DFT and BOLS approximations were carried out to study the electronic and optical properties of different sizes of black phosphorus nanoribbons (PNRs) with either zigzag- or armchair-terminated edges. PNRs exhibit a nearly direct bandgap, the size of which is strongly increased because of quantum effects. Meanwhile, the bandgap energies of these two kinds of edge PNRs reveal an excellent size dependency. We reconcile the size-dependence of the bandgap energy shifts for PNRs with respect to stimulated bond relaxation, and quantification of the bond length, the bond strength, and the bond nature index. Our calculations suggest that atomic under-coordination shortens the length and increases the stiffness of the P-P bond, which widens the bandgap.
Electronic fitness function (EFF, achieved by the electrical transport properties) as a new quantity to estimate thermoelectric (TE) performance of semiconductor crystals is usually used for screening novel TE materials. In recent years, because of the high EFF values, an increasing number of two-dimensional materials have been predicted to have the potential for TE applications via high-throughput calculations. Among them, the GeS2 monolayer has many interesting physical properties and is being used for industrial applications. Hence, in this work, we systematically investigated the TE performance, including both electronic and thermal transport properties, of the GeS2 monolayer with first-principles calculations. The results show that the structure of the GeS2 monolayer at 700 K is thermally unstable, so we study its TE performance only at 300 and 500 K. As compared with other typical TE monolayers, the GeS2 monolayer exhibits excellent electronic transport properties but a relatively high lattice thermal conductivity of 5.71 W m−1 K−1 at 500 K, and thus an unsatisfactory ZT value of 0.23. Such a low ZT value indicates that it is necessary to consider not only the electron transport properties but also the thermal transport properties to screen the thermoelectric materials with excellent performance through high-throughput calculations.
Understanding the physical mechanism behind atomic-size dependence of the bandgap, phonon frequency, and mechanical strength in various monolayered MA2Z4 is of crucial importance for their electronic and photoelectronic applications. The density functional theory calculation results confirm that these physical quantities gradually decrease with the increasing periodicity of the atomic size (or radius) of the A or Z of MA2Z4. In order to clarify the common origin of the atomic-size dependence of these quantities, we establish these quantities as functions of bond length and bond energy by developing a bond relaxation theory approach. Theoretical reproduction of periodic trends confirms that bond expansion and energy weakening dominate their atomic-size dependence. The proposed approach is not only helpful to understand the physical origins of atomic-size dependence in different MA2Z4 monolayers but also can be extended to study the periodic trends of the related physical properties in other systems.
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