Ferroelectricity And Piezoelectricity

[Doris Joo]

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Ferroelectricity and piezoelectricity in TmFe2O4. dc voltage dependence of (a) phase of polarization and (b) strain along the ch axis of TmFe2O4 as a function of temperature obtained by applying the dc electric field parallel to the ch axis of TmFe2O4. (c) Inversion of spontaneous polarization along the ch axis of TmFe2O4 demonstrated by SNDM measurements at 303 K. The left and right figures show the domain structures before and after the application of dc voltage of 10 V parallel to the ch axis in the area surrounded by the dotted lines (right figure). The red and blue regions represent the domains where the directions of the spontaneous polarizations are opposite to each other.

Evaluation of the dielectric properties in TmFe2O4. (a) dc electric field dependence of piezoelectric coefficient, d33, a component corresponding to a strain along the ch axis with a dc electric field parallel to the ch axis of TmFe2O4. (b) dc electric field dependence of capacitance and dielectric loss at 303 K. The electric field was applied parallel to the ch axis.

Crystal structure of TmFe2O4. (a) [001]-zone axis bright-field pattern of CBED. (b) Crystal structure model for the ferroelectric phase of TmFe2O4. (c) [100]-zone HAADF-STEM image. Inset shows the crystal structure refined by using single crystal XRD data.

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Two-dimensional (2D) layered materials with a non-centrosymmetric structure exhibit great potential for nano-scale electromechanical systems and electronic devices. Piezoelectric and ferroelectric 2D materials draw growing interest for applications in energy harvesting, electronics, and optoelectronics. This article first reviews the preparation of these functional 2D layered materials, including exfoliation methods and vapor phase deposition growth, followed by a general introduction to various piezo/ferro-electric characterization methods. Typical 2D piezoelectric and ferroelectric materials and their electronic properties, together with their potential applications, are also introduced. Finally, future research directions for 2D piezoelectric and ferroelectric materials are discussed.

Since the successful isolation of graphene by Novoselov et al. in 2004,1 worldwide scientific efforts have been focused on wide range of two-dimensional (2D) layered materials, driven by the fundamental interest and their potential applications. Atomically thin 2D materials exhibit a wide range of unique electrical,2,3 optical,4,5 mechanical6, and thermal7 properties, which do not exist in their bulk counterparts, and the outstanding advantages of properties enlighten the development in light-weight and high-performance multifunctional applications.8 Particularly, the 2D layered materials with non-centrosymmetric structure have great potential for nanoscale electromechanical systems and electronic device applications. Among them, piezoelectric and ferroelectric 2D materials have drawn a growing interest in recent years.9,10,11,12,13,14,15,16,17

Exfoliation method is a top-down approach to prepare atomically thin 2D materials from the bulk crystals constructed by weak van der Waals interlayer interaction, which was applied to produce graphene in 2004.1 In general, it can be realized by mechanical or chemical approaches.

Ion intercalation method, the other way of chemical exfoliation, was first used in 1986 by Joensen et al.32 to isolate individual MoS2 sheets. The method is to widen the interlayer space by intercalated lithium, as well as the H2 which evolves due to the reaction between water (or methanol, ethanol, and isopropanol) and intercalated lithium. By this method, monolayer MoS2 sheets were successfully achieved. But this process usually requires a relatively high temperature and long reaction time (several days). Moreover, the 2H crystal structure of MoS2 sheets usually becomes a mixture of 2H and 1T forms, and the low controllability of intercalation process leads to either a low yield of layered sheets with less intercalation, or decomposition of MoS2 to metal nanoparticles and Li2S with excess intercalation.33

The ion intercalation was improved later by introducing the concept of electrochemical process, which exploits a lithium foil as the anode and the objective layered bulk material, such as MoS2, WS2, or graphite, as the cathode to make a battery test cell (Fig. 1).34,35 By in situ monitoring and adjusting the measurable galvanostatic discharge rates, the insertion content of Li can be accurately controlled where the production yield of monolayer or few-layer sheets could reach as high as 92%.34 Moreover, the exfoliation by the electrochemical process can be achieved at room temperature within several hours, significantly improving the production efficiency.

Compared with the mechanical exfoliation, the yield of 2D materials by chemical exfoliation is significantly improved. However, the disadvantages of the chemical methods are obvious. The products degraded due to the change of the lattice structure,33 Hence, it is necessary to perform post-treatment to reconstruct the layer structure.36 Also, because the sonication is required, the 2D materials sheets obtained are non-uniform small in lateral dimension,37 which makes the large-scale device fabrication challenging.

Vapor phase deposition is a technique to prepare atomically thin flakes or films by directly depositing desired vapor-phased compounds with or without chemical reaction to form layers on specified substrates. By this technique, 2D material preparation can be realized with good crystallinity, high layer controllability, and large-area uniformity. Generally, vapor phase deposition includes three main approaches: metal transformation by sulfurization or selenization, thermal decomposition of precursors, physical vapor deposition (PVD), and chemical vapor deposition (CVD).

Piezoelectricity is a property of a material that generates electric potential upon an applied mechanical stress, whereas inverse-piezoelectricity is a property of a material that generates mechanical deformations upon an electric field. The basic principle of (Piezoelectric force microscopy) PFM is using the inverse-piezoelectric effect: an ac field is applied onto the sample surface through the AFM probe, which results in the surface deformation (vertical expansion/retraction and lateral torsion) of the sample.44 The amplitude/phase of this subtle deformation could be used to characterize the strength/direction of the dipole. PFM has been widely used to characterize the piezoelectric and ferroelectric materials, such as quantitative determination of piezoelectric coefficient,45 the ferroelectric domain imaging and domain switching dynamic behaviors.46 Specifically, a combined local butterfly-like amplitude switching loop and 180 phase reversal loop under the external electric bias, together with the switched domains induced by the opposite voltages are usually utilized to confirm the ferroelectric properties of the material. However, many non-ferroelectric materials could also show similar behaviors due to the coulombic electrostatic interaction between the AFM tip/cantilever and the charged sample surface, or the electrical driven ion accumulation/depletion induced sample deformation.47,48 These effects can be largely avoided by producing a homogeneous electric field through a top metal electrode, by advanced voltage pulse protocol, and through combined characterization with other tools such as Kelvin probe force microscopy.44 Many excellent reviews have been published over the years on the PFM characterization,44,48,49,50,51 and the reader is referred to these reviews for further information.

Besides, the demands for small-scale and diverse-functional devices are particularly urgent. In these regards, 2D piezoelectric nanomaterials become exceedingly intriguing due to their ultrathin geometry, excellent electromechanical response, and other unique physical properties.9,62 Furthermore, they are expected as the promising candidates and platforms for innovative design and development of future nanomechanical system, self-adaptive nanoelectronics/optoelectronics, and smart robotics.

Layered hexagonal boron nitride (h-BN) is a wide-band insulator and theoretically calculated to possess planar piezoelectric properties in 2009.67 The boron and nitride atoms in h-BN crystals also arrange in a hexagonal array (the left panel of Fig. 3b). Unlike its counterpart, graphene, monolayer h-BN keeps strong in-plane piezoelectricity due to the alternating arrangement of boron and nitride atoms in the hexagonal vertex site. 1n 2013, Tony F. Heinz et al. utilized optical second-harmonic generation (SHG) to demonstrate the non-centrosymmetric structure of h-BN (the right panel of Fig. 3b),68 indirectly verifying the existence of h-BN piezoelectricity. Note that along the directions of three-fold symmetry axis, h-BN always exhibits distinct SHG intensity and the SHG signals also show layer dependent effect in virtue of the special crystal structure.

In addition, some special material can exhibit intricate piezoelectricity by the coupling of in-plane and out-of-plane piezoelectricity. Recently, Janus MoSSe monolayer with broken out-of-plane symmetry is innovatively synthesized by the plasma stripping and thermal selenization.74 In this structure, one S-atom layer in monolayer MoS2 is artificially replaced by the Se-atom layer as shown in Fig. 3f, exhibiting the out-of-plane piezoelectricity that will not appear for the pristine monolayer MoS2. PFM signal has been used to characterize the vertical piezoelectricity as shown in the bottom panel of Fig. 3f. Moreover, such Janus monolayer should also possess the in-plane piezoelectric effect owing to the similar atom planar projections with TMDC family. Interestingly, CVD synthesized rhombohedral α-In2Se3 flakes have been demonstrated as in-plane and out-of-plane ferroelectric, from which we can scientifically infer that this material should also keep the in-plane and out-of-plane piezoelectricity (because a ferroelectric material must possess the piezoelectric effect).

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