Persistent Electronic Conduction in 12CaO7Al 2 O 3 Thin Films Produced by Ar Ion Implantation: Selective Kick-Out Effect
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E2.7.1
Persistent Electronic Conduction in 12CaO.7Al2O3 Thin Films Produced by Ar Ion Implantation: Selective Kick-Out Effect Leads to Electride Thin Films Masashi Miyakawa1, Katsuro Hayashi1, Yoshitake Toda1,2, Toshio Kamiya1,2, Masahiro Hirano1, and Hideo Hosono1,2 1
Hosono Transparent Electro-Active Materials Project, Exploratory Research for Advanced
Technology (ERATO), Japan Science and Technology Agency, KSP C-1232, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan. 2
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan.
ABSTRACT A new method to convert 12CaO.7Al2O3 (C12A7) thin films to electronic conductor by hot Ar+ ion implantation has been developed and its mechanism is discussed. It was found that hot Ar+ ion implantation extruded free O2- ions in C12A7 films by kick-out effects at fluences higher than 1×1017 cm-2, which left electrons in the cages embedded in C12A7 crystal and produced high concentration F+-like centers (~1.4×1021 cm-3). The resulting films show coloration and persistent electronic conduction with conductivities up to ~1 Scm-1. On the other hand, fluences less than 1×1017 cm-2 kept the films transparent and insulating.
INTRODUCTION We have found that 12CaO.7Al2O3 (C12A7) may exhibit novel functions by substituting so-called ‘free O2- ions’ with other anions, which includes strong oxidation capability and persistent light-induced electronic conduction [1,2]. These properties arise from the unique crystal structure of C12A7 that is composed of a positively charged [Ca24Al28O64]4+ framework containing twelve crystallographic cages (~0.4 nm in inner diameter) and two extra O2- ions (referred to as free O2- ion) occupying two different cages. These free O2- ions can be substituted with O- or H- ions, which leads to the appearance of the above-mentioned novel properties. The free O2- ions are also replaceable with electrons through chemical reductions by thermal annealing with metal Ca in vacuum (Ca-treatment) [3]. The resultant material is represented as C12A7:e-, which is a new class of an electride [4]. It has advantages over conventional organic electrides as it is thermally and chemically stable even at room temperature in air. The electrides are expected to show novel properties originating from these unique structures such associated with cooperative motion of the high-density semi-delocalized electrons and the flexible cage framework.
E2.7.2
However, the Ca-treatment to produce C12A7:e- electride has been limited to bulk samples because it forms CaO layer at the sample surfaces which has to be removed by mechanism polishing. Therefore we needed to develop a new method applicable to thin films. In this work, we have examined an alternative approach to produce polycrystalline (p-) C12A7:e- thin films. It was found that a physical process, Ar+ ion implantation, produced C12A7:e-, driven by directly kicking out the free O2- ions from the cages through nuclear collisions.
EXPERIMENTAL DETAILS
Thin films of p-C1
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