Weak screening of a large dipolar molecule adsorbed on graphene

We compare the electronic structure of a quinonoid zwitterionic type molecule adsorbed on both gold and graphene on copper substrates. This (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C6H2(center dot center dot center dot NHR)(2)(center dot center dot center dot O)(2) where R = n-C4H9, film is made of small molecules with a large intrinsic dipole of 10 Debyes. We find that the photoemission and inverse photoemission final states are well screened for these dipolar molecules on gold. This is not observed when they are adsorbed on graphene on copper. This weaker screening results in a larger highest occupied molecular orbital to lowest unoccupied molecular orbital gap for the molecules on graphene. (C) 2012 Elsevier Ltd. All rights reserved.