Modulation of opto-electronic properties of the functionalized hexagonal boron nitride nanosheets with tunable aryl alkyl ionic liquids (TAAILs): Defect based analysis

Abstract

Abstract The mono-vacant defects based on omission of the one nitrogen or boron (VB or VN) atom could potentially allow for dramatic tuning of the both optical and electronic properties of hexagonal boron nitride nanosheets (h-BN). We compare the physisorption behavior of [MPhIM][BF4], [MPhTAZ1][BF4], [MPhTAZ2][BF4] and [MPhTTAZ][BF4] tunable aryl alkyl ionic liquids (TAAILs) on pristine and defective h-BN surfaces (h-BN, h-BN-VB and h-BN-VN) using density functional theory (DFT) calculations at M06-2X/cc-pVDZ level of theory in gas phase. The presence of such VN and VB defects increases the adsorption strength of the TAAILs on the surfaces abaut1–4 kcal mol−1 due to the lowering of the energy band in the defective surfaces. A more negative electrostatic potential at the B vacancy-site makes h-BN-VB surface more reactive than h-BN-VN surface enabling more interactions with corresponding cations of the TAAILs. [MPhTAZ2][BF4] TAAILs adsorb energetically more favorably on h-BN, h-BN-VB and h-BN-VN surfaces than other TAAILs while [MPhIM][BF4] TAAIL more prefer to adsorb on h-BN-VB and h-BN-VN surfaces than on h-BN surface. Upon adsorption of the TAAILs on the h-BN, h-BN-VB and h-BN-VN surfaces a decrease in LUMO orbital energy level of the corresponding complexes in comparison to the isolated surfaces is observed (in order [MPTTAZ][BF4] > [MPTAZ2][BF4] > [MPTTAZ1][BF4] > [MPIM][BF4] and h-BN > h-BN-VN > h-BN-VB) that it leads to a decrease in HOMO-LUMO gap energy (Eg) values. Results revealed that the electrical conductivity of the h-BN surface increases due to the presence of VB and VN defect sites in the h-BN surface (h-BN-VN > h-BN-VB). Time-dependent DFT (TD-DFT) calculations reveal that the absorption spectra of mono vacant surfaces undergo red shifts upon defect formation (from 186 nm in h-BN to 304 and 296 nm in h-BN-VB and h-BN-VN) and for h-BN-VN surface, a second absorption peak at 421 nm is observed. The absorption peak of the h-BN-VB (h-BN-VN)⋯TAAILs complexes shows that they absorb visible light in the range of 276–455 nm, consistent with the greater interaction energies and lower gap energies of their complexes in comparison to the h-BN⋯TAAILs complexes. Generally, it is concluded that the modulation of optoelectronic properties of TAAIL-functionalized h-BN surfaces is largely provided by the defects and to a lesser extent by the choice of TAAIL.