Citation: Pudza, I.; Pudzs, K.; Tokmakovs, A.; Strautnieks, N.R.; Kalinko, A.; Kuzmin, A. Nanocrystalline CaWO 4 and ZnWO 4 Tungstates for Hybrid Organic–Inorganic X-ray Detectors. Materials 2023, 16, 667. https:// doi.org/10.3390/ma16020667 Academic Editor: Marco Girolami Received: 13 December 2022 Revised: 4 January 2023 Accepted: 6 January 2023 Published: 10 January 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 Nanocrystalline CaWO 4 and ZnWO 4 Tungstates for Hybrid Organic–Inorganic X-ray Detectors Inga Pudza 1, * , Kaspars Pudzs 1 , Andrejs Tokmakovs 1 , Normunds Ralfs Strautnieks 1 , Aleksandr Kalinko 1,2 and Alexei Kuzmin 1 1 Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia 2 Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany * Correspondence: inga.pudza@cfi.lu.lv Abstract: Hybrid materials combining an organic matrix and high-Z nanomaterials show potential for applications in radiation detection, allowing unprecedented device architectures and functionality. Herein, novel hybrid organic–inorganic systems were produced using a mixture of tungstate (CaWO 4 or ZnWO 4 ) nanoparticles with a P3HT:PCBM blend. The nano-tungstates with a crystallite size of 43 nm for CaWO 4 and 30 nm for ZnWO 4 were synthesized by the hydrothermal method. Their structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The hybrid systems were used to fabricate direct conversion X-ray detectors able to operate with zero bias voltage. The detector performance was tested in a wide energy range using monochromatic synchrotron radiation. The addition of nanoparticles with high-Z elements improved the detector response to X-ray radiation compared with that of a pure organic P3HT:PCBM bulk heterojunction cell. The high dynamic range of our detector allows for recording X-ray absorption spectra, including the fine X-ray absorption structure located beyond the absorption edge. The obtained results suggest that nanocrystalline tungstates are promising candidates for application in direct organic–inorganic X-ray detectors. Keywords: tungstates; hybrid organic–inorganic X-ray detectors; X-ray sensing 1. Introduction Nowadays, developing new radiation detectors based on nanomaterials is an active field of research [1]. Among the different types of detectors, hybrid organic–inorganic sys- tems for X-ray detection have attracted considerable attention during the last ten years [2]. The strong advantages of such systems are the combination of relatively inexpensive, easy-to-manufacture, flexible, and low-bias voltage (<10 V) organic semiconductors with high-Z inorganic (nano-)compounds [1,3–5]. The latter provide a large X-ray cross-section and control of spectral selectivity, which improve the absorption efficiency and sensitivity while maintaining the beneficial physical properties of the host organic matrix [2]. Various inorganic materials have been proposed for use in hybrid organic–inorganic systems for X-ray detection [2]. However, the search for the best system(s) that can be used in everyday applications remains one of the most important and challenging tasks in the field [6,7]. Here, we propose tungstates with a general chemical formula of AWO 4 [8] (where A is a divalent ion, for example, Ca, Sr, Ba, Pb, Ni, Zn, or Cd) as a new class of materials for use in hybrid organic–inorganic systems for direct-conversion X-ray detection. Using tungstates opens up a wide range of possibilities for solving a specific problem by optimizing their chemical composition and degree of crystallinity. This is convenient for producing hybrid systems but has not been exploited until now. Another advantage of tungstates for use in X-ray detectors is the high Z of tungsten (Z = 74) and the possibility of selecting the Z number of the second metal ion in a wide range. This allows for optimizing the absorption efficiency in a certain range of X-ray energies. Materials 2023, 16, 667. https://doi.org/10.3390/ma16020667 https://www.mdpi.com/journal/materials