Yuta Arata1,Hiroyuki Nishinaka1,Minoru Takeda1,Masahiro Yoshimoto1
Kyoto Institute of Technology1
Yuta Arata1,Hiroyuki Nishinaka1,Minoru Takeda1,Masahiro Yoshimoto1
Kyoto Institute of Technology1
Modulations of the physical properties of materials by doping, alloying, and straining play an important role in electronics and optoelectronics. Recently, it has been demonstrated that the various properties of single nanowires were modulated by strain [1, 2]. However, the difficulty of bending a single nanowire is a significant barrier to applying devices. On the other hand, thin films deposited on flexible substrates can be simply bent. However, conventional flexible substrates, amorphous organic compounds such as polyimide (PI) and polyethylene terephthalate (PET), do not allow epitaxial growth of single-crystal thin films. It has been demonstrated to transfer the thin films to the flexible substrates; however, complicated processes such as epitaxial lift-off and sacrificial layer etching were required [3, 4]. We utilized synthetic mica substrates to directly grow epitaxial oxide thin films via van der Waals epitaxy [5]. The synthetic mica substrates can be thinned and bent easily by cleaving. We used the cleaved synthetic mica to bend the thin film and the strain applied by bending to modulate the properties.<br/>We grew ZnO thin films on the synthetic mica substrates and demonstrated the modulation of optical properties by bending strain. ZnO thin films were grown by mist chemical vapor deposition (CVD) using a solution of zinc acetylacetonate (Zn(C<sub>5</sub>H<sub>7</sub>O<sub>2</sub>)<sub>2</sub>) dissolved in a mixture of methanol and de-ionized water as a precursor. Epitaxial ZnO thin film was grown on synthetic mica at a growth temperature of 400 °C. To investigate the impact on the optical properties by strain, we measured photoluminescence (PL) of the ZnO thin film with and without bending. By bending with a radius of curvature of 5 mm, the peak energy was red-shifted to 0.02 eV compared with an unbent state. Thus, we have successfully modulated the PL peak of the ZnO thin film with the bending strain.<br/>In addition, we grew epitaxial VO<sub>2</sub> thin film on the synthetic mica and demonstrated modulation of the metal-insulator transition temperature by the bending strain. We utilized SnO<sub>2</sub> buffer layers to grow epitaxially VO<sub>2</sub> thin films. VO<sub>2</sub> and SnO<sub>2</sub> layers were grown by mist CVD using vanadyl acetylacetonate (VO(C<sub>5</sub>H<sub>7</sub>O<sub>2</sub>)<sub>2</sub>) and tin chloride pentahydrate (SnCl<sub>4</sub> 5H<sub>2</sub>O) as precursors, respectively. Epitaxial VO<sub>2</sub> thin film were grown on the SnO<sub>2</sub> buffer layer on the synthetic mica at a growth temperature of 500 °C. Using the epitaxial VO<sub>2</sub> thin film with bendable, we measured the electrical resistance as a function of temperature with and without bending. VO<sub>2</sub> thin film exhibited metal-insulator transitions both with and without bending. By bending with a radius of curvature of 5 mm, the metal-insulator transition temperature increased by 2.5 °C compared to the unbent state.<br/>In the symposium, we will discuss the magnitude of strain with bending and detailed measurements of PL and metal-insulator transition characteristics.<br/><br/>[1] B. Wei <i>et al.</i>, Nano Lett., <b>12</b>, 4595 (2012). [2] M. Zapf <i>et al.</i>, Nano Lett., <b>17</b>, 6637 (2017). [3] L. Shen <i>et al.</i>, Adv. Mater., <b>29</b>, 1702411 (2017). [4] W. H. Dong <i>et al.</i>, ACS Appl. Mater. Interfaces, <b>11</b>, 30477 (2019). [5] Y. Bitla and Y. -H. Chu, Nanoscale, <b>12</b>, 18523 (2020).