Changes in optical properties of conjugated molecules and polymers upon adsorption onto graphene

ABSTRACT

We compared the changes in optical transmission spectrum induced by self-assembly on CVD graphene of (i) a model squarylium dye, namely n-butyl-squarylium dye (1,3-Bis[4-(dibutylamino)phenyl]-2,4-squaraine) and (ii) a conjugated polymer, poly(3-hexyl thiophene). We analyse the results based on Scanning Tunnelling Microscope images. The self-assembly of the zwitterionic dye is controlled by electrostatic interactions. A strong red-shift is observed and is explained in terms of electrostatic interactions with graphene. The self-assembly of polyalkylthiophenes presents a large red shift due to a strong alignment and planarization of the conjugated π-electron system and the formation of a new molecule-graphene charge-transfer exciton state.

KEYWORDS:

• Graphene
• optical spectroscopy polythiophene
• scanning-tunnelling microscopy
• self-assembly
• squarylium

Acknowledgement

We thank P. Raimond (Technological-Research Division, CEA/Saclay) for the synthesis of SQR molecules.

Funding

The authors acknowledge the support of the French Agence Nationale de la Recherche (ANR) under grants ANR-13-BS100006 (project SAMPLE) and ANR-14-CE26–0016 (project SMAL'LED). This work has been supported by a Ph.D. joint funding from CEA and Triangle de la Physique.