SPM moves forward: Imaging the charge in a single-molecule.

In experimental science, and I would imagine in other fields, there are often very clear trailblazers. The guys who seem to sneeze out high-impact publications that push the boundaries of what is possible, and usually with data so beautiful you could weep. The kind of guys who; after you have devised an amazing experiment and just begin to get the resources together for it, publish the same idea the next week with the sort of ease that makes you suspect they had just ‘knocked it out’ on a lazy Sunday afternoon.

In the field of Scanning Probe Microscopy (SPM) one such trailblazer is Leo Gross.

Scanning Probe Microscopy is the name given to a group of scientific instruments that can allow us to actually see the atoms on a material’s surface.

I’d like you to take a moment to consider that. We can actually make devices that allow us to see atoms. Until the beginning of the 20th century the very existence of atoms was doubted by the majority of the scientific community at the time. In the 1980’s, the first microscopes capable of seeing these elusive building blocks were invented. Now, thanks to Leo Gross, the technology moves forward again.

These microscopes do not work in the way expected by most people when you say the word “microscope”. It does not use light and there are no lenses. A common analogue to introduce people to the field is to consider an old-fashioned record player. Like the record player, a scanning probe microscope slides a sharp tip across the sample to “feel” the atoms and molecules upon it. I write “feel” in quotation marks as the actual mechanism of detecting the atoms differs between instruments and the exact needs of the experiment.

The two big boys of the STM world are the Scanning Tunnelling Microscope (STM) and the Atomic Force Microscope (AFM). Those interested should follow the links for the full details on how these devices work. For some time there was a little bit of smugness on the side of STM users as, their instrument was the first to allow atomic resolution and continued to do so reliably as a matter of course, whereas the AFM had to be worked at from its invention to yield its first pictures of atoms.

In 2009, Leo Gross spun that around, by taking picture of molecules that actually look like the diagrams you would draw in Chemistry class.  The details of this can be found in this IBM press release from the time and this Science paper. The key to this coup for AFM is that STM is, by its nature, an electrical measurement, a very-sensitive and glorified current detector. As such its ultimate resolution is limited by the electron states surrounding the molecule like a cloud. AFM does not have this limit, as its name suggests it works by “feeling” the various forcefields that surround the atoms in the molecules as so show them as they “are”. This difference is shown below in the image from the Science paper. A) Shows the chemical model of the pentacene molecule B) Shows the STM image and C) Shows the AFM image of the same molecule. Note the amazing similarity between A and C and the comparative “Fuzziness” of B.

Not content to leave it there:  Leo Gross’ group at IBM has been continuing to push the envelope as to what the AFM can do, now they have applied their uncanny ability to take amazing, high-resolution images to an AFM spin-off: Kelvin Probe Force Microscopy (KPFM). With this they have imaged the distribution of charges within a single-molecule. The story can be read for the public at the BBC and the full article is published in Nature Nano article for those with access

Kelvin-Probe uses the fact that when two metals are brought together a voltage/contact potential forms between them, due to the difference is workfunctions (how easy it is to pull the electrons out) of the metals. In KPFM, this voltage is formed between the sample and the tip of the probe which responds to the localised change. A feedback circuit is then used to apply its own voltage between tip and sample, countering the effect. The instrument’s record of how much voltage is used to cancel out the contact potential is recorded with sub-nanometer resolution and used to form an image.

To test the technique, Gross’ group studied a molecule called napthalocyanine, which has been shown to behave as a molecular switch. The molecule is shaped like a + sign, with four distinct arms and hydrogen atoms can be switched from one set of arms to another in a controlled way. In one state, the KPFM images show charge concentrated on two opposing arm (call them, North and South). After the switch is ‘pressed’ the images now show the charge concentrated on the other two arms ( East and West) having rotated by 90 degree around the molecule. The observed images matched very well with the distribution of charges predicted by theory (DFT calculations).

Still not satisfied, they went further by improving the resolution of their images by refining their AFM probe by picking up a single carbon monoxide molecule on the end of their tip to use as a much smaller, more refine and more precise probe in their experiment.

Long-blog post short, they can now see where the charge is gathered within on napthalocynanine molecule, which parts hold the positive charges and which the negative.  This knowledge is of great interest to scientist working on nanoelectronics devices. In such materials, especially those made from organic molecules, their function depends on the  separation of charge and the dynamics its storage and transport. In particular, these processes are critical to the operation of solar cells and their natural analogue the photosynthetic proteins in some plants and bacteria. The ability to actually see these effects is a major step-forward in both the development of SPM and future electronics.

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