J. Phys. Chem. 1994,98, 8169-8172
8169
Resonant Tunneling in Metal Phthalocyanines Ursula Mazur' and K. W. Hipps Department of Chemistry, Washington State University, Pullman, Washington 991 64-4630 Received: January 26, 1994; Zn Final Form: April 27, 1994"
Inelastic electron tunneling spectra obtained from A1-AIOx-MPc-Pb junctions (M = Cu, Mg, Zn, or Co) are presented. Improved experimental methods allow us to report both the vibrational and electronic spectra taken in both bias directions. The spectra are highly bias direction dependent, and an intense strong feature seen in Al- bias spectra is interpreted as due to resonant tunneling or resonance-like enhanced conduction via the eg(x*) LUMO. No evidence for direct d orbital participation in the resonance process is observed, but excitation of transitions involving the d orbitals in CoPc are observed as inelastic features in the Al+ bias spectrum.
Introduction Tunneling spectroscopy has been used to study electronic and vibrationalstates of M'-I-X-M (metal-insulator-adlayer-metal) junctions.I-l2 Until recently, however, experimental tunneling spectra have been interpreted solely in terms of inelastic excitations of molecular states by tunneling electrons. Thus, the acronym IETS (inelastic electron tunneling spectroscopy)is often used for the technique. In previous paper~,~.8 we found that tunneling spectra of CuPc vary with both bias direction and top metal. Lead, silver, and thallium (M) topped junctions gave very different spectra when the base (M') electrode, aluminum, was biased negatively and similar but not identical spectra when aluminum was positive. Using Raman spectroscopy applied to functioning diodes, we demonstrated that the anomalies seen in the tunneling spectrum are not the result of static electrochemical reduction of the CuPc layer. Following Persson and Baratoff s theoretical analysis,l3-I5we suggested that some form of resonant tunneling produces the unusual features observed in the CuPc tunneling spectrum. We argued for, but could not prove that, the ?r* LUMO was the only significant participant in this process and that the metal d orbitals-specifically the half-filled bl, orbital-were not involved. Tunneling spectra for A1-A10,-MPc-Pb junctions have been reported several The early are important because they established that electronic transitionscan be observed by IETS. In the early work, however, there were troubling aspects about, and inconsistencies between, the reported MPc spectra. First and foremost, the spectra did not have the expected bias voltage-dependentsymmetry. As is predicted theoretically,6and observed experimentally for electronic and vibrational transitions,IlJ2 bands seen with A1 biased negatively should also occur (reduced in intensity) when A1 is biased positively. MPc spectra often do not show this symmetry.'" In one case: the authors even observed scan history-dependentbands for aluminum biased positively. In the present article we report and interpret the tunneling spectra obtained from CuPc, MgPc, ZnPc, and CoPc. Complete spectra in the bias region extending from -1.5 to 1.5 eV are reported. On the basis of these results, we will show that MPc metal d orbitals can participate in the inelastic tunneling process but play no direct role in the resonance-like structure.
cyanines were deposited from a baffled W box source (ME-1 from R.D. Mathis Inc.). The method of device preparation has been described e l s e ~ h e r e . ~ - ~ Typically 100 nm of aluminum was deposited on each of three Corning glass microscope slides at a pressure of
