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Temperature Independence of Orbital Mediated Tunneling in Cobalt(II) Phthalocyanine Brett Gyarfas,† Bryan Wiggins, and K. W. Hipps* Materials Science Program and Department of Chemistry, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: June 7, 2010; ReVised Manuscript ReceiVed: July 8, 2010
The effect of a large (∼200 K) temperature change on orbital mediated electron tunneling spectra is measured for the first time. Tunneling spectra obtained from CoPc throughout the 300 to 100 K region show no definitive change in integrated intensity. The integrated intensity of the Pc ring LUMO normalized tunneling band, d ln(I)/d ln(V)-V, is 0.28 ( 0.04 V. The ring HOMO and cobalt metal centered orbitals are not separately resolved and give a total integrated intensity of 0.24 ( 0.04 V. Significant temperature dependence was observed in the I(V) curves of bare and CoPc covered Au(111) and is primarily attributed to a change in the work function with temperature and the associated change in tip-sample separation at fixed set point. Introduction
Experimental Section
Over the last several years there has been considerable interest in electron tunneling through molecular orbitals. This interest includes configurations ranging from single molecules chemically bound to two electrodes (single molecule wires),1-8 through molecules covalently bound at one end with through-solvent tunneling to the second electrode,9-15 to the case of physisorbed molecules interacting with the tip in an STM.16-24 Of particular interest to us are elastic quasiresonance tunneling events that utilize orbitals of adsorbed species that lead to well-defined spectra of ionization and affinity states. These we have identified as orbital mediated tunneling spectra, OMTS.23,25 In recent years, these spectra have been reported at cryogenic temperatures,22,26-30 over relatively short temperature ranges (e.g.; 5 to 40 K),31 and at or near room temperature.24,25,32,33 Theories of these OMT transitions suggest various temperature dependences, from relatively mild line shape changes15 to large vibronically activated changes in intensity.14,34 These theories include the internal vibrational structure of the molecule and the reorganization of the surroundings associated with the passage of charge. They generally fall into two categories: those in which the electron residence time on the molecule is so short that no vibrational relaxation occurs, and those in which a fully thermally relaxed transient ion is formed that subsequently exchanges charge with the final electrode. Thus, there are sound reasons to question the transferability of OMTS taken at low temperature to that obtained at room temperature. To our knowledge, this issue has never been experimentally tested. The work closest to this is a study by Allara8 in which the temperature dependence of nonresonant super exchange tunneling was measured over a broad temperature range. The present article reports the first study of the intensity of orbital mediated tunneling spectra as a function of temperature over a large (∼200 K) temperature range. CoPc physisorbed on Au(111) was chosen because high-resolution STM imaging and spectroscopy have been reported and are well understood.22,26 * To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: The Biodesign Institute, Arizona State University.
Scanning Tunneling Methods. Scanning tunneling microscopy (STM) and a STM based spectroscopy, orbital mediated tunneling spectroscopy (OMTS), analysis was done in UHV with a commercial variable-temperature microscope (model UHV300) and control electronics (model SPM100) from RHK Technology.35 Sample temperature was measured with a type K thermocouple in direct contact with the sample. Residual gas analysis of the STM chamber typically detected H2 at ∼1 × 10-10 Torr and very low levels of H2O, CO, and CO2, all below 5 × 10-11 Torr. These pressures were reached with a 500 L/s ion pump assisted by a Ti sublimation pump. The STM vacuum chamber was mounted on air legs and housed in a low vibration laboratory at Washington State University. STM tips were prepared from 0.25 mm Pt0.8Ir0.2 wire (purchased from California Fine Wire Co.36). Pt-Ir tips were prepared by mechanical cutting. Generally, several tips were made at a time and then loaded into the STM chamber where their suitability was determined by imaging Au(111). I-V and dI/dV data were simultaneously acquired. Normalized orbital mediated tunneling spectra (OMTS) are defined as (dI/dV)/(I/ V) and were extracted from the I-V and dI/dV curves. The I-V curves were taken directly from the preamp while dI/dV was obtained by using lock-in amplification. The modulation frequency and modulation voltage were 9.2 kHz and 14 mV rms, respectively. To check the calculated conversion factor for lockin signal to dI/dV, we also numerically differentiated some of the I(V) curves for comparison. While the numerical derivatives were much noisier, the two dI/dV methods yielded the same size peaks. The reported spectra represent averages taken over several adjacent molecules. Typically, several groups of 30 spectra collected over a molecule were averaged with results from different molecules on a sample. With the somewhat dull tips used in this study, it was impossible to image the internal structure of CoPc and the spectra also showed no qualitative positional dependence. Two separate runs with different samples, blanks, and tips were made and yielded similar results. The set point for all experiments was 330 pA with a bias of -0.4 V. Note that in the RHK STM design only the sample is cooled and the tip remains near 300 K. STM/STS Sample Preparation. A dedicated cryopumped (Cryotorr 8, 1500 L/s, CTI-Cryogenics) chamber was used for
10.1021/jp105230u 2010 American Chemical Society Published on Web 07/21/2010
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Figure 1. Structure of cobalt(II) phthalocyanine, CoPc.
preparation of Au(111)/mica samples by vapor deposition. This chamber reached a base pressure of 4 × 10-10 Torr without baking. Mica substrates were cut from 1 × 4 cm sheets purchased from Ted Pella37 (Pelco #54). Freshly cleaved mica was heated at 485 °C for 36 h to clean the surface and then the temperature was reduced to 360 °C for gold deposition. Gold splatters (99.999%, Cerac, Inc.38) were evaporated from resistively heated tungsten boats (ME5-.005W, R.D. Mathis Co.39) that consisted of a 0.25“×.005” W strip with a dimple in the center. With these boats, the pressure could be maintained below 3 × 10-9 Torr while evaporating gold at rates of ∼0.7 Å/s over a period of ∼1 h. The deposition rate was monitored with a quartz crystal monitor (QCM). About 120 nm of gold was deposited. The resulting Au film was allowed to cool for at least 6 h before removing and immediately mounting on RHK sample holders and inserting into the RHK deposition chamber that is attached to the RHK STM chamber. Some substrates were also hydrogen flame annealed before loading into the RHK chamber. Organic thin films were prepared by vapor deposition onto the Au(111)/mica substrates in the deposition chamber portion of the RHK vacuum system. These samples were then transferred to the attached STM analysis chamber. CoPc was purchased from Strem Chemicals40 and doubly sublimed in a quartz sublimation apparatus before use. The molecular structure of CoPc is shown on the left side of Figure 1, while the space filling CPK model is shown on the right. STM based orbital mediated tunneling (STM-OMTS) was obtained from films of CoPc on Au that were nearly 1 monolayer thick.
Figure 2. Constant current STM image of CoPc on Au(111) obtained at -0.40 V and 330 pA set point and a temperature of 290 K.
Results and Discussion A typical STM image obtained from the samples studied is shown in Figure 2. Unlike images we have observed using very sharp tips, no internal structure of the CoPc is observed.22,24 On the other hand, it is easy to identify single molecules and defects in the monolayer. We deliberately chose to use tips that gave this quality of image because we wanted to acquire molecular average spectra rather than site resolved ones. Image drift at room temperature was such that acquiring a sufficient number of spectra to provide reasonable signal-to-noise (hundreds) at one site on one molecule was impossible. Thus we chose to average over an entire molecule for a period consistent with the drift rate, move to another molecule and average, and repeat until a reliable molecule independent average was obtained. This approach would have been impossible had we used a sharper tip. Representative averaged tunneling spectra are reported in Figure 3. The peak near +1.2 V bias (Figure 3) is due to orbital
Figure 3. Normalized tunneling spectra at 290 and 113 K. Solid lines are of CoPc on Au(111) while broken lines are of clean Au(111) at the same temperatures.
mediated tunneling (OMT) through the LUMO of CoPc, while the complex structure with a peak near -0.65 V is a combination of a low bias shoulder due to the metal ion and a more intense peak due to the highest occupied Pc ring orbital.22,24,26 With a sharp tip at lower temperature these negative bias structures
Temperature Independence of Orbital Mediated Tunneling TABLE 1: Integrated areas of d ln(I)/d ln(V)-V Spectra for Indicated Bands and Temperatures
J. Phys. Chem. C, Vol. 114, No. 31, 2010 13351 TABLE 2: Parameters Used with Eqs 2 and 3 in Calculating the Smooth Curves Shown in Figure 5
band max (V)
temp, K
area (V)
curve
temp, K
φ, V
s, Å
-0.70 -0.66 +1.22 +1.20
290 113 290 113
0.27 ( 0.04 0.29 ( 0.04 0.26 ( 0.04 0.22 ( 0.04
blue blue red red
290 135 0 0
4.85 6.00 4.85 6.00
10.01 9.00 10.01 9.00
K. Simmons41 predicted the temperature dependence of metal-insulator-metal junctions assuming only the broadening of the electron energy distribution near the Fermi surface. For a simple rectangular barrier, he predicted that the percentage change in current between 0 K and T (in K) would be
are resolved and contribute differently at different positions over the molecule.22,24,26 It is immediately evident from Figure 3 that the integrated intensities and band shapes of the OMTS are not significantly affected by this 177 K temperature change. To our surprise, however, the nonresonant tunneling background does change significantly. To demonstrate that the background change was not associated with the CoPc adlayer, we also collected the normalized tunneling spectra obtained from a clean Au(111) substrate with the same tip at the appropriate temperatures (broken curves). This change in background is in fact the major source of error associated with determining the integrated areas as a function of temperature. Using our best estimates of baselines, we found the integrated areas under the curves to be as reported in Table 1. While there appears to be a slight increase in area of the 113 K complex of peaks maximizing near -0.65 V, the differences are statistically insignificant. Similarly, the LUMO peak area is statistically unchanged upon cooling from 290 to 113 K. We note that small shifts in peak position are likely due to the background correction problem and also due to the fact that the negative bias band is actually at least two transitions superimposed. On the basis of our results, the OMTS of CoPc is temperature independent to room temperature. Most all of the models proposed for OMTS predict a small narrowing of the OMTS band with decreasing temperature. Part of that narrowing is due to a sharpening of the Fermi levels of substrate and tip. In our experiment, only the sample is cooled resulting in a narrowing of the energy distribution for the substrate, only. Moreover, most of the width of the lines is intrinsic width associated with the change in geometry of the molecule upon ion formation. Thus, it is not surprising that the OMTS bands in Figure 3 change little with cooling. Because the change in nonresonant tunneling was a significant factor in quantification of the OMTS data, and because its size was a bit of a surprise, we looked more carefully at the I(V) behavior of a Au(111)-vacuum-PtIr tip tunnel structure. Figure 4 displays the I(V) curves obtained at 290 and 135 K with the same substrate and tip. It is at least intuitively correct that more current flows as a function of bias at 290 K than flows at 135
where s is the barrier width in angstroms, T is the temperature in Kelvin, φ1 and φ2 are the work functions for metal 1 and metal 2 in volts, and V is the bias voltage in volts. Taking each φ to be 5 V and s to be 10 Å, eq 1 predicts a %Change between 0 and 300 K of (6 × 10-7)(3000)2/(10 - |V|)%, or 5.4/(10 |V|)%. Thus, the percentage change in the tunneling current at a bias of 2 V or less is predicted to be less than 1%. Even if the tip and Au were heavily contaminated, the work function would be no less than 1 eV and the percentage change below 1 V would still be less than 5%. Moreover, Simmons predicted that the slope of the %Change curve would become negative at a voltage near the mean barrier height. Thus, so long as the %Change is increasing with V, the barrier height is greater than the applied voltage (2 V in this case). Letting the work functions take their smallest reasonable value, eq 1 predicts a maximum percentage change of about 2.5% at 2 V bias. The measured %Change derived from data in Figure 4 is presented as the broken line in Figure 5. The region near zero bias has been removed because of errors associated with the very small values of numerator and denominator. The measured %Change is a factor of 10 (or more) larger than that predicted by eq 1. Thus, neither in size nor in functional form is the temperature dependence consistent with simple thermal broadening of the Fermi edge. Of course, Simmons formula was derived for a metal-insulator-metal junction where the insulator was thought of as a rigid chemical species so that neither the work function nor the barrier thickness changed with temperature. As it turns out, neither assumption is appropriate in the STM environment.
Figure 4. I(V) curves obtained from a Au(111) surface at 290 K and at 135 K with the same Pt0.9Ir0.1 tip.
Figure 5. Percentage change in I(V) curves obtained from a Au(111) surface at 290 and 135 K with the same Pt0.9Ir0.1 tip (broken curve). Also shown are plots of predicted curves based on eqs 2 and 3. The solid blue curve has the explicit temperature difference while the solid red curve does not. Both utilize the parameters shown in Table 2.
%Change ) (6 × 10-7)(sT)2 /(φ1 + φ2 - |V|)%
(1)
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The work functions of metals are known to be temperature dependent, both because of intrinsic properties and because of surface adsorption.42-47 In our case, the temperature-dependent surface adsorption of hydrogen or carbon monoxide (the primary residual gases at the experimental pressure) must be considered in addition to intrinsic work function changes of the Au(111)/ mica substrate. Any, or all, of these mechanisms can lead to a change in the work function, φ, of the surface. Temperaturedependent changes in work function, at constant tip-surface separation, would lead to significant changes in tunneling current. Unfortunately, most tunneling experiments (including those reported here) are performed at a fixed set point (fixed value of current and voltage). Thus, as the work function changes at a fixed set point, so does the distance between the tip and the surface. Any analysis of the I(V) temperature dependence must take this into account. The simplest way to do this is to use Simmons full formula for the tunneling current with both the work function and barrier width as adjustable parameters at each temperature, but constrained to give the same set point current as T varies. The appropriate formula at low to intermediate bias for an asymmetric junction is:41
[( )[ (
)[
(
(
V exp -1.02s 2 V V φ+ φ+ exp -1.02s 2 2
J(V, φ, s, T) ) A φ -
(
(
φ-
V 2
))]
-
))]][1 + 6 × 10
-9
(sT)2 2φ - V
]
(2)
where the work function and voltage are expressed in volts, s is the barrier thickness in angstroms, and T is the temperature in Kelvin. As mentioned above, φ and s may be temperature dependent and are interrelated through the requirement that:
J(Vsp,φ(T1),s(T1), T1) ) J(Vsp,φ(T2),s(T2), T2)
(3)
and Vsp is the set point voltage. The above formula is a rather simple approximation having many failings that include the facts that it does represent the true shape of the potential barrier, it is a one-dimensional theory, it does not take into account variations in density of state of the metals, and it is based on the WKB approximation. Thus, we did not attempt to exactly fit all parameters, but instead chose to find a reasonable set of values that would approximate the experimental results. Figure 5 displays the calculated %Change obtained from such a choice of parameters inserted into eqs 2 and 3. The solid curves are calculated by using the parameters given in Table 2. We note that these are not unique values because other combinations of φ and s can give similar fits. The blue curve results when the actual upper (290 K) and lower (135 K) temperatures are inserted. On the basis of our earlier discussion, we suspected that the explicit T dependence would have a very small effect for temperatures below 400 K. Thus, we recalculated assuming that the explicit temperature was 0 K (red curve in Figure 5). It is apparent from Figure 5 that the majority of the temperature dependence in the I(V) curves comes from the implicit temperature dependence of the work function and the associated change in the tip-sample separation. At this point, it is worth returning to the apparent temperature independence of the OMTS. Why is it not strongly affected by the same changes in s? If the OMTS are reported as dI/dV-V spectra, it certainly will be! However, the normalization technique used here to report OMTS is known to factor out much of the exponential work function dependence and also
the tip distance dependence in cases where there is no specific tip-adsorbate interaction.48-51 On the other hand, it should be pointed out that the error bars on the integrated intensity are large enough to hide small changes in intensity. Moreover, the fact that the OMTS of CoPc is at most weakly temperature dependent certainly does not mean that all molecular systems will have temperature-independent OMTS in this temperature interval. Conclusions Orbital mediated tunneling spectral integrated intensities of CoPc are temperature independent up to 300 K within our experimental error of 15%. The main source of error is the strong temperature dependence of the I(V) curve associated with the Au-vacuum-tip structure. We demonstrate that the origin of this baseline variation can be attributed to thermally induced changes in the work function and the associated changes in tip-sample separation caused by the use of a single fixed set point in the measurements. The normalized OMTS is less sensitive to this problem. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant CHE0555696. We thank the NSF for its support. References and Notes (1) Chen, F.; He, J.; Nuckolls, C.; Roberts, T.; Klare, J.; Lindsay, S. Nano Lett. 2005, 5, 503–506. (2) Perez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M.; Oleynik, I.; Tao, N. J. Nat. Chem. 2009, 1, 635–641. (3) Wang, W.; Takhee, T.; Reed, M. J. Phys. Chem. B 2004, 108, 18398–18407. (4) Chen, F.; Tao, N. J. Acc. Chem. Res. 2009, 42, 429–438. (5) Lindsay, S. J. Chem. Ed. 2005, 82, 727–733. (6) Ghosh, A.; Datta, S. J. Comput. Electron. 2003, 1, 515–525. (7) Tao, N. J. Chem. Educ. 2005, 82, 720–726. (8) Selzer, Y.; Cabassi, M.; Mayer, T.; Allara, D. L. Nanotechnology 2004, 15, S483–S488. (9) Saloman, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V.; Frisbie, D. AdV. Mater. 2003, 15, 1881–1890. (10) Han, W.; Durantini, E.; Moore, T.; Moore, A.; Gust, D.; Rev, P.; Letherman, G.; Seely, G.; Tao, N.; Lindsay, S. J. Phys. Chem. B 1997, 101, 10719–10725. (11) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066–4069. (12) Schmickler, W. Interfacial Electrochemistry; Oxford University Press: Oxford, UK, 1996; Chapter 19. (13) Scmickler, W. Surf. Sci. 1993, 295, 43–56. (14) Zhang, J.; Chi, Q.; Kuznetsov, A.; Hansen, A.; Wackerbarth, H.; Christensen, H.; Andersen, J.; Ulstrup, J. J. Phys. Chem. B 2002, 106, 1131– 1152. (15) Sumi, H. J. Phys. Chem. B 1998, 102, 1833–1844. (16) Repp, J.; Meyer, G. Phys. ReV. Lett. 2005, 94, 026803-1026803-4. (17) Qiu, X.; Nazin, G.; Ho, W. Phys. ReV. Lett. 2004, 92, 206102-1– 206102-4. (18) Naitoh, Y.; Rosei, F.; Gourdon, A.; Laegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. J. Phys. Chem. C 2008, 112, 16118–16122. (19) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465–1471. (20) Miura, A.; Chen, Z.; Uji-i, H.; De Feyter, S.; Zdanowska, M.; Jonkheijm, P.; Schenning, A.; Meijer, E.; Wurthner, F.; De Schryver, F. J. Am. Chem. Soc. 2003, 125, 14968–14969. (21) Toerker, M.; Fritz, T.; Proehl, H.; Gutierrez, R.; Grossman, F.; Schmidt, R. Phys. ReV. B 2002, 65, 245422-1–245422-8. (22) Barlow, D.; Scudiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413– 4421. (23) Hipps, K. W.; Scudiero, L. J. Chem. Educ. 2005, 82, 704–711. (24) Lu, X.; Hipps, K. W.; Wang, X.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197–7202. (25) Hipps; K. W. In Handbook of Applied Solid State Spectroscopy; Vij, D. R., Ed.; Springer Verlag: New York, 2006; ISBN 0-387-32497-6. (26) Takada, M.; Tada, H. Mol. Cryst. Liq. Cryst. 2006, 455, 93–97.
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