▎ 摘　　要

Using hexagonal boron nitride (hBN) as a substrate for graphene has shown faster carrier cooling which makes it ideal for high-power graphene-based devices. However, the effect of using boron-isotope-enriched hBN has not been explored. Herein, femtosecond pump-probe spectroscopy is utilized to measure and compare the time dynamics of photo-excited carriers in graphene-hBN heterostructures for hBN with the natural distribution of boron isotopes (20% B-10 and 80% B-11) and hBN enriched to 100% B-10 and B-11. The carriers cool down faster for systems with isotopically pure hBN substrates by a factor of approximate to 1.7 times. This difference in relaxation times arises from the interfacial coupling between carriers in graphene and the hBN phonon modes. The results show that the boron isotopic purity of the hBN substrate can help to reduce the hot phonon bottleneck that limits the cooling in graphene devices.