Investigations Of Thermoelectric Performance And Excited Carriers In Novel Two-dimensional Multinary Semiconductors

With the progress in material synthesis and characterization,two-dimensional nohexagonal carbon configurations are a current focus in thermal transport,thermoelectric,and optoelectronic.In this study,using the evolutionary algorithm USPEX structural searches method,Boltzmann’s equation approach,and nonadiabatic molecular dynamics simulations.We identify that novel two-dimensional BCN biphenylene and Haeckelite（8|4）Ga N-Zn O stabilizes and investigates the thermal,optical,and excited carriers dynamics.The innovative results are as follows:（1）Carbon biphenylene has stimulated substantial researches because of extraordinary properties introduced by the metallic character,e.g.,the ultrahigh electron thermal transport.Here,inspired by the synthesis of carbon biphenylene（Fan et al.,Science 372,852-856（2021））,we identify the stability of C_x（BN）_1-x biphenylene as CBN and C4BN semiconductors with four-,six-,and eight-membered periodic rings of irregularly-sp²-hybridized atoms via structural searches.Unexpectedly,we confirm that CBN biphenylene exhibits a peculiar funnel-shaped band structure,which is a direct consequence of the delocalization/localization ofπbonds formed by B-p_z,N-p_z or C-p_z electrons.The novel band structure greatly improves the thermoelectric performance by enhancing the power factor,although the lattice thermal conductivity is relatively large after including four-phonon scattering resistance because of low atomic masses.The similar behaviors are absent in C₄BN biphenylene because of the localization ofπbonds formed by C-p_z electrons,although with a stronger anharmonicity and thus a lower lattice thermal conductivity.The anomalous power factor can be explained by the constantτapproximation:the p-type doping controls the carrier group velocities and thus realizes the tunability of tensor ratio K₁/K₀.Our analysis suggests that the funnel-shaped electronic structure could be reproduced in two-dimensional semiconductor systems with the small electronegativity difference and the comparable stoichiometry.Our work realizes the thermoelectric improvement through controlling the shape of band structure,which provides new insights for designing promising two-dimensional thermoelectric materials.（2）Low internal electrostatic field is usually required to improve optical performance,however,it is not the case in two-dimensional haeckelite（8|4）Ga N-Zn O with more excellent physical properties than binary counterparts.By performing nonadiabatic molecular dynamics simulations,we ascribe the superior limit of improvement of light absorption to the convergence of electron-hole recombination time when the thickness of 8|4 phase exceeds a critical value,which arises from the competition between the nonadiabatic coupling and the quantum decoherence.We show that the nonadiabatic coupling continuously becomes weak because of the reduced nucleus velocity by increasing thickness.We further demonstrate that the quantum decoherence is first accelerated and then decelerated because of the thickness dependent electron-phonon coupling controlled by the peculiar in-plane A′and A″phonon modes.Our study clarifies the debated issue with regard to light absorption,which provides useful guidance for further understanding the optical properties in two-dimensional polar semiconductors.