Nonpolar Axis Research Articles

Anisotropic Strain-Tailoring Nonlinear Optical Response in van der Waals NbOI2.

Two-dimensional (2D) NbOI2 demonstrates significant second-harmonic generation (SHG) with a high conversion efficiency. To unlock its full potential in practical applications, it is desirable to modulate the SHG behavior while utilizing the intrinsic lattice anisotropy. Here, we demonstrate direction-specific modulation of the SHG response in NbOI2 by applying anisotropic strain with respect to the intrinsic lattice orientations, where more than 2-fold enhancement in the SHG intensity is achieved under strain along the polar axis. The strain-driven SHG evolution is attributed to the strengthened built-in piezoelectric field (polar axis) and the enlarged Peierls distortions (nonpolar axis). Moreover, we provide quantifications of the correlation between strain and SHG intensity in terms of the susceptibility tensor. Our results demonstrate the effective coupling of orientation-specific strain to the anisotropic SHG response through the intrinsic polar order in 2D nonlinear optical crystals, opening a new paradigm toward the development of functional devices.

Bulk Photovoltaic Effect Along the Nonpolar Axis in Organic-Inorganic Hybrid Perovskites.

The origin of the bulk photovoltaic effect (BPVE) was considered as a built-in electric field formed by the macroscopic polarization of materials. Alternatively, the "shift current mechanism" has been gradually accepted as the more appropriate description of the BPVE. This mechanism implies that the photocurrent generated by the BPVE is a topological current featuring an ultrafast response and dissipation-less nature, which is very attractive for photodetector applications. Meanwhile, the origin of the BPVE in organic-inorganic hybrid perovskites (OIHPs) has not been discussed and is still widely accepted as the classical mechanism without any experimental evidence. Herein, we observed the BPVE along the nonpolar axis in OIHPs, which is inconsistent with the classical explanation. Furthermore, based on the nonlinear optical tensor correlation, we substantiated that the BPVE in OIHPs is originated in the shift current mechanism.

Bulk Photovoltaic Effect Along the Nonpolar Axis in Organic‐Inorganic Hybrid Perovskites

AbstractThe origin of the bulk photovoltaic effect (BPVE) was considered as a built‐in electric field formed by the macroscopic polarization of materials. Alternatively, the “shift current mechanism” has been gradually accepted as the more appropriate description of the BPVE. This mechanism implies that the photocurrent generated by the BPVE is a topological current featuring an ultrafast response and dissipation‐less nature, which is very attractive for photodetector applications. Meanwhile, the origin of the BPVE in organic–inorganic hybrid perovskites (OIHPs) has not been discussed and is still widely accepted as the classical mechanism without any experimental evidence. Herein, we observed the BPVE along the nonpolar axis in OIHPs, which is inconsistent with the classical explanation. Furthermore, based on the nonlinear optical tensor correlation, we substantiated that the BPVE in OIHPs is originated in the shift current mechanism.

Applied in-plane strain effects on the polarization response of ferroelectric hafnium zirconium oxide thin films

The influence of biaxial stress on the maximum and remanent polarizations of 10 nm thick hafnium zirconium oxide thin films in metal–ferroelectric–metal capacitor structures has been quantified. In the as-prepared state with a nominal biaxial tensile strain of 0.20% and no applied extrinsic stress, remanent and maximum polarizations of 7.6 and 13.1 μC/cm2, respectively, were measured using a 2 MV/cm applied electric field. Reducing the intrinsic strain by 0.111% through the application of a compressive uniaxial stress results in a decrease in the remanent and maximum polarizations to 6.8 and 12.2 μC/cm2, respectively. The polarization dependence on strain is nearly linear between these values. The observed variation in polarization with strain is consistent with strain impacting ferroelastic switching whereby in-plane tension increases the fraction of the short polar axis orienting out-of-plane, hence increasing out-of-plane polarization. In contrast, reducing the in-plane strain through compression results in an increase in the fraction of the long non-polar axis orienting out-of-plane, thereby decreasing out-of-plane polarization.

Orientation effects and links between hydrostatic parameters in piezo-active 2–0–2 composites

The paper reports results on the large hydrostatic parameters of novel three-component 2–2-type composites based on [011]-poled relaxor-ferroelectric single crystals. The role of the rotation of the main crystallographic axes X and Y in forming the considerable hydrostatic piezoelectric response is studied. The composite consists of the single-crystal and 0–3 corundum ceramic/polyethylene layers that are regularly arranged along the non-polar axis. 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 and 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 single crystals poled along [011] of the perovskite unit cell are considered as main piezoelectric components of the composites with 2–0–2 connectivity. Links between the hydrostatic parameters (piezoelectric coefficients and squared figure of merit and electromechanical coupling factor ) are analysed by taking into account specifics of the microgeometry and elastic anisotropy of the 0–3 layer. Due to the large values of the aforementioned hydrostatic parameters, the studied 2–0–2 composites are of interest for hydroacoustic and piezoelectric energy-harvesting applications.

Orientation dependence of dielectric and piezoelectric properties of tetragonal relaxor ferroelectric single crystals by alternate current poling

Piezoelectric and dielectric properties of [001] and [110]-oriented tetragonal Pb(In1/2Nb1/2)-Pb(Mg1/3Nb2/3)-PbTiO3 (PIN-PMN-PT) single crystals (SCs) after alternate current poling (ACP) were investigated. Compared with direct current poling (DCP), the piezoelectric and dielectric properties of [001]-oriented samples have no improvement, while the piezoelectric coefficient d33 and free dielectric constant ε33T/ε0 of [110]-oriented samples increase by 32% and 26%, respectively. The domain was observed on (001) and (11¯0) faces of [110]-oriented samples. The enhancement of the piezoelectric performance is attributed to the high density of domain wall, increase of distorted regions between 90° domains, and monoclinic phase. Compared with DCP, both intrinsic and extrinsic contributions enhance after ACP though the Rayleigh analysis. This work indicates that ACP is an effective way to enhance piezoelectric properties of non-polar axis oriented relaxor-PT ferroelectric SCs.

Domain orientations and piezoelectric properties in novel 2–2-type composites with two single-crystal components

The present paper is concerned with the piezoelectric properties and related parameters of the 2–2-type parallel-connected composites wherein rotations of the main crystallographic axes in ferroelectric single crystal components are analyzed. The composite consists of layers of two types that are regularly arranged along the non-polar axis. The Type I layer is a single domain 0.63Pb(Mg1/3Nb2/3)O3–0.37PbTiO3 single crystal (either tetragonal in Composite I or orthorhombic in Composite II). The Type II layer is a Li2B4O7 single crystal/polyethylene medium with 0–3 connectivity. The composite as a whole is characterized by 2–0–2 connectivity. The following parameters of the composite are studied by taking into account the polarization orientation effect in the Type I layer and the microgeometry of the composite in the Type II layer: piezoelectric coefficients and their hydrostatic analogs and hydrostatic squared figure of merit piezoelectric anisotropy factors / and / It is shown that large values of the studied piezoelectric coefficients, hydrostatic parameters and anisotropy factors make Composite II more preferable in comparison to Composite I for hydroacoustic, piezoelectric sensor and energy-harvesting applications.

Unravelling a solution-based formation of single-crystalline kinked wurtzite nanowires: The case of MnSe

The search for a novel strategy to sculpt semiconductor nanowires (NWs) at the atomistic scale is crucial for the development of new paradigms in optics, electronics, and spintronics. Thus far, the fabrication of single-crystalline kinked semiconductor NWs has been achieved mainly through the vapor−liquid−solid growth technique. In this study, we developed a new strategy for sculpting single-crystalline kinked wurtzite (WZ) MnSe NWs by triggering the nonpolar axial-oriented growth, thereby switching—at the atomistic scale—the NW growth orientation along the nonpolar axes in a facile solution-based procedure. This presents substantial challenges owing to the dominant polar c axis growth in the solution-based synthesis of one-dimensional WZ nanocrystals. More significantly, the ability to continuously switch the nonpolar axial-growth orientation allowed us to craft the kinking landscape of types 150°, 120°, 90°, and 60°. A probabilistic analysis of kinked MnSe NWs reveals the correlations of the synergy and interplay between these two sets of nonpolar axial growth-orientation switching, which determine the actual kinked motifs. Furthermore, discriminating the side-facet structures of the kinked NWs significantly strengthened the spatially selected interaction of Au nanoparticles. We envisage that such a facile solution-based strategy can be useful for synthesizing other single-crystalline kinked WZ-type transition-metal dichalcogenide NWs to develop novel functional materials with finely tuned properties.

Electromechanical properties and anisotropy of single- and multi-domain 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Complete sets of elastic, piezoelectric, and dielectric constants of 0.72Pb(Mg(1∕3)Nb(2∕3))O(3)-0.28PbTiO(3) single crystal poled along [111](c) (single domain) as well as non-polar axes [001](c) and [011](c) (multidomain) have been measured under natural conditions. These data allowed us to evaluate accurately the extrinsic contributions to the superior piezoelectric properties. Very large extrinsic contributions to the unusual anisotropies in multidomain crystals are confirmed. We found that the instability of domain structures is the origin of the low mechanical quality factor Q for the multidomain relaxor-based ferroelectric single crystals. Our results can provide useful guidance in future design of domain engineered materials.

Control of domain configurations and sizes in crystallographically engineered ferroelectric single crystals: Phase field modeling

Computer simulation is performed to study the mechanisms for controlling domain configurations and sizes in crystallographically engineered ferroelectric crystals. It is found that minimal domain sizes and highest domain wall densities are obtained with intermediate electric field applied along nonpolar axis of ferroelectric crystals, while lower and higher fields produce coarser domains, and temperature also plays important role in domain size control. The simulation shows that selection of polar domain variants by external electric field during nucleation stage of ferroelectric phase transition significantly affects subsequent domain growth and evolution kinetics, controlling the formation and sizes of lamellar domains.

First principles study of the responses to equibiaxial in-plane tensile stresses in tetragonal BaTiO 3

Tetragonal BaTiO 3 under equibiaxial in-plane tensile stresses is investigated from first principles calculations. Our results show that this material transfers to the paraelectric phase at the critical stress of about 13 GPa. We also find the great enhancement of piezoelectricity close to the phase-transition region, due to large atomic displacements induced by stress. We also show that under the loading of applied stress, this material becomes markedly hard along the polar axis, especially at the phase-transition stress, but becomes soft along the nonpolar axes.

Influence of Uniaxial Tensile Stress on the Mechanical and Piezoelectric Properties of Short-period Ferroelectric Superlattice

Tetragonal ferroelectric/ferroelectricsuperlattice under uniaxial tensile stress along the c axis is investigated from first principles. We show that the calculated ideal tensile strength is 6.85 GPa and that the superlattice under the loading of uniaxial tensile stress becomes soft along the nonpolar axes. We also find that the appropriately applied uniaxial tensile stress can significantly enhance the piezoelectricity for the superlattice, with piezoelectric coefficient d33 increasing from the ground state value by a factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the enhancement of piezoelectricity is discussed.

Effects of orientation and composition on the extrinsic contributions to the dielectric response of relaxor-ferroelectric single crystals

The dielectric response of (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-x%PT,4.5≤x≤12) single crystals is reported for ac field amplitudes up to ∼2 kV/cm at 1 kHz. The Rayleigh law is used to study the dielectric nonlinear response. It is found that the extrinsic contributions are extremely anisotropic. Rhombohedral crystals poled along [001]C showed enhanced extrinsic contributions to their dielectric response with respect to those poled along [111]C, while in tetragonal crystals, the enhancement was observed for poling along [111]C rather than along [001]C. This is attributed to “domain engineering” or higher domain wall concentrations in ferroelectric crystals poled along nonpolar axes.

Field-induced inter-ferroelectric phase transformations and domain mechanisms in high-strain piezoelectric materials: insights from phase field modeling and simulation

Large, reversible and anhysteretic strain induced by external field is desired for multifunctional materials used in sensors, actuators and transducers. However, these desired attributes usually compromise each other, leading to trade-offs in materials’ properties and limiting their applicability. This paper focuses on field-induced inter-ferroelectric phase transformations. Some fundamental principles and domain mechanisms are systematized based on insights learned from phase field modeling and simulations, whose synergistic operations are expected to provide unique combination of large, reversible, and anhysteretic strain attributes. These working principles are: (i) Field-induced inter-ferroelectric structural phase transformation to achieve large strain; (ii) Field-induced stable-metastable phase transformation to maximize reversibility; (iii) Heterogeneous nucleation-and-growth process at domain walls to enhance low-field responses; (iv) Deactivation of domain wall motion by applying external fields along nonpolar axes to minimize hysteresis; (v) Domain wall broadening mechanism and domain size effect to exploit nanoscale engineered domain microstructures; and (vi) Bridging domain mechanism and phase coexistence to promote ferroelectric shape memory effects. It is shown that special initial domain microstructures and preferred evolution kinetic pathways can be achieved by crystallographic domain engineering technique, which allow multiple principles to work together without compromising one another. Due to the commonalities and fundamental interrelations among ferroelectric, ferromagnetic and ferroelastic materials, the gained understanding of thermodynamic and kinetic principles has general implications to displacive phase transformations in ferroic materials and is helpful for design of new functional materials with advanced field-induced strain properties.

Optical properties of GaN/AlN quantum dots

We present here a review of the peculiar optical properties of GaN/AlN quantum dots. These systems show unusually large exciton binding energies and band-offsets. Moreover, when grown along the (0001) axis in the wurtzite phase, the optical properties are dominated by huge on-axis internal electric fields, leading to a very low oscillator strength and complex dynamical behavior. It is also possible to grow GaN quantum dots in the cubic phase or along nonpolar axis of the wurtzite cell. We discuss properties of ensembles of quantum dots, as well as of single quantum dots studied by micro-photoluminescence. To cite this article: P. Lefebvre, B. Gayral, C. R. Physique 9 (2008).

Domain wall broadening mechanism for domain size effect of enhanced piezoelectricity in crystallographically engineered ferroelectric single crystals

Computer modeling and simulation reveal a domain wall broadening mechanism that explains the domain size effect of enhanced piezoelectric properties in domain engineered ferroelectric single crystals. The simulation shows that, under electric field applied along the nonpolar axis of single crystal without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. This mechanism plays a significant role in the vicinity of interferroelectric transition temperature and morphotropic phase boundary, where energy difference between stable and metastable phases is small. Engineered domain configuration fully exploits this domain wall broadening mechanism.

Contributions to the Piezoelectric Effect in Ferroelectric Single Crystals and Ceramics

The piezoelectric effect in ferroelectric single crystals and ceramics is investigated considering intrinsic (lattice), and extrinsic (originating mainly from displacement of domain walls) contributions. The focus of the study of intrinsic properties is on piezoelectric anisotropy, which was examined using the Landau–Ginsburg–Devonshire phenomenological theory. It is shown that the enhanced piezoelectric response along nonpolar directions, observed in many perovskite systems, is a consequence of the flattening of the Gibbs free energy profile. This flattening is common for temperature‐, composition‐, and external field‐induced enhancement of the piezoelectric properties along nonpolar axes. A brief review of recent advances in understanding the origins of the piezoelectric nonlinearity, hysteresis, and frequency dispersion is also given.

Correlation between dielectric anisotropy and positive or zero transverse piezoelectric coefficients in perovskite ferroelectric single crystals

The transverse piezoelectric coefficient (d31⋆) has been calculated for tetragonal barium titanate (BT) and lead titanate (PT) cut along nonpolar axes, over a range of temperatures, using the phenomenological Landau-Ginzburg-Devonshire theory. It is shown that negative values of d31⋆ are favored when the transverse dielectric susceptibility η11 and shear coefficient d15 are large, such that polarization rotation is more significant than the collinear piezoelectric effect. This occurs in tetragonal BT close to its ferroelectric-ferroelectric phase transition to an orthorhombic phase. In PT, however, where no phase transition occurs, d15 is small and polarization “extension” due to d33 dominates: small or positive values of d31⋆ become prominent. This extends a previous result found for polycrystalline modified PT ceramics.

Nanoscale polarization relaxation in a polycrystalline ferroelectric thin film: Role of local environments

In this letter, we report on the study of nanoscale polarization relaxation phenomena in polycrystalline PbZr0.4Ti0.6O3 films. Piezoresponse force microscopy (PFM) images of the as-grown sample reveal grains with a range of contrast, from fully white to gray to fully black. It is shown that this local change in the contrast (magnitude) of the piezoresponse from grain to grain can be attributed to the crystallographic orientation within each grain. PFM-based relaxation experiments show that the rate of relaxation is different for each grain, furthermore it is strongly dependent on the tilt of individual crystallographic orientation with respect to the polar axis. Strongly tilted away nonpolar axis grains show a much stronger decay of the polarization compared to polar axis-oriented grains. Therefore, for an ensemble of grains under a common top electrode, the relaxation events would first take place in grains, which are nonpolar axis oriented.

Structure and properties of GaN films grown on single crystalline ZnO substrates by molecular beam epitaxy

GaN films were grown on ZnO single crystal substrates with various orientations by a molecular beam epitaxy method. Particularly, applicability of ZnO substrates for growth of GaN along non-polar axis was examined by using ZnO substrates with (1 1 2¯ 0) or (1 0 1¯ 0) orientation and effect of lattice matched (In,Ga)N buffer layer on crystallinity of GaN films was also investigated. Epitaxial GaN films were obtained regardless of the absence of (In,Ga)N buffer layer, when ZnO substrates with c-face were used. In contrast, GaN films deposited on (1 1 2¯ 0) or (1 0 1¯ 0) face of ZnO exhibited complicated manner of lattice orientation. Epitaxial growth of GaN films along non-polar axis was achieved only when the GaN films were grown on (In,Ga)N buffer layer deposited on ZnO substrates of (1 1 2¯ 0) orientation.