Peer review of: Strain-induced room-temperature ferroelectricity in SrTiO3 membranes (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In the manuscript by Xu et al., the authors report the observation of room-temperature ferroelectricity 
in uni-axial strained SrTiO3 membranes. By SHG and PFM, the authors observed 180° ferroelectric 
domains at room temperature in SrTiO3 membranes with large strain. Moreover, the authors also 
performed DFT and molecular dynamics simulations to explore the nature of the ferroelectric phase 
transition. Unlike the films on rigid substrates, this work demonstrates the continuous strain 
engineering of STO up to a record high value and explores the mechanism of intrinsic ferroelectricity 
in strained STO, which is of highly interest for the community. I would recommend publication after 
the authors address the following concerns/questions. 

1. Currently, the strain is characterized by the displacement of the gold markers measured by optical 
microscopy. As the strain can be partially relaxed with microscopic cracks which may not be detected 
by optical microscopy or missed by microscopic PFM measurements. It is important to have a direct 
XRD characterization of the real strain and a,b,c lattice constants. Especially, the 2.5% strain is a 
record high value for SrTiO3 and should be confirmed by direct measurements of the lattice constants. 

2. How does the STO membranes start to from cracks as the strain increases? Is that a sudden or 
gradual change? It would be great if the authors can discuss how the ferroelectric domain changes 
beyond the maximum strain without forming cracks? 

3. Are the measurements of different strained state performed on the sample sample or on different 
samples? 

4. There is a very relevant recent paper that demonstrated the giant uniaxial strain applied on 
freestanding PbTiO3 membranes (Adv. Mater. Interfaces 1901604 (2020)). 



Reviewer #2 (Remarks to the Author): 

Using the PFM, SHG and first principles calculations, authors study the freestanding layers of SrTiO3 
and claim, “Room temperature robust ferroelectricity in strained STO”. While PFM measurements and 
GLD calculations shows above room temperature FE, given the controversy surrounding PFM 
measurements and FE switching, I am afraid the claim of “robust FE above room temperature Tc” is 
rather weak. Ohter than that article is well written and overall a good read. 
Few more technical comments: 
Why induced FE is dynamic, does it decay as function of time? 
How big is the tensile stress that is applied to keep the membrane undeformed along the other 
orthogonal in-plane direction; how the behavior changes if no such stress is applied. 
Atoms does not have labeling in Fig 1. 
Arrow in Fig 4c are too small and hard to see. 


Reviewer #3 (Remarks to the Author): 
The current manuscript reports the room-temperature ferroelectricity induced by uniaxial tensile strain 
in SrTiO3, which is a typical centrosymmetric and paraelectric material. The epitaxial heterostructures 
of STO films with the Sr2CaAl2O6 sacrificial buffer layer were grown to obtain the freestanding STO 
membranes. And by transferring the membranes onto a stretchable polymer sheet, a continuously 
tunable uniaxial strain could be applied. The in-plane ferroelectric polarizations along the tensile strain 
direction and the 180-degree domains were observed with the piezoresponse force microscopy, the 
symmetry lowering of the structure characterized with the second harmonic generation measurements, 
the energy stability of the strain-driven ferroelectric phase verified by the density functional theory 
calculations, while the nature of the ferroelectric phase transition was illustrated with molecular 
dynamics simulations. All these comprehensive investigations are solid and convincing. However, the 
presentation of the manuscript should be improved before being acceptable for publishing on Nature 
Communications. 

1. It is an important method to apply the continuously tunable uniaxial strain to freestanding 
membranes and investigate the new behaviors being induced. To demonstrate its advantage, the 
unique response of STO should be clarified. For example, besides Tc, how do other ferroelectric 
characters (polarization strength, coercive field or domain structure) change upon the variation of 
strain? As a prospect, how would the anisotropy associated with the uniaxial strain be connected to 
potential applications? 

2. The symmetry of the low-temperature ferroelectric phase was determined by SHG measurements. 
It would be more convincing if the results of other independent lattice measurements are given, e.g. 
the quantitative analyses of x-ray diffractions. By the way, the data are of high-quality – does this 
suggest the high quality of the interface between the STO membrane and the polyimide sheet? The 
authors are encouraged to comment on this. 

3. The lattice structure of the low-temperature phase was determined by DFT calculations. The detail 
information, e.g. the cif file, should be presented and clearly described. And, how do the relaxed 
structures change with the uniaxial strain? 

4. The discussions on Fig. 3b. What does “Distribution” refer to exactly? Under what strain the results 
were obtained – it should be specified (2% tensile strain along [100] direction) in the main text. 

5. The MD results indicate the order-disorder character of the ferroelectric transition. There exist 
different polarization orientations as the temperature rising close to Tc. How can the MD results be 
correlated to the PFM and DFT results? How do the displacive and order-disorder mechanisms 
cooperate or compete?