Unveiling the Thermoelectric Properties of Group III-Nitride Biphenylene Networks

TL;DR

The paper tackles the thermoelectric performance of non-benzenoid 2D group-III nitride biphenylene networks by combining first-principles DFT (PBE-GGA and HSE06) with non-equilibrium Green's function transport and phonon analysis in a ballistic framework. It uses Landauer formalism to compute electronic and phonon transmissions, along with MD and phonon dispersion to establish stability, revealing strong transport anisotropy between armchair and zigzag directions. The key finding is that InN-BPN exhibits the most promising p-type thermoelectric performance, achieving a $zT$ of 2.33 at 800 K along the zigzag direction, aided by a sharp rise in p-type transmission near the valence-band edge and a very low lattice thermal conductance $κ_{ph}$. These results illuminate design principles for high-performance thermoelectrics in 2D non-benzenoid lattices and highlight the critical roles of valence-band dispersion and directional transport.

Abstract

After the synthesis of the carbon biphenylene network (C-BPN), research has increasingly focused on adapting elements from other groups of the periodic table to this lattice structure. In this study, the direction-dependent electronic, thermal, and thermoelectric (TE) properties of semiconducting group-III (group-III = B, Al, Ga, In) nitride biphenylene networks are investigated using the non-equilibrium Green's function formalism in combination with first-principles calculations. Phonon spectra and force field molecular dynamics (MD) simulations were used to asses the dynamically and thermally stable structures. At room temperature, the lowest phonon thermal conductance values are obtained for InN-BPN, with $κ_{\mathrm{ph}}$ = 0.12 nW/K/nm and $κ_{\mathrm{ph}}$ = 0.21 nW/K/nm along the armchair and zigzag directions, respectively. The nearly dispersionless valence-band region between the $Γ$--$X$ symmetry points causes a sharp increase in the $p$-type electronic transmission, which significantly enhances the $p$-type thermoelectric figure of merit, $zT$. Among the investigated group-III nitride BPNs, InN-BPN exhibits the best performance, with a $p$-type $zT$ value of 2.33 in the zigzag direction at 800 K.

Figures (10)

• Figure 1: (a) Prototype of the top and side view of the optimized crystal structure, (b) Unit cell of the structures and bond lengths between atoms (c) Electron localization function (ELF) of BN, AlN, GaN and InN biphenylenes, respectively.
• Figure 2: Evolution of (a) total energy and (b) the fraction of III-fold coordination (i.e., the number of Al, Ga, In, or B atoms surrounding N within its first coordination shell) in AlN, GaN, InN, and BN as the temperature increases from 1 to 2000 K over 6 ns. (c) Representative structural snapshots of AlN, GaN, InN, and BN at selected temperatures during the heating process.
• Figure 3: (a) Fluctuations in total energy (eV per unit cell) and (b) the fraction of III-fold coordination of N in GaN, InN, and BN equilibrated at 400 K, 400 K, and 800 K, respectively, over a 10 ns simulation.
• Figure 4: Calculated electronic band structures based on PBE and PBE+HSE06 functionals and obtained projected density of states using PBE+HSE06 functionals of group-III nitride biphenylenes.
• Figure 5: Calculated phonon branches and projected density of states based on x, y and z movements of BN, AlN, GaN, InN biphenylenes, respectively.