多鐵材料具有多元的有序參量共存以及強烈的電磁耦合，而在介面處由晶格、電荷、軌道及自旋的交互作用引發磁性或電子結構之間的變化因而具有豐富的物理特性，例如鐵電材料中鐵電域壁的準二維導電特性。在本實驗中，我們利用脈衝雷射沉積藉由獨立自支撐(Freestanding)的技術結合材料堆疊(Twistronics)的概念透過特定角度的堆疊或扭轉來製作旋轉轉移基板，並將鐵電材料BiFeO3 磊晶於此旋轉轉移基板，製作出具有不同晶格結構的材料堆疊系統，藉由原生域與自支撐域的晶相差異而在邊界處形成側向扭轉接合結構，稱之為側向磊晶(Lateral epitaxy)，而此介面為本實驗的研究重點。我們利用X 光繞射儀、X 光吸收光譜以及穿透式電子顯微鏡分原生域與自支撐域的晶格結構，並藉由導電度原子力顯微鏡發現BiFeO3 在介面處出現導電特性的變化，由絕緣態轉變為半導體態，且此導電通道寬度僅數奈米。我們透過改變BiFeO3 的氧空缺濃度和觀察極化與結構不連續的邊界條件對導電度的影響，並利用升溫導電度原子力顯微鏡系統量測導電度與溫度關係，發現的導電特性是由熱振動所致，因此我們認為BiFeO3 側向扭轉接合處的導電行為是由氧空缺主導。

Investigation of correlated oxide systems has become popular in condensed-matter research because of its intriguing coupling between different order parameters. The interplay among degrees of freedom - lattice, charge, orbital and spin play an important role to generate many physical properties, such as the electrical conduction at ferroelectric domain wall. Enhanced electrical conductivity in ferroelectric domain walls has been reported in several materials in recent years. In our research, we demonstrate a new homostructure called lateral epitaxy for ferroelectric BiFeO3(BFO) thin film grown on the twist substrate as a new types of complex oxide interface by the combination of freestanding technique and twist stacking. To understand this novel homostructure, we carry out the systematic experiments on the interface by Conductive AFM, X-ray diffraction, X-ray absorption spectra and transmission electron microscopy and present the observation of an electronic conductor–insulator transition. The phenomenon of local conduction at the interface was observed at the nanoscale. As the result of the trend of current versus temperature measurements at the interface, the dominant conduction mechanism should be the oxygen vacancies.

1 Nataf, G. et al. (2020). Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials. Nature Reviews Physics, 2(11), 634-648.2-1
2 Eerenstein, W. et al. (2006). Multiferroic and magnetoelectric materials. nature, 442(7104), 759-765.
3 Onishi, H., & Hotta, T. (2004). An orbital-based scenario for the magnetic structure of neptunium compounds. New Journal of Physics, 6(1), 193.
4 Mitsui, T. et al. (1977). An Introduction to the Physics of Ferroelectrics. Gordon and Beach Science Publishers, New York.
5 Tybell, T. et al. (2002). Domain Wall Creep in Epitaxial Ferroelectric Pb (Zr0.2Ti0.8)O3 Thin Films. Physical review letters, 89(9), 097601.
6 W. D. Callister, Jr., “Material Science And Engineering”, John Wiley and Sons Inc., London, (2007).
7 Zavaliche, F. et al. (2005). Polarization switching in epitaxial BiFeO3 films. Applied physics letters, 87(25), 252902.
8 Streiffer, S. K. et al. (1998). Domain patterns in epitaxial rhombohedral ferroelectric films. I. Geometry and experiments. Journal of applied physics, 83(5), 2742-2753.
9 Chu, Y. H. et al. (2007). Controlling magnetism with multiferroics. Materials Today, 10(10), 16-23.
10 Chen, L. Q. (2002). Phase-field models for microstructure evolution. Annual review of materials research, 32(1), 113-140.
11 Schlom, D. G. et al. (2007). Strain tuning of ferroelectric thin films. Annu. Rev. Mater. Res., 37, 589-626.
12 Sando, D. et al. (2014). BiFeO3 epitaxial thin films and devices: past, present and future. Journal of Physics: Condensed Matter, 26(47), 473201.
13 Zeches, R. J. et al. (2009). A strain-driven morphotropic phase boundary in BiFeO3. science, 326(5955), 977-980.
14 Christen, H. M., et al. (2011). Stress-induced R− M A− M C− T symmetry changes in BiFeO3 films. Physical Review B, 83(14), 144107.
15 Zhang, J. X. et al. (2011). Large field-induced strains in a lead-free piezoelectric material. Nature nanotechnology, 6(2), 98-102.
16 Qi, X. et al. (2005). Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3. Applied Physics Letters, 86(6), 062903.
17 Yang, H. et al. (2007). Temperature-dependent leakage mechanisms of Pt∕ BiFeO3∕ SrRuO3 thin film capacitors. Applied Physics Letters, 91(7), 072911.
18 Pabst, G. W. et al. (2007). Leakage mechanisms in BiFeO3 thin films. Applied Physics Letters, 90(7), 072902.
19 Seidel, J. et al. (2009). Conduction at domain walls in oxide multiferroics. Nature materials, 8(3), 229-234.
20 Yang, C. H. et al. (2009). Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films. Nature materials, 8(6), 485- 493.
21 Sze, S. M. et al. (2021). Physics of semiconductor devices. John wiley & sons.
22 Nataf, G. F. et al. (2020). Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials. Nature Reviews Physics, 2(11), 634-648.
23 Farokhipoor, S., & Noheda, B. (2012). Local conductivity and the role of vacancies around twin walls of (001)− BiFeO3 thin films. Journal of Applied Physics, 112(5), 052003.
24 Farokhipoor, S., & Noheda, B. (2011). Conduction through 71 domain walls in BiFeO3 thin films. Physical Review Letters, 107(12), 127601.
25 Rojac, T. et al. (2017). Domain-wall conduction in ferroelectric BiFeO3 controlled by accumulation of charged defects. Nature materials, 16(3), 322-327.
26 Li, L. et al. (2013). Atomic scale structure changes induced by charged domain walls in ferroelectric materials. Nano letters, 13(11), 5218-5223.
27 Vasudevan, R. et al. (2012). Domain wall geometry controls conduction in ferroelectrics. Nano letters, 12(11), 5524-5531.
28 Crassous, A. et al. (2015). Polarization charge as a reconfigurable quasi-dopant in ferroelectric thin films. Nature nanotechnology, 10(7), 614-618.
29 Sluka, T. et al. (2012). Enhanced electromechanical response of ferroelectrics due to charged domain walls. Nature communications, 3(1), 1-7.
30 Bakaul, S. R. et al. (2016). Single crystal functional oxides on silicon. Nature communications, 7(1), 1-5.
31 Lu, D. et al. (2016). Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers. Nature materials, 15(12), 1255-1260.
32 Smith, H. M., & Turner, A. F. (1965). Vacuum deposited thin films using a ruby laser. Applied Optics, 4(1), 147-148.
33 Dijkkamp, D. et al. (1987). Preparation of Y‐Ba‐Cu oxide superconductor thin films using pulsed laser evaporation from high T c bulk material. Applied Physics Letters, 51(8), 619-621.
34 Johnson, R. D., Terada, N., & Radaelli, P. G. (2012). Comment on “spin-canting-induced improper ferroelectricity and spontaneous magnetization reversal in SmFeO3”. Physical review letters, 108(21), 219701.
35 曾賢德、果尚志,“奈米電性之掃描探針量測技術”, 物理雙月刊(廿五卷五期), (2003).
36 Robinson, J. W. et al. (2005). Undergraduate instrumental analysis.
37 Tan, Z., Budnick, J. I., & Heald, S. M. (1989). Structural parameter determination in fluorescence EXAFS of concentrated samples. Review of scientific instruments, 60(6), 1021-1025.
38 Tröger, L. et al. (1992). Full correction of the self-absorption in softfluorescence extended X-ray-absorption fine structure. Physical Review B, 46(6), 3283.
39 Gilbert, B. et al. Multiple Scattering Calculations of Bonding and Xray Absorption Spectroscopy of Manganese Oxides. The Journal of Physical Chemistry A 107, 16, 2839-2847 (2003).
40 Chen, J.-M. et al. A Complete High-to-Low spin state Transition of Trivalent Cobalt Ion in Octahedral Symmetry in SrCo0.5Ru0.5O3-δ. Journal of the American Chemical Society 136, 4, 1514-1519 (2014).
41 Hu, Z. et al. Different look at the spin state of Co(3+) ions in a CoO(5) pyramidal coordination. Phys Rev Lett 92, 20, 207402 (2004).
42 Burnus, T. et al. X-ray absorption and x-ray magnetic dichroism study on Ca3CoRhO6 and Ca3FeRhO6. Physical Review B 77, 20, 205111(2008).
43 Chen, C. T. et al. Out-of-plane orbital characters of intrinsic and doped holes in La2-xSrxCuO4. Physical Review Letters 68, 16, 2543- 2546 66(1992).
44 Haverkort, M. et al. Magnetic versus crystal field linear dichroism in NiO thin films. Physical Review B 69 (2003).
45 Wolf, S. G., & Elbaum, M. (2019). CryoSTEM tomography in biology. Methods in cell biology, 152, 197-215.
46 Hermes, I., & Stomp, R. Stabilizing the piezoresponse for accurate and crosstalk-free ferroelectric domain characterization via dual frequency resonance tracking.