1/16 Surfaces and Interfaces Fouad MAROUN Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique CNRS Palaiseau, France
2/16 Outline Definition of surfaces and interfaces and 3 examples in every day's life How to investigate surfaces and interfaces at the microscopic scale: theoretical and experimental issues + 2 examples (STM, SPSTM) A detailed example where surfaces and interfaces are involved: Co islands and ultrathin films on Au; influence of morphology, surface chemistry and electric field on the magnetic properties overview of research subjects related to surfaces and interfaces in Ile-de-France
3/16 Definition of surfaces and interfaces Surface Surface atoms and geometry bring the new property Interface The new property induced by the junction of two media surface atoms have less neighbors Interface atoms have different environment Presence of steps Presence of strain
Are surfaces and interfaces useful in every day s life? Surface Example with chemical properties: catalysis Interface Examples with electrical properties: transistor Different surface sites Solar cell Different atomic ensembles chemical reactivity depends on the atomic roughness and local environment electrical properties depend on the interface between two semiconductors, semiconductor/metal, semiconductor/oxide, ICFP - 10 October 2012 4/16
5/16 How to study surfaces and interfaces at the sub-µm scale? Theoretically Experimentally Need to calculate with an appropriate cell size and calculation precision: 1) large cells to account for surface reconstruction, steps, strain, corrugation due to lattice misfit, 2) small cells in the case of surface covered by a molecular film, in the absence of long range structural modulation Need to characterize at an appropriate scale: 1) sub-µm scale for optical properties 2) sub-µm to nm scale for electrical properties 3) µm to atomic scale for magnetic properties 4) atomic scale for catalytic properties Available calculation methods: 1) Density Functional Theory (small cell size) 2) Monte Carlo (large cell size but dependent on the choice of the appropriate interaction potentials) 3) Molecular Dynamic simulations (dedicated to structural determination) 4) Micromagnetic simulations (large cell) 5) Electromagnetic simulations (large cell) Available techniques for property characterization: 1) Scanning Near Field Microscopy 2) Electric Field Microscopy, Scanning Tunneling Spectroscopy 3) Magnetic Force Microscopy, Spin Polarized Scanning Tunneling Microscopy 4) Scanning Tunneling Microscopy Available techniques for morphology characterization: 1) Atomic Force Microscopy 2) Shear Force Microscopy 3) Scanning Tunneling Microscopy 4) Electron Microscopy (SEM, TEM)
Scanning tunneling microscopy (STM) Scanning tunneling microscopy φ Tunneling current φ I Aexp α d Contrast: electronic density of states, topography for conductors Resolution: < 0.1 nm in all directions Spectroscopy and Spin Polarized STM using a magnetic tip ICFP - 10 October 2012 Wiesendanger group, Hamburg IBM Fe atoms anti-ferromagnetically coupled 6/16
7/16 Examples of surface and interface induced morphology or property Surface Interface Au(111) surface reconstruction Magnetization easy axis rotation 20 nm 5 nm 23 atoms B Co Au Magnetization (a.u.) 0.01 0.00-0.01-2000 -1000 0 1000 2000 Magnetic Field (Oe) 22 atoms Influence of the steps Au Co Au Magnetization (a.u.) 0.01 0.00-0.01-2000 -1000 0 1000 2000 Magnetic Field (Oe)
8/16 Morphology of cobalt deposited on Au(111) in vacuum: thickness < 1.6 ML Morphology 20 nm Surface reconstruction and steps greatly influences the cobalt island density Magnetism NO ferromagnetic order at room temperature for cobalt thickness < 1.6 monolayers
Morphology of cobalt deposited on Au(111) in electrochemistry: thickness < 1.6 ML 0.2 ML 0.6 ML 0.8 ML 5 nm 50 nm Large Co islands little interaction with the surface reconstruction Co Au 1 ML Co Magnetization (a.u.) 0.002 0.000-0.002 Ferromagnetic order at room temperature for cobalt thickness well below 1.6 ML Morphology greatly influences magnetic properties -2000-1000 0 1000 2000 Magnetic Field (Oe) ICFP - 10 October 2012 9/16
10/16 Morphology of cobalt deposited on Au(111) in vacuum: thickness > 1.6 ML Vacuum 2 ML 4 ML 1.0 Mr/Ms 0.5 Electrochemistry ~2 ML ~4 ML Mr/Ms 0.0 1.0 0.5 0.0 0 1 2 3 4 5 Co thickness (ML) 20 nm 20 nm Difference due to surface chemistry
11/16 Surface chemistry and magnetization orientation Co Au Vacuum Co Au Electrochemistry HH HHHH H H? replacing H on the cobalt surface Surface chemistry changes drastically magnetic properties It allows controlling the magnetization orientation
12/16 E Electric field and magnetization orientation HH HHHH H H E 4 Co(1.7 ML)/Au Magnetization (a.u) 2 0-2 -1.6 V -1.15 V -4-1000 -500 0 500 1000 Magnetic Field (Oe) Electric field changes drastically magnetic properties It allows controlling the magnetization orientation
13/16 Where could you do physics of surfaces and interfaces in Ile de France?
14/16 Where could you do physics of surfaces and interfaces in Paris? Laboratory INSP: (www.insp.jussieu.fr) growth and alloying magnetism catalysis Laboratory MPQ: (www.mpq.univ-paris7.fr) growth and magnetism alloy formation at the atomic scale
15/16 Where could you do physics of surfaces and interfaces in the south of Paris? PMC Polytechnique: growth and magnetism alloy formation at the atomic scale PICM Polytechnique: growth of nanostructures LSI Polytechnique DFT calculations Synchrotron SOLEIL: structure and magnetism with X rays LPS Orsay: (www.lps.u-psud.fr) Low dimensional physics carbon nanotubes magnetism CEA Saclay: Supraconductors molecules on surfaces DFT calculations for magnetism ISMO Orsay: (www.ismo.u-psud.fr) molecules on surfaces surface reactivity IEF Orsay: (www.ief.u-psud.fr) nanostructures, semiconductors ICMMO Orsay (www.icmmo.u-psud.fr) Monte Carlo, Molecular Dynamics for growth
16/16 Take home message Watch out, surfaces and interfaces are everywhere!