Phase Separation Prevents the Synthesis of VBi₂Te₄ by Molecular Beam Epitaxy

Intrinsic magnetic topological insulators (IMTIs) have a non-trivial band topology in combination with magnetic order. This potentially leads to fascinating states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. One of the theoretically predicted IMTIs is VBi2Te4, but experimental evidence of this material is lacking so far. Here, we report on our attempts to synthesise VBi2Te4 by molecular beam epitaxy (MBE). X-ray diffraction reveals that in the thermodynamic phase space reachable by MBE, there is no region where VBi2Te4 is stably synthesised. Moreover, scanning transmission electron microscopy shows a clear phase separation to Bi2Te3 and VTe2 instead of the formation of VBi2Te4. We suggest the phase instability to be due to either the large lattice mismatch between VTe2 and Bi2Te3 or the unfavourable valence state of vanadium.</p

Adding magnetism to Bi2Te3/Bi2 multilayers

Bi2Te3 is a 3D topological insulator with a single Dirac cone on the surface [1]. This surface state can be gapped by means of magnetic doping, resulting in the quantum anomalous hall state [2]. Recently, there has been an increasing interest in natural superlattices containing Bi2Te3, such as Bi4Te3 [3]. This compound consists of alternating Bi2 and Bi2Te3 layers. The exact topological nature of these compounds is still under debate. Here we fabricated the Bi4Te3 thin films with molecular beam epitaxy and characterized the films with X-ray diffraction, transmission electron microscopy and electrical transport measurements. We show that these films exhibit weak antilocalization, hinting towards the presence of a 2D surface state. Furthermore, we magnetically doped the films with V and observed signatures of magnetism in electrical transport measurements. [1] Y. L. Chen, et al., Science, 325, 5937. (2009). [2] C.-Z. Chang, et al, Science, 340, 6129. (2013)[3] D. Nabok, et al, Phys. Rev. Mat. 6, 034204. (2022)<br/

X-ray standing wave characterization of the strong metal–support interaction in Co/TiOx model catalysts

The strong metal–support interaction (SMSI) is a phenomenon observed in supported metal catalyst systems in which reducible metal oxide supports can form overlayers over the surface of active metal nanoparticles (NPs) under a hydrogen (H2) environment at elevated temperatures. SMSI has been shown to affect catalyst performance in many reactions by changing the type and number of active sites on the catalyst surface. Laboratory methods for the analysis of SMSI at the nanoparticle-ensemble level are lacking and mostly based on indirect evidence, such as gas chemisorption. Here, we demonstrate the possibility to detect and characterize SMSIs in Co/TiOx model catalysts using the laboratory X-ray standing wave (XSW) technique for a large ensemble of NPs at the bulk scale. We designed a thermally stable MoNx/SiNx periodic multilayer to retain XSW generation after reduction with H2 gas at 600°C. The model catalyst system was synthesized here by deposition of a thin TiOx layer on top of the periodic multilayer, followed by Co NP deposition via spare ablation. A partial encapsulation of Co NPs by TiOx was identified by analyzing the change in Ti atomic distribution. This novel methodological approach can be extended to observe surface restructuring of model catalysts in situ at high temperature (up to 1000°C) and pressure (≤3 mbar), and can also be relevant for fundamental studies in the thermal stability of membranes, as well as metallurgy

Nanoscale work function contrast induced by decanethiol self-assembled monolayers on Au(111)

In this paper, we obtain maps of the spatial tunnel barrier variations in self-assembled monolayers of organosulfurs on Au(111). Maps down to the sub-nanometer scale are obtained by combining topographic scanning tunneling microscopy images with dI/dz spectroscopy. The square root of the tunnel barrier height is directly proportional to the local work function and the dI/dz signal. We use ratios of the tunnel barriers to study the work function contrast in various decanethiol phases: the lying-down striped β phase, the dense standing-up φ phase, and the oxidized decanesulfonate λ phase. We compare the induced work function variations too: the work function contrast induced by a lying-down striped phase in comparison to the modulation induced by the standing-up φ phase, as well as the oxidized λ phase. By performing these comparisons, we can account for the similarities and differences in the effects of the mechanisms acting on the surface and extract valuable insights into molecular binding to the substrate. The pillow effect, governing the lowering of the work function due to lying-down molecular tails in the striped low density phases, seems to have quite a similar contribution as the surface dipole effect emerging in the dense standing-up decanethiol phases. The dI/dz spectroscopy map of the nonoxidized β phase compared to the map of the oxidized λ phase indicates that the strong binding of molecules to the substrate is no longer present in the latter.Fil: Tsvetanova, Martina. University of Twente; Países BajosFil: Oldenkotte, Valent J. S.. University of Twente; Países BajosFil: Bertolino, María Candelaria. University of Twente; Países Bajos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Gao, Yuqiang. University of Twente; Países BajosFil: Siekman, Martin H.. University of Twente; Países BajosFil: Huskens, Jurriaan. University of Twente; Países BajosFil: Zandvliet, Harold J. W.. University of Twente; Países BajosFil: Sotthewes, Kai. University of Twente; Países Bajo

Collective phenomena on metallic surfaces studied with scanning tunneling microscopy and low energy electron microscopy

This thesis is devoted to the study of collective phenomena on metallic surfaces. Great part of the research was done on molecular self-assembled monolayers (SAMs), which we studied with scanning tunnelling microscopy (STM). These films are very exciting to measure at the nanoscale. We focused on the dynamics at the boundary between molecular phases, on mapping the nanoscale properties of phases, and on trying to understand the mechanisms for the SAM formation by exploring less-well studied surfaces and molecular coverage regimes. Both Au(111) and Au(001) substrates were deployed. We used the decanethiol molecule as the SAM building block. When STM was not sufficient, density functional theory (DFT) calculations were also performed. Our analysis on the phase boundary dynamics was also incorporated in a more general anisotropic triangular lattice framework. Additionally, we wished to extend our collective phenomena framework by deploying low energy electron microscopy (LEEM) to study dynamics on the surface. However, SAMs are prone to electron-induced damage. Therefore, we alternatively looked at the Bi-modified Ni(111) surface. Bi deposition on the Ni(111) and Cu(111) surfaces has already shown to lead to a variety of dynamic processes. Therefore, the Bi/Ni system was a perfect playground for our study, with the added value that we can further expand our knowledge of its surface phase diagram. We obtained an ultrathin 3-fold symmetric Bi wetting layer and demonstrated a reproducible phase transition between an ordered and a disordered state, which occurs when heating-up the system. Upon cooling down, the original structure could be restored

Free energy of domain walls and order-disorder transition in a triangular lattice with anisotropic nearest-neighbor interactions

We have derived exact expressions for the domain wall free energy along the three high-symmetry directions of a triangular lattice with anisotropic nearest-neighbor interactions. The triangular lattice undergoes an order-disorder phase transition at a temperature Tc given by e-(ϵ1+ϵ2)/2kTc+e-(ϵ2+ϵ3)/2kTc+e-(ϵ3+ϵ1)/2kTc=1, where ϵ1, ϵ2, ϵ3 are the nearest-neighbor interaction energies, and ϵ1+ϵ2>0, ϵ2+ϵ3>0, ϵ3+ϵ1>0. Finally, we have derived expressions for the thermally induced meandering of the domain walls at temperatures below the phase transition temperature. We show how these expressions can be used to extract the interaction energies of two-dimensional systems with a triangular lattice

Adding magnetism to Bi2Te3/Bi2 multilayers

Bi2Te3 is a 3D topological insulator with a single Dirac cone on the surface [1]. This surface state can be gapped by means of magnetic doping, resulting in the quantum anomalous hall state [2]. Recently, there has been an increasing interest in natural superlattices containing Bi2Te3, such as Bi4Te3 [3]. This compound consists of alternating Bi2 and Bi2Te3 layers. The exact topological nature of these compounds is still under debate. Here we fabricated the Bi4Te3 thin films with molecular beam epitaxy and characterized the films with X-ray diffraction, transmission electron microscopy and electrical transport measurements. We show that these films exhibit weak antilocalization, hinting towards the presence of a 2D surface state. Furthermore, we magnetically doped the films with V and observed signatures of magnetism in electrical transport measurements. [1] Y. L. Chen, et al., Science, 325, 5937. (2009). [2] C.-Z. Chang, et al, Science, 340, 6129. (2013) [3] D. Nabok, et al, Phys. Rev. Mat. 6, 034204. (2022

Low coverage disordered decanethiol monolayers on Au(001): A conjecture regarding the formation of Au-adatom-molecule complexes

We present a scanning tunneling microscopy study of decanethiol on Au(001) in the low coverage regime. As expected, the hex reconstruction is lifted, however, no ordered decanethiol phases form. We observe large areas free of Au islands and covered with a disordered decanethiol phase. I(t) spectroscopy measurements suggest that this disordered phase is dynamic and most likely comprises diffusing Au adatoms, decanethiol molecules, and/or Au-adatom-decanethiol molecule complexes. We have performed density functional theory calculations and show that the activation barrier for diffusion is lower when Au-adatom-molecule complexes are considered in comparison to the case of bare molecule. These findings suggest that although no vacancy pits form on Au(001), Au-adatoms expelled during the lifting of the hex reconstruction may be still important for the diffusion of thiol molecules on this surface

Structural Stability of Physisorbed Air-Oxidized Decanethiols on Au(111)

Kap, Ozlem/0000-0002-7609-4272; Zandvliet, Harold/0000-0001-6809-139X; Varlikli, Canan/0000-0002-1081-0803WOS: 000538758700027We have studied the dynamic behavior of decanethiol and air-oxidized decanethiol self-assembled monolayers (SAMs) on Au(111) using time-resolved scanning tunneling microscopy at room temperature. The air-oxidized decanethiols arrange in a lamellae-like structure leaving the herringbone reconstruction of the Au(111) surface intact, indicating a rather weak interaction between the molecules and the surface. Successive STM images show that the air-oxidized molecules are structurally more stable as compared to the nonoxidized decanethiol molecules. This is further confirmed by performing current-time traces with the feedback loop disabled at different locations and at different molecular phases. Density function theory calculations reveal that the diffusion barrier of the physisorbed oxidized decanethiol molecule on Au(111) is about 100 meV higher than the diffusion barrier of a chemisorbed Au-decanethiol complex on Au(111). A two-dimensional activity map of individual current-time traces performed on the air-oxidized decanethiol phase reveals that all the dynamic events take place within the vacancy lines between the air-oxidized decanethiols. These results reveal that the oxidation of thiols provides a pathway to produce more robust and stable self-assembled monolayers at ambient conditions