5942
Langmuir 1996, 12, 5942-5946
Measuring the Surface Stresses in an Electrochemically Deposited Metal Monolayer: Pb on Au(111) T. A. Brunt,† T. Rayment,*,† S. J. O’Shea,‡ and M. E. Welland‡ Department of Chemistry, University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., and Engineering Department, University of Cambridge, Cambridge CB2 1PZ, U.K. Received June 10, 1996. In Final Form: August 13, 1996X An atomic force microscope cantilever has been used as a bending-beam sensor to measure surface stress changes which occur during electrochemical processes. The mechanical properties of the lever and the sensitivity of the detection system mean that the sensor is both fast and sensitive. Surface stress changes presented for the electrochemical deposition and stripping of a Pb monolayer on an Au(111) surface show features which match peaks in the cyclic voltammogram and can be understood by reference to the known surface structure determined by STM, AFM, and grazing incidence XRD. There is a pronounced reduction in the stress derivative at the potential corresponding to the rotational phase transition of the lead monolayer. In the electrocompression region which follows monolayer formation, there is an essentially linear increase in compressive stress which may be modeled to within 50% accuracy by a simple linear elastic model.
Introduction At surfaces and interfaces the change in the atomic environment from the bulk means that the surface region is frequently subjected to large surface stresses.1 This is a consequence of the tendency to increase the electron density in the surface region, compared to the hypothetical bulk termination structure. Bulk termination surface arrangements are therefore subject to tensile surface stresses,2 and this may lead to surface reconstructions. The deposition of an adsorbate can dramatically alter the surface stress. This type of adsorbate-induced surface stress change is well-documented in the UHV environment. For instance stress changes have been measured for adsorption and reconstruction processes at both metal3,4 and semiconductor5,6 surfaces. In this work we have measured in situ the surface stress changes associated with the reversible electrochemical deposition of a metal monolayer. The lateral forces which give rise to the surface stress are different at the electrode/ electrolyte interface from those at the metal/vacuum interface. For instance, there are long range lateral electrostatic forces due to the charges present on and around the electrode surface. These forces give rise to the electrocapillary variation in surface energy with applied electrode potential.7,8 For a particular metal electrode surface there is a certain potential at which the electrode is unchargedsthis is termed the potential of zero charge (pzc). As the potential is varied from this value in either direction, there is an increase in the compressive surface stress, due to repulsive Coulombic forces between like charges. This effect occurs without * To whom correspondence should be addressed. † Department of Chemistry. ‡ Engineering Department. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Ibach, H. J. Vac. Sci. Technol. A 1994, 12, 2240. (2) Needs, R. J.; Godfrey, M. J.; Mansfield, M. Surf. Sci. 1991, 242, 215. (3) Grossmann, A.; Erley, W.; Ibach, H. Surf. Sci. 1994, 313, 209. (4) Grossmann, A.; Erley, W.; Ibach, H. Surf. Sci. 1995, 337, 183. (5) Schell-Sorokin, A. J.; Tromp, R. M. Phys. Rev. Lett. 1990, 64, 1039. (6) Martinez, R. E.; Augustyniak, W. M.; Golovchenko, J. A. Phys. Rev. Lett. 1990, 65, 1035. (7) Fredlein, R. A.; Damjanovic, A.; Bockris, J. O’M. Surf. Sci. 1971, 25, 261. (8) Raiteri, R.; Butt, H.-J. J. Phys. Chem. 1995, 99, 15728.
S0743-7463(96)00564-1 CCC: $12.00
necessarily being accompanied by any specific adsorption of material at the electrode surface and is therefore always present in any electrochemical stress measurements. The electrocapillary curves of several different electrode surfaces have been measured, with most work concentrating on the Hg, Au, and Pt surfaces.7,8 The stress changes associated with adsorption processes can also be measured. For instance, Haiss and Sass9 have used scanning tunneling microscopy (STM) to measure stress changes associated with metal monolayer formation. Although the magnitude of the stress changes is similar for both electrocapillary curves and monolayer deposition processes, the monolayer stress changes usually occur over a much narrower potential range and hence the gradient of the stress change with respect to potential is much greater for these processes. Thus it is possible to distinguish the electrocapillary and deposition stress changes. In electrochemical deposition of metals, it is commonly observed that one or more metal monolayers are deposited prior to bulk metal deposition. This process has been termed underpotential deposition (UPD), because it occurs at a potential positive of the reversible Nernst potential for bulk metal formation. The UPD process has been extensively studied, initially by electrochemical methods10,11 and more recently by in situ structural techniques. In particular, scanning tunneling microscopy,12 atomic force microscopy (AFM),13 grazing incidence X-ray scattering (GIXS),14 and extended X-ray absorption fine structure (EXAFS)15 have all yielded atomic level structural information. Two general types of monolayer structure have been found: open commensurate structures with strongly coadsorbed anions (for example Ag on Au(111)16 and Cu on Au(111)17) and close-packed incom(9) Haiss, W.; Sass, J. K. J. Electroanal. Chem. 1995, 386, 287. (10) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 1. (11) Swathirajan, S.; Bruckenstein, S. Electrochim. Acta 1983, 28, 865. (12) Mu¨ller, U.; Carnal, D.; Siegenthaler, H.; Schmidt, E.; Lorenz, W.; Obretenov, W.; Schmidt, U.; Staikov, G.; Buevski, E. Phys. Rev. B 1992, 46, 9754. (13) Chen, C.; Washburn, N.; Gerwirth, A. J. Phys. Chem. 1993, 97, 9754. (14) Toney, M. F.; Gordon, J. G.; Samant, M. G.; Borges, G. L.; Melroy, O. R.; Yee, D.; Sorensen, L. B. J. Phys. Chem. 1995, 99, 4733. (15) Samant, M. G.; Borges, G. L.; Gordon, J. G.; Blum, L.; Melroy, O. R. J. Am. Chem. Soc. 1987, 109, 5970. (16) Chen, C.; Vesescky, S. M.; Gerwirth, A. A. J. Am. Chem. Soc. 1992, 114, 451.
© 1996 American Chemical Society
Surface Stresses in a Metal Monolayer
Langmuir, Vol. 12, No. 24, 1996 5943
mensurate adlayers in which the adlayer structure is similar to the close-packed bulk plane (for example Pb, Tl, and Bi on Au(111) and Ag(111)14). In this paper we have measured the surface stress changes associated with an incommensurate system (Pb on Au(111)) and compared the results with the existing monolayer strain data.14 The qualitative form of the surface stress changes for this system is very different from those associated with the commensurate system Ag on Au(111), which are described elsewhere.18 Experimental Section The stress sensor is of the bending-beam type and consists of an optical deflection AFM in which the Au-coated cantilever itself constitutes the working electrode. The experimental arrangement has been described in detail elsewhere.18,19 The high sensitivity of the detection system and the low force constant of the AFM cantilevers means that the system is a highly sensitive stress sensor. It is clear from the results presented in this work that surface stresses can be measured with submonolayer sensitivity. In addition the high resonant frequency of the cantilevers means that the sensor has a response time of ∼1 ms. This allows fast surface processes to be studied. The Si3N4 (Park Scientific) cantilevers used are stripped of their Au reflective coating in aqua regia, and the electrode surface is deposited by thermal evaporation of Au at 270 °C. This yields a well-ordered (111) oriented Au film. The crystallinity of the film was verified by X-ray diffraction. The electrolytes used in this work are 1 mM Pb(ClO4)2 (99% Johnson Matthey) in 0.1 M HClO4 (70% Aristar grade BDH) and also 10 mM Pb(ClO4)2 in 0.1 M HClO4. Throughout this work a freshly cut Pb wire sealed in a glass tube was used as the reference electrode, and all potentials are quoted with respect to the Pb/Pb2+ couple. Electrochemical control was provided by an Eco-Chimie PSTAT 10 potentiostat, which facilitated the simultaneous measurement of the current and stress as a function of electrode potential. The deflection of the free end of the cantilever is measured using a standard optical deflection AFM arrangement. The focused optical beam is aligned so that it strikes the free end of the cantilever, and the reflected beam impinges upon a linear photodiode detector. The deflection of the free end of the lever can be related to the geometry of our particular detection arrangement and the difference between the photocurrents from the two ends of the photodiode. The surface stress can be determined from Stoney’s equation,20 which relates the deflection of a cantilever to the difference in the surface stress in its two faces:
σ1 - σ2 )
Et2 6(1 - ν)R
(1)
where σ1 and σ2 represent the surface stresses in the two faces of the cantilever and E, t, ν, and R are the Young’s modulus, thickness, Poisson ratio, and radius of curvature of the cantilever, respectively. The Au-coated Si3N4 cantilever is a composite cantilever, so it is necessary to consider the effect of the Au coating on the mechanical properties of the cantilever. Since the Au coating is thin compared to the Si3N4 coating, the elastic properties can be calculated simply using the method of Sader et al.21 The presence of one monolayer of Pb on the lever at some stages of the experiment has a negligible effect on the mechanical properties of the cantilever. For our particular levers the appropriate values for E and ν are 1.47 × 1011 N m-2 and 0.31, respectively. In our particular case the stress change can be calculated from18 (17) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Yee, D.; Sorensen, L. B. Phys. Rev. Lett. 1995, 75, 4472. (18) Brunt, T. A.; Chabala, E. D.; Rayment, T.; O’Shea, S. J.; Welland, M. E. Accepted by J. Chem. Soc., Faraday Trans. (19) O’Shea, S. J.; Welland, M. E.; Brunt, T. A.; Ramadan, A. R.; Rayment, T. J. Vac. Sci. Technol., B 1996, 14, 1383. (20) Maissel, L. I.; Francombe, M. H. An Introduction to Thin Films; Gordon and Breach Science Publishers, New York. (21) Sader, J. E.; Larson, I.; Mulvaney, P.; White, L. R. Rev. Sci. Instrum. 1995, 66, 3789.
Figure 1. Pb UPD on Au(111); 1 mM Pb(ClO4)2 in 0.1 M HClO4; 50 mV s-1; solid line, current; dashed line, surface stress.
∆σ )
[
∆(A - B) Et2L 24(1 - ν)sl (A + B)on - (A + B)off
]
(2)
in which ∆σ is the change in surface stress of the electrode surface, L is the length of the photodiode detector (5 mm), s is the leverphotodiode separation (12.0 mm), l is the effective length of the lever (typically 300 µm), and A and B are the measured photodiode output voltages, which are proportional to the two photocurrents from the two ends of the photodiode. The subscripts “on” and “off” refer to the value of the (A + B) signal when the optical beam strikes the free end of the cantilever or misses the cantilever, respectively. This is necessary to calibrate the stress changes correctly.18
Results and Discussion Figure 1 shows a cyclic voltammogram for the Pb UPD process on Au(111). There are three features in both the deposition and the stripping scans in the cyclic voltammogram. The first peak (1) is seen as a broad shoulder in deposition at +370 mV. The corresponding stripping peak (1′) is more well-defined and is at a potential of +500 mV. The next two features are both associated with the principal UPD peak, and both occur between +150 and +250 mV in the deposition scan (2 and 3) and between +200 and +300 mV in the stripping scan (2′ and 3′). This is in reasonable agreement with the voltammetry in the literature.10,13 The splitting of the main UPD feature into two peaks is strongly dependent upon the scan rate, and at scan rates greater than 100 mV s-1 the two peaks coalesce. The surface stress is also recorded as a function of potential. Despite the fact that the current curve shows some scan rate dependence, the stress curve shows minimal variation with scan rate. This suggests that the monolayer structure and its interaction with the underlying substrate surface are the same irrespective of the form of the voltammogram. That is, the surface stress is a more sensitive probe of monolayer formation than the current. For example, if the scan rate is reduced so that the monolayer current peaks are no longer visible above the background current, the stress changes still exhibit the same variation with applied potential, indicating that monolayer adsorption and desorption is still occurring. In the deposition scan there is a gradual reduction in the compressive stress as the potential is scanned from +800 to +400 mV. In this potential region no Pb is deposited onto the Au surface, so the stress change arises from the electrocapillary curve of the Au(111) electrode. We have measured the potential of zero charge (pzc) of the Au electrodes, and it is ∼ -50 mV with respect to the Pb/Pb2+ reference electrode; so one would expect the compressive stress to gradually decrease in the potential
5944 Langmuir, Vol. 12, No. 24, 1996
Figure 2. Current (solid line) and surface stress derivative (dashed line) showing the correlation between peaks in the deposition current and maxima and minima in the surface stress derivative. These data are taken from the deposition scan of Figure 1.
region prior to Pb deposition (i.e. between 800 and 400 mV in the cathodic scan direction). In the region of the pzc, the electrocapillary stress changes are small in relation to the Pb-induced stress changes (i.e. ∼0.1 N m-1 over a 200 mV range.8,18 As the potential is reduced further, the compressive stress starts to increase due to deposition of Pb on the surface. Studies using AFM13 and STM22,23 suggest that the broad shoulder in the current at a potential of +400 mV is associated with the onset of Pb deposition, initially at step edges and then as closepacked islands on the (111) terraces. Formation of the close-packed incommensurate monolayer which corresponds to the main monolayer deposition peak at around +220 mV is thought to take place via coalescence of these islands. In the potential region prior to the main deposition peak, all the islands are thought to have the same incommensurate structure as the full monolayer. This means that one would expect the surface stress to increase in a monotonic fashion with increasing coverage. This is demonstrated in both the variation of surface stress with potential and in the surface stress derivative with respect to potential (Figure 2). It can be seen that the rate of compressive stress increase (Figure 2) has a maximum value at the potential corresponding to the sharp deposition current peak at +220 mV. This sharp current peak corresponds to the formation of a full close-packed monolayer.13,14,23 The maximum in the stress derivative is followed closely by a minimum, the position of which corresponds to the second peak in the voltammogram at a potential of +160 mV. This corresponds to the potential at which Toney et al.14 observe a discontinuous change in the rotation angle of the incommensurate monolayer from 0° to 2.5°. We observe this rotational phase transition in the surface stress. Finally in the potential region between UPD and bulk deposition, there is a reasonably linear increase in the compressive stress as the potential is reduced toward the bulk Pb region. This latter observation agrees well with the monolayer strain measurements of Toney et al.14 made on this system. Using grazing incidence X-ray scattering (GIXS), they observed a continuous decrease in the Pb monolayer lattice parameter in the potential region (22) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. 1992, 271, L338. (23) Green, M. P.; Hanson, K. J.; Carr, R.; Lindau, I. J. Electrochem. Soc. 1990, 137, 3493. (24) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897.
Brunt et al.
Figure 3. Surface stress (solid line) and nearest neighbor spacing (points) as a function of potential. The stress data are taken from Figure 1, and the nearest neighbor spacings are from ref 14.
between the main monolayer peak and the onset of bulk deposition. This potential-driven reduction in the monolayer lattice parameter has also been seen for other incommensurate metal monolayer systems, specifically Tl and Bi on Au(111) and Pb, Tl, and Bi on Ag(111),14 as well as for halide adlayers on Au(111).23 This effect has been termed ‘electrocompression’.24 In Figure 3 the stress measurements for this electrocompression region are plotted along with the lattice parameters (i.e. the monolayer strain) measured by Toney et al.14 The principle difference between the two sets of data is the pronounced hysteresis in the stress curve. This is a quite general feature of all the stress curves that we have measured and does not diminish on reduction of the potential scan rate. However, Toney et al. do not see any hysteresis in their measured lattice parameters depending upon the scan direction. One reason for this difference could be that our stress measurements are comparatively rapid, whereas by necessity the GIXS measurements are static (i.e. X-ray data are collected at a fixed value of potential). The possibility of surface alloy formation could also give rise to these different dependencies upon scan direction, again because of the different time scales of the two measurements. Very slow scan rates are difficult in our experiments because of thermal drift, so slow surface processes could be responsible for the hysteresis in the surface stress which is not present in the nearest neighbor spacing data. It has been shown using thin layer electrochemical cells10 that the Pb species in the completed monolayer is fully discharged, so it is reasonable to assume that the surface stress changes in the 0 < V < 150 mV region are solely due to the repulsive forces between the surface Pb atoms. The extent of agreement between the stress and strain curves can be estimated by treating the Pb monolayer as an isotropic elastic medium characterized by the elastic constants of bulk Pb. If we assume that Hooke’s Law is obeyed, then we may treat the Pb monolayer as a free-standing elastic film. In this case the stress σ and strain are linearly related via the biaxial modulus, Y, and the film thickness t.25
)-
σ Yt
(3)
However, we can only measure changes in the surface stress rather than absolute values, so in the results (Figures 1, 3, and 4) the zero of stress is arbitrary. In (25) Cammarata, R. C. Prog. Surf. Sci. 1994, 46, 1.
Surface Stresses in a Metal Monolayer
Langmuir, Vol. 12, No. 24, 1996 5945
other words eq 3 should be replaced by an equivalent expression involving the changes ∆ and ∆σ:
∆ ) -
∆σ Yt
(4)
The strain can be written in terms of the lattice parameter d:
)
d - d0 d0
(5)
where d is the strained nearest neighbor spacing and d0 the unstrained (bulk) nearest neighbor spacing. The strain change can be written
∆ )
∆(d - d0) d0
(6)
Over the potential range 0 < V < 150 mV, the change in nearest neighbor spacing ∆(d - d0) is 0.07 Å (taken from the GIXS data14) with the unstrained nearest neighbor spacing, d0, taken as 3.50 Å. This corresponds to a measured strain change of 2.0% over this potential region, calculated from eq 6. Using the biaxial modulus value of bulk Pb of 1.986 × 1010 Nm-2 25,26 and taking a value of the effective film thickness of 3.50 Å (equal to the unstrained nearest neighbor spacing), then the strain change calculated from eq 4 using the data of Figure 1 is 3.1%. Although these values are of the same order of magnitude, clearly a more careful calculation of the surface strain is required. It must be emphasized that both Y and t are not accurately known for a monatomic film because of the differences in the mechanical properties of a monolayer from those of the bulk material. This makes the simple comparison outlined above difficult to evaluate. For example, it has been shown that biaxial moduli of thin films can differ from the bulk value by as much as 50%.27 In addition, it is necessary to consider the effects of the substrate surface, which is likely to strongly influence the physical properties of the monolayer; i.e., the assumption that the monolayer is free-standing needs to be addressed. The stress data can also be considered qualitatively in terms of the strengths of the various lateral and vertical surface interactions between the Pb and Au species. There is clearly competition between maximizing the favorable Pb-Au interaction by compressing the monolayer at the expense of increasing the compressive stress and hence the strain energy in the monolayer. This has been discussed in terms of effective medium theory,14 which asserts that the monolayer compresses to compensate for its constituent atoms’ lack of coordination to neighboring species. The continuous compression of the monolayer with decreasing potential can be explained in terms of a thermodynamic driving force28 for the 2-D compression. Consider a hypothetical uncompressed monolayer in equilibrium with the Pb ions in solution at a potential more negative than the monolayer equilibrium potential; i.e., the monolayer effectively experiences a cathodic overpotential. This means that the concentration of Pb within the monolayer is lower than that required to maintain equilibrium with the ions in the electrolyte. Hence there is a driving force for incorporation of more (26) Lide, D. R., editor in chief. Handbook of Chemistry and Physics, 75th ed.; C.R.C. Press: Boca Raton, FL, 1994. (27) Cammarata, R. C.; Sieradzki, K. Phys. Rev. Lett. 1989, 98, 897. (28) Melroy, O. R.; Toney, M. F.; Borges, G. L.; Samant, M. G.; Kortright, J.; Ross, P.; Blum, L. J. Electroanal. Chem. 1989, 258, 403.
Figure 4. Pb UPD and bulk deposition on Au(111); 10 mM Pb(ClO4)2 in 0.1 M HClO4; 50 mV s-1; solid line, current; dashed line, surface stress.
Pb species into the monolayer, which compresses as the potential is reduced. This is seen in the stress data of Figure 1. On reversal of the potential scan direction, the compressive stress in the monolayer is lifted, although, as has been mentioned earlier, there is some hysteresis in the stress change on reversal. Qualitatively, the stress changes throughout this anodic scan are similar to those in the cathodic scan, although the hysteresis becomes even more marked in the potential region positive of the main monolayer stripping peak. Not only is hysteresis apparent in the stress-voltage curve, but the change in stress during stripping is considerably smaller than the corresponding deposition change. It is interesting to note that even though the voltammogram suggests that most of the Pb has been stripped off at +0.3 V, the residual adsorbed Pb still causes a large stress (∼0.1 N m-1). These observations are consistent with our previous conjecture, concerning the lack of hysteresis in the GIXS strain data, that the system is in some sense only reversible on long time scales. For faster potential scan rates, a slow desorption rate means that adsorbates can remain on the surface at potentials appreciably removed from the true equilibrium potential. This hysteresis matches the electrochemically irreversible nature of the adsorption of Pb on the Au (111) surface observed in this potential range, i.e. the hysteresis in peaks 1 and 1′ in the voltammogram. Pb remains on the surface well after the main monolayer stripping peak, and this gives rise to a considerable stress. Irreversible adsorption of Pb on the Au(111) surface has also been observed using both the electrochemical quartz crystal microbalance (EQCM)29 and the rotating ring disk electrode (RRDE).30 If the potential is scanned to +0.8 V, then all the Pb is stripped from the electrode surface and the surface stress returns to its original value. Figure 4 shows the stress change associated with depositing and stripping bulk Pb. In the monolayer region the current and stress changes are the same as those in Figure 1. On continuing to scan into the bulk region, the compressive stress continues to increase reasonably linearly. On reversal of the potential scan direction, there is initially little change in the stress, but then the compressive stress is reduced as the bulk Pb is stripped from the electrode surface. Once again there is pronounced hysteresis in the stress curve on reversal of the scan direction. Pb is known to grow in a Stranski-Krastanov growth mode in the bulk region,31,32 which is favored (29) Hepel, M.; Kanige, K.; Bruckenstein, S. Langmuir 1990, 6, 1063. (30) Vicente, V. A.; Bruckenstein, S. Anal. Chem. 1973, 45, 2036.
5946 Langmuir, Vol. 12, No. 24, 1996
Figure 5. Current (solid line) and surface stress derivative (dashed line) for deposition scan of Figure 4, showing that the main stress changes are associated with monolayer rather than bulk deposition.
because of the lattice misfits both between the substrate and the compressed UPD monolayer and between this monolayer and the unstrained bulk phase growing on top of it. In Figure 4 it can be seen that the compressive stress continues to increase until the onset of bulk crystallite formation because of the potential-driven thermodynamic driving force for electrocompression described above. The bulk crystallites grow with the bulk Pb lattice parameter and consequently would be expected to be relatively stress-free.30 Furthermore, the area of contact between the crystallites and the underlying surface may be small because of the geometry of the crystallites. This means that any stresses present in the crystallites will be poorly coupled to the cantilever. This is shown in Figure 4, where the stress changes around -0.1 V are relatively small but the current shows that large amounts of Pb are being deposited. This is seen more clearly in Figure 5, in which the stress derivative is plotted and approaches zero in the potential region around -0.1 V. Despite the fact that large amounts of material are being deposited in the bulk deposition region (several monolayer equivalents), the associated stress changes are small, showing that the bulk crystallites are indeed relatively stress-free. For a similar electrochemical system, the UPD of Pb on Ag(111), Melroy et al.28 observed that if a sufficient amount of bulk Pb was deposited (∼5 monolayer equivalents), then the underlying UPD monolayer would restructure to the bulk Pb geometry. If any restructuring of the monolayer did occur, then we would expect to see some change in the surface stress. However, there is no indication of any restructuring of the monolayer associated with the deposition of up to 10 monolayer equivalents of bulk Pb. Conclusions We have directly measured the surface stress changes associated with monolayer deposition and stripping. It (31) Staikov, G.; Budevski, E.; Obretenov, W.; Lorenz, W. J. J. Electroanal. Chem. 1993, 349, 355. (32) Obretenov, W.; Schmidt, U.; Lorenz, W. J.; Staikov, G.; Budevski, E.; Carnal, D.; Mu¨ller, U.; Siegenthaler, H.; Schmidt, E. J. Electrochem. Soc. 1993, 140, 692.
Brunt et al.
is clear from the data that the stress changes vary in a complicated way with the applied electrode potential. These stress changes can be discussed in terms of the different adlayer structures present13,14 in different regions of the potential cycle. 1. During the initial stages of Pb deposition, associated with the broad feature in the voltammogram (+370 mV in deposition), the Pb is deposited in incommensurate islands. The compressive stress in this region increases as the amount of Pb on the surface increases. 2. The rate of stress increase is a maximum at the main monolayer deposition peak (+220 mV in the deposition scan). This corresponds to the formation of a completed incommensurate monolayer, which is compressively stressed. 3. At the third peak in the voltammogram (+160 mV in deposition) there is a pronounced reduction in the derivative of the stress with respect to potential. This is despite the fact that more Pb is being adsorbed. This change in the stress derivative may be related to a rotational phase change in the monolayer. It has been shown14 that the rotation angle changes discontinuously from 0° to 2.5° at this potential (+160 mV). 4. As the potential is scanned further toward the bulk deposition region, the compressive stress increases linearly. This corresponds to a continuous compression of the completed monolayer which occurs so that more Pb can be incorporated into the monolayer. A linear decrease in the monolayer nearest neighbor spacing in this potential region has also been observed using GIXS.14 The changes in nearest neighbor spacing have been related to the measured stress changes using a simple linear elastic model for the mechanical behavior of the monolayer. The strain changes calculated from our stress data agree with the measured values14 to within a factor of ∼50%. This discrepancy most probably arises from the assumptions present in this treatment and uncertainties in the appropriate values for the thickness and elastic constant of the monolayer. 5. On reversal of the potential scan direction, the same qualitative features are apparent in the variation of the stress with potential, although there is a large hysteresis. There is some degree of hysteresis associated with all three pairs of current peaks, particularly the broad peak (1/1′). Irreversible adsorption of Pb in this potential region has been detected using other techniques (EQCM29 and RRDE,30 and we propose that differences in the amount of Pb adsorbed at a given potential give rise to a significant (∼0.1 N m-1) difference in the stress in the different scan directions. 6. The stress changes associated with the deposition of bulk Pb are smaller than those which have been described for monolayer processes, despite the fact that large amounts of Pb are deposited during bulk deposition. The compressive stress is associated with the lattice mismatches between the Au surface, the compressively strained Pb monolayer, and the unstrained bulk Pb. Consequently, the bulk Pb grows in a Stranski-Krastanov mode.31 This results in the formation of three-dimensional crystallites which appear to be largely stress-free. LA960564+
