Ferroelectricity

Ferroelectricity usually exists in a special class of dielectric materials. The spatial inversion symmetry of these materials is broken, so that the positive and negative charge centers of the system are separate, resulting in a spontaneous polarization. The direction of this spontaneous polarization can be reversed by the external electric field, resulting in a hysteresis loop^1-2 similar to ferromagnetic materials, so this kind of property is called ferroelectric in analogy with ferromagnetism. In ferroelectric materials, the two electrical states of the electric polarization vectors before and after flipping can correspond to “0” and “1” in binary language respectively. Because the electrical state can be read and written repeatedly and preserved for a long time, ferroelectric materials are widely used in many fields, such as ferroelectric memory, ferroelectric tunnelling junction(FTJ)³, ferroelectric capacitor and ferroelectric diode. The traditional ferroelectric materials are mainly ABO₃ type perovskite materials⁴.

It shows the behaviour of a ferromagnetic core graphically as the relationship between B and H is non-linear. Starting with an unmagnetised core both B and H will be at zero, point 0 on the magnetisation curve¹

hysteresis loop

(a) ABO3 cubic perovskite structure showing (b) the uniform strain in the crystalline cell with ferroelectric polarization and (c) flexoelectric induced polarization due to strain gradient⁴

ABO3 type perovskite

Energy band diagrams of metal/ferroelectric (EE)/correlated electron oxide (CEO) FTJs for two polarization directions, in which the tunneling probability is controlled by the direction of the polarization, yielding either a hole accumulated (top) or a hole depleted (bottom) state in the CEO layer³

FTJ models

Under the general trend of miniaturization and integration of electronic equipment, exploring the new ferroelectric materials with small size and foldable properties has become a hot topic in the low dimensional ferroelectricity fields. In traditional ferroelectric materials (ABO₃-type perovskite), when the thickness of the material reduce to several nanometers, the surface depolarization effect is strong, so that the ferroelectric property significantly weakened or even disappeared, corresponding to the ferroelectric critical size. Therefore, how to design and prepare high performance ferroelectric devices with small size has become one of the major challenges in this field.

Bi2TeO5 ferroelectric domain

Molecule electret

Nature Nanotechnology

A Gd@C82 single-molecule electret

Electrets are a class of materials that can be compared to permanent magnets. They can be used for information storage, as well as for static earphones and microphones. It has long – lasting properties. The feature of electret was first discovered by Gray in 1732. In 1892, Heaviside combined electr-et (electret) with the phrase electric and magnet firstly, and clearly put forward the concept of electret⁵.

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Fig.1 Single-electron transport of the Gd@C82 SMD

Specifically, they created a gap of about 1 nm on a 50 nm wide metal wire by using the electromigration at a low temperature of 1.6 K (about -271.6 ℃), and successfully constructed several Gd@C82 single-molecule devices (as shown in Fig. 1a). Then a source-drain voltage value was set close to zero (2 mV) . By changing the gate voltage Vg and recording the source-drain current I_ds at different gate voltage, two sets of spectral lines are obtained, corresponding to two device states (state 1 and state 2). As shown in Fig. 1b, these two states can be switched between each one by changing the gate voltage, showing two sets of different transport characteristics in the same single molecule device.

Fig.2 Density functional theory calculations revealing the SME physics

These two states probably correspond to two molecular configurations, but this configuration change is difficult to be directly observed by experiments, by using the first-principles calculation, it was found that Gd atom in the Gd@C82 molecule was located at the two most stable adjacent adsorption sites on the C82 cage, with an energy difference of ~ 6 meV (Fig. 2a). It can be seen that the positive and negative charge centers of Gd@C82 molecules do not coincide, that is, the molecule has a non-zero electric dipole moment.

Gd atom moves between two stable adsorption sites, which can change the direction of the electric dipole moment , so that the relative stability of the two adsorption sites can be regulated by the electric field. The calculation shows that Gd atom can move between the two sites under the electric field as long as the energy barrier of ~11 meV is overcome (Fig. 2). we can flip of the electric dipole moment at the level of a single molecule, i.e. the device is a monatomic (Gd) information memory.

Sliding ferroelectricity

Fig.3 MoS₂/WS₂ heterobilayers grown by CVD method

Science

Ferroelectricity in Untwisted Heterobilayers of Transition Metal Dichalcogenides

Collaborators used chemical vapor deposition (CVD) to grow a untwisted MoS₂/WS₂ heterobilayers with a thickness of only about 1 nm. This heterobilayers has two stacking structures of 2H and 3R, both of which break the out-of-plane inversion symmetry(Fig.3). The PFM results show that the material has the out-of-plane ferroelectric property with the obvious ferroelectric hysteresis loops (Fig.4). The piezoelectric coefficient d₃₃ is 1.95-2.09 pm/V. This value is about 6 times higher than that of monolayer α-In₂Se₃, which has the highest out-of-plane polarization among previously known 2D ferroelectric materials⁶.

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Fig.4 Ferroelectric characterization of MoS₂/WS₂ heterobilayers

Fig.5 The origin of the MoS₂/WS₂ heterobilayers’ ferroelectric behaviours

In addition, the ferroelectric thin film was constructed into a ferroelectric tunnelling junction(FTJ), shows an considerable on/off ratio(~10³). By using the first-principles calculation. We had explored a reliable method to calculate the piezoelectric coefficient of 2D materials , and the calculated values of 2.28~2.40 pm/V are obtained, which are consistent with the experimental results. The calculation results show that the non-zero out-of-plane polarization comes from the interlayer charge transfer (Fig.5), and under the external electric field, the direction of interlayer charge transfer can be flipped by overcoming the 16 meV/f.u energy barrier, so that the polarization direction can be reversed through the in-plane sliding, indicating that the heterobilayers is an out-of-plane ferroelectric thin film.

REFERENCES

1.https://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html

2.Li, B., Wan, Z., Wang, C. et al. Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order. Nat. Mater. 20, 818–825 (2021)

4. R. Tararam, I. K. Bdikin, N. Panwar, J. A. Varela, P. R. Bueno, and A. L. Kholkin , “Nanoscale electromechanical properties of CaCu3Ti4O12 ceramics”, Journal of Applied Physics 110, 052019 (2011)

5. Zhang, K., Wang, C., Zhang, M. et al. A Gd@C82 single-molecule electret. Nat. Nanotechnol. 15, 1019–1024 (2020). https://doi.org/10.1038/s41565-020-00778-z

6. (1) Rogée, L.; Wang, L.; Zhang, Y.; Cai, S.; Wang, P.; Chhowalla, M.; Ji, W.; Lau, S. P. Ferroelectricity in Untwisted Heterobilayers of Transition Metal Dichalcogenides. 2022, 7

Ji Group@Renmin University