Imaging and dynamics of light atoms and molecules on graphene.

Observing the individual building blocks of matter is one of the primary goals of microscopy. The invention of the scanning tunnelling microscope revolutionized experimental surface science in that atomic-scale features on a solid-state surface could finally be readily imaged. However, scanning tunnelling microscopy has limited applicability due to restrictions in, for example, sample conductivity, cleanliness, and data acquisition rate. An older microscopy technique, that of transmission electron microscopy (TEM), has benefited tremendously in recent years from subtle instrumentation advances, and individual heavy (high-atomic-number) atoms can now be detected by TEM even when embedded within a semiconductor material. But detecting an individual low-atomic-number atom, for example carbon or even hydrogen, is still extremely challenging, if not impossible, via conventional TEM owing to the very low contrast of light elements. Here we demonstrate a means to observe, by conventional TEM, even the smallest atoms and molecules: on a clean single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon can be seen as if they were suspended in free space. We directly image such individual adatoms, along with carbon chains and vacancies, and investigate their dynamics in real time. These techniques open a way to reveal dynamics of more complex chemical reactions or identify the atomic-scale structure of unknown adsorbates. In addition, the study of atomic-scale defects in graphene may provide insights for nanoelectronic applications of this interesting material.