Field-doping of C60 crystals: Polarization and Stark splitting

TL;DR

This paper explores how applying strong electric fields to C60 crystals causes polarization and Stark splitting, significantly altering their electronic structure and impacting their behavior in field-effect devices.

Contribution

The study introduces a multipole expansion approach and ab initio calculations to analyze Stark effects and molecular responses in C60, extending results to arbitrary orientations.

Findings

Multipole components lift degeneracy of HOMO and LUMO levels.

Splittings add up when charge is induced, modifying electronic structure.

Charge injection in C60 devices significantly alters their electronic properties.

Abstract

We investigate the possibility of doping C60 crystals by applying a strong electric field. For an accurate description of a C60 field-effect device we introduce a multipole expansion of the field, the response of the C60 molecules, and the Stark splitting of the molecular levels. The relevant response coefficients and splittings are calculated ab initio for several high symmetry orientations. Using a group theoretic analysis we extend these results to arbitrary orientations of the C60 molecules with respect to the external field. We find that, surprisingly, for the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively, the two leading multipole components lift the degeneracy of the molecular level in the same way. Moreover the relative signs of the splittings turn out to be such that the splittings add up when the external field induces charge into the respective level. That means that when charge carriers are put into a level, its electronic structure is strongly modified. Therefore, in general, in C60 field-effect devices charge is not simply put into otherwise unchanged bands, so already because of this their physics should be quite different from that of the alkali-doped fullerenes.