IBM scientists use the STM to image molecules in liquid

Nirmalraj in the Noise Free Lab
Since the first microscope was invented, researchers and scientists around the world have searched for new ways to stretch their understanding of the microscopic world. In 1981, two IBM researchers, Gerd Binnig and Heinrich Rohrer, broke new ground in the science of the very, very small with their invention of the scanning tunneling microscope (STM).

Like no instrument before it, Binnig and Rohrer’s invention enabled scientists to visualize the world all the way down to its molecules and atoms. The STM was recognized with the Nobel Prize in Physics in 1986 and is widely regarded as the instrument that opened the door to nanotechnology and a wide range of explorations in fields as diverse as electrochemistry, semiconductor science, and molecular biology.

In a new paper appearing today titled "Capturing the embryonic stages of self-assembly - design rules for molecular computation" in Nature Scientific Reports, IBM scientists in Zurich are adding a new chapter to the STM's legacy by reporting on a new methodology for the reliable extraction of incredibly high resolution images of the swarming behavior of molecules in situ or which translates to "on site".

I spoke with the lead author of the paper Dr. Peter Nirmalraj about his research and what's next.

Q. Why has it taken so long to use the STM for in situ imaging in liquid? 

Peter Nirmalraj (PN): In situ STM imaging (imaging in liquids at room-temperature) has been around for the last 20 years, however extracting high resolution data comparable to UHV STM standards has remained a challenge, mainly due to the electrochemical congestion and external noise interference occurring when performing such measurements in liquids at room temperature.

Q. How did you come up with the idea to make it work?

PN: We have previously demonstrated how to control molecular motion for stable electrical readouts. 

In the current work we employ the same STM tool capable of measuring dynamics of individual molecules at the liquid-solid interface, with excellent spatio-temporal sensitivity. In particular we chose electrically inert and low vapor pressure liquids as the medium, that does not interfere with the tunneling mechanism. The entire setup is located within our state-of-the-art noise free laboratories located in the Binnig and Rohrer Nanotechnology Center which immensely aids such nanoscopic measurements in liquids.

The question we asked ourselves, can we record in real-time the evolution of an organic molecular layer and in particular capture the very early-stages of self-assembly rather than only imaging a fully packed thermodynamically stable molecular matrix? 

Information obtained from such carefully designed experiments can and have provided deeper insights on the fundamentals of molecular self assembly, which is central in molecular computation and in refining step-by-step equilibration rules for agent-based algorithms (algorithmic self-assembly).

Q. Why did you choose C60 molecules for the molecular solution?

PN: Fullerenes are a well studied class of molecules. This makes it easier to calibrate our STM tool as the dimensions and intermolecular packing arrangements of this system is known from both theory and previous STM studies. More importantly fullerenes are compatible with our solvent of choice, n-tetradecane and form a stable molecular solution (molecules are well dispersed with minimal treatment and does not aggregate in solution). Currently we are exploring other molecules with different dimensions from porphyrins to ferrocene.

Q. Could this technique be used for healthcare, to image bacteria and and viruses for example?

PN: Yes, biomolecules which are generally not highly rigid and less conductive in nature can be imaged using STM when drop casted from a liquid-phase onto conductive metal surfaces. However, the solvents that generally support biomolecules are polar which necessitates an additional step for insulating the STM probe, minus the apex, to minimize parasitic currents during tunneling. Such measurements also need to be performed under closed-liquid cells with larger volume to contain rapid solvent evaporation.

Q. What's next for your research?

PN: The next step would be to investigate naturally occurring pattern formation and understand better the local structural dynamics of organic molecules such as porphyrins as they evolve from a disordered phase to an energetically stable stage and verify their internal response to external electrical pulses. 

Large data sets generated from such studies will then be directed towards constructing predictive algorithms for assembly of new 2D and 3D molecular configurations and for testing basic logic gate operations in molecular layers.

Follow the authors on Twitter at @stm_pnn and @HeikeRiel

This research is partly funded by the Marie Curie Actions-Intra-European Fellowship (IEF-PHY) under grant agreement N° 275074 “To Come” within the 7th European Community Framework Programme.

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