3, Haldor Topsoe A/S, Lyngby, , Denmark
1, SuperSTEM Laboratory, Warrington, , United Kingdom
The introduction and tremendous advances in recent years in the capabilities of dedicated holders for testing materials in situ under a variety of external stimuli has enabled a wide range of ground-breaking studies. Nevertheless, however advanced this new instrumentation may be, the fundamental differences in the electron microscopy operating conditions between in situ conditions and those typically applied to ultra-high resolution (spatial and energy) work mean that it is still difficult to reconcile both types of information in a single experiment. Here a multi-length-scales correlative microscopy approach is used to combine high resolution scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) spectrum imaging and in situ testing on the same nano-objects. In particular, we demonstrate how the addition or removal of native semiconductor material at the edge of a nanocontact can be used to determine the electrical transport properties of metal−nanowire interfaces. While the transport properties of as-grown Au nanocatalyst contacts to semiconductor nanowires are well-studied, there are few techniques that have been explored to modify their electrical behaviour. The approach taken here used an iterative analytical process that directly correlates multiprobe in situ transport measurements with subsequent aberration-corrected scanning transmission electron microscopy on the same nano-objects to study the effects of chemical processes that create structural changes at the contact interface edge. A strong metal−support interaction that encapsulates the Au nanocontacts over time, adding ZnO material to the edge region, gives rise to ohmic transport behaviour due to the enhanced quantum mechanical tunnelling path. Removal of the extraneous material at the Au−nanowire interface eliminates the edge-tunnelling path, producing a range of transport behaviour that is dependent on the final interface quality. These results demonstrate chemically driven processes that can be factored into nanowire-device design to select the final properties. Another application of this approach combines atomic-resolution TEM with EELS to examine the surface structure and oxidation state of VOx, supported on anatase TiO2 nanoparticles, in situ, under alternating oxidizing and reducing environments. A two-step approach where chemical mapping by means of STEM-EELS spectrum imaging was used to address the homogeneity of the VOx distribution, while in situ TEM imaging was used to monitor the structural response to changes in the surrounding gas environment, reveals reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions.