J. Phys. Chem. B 1999, 103, 9721-9727
9721
Orbital-Mediated Tunneling, Inelastic Electron Tunneling, and Electrochemical Potentials for Metal Phthalocyanine Thin Films Ursula Mazur* and K. W. Hipps Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: July 12, 1999; In Final Form: August 20, 1999
Orbital-mediated electron tunneling through both occupied and unoccupied orbitals of metal phthalocyanines imbedded in metal-insulator-metal tunnel junctions is reported and discussed in terms of transient oxidation and reduction of the molecular species. Electrochemical oxidation and reduction potentials for the solution phase molecular systems are compared to the orbital-mediated tunneling spectroscopy (OMTS) data and a strong correlation is observed. These results are consistent with a simple model. It is demonstrated that different redox sites and states within a given molecule within a thin film will shift in unique ways relative to the equivalent processes in solution. Inelastic scattering from phthalocyanine π-π* transitions and metal centered d-d transitions are also observed.
Introduction Focus here is placed on thin films of electroactive molecules in solid state devices. Such structures are playing a progressively larger role as the era of molecular electronics and nanostructured materials comes into being. The nature of the organic-metal and inorganic complex-metal buried interface will be of progressively greater importance as smart, advanced, and biotechnological materials enter the electronics marketplace. Keys to understanding these interfaces are the oxidation and transition states, the location of the conduction band, and the bias dependence and mechanism of electrochemical processes occurring in the immediate vicinity of that interface. Knowledge of electron affinity and oxidized states of the molecular species at the interface is central to an understanding of these properties and processes. The primary tool for measuring gas-phase electron affinity levels, electron transmission spectroscopy (ETS), is a technique that can be applied only to negative affinity states.1-3 Inverse photoemission spectroscopy (IPS) has been successfully applied to the electron affinity levels of metals, semiconductors, and diatomic molecules chemisorbed on metal surfaces.4,5 Applications to larger systems have been few.6-8 One reason for this dearth of applications is that most compounds cannot survive the experiment; only a few minutes of electron beam exposure (at currents required to generate usable signals) is sufficient to destroy most organic compounds. The most commonly used measure of condensed phase electron affinity is the half-wave reduction potential measured in nonaqueous solvents, E1/2. Several authors have presented extensive correlation’s between the gas-phase oxidation and reduction potentials, and the associated half-wave potentials measured electrochemically, and some have suggested methods for estimating values in the solid state.9-11 Given the strong solvent dependence of some of these potentials, and the importance of coordination state of the metal ion in metal complexes, these estimates easily can be off by 1 eV. What is needed is a technique that (a) can be applied to * Corresponding author. E-mail:
[email protected].
positive affinity levels, (b) does not damage the molecular species studied, (c) can be applied to both chemisorbed and physisorbed layers, and (d) provides oxidation state information about buried interfaces to compare with ultraviolet photoelectron results obtained from films with an exposed surface. We have recently demonstrated that orbital-mediated tunneling spectroscopy (OMTS) is a technique capable of easily and rapidly providing the locations of electron affinity levels in adsorbed species and thin films.12,13 A model for these reduction-like processes that made use of solution phase electrochemical potentials was presented and demonstrated to provide remarkably accurate predictions of the positions of the OMTS bands.14 Recent OMTS work has been performed on metal-insulatoradsorbate-metal (M-I-S-M′) tunnel diodes12-14 typical of inelastic electron tunneling spectroscopy (IETS)15 and in the solution phase STM environment.16 In either configuration, the basic physics should be the same, although oxide or solvent chemistry will complicate the analysis relative to vacuum STM work. When the bias across the junction is such that the lefthand metal Fermi surface matches the energy of a vacant molecular orbital in the S layer, as shown in Figure 1, a large increase in conductance occurs due to orbital-mediated electron transfer. In the opposite bias, any orbital-mediated process involves occupied orbitals. The resulting orbital-mediated tunneling transitions are easily observed by recording the normalized conductance derivative, (dσ/dV)/σ, versus the applied bias. The history of orbitally-mediated tunneling processes was recently reviewed, and a connection was made between OMTS and apparent heights seen in the STM.14 It was stated that oxidative processes should occur in OMTS but would be generally less intense and should also have greatest intensity for occupied orbitals nearest to the Fermi energy, Ef. In this paper, for the first time, both cation and anion (HOMO and LUMO) mediated tunneling will be reported. Experimental Section All metal phthalocyanines (MPcs) were purchased as highpurity compounds from Aldrich Chemical. Because most of the
10.1021/jp9923419 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/14/1999
9722 J. Phys. Chem. B, Vol. 103, No. 44, 1999
Mazur and Hipps
Figure 1. Schematic view of elastic tunneling in an M-I-S-M′ device and a representative orbital-mediated tunneling spectrum showing the band resulting from tunneling via the lowest unoccupied orbital of tetracene. Spectrum acquired at 4 K.
impurities have a higher vapor pressure than the MPcs, all the compounds were twice presublimed and then sublimed before use. By presublimed is meant that the compounds where heated to about 300 °C in a sublimation vessel for several hours. The coldfinger was then removed and cleaned thoroughly. Metals used were >99.99% purity. Metals were resistively deposited from W wire (Al) or from Mo boats (Pb). Phthalocyanines were incorporated into the [Al-AlOx-MPc-Pb] tunnel diodes by one of two methods. They were either vapor deposited onto the oxide layer, or they were spin-doped from dichloromethane solution. The majority of diodes studied were formed by deposition of MPc from either a baffled W box source (ME-1 from R. D. Mathis, Inc.) or a quartz crucible; no difference in the resulting films was observed. This vapor deposition method of device preparation was as follows. Typically, 100 nm of aluminum was deposited on each of three corning glass microscope slides at a pressure of
