Imaging atoms on 2D atomic crystals in liquids

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IBM scientist Peter Nirmalraj

Peter Nirmalraj in­vest­i­gates the prop­er­ties of 2D lay­ered ma­ter­ials using a C60 func­tion­a­lized met­al STM probe in the noise-free labs at the Binnig and Rohrer Nano­tech­no­logy Cen­ter. (Source: Marcel Begert, IBM Research–Zurich)

More than 35 years after IBM Nobel laureates Gerd Binnig and Heinrich Rohrer invented the scanning tunneling microscope (STM), IBM scientists in Zurich have achieved another breakthrough in the field of atom-by-atom imaging and metrology. But this time it’s in liquids.

In collaboration with scientists from the University of Limerick, École Polytechnique Fédérale de Lausanne, and the University of Massachusetts–Amherst, IBM scientists Peter Nirmalraj, Bernd Gotsmann, and Heike Riel have engineered and successfully demonstrated the operation of a robust molecular STM probe — in a liquid ecosystem at room temperature to analyze emerging 2D layered materials.

This work marks a first-time technical achievement. Previously, imaging of low-dimensional materials such as organic molecules and 2D materials using a molecular STM probe with spectacular spatial resolution was achieved under ultrahigh vacuum (UHV), often at cryogenic conditions.

The paper entitled “A robust molecular probe for Ångstrom-scale analytics in liquids,” which appeared today in Nature Communications, outlines the research and its findings.

I spoke with Peter, who specializes in molecular surface science and scanning probe microscopy in liquids, to learn more about his research.

Which element of the probe’s design enabled the high resolution you achieved in your paper?

Peter Nirmalraj: We went from using a non-functionalized gold STM probe to chemically terminating the apex of the STM probe with a single carbon-60 (C60) molecule, which reduces the reactivity of the probe apex and can enhance the spatial information content of the material under investigation. Until now, this level of control and scope in information atom-by-atom on 2D materials was not trivial to achieve in liquids at room temperature.

What is particular to imaging under standard laboratory conditions, as opposed to the common UHV and cryogenic conditions?

PN: The main challenge lies in the stability of the single molecule at the apex of the probe’s tip. Imagine an inverted mountain and place a cherry at its peak — that is the scale we are concerned with. Under cryogenic conditions, the contact is much more stable because it has minimal fluctuations, but at room temperature, the molecule is energetically and dynamically active. This tends to result in an unstable molecular STM probe complex. Here, we show that a delicate probe at room temperature can be stabilized in high-density liquids, which are capable of minimizing the motion of the molecule anchored around the STM metal probe apex.

From left to right: Fullerene-terminated gold STM probe. A single-atomic hexagon of monolayer graphene showing the carbon atomic sites within graphene lattice. Atomic structure of 2D molybdenum disulfide, where the atomic species can be selectively analyzed.


The World Economic Forum named 2D materials one of the top 10 emerging technologies of 2016. In this context, what is the significance of the high resolution with which you can conduct atom-by-atom imaging of 2D materials in liquids?

PN: A better understanding of the properties of 2D materials mined under practical conditions will become decisive if robust devices based on such exciting materials are to be realized. Accurate knowledge of 2D materials’ ambient compatibility, environmental robustness, and electronic properties would serve as a great benefit to manufacturers of devices such as thin-film transistors or transparent and flexible electronic devices based on 2D materials.

“Our technique allows faster and more reli­able struc­tur­al and elec­tron­ic fin­ger­print­ing of a rap­id­ly grow­ing body of 2D ma­ter­i­als.”

—Peter Nirmalraj, IBM Research scientist

In scaling up the characterization of these materials, we successfully combined the highest possible resolution to date under experimentally challenging conditions. Bridging this gap gives the information great value and has direct implications in 2D materials-based device engineering.

What steps must be taken next to advance imaging at liquid–solid interfaces?

PN: The next test lies in applying this technique to resolve single molecular elements with submolecular resolution. From both an experimental and theoretical standpoint, we need to understand more about the coupling mechanisms between the molecule and the tip in the presence of the encompassing liquid medium, and about the electronic and structural impact of the molecule by exploring the observed improvements in spatial contrast.

About the author: Millian Gehrer is a summer intern at IBM Research – Zurich, where he is interviewing scientists to learn more about their work and motivations. In the fall, he will begin studying Computer Science as an undergraduate at Princeton University.

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