Polar oxide surfaces and ultra-thin filmslifan.insp.upmc.fr/IMG/pdf/Noguera.pdf · J. Goniakowski ,...

Claudine Noguera

Institut des Nanosciences de Paris , CNRS UMR 7588, Université Pierre et Marie Curie (Paris VI)

Campus de Boucicaut, 140 rue Lourmel, 75015 Paris

Polar oxide surfaces and ultra-thin films

1) Presentation of my research group2) Polar surfaces of dielectric materials3) Polarity at the nanoscale: ultra-thin oxide films

Outline:

1st LIFAN workshop Buenos Aires November 23-24 2009

Jacques Jupille

Jacek Goniakowski

Fabio Finocchi


Reasearch group: Oxides in low dimensions

Ab initio

DFT, DFPTGGA+U

Many-body empirical

SMA + PES

Continuous model

Rate equations

Quantum semi- empirical

INDO O(N)

Oxides in contact with water

Oxides surfaces and thin films, effects of polarity

Growth and epitaxy of oxide-supported metal films

Towards more and more complex systems : increasing sizes

surface reconstructions, complex interfaces; nanoscale objectscontact with the environment :

humid atmosphere, contact with aqueous solution, thermodynamic phase diagramsdynamical effects

growth, dissolution, precipitation

Kinetic Monte Carlo

Rate equations

Multi-technique

apparatus for surface

characterizatio

IR-Ellipsometry

under gas pressure

Synchrotron techniques: GIXS and

GISAXS under pressure (ESRF)

Vibrational spectroscopy in

UHV(HREELS)

NanoparticleSynthesis


Oxides in low dimensions

Fine electronic, structural, and dielectric properties

Average island size: 6±1 nm

MgO/Ag(100)

Polarity at surfaces and interfaces: electrostatic

coupling structure-charge:

Ab initio

DFT, DFPTGGA+U Continuous

model

Rate equations

Quantum semi- empirical

HF- O(N)

Structure and growth ofOxide nano-objects

Nucleation and growthin aqueous solutions

Applications to geochemistry:Water-rock interaction

Clay formation

My own research fields


J.Goniakowski, F. Finocchi, C. Noguera, Rep. Prog. Phys. 71 (2008) 016501

Polar surfaces of dielectric materials

Polar materials: ferroelectric materialsPolar surfaces in non-polar materials

C. A. Coulomb(1736-1806)

M. Faraday(1791-1867)

Concept of electrostatic field

1993: modern theory of polarization; link between macroscopic electrostatics

and quantum theory

24'

RQQF


Application of classical electrostatics lawsto a modern problem

bulk surface

= 0 B + S =

Depolarizing field due to the polarization surface charge density:

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.

PbO 2O2- Ti4+ PbO 2O2- Ti4+

Not periodic !!

Classical electrostatics of dielectric materials:depolarization field and surface charge density

bulk surface1st LIFAN workshop Buenos Aires November 23-24 2009

Lead titanate= ferroelectric

Rock-salt structure (eg. MgO)

(110)(100) (111)

Type 1 Type 3

≠ 0 , ≠ 0

Type 2

Polar orientation = charged atomic layers + non-zero dipole moment

in the repeat unit

Polar surfaces of non-polar materials:Classification of compound surfaces

Type 1 Type 3Type 2

P.W. Tasker, J. Phys. C: Solid State Phys. 12 4977 (1979)


Non-stoichiometry (reconstructions) & adsorption of charged species

H+ H+ H+ H+

Modification of the electronic structureSurfaces may be stoichiometric or not

M M M M

It is necessary to modify the charge

density in the surface layers

modification of the atom density modification of chargesand/or

Mechanisms of polarity compensation

(charge density= atom density x charge)


Compensating charge R1/(R1+R2)

R1=rumplingNo linear component of the electrostatic potential

R2 VR1

4NR1

V and P grow with N

Modification of the electronic structure(charge modification)

Surface metallization but 5 J/m2 surface energy=> never observed!

MgO(111)

J. Goniakowski , C. Noguera, PRB 60, 16120 (1999)

Change of oxidation state

Plan OPlan Mg

-2 +2 -2 +2

Plan (111)

Atome Mg Atome O

Compensating charges R1/(R1+R2)=/2


Stabilization of (1x1)-MgO(111) by a metal/oxide interface

Internal oxidation of a Cu Mg alloy

J. Goniakowski, C. Noguera, PRB 60, 16120 (1999); PRB 66, 85417 (2002).

Metal adhesion (case of Pd/MgO interface):(111): Eadh ~ 5 J/m2

(100): Eadh ~ 1 J/m2

D . Imhoff et al., Eur. Phys. J. AP 5 9 (1999).

Compensation by adsorption of foreign atoms


Compensation by non-stoichiometry in the surface layers

Atom desorption allowing to preserveinsulating character and surface charges close

to bulk ones

Vacancy ordering(reconstruction) leading to

low energy facets: SrTiO3(110)

Bottin et alSS (2004)

H. Bando et al., JVST B 13 (1995) 1150

(2x6) reconstruction model


Compensation by non-stoichiometry in the surface layers

Atom desorption allowing to preserve insultingcharacter and surface charges close to bulk

ones

Charge compensation without ordering(magic triangles on ZnO(0001)

Diebold et al : ZnO(0001)-Zn

Triangular islandsheight = Zn-O double layer

With oxygen edges

28 oxygens21 zincs

( -7 Zn = - 28 / 4)

Sequence : Zn / O / Zn / O…..R1/(R1 + R2) ~ ¼


R. Hacquart and J. Jupille, Chem. Phys. Lett. 439 (2007) 91

F. Finocchi and J. Goniakowski, Surf. Sci. 601 (2007) 4144.

Stabilization of MgO(111) by dissociative water adsorptionAfter 7 days in water

MgO smokes

Modification of the surface charge by adsorption of charged species

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 = -1Q = +2Q = -2Q = +2Q = -2Q = +1

150 K

400 K

700 K


Summary I:

Polarity compensation may be achieved:by modification of the number of surface ions (non-stoichiometry), by adsorption of charged speciesby adsorption of species which get charged without an important energy costby 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 verydifferent from what exists at non-polar surfaces.

Consequences on adsorption and reactivity properties

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 at the nano-scalePart Two:

J. Goniakowski, C. Noguera, L. Giordano, PRL 93, 215702 (2004); PRL 98, 205701 (2007).J. Phys. Condensed Matter 20 (2008) 264003

The condition for polarity compensation has been established in the limit of infinite sizeAt the nanoscale: N does not go to infinity

there exists no « bulk »Can we still talk of polarity?What is the electrostatic behavior ???

Rumpling (Å)

1 ML MgO(111)

Bulk-like R1/(R1+R2) produces huge V and D1st LIFAN workshop Buenos Aires November 23-24 2009

NaCl Structure

h-BN Structure

ZnS Structure

Nanometric MgO(111) layers of polar orientationstructural stability

First principles study of (1x1) unsupported filmsLocal hexagonal symmetry in surface layers

The structural ground state is size dependent

At low thickness, the most stable structure is not rocksalt


3.00 A

3.49 A

Rocksalt (B1)

h-BN (Bk)

CsCl(B2)

ZnS(B3)

Non-polar

Structural phase diagram of bulk MgO

Wurtzite(B4)


Structure NaCl

Structure h-BN

No dipole moment

neutral layers

NON-POLARStructure ZnS

Structural stability of MgO(111) films

Competition between :bulk cohesion energy which stabilizes rocksalt structure: EB=ENaCl-EhBN<0surface energy which favors non-polar surfaces Es

the electrostatic cost, associated to polarity, although finite, is high

h-BN (0001) structure

Confirmed by simulations of deposited MgO/Ag(111)


Ag-supported MgO(111) ultra-thin films



Same result for ZnO(0001) et NaCl(111)

Surface X ray diffraction and STM


POLAR Uncompensated but

strong rumpling reduction

J. Goniakowski, C. Noguera, L. Giordano, PRL 93, 215702 (2004)PRL 98, 205701 (2007).

POLAR Compensated by

metallization

NOT POLARWhole structural transformation

rock-salt

zinc blende

h-BN


Three generic behaviors for unsupported (1x1)-MgO(111) films


MgO(111)/Me(111) and FeO(111)/Me(111): charge transfer and rumplingon monolayers: ultimate size reduction

An interfacial charge transfer takes placefunction of the metal electro-negativity

A rumpling occurs in response to theinterfacial charge transfer

(opposite dipoles)

film

+

__

Me MgO

E

Me MgO

E

Me MgO

E

film

inte

rfac

e + +

_e-

_

++_

e-e-

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

CT dipole Rumplingdipole


STM topographic image 4500 mV, 0.1 nA

Calculated map of averaged electrostatic potential above the surface.

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.

L. Giordano, G. Pacchioni, J. Goniakowski, N. Nilius, E. D. L. Rienks, H.-J. Freund, Phys. Rev. B 76, 075416 (2007)

Large interf. distance

Small charge transferSmall positive rumpling

top

top

hcp

fcc

hcp/fccSmall interf. distance

Larger charge transferLarge positive rumpling

FeO

FeO(111)/Pt111): charge transfer and rumplingModulation of the film structure


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 adsorptionthis yieds a wide range of structural and electronic configurationsit allows to obtain templates for nano-object growthspecific 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 polarityelectronic 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 rumplingthis exists independently of the layer orientation


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Polar oxide surfaces and ultra-thin filmslifan.insp.upmc.fr/IMG/pdf/Noguera.pdf · J. Goniakowski ,...

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