Langmuir 1992,8, 2311-2316
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Differential Conductance Tunneling Spectroscopy in Electrolytic Solution R. S. Robinson* Bell Communications Research, Red Bank, New Jersey 07701
Cindra A. Widrig Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300 Received March 24,1992. In Final Form: June 8, 1992 We report the first application of the differential conductance tunneling spectroscopy (DCTS) mode of a acaming tunnelingmicroscope (STM)for in eitu examinationof an electrochemicalinterface. Tunneling spectroscopy gives information complementary to the topographicinformation obtained with an STM and is an extremely powerful probe of surface electronic properties, including the chemical nature of the surface. Here, in situ spectroscopy was facilitatedby exposing insulated STM tips to a methanolicsolution of octadecanethiol. This permitted modulation of the tip bias voltage for spectroscopic measurements without excessive capacitive background current. Topographic and DCTS images of a platinum film immersed in electrolyte are compared.
Introduction Scanning tunneling microscopy (STM)1-3has provided reeearchers with high-resolution,three-dimensional images of surface structure in various samplingenvironments and has been widely applied to the study of surfaces in contact with liquids' and under electrochemical control.s@ Additionally, early reports3*'noted the promise of STM for investigationsof surface electronicproperties, as a spatiallyresolved adjunct to classicalmetal-xide-metal tunneling spectroscopy .8,9 A number of such studies, conducted in vacuum and air, have since been (1) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982,49, 57. (2) B i ~ i gG.; , Rohrer, H.Surf. Sei. 1983,126,236. (3) Tersoff, J.; Hanema, P. K. J . Appl. Phys. 1987,61, R1. (4) Sonnenfeld,R.; Hausma, P. K. Science 1986,232,211. Liu, H.-Y.; Fan, F. R.; Lin, C. W.; Bard, A. J. J . Am. Chem. Soc. 1986,108, 3838. fhnnenfeld, R.; Schardt, B. C. Appl. Phys. Lett. 1986,49,1172. Schneir, J.; hnnenfeld, R.; Hanema,P. K.; Tersoff,J. Phys. Rev. B 1986,34,4979. Drake,B.; Sonnenfeld,R.; Schneir,J.; Hnnamn,P. K. Surf. Sci. 1987,181, 92. Sonnenfeld,R.; Schneir, J.; Drake, B.; Hansma, P. K.; Aspnes, D. E. Appl. Phys. Lett. 1987,50,1742. Schneir, J.; Eling~,V.; Hansma, P. K. J . Ekctrochem. Soc. 1988,135,2774. Itaya, K.; Sugawara, S.; Higaki, K. J. Phys. Chem. 1988,92,6714. Zhang, X.G.; Stimming,U.Corros. Sci. 1990,30,951. Itaya, K.; Sugawara, S.; Sashikata,K.; M y 4 N. J . Vac. Sci. Technol. A 1990,8,515. (5) Lustenberger, P.; Rohrer, H.;Christoph, R.; Siegenthaler, H. J. Ekctrwnal. Chem. 1988,243,225. Green, M.P.; Richter, M.;Xing, X.;
Scherson,D.; Haneon, K. J.; Ross,P. N.; Carr, R.; Lindau, I. J. Microsc. 1988,152,Pt 3,823. Gewirth, A. A.; Bard, A. J. J. Phys. Chem. 1988,92, 5663. Lev, 0.;Fan, F. R.; Bard, A. J. J. Ekctrochem. SOC.1988,135,783. Wiechers, J.;Twomey,T.;Kolb,D. M.;Behm,R. J. J.Ekctroana1. Chem. 1988,248,451. Trevor, D. J.; Chideey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989,62,929. Green, M.P.; Haneon, K. J.; Cam, R.; Lindau, I. Phys.Rev.Lett. 1990,137,3493. Trevor, D. J.;Chideey,C.E.D. J . Vac. Sei. Technol. B 1991, 9, 964. Robinson, R. S. J. Vac. Sei. Technol. A 1990,8,511. (6) Robinson, R. S.J . Microsc. 1988,152, Pt 2,541. Robinson, R. S. J . Ekctrochem. Soc. 1989,136,3145. (7) Garcia, N. ZBM J . Res. Deu. 1986,30,533. Arvia, A. J. Surf. Sci. 1987,181,78. Tomita, E.; Matauda, N.; Itaya, K. J . Vac. Sci. Technol. A 1990,8,534. (8) Wolf, E. L. Principles of Tunneling Spectroscopy;Oxford P r a : New York, 1985. hcnneling (9) Jaklevic,R.C.;Lambe,J.Phys.Reu.Lett.1966,17,1139. Spectroscopy; Hansma, P. K., Ed.; Plenum Press: New York, 1982. (10) Binnig, G. Bull. Am. Phys. Soc. 1986,30,251. Gimzewski, J. K.; Mijller, R. Phys. Rev. B 1987,36, 1284. (11) Becker, R S.; Golovchenko, J. A.; Hamann, D. R.; Swartzentruber, B. S. Phys. Rev. Lett. 1985,55,2032. (12) Tromp, R. M.;Hamers, R. J.; Demuth, J. E.Phys. Rev. B 1986, 34.1388.
STM topographic images reflect the integral of the density of tunneling states over electron energiw between the Fermi levels of the tip and substrate. For small applied biae, this approximately correspondsto the surface charge density,9J2J' reflecting the surface geometry. However, the electronic properties of the surface may strongly influence the imagea and can be examined. Tunneling spectroscopy concerns the relationship between the tunneling current signal ( i d and the tipeurface separation ( 8 ) or the bias voltage (VB).Examination of the iT vs s relationship allow the measurement of local barrier heights' and the dependence of iT on VB reflects the local density of states. If the bias voltage is ramped, the density of electronictunneling s t a h available at M e r e n t electron (13) Hamers,R.J.;Tromp,R.M.;Demuth,J.E.Phys.Rev.Lett.1986, 56,1972. (14) Kaiser, W. J.; Jaklevic, R. C. ZBM J . Res. Dev. 1986,30, 411. (15) Kainer, W. J.; Jaklevic, R. C. Surf. Sci. 1987,181,55. Gar& R.; Sbenz,J. J.; Soler, J. M.;Garcia, N. Surf.Sei. 1987, 181,69. Becbr, J. Surf. Sci. 1987,181,200. Feenstra, R. M.;Stroecio, J. A; Fein, A. P. Surf. Sci. 1987,181,295. Hamera, R. J.; Tromp, R. M.;Demuth, J. E. Surf. Sci. 1987,181, 346. (16) Van de Wde, G. F. A.; Van Kempen, H.; Wyder, P.; F l i p , C. J. Surf. Sei. 1987,181, 27. (17) Humbert,F.; Salvan, F.; Mouttat, C. Surf. Sei. 1987, 181, 307. (18) Bando, H.; et al. J . Vac. Sci. Technol. A 1988,6,344. Stroecio,
J. A.; Feenstra, R. M.;Ne-, D. M.;Fein, A. P. J . Vac. Sci. Technol. A 1988,6,499. Hamera, R. J.; Avouria, Ph.; Bozso, F.J. Vac.Sei. Technol. A 1988,6,512. Hamers, R. J.; Demuth, J. E. J . Vac. Sei. Technol. A 1988,6,512. (19) Wolkow,R.; Avouria,Ph. J.Microsc. 1988,152, Pt 1,167. Avouria, Ph.; Wolkow, R. Phys. Rev. B 1989,39,5091. Kuk, Y.; Silverman,P. J. J . Vac.Sci. Technol. A 1990,8,289. Yagi, A,; et aL J . Vac.Sci. Technol. A 1990,8,336. Bando, H.; et aL J . Vac. Sci. Technol. A 1990,8,479. okamura, k; Miyamwa, K.; Goehi,Y. J . Vac. Sci. Technol. A 1990,8, 625. Fan, F.-R., Bard, A. J. J. Phys. Chem. 1990,94,3761. (20) Coombs, J. H.; Cimzeweki, J. K. J. Microsc. 1988,152, Pt 3,841. ( 2 1 ) H a , H. R.; Robinson, R. B.; Dynes, R. C.; Valles, J. M.,Jr.; W d , J. V. Phys. Rev. Lett. 1989, 62, 214.
(22)Stroscio,J.A.;First,P.N.;Dragoeet,R.k;Whitman,L.J.;Pierce,
D. T.; hlotta, R. J. J. Vac. Sei. Technol. A 1990,8,2&1. (23) Nishikawa, 0.; et al. J . Vac. Sci. Technol. A 1990,8,421. (24) Strecker, H.; Stahl,C.; Starke, H. J . Vac. Sci. Technol. 1990,8, 618.
(25) Everson, M.P.;Davis, L. C.; Jaklevic, R. C.; Shen, W. J . Vac.Sci. Technol. B 1991,9,891. (26) Chen, T.; T a m e r , S.; Tucker, J. R.; Lyding, J. W.; VanHarlingen, D. J. J . Vac. Sci. Technol. B 1991,9, 1OOO. (27)Tersoff, J.; Hamann, D. R. Phys. Rev.B 1986,31,2. Lang, N. D. Phys. Rev. Lett. 1986,56,1164. (28) Selloni, A.; Carnevali, P.; Tossati, E.; Chen, C. D. Phys. Rev. B 1986,31,2602.
0 1992 American Chemical Society
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2312 Langmuir, Vol. 8, No. 9, 1992
energies can be obtained from the derivativeof the currentvoltage curve. In “scanning tunneling spectroscopy,” complete current-voltage curves can be recorded for each topographic data point,13t22p25v26 and a daunting amount of data collected. Alternatively, the density of tunneling states can be obtained at discrete bias voltBges.11,14,16,21,24 If the bias voltage is modulated much faster than the time constant of the STM’s tip-positioning constant-current feedback circuit, a modulation is induced in the tunneling current. For a small amplitude bias modulation this signal corresponds to the density of tunneling states at the bias voltage13J7*20*23*29 and forms the basis of differential conductance tunneling spectroscopy (DCTS). The optimum modulation amplitude is limited by thermal smearing, with a maximum practical resolution of =2kT (36 mV) at ambient temperature. The DCTS image can be interpreted as the variation in the density of tunneling states over the surface, at the applied bias voltage. Uncertainty in the lateral tip position during the spectroscopic measurement is eliminated because the topographic and spectroscopic images are recorded simultaneously. Tunneling spectroscopy has provided important insights into the electronicnature of surfacesand their interactions with adsorbates and defects. Unfortunately, the utility of this powerful technique for similar investigations of surfaces immersed in liquidshas gone untapped. Although bias-dependent spectroscopictechniques have been used to study surfaces in vacuum and air, no spectroscopic imaging studies of substrates immersed in liquids have appeared. This is probably due to experimental difficulties. Even under optimum (vacuum) conditions, the spectroscopicsignal (variation in density of states) is only a few percent of the tunneling current, and background signals or noise can prohibit its measurement. In an electrolytic environment, the comparatively low impedance between the tip and sample arising from electrochemical double-layer capacitance causes troublesome reactive background currents at useful modulation frequencies. The usual methods for insulating tunneling tips for topographic imaging in electrochemical environment.^^^^^^^ are ineffective for eliminating the background current. However, using a novel tip preparation method, we have reduced background currents and succeeded in applying tunneling spectroscopy to surfaces immersed in electrolytes. We report the first spectroscopictunneling experiments carried out in an electrolyte solution.
Experimental Section The microscope tips were etched from 0.25 mm diameter Ir wire (Johnson-Matthey) in an aqueous solution of 3.2 M CaCl2 and 0.3 M HC132plus 1 mM Triton X-100 surfactant. After etching through a t 26 V ac (60 Hz), which takes =15 min, the remaining wire was brought into contact with the solution and etched for 2 s a t 6 V. The added Triton X-100 produced a relatively blunt profile that rapidly tapered to a whisker of Ir, as shown in Figure 1. The tips were rinsed with water, then methanol, dried in air, and coated with melted Apiezon “W”wax from an electrically heated Nichrome wire loop.31 After cooling, the coated tip was immersed in a saturated solution of octadecanethiol (Aldrich) in methanol for 1-2 h. (29) Gundlach, K. H.Solid State Electron. 1986,9,949. (30) Wightman, R. M. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1988, Vol. 15. Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989,61,1630. Penner, R. M.; Heben, M. J. Longin, T. L.; Lewis, N. S. Science 1990,250, 1118. (31) Heben, M.J.; Dovek, M.M.; Lewis,N. S.; Penner, R. M.; Quate, C. F. J. Microsc. 1988, 152, Pt 3, 651. (32) Grigg, D. A.; Russell, P. E.; Griffith, J. E.; Valise, M.J.; Fitzgerald, E. A. To be submitted for publication.
Figure 1. Optical micrograph of Apiezon-coated Ir tip, etched as explained in the text. For DCTS imaging, the STM bias voltage was modulated a t an amplitude of 30-75 mV (5-12.596 modulation for a 600-mV bias amplitude) a t 42 kHz. This frequency was high enough to ensure that the distance between the tip and substrate was not affected by interaction of the bias modulation and the microscope feedback. A PAR 5301 lock-in amplifierwith Model 5316preamp and 10-ms time constant was used to amplify the spectroscopic signal. Two other types of lock-in amplifiers used were inferior for this application, due to their slow recovery from transient overloads induced by the comparatively noisy tunneling current. The spectroscopic images were corrected for the signal delay introduced by the lock-in filtering. The capacitive background signal is out of phase with the (resistive-appearins)spectroscopic signal. To discriminate against remaining capacitive background currents, the lock-in was tuned for maximum signal with the tip e60 nm from the surface. The quadrature output of the lock-in is zero with the tip out of tunneling range, so selecting this signal keeps the DCTS signal completely free of contamination and interference from background current. The disadvantage of this method is that it is only possible to determine the relative variation in DCTS signal over the surface, not the absolute magnitude, because the maximum in DCTS signal lies between the background maximum and the quadrature signal. Since our goal was to distinguish electronically different areas of the surface, this limitation was not a problem. In principle this technique could be used with conventionally coated tips but the sensitivity of the lock-in preamp would have to be reduced to a level which would prevent detection of the signal. The substrates consisted of 2000 A of evaporated Pt over a 300-ATi buffer layer on glass microscope slides. After the samples were removed from the evaporator, the substrates were protected with a mechanically strippable polymer layer,= giving better STM images than similarsamples exposed to air for several hours before imaging. Images and the current vs voltage (i-V) behavior of tips were recorded in an aqueous solution of 1.0 M HClOd + 0.8 M CH3COOH + 0.8 M (CH&00)2Pb (images) or 1.6 M CHaCOONa (images and i-Vcurves). These electrolyteswere chosen to permit in situ deposition of PbO2 on the substrates for comparison with DCTS images of the bare substrate, and with a blank (substituting Na for Pb). i-V curves were recorded a t a bias scan rate of 5 mV s-*, and the substrate surface area was -1 cm2. (33) ‘Opti-clean”; Optisources, Oceanside, CA. (34) Sexton, B. A.; Cotterill, G. F.; Fletcher, S.; Home, M. D.J. Vac. Sci. Technol. A 1990,8, 544.
DCTS in Electrolytic Solution
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Current (PA)
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. . . . , . . . . -0.5
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..,:
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. . . . .j
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Figure 2. Current (dashed curves) and capacitance vs bias for Apiezon and Apiezon + OT coated tips in electrolyte. The jump in capacitanceat 4 . 2 5 V occurred while the bias was held for the background current measurement of Figure 3. The STM used here differs substantially from commercial .~~ instruments; details of its design are given e l ~ e w h e r e . ~The software and hardware were configured to permit simultaneous acquisition of spectroscopic and topographic images. For each pixel in the topographic image, the spectroscopicsignal from the lock-in amplifier was also recorded. To help identify possible artifacts, the images were recorded in both directions of tip motion in x (the fast scan direction)and y . To help excludecontaminants and maintain a reproducible sample environment,a stream of Ar gas was passed through activated carbon and 200-nm membrane filters and directed through the STM scanner tube onto the sample.
Results and Discussion The dc background current, idc, for Apiezon-coated Ir tips was measured in electrolytewith the tip biased relative to the platinum substrate. An i-Vcurve is shown in Figure 2. idc was low, less than 100 pA for VB between -0.5 and 0.0 V. idc increased at VB positive or negative of -0.4 V, but its magnitude was typically less than 200 PA. Larger potential excursions (outside a -1.0 to 0.5 V window) irreversiblydamaged the tip insulation resulting in higher background currents. Topographic images of Pt immersed in electrolyte were easily obtained with these tips, but the bias modulation needed for the spectroscopic measurements resulted in large background currents, iac. The magnitude of iac depends on the tip capacitance, which is also plotted in Figure 2. The capacitance was estimated from the relationship C = i a J ( 2 ~ f dV) with f the modulation frequency (42 kHz) and dV = 300 pV. The tip capacitance was lowest at bias voltages less than -0.4 V, but a typical value of 250 fF leads to an unacceptable iac of =2 nA with 30 mV bias modulation. Occasionally, tips were produced with capacitance as low as 100 fF and idc of =20 PA, but such tips gave inferior images; it is probable in these cases that the apex of the tip was coated with an imperfectlayer of wax, with the remaining electrochemical activity occurringat pinholes. Both idc and iacwere reduced by immersing tips in octadecanethiol (OT) solution after Apiezon coating. The background for OT-coated tips over the same range of VB is also shown in Figure 2. idc showed the greatest improvement at VB < -0.5 V, with modest improvement elsewhere. More importantly, however, the lower capacitance gave iac of only =170 pA for 30-mV bias (35) Robinson, R. S. J . Microsc. 1988,152, Pt 2,387. Robinson, R. S. J . Comp. Assist. Microsc. 1990,2,53. Robinson, R. S.; Kimsey, T. H.; Kimsey, R.J. Vac. Sci. Technol. R 1991,9,631. Robinson, R. S.; Kimsey, T. H.; Kimsey, R. Reu. Sci. Instrum. 1991,62, 1772.
-
10 ms
Time
Figure 3. ac background current resulting from a 30-mV, 42kHz bias modulation for Apiezon and Apiezon + OT coated tips in electrolyte. The measurement bandwidth was 100 kHz, and system noise measured with the tip out of the electrolyte.
-5
I
PA
Time
'
- 0.6 V I
20 -5 O
Time
'
y 20 IF
0.0 v -25 -5
Figure 4. Drift in background current and capacitance (circles) over 21 min at each of four different bias voltages, with an Apiezon + OT coated tip. The curvesat -0.3 V were recorded shortly afer immersing the fresh tip in electrolyte.
modulation. A comparison of iac for different tip preparation methods is shown in Figure 3. Without the OT coating, tunneling currents of moderate magnitude are overwhelmed by the capacitive component. Furthermore, the overall noise level is also improved with OT coating; the high-frequency spikes above the background for VB = -0.25 V (Apiezon)are absent. The OT coatingprocedure was less effective with tips previously immersed in electrolyte. The long-term stabilityof a treated tip is shown in Figure 4. The capacitance and dc background current were recorded at four bias voltages for a total of 84 min. The measurement at -0.3 V was made after scanning the bias of a freshly prepared tip to -1.0 V, and shows the initial decay of the background capacitanceobserved with a newly prepared tip. After settling, the behavior of the drift was similar to that at -0.6 V. Ten minutes after changing the bias voltage, the drift in idc for VB between -0.9 and M.3 V was less than 20 fA mi&. The magnitude of idc was also low, less than 1 PA. The current and capacitance were stable over the life span of the tips, which ranged from several hours to 3 days. At more positive VB, the background current and capacitance were higher, and irreversible deterioration of the coating occurred if VB was rapidly scanned positive of 0.0 V; however, a low background could be maintained provided that VB was cycled slowly (55 mV s-*) and repeatedly to increasingly positive values. For each successive cycle, the scan was reversed when iac began to increase. In this manner VB could be brought to +0.5 V, giving stable values of ia, and idc over VB = -1.0 to 0.5 V.
2314 Langmuir, Vol. 8, No. 9, 1992
Robinson and Widrig
Differential Conductance Image
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20 i 0 60 80 100 120
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Figure 5. STM and DCTS images of Pt film in electrolyte taken with an OT-coated tip. Images taken in different fast-scan (x as displayed) directions are shown. A planar background was subtracted from the topographic images and the DCTS images are raw data: bias, -0.6 V; tunneling current, 200 PA; bias modulation, 75 m V at 42 kHz; gray scale: topograph, 10 nm; DCTS, 730 fA.
The improved insulation of the tip is probably due to the formation of a layer of OT on portions of the Ir tip that remain exposed subsequent to Apiezon coating. OT forms ~ ~these passivatingmonolayers on Au, Ag, Pt, and C U . For metals, it has been shown that the sulfur head group of the molecule binds strongly to the metal and its hydrocarbon tail is extended from the surface. Hydrophobic interactions between the tail groups of neighboring adsorbed molecules help induce the formation of an ordered hydrocarbon layer at the metal/solution interface. The decrease in the capacitance observed upon coating the Ir tips is consistent with the formation of a similar monolayer.37 Topographic images obtained in situ with these tips were indistinguishable from images taken with tips coated only with Apiezon indicating that the OT layer does not interfere with the imaging mechanism. It is possiblethat the OT layer is removed from the apex of the tip by mechanical interactions with the surface during scanning. Topographic and DCTS images of Pt in 1.0 M HC104 + 0.8 M CH3COOH + 0.8 M (CH&00)2Pb are shown in Figure 5. The topographic and DCTS signals were averaged 64 times at each data point while scanning the 424 X 424 point images. The images were distorted by thermal drift of the microscope field of view, because of the slow scan rate needed for averaging - - the DCTS signal. (36) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87, and references therein. Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J . Am. Chem. SOC.1991,113,2370. (37) Porter, M. D.; Bright,T. B.; Allara,D. L.; Chidsey,C. E. D. J. Am. Chem. SOC.1987,109,3559. Widrig, C. A.; Chung, C. K.; Porter, M. D. J . Electroanal. Chem. 1991,310, 335.
At faster scan rates image distortion is eliminated, but the DCTS image is noisier. A planar surface was subtracted from the topographic images to remove the effect of sample tilt and improve contrast, but the DCTS images are raw, unfiltered data, normalized to the gray scale. Similar images were obtained in electrolyte with Na substituted for Pb, in distilled water, and in the ambient environment. As shown in Figure 5, the DCTS signal is independent of the fast scan direction of the tip, ruling out artifacts caused by motion of the tip. We believe these to be the first such spectroscopic images obtained in solution. Figure 6 is a color-keyed overlay of the DCTS and topographic images. For Figure 6, each pair of DCTS and topographic scans of Figure 5 were averaged, resulting in two images. The averaged topograph was shaded with a gray scale corresponding to the absolute value of the gradient of the surface along the 3'' direction. This type of shading pictorially simulates a metallic surface illuminated by an overhead artificial white light source, as viewed from the direction of the illumination. This also emphasizes the surface gradient. The DCTS image was converted to a color representation and applied to the topographic images as an overlay. Red and blue regions correspond to relatively high and low signal at VB for the image (-0.6 V). The variation in the differential current over this image was 9.7 pA V-l. The range of dildV observed for images of polycrystalline Pt at -0.6 v in air, water, and both of the above electrolytes (for 12 samples and =lo0 images) was from 4.7 to 18 pA V-*. The lower DCTS signals were obtained at 75 mV amp1itude, as be expected: at the extreme of 100% modulation (600 mV) the DCTS
DCTS in Electrolytic Solution
Langmuir, Vol. 8, No. 9, 1992 2315
Y *
L-
P
Figure 6. Color-keyed picturial overlay of averaged forward and back DCTS images onto averaged forward and back topographic images. The topographic image is gradient shaded (see text). Red corresponds tu higher DCTS signal, blue lower, with color saturation proportional to DCTS signal amplitude.
signal would drop to zero (representing the variation in average current amplitude, which is held to zero by constant-current feedback). Electrochemically deposited PbO:! filmsu with surface topographies comparable to the Pt were also examined after electrochemical deposition
induction by tip anisotropy. There are several possible ways in which the observed signals may arise. An obvious instrumental artifact is a variation in the tunnel gap resulting from feedback mistracking, causing a higher signal while moving up features and the opposite when moving down features. This would depend on the surface gradient in the fast scan direction and is ruled out by the similarity of the DCTS images scanned in different directions. The higher signal invalleys is not due to a possible higher area overlap of tip and surface, because this would affect the average current, and the average current is maintained at a constant value by the STM feedback, as it is anywhere on the surface. Also,if the signals depended only on topography, the DCTS signal for P t and PbO:! would be closer in magnitude. Other
sources of the signal to be considered include changes in tunneling conductance induced by surface plasmon^,^ field emission-type enhancement effects at regions with small radii of curvature,~variationsin surfaceelectricpotentialm or temperaturetl and tip electronic effects. Most of these e variations in the electric potential between d surface and can be ignored because of their agnitude. Tip electronic effeds are minimized tunnel barrier weightind2assures the dominance of sample electronic structure when tunneling into unoccupied states. However, two favored explanations of the conductance variation involve distortion of electronic band structure at disordered regions on the surface or different work functions (at different crystal faces or caused by adsorbed impurities). First, distortion of electronic band structure could account for the variation, as shown by Everson et al.% They reported lower tunneling conductance at steps on Au(ll1) at a bias voltage centered on the surface state (38) Miiller, R.; Albrecht,U.; Boneberg, J.; Koslomki, B.;Leiderer,P.; Dransfeld,K. J. Vac.Sci. Technol.B 1991,9,506. Akari, S.;Lux-Steiner, M. Ch.; V W ,M.; Stachel,M.; Dransfeld, K. J. VUC.Sci. Technol.B 1991, 9, 561. (39) Neidermann, Ph.; Renner, Ch.; Kent, A. D.; Fischer, 0. J. Vac. Sci. Techml. A 1990,8,594. (40)Miiller, R.; Baur, C.; Esslinger, A.; Kiirz, P. J. VUC.Sci. Technol. B 1991,9,609. (41) Weaver, J. M.; Walpita, L. M.; Wickramwinghe, H. K. Nature 1989,342,783. W i , C . C.;Wickramasinghe,H. K. J. Vac.Sci. Techno!. B 1991,9, 537. (42) Klitsner, T.; Becker, R. 5.; Vickers, J. S. Phys. Rev. B 1990,41, 3837.
2316 Langmuir, Vol. 8, No. 9, 1992 energy. The lower conductance resulted from a repulsive tunneling barrier at steps, quenching the surface state. (Asthey pointed out, this effect would only cause a ca. 1% change in an STM topographic image, but the sensitivity to the surfacestate is greatly enhanced by the spectroscopic mode of operation). In our case, a higher density of steps and surface disorder is expected at the grain boundaries of a polycrystalline film, but a drop in conductance in those regions would only be expected if the bias were properly tuned to the surface state. For biases away from the surface state the opposite behavior would be expected. Even neglectingsurfacestates, distortion of band structure at disordered regions (e.g., near grain boundaries) is expected and would be manifested as changes in differential conductance.43Second, a variation in work function of ca. 1eV for different crystal faces is possible and may (43) Jaklevic, R. C.; Lambe, J. Phys. Reu. B 1975,12,4146. (44)Kirtley, J., Washburn, S., Brady, M.J. Phys. Reo. Lett. 1988,60, 1546. Pelz, J. P.; Koch,H. R. Rev. Sci. Znstrum. 1989,60,301.
Robinson and Widrig affect the barrier height, and therefore tunneling current, by sufficient magnitude to be observed.44 Adsorbed impurities have also been suggested as the cause of low barrier heights measured with STM in air and liquids. Adsorbates have been spectroscopically identified in vacuum STM studies. It is possible that impurities preferentially adsorb at disordered regions, giving rise to the observed current variation. Improved interpretation of the signal would entail characterization at different bias voltages and the use of other substrates. Here, we have demonstrated that our tip preparation methods allow tunneling spectroscopy to be performed in electrolytic media. The validity of our approach is independent of substrate, and a more detailed investigation of the nature of the DCTS signal in electrolyte with other systems is envisioned. Acknowledgment. We are grateful to P. England, S. Fletcher, M. P. Green,L. H. Greene, K. Luther, R. J. Warmack, and E. L. Wolf for helpful suggestions.
