German researchers have captured clear images of intermolecular bonds for the first time using a modified form of scanning tunnelling microscopy (STM). The technique could help scientists studying the functionalisation of surfaces with molecules to develop new materials including semiconductors and fuel cells.

Stefan Tautz and colleagues at the Jülich Research Centre in Germany first demonstrated the technique  in 2008 when they produced images of  various phases of the aromatic hydrocarbon molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on a gold surface.¹ By dousing the tip of a low temperature scanning tunnelling microscope with liquid hydrogen, images of the molecule's internal structure were produced that looked remarkably like textbook drawings of atomic structures. 

Now, the team have gone a step further and optimised the scanning tunnelling hydrogen microscopy (STHM) technique to image the types of interactions occurring between PTCDA molecules on a gold surface.² 'This has been impossible up to now with any method that I am aware of,' says Tautz.   In the latest experiments, the team condensed deuterium at temperatures between 5 and 10 Kelvin at the microscope's tip and doped the gold surface with potassium. 

The resulting images produced much greater contrast than an STM with a bare tip to reveal local, noncovalent intermolecular interactions of a PTCDA molecular layer. The team managed to resolve two bonds (O · · H-C and O · · K) which appear as thin lines or as sharp boundary lines between areas of different brightness.   'Although the mechanism is still not completely clear, we seem to have a handle now to actually visualise these interactions directly,' says Tautz, 'That certainly makes a big difference for studying the functionalisation of surfaces with molecules if this can be confirmed for other systems.' 

Leo Gross at IBM Research in Zuruich, Switzerland, who earlier this month reported a new technique to image molecular structures using atomic force microscopy, thinks the new study is an important development. The images obtained by the STHM technique show remarkable intramolecular resolution and greatly advance the investigation of molecular monolayer structures,' he comments. 

'I think it has the potential of becoming a method which is complimentary to ordinary STM but which can also be carried out on the same instrument which is a very big advantage,' Tautz adds. 'If you have the instrument already in your lab then it is very cheap and simple thing to do.' 

James Urquhart 

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