- Email: [email protected]
Determination of cluster spacings in the FIM: iridium dimers on W(110)
~X160 Surface Science 246 (1991) 1 12 North-Holland
Direct observations of the surface diffusion of atoms and clusters * Gert Ehrlich
* *
('oordinated Science Laboratory and Department of Materials Science and Engineering, Universi O, o] Illinois" at Urbana-Champaign, Urbana, IL 61801, USA Received 27 July 1990; accepted for publication 27 August 1990 Diffusion of metal atoms on highly perfect planes was one of the first surface phenomena to be examined on the atomic level. This was made possible through the unique capability of the field ion microscope to routinely reveal individual adatoms. Despite subsequent advances in observational techniques, such as the development of high-resolution electron microscopes and the scanning tunneling microscope, current understanding of the diffusion behavior of metal atoms is still based primarily upon field ion microscopic studies. The results of this overall effort are briefly surveyed, with special emphasis upon factors important in crystal growth: structural specificities in diffusion, mobility during deposition, the mechanisms of atomic jumps, and the motion of clusters. From the data now available it appears that surface diffusion is a more complicated and interesting process than anticipated.
Surface Science 246 (1991) 13-24 North-Holland
13
Diffusion of Ir-dimers on Ir(ll0) surfaces by atomic-exchange and atomic-hopping mechanisms Tien T. Tsong and Chong-lin Chen Physics Department, The Pennsylvania State University, University Park, PA 16802, USA Received 29 July 1990: accepted for publication 27 August 1990 On the lr(ll0) 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 lr and Ir z is found to be the (1 x 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 j u m p s extend only to the nearest-neighbor sites. For single lr-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 lr-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(ll0) surface an Ir-dimer diffuses by concerted j u m p s of the two atoms, and the (1 x 2) surface reconstruction of this surface is achieved by diffusion of small atomic clusters.
Surface Science 246 (1991) 25 30 North-Holland
25
Determination of cluster spacings in the FIM: iridium dimers on W(ll0) * Robert S. Chambers Coordinated Science Laboratory and Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 31 July 1990; accepted for publication 27 August 1990 The field ion microscope normally does not provide a reliable indication of distances; nevertheless, by mapping out the binding sites for a single adatom and relying upon the known lattice spacings, it has in general been possible to circumvent this difficulty. Problems still remain, however, in arriving at unequivocal results for the interatomic spacings in clusters. A combination of methods, involving mapping of the exact sites at which a cluster is bound, computer-based image processin$, together with diffusion measurements and observation of field effects, can usually yield concordant information about spacings, at least in clusters containing only a small number of atoms. These techniques are illustrated using as an example the spacing between atoms in Ir 2 on W(110), about which there is still considerable disagreement.
Direct observations of the surface diffusion of atoms and clusters * Gert Ehrlich
* *
('oordinated Science Laboratory and Department of Materials Science and Engineering, Universi O, o] Illinois" at Urbana-Champaign, Urbana, IL 61801, USA Received 27 July 1990; accepted for publication 27 August 1990 Diffusion of metal atoms on highly perfect planes was one of the first surface phenomena to be examined on the atomic level. This was made possible through the unique capability of the field ion microscope to routinely reveal individual adatoms. Despite subsequent advances in observational techniques, such as the development of high-resolution electron microscopes and the scanning tunneling microscope, current understanding of the diffusion behavior of metal atoms is still based primarily upon field ion microscopic studies. The results of this overall effort are briefly surveyed, with special emphasis upon factors important in crystal growth: structural specificities in diffusion, mobility during deposition, the mechanisms of atomic jumps, and the motion of clusters. From the data now available it appears that surface diffusion is a more complicated and interesting process than anticipated.
Surface Science 246 (1991) 13-24 North-Holland
13
Diffusion of Ir-dimers on Ir(ll0) surfaces by atomic-exchange and atomic-hopping mechanisms Tien T. Tsong and Chong-lin Chen Physics Department, The Pennsylvania State University, University Park, PA 16802, USA Received 29 July 1990: accepted for publication 27 August 1990 On the lr(ll0) 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 lr and Ir z is found to be the (1 x 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 j u m p s extend only to the nearest-neighbor sites. For single lr-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 lr-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(ll0) surface an Ir-dimer diffuses by concerted j u m p s of the two atoms, and the (1 x 2) surface reconstruction of this surface is achieved by diffusion of small atomic clusters.
Surface Science 246 (1991) 25 30 North-Holland
25
Determination of cluster spacings in the FIM: iridium dimers on W(ll0) * Robert S. Chambers Coordinated Science Laboratory and Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 31 July 1990; accepted for publication 27 August 1990 The field ion microscope normally does not provide a reliable indication of distances; nevertheless, by mapping out the binding sites for a single adatom and relying upon the known lattice spacings, it has in general been possible to circumvent this difficulty. Problems still remain, however, in arriving at unequivocal results for the interatomic spacings in clusters. A combination of methods, involving mapping of the exact sites at which a cluster is bound, computer-based image processin$, together with diffusion measurements and observation of field effects, can usually yield concordant information about spacings, at least in clusters containing only a small number of atoms. These techniques are illustrated using as an example the spacing between atoms in Ir 2 on W(110), about which there is still considerable disagreement.