TY  - JOUR
A1  - Sorgenfrei, Nomi
A1  - Giangrisostomi, Erika
A1  - Jay, Raphael Martin
A1  - Kühn, Danilo
A1  - Neppl, Stefan
A1  - Ovsyannikov, Ruslan
A1  - Sezen, Hikmet
A1  - Svensson, Svante
A1  - Föhlisch, Alexander
T1  - Photodriven transient picosecond top-layer semiconductor to metal phase-transition in p-doped molybdenum disulfide
JF  - Advanced materials
N2  - Visible light is shown to create a transient metallic S-Mo-S surface layer on bulk semiconducting p-doped indirect-bandgap 2H-MoS2. Optically created electron-hole pairs separate in the surface band bending region of the p-doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H-semiconductor to 1T-metal phase-transition of the surface layer. Electron-phonon coupling of the indirect-bandgap p-doped 2H-MoS2 enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p-doped 2H-MoS2. In particular, light-induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H-MoS2. Generally, it opens up for a way to control not only the surface of p-doped 2H-MoS2 but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time-resolved electron spectroscopy for chemical analysis with photon-energy-tuneable synchrotron radiation.
KW  - catalysis
KW  - dichalcogenides
KW  - hydrogen evolution reaction
KW  - phase transitions
KW  - photoelectron spectroscopy
Y1  - 2021
U6  - https://doi.org/10.1002/adma.202006957
SN  - 0935-9648
SN  - 1521-4095
VL  - 33
IS  - 14
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Haverkamp, Robert
A1  - Sorgenfrei, Nomi
A1  - Giangrisostomi, Erika
A1  - Neppl, Stefan
A1  - Kühn, Danilo
A1  - Föhlisch, Alexander
T1  - Directional charge delocalization dynamics in semiconducting 2H-MoS2 and metallic 1T-LixMoS2
JF  - Scientific reports
N2  - The layered dichalcogenide MoS2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS2 to metallic 1T-LixMoS2. With the core hole clock approach at the S L1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane () and out-of-plane (perpendicular to) for 2H-MoS2 as tau 2H,=0.38 +/- 0.08 fs and tau 2H,perpendicular to =0.33 +/- 0.06 fs and for 1T-LixMoS2 as tau 1T,=0.32 +/- 0.12 fs and tau 1T,perpendicular to =0.09 +/- 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-LixMoS2 by lithiation accelerates the in-plane charge transfer by a factor of similar to 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of similar to 3.7 upon lithiation occurs due to S-Li coupling.
Y1  - 2021
U6  - https://doi.org/10.1038/s41598-021-86364-2
SN  - 2045-2322
VL  - 11
IS  - 1
PB  - Macmillan Publishers Limited, part of Springer Nature
CY  - London
ER  -