Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

Abstract

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solidstate 10 BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched singlecrystal h-BN. Phonon frequency and lifetime are measured in the 80-600-K temperature range for 10 B-enriched, 11 B-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10 B-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

Influence of isotopic substitution on the anharmonicity of the interlayer shear mode of h -BN

“…16 Calculation details and the underlying theory used to evaluate the thermal expansion, anharmonicity, and isotopic disorder effects are given in Ref. 13.…”

“…12 Phonon studies on isotopic dependence are even more scarce. In a recent publication, 13 we analyzed the isotopic effects on the phonon anharmonic decay of the high-energy Ramanactive mode of h-BN. Changes in the main decay pathways were established as a result of the sizable isotopic shift of the upper optical phonon branches.…”

“…Changes in the main decay pathways were established as a result of the sizable isotopic shift of the upper optical phonon branches. 13 Since the isotopic shifts are very small for the low lying branches, such dramatic effects are not expected in the low-energy Raman active mode. Nevertheless, the interlayer shear mode is particularly sensitive to the van der Waals forces and may be influenced by subtle isotopic effects.…”

“…9 The predominant role of the 4-phonon scattering mechanism of quartic anharmonicity in layered compounds has been well established for the high-frequency mode both in graphite 14,15 and h-BN. 13 However, since phonon anharmonic decay and isotopic disorder have also a significant influence on the high-frequency mode, it is difficult to single out the effects of isotopic substitution on this anharmonic mechanism from the analysis of the experimental temperature dependence data. In contrast, both the phonon anharmonic decay and isotopic disorder effects are negligibly small for the E low 2g mode, which makes it possible to directly relate the variations in the temperature dependence of the E low 2g frequency with the isotopic mass to variations in the 4-phonon scattering contribution.…”

Rotationally aligned hexagonal boron nitride on sapphire by high-temperature molecular beam epitaxy

“…It has also recently been shown to host point defects that function as bright, room temperature single photon sources, which can enable technologies such as quantum cryptography and precision sensing [14,15]. On top of these potential applications, the strong electron-phonon coupling and hyperbolic dispersion of hBN make it an exciting platform to study rich new physical phenomena such as phonon-polaritons and the effects of isotopes [12,[16][17][18][19][20].…”