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Current–Voltage Characteristics of a Homologous Series of Polycyclic Aromatic Hydrocarbons
Author(s) -
Böhme Thilo,
Simpson Christopher D.,
Müllen Klaus,
Rabe Jürgen P.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200601249
Subject(s) - chemistry , aromaticity , homologous series , alkyl , molecule , spectroscopy , monolayer , dipole , substrate (aquarium) , carbon fibers , scanning tunneling microscope , homo/lumo , analytical chemistry (journal) , crystallography , photochemistry , materials science , organic chemistry , nanotechnology , biochemistry , physics , oceanography , quantum mechanics , composite number , composite material , geology
A novel alkyl‐substituted polycyclic aromatic hydrocarbon (PAH) with D 2 h symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic π system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode‐like current versus voltage ( I – V ) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current–voltage ( I – V ) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO–LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction.