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Page 2 of 7 Feng et al. Microstructures 2023;3:2023017 https://dx.doi.org/10.20517/microstructures.2023.01

INTRODUCTION
Two-dimensional (2D) tin in honeycomb lattice, also known as stanene, has attracted enormous research
attention in the field of condensed matter physics as a candidate for room temperature quantum spin Hall
(QSH) insulator and recently discovered topological superconductor . The arrangement of 2D lattice and
[1-3]
degree of buckling have been playing vital roles in the structure stability, bonding configuration, and
electronic band structures of 2D tin, as exemplified by a number of epitaxially grown stanene systems, such
as planar stanene on Cu(111), strained stanene on Sb(111), and stanene Au(111) [4-12] . Aiming at either
pursuing superior or new physical properties, allotropes of 2D tin beyond planar and buckled honeycomb
structures have been widely predicted. 2D tin with dumbbell structure and staggered layered dumbbell
structure is predicted to be a stable topological insulator and topological Dirac semimetal, respectively [13,14] .
Besides, 2D tetragonal tin with repeated square and octagon rings is predicted to be a node line semimetal
[15]
protected by the combination of spatial inversion and time-reversal symmetries . Double-layer 2D tin with
a square lattice has also been proposed to be favorable for free-standing films . In contrast to the intensive
[16]
theoretic exploration, experimental construction of 2D allotropes beyond honeycomb and buckled
honeycomb structures, however, are rarely reported for either tin or other 2D elemental single layers [17-20] ,
which requires subtle interaction and proper lattice-matching between the substrate and the elemental layer.

In this work, two different 2D tin allotropes with quasi-periodic lattice and square-like lattice are fabricated
on Al(111) by epitaxial growth method at different tin coverages. Scanning tunneling microscopy (STM)
and scanning tunneling spectroscopy (STS) studies reveal their atomic structures, electronic properties, and
the degree of interaction with the substrate. We find that in the sub-monolayer region, tin overlayer prefers
the quasi-periodic lattice, in which three-coordinate and four-coordinate tin atoms coexist. While at 1
monolayer (ML) and above, the quasi-periodic lattice transforms to the square-like lattice with four-
coordinate tin atoms, which exhibits apparently enhanced decoupling with the substrate.

MATERIALS AND METHODS
The growth of tin on Al(111) and STM measurements were carried out in the ultra-high vacuum (>1 × 10 -10
Torr). The Al(111) single crystal with a well-polished surface was purchased from Mateck, GmbH. The
Al(111) single crystal was treated in the ultra-high vacuum condition by cycles of sputtering and annealing.
The process of epitaxial growth is illustrated in Figure 1A, in which Sn atoms were evaporated from a
home-made crucible onto Al(111) at 293 K. The as-grown samples were then in-situ transferred to STM
chamber for characterization at 77 K. During the STM and STS tests, the voltage was applied to the sample.
All the STM images were acquired at constant current mode. The dI/dV measurements were acquired with
a lock-in technique with a sample voltage modulation of 10 mV at 937 Hz. The STM images were analyzed
using WSxM software .
[21]

RESULTS AND DISCUSSION
Figure 1B and C show two typical surface topographies after deposition of sub-monolayer tin on Al(111)
surface. The majority of islands are found on the same atomic layer of Al(111), between which an apparent
boundary can be observed as indicated by the white dashed line in Figure 1B. In addition, several islands
that are not adjacent to the step edges can also be found [Figure 1C]. These two types of islands turn out to
show the same superstructure, as shown in Figure 1D, which exhibits hexagonal structure with a periodicity
of around 1.5 nm and directions of +/- 19° with respect to Al[1 0] direction. Therefore, they are assigned to
be Sn-I phase. Moreover, during the scanning, the Sn-I islands can easily be scratched at extremely low
sample bias, here, 5 mV, as shown in Figure 1E. The apparent height measurement along a phase I island
[Figure 1C] and the scratched area [Figure 1E] are shown in Figure 1F and G, respectively, in which the
same apparent height is verified for the scratched area and the lower terrace. This indicates that Sn-I islands