Two-dimensional III-V binary compounds are considered as high-performance optoelectronic materials due to their tunable bandgap and unique photoelectric properties. In this research, the effects of strain engineering on the electronic properties and optical properties of hexagonal boron phosphide monolayer have been systematically studied by using first principles calculations. The bandgap is enlarged monotonously while the direct bandgap character remains as the strain increases from -10% (compression) to +10% (tension), suggesting its application prospect in flexible electronics. Interestingly, the hexagonal boron phosphide monolayer exhibits a large optical absorption coefficient in both visible and ultraviolet regions and could reach 1.3 × 106 cm-1 in ultraviolet region. As compressive strain gets larger, the main peak of dielectric function as well as the edge of optical absorption appear redshift. In addition, the absorption spectrum broadens in visible light region and the light absorption intensity becomes larger in ultraviolet range with increasing compressive strain. The calculated optical properties prove that hexagonal boron phosphide monolayer is a suitable material as ultraviolet-visible dual band photodetectors and a potential auxiliary material for quantum cutting.
A series of two-dimensional TiX2 (X=Cl, Br, I) monolayers and their corresponding van der Waals heterostructures were predicted by the first-principle calculations. The dynamic and thermodynamic stability of TiX2 (X=Cl, Br, I) monolayers are confirmed by the phonon spectra and molecular dynamics simulations, respectively. The Heyd-Scuseria-Ernzerhof-based band values are 0.311-0.989 eV, showing the tunable transition probability between valence and conduction band. Additionally, the significant visible-light absorption coefficient (~ 105 cm-1) and high power conversion efficiency (~12%) of TiBr2/TiCl2 heterojunctions provide promising potentials for solar cells.
The dynamic population processes of infrared radiation in dysprosium-doped different host materials (LaF3, Y2O3, YAlO3 and silicate glass) are theoretically investigated. The radiation and non-radiation transition rates of each energy level are calculated using Judd-Ofelt (J-O) theory and according to “energy-gap law”. It is demonstrated that the non-radiative transition rate increases significantly as the phonon energy increases, indicating that the choice of host materials has a great influence on the infrared transition processes. By solving the rate equations we establish, it is found that the population profiles of the same energy levels are almost the same, but the time to reach equilibrium population varies greatly among different materials. The population probability of 6H9/2 and 6H11/2 energy levels increases first and then decreases, whereas that of 6H13/2 and 6H15/2 monotonically increases or decreases with time. The excited state 6H13/2 has a quite long decay lifetime of 38.97 ms in dysprosium-doped LaF3, which is a good metastable state for mid-infrared emission.. These results are helpful to the material selection and application of infrared lasers.
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