Supplementary Material for
Mechanical Switching of Ferroelectric Polarization in Ultrathin BaTiO3 Films: the Effects
of Epitaxial Strain
Zheng Wen,1,2,a) Xiangbiao Qiu,1 Chen Li,1 Chunyan Zheng,2 Xiaohui Ge,2 Aidong Li,1 and Di
Wu1,a)
1
National Laboratory of Solid State Microstructures and Department of Materials Science and
Engineering, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093,
China
2
College of Physics, Qingdao University, Qingdao 266071, China
a)
Author to whom correspondence should be addressed. Electronic mail: zwen@qdu.edu.cn and
diwu@nju.edu.cn
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FIG. S1. RHEED intensity oscillations recorded in situ during the growth of BTO layers on
SRO-buffered DSO (a) and STO (b) substrates. The insets in (a) and (b) are surface
morphologies of the substrates (left) and the BTO ultrathin films (right), respectively. The scale
bars represent 1 μm in length.
FIG. S2. Two-dimensional diffraction intensity mapping around the (103) reciprocal spot of the
STO substrate, demonstrating coherent growth of a BTO film on the STO substrate.
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FIG. S3. Surface morphology of the BTO/SRO/DSO heterostructure acquired before (a) and
after (b) the mechanical writing. The scale bar represents 500 nm in length.
As shown in Fig. S3, there is no significant difference observed in the surface morphology
of the BTO/SRO/DSO heterostructure recorded before and after the mechanical writing with the
loading force discretely increasing from ~800 to ~2150 nN. The AFM image is found to blur
after the mechanical writing. This is probably because the tip may be worn due to the large
contact forces.
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FIG. S4. Phase images of a domain written mechanically (a) and erased electrically (c), over the
area indicated by the dotted square, in a BTO ultrathin film deposited on the SRO-buffered DSO
substrate. Corresponding tunneling current mapping are shown in (b) and (d). Insets are current
signals along the solid lines in (b) and (d). The scale bar represents 500 nm in length.
As demonstrated in Fig. S4, the mechanically-written domains can be switched electrically,
using the BTO/SRO/DSO as an example. By applying a 1800 nN mechanical force within an
upward domain electrically patterned by a -4.0 V bias, the polarization in the dotted squared in
Fig. S4(a) is switched downward. This central mechanically-written downward domain can be
switched upward again by scanning a -4.0 V biased tip, as shown in Fig. S4(c). In fact, the
conductive-tip/BTO/SRO heterostructure functions as a ferroelectric tunnel junction, in which
the tunneling current distribution (Fig. S4(b)) coincide exactly with the domain structure (Fig.
S4(a)). The downward and upward domains correspond to a low and a high tunneling resistance
level, respectively, in the tip/BTO/SRO tunnel junction on DSO substrate. When the central
downward domain written mechanically in the dotted square is switched upward by a -4.0 V tip
bias, the high tunneling current is shut off correspondingly, as shown in Figs. S4(c) and (d).
Current profiles along the solid lines after the mechanical writing and after the electrical erasing
are shown in the insets for clarity.
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