (MAE) we found that the easy axes are along the z direction (perpendicular) irrespective of the stacking. The electronic band structure was calculated and found a semiconductor nature in both stackings with an indirect band gap of 1.71 and 1.68 eV for AB and AA stacking respectively. Then we apply an external perpendicular pressure and found a transition from FM to AFM ground state. Besides, a linear enhancement in the band gap, we also found a linear increase in the MAE with the perpendicular easy axes remains preserved. Now in the next section of chapter 3, we investigated the strain effect over the monolayer VI3 on the structural, electronic, and magnetic properties. Recently, another FM material called vanadium-tri iodide (VI3) has been also reported which had a Mott insulator nature and exist in the same crystal symmetry as of the CrI3. Further, it has been proposed that the 2D FM state can be observed in VI3. Indeed, the bulk VI3 displays the Mott insulator nature and also has a structural transition from the monoclinic to the rhombohedral structure at 79 K. Moreover, the magnetic phase transition from non-magnetic state to FM state was observed at 50 K. It has also been known that the VI3 can be easily cleaved into few layer thicknesses. Here in this thesis we also explored the structural, electronic and magnetic properties of this newly predicted 2D FM material. We found that the lattice constants and band gap of the pristine monolayer VI3 are both larger than the bulk structure. We also explore the magnetic anisotropy (MAE), and the calculated value for pristine monolayer is 0.29 meV/cell along the z-axes as in the bulk. Using the Metropolis Monte Carlo (MC) simulations, we also obtained that the monolayer had a Curie temperature of 47 K in the pristine monolayer. Here, we also investigate the effect of biaxial tensile/compressive strain over the magnetic properties of the material. The Tc was increased/decreased in the tensile/compressive strain while the same behavior has been explored in the electronic and magnetic anisotropy also. In section 4.1 of chapter 4 in this thesis, we investigated the electronic transport properties of the tilted phosphorene nanoribbons in three different ways called self, hydrogen and oxygen passivated nanoribbons. For this purpose, we use the non-equilibrium Green’s function (NEGF) technique under the state-of-the-art of density functional theory (DFT). We computed the electronic band structures and the transmission spectrum of all the three systems at zero and high external applied bias, where we found that the current is purely bias dependent through these ribbons. Further, we created a two-terminal device from the nanoribbons and explore the effect of external bias on the nanoribbons by varying the external bias. The current dependency on the bias basically originates from the interaction on the near states around the Fermi level in the scattering region. Shifting of the electrodes energy states at different applied biases and the change of the transmission spectrum at these biases have also been analyzed. We also noticed that at zero bias there is no current in all the three systems but as the external bias exceeded the band gap of each system current is linear starts increases, which simply shows the current dependency over the external bias in these systems. Shifting of the electrodes energy states at different applied bias and the change of the transmission spectrum at these biases have also been analyzed. Similarly, in section 4.2 of the chapter, we further investigated the effect of vacancy defects in the hydrogen passivated tilted phosphorene nanoribbons using the same technique of DFT in the nonequilibrium Green’s function implemented in the TranSIESTA code. In this work, we explore the position dependency of defects and investigate the transport properties of the hydrogen passivated tilted phosphorene nanoribbons. From the results, we found that how the structural imperfections affect the structural, electronic and transport properties of the hydrogen tilted phosphorene nanoribbons at different applied biases. Interestingly, in the case of hydrogen passivated tilted phosphorene nanoribbons, we found around five times enhancement in current for different site vacancies in the scattering region of the nanoribbons. The reason behind the enhancement in current is not only the availability of more and more states in the scattering region but also the appearance of transmission channels around the fermi level. The availability of more and more states in the scattering region was further confirmed by investigating the bias dependent density of states of the scattering region. In chapter 5 of this thesis, we investigate the effect of different gas adsorption over the g-C4N3 nanosheet. In this work, we use the first principles calculations for the selection of multiple toxic gases, by exploring the transport properties of H2, N2, O2, CO, CO2, NO2, and NH3 gas molecules on two dimensional (2D) graphitic carbon nitride (g-C4N3) substrate. Usually, gas sensing study has been performed by investigating the bias dependent I-V curve characteristic and the spin dependency was not considered because the non-magnetic 2D materials have been utilized for gas sensing properties and also the adsorbed gas molecule itself non-magnetic. Therefore, in this work for the first time, we explore the effect of half-metallic 2D ferromagnetic (FM) material called g-C4N3. Where we found that homo-nuclear gas molecules such as H2, N2, and O2 with a very weak adsorption energy of less than 0.1 eV. Interestingly, the typical toxic gas molecules have a large amount of charge transfer and also having large adsorption energies. By calculating the I-V curve, we found an appreciable current difference in different gas molecules over their adsorption on the layer. Despite the extensive studies and many outstanding results, the particular substrate material shows a good selectivity for a specific gas molecule. If we can propose that the multiple toxic gases (CO, NO2, and NH3) can be distinguished at the same time with one substrate material, then it will be highly desirable for device application. In this report, we aim to explore this issue using the 2D half-metallic g-C4N3. Simply our results show that the 2D g-C4N3 layer can be a superior substrate material for gas sensing for multiple toxic gases.