arXiv subjects

Jan Ingo Flege

Publications and source records attributed to Jan Ingo Flege.

6 recordsLinked to original sources

arXiv2025

Rubidium intercalation in epitaxial monolayer graphene

Alkali metal intercalation of graphene layers has been of particular interest due to potential applications in electronics, energy storage, and catalysis. Rubidium (Rb) is one of the largest alkali metals and the one less investigated as intercalant. Here, we report a systematic investigation, with a multi-technique approach, of the phase formation of Rb under epitaxial monolayer graphene on SiC(0001). We explore a wide phase space with two control parameters: the Rb density (i.e., deposition time) and sample temperature (i.e., room- and low-temperature). We reveal the emergence of $(2 \times 2)$ and $(\sqrt{3} \times \sqrt{3})$R30° structures formed by a single alkali metal layer intercalated between monolayer graphene and the interfacial C-rich reconstructed surface, also known as buffer layer. Rb intercalation also results in a strong n-type doping of the graphene layer. Progressively annealing to high temperatures, we first reveal diffusion of Rb atoms which results in the enlargement of intercalated areas. As desorption sets in, intercalated regions progressively shrink and fragment. Eventually, at approximately 600°C the initial surface is retrieved, indicating the reversibility of the intercalation process.

cond-mat.mtrl-sci↗

arXiv2021

Identifying the mechanism of single-domain single-layer MoS2 growth on Au(111)

The nucleation and growth of single-layer molybdenum disulfide single domain islands is investigated by in situ low-energy electron microscopy. We study the growth of micron-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both island and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free single-domain single-layer islands of several micrometers in size, which is likely transferable to other transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2020

The Breakdown of Mott Physics at VO$_2$ Surfaces

Transition metal oxides such as vanadium dioxide (VO$_2$), niobium dioxide (NbO$_2$), and titanium sesquioxide (Ti$_2$O$_3$) are known to undergo a temperature-dependent metal-insulator transition (MIT) in conjunction with a structural transition within their bulk. However, it is not typically discussed how breaking crystal symmetry via surface termination affects the complicated MIT physics. Using synchrotron-based x-ray spectroscopy, low energy electron diffraction (LEED), low energy electron microscopy (LEEM), transmission electron microscopy (TEM), and several other experimental techniques, we show that suppression of the bulk structural transition is a common feature at VO$_2$ surfaces. Our density functional theory (DFT) calculations further suggest that this is due to inherent reconstructions necessary to stabilize the surface, which deviate the electronic structure away from the bulk d$^1$ configuration. Our findings have broader ramifications not only for the characterization of other "Mott-like" MITs, but also for any potential device applications of such materials.

cond-mat.str-el↗

arXiv2020

Massively strained VO2 thin film growth on RuO2

Strain engineering vanadium dioxide thin films is one way to alter this material's characteristic first order transition from semiconductor to metal. In this study we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78 % occurring in the VO$_2$/RuO$_2$ system along the c axis of the rutile structure. We have grown VO$_2$ thin films on single domain RuO$_2$ islands of two distinct surface orientations by atomic oxygen-supported reactive MBE. These films were examined by spatially resolved photoelectron and x-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO$_2$ films to grow indeed fully strained on RuO$_2$(110), exhibiting a previously unreported ($2\times2$) reconstruction. On TiO$_2$(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO$_2$(100) on the other hand, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions.

cond-mat.mtrl-sci↗

arXiv2018

Growth and Structure of Singly-Oriented Single-Layer Tungsten Disulfide on Au(111)

We present a complete characterisation at the nanoscale of the growth and structure of single-layer tungsten disulfide (WS$_2$) epitaxially grown on Au(111). Following the growth process in real time with fast x-ray photoelectron spectroscopy, we obtain a singly-oriented layer by choosing the proper W evaporation rate and substrate temperature during the growth. Information about the morphology, size and layer stacking of the WS$_2$ layer were achieved by employing x-ray photoelectron diffraction and low-energy electron microscopy. The strong spin splitting in the valence band of WS$_2$ coupled with the single-orientation character of the layer make this material the ideal candidate for the exploitation of the spin and valley degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2013

Adsorbate-Mediated Growth of Rare-Earth Oxides on Silicon

Ultrathin cerium oxide films have been deposited on chlorine, gallium, and silver passivated Si(111) by reactive molecular beam epitaxy in a comparative study. The crystallinity of these films has been characterized by x-ray standing waves while the oxidation state of the rare-earth oxide (REOx) films and the chemical interface composition have been revealed by hard x-ray photoelectron spectroscopy. The use of Cl as passivating agent results in the epitaxial growth of highly crystalline REOx films with the RE metal in the 3+ oxidation state while effectively suppressing silicate and silicon oxide formation at the interface. In contrast, Ga and Ag preadsorption yield films of inferior quality, in the case of Ag of even lower crystallinity than without passivation. Further investigations show that Cl-passivation also results in ultrathin lanthana films of superior quality, which facilitate the growth of well-ordered ceria on lanthana REOx multilayers.

cond-mat.mtrl-sci↗