Magnetoelectric Coupling in Room Temperature Multiferroic Ba2EuFeNb4O15/BaFe12O19 Epitaxial Heterostructures Grown by Laser Ablation.

Multiferroic thin films are a promising class of multifunctional materials, since they allow the integration of multiple functionalities within a single device. In order to overcome the scarcity of single phase multiferroics, it is crucial to develop novel multiferroic heterostructures, combining good ferroelectric and ferromagnetic properties as well as a strong coupling between them. For this purpose, Ba2EuFeNb4O15/BaFe12O19 multiferroic magnetoelectric bilayers have been epitaxially grown on niobium doped SrTiO3 (100) single crystal substrates by pulsed laser deposition. The simultaneous presence of both ferroelectric and magnetic properties-due, respectively, to the Ba2EuFeNb4O15 and BaFe12O19 components-was demonstrated at room temperature, attesting the multiferroic nature of the heterostructure. More interestingly, a strong magnetoelectric coupling was demonstrated (i) by manipulating the ferroelectric properties via an external magnetic field, and conversely, (ii) by tuning the magnetic properties via an external electric field. This strong magnetoelectric coupling shows the high interdependence of both ferroic orders in the Ba2EuFeNb4O15/BaFe12O19 heterostructure, mediated by elastic (epitaxial) strain at the interfaces.

Phonon confinement and particle size effect on the low-frequency Raman mode of aurivillius phase Bi4Ti3O12 powders.

We report the systematic measurements in bismuth titanate powders of Raman frequency shift, ω and full width at half maximum (FWHM), Γ of optical phonons at q = 0 obtained between ∼300 K and 673 K in air. Both the particle size and phonon confinement effects are reasonably satisfactory to explain the Raman peak shift and asymmetric broadening observed in the ferroelectric soft phonon mode at 42 cm-1. It is shown that the lattice parameter varies as particle size x, and its contribution to size-dependent Raman shift and broadening of linewidth follows ω ∝ x -0.73 and Γ âˆ x -0.38 law, respectively. Moreover, a single phonon coupling term corresponding to a three-phonon anharmonic process is sufficient to describe the phonon coupling decay culminating from the softening of this strongly overdamped phonon mode.

Low-cost photodetector architectures fabricated at room-temperature using nano-engineered silicon wafer and sol-gel TiO2 - based heterostructures.

In the last decades, significant research has been done on the nanocrystalline forms of titanium dioxide (TiO2). Amorphous TiO2 has not been studied intensively despite being significantly less expensive compared to crystalline TiO2. This study reveals significant improvement in UV-VIS photodetection properties from heterostructures fabricated in ambient environment using n-type silicon nanowire arrays and amorphous TiO2 sol-gel. Our ultra-low-cost UV-VIS photodetectors can cover a wide range of applications. We report fast rise/decay time constants of 0.23 ms/0.17 ms and high responsivity up-to 6.0 A/W in the UV and 25.0 A/W in the visible range under low (1 V) external bias. The large surface area due to the nanowire array architecture leads to 2 orders of magnitude enhancement in photo-response. Besides the final electrode deposition, the entire device fabrication is performed using low-cost, all solution-based methods in ambient conditions. These low-cost UV-Visible broadband photodetectors can potentially serve a wide range of applications.

Highly Efficient and Ultrasensitive Large-Area Flexible Photodetector Based on Perovskite Nanowires.

Hybrid organic-inorganic perovskites have shown exceptional semiconducting properties and microstructural versatility for inexpensive, solution-processable photovoltaic and optoelectronic devices. In this work, an all-solution-based technique in ambient environment for highly sensitive and high-speed flexible photodetectors using high crystal quality perovskite nanowires grown on Kapton substrate is presented. At 10 V, the optimized photodetector exhibits a responsivity as high as 0.62 A W-1 , a maximum specific detectivity of 7.3 × 1012 cm Hz1/2 W-1 , and a rise time of 227.2 µs. It also shows remarkable photocurrent stability even beyond 5000 bending cycles. Moreover, a deposition of poly(methyl methacrylate) (PMMA) as a protective layer on the perovskite yields significantly better stability under ambient air operation: the PMMA-protected devices are stable for over 30 days. This work demonstrates a cost-effective fabrication technique for high-performance flexible photodetectors and opens opportunities for research advancements in broadband and large-scale flexible perovskite-based optoelectronic devices.

Hysteresis-Free 1D Network Mixed Halide-Perovskite Semitransparent Solar Cells.

The morphology of hybrid organic-inorganic perovskite films is known to strongly affect the performance of perovskite-based solar cells. CH3 NH3 PbI3-x Clx (MAPbI3-x Clx ) films have been previously fabricated with 100% surface coverage in glove boxes. In ambient air, fabrication generally relies on solvent engineering to obtain compact films. In contrast, this work explores the potential of altering the perovskites microstructure for solar cell engineering. This work starts with CH3 NH3 PbI3-x Clx , films with grain morphology carefully controlled by varying the deposition speed during the spin-coating process to fabricate efficient and partially transparent solar cells. Devices produced with a CH3 NH3 PbI3-x Clx film and a compact thick top gold electrode reach a maximum efficiency of 10.2% but display a large photocurrent hysteresis. As it is demonstrated, the introduction of different concentrations of bromide in the precursor solution addresses the hysteresis issues and turns the film morphology into a partially transparent interconnected network of 1D microstructures. This approach leads to semitransparent solar cells with negligible hysteresis and efficiencies up to 7.2%, while allowing average transmission of 17% across the visible spectrum. This work demonstrates that the optimization of the perovskites composition can mitigate the hysteresis effects commonly attributed to the charge trapping within the perovskite film.

Effect of the gold crystallinity on the enhanced luminescence signal of scanning probe tips in apertureless near-field optical microscopy.

The effect of gold tip crystallinity on their spectral amplification characteristics, monitored through the luminescence enhanced by surface plasmon resonance (SPR), is investigated experimentally. As the tip radius increases, the grains composing polycrystalline tips become larger, resulting in a blueshift of the emission while a redshift of the SPR was predicted for monocrystalline gold. This reveals that the effect of the grain size, a parameter that has not been considered so far, is dominant over that of the tip radius. This study is significant to apertureless scanning near-field optical microscopy, where the gold tip emission defines the spectral antenna range.

Epitaxially stabilized thin films of ε-Fe2O3 (001) grown on YSZ (100).

Epsilon ferrite (ε-Fe2O3) is a metastable phase of iron(III) oxide, intermediate between maghemite and hematite. It has recently attracted interest because of its magnetocrystalline anisotropy, which distinguishes it from the other polymorphs, and results in a gigantic coercive field and a natural ferromagnetic resonance frequency in the THz range. Moreover, it possesses a polar crystal structure, making it a potential ferroelectric, hence a potential multiferroic. Due to the need of size confinement to stabilize the metastable phase, ε-Fe2O3 has been synthesized mainly as nanoparticles. However, to favor integration in devices, and take advantage of its unique functional properties, synthesis as epitaxial thin films is desirable. In this paper, we report the growth of ε-Fe2O3 as epitaxial thin films on (100)-oriented yttrium-stabilized zirconia substrates. Structural characterization outlined the formation of multiple in-plane twins, with two different epitaxial relations to the substrate. Transmission electron microscopy showed how such twins develop in a pillar-like structure from the interface to the surface. Magnetic characterization confirmed the high magnetocrystalline anisotropy of our film and revealed the presence of a secondary phase which was identified as the well-known magnetite. Finally, angular analysis of the magnetic properties revealed how the presence of twins impacts their azimuthal dependence.

Permittivity imaged at the nanoscale using tip-enhanced Raman spectroscopy.

Localized surface plasmon resonances are the dominating contribution to the optical enhancement and the lateral resolution in tip-enhanced Raman spectroscopy. This well studied phenomenon may give access to more information about the sample than the enhanced Raman spectra alone due to its sensitivity to the permittivity of the tip environment. In this work, the effects of the permittivity of the sample on the properties of localized surface plasmon resonance are studied through the amplified signal of the luminescence of gold tips.

Ferroelectric switching in Bi4Ti3O12 nanorods.

We report the piezoelectric and ferroelectric properties of individual one-dimensional objects made of Bi(4)Ti(3)O(12) (BiT). The nanorods and nanowires investigated in this study were fabricated by a two-step process: 1) preparation of reactive templates using hydrothermal-like synthesis and colloidal chemistry and 2) transformation of the reactive templates in Bi(4)Ti(3)O(12) by solid-state reaction, overcoming the morphological instability problem of 1-D templates. Using piezoresponse force microscopy (PFM) with both out-of-plane and in-plane detection capability, we show that both types of objects exhibit strong piezoelectric activity and good switching ferroelectric behavior. Analysis of the PFM hysteresis loops obtained revealed that the coercive voltage of the in-plane PFM signal can be either equal to or different from that of the out-of-plane response. We associate these situations with two types of polarization switching mechanisms: direct 180° switching, and via rotation of polarization, resulting from the independent switching of the components along the a- and ccrystallographic axes. In a few instances, we observe a negative piezoelectric coefficient, which we explain by the specific shape of the piezoelectric surface of Bi(4)Ti(3)O(12).

Imaging, core-loss, and low-loss electron-energy-loss spectroscopy mapping in aberration-corrected STEM.

High-angle annular dark-field and annular bright-field imaging experiments were carried out on an aberration-corrected transmission electron microscope. These techniques have been demonstrated on thin films of complex oxides Ba3.25La0.75Ti3O12 and on LaB6. The results show good agreement between theory and experiments, and for the case of LaB6 they demonstrate the detection of contrast from the B atoms in the annular bright-field images. Elemental mapping with electron-energy-loss spectroscopy has been used to deduce the distribution of Cr and Fe in a thin film of the complex oxide Bi2(Fe1/2Cr3/2)O6 at the unit cell level and the changes in the near-edge structure within the inequivalent regions in the crystalline unit cell. Energy-filtered images in the low-loss region of the energy-loss spectrum show contrast and resolution consistent with the modulation of the signals from elastic scattering. High-resolution contrast, mediated by phonon scattering, is observed for interband transitions. The limitations in terms of detection and signal are discussed.

Two-dimensional nanoscale structural and functional imaging in individual collagen type I fibrils.

The piezoelectric properties of single collagen type I fibrils in fascia were imaged with sub-20 nm spatial resolution using piezoresponse force microscopy. A detailed analysis of the piezoresponse force microscopy signal in controlled tip-fibril geometry revealed shear piezoelectricity parallel to the fibril axis. The direction of the displacement is preserved along the whole fiber length and is independent of the fiber conformation. It is shown that individual fibrils within bundles in skeletal muscle fascia can have opposite polar orientations and are organized into domains, i.e., groups of several fibers having the same polar orientation. We were also able to detect piezoelectric activity of collagen fibrils in the high-frequency range up to 200 kHz, suggesting that the mechanical response time of biomolecules to electrical stimuli can be approximately 5 micros.

The structural origin of second harmonic generation in fascia.

Fascia tissue is rich in collagen type I proteins and can be imaged by second harmonic generation (SHG) microscopy. While identifying the overall alignment of the collagen fibrils is evident from those images, the tridimensional structural origin for the observation of SHG signal is more complex than it apparently seems. Those images reveal that the noncentrosymmetric (piezoelectric) structures are distributed heterogeneously on spatial dimensions inferior to the resolution provided by the nonlinear optical microscope (sub-micron). Using piezoresponse force microscopy (PFM), we show that an individual collagen fibril has a noncentrosymmetric structural organization. Fibrils are found to be arranged in nano-domains where the anisotropic axis is preserved along the fibrillar axis, while across the collagen sheets, the phase of the second order nonlinear susceptibility is changing by 180 degrees between adjacent nano-domains. This complex architecture of noncentrosymmetric nano-domains governs the coherent addition of 2ω light within the focal volume and the observed features in the SHG images taken in fascia.

Growth, structure, and properties of BiFeO3/-BiCrO3 films obtained by dual cross beam PLD.

The properties of epitaxial Bi(2)FeCrO(6) thin films, recently synthesized by pulsed laser deposition, have partially confirmed the theoretical predictions (i.e., a magnetic moment of 2 micro(B) per formula unit and a polarization of approximately 80 microC/cm(2) at 0 K). The existence of magnetic ordering at room temperature for this material is an unexpected, but very promising, result that needs to be further investigated. Because magnetism is assumed to arise from the exchange interaction between the Fe and Cr cations, the magnetic behavior is strongly dependent on both their ordering and the distance between them. We present here the successful synthesis of epitaxial Bi(2)Fe(x)CryO(6) (BFCO x/y) films grown on SrTiO3 substrates using dual crossed-beam, pulsed-laser deposition. The crystal structure of the films has different types of (111)-oriented superstructures, depending on the deposition conditions. The multiferroic character of BFCO (x/y) films is proven by the presence of both ferroelectric and magnetic hysteresis at room temperature. The oxidation state of Fe and Cr ions in the films is shown to be 3+ only, and the difference in macroscopic magnetization with Fe/Cr ratio composition could only be due to ordering of the Cr(3+) and Fe(3+) cations to the modification of the exchange interaction between them.

Higher-order electromechanical response of thin films by contact resonance piezoresponse force microscopy.

Piezoresponse scanning force microscopy (PFM) has turned into an established technique for imaging ferroelectric domains in ferroelectric thin films. At least for soft cantilevers, the piezoresponse signal is not only dependent on the elastic properties of the material under investigation but also on the elastic properties of the cantilever. Due to this dependency, the cantilever response and, therefore, the measured properties depend on the frequency of the small alternating current (AC) testing voltage. At the contact resonance, the cantilever response is maximum, and this increased sensitivity can be used to detect very small signals or to decrease the voltage applied to the sample. We have shown that by using the hysteretic ferroelectric switching, it is possible to separate the signal into its components (viz. electromechanical and electrostatic contributions). Additionally, the measurement frequency can be tuned such that the second and third harmonics of the electromechanical response can be detected at the cantilever resonance, providing information about the higher-order electromechanical coefficients. We assume that this nonlinear behavior seen in local and macroscopic measurements is rooted in the nonlinearity of the dielectric permittivity. Our results are of crucial importance for the study of ferroelectric and electromechanical properties of nanostructures.