1Surface structure of cleaved (001) USB2 single crystal surface Page: 4 of 17
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Single crystals of USb2 were grown in Sb flux growth [17]. The crystals were prepared for
STM imaging using the following procedure: the crystals were cleaved between a-b planes to
obtain thin flat crystals, which were then attached to STM platens using conductive silver epoxy;
a small piece of sapphire substrate was epoxied to the top surface of each crystal using
TorrsealTM; and a short titanium post was attached to the sapphire substrate to facilitate in situ
UHV cleaving. The freshly cleaved surfaces were characterized using an Omicron UHV-STM
operating under a pressure of about 5x1010 Torr. Due to unreliable sample cleaving and the
presence of surface contaminants, even on a freshly cleaved crystal, atomic resolution STM
images were achieved for only two out of 14 different USb2 samples. Imaging was carried out
using cut Pt-Ir tips under typical tunneling conditions of 1V sample bias voltage and 0.125nA
tunneling current. Both filled and empty state images were taken on one of the samples to help
understand the origin of the observed surface features. A freshly prepared pristine Si(001)-2x1
surface was used to calibrate the piezoelectric scanner to correct the observed atomic spacing for
the USb2 crystals. All measurements were conducted at room temperature.
USb2 has a PbFCl or anti-Cu2Sb (P4/nmm) structure with a = b = 0.4270 nm, c = 0.8748 nm
[10] (c = 0.8741 nm, corrected value as per recent discussion with Tomasz Durakiewicz), and c/a
= 2.049. In this tetragonal layered structure, it is expected to cleave between a-b planes, which
have a square arrangement of atoms in the plane perpendicular to the long axis along [001].
Figure 1 shows two different resolution images collected from one of the samples. The STM
images reveal that the atoms on the cleaved surface are arranged in a square pattern, as expected
for the sample cleaving between a-b basal planes. The measured distance between rows of atoms
in the plane, which has been corrected using measurements from a Si(100)-2x1 surface, is
0.441 0.006 nm. This value is slightly larger than the published bulk value mentioned above of
0.4270 nm [10]. Even though we do not know the source of this discrepancy, this difference
might be used effectively for the level of the accuracy (+0.014 nm) for the current experiment.
Although the square atomic arrangement is clearly visible, the surface contains a significant
number of what appear to be missing atoms, primarily in rows corresponding to the <100> and
<010> crystal directions with equal probability. The most common feature is a single atom
vacancy, followed by two to three adjacent vacancies. In order to distinguish vacancies from the
presence of another type of atom (e.g. contaminant or oxygen) whose DOS differs significantly
from that of the majority of atoms on the surface (in this case either U or Sb), filled and empty
state images were taken simultaneously (figure 2). As can be seen in figure 2, the filled and
empty state images are nearly indistinguishable, suggesting that the darker features are most
likely vacancies rather than differences in the DOS between atomic species. There is no
significance to the approximately 450 rotation of the rows of atoms and "vacancies" from those
in figure 1 since the samples were not oriented when epoxied to the STM platens. Even though
the individual atoms are not as clearly visible as in figure 1, the rows of atoms and "vacancies"
are aligned at approximately 900 angles, and the atomic spacing is the same as in the first sample.
Unlike the crystal shown in figure 1, this area of the second sample had a nearly equal number of
atoms and "vacancies" The topographic height difference between the surface atoms and the
bottom of the dark features in both sets of images is 0.080+0.014 nm, which is discussed below.
Calculations3
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Chen, Shao-ping. 1Surface structure of cleaved (001) USB2 single crystal surface, article, January 1, 2009; [New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc932171/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.