Suppression of intervalley scattering and enhanced phonon anharmonic interactions in 2D Bi_(2)TeSe_(2):Crystal-field symmetry and band convergence

The electron-phonon(el-ph)and phonon-phonon interactions play a key role in determining electronic and thermal transport properties,respectively,in promising two-dimensional(2D)semiconductor de-vices.In this study,we investigated el-ph interactions using Wannier-Fourier interpolation method and renormalized phonon scattering considering finite-temperature effects in Bi_(2)TeSe_(2)monolayer.The re-sults show that the optical phonon modes dominate the carrier scattering,where level repulsion induced by crystal field splitting and spin-orbit coupling(SOC)effect effectively suppresses intervalley scattering,leading to high hole mobility.Moreover,the strong anharmonicity in Bi_(2)TeSe_(2)monolayer results in the temperature-dependent softening of its optical phonons,along with a corresponding variation in interatomic force constants(IFCs).As a result,the lattice thermal conductivity is remarkably low and exhibits weak temperature dependence.Finally,the predicted dimensionless thermoelectric figure of merit exceeds unity in the range of 200-800 K,indicating the potential of Bi_(2)TeSe_(2)monolayer for thermoelectric applications.This work provides new insights into the elimination of intervalley scat-tering by crystal field splitting and SOC effects,and paves the way for the evaluation of thermoelectric properties of materials with complex scattering mechanisms and strong anharmonicity.

Effects of crystal field on photoluminescence properties of Ca_2Al_2SiO_7:Eu^(2+) phosphors

A series of single-phased Ca2Al2SiOT：EU2＋phosphors were synthesized by the solid-state reaction. Their structure and photoluminescence properties were investigated by the X-ray powder diffraction （XRD） and excitation and emission spectra in detail. The emission spectra of Ca2Al2SiO7：Eu2＋ phosphors consisted of blue and green band located at 419 and 542 nm, respectively. The relative intensities of the blue and green emission changed with Eu2＋ concentration and were sensitive to the excitation wavelength. The unique photoluminescence property originated from the 4f^7→4f65d transition of Eu2＋ at different energy levels, on which the effect of the crystal field strength was con- sidered to be tailed by adjusting the host composition.

Band convergence boosted high thermoelectric performance of Zintl compound Mg_(3)Sb_(2) achieved by biaxial strains

Mg_(3)Sb_(2) as a Zintl compound is a promising thermoelectric material with the intrinsically low lattice thermal conductivity and excellent n-type electrical properties,but its p-type electrical transport properties are poor.Here,the thermoelectric performance of Mg_(3)Sb_(2) under the effect of biaxial strain is investigated by using first-principles method and Boltzmann transport theory.The application of biaxial strain enables tuning the band structure of Mg_(3)Sb_(2) in such a way that the band degeneracy of both the conduction band and valence band increases.As the biaxial strain increases,the Seebeck coefficient of ptype Mg_(3)Sb_(2) has a remarkable increase,leading to a significant improvement in power factor.This is mainly ascribed to the achievement of valence band orbital degeneracy.Meanwhile,the lattice thermal conductivity exhibits very slight biaxial strain dependence within the strain range considered in this work,which increases from 1.28 to 1.62 W m^(-1) K^(-1) at 300 K.Finally,the highest ZT of p-type Mg_(3)Sb_(2) at 700 K can be up to 2.6 along the in-plane direction under-2.5%biaxial strain,which is almost three times that of the unstrained counterpart.The realization of high thermoelectric performance of p-type Mg_(3)Sb_(2) will promote its practical applications as thermoelectric generators.