This is a guest post by Quanmin Guo, Senior Lecturer in the School of Physics and Astronomy at the University of Birmingham
I was brought to the attention of the debate around stripy nano-particles fairly recently. Since I have been working with alkanethiol monolayers on Au(111) for a number of years, naturally I became interested in the arguments put forward by both sides. Incidentally, I imaged thiol-passivated gold nanoparticles in 1999. I have to say that the quality of our images was poor (A few images can be found on PCCP Vol. 4, 2002), certainly not better than those of Stellacci. The poor quality of the images was partly due to the equipment that we used, being noisier that we would have liked; and partly due to the fact that STM does not cope well with non-flat surfaces. While the debate at the moment focuses on whether stripes have been observed or not on gold nano-particles, I would like to address another relevant issue here first, are we expected to see stripes at all? The answer is yes, if the nanoparticle presents a (111) facet to the STM tip. The figure (Surf. Sci. 2011, Vol. 605, 1016) below shows an STM image from an octanethiol covered Au(111) surface.
In this image you can see a dense phase and a less dense “striped phase”. Stripes within alkanethiol SAMs on Au(111) corresponding to ≤ 80% of saturation coverage are frequently observed with single component thiols. Also in the image, there are three islands. These islands are formed by post-deposition of Au onto the SAM. The Au atoms dive through the SAM and form one atomic layer tall islands. So the newly formed Au islands are “inserted” between the initial gold substrate and the SAM. If we call these islands “nano-particles”, there you are, we have striped nano-particles. The formation of stripes is always linked to a less than saturation coverage. Under saturation coverage, the Au islands are capped by a layer of “close-packed” molecules. The nanoparticles used by Stellacci are not single layer gold islands, they are more 3-D like. If you deposit such nano-particles onto a gold substrate, they may become flatted over time due to atomic diffusion. If the particles assume a plate-like structure, and at the same time allowing some thiolate to migrate from the particle to the flat Au substrate, then stripes may appear on top of the plate.
However, the stripes reported by Stellacci seem to come from a different origin: phase separation of hydrophilic thiol from hydrophobic thiol, for example. It is my personal view that phase separation would occur so each facet is covered by a single type of thiol because the particle is so small. Under non-equilibrium conditions, there might be some random mixing. Along the stripes shown in the above figure, the distance between the dots is 0.5 nm which is also the distance reported by Stellacci. From the STM images of Stellacci and colleagues, one really have great difficulty in seeing any features of stripes. If you examine the STM images in the figure above, you can see the stripy feature are not very regular and between rows there are disordered molecules. Nevertheless, the observation of stripes in this case is beyond any doubt, and there is no need to perform power spectrum analysis.
I am not trying to criticize anybody here, but in their recent Langmuir paper, Stellacci et al included an STM image of thiol covered flat Au surface (Fig. 4). Even for a flat sample, their image showed no molecular resolution. This makes me worried. It is understandable that Stellacci is eager to show some new results to make his initial observation credible. I think he needs to produce better STM images. At this point, I would like to say that the theoretical modeling around the stripy particles is immature. The long-standing view that alkanethiol SAMs consist of thiolate (-SR) directly attached to a Au surface via the hollow, bridge or bridge/hollow site is on the way out. Thiloalte are paired on the surface in the form of Au-adatom-dithiolate (RS-Au-SR). This linkage/ paring of the thiolate has an important influence on the structure of the SAMs.