Imaging of isotope diffusion using atomic-scale vibrational spectroscopy

The spatial resolutions of even the most sensitive isotope analysis techniques based on light or ion probes are limited to a few hundred nanometres. Although vibrational spectroscopy using electron probes has achieved higher spatial resolution^1-3, the detection of isotopes at the atomic level⁴ has been challenging so far. Here we show the unambiguous isotopic imaging of ¹²C carbon atoms embedded in ¹³C graphene and the monitoring of their self-diffusion via atomic-level vibrational spectroscopy. We first grow a domain of ¹²C carbon atoms in a pre-existing crack of ¹³C graphene, which is then annealed at 600 degrees Celsius for several hours. Using scanning transmission electron microscopy-electron energy loss spectroscopy, we obtain an isotope map that confirms the segregation of ¹²C atoms that diffused rapidly. The map also indicates that the graphene layer becomes isotopically homogeneous over 100-nanometre regions after 2 hours. Our results demonstrate the high mobility of carbon atoms during growth and annealing via self-diffusion. This imaging technique can provide a fundamental methodology for nanoisotope engineering and monitoring, which will aid in the creation of isotope labels and tracing at the nanoscale.