GENERAL DISCUSSION Dr. H. Wilman (Imperial College, London) said: There is presumably much uncertainty in the estimation of the current density in the electrodeposition on to very small convex tips of metal wires used in ion-microscope experiments. I wonder if Farr can state what was the order of magnitude of the “ high ” and “ low ” current densities used by Rendulic and Muller (his ref. (21)), to whom he refers, regarding epitaxy in cathodic processes? It is also likely that the tendency to cracking of epitaxial layers strained by some misfit would be increased when the deposits are made on such substrates of very small radius of curvature, and also subjected to the high field stresses in the ion microscope, as Farr indicates. I find it difficult to reconcile fig. 2(a) with the statement in the legend that “ a (100) region lies centrally, .. .”. Although there is some degree of symmetry about a horizontal line through the centre of the photograph, and also about a vertical line through it, there does not seem to be 4-fold symmetry of the pattern features about the central region of the photograph, to give square dispositions of similar 4-fold repeated features. Can the authors clarify this? Nanis and Javet (their ref. (20)) show a clear pattern of 4-fold symmetry from an iridium tip in a (100) orientation, and it differs clearly from this. I would also emphasize that the “ liquid-like ” coalescence observed by Pashley, Matthews and others in the growth of island crystal nuclei during condensation of vapour in vacuum, was under conditions of high atomic mobility on substrates in vacuum at considerably raised temperatures-about 400°C.It is unlikely that such high mobilities and coalescence effects will occur in electrodeposition except at com- parable temperatures in non-aqueous electrolytes (molten salts) or at very high current densities. With respect to the use by Farr and Rowe of the term “pseudomorphism”, I would stress that this term is not normally confined to cases of strict conformity of lattice spacings of the deposit crystal with those of the substrate crystal at the interface -if indeed any such cases of exact correspondence are known in practice. “ Pseudo- morphism” denotes that the deposit atoms, under the forces exerted by the 2- dimensional array of substrate surface atoms (with some contribution from lower layers), must take up positions tending towards the consecutive potential troughs of the substrate surface, but not in general exactly at the minima, since the deposit atoms are also acted on by the forces of cohesion with other laterally adjacent deposit atoms.It therefore seems inappropriate to say that “ pseudomorphism is, however, no longer regarded as necessary for epitaxy ”. I have also a query on the results under the heading Electrochemical Experiments in their paper, where evidence was quoted for deposition of copper at electrode potentials anodic to the equilibrium potential on some substrates. Were the metal substrates in these cases polycrystalline? If so, their surfaces would in general be uneven and consist of elements of different crystallographic forms on different crystal grains, on which presumably the equilibrium potentials would be also different, so that deposition would occur on some surface elements while still not on others.Dr. 6. P. G. Farr (Birmingham University) said: Most potential sweep experiments on electrodeposition so far reported have involved polycrystalline cathodes. There is general agreement (see our ref. (40)) with Wilman’s suggestion of initially localized 222GENERAL DISCUSSION 223 deposits on particular surface elements. Our own work on single crystals suggests that even here a variety of initial growth sites is normally available (fig. 1). One may question the use of the term “equilibrium potential” at the early stages of formation of the solid-solid interface. In our review we refer to the equilibrium potential as, for example, that of the Cubulk/Cu2+ electrode and we accept that an “ under-voltage ” is an indication of the adsorption energy between depositing atoms and a foreign substrate. One might expect a spread in under-voltage across a sub- strate surface, a dependence on surface preparation, and some variation as growth proceeds beyond a monolayer until the deposit is indistinguishable from bulk metal.Dickson, Jacobs and Pashley (ref. (27) of our paper) interpret the similarity in the early stages of electrolytic growth of gold on (111) silver substrates to growth by evaporation in terms of a surface migration of gold over silver and over gold islands. Farr and Loong have not observed liquid-like coalescence of copper or nickel nuclei on Ag.These metals grow rather differently from each other, copper particularly at discrete nuclei and nickel in continuous layers on the silver substrate with typical misfit dislocations. (This work will be published in detail elsewhere.) In the first part of our paper we mention some deficiencies in the characterization of adspecies from the electrode kinetics of aqueous systems. A further discussion of the role of adspecies may be obtained from ref. (2), (3) in our paper. In some early work, Gerischer found it necessary to ascribe 30-40 % ionic character to silver adspecies on silver and suggested that this was due to a strong interaction between adsorbed atoms with polar water molecules.1 suggest that at very high current densities the growth process is governed by the rate of charge transfer rather than the rate of nucleation or the surface diffusivity of an adspecies.The uncertainty in estimating a current density in electrodepositing on to a field ion microscope tip was recognized by Rendulic and Muller (ref. (21) of our paper) who estimated a mean current density i, between 0.1 A/cm2 (low) and 10 A/cm2 (high) at 50°C. They mention that the epitaxial layer of Pt on Ir ruptured easily at a thickness of 100-200 A ; the corresponding non-epitaxial layer on W was much more stable. However, the cracks within the epitaxial Pt were not thought to be the result of field stress. The apparent difference between our field-ion image (our fig. 2a) of a (100) oriented iridium tip and that shown by Nanis and Javet (ref. (20), fig.4) results from a rotation of about 45” in the plates as printed and, more important, a disparity in the dimensions of the tips. Our tip was smaller than that of Nanis and Javet and so there are fewer rings, or atomic ledges, between the central (002) plane and the next significant four (113) poles. In both cases the (111) planes referred to are at the peripheries of the images and in ours only one (1 11) plane is imaged, i.e., at the right- hand side of fig. 2a. Both illustrations show the “ zone decoration ” characteristic of the region around the (002) plane, i.e., bright spots on the images running from (002) towards { 113). It will be appreciated that in the field ion microscope an image of a 3-dimensional surface is projected on to a 2-dimensional screen and this projec- tion may not be simple; furthermore, our image is truncated compared witn that of Nanis and Javet, because our tip was asymmetric.Some authors have adopted a more restricted use of the term “ pseudomorphism ” than does Wilman. In our paper we have followed Pashley (ref. (25) of our paper), who accepts that pseudomorphism prevails where lattice misfits can be accommodated by elastic strain alone. However, this allows the “ basal plane pseudomorphism ” Various authors H. Gerischer and H. Fischer, 2. Elektrochem., 1957, 61, 1159. see, e.g., K. Vetter in Electrochemical Kinetics, Academic Press, 1967, p. 325.224 GENERAL DISCUSSION of Finch and Quarrell to be a rather frequent phenomenon.Basal plane pseudo- morphism would be expected to be lost more rapidly as a deposit thickened if the lattice misfit were considerable or if there were differences in crystal symmetry. For Ni (100) on Ag (100) where there is a misfit of some 15 %, Loong and Farr have observed increasing numbers of typical misfit dislocations in thickening films and suggest that partial coherence may occur in the initial stages of growth. For Ni on Ag it would not be expected (see ref. (25) of our paper) that pseudomorphism in Pashley's sense would be retained beyond the basal plane. It is thought that most solid precipitates within bulk metal have some coherence with the matrix.2 Unusual crystal forms and particular precipitate morphologies may result. Clearly, epitaxy may often be a result of basal plane pseudomorphism.Occasionally it may be that where deposition conditions or crystallographic factors do not permit this, a close-packed island of depositing atoms may form a 2-dimen- sional nucleus, so that a highly textured deposit may grow without there having been initial coherence. This may explain the [ l l l ] oriented Pt crystals obtained by Rendulic and Muller (ref. (21) of our paper) on Ir. We thank Wilman for reminding us of the review by K. Lawless in Physics of Thin Films, 1967, 4 ; that by K. L. Chopra (in Thin Film Phenomena, McGraw-Hill (1969) p. 193) is also relevant. Prof. M . W . Roberts (Bradford University) said : With reference to Wilman's remark concerning the growth model, there is now evidence from Bassett's field-ion work that dimers, trimers and " long chain " monomers are involved in growth of one metal on another.The predominance of any one type of growth nucleus (if I recall correctly) depended on the nature of the adatom, and also possibly on the im- pingement rate (i.e. , Wilman's current density). Dr. H . Wilman (Imperial College, London) said : Can O'Sullivan and Oxley state the mean thickness of their electrodeposited films under discussion? The form of expression used to define the epitaxial orientation of the b.c.c. alloy deposits seems unnecessarily complicated and confusing. This expression is indeed the same as is stated by Jones who, however, made it clear that $ is the azimuthal angle between the two sets of orthogonal b.c.c. (110) orientations on the Cu (001) substrate face (e.g., the angle between the radii to the innermost strong pairs of arcs in patterns such as fig.(b) of plate 1 of the paper of O'Sullivan and Oxley). Jones also stated that this angle was observed to lie between 15 and 18", so that the value of f (19'30'- $)/Z was only between 0 and about 2.5". It thus would be much clearer to define these epitaxial orientations simply as, in the mean : (1 10)[1T1] alloy 11 (100)[011] or [Oil] Cu in some deposits, but in others differing from this by up to about 2.5" in azimuthal orientation, in both cases there being a few degrees spread of azimuthal orientation about the mean, as shown by arcing in the pattern. In such electron diffraction patterns obtained by transmission through isolated thin films there is often some ambiguity as to the interpretation.An arcing of the diffractions can arise as a result of cylindrical curvatures (or bending) of the film. An appreciable inclination of the mean film plane away from the setting normal to the electron beam could cause a small change in the observed angle $ between the radii to the arc centres concerned, like the above small deviations, even if the ideal G. I. Finch and A. G . Quarrel], Proc. Roy. SOC. A, 1933, 141, 398. see, e.g., A. Kelly and R. B. Nicholson, Prog. Materials Sci., 1963, 10, 151 ; also the A. S. M. Seminar on Phase Transformations, ed. H. I. Aaronsen (1968)).GENERAL DISCUSSION 225 epitaxial b.c.c. (1 10) orientation stated above were strictly followed. The four-fold symmetry of the arc pattern would, however, be slightly impaired in this case and this should be an indication that the film had become tilted away from the normal setting.Similarly, such an inclination from the normal setting could suggest azimuthal disorientations or displacements of the kind concluded here, even if the disorientation were really about an axis in the film plane, as we have observed in some cases by grazing-incidence electron diffraction of films in situ on their substrates (see my paper with Verma, this Discussion). I am also not clear as to the reference to the case of " when 11/ = 90". . .". Did the authors in fact observe such (1 10) orientations in their electrodeposited b.c.c. alloys? As a further comment, ref. (2) is evidently incorrect and should be presum- ably Phil.Mag., 1965, 11, 993. Dr. J, M. O'Sullivan (Stainless Equipment Co.) said: The mean thickness of alloy foils used in our experiments was lOOOA. This was measured by application of Faraday's laws of electrolysis and direct weighing of cathodes. We are grateful to Wilman for his suggestion of an alternative and simpler method of expressing the interface crystallography between b.c.c. deposits and the copper substrate. His remarks about arcing and the effects of inclination of the mean film plane from a setting normal to the electron beam are of general application to transmission diffraction work, and it is salutary to be reminded of them. " When $ = 90" . . ." should have read ". . . when 11/ = 19'30'. This has been corrected in the present paper. Ref. (2) is indeed incorrect.Prof. W. C. Wake (City University) said: I have a few comments on the methods used by Roberts et al. for characterizing the deposited and bombarded surfaces. The use of Zisman's critical surface tension can be pushed too far by way of inter- pretation. For example, it is usually believed that yc will decrease with increasing density of packing but recent work at Brunel University has shown this is not so.' Using highly branched compounds of the type CH3 I CH3-C-CH3 CH3 1 I CH3-C-CH and rather simpler fluorinated derivatives, it was found that yc decreased with de- creasing density. The point is that under electron bombardment highly branched structures are likely and yc will be accordingly influenced. M. A. Parrish, unpublished work for M. Tech (1970).226 GENERAL DISCUSSION Prof.M. W. Roberts (Bradford University) said: Thank you for the information regarding the work at Brunel University. I agree that the correlation between yc and molecular environment (fig. 2) should be used cautiously. Nevertheless, if by controlling the chemistry we can predict surface characteristics, this is a step in the right direction. The present paper illustrates how control of some parameters (e.g. , the nature of the monomer, the substrate temperature, the nature of negative ions) leads to distinct surface characteristics as reflected by yc. It is also worth mentioning that the ll term (the spreading pressure) can not, as is often assumed, be equated to zero. The next stage is to obtain direct information on the molecular nature of the sur- face and we have had recently some success in this direction.Films formed from C,F,(g) and a substrate temperature of - 300 K have been shown to contain little, if any, fluorine. The films are therefore essentially composed of a carbon lattice and not built around CF as suggested in the paper. The electron spectroscopy informa- tion l is compatible with our mass-spectrometric studies in that CF ions were the pre- dominant gas phase ions observed. Prof. W. C. Wake (City Uiziversity) said: If the net result is carbon deposition then it is known that the normal alkanes used for determining yc show a mobile and not a fixed site (Langmuir) type adsorption. This I demonstrated some years ago by calculation of the appropriate entropies of adsorption.In this case the spreading pressure term in the Young equation would be appreciable and must influence the contact angle. Prof. R. Sh. Mikhail (Ain Shams University, Egypt and University of Salford) said: There is an unfortunate use in nomenclature when the authors used the terms “ high energy ” and “ low energy ” surfaces. The surface may have a weak interaction with one adsorbate and a strong interaction with another adsorbate, and it seems quite inappropriate to describe the surface itself to be low energy or high energy when this energy is a characteristic of the bond and not the surface. Prof. M. W. Roberts (Bradford University) said: Zisman drew a distinction between different solid surfaces on the basis of the observed yc (the critical surface tension, i.e., the value of y l v at cos 8 3 1 .O) using a series of non-polar liquids.Values below 100dyncm-l he referred to as “low energy ” surfaces. First, it is clearly an arbitrary definition. Secondly, as we have mentioned in our paper and else- where, the measured yc reflects not only the molecular nature of the substrate but how that substrate is perturbed by adsorption from the liquid phase. Furthermore, on this basis we put forward some tentative suggestions as to the orientation of the adsorbed molecules on the polymer film surfaces. It should be emphasized that yc should not be equated with ys the surface energy of the solid. Prof. W. C. Wake (City University) said: With regard to the paper by Nash et al., degradation by mechanical or ultrasonic means from high molecular weight to a limiting low molecular weight has been identified in morphological terms by Schoon and his fellow workers at Wurzburg. Using electron microscopy and a variety of ’ M. W. Roberts and C. R. Brundle, unpublished work. W. J. Murphy, M. W. Roberts and J. R. H. Ross, J.C.S., Faraday Trans. I, 1972, 68, 1190. Th. G. F. Schoon and R. Kretschmer, KoIIoidZ. 2. Polymer, 1964,197,51 ; Th. G. F. Schoon, British Polymer J,, 1970, 2, 86 ; Th. G. F. Schoon and G . Rieber, Angew. Makromol. Chem., 1971, 15, 263.GENERAL DISCUSSION 227 sample preparative techniques, they have shown that bulk polymer contains structures not apparent in solution studies, that there are several stages of ordered structure, and that a fundamental structural unit exists the size of which corresponds roughly to the limiting low molecular weight identified in the work of Nash et al. The views of Schoon et al. differ from those of Nash et al. in that they postulate a structure which is broken into pre-existing units rather than a mechanism determining a zone size. A recent paper by P. H. Geil of the Case Institute of Cleveland comes to similar conclusions with respect to polyvinyl chloride.