Effects of Vacancy Defects and Dimensionality on the Structure and Electronic Properties of B4C3

Background: Monolayer B4C3 represents a novel functional material with appealing performance for photocatalysis, lithium-ion battery anodes and hydrogen storage applications. Vacancy defects and structural dimensional modulation are effective strategies to tailor the electronic structures and physicochemical performances of semiconductor materials. However, studies of B4C3 in these respects are still inadequate, restricting its further practical utilization. Methods: The effects of vacancy defects and dimensionality (two-dimensional monolayer vs. one-dimensional nanoribbon) on the structure and electronic properties of B4C3 were investigated using density functional theory. Three defective B4C3 models (B monovacancy, C monovacancy, B–C divacancy) were constructed, along with bare and hydrogen-passivated B4C3 nanoribbons. Structural stability and electronic properties were systematically analyzed through formation energy, binding energy, band structure, density of states, charge density difference and Hirshfeld population. Results: Pristine B4C3 is a direct-band-gap semiconductor (1.997 eV). Vacancies induce lattice reconstruction and band gap reduction: C monovacancy decreases the band gap to 1.271 eV, while B and B–C vacancies render B4C3 metallic, and B monovacancy possesses the highest thermodynamic stability. All bare B4C3 nanoribbons (B4C3NRs) exhibit metallicity, whereas hydrogen passivation restores their semiconducting properties. The band gap of passivated B4C3NRs declines with increasing width and stabilizes when the width of the nanoribbon is large enough. Conclusions: The introduction of vacancy defects reduced the band gap, thereby increasing conductivity, while dimensionality reduction induced a semiconductor-to-metal transition. The hydrogen-passivation of the nanoribbon edge atoms caused a transition to semiconducting behavior irrespective of nanoribbon width. With increasing nanoribbon width, the band gap decreased and stabilized at a certain value.