Jack M. Blakely

Equilibrium segregation of carbon to a nickel (111) surface: A surface phase transition

Abstract Equilibrium segregation of carbon to the (111) surface of carbon-doped nickel single crystals has been studied by Auger spectroscopy. At least three distinct equilibrium states of the surface occur, viz: A, a high-temperature dilute carbon phase, B, a condensed graphitic monolayer, separated from A by a sharp transition with temperature and G, a multilayer epitaxial graphite precipitate. State B persists in equilibrium about 100K above the observed equilibrium temperature for dissolving graphite into the nickel crystal. Our interpretation attributes this to (∼ 0.05 eV) stronger binding of carbon to this monolayer phase than to bulk graphite. We believe the observed equilibrium A-B transition is the first equilibrium surface phase transition detected in segregation studies.

Carbon monolayer phase condensation on Ni(111)

A condensed monolayer of carbon on the (111) surface of C-doped nickel single crystals has been observed at different bulk doping levels in the range of 10−1 to 1 at%. This phase persists over a range of temperatures of more than 100 K; it is bounded at high temperatures by the formation of a dilute carbon phase at TS and at a lower temperature, TP, by the formation of a thick graphite precipitate. The ratio TSTP is about 1.11 at all doping levels investigated. The dependence of the precipitation and the phase transition temperatures on the bulk doping level allow the binding energy per carbon atom in the monolayer phase relative to that in the bulk solution to be determined; it is approximately 10% greater than for bulk graphite with a value of 0.55 eV. The partial atomic entropy in the monolayer phase was also determined; it is about 0.03 k greater than for bulk graphite.

Carbon segregation to single crystal surfaces of Pt, Pd and Co

Abstract Results are reported on the surface segregation behaviour of carbon from dilute solid solutions in Pt, Pd and Co. With Pt(100) no preferential surface segregation was observed; this is similar to previous results for Pt(111). For Pd(lOO), Pd(111) and Co(0001) segregation was observed with evidence for a surface phase transition of the type previously reported for Ni(111). These observations suggest that the strong carbon-carbon interactions within a graphite monolayer are of more importance in producing the transition than a good epitaxial fit to the substrate. A comparison of the kinetics of carbon segregation to Co(0001) with those predicted by a simple diffusion model suggest that surface processes such as nucleation or lateral diffusion may play important roles.

Segregation isosteres for carbon at the (100) surface of nickel

Abstract Measurements of segregation isosteres are reported for C to Ni(100). The model independent isosteric heat of segregation is found to be in agreement with the value derived from coverage versus temperature data using the Langmuir model. Results for other nickel surfaces are summarized and compared with predictions using the empirical bond energy bond order scheme. The relative success of this scheme supports the suggestion that the carbon-surface bonding is to be regarded as arising from localized interactions.

Origin of equilibrium space charge potentials in ionic crystals

Abstract A simple model is constructed which illustrates the origin of ionic space charge regions at surfaces of ionic crystals of the AgCl and NaCl type. The model differs from previous treatments by considering explicitly the binding states of ions on surfaces and the surface density of such states. The problem is then formally analogous to that of an intrinsic semiconductor with a fixed density of surface electronic states. The space charge potential difference is determined by the occupation statistics of the surface ionic states. The model predicts significant differences from previous calculations of the temperature variation of the surface space charge potential.

The kinetics of oxygen adsorption on the (112) and (110) planes of tungsten

Abstract The kinetics of oxygen adsorption on the (112) and (110) planes of tungsten at coverages below a monolayer have been studied in detail by LEED and work function measurements during adsorption. On both surfaces the oxygen appears to adsorb first in a weakly bound molecular state and chemisorption occurs at the edges of growing islands of previously chemisorbed oxygen atoms. The sticking coefficient is found to be independent of coverage on the (112) surface indicating that the inter-island distance is sufficiently small that every adsorbed molecule reaches an island in its diffusion lifetime. On the (110) surface the sticking coefficient is found to be proportional to the square-root of the coverage indicating that the islands are so far apart that only those molecules adsorbed sufficiently close to the edge of an island become chemisorbed. The question of reconstruction of substrate atoms is considered in some detail in view of these and other recently published1) findings. It is shown that the experimental data is consistent with a simple chemisorption model which does not involve reconstruction.

A study of facetting of tungsten single crystal surfaces

Abstract Thermal facetting of the (100), (112), and (111) surfaces of tungsten to {110} planes is observed in the presence of oxygen at pressures between 10−8 and 10−6 Torr using a vibrating capacitor work function probe in conjunction with low energy electron diffraction (LEED). The (110) surface is stable against facetting under the same conditions. The work function of the facetted surfaces for the three planes indicated is the same as that of an oxygen covered (110) surface and is equal to 6.45 eV. A (112) surface containing periodic {110} facets only 20 A wide has the same 6.45 eV work function as an oxygen covered (110) plane. A diffraction analysis is presented which allows the determination of the indices of the facet planes from the voltages at which very intense primary maxima occur and the symmetry of the pattern at intermediate voltages. For simple facet planes, the primary maxima correspond to bulk Laue reflections. The kinetics of facet growth on the (112) surface are characterized by a low temperature W-O coupled diffusion controlled region and a high temperature region controlled by oxygen supply to dissociation sites.

Surface self-diffusion on Si from the evolution of periodic atomic step arrays

Abstract The development, during annealing, of periodic atomic step arrays associated with etched grating structures on Si(001) has been monitored by optical methods and by STM. The grating amplitudes decay exponentially with time in the temperature range from 800 to 1100°C with characteristic decay constants that scale approximately as the inverse fourth power of the grating period; this indicates the dominance of surface diffusion as the mass transport mechanism. The activation energy for Si surface self diffusion is found to be ~2.3 eV. Various detailed atomic mechanisms of diffusion are possible; comparing with other relevant data and calculations, our experimental value is consistent with an adatom transfer process. The details of the atomic step morphologies as determined by STM for these grating structures are described. The interactions of the atomic steps during decay are related to the curvature dependent driving forces for mass transfer.

Chlorine adsorption on the low index surfaces of silver: Energetics and structures

The interaction of Cl2 with Ag single crystal surfaces has been studied over a range of crystal temperature and gas pressure. Observations have been made for the (111), (100) and (110) surfaces. Measured adsorption isobars for Ag(100) were used to obtain isosteric heats of adsorption; values ranged from ~ −60 kcal/mole (of Cl2) at low coverage to ~ −70 kcal/mole near saturation. The structure formed by Cl2 adsorption on (100) is believed to be a simple overlayer. For (110) and (111) the values obtained for the heat of adsorption were ~ −55 kcal/mole. On Ag(111) an epitaxially oriented AgCl(111) is believed to form.

Space charge regions at silver halide surfaces: Effects of divalent impurities and halogen pressure

Abstract The model first proposed by Poeppel and Blakely to account for equilibrium space charge regions in ionic crystals is extended to include effects of mobile divalent impurities. Equations are developed that account for impurities segregating to free surfaces of crystals of the AgCl type. Numerical computations have been performed and the results indicate the variation with temperature of the potential difference across the surface space charge region. The importance of accounting for the density and energy of surface sites is demonstrated. The calculation includes effects of varying the doping level of the impurities as well as the binding energy of the impurity-vacancy complex. The effect of a chlorine atmosphere on intrinsic crystals is discussed and the variation of the chemical potential of the silver ions with Cl 2 pressure is presented.