Tuning Ultrafast Charge Carrier Dynamics of Monolayer Graphene using Substrates

As monolayer graphene comprises only one single atomic layer, its physical and chemical properties could be easily tuned by the underlying substrate. However, it is still unknown how the distinct substrate influences the charge carrier dynamics of graphene, which is crucial for its optoelectronic and electrical properties. In this work, we investigated the charge carrier dynamics in monolayer graphene supported by hexagonal boron nitride (hBN), octadecyltrichlorosilane self-assembled monolayers (OTS SAMs), and glass using transient absorption microscopy. It is found that the charge carrier relaxation was tuned by these three substrates. The charge carriers decayed fastest on hBN, slowest on glass, and the decay rate was somewhere in between that on hBN and glass on OTS SAMs. The two-exponential fitting shows that this substrate-dependent charge carrier decay dynamics originated from the distinct hot phonon lifetime. Smaller electron-hole charge fluctuations, lower p-doping level, and larger thermal conductance are helpful for cooling the hot phonons and hence accelerating the charge carrier relaxation in monolayer graphene on hBN. These findings provide ideas for developing novel substrates to improve the performance of graphene-based electronic devices.