Tuning the Direct and Indirect Excitonic Transitions of h-BN by Hydrostatic Pressure

Abstract

The pressure dependence of the direct and indirect bandgap transitions of hexagonal boron nitride is investigated using optical reflectance under hydrostatic pressure in an anvil cell with sapphire windows up to 2.5 GPa. Features in the reflectance spectra associated with the absorption at the direct and indirect bandgap transitions are found to downshift with increasing pressure, with pressure coefficients of −26 ± 2 and −36 ± 2 meV GPa –1 , respectively. The GW calculations yield a faster decrease of the direct bandgap with pressure compared to the indirect bandgap. Including the strong excitonic effects through the Bethe–Salpeter equation, the direct excitonic transition is found to have a much lower pressure coefficient than the indirect excitonic transition. This suggests a strong variation of the binding energy of the direct exciton with pressure. The experiments corroborate the theoretical predictions and indicate an enhancement of the indirect nature of the bulk hexagonal boron nitride crystal under hydrostatic pressure.

Excitons under strain: light absorption and emission in strained hexagonal boron nitride

“…In both cases, the G 0 W 0 correction is just a rigid shift of the DFT energies with respect to strain. A similar cell-independent shift was also found in the case of hydrostatic pressure [14]. The different behaviors of the direct/indirect gap originate from the different locations of the points in the Brillouin zone that form these gaps.…”

“…This has been shown for bulk Ge [7,8], and more recently in low dimensional transition metal dichalcogenides [9][10][11]. In the case of h-BN , different experiments were performed to study the effect of an hydrostatic pressure on the structural, vibrational [12,13] and optical properties [14]. The effect of strain on phonons and Grüneisen parameters was investigated experimentally in exfoliated h-BN with various thicknesses [15], while theoretically biaxial tensile strain was considered for the mono-and bilayer h-BN [16,17] as well as its role on hexagonal boron nitride quantum emitters [18].…”

Excitons under strain: light absorption and emission in strained hexagonal boron nitride

“…In both cases, the G 0 W 0 correction is just a rigid shift with respect to strain. A similar cell-independent shift was also found in the case of hydrostatic pressure [14]. The reason for the different behaviors of the direct/indirect gap originates from the different locations of the points in the Brillouin zone that form these gaps.…”

“…the orbitals close to M/M are also affected by the interlayer interaction, while the states at K are protected for symmetry reasons [24]. The interlayer interaction changes with the distance between the planes which varies linearly with compression/stretching of the system, and this effect contributes to the in-plane changes [14].…”

“…This has been shown for bulk Ge [7,8], and more recently in low dimensional transition metal dichalcogenides [9][10][11]. In the case of h-BN , different experiments were performed to study the effect of an hydrostatic pressure on the structural, vibrational [12,13] and optical properties [14]. The effect of strain on phonons and Grüneisen parameters was investigated experimentally in exfoliated h-BN with various thicknesses [15], while theoretically biaxial tensile strain was considered for the mono-and bilayer h-BN [16,17] as well as its role on hexagonal boron nitride quantum emitters [18].…”