Tunable band gaps in bilayer graphene-BN heterostructures

We investigate band gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.     

Science and technology of ferroelectric films and heterostructures for non-volatile ferroelectric memories

We present in this article a review of the status of thin film ferroelectric materials for nonvolatile memories. Key materials issues relevant to the integration of these materials on Si wafers are discussed. The effect of film microstructure and electrode defect chemistry on the ferroelectric properties relevant to a high density nonvolatile memory technology are discussed. The second part of this review focuses on approaches to integrate these capacitor structures on a filled poly-Si plug which is a critical requirement for a high density memory technology. Finally, the use of novel surface probes to study and understand broadband polarization dynamics in ferroelectric thin films is also presented.     

Interlayer relaxation in group-III-nitride-based heterostructures according to X-Ray diffraction data

The structure perfection in two samples of the InN-GaN bilayer heterosystem, grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, has been studied by the X-Ray diffraction techniques. Components of the microdistortion tensor were determined from an analysis of the broadening of diffraction peaks measured in various geometries. These data were used to evaluate the densities of various dislocation families in each layer of the heterosystem and to trace a change in the dislocated structure from the lower (GaN) to upper (InN) layer. A difference in the behavior of dislocations in the two samples grown by different methods (MBE versus MOVPE) suggests that different mechanisms of relaxation of the elastic stresses between InN and GaN layers are operative in these cases.     

Interlayer coupling in peroxitonic model of superconductivity

The unusual ESR spectra of YBa₂Cu₃O_7−x is ascribed to a Cu²⁺ ion, whose one ligand in CuO₂ plane is involved in a peroxiton O⁻−Cu⁺−O⁻ bond. The analysis shows that the presence of a ligand in O⁻ substantially increases the energies ofd-hole orbitals. Since this increase for YBa₂Cu₃O_7\t-\gd, orbital is much larger than that ford ₃ z ²−r ² orbital, their separation is drastically reduced and there occurs a significant orthorhombic mixing between the two. This charge transfer from a planard _x ²− _r ² orbital to ad ₃ z ²−r ² orbital, which lies mainly perpendicular to the plane, provides a mechanism for interlayer coupling. The effect of this coupling on superconducting transition temperature is discussed.     

III-Nitrides growth and AlGaN/GaN heterostructures on ferroelectric materials

The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb₃O₈) along with lithium niobate (LiNbO₃), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.     

Relaxor ferroelectric thin-film heterostructures: Scaling of dielectric properties

An experimental study of the dielectric behavior of relaxor ferroelectric (RFE) thin-film capacitor heterostructures and a separate analysis of the dielectric properties of the RFE films are presented. Epitaxial heterostructures of RFE PbMg_1/3Nb_2/3O₃, PbSc_0.5Nb_0.5O₃, PbMg_1/3Nb_2/3O₃-PbTiO₃ and PbSc_0.5Nb_0.5O₃-PbTiO thin films (100–500 nm) with La_0.5Sr_0.5CoO₃ bottom and Pt top electrodes were fabricated by in situ pulsed laser deposition on MgO (1 0 0) and LaAlO₃(1 0 0). Dielectric properties of the heterostructures were studied as a function of frequency (10²–10⁶ Hz), temperature (77–725 K), and amplitude of applied a.c. field (10²–10⁶ V m^–1). The contribution of the film/electrode interfaces to the properties of the heterostructures was evaluated using a model of a series capacitor connection of the film and passive interface layers. The characteristics of the interface layers were experimentally found from the temperature evolution of the dielectric response of the heterostructures, and the true properties of the films were reconstructed. In the films, all typical features of RFE were found to be essentially similar to those in single crystals. Also, it was shown that in heterostructures, both a relaxor-like behavior and an apparent dielectric nonlinearity can be determined by the film/electrode interface rather than by the films.     

Pyroelectric properties of a ferroelectric superlattice with surface transition layers

The transverse Ising model within the framework of the mean-field theory is used to investigate a ferroelectric superlattice composed of two different alternating ferroelectric slabs A and B, with the surface transition layer within each slab and an antiferroelectric interfacial coupling between two slabs. The combined influence of the surface transition layer and antiferroelectric interfacial coupling on the pyroelectric properties is discussed in detail. The results show that the pyroelectric properties of a ferroelectric superlattice present some interesting phenomena because of the existence of surface transition layer.     

Electroacoustic waves of a moving domain wall superlattice in a ferroelectric crystal

The dispersion properties of electroacoustic wave modes confined by a superlattice of 180° domain walls uniformly moving in a tetragonal ferroelectric crystal are considered. It is shown that the manifold of partial electroacoustic interfacial waves in the superlattice is restricted to the first allowed band, the configuration of which in the plane of spectral variables can significantly vary under the action of moving domain walls. For the partial electroacoustic interfacial waves with the Bloch wavenumbers χ π/d (where d is the lattice half-period), the motion of domain walls is predicted to result in splitting of the modes of a static superlattice into pairs, which is invariant with respect to reversal of the direction of motion.     

Sulfur-vacancy-tunable interlayer magnetic coupling in centimeter-scale MoS2 bilayer

Endowing bilayer transition-metal dichalcogenides(TMDs)with tunable magnetism is significant to investigate the coupling of multiple electron degrees of freedom...     

Device performance of ferroelectric/correlated oxide heterostructures for non-volatile memory applications

Ferroelectric field effect devices offer the possibility of non-volatile data storage. Attempts to integrate perovskite ferroelectric materials with silicon semiconductors, however, have been largely unsuccessful in creating non-volatile, nondestructive read memory elements because of difficulties in controlling the ferroelectric/semiconductor interface. Correlated oxide systems have been explored as alternative channel materials to form all-perovskite field effect devices. We examine a non-volatile memory using an electric-field-induced metal-insulator transition in PbZr(0.2)Ti(0.8)O(3)/La(1 - x)Sr(x)MnO(3) (PZT/LSMO), PZT/La(1 - x)Ca(x)MnO(3) (PZT/LCMO) and PZT/La(1 - x)Sr(x)CoO(3) (PZT/LSCO) devices. The performance of these devices in the areas of switching time and retention are discussed.     

Large tuning in the electrowetting behaviour on ferroelectric PVDF-HFP/Teflon AF bilayer

Electrowetting (EW) response on a dielectric depends on its permittivity value, Young contact angle and voltage amplitude. We present a large change in EW contact angle, from 163° to 80°, on the bilayer dielectric made up of ferroelectric PVDF-HFP with a thin layer of fluoropolymer. The thickness values of both layers were separately optimized for high effective capacitance essential for the large EW response. It reveals that the bilayer with?~?500 nm thick PVDF-HFP layer and?~?50 nm thin layer of Teflon results in the maximum value of effective dielectric constant, ε ≈ 8. Besides this gain, dc-voltage EW response exhibits hysteresis mainly due to polarization in the ferroelectric layer such that, hysteretic offset voltage was found to depend on the applied voltage amplitude and thickness of the dielectrics. Finally, bilayer was subjected to ac-voltage EW in silicone oil for ambient temperature ranging from ??25 to 70 °C. The consistent EW response in this ambient without any degradation/delamination of polymer surface confirmed the durability of the bilayer on the transparent ITO electrodes.

     

Sulfur-vacancy-tunable interlayer magnetic coupling in centimeter-scale M0S2 bilayer

Endowing bilayer transition-metal dichalcogenides(TMDs)with tunable magnetism is significant to investigate the coupling of multiple electron degrees of freedom(DOFs).However,effectively inducing and tuning the magnetic interaction of bilayer TMDs are still challenges.Herein,we report a strategy to tune the interlayer exchange interaction of centimeter-scale MoS2 bilayer with substitutional doping of Co ion,by introducing sulfur vacancy(V_s)to modulate the interlayer electronic coupling.This strategy could transform the interlayer exchange interaction from antiferromagnetism(AFM)to ferromagnetism(FM),as revealed by the magnetic measurements.Experimental characterizations and theoretical calculations indicate that the enhanced magnetization is mainly because the hybridization of Co 3d band and Vs-induced impurity band alters the forms of interlayer orbital hybridizations between the partial Co atoms in upper and lower layers,and also enhances the intralayer FM.Our work paves the way for tuning the interlayer exchange interaction with defects and could be extended to other two-dimensional(2D)magnetic materials.     

Interlayer coupling in anisotropic/isotropic van der Waals heterostructures of ReS<Subscript>2</Subscript> and MoS<Subscript>2</Subscript> monolayers

In-plane symmetry is an important contributor to the physical properties of two-dimensional layered materials, as well as atomically thin heterojunctions. Here, we demonstrate anisotropic/isotropic van der Waals (vdW) heterostructures of ReS₂ and MoS₂ monolayers, where interlayer coupling interactions and charge separation were observed by in situ Raman-photoluminescence spectroscopy, electrical, and photoelectrical measurements. We believe that these results could be helpful for understanding the fundamental physics of atomically thin vdW heterostructures and creating novel electronic and optoelectronic devices.

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Epitaxial growth and interface strain coupling effects in manganite film/piezoelectric-crystal multiferroic heterostructures

We constructed multiferroic structures by epitaxially growing colossal magnetoresistive La_0.7Sr_0.3MnO₃ (LSMO) thin films on piezoelectric single-crystal substrates of composition 0.67Pb(Mg_1/3Nb_2/3)O₃–0.33PbTiO₃ (PMN-PT). Due to the efficient elastic coupling at the interface, the electric-field-induced piezoelectric strain (?_piezo) in the PMN-PT substrate is effectively transferred to the LSMO film, giving rise to a remarkable modulation of the lattice strain, resistivity, and Curie temperature T_C of the LSMO film. Particularly, it was found that the magnetic field has an opposite effect on the strain-tunability of resistivity above and below T_C. Moreover, we found that the resistivity of the film is most sensitive to ?_piezo near T_C and becomes less sensitive to ?_piezo when the temperature is lower or higher than T_C. These, together with the well fitted resistivity data into a phenomenological model based on coexisting phases, demonstrate that the phase separation is crucial to understand the strain-mediated multiferroic properties in manganite film/PMN-PT structures.     

Electrostatic coupling and local structural distortions at interfaces in ferroelectric/paraelectric superlattices

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.     

高度取向的(Pb0.5Sr0.5)TiO3/LaNiO3异质结构铁电薄膜性能

用sol-gel方法在Pt(111)/Ti/SiO2/Si(100)衬底上制备了具有LaN iO3(LNO)缓冲层的(Pb0.5Sr0.5)TiO3(PSrT50)/LNO/Pt异质结构铁电薄膜。X射线分析发现PSrT50薄膜在(100)方向高度取向,同时扫描电镜图像显示薄膜结构致密、表面平整。通过和直接在Pt上制备的、相同厚度的PSrT50薄膜比较,PSrT50/LNO/Pt结构薄膜在室温下具有更大的剩余极化(Pr=4.5μC/cm2)和更高的介电常数(rε=850)。同时,漏电流机理分析表明,PSrT50/LNO/Pt结构薄膜在低电场作用下呈现Pool-Frenkel发射效应,在高电场作用下则表现为空间电荷限制电流。     

Effect of strain on ferroelectric and magnetic behavior in Pb(Zr0.52Ti0.48)O3-based magnetoelectric heterostructures

In this paper, the "sandwich" structured magnetoelectric composite films of Pb(Zr0.52Ti0.48)O3/ NiFe2O4/Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.52Ti0.48)O3/CoFe2O4/Pb(Zr0.52Ti0.48)O3 are epitaxially grown on SrRuO3/SrTiO3 substrates by pulsed-laser deposition. The crystalline quality and microstructures of these heterostructures are investigated by X-ray diffraction technique. The effects of strain on the ferroelectric, magnetic and magnetoelectric coupling properties of these thin films are systematically studied. The results show that the strain effect induced by lattice mismatch between the ferroelectric/ferromagnetic layers plays an important role in the ferroelectric and magnetic properties of these composite films. Compared to the strained Pb(Zr0.52Ti0.48)O3/ CoFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure, improved ferroelectric properties with an out-of-plane polarization (2P(r)) of 34.2 microC/cm2 and electric coercivity field of 158 kV/cm are obtained in the strain-free Pb(Zr0.52Ti0.48)O3/NiFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure. The ME measurement results not only show that the strain induced by lattice mismatch has great influence on the ME behavior, but also provide an understanding of the multilayers with full control over the interface structure at the atomic-scale.     

Magnetic, ferroelectric and magnetoelectric coupling properties of Fe-implanted BaTiO3 films

BaTiO₃ (BTO) nano-films were implanted with iron by means of ion-implantation. According to the results of X-ray diffraction (XRD) and Raman analysis, all the films are single-phase with tetragonal structure. Upon the increase of Fe dosage, on one hand, there is a decrease in c/a ratio of the unit cells. As the grain size and surface roughness of the Fe-implanted films decrease, the well crystallized film was obtained. Consequently, both decreasing of coercivity and boosting of magnetization were observed simultaneously. With the decline of the grain size and surface roughness during thermal treatment of the Fe-implanted films, there is an improvement of Fe distribution across the BTO matrix. Consequently, there is a decrease of coercivity and rise of magnetization. Upon the decrease of Fe dosage, on the other, there is a decline in magnetization, and a ferroelectric hysteresis loop is observed at a Fe dosage of 3.0 × 10¹⁴ cm^− 2. Additionally, the dependence of capacitance on the applied magnetic field and its frequency were also monitored.