#IRON ATOMIC NUMBER FREE#

In this paper, we seek to resolve with atomic resolution the three-dimensional shape, atomic arrangement and structural transformation of ultrafine Fe clusters on graphene supports by using monochromated and aberration-corrected low-voltage TEM. Until now, there is no clear-cut TEM observation showing direct evidence of the atomic structure of ultrafine clusters. Although high resolution TEM imaging benefits from the high-energy electron source and the uniform and low background of amorphous substrates, the high transferred energy and the unknown surface structure of the support significantly affect the behavior of ultrafine clusters and introduce extra complexity in resolving atomic structures under electron irradiation. Some argue that it occurs through a melting-recrystallization process, whereas others suggest that it is through solid-solid transformation 1, 3, 7, 8, 9, 10, 11. The consequential morphology change has long been the subject of debate. Heat absorption from inelastic electron-nucleus scattering leads to structural instability and fluctuations via atomic rearrangement. The consequences of these factors are further exaggerated by the excitation from the high-energy (beyond 200вЂ…keV) incident electron beam. Besides, the elastic strain induced through the contact also causes structural modifications to the small energy barriers in various configurations 5, 6, 7, 8, 9, 10.

#IRON ATOMIC NUMBER FREE#

Because of the active nature of low-coordinated particles 14, 15, the free energy may be overwhelmed by the chemical bonding to the support during TEM imaging, which leads to substantial changes in the atomic arrangement 2, 5, 8, 9, 12, 16. However, unambiguous determination of the three-dimensional (3D) atomic structure of ultrafine clusters remains a challenge. Clusters with diameters larger than 3вЂ…nm have been extensively studied by atomic resolution transmission electron microscopy (TEM) over the past few decades 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The starting point to explore these properties is to unravel the atomic structures of the ultrafine clusters. Ultrafine clusters, containing less than a few hundred atoms (diameters of 1вЂ“3вЂ…nm), have size-dependent effects in catalytic processes and electronic structures 1, 2.

The techniques introduced here will be of use in investigations of other clusters or even single atoms or molecules. However, preferential carving, as observed by other research groups, can be realized only when Fe clusters are embedded in graphene.

No interaction was observed between Fe atoms or clusters and pristine graphene. Our observations differ from observations from earlier experimental study and theoretical model, such as icosahedron, decahedron or cuboctahedron. These clusters prefer to take regular planar shapes with morphology changes by local atomic shuffling, as suggested by the early hypothesis of solid-solid transformation. Here, we describe the stable close-packed structure of ultrafine Fe clusters for the first time, thanks to the superior properties of graphene, including the monolayer thickness, chemical inertness, mechanical strength, electrical and thermal conductivity. Unraveling the atomic structures of ultrafine iron clusters is critical to understanding their size-dependent catalytic effects and electronic properties.

Iron atomic number