Nucleation and growth of supported clusters at defect sites: Pd/MgO(001)

Abstract

Nucleation and growth of Pd on cleaved MgO͑001͒ surfaces were studied by variable-temperature atomic force microscopy in the temperature range 200-800 K. Constant island densities (ϳ3ϫ10 12 cm Ϫ2 ) were observed over a wide temperature range, indicating nucleation kinetics governed by point defects with a high trapping energy. These results are compared to a rate equation model that describes the principal atomistic nucleation and growth processes, including nucleation at attractive point defects. Energies for defect trapping, adsorption, surface diffusion, and pair binding are deduced, and compared with recent ab initio calculations.Metal aggregates supported on oxide surfaces have many practical applications due to their catalytic, magnetic, and electrical properties. Consequently, fundamental studies have been carried out on a range of model systems.1 Although the main microscopic steps governing nucleation and growth of the films are now understood, detailed characterization of these processes has proven difficult. In particular, little is known about the energies involved. In recent years, ab initio calculations of the binding of metal atoms and clusters to oxide surfaces have progressed, generating further stimulus for experimental determination of the relevant interactions. On the other hand, a much more complete understanding has been achieved for the case of metallic substrates. In large part, this is due to the application of variable-temperature scanning tunneling microscopy ͑STM͒ for in situ imaging of the nucleation and growth stages, compared to analytic models and numerical simulations. In this work, we adopt a similar approach to metal growth on insulating oxide surfaces. This allows us to determine the principal energies governing nucleation and growth, and in particular to consider the influence of defects. On oxides, defects like vacancies or steps are present even at wellprepared single-crystal surfaces. Frequently the nucleation and growth behavior on these substrates is dominated by the defects. Early transmission electron microscopy studies of metal growth on alkali halide and alkaline earth surfaces 3 indicated defect nucleation in some cases, but relatively little is known about the interaction between defects and adsorbed metal atoms.We have studied the growth of Pd on MgO͑001͒ surfaces, utilizing variable-temperature atomic force microscopy ͑AFM͒ to image the deposits. This is one of the most explored metal/oxide systems, being a model for supported metal catalysts. Palladium grows in three-dimensional clusters, similar to most metal/oxide systems, as the surface energy is usually higher for metals than for oxides.1 We demonstrate that the nucleation kinetics in a large temperature range is determined by attractive point defects. With the help of a simple rate equation model we deduce the relevant interaction energies for the Pd/MgO͑001͒ system. It is expected that the understanding of nucleation at defects can be explored to produce novel nanostructures. The experiments were performed...

Self-Organization of Gold Atoms on a Polar FeO(111) Surface

“…Surprisingly, the ratio between small Au clusters and isolated adatoms remains extremely low, even when the metal coverage approaches 0.012 ML. This apparent blocking of the Au aggregation on the FeO film is in sharp contrast to the nucleation behavior observed on nonpolar oxide surfaces and indicates substantial repulsion between the adatoms [1,2,23].…”

“…A comparatively low aggregation tendency is not observed for metals on nonpolar surfaces, where clustering sets in at much smaller nucleation densities [1,2,23]. On the other hand, self-organization is a common behavior for alkali atoms on metal and semiconductor surfaces, where the adsorption process is accompanied by an electron transfer to the support.…”

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…Furthermore, there is experimental evidence that metal nucleation often preferentially occurs at defects rather than at regular sites of well-ordered terraces of oxide surfaces. [6][7][8] Among these sites, oxygen vacancies or color centers, neutral F s (left after removal of an O atom) and charged F s + (formed when an O À anion is missing) are considered to Abstract: Cu 4 , Ag 4 , and Au 4 species adsorbed on an MgOA C H T U N G T R E N N U N G (001) surface that exhibits neutral (F s ) and charged (F s + ) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas-phase rhombic-planar structure of the coinage metal tetramers is only moderately affected by adsorption.…”

Imaging the real shape of nanoclusters in scanning force microscopy

“…The force microscope in its dynamic mode ͑dynamic SFM͒ offers the possibility to image surfaces with a true atomic resolution, [11][12][13] and indeed, to a greater extent in recent years, the dynamic SFM has been used for the imaging of nanoclusters on many types of surfaces. [14][15][16][17][18][19][20][21][22][23][24] However, also in dynamic SFM, the tip-surface convolution strongly reduces the resolution during imaging of the nanoclusters in the standard topography mode. 19,25,26 Furthermore, the scanning speed is limited and does not exceed speeds much higher than ϳ1 − 2 Hz, which does not permit time-dependent measurements on short time scales--a particular problem for biophysical SPM studies.…”