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We report on a coupled experimental and theoretical study of electronic and optical properties of quaternary Tl 2 CdGeSe 4 selenide. The experimental results reveal p-type electrical conductivity, the band gap energy of E g ​= ​1.52 ​eV for this compound and its non-direct nature. The latter experimental findings are confirmed theoretically by first-principles calculations based on density functional theory (DFT). The DFT calculations present that Se 4p states give principal contributions to the top of the valence band of Tl 2 CdGeSe 4 , whereas the central portion of the band is composed from Se 4p, Ge 4p and Cd 5s states, its lower portion is due to the input of Tl 6s states and the band bottom is dominated by Ge 4s states with smaller Se 4p states contributions. To verify the DFT calculations the XPS spectrum of valence electrons was measured for Tl 2 CdGeSe 4 crystal and XPS core-level spectra were studied to evaluate charge states of the composing atoms and features of the chemical bonding. The present theoretical data of calculations of the principal optical constants reveal that Tl 2 CdGeSe 4 selenide is a very promising semiconductor for optoelectronics. The Tl 2 CdGeSe 4 crystal was found to be rather stable with respect to treatment its surface with middle-energy Ar ​+ ​ions. The crystal surface of Tl 2 CdGeSe 4 demonstrates comparatively low hygroscopicity. Band dispersion along particular paths defined by high-symmetry k points of the first Brillouin zone for Tl 2 CdGeSe 4 adjusted with total densities of states of atoms constituting this compound. [Display omitted] • Electronic structure of Tl 2 CdGeSe 4 crystal was studied by XPS. • Tl 2 CdGeSe 4 possesses p-type of electrical conductivity. • Energy band gap of Tl 2 CdGeSe 4 is equal to 1.52 ​eV at room temperature. • Calculations within TB-mBJ ​+ ​SOC ​+ ​U model are in agreement with XPS measurements. • Main optical properties of Tl 2 CdGeSe 4 are calculated from first principles. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we study the difference in electronic and optical properties of bulk and monolayer zirconium sulfide by applying the APW + lo method in the framework of density functional theory. All calculation is performed at the energy level of visual light and higher ranging from 0 eV to 15 eV. Our results demonstrates that except for the underestimated band gap like other GGA calculation, the remain properties like dielectric function, the reflectivity, absorption and loss energy are close to experiment. The valence band is constructed by mainly sulfur s / p -states and the lower portion of zirconium s / p / d -states. The conduction band is mostly donated by zirconium d -state. In contrast with bulk structure, the valence band maximum in monolayer has the triple peak at Γ point, making its monolayer be more sensitive to light absorption. The dielectric function has the highest peak at about 1.5–2.5 eV with remarkable anisotropy, beyond this level to the ultraviolet region the anisotropy decreases and almost disappears at energy larger than 10 eV. The absorption is at 106 x 10 4 cm -1 for energy range 5–10 eV, while the reflectivity is at its highest value of 30 %–50 % in the energy range from 0 to 8 eV. The energy loss of monolayer is higher than those of bulk. For optical and electronic properties, the monolayer show sharper peaks and their clear separation indicate the progressive application of monolayer zirconium sulfide. Highlight • Monolayer ZrS 2 has new interband transitions at energy levels of 4-7 eV and 7-10 eV. • For energy ranging from -14 eV to 6 eV, the dielectric constants, energy loss, and reflectance are close to experimental values. • ZrS 2 is highly anisotropic to the parallel and perpendicular components of electric field at energy lower than 10 eV. • The energy loss L (ω) in monolayer is higher than in bulk. • Our results give an explanation on perspective optical properties of monolayer ZrS 2 , which can be helpful for further design and application of this material. [ABSTRACT FROM AUTHOR]