Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Atom-resolved AFM studies of the polar MgAl2O4 (001) surface
Morten K. Rasmussen, Jeppe V. Lauritsen, and Flemming Besenbacher
Interdisciplinary Nanoscience Center (iNANO), University of Aarhus, Denmark
P.II-24
Magnesium-aluminate spinel (MgAl2O4) has many interesting properties and
applications, among other things the use as a support for catalysts. In particular, MgAl2O4
in a porous form is a support material for the important Ni-based catalyst used for steam
reforming of methane, which today is the most common method of producing bulk
hydrogen. A study of the clean spinel is thus of very great importance to understand the
role of the support, and still almost no surface science studies on the MgAl2O4 surface
have been presented due to the insulating nature of the material. When constructing a
surface model one must consider the fact that the spinel (001) surface is polar and the
surface consequently has to be modified somehow to cancel out or compensate for this.
Additionally, it is well known that the AFM tip can pick up material from the surface,
and this material then can change the polarity of the nano-apex which leads to variations
in the AFM contrast seen in atom-resolved images and reveals the individual sub lattices
[1, 2]. In this study we observe from atom-resolved AFM images that the MgAl2O4 (001)
surface may be imaged in two distinctly different contrast modes revealing the anion or
cation sub-lattice. Tentative structural characterizations from the AFM studies suggest a
stoichiometric bulk like termination. Stabilization mechanism may then include
relaxations in the crystal structure below the topmost layer which cannot be probed by
AFM, hence surface x-ray diffraction is used to complement the AFM data.
a) b)
Figure 1: (a) Atom resolved non-contact AFM image (4nm×4nm) of the clean MgAl2O4 (001)
surface imaged under UHV conditions together with (b) a ball model of the surface. The surface
appears to be Mg-terminated and near stoichiometric with a low atomic defect density less than
0.1ML.
[1] Lauritsen, J.V., et al., Chemical identification of point defects and adsorbates on a metal oxide
surface by atomic force microscopy. Nanotechnology, 2006. 17(14): p. 3436.
[2] Enevoldsen, G.H., et al., Noncontact atomic force microscopy studies of vacancies and hydroxyls
of TiO[sub 2](110): Experiments and atomistic simulations. Physical Review B (Condensed
Matter and Materials Physics), 2007. 76(20): p. 205415-14.
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