Excitonic recombinations in $h-\mathrm{BN}$: From bulk to exfoliated layers

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, 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 trapped 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 replicas 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.

Figure 1

Images of the bulk material: (a) SEM image of the analyzed crystallite. (b), (c) Corresponding monochromatic CL images recorded at (b) 5.76 eV (215 nm), centered on the main S line emission, and at (c) 5.46 eV (227 nm), centered on the main D line emission. (d) Map of the $D/S$ ratio between the structural defect-related and the intrinsic excitonic recombinations, extracted from a $64\times 48$ CL hyperspectral mapping.

Figure 2

CL spectra of the bulk h-BN materials in the near-band-edge region: At the top is a reference spectrum of a HPHT high-quality crystal (Ref. [34]), compared to #1, registered in the grain boundary area delimited by the red rectangle (averaged over 15 spectra) and to #2 on the main grain of the crystallite in the area delimited by the green square (averaged over 240 spectra), as labeled in Fig. 1. The specimen temperature is about 20 K.

Figure 3

AFM images of exfoliated h-BN layers transferred onto a SiO${}_2$/Si substrate: (a) sample A; (b) sample B; (c) sample C. The image size unit is the micrometer. (d) Profile taken across the part of sheet A that is folded back, along the arrow shown in (a).

Figure 4

Near-band-edge exciton recombinations from exfoliated h-BN layer A (6 ML), B (8 ML), and C (10 ML) compared to the normalized reference spectrum (from the HPHT crystal). The zero of the CL spectra was shifted for clarity.

Figure 5

$D/S$ ratio in the three exfoliated layers, compared with the bulk material, consisting of the crystallites from which the layers were exfoliated.