V. Bostanov

Form and step distance of polygonized growth spirals

Abstract The forms and the step distances of polygomized spirals of growth arising from emergence points of screw dislocations are calculated taking into account that the propagation rate of steps depends on step length. A rather unexpected result is obtained showing that the ratio between the step distance d and the radius ρ c of the inscribed circle in the geometrical form of the nucleus is independent of the geometrical form (i.e. number of corners) of the spiral and equal to approximately 19, very near to the value of this ratio for a circular spiral calculated by Cabrera and Levine.

STM studies of real and quasi-perfect silver single crystal surfaces used in electrochemical experiments

Abstract Real and quasi-perfect silver single crystal surfaces, which have been used intensively in different electrochemical investigations, are studied by STM to determine the surface profile on an atomic level. The measurements were carried out on chemically polished macroelectrodes and electrolytically grown microelectrodes in contact with air as well as under potentially controlled electrochemical conditions. The real macroelectrodes show a much higher surface corrugation than the quasi-perfect microelectrodes which have large atomically flat terraces separated by monoatomic steps. The possibility to observe the dynamics of monoatomic steps under anodic and cathodic polarization conditions on quasi-perfect single crystal surfaces is demonstrated. Nucleation and crystal growth processes are observed by means of in situ STM investigations of lead electrodeposition on Ag(111) macroelectrodes. The results demonstrate the possibility to study fundamentally the initial steps of electrocrystallization by in situ STM under electrochemical well-defined conditions.

STM studies in underpotential—overpotential metal deposition

Abstract The deposition of lead on “real” silver single crystal faces in the so-called underpotential ( upd ) and overpotential ( opd ) ranges was studied by in situ STM with atomic resolution. The results are compared with previous electrochemical, morphological, and in situ EXAFS and GIXS investigations. The STM images on bare substrates show regular hexagonal and quadratic lattices for the (111) and (100) planes, respectively, without any reconstructions. At high coverages in the upd range, well-ordered lead adlayers are observed on both substrate orientations. The lead monolayer on Ag(111) was found to be incommensurate, close packed, and rotated by about 4–5° with respect to the substrate. This agrees with previous GIXS measurements and indicates internal compressive strain within the adlayer. In contrast, lead on Ag(100) forms a superlattice structure which has a much lower adatom density than a hexagonal close-packed layer. The symmetry and interatomic distances are close to a c (2 × 2)Pb overlayer. In the opd range, 3-D lead crystallites are formed. On both substrate orientations, the contact plane of the lead crystallites is the close-packed (111) plane. At low overpotentials, the lead crystallites on Ag(111) grow epitaxially, rotated by about 5° with respect to the substrate. On Ag(100), two different orientations of lead crystallites in parallel to both [110] crystallographic directions of the substrate are observed. The nucleation rate of lead depends not only on the overpotential, but also on the initial polarization state in the upd range using an upd  opd transient technique. Nucleation and growth may proceed either inside the imperfect and strained upd -adlayer or on top of it according to the Stranski-Krastanov mechanism.

Unified description of the rate of nucleation-mediated crystal growth

Abstract The steady-state rate of nucleation-mediated growth of crystals is described by a unified expression combining the known equations for mononuclear and polynuclear growth mechanisms. The newly developed description is verified by means of a Monte Carlo computer simulation and comparison with the results of previous simulations is made.

Polynuklearer wachstumsmechanismus von schraubenversetzungsfreien kristallflächen (100) bei der elektrokristallisation des silbers

Zusammenfassung Das polynukleare Wachstum bei der Elektrokristallisation des Silbers an schraubenversetzungsfreien (100)-Flachen wird experimentell untersucht. Der Verlauf der experimentellen potentiostatischen strom—Zeit-Transienten stimmt recht gut mit der Theorie uberein. Es wird gezeigt, daβ die “Alterung” der Kristalloberflache einen starken Einfluβ auf die Wachstumkinetik hat. Aus der experimentellen Strom—Zeit-Anhangigkeit wird die Keimbildungsgeschwindigkeit als Funktion von der Uberpannung an “aktivierten” und “gealterten” Kristalloberflachen bestimmt.

Electrocrystallization of silver: Growth kinetics of screw-dislocation-free (100) crystal faces

Abstract At given conditions, especially at higher supersaturation, the growth rate of a close packed, perfect crystal face depends on the formation rate of two-dimensional nuclei and on the propagation rate of the monoatomic layers. This multinuclear multilayer growth as well as the advancement rate of growth steps have been studied experimentally on electrocrystallization of silver. The advancement rate of mono- and polyatomic growth steps has been measured on screw dislocation-free (100) crystal faces. For low overvoltages a linear dependence of the rate on overvoltage has been found. A strong influence of the surface condition of the crystal face — “fresh” or “aged” on the step advancement rate has been established. It was also found that on a “fresh” surface mono- and polyatomic steps advance with the same rate. The average monoatomic step spacing of the polyatomic step has been determined. The kinetic constants of the step growth rate are established and a conclusion regarding the mechanism of electrolytic deposition of silver is drawn. The initial current—time curves were recorded on applying potentiostatic pulses on a perfect crystal face. The shape of these curves coincides very well with those theoretically calculated for the cases of multinuclear growth. On the basis of the theoretical dependences, one can determine from these curves the formation rate J of two-dimensional nuclei at a given overvoltage η since the rate of step advancement is known. A linear dependence of log J on 1/η has been established. The values of the pre-exponential term in Volmers equation and the specific edge energy of the two-dimensional nucleus have been determined. The surface condition of the crystal face influences strongly also the process of two-dimensional nucleation.

Monolayer formation by instantaneous two-dimensional nucleation in the case of electrolytic crystal growth

Abstract The formation kinetics of monoatomic layers on a (100) screw-dislocation free face of a silver single crystal is investigated experimentally by means of the double-pulse potentiostatic method. Two-dimensional nuclei are formed with an initial pulse at a higher overpotential and are grown at a lower overpotential to complete a monolayer. The current transients recorded are in good agreement with the theoretical model of monolayer deposition by instantaneous two-dimensional nucleation and growth. From each transient the number of nuclei participating in the monolayer formation and the nucleation rate are determined. From sets of transients with different nucleation pulses the time dependence of the number of nuclei and the overpotential dependence of the nucleation rate are obtained. The kinetics of two-dimensional nucleation is found to be stationary. The overpotential dependence of the nucleation rate is in good agreement with the theory, giving the possibility of evaluation of kinetic parameters.

Monte Carlo simulation of polynuclear mechanism of crystal growth

Abstract The polynuclear growth mechanism of crystals is studied by means of a modified Monte Carlo simulation technique. An attempt is made to bring the simulation model to correspond to the growth conditions of the polynuclear mechanism as required by theory. A representative transient of the growth rate is obtained and compared with existing simulation studies of the polynuclear growth mechanism.

Adsorption of ClO4−, NO2− and NO3− on the (0001) face of a cadmium single crystal

The capacitance-potential dependence of an electrolytically grown (0001) face of cadmium in dilute solutions of ClO4−, NO2− and NO3− is studied. A weak specific adsorption increasing in the order ClO4− < NO2− < NO3− is observed. An influence of the electrode on the electroreduction of NO2− and NO3− is also found.

Zero charge potential and structure of the electric double layer of the (0001) face of a cadmium single crystal in NaF solution

Abstract The electric double layer of the (0001) face of a cadmium single crystal grown electrolytically in a Teflon capillary has been studied in a solution of NaF. A value of −0.99 ± 0.01 V (vs. SCE) was obtained for the zero charge potential (pzc). This value is compared with literature data for other cadmium faces and polycrystalline cadmium. It is shown that the pzc is practically independent of the crystallographic orientation. The capacity of the inner layer of the (0001) cadmium face is calculated and compared with that of mercury and of separate silver faces. It is concluded that the hydrophilirity of the (0001) face of cadmium is similar to that of mercury and much lower than that of the (111) and (100) faces of silver.