Obtaining pure group IV 2D films on well-behaved substrates is at present a major goal in materials science and of great interest for the associated industries. This goal still represents a challenge in surface science because often these materials tend to form alloys. As a consequence, some of the proposed 2D films resulted in topics of controversy regarding the top-layer elemental composition and interpretation of the honeycomb patterns measured by STM. Very recently, germanene on Al(111) was proposed to be a system having a larger gap than silicene and a quantum-spin Hall effect. This system was studied by several techniques including scanning tunnel microscopy, low-energy electron diffraction, photoemission, and density functional theory. None of the techniques used until now have the capability to detect unambiguously the presence of substrate atoms within the ultrathin film (i.e., separated from the corresponding substrate), thus leaving open the question of the composition or purity of the layer. Here we follow previous guidelines to grow a Ge film on Al(111) with the expected 3 × 3 arrangement that was assumed to be characteristic of germanene, and then we study in situ the properties of the films with ion scattering and recoiling spectrometry, a technique particularly suited for determining the elemental composition of the last surface layer. Our results unambiguously show the formation of a mixture of well-ordered Ge and Al atoms for all of the temperatures and conditions tested, in clear disagreement with the pure single germanene layer proposed in previous works. These conclusions led us to investigate by DFT calculations other possible structures compatible with our present results and the previously reported ones. The most favorable alloyed structures obtained by DFT were then compared with new I–V low-energy electron diffraction curves, and from this comparison, a top surface model composed of five Ge atoms and three Al atoms is proposed to replace the germanene model.
The synthesis of antimonene and related 2D Sb films on top of metallic substrates has recently become a very active subject due to the strong spin−orbit coupling and the envisaged topological properties of this novel material. Gold has been used as a standard substrate for other 2D films, but in the case of Sb, no success has yet been reported mainly due to the formation of AuSb 2 surface alloy. In this work, we have used low energy electron diffraction, time-of-flight direct recoil spectroscopy, X-ray photoemission spectroscopy, and scanning tunneling microscopy together with density functional theory calculations to provide a full characterization of the growth and the structural properties of Sb deposited on Au(111). We show that the controlled deposition of Sb on Au(111) at room temperature results in a surface alloy at submonolayer coverage, but around 1 ML a pure 2D Sb film with a ( ) 3 0 1 2 commensurate structure is achieved. The obtained 2D material has high stability and can be produced not only by direct deposition of Sb over the clean Au substrate but also by starting from a thicker Sb film followed by annealing of the sample at about 500 K, which is easier and reliable for potential applications.
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