Atomic scale microstructure and properties of Se-deficient two-dimensional MoSe2.
We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe2-x by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during in situ removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe2 suggest routes toward engineering the properties of 2D transition metal dichalcogenides by deviations from the stoichiometric composition.
†Group of Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, University of Ulm, 89081 Ulm, Germany.
Ossi
O
Lehtinen
‡COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 Aalto, Finland.
Hannu-Pekka
HP
Komsa
§Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Artem
A
Pulkin
⊥Electrical Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Michael Brian
MB
Whitwick
⊥Electrical Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Ming-Wei
MW
Chen
†Group of Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, University of Ulm, 89081 Ulm, Germany.
Tibor
T
Lehnert
†Group of Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, University of Ulm, 89081 Ulm, Germany.
Michael J
MJ
Mohn
§Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Oleg V
OV
Yazyev
⊥Electrical Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Andras
A
Kis
†Group of Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, University of Ulm, 89081 Ulm, Germany.
Ute
U
Kaiser
∥Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 Aalto, Finland.
Arkady V
AV
Krasheninnikov
2D TMD; CVD; DFT; HRTEM; MBE; defects; microstructure; stoichiometry