Insight into the optoelectronic and thermoelectric nature of NaLiX (X = S, Se, Te) novel direct bandgap semiconductors: a first-principles study

Abstract

In the present work, an in-depth study was conducted on the structural, optoelectronic, and thermoelectric nature of novel ternary-type NaLiX (X = S, Se, Te) chalcogenides. The WC-GGA exchange correlation and the most accurate TB-mBJ method within the framework of density functional theory were used to calculate the electronic properties. Bandgap calculations with TB-mBJ potential show a direct band transition nature with gap values of about 3.59 eV for NaLiS, 2.87 eV for NaLiSe, and 2.73 eV for NaLiTe. Decreasing bandgap values in the order of NaLiS → NaLiSe → NaLiTe are attributed to the reduction of electronegativity while going from S to Te anions in these materials. The wide energy bandgaps in these materials reveal that the bonds involved have a strong covalent nature. Our calculated density of states plots shows a similar overall trend, indicating that the top of the valence bands in all these materials originate primarily from the p-states of the chalcogen anions. The computed electron effective masses decrease in the order of NaLiS → NaLiSe → NaLiTe, which implies that the flatness of the energy band at the CBM decreases at the Γ-point. Additionally, the linear optical constants, such as the complex dielectric function, refractive index, electron energy loss function, absorption coefficient, and reflectivity spectra of these novel chalcogenides were computed and examined in detail for their possible applications in optoelectronic devices. The thermoelectric transport properties were also computed, and the attained results are discussed in detail, which suggest these materials are efficient for thermoelectric device applications. The current work essentially supports the advancement of discrete and integrated semiconductor device applications.