Anomalous Raman signals in multilayer hexagonal boron nitride grown by chemical vapour deposition on metal foil catalysts.

Hexagonal boron nitride (hBN), a two-dimensional (2D) wide bandgap material, serves as an ideal insulating substrate and a protection layer for other 2D materials, such as graphene and transition metal dichalcogenides (TMDs). Here, we report for the first time the emergence of an anomalous Raman peak in single-crystal, multilayer pyramidal hBN grains grown on Fe-Ni alloy foil by chemical vapour deposition (CVD). This peak is located near the characteristic E_2g band (1367 cm^-1) and shifts to higher wavenumbers with the increasing number of hBN layers, peaking at ∼1415 cm^-1 at the centre of hBN grains. The appearance of this Raman peak is attributed to a blue shift of the E_2g phonon caused by compressive strain induced during the cooling step in the CVD process. Triangular hBN grains are epitaxially grown on the alloy catalyst and hence are strongly affected by the volume change of the Fe-Ni alloy catalyst and by lateral compression induced by the steps of the Fe-Ni surface. The maximum strain calculated from the peak shift is -1.23%, which is much higher than the values previously reported for strained hBN, indicating a strong impact of the metal catalyst on the growing hBN structure. These results demonstrate the feasibility of strain engineering in hBN via CVD growth.