Defect-assisted photoluminescence in hexagonal boron nitride nanosheets

Abstract

The development of functional optoelectronic applications based on hexagonal boron nitride nanosheets (h-BNNs) relies on controlling the structural defects. The fluorescent emission, in particular, has been observed to depend on vacancies and substitutional defects. In the present work, few-layer h-BNNs have been obtained by sonication-assisted liquid-phase exfoliation of their bulk counterpart. The as-prepared samples exhibit a weak fluorescent emission in the visible range, centred around 400 nm. Tailored defects have been introduced by oxidation in air at different temperatures. A significant increase in the fluorescent emission of the oxidated h-BNNs has been observed with maximum emissive intensity for the samples treated at 300 • C. A further increase in temperatures (>300 • C) determines a quenching of the fluorescence.We investigated, by means of detailed microscopic and spectroscopic analysis, the relationship between the optical properties and defects of h-BNNs. The investigation of the optical properties as a function of treatment temperature highlights the critical role of hydroxyl groups created by the oxidation process. Only h-BN exfoliated in water allows introducing OH groups with consequent enhancement of fluorescence emission. Quantum chemical calculations support the experimental findings.

Hydroxylated boron nitride materials: from structures to functional applications

“…The treatment can generate some other defects in HO-BNs. If the temperature reaches 1500°C, the products have to be hydroxylated by the above methods [47][48][49][50][51][52][53] h-BNNSs Sonication-assisted exfoliation It is a facile method to directly exfoliate BN bulk into HO-BNNSs The hydroxyl degree is relatively low For a high hydroxyl degree, some additives have to be introduced, such as, acid, base, salt, etc [31,[55][56][57][58][59][60][61][62][63] Ball-milling-assisted exfoliation Situations similar to ''sonication-assisted exfoliation'' [64][65][66][67][68][69][70][71][72] Hummer's-like routes The use of strong oxidants (i.e., H 2 SO 4 , H 2 O 2 , KMnO 4 , and their mixtures) requires an extra attention Sometimes the oxidization process can lead to the presence of a few by-products of BN dots [73][74][75][76] Multi-step functionalization method It can be divided into two stages: one is from BN to BNNS and then to HO-BNNS, the other is from BN to HO-BN and then to HO-BNNS [82][83][84][85][86][87][88][89][90][91][92] Bottom-up routes It generally requires high temp...…”

“…One-pot sonication-assisted exfoliation h-BNs, hydroxylated nanosheets can be directly prepared by thermal oxidation of BNs in air, typically at 1000°C, followed by treatment in water [54]. Another simple way to obtain HO-BNNSs is to exfoliate h-BNs using water as dispersant solvent by sonication without any additional use of surfactants or organic functionalization [31,[55][56][57][58][59]. Sonication process can provide an intensive longitudinal shear force leading to complete exfoliation of the bulk into h-BNNSs with few layers and reduced lateral sizes, with attached OH groups ( Fig.…”

“…HO-BNNSs are partially oxidized in the hydrolysis process [57] and show strong affinity toward proteins by Lewis acid-base interactions, confirming that surface-organic-free BNNSs have a high potential in biological applications. Very recently, our research group has found that HO-BNNSs possess a weak photoluminescent emission in a visible range, centered at * 400 nm [58]. Tailored defects can be further introduced through thermal treatment in air, and a maximum emissive intensity is achieved at 300°C due to the role of hydroxyl groups created by the oxidation process (see Fig.…”

“…3D PL excitation-emission-intensity spectra of HO-BNNSs after treatment in air with an increasing temperature. Reproduced from Ref [58]. with permission of Copyright (2020) IOP Publishing.…”

Boron vacancy: a strategy to boost the oxygen reduction reaction of hexagonal boron nitride nanosheet in hBN–MoS2 heterostructure

“…In addition, cavitation induces structural defects, surface modification of the basal plane via B-OH bond formation promotes the layer separations from bulk hBN counterpart. 40,41 The OH functionalization weakens the B-N double bonds (due to p-p overlap)…”