1. Surfaces of Metal Oxides, Studied at the Atomic Scale
Ulrike Diebold
Institute of Applied Physics
TU Wien
Vienna, Austria

2. Why Study Metal Oxides?
They are there.
They are useful.

3. Metals
Normally present as an oxide/with a surface oxide

4. TiO2 & Li-ion batteries Optical Properties Biocompatibility
Heterogeneous
Catalysis (support & active catalyst)
Gas sensing
Photocatalyst
Dye-sensitized solar cells
Li-ion batteries
Memristor
Optical Properties
Biocompatibility
TiO2
U. Diebold,
“The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53

5. The surface chemistry of metal oxides is heavily influenced, and sometimes even dominated, by defects.
V.E. Henrich & P.A. Cox
(Cambrigdge University Press, 1993)

6. Scanning Tunneling Microscopy* (STM)
&
Main Tool:
Scanning Tunneling Microscopy* (STM)
Kizuka et al.,
Phys. Rev. B 55, R7398 (1997)
Credits: Forschungszentrum Jülich
*G. Binnig and H. Rohrer, 1982, Nobel prize 1986

7. Scanning Tunneling Microscopy* (STM)
&
Main Tool:
Scanning Tunneling Microscopy* (STM)
Dulub et al., Science 2007
Need theory for interpretation
Density Functional Theory (DFT)

8. Scanning Tunneling Microscopy* (STM)
&
Main Tool:
Scanning Tunneling Microscopy* (STM)
Dulub et al., Science 2007

9. &
*Flat, large single crystals. Ultrahigh Vacuum (10-11 mbar), low temperature (6K K)

10. TiO2- Color Scheme (Rutile Samples)
M. Li, et al., Journal of Physical Chemistry B 104 (20) (2000)4944
TiO2- Color Scheme (Rutile Samples)
Samples heated in furnace to different temperatures
(Ar with 20 ppm O2
~4x10-3 Torr)

11. Diebold, Li, and Schmid Annual Rev. Phys. Chem 2010
TiO2 Rutile (110): Model & Scanning Tunneling Microscopy
Diebold, Li, and Schmid Annual Rev. Phys. Chem 2010 OH
Ti5c VO
O2c
O3c
[001]
[110]
Empty-states STM image
Vsample=+0.8 V, const. height, T=78 K
S.-C. Li, et al., JACS 130, 9080 (2008)

12. TiO2-x + O2 O2
Wendt et al. Science 2007
… see groups of Henderson, Iwasawa, Bowker, Yates, Thornton, Besenbacher, Wendt, Dohnalek, Kummel, Selloni, Hammer, …

13. Ph. Scheiber et al., Phys. Rev. Lett., 105 (2010) 216101
Vsample=+1.8 V,
I = 0:03 nA,
Tsample = 17 K)

14. Adsorption of Oxygen on TiO2
&
Adsorption of Oxygen on TiO2
M. Haruta, CatalysisToday 36 (1997)153
Heterogeneous
Catalysis (support & active catalyst)
Gas sensing
Biocompatibility
Photocatalyst
M. Batzill and UD, Progr. Surf. Sci. 2005

15. Ongoing work and future challenges:
Defects underneath the surface

16. Interaction of O2 with (subsurface) O vacancy
Uli Aschauer, Annabella Selloni:
Interaction of O2 with (subsurface) O vacancy
First-Principles Molecular Dynamics at T = 200 K*
‘bridging peroxo’:
O22- replaces lattice O
Movie available: Supplement to DOI: /science
*Car-Parrinello, timestep of 5 au, fictitious electron mass 500 au

17. Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:
Uli Aschauer, Annabella Selloni:
Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:
‘bridging peroxo’:
O22- replaces lattice O:
Vo+ O22- → O22- bridging
Experiment:
VO + O2 → O22- bridging
Movie available: Supplement to DOI: /science

18. STM tip: Pulling O vacancies back to the surface
Setvin et al. Science, 341 (2013) 988
1115
+1.2 V, 0.1 nA
1111
+1.0 V, 0.1 nA (78 K)
1014
+5.2 V, 0.7 nA
Dopants (1% Nb)
+0.9 V, 0.12 nA (6 K)
2140

19. STM - tip: induced VO migration
M. Setvin et al. Science, 341 (2013) 988
Phys. Rev. B. 91 (2015)
STM - tip: induced VO migration -
-5.2V
Memristor E + VO
Charges positively because of tip-induced band bending – reversal of energetics.
Ballistic electrons – overcoming kinetic barriers
D. B. Strukov et al., Nature 453, 80 (2008)
J. O. Lee et al., Nat. Nanotechnology 8, 36 (2013)

20. Ongoing work and future challenges:
Defects underneath the surface
What happens outside vacuum?

21. TiO2(110): STM, in-situ, in high-purity water
450 x 450 nm2, Vtip = V, Itunnel = 1 nA
G. Serrano, B. Bonanni, M. Di Giovannantonio, T. Kosmala, M. Schmid, U. Diebold, A. Di Carlo, J. Cheng, J. VandeVondele, K. Wandelt and C. Goletti “Molecular Ordering at the Interface Between Liquid Water and TiO2 Rutile (110)”, Adv. Mater. Interf. (2015) c

22. TiO2(110): STM, in-situ, in high-purity water
450 x 450 nm2, Vtip = V, Itunnel = 1 nA).
10 x 10 nm2, Vtip = V, Itunnel = 1 nA).
G. Serrano, B. Bonanni, M. Di Giovannantonio, T. Kosmala, M. Schmid, U. Diebold, A. Di Carlo, J. Cheng, J. VandeVondele, K. Wandelt and C. Goletti “Molecular Ordering at the Interface Between Liquid Water and TiO2 Rutile (110)”, Adv. Mater. Interf. (2015) c

23. Ongoing work and future challenges:
Defects underneath the surface
What happens outside vacuum?
More complex materials (ternary systems, perovskites)

24. Challenge: Mastering Complexity (Surface Preparation)

25. SrTiO3 Ti evaporator Sr evaporator O2 leak valve Chemical Potential
m(O2)
m(Sr)
m(Ti)

26. Surface Phase Diagram of SrTiO3(110)
[110]
[001]
We choose the (110) direction because surface structures can be prepared much more easy than at the (001) surface. On the (110), a series of reconstructions can be changed into each other reversibly by adjusting the surface stoichiometry. This means If you prepare you surface by sputtering and annealing then one of these reconstructions is usually formed. By depositing Ti or Sr onto the surface and annealing in 10E-6 mbar O2 any of these other structures can be created.
Deposition of Sr or Ti and annealing in 10-6 mbar O2
Surface stoichiometry drives the reconstructions
Wang et al., Phys. Rev. B 83, (2011)
Wang et al., Appl. Phys. Lett. 100, (2012)

27. Ongoing work and future challenges:
Defects underneath the surface
What happens outside vacuum?
More complex materials (ternary systems, perovskites)
Insulators
Atomic Force Microscopy with atomic resolution (Q+ sensor)

28. Pentacene/2 ML NaCl/Cu(111)
Gerhard meyer has demonstrated that it is possible to resolve the chemical structure of a molecule using nc-afm
Special tip preparation required: CO terminated tip. Chemically inert, sharp, p-wave character
The contrast mechanism is pauli repulsion between electrons in the tip and the electrons in the molecule
L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer: Science 325, 1110–4 (2009)

29. Alex Riss, et al (Mike Crommie group)
T>90C
AFM: -200mV, 60pm, Q=80000
Afm measurements to resolve chemical structure. You can directly see the bonds between atoms. Triple bonds are clearly resolved, too. This allows assignment of structures.
The reactions preserve the number of C and H atoms.

30. Thank you…

31. Thank you…

32. Thank you…
Many, many collaborators, former students and post-docs, coworkers and friends, ...