Abstract
A major challenge in molecular investigations at surfaces has been to image individual molecules, and the assemblies they form, with single-bond resolution. Scanning probe microscopy, with its exceptionally high resolution, is ideally suited to this goal. With the introduction of methods exploiting molecularly-terminated tips, where the apex of the probe is, for example, terminated with a single CO, Xe or H2 molecule, scanning probe methods can now achieve higher resolution than ever before. In this review, some of the landmark results related to attaining intramolecular resolution with non-contact atomic force microscopy (NC-AFM) are summarised before focussing on recent reports probing molecular assemblies where apparent intermolecular features have been observed. Several groups have now highlighted the critical role that flexure in the tip-sample junction plays in producing the exceptionally sharp images of both intra- and apparent inter-molecular structure. In the latter case, the features have been identified as imaging artefacts, rather than real intermolecular bonds. This review discusses the potential for NC-AFM to provide exceptional resolution of supramolecular assemblies stabilised via a variety of intermolecular forces and highlights the potential challenges and pitfalls involved in interpreting bonding interactions.
Highlights
The ability to see a single atom, for many, was considered something of an impossible dream, with the smallest units of stable matter, that is, a unit still retaining identifiable chemical properties, remaining an abstract notion
That was until the invention of the field ion microscope (FIM) [1,2] and later the scanning tunnelling microscope (STM) [3,4,5], whose exceptional spatial resolution lets us routinely visualise the atomic nature of materials in real space, allowing us to directly see single atoms
As the number of methods, substrate materials and molecular tip terminations available to achieve submolecular resolution increases, and ever more research groups overcome the technical difficulties associated with its implementation, adoption of the technique is reaching a tipping point, where we are seeing the beginnings of a huge expansion of submolecular non-contact atomic force microscope (NC-AFM) that will provide unprecedented insights into a vast array of molecular and supramolecular systems
Summary
The ability to see a single atom, for many, was considered something of an impossible dream, with the smallest units of stable matter, that is, a unit still retaining identifiable chemical properties, remaining an abstract notion. Similar to the breakthrough that STM provided for imaging single atoms of crystal surfaces, NC-AFM (and the scanning tunnelling hydrogen microscopy (STHM) [14] variant of STM, as discussed later) can directly provide beautiful real space images of the atomic structure of individual molecules, revealing a vivid appearance sharing an amazing similarity to school textbook ball-and-stick drawings. This unparalleled capability has led to an explosion of interest, fettered only by the difficulty in instrument operation and in achieving the highly-controlled environments required. The review proceeds to discuss recent exciting work investigating assemblies of molecules stabilised via intermolecular forces, with a particular focus on hydrogen bonding, where the question is posed: can NC-AFM resolve and uniquely identify single intermolecular bonds? The many potential challenges in answering these questions are discussed within the context of a larger number of supramolecular structures stabilised through a variety of intermolecular interactions
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