Atomic and electronic structures of two-dimensional layers on noble metals
Sammanfattning: Two-dimensional (2D) materials, in the form of a single atomic layer with a crystalline structure, are of interest for electronic applications. Such materials can be formed by a single element, e.g., by group IV or group V elements, or as a 2D surface alloy. As these materials consist of just a single atomic layer, they may have unique properties that are not present in the bulk. The (111) surfaces of the noble metals Ag and Au are important for the preparation of several 2D materials. To investigate the atomic and electronic structures, the following experimental techniques were used in this thesis: angle resolved photoelectron spectroscopy (ARPES), scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The 2D structures studied in this thesis include arsenene (an As analogue to graphene) and As/Ag(111), Sn/Au(111), and Te/Ag(111) surface alloys.Arsenene has been thoroughly investigated theoretically for many years and several interesting properties important for next generation electronic and optoelectronic devices have been described in the literature. This thesis presents the first experimental evidence of the formation of arsenene. A clean Ag(111) surface was exposed to arsenic in an ultra-high vacuum chamber at an elevated substrate temperature (250 to 350 °C ). The resulting arsenic layer was studied by LEED, STM and ARPES. Both LEED and STM data resulted in a lattice constant of the arsenic layer of 3.6 Å which is consistent with the formation of arsenene. A comparison between the experimental band structure obtained by ARPES and the theoretical band structure of arsenene based on density functional theory (DFT), further verified the formation of arsenene.The As/Ag(111) surface alloy was prepared by exposing clean Ag(111) to arsenic followed by heating to 400 °C. This resulted in an Ag2As surface alloy which formed by the replacement of every third Ag atom by an As atom in a periodic fashion. LEED showed a complex pattern of diffraction spots corresponding to a superposition of three domains of a reconstruction described by a unit cell. STM images revealed a surface with a striped atomic structure with ridges characterized by a local √3 × √3 structure. ARPES data showed three alloy related bands of which one can be associated with the √3 × √3 structure on the ridges. This band shows a split in momentum space around the point along the direction of a √3 × √3 surface Brillouin zone in similarity with a Ge/Ag(111) surface alloy.Sn/Au(111) surface alloys can be prepared with different periodicities. An Au2Sn phase characterized by a √3 × √3 periodicity and an Au3Sn phase with a 2 × 2 periodicity are formed containing 0.33 and 0.25 monolayer of Sn, respectively. The clean Au(111) surface itself, shows a complex reconstruction, the so called herringbone structure, that can be viewed as a zig-zag pattern of stripes described by a 22 × √3 unit cell. The replacement of Au atoms by Sn results in change of the periodicity of the herringbone structure to 26 × √3 and ≈ 26 × 2√3 for the Au2Sn and Au3Sn surface alloys, respectively. Furthermore, the local 1 × 1 periodicity of clean Au(111) is replaced by a √3 × √3 and a 2 × 2 periodicity as is clear from STM images of the respective cases. ARPES data are presented for the Au2Sn surface alloy, which reveal an electronic band structure with similarities to other striped surface alloys. In particular, the split in momentum space around the point of a √3 × √3 surface Brillouin zone is observed also for Au2Sn.A Te-Ag binary surface alloy can be formed by evaporating 1/3 monolayer of Te onto a clean Ag(111) surface followed by annealing. After this preparation, LEED showed sharp √3 × √3 diffraction spots that is evidence for a well-ordered surface layer. ARPES data revealed two distinct electronic bands that followed the √3 × √3 periodicity. One of these bands showed a small spin-split of the Rashba type. The experimental band structure was compared with the theoretical bands of several atomic models of Te induced structures on Ag(111). An excellent fit was obtained for a Te-Ag surface alloy with a planar honeycomb structure, with one Te and one Ag atom in the unit cell. A semiconducting electronic structure of the Te-Ag surface alloy was inferred from the ARPES data in agreement with the ≈0.7 eV band gap predicted by the DFT calculations.
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