'Stand-up molecule' defies scientific wisdom


Rebecca Pool

Monday, July 2, 2018 - 15:45
Image: Upright PTCDA molecule on a silver platform (left); normally the molecule is deposited flat against the flat layer of silver atoms (right) [Forschungszentrum Jülich/T. Esat]
Germany-based researchers have used scanning probe microscopy to stand a large planar aromatic molecule on a pedestal of two silver adatoms. 
A key challenge to fabricating novel devices is to overcome the attraction of molecules to the underlying surface.
However, by orienting the platelet-shaped PTCDA molecule in this way, Forschungszentrum Jülich researchers have opened the door to novel molecular fabrication methods and the free creation of nanoscale structures. 
"Until now, it was assumed that the molecule would revert back to its favoured position and lie flat on the surface," points out Dr Taner Esat from Professor Ruslan Temilov's Low Temperature Scanning Probe Microscopy Group.
"But that is not the case. The molecule is surprisingly stable in the upright orientation," he says. "Even when we push it with the tip of the microscope, it does not fall over; it simply swings back up again. We can only speculate as to the reason for this."
Dr Taner Esat (left) and Dr Ruslan Temirov, [Forschungszentrum Jülich / S. Kreklau]
As part of the experiment, the researchers first prepared an atomically clean silver surface in an ultrahigh vacuum and then deposited PTCDA molecules across it. 
The sample was then moved into the microscope, cooled to 5K, with silver atoms then evaporated onto the sample surface.
Using a PtIr wire, sharpened with a focused-ion beam, as the SPM tip, the researchers were able to attach two silver atoms to the edges of a PTCDA molecule, which was then lifted up until it stood upright on its tiny 'silver platform'. 
The researchers have demonstrated that the 'stand-up molecule' can be used as an electron source - to emit single electrons.
As Esat highlights in Nature: "This atypical and surprisingly stable upright orientation of the single molecule, which under all known circumstances adsorbs flat on metals, enables the system to function as a coherent single-electron field emitter."
"We anticipate that other metastable adsorbate configurations might also be accessible, thereby opening up the third dimension for the design of functional nanostructures on surfaces," he adds.
Research is published in Nature.
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