The impact of electron-phonon coupling on the figure of merit of Nb2SiTe4 and Nb2GeTe4 ternary monolayers

Colossal figure of merit and compelling HER catalytic activity of holey graphyne

Herein, we have conducted a comprehensive study to uncover the thermal transport properties and hydrogen evolution reaction catalytic activity of recently synthesized holey graphyne. Our findings disclose that holey graphyne has a direct bandgap of 1.00 eV using the HSE06 exchange-correlation functional. The absence of imaginary phonon frequencies in the phonon dispersion ensures its dynamic stability. The formation energy of holey graphyne turns out to be - 8.46 eV/atom, comparable to graphene (- 9.22 eV/atom) and h-BN (- 8.80 eV/atom). At 300 K, the Seebeck coefficient is as high as 700 μV/K at a carrier concentration of 1 × 10¹⁰ cm^-2. The predicted room temperature lattice thermal conductivity (κ_l) of 29.3 W/mK is substantially lower than graphene (3000 W/mK) and fourfold smaller than C₃N (128 W/mK). At around 335 nm thickness, the room temperature κ_l suppresses by 25%. The calculated p-type figure of merit (ZT) reaches a maximum of 1.50 at 300 K, higher than that of holey graphene (ZT = 1.13), γ-graphyne (ZT = 0.48), and pristine graphene (ZT = 0.55 × 10^-3). It further scales up to 3.36 at 600 K. Such colossal ZT values make holey graphyne an appealing p-type thermoelectric material. Besides that, holey graphyne is a potential HER catalyst with a low overpotential of 0.20 eV, which further reduces to 0.03 eV at 2% compressive strain.

Anisotropic Infrared Response and Orientation-Dependent Strain-Tuning of the Electronic Structure in Nb2SiTe4

Two-dimensional materials with tunable in-plane anisotropic infrared response promise versatile applications in polarized photodetectors and field-effect transistors. Black phosphorus is a prominent example. However, it suffers from poor ambient stability. Here, we report the strain-tunable anisotropic infrared response of a layered material Nb₂SiTe₄, whose lattice structure is similar to the 2H-phase transition metal dichalcogenides (TMDCs) with three different kinds of building units. Strikingly, some of the strain-tunable optical transitions are crystallographic axis-dependent, even showing an opposite shift when uniaxial strain is applied along two in-plane principal axes. Moreover, G₀W₀-BSE calculations show good agreement with the anisotropic extinction spectra. The optical selection rules are obtained via group theory analysis, and the strain induced unusual shift trends are well explained by the orbital coupling analysis. Our comprehensive study suggests that Nb₂SiTe₄ is a good candidate for tunable polarization-sensitive optoelectronic devices.