Delving into the bandgap tuning and nonlinear optical properties of hydrothermally synthesized pristine and boron doped molybdenum trioxide nanorods†

Abstract

Nanostructures undergo significant changes in their electronic structure due to defects and disorders, which affect their electronic, structural, nonlinear and linear optical properties. This work highlights defect engineering as an effective tailoring approach to tune the structural and nonlinear and linear optical characteristics of boron-doped MoO₃ (B–MoO₃) nanorods. The degree of boron (B) doping in the MoO₃ lattice influences the bandgap and defect-tunable luminescence through the introduction of intermediate defect states. The pristine and doped MoO₃ were synthesized using the hydrothermal method. Structural and morphological characterization techniques like XRD, Raman spectroscopy and FESEM were utilized to confirm the synthesis of the samples. The XPS analysis shows that defects like Mo interstitials, Mo vacancies and oxygen vacancies cause bandgap narrowing and PL intensity quenching. Optical properties were studied using UV-Vis absorption spectroscopy, revealing the inverse relationship between bandgap and the Urbach energy. Additionally, it was found that the phonon lifetime of the samples decreases with increasing doping concentration of B. The nonlinear optical investigation revealed that the third-order nonlinear optical parameters like saturation intensity (I_s), nonlinear absorption coefficient (β) and optical limiting threshold (OLT) were improved with an increase in doping concentration and can be correlated with the impact of point defects that develop in the material. Specifically, the β value increased from 2.74 × 10⁻¹⁰ to 5.19 × 10⁻¹⁰ m W⁻¹, I_s decreased from 3.81 × 10¹² to 9.6 × 10¹¹ W m⁻², and the OLT value decreased from 3.12 × 10¹³ to 1.45 × 10¹³ W m⁻² with the increase in doping concentration.