Inelastic electron tunneling: an alternative molecular spectroscopy

FEATURE ARTICLE. Inelastic Electron Tunneling: An Alternative Molecular Spectroscopy. K. W. Hipps' and Ursula Mazur. Department of Chemistry, Washingt...
4 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. 1993,97,1803-1814

7803

FEATURE ARTICLE Inelastic Electron Tunneling: An Alternative Molecular Spectroscopy K. W.Hipps’ and Ursula Mazur Department of Chemistry, Washington State University, Pullman. Washington 99164-4630 Received: March 5, 1993; In Final Form: April 29, 1993

The basic theoretical and experimental concepts required for an understanding of inelastic electron tunneling spectroscopy (IETS) are presented. While most of the applications of IETS to date have centered on surface chemical analysis, the thrust of the present review is to present IETS as an alternative molecular spectroscopy. Comparison of IETS, IR, and Raman data obtained in the vibrational region of the spectrum and of IETS and absorption and reflectance data taken in the electronic region of the spectrum will be made. The difference in selection rules in IETS and in optical spectroscopy is emphasized. Numerous examples of optically forbidden transitions observed as strong bands in IETS are presented. Spin and orbitally forbidden electronic transitions in the IETS are often as strong as, or stronger than, their optically allowed counterparts. We will identify those spectral features that are unique to the tunneling environment and cannot be associated with normal molecular spectra. Finally, we will give a brief introduction to some new innovations that may make tunneling spectroscopy a more useful technique for the nonspecialist.

I. Introduction

W h y Study IETS? Inelastic electron tunneling spectroscopy is an all electronic spectroscopy that has been extensively reviewed.’“ It has been successfully applied to problems in surface chemistry and catalysis,l-1° adhesion and corrosion,ldJ1J2 and molecular ~ibrational~-~J3-~l and electroni~22-~~ spectroscopy.The motivation for tunneling studies is based on several unusual properties of the electron-molecule scattering process. The advantagesof IETS include the following: (1) ultrahigh sensitivity is observed-less than 10” molecules are required to provide a spectrum;3.33J4 (2) overtone and combination bands are exceptionally weaklJ,ls,25,3~3*-thus it is easier to identify fundamentals in IETS than in IR or Raman; (3) optically forbidden transitions may be observed as strong band~;I3-15Jl-3~ (4) when IETS is used to study chemisorption on oxide surfaces, the oxide bands are generally less intensethan adsorbate bands-it is possible to obtain adsorbate spectra in the “IR opaque” regions of the oxide spectrum.3-5 In addition to these advantages, IETS also has important features which are necessary for any viable spectroscopy. They are as follows: (1) resolution can be better than 5 cm-1; (2) a broad spectral range is observed (50 to above 19 000 cm-1); (3) similar tunneling, Raman, IR, and near-IR band positions and widths a r e observed for similar samp~~~~l,13,l5,16.1~20,2~28,3~5

The focus of the present article is on a single aspect of IETS. Namely, we will consider its application as a molecular spectroscopy. Thus, we will be concerned with small molecules and ions wherein the vibrational and/or electronicstates are relatively well-known. Comparisonof IETS, IR, Raman, and other optical spectra will be used to exemplify the notion that IETS is a technique complementary to the more common photon spectroscopies. More properly stated, IETS is a matrix isolation technique for performing molecular spectroscopy. The molecule of interest is trapped (either physically or chemically) within the insulator/top-metal interface of the tunnel diode. As we shall see, the interaction between the molecule of interest and the IETS environment sometimes leads to chemicalchanges in the molecule. Thus, any study of molecular IETS must include a component

which clearly identifies the chemical form of the trapped species. The significance of this will become clear in the following sections. What Is IETS? By electron tunneling, we mean the motion of an electron from one classically allowed region to another through a region where the electron is classically forbidden to exist. The forbiddenor barrier regionis one in which the potential energy, U,is greater than the total classical energy, E. If the particle moves from one allowed region to another, it must tunnel through the potential barrier. Of course, there is no such rigid prohibition in quantum mechanics. An electron of energy E impinging on the potential barrier from the left in Figure 1A has an exponentially damped probability of penetrating the barrier a distance z. If U and E are both large (several electronvolts) and d is of the order of a few nanometers,there is a finite likelihood of the incident electron emerging into the classically allowed region 111. Since the incident and emergent energies are the same, this is an elastic process. Quantum mechanics p r e d i ~ t s l .that ~ . ~ the transmission probability will be proportional to exp(-Ad(U E)1/2),where A is about 1.0 eV-l/2 A-1. This elastic tunneling process is easily realized in the form of a metal-insulator-metal (M-I-M) tunnel diode. Tunneling spectroscopy studies are carried out on M-I-M’ tunnel diodes, shown schematically in Figure 1B. Typically, 0.1 pm of metal, M, is deposited on a clean microscope slide to form a film 1 mm wide by 3 cm long. An insulating film on the order of 2 nm in thickness is either grown or deposited on that metal film. At this point in the conventional fabrication the material of interest, X,is adsorbed onto the oxide at a coverage ranging between 0.01 and 1.0 monolayers. The device is completed by depositing a second metal, M’, strip (typically 1 mm X 10 mm X 0.2 pm) at right angles to the first and crossing over it. Thus, a M-I-X-M‘ sandwich is formed wherein the insulator and adsorbate act as a fixed thickness barrier to conduction. This device can be related to the simple model shown in the lower part of Figure 1. Consider the upper diagram in Figure 2. The free. electron theory can be used to model the metals, and the insulator may be treated as a vacuum space. The electron states are filled up to the Fermi energy, Et, and the work function of the metal is given by 4. As the two metals and insulator are brought together, electrons flow via tunneling until the Fermi

0022-3654/93/2091-1803$04.00/0 0 1993 American Chemical Society

Hipps and Mazur

1804 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993

An M-I-M'

of anthracene are also shown on Figure 2, as are a few schematic ground vibrational levels. The weak interaction between the top metal and anthracenecauses a shift in anthracene's zero of energy but does not produce significant changes in its level spacing-4.e. its electronic and vibrational spectra are not much changed. This is not true for every molecule/metal combination, but the body of IETS literature'" and recent Raman e x p e r i m e n t ~ l ~ . ~ ~ ~ ~ ~ indicate that it is usually true. Suppose, as in Figure 2, that a small positive voltage, V, is applied to the top (Pb) electrode. Because the electric field inside a metal must vanish, all the potential drop occurs within the barrier. Because the molecule, X,is closer to Pb than to Al, its potential almost moves with that of Pb. At this point a band of electronseVwide has sufficient energy to tunnel intoempty states of the M'(Pb) electrode. If the electrons do so without loss of energy, it is elastic tunneling. Equation 1 approximates theelastic tunneling current at 0 K for small V. The average barrier height is 6 = (c#IM @M/ - eV)/2. For a typical IETS diode, 4 = 4 eV and d = 2 nm. For values of Vlarger than about 0.7 V the current becomes nonlinear in Vbecause of the voltage dependence of the effective barrier height.

tunnel diode

+

__ reglon z-8

II z=d

Figure 1. Schematic representation of a tunnel diode (B) and a model

of the tunneling process (A).

Elas, V

Blas, V

Figure 2. Energy diagram for a model Al-Al203-anthracenePb tunnel

diodeshowingeiasticandinelastictunnelingchannels (top). The hatched region representsthe filled states of the top and bottom metal electrodes. Theareain thecenterrepresents the insulatorandadsorbate. The HOMO (r)and LUMO (r*)orbitals of an anthracenelike adsorbate are shown and a few schematic vibrational levels are indicated. Energy loss {equilibration)for the tunneling electron occurs through a cascade process In the metal electrode. Also shown (bottom) are the I-V curve, conductancbVcurve,and IETS band that would result from an inelastic tunneling channel opening at eV = hv. surfaces are matched. If the left hand metal electrode is taken as reference, the right hand electrode develops a net charge of sign and magnitude determined by the difference in work functions. Once the Fermi energies are matched, no net current flows. The hatched regions in Figure 2 represent completely filled metal orbitals. Figure 2 is drawn for a typical tunnel junction, something like an Al-A120j-anthracene-Pb diode. The HOMO and LUMO

z = CVexp(-Aci$1/2) (1) In addition to elastic tunneling, there are other tunneling mechanisms which may contribute to the current. IETS is based upon inelastic scattering as is also shown in Figure 2. The moving electronic charge interacts with the timevarying molecular dipoles (electronic or vibrational) to induce excitation of the molecule in the barrier with concomitant loss of energy by the electron. If the applied voltage is less than hv/e, the inelastic channel is closed because thefinalstates arealreadyfdled. At V= hvletheinelastic channel opens. Further increases in Vresult in additional possible final states with an associated increase in current due to this channel. As is depicted in the lower half of Figure 2, there is a break in the Z(V) curve at V = hu/e. If one measures the conductance, dZ/dV, the opening of the inelastic channel is signalled by a step. Plotting d2Z/dV versus Vproduces a peak a t V = hv/e. Both vibrational and electronic transitions may be observed as peaks in the dzZ/dVZ versus V plots. The width of these peaks depends upon the sharpness of the onset of the inelastic process-the thermal distribution of electron energies about Ef. Thus, the IETS line width depends on temperature and is about 3.5 T cm-l K-1.47 Because of this, vibrational IETS is most often performed below 5 K. In its simplest form, an IET spectrum is a plot of d*I/dV versus V. It turns out that using d2Z/dV2/(dZ/dV) as they axis provides spectra having flatter base lines and is most appropriate for high bias ~ o r k . These ~ ~ , are ~ ~called normalized tunneling intensities (NTI). Simple tunneling spectra are measured by applying both a variable bias, V, and a small modulation component, 5,at frequency,$ A lock-in amplifier is used to detect the 2fsignal which is proportional to d2Z/dV2, The bias voltage may be converted to the more conventional wavenumbers through the factor of 8066 cm-1 V-l. The instrumentation required for obtaining normalized intensities, NTI, is a bit more complex48 but is similar. The amplitude of the modulation determines the observed signal strength and resolution. The signal increases as V t but the experimental line width is proportional to 5 . 4 7 11. Experimental Methods

Sample Preparation. Methods for making M-I-X-M' tunnel diodes have been extensively described.l4 We will only outline the procedure. Typically, a high vacuum chamber is used. A thin strip of metal, most often A1 or Mg, is deposited on a flat substrate (typically a Pyrex microscope slide) to form the metal strip described in the previous section (see Figure 1B). The insulator is then either deposited on the metal (M) strip, or grown on it. Most often, the insulator is a native oxide. At this point

Feature Article

The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7805

4

+

I

[

Ramp (bias)

/

f l

I

/

@

0 OVM 0

substrate Figure 3. Block diagram of a simple d2V/dP IETS spectrometer.

8000

16000

Energy (cm-’

)

Figure 4. Comparison of constant resolution (top) and d2V/dP spectra obtained from the same [AI-A1203-(EbN)2CoCl,-Pb]junction under the same conditions.

in the preparation, the molecule of interest is adsorbed on the insulator. This can be done by direct vapor deposition for i cos (271-8). Substituting into eq 2, we find molecular species, or by adsorption from solution for either molecular or ionic species. In either case, the junction is said to V(Z) = V(1b) (dV/dZ) i COS (2rf) be doped with the molecule of interest, X. Because of the (1/2)(dZV/dZ’)(i cos ( 2 4 ) ) ’ ... (3) exponentialdependenceof the device resistance on barrier width, Expanding the expression in cos (27rft)’, we find a term in cos the barrier is typically less than about 8 nm wide. In order to (271-(2f)t),namely therms amplitude detected by the lock-in tuned satisfy this requirement, the molecules studied must not be too to 2J is large and the method of doping must not leave crystallites or molecular aggregates on the surface. In thecase of solution phase Vz, = -( 1/4)(d2V/dZz)i2/21/2 (4) doping, it has been found that spin doping can produce uniform The above quantity may be converted to dzZ/dVz through the adsorbed layers of the appropriate thicknessprovided the solution relationship concentration is adjusted appropriately. As a starting point, 1 g/L is often used. d’Z/dV’ = -(dzV/dZz)(dZ/dV)3 = -(d’V/dZ’)u’ (5) Once the doping process is complete, the top electrode (most where the conductance of the device at a particular bias is u, often Pb, T1, Sn, or Ag) is deposited. In practice, a number of where u = (dZ/dV). Since the conductance is nearly constant M’ metal strips are laid across the M-I-X strip to form four or throughout the vibrational region of the spectrum, there are no more tunnel junctions on the same substrate. As shown in Figure qualitative differences between d2Z/dVz vs V and dzV/dP vs V 1, four terminal electrical contact is then made either through spectra. the use of In solder or mechanical contacts. The entire substrate The rms modulation voltage across the junction is immersed in cryogen (usually liquid He) and the tunneling V,= i/(1.414u) (6) spectrum measured. Spectrometer Design. There are no commercial tunneling is the experimental equivalent of the slit width in an optical spectrometer. A 2-fold reduction in modulationvoltage increases spectrometers. Tunneling studies must, therefore, be performed the resolution by a factor of 2 and decreases the signal by a factor with homemade instruments. A number of different designs are of four. If neither metal is superconducting, the half-width at described in the l i t e r a t ~ r e . ’ The ~ . ~simplest ~~ design appropriate l / e height of an IETS band is given (in cm-l) b P 7 for the vibrational region of the spectrum (0-500 mV or 0-4000 cm-1) is shown in Figure 3. This instrument14is relatively easy r = i ( 2 . 2 9 ~ ) ~(7.07~~)’ r;ll/’ (7) to build provided that an ultralow distortion oscillator, high input where T is in Kelvin, V ,is in millivolts, and ro is the intrinsic impedance digital DC voltmeter (DVM), and lock-in amplifier half-width of the line in reciprocal centimeters. When one or are available. Theresistor, RIis set toabout 100 times thejunction both metals is superconducting, the apparent line narrows and impedance. The inductors, L, must provide high impedance at shifts slightly to higher energ~.’~,54 In analogy with conventional fand 2J The capacitors prevent the lock-in and oscillator from optical spectroscopy, the oscillator and ramp together act as the providing a low impedance DC path to ground. This particular source whose “slit width” is determined by the modulation spectrometer actually measures dZV/dP. As has been discussed amplitude. The lock-in measures signal intensity, and the DVM by numerous authors, dW/dIZ and dzZ/dVz spectra are essentially provides precise values for the energy (cm-1) axis through the equivalent in the vibrational region of the spectrum since they conversion, 1 mV = 8.066 cm-1. are related by a term that is nearly constant.~~5.48~49~5z~~~~~8 At the greater bias voltages typical of electronic tunneling, the To see how this type of spectrometer works, consider a Taylor spectrometer shown in Figure 3 is not appropriate since the series expansion of the voltage across the device about the bias conductance increases rapidly above about 0.7 V and the lock-in output is scaled as (l/u3). The lower trace of Figure 4 current, zb. demonstrates how rapidly intensity is lost at high voltages in a V(Z) = V(ZJ (dV/dZ)(Z-ZJ + normal d2V/dP spectrum. A much improved spectrometerresults when an ac volt meter is added to monitor the rms modulatioin (l/2)(d’V/dZ2)(Z- Zb)’ ... (2) voltage, V,, across the tunnel junction and a feedback loop is The total current trough the device is the sum of the bias current inserted to maintain constant modulation voltage and, therefore, constant r e s o l ~ t i o n .The ~ ~ constant resolution spectrum of the provided by the ramp and the modulation current, i. Z = zb +

+

+

+

+

+

+

7806 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993

lb\

I 1

- 10.000

I

1

I

I

0 Energy (crn-’

10,000 )

Figure 5. Magnitude of the constant resolution tunneling spectra of two different junctions as a function of bias polarity. The lower spectrum was obtained from an Al-A1203-(en)silancPb junction. The upper spectrum was obtained from a similar device but with Co*+ complexed to the (en)silane.

same device as used to obtain the d2V/dP spectrum is also shown (offset slightly) in Figure 4. Constant resolution spectra offer several experimental advantages. The most obvious is that the experimentalresolution remains constant throughout the spectral range studied rather than progressively decreasing. Since the intrinsic widths of electronic bands are generally much greater than vibrational bands, the improvement in resolution with bias voltage predicted by eq 6 has no practical value. Since the signal level varies as the square of the spectrometer of Figure 3 actually introduces a significant decrease in signal to noise at higher bias voltages. The second benefit of the constant resolution spectrometer is made clear by comparing the two types of spectra presented in Figure4. Thesignaldetected by the lock-in in constant resolution mode is

v,

V2/=(Vf/1.68)2(du/dV)/a (8) This output is directly related to (du/dV)/u, the “normalized tunneling intensity” or NTI. It is a much more useful quantity than dzV/dP at high bias because the elastic background increases monotonically and almost linearly instead of maximizing and then decreasingsharply. In Figure 4, for example,thevery strong transition near 12 000 cm-l is not visible in the dzV/dP spectrum. The third benefit of constant resolution spectra is the relative ease of reporting a quantitative measure of transition intensity, 6a/a,the overall change in conductance associated with an inelastic channel scaled by the overall conductance. If the total changein conductance, 6u,is small, then the area under an inelastic band in the NTI spectrum is (9)

+

where IY = ue ui, the sum of elastic and inelastic tunneling terms. In order to determine 6a/a from d2V/d12 or d2Z/dVz spectra, the conductance must be separately measured and recorded. We will see later that 6u/a5 0.8%for most vibrational bands and 8a/u I10% for most electronic transitions. Bias Asymmetry. Inelastic structure should and does appear in both bias directions. Typically, the spectrum of an M-I-XM’ junction is only measured with M’ relatively positivethe “normal bias direction”. Figure 5 , for example, shows the tunneling spectra of two differently doped A1-A12O3-X-Pb junctions recorded with the Pb electrode positive (AI-) and in reverse bias.24 Note that all the vibrational bands on the normal bias (A1 electrode negative) side are more intense. Spectrum 5b includes a strong d-d electronic t r a n ~ i t i o n .Even ~ ~ for this strong

Hipps and Mazur and broad electronic feature, the reverse bias (AP) spectrum is weaker than the normal bias side. This bias dependence is a natural consequence of the asymmetry in the barrier and the exponential dependence of tunneling probability upon barrier height and width.35J7*56.57 An electron tunneling from the A1 electrode to the Pb electrode (normal bias) does not lose energy by inelastic scattering with the adsorbate until it is near the Pb electrode. An electron tunneling in the opposite direction, however, will lose energy while still facing a significant width of barrier. According to the transition probability shown in Figure 1, the probability of inelastically tunneling in the reverse direction (from Pb to Al) is less by a factor of roughly exp[-d(&/2 - (4 - eV)1/2)]. There are only a few known cases where the IETS does not show this bias direction dependence and most tunneling spectra are only recorded in the Al-, or normal, bias direction. It is also of interest to note the intense structure due to Pb phonons in Figure 5 . These intense features actually appear smaller than they are due to the coarse (4 mV) data acquisition interval. These spectra were taken at 4 K where Pb is superconducting. The very strong electron-phonon coupling completely dominates the region from about -260 to +260 cm-1. When it is desirable to study dopant bands in this region, it is possible to quench the superconductivity,and therefore most of the Pb phonon IETS, by applying a magnetic field with an inexpensive CoSm magnet.

III. The Vibrational Region of tbe Spectrum

Theory of Vibrational IETS Intensities. There is vastly more known about the intensities of vibrational transitions in IETS than is known about electronictransitions. Since the first efforts of Lambe and Jaklevic and of Scalapino and Marcus, the sophisticationand scope of attempts to predict tunneling spectra has increased ~ignificantly.l.l~.35.5~,~~~ To date, all of the attempts to calculate completemolecular spectra have utilized the transfer Hamiltonian formalismfirst introduced by Kirtley, Scalpino,and Hansma (KSH).56 In this section we will discuss efforts to calculate vibrational tunneling intensities for real molecules with emphasis on qualitative concepts that may be useful in predicting intensities for any molecular IETS. Sadly, the state of the theory is such that no complete molecular spectrum has been accurately predicted. We will not come away with selection rules, but rather will find selection preferences. In the original KSH formulation, the interaction potential between the tunnelingelectron and a molecule is a sum of Coulomb terms between the electronand partial charges localized on atoms. Images of these charges in both metals were included and play an important qualitative and quantitative role in understanding IETS intensities. These image charges lead to molecule-surface orientation dependentdipole interferences. For example,a dipole oscillating normal to the metal surface has greatest intensity when it is near the surface and least intensity if it is imbedded in the insulator midway between the electrodes. A dipole near the metal surface and oscillating parallel to it has small intensity, but the intensity increases if it is embedded deep in the insulator. KSH predicted that Raman and IR active modes should have similar intensity and that optically forbidden modes should be observed by IETS. Forbidden vibrational transitions can be observed because the relative phasing of the oscillating partial charges (and their images) is spatially inhomogeneous and breaks the optical selection rules. Off axis scattering, scattering in which the momentum of the electron parallel to the interface changes, was shown to play a qualitatively and quantitatively significant role. According to KSH, electronsexciting vibrationaltransitions have a weighted average scattering angle of 7 degrees. Thus the “normal component” selection rule associated with optical processes near metal surfaces is weakened. For the surface OH stretch, a value of 6u/a = 0.5% was calculated and compared well to typical experimental values of 0.4%. KSH postulated

Feature Article

The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7807

that optical selectionrules would be least appropriatefor predicting the vibrational IETS of large molecules. Kirtley and Hall" modified the KSH theory using a dipole potential Hamiltonian and applied it to the calculation of the IETS of the ion, CH3S03-. Comparisonof theoretical predictions with experimental data indicated that the ion is adsorbed with the C S bond oriented normal to the surface. Dipole derivatives obtained from IR measurements were of the same order as those obtained from the IETS intensities. Correct ordering of the relative intensities was not obtained. The optically forbidden A2 mode was not considered, almost certainly because it lies within the Pb phonon structure and cannot be observed under the experimental conditions used. All the assigned bands were fundamentals, and the strongest band in the spectrum had 6u/u = 0.11%. Godwin and co-workerss9utilized the KSH theory to calculate the intensitiesof the fundamentalsin formicacid and its deuterated form on alumina. This combined theoretical and experimental IETS study of the formate ion (the stable form of formic acid on alumina) gave intensities of the correct order of magnitude. Hipps and Knochenmuss proposed further modifications to the KSH theory.61 The physical model was expandedto includeterms involving charge migration during molecular vibrations. The authors concluded that the direct use of dipole derivatives in estimating KSH theory parameters is not even approximately correct for ionicspecies. Yang and Whitd2used the KSH theory to perform a partial charge calculation on the vibrational IETS of thiourea. Of the 18 fundamentals of the free molecule, they identified 12 IETS bands as fundamental transitions. In the region below 2000 cm-l, all molecular IETS bands are clearly fundamentals. They calculate relative intensities for 7 of the 12 fundamentals. They found that calculated intensities for normal vibrations involving hydrogen did not agree with experiment. The authors believe this is because the bond charges do not follow the physical motions of the hydrogen atoms. ExperimentalComparisonof VibrationalIETS, IR, and Raman. Early in the development of IETS, Simonsen, Coleman, and Hansma reportedthat the tunnelingspectrum of anthracene shows both IR only and Raman only active bands with equal intensity while a small number of strong IR and Raman active bands do not appear.18 They also compared IR and Raman spectra of benzoic acid adsorbed on bulk alumina with their IETS of benzaldehyde on alumina (both form benzoates). They found that, "the agreement in peak positions is amazingly good-the agreement between the tunneling peak positions and those from IR and Raman data is as close as between IR and Raman positions." Cass, Strauss, and Hansma studied long chain fatty acids by IETS.19 They found that IR and Raman intensities of bands characterized by different wave vector, k, values decrease as k increases, but that the tunneling intensities of these modes are similar. They attributed this to the short wavelength of the tunneling electron relative to an IR or Raman photon. The first focused experimental study of selection preferences was the high resolution tunneling study of anthracene in AlA1203-Pb junctions.45 Anthracene has 66 fundamentals and about 37 bands were observed in the IETS below 1600 cm-l, all of which were assigned as fundamentals. The normal mode analysis for anthracene gives 12A1, + 5Al, + 11B1, + 6B1, + 4Bzg 11B2, 6B3, 1lB3, modes. Of these, 33 are Raman active (AI*, Blp, Bl,, Bjg), 28 are IR active (B1, B2,, B3u), and 5 are optically inactive out of plane motions (AlU). Kirtley and Hansma45 concluded that (1) band positions in IETS are in good agreement with Raman and IR, (2) Raman and IR active modes appear with roughly the same intensity, (3) no conclusions could be drawn about orientation selection rules, and (4) anthracene's optically inactive modes have very small intensity. Hall and Hansma39 measured and assigned the IETS spectra of methanesulfonate (CHaSOj-) and trifuoromethanesulfonate. Assuming C3, symmetry for the adsorbed CH3SO3-, rvib = 5A1

+

+

+

-1

K4Fe(CN)6*3H20

inKBr

30

ENERGY (cml) Figure 6. Comparisonof IR, IETS,and Raman spectra ofthe ferrocyanide ion obtained under different conditions.

0

1000

2000

Energy (an-')

Figure 7. IETS,Raman, and IR spectra obtained from CsPCP.

+

A2 + 6E. Both A1 and E modes are IR and Raman active while the A2 mode is optically inactive (forbidden). The A2 mode could not be observed since it occurs below the spectral window used in these studies. All the other fundamentals were observed and reported. The reported tunneling intensities were not a simple linear combination of IR and Raman spectra. The authors said: "In fact, no measured tunneling intensities have ever been reproduced as a simplelinear combinationof infrared and Raman intensities." The relationship between IETS, IR, and Raman spectroscopy is illustrated in Figure 6. The ferrocyanide ion is octahedral in solution but is slightly distorted in the potassium half-hydrate ~ r y s t a l l so ~ *the ~ ~ Raman spectrum shows more bands than expected. It is clear from Figure 6 that the IETS containsdifferent information than either the Raman or IR spectrum and is not a linear combination of both. The line widths in IETS are very similar to those in the IR and are slightly broader than the Raman lines. In a later section we will consider further the IETS of severalhexacyanides, showing that opticallyforbiddentransitions are strongly allowed in the tunneling spectrum. For the present, we note that all of the tunneling spectral bands correspond to fundamental vibrations in the octahedral ion. Another comparison of the results obtained from IR, Raman, and IETS studiesof the same speciesis shown in Figure 7.Infrared, Raman, and tunneling spectra of the pentacyanopropenide ion ( P C P ) all compare favorably in terms of peak positions but have significantly different intensity distribution^.^^ P C P has Cb symmetry and is shown in Figure 7. The IETS has a similar number of bands as does the Raman, but the IETS bands are more uniform in intensity. The IR spectrum, on the other hand,

Hipps and Mazur

7808 The Journal of Physical Chemistry, Vol. 97, No. 30, 199'3

has fewer strongbands than does the tunneling spectrum. Another point to note here is the strong broad feature near 900 cm-l in the tunneling spectrum. This band is due to A 1 4 motion in the oxide barrier and is present in every tunneling spectrum obtained from an Al-Al203-Pb tunnel diode. It is prominent in this particular spectrum because the P C P tunneling spectrum is a weak one. The above comparisons, and many similarones in the literature, indicatethat tunneling data can be usefully combined with Raman and IR spectra taken from conventional samples, e.g. KBr pellets. None the less, IETS is a matrix isolation technique and the presence of an oxide substrate and a metal overlayer might produce severe chemical changes in the molecule of interest (for example, see ref 64). We know, for example, that organic acids generally adsorb on alumina as carboxylate ions. Moreover, the optical selection rules might be sufficiently modified by the simple reduction in symmetry associated with the diode structure to account for all the intensity variationsseen between optical spectra of conventional samples and tunneling spectra. To address these issues requires several different experimental approaches. The first step in any tunneling study must be a careful determination of what species is actually present in the diode. To this end, tunneling spectra obtained from isotopically substituted species and thorough comparison of IR and Raman spectra with tunneling spectra are invaluable. We have also found it very helpful to determine what, if any, reactions occur between conventional high surface area oxides and the molecular species of interest. The effect of the tunneling environmenton band positions and optical selection rules has been studied. The common feature in this work is that some sort of enhanced Raman spectroscopy was used to probe adsorbatesin the tunnelingenvironment. Kirtley's" used surface-enhanced resonance Raman scattering to study 4-pyridine-carboxylic acid chemisorbed in a Al-Al203-Ag junction. The intensity patterns for IETS and resonant Raman spectra were quite different, indicating that the local environment of the ion does not account for the difference in IETS and optical selection rules. Both IR and Raman active modes are observed in the IETS with no significant peak shifts, but the IETS spectrum has significantly different intensities. While not all the IETS lines are seen in the resonance Raman spectrum, all the Raman lines have counterparts in the IETS spectrum. The intrinsic electronicresonance enhancement of the Raman spectrum of P C P was used by Hipps and Keder to probe the tunneling environment.42 A1-A1203 structures were used as substrates. These were made exactly as for Al-Al203-Pb diodes, but no top metal was deposited. Raman spectra obtained from a single monolayer of the pentacyanopropenide anion ( P C P ) adsorbed on the alumina surface of these incomplete tunnel diodes were compared to the IETS obtained from the completed junctions. While both A1 and B2 in-plane motions appear in the Raman spectrum of the ion in solution, only the AI modes appear strongly in the monolayer spectra seen in Figure 8. In contrast, the IETS spectrum contains A1 and B2 in-plane modes as well as out-of-plane motions at 453 and 468 cm-1.65 Comparison of IR and Raman spectra taken from pure solid CsPCP with IETS data shows that theout-of-plane motions are weak in both photon techniques but strong in IETS.42 Most recently, the resonance-enhanced Raman scattering of metal phthalocyanines (MPcs) was used to probe completed tunnel diodes.16 Raman spectra of A1-A1203-MPc-M' tunnel diodes, where M = Cu or H2 and M' = Ag or Pb, were reported. No evidence was found for chemical modification of either CuPc or H2Pc at any step in the fabrication process. No shifts in band positions were observed that amounted to more than their experimental uncertainty of -+2 cm-1.16 In this study Raman spectra were taken of working tunnel diodes while they were under bias. By observation of the Raman spectra of functioning

1

400

Raman v s IETS

550

I

700

Energy (an-')

Figures. Spectra obtainedfromaboutonemonolayerofCsPCPadsorbed on alumina. The broken curve is the IETS while the solid curve is the

surface Raman spectrum. tunnel diodes, it was demonstrated that no modification of the MPc Raman spectrum occurs in the bias range -1.3 to 1.3 V. Thus, despite a local field of about 3 X 106 V/cm, Stark shifts of less than 1 2 cm-l occurred. The Raman spectra provide solid evidence that CuPc and HzPc are not significantly modified by being deposited onto the alumina layer, by being coated with Ag or Pb, or by application of a static potential of up to 1.3 V across the device. Even the Stark shift is small. Intensityof Combinationand OvertoneBands. There are sound theoretical reasons to expect that overtone bands should be very weakin IETS.lJs To our knowledge, there has been no theoretical investigationof the intensities of combination bands in tunneling spectra. To be sure, there are experimental papers that contain tunneling band assignments that include assignments as combination and overtone bands. Most of these are unreliable in that they either were made by simply choosing the energetically nearest equivalent IR or Raman band assignment or were used when the real issue was chemical composition. Consider two examples. The best known overtone band in all of IETS was the overtone of the A1-0 motion that occurred near 1900 cm-I. In the mid 19809, however; Adler36and Gauthier3' showed that this band was actually a fundamental motion of AI-H! Another example comes from the otherwiseexcellent work of Yang et a1.62 In their study of thiourea, they suggest that unspecified combination bands may be responsible for the "extra bands" seen in the N H stretching region of the tunneling spectrum. In the NH stretching region no individual bands are resolved and it is difficult to determine how many transitions occur near 3300 cm-I. A more likely explanation for the observed lumpy band is that the N H region is complex because of inhomogeneity in adsorption sites. The observed texture might also be due to coupling between lattice and internal motions. Is there any hard evidence for the existence of overtone or combinationbands? Todate, the answer is: "Overtones dooccur, but they are very weak. Combination bands are seldom, if ever, observed." Kirtley, for example, says that overtones are about a factor of 200 weaker than fundamentals in the case of the benzoateion.33 Ramsier, Henriksen, and Gent38 identify a single clear overtone in the tunneling spectrum of the phosphite ion (HP032-). The fundamental associated with this overtone is a very strong P-H bending band and occurs at 1034 cm-l. This overtone is about I/sOth of the height of the fundamental. All the other bands in the spectrum appear to be fundamentals. In our own work, we have seen little clear evidence for combination and overtone bands. An exception is the intense CH stretch in the tetraethylammonium ion (2971 cm-l).26 This band has a very weak overtone at 5940 cm-1. The integrated intensity of the overtone is about l/mth of the fundamental band.

+

Feature Article

The Journal of Physical Chemistry, Vol. 97, NO. 30, 1993 7809

TABLE I: Observed’ and Calculated Positions of Vibrational Fundamentals for Potassium Ferrocyanide Trihydrate Based on a Ten-Parameter Potential Functionb A(W) A(15N) mode obsd calcd obsd calcd obsd calcd obsd in 2088 -45 -47 2090 -29 -30 Raman *I, VI -7 -7 -7 -8 Raman 385 386 v2 2055 -44 -41 -29 -29 Raman, IETS(?) E8 v3 2059 v4 392 393 -8 -1 -8 -8 Raman 355 -9 -10 -1 -3 IETS TI, v5 354 T1u V6 2044 2049 41 -45 -35 -30 IR, IETS Vl 586 588 -1 1 -13 -1 -3 IR, IETS vu 415 413 -7 -6 -6 -5 IR ~~

83

v9

T28 Tzu

-2

511

511 113

-18

-17 -1

-1

-2 -3

Raman, IETS

v12

414

415 61

-16

-15

0

-3 -2

IETS

VI3 a

-1

v10 v11

-1

Based upon IR, Raman, and IETS data. Mean percentage error = 0.25%. A ( W ) = v ( W ) - v ( W ) and A(I5N) = v(l5N) - v(14N).

IV. Forbidden Vibrational Modes Importance of Bands Forbidden in Both IR and Raman. Understanding the forces that produce molecules from atoms is a primary goal of physical chemistry. These forces are most easily studied by vibrational spectroscopy wherein the observed fundamental frequencies can be related to the forces that hold a molecule near its equilibrium geometry. Analysis of overtones and combination bands extends our knowledge to regions somewhat distant from the equilibrium configuration. The analysis of these vibrational data yields force constants (or compliances) that provide information about the electronic structure of molecules. The methods by which this analysis may be performed are well-known and have been extensively applied.66-68 Although Raman and IR methods have provided a rich harvest of molecular information, they are limited by the selection rules governing the absorption and scattering of light. For example, consider the ferrocyanide ion. The octahedral Fe(CN)6-3 ion has 13 fundamentals, rd,, = 2A1, + 2E, + TI, + 4T1, + 2T2, + 2Tzu, six of which are Raman active AI,, 2E,, 2T2,), four are IR active (4T1,), and three are totally inactive, or forbidden (TI,, 2Tzu). Thus, a large fraction of the ion’s spectrum, and the correspondinginformation about the molecular forces present in the ion, is not availableto the optical spectroscopist. This situation is repeated in many different classes of ions and compounds. Of the 47 point group character tables listed by 27 contain irreducible representations that do not transform as either IR or Raman active. Thus, molecules of sufficient size belonging to these 27 point groups would have fundamental vibrations that could not be directly observed with photons. These forbidden bands sometimes may be inferred from the positions of combinations and overtones, but these assignmentsare often ambiguous. Because symmetry factors the vibrational Hamiltonian, inability to assign these forbidden modes is equivalent to ignorance about certain aspects of the molecular potential. What is needed is a new vibrational spectroscopy complementary to IR and Raman but governed by different selection rulesselection rules that allow optically forbidden fundamental transitions to be observed with ease. In the following subsection we will demonstrate that IETS can often satisfy this need. Optically forbidden transitions can appear as strong bands in IETS. The absence of strong combination and overtone bands significantlysimplifies the assignmentsof the observed tunneling bands. On the opposite side of the ledger is the conspicuous absence of clear symmetry based selection rules. This deficit is not a significant problem so long as IETS is viewed as complementary to conventional IR and Raman studies with all of the optically active bands assigned by conventional means.

N

b H

VI 2

W E

z

H

300

500

7

ENERGY (cm-1) Figure 9. Tunneling, Raman, and IR spectra of the ferrocyanide ion. The Raman spectrum was taken from solid &Fe(CN)c3H20.

Forbidden Vibrational Modes Observed by IETS. To our knowledge, optically forbidden modes were first observed as strong tunneling bands in the spectrum of the ferrocyanide ion.13.14,20 Table I identifies the 13 normal coordinates of this ion in terms of both mode number and symmetry. Also given in Table I is the method by which a particular fundamental was observed. Figure 9 presents the metal-cyanide region of the vibrational spectrum as obtained from IR, Raman, and IETS. The Raman and IR data were obtained from solid &Fe(CN)e3H20, a crystalline environment that differs somewhat from octahedral. This reduction in symmetry can be seen in the presence of a weak v7 band (IR active) in the Raman spectrum. Vibrational spectra (IR, Raman, and IETS) of 13C and 15N isotopomers of the ferrocyanide ion were measured and analyzed.13J4 Although the CN stretching motions in both the IR and IETS appear as a single unresolved band, well-resolved metalcyanide bands are found in all three methods. Four prominent tunneling transitions in the region between 300 and 700 cm-1 are assigned to bending motions; two of these (v5 and v12) are optically forbidden modes. Using the peak positions and isotopic shifts reported in Table I, a 10 parameter valence-forcepotential was fit to the data. The band positions and isotopic shifts calculated from this potential are also reported in Table I. The data presented in Table I allow the four force constants associated with the forbidden modes to be determined with precision. The vibrational spectra of Ru(CN)& and OS(CN)6’ were also reported and assigned.13 Here again the four fundamentals

Hipps and Mazur

7810 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993

TABLE II: Etectronic Transitioas in Rare-Earth Oxidea and Oscillator Strength As Measured by IETS and by Optical SpeCtrOSCODY29

u Raman

motions of A, symmetry. Graves and White report that Raman and IR active modes are seen with similar intensities in the IETS. Moreover, two of thestrong peaks (456,858 cm-l) andone strong shoulder (949 cm-1) are assigned to optically forbidden modes. A weak shoulder at 731 cm-l is assigned as a fourth A, mode. V. Electronic Spectroscopy by IETS

0

400

800

Wavenumber Figure 10. Tunneling (bottom),IR (middle),and solution phase Raman (top) spectra of the C(CN)j- ion.

seen in the metal-ligand region of the IETS correspond to one Raman active (v10),one IR active (9), and two inactive (us and v12)modes. They are all bending modes. While the internal CN stretches are observed as a weak broad band in the IETS, the metal-CN stretches were not observed. As is the case for Fe(CN)&-, the tunneling line widths are similar to those in the IR. The solution Raman bands are wider than in the solid state and are comparable to the tunneling widths. An interesting consequence of the change in bonding upon going from iron to ruthenium is the order reversal of and v12. In the osmium salt, v12 isi almost intermediate between vl0 and v7. These changes are paralleled by a collapse in the ~2 to v10 spacing. The tricyanomethanideion, C(CN)3-, is a planar D3hsymmetry ion that behaves chemically like a halide. Thevibrational analysis gives rib= 2At1 At2 4E’ 2Aft2+ E“. Of these, the AtI and E” modes are only Raman active, the E’ modes are both IR and Raman active, the A”2 modes are IR active only, and the A’2 mode is inactive in both IR and Raman.15 Since the inactive mode is a bending motion, it is expected to occur in the region below 800 cm-l. This region of the IR, Raman, and tunneling spectrum is shown in Figure 10, wherein are shown the IETS, solid-state IR, and solution phase Raman spectra.15 Note that the low-frequency A”2 mode could be observed in the tunneling spectrum only because the superconductivityof the Pb electrode was quenched by a static magnetic field. In Figure 10, the IETS lines are clearly broader than the IR or Raman bands, but this is an instrumental artifact. The IETS spectrum was recorded at 4.2 K and 2 mV (rms) modulation. The line width is therefore almost completely determined by the modulation voltage. The assignments in Figure 10 were made based upon data acquired by all three spectroscopic methods. Moreover, the peak positions from three different isotopomers (the natural abundance, 13C enriched, and 15Nenriched species) were measured and an 11 parameter empirical force field was fitted to the data. The authors13 found that this procedure provided a f 2cm-l standard deviation fit to all the normal modes of the ion. Note that the inactive A’2 mode appears as a medium intensity band in the tunneling spectrum,making it easy to identify and to evaluate isotopic shifts. One further point of interest is the band near 300 cm-l in the IETS. This is a vibrational mode of the aluminum electrode and can be seen in Figure 10 only because the tricyanomethanide adsorbs only weakly. Graves and White analyzed the IETS of phenazine:’ a planar D2h symmetry ion having rib = 1lA, + 5A, + 5B1, + 10B1, + 4Bz8 + 10B2, + 10B3, + 5B3,. Of these modes, 30 are only Raman active (llA,, 5B1,, 4Bzs, lOB,,); 25 are only IR active (10B1,, 10B2., 5B3,); and 5 are optically inactive out of plane

+

+

+

Selection Rules in Electronic IETS. The good news is also the bad news-there seem to be no clear selection rules for electronic transitions in IETS. One can, of course, make a simple spin conservation argument to show that electron scattering should be allowed so long as AS = f 1 or 0. Since photon processes cannot change S, this alone is motivation for performing IETS as a molecular electronic spectroscopy. It turns out that electricdipole forbidden transitions can also be observed as strong bands in IETS. In later subsections we will present experimental evidence that IETS can be an effective tool for electronic spectroscopy in the near-IR region of the spectrum. Here, we will consider the meager theoretical basis for understanding electronic IETS. De Cheveigne and co-workers commented on the absence of a workable theory for electronic transitions in inelastic electron tunneling.28 They identifiedthe following difficultiesin observing electronicIETS: (1) the 3-nm A1203barrier (themost frequently used insulator in the M-I-X-M’ structure) will breakdown under a bias of about 2.5 V. (2) At high bias, the conductance varies exponentially with bias making it more difficult to observe small changes in conductance. (3) The inelastically scattered electron must tunnel through a much higher barrier than most elastically tunneling electrons, making 6a/a smaller as V increases. They predict that tunneling spectroscopy should be less sensitive to high energy transitions than to low ones. However, they note that the electron-electronic state interaction appears to be many orders of magnitude larger than the electron-vibrational state interaction.28 It is now known that this last factor can produce overall values of 6a/a approaching 20% (vide infra). An additional factor that may, for specific adsorbates, affect both intensities and band shapes is the interaction of the adsorbate with either the oxide substrate or the top metal electrode. Adane et al.29made an intensity analysis of inelastic tunneling due to excitation of electronic transitions in rare-earth oxides in junctions of the type M-M203-Pb, where M = Ho or Er. They sought td understand why the IETS was about 13 times more intensethan expected based upon the opticallymeasured oscillator strength.29 Experimental intensities, band positions, and assignments are shown in Table 11. Two different theoretical mechanisms were considered in order to account for this factor of 13. These were (1) mixing of the 5d configuration into the 4f configuration by the internal electric field of the junction, and (2) electricquadrupoleterms in the electron-ion interaction. They found that the electric field mixing was not sufficient to account for the IET intensities but that the quadrupole term is sufficient. They suggest that even higher order terms may be important in the electron4ectronic state inelastic scattering process. This same group later noted that off-axis scattering could contribute significantlyto AS = f 1 transitions.22.27 This observation,coupled with KSH’s determinations that off-axis scattering is significant even for vibrational transitions,may be important for the eventual development of a working theory of electronic IETS. Allowed Electronic TransitimObserved by IETS Historically, the first electronic transition observed by tunneling spectroscopy was a spin forbidden one.30 It is much easier to believe the

Feature Article

The Journal of Physical Chemistry, Vol. 97, No. 30, 993 7811

TABLE IIk Electronic Transitions Observed by IETS by de Cheveime et a l . 2 7 9 energy molecule transition (cm-1) maxa CUPC PbS-T 9300 &carotene S-T 10500 Pbpentacene S-T 6500 Pb+ S-S 15000 Pb+ xenocyanine s-s 11OOo b tetracyanine S-S 10500 Pbbis((4-dimethyl-

amino)dithio-

S

-

S

1.3 eV

b

' \\

I

I

\

Mg/MgO

benzy1)nickel

\

Polarity of the device in which the electronicIETS band is strongest. Bipolar data was not given or each side given as a different transform.

\

I I

TABLE Iv: Electronic Transitions in Cobalt(II) Tetrahedral Complexes Observed by IETSW ion transition energy (cm-') &u/u near-IR activity CoBr4s 4A2-4T2 2900 1.0% no COB$4A2-4TI 5000 0.4% Yes coCb23000 2.3% no 4A2-4T2 c0Cbs 5400 1.0% Yes 4A2-4T1 Co(NCS)42- 4A2 4T2 4000 2.7% no 8200 -13% Yes C O ( N C S ) ~4Az ~ 'TI

-

, ,,A \ \v

J

I ,\\'

I

I

4000

2000

I

I

I

6000

E n e r g y (cm-l) Figure 11. IETS (broken curve) and diffuse reflectanceIR (solid curve) spectraof the cobalt(I1)tetrachlorideion. Difference spectra were taken interactively to minimize the EGN+ ion vibrational modes.

+

tetrabromocobaltatd Ill electronic assignments of forbidden transitions, however, after one has had the opportunity to study the IETS of transitions that can also be observed by conventional means. Thus, we will begin our discussion of electronicstate tunnelingspectroscopy by comparing bands seen both in IETS and in optical spectroscopy. I The French group studied the IETS of spin-allowed ~4' t I transitions in organic and metal complex dyes.22.27.28A summary of their observations is presented in Table 111. They found that the positions and widths of the bands they observed in IETS agreed well with optical absorption spectra but that a significant amount of vibronic structure had been lost. This was attributed to interactions between the adsorbed molecule and the tunneling environment. Typical tunneling intensities are about 10 times larger for these electronic transitions than for vibrational ones ( h / a a 5%). A curious feature of the data reported in Table I11 is the bias dependence of the intensity. In some cases, the 2880 4088 6088 reverse polarity (Pb)actually gave larger electronic intensities Energy (cm-' ) than did the normal polarity. Figure 12. IETS (broken curve) and diffuse reflectance IR (solid curve) Because all the early electronicstate tunneling studies resulted spectraof the cobalt(I1)tetrabromideion. Difference spectrawere taken in wide unstructured bands, it was thought that all electronic interactively to minimize the EGN+ ion vibrational modes. state tunneling spectra would be unstructured. This was cleanly disproved when the spectra of the tetraethylammonium, Et4N+, over a total width of about 900 cm-l, considerably less than the salts of several cobalt(I1)tetrahedral complexeswere reported.z,26 observed band width. Thus, the energy spread in the observed A listing of transition energies, assignments, and intensities is band structure is too large (and too complex) to be due to purely given in Table IV. For example, Figure 1 1 contrasts the near-IR electronic transitions.25326 diffuse reflectanceand IETS spectra of the cobalt(I1) tetrachloride In order to account for the observed structure, electroniw complex. Figure 12 is the corresponding spectra for the vibrational (vibronic) coupling must contribute to the transition tetrabromide ion. These are difference spectra, wherein the mechanism. The 4Az 4T1 transition is unusual in that any spectrum of Et4NC1, or EtrNBr, was used to remove most of the vibrational excitation may couple with the electronic transition spectral features associated with the anion. The oscillations near without changing its allowedness. One expects, therefore, that 3000 cm-1 in the IR and tunneling spectra (Figures 1 1 and 12) each spin-orbit component would be accompanied by significant are the result of a slight shift in the CH stretch upon complexation. vibronic structure and that the observed width would be much Since the residuals from subtraction of the IR spectrum of Et4larger than predicted by spin-orbit coupling alone. Of course, NCl from that of (Et4N)~CoC14are of comparable peak intensity if the vibronic coupling becomes strong enough, Jahn-Teller to the total peak intensity of the 4Az 4T1electronic transition, interactions may also contribute to the structure.70 Two further it is clear that the 4A2 4T1transition in the IR is not much points should be noted. The electronic transitions in these stronger than a typical IR vibrational band. compounds are most intense in the normal bias direction, as expected. Also, the integrated intensities of the tunneling An exciting feature of Figure 1 1 is the structuring seen on the transitions increase as a function of ligand in the series SCN-> allowed 4A2 4T1transition. Feature for feature the IETS and C1- > B r . optical diffuse reflectance spectra agree, thus laying to rest the Figure 5 (upper trace) is a spectrum obtained from a collection notion that electron state IETS is always unstructured.25s26The of cobalt(I1) complexes that were formed as part of the device origin of the structure is interesting. The spin-orbit interaction fabrication. Alumina-immobilizedN-(2-aminoethyl)-3-aminois expected to split the transition into four components spread

4

I

-

-

-

-

Hipps and Mazur

7812 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993

intensities measured. As can be seen from Table 111,even though the transitions are 1.1 eV (8500 cm-1) apart and even though one transition is strongly allowed and the other strongly forbidden, the tunneling intensities only differ by a factor of 4. Forbidden d-d transitions have been observed in the IETS of cobalt(I1) tetrahedral complexes.25~26As noted previously, allowed d-d transitions are also observed by IETS. Spectra showing both allowed and forbidden transitions in cobalt(I1) tetrahalides are displayed in Figures 11 and 12. The variation of intensity with ligand in these complexes is quite interesting. Although the orbitally forbidden transition intensity increases in the same way as does the allowed transition (Table IV), SCN- > C1- > B r , it increases much more slowly.25 As a consequence, the forbidden propyl) trimethoxysilane, (en)silane, provides an ethylenediaminetransition is much more intense than the allowed one in the bromide like surface functional group that can bind cobalt. On the basis complex, but much less intense in the thiocyanate. The ratios of of the observed d-d electronic transitions, the complexation of intensities of forbidden and allowed bands in the halide complexes, (en)silane with Cox2 in dimethyl sulfoxide (DMSO)produces however, appear to be similar (0.4/1.0 in the bromide and 1.0/ tetrahedral, tetragonal, and (to a lesser extent) trigonal cobalt 2.3 in the chloride). We also note that charge transfer transitions complex species.24 At all coverages, the IETS contains a fixed (d T * ) have been reported by Luth,31J2but the intensities were contributionfrom the spectrum of the tetrahedral COX^^- species. not provided. The 4Az 4T1(P) transition in C O C ~and ~ ~ in - CoBfl2- were Review of Tables 11-V shows that a variety of forbidden observed by IETS at 14 000 and 13 400 cm-l, respectively. The electronic transitions have been observed by IETS. Not only are intensity of this transition in the IETS (6u/a = 5% for C O C ~ ~ ~ - ) spin forbidden transitions observed, but so are spin-allowed but is about 5 times that of the lower energy4A2-4T1(F) band near electric dipole forbidden ones such as the 4Az 4Tztransition 5500 cm-I. The ratio of IR intensities (optical intensities) for the in cobalt(I1) tetrahalides. Moreover, the intensities of the same bands is 1O:l. As the (en)silane coverage approached a electronic bands reported in Tables I11 and IV are enlightening. monolayer, a very strong (6a/u = 11%) broad band grows in There appears to be no clear difference in tunneling intensities centered near 8000 cm-I. This band was assigned as the 2Al, for bands that are spin allowed, orbitally allowed, totally allowed, ZBzr transition in a square planar low-spin Co(en)2+complex; and totally forbidden. This may, in part, be due to the fact that but, an admixture of 2A1, 2E, transition may'be present. A until recently tunneling spectroscopists only chose to assign bands band near 9700 cm-1 also grows in with increasing (en)silane that showed clearly in d2V/dP spectra. As indicated by the very coverage and is believed to result from a six-coordinate pseudosmall intensities of the rare-earth oxide f-f bands, there probably D3 complex. is a very broad spectrum of possible electronic state intensities. Forbidden Electronic Transitions Observed by IETS. The first We can be sure that optically forbidden transitions can, and often electronic transition observed by IETS was the singlet to triplet do, appear as strong bands in the IETS. T-T* transition of copper phthal~cyanine.~~ A more modern Just as in the case of optically allowed transitions, optically version of the spectrum is presented in Figure 12.23 As indicated forbidden transitions can also show vibronic structuring. Consider in Table 111, this spin forbidden transition occurs near 9300 cm-I for example the electric dipole forbidden 4A2 4T2transition of with intensity 1-2 orders of magnitude larger than the correCoBrd2- (shown in Figure 12). This transition has never been sponding vibrational bands (just discernible in Figure 12). As observed by any other method. While thecentral band is obscured was the case for the spin-allowed transition in tetracyanine, the by the C H stretching band, the electric dipole forbidden 4Az electronic band appears more strongly in reverse bias. In fact, 4Tztransition shows three well-defined sub bands in IETS.25 Under there is no sign of it where it is expected to be strongest, in the favorable conditions, therefore, we can expect to probe excitedAl- bias direction. We believe that this violation of the bias state vibronic structure by IETS. polarity rule is the result of resonant t ~ n n e l i n g . ~It~isqthought ~~*~ What Is NOT Electronic State IETS? The most common that the LUMO of the T system lies about 0.5 eV above the feature observed in tunneling that might appear to be an electronic Fermi surface of the top metal electrode. Only a small forward transition but is not is the quantum size effect structure near 0.83 bias is required to bring that LUMO into resonance with the V (6700 cm-1).72,73For the majority of tunnel junctions this Fermi surface of the opposite (A1 or Mg) electrode. A reverse feature appears as a simple dip. It is easily identified by two bias, on the other hand, never brings the LUMO into resonance characteristics-it moves with temperature and it appears only with the opposite electrode, and only a very large bias can force in the normal bias direction (Pb+). Under unusual circumstances, it into resonance with the top (Pb) electrode. the QSE structure can become large and oscillatory,73 but it can Luth, Roll, and E ~ e r t ~ I .reported 32 the d2V/dP electronic state still be identified by the above characteristics. spectra of several metal phthalocyanine films. They reported Trap-mediated resonant tunneling can also lead to unusual spectra obtained from Al-Al203-X-Pb devices in both polarities. features in the IETS46J4 This phenomena is rare but should be A summary of their results appears in Table V. As discussed in easy to identify since it produces a derivative like line shape in section 11, it is very difficult to apply the d2V/dP method to the tunneling spectrum. Surface states near the top metal band transitions occurring above about 0.7 eV. For example, they did edge may also produce structure in the tunneling spectruma75 not observe the T-a* spin forbidden transition in ZnPc even though it is very clear in the constant resolution ~pectrum.~l Thus, the work of Luth et al.31,32 is very important in terms of demonstrating VI. Things to Come that IETS can be applied to a range of metal phthalocyanines, Infusion Doping. There are several reasons why the normal but the completeness and precision of the entries in Table V are procedure for preparing tunnel diodes might not seem attractive questionable. In addition to the phthalocyanines, p-carotene,z7 p e n t a ~ e n e , ~ ~ to the nonspecialist. A few are as follows: (1) The vacuum steps requires equipment and expertise that and NH-rhodamine-merocyanineg1 all have S T transitions are not available. that have beenobserved by IETS. Of these, pentaceneis especially interesting because both the S T and S S transitions arising (2) Your sample desorbs from the insulator during top metal from the same T-T* excitation have been observed and their deposition.

TABLE V Assignments and Energies of Electronic Transitions Observed in IETS by Luth, Roll, and Ewert31J2 molecule transition energy (cm-1) max pol CUPC S-T 9300 PbH~Pc S-T 9700 Pb+ impurity 5500 PbS-+T 9700 Pb+ FePc d-r* 5100 PbCOPC S-T 10900 Pb+ d d r * 6000 Pb+ NiPc S-T 11300 Pb+ ZnPc d-r* 5200 Pb+ 12100 Pb+ NH-rhodaminemerocyanine S T

-

-

-

-

-

-

-

-

- -

-

Feature Article

The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7813

1

-12,000

i

-6,000

i

v

i

0.0

I

I

6,000

1

12,008

Energy (cm- ) Figure 13. Magnitude of the constant resolution tunneling spectrum of copper phthalocyanine taken at 4 and 77 K.

(3) Your sample reacts with the topmetalduring thedeposition. (Rare, but it happens.) (4)Your sample is short lived and must be rapidly cooled if it is to survive. All of these concernswould be answered if prefabricated tunnel diodes with permeable top electrodes were available. You could open a “blister pack”, place a drop of solution on the junction, wire up the device, immerse it in helium, and then measure the spectrum. Suddenly, IETS is much easier! While we have not reached this state, we are almost there. It was known that this kind of post fabrication doping (infusion) was possible with molecules as large as pyridine.7678 Unfortunately, the reliable electrode materials were highly reactive with atmospheric water vapor. What was needed was a chemically and physically stable electrode having well defined porosity. Conceptually, the device would look like the one in Figure 1, but it would have a porous gold top electrode that would admit dopant molecules into the barrier region of the diode. We recently demonstratedthat such structures can be made.79.80 A prescription is now available for growing a porous gold film that is permeable to water, formic acid, acetic acid, and thiocyanate ion.80 There is still a lot to be learned about the pore size distribution in these metal films and how changes in the film fabrication process affect that distribution. There are also questions remaining about the long term stability of the pores. Nevertheless, it is now clear that our “blister pack” scenario is a real possibility for the future. Tunneling at Higher Temperatures. In section I1 we discussed the role of temperature on the tunneling line shape and indicated that virtually all molecular tunneling spectroscopy is presently performed at helium temperature. We suggested that, for electronic state studies, this is usually unnecessary. For almost all electronicstate tunneling liquid nitrogen will suffice. To make this point graphically, consider Figure 13. The top spectrum was taken at 4 K using 8 mV rms modulation. Thus, the line width in the upper spectrum was determined by the modulationvoltage. The same junction was then placed in liquid nitrogen and the lower spectrum in Figure 13 was recorded. Clearly, one would not like to attempt vibrational studies at this temperature. However, the thermal width does not significantly distort the U-U* electronic transition. For the highest resolution electronic state tunneling studies, those analyzing vibronic structure, it may be desirable to reduce the measurement temperature to 30 K or so. This temperature region is easily and relatively inexpensivelyachieved using a closedcycle refrigerator. In the future, we expect to see a significant portion of tunneling spectra obtained within a closed-cycle refrigerator or at liquid nitrogen temperature.

IETS in a STM. The structure of a scanning tunneling microscope, reduced to its essentials, is the same as a very small tunnel diode. The tip constitutes the bottom electrode and the metal sample support is the top electrode. Adsorbates on the sample support are equivalent to the molecular species in IETS. There are no fundamental reasons why the types of spectra discussed above could not be obtained from a very few molecules, perhaps even a single molecule at a specific site on a metal. As first pointed out by Binnig et al.,81 the limiting factor in doing IETS within the STM is that (at the moment) the top electrode is too shaky. Since the current through the barrier depends exponentiallyon the barrier width, small changes in barrier width caused by tip to sample vibrations produce large changes in conductance. These conductance changes mask the conductance changes associated with the inelastic tunneling process. In order to make STM-based IETS viable, we must either decrease the motion of the tip relative to the sample or we must increase the conductance change associated with the inelastic channel. Initially, both will be required. The STM can be reduced in size and made more rigid to isolate the tip and sample from the dominant low frequencyvibrations. Accordingto Binnig, current changes (6u/u) of a few percent are detectable by lock-in techniques in microscopes having 0.002-nm gap stability. Kuk and Silverman,82reported on a room temperature STM having a gap stability approaching 0.0001 nm. If this stability is inserted into Binnig’s formula for the changein current through the STM,80 this is stable enough for measuring 6u/u = 0.2% for a 4-eV barrier height. While this stabiliity has yet to be achieved in a cryogenic microscope, there appear to be no fundamental barriers to reaching it. The ideal initial sample for IETS in an STM is one showing a strong electronic state transition. These have both a large conductance change and can be measured with high modulation amplitude (better signal to noise) and relatively high (77 K) temperature. In the near future we expect to see electronic state IETS being performed in the scanning tunneling microscope. Acknowledgment. We would like to thank our present and past postdoctoral and research students, both graduate and undergraduate, for their assistance. We also thank the Petroleum Research Fund (Grant 25763-AC3), the United States Environmental Protection Agency (Grant R-8 16329-02-0), and the National Science Foundation (Grant DMR 9201767) for their financial support. References and Notes (1) Kirtley, J. ACS Symp. Ser. 1980, 137, 217. (2) Reviere, J. C. Surface Analytical Techniques; Clarendon: Oxford, England, 1990; Chapter 20. (3) Hansma, P. K. TunnelingSpectroscopy; Plenum Press: New York, 1982. (4) Weinberg, W. H. Vib. Spectra Struct. 1982, 11, 1.

( 5 ) Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Madey, T. E., E&.; Plenum Press: New York, 1987. (6) Walmsley, D. G.; Tomlin, J. L. Prog. Surf. Sci. 1985, 18, 247. (7) Sleigh, A.; Adkins, C.; Phillips, W. A. Vacuum 1988, 38, 301. (8) de Cheveigne, S.;Gauthier, S.;Guinet, C.; Lebrun, M.; Klein, J.; Belin, M. J. Chem. SOC.Faraday Trans. 2 1985, 81, 1375. (9) Higo, M.; Hayashi, H.; Kamata, S.Appl. Surf.Sci. 1988,32, 338. (10) Hipps, K. W.; Dunkle, E. A.; Mazur, U. Lungmuir 1986, 2, 528. (1 1) Comyn, J.; Horley, C.; Pritchard, R.;Malik, R.Adhesion (London) 1987, 11, 38. (12) White, H. W.; Godwin, L. M.; Ellialtioglu, R. Adhesion 1981, 13, 177. (13) Hipps, K. W.; Williams, S.D.; Mazur, U. Inorg. Chem. 1984, 23, 3500. (14) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1980,84, 3162. (15) Hipps, K. W.; Aplin, A. T. J. Phys. Chem. 1985, 89, 5459. (16) Hoagland, J. J.; Dowdy, J.; Hipps, K. W. J. Phys. Chem. 1991, 95, 2246. (17) Kirtley, J.; Hall, J. T. Phys. Rev. B 1980, 22, 848. (18) Simonsen, M. G.; Coleman, R. C.; Hansma, P. K. J. Chem. Phys. 1974,61, 3789. (19) Cass, D. A.; Strauss, H.L.; Hansma, P. K. Science 1976,192,1128.

7814 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 (20) Hipps, K. W.; Mazur, U.; Pcarce, M. S.Chem. Phys. &ti. 1979.68, 433. (21) Graves, R. J.; White, H. W. Spectrochim. Acra 1987,43A, 107. (22) de Cheveigne, S.;Klein, J.; Leger, A. In ref 7,Chapter 4. (23) Hipps, K. W. J . Phys. Chem. 1989,93,5958. (24) Hipps, K. W.; Mazur, U. Surf.Sci. 1989,207,385. (25) Hipps, K. W.; Mazur, U. J . Phys. Chem. 1987,91, 5218. (26) Hipps, K. W.; Mazur, U. JACS 1987,109,3861. (27) de Cheveigne, S.;Klein, J.; Leger, A,; Belin, M.; Defoumeau, D. Phys. Rev. B 1977,15, 750. (28) de Cheveigne, S.;Leger, A,; Klein, J. Proc. 14th Inter. Conf. Low Temp. Phys. 1975,3,491. (29) Adane, A,; Klein, J.; Leger, A.; Belin, M.; Defourneau,D. Phys. Rev. B 1984,29,4443. (30) Leger, A,; Klein, J.; Belin, M.; Defourneau, D. SolidStare Commun. 1972,1I,1331. (31) Luth, H.; Roll, U.; Ewert, S.Phys. Rev. B 1978,18,4241. (32) Roll, U.;Ewert, S.;Luth, H. Chem. Phys. Lett. 1978,58,91. (33) Langan, J. D.; Hansma, P. K. Surf.Sci. 1975,52,211. (34) Cederberg, A. A. Surf.Sci. 1981,103, 148. (35) Kirtley, J.; Soven, P. Phys. Rev. B 1979,19, 1812. (36) Igalson, J.; Adler, J. 0. Phys. Rev. B 1983,28,4970. (37) Gauthier, S.;De Cheveigne, S.;Klein, J.; Belin, M. Phys. Rev. B I&, 29, 1748. (38) Ramsier, R. D.; Henriben, P. N.; Gent, A. N. Surf.Sci. 1988,203, 72. (39) Hall, J. T.; Hansma, P. K. Surf.Sci. 1978,71, 1. (40) Tsang, J. C.; Kirtley, J. R.; Theis, T. N.; Jha, S.S.Phys. Rev. B 1982, 25,5070. (41) Mazur, U.;Hipps, K. W. J. Phys. Chem. 1982,86,2854. (42) Hipps, K. W.; Kcder, J. W. J . Phys. Chem. 1983,87,3186. (43) Hipps, K. W.; Dowdy, J.; Hoagland, J. J. Lungmuir 1991, 7,5. (44) Hipp, K. W.; Hoagland, J. J. Lungmuir 1991, 7 , 2180. (45) Kirtley, J.; Hansma, P. K. Surf.Sci. 1977,66, 125. (46) Gundlach, K. H. Fesrkorperprobleme 1971,11, 237. (47) Hipps, K.W.; Peter, S.L. J. Phys. Chem. 1989,93,5717. (48) Hipps, K. W.; Mazur, Ursula. Rev. Sci. Instrum. 1988,59, 1903; 1987,58,265. (49) Khanna, S.K.; Lambe, J. Science 1983,220, 1345. (50) Dargis, A. B. Rev. Sci. Instrum. 1981,52.46. (51) Colley, S.;Hansma, P. Rev. Sci. Instrum. 1977,48, 1192. (52) Gajda, G. J.; Weinberg, W. H. Rev. Sci. Instrum. 1985, 56,700. (53) Huriuchi, T.; Ebisawa, F.; Tabei, H. Rev. Sci. Insrrum. 1989,60, 993. (54) Kirtley, J. R.; Hansma, P. K. Phys. Rev. B 1976,13,2910.

Hipps and Mazur (55) Brown, N. M. D. In Spectroscopy ofburfaces; Clark, R., Hester, R.

E.,Eds.;John Wiley and Sons: New York, 1988;p 215.

(56) Kirtley, J.; Scalpino. D. J.; Hansma, P. K. Phys. Rev. B 1976,14, 3 177. (57) Muldoon, M. F.;Dragwt, R. A.; Coleman, R. V.Phys. Rev. B 1979, 20,416. ( 5 8 ) Lambe, J.; Jaklevic, R. C. Phys. Rev. 1968,165,821. (59) Godwin, L. M.; White, H. W.; Ellialtioglu, R. Phys. Rev. B 1981, 23,5688. (60) Gauthier,S.; Klein, J.; Leger, A,; DeCheveigne,S.; Guinet, C.; Belin, M.; Defourneau, D. Phys. Rev. B 1982,26,3622. (61) Hipps, K. W.; Knochenmuss, R. J . Phys. Chem. 1982,86,4477. (62) Yang, W. J.; White, H. W. Surf. Sci. 1982,118, 303. (63) Scalapino, D. J.; Marcus, S.M. Phys. Rev. Lert. 1967,18. 459. (64) Mazur, U.;Williams, S.D.; Hipps, K. W. J. Phys. Chem. 1981,85, 2305. (65) Hipp, K. W.; Geiser, U.; Mazur, U.; Willett, R. D. J . Phys. Chem. 1984,88,2498. (66) Jones, L. H. Inorganic VibralioMl Spectroscopy; Marcel Dekker: New York, 1971. (67) Wilson, E.; Decius, J.; Cross, P. Molecular Vibrations;Dover: New York, 1955. (68) Decius, J.; Hexter, R. Molecular Vibrations in Crystals; McGraw: New York, 1977. (69) Cotton, F. A.Chemical Applicationsof Group Theory;Wiley: New York, 1971. (70) Harada, M.; Sakatsume, S.;Tsujikawa, I. J. Phys. Sot. Jpn. 1973, 37, 1359. (71) Mazur, U.;Hipps, K. W. Manuscript in preparation. (72) Jacklevic, R.; Lambe, J.; Kirtley, J.; Hansma, P. K. Phys. Rev. B 1977,15,4103. (73) Hipp, K. W.; Susla, P.; Dunkle, E. J. Phys. Chem. 1986,90,3898. (74) Esaki, L.; Stiles, P. J.;Chang, L. L. Phys. Rev. Leu. 1968,20,1108. (75) Lambe, J.; Thakoor, A. P.; Khanna, S.K. J . Vac. Sci. Technol. A 1984,2,792. (76) Jaklevic, R. C.; Gaertner, M. R. Applications of Surface Science 1978,I , 479. (77) Nelson, W. J.; Walmsley, D. G.; Bell, J. M. ThinSolid Films 1981, 79,229. (78) Bellingham, J. R.; Adkins, C. J.; Phillips, W. A. Thin Solid Films 1991, 198,85. (79) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1992,96,1160. (80) Mazur, U.; Wang, X. D.; Hipps, K. W. AMI. Chem. 1992,64,1845. (81) Binnig, G.; Garcia, N.; Rohrer, H. Phys. Rev. B 1985,32, 1336. (82) Kuk, Y.; Silverman, P. J. Rev. Sci. Instrum. 1989,60, 165.