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Langmuir 1993,9,3471-3477
Electric Field Induced Phenomena in Scanning Tunneling Microscopy: Tip Deformations and Au( 11 1) Surface Phase Transitions during Tunneling Spectroscopy Experiments Joachim Hossick Schott Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Henry S. White' Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received June 14,1993.I n Final Form: September 7,199P We report electric field induced phase transitions of Au(ll1) surfaces and electric field stress induced elongation of Au tips during current voltage measurements with a scanning tunneling microscope (STM) in air. Transitions between the reconstructed 4 3 X 22 and the unreconstructed 1X 1phase of the Au(ll1) surface are attributed to changes in the electronic surface excess charge density induced by the electric field between the tip and sample. Elongations of STM tips during tunneling spectroscopy give rise to a hysteresis in the current-voltage response at tip-to-sample biases greater than -0.5 V and frequently result in the formation of a mechanical tip-sample contact.
Introduction The measurement of the tunneling current, I, as a function of the applied bias voltage, V, between a spatially fixed tip and a sample using the scanning tunneling microscope (STM) has received a great deal of attention from various research groups in recent years. Analyses of I vs V curves (and/or their first and second derivatives) are used to characterize the electronic structure of the sample surface (e.g., the energetics and local densities of electronic surface and bulk stated-'), as well as to investigate fundamental aspects of tunneling phenomena (e.g., Coulomb blockade induced single electron tunnelin9pB). The analysis of tunneling spectroscopy (TS) data obtained in STM experiments generally relies on the assumption that both the tip and the sample surface remain mechanically stable during the I-V measurement. However, we have shown in a preceding paperlothat the electric field between a Pt(70%)-Ir(30%)-tip and a Au(ll1) surface can lift and induce the d3 X 22 reconstruction of this surface at bias voltages commonly used in TS and STM experiments. Hence, since the atomic structure of the sample surface can change as a function of the applied bias, it seems reasonable to anticipate that the tip structure can change, too. This speculation motivated the present study. Very little has been reported concerning the effects of the electric field on the mechanical stability of the tip and ~~
a Abstractpublishedin Advance ACSAbatracts, October 15,1993.
(1)Binnig, G.; Frank, K. H.; Fuchs, H.; Gracia, N.; Reihl, B.; Rohrer, H.; Salvan, F.; Williams, A. R.Phya. Rev. Lett. 1985,55 (9), 991. (2)Feenstra, R. M.; Martesson, P. Phya. Rev. Lett. 1988,61, (4),447. (3)Firet, P. N.; Stroacio, I. A.; Dragoset, R.A.; Pierce, D. T.; Celotta,
R.J. Phya. Rev. Lett. 1989, 63 (13),1416. (4)Kuk, Y.;Silverman, P. J. J. Vac. Sci. Technol. A 1990,8 (l),289. (5)Kaiser, W. J.;Jaclevic, R. C.JBMJ.Rea.Develop. 1986,30(4),411. (6)Everson, M. P.; Jaclevic, R.C.; Shen, W. J. Vac. Sci. Technol., A
--
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1990.8 - -,- (fib. -,, 3662. - - - -.
(7) Everson, M. P.; Davis, L. C.; Jaclevic, R. C.; Shen, W. J. Vac. Sci. Technol., B 1991, 9 (2),891. (8)van Bentum,P. J. M.; van Kempen, H.; van der Leemput, L. E. C.; Teunieeen, P. A. A. Phya. Rev. Lett. 1989, 60 (4),801. (9)Wilkins, R.; Ben-Jacob, E.; Jaclevic, R. C. Phya. Rev. Lett. 1989, 63 (7),801.
0743-7463/93/2409-3471$04.00/0
sample in STM,lO-13despite the fact that high field pulses are increasingly used in STM to modify surfaces on the nanometer scale.I4 Yet it is known from field-ionmicroscopy studies that a field of 1VIA, which is easily realized in an STM, can result in mechanical stresses which can deform or even rupture the tip.l5J6 In this paper, we present a detailed investigation of I-V characteristics in tunneling spectroscopy experiments performed with PtIr and Au tips on Au(ll1) surfaces. We show that the field induced deformation of the STM tip during TS measurements has a significant effect on the shape of the measured I-V characteristics. N
Experimental Section Our experimenta were performed in air with a Digital Instruments Nanoscope I1 scanning tunneling micr0scope,1~in which the tip is held at virtual ground and the bias voltage Vis applied to the sample. All images reported here were recorded in the constant-currentmode using a scan rate of 8.6 Hz and consist of 400 X 400 data points. Tunnelingspectroscopy (TS)experimente were performed by measuring the tunnelingcurrentZas a functionof the bias voltage V . The tunneling current is recorded after current-voltage conversion across a 1M Qresistor. The tip is placed in the center of a surface area which is imaged before and after each current voltage measurement. Typical scan times were -0.5 8. All Z-V data are reported without filtering, smoothing, or averaging. Before the actual Z-V measurement, a tip-sample distance s is established by adjusting the feedback loop of the instrument with a programmable gap resistancefL = VJZut. The feedback is then disabled, and the tunneling current is measured as a function of the ramp voltage. The set of TS measurements reported in the section Field Induced Surface Changes on Au(ll1) was recorded with the TS (10)Hmick Schott, J.; White, H. S . Langmuir 1992,8, 1955. (11) Stroscio, J. A.;Eigler, D. M. Science 1991,254, 1319. (12)Whitman, L. J.; Stroscio, J. A,; Dragoset, R.A,; Celotta, R. J. Science 1991,251, 1206. (13)Mamin,H.J.; Guethner, P. H.; Rugar,D. Phya. Rev. Lett. 1990, 65 (19),2418. (14)Quate, C. F. In Highlights in Condensed Matter Physics; Esaki, L.,Ed.; Plenum Press: New York, 1991. (15)Wagner, R. Field Zon Microscopy in Materials Science; Springer-Verlag: Berlin, Heidelberg, New York, 1985. (16)Birdseye, P. J.; Smith, D. A. Surf. Sci. 1970,23, 198. (17)Digital Instruments Inc., Nanoscope I1 Manual.
0 1993 American Chemical Society
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3472 Langmuir, Vol. 9, No. 12,1993 computer program as installed in the Nanoscope I1 scanning tunneling microscope. This program generates the triangleshaped voltage ramp. The program, however, records I-V data only duringthe second half-cycle of the ramp, which starts at the positive limit of the selected voltage scan window. The I-V measurements reported in the section ElectricField Induced Tip Changes were recorded with an r y recorder, after passing the current-proportional voltage signal through a 1:l operationalamplifier. The bias voltage was also recorded after passing the signal through a 1:loperationalamplifier. This way, I-V curves corresponding to both half-cycles of the ramp could be registered. Since high current levels were measured in these experiment, the 1-MQ resistor was replaced by a 10 ki2 f 1% resistor in order to increase the current limit. 4 3 X 22 reconstructed Au(ll1) samples were prepared according to the procedure given by Hsu and Cowley.l* One end of a -2-cm piece of Au wire was melted to form a sphere of 1-2." diameter in a hydrogen/oxygen flame. Upon cooling in air or argon, highly reflective facets appear on the sphere which proved to be atomically flat in STM. Further annealing of a sphere in a cooler, hydrogen-rich flame for about 2 min typically results in a highly ordered 4 3 X 22 surface within the faceted area. Unannealed surfaces typically yield unreconstructed Au(111)surfaces. Tips were mechanically cut from Pt(70%)-Ir(30%)wire (0.1." diameter, Aesar/JohnsonMathey)and gold wire (99.999% purity, 50-pm diameter,Aesar/JohnsonMathey). Before cutting, the wires were heated to a white and red glow, respectively, in a hydrogen flame to minimize organic surface contaminants.
Results and Discussion Field Induced Surface Changes on Au(ll1). The motivation for this TS study is based on our previously reported observationlo that the d 3 X 22 reconstruction of the Au(ll1) surface can be lifted and induced as a function of the bias voltage V during STM imaging. The reconstructedd3 X 22 Au(ll1) surface is imaged in STM as parallel pairs of corrugation lines (height -0.2 A) running in the (112) direction with a pair-to-pair separation of -63 A.1s21 Long-range features of this reconstruction include domain structures separated by -250 A and rotated f120' with respect to each other, i.e., zigzag patterns of parallel lines (''herringbone- pattern).20 The reconstruction can be rationalized with a tensile surface stress, arising from charge redistributions of mainly sp electrons at the surface. This stress causes uniaxial compression of the topmost layer interatomic bond distances by about 5% in the (110) direction. In brief, the results from our previous worklo can be summarized as follows: the d 3 X 22 reconstruction features are stable and unaffected by continuous STM imaging over a wide range of negative and low to moderate positive sample biases (V I -+0.5 V). Scanning with a positive sample bias V of -1.5 V, however, completely lifts the reconstruction within the geometrical scan window. Negative biases V I-0.5 V induce a transition from the 1 X 1 to the d 3 X 22 reconstruclkd phase. We concluded that the sign and magnitude of the induced surface excess charge during the STM experiment determine the relative stability of the respective surface phases. Other researchers arrived at the same on the basis of (18) HEU,T.; Cowley, J. M. Ultramicroscopy 1983, fl, 126. (19) Wbll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989,39 (111, 7988. ' (20) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Rev. B 1990, 42 (16), 9307. (21) H a h , W.; Lackey, D.; Sass, J. K.; Besocke, K. H.J. Chem. Phys. 1991, 95, (3), 2193. (22). Gao,. X.:. Hamelin,. A.:. Weaver. M. J. J. Chem. Phvs. 1991.95 . (9). . .. 6993. (23) Tao, N. J.; Lindsey, S. M. Surf. Sci. Lett. 1992,274, L W . (24) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaaca, H. S. Science 1992,255, 1416.
electrochemicalexperiments, in which the Au(ll1) surface is immersed in an electrolyte and the potential drop is established by the electrochemical double layer. Figures 1and 2 demonstrate that these phase transitions can also be observed in combined STM/TS experiments when the bias during the I-Vmeasurement is scanned to sufficiently positive or negative values. For example, Figure l a shows a STM image of a d 3 X 22 reconstructed Au(ll1) surface area obtained with a moderate bias of -125 mV. Performing an I-V measurement (Figure lb) in the center of the imaged area leaves the surface unchanged when the sample bias during the I-V measurement is negative (e.g., 0 to -1 V in Figure lb), as is evidenced by the image shown in Figure IC,which was taken immediately after the I-V measurement shown in Figure lb. Voltage excursions to higher positive sample biases (e.g., 0 to +1V in Figure Id), however, partially lift the reconstruction in the area of the tip position. This can be observed in the image shown in Figure le, taken immediately after the I-V scan shown in Figure Id. This latter image is noisier than the images shown in Figure la,c. This is to be expected since the d 3 X 22 1X 1transition requires transport of surface atoms in the transition area as demonstrated in the previous study.1° Figure 2 shows the inverse phenomenon measured on a different sample with a different tip. First, a small unreconstructed area was created on the d 3 X 22 surface (Figure 2a) by applying positive sample biases in an I-V experiment performed before taking the image. Two I-V measurements at large negative biases were then made over the same surface section (Figure 2b,d). Images recorded following the I-V measurements show new reconstruction features in the previously unreconstructed area (Figure 2c,e). In these experiments, bias excursions to voltages less negative than -0.5 V do not result in any significant surface changes. Tunneling spectroscopy measurements utilizing both negative and positive bias voltages clearly also affect the surface (Figure 2f-i). However, the effect appears to be merely a movement of reconstruction features rather than a phase transition. Structural changes in the surface during the TS experiment must also affect the I-Vresponse. For example, if a corrugation line on the d 3 X 22 surface of height -0.2 A moves under the apex of the tip, the corresponding change in the geometrical tunneling barrier will produce achange in the measured tunneling current. A comparison between the I-Vcurves in Figure lb,d shows that the curve obtained upon inducing the d 3 X 22 1 X 1transition under the tip (Figure Id) is considerably noisier than the curve obtained upon ramping negative biases (Figure lb), which leaves the surface unchanged. We conclude that the noise observed in the I-V curves shown in Figure 2 is due to lifting, creation, or movement of reconstruction features under the tip. Electric Field Induced Tip Changes. In conaideration of the above-described field induced changes in the Au(ll1) surface, it is relevant to consider how the electric field affects the mechanical stability of the tip. We speculate that the mechanical force exerted by the electric field between the tip and sample may result in a physical elongation of the tip. To test this, we performed I-V measurements with a Au tip positioned above an unreconstructed Au(ll1) surface (Figure 3). After imaging the surface area shown in Figure 3a, the tip was placed in the center of this area and an I-V measurement was performed. During the initial half-cycle of the voltage ramp (Figure 3b) the current rises in a strongly nonlinear fashion to very high levels (-50 pA) a t positive bias
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Bias Voltage (V) Figure 2. Unfiltered STM images and current-voltage curves obtained in sequential order on a 4 3 X 22 reconstructed Au(ll1) surface. (a) Initial image with an unreconstructed section in the center generated as shown in Figure ld,e. (b) Current-voltage curve for negatiye bias voltage excursions. (c) Image of the surface after the I-V measurement shown in (b); new reconstruction features are forming. (d) Current-voltage curve for more negative bias voltage excursions. (e) Image of the surface after the I-Vmeasurement shown in (d), showing more new reconstruction features which seem to center around the tip location during the I-V measurement. (0 Current-voltage curve for bipolar bias voltage excursions. (g) Image of the surface after the I-V measurement shown in (0. (h) Current-voltage curve for higher bipolar voltage excursions. (i) Image of the surface after the I-V measurement shown in (h). I d = 1nA, Vmt = 0.15 V for all images and I-V curves.
voltages around -+0.5 V. Upon reversal of the voltage scan direction at +0.95 V, the current follows almost exactly the 10-kR characteristics of the current limiting resistor in series with the tip, indicating that an ohmic contact has been formed between the tip and sample. Thus, upward and downward voltage scans yield a hysteresis between the correspondingcurrent traces. Imagingthe surface after this experiment (Figure 3c) reveals that a mound has been formed under the tip during the I-V measurement. We emphasize that these results are representative for a set of -50 TS measurements performed with different Au tips on unreconstructed and reconstructed Au( 111)surfaces in the scan voltage range hl V. On average, every second TS experimentin this scan voltage range and under similar set-point conditions results in the formation of a mound on the surface, similar to the one shown in Figure 3c. TS measurements which leave the surface unchanged
also yield strongly nonlinear I-V curves and a hysteresis between the current traces corresponding to upward and downward voltage scans (not shown here). However, in these cases, the tunneling gap resistances are not quite as low as in Figure 3b and the hysteresis is less pronounced. Upon using scan voltage windows as large as f2.5 V and similar set-point conditions, every single TS experiment results in the formation of a mound on the surface and the shape of the I-V curves is similar to the one shown in Figure 3b. The onset of electron field emission, which may be can expected in the bias voltage region above -0.5 neither account for the observed hysteresis between upward and downward voltage scans nor explain the formation of a mound under the tip. On the basis of our V , 2 6 9 2 6
(25) Simmons, J. G.J. Appl. Phys. 1963,34,1793. (26) Coombs, J. H.;Pethica, J. B. IBM J. Res. Deu. 1986,30 (5), 455.
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sample distance allows for the onset of strong adhesive forces,n*28possibly accompanied by thermal instabilities of the tip apex.% If the tip touches the sample, adhesive forces will fuse the tip to the sample. Therefore, the tipsample contact is maintained upon reversing the voltage scan direction. The contact is opened, and a mound is formed on the surface when the feedback loop of the instrument is reactivated after completion of the voltage ramp, since the feedback loop will retract the tip from the sample in order to reestablish the set point resistance. Mamin et al.13 reported mound formations on a Au(111)surface in STM experiments in air using a Au tip upon applyingvoltage pulses of 3-5-V pulse height. These researchers suspected that a field evaporation mechanism causes gold deposition from the tip on the surface. Field stress induced tip elongation and subsequent contact formation were excluded in Mamin et al.'s study because the voltage pulse duration was on the order of some 100 ns, which makes a plastic response of the tip seem unlikely. In our case, however, the bias voltage is ramped sufficiently slow (- 1s) to allow for a plastic deformation. Assuming a parallel plate geometry and a tip-sample distance of 5 A,we estimate the local electric field E at a bias voltage of 1 V in our STM experiment to be -0.2 V/A, which results in a combined normal and shear stress at the tip apex c = 0.5e@ = 2 X lo9 N/m2 (€0 = vacuum dielectric constant). A more realistic geometry based on a sharp tip would yield an even larger estimate of the local electric field. Thus, our calculation suggests that the stresses encountered in our experiments are at least of the order of the maximum tensile stress of polycrystalline gold (- 1 X lo8 N/m2). Therefore, tip deformations might be expected even for polycrystalline tip materials with higher elastic moduli and correspondingly higher tensile strengths, e.g., Pt, Ir, and W. A largely defect free, crystalline tip apex, however, should be stable even at much higher field ~trengths.1~ This may explain the different shapes of the I-Vcurves in Figures 1and 2: whereas the curves in Figure 2 are linear over the entire measured bias voltage window (-3 V 5 V I+3 V), those in Figure 1are linear only in a smaller voltage range around 0 V. A strong nonlinear behavior is observed in Figure 1at larger bias voltages, V > 10.5 VI.
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Conclusions
Figure 3. Current-voltage curves and STM images of an unreconstructed Au(ll1) surface obtained with a Au tip. (a) STM image of a Au(ll1) surface area with a monoatomic step. V , = -100 mV,,I = 4.5 nA. (b) I-V curve for the Au/Au STM tunneling junction in the scan voltage range *1 V, obtained by placing the tip in the center of the area shown in (a). Set-point values same as in (a). The arrows indicate the direction of the voltage scan. Scanningproceeds initially toward positive voltages. (c) STM image of the area shown in (a), after the TS experiment yielding the I-V curve shown in (b). Set-point values same as in (a).
speculation,however, the interpretation of the result shown in Figure 3 is straightforward. The mechanical stresses exerted by the electric field between the tip and sample elongate the tip at bias voltages larger than +0.5 V. The field induced stress increases until the decreasing tip-
Our experiments demonstrate that the electric field in an STM can alter both the tip and the sample surface structures. In particular, we have shown that the electric field between the tip and sample can induce transitions between the 4 3 X 22 and the 1X 1phases of the Au(ll1) surface duringI-Vmeasurements. Therefore, meaningful I-V measurements (and especially dl/d V measurements) should always be checked by imaging the probed surface areas. Tip deformations should be considered when interpreting tunneling spectroscopy experiments performed with an STM. Elastic tip deformations caused by adhesion forces were suspected by Wintterlin et aL30 and Doyen et al.31 to explain the atomically resolved STM topographs of Al(111). Thus, tip deformations may be of fundamental importance in STM. (27) Diirig, U.; Ziiger,0.;Pohl,D. W.Phys.Reu.Lett. 1990.65 (31,349. (28)Pethica, J. B.; Oliver, W. C. Phys. Scr. 1984,19A, 61. (29) Tsong, T. T., as quoted in ref 11. (30) Wintterlin, J.; Wiechers, J.; Brune, H.; Gritech, T.; HQlfer,H.; Behm, R. J. Phys. Rev. Lett. 1989,62 (l),59. (31)Doyen, G.; Koetter, E.; Barth, J.; Drakova, D. In Scanning Tunneling Microscopy and Related Methods; Behm, R. J., Garcia, N., Roher, H., Eds.; Nab Asi Series E 184; Kluwer Academic Publishers:
Dordrecht, Boston, 1990.
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It is interesting to speculate about the reason for the instabilities of the apparent barrier height frequently where the tip reported in electrochemical STM and the sampleare immersed in an electrolyte. We suspect that these instabilities are also, at least partly, related to tip deformation effects (other possible mechanisms are ~). the tip as suggested by Sass and G i m ~ e w s k i ~Picturing
a "model ion", we may assume that a solvation shell of solvent molecules forms around the tip apex. As in the dissolution process of salts in solvents, this solvation shell might lower the interatomic bond energy in the tip apex region. Consequently, a solvated tip apex may be more susceptible to plastic deformations, caused by either electrostatic or adhesive forces between the tip and sample.
(32) Binggeli, M.; Carnal, D.; Nyffenegger, R.; Siegenthaler, H.; Chrietoph, R.; Rohrer, H. J. Vuc. Sci. Technol., B 1991, 9, (4), 1986. (33)Gimzewieki,J.K.;Sass, J. K. InProc.ICTPConf. on Cond.Matter Physics Aspects of Electrochemistry,Trieste, Italy, 1990, Tosi, M. P., Komyshev, A. A. as. World ; Scientific: Singapore;Teaneck,NJ, 1991.
Acknowledgment. Thiswork was funded by the Office of Naval Research. J.H.S.would like to thank the Deutsche Forschungsgemeinschaft for support under Grant No. Sch 428/1-1.
