Polar oxide surfaces and ultra-thin films

Transcription

1 Polar oxide surfaces and ultra-thin films Claudine Noguera Institut des Nanosciences de Paris, CNRS UMR 7588, Université Pierre et Marie Curie (Paris VI) Campus de Boucicaut, 140 rue Lourmel, Paris Outline: 1) Presentation of my research group 2) Polar surfaces of dielectric materials 3) Polarity at the nanoscale: ultra-thin oxide films

2 Reasearch group: Oxides in low dimensions Jacek Goniakowski Jacques Jupille Fabio Finocchi

3 Oxides in low dimensions Oxides surfaces and thin films, effects of polarity Growth and epitaxy of oxide-supported metal films Oxides in contact with water Towards more and more complex systems : increasing sizes surface reconstructions, complex interfaces; nanoscale objects contact with the environment : humid atmosphere, contact with aqueous solution, thermodynamic phase diagrams dynamical effects growth, dissolution, precipitation Ab initio DFT, DFPT GGA+U Quantum semi- empirical INDO O(N) Many-body empirical SMA + PES Kinetic Monte Carlo Rate equations Continuous model Rate equations Multitechnique apparatus for surface characterizatio Vibrational spectroscopy in UHV (HREELS) IR- Ellipsometry under gas pressure Nanoparticle Synthesis Synchrotron techniques: GIXS and GISAXS under pressure (ESRF)

4 My own research fields Polarity at surfaces and interfaces: electrostatic coupling structure-charge: Ab initio DFT, DFPT GGA+U Structure and growth of Oxide nano-objects Quantum semi- empirical HF- O(N) Nucleation and growth in aqueous solutions Applications to geochemistry: Water-rock interaction Clay formation Continuous model Rate equations MgO/Ag(100) Fine electronic, structural, and dielectric properties Average island size: 6±1 nm

5 Polar surfaces of dielectric materials Application of classical electrostatics laws to a modern problem 1993: modern theory of polarization; link between macroscopic electrostatics and quantum theory Polar materials: ferroelectric materials Polar surfaces in non-polar materials C. A. Coulomb ( ) F QQ' 2 4 R M. Faraday ( ) Concept of electrostatic field J.Goniakowski, F. Finocchi, C. Noguera, Rep. Prog. Phys. 71 (2008)

6 Classical electrostatics of dielectric materials: depolarization field and surface charge density Lead titanate= ferroelectric PbO 2O 2- Ti 4+ PbO 2O 2- Ti 4+ In absence of an external field even polar materials do not display a net dipole moment because the intrinsic dipole moment is neutralized by "free" electric charge that builds up on the surface. Not periodic!! Depolarizing field due to the polarization surface charge density: bulk surface bulk surface = 0 B + S =

7 Polar surfaces of non-polar materials: Classification of compound surfaces Rock-salt structure (eg. MgO) (100) (110) (111) Type 1 Type 2 Type 3 Type 1 Type 2 Type 3 0, 0 Polar orientation = charged atomic layers + non-zero dipole moment in the repeat unit P.W. Tasker, J. Phys. C: Solid State Phys (1979)

8 Mechanisms of polarity compensation It is necessary to modify the charge density in the surface layers Compensating charge R 1 /(R 1 +R 2 ) R 1 =rumpling R 1 R 2 V 4NR 1 No linear component of the electrostatic potential (charge density= atom density x charge) V and P grow with N modification of the atom density Non-stoichiometry (reconstructions) & adsorption of charged species H+ H+ H+ H+ and/or modification of charges Modification of the electronic structure Surfaces may be stoichiometric or not M M M M

9 MgO(111) Modification of the electronic structure (charge modification) Plan (111) Plan O Plan Mg Compensating charges R 1 /(R 1 +R 2 )= / Atome Mg Atome O Surface metallization but 5 J/m 2 surface energy=> never observed! Change of oxidation state J. Goniakowski, C. Noguera, PRB 60, (1999)

10 Compensation by adsorption of foreign atoms Stabilization of (1x1)-MgO(111) by a metal/oxide interface Internal oxidation of a Cu Mg alloy Metal adhesion (case of Pd/MgO interface): (111): E adh ~ 5 J/m 2 (100): E adh ~ 1 J/m 2 D. Imhoff et al., Eur. Phys. J. AP 5 9 (1999). J. Goniakowski, C. Noguera, PRB 60, (1999); PRB 66, (2002).

11 Compensation by non-stoichiometry in the surface layers Atom desorption allowing to preserve insulating character and surface charges close to bulk ones Vacancy ordering (reconstruction) leading to low energy facets: SrTiO 3 (110) (2x6) reconstruction model Bottin et al SS (2004) H. Bando et al., JVST B 13 (1995) 1150

12 Compensation by non-stoichiometry in the surface layers Atom desorption allowing to preserve insulting character and surface charges close to bulk ones Charge compensation without ordering (magic triangles on ZnO(0001) Diebold et al : ZnO(0001)-Zn Sequence : Zn / O / Zn / O.. R 1 /(R 1 + R 2 ) ~ ¼ Triangular islands height = Zn-O double layer With oxygen edges 28 oxygens 21 zincs ( -7 Zn = - 28 / 4)

13 Modification of the surface charge by adsorption of charged species MgO smokes After 7 days in water Stabilization of MgO(111) by dissociative water adsorption OH - OH - OH - OH - Mg ++ Mg ++ Mg ++ Mg ++ O -- O -- O -- O -- Mg ++ Mg ++ Mg ++ Mg ++ O -- O -- O -- O -- H + H + H + H + Q = -1 Q = +2 Q = -2 Q = +2 Q = -2 Q = K R. Hacquart and J. Jupille, Chem. Phys. Lett. 439 (2007) K F. Finocchi and J. Goniakowski, Surf. Sci. 601 (2007) K

14 Summary I: All polar surfaces have to be compensated. The electrostatic condition cannot be by-passed (N ) And polarity compensation cannot be obtained by processes other than modification of charge density Polarity compensation may be achieved: by modification of the number of surface ions (non-stoichiometry), by adsorption of charged species by adsorption of species which get charged without an important energy cost by modification of the number of electrons in the surface layers: metallization or change of oxidation state This compensation is accompanied by structural and/or electronic characteristics, which are very different from what exists at non-polar surfaces. Consequences on adsorption and reactivity properties

15 Part Two: Polarity at the nano-scale The condition for polarity compensation has been established in the limit of infinite size At the nanoscale: N does not go to infinity there exists no «bulk» Can we still talk of polarity? What is the electrostatic behavior??? 1 ML MgO(111) J. Goniakowski, C. Noguera, L. Giordano, PRL 93, (2004); PRL 98, (2007). J. Phys. Condensed Matter 20 (2008) Rumpling (Å) Bulk-like R 1 /(R 1 +R 2 ) produces huge V and D

16 Nanometric MgO(111) layers of polar orientation structural stability First principles study of (1x1) unsupported films Local hexagonal symmetry in surface layers NaCl Structure ZnS Structure h-bn Structure The structural ground state is size dependent At low thickness, the most stable structure is not rocksalt

17 Structural phase diagram of bulk MgO Rocksalt (B1) ZnS (B3) h-bn (Bk) Wurtzite (B4) Non-polar 3.00 A 3.49 A CsCl (B2)

18 Structural stability of MgO(111) films h-bn (0001) structure Structure NaCl Structure ZnS neutral layers No dipole moment NON-POLAR Confirmed by simulations of deposited MgO/Ag(111) Structure h-bn Competition between : bulk cohesion energy which stabilizes rocksalt structure: E B =E NaCl -E hbn <0 surface energy which favors non-polar surfaces E s the electrostatic cost, associated to polarity, although finite, is high

19 Ag-supported MgO(111) ultra-thin films

20 Structural stability of MgO(111) films Same result for ZnO(0001) et NaCl(111) Surface X ray diffraction and STM

21 Structural stability of MgO(111) films Three generic behaviors for unsupported (1x1)-MgO(111) films rock-salt POLAR Compensated by metallization zinc blende POLAR Uncompensated but strong rumpling reduction h-bn NOT POLAR Whole structural transformation J. Goniakowski, C. Noguera, L. Giordano, PRL 93, (2004) PRL 98, (2007).

22 MgO(111)/Me(111) and FeO(111)/Me(111): charge transfer and rumpling on monolayers: ultimate size reduction E _ + An interfacial charge transfer takes place function of the metal electro-negativity e - Me MgO E _ film Me MgO E e - Me MgO A rumpling occurs in response to the interfacial charge transfer (opposite dipoles) + _ interface+ _ CT dipole e - + _ + film Rumpling dipole Identical result on MgO(100)/Me(100)!!! polarity is not relevant at this scale (both unsupported (100) and (111) layers are flat) similar electrostatic mechanism of competition between charge transfer and rumpling dipoles

23 FeO(111)/Pt111): charge transfer and rumpling Modulation of the film structure Fe O top fcc hcp Modulation of the surface potential observed experimentally is driven principally by the local atomic structure of the FeO layer: its rumpling and its adsorption height. Calculated map of averaged electrostatic potential above the surface. top Large interf. distance hcp/fcc Small interf. distance top fcc hcp Small charge transfer Small positive rumpling Larger charge transfer Large positive rumpling STM topographic image 4500 mv, 0.1 na L. Giordano, G. Pacchioni, J. Goniakowski, N. Nilius, E. D. L. Rienks, H.-J. Freund, Phys. Rev. B 76, (2007)

24 Conclusion Electrostatic effects strongly drive polar surface and thin film properties at all sized: At semi-infinite surfaces: it is the dominant interaction and polarity HAS to be compensated (N ) various ways of surface compensation: non-stoichiometry, change of oxydation state, charged species adsorption this yieds a wide range of structural and electronic configurations it allows to obtain templates for nano-object growth specific surface electronic states available for reactivity Ultra-thin «polar» films: electrostatic energy competes with other energy terms: elastic energy: efficient to decrease rumpling (uncompensated polarity) cohesion energy allows change of cristallographic structure to avoid polarity electronic excitation energy: case of non-stoichiometric layers «Polar» monolayers: no specific signature of polarity but competition between rumpling and charge transfer dipoles the interfacial dipole induces a rumpling this exists independently of the layer orientation