Microscope Technique Promises Stunningly High-Resolution Look at Molecules

Scanning tunneling microscopy, or STM, already allows us to see the boundaries of small organic molecules with remarkable precision: The molecule in the top picture above, perylene tetracarboxylic dianhydride, consists of just 26 carbon atoms, eight hydrogen atoms and six oxygen atoms. But an emerging technique for microscope imaging, so-called scanning tunneling hydrogen microscopy (STHM), allows us to see molecules at even greater resolution, such that their internal bonds between atoms are revealed, as you can see in the middle picture above.

Scientists first published pictures of STHM-imaged molecules in 2008, but they didn’t fully understand where they came from. Now, a recently published paper explains how the process works:

A scanning tunneling microscope works by bringing a conducting tip within a nanometer — one billionth of a meter — of a molecule being observed and applying voltage, which creates a vacuum through which electrons flow, allowing scientists to gather information about the molecular surface. (The Wikipedia article on STMs explains in greater detail how this all works.) Scientists at Germany’s Jülich Research Center discovered previously that coating the tip of an STM with hydrogen would allow them to see molecules’ inner structure; part of what makes this technique attractive for researchers is that it doesn’t require expensive new equipment but can be done with existing STMs.

But how does this work? The Jülich team’s latest set of experiments, which entailed replacing the hydrogen with heavy hydrogen, or deuterium, suggests that the key is the Pauli exclusion principle, which states that two electrons cannot occupy the same quantum state at the same time.

Physical Review Focus:

Normally the conductance through an STM junction increases exponentially as the tip approaches the surface. In the team’s experiment, however, the conductance grew relatively slowly and sometimes decreased as the tip approached the surface, over a range of about an ångström. The measurements from this range yielded the high-resolution images.

The researchers explain these effects in terms of the repulsion between electrons associated with the Pauli exclusion principle, which says that two electrons can’t be in the same state and location at the same time. As the tip approaches, the electrons in the deuterium molecule are repelled from the surface, and their orbitals begin to overlap with those of the tip. So the electronic states in the tip rearrange, which changes the conductance through it.

Temirov explains that conventional STM can only probe electronic states in the sample that are within a limited range of energies, because the energy of the detected states depends on the applied voltage. With STHM the conductance also depends on the repulsion generated by Pauli exclusion, which affects states of all energies, so it gives a more complete picture.

As useful as bond-level molecular imaging may prove to be, there may be even higher resolution imaging coming further down the line; according to Azonano, the Jülich team is “planning to calibrate the measured current intensity as well,” which would allow scientists to directly determine the types of individual atoms within molecules.

(Physical Review Focus via Evil Mad Links; Azonano)