Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc Structure determination of ultra-flat stanene on Cu(111) using low energy electron diffraction Rezwan Ahmed ⁎ , Takeshi Nakagawa, Seigi Mizuno Department of Molecular and Material Sciences, Kyushu University, Kasuga, Kasugakoen, 6-1, Fukuoka, 816-8580, Japan ARTICLEINFO Keywords: Stanene Cu(111) LEED 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 (LEED). The structural analysis of Cu{111}-p(2 × 2)-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 Sn atoms forming a honeycomb structure. Our results using quantitative LEED 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. 1. Introduction The two-dimensional (2D) group-14 materials of carbon, silicon, ger- manium, tin, and lead which are termed respectively as graphene, silicene, germanene, stanene, and plumbene—have a single layer of atoms orga- nized in a honeycomb like lattice that possesses striking physical proper- ties due to its unique structure [1–3]. Some of these materials are pre- dicted to exhibit band gap due to spin orbit coupling, which makes them 2D topological insulators [4–9]. Graphene has been extensively studied in the past for its relative ease of fabrication; however, in the last few years, experimental studies of other 2D materials as we move down the same group have taken the limelight due to their stronger spin-orbit coupling, which results in additional intriguing properties in the field of solid state physics [9–12]. Among them, stanene is considered to be highly promising due to its exclusive enriched properties such as topological super- conductivity, a near-room-temperature quantum anomalous Hall effect, giant magnetoresistance, and enhanced thermoelectricity [7,8,11,13,14]. Moreover, the low buckling structure of stanene is predicted to be more stable and also expected to represent large gap in quantum spin hall state as suggested by density functional (DFT) calculations [9,15]. The suc- cessful growth and realization of 2D materials largely depends upon per- fect selection of underlying substrates, as proper interaction between the substrate and 2D material is necessary. The growth of stanene was elusive, and it has only been in recent years that there have been several reports of successful fabrication of stanene in different substrates, gradually leading to the realization of many of these interesting properties. One of the first experimental realizations of stanene was reported by Zhu et al., who grew it on Bi 2 Te 3 substrate using molecular beam epitaxy and the growth adopts Vollmer-Weber (island) growth mode [16]. Later on, stanene was further successfully reported on substrates like Sb(111), InSb(111), Bi(111), Au(111), Ag(111), and PbTe(111) [17–22]. Besides grown on diverse substrates, the other distinct structural features of as grown stanene for each of these cases is the presence of different degrees of buckling between the Sn atoms. The DFT calculations suggest that stanene becomes more stable as buckling is reduced [9]. But due to the weak interaction of π bonds, the honeycomb structure is buckled, al- lowing π−σ interactions that stabilize the structure [23,24]. The adsorption of Sn on Cu(111) substrate in previous studies was found to form a surface alloy at or above room temperature due to the high solubility of Sn on the Cu surface [25,26]. The realization of 2D tin atoms on Cu(111) substrate at low temperature was first addressed by Xihui et al., who proposed single tin atom per unit cell of p(2 × 2) structure [27]. However, recently Deng et al. have observed the epi- taxial growth of stanene on Cu(111) having honeycomb structure and an unusually ultra-flat zero-buckling geometry using STM, which is supported by DFT calculations [28]. Interestingly, the ultra-flat stanene represents topological band inversion which may pave opportunities for exploring device applications and understanding fundamentals of to- pological physics and thus it is important to conclusively determine its structure. Besides, the growth of ultra-flat stanene on Cu(111) also suggests the importance of the underlying interaction of substrates with stanene in stabilizing the ultra-flat 2D structure, which has not been reported on substrates other than Cu(111) to date. However, the in- teraction and relaxation of the underlying substrate was not in- vestigated in previous reports, and can be an important factor for un- derstanding the overlayer structure with a zero buckling nature. https://doi.org/10.1016/j.susc.2019.121498 Received 4 July 2019; Received in revised form 3 September 2019; Accepted 10 September 2019 ⁎ Corresponding author. E-mail address: ahmed.rezwan.447@s.kyushu-u.ac.jp (R. Ahmed). Surface Science 691 (2020) 121498 Available online 10 September 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved. T