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Caption : figure s1. (a) survey xps spectra for vox, vom and vorta.

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Caption : figure s2. schematic illustration of the electroforming process in the vorta.

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Caption : figure s3. crest value (ic) and trough value (it) of each sample.

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Caption : figure s4. detail information of the i-v curve for vom and vorta around the turn-on voltage.

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Caption : figure s5. left: xrd pattern for ito substrate, bottom vo<sub>2</sub>(m) layer and two layer structure vos. right: tem image of two layer structure vos.

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Caption : figure s6. raman spectra of unformed and formed area for vorta and vos.

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Caption : figure s7. raman spectra for the vorta and vos in the range of 175-215 cm<sup>-1</sup>.