Epitaxial KNbO 3 :Yb 3þ ,Er 3þ nanopattern for enhanced upconversion photoluminescence Heeyeon Park a , Kyu-Tae Lee b , Soon-Hong Kwon c , In Hwan Ahn b , Byunghoon Kim b , Doo-Hyun Ko b, ** , Woong Kim a, * a Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea b Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi,17104, Republic of Korea c Department of Physics, Chung-Ang University, Seoul, 06974, Republic of Korea article info Article history: Received 22 March 2019 Received in revised form 2 September 2019 Accepted 10 September 2019 Available online 11 September 2019 Keywords: Upconversion Solegel Epitaxy Nanopattern Nanoimprint abstract Nanopatterned epitaxial films of upconversion (UC) materials are desirable for various applications such as display lighting, waveguides, and optical imaging. In this study, we demonstrate that a nanopatterned epitaxial film of a UC material can be fabricated by combining a solegel process and nanoimprint lithography. An epitaxial KNbO 3 :Yb 3þ ,Er 3þ film is grown on a lattice-matched SrTiO 3 single-crystal substrate, which exhibits an approximately 70 times enhanced UC photoluminescence (PL) intensity compared to that of a non-epitaxial KNbO 3 :Yb 3þ ,Er 3þ film grown on a Si substrate. Moreover, the introduction of a nanopattern enhances the UCPL intensity ~20 times compared to that of a planar film with the same volume of material. Our study paves the way for a better fundamental understanding and expansion in the application of UC materials. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Functionality of up-conversion (UC) materials, which absorb multiple photons of near-infrared (NIR) light and then emit a photon of visible or NIR light with a shorter wavelength, has been widely investigated since the recognition of the UC concept in the 1960s [1‒2]. Compared with the conventional luminescent mate- rials, UC luminescent materials have promising characteristics such as the augmented use of a wide-range NIR light, large anti-Stokes shift, significant light penetration depth in tissues, and high signal-to-noise ratio [3‒5]. Considering these advantages, UC ma- terials have been extensively investigated for applications in NIR detectors, temperature sensors, full-color displays, solar cells, bio- imaging systems, etc [6‒10]. Recent studies on UC materials have focused mainly on nanocrystals, particularly owing to their biomedical applications [11‒15]. It is desirable to investigate the UC materials in the form of thin films for optical device applications such as displays, waveguides, optical sensing, and imaging [16‒18]. Although thin films of UC nanocrystals can be prepared, usually they are embedded in an optically inactive matrix and their crystal orientations are random, which hinders the full utilization of their excellent optical properties [19,20]. The solegel method has various advantages for the thin-film fabrication of UC materials: i) it is suitable for a simple control of dopant concentrations (activator, sensitizer) and/or host matrix stoichiometry for a high UC quantum yield or emission color tuning [21 ,22], ii) the synthesized UC materials have high homogeneities and purities, which is important as nonuniform distributions of dopants and defects could lead to a decrease in UC photo- luminescence (PL) intensity [23,24], iii) the crystal orientation of the UC film can be tuned over a large area by an epitaxial growth under ambient atmosphere [25], and iv) large-scale nanopatterns with various shapes and dimensions can be simply fabricated by soft lithography [26]. To fully exploit the potentials of the solegel- based UC thin films, extensive studies on their fabrication and an- alyses of their optical properties are indispensable. Ytterbium ions (Yb 3þ ) and erbium ions (Er 3þ ) are important and popular UC dopants perhaps because of their high UC efficiency. They also have been used in various other applications such as all optical switching [27 ,28]. On the other hand, KNbO 3 is an appro- priate host matrix owing to its potential applicability in nonlinear * Corresponding author. ** Corresponding author. E-mail addresses: dhko@khu.ac.kr (D.-H. Ko), woongkim@korea.ac.kr (W. Kim). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2019.152238 0925-8388/© 2019 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 813 (2020) 152238