Room temperature exciton formation and robust optical properties of CVD-grown ultrathin BiOSe crystals on arbitrary substrates.

The appealing success of non-van der Waals (non-VdW) two-dimensional (2D) bismuth oxyselenide (BiOSe) crystals in optoelectronics provides an exciting avenue to investigate their fundamental physical properties. To date, the majority of efforts have focused on understanding the properties of 2D BiOSe, usually grown on a mica substrate. However, a gap exists in realizing the origin of photoluminescence (PL) of new age non-VdW BiOSe at visible and near-infrared (NIR) wavelengths and the effect of growth substrates on the structure and optical properties. Herein, we report that the formation of multiple excitons in momentum valleys is responsible for broadband absorption and visible PL from a few layer thick 2D BiOSe. The effect of growth substrates on the structure and optical properties is investigated in detail. Our studies unfold that the growth substrates (mica, sapphire, quartz, SiO, glass) introduce strain/doping in chemical vapor deposition (CVD)-grown BiOSe crystals, and consequently, the morphology, lattice constant, absorption coefficient, optical bandgap, refractive index, and PL properties are modulated. In addition, the possible direct/indirect multiple exciton formation at the valence band to the conduction band at different symmetry points of BiOSe is analyzed from experimental data on different growth substrates and corroborated with the density functional theory (DFT) calculation of the electronic band structure. Furthermore, temperature-dependent photo-carrier dynamics discloses an A/Γ-exciton activation energy of 209.6 meV in BiOSe. These findings are significant for the futuristic optoelectronic applications of BiOSe and the choice of growth substrates on directly fabricated nanodevices.