Observation of dynamical crater-shaped charge distribution in the space–time imaging of monolayer graphene
A better understanding of charge carrier dynamics in graphene is key to improvement of its electronic performance. Here, we present direct space–time visualization of carrier relaxation and diffusion in monolayer graphene using time-resolved scanning electron microscopy techniques. We observed striking fluence-dependent dynamic images, changing from a Gaussian shape to a novel crater-shaped pattern with increasing laser fluence. Such direct observation of dynamic changes in spatial charge distribution is not readily available from the conventional spectroscopic approaches, which reflect essentially overall effective carrier temperature and density. According to our analysis, for this crater-shaped carrier density to occur in aggregated electron–hole pairs in the high fluence regime there exists an unconventional Auger-assisted carrier recombination process to provide effective relaxation channels, most likely involving emission of optical phonons and plasmons, which is dynamically accessible due to a strong temporal overlap among them. The presented model allows us to successfully account for these unexpected phenomena and to quantitatively analyze the observed spatiotemporal behavior.
Additional Information© 2018 The Royal Society of Chemistry. The article was received on 28 Jan 2018, accepted on 12 Apr 2018 and first published on 13 Apr 2018. This work was supported by the National Science Foundation Grant DMR-0964886 and the Air Force Office of Scientific Research Grant FA9550-11-1-0055 for research performed in The Gordon & Betty Moore Center for Physical Biology at the California Institute of Technology. J. C. acknowledges support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03034395). We thank John S. Baskin for valuable technical support with beam shape characterization and discussions, Ebrahim Najafi and Heng Li for experimental assistance to confirm data reproducibility. We also thank Professor George R. Rossman for access to the micro-Raman equipment. We acknowledge useful discussions with Professors Jisoon Ihm and Young-Woo Son. There are no conflicts to declare.
Supplemental Material - c8nr00789f1_si.pdf