Sliding ferroelectricity

Sliding ferroelectricity

by | Mar 27, 2023 | 2D ferroelectric

Fig.3 MoS2/WS2 heterobilayers grown by CVD method

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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
2D Magnets

2D Magnets

by | Feb 24, 2023 | Research

2D magnets
CrI3 bilayer、CrX2(X=S,Se,Te)…
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Two-dimensional magnets

Two-dimensional magnetic materials with magnetic anisotropy can form magnetic order under limited temperature in the single-layer limit. Their macroscopic magnetism is closely related to the number of layers and stacking orders, and their magnetic exchange can be regulated by various external fields. These novel properties have endowed 2D magnetic materials with rich physical connotation and potential application value, which has attracted wide attention of researchers1.

Currently, two-dimensional magnetic materials reported in experiments mainly include transition metal halides, transition metal sulfides, transition metal carbonitides, transition metal phosphorus sulfides, Mn-Bi-Te family, etc. When the system degree is reduced to the single-layer limit, they usually exhibit significantly different magnetic behavior from that of bulk and less layered samples. These single-layer and few-layer magnetic materials are more easily regulated by external means, such as magnetic field, electric field, strain, light, etc. Therefore, the rich and highly adjustable magnetic phenomena of these two-dimensional magnetic materials indicate that they have broad development prospects in the application of two-dimensional devices.

Although a considerable number of intrinsic two-dimensional magnetic materials have been discovered, the related research on them is still in its infancy. The discovery of new mechanisms of magnetic coupling and magnetic regulation of two-dimensional magnetic materials and the design of intrinsic two-dimensional magnetic materials with high magnetic ordered transition temperatures are still challenging frontier scientific issues.

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Ji Group@Renmin University

2D ferroelectricity

2D ferroelectricity

by | Feb 24, 2023 | Research

Low dimensional ferroelectric materials
Bi2TeO5、Molecule electret、C82@Gd、Sliding ferroelectricity、vdW heterostructures…
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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 loop1-2 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.

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Ji Group@Renmin University

2D multiferroics

2D multiferroics

by | Feb 24, 2023 | Research

2D multiferroics
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Basic concept of multiferroic materials

Fig.1 Multiferroic family tree[1]

Multiferroics exhibit more than one primary ferroic ordering, including ferromagnetism, ferroelectricity, or ferroelasticity in the same phase[1,2]. The terminology is usually extended to include non-primary orderings such as antiferromagnetism, antiferroelectricity, as well as composites of individual ferroics. The concept often used today is referred to magnetoelectric materiasl combing ferroelectric and magnetic behaviors (Fig. 1);. Such a combination of ferroic orders in multiferroics can lead to coupling between them and provide an efficient route to control magnetism by electric fields or vice versa.

Types of multiferroics

Fig.2 Types of multiferroicity[3]

Multiferroics can be classified into two categories: type-I mulitferroics refer to the ferroelectricity does not have magnetic origin, which usually leads to an indirect and weak magnetoelectric coupling; Type-II multiferroics are induced by some specific magnetic orders with a strong magnetoelectric coupling but a small electric polarization and thus low transition temperature (TC). The two types are depicted in Fig. 2[3]

Several mechanisms in multiferroics

Ferroelectricity may be driven by electronic lone pairs, geometric effects, charge order or magnetism. In the first three classes, electric polarization and magnetism occur independently, i.e. the Type-I. In the last one, electric polarization and magnetic order emerge jointly, i.e. the Type-II.

Fig.3 Mechanisms in multiferroics[3]

Lone pairs mechanism

The lone-pairs mechanism is based on spatial asymmetry caused by anisotropic distribution of unbonded valance electrons around the host ion (see Fig. 3a). In this case the material is a single-phase, containing two cations, with one provides the ferroelectricity and the other the magnetism. This mechanism originates from room-temperature ferroelectricity in BiFeO3. A pair of Bi3+ valance electrons in the 6s orbital in BiFeO3 is not involved in sp hybridization and creates a local dipole, which yields a spontaneous polarization[4] of ~100 μC/cm2 below the Tc = 1,103 K[5]. The antiferromagnetism comes from the 3d electrons of Fe ions.

Geometry ferroelectricity

In geometrically driven ferroelectrics, a small A-site cation permits rotations of its surrounding coordination polyhedral (Fig. 3b), which are either polar in their own right (for example, in BaNiF4[6]) or couple to a secondary polar distortion (as in YMnO3[7]). In this case, the polarization is both robust and small in magnitude. In h-RMnO3 (R=Sc, Y, In or Dy-Lu[8,9]), unit-cell tripling drives the emergence of a ferroelectric order at Tc ≥ 1,200 K with a polarization of 5.6 μC/cm2, followed by a magnetic ordering at the Neel temperature (TN ≤ 120 K).

Charge ordering

Valance electrons can be distributed non-uniformly around their host ions in the crystal lattice to form a periodic superstructure (Fig. 3c). For example, the Fe atoms in LuFe2O4 may form a superlattice with an alternating sequence of Fe2+ and Fe3+ ions[10]. This kind of charge ordering might be the source of an electric polarization.

Magnetic ordering

In magnetically induced ferroelectrics, a non-collinear magnetic ordering, such as a spin spiral, breaks the inversion symmetry and induces a spontaneous polarization. These magnetically induced, so-called improper ferroelectric materials are totally different from displacive ferroelectrics, in which magnetic ordering is inhibited. The most intensely discussed mechanism is the inverse Dzyaloshinskii–Moriya (DM) interaction. Spin-orbit coupling (SOC) is crucial for the inverse DM interaction. The polarization is essentially determined by the optimization of the spin configuration from the point of view of anitsymmetry exchange, expressed by the antisymmetric product Si × Sj of neighboring spins Si,j (see Fig. 3d). The polarization vector is P ∝ eij × ( Si × Sj), where eij is the unit vector connecting neighboring spins. Multiferroicity of this type was first found in Cr2BeO4[11].

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Ji Group@Renmin University

STEM beam effect

STEM beam effect

by | Feb 24, 2023 | Research

STEM beam effect
Electron beam、Black phosphorus…
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Scanning Transmission Electron Microscope (STEM)

Manipulating and engineering atom dynamics with single atom precision has long been the ultimate goal in nanoscience and nanotechnology [1]. Direct atom manipulation or beam induced structural evolution by scanning transmission electron microscopy (STEM) has several advantages, i.e. much faster speed of atom manipulation (a few tens of seconds) and manipulation at room temperature [1]. STEM has been applied in various research, such as, structural defects [2], phase transitions [3], electron beam lithography [4], assembly of atoms [5] and single-atom migration [6].

Schematic of scanning Transmission Electron Microscope (STEM) System

STEM

When electrons pass through atomically thin 2D materials, energy can be either elastically or inelastically transferred to the targeted atom. During an elastic collision, impinging electrons are scattered to high angles by the recoiling nucleus and energy and momentum are both conserved. During the inelastic scattering process, the incident electron interacts with the electronic system of the target atom resulting in electronic excitation and electrostatic charging [1]

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Ji Group@Renmin University