Novel bora- and aza-triangulene-based graphyne and graphdiyne: Stable and stretchable narrow-gap semiconductors explored by first principles and machine learning

Following the recent experimental synthesis of the aza-triangulene kagome lattice (NTA-GYN), we present the design and a comprehensive theoretical investigation of a novel family of organic frameworks. Motivated by this breakthrough and the availability of bora-triangulene molecules, we propose the boron-analogous BTA-GYN lattice, alongside two graphdiyne-inspired networks, NTA-GDY and BTA-GDY. Using first-principles density functional theory calculations combined with machine learning interatomic potentials, we next systematically investigate their structural stability, electronic, optical, thermal, and mechanical properties. Hybrid functional (HSE06) results reveal that these frameworks are direct band-gap semiconductors at the Γ point, exhibiting relatively narrow band gaps of 0.57, 0.88, 0.43, and 0.63 eV for NTA-GYN, BTA-GYN, NTA-GDY, and BTA-GDY monolayers, respectively. Notably, the emergence of flat electronic bands near the band edges leads to large effective carrier masses, giving rise to heavy electrons in the nitrogen-based monolayers and heavy holes in their boron-based counterparts. Optical analyses indicate pronounced absorption in infrared and ultraviolet regions, suggesting potential suitability for specialized optoelectronic applications. Furthermore, the studied lattices exhibit intrinsically low lattice thermal conductivities (below 4 W/m·K at 300 K) and robust mechanical performance, with tensile strengths exceeding 18 GPa. Among them, NTA-GDY demonstrates an exceptional balance of high stretchability and mechanical strength. Overall, the synergy of tunable electronic properties, distinct optical signatures, and mechanical resilience positions these triangulene-based graphyne and graphdiyne monolayers as promising candidates for next-generation optoelectronic and energy-related technologies.