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

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 A , 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.