Electric field induced phase transitions in scanning tunneling

Langmuir 1992,8, 1955-1960

1955

Electric Field Induced Phase Transitions in Scanning Tunneling Microscopy Experiments on Au( 11 1) Surfaces Joachim Hossick Schott and Henry S. White* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received March 13,1992. I n Final Form: May 26,1992 Phase transitions of charged Au(ll1) surfaces during scanning tunneling microscopy imaging in air are reported. The reversible appearance and disappearance of corrugation line pairs of 0.2 A height and -63 A periodic spacing as a function of the bias voltage is attributed to electric-field induced transitions of the surface layer between the 1 X 1and the 4 3 X 22 phases. The results indicate that the sign (positive or negative) of the electronic surface excess charge is the controlling factor in determining the relative stability of the two surface phases. The estimated excess charge threshold values for the appearance (--I rC/cm2) and lifting (-+3 pC/cm2) of the 4 3 X 22 reconstruction features are compared to corresponding values obtained in electrochemical investigations.

Introduction Electric field induced rearrangements of surface layer atoms have been a topic of research in field ion microscopy' and related theoretical studies for a number of In scanning tunneling microscopy (STM) experiments, it has recently been shown that the electric field between the tip and substrate can modify the spatial structure of adsorbates5 on semiconductors, as well as the first layer atomic structure of clean semiconductor surfacese6In this report, we provide evidence for reversible, and localized electric field induced transitions of the (111)surface of Au between the d3 X 22 reconstructed and the 1 X 1 unreconstructed phases during STM imaging. This type of tip-sample interaction is of considerable interest for modifying surfaces a t the nanometer scale and may also serve to elucidate aspects of chemically and thermally driven surface reconstructions. Recently observed surface phenomena on Au in STM experiments, including the formations of mounds and pits under the as well as the movement of steps,l0may also be related to tip-induced effects. Au(ll1) is the only hexagonal closed packed surface known to reconstruct, as has been inferred earlier from LEED," He-atom scattering,12 RHEED,13 and X-ray14 data. The exact atomic positions in the unit cell of the d3 X 22 reconstruction, however, were revealed only (1) Milller,E. W.;Tsong,T.T.Field1onMicroscopy; AmericanElsevier Publishing Company, Inc.: New York, 1969. (2) McMullen, E. R.; Perdew, J. P. Solid State Commun. 1982,44 (6), 945. (3) McMullen, E. R.; Perdew, J. P. Phys. Rev. B. 1987,36 ( 5 ) , 2598. (4) Kiejna, A. Solid State Commun. 1984,50 (41, 349. (5) Whitman,L. J.;Stroscio,J. A.;Dragoeet,R. A.;Celotta,R.J. Science 1991,251, 1206. (6) Lyo, I. W.; Avouris, P. Science 1991, 253, 173. (7) Emch, R.;Nogami, J.; Dovek, M. M.; Lang,C. A.; Quate, C. F. J . Microsc. 1988, 152, 129. (8) Emch, R.;Nogami, J.; Dovek, M. M.; Lang, C. A.; Quate, C. F. J . Appl. Phys. 1989, 65 (l),79. (9) Sommerfeld,D. A.; Cambron,R. T.;Beebe, T. B.,Jr. J.Phys. Chem. 1990,94,892. (10) Holland Moritz, E.; Gordon, J., II; Borges, G.; Sonnenfeld, R. Langmuir 1991, 7, 301. (11) van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Biberian, J. P.; Kesmodel, L.; Barbs, I.; Somorjai, G. Surf. Sci. 1981, 103, 189. (12) Harten, U.;Lahee, A. M.; Toennies, J. P.; WMl, Ch. Phys. Reu. Lett. 1986,54, 2619. (13) Melle, E.; Menzel, E. 2.Naturforsch. 1978, 33A, 282. (14) Huang, K. G.; Gibbs, D.; Zehner, D. M.; Sandy, A. R.; Mochrie, S. G. J. Phys. Rev. Lett. 1990, 65 (23), 3313.

recently by Woll et al.15 and Barth et al.16 in STM studies performed in ultrahigh vacuum UHV. Subsequently, Haiss et al." showed that the d 3 X 22 surface can be imaged in air and in weakly polar organic solvents. In brief, the reconstruction features on the Au(ll1) surface are im ed in STM as parallel pairs of corrugation lines (-0.2 height) running in the (112) direction with a pair-to-pair separation of -63 A. The reconstruction can be rationalized with a tensile stress, arising from charge redistributions of mainly sp-electrons a t the surface. This stress causes uniaxial compression of the topmost layer interatomic bond distances by about 5 % in the (110) direction. Barth et al.16 also reported new long range features of this reconstruction on large facets, showing domains separated by -250 A and rotated f120° with respect to each other, i.e., zigzag patterns of parallel lines ("herringbone" pattern). We reproduced all of the structures known to occur on the reconstructed Au(ll1) surface in our study, which was performed with the sample being exposed to laboratoryair. In addition, however, our results demonstrate that the d 3 X 22 reconstruction can be lifted and induced during STM investigations by controlled variations of the tip-sample bias. Our observations are relevant to recent electrochemical studies of Au reconstructionsleZ0 since the electric fields involved in STM are of similar magnitude as those observed in the electrochemical double layer. Gao et al.l8J9 reported results of in situ STM studies, revealing reversible potential induced reconstructions on Au(100)18 and Au(lll)lg electrodes in aqueous electrolytes. These researchers speculate that the stresses induced by excess electronic surface charge drive the reconstruction a t the metal/ electrolyte interface, a conclusion supported by X-ray scattering results of Wang et al.,20 who also observed reversible potential-induced transitions between the 1 X 1 and the d 3 X 22 reconstructed phase for Au(ll1) electrodes immersed in an electrolyte solution. We show (15) Woll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Reu. B. 1989, 39 (ll),7988.

(16) Barth, J. V.;Brune, H.; Ertl, G.; Behm, R. J. Phys. Reu. B 1990,

42 (151. 9307. ._ --, - - , 7

(17) Haiss, W.; Lackey, D.; Sass,J. K.; Besocke, K. H. J. Chem. Phys. 1991, 95 (3), 2193. (18) Gao, X.;Hamelin, A.; Weaver, M. J. Phys. Reu. Lett. 1991,67 (5), 618. (19) Gao, X.;Hemelin, A.; Weaver, M. J. J . Chem. Phys. 1991,95 (9), 6993. (20) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. Science, in press.

0743-746319212408-1955$03.00/00 1992 American Chemical Society

1956 Langmuir,

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Figure 1. STM images of an annealed Au(ll1) surface showing parallel line pairs of the d3 X 22 reconstruction: (a and b) bias = +lo0 mV, current = 3 nA, unfiltered; (c) bias = +40 mV, current = 7 nA, band pass filtered).

that the surface excess charge required to induce and lift the reconstructed phase in STM experiments is of similar magnitude and sign as in previous electrochemical experiments. Experimental Section Au(ll1) surfaces were imaged with a Nanoscope I1 scanning tunneling microscope21in which the tip is held a t virtual ground and the bias voltage v b is applied to the sample. Experiments were performed in air using mechanically cut Pt(70%)-Ir(30%) tips. All STM images were recorded in the constant-current mode using a scan rate of 8.6 Hz and consist of 400 X 400 data points. Imageswere typically low-pass filtered once, unless stated otherwise in the figure captions. The Au surfaces were prepared according to the procedure given by Hsu and Cowley.22 A 2-cm piece of Au wire (99.999% purity, 0.5 mm diameter, Aesar/Johnson Mathey) was flame cut. Au spheres (1-2 mm diameter) were formed by melting one end of the wire in a HJ02 flame. Upon cooling in Ar or air, highly reflective, optically flat facets appear on the sphere. Some spheres (21) Digital Instruments, Inc. Nanoscope I1 Manual. (22) Hsu, T.; Cowley, J. M. Ultramicroscopy 1983, 22, 125.

were further annealed in a cooler, hydrogen-rich flame. Au spheres that were not annealed typically displayed large, atomically flat but unreconstructed areas on the facet, while the annealed spheres typically exhibited large reconstructed areas. The Au spheres were mounted in a home-built specimen holder, allowing for orientation of a facet for STM studies.

Results and Discussion Some of the well-known 4 3 X 22 reconstruction features obtained on the facet of a carefully annealed sphere are documented in Figure 1. Pairs of corrugation lines with a pair-to-pair separation of -63 A (-0.2 A height) are observed which can be atomically resolved (e.g., Figure IC).The "herringbone" pattern, which results from f120° rotations of reconstruction domains, was also routinely observed but is not shown. All features are in good agreement with previous work.15-*7 The features of the reconstructed surface are stable and unaffected by continued STM scanning over a wide range of negative values of v b (sample vs tip) and for low to moderately positive biases ( v b 5 +-0.5 v). However, the characteristic d3 X 22 features disappear when the

Electric Field Induced Phase Transitions

Langmuir, Vol. 8, No. 8, 1992 1957

surface is scanned with high positive v b values, as shown in the series of images presented in Figure 2a-d. In this experiment, v b was increased by 200-mV increments between consecutive images. Upon an increase of v b to -+1.4 V (Figure 2c) the d 3 X 22 reconstruction features are lifted almost completely (after -1 min of scanning) within the scan window of 70 nm X 70 nm, the center of which is located within the scan window of Figure 2a, in the area below the step (see cross-mark). Parta d and e of Figure 2, recorded at moderate values of v b and after scanning at high positive values of v b , demonstrate that the l i f t i i of the reconstruction is largely confined to within the area scanned with high positive v b values. The center of the image in Figure 2d is not quite identical to the center of the image in Figure 2a, as can be seen by the relative position of the step. This was corrected for in the image shown in Figure 2e, which was taken after 4 min of continuous scanning with a moderate bias of -100 mV. Corrugation lines associated with the d 3 X 22 phase reappear, when the 150nm X 150 nm area is scanned with high negative values of v b , as shown in parta f and g of Figure 2. A comparison of the image in Figure 2h, which was taken at a bias of -100 mV, following the high negative bias scans, with the initial image in Figure 2a reveals that a few new reconstruction features have been generated during the various imaging cycles, including a U-shaped termination of reconstruction lines above the step. These results indicate that the d 3 X 22 reconstruction features on Au(ll1) can be lifted and induced as a function of sample-tip bias and, hence, as function of surface excess charge. Since only minor changes in the surface topography of either the reconstructed or the unreconstructed surface occur during scanning with low to moderate values of v b (Ivbl I 0.1 V), and since the possibility of local heating effects under the tip can be ruled we conclude that the transitions between the reconstructed and unreconstructed surface shown in Figure 2 are driven by the electric field between tip and sample at high negative and positive biases. Although the feedback system of the instrument retracts the tip from the surface upon increasing the bias voltage in order to keep the tunnel current constant, the electric field is expected to increase since it is inversely proportional to the tip-sample distance, d, whereas the tunneling current is proportional to e4. In turn, the tip retraction should decrease the likelihood of mechanical interactions with the surface. Assuming an average tip-sample distance of 5 A, a parallel plate geometry, and a v b value of +1 V, we estimate the magnitude of the electric field between the tip and sample to be +0.2 X 101OV/m,correspondingto a positive excess charge density of -+2 pC/cm2 (based on the value of the vacuum dielectric constant). Since we observe the lifting of the d 3 X 22 phase at v b -+1.4 V, our experiments suggest that a positive excess charge of -+3 pC/cm2(corresponding to an electric field of -+0.3 X 1Olo V/m) is required to lift the reconstruction (Figure 2c). Similarly, a negative surface excess charge of --1 pC/cm2 ( ~ - 0 . 1 X 1O1O V/m) appears necessary to induce the transition from the unreconstructed to the reconstructed phase (Figure 2e,f). Precise values of the magnitude of the fields and surface charges, however, are difficult to determine since the absolute value of the tip-sample separation and the tip geometry are not known. Generally, metal surface reconstructions are thought to be driven by forces on the topmost layer metal ion cores 0

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(23) Persson, B. N. J.; Demuth, J. E. Solid State Commun. 1987,57 (9),769.

which arise from electronic charge redistributions in the surface layer.24-27Details of this charge redistribution are still a topic of controversy; thus, it is not yet possible to predict the direction of the surface layer atom relaxation. In an attempt to reconcile apparently contradicting experimental findings, Heine and Marks24argued that both expansion and contraction of interatomic bonds in the topmost layer are possible for the uncharged Au(ll1) surface. Both in-plane contraction and an outward force normal to the surface should occur on an atomically flat surface, whereas expansive forces should emerge in the vicinity of a step. The situation is more complicatedwhen excess charge resides on the surface. Gies and GerhardW have theoretically shown that the density in the electron tail above the jellium edge is increased (decreased) when a negative (positive) electric field is applied; i.e., negative (positive) charge resides on the surface. Changes in the surface electron density should cause the atomic positions in the topmost layer to shift, as has been theoretically shown by Kiejna4 and McMullen and Perdew.2 In the STM experiment, these shifts should be localized to the surface region directly beneath the tip. Following previous arguments24that the interatomic bonds in the Au surface represent a bifurcation (a physical system with two or more energy minima) and that both the unreconstructed and the reconstructed surface each represent one of those minima, the effect of varying the local electric field may be to switch the system between the two phases. Since the topmost layer atom density of the reconstructed surface is increased by about 5 % versus the unreconstructed, bulk lattice terminated surface, mass transport is required for transitions out of or into the reconstructed phase. The phase transition sequence shown in Figure 2 was performed near an atomic step. We suspect, that the excess atoms from the d 3 X 22 1 X 1transition (Figure 2a-d) diffuse to the step. Similarly, we propose that the additional material needed to facilitate the transition 1X 1 d 3 X 22 diffuses from the step area into the area of the phase change (Figure 2e-h). In a different experiment (Figure 3), performed on an initially unreconstructed surface, we observed step shape changes and the occurrence of nanometer size holes upon inducing the reconstruction under high negative field conditions. Figure 3a shows an image of a largely unreconstructed area recorded at a v b of -0.1 V. With an increase of the bias voltage between sample and tip from -0.1 to -1.0 V (Figure 3b,c), holes appear on the terrace to the right of the step, corrugation lines begin to form, and the shape of the monoatomic step in the middle of the picture changes slightly. Upon decreasing v b back to -0.1 V, the newly created corrugation lines persist on the surface (Figure 3d), apart from some further ordering (Figure 3e). The formation of holes during the reconstruction of the surface, as well as the fact that the d 3 X 22 reconstruction can be reversibly lifted and induced on the same area (Figure 2)) supports ealier proposals24that Au allows for both inplane contraction and in-plane expansion of interatomic bonds. However, more detailed and careful measurements are needed to quantitatively evaluate mass transport during the reconstruction process. It is interesting to note that the dependence of the Au(111)d3 X 22 reconstruction on the sign and magnitude of the local electric field between tip and sample closely

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(24) Heine, V.; Marks, L. D. Surf. Sci. 1986, 165, 65. (25) Dodson, B. W. Phys. Rev. Lett. 1987, 60 (22), 2288. (26) Needs, R. T.;Godfrey, M. T.;Mansfield, M. Surf. Sci. 1991,242, 215. (27) Chen, S. P.; Voter, A. F. Surf. Sci. Lett. 1991, L107. (28) Gies, P.; Gerhardta, R. R. Phys. Rev. B 1985,31 (lo), 6843.

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Figure 2. Bias-dependent disappearance and appearance of reconstruction features on the Au( 111) surface: (a) -100 mV,7.5 nA, (b)+600 mV,7.5 nA; (c) +1400 mV,7.5 nA; (d) -100 mV,7.5 nA, (e) -100 mV,7.5 nA; (f) -400 mV,7.5 nA; (g) -800 mV,7.5 nA, (h) -100 mV,7.5 nA. All images low-pass filtered once. The contrast a t the monoatomic step in the middle of Figure 2a is slightly distorted because the step is nearly parallel to the scan direction and the low pass filtering routine averages over adjacent lines. resembles the behavior of the Au( 111)reconstruction in Gao et al.19 and Wang et a1.m report that a negative surface

electrochemical environments, where the electric field is established by the potential drop in the double layer. Both

excess charge density of --lo pC/cm2is necessaryto drive the surface into t h e d 3 X 22 phase. Positioe excess charge

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Electric Field Induced Phase Transitions

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Figure 3. Bias-dependent development of reconstruction features on a largely unreconstructed Au(ll1) surface: (a) -100 mV, 8 &, (b) -500 mV, 8 nA; (c) -1OOO mV, 8 nA; (d) -100 mV,8 nA; (e) -100 mV, 8 nA. Image processing software was used to adjust the average heights of the terraces (right and left of the monoatomic step) in order to enhance the contrast associated with the reconstruction lines.

of -+lo pC/cm2 is reported to lift the reconstruction in the study of Wang et aL20 (no value is given in the study of Gao et al.;19 however, it is stated that the reconstruction is lifted at potentials positive versus the potential of zero charge (pzc)). Thus, the magnitude of the surface charge density required to induce transitions between the 1 X 1

and the 4 3 X 22 phases differs slightly between the electrochemical s t u d i e ~ land ~ *our ~ ~STM study. The sign of the charge, however, is identical in both types of experiments. Therefore, both sets of results suggest that the relative stabilities of the two phases of the Au(ll1) surface are determined by the sign and the magnitude of

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excess electronic surface charge densities. One important difference, however, is that the 4 3 X 22 reconstruction is apparently not observed in electrochemicalstudies when the electrode is uncharged, i.e., at the P Z C , whereas ~ ~ ~ ~ it is clearlyobserved by STM on thermally annealed samples in both air and vacuum. In our preliminary STM experiments on Au(ll1) samples immersed in triply distilled water, we obseved only lgrgely unreconstructed surfaces under bias and current conditions where the 4 3 X 22 phase is routinely observed in air. Gao et al.19 have suggested that the water dipoles in the inner Helmholtz layer might be responsible for the absence of the reconstruction at the pzc, consistent with the above mentioned experimentalresults. Using the reversibleAu(111)surface phase transitions as an indicator, more detailed STM work on samplesimmersed in electrolytesmight help to elucidate fundamental aspects of the electrochemical double layer. Approaching the end of our discussion, we address the possible role that contaminants might play in our experiments. The observation of reconstruction patterns which are known to form only when the surface exhibits a high degree of cleanliness, leads us to the assumption that we are dealing with clean Au. Clearly, we cannot rule out the presence of a mobile (and therefore undetected) adsorbate layer on the surface. However, this should not affect the basic physics suggested in our proposed mechanism. We do rule out the possibility of mechanical tip-sample interactions as the driving force for the observed phase

transitions in our experiments since the probability of this kind of interaction is higher when the tip is very close to the surface, i.e., when the surface is scanned with low bias voltages. However, we did not observe any surface phase transitions when the surface was scanned under low bias voltage conditions. Also we note that the phenomena reported here were reproduced on different samples and with different tips. Finally, we note that excess surface charge may also be responsible for the 1X 1 1X 2 reconstructions observed upon alkali adsorption on some metallic (110) substrates, e.g., Cu(ll0) (for an overview on these phenomena, see BonzelB). Charge transfer between the adsorbed alkali atom and the metal increases the local surface electronic charge,a situation not too dissimilarfrom that encountered in our STM experiment (with the adsorbed alkali ion replacing the STM tip in a gedanken experiment). The conceptualsimilarityof these processes, in conjuctionwith the present demonstration of sample-tip bias induced surface phase transitions, suggestsinteresting applications of STM in fundamental investigations of reconstruction dynamics. Acknowledgment. This work was funded by the Office of Naval Research. J.H.S. thanks the Deutsche Forschungsgemeinschaft for support under Grant No. Sch 4281

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1-1. (29) Bond,

H.P.Surf. Sci. Rep. 1987, 8,43.