6684
J. Phys. Chem. 1995,99, 6684-6688
Resonant Tunneling Bands and Electrochemical Reduction Potentials Ursula Mazur and K. W. Hipps* Washington State University, Pullman, Washington 99164-4630 Received: December 6,1994; In Final Form: February IO, 1995@
The recent observations of resonant tunneling through unoccupied orbitals of molecular species imbedded in metal-insulator-metal tunnel junctions are discussed in terms of transient reduction of the molecular species. Electrochemical reduction potentials for the solution phase molecular systems are compared to the orbitalmediated tunneling spectroscopy (OMTS) data, and a strong correlation is observed. A simple model is proposed that accounts for this correlation. This model also explains previous observations of spontaneous and permanent reduction of certain compounds in the tunnel junction environment. The model should be equally applicable to resonant tunneling and to spontaneous reduction observed in the scanning tunneling microscope. Based on the view presented here, orbital-mediated tunneling spectroscopy is a new method for providing reduction potentials and affinity levels in adlayers and thin solid films.
Introduction Photoelectron spectroscopy has provided a wealth of information about the filled orbitals of molecular and solid state materials. This information has been used to stimulate and verify theoretical models predicting a wide range of physical phenomena. The locations of unfilled orbitals are of similar importance, but our experimental knowledge of these is far poorer. Unoccupied orbital energies, electron affinities, and electron attachment energies are related quantities of fundamental importance to virtually every aspect of modem science and technology. A few examples of their importance in condensed or adsorbed state are indicated below. (1) “The electron affinity of a molecule is the definitive measure of its ability to act as an electron acceptor,” and it is thus of great importance in designing and understanding modem electronic materials. (2) The nature of the organic-metal and organometallicmetal 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 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 the unoccupied orbitals of the molecular species at the interface is central to an understanding of these properties and processes. (3) Rational design of molecular electronic devices such as organic semiconductors requires a knowledge of the energy of the unoccupied orbitals in several classes of organic compounds. In particular, the conduction band is determined by the adiabatic electron affinity of the solid. (4) Unoccupied orbital energies are of special importance in discussions of ligand field splitting, through space interactions, d-orbital bonding, and back-bonding. These latter topics are central to an understanding of organometallic and transition metal complexes that are the key actors in heterogeneous catalysis, homogeneous catalysis, and biological activity associated with metal centers. ( 5 ) Scanning tunneling microscopy (STM)theory rationalizes the apparent height of molecular images in terms of the energy and spatial distribution of unoccupied orbitals. Spontaneous reduction of molecular species in the tunnel junction environ@
Abstract published in Advance ACS Abstracts, April 1, 1995.
ment is also governed by the electron affinity of the molecule in the adsorbed state. Thus, a predictive knowledge of the true oxidation state and the location of low-lying vacant orbitals of the species studied by STM are very much needed. The primary tool for measuring electron affinity levels, electron transmission spectroscopy (ETS), is a gas phase technique that can be applied only to negative affinity state^,^-^ i.e., those lying above the vacuum level. Inverse photoemission spectroscopy (IPS) has been successfully applied to the electron affinity levels of metals, semiconductors, and diatomic molecules chemisorbed on metal surface^.^^^ Applications to larger systems have been f e ~ . ~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. What is needed is a technique that (a) can be applied to positive affinity levels, (b) does not damage the molecular species studied, and (c) can be applied to both chemisorbed and physisorbed layers. 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 fi1ms.l0J1 Electron tunneling mediated by vacant gap states was first experimentally observed in field-emission studies.l2 The quantum size effect observed in M-I-M’ (metal-insulator-metal) tunnel diodes is due to size quantization in the top metal, M’, and is based on a resonant tunneling process.13J4 Studies of quantum well structures composed of semiconductor-metal and semiconductor-semiconductor multilayer structures began in the mid-1970s,l5 and the conductance increases observed when size-quantization states are resonant with the energy of tunneling electrons are still of intense interest.I6 Resonant tunneling due to adsorbate states in M-I-S-M’ devices (where S is a molecular adsorbate layer on the order of one monolayer thick) was originally thought to never occur or to occur so rarely that no definitive examples had been 0b~erved.I~ In 1989, we first suggested that orbital-mediated tunneling in this M-I-S-M’ environment might be more common than thought and prwided an experimental example, Al-AhO3-copper phthalocyaninePb.18 Thereafter, a number of anomalous spectral effects seen in the vibrational and electronic regions of the tunneling spectrum were associated with the resonant proces~es.’~ Unlike the resonant tunneling observed in quantum wells and dots,
0Q22-3654/95/2099-6684$09.oO/o 0 1995 American Chemical Society
Tunneling Bands and Reduction Potentials
J. Phys. Chem., Vol. 99, No. 17, 1995 6685
CA1-A1203-tetracene-Pbl
' I
positive bias = 1 ,Ovolt
Energy diagram for an M-I-S-M' tunnel diode 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. Note that 12 000 cm-' is 1.488 V.
Co p h t h a l o c y a n i n e
electronic
-12,000
0
12,000
Energy (cm-1 1 Figure 2. Orbital-mediated tunneling spectrum of cobalt(II) phthalocyanine taken at 77 K showing both electronic IETS and an intense resonance tunneling band. For the OMTS band, the relative change in conductance, &J/u, is more than 20%. Note that 12 OOO cm-I is 1.488 V.
orbital-mediated tunneling in the M-I-S-M' environment involves the molecular orbitals of physisorbed adlayers rather than collective states of epitaxial crystalline layers. This sort of interaction is believed to be an important requirement for imaging adsorbate molecules in the scanning tunneling microscope.2°~21In the case of scanning tunneling microscope (STM) studies, resonant tunneling has been cleanly demonstrated only for crystalline layers or where an adlayer chemically modifies a crystalline substrate to the point where the density of surface states significantly changesz2 In what follows we will identify spectroscopic studies of adsorbate molecular orbitals through resonant tunneling as orbital-mediated tunneling spectroscopy (OMTS). Recent OMTS worklOJ1 has focused on tunneling in the M-I-S -M' diode typical of inelastic electron tunneling s p e c t r o ~ c o p ybut , ~ ~the method should be equally applicable to
the scanning tunneling microscope environment. If performed in an STM, M' would be the metal substrate, S the adsorbed layer, I the gap, and M would be the metallic tip. When the bias across the junction is such that the left hand 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 resonant electron transfer. In the opposite bias, any resonance process involves occupied orbitals. The resulting resonant tunneling transitions are easily observed by recording the normalized conductance derivative, (du/dV)/u, vs the applied bias. We have used this method to identify the lower unoccupied orbitals in several systems. The positions of the bands obtained from thin films of polyacenes,Lo for example, are consistent with Huckel MO predictions and provided an experimental value of /3 = 2.9 eV. Because this is a resonant process, the electron energies are no higher than that required to form the affinity level. Thus, there is little or no excess energy available to cause decomposition of the molecular species, and these spectra can be rerun for days with no discernible change in results. These OMT spectra are remarkably temperature independent. We see little difference in bandwidth or position for spectra measured in the 4 to 200 K temperature range. Most of our OMT spectra were obtained at 77 K.lO,ll This insensitivity to temperature suggests that small amounts of thermal motion have little effect on the transition mechanism. It is also consistent with the intrinsic widths of the OMTS bands. There are nonOMTS features in the tunneling spectrum presented in Figure 1 that are often observed in spectra taken below about 10 K. These are due to inelastic tunneling events such as C-H and 0-H vibrational excitations (2900 and 3500 cm-', respectively) and to the band gap generated by superconductivity of the Pb top electrode (near 10 cm-'). These nonOMTS processes are well-~nderstood.~~ Tunneling spectra taken at 77 K and above show much less fine structure and are dominated by electronic processes such as OMTS and inelastic excitations of electronic states. A typical example of a 77 K spectrum is shown in Figure 2.
Discussion One model that can account for resonant tunneling bands of the type shown in Figures 1 and 2 is proposed here. It is
Mazur and Hipps
6686 J. Phys. Chem., Vol. 99, No. 17,1995 proposed that the tunneling electron produces a short lived anion of S, S-. This anion then rapidly decays back to the parent, S, through the emission of an electron into the huge manifold of vacant states of the adjacent metal, M’. This view is supported by two independent pieces of data. First, as shown in Figure 1 for tetracene, all the OMTS bands we have observed have been scan direction Thus, no charge buildup (on a time scale of seconds) is associated with these bands. Second, resonance Raman spectra obtained from functioning M-I-SM’ diodes (S = a metal phthalocyanine) while under bias show no changes with applied voltage over a range greater than that required to observe the OMT band in the same device^.'^^^^ These Raman results show that if any S- is forming, its concentration has a very small steady state value. To see this, note that OMT bands produce relative conductance changes (do/ a) from 10 to 30%-changes so large that they are easily seen in the conductance curve. The typical tunnel junction will have a current of about 1 mA/mmz at the voltage associated with an OMTS band. This current flows through an average lnm layer of molecular species for a total electron flux of about 1300 e/(molecule.s) (assuming a 200 amu molecular mass and a density of 1.6 g/cm3). Taking the resonant tunneling current to be about 10% of the overall current, this corresponds to 130 resonant-tunneling-electrons/(moleculas). Assuming that a steady state concentration of anions of less than 5% is implied by the above cited Raman studies, the average lifetime of the anions must be less than 400 ps. Thus, we imagine a real (as opposed to virtual) reduction having a lifetime longer than a few vibrational periods but less than 400 ps and probably less than nanoseconds. In the present model, the OMT bands are due to electron affinity levels (more precisely, vertical attachment energies) that is, energy levels associated with the addition of an electron to a molecular or ionic sample, S. If the electronic and vibrational quantum numbers of the molecular species are n and v, respectively, the process of excitation to an affinity level can be represented as
S(n,v)
+ e- - S-(n’,v’)
This process should be clearly distinguished from that occurring in optical excitation by a photon, wherein
+ hv - S(n’,v’)
S(so1ution)
+ e-(vac) - S-(solution)
(4)
These values can be computed from those referenced to sce by adding 4.71 V to E l ~ ( s c e ) . ~In~ what - ~ ~ follows, we identify these vacuum state referenced potentials as E112, with Ell2 = 4.7 1 E1/2(sce). Electron affinities are conventionally positive when the anion formation releases energy. Solid state convention, on the other hand, often places the Fermi level at the zero of energy. Because these conventions differ and can introduce significant confusion, we will use the scale convention depicted diagrammatically in Figure 3. This diagram is based on one presented by Loutfy et al.27 Note that P is defined to have a negative sign for exothermic processes but that A and E112 have the opposite convention. With these conventions, the following useful relations result.
+
+ P,A, = E,, + P,A, = E,,, + (Ps- - P,-) A , = A,
The lowest affinity level (electronic and vibrational) measured relative to the electron at rest at infinity (the vacuum state) defines the adiabatic electron affinity for the molecule or ion (conventionally, electron affinities are positive for final states below the vacuum state and negative for those above it). Higher affinity levels represent transitions to vibrationally, or electronically and vibrationally, excited states of the reduced species. In what follows, we will assume that the initial state of the unreduced form is the ground state. Thus, excitation to a general affinity level (or anion state) can be written as
S(g,O)
and the energy of “the unoccupied orbital” are different for electron attachment than for optical excitation. To some extent, therefore, the meaning of the term “unoccupied orbital” depends upon the context. If a large amount of nuclear rearrangement occurs upon reduction, the equilibrium geometry of the reduced species, S-, will be very different from that of the parent species, S. Thus, one distinguishes the adiabatic electron affinity from the affinity levels that result from vertical electron attachment energies. The energy produced upon addition of an electron to the molecule or ion, S, at its equilibrium geometry (not allowing S- to relax) is the vertical attachment energy and can be significantly different (as much as 0.5 eV) from the adiabatic affinity. The most commonly used measure of condensed phase electron affinity is the half-wave reduction potential measured in nonaqueous solvents, El/,. Often these values are tabulated relative to the standard calomel electrode (sce). For our purposes, we want the reduction potentials referenced to an electron in the vacuum state. That is, we need the potential for the half reaction
(3)
In an orbital picture, both processes 2 and 3 involve the same highest occupied molecular orbital (HOMO) of S and formally the same unoccupied molecular orbital, but the charge state and occupation numbers of the products are very different. In the case of affinity level excitation, the HOMO occupation is unchanged and the overall charge is now more negative by 1. In the optical excitation case, the final state has the HOMO occupation decreased by 1 and the overall charge state is unchanged. Thus, the spatial distribution of electron density
(7)
P,- typically varies from -2.1 to -2.5 eV,27,28and Pc- can range from -1.6 eV to about -2.2 eV for organic molecular crystal^.^^^^^ Loutfy, for example, found that Pc- 0.86Psfor a series of electron acceptors.27 Thus, we might expect A , to lie between E1/2 and E112 - 0.4 eV. While the first reduction potential provides an estimate for A, (assuming it is a reversible process and is not due to some chemical process other than (3)), the second and higher reduction potentials generally do not provide the spectrum of single electron affinity levels. Rather, they provide information about 2,3, and higher electron reduction processes and, therefore, depend on the electron pairing energy. Thus, the utility of solution phase reduction potentials for estimating solid state affinity levels is, at best, limited to the lowest affinity level. OMTS, on the other hand, should be useful for determining all affinity levels up to the vacuum level. When a metal is in equilibrium with a thin crystalline layer of S having electron affinity A, (neglect for the moment possible differences in the film affinity, and the crystal affinity, A,), an interesting and important observation follows naturally from Figure 3. If A, is greater than Ef,then the parent molecule will spontaneously reduce! This observation has wide implica-
-
Tunneling Bands and Reduction Potentials
,
0.0 L -
Vacuum level
J. Phys. Chem., Vol. 99, No. 17,1995 6687
I +4.@ +3 0
+2 0 +
6.0 5’0
1
r?
> v
U
1.0
c ro m
00
-I
-lQ
0
Figure 3. Energy level diagram for reduction. A, and A, are the gas and crystalline phase electron affiities, respectively. Eln is the reduction potential referred to the vacuum state (left hand scale) and is taken as A,, the solution phase affinity. Efis the Fenni level of a typical metal. P,- is the solvation energy of the reduced species (S-)relative to the parent (S)and is taken to be a negative quantity. The corresponding position in the OMT spectrum is shown on the right and corresponds to the affinity of the adsorbate film modified by interaction with the top metal, Af-m.
tions for all kinds of device technology. Any time an organic or inorganic species comes into contact with a metal, the potential for spontaneous reduction must be considered. For example, in STM studies of molecules on metals, the species studied might be a reduction product rather than the originally doped molecule. This problem is aggravated by the fact that the resulting molecular anion can undergo further reactions with oxygen or water if the interface is exposed to air. Additionally, very thin layers of adsorbates may have their affinity levels shifted and broadened by the image field in the metal. Thus, an additional polarization term is applicable to very thin films, and the resulting affinity level is denoted by Af-, in Figure 3. Despite these details, a simple test of reduction stability can be applied as a first approximation. If 9 is the work function of a metal, a species with E112 1 q5 will spontaneously reduce upon electrical contact with that metal. We believe that this is the origin of the observed reduction of TCNQ, TCNE, and ferricyanide (as indicated in Table 1) in the Al-Al203-S-Pb tunnel junction environment. Previous studies have suggested that this reduction is due to redox reactions between the adsorbate and hydroxyl groups present on the oxide On the basis of the trend observed in Table 1, it now appears that these species would have been reduced by contact with the top metal even if no oxide was present. Now consider a molecular adsorbate in a metal-insulatormetal structure such as a tunnel diode or an STM tip adjacent to a metal surface that supports the adsorbate. Ef, shown in Figure 3, is then the work function of the M-I-S-M’ structure and is roughly the average of the work functions of the two metals. When the bias between the metals is such that M’ is positive and is equal to the difference in energy between Ef and the film affinity, eV = Ef - Af-,, the transient reduction of S becomes energetically possible and the current through the junction increases significantly. The onset of this resonance process is signaled by an extremely strong band seen in the plot of (doldV)la vs V , the OMT spectrum. On the basis of Figure 3, we would therefore expect a strong correlation between the first OMTS band position observed from a species, S , and the reduction potential of that species. A comparison of these values for several different materials is given in Table 1. Since all the data reported in Table 1 were taken from Al-Al203S-Pb junctions, the mean value of the Fermi energy was taken
TABLE 1: Standard Reduction Potentials (in Nonaqueous Solvents) and Adjusted First Orbital-Mediated Tunneling (OMT) Band Position (in v ) in a Variety of Compounds molecule 4.2 - OMT band max Ell2 pyridine 1.99 biphenylene 2.46 coronene 2.62 2.61‘ s-triazine 2.66 anthracene 2.80 2.84‘ perylene 3.00 3.Olc tetracene 3.12 3.21‘ Zn tetrabenzoporphine 3.24 3.2gb indene 3.26 pentacene 3.40 3.57‘ Co(acac)3 3.47 Cu phthalocyanine 3.9od Cu(II) phthalocyanine tetrasulfonate 3.98 4.28 [C0(en)3~+1 TCNE 4.50 spontaneous reductione TCNQ 4.56 spontaneous reductiod ferricyanide 4.80 spontaneous reductiod Values of reduction potentials were taken from refs 8 and 34-36 and then corrected to the vacuum state as indicated in the text. Tunneling spectroscopy data are taken from refs 10, 11, 30-33, and 37. Reference 37. Reference 10. * Reference 11. e References 30 and 31. f References 31 and 32. Reference 33. as 4.2 eV.26 Electrochemical data were taken from the literat~re.~~-~~ The band maximum in the OMT spectrum (after baseline correction) is used here because it is easily defined. It is not clear that this is the “best” indicator of the location of the affinity level since the line shape of these transitions has not been characterized fully. As we discuss in previous papers,lOJ1the true line shape almost certainly has a negative high-energy tail that combines with the rising conductance of the junction. Moreover, the band maximum may be more closely related to the vertical electron attachment energy rather than to the adiabatic value (corresponding to the electrochemical reduction potential). The vertical attachment energies are always greater than the adiabatic values. For the present purposes, however, we only require estimates accurate to a few tenths of an electrovolt and that have the correct qualitative trend. For these purposes we believe the band maximum suffices. The agreement between the solution phase reduction potentials and the first OMT band positions is extremely good. We would not expect the agreement to be so good for all materials. As indicated earlier, affinities in the solid state are expected to differ from those in solution. Moreover, there can be shifts in the affinity of a thin film relative to that of a microcrystalline solid due to interactions with the support. For the materials and film thicknesses studied to date, a fortuitous cancellation of polarization terms and differences between vertical and adiabatic affinities has resulted in OMT bands lying very near A,. As the thickness of the adlayer increases, we would expect the OMT band to move from Ef - Af-, toward Ef - A, as the surface state contributions decrease relative to those of the bulk. It should also be noted that the apparent maximum observable reduction potential is set by Ef of the M-I-M’ structure. The value of 4.2 eV used in Table 1 was derived from the mean work functions of A1 and Pb, the metals used in the experiments reported in Table 1. Other metal combinations can be selected that produce a much larger value of Ef and therefore provide the opportunity for observing larger reduction potentials. A Pt substrate paired with a Pt tip, for example, should have Ef near 5.2 eV. A different issue arises when one considers the situation when the direction of the applied bias is changed. In the reverse bias
Mazur and Hipps
6688 J. Phys. Chem., Vol. 99, No. 17, 1995 case, it is the filled levels of S that can contribute to structure in the OMTS. The first band of this type would formally occur when the applied (reverse) bias is near the first oxidation potential of S referenced to Ef. The physical situation here is really very different. Because the tunneling process depends exponentially upon the negative square root of the barrier height, most of the tunneling current comes from the Fermi surface of the more negative metal. In reverse bias, therefore, the appearance of additional states well below the Fermi level would have little effect on the tunneling current. Thus, only materials with oxidation potentials near Efshould be effective in providing OMTS bands in reverse polarity. Until recently, we had only observed reduction (LUMO) mediated processes. We now have found cases where both HOMO- and LUMO-mediated tunneling occur.37 Moreover, one of the reviewers of this paper provided us with a preprint wherein HOMO (only) mediated resonant tunneling in Au-(Langmuir-Blodgett film)-Au junctions is reported.38 In this latter case, the HOMO’S are within 0.5 eV of the junction Fermi energy.
Conclusions The proposed model of orbital-mediated resonant tunneling provides an easy method for predicting the location of the lowest energy OMT band. It also provides a simple rule of thumb for when adsorbates will spontaneously and permanently reduce when in contact with a metal. The model also indicates that the effects of metal-adlayer interaction on unoccupied orbitals can be quantified by following the OMTS band position as a function of adlayer thickness. This electrochemical model does not address the issue of orbital-mediated tunneling through higher affinity states nor is it expected to work well when (1) there are large differences in the equilibrium geometry of the neutral and anionic species, ( 2 ) the electrochemical data are obtained from irreversible processes, (3) the polarization energy in the solid, Pc-, differs markedly from that in the particular solution, Ps-, used for the electrochemical studies, or (4) there are specific chemical reactions between the adlayer and the materials it touches. It appears, however, that these exceptions are sufficiently weak or rare that the simple picture works well for many molecular materials. An exciting consequence of the model is that electrochemical oxidation and reduction potentials for thin films and interfacial materials can now be directly determined through orbital-mediated tunneling spectroscopy.
Acknowledgment. We thank the Petroleum Research Fund of the American Chemical Society for their financial support in the form of Grant PRF-25763-AC6. References and Notes (1) Briegleb, G. Elecktronen-Donator-Acceptor-Komplexe;Springer Verlag: Berlin 1961.
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