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

P.II-22
Simultaneous NC-AFM/STM Imaging of the Surface Oxide Layer on
Cu(100) and Identification of Lattice Sites
Mehmet Z. Baykara 1 , Todd C. Schwendemann 1,2 , Eric I. Altman 2 , Udo D. Schwarz 1
1
Department of Mechanical Engineering and Center for Research on Interface Structures and Phenomena
(CRISP), Yale University, New Haven, USA
2
Department of Chemical Engineering and Center for Research on Interface Structures and Phenomena
(CRISP), Yale University, New Haven, USA
Exposure of Cu(100) to molecular oxygen at elevated temperatures leads to the (2√2 ×
√2) R45 o missing row reconstruction pictured in Fig. 1a, where oxygen atoms are found
to sit nearly co-planar with the Cu atoms in the outermost layer [1]. Since O and Cu
atoms occupy sublattices with different symmetries, this surface represents an ideal
model where atoms responsible for contrast formation in NC-AFM and STM imaging
modes can be identified by symmetry alone. Using our homebuilt low temperature,
ultrahigh vacuum atomic force microscope [2], we have obtained simultaneously
recorded NC-AFM and tunneling current images on the oxidized Cu(100) surface.
Comparing the visual distinctions between the two images, bright features in the images
are assigned to specific locations on the surface. In particular, metal tips are found to
interact strongly with bridge sites between two under-coordinated oxygen atoms, leading
to a rectangular unit cell contrast in high-resolution NC-AFM images (Fig. 1b), whereas
simultaneously collected tunneling current data exhibit an elongated hexagonal unit cell,
which is assigned to surface Cu atoms (Fig. 1c). These measurements demonstrate the
ability of multi-dimensional, atomic-resolution scanning probe microscopy to identify
adsorption sites on oxide surfaces.
Figure 1: a) Model of the (2√2 ×√2) R45 o O-induced reconstruction of Cu(100). The grey balls
represent O, the light orange surface Cu, and the darker orange second layer Cu. b) NC-AFM
image acquired with a frequency shift of -0.95 Hz at T = 6 K and c) simultaneously acquired
tunneling current image collected at a sample bias of -0.4 V.
[1] M. C. Asensio et al, Surf. Sci. 236, 1 (1990).
[2] B. J. Albers et al, Rev. Sci Instrum. 79, 033704 (2008).
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