  Citation: Shafiqul, I.M.; Deep, R.; Lin, J.; Yoshida, T.; Fujita, Y. The Role of Nitrogen Dopants in ZnO Nanoparticle-Based Light Emitting Diodes. Nanomaterials 2022, 12, 358. https://doi.org/10.3390/ nano12030358 Academic Editor: Iván Mora-Seró Received: 28 December 2021 Accepted: 20 January 2022 Published: 22 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). nanomaterials Article The Role of Nitrogen Dopants in ZnO Nanoparticle-Based Light Emitting Diodes Islam Mohammad Shafiqul 1, *, Raj Deep 2 , Jie Lin 3 , Toshiyuki Yoshida 2 and Yasuhisa Fujita 2,3, * 1 Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan 2 Graduate School of Natural Science and Technology, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan; n21d103@matsu.shimane-u.ac.jp (R.D.); yosisi@riko.shimane-u.ac.jp (T.Y.) 3 S-Nanotech Co-Creation. Co., Ltd., 1060 Nishikawatsu, Matsue 690-0823, Japan; linjie3000@gmail.com * Correspondence: s199802@matsu.shimane-u.ac.jp (I.M.S.); fujita@riko.shimane-u.ac.jp (Y.F.); Tel.: +81-0852-32-6257 (Y.F.) Abstract: In this work, nitrogen-doped ZnO nanoparticles were synthesized in various conditions by the gas evaporation method with DC arc plasma. Nitrogen concentrations of 6.38 × 10 18 cm −3 to 2.6 × 10 19 cm −3 were obtained at a chamber pressure of 150 torr, using arc currents of 20 A to 70 A. The intensities of local vibrational modes at 275 cm −1 and 581 cm −1 in the Raman spectra of ZnO nanoparticles showed a dependency on the nitrogen concentration in the ZnO nanoparticles. The ratios of donor–acceptor pair and exciton emissions in the photoluminescence spectra of nitrogen- doped ZnO nanoparticles, and the electroluminescence of light-emitting diodes based on these nanoparticles, were nearly proportional to the Raman peak’s intensity at 275 cm −1 . The results indicated that the nitrogen dopants in the ZnO nanoparticles were acting as an acceptor. Keywords: ZnO nanoparticles; nitrogen doping; electroluminescence; photoluminescence; light- emitting diodes 1. Introduction ZnO is currently of great interest for the development of novel solid-state lighting devices. ZnO has a wide bandgap of 3.37 eV and a stable exciton binding energy of 60 meV for light emission in the near-UV spectral range at room temperature [1,2]. One hurdle in ZnO light device development is the difficulty in fabricating p-type ZnO. This difficulty limits the application of ZnO in common optical devices, such as LEDs, that require precise doping. To date, protocols that achieve reproducible and stable p-type ZnO have not been developed. One reason for the slow development is due to the defects in ZnO, such as oxygen vacancy (V O ), zinc interstitial (Zn i ), etc., and that there are few candidates for shallow acceptors [3]. The most reliable dopants for p-type ZnO are group V elements, such as phosphorus (P) [4], arsenic (As) [5], antimony (Sb) [6], and nitrogen (N) [7]. Nitrogen is the most suitable p-type dopant due to its atomic size being similar to that of oxygen. The behavior of nitrogen dopants in ZnO has been discussed by theoretical calculations, such as the ab initio electronic bandstructure method [8] and density functional theory [9]. Incor- porating nitrogen dopants and co-dopants into ZnO has previously been reported [10–12]. Some reports have shown nitrogen-doped p-type ZnO- or ZnMgO-based LEDs with single- crystal films using epitaxial growth technologies [13–15]. Unfortunately, single-crystal substrates and epitaxial growth technologies require strick fabrication controls and are currently not cost-effective. On the other hand, the fabrication of scalable LEDs using nanoparticles (NPs) is inexpensive and can be fabricated in atmospheric conditions. Currently, nitrogen-doped ZnO NPs are a unique and attractive issue. Various fab- rication methods have been developed for synthesizing nitrogen-doped ZnO NPs, such as radio frequency (RF) thermal plasma, hydrothermal-ammonolysis, and Nd:YAG laser Nanomaterials 2022, 12, 358. https://doi.org/10.3390/nano12030358 https://www.mdpi.com/journal/nanomaterials