arXiv subjects

Tomáš Skála

Publications and source records attributed to Tomáš Skála.

6 recordsLinked to original sources

arXiv2024

Growth of bilayer stanene on a magnetic topological insulator aided by a buffer layer

Stanene, a two-dimensional counterpart to graphene, has the potential to exhibit novel quantum phenomena when grown on a magnetic topological insulator (MTI). This work demonstrates the formation of up to bilayer stanene on 30\% Sb-doped MnBi$_{2}$Te$_4$ (MBST), a well known MTI, albeit with a buffer layer (BL) in between. Angle-resolved photoemission spectroscopy (ARPES), when combined with density functional theory (DFT), reveals stanene related bands such as two hole-like bands and an inverted parabolic band around the $\overlineΓ$ point. An outer hole-like band traverses the Fermi level (\ef) and gives rise to a hexagonal Fermi surface, showing that stanene on MBST is metallic. In contrast, a bandgap of 0.8 eV is observed at the $\overline{K}$ point. We find that DFT shows good agreement with ARPES only when the BL and hydrogen passivation of the top Sn layer are considered in the calculation. Scanning tunneling microscopy (STM) establishes the honeycomb buckled structure of stanene. A stanene-related component is also detected in the Sn $d$ core level spectra, in addition to a BL-related component. The BL, which forms because of the chemical bonding between Sn and the top two layers of MBST, has an ordered crystal lattice with random anti-site defects. The composition of the BL is estimated to be Sn:Te:Bi/Sb $\approx$ 2:1:1 from x-ray photoelectron spectroscopy. Low energy electron diffraction shows that the lattice constant of stanene is marginally larger than that of MBST, and the STM result aligns with this. The BL bridges this disparity and provides a platform for stanene growth.

cond-mat.mtrl-sci↗

arXiv2024

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain a desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4 carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while mediating energy level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. DFT calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

cond-mat.mtrl-sci↗

arXiv2022

Tunable Energy Level Alignment in the Multilayers of Carboxylic Acids on Silver

The precise energy level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. One of the ways is to include interlayers that mediate the energy level alignment, i.e., charge injection layers (CILs). Here we introduce the monolayer thick CILs based on the aromatic carboxylic acids that can induce the energy level shift in the subsequent layers by up to 0.8 eV. By gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy level shift in the range of 0.5 eV. We reveal that the position of both the work function and energy-level position in the CIL increases linearly with the density of induced dipoles. The energy level position of subsequent layers changes in the same way as the CIL. Our results thus connect the energy alignment quantities, i.e., energy level positions of both CIL and subsequent layers and sample work function. The high tunability would allow precise tuning of the active layers deposited on the CIL, which marks the path towards efficient charge injection layers on metal electrodes.

cond-mat.mtrl-sci↗

arXiv2022

Adatom and nanoparticle dynamics on single-atom catalyst substrates

Single-atom catalysts represent an essential and ever-growing family of heterogeneous catalysts. Recent studies indicate that besides the valuable catalytic properties provided by single-atom active sites, the presence of single-atom sites on the catalyst substrates may significantly influence the population of supported metal nanoparticles coexisting with metal single atoms. Treatment of ceria-based single-atom catalysts in oxidizing or reducing atmospheres was proven to provide precise experimental control of the size of the supported Pt nanoparticles, and, correspondingly, control of catalyst activity and stability. Based on dedicated surface science experiments, ab-initio calculations and kinetic Monte-Carlo simulations we demonstrate that the morphology of Pt nanoparticle population on ceria surface is a result of a competition for Pt atoms between Pt single-atom sites and Pt nanoparticles. In oxidizing atmosphere, Pt single-atom sites provide strong bonding to single Pt atoms and Pt nanoparticles shrink. In reducing atmosphere, Pt single atom sites are depopulated and Pt nanoparticles grow. We formulate a generic model of Pt redispersion and coarsening on ceria substrates. Our model provides a unified atomic-level explanation for a variety of metal nanoparticle dynamic processes observed in single-atom catalysts under stationary or alternating oxidizing/reducing atmospheres, and allows to classify the conditions when nanoparticle ensembles on single-atom catalysts substrates can be stabilized against Ostwald ripening.

cond-mat.mtrl-sci↗

arXiv2022

The Role of Phase Stabilization and Surface Orientation in 4,4'-Biphenyl-Dicarboxylic Acid Self-Assembly and Transformation on Silver Substrates

Molecular functionalization of nanoparticles and metallic substrates can be used to tune their properties for specific applications. However, polycrystalline substrates and nanoparticles exhibit surface planes with distinct crystallographic orientations. Therefore, the development of reliable strategies for molecular functionalization requires knowledge of the role of the surface plane orientation in the growth kinetics, structure, and properties of the molecular layer. Here, we apply a multiscale analysis to investigate the self-assembly of 4,4'-biphenyl-dicarboxylic acid (BDA) on Ag(111) and critically discuss the difference to Ag(100). Whereas the structural motifs for intact and fully deprotonated BDA are similar on both surfaces, the intermediate phases comprising partially deprotonated BDA differ in the structure and chemical composition. A real-time view of the phase transformations enables us to present a generalized picture of the phase transformations between the self-assembled molecular phases on the surfaces and underline important features such as the phase stabilization of the chemical composition and the mechanism of the related burst transformation. The influence of the substrate orientation on the structure of molecular layers and phase transformations provides the necessary background for developing functionalization strategies of the substrates displaying multiple surface planes and kinetic models for the growth near thermodynamic equilibrium.

cond-mat.mtrl-sci↗

arXiv2019

Step-edge assisted large scale FeSe monolayer growth on epitaxial Bi2Se3 thin films

The interest in Fe-chalcogenide unconventional superconductors is intense after the critical temperature of FeSe was reported enhanced by more than one order of magnitude in the monolayer limit at the interface to an insulating oxide substrate. In heterostructures comprising interfaces of FeSe with topological insulators, additional interesting physical phenomena is predicted to arise e.g. in form of {\it topological superconductivity}. So far superconductive properties of Fe-chalcogenide monolayers were mostly studied by local scanning tunneling spectroscopy experiments, which can detect pseudo-gaps in the density of states as an indicator for Cooper pairing. Direct macroscopic transport properties which can prove or falsify a superconducting phase were rarely reported due to the difficulty to grow films with homogeneous material properties. Here we report on a promising growth method to fabricate continuous carpets of monolayer thick FeSe on molecular beam epitaxy grown Bi$_2$Se$_3$ topological insulator thin films. In contrast to previous works using atomically flat cleaved bulk Bi$_2$Se$_3$ crystal surfaces we observe a strong influence of the high step-edge density (terrace width about 10~nm) on MBE-grown Bi$_2$Se$_3$ substrates, which significantly promotes the growth of coalescing FeSe domains with small tetragonal crystal distortion without compromising the underlying Bi$_2$Se$_3$ crystal structure.

cond-mat.mtrl-sci↗