The beauty of ambient STM
So I actually started my scanning probe microscopy career with ambient scanning tunneling microscopy (STM) in the basement of Columbia University’s Havemeyer Hall in George Flynn’s lab. We were studying long chain hydrocarbon self-assembled monolayers at the phenyloctane-graphite interface, and would see amazing structures and resolution as in this image of a self-assembled monolayer of hexadecanoic acid below (those dark red filled in circular groups are the hydrogen bonded carboxylic acid groups):
STM image of Hexadecanoic acid
Collecting STM images involved a lot of patience, as well as a hat and sweater as we kept the lab at 60⁰F to try and promote precipitation of the molecules on the surface. To this day, STM imaging of atomic resolution on a graphite surface with practically no effort brings the same thrills as those long cold days in graduate school. Although STM produced amazing images (and also was the pioneering mode in the scanning probe microscopy family!), it was pretty limited in what it could image since it needs a conducting or semiconducting surface. At ambient conditions exposed to oxygen, not a whole lot fits that bill.
Going into industrial research, I quickly segued into atomic force microscopy (AFM) which is far more versatile and can handle a much wider range of materials in ambient conditions. AFM doesn’t care about the electronic properties of the material.
And so most of STM research remained in the purview of ultra-high vacuum scientists, who in this environment could clean a surface and keep it pristine. Binnig, Rohrer, Gerber, and Weibel used UHV STM to successfully image the silicon 7 x 7 surface in 1983 (!), a canonical sample for surface scientists, and published their results in Physical Review Letters.
Silicon 7 x 7 image, using STM, c. 1983
So I was very happy to see ambient STM images making a splash recently with some low current STM images from Oxford Instruments Asylum Research. How low? Well, typical STM currents for ambient liquid-based measurements were in the hundreds of picoAmps coupled with a bias voltage of hundreds of mV to ~1V. These measurements are now being down with a few hundred femtoAmps of current! The results are pretty – see below images of metalloporphyrin crystals on an HOPG surface, in this case, nickel octaethylporphyrin where the porphyrin ring is successfully imaged. Full reference to these images can be found here.
Nickel Porphyrin images
Keep in mind that another challenge – or benefit – of STM is that it is not actually measuring topography, but rather measuring tunneling current, which depends on the electronic structure of the surface. Thus the source of contrast is due to the local density of states at the Fermi level of the sample surface at the position probed by the tip. This can make interpretation of STM images more difficult, especially if you are after topography. On the other hand, you are getting a lot more information than just topography!
STM will remain a critical tool to physical and materials scientists as its information and resolution are valuable and hard to attain, if not impossible, with other methods. Ambient STM will also remain somewhat limited because of the constraint of a conducting or semiconducting surface, which is hard to find in the presence of oxygen. UHV STM is of course much more flexible then in terms of samples, but having to work in UHV requires yet another level of expertise, patience, and cost. But certainly if you have a system that cooperates with ambient STM’s constraints, the results are worthwhile and breathtaking!
Dalia Yablon, PhD