Excitonic recombinations in hBN: from bulk to exfoliated layers

TL;DR

This study investigates excitonic recombination in hexagonal boron nitride (h-BN), revealing that luminescence persists in exfoliated layers with minimal energy shifts, and links defect presence to recombination intensity.

Contribution

It provides new insights into the excitonic properties of h-BN from bulk to few-layer structures, combining cathodoluminescence analysis with exfoliation techniques.

Findings

Defect presence correlates with recombination intensity.

Luminescence remains strong down to six atomic layers.

Exciton energies are similar in bulk and exfoliated h-BN.

Abstract

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First the link between the presence of structural defects and the recombination intensity of bound-excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replica of exciton recombinations. Second the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations indicating extremely compact excitons in h-BN.