Ir Adatoms Research Articles

Modeling of Ir adatoms on Ir surfaces.

We used the embedded-atom method potential to study the structures, adsorption energies, binding energies, migration paths, and energy barriers of the Ir adatom and small clusters on fcc Ir (100), (110), and (111) surfaces. We found that the barrier for single-adatom diffusion is lowest on the (111) surface, higher on the (110) surface, and highest on the (100) surface. The exchange mechanisms of adatom diffusion on (100) and (110) surfaces are energetically favored. On all three Ir surfaces, ${\mathrm{Ir}}_{2}$ dimers with nearest-neighbor spacing are the most stable. On the (110) surface, the ${\mathrm{Ir}}_{2}$ dimer diffuses collectively along the 〈110〉 channel, while motion perpendicular to the channel walls is achieved by successive one-atom and correlated jumps. On (111) surface, the ${\mathrm{Ir}}_{2}$ dimer diffuses in a zigzag motion on hcp and fcc sites without breaking into two single atoms. On the (100) surface, diffusion of the ${\mathrm{Ir}}_{2}$ dimer is achieved by successive one-atom exchange with the substrate atom accompanying by a 90\ifmmode^\circ\else\textdegree\fi{} rotation of the ${\mathrm{Ir}}_{2}$ dimer. This mechanism has a surprisingly low activation energy of 0.65 eV, which is 0.14 eV lower than the energy for single adatom exchange on the (100) surface. Trimers were found to have a one-dimensional (1D) structure on (100) and (110) surfaces, and a 2D structure on the (111) surface. The observed abrupt drop of the diffusion barrier of tetramer, ${\mathit{I}}_{{\ensuremath{\gamma}}_{4}}$ on the Ir (111) surface was confirmed theoretically. \textcopyright{} 1996 The American Physical Society.

First-Principles Study of Domain Evolution of IR on IR(111)

ABSTRACTWe have calculated the effective cluster interactions (ECI) which govern the ordering of Ir adatoms on the Ir(111) surface. The computations are based on a tight-binding Hamiltonian in which no adjustable or experimentally determined parameters were introduced. Both atoms adsorbed in ‘bulk’ sites (i.e. continuing the fee lattice) and those in ‘surface’ sites (i.e. producing hep stacking) are considered. We use this formalism to determine the relative stability of various adsorption sites and cluster shapes at zero temperature. The overall trends are in excellent agreement with the experimental results found by Ehrlich and co-workers. Next, we employ these ECI in Monte Carlo simulations of the kinetics of domain growth and evolution. Specifically we analyze the effect of diffusion barriers and the competition between the ordering tendencies of the system and entropic effects. Typical ‘snapshots’ in a range of temperatures and coverages are discussed.

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion of single adatoms approaching both descending and ascending steps on Ni(111), (110) and (100) has been investigated with the embedded atom method (EAM) and molecular statics (MS). (1) It was found that there exists a “forbidden” region near both descending and ascending steps on Ni(111). Adatoms have to overcome a slightly higher energy barrier to get into the “forbidden” region, which extends 2–3 nearest-neighboring spacings from the steps. This is consistent with FIM experiments for incorporation of Ir adatoms into ascending steps of Ir clusters. (2) Detailed calculations for incorporation of adatoms over descending steps of type B on Ni(111) have revealed that exchange diffusion, in which an adatom replaces the position of an atom in the step and exchange their roles, is energetically favored over ordinary jumps and is the dominant diffusion mechanism at the descending steps. This is consistent with recent FIM experiments of W adatom diffusion at Ir cluster edges of type B on Ir(111). (3) Exchange diffusion was also found to be favored over direct jumps at the descending steps on Ni(110) and Ni(100).

Relative stability of ternary adsorption sites on (111) fcc and (0001) hcp transition-metal surfaces

In order to shed some light on the problem of crystal growth we here present a theoretical study of the relative stability of ternary adsorption sites on a close-packed surface of a hcp or fcc transition metal. We find that on Ir(111) the relative stability of the normal and fault sites varies with the nature of the adatom, which is in very good agreement with experiments: a single Ir adatom prefers the fault site, while an adatom with a larger d-band filling is more stable at a normal site. A similar calculation is carried out on Ru(0001) and predictions are made concerning the relative stability of these two adsorption sites.

Atomic replacement and adatom diffusion: Re on Ir surfaces.

The behavior of vapor-deposited Re adatoms on Ir surfaces has been studied with the field ion microscope (FIM). On the Ir{113} surface, diffusion of Re adatoms occurs by atomic hopping along the surface channels above \ensuremath{\sim}250 K. On the Ir{001} and {110} surfaces, atomic replacement of Re adatoms with substrate atoms will occur around 300 and 250 K, respectively; thus, diffusion of Re adatoms will not occur. Instead, the atomic replacement will induce self-diffusion of these Ir surfaces. On the Ir{110} surface, an atomic replacement takes one step, whereas on the Ir{001} surface, it takes two steps: first, an Ir atom is displaced out of the substrate to form a Re-Ir dimer-vacancy complex near 240 K. Around this temperature, the dimer can also change its orientation by 90\ifmmode^\circ\else\textdegree\fi{} without moving to a new site. Above 280 K, the complex can dissociate. Upon dissociation, the Re atom will incorporate into the substrate lattice and diffusion of the Ir adatom will occur. These elementary atomic steps can be seen directly in the FIM and the activation energies of these steps have also been measured. For heterosystems, this low-temperature atomic replacement is a mechanism of single atomic site alloying of the top surface layer. It will also induce self-diffusion of the substrate. It can already occur at or below room temperature for refractory metals.

Self-diffusion on reconstructed and nonreconstructed Ir surfaces

Using the field ion microscope, we have studied random walk diffusion of single Ir atoms and small Ir atomic clusters on Ir surfaces. On the (1×1) Ir {001} surface, diffusion of Ir adatoms occurs by an atomic replacement mechanism along the 〈001〉 directions. On the (1×1) Ir {110} surface, self-diffusion occurs by atomic hopping along the 〈110〉 directions and by atomic replacement along the 〈001〉 directions. On the (1×2) reconstructed Ir {110} surface, self-diffusion occurs only by atomic hopping along the doubly spaced [1̄10] surface channels. Within the experimental accuracy, the pre-exponential factors are all in the 10−3 cm2/s range. Energies needed for these atomic replacements and the barrier heights for these atomic hoppings are also derived. Diffusion of diatomic clusters on the (1×1) Ir {110} surfaces has also been studied.

Diffusion of Ir-dimers on Ir(110) surfaces by atomic-exchange and atomic-hopping mechanisms

On the Ir(110) surface, single Ir adatoms and Ir-dimers can diffuse along the [110] surface channels as well as across these channels. The latter diffusion most probably occurs by an atomic-exchange mechanism. The visited-site-lattice for both Ir and Ir 2 is found to be the (1 × 1) net of the substrate lattice. From a measurement of the two-dimensional displacement distributions and comparing them with Monte Carlo simulations we conclude that atomic jumps extend only to the nearest-neighbor sites. For single Ir-adatoms, diffusion across the channels is easier than along the channels. For Ir-dimers the along-channel diffusion is easier. The energies needed for single Ir-atoms and Ir-dimers to hop along the channel and to displace across the channel by atomic exchanges are derived. The binding energy of the dimer is measured to be 1.10 ± 0.11 eV which is exceptionally high for a metallic dimer on a metal surface. This large dimer binding energy leads us to believe that on the Ir(110) surface an Ir-dimer diffuses by concerted jumps of the two atoms, and the (1 × 2) surface reconstruction of this surface is achieved by diffusion of small atomic clusters.

Self-diffusion on the reconstructed and nonreconstructed Ir(110) surfaces.

On the (1\ifmmode\times\else\texttimes\fi{}1) Ir(110) surface the diffusion of single Ir adatoms is two dimensional; i.e., an adatom can diffuse along the [11\ifmmode\bar\else\textasciimacron\fi{}0] surface channel by atomic hopping as well as across the channel by atomic replacement. The energy needed for an Ir adatom to hop along the channel is 0.80\ifmmode\pm\else\textpm\fi{}0.04 eV, or 0.09 eV larger than needed to replace a lattice atom in the cross-channel jump. In contrast, Ir adatom diffusion on the (1\ifmmode\times\else\texttimes\fi{}2)-reconstructed surface, with double-space [11\ifmmode\bar\else\textasciimacron\fi{}0] channels, is one dimensional; i.e., an adatom can only hop along these channels. The activation energy, 0.86\ifmmode\pm\else\textpm\fi{}0.03 eV, is higher than that for either cross-channel or along-the-channel diffusion on the nonreconstructed surface.

Displacement distribution and atomic jump direction in diffusion of Ir atoms on the Ir(001) surface.

On the Ir{001} surface an Ir adatom is surprisingly found to jump along the 〈001〉 direction instead of along the smoother closely packed 〈110〉 atomic channel of the substrate, probably by an atomic exchange mechanism. The sites visited by one diffusing adatom therefore form a c(2\ifmmode\times\else\texttimes\fi{}2) net of the substrate lattice. The displacement distributions derived are consistent with those expected from a discrete random walk with the atomic jump length of the nearest-neighbor distance of the c(2\ifmmode\times\else\texttimes\fi{}2) net.

Diffusion of Ir-dimers on Ir(110) surfaces by atomic-exchange and atomic-hopping mechanisms

Abstract On the Ir(110) surface, single Ir adatoms and Ir-dimers can diffuse along the [110] surface channels as well as across these channels. The latter diffusion most probably occurs by an atomic-exchange mechanism. The visited-site-lattice for both Ir and Ir 2 is found to be the (1 × 1) net of the substrate lattice. From a measurement of the two-dimensional displacement distributions and comparing them with Monte Carlo simulations we conclude that atomic jumps extend only to the nearest-neighbor sites. For single Ir-adatoms, diffusion across the channels is easier than along the channels. For Ir-dimers the along-channel diffusion is easier. The energies needed for single Ir-atoms and Ir-dimers to hop along the channel and to displace across the channel by atomic exchanges are derived. The binding energy of the dimer is measured to be 1.10 ± 0.11 eV which is exceptionally high for a metallic dimer on a metal surface. This large dimer binding energy leads us to believe that on the Ir(110) surface an Ir-dimer diffuses by concerted jumps of the two atoms, and the (1 × 2) surface reconstruction of this surface is achieved by diffusion of small atomic clusters.

Extraction of IrIr adatom interaction strengths on Ir(100) from field ion microscopy of iridium cluster structures

The interaction energetics of iridium atoms self-adsorbed on Ir(100) are inferred from field ion microscopy observations which show that for 5 or fewer adatoms the most stable cluster configuration is a linear chain oriented in a 〈110〉 surface direction, while for 6 or more adatoms certain two-dimensional islands are the most stable. The critical number of adatoms beyond which island growth is preferred, together with the observed stable island configurations for 6 and 7 atoms, imply relatively tight bounds on possible lattice gas interaction parameters once reasonable assumptions are made regarding their falloff with adatom separation. The third near-neighbor interaction must be attractive and non-negligible, indicating that relatively long-range interactions between the Ir adatoms are present. In addition, configurations involving second near-neighbor interactions must be repulsive if the observed cluster configurations are to be explained.

ABSTRACT : THE STRUCTURE OF ADATOM NUCLEI ON METAL CRYSTAL PLANES

THE STRUCTURE OF ADATOM NUCLEI ON METAL CRYSTAL PLANES P. Schwoebel, G. Kellogg To cite this version: P. Schwoebel, G. Kellogg. ABSTRACT : THE STRUCTURE OF ADATOM NUCLEI ON METAL CRYSTAL PLANES. Journal de Physique Colloques, 1988, 49 (C6), pp.C6-265-C6265. . HAL Id: jpa-00228142 https://hal.archives-ouvertes.fr/jpa-00228142 Submitted on 1 Jan 1988 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinee au depot et a la diffusion de documents scientifiques de niveau recherche, publies ou non, emanant des etablissements d’enseignement et de recherche francais ou etrangers, des laboratoires publics ou prives. JOURNAL DE PHYSIQUE Colloque C6, suppi6ment au noll, Tome 49, novembre 1988 ABSTRACT : THE STRUCTURE OF ADATOM NUCLEI ON METAL CRYSTAL PLANES(l) THE STRUCTURE OF ADATOM NUCLEI ON METAL CRYSTAL PLANES(l) P.R. SCHWOEBEL(~) and G.L. KELLOGG Sandia National Laboratories. Albuquerque, NM 87185, U.S.A. The geometrical structure of adatom nuclei involved in the nucleation of a subsequent crystal plane is of fundamental interest as it reflects the energetics of adatom-adatom and adatom-surface interactions. Investigations of adatom nuclei structure by field ion microscopy have yielded some very interesting results. ~assettl and Fink and ~hrlich~ have observed that on W(110) the heterogeneous nucleation of Ni, Pd, Ir, and Pt adatoms initially preceeds by the formation of stable linear chain nuclei oriented in one of the close-packed directions of the substrate. Bassett found that only beyond a critical chain length of roughly 4 Ni and 8 Pd atoms will the nucleus assume a two-dimensional island configuration. In addition, Fink and Ehrlich found that a two-dimensional island configuration of 12 Ir adatoms on W(110) is metastable and reconfigures into linear chain nuclei with heating. In the course of our research we have observed chain forming nuclei in other heterogeneous systems. For example, with Pd on Ta(llO), a linear chain oriented in the close-packed direction on the surface is the most stable structure for nuclei of 8 or fewer adatoms; only for 9 or more adatoms is a two-dimensional island configuration stable. Similarly, mixed metal clusters of Pt and Pd on W(110) were also found to initially nucleate in the form of stable linear chains. Further experiments produced the interesting result that the formation of stable linear chain nuclei is not limited to heterogeneous systems3. In particular, for the selfadsorption of Ir on Ir(100), linear chYin nuclei oriented in a close-packed surface direction are the most stable configuration for 5 or fewer adatoms. Two-dimensional islands are only the most stable configuration for 6 or more Ir atoms. These facts are evidenced by the direct observation of transformations of metastable two-dimensional islands of 5 adatoms to stable linear chains, and metastable 6 adatom linear chains to stable two-dimensional islands. In contrast, with Ir on Ir(lll), two-dimensional island island formation begins immediately at the tri-atomic nucleus for which a triangular shape is the most stable. To interpret such chain to island transformations in homogeneous systems, we have modeled the Ir on Ir(100) system as a lattice gas4. The critical chain length of 5 Ir atoms, together with the observed stable island geometries for 6 and 7 adatoms are found to imply relatively tight bounds on the adatom-adatom interaction energetics once reasonable assumptions are made regarding falloff with distance. Specifically, the effective second near-neighbor interaction must be repulsive and of a magnitude lying between approximately 1/4 and 1/3 that of the nearest-neighbor attraction. In addition, the effective third nearneighbor interaction must be attractive and have a magnitude between roughly 1/6 and 2/7 that of the nearest-neighbor attraction. The fact that the third near-neighbor interaction cannot be considered to be negligible if the experimental results are to be explained, provides rather direct evidence that relatively long-range interactions exist between the Ir adatoms on Ir(100). Such considerations of adatom cluster geometries are suggestive of further experiments which may provide substantially more detail regarding the sign and magnitude of adatom-adatom interactions on surfaces. (1) This work was supported by the U. S. DOE under contract i/ DE-AC04-76DP00789 (2) Associated Western Universities Post-Doctoral Appointee 1) D.W. Bassett, Thin Solid Films 48, 237 (1978). 2) H.W. Fink and G. Ehrlich, Surface Science 110, L611 (1981). 3) P.R. Schwoebel and G.L. Kellogg, Phys. Rev. Lett., submitted for publication. 4) P.R. Schwoebel, Peter J. Feibelman, and R.L. Schwoebel, to be published. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988645

Structure of iridium adatom cluster nuclei on Ir(100).

A transition from chain- to island-type structure is observed in Ir adatom cluster nuclei on Ir(100). For five or fewer adatoms the stable configuration is a nucleus in the form of a linear chain, whereas for six or more adatoms a two-dimensional island configuration is stable. This unusual transformation indicates the presence of relatively long-range interactions between the Ir adatoms.

Mechanism for the self-diffusion of Au and Ir adatoms on Pt(110) surface

A molecular dynamics computer technique was used to simulate the diffusion of a Au and and Ir adatom on the Pt(110) surface. Details of the exchange mechanism associated with cross-channel diffusion were observed. For the Au adatom at the low temperature, the expected channel diffusion occurred, whereas at the higher temperature a temporary exchange of the Pt wall atom by the Au adatom was seen. In the Ir case at the low temperature no exchange was observed, although there was a partial displacement of the wall atom by the adatom. At the higher temperature, the exchange mechanism was observed. The results are consistent with experimental observations and also indicate the importance of the stability of the channel wall atoms (via their thermal motion) on the occurrence of the exchange mechanism.

Pair interaction of metal atoms on a metal surface

Pair interactions of tungsten and iridium adatoms on the W {110} plane are studied by measuring two-dimensional pair distributions with two adatoms on a plane. Each distribution contains from 600 to 950 field-ion-microscopy observations. Pair energies over a distance range of \ensuremath{\sim}2.5 to \ensuremath{\sim}50 \AA{} are derived by comparing the experimentally measured pair distributions with the calculated pair distributions for two noninteracting atoms. It is found that Ir-Ir pair interaction exhibits an attractive region at \ensuremath{\sim}5 \AA{} and a repulsive region around 8 \AA{}. If an oscillatory structure exists, its amplitudes decay already to less than \ensuremath{\sim}10 meV beyond 10 \AA{}. The plane edge seems to repel Ir adatoms with a weak long-range force. The W-Ir interaction at a short range is weaker than the Ir-Ir interaction. However, the interaction extends to larger distances. From \ensuremath{\sim}950 observations at 330 K with two adatoms, we derive a pair energy which exhibits two attractive and two repulsive regions, thus strongly suggesting an oscillatory structure. The pair energies derived beyond 25 \AA{} are erratic for both Ir-Ir and W-Ir interactions, most probably because of the limited amount of data available. However, this work represents the first time statistically reliable amounts of data have been obtained for two-dimensional pair distributions with only two adatoms on a plane. The nonmonotonic behaviors of adatom-adatom interaction on the smooth W {110} plane are clearly established.

Diffusion of single adatoms of platinum, iridium and gold on platinum surfaces

Diffusion of single adatoms of platinum iridium and gold on platinum surfaces has been studied experimentally using field ion microscopy and theoretically using Morse potentials to describe the adatom bonding. The order of increasing activation energies for the surfaces studied was (111), (113), (011), (133), (001) and calculated activation energies agreed satisfactorily with the experimental values. On (113) and (011) platinum the activation energies for the adsorbates increased in the order Au, Pt, Ir. The calculations show that this variation arises largely from differences in the adsorbate binding energy, with the differences in adatom size having little effect. Diffusion of Au on (011) platinum occurred only along the direction of the surface channels, as is to be expected from the surface structure. In contrast, Pt and Ir adatoms were found to diffuse in two dimensions on (011). Since direct inter-channel jumps by adatoms should require large activation energies, it is proposed that the observed inter-channel adatom transfers proceed by a surface vacancy mechanism.

Surface atom displacement processes

Progress in field ion microscope studies of adatom displacements on metal surfaces is reviewed. It is concluded that of the displacement processes that contribute to surface diffusion only displacements between low-coordination (terrace) sites are well characterised. Procedures and preliminary results of FIM studies of adatom displacement over steps are described. Activation energies measured for passage of Ta, W, Re, Ir and Pt adatoms across (110) W steps are found to equal activation energies for diffusion over (110) W, despite the highly reflecting character of the step for all the adsorbates except Pt. Displacements of adatoms interacting with other adatoms are discussed. Results presented show that interaction of transition metal adatoms forming close-packed dimers on (110) W is rather weak, with a minimum interaction energy [− U( r) < 4kJ/mol] for Re 2 corresponding to a very weak attraction for Re adatoms 0.27 nm apart.

Field ion microscope studies of transition metal adatom diffusion on (110), (211) and (321) tungsten surfaces

The diffusivity of single atoms of Ta, Mo, W, Re, Ir, and Pt, adsorbed on clean tungsten surfaces, was studied by field ion microscopy. Activation energies for diffusion of third transition series metals on (110), (211), and (321) surfaces increase with atomic number to Re, then decline, confirming the importance of 5d-electrons in the adatom bonding. Except for Re, adatom diffusion on (211) surfaces at low temperatures is characterized by low activation energies and very low values of D0 in the range 10−5-10−7 cm2 s−1. This is consistent with a facilitation of adatom displacement by correlated substrate motion. For all adsorbates, activation energies are larger for diffusion on (110) than on (211) surfaces, contrary to the predictions of pair-bond model calculations. This, and the restricted and directional bonding in Re, Ir, and Pt adatom clusters on (110) surfaces, are consistent with more localized bonding of adatoms on (110) than on (211) or (321) tungsten surfaces.