Chapter 3
C60 on WO2/W(110): From Self-Assembly to Phase Transitions and Charge States

Much excitement in the field of C60 was based on the prospects of using these remarkable molecules for constructing nano-electronic devices [14–20]. However, 25 years have passed since the discovery of C60 and these expectations are yet to be fulfilled. The stability of such molecular electronic devices is one of their main drawbacks, and nanomechanical motion can result in the generation of noise in C60-based devices [20]. A natural approach to constructing such devices is through frameworks based on a monolayer of C60 molecules.

In this chapter, by using STM, LEED and DFT calculations, we focus on the molecular self-assembly of C60 on the WO2/W(110) surface in the submonolayer to monolayer regimes in order to reveal the conformational behaviour of C60 molecules. DFT is utilised to obtain information about the local density of states.

As the monolayer film is cooled from room temperature down to 78 K, it undergoes two phase transitions which are accompanied by several different types of molecular movement. Close to the first phase transition at 259 K, some molecules switch between at least two charge states, becoming alternately “bright” or “dark” as they gain or lose electrons. These states are elucidated using systematic STM studies, and confirmed by DFT calculations. The results of this work yield important information on the electronic and structural properties of C60 molecules adsorbed on the WO2/W(110) surface.

 3.1 Experimental details
 3.2 C60
  3.2.1 Possible C60 orientations
 3.3 WO2/W(110)
 3.4 Growth and topography of the C60 film
 3.5 Rotational transitions in the C60 thin film
  3.5.1 Temperature dependence
  3.5.2 Molecular transitions between different states
  3.5.3 DFT calculations of C60 orientation & spinning molecules
  3.5.4 Phase transition
  3.5.5 Kinetic transition
 3.6 Charge transfer and rotation
  3.6.1 Switching time-evolution
  3.6.2 Density of states of C60
  3.6.3 The Fukui function of charge states
 3.7 Conclusions