Characterization of the Interactions between Alq3 Thin Films and Al Probed by Two-Color Sum-Frequency Generation Spectroscopy

We present the investigation of the vibrational and electronic states of tris(8-hydroxyquinoline) aluminum (Alq3)/Al (Alq3 on Al) interfaces by using two-color infrared-visible sum frequency generation (SFG) spectroscopy. The visible wavelength dependence of the SFG spectra of the 2 nm thick Alq3/Al consists of the vibrational bands derived from the Alq3 at the Al interfaces. The intensities of the peaks derived from the ring stretching modes of the quinolate ligands were significantly enhanced due to the double resonance effect. In contrast, the SFG electronic spectrum obtained from the output photon energy dependence of the SFG peak amplitudes derived from the CC bands of the Al on Alq3 interfaces does not show the wavelength dependences, indicating that the electronic-resonance associated with the π–π* transitions in the quinolate rings are almost vanished at the Al deposited on the Alq3. The disappearance of the electronic-resonance of the CC stretching modes must be caused by the perturbation of the HOMO and LUMO of pristine Alq3 by the interaction with the Al. The spectral features of the two-color SFG spectra of the Al/LiF/Alq3 system show quite different behavior from those of Alq3/Al and Al/Alq3. The shift of the CC stretching modes toward lower frequencies is indicative of the formation of the Alq3 anionic states upon reaction with Li at the interface. Additional broad bands around 1335 and 1450 cm–1, which show the weak excitation wavelength dependence, might be due to the existence of the Li-reacted graphitic carbon-like Alq3.