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High-Entropy Oxides: Advanced Research on Electrical Properties

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High-Entropy Oxides: Advanced Research on Electrical Properties coatings Article High-Entropy Oxides: Advanced Research on Electrical Properties Haoyang Li 1, Yue Zhou 1, Zhihao Liang 1, Honglong Ning 1,* , Xiao Fu 1, Zhuohui Xu 2, Tian Qiu 3, Wei Xu 1, Rihui Yao 1,* and Junbiao Peng 1 1 Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; [email protected] (H.L.); [email protected] (Y.Z.); [email protected] (Z.L.); [email protected] (X.F.); [email protected] (W.X.); [email protected] (J.P.) 2 Guangxi Key Lab of Agricultural Resources Chemistry and Biotechnology, Yulin Normal University, Yulin 537000, China; [email protected] 3 Department of Intelligent Manufacturing, Wuyi University, Jiangmen 529020, China; [email protected] * Correspondence: [email protected] (H.N.); [email protected] (R.Y.) Abstract: The concept of “high entropy” was first proposed while exploring the unknown center of the metal alloy phase diagram, and then expanded to oxides. The colossal dielectric constant found on the bulk high-entropy oxides (HEOs) reveals the potential application of the high-entropy oxides in the dielectric aspects. Despite the fact that known HEO thin films have not been reported in the field of dielectric properties so far, with the high-entropy effects and theoretical guidance of high entropy, it is predictable that they will be discovered. Currently, researchers are verifying that appropriately increasing the oxygen content in the oxide, raising the temperature and raising the pressure during preparation have an obvious influence on thin films’ resistivity, which may Citation: Li, H.; Zhou, Y.; Liang, Z.; be the guidance on obtaining an HEO film large dielectric constant. Finally, it could composite a Ning, H.; Fu, X.; Xu, Z.; Qiu, T.; Xu, metal–insulator–metal capacitor, and contribute to sensors and energy storage devices’ development; W.; Yao, R.; Peng, J. High-Entropy alternatively, it could be put into application in emerging thin-film transistor technologies, such as Oxides: Advanced Research on those based on amorphous metal oxide semiconductors, semiconducting carbon nanotubes, and Electrical Properties. Coatings 2021, organic semiconductors. 11, 628. https://doi.org/10.3390/ coatings11060628 Keywords: high-entropy oxides; thin films; dielectric constant; oxygen content Academic Editor: Aivaras Kareiva Received: 24 April 2021 Accepted: 17 May 2021 1. Introduction Published: 24 May 2021 In order to further explore the central region of the multi-component metal phase diagram (Figure1) in metallurgy [ 1], Cantor [2] et al. found that, under Gibbs phase Publisher’s Note: MDPI stays neutral rule, approximately equimolar mixing could yield the permitted number of phases, even with regard to jurisdictional claims in for single-phase alloys. Then, Murty et al. [3] explicitly defined the concept of high- published maps and institutional affil- entropy alloys (HEAs), that is, alloys containing at least five principal elements, each iations. having an atomic percentage between 5% and 35%. Due to its high hardness [4,5], high stability [6], high wear resistance [7,8], oxidation resistance [9,10] and other excellent properties, HEAs have attracted worldwide scientists’ attention. So far, research on HEAs and theoretical systems has been relatively complete. Inspired by the remarkable properties Copyright: © 2021 by the authors. of them, researchers speculated that oxides could also exhibit better performance under Licensee MDPI, Basel, Switzerland. the influence of high entropy [11]. The first time applying the concept of high entropy This article is an open access article to oxides was in 2015, when Rost et al. [12] successfully prepared an entropy-stabilized distributed under the terms and crystal (MgCoNiCuZn)O by filling a single sublattice with five different cations, and first conditions of the Creative Commons proposed the concept of “Entropy-Stabilized Oxides”. Bérardan et al. [13,14] followed Attribution (CC BY) license (https:// up with a study of its electrical conductivity and dielectric coefficient and renamed it creativecommons.org/licenses/by/ “high entropy oxides” (HEOs). Since then, more types of remarkable properties have been 4.0/). Coatings 2021, 11, 628. https://doi.org/10.3390/coatings11060628 https://www.mdpi.com/journal/coatings Coatings 2021, 11, x FOR PEER REVIEW 2 of 17 Coatings 2021, 11, 628 2 of 17 entropy oxides” (HEOs). Since then, more types of remarkable properties have been dis- covered whilediscovered more scientists while conduct more scientists research conduct on them. research Due to on the them. impacts Due of to high the impactsen- of high tropy on thermodynamicsentropy on thermodynamics and structure, andthe properties structure, the of HEOs properties are different of HEOs from are different those from those of ordinary oxides.of ordinary Known oxides. excellent Known properties excellent range properties from magnetic range from properties magnetic [15,16] properties to [15,16] electrochemicalto electrochemical properties [17,18], properties from catalytic [17,18], fromproperties catalytic [19] properties to thermal [19 conductive] to thermal conductive properties [20],properties etc. In particular [20], etc. In, HEOs particular,’ electrical HEOs’ properties, electrical specifically properties, the specifically dielectric the dielectric property, attractproperty, scientists attract’ attention. scientists’ It is attention. reported It that is reported it can achieve that it a can coloss achieveal dielectric a colossal dielectric constant, higherconstant, than 4800 higher [13]. than As 4800 the trend [13]. Astowards the trend the towardsminiaturization the miniaturization of today’s and of today’s and future electronicfuture devices electronic brings devices an increase brings in an the increase tunneling in the current, tunneling materials current, with materials high with high permittivity permittivityare potentially are of potentiallygreat value. of Therefore, great value. HEOs Therefore, with a colossal HEOs dielectric with a colossal con- dielectric stant satisfy constantrecent electronic satisfy recent devices electronic’ requirements devices’ and requirements could be the and key could to besolving the key the to solving the current bottleneck.current Moreover, bottleneck. the Moreover, mixing the of mixingvarious of elements various elementsallows for allows the design for the designof of more more materialsmaterials with distinctive with distinctive properties, properties, and its and variety its variety and andflexibility flexibility meet meet modern modern materials’ materials’ requirements.requirements. Significantly, Significantly, many many HEOs HEOs are are based based on onmagnetic magnetic transition transition metal metal ions; this ions; this is becauseis because electrical electrical and and magnetic magnetic properties properties of ferrite of ferrite or manganite or manganite are strongly are strongly related related to theirto theirbond bond lengths lengths and bo andnd bondangles angles,, which which can be can easily be easily changed changed by paramag- by paramagnetic or netic or diamagneticdiamagnetic cation cation substitutions substitutions [21,22] [21,22. At]. Atthe the same same time, time, the the disorder disorder of of the the system can system can bebe further further increased byby controllingcontrolling the the concentration concentration of of doping doping cations cations and and their oxidation their oxidationstate state [23 [23]], which, which makes makes it easierit easier for for researchers researchers to to regulate regulate HEOs’ HEOs’ properties proper- according ties accordingto to the the demand. demand. However, However, the the fabrication fabrication temperature temperature of of HEOs HEOs is is usually usually high, and the high, and themechanism mechanism behind behind it isit notis not yet yet clear, clear, so it so is necessaryit is necessary to conduct to conduct more studiesmore in order to studies in ordereffectively to effectively guide theguide design the design and preparation and preparation of materials. of materials. Figure 1. Sketch of metalFigure phase diagram 1. Sketch [1] of. metal phase diagram [1]. In our study,In we our first study, introduce we first the introduce definition the of definition high entropy of high and entropyroughly and classify roughly classify HEOs accordingHEOs to accordingelements or to elementsstructures. or Then structures., we discuss Then, the we electrical discuss the properties electrical of properties of HEOs, followedHEOs, with followed that of HEO with films. that of Remarkably, HEO films. Remarkably,HEO films are HEO usually films deposited are usually on deposited on a monocrystallinea monocrystalline silicon sheet siliconor a glass sheet substrate or a glass by substratedifferent bysputtering different means sputtering in the means in the oxygen atmosphere.oxygen atmosphere.Finally, we advance Finally, wefuture advance potential future applications potential applicationsas gate dielectrics as gate dielectrics after presentingafter current presenting mature current applications. mature applications. 2. High Entropy2. High Entropy In statistical physics,In statistical according physics, to accordingthe Boltzmann to the formula Boltzmann and formulathe additivity and the property additivity property of entropy, weof entropy,can calculate we can the calculate configurational the configurational
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