Cairo University

MTPR Journal


Dynamic of molecules in ultrathin C60 film

Bozhko S.I.
Institute of Solid State Physics RAS
Vol./Issue: 19 , id: 261

The motion of single atoms or molecules plays an important role in nanoscale engineering at the single atomic or molecular scale. Effect of molecule center mass displacement on conductivity of C60 based junction has been previously reported [1].Understanding of molecular motion is crucial to further progress in molecule-based nano-electronic devices.The symmetry and deviations from a spherical shape of the C60 molecule generate the rotational degrees of freedom which often determine physical and structural properties of compounds, C60 based clusters and crystal. Scanning tunneling microscopy (STM) is one of the most promising techniques for imaging of prototype molecular devices and testing their properties. We employed STM to reveal switching of individual C60 between different orientations within a single molecular layer grown on the WO2/W (110)surface(Fig. 1a-e). Switching of the molecule between orientations resulted in a telegraph noise in tunneling current (Fig. 1d) or in Z position of STM probe. Statistics of switching has been used to determine energy gap and potential barrier height between two adjacent orientations of the molecule. Rotational transitions in single molecular layer of C60 were also studied. STM experiments were performedin temperature range 80K-320K.Rotational first order phase transition at TC=260Khas been established [2].Above that temperature molecules continuously rotate around their centres of mass whereas below TC the rotational degree of freedom is suppressed and C60 molecules undergo thermally activated switches between closely positionedin energy orientations. The rate of jumps rapidly decreases with decreasing temperature and at 220 K the molecules stay in each state for longer than the time of a possible experiment. Therefore a kinetic glassy transition can be identified at 220 K.The glassy transition and nonexponential relaxation in solid C60 are due to the freezing of weakly correlated orientations of nearest-neighbor molecules.The large number of different molecular orientations observed in the film results in an averaging-out of the interaction potentials and should cause Arrhenius-like relaxation processes. However, STM experiments reveal correlations in the nanomotion of the C60 molecules that suggest arguments in favor of a constrain-dynamic scenario.The observation of a glassy transition at 220 K reveals a nonexponential relaxation in the C60 monolayer. The Kauzmann temperature was estimated to be 45 K.Link of dynamics of individual molecules to the rotational transitions in the overall film discussed in a frame of mean field theory.

[1]Hongkun Park,, Nanomechanical oscillationsin a single-C60 transistor,Nature, 407, 57, 2000; N.Neel, Two-level conductance fluctuations of a single-molecule junction, Nano letters, 2011, 11, 3593
[2] Sergey I. Bozhko Rotational transitions in a C60monolayer on the WO2/W(110) surface, Phys. Rev. B. 84,195412, (2011)