Since the discovery of graphene, two-dimensional materials have attracted much attention due to their unique structure and novel properties. In particular, single element two-dimensional materials have become a research hotspot because of their simple structure, easy analysis and property control. Recently, Professor Li Hui of Beijing University of chemical technology and Professor Niu TianChao of Nanjing University of science and technology cooperated to summarize the latest research progress of two-dimensional tin ene, including the basic structure and properties of tin ene, the epitaxial growth of tin ene on different substrates and the regulation of properties, etc., and discussed the existing difficulties and challenges in application.
The two-dimensional tin ene is a monolayer of the fourth group element SN. Tin tends to form α phase in epitaxial growth, and α - Sn film is a folded honeycomb structure similar to graphene (Fig. 1a). Surface functionalization and external stress can change the band gap size of snene. For example, if the surface functionalization of snene is carried out by hydrogen or halogen, the band gap of snene will expand with the increase of lattice constant (Fig. 1b). After surface functionalization, tin-ene is expected to be a quantum spin Hall insulator with a band gap of 0.3ev. Under certain compression strain, α - Sn bulk crystal can be transformed into a three-dimensional topological Dirac semimetal (TDS).
Fig. 1. (a) top view and side view of single and modified tin ene; (b) theoretical calculation of lattice constant and band gap of modified tin ene
Due to the lack of corresponding bulk crystals, it is difficult to obtain the isolated tin ene by mechanical stripping or liquid phase stripping, so suitable substrates are needed to epitaxial grow tin ene. Recently, the study of snene and its heterojunction shows that the electronic properties of snene grown on different substrates are different, and the substrate has an important influence on the properties of snene. Tin olefins have been successfully grown on Bi2Te3 (111), PbTe (111), InSb (111), Sb (111), Ag (111), Au (111) and Cu (111) substrates.
Fig. 2. (a) large area STM of single-layer tin ene on Bi2Te3 (111); (b) atom resolution STM of single-layer tin ene on Bi2Te3 (111); (c) experimental ARPES of single-layer tin ene on Bi2Te3 (111) in the direction of γ - m-m-k-m-k; (d) large area STM of single-layer tin ene on PbTe (111); (E) atom resolution STM of single-layer tin ene on PbTe (111); (f) superconducting transition temperature and tin layer thickness on PbTe Degree relation
In 2015, Professor Jia Jinfeng's research group of Shanghai Jiaotong University first epitaxed and grown a single layer of snene (Fig. 2a-b) on Bi2Te3 (111) substrate with a lattice constant of 0.44nm. The ARPES spectrum measured in the experiment proved the weak coupling between snene and Bi2Te3 (111) substrate, and the electron transfer effect from snene to Bi2Te3 (111) substrate (Fig. 2C). Although the lattice of SNE and Bi2Te3 match, the valence band of SNE is hybridized with the conduction band of the substrate to form a metal interface state. Therefore, it is very important to find a suitable substrate which can stabilize and restore the intrinsic properties of snene. Theoretical calculations show that the epitaxial tin ene on PbTe (111) substrate is quantum spin Hall insulator. The team of Professor Xue Qikun of Tsinghua University successfully obtained the epitaxial growth of tin ene on PbTe (111) substrate by low-temperature molecular beam epitaxy (Fig. 2d-e), and with the increase of the number of tin ene layers, the tin ene showed superconductivity (Fig. 2f).
Fig. 3. (a) wetting layer at the initial stage of Sn deposition on InSb (111); (b) single layer of tin ene appears on the wetting layer after increasing covering amount and annealing at 493k; (c) ARPES spectrum of single layer of tin ene on InSb (111) before and after K doping; (d) STM diagram of atomic resolution of single layer of tin ene on sb (111); (E) left: Calculation of energy band structure of tin ene with spin orbit coupling including three layers of single layer of sb (111), Right: Calculation of the density of states of tin ene
InSb (111) has good lattice matching and large band gap, which is another suitable substrate for the growth of snene (Fig. 3a). The scanning tunneling micrograph of SNE on InSb (111) shows a band gap of 0.2eV (Fig. 3b). In another study, ARPES spectra showed a band gap of 0.29 EV, which is larger than the theoretical predicted band gap (Fig. 3C). In addition, semi metallic sb (111) also has the possibility of epitaxial phosphorene growth. The lattice constant (0.43 ± 0.01 nm) of sn-ene epitaxial grown on sb (111) is slightly smaller than that predicted by theory (0.468 nm), indicating that sb (111) has a certain compressive stress on sn-ene, thus adjusting its properties (Fig. 3D). Tin ene becomes metallic on sb (111), but there is a band gap of about 0.2 EV at point K (Fig. 3e).
In addition to considering the electronic properties of tin ene, another key problem is how to achieve large-scale and high-quality preparation of tin ene. Tin can form stable alloys with many metals, including Cu, Ag, Au, and these alloy surfaces can be used as substrates for the growth of tin ene (Fig. 4a-c). In this paper, the growth mechanism of tin on the surface of noble metal (111) and the effect of the surface alloy on tin extension are discussed. On the surface of precious metal, tin deposition on the substrate does not directly form tin ene, but first forms surface alloy, and then grows tin ene on the alloy surface (Fig. 4d-e). Under certain conditions, for example, the deposition of tin on a 200K Cu (111) substrate can avoid the formation of surface alloy and directly form a uniform Sn film (Fig. 4F).
Fig. 4. (a) STM of ag2sn (111); (b) STM of au2sn (111); (c) STM of cu2sn (111); (d) STM of snene on ag2sn (111); (E) STM of snene on au2sn (111); (f) STM of snene on Cu (111) substrate
Under suitable strain, multilayer α - Sn can be transformed into TDS. The unique electronic structure of TDS can not only produce many nontrivial properties, but also be the adjacent states of various quantum states. Ideally, the TDS state can be achieved by adjusting the spin orbit coupling strength to a quantum critical point from a conventional insulator to a topological insulator (Fig. 5a). Therefore, TDS material is an important platform for the study of quantum phase transition. The topological phase of α - Sn is due to its large spin orbit coupling, while the substrate affects the electronic properties of snene. The band gap of α - Sn is largely affected by the applied plane stress (Fig. 5b).
Figure 5. (a) the model describes the generation of TDS state at the quantum critical point during the transition from regular insulator to topological insulator; (b) the relationship between band gap and strain of α - Sn
The interesting properties of tin ene provide many possibilities for its application, but at present, most of them are theoretical predictions. Recent experiments on tin ene and other two-dimensional materials show that the theoretical calculation is helpful to predict the suitable substrate. There are still many key problems in the application and integration of devices about the application of tin ene, so we need to understand the growth mechanism details of different substrates more comprehensively, which is the key to realize the controllability and repeatability of manufacturing methods in practical applications.
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