polar oxide surfaces and ultra-thin filmslifan.insp.upmc.fr/img/pdf/noguera.pdf · j. goniakowski ,...
TRANSCRIPT
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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)
1st LIFAN workshop Buenos Aires November 23-24 2009
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)
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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)
1st LIFAN workshop Buenos Aires November 23-24 2009
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)
1st LIFAN workshop Buenos Aires November 23-24 2009
Ag-supported MgO(111) ultra-thin films
1st LIFAN workshop Buenos Aires November 23-24 2009
Structural stability of MgO(111) films
Same result for ZnO(0001) et NaCl(111)
Surface X ray diffraction and STM
1st LIFAN workshop Buenos Aires November 23-24 2009
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
Structural stability of MgO(111) films
Three generic behaviors for unsupported (1x1)-MgO(111) films
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009
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
1st LIFAN workshop Buenos Aires November 23-24 2009