Citation: Buryi, M.; Babin, V.; Neykova, N.; Wang, Y.-M.; Remeš, Z.; Ridzo ˇ nová, K.; Dominec, F.; Davydova, M.; Drahokoupil, J.; Chertopalov, S.; et al. Changes to Material Phase and Morphology Due to High-Level Molybdenum Doping of ZnO Nanorods: Influence on Luminescence and Defects. Materials 2023, 16, 3294. https://doi.org/ 10.3390/ma16093294 Academic Editor: Federico Cesano Received: 10 March 2023 Revised: 14 April 2023 Accepted: 17 April 2023 Published: 22 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Changes to Material Phase and Morphology Due to High-Level Molybdenum Doping of ZnO Nanorods: Influence on Luminescence and Defects Maksym Buryi 1,2 , Vladimir Babin 1, *, Neda Neykova 1,3 , Yu-Min Wang 4 , Zden ˇ ek Remeš 1 , Katarína Ridzo ˇ nová 1,5 , Filip Dominec 1 , Marina Davydova 1 , Jan Drahokoupil 1 , Sergii Chertopalov 1 , Lucie Landová 1,3 and Ognen Pop-Georgievski 4 1 FZU—Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Prague, Czech Republic 2 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 120 00 Prague, Czech Republic 3 Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, 166 27 Prague, Czech Republic 4 Department of Chemistry and Physics of Surfaces and Interfaces, Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovský sq. 2, 162 06 Prague, Czech Republic 5 Faculty of Mathematics and Physics, Institute of Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic * Correspondence: babinv@fzu.cz Abstract: The influence of Mo on the electronic states and crystalline structure, as well as morphology, phase composition, luminescence, and defects in ZnO rods grown as free-standing nanoparticles, was studied using a variety of experimental techniques. Mo has almost no influence on the luminescence of the grown ZnO particles, whereas shallow donors are strongly affected in ZnO rods. Annealing in air causes exciton and defect-related bands to drop upon Mo doping level. The increase of the Mo doping level from 20 to 30% leads to the creation of dominating molybdates. This leads to a concomitant drop in the number of formed ZnO nanorods. Keywords: ZnO nanorods; molybdenum doping; morphology; luminescence; electron paramagnetic resonance 1. Introduction Zinc oxide (ZnO) is a well-known optically active substance, typically appearing as bulk crystals, thin films, and nanoparticles [1–3], with a wide range of applications, including medicine [4], in particular, drug delivery [5], and wound scaffolding [6]. An- other niche implementation is in scintillators. The well-known representatives are ZnO:Cu as the material for cathodoluminescence screens [7], whereas ZnO:Ga has the potential to be applied in alpha particle scintillation screens [8]. There are also common applica- tions such as photocatalysis, electrocatalysis, gas or biological substances sensing, and Li-ion batteries [9–13]. Due to its strong photocatalytic properties, ZnO is also suitable for waste solidification/stabilization [14,15]. Historically, ZnO nanoparticles have been used in optoelectronic devices [16]. Since ZnO nanoparticles possess ultrafast excitonic luminescence (strongly below 1 ns), having a maximum of around 380 nm [17–19], another implementation of ZnO is the time of flight positron emission tomography [20]. Typi- cally, ZnO nanorods are hexagonal and Wurtzite-like [21]. The free-standing particles, which are most commonly grown on the random nucleation seeds (NRPs), are considered in [17,22,23]. In most cases, the hydrothermal method, the simplest and cheapest one, is used [24,25]. There are several works dedicated to free-standing hydrothermally grown ZnO:Mo nanorods [23,26–28]. In particular, excitonic emission was found to be very sensi- tive to plasma treatment, X-ray irradiation, and annealing in air [23,27,29]. It was especially Materials 2023, 16, 3294. https://doi.org/10.3390/ma16093294 https://www.mdpi.com/journal/materials