Physics and Chemistry of ABO3 Nanostructures from First Principles

Ghanshyam Pilania

Chemical, Materials & Biomolecular EngineeringInstitute of Materials Science

University of Connecticut

Principal Advisor: Prof. R. Ramprasad Associate Advisor: Prof. P. Gao Associate Advisor: Prof. G. Rossetti, Jr.

Ph.D. Dissertation Proposal

ABO3-type Perovskite structure

A

B

O

Novel polarization states in ABO3 nanowires

(p,T) surface phase diagrams of ABO3 surfaces

“Vortex” v/s “axial” polarization states

Effect of size, surface termination and axial strain on the polarization states

Outline

Methodology to construct surface phase diagrams

Calculated (p,T) surface phase diagrams for LaMnO3 and PbTiO3 (001) surfaces

Remaining work

Impact of work

Novel polarization states in ABO3 nanowires

Ferroelectricity in bulk perovskites

Ferroelectricity: a collective phenomena

A balance between long range Coulombic force (favor ferroelectric state)short range repulsive forces (resist ferroelectric state)

Dipole moment per unit volume = Polarization

T

Tc

Ferroelectric

Paraelectric

ABO3 perovskite

Energy

P

Paraelectric state

ABO3 perovskite Ferroelectric Well

Energy

P

Paraelectric state

Energy

P

Ferroelectric state

Energy

P

Ferroelectricity in NanostructuresThin film

Depolarizing Field

+++++++++

- - - - - - - - -

Bulk

Aguado-Puente et al. (PRL, 2008)

Ferroelectricity in Nanostructures

P

+ +

+

- -

-

Depolarizing Field

P

No depolarizing Field

No depolarizing Field

Closure domain

Prosendeev & Bellaiche (PRB 2007)

PFM results indicate possible presence of non-rectilinear polarization in PZT nanodots

Rodriguez et al (Nanoletters, 2009)

ABOABO33 Nanowires – Our DFT Study Nanowires – Our DFT Study

2x2-AO-terminated nanowire

2x2-BO2-terminated nanowire

AO-plane

BO2-plane

AO-plane

BO2-plane

Construction of ABO3 nanowires

BaTiOBaTiO33 Nanowires – Our DFT Study Nanowires – Our DFT Study

Axial polarization instability above 1.2 nm

ferroelectricparaelectric 4x4-TiO2

P

4x4-BaO

τ=rxP

Vortex polarization instability above 1.6 nm

Geneste et. al, APL 88, 112906 (2006);

Spanier et al, Nano Lett. 6, 735 (2006)

0.8 nm

Off-axis Polarization in BaTiO3 nanowires

BaTiOBaTiO33 Nanowires Nanowires – Experimental Study– Experimental Study

PbTiOPbTiO33 Nanowires – Our DFT Study Nanowires – Our DFT Study

c (Å)

Fa

Fa

FaFa

P

1x1 to 4x4-PbO

Fv

Shimada et al, PRB 79, 024102 (2009)

c tetragonal Bulk

acubic Bulk

P

P

P

4x4-TiO2

τ=rxP

Unit cell decomposed

dipole moments

PbTiOPbTiO33 Nanowires vs. Terminations Nanowires vs. TerminationsStrain-induced phase transition: vortex Strain-induced phase transition: vortex axial polarization axial polarization

4x4-TiO2-terminated nanowire

[001]

Axial compressive StrainAxial Tensile Strain

4x4-PbO-terminated nanowire

Four possible switchable polarization statesVortex (clockwise/counter-clockwise), Axial

(positive/negative)

PbTiOPbTiO33 nanowires display switchable rectilinear (axial) and nanowires display switchable rectilinear (axial) and

non-rectilinear (vortex) polarization configurationsnon-rectilinear (vortex) polarization configurations

Control of polarization statesControl of polarization statesaxial Strain and surface terminationsaxial Strain and surface terminations

(T, p) surface phase diagrams of ABO3 systems

Flexibility

Versatility

Less expensive

Thermal stability

Excellent oxygen exchange properties

Why are they important?

Perovskite Surfaces in Catalysis

R. J. H. Voorhoeve, D. W. Johnson, Jr., J. P. Remeika, P. K. Gallagher

SO4-2

Dead site Active site

Sulfur poisoning

26 MARCH 2010 VOL 327 SCIENCEChang Hwan Kim, Gongshin Qi, Kevin Dahlberg, Wei Li

Perovskite Surfaces in Catalysis

Suprafacial v/s Intrafacial

22

1OO Surface-O*↔ Surface + ½

O2 (g)

Cubic LaMnO3 and PbTiO3 surface phase diagrams

+ N/2 O2

Cubic LaMnO3 and PbTiO3 surface phase diagrams

(1x1) AO-terminated (1x1) BO2-terminated

Formation Energies

Cubic LaMnO3 and PbTiO3 surface phase diagrams

A

Relaxed geometries for most favored adsorption sites

Cubic LaMnO3 and PbTiO3 surface phase diagrams

Perovskite surfaces in contact with O2 (g)

000 2

222ln),(),0(),(

p

pTkpTpKTpT OBOOO

Assuming ideal gas behavior for O2

22

1OO

Surface-O*↔ Surface + ½ O2 (g)

Surface phase diagrams for surfaces in contact with O2

PbTiO3 (001) TiO2-terminated

log

PO

2

100% O ad-atom coverage

Partial O vacancycoverage

Partial coverage of O ad-atom

Clean surface

100% O vacancy

T (K)

LaMnO3 (001) MnO2-terminated

100% O vacancy

Partial coverage ofO ad-atom

Partial O

vacancy

coverage

100% O ad-atom coverage

log

PO

2

T (K)

Remaining Work

Electric field response of the vortex polarization state in PbTiO3 nanowires

Efield ?

Dielectric tensor of ferroelectric nanowires

4x4-PbO terminated nanowire (axial polarization)

4x4-TiO2 terminated nanowire (vortex polarization)

Effect of surface passivation (by various species such as –OH, H, -CH3 etc.) on polarization states in PbTiO3 nanowires

Thermodynamics of environment dependent interaction of various gases on the (001) surface of ABO3 type perovskites

NO, NO2, N2, O2 (gases)

Adsorption site Equilibrium geometry Electronic structure Energetics

Kinetics ??

Remaining Work

Impact of Work

0 1 0 0

Non volatile Ferroelectric memory

Potential to increase present memory storage density by five order of magnitude

How to shrink the hard drive?!!

Impact of Work

DeNOx processes

NO + CO + unburned hydrocarbons

catalyticconvertercatalytic

converter

CO CO2

NOxN2 + O2

CnHm CO2+H2O

List of PublicationsG. Pilania, S. P. Alpay and R. Ramprasad, "Ab initio study of ferroelectricity in BaTiO3 nanowires", Phys. Rev. B 80, 014113(1)-014113(7)- (2009).

G. Pilania, D. Q. Tan, Y. Cao, V. S. Venkataramani, Q. Chen and R. Ramprasad, "Ab initio study of antiferroelectric PbZrO3 (001) surfaces", J. Mater. Sci. 44, 5249-5255 (2009).

G. Pilania, T. Sadowski and R. Ramprasad, "Oxygen adsorption on CdSe Surfaces: A case study of asymmetric anisotropic growth through Ab initio computations", J. Phys. Chem. C. 113(5), 1863-1871 (2009).

J. D. Doll, G. Pilania, R. Ramprasad and F. Papadimitrakopoulos, "Oxygen-Assisted Unidirectional Growth of CdSe Nanorods Using a Low-Temperature Redox Process", Nano Lett., 10 (2), 680-685 (2010).

G. Pilania and R. Ramprasad “Vortex -Polarization Instability in PbTiO3 nanowires”, under review.

G. Pilania and R. Ramprasad “Thermodynamics of environment dependent oxygen adsorption and vacancy formation on cubic PbTiO3 and LaMnO3 (001) surfaces”, In preparation.

AcknowledgmentsAcknowledgments

Group Members :

Ning, Tang, Tom, Hong, Satyesh, Chenchen, Yenny

Committee members:

Profs. Rampi Ramprasad, Puxian Gao and George A. Rossetti, Jr. Profs. Rainer Hebert and Pamir S. Alpay

Computational resources:

IMS computation clusters; SGI supercomputer in SoE and Teragrid

Funding:

NSF & ONR

Thanks!Thanks!

Back-up slides

[001]

4x4-TiO2-terminated nanowire

4x4-TiO4x4-TiO22 terminated Nanowire terminated Nanowire Atomic relaxations in the vortex stateAtomic relaxations in the vortex state

Cubic LaMnO3 and PbTiO3 surface phase diagrams∆

γ=

Effect of vibrational free energy

(1x1)-MnO2-terminated (001) LaMnO3 surface

O ad-atoms

% change in ∆γ

T (k)

O vacancies

% change in ∆γ

T (k)