The best of both worlds – A novel approach to macromolecular STM studies

Abstract

The advent of scanning tunnelling microscopy (STM) and its younger sibling atomic force microscopy (AFM) have fundamentally changed the way we investigate phenomena on the molecular scale. Direct visualisation is routine and atomic and molecular level imaging is an expectation of most researchers, if not their supervisors. It might therefore be surprising to learn that whole classes of systems cannot be readily visualised using either technique. AFM is in principle the more versatile technique and ultrahigh vacuum (UHV) studies have demonstrated atomic resolution on surfaces [1] and more recently an ability to distinguish between different surface atoms [2]. However, without significant advances in force sensor technology [3] the prospect for routine molecular (not to mention sub-molecular) imaging is poor. STM, the elder sibling, is itself not without limitations. Apart from the obvious requirement of conducting substrates, it is also important to minimise resolution loss due to unwanted mixing between molecular and substrate electronic levels. In the quest to image large molecular and polymer systems using STM most researchers are faced with deciding between an ambient or UHV approach, including the oftentimes difficult task of developing appropriate sample handling and preparation procedures. Ambient STM studies typically employ gold or graphite substrates and for which samples are typically deposited directly from solution. This approach works well for a broad range of soluble molecular and polymer systems but imaging is often unstable and contamination and artefacts can be problematic [4]. The alternative UHV approach offers more stable tunnelling conditions, together with an atomically clean substrate lattice that provides an in-situ reference for structure analysis. The latter approach, however, is not amenable to systems with ultra-low vapour pressures or which decompose or denature at elevated preparation temperatures. On page XXX of this issue Laracuente and co-workers [5] describe an innovative approach to this problem that combines the best of UHV STM imaging with solution-based sample processing. Starting from a UHV prepared Si(100) surface, the authors formed a passivated H-termination by exposure to atomic hydrogen [6]. H-terminated Si(100) surfaces are stable, exceptionally well ordered and are routinely imaged with atomic resolution [7]. Transferring the H-terminated Si(100) substrate to a nitrogen-purged load-lock, samples were then drop-cast from solution onto the surface, and quickly transferred back to the STM for analysis. CDCl3 was found to be the most suitable solvent for their particular system (a pentiptycene-based polymer) and was chosen following a careful study of the contamination and oxidation levels on the H-terminated Si(100) surface after treatment with different solvents [5]. The beauty of this approach is that it overcomes the vexing problem of sample deposition by eliminating the need for Knudsen cells or even more sophisticated pulsed valve delivery systems, while at the same time provides the full benefits of