1. Physics and Chemistry of ABO 3 Nanostructures from First Principles Ghanshyam Pilania Chemical, Materials & Biomolecular Engineering Institute 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

2. ABO 3 -type Perovskite structure A B O

3. Novel polarization states in ABO 3 nanowires (p,T) surface phase diagrams of ABO 3 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 LaMnO 3 and PbTiO 3 (001) surfaces Remaining work Impact of work

4. Novel polarization states in ABO 3 nanowires

5. 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 ABO 3 perovskite Energy P Paraelectric state ABO 3 perovskite Ferroelectric Well Energy P Paraelectric state Energy P Ferroelectric state Energy P

6. Ferroelectricity in Nanostructures Thin film Depolarizing Field +++++++++ - - - - - - - - - Bulk Aguado-Puente et al. (PRL, 2008) C. Kittel, Phys. Rev. 70, 965 1946. Ferromagnetic closure domains

7. 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)

8. ABO 3 Nanowires – Our DFT Study 2x2-AO-terminated nanowire 2x2-BO 2 -terminated nanowire AO-plane BO 2 -plane AO-plane BO 2 -plane Construction of ABO 3 nanowires

9. BaTiO 3 Nanowires – Our DFT Study Axial polarization instability above 1.2 nm ferroelectricparaelectric 4x4-TiO 2 P 4x4-BaO τ =rxP Vortex polarization instability above 1.6 nm Geneste et. al, APL 88, 112906 (2006);

10. Spanier et al, Nano Lett. 6, 735 (2006) 0.8 nm Off-axis Polarization in BaTiO 3 nanowires BaTiO 3 Nanowires – Experimental Study

11. PbTiO 3 Nanowires – Our DFT Study c (Å) FaFa FaFa FaFa FaFa P 1x1 to 4x4-PbO FvFv Shimada et al, PRB 79, 024102 (2009) c tetragonal Bulk a cubic Bulk P P P 4x4-TiO 2 τ =rxP Unit cell decomposed dipole moments

12. PbTiO 3 Nanowires vs. Terminations Strain-induced phase transition: vortex  axial polarization 4x4-TiO 2 -terminated nanowire [001] Axial compressive Strain Axial Tensile Strain 4x4-PbO-terminated nanowire Four possible switchable polarization states Vortex (clockwise/counter-clockwise), Axial (positive/negative)

13. PbTiO 3 nanowires display switchable rectilinear (axial) and non-rectilinear (vortex) polarization configurations Control of polarization states axial Strain and surface terminations

14. (T, p) surface phase diagrams of ABO 3 systems

15. Flexibility Versatility Less expensive Thermal stability Excellent oxygen exchange properties Why are they important? Perovskite Surfaces in Catalysis

16. R. J. H. Voorhoeve, D. W. Johnson, Jr., J. P. Remeika, P. K. Gallagher SO 4 -2 Dead site Active site Sulfur poisoning 26 MARCH 2010 VOL 327 SCIENCE Chang Hwan Kim, Gongshin Qi, Kevin Dahlberg, Wei Li Perovskite Surfaces in Catalysis

17. Suprafacial v/s Intrafacial

18. Surface-O* ↔ Surface + ½ O 2 (g) Cubic LaMnO 3 and PbTiO 3 surface phase diagrams

19. + N/2 O 2 Cubic LaMnO 3 and PbTiO 3 surface phase diagrams

20. (1x1) AO-terminated(1x1) BO 2 -terminated Formation Energies Cubic LaMnO 3 and PbTiO 3 surface phase diagrams A

21. Relaxed geometries for most favored adsorption sites Cubic LaMnO 3 and PbTiO 3 surface phase diagrams

22. Perovskite surfaces in contact with O 2 (g) Assuming ideal gas behavior for O 2 Surface-O* ↔ Surface + ½ O 2 (g)

23. Surface phase diagrams for surfaces in contact with O 2 PbTiO 3 (001) TiO 2 -terminated log P O2 100% O ad-atom coverage Partial O vacancy coverage Partial coverage of O ad-atom Clean surface 100% O vacancy T (K) LaMnO 3 (001) MnO 2 -terminated 100% O vacancy Partial coverage of O ad-atom Partial O vacancy coverage 100% O ad-atom coverage log P O2 T (K)

24. Remaining Work Electric field response of the vortex polarization state in PbTiO 3 nanowires E field ? Dielectric tensor of ferroelectric nanowires 4x4-PbO terminated nanowire (axial polarization) 4x4-TiO 2 terminated nanowire (vortex polarization) Effect of surface passivation (by various species such as –OH, H, -CH 3 etc.) on polarization states in PbTiO 3 nanowires

25. Thermodynamics of environment dependent interaction of various gases on the (001) surface of ABO 3 type perovskites NO, NO 2, N 2, O 2 (gases) Adsorption site Equilibrium geometry Electronic structure Energetics Kinetics ?? Remaining Work

26. 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?!!

27. Impact of Work DeNO x processes NO + CO + unburned hydrocarbons catalytic converter catalytic converter CO CO 2 NO x N 2 + O 2 CnHmCnHm CO 2 +H 2 O LaCoO 3 (○) La 0.9 Sr 0.1 CoO 3 (●) LaMnO 3 (□) La 0.9 Sr 0.1 MnO 3 (■) commercial DOC (▲)

28. List of Publications G. Pilania, S. P. Alpay and R. Ramprasad, "Ab initio study of ferroelectricity in BaTiO 3 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 PbZrO 3 (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 PbTiO 3 nanowires”, under review. G. Pilania and R. Ramprasad “Thermodynamics of environment dependent oxygen adsorption and vacancy formation on cubic PbTiO 3 and LaMnO 3 (001) surfaces”, In preparation.

29. Acknowledgments 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

30. Thanks!

31. Back-up slides

33. [001] 4x4-TiO 2 -terminated nanowire 4x4-TiO 2 terminated Nanowire Atomic relaxations in the vortex state

34. Cubic LaMnO 3 and PbTiO 3 surface phase diagrams ∆γ=∆γ=

35. Effect of vibrational free energy (1x1)-MnO 2 -terminated (001) LaMnO 3 surface O ad-atoms % change in ∆ γ T (k) O vacancies % change in ∆ γ T (k)