Isotopic Effect of Carrier Relaxation in Graphene-hBN Heterostructures*

In atomically thin systems, the choice of substrate plays an important role in the relaxation of photo-excited carriers. In previous work, hexagonal boron nitride (hBN) substrates have been shown to improve the thermal relaxation rates of carriers in graphene as compared to silicon oxide substrates. Naturally occurring boron contains a mixture of two isotopes with atomic masses 10 and 11 with abundances of 20% and 80% respectively. Theoretical studies have predicted a higher thermal conductivity with higher isotopic purity of hBN, due to reduced phonon scattering from isotopic defects. We utilize femtosecond pump-probe spectroscopy to observe the time dynamics of photo-excited carriers in graphene-hBN heterostructures for both natural and isotopically pure hBN.