NLO-Based Nitrobenzene Sensing Using Defective BN Nanosheets: A DFT Study

Abstract

This is a study to survey the structural, electronic, and nonlinear optical (NLO) characteristics of pristine and defective boron nitride (BN) nanosheets, along with their adsorption tendencies and interactions with nitrobenzene (NB). Employing density functional theory (DFT) and time-dependent DFT (TD-DFT), the effects of carbon substitution (CB, CN, CBN) and vacancy defects (VB, VN) on the energy gap, charge distribution, and optical characteristics have been examined. Defect engineering has been demonstrated to greatly decrease the HOMO-LUMO gap, improve charge polarization, and cause a notable red shift in optical absorption. At the same time, VN nanosheets broaden photo-response into the visible spectrum. NBO analysis indicates that the adsorption of NB is influenced by different mechanisms: robust dative n_O→n_B^* bonding in CB-NB aligns with the maximum adsorption energy, whereas notable stabilization in CBN-NB and VN-NB stems from key non-covalent interactions, highlighted by the difference between elevated adsorption energy and minimal charge-transfer energy. The complexation with NB significantly boosts the first and second hyperpolarizabilities (β, γ) of certain complexes, especially VN-NB, which shows an increase exceeding 6000% in β value. This study demonstrates that precise defect engineering not only adjusts the electronic and optical characteristics of BN nanosheets but also significantly changes the nature of molecular adsorption, providing a systematic design approach for enhanced optoelectronic and sensing materials.