Anisotropy of the Electric Field Gradient in Two-Dimensional α-MoO<sub>3</sub> Investigated by <sup>57</sup>Mn(<sup>57</sup>Fe) Emission Mössbauer Spectroscopy
Juliana Schell,
Dmitry Zyabkin,
Krish Bharuth-Ram,
João N. Gonçalves,
Carlos Díaz-Guerra,
Haraldur P. Gunnlaugsson,
Aitana Tarazaga Martín-Luengo,
Peter Schaaf,
Alberta Bonanni,
Hilary Masenda,
Thien Thanh Dang,
Torben E. Mølholt,
Sveinn Ólafsson,
Iraultza Unzueta,
Roberto Mantovan,
Karl Johnston,
Hafliði P. Gíslason,
Petko B. Krastev,
Deena Naidoo,
Bingcui Qi
Affiliations
Juliana Schell
European Organization for Nuclear Research (CERN), 1211 Geneva, Switzerland
Dmitry Zyabkin
Chair Materials for Electrical Engineering and Electronics, Institute of Materials Science and Engineering, Institute of Micro and Nanotechnologies MacroNano<sup>®</sup>, TU Ilmenau, Gustav-Kirchhoff-Strasse 5, 98693 Ilmenau, Germany
Krish Bharuth-Ram
School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4001, South Africa
João N. Gonçalves
CICECO—Aveiro Institute of Materials and Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Carlos Díaz-Guerra
Departmento Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
Haraldur P. Gunnlaugsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
Aitana Tarazaga Martín-Luengo
Quantum Materials Group, Institute for Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria
Peter Schaaf
Chair Materials for Electrical Engineering and Electronics, Institute of Materials Science and Engineering, Institute of Micro and Nanotechnologies MacroNano<sup>®</sup>, TU Ilmenau, Gustav-Kirchhoff-Strasse 5, 98693 Ilmenau, Germany
Alberta Bonanni
Quantum Materials Group, Institute for Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria
Hilary Masenda
School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa
Thien Thanh Dang
Institute for Materials Science and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany
Torben E. Mølholt
European Organization for Nuclear Research (CERN), 1211 Geneva, Switzerland
Sveinn Ólafsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
Iraultza Unzueta
Department of Applied Mathematics, University of the Basque Country (UPV/EHU), Torres Quevedo Ingeniaria Plaza 1, 48013 Bilbao, Spain
Roberto Mantovan
CNR-IMM, Unit of Agrate Brianza, Via Olivetti 2, 20864 Agrate Brianza (MB), Italy
Karl Johnston
European Organization for Nuclear Research (CERN), 1211 Geneva, Switzerland
Hafliði P. Gíslason
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
Petko B. Krastev
Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria
Deena Naidoo
School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa
Bingcui Qi
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5–5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system.
