The structure and formation of hydrogen-bonded molecular networks on Au(111) surfaces revealed by scanning tunnelling and torsional-tapping atomic force microscopy

摘要

The comment on our recent paper has raised a number of interesting issues to which we would like to provide a response. First, we would like to reiterate that the main focus of our article was to analyse the PTCDI-melamine network using two complementary scanning probe techniques; namely scanning tunnelling microscopy (STM) (as employed in the work of Madueno et al) and atomic force microscopy (AFM). To date, publications on this molecular system have relied on STM, either in UHV or ambient environments, as the only microscopy method. Although, on this system, STM typically delivers superior lateral resolution compared to AFM, its reliance on the formation of a tunnelling current across a nanoscale gap to an underlying conducting substrate means that it provides little insight into the structure of thicker insulating films formed, for example, by the presence of multilayers, and is inherently limited in this respect It has been known, for example, since the late 80's that STM will penetrate multilayers of insulating liquid crystal material to image an ordered interface on graphite, which hence can appear as an ordered monolayer at that surface. In these cases, the STM travelled through layers of material until it reached a pre-set current value between the tip and conducting substrate. In contrast, AFM has the potential to interrogate the top surface of thicker films as no such current is required, only a force interaction between the probe and the analysed surface. A previous study from our laboratory on a similar network to that of PTCDI-melamine formed by trimesic acid on HOPG surface clearly demonstrated this point AFM showed the presence of 4 layers, whereas STM only observed one. In conclusion, we believe that the use of STM alone makes the unequivocal assignment of monolayer film formation problematic.