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Abstract:
Ultrahigh vacuum (UHV)-deposited films of the mixed phase of
tetramethoxypyrene and tetracyanoquinodimethane (TMP(1)-TCNQ(1)) on gold
have been studied using ultraviolet photoelectron spectroscopy (UPS),
x-ray diffraction (XRD), infrared (IR) spectroscopy, and scanning
tunneling spectroscopy (STS). The formation of an intermolecular
charge-transfer (CT) compound is evident from the appearance of new
reflexes in XRD (d(1)=0.894 nm and d(2)=0.677 nm). A softening of the CN
stretching vibration (redshift by 7 cm(-1)) of TCNQ is visible in the IR
spectra, being indicative of a CT on the order of 0.3e from TMP to TCNQ
in the complex. Characteristic shifts in the electronic level positions
occur in UPS and STS that are in reasonable agreement with the
prediction of density-functional theory (DFT) calculations (GAUSSIAN03
with hybrid functional B3LYP). STS reveals a highest occupied molecular
orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the CT
complex of about 1.25 eV being much smaller than the gaps (>3.0 eV) of
the pure moieties. The electron-injection and hole-injection barriers
are 0.3 eV and 0.5 eV, respectively. Systematic differences in the
positions of the HOMOs determined by UPS and STS are discussed in terms
of the different information content of the two methods.
