Post-transition metal oxides ...

… transparent, electrically conducting, and with intriguing surfaces

We are also interested in non-transition metal oxides, specifically in the ones with a full d-shell: ZnO, SnO₂, and In₂O₃. These materials have a band gap of the order of 3 eV (meaning they are optically transparent) and an s-like conduction band (which implies that they conduct quite well when they are doped a little). The combination of these properties is as rare as it is useful; so-called transparent optical conductors are needed in virtually any technology that combines light with electronics. The conductivity of these materials changes easily when they are exposed to oxidizing or reducing gases, a property which is richly exploited in chemical sensing. And, as is true for most metal oxides, ZnO, In₂O₃, and SnO₂ play a role in catalysis. Again, surfaces and interfaces are critical wherever these materials are used, so we want to learn more about them.

With STM we have looked at all the low-index surfaces of ZnO [1], a material with applications from the rubber and concrete industries to corrosion protection, and also a component of many sun lotions: Its bandgap is just conveniently wide to absorb UV light while transmitting the visible spectrum. Of particular interest are the (0001)-oriented basal planes, as these are strictly polar - but polar surfaces should not exist because it would cost almost infinite energy to create them! Together with Georg Kresse from the University of Vienna we have found out how the Zn-terminated surface deals with its polarity: It forms many small islands with O-terminated step edges, providing additional negatively charged oxygen ions, compensating for the excess positive charge of the zinc [2].

SnO₂ is the prototypical material used in gas sensing; how its surfaces are used in this and in various other applications is summarized in this review [3]. The SnO₂(011) surface is a particularly nice model system – when oxygen is removed or added, the surface does not 'reconstruct', i.e., it keeps its symmetry. In addition, the surfaces of SnO₂ nanowires have mostly this orientation [4].

The most common transparent optical conductor to date is In₂O₃; usually it is doped with Sn and called ITO. Among other applications, e.g., for solar cells, it is used for transparent electrodes of LCD screens like the one you are probably sitting in front of. For our first STM experiments we grew epitaxial thin films [5, 6]; recently we have started to work on In₂O₃ single crystals [7]. More exciting results still to come!

1. O. Dulub, L. A. Boatner, U. Diebold
   STM study of the geometric and electronic structure of ZnO(0001)-Zn, (000-1)-O, (10-10), and (11-20) surfaces
   Surf. Sci. 519, 201-217 (2002) ⋅ full text*
2. O. Dulub, U. Diebold, G. Kresse
   Novel Stabilization Mechanism on Polar Surfaces: ZnO(0001)-Zn
   Phys. Rev. Lett. 90, 016102 (2003) ⋅ full text
3. M. Batzill, U. Diebold
   The surface and materials science of tin oxide
   Prog. Surf. Sci. 79, 47-154 (2005) ⋅ full text*
4. K. Katsiev, A. Kolmakov, M. Fang, U. Diebold
   Characterization of individual SnO2 nanobelts with STM
   Surf. Sci. 602, L112-L114 (2008) ⋅ full text*
5. E.H. Morales, Y. He, M. Vinnichenko, B. Delley, U. Diebold
   Surface structure of Sn-doped In2O3(111) thin films by STM
   New J. Phys. 10, 125030 (2008) ⋅ full text
6. E.H. Morales, U. Diebold
   The structure of the polar Sn-doped indium oxide (001) surface
   Appl. Phys. Lett. 95, 253105 (2009) ⋅ full text
7. D. R. Hagleitner, M. Menhart, P. Jacobson, S. Blomberg, K. Schulte, E. Lundgren, M. Kubicek, J. Fleig, F. Kubel, C. Puls, A. Limbeck, H. Hutter, L. A. Boatner, M. Schmid, and U. Diebold
   Bulk and surface characterization of In2O3(001) single crystals
   Phys. Rev. B 85, 115441 (2012) ⋅ full text

* Please note: access to full text (PDF files) of some articles is restricted due to copyright reasons.