Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA23 / P35
Structure determination of Sn on Cu(111) using LEED
Rezwan Ahmed, Takeshi Nakagawa, Seigi Mizuno
Department of Molecular and Material Sciences, Kyushu University, Kasuga, Kasugakoen, 6-1, Fukuoka
816-8580, Japan
Email: r_ahmed17@yahoo.com
Abstract: The 2D hexagonal structure of tin atoms which is termed stanene is structurally determined on a Cu(111)substrate using low energy electron diffraction. The structural analysis of Cu{111}-p(2x2)-Sn at a coverage of 0.5 at low
temperature reveals the relaxation of underlying Cu atoms, which stabilizes the formation of almost zero-buckled Snatoms forming a honeycomb structure. Our result using quantitative low energy electron diffraction conclusively reveals
an ultra-flat stanene structure that complements well with the previous calculations. The detailed structural analysis
presented in this article is expected to give in-depth information for characterizing the properties of as-grown stanene.
Keywords: Stanene; Cu(111); LEED
1. INTRODUCTION
The two dimensional (2D) group IVA materials of carbon,silicon, germanium, tin and lead which are termed asgraphene, silicene, germanene, stanene and plumbene
respectively having single layer of atoms organized in ahoneycomb like lattice possesses striking physicalproperties due to its unique structure. [1]–[3] All thesematerials are predicted to exhibit band gap due to spin
orbit coupling which makes them 2D topologicalinsulators. [4]–[9] Graphene has been extensively studied
in the past for its relative ease of fabrication however, inthe last few years experimental studies of other 2D
materials as we move down the same group have taken
the limelight due to the stronger spin-orbit coupling
which results in additional intriguing properties in thefield of solid state physics. [9]- [12] Among them,stanene is considered to be highly promising 2D material
due to its exclusive enriched properties such as
topological superconductivity, near-room temperature
quantum anomalous Hall (QAH) effect, giant
magnetoresistance and enhanced thermoelectricity. [7][8], [11]-[13][14] The successful growth and realizationof 2D materials largely depend on the perfect selection ofunderlying substrates, as proper interaction between
substrate and 2D material is necessary. The growth ofstanene was elusive until in recent years there are several
reports of successful fabrication of stanene in different
substrates which have gradually led to the realization ofmany of these interesting properties.
One of the first experimental realization of thetheoretically predicted stanene was reported by Zhu et al.
where the 2D material was grown on Bi2Te3 using
molecular beam epitaxy and adopts Vollmer-Weber(island) growth mode. [15] Later on, stanene was further
successfully reported on substrates like Sb(111),InSb(111), Bi(111), Au(111), Ag(111) and PbTe(111).
[16][17][18][19][20][21] Besides grown on diverse
substrates, the other distinct structural features of asgrown stanene for each of these cases is the difference ofbuckling between the Sn atoms. The DFT calculations
suggest that stanene becomes more stable as the buckling
is reduced. [9][22][23].
The study of ultrathin Sn films on Cu(111) substrate iswell studied which generally forms a surface alloy at orabove room temperature due to the high solubility of Snon Cu. [24][25] The realization of 2D tin atoms onCu(111) substrate at low temperature was first addressedby Xihui et al. proposing one atom per unit cell of p(2x2)structure. [26] However, recently Deng et al haveobserved the epitaxial growth of stanene on Cu(111)having honeycomb structure and an unusually ultra-flatzero-buckling geometry using STM and supported by
DFT calculations.[27] This suggests the importance ofunderlying interaction of substrates with stanene instabilizing the ultra-flat 2D structure. However, theinteraction and relaxation of the underlying substrate is
not investigated in previous reports which can be an
important factor in understanding the overlayer structurewith zero buckling nature.
In this report we have investigated the structuralparameters of stanene on Cu(111) more distinctively
using quantitative tensor low energy electron diffraction
which is sensitive to the surface and provides detailed
structural parameters compared to STM alone. [25]-[26]Because the substrate plays an important role isstabilizing the zero-buckling stanene structure, we triedto investigate the role of relaxation of first few layers ofCu atoms and the bonding between the adsorbed Sn
atoms at both fcc and hcp site with that of the underlying
Cu atoms. This detailed structural analysis can pave away for understanding the interactions of Cu with the Sn
atoms arranged in honeycomb structure and hence canhelp for further characterizing the properties of as grownstanene.
2. Experimental Procedure
The experiments were performed in an ultra-highvacuum chamber with a base pressure of 5 x 10-8 Pa,
which is equipped with four grid LEED system including
in situ sample preparation and adsorbate (Sn) deposition.
At first the Cu(111) substrate was cleaned with several
cycles of Ar+ ion sputtering (1 kV, 7 µA, 15 min) andsubsequent annealing at 800 K until a sharp (1x1) LEED
pattern was recorded. The sample was pre-cooled withliquid N2 until the temperature reaches ~150 K before theSn was evaporated by thermal heating from a tantalum
tube onto the sample. The deposition is sequentiallymonitored until p(2x2) structure is obtained in the LEED
pattern. The sample was further cooled until 130 K withliquid N2 when the p(2x2) structure is sharped which- 61 -

1670 eV having 9 symmetrically inequivalent beams.
The numerical parameters of the lateral displacement andcorresponding vertical displacement of the Sn andunderneath Cu atoms alongside the error ranges are listed
in Table 1. All the lateral displacements were measured
relative to the ideal three-fold hollow sites while thevertical displacement is calculated relative to the ideal
first layer Cu atoms.

were observed using a digital charge-coupled device
camera with computer-controlled data acquisition
system. [30] The intensity for nine inequivalent spots ofSn-p(2x2)-Cu(111) structure having three fold rotational
symmetry is recorded for an energy range of 70-450 eV.
The Barbieri/Van Hove symmetrized automated tensor
LEED package was used to calculate the theoretical I(E)curves of the structure models to determine the atomicpositions. [31] The calculations for the atomic scattering
was performed by considering 13 phase shifts(lmax = 12) whereas the imaginary part of the innerpotential (Voi) was fixed to −5.0 eV and the real part wasdetermined through theoretical-experimental matching
and attributed by minimizing Pendry’s reliability factor(Rp). [32] The error bars on the structural parameters
were calculated from the variance of Rp,
ΔR = Rmin(8|Voi|/ΔE)1/2, where Rmin is the minimum
Rp value and ΔE is the total energy range of theexperimental I(E) curves. [32]3. Results and Discussion
In order to determine the structural model of p(2x2)Sn we have analyzed ten structures. All the structuralmodels represent p3m1 symmetry. Each of the structures
are optimized however, the first model having
honeycomb structure indicates the lowest RP factor
which was also proposed by Deng et al. [27] Further
optimization of the Debye temperatures of Sn, Cu, andstructural parameters led to more reduced RP factor of0.14 for model 1. The detailed structural parameters forthe model 1 with top and cross sectional views after
optimization is shown in Fig. 1 with Debye temperaturesof Sn, first layer Cu, second layer Cu, third layer Cu andoperating temperature of 90, 230, 250, 300 and 150 K
respectively. The comparison between the calculated I(E)curves of the best fit model with the experimental curves
is shown in Fig. 2 which represents total energy range ofFig. 2 Comparison between the experimental (red line) andbest fit calculated (black line) LEED I(E) spectra forCu(111)-p(2x2)-Sn structure.

The coverage of the optimized honeycomb structure is
0.5 in a unit cell with one Sn atom in the face-centeredcubic hollow site while the other in the hexagonal closepacked hollow site denoted as Sn1 and Sn2 respectively
from hereon. In the previous study with STM, Deng et al.
showed no buckling in the stanene with two Sn atoms
having the same apparent height of 1.8 Å relative fromthe first Cu layer. [27] It is quite interesting as the Sn
atoms are positioned in two different sites so they are
expected to have different interactions with surrounding
Cu atoms which would result in different adsorption
heights for optimizing the energy. The height between
the two Sn atoms in our optimized calculation shows 0.02
Å however, the difference is smaller than the error range
which dismiss this difference into consideration.
One common structural phenomenon observed fromall the calculations that led to the optimized structure isthat the Sn1 which is in the FCC position has always
dipped to the Cu surface more compared to the Sn2 in

Fig. 1 Top and cross sectional views of the optimized
honeycomb stanene structure Cu(111)-p(2x2) (Model 1 inFig. 1). Atomic size ratios between the Cu and Sn atoms areroughly reflected. Sn1 indicates Sn atom in the fcc hollow
site, Sn2 indicates Sn atom in hcp hollow site. There arefour Cu atoms in the unit cell. Cu1-1 and Cu1-2 notations
indicate first layer Cu and their corresponding number. Thesame is true for the second and third layer Cu atoms.

- 62 -HCP site. On contrary, interestingly the average bondlength of Sn1 is longer than the Sn2 with corresponding
first layer Cu atoms as shown in Table 2. To interpret thesituation, we analyzed the structural parameters andfound that this is due to the lateral displacement of the Cu
atoms relative to their positions which is numerically
Table 1 Structural parameters for the optimized best fit structure
for p(2x2)-Sn structure. Atoms and their numbers are referenced
in Fig. 2. The height is calculated from the starting position offirst Cu layer where 0 indicates no displacement of atoms in thespecified direction due to symmetry requirement.

We also determined the overall relaxation in [111]direction of the Sn adsorbed Cu(111) surface with respect
to clean Cu(111) surface to understand the contributionof the substrate relaxation in stabilizing the flat stanenestructure. The interlayer distance between the first andsecond Cu layer is 2.08 Å whereas it is 2.05 Å between
the second and third layers compared to the bulk value of
2.08 Å. [33] It is quite apparent that instead ofcompression between the first two layers with respect tothe interlayer distance between the second and third layer
which is present in clean Cu(111), the adsorption of Sn
as an overlayer has an opposite effect on Cu(111) surface.
This is due to the fact that Cu1-1 atom has verticallytranslated upward by 0.06 Å to neutralize the lateraldisplacement of the other Cu atoms in first layer which
in turn optimized the buckling effect between the two Snatoms. Therefore, from the above discussion it is clearly
evident that the underlying Cu atoms provoke theformation of ultra-flat stanene by subsidizing the overallrelaxation on the surface. These detail numerical data of
the structure obtained using experimental QLEED
analysis is expected to provide important information forthe characterization of the properties of stanene as wellas understanding the role of substrate in stabilizing other2D materials.
4. REFERENCES[1]V. M. Apalkov and T. Chakraborty, “Tunability ofthe fractional quantum Hall states in buckled Diracmaterials,” Phys. Rev. B - Condens. Matter Mater.
Phys., vol. 90, no. 24, pp. 1–6, 2014.
[2] H. Wu and F. Li, “First-Principles Calculation on
Geometric, Electronic and Optical Properties of Fully
Fluorinated Stanene: a Large-Gap Quantum Spin Hall
Insulator,” Chinese Phys. Lett., vol. 33, no. 6, p.
67101, Jun. 2016.
[3] T. Taniguchi et al., “Observation of the quantum spin
Hall effect up to 100 kelvin in a monolayer crystal,”
Science (80-. )., vol. 79, no. January, pp. 76–79, 2018.
[4] S. Sadeddine et al., “Compelling experimental
evidence of a Dirac cone in the electronic structure ofa 2D Silicon layer,” Sci. Rep., vol. 7, pp. 1–7, 2017.
[5] M. E. Dávila, L. Xian, S. Cahangirov, A. Rubio, andG. Le Lay, “Germanene: A novel two-dimensional
germanium allotrope akin to graphene and silicene,”
New J. Phys., vol. 16, 2014.
[6] J. Gao, G. Zhang, and Y. W. Zhang, “ExploringAg(111) substrate for epitaxially growing monolayer
stanene: A first-principles study,” Sci. Rep., vol. 6, no.
April, pp. 1–8, 2016.
[7] Y. Fang et al., “Quantum spin hall states instanene/Ge(111),” Sci. Rep., vol. 5, no. 111, pp. 1–8,2015.
[8] J. Wang, Y. Xu, and S. C. Zhang, “Two-dimensionaltime-reversal-invariant topological superconductivity
in a doped quantum spin-Hall insulator,” Phys. Rev.
B - Condens. Matter Mater. Phys., vol. 90, no. 5, pp.
1–5, 2014.
[9] Y. Xu et al., “Large-gap quantum spin hall insulatorsin tin films,” Phys. Rev. Lett., vol. 111, no. 13, pp. 1–5, 2013.
[10] Y. Xu, Z. Gan, and S. C. Zhang, “Enhanced
thermoelectric performance and anomalous seebeckpresented in Table 1. The lateral and verticaldisplacement of the first layer Cu atoms also have animpact on the subsequent underneath Cu layers as can beseen from the Table 1. Similar to the first layer Cu atoms
the second layer Cu atoms also have a lateral and verticalrelaxation to stabilize the structure which is carriedforward to the third Cu layer as well.
The bond length between the Sn atoms in our LEEDcalculation is 2.95 Å which is comparable to the bulk α-

Sn and freestanding stanene which also complements
well with the previous study. [9][27] The height between
the Sn and the first layer Cu atoms is 2.17 ± 0.28 Å which
complements well with the theoretical adsorption heightof stanene on Cu(111) but is little higher than the STM
line profile of 1.8 Å. However, the average bond length
between the Sn1 and the first layer Cu atoms is 2.75 Åwhich is comparable with the bulk Sn-Cu bond of 2.73 Å.
Therefore, we can deduce that our average calculated
height between the Sn and first layer Cu atoms isreasonable.

- 63 -effects in topological insulators,” Phys. Rev. Lett.,
vol. 112, no. 22, pp. 1–5, 2014.
[11] A. Molle, J. Goldberger, M. Houssa, Y. Xu, S. C.
Zhang, and D. Akinwande, “Buckled twodimensional Xene sheets,” Nat. Mater., vol. 16, no. 2,pp. 163–169, 2017.
[12] S. Rachel and M. Ezawa, “Giant magnetoresistanceand perfect spin filter in silicene, germanene, andstanene,” Phys. Rev. B - Condens. Matter Mater.
Phys., vol. 89, no. 19, pp. 1–6, 2014.
[13] M. Liao et al., “Superconductivity in few-layerstanene,” Nat. Phys., vol. 14, no. 4, pp. 344–348,2018.
[14] S. Nigam, S. Gupta, D. Banyai, R. Pandey, and C.
Majumder, “Evidence of a graphene-like Sn-sheet ona Au(111) substrate: Electronic structure andtransport properties from first principles calculations,”
Phys. Chem. Chem. Phys., vol. 17, no. 10, pp. 6705–6712, 2015.
[15] F. F. Zhu et al., “Epitaxial growth of twodimensional stanene,” Nat. Mater., vol. 14, no. 10, pp.
1020–1025, 2015.
[16] C. Z. Xu et al., “Gapped electronic structure ofepitaxial stanene on InSb(111),” Phys. Rev. B, vol. 97,no. 3, pp. 1–5, 2018.
[17] J. Yuhara et al., “Large area planar stanene
epitaxially grown on Ag(1 1 1),” 2D Mater., vol. 5,
no. 2, 2018.
[18] Y. Zang et al., “Realizing an Epitaxial Decorated
Stanene with an Insulating Bandgap,” Adv. Funct.
Mater., vol. 28, no. 35, pp. 1–7, 2018.
[19] J. Gou et al., “Strain-induced band engineering inmonolayer stanene on Sb(111),” Phys. Rev. Mater.,vol. 1, no. 5, pp. 1–6, 2017.
[20] M. Maniraj et al., “A case study for the formation ofstanene on a metal surface,” Commun. Phys., vol. 2,no. 1, pp. 1–9, 2019.
[21] Y.-H. Song et al., “High-buckled R3 stanene with
topologically
nontrivial
energygap,”
arXiv:1707.08657 [cond-mat.mtrl-sci], 2017.
[22] S. R. Song, J. H. Yang, S. X. Du, H. J. Gao, and B.
I. Yakobson, “Dirac states from px,y orbitals in thebuckled honeycomb structures: A tight-binding
model and first-principles combined study,” Chinese
Phys. B, vol. 27, no. 8, 2018.
[23] A. Molle, J. Goldberger, M. Houssa, Y. Xu, S. C.
Zhang, and D. Akinwande, “Buckled twodimensional Xene sheets,” Nat. Mater., vol. 16, no. 2,pp. 163–169, 2017.
[24] J. F. Aguilar, R. Ravelo, and M. I. Baskes,“Morphology and dynamics of 2D Sn-Cu alloys on(100) and (111) Cu surfaces,” Model. Simul. Mater.
Sci. Eng., vol. 8, no. 3, pp. 335–344, 2000.
[25] S. H. Overbury and Y. S. Ku, “Formation of stable,two-dimensional alloy-surface phases: Sn on Cu(111),Ni(111), and Pt(111),” Physical Review B, vol. 46, no.
12. pp. 7868–7872, 1992.
[26] X. Liang, J. H. Deng, L. J. Fan, Y. W. Yang, and D.
A. Luh, “Nonalloying surface reconstructions ofultrathin Sn films on Cu(111) investigated withLEED, XPS, and photoelectron extended finestructure analysis,” Phys. Rev. B - Condens. Matter
Mater. Phys., vol. 84, no. 7, pp. 1–6, 2011.

[27] J. Deng et al., “Epitaxial growth of ultraflat stanene
with topological band inversion,” Nat. Mater., vol. 17,no. 12, pp. 1081–1086, 2018.
[28] M. A. VANHOVE J CERDA P SAUTET M.-L
BOCQUET M SALMERON, “Surface structure
determination,” Vacuum, vol. 31, no. 10–12, pp. 399–405, 1981.
[29] K. Heinz and L. Hammer, “Combined application of
LEED and STM in surface crystallography,” J. Phys.
Chem. B, vol. 108, no. 38, pp. 14579–14584, 2004.
[30] S. Mizuno, H. Tochihara, A. Barbieri, and M. A.
Van Hove, “ Completion of the structural
determination of and rationalization of the surfacestructure sequence (2×1)→(3×3)→(4×4) formed
on Cu(001) with increasing Li coverage,” Phys. Rev.
B, vol. 52, no. 16, pp. 658–662, 1995.
[31] M. A. Van Hove et al., “Automated determinationof complex surface structures by LEED,” Surf. Sci.
Rep., vol. 19, no. 3–6, pp. 191–229, 1993.
[32] J. B. Pendry, “Reliability factors for {LEED}
calculations,” J. Phys. C Solid State Phys., vol. 13, no.
5, pp. 937–944, Feb. 1980.
[33] S. Review, W. F. Chung, and Q. Cai, “Cu ( 111 )Surface Relaxation by Vleed,” no. August, 1995.

- 64 -