Electrostatic Properties of Organic Monolayers on Silicon Oxides Studied by Kelvin Probe Force Microscopy

Abstract

The vacuum level (VL) shifts of pentacene (PEN), perfluoro-PEN (PFP), α,ω-dihexylsexithiophene (DH-6T) and N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) monolayers on SiO2 terminated with various chemical species were studied by Kelvin probe force microscopy (KPFM). The PEN and PFP monolayers deposited on OH– and CH3–SiO2 showed a downward VL shift, while those on CF3–SiO2 showed an upward VL shift. The VL shift is considered to originate not from the charge transfer but from the dipole induced in molecules by surface charges. The KPFM of the DH-6T and PTCDI-C13 monolayers on OH–SiO2 revealed that the alkyl chains between the π-system and SiO2 reduce the effect of surface charges. The structural asymmetry of these molecules causes the VL shift.