Tuning Structural, Electronic,Magnetic and Optical Properties of 2D-WX2 under Biaxial Strain and Substitutional Doping in First Principles Method

Abstract

Two dimensional (2D) WX2 for X= S, Se and Te in various supercell sizes are considered in monolayer and heterobilayer. The structural, electronic, magnetic, and optical properties of 2D- WX2 are investigated using density functional theory (DFT) with respect to a plane wave ultra-soft Pseudopotentials (PW-USPPs) and normconserving pseudopotentials (NCPPs) in a generalized gradient approximation (GGA) and the Hubbard correction (GGA + U) with the implementation of quantum espresso package. The optimized lattice parameters have been investigated interactively in each type of material. The band gaps for monolayer WTe2 are investigated for unstrained, 2% biaxial compressive and tensile strain to the slab of the mono layer using GGA and GGA+U approximations. In GGA+U approximation, the energy band gap becomes slightly wider than with that of the GGA approximation. Monolayer WTe2 under biaxial compression exhibits an expanded energy band resulting in blue shift and in biaxial tension red shift by narrowing of the energy band gap in comparison to the equilibrium. Pristine WSe2 monolayer is identified as a nonmagnetic direct band gap semiconductor with a band gap of 1.55 eV. Upon substituting Mn for W in the WSe2 monolayer, the resulting structure exhibits enhanced stability, indicated by a negative formation energy. The Mn doped monolayer of WSe2 has slightly shorter bond length with 2.4294 ˚ A than the pure with 2.5405 ˚ A for the Mn-Se and W-Se respectively. The doped system becomes FM and total magnetic mo ment within the nearest neighboring interactions of the impurity atoms increases notably and it is attributed to the electron-correlation effect in the high spin state under the GGA+U approx imation. However, this correlation effect proves insignificant on the total magnetic moment for the second nearest neighboring interactions, yielding consistent outcomes in both GGA and GGA+U approximations. The transition of FM to AFM state occurs above room temperature for low impurity concentration, 443 k and 450 k for 8% and 12.5% substitution with Mn re spectively. This indicates long-range FM ordering and crucial for high-temperature 2D-diluted magnetic semiconductors. However, at high concentrations of impurity atoms, the temperature drops below room temperature (220 K in 22.2%) in the doped monolayer WSe2, showing weak magnetic interaction. Lastly, the optical property of a doped system is studied by applying polarization in perpendicular and parallel directions to the plane of the monolayer. For two-dimensional WS2/WSe2 hetero structure lattice constant and the vertical interlayer distance between slabs are a=3.28 ˚ A andc=13.14 ˚ Arespectively. The bandgapsforWS2/WSe2 hetero structure are investigated for unstrained, (0-4)% biaxial compression and tension using GGA approximations. Direct band gap of 0.51eV is obtained for unstrained heterostructure. The band gap value of the heterobilayer is affected by the application of strain. When the heterobilayer is imposed to biaxial compression the band gap value increases. However, the tensile stress results in reducing the band gap value of the heterobilayer and the system attains its metallic or semimetalic state at 4% biaxial tensile strain. Tunability of the band gap is very viii crucial in developing photovoltaic and optoelectronic devices. The strained heterostructure shows different optical property when it is under biaxial tension and compression due to the change in the values of the band gap. The heterobilayer has also high absorption coefficient in the visible light spectrum which makes it a promising material for photovoltaic applications.