Monolayer structures of 4,4′ bipyridine on graphite at sub-monolayer coverage

Abstract

The formation of crystalline monolayers of 4,4′ bipyridine at submonolayer coverage have been characterized by synchrotron X-ray diffraction. The experimentally determined monolayer unit cell exhibits a small temperature dependence, such that the monolayer structure at higher temperatures has a square unit cell, while at lower temperatures the unit cell becomes very slightly rectangular. Both structures have the same plane group, Pgg. The experimentally determined lattice parameters are a = 11.42 Å, i = 11.42 Å and v (=90° and a = 11.26 Å, b = 11.45 Å, v = 90° for the high and low temperature structures, respectively. There are two molecules per unit cell with the pyridine units of each molecule in a plane parallel to the graphite substrate, although we are not particularly sensitive to this orientation. Interestingly, the monolayer does not melt even at the highest temperatures accessible on the device, 445 K, well above the bulk melting point. This is a very unusual behaviour particularly at these submonolayer coverages.

Keywords:

• monolayers
• synchrotron
• physisorption

Acknowledgements

We thank the BP Institute, Diamond Light Source, and the University of Cambridge for financial support and facilities. Thanks to the DLS staff and scientists Chiu Tang and Stephen Thompson for beam time and technical support. This work was carried out with the support of the Diamond Light Source. The Herchel Smith find is acknowledged for a research fellowship (TF). Steven De Feyter is thanked for helpful discussions.