arXiv subjects

Jeppe Vang Lauritsen

Publications and source records attributed to Jeppe Vang Lauritsen.

4 recordsLinked to original sources

arXiv2026

Photoelectron and electron microscopy investigation of laboratory grown Mg-silicate space dust analogues

The atomic structure at the surface of interstellar silicate dust particles likely plays a key role in a variety of chemical processes occurring in star- and planet-forming regions of the interstellar medium (ISM). Here, we use the photoelectric effect to characterize, in situ, the local chemical structure of Mg-silicate nanoparticulate films, and ex situ electron microscopy to trace changes to morphology resulting from different starting compositions. Nanoparticulate films are prepared via co-deposition of Si, Mg and O atoms on a graphitic substrate under ultra-high vacuum (UHV) and are characterised with X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure measurements. Analysis, supported by density functional theory calculations, shows that adjusting the Mg-to-Si ratio from 3.5 to 1.8 changes particle composition from a mixture of MgO and Mg-silicate towards predominantly Mg-silicate. Annealing in UHV also pushes the composition towards Mg-silicate, however, non-stoichiometric chemical motifs are always observed, presenting a distribution of potentially catalytically active sites. Electron microscopy images show composition-dependent film morphologies that restructure differently upon thermal annealing. The Mg-silicate nanoparticulate films presented here are characterised as a 2D version of ISM analogue dust particles prepared via laser ablation and are therefore a more readily suitable model for surface science investigations into the catalytic properties of interstellar Mg-silicate dust.

astro-ph.GA↗

arXiv2018

Size-dependent phase transitions in MoS2 nanoparticles controlled by a metal substrate

Nanomaterials based on MoS2 are remarkably versatile; MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS2 nanoparticles (NPs) are catalytically active, and extended MoS2 systems show different character (direct or indirect gap semiconducting, or metallic) depending on their thickness and crystallographic phase. In this work, we show how particle size and interaction with a metal surface affect the stability and properties of different MoS2 NPs and the resulting phase diagrams. By means of calculations based on the Density Functional Theory (DFT), we address how support interactions affect MoS2 nanoparticles of varying size, composition, and structure. We demonstrate that interaction with Au modifies the relative stability of the different nanoparticle types so that edge terminations and crystallographic phases that are metastable for free-standing nanoparticles and monolayers are expressed in the supported system. These support-effects are strongly size-dependent due to the mismatch between Au and MoS2 lattices, which explains experimentally observed transitions in the structural phases for supported MoS2 NPs. Accounting for vdW interactions and the contraction of the Au(111) surface underneath the MoS2 is further found to be necessary for quantitatively reproducing experimental results. This work demonstrates how the properties of nanostructured MoS2 and similar layered systems can be modified by the choice of supporting metal.

cond-mat.mtrl-sci↗

arXiv2015

Growth and electronic structure of epitaxial single-layer WS$_2$ on Au(111)

Large-area single-layer WS$_2$ is grown epitaxially on Au(111) using evaporation of W atoms in a low pressure H$_2$S atmosphere. It is characterized by means of scanning tunneling microscopy, low-energy electron diffraction and core-level spectroscopy. Its electronic band structure is determined by angle-resolved photoemission spectroscopy. The valence band maximum at $\bar{K}$ is found to be significantly higher than at $\barΓ$. The observed dispersion around $\bar{K}$ is in good agreement with density functional theory calculations for a free-standing monolayer, whereas the bands at $\barΓ$ are found to be hybridized with states originating from the Au substrate. Strong spin-orbit coupling leads to a large spin-splitting of the bands in the neighborhood of the $\bar{K}$ points, with a maximum splitting of 419(11)~meV. The valence band dispersion around $\bar{K}$ is found to be highly anisotropic with spin-branch dependent effective hole masses of $0.40(02)m_e$ and $0.57(09)m_e$ for the upper and lower split valence band, respectively. The large size of the spin-splitting and the low effective mass of the valence band maximum make single-layer WS$_2$ a promising alternative to the widely studied MoS$_2$ for applications in electronics, spintronics and valleytronics.

cond-mat.str-el↗

arXiv2014

Electronic Structure of Epitaxial Single-Layer MoS$_2$

The electronic structure of epitaxial single-layer MoS$_2$ on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39$\pm$0.05)~eV direct band gap at $\bar{K}$ with the valence band top at $\barΓ$ having a significantly higher binding energy than at $\bar{K}$. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at $\bar{K}$ is found to be lifted by the spin-orbit interaction, leading to a splitting of (145$\pm$4)~meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the strength of the potassium doping is $k$-dependent, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

cond-mat.mtrl-sci↗