Heterostrain-enabled ultrahigh electrostrain in lead-free piezoelectric.

Piezoelectric materials provide high strain and large driving forces in actuators and can transform electrical energy into mechanical energy. Although they were discovered over 100 years ago, scientists are still searching for alternative lead-free piezoelectrics to reduce their environmental impact. Developing high-strain piezoelectric materials has been a long-term challenge, particularly challenging for the design of high-strain polycrystalline piezoelectrics containing no toxic lead element. In this work, we report one strategy to enhance the electrostrain via designing "heterostrain" through atomic-scale defect engineering and mesoscale domain engineering. We achieve an ultrahigh electrostrain of 2.3% at high temperature (220 °C) in lead-free polycrystalline ceramics, higher than all state-of-the-art piezoelectric materials, including lead-free and lead-based ceramics and single crystals. We demonstrate practical solutions for achieving high electrostrain in low-cost environmentally piezoelectric for various applications.

Tuning the band gap and carrier concentration of titania films grown by spatial atomic layer deposition: a precursor comparison.

Spatial atomic layer deposition retains the advantages of conventional atomic layer deposition: conformal, pinhole-free films and excellent control over thickness. Additionally, it allows higher deposition rates and is well-adapted to depositing metal oxide nanofilms for photovoltaic cells and other devices. This study compares the morphological, electrical and optical properties of titania thin films deposited by spatial atomic layer deposition from titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl4) over the temperature range 100-300 °C, using the oxidant H2O. Amorphous films were deposited at temperatures as low as 100 °C from both precursors: the approach is suitable for applying films to temperature-sensitive devices. An amorphous-to-crystalline transition temperature was observed for both precursors resulting in surface roughening, and agglomerates for TiCl4. Both precursors formed conformal anatase films at 300 °C, with growth rates of 0.233 and 0.153 nm s-1 for TiCl4 and TTIP. A drawback of TiCl4 use is the HCl by-product, which was blamed for agglomeration in the films. Cl contamination was the likely cause of band gap narrowing and higher defect densities compared to TTIP-grown films. The carrier concentration of the nanofilms was found to increase with deposition temperature. The films were tested in hybrid bilayer solar cells to demonstrate their appropriateness for photovoltaic devices.

Dielectric films for high performance capacitive energy storage: multiscale engineering.

Dielectric capacitors are fundamental components in electronic and electrical systems due to their high-rate charging/discharging character and ultrahigh power density. Film dielectrics possess larger breakdown strength and higher energy density than their bulk counterparts, holding great promise for compact and efficient power systems. In this article, we review the very recent advances in dielectric films, in the framework of engineering at multiple scales to improve energy storage performance. Strategies are summarized including atomic-scale defect control, nanoscale domain and grain engineering, as well as mesoscale composite design. Challenges and remaining concerns are also discussed for further performance improvement and practical application of dielectric films.

Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films.

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (-3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

Design of a Vertical Composite Thin Film System with Ultralow Leakage To Yield Large Converse Magnetoelectric Effect.

Electric field control of magnetism is a critical future technology for low-power, ultrahigh density memory. However, despite intensive research efforts, no practical material systems have emerged. Interface-coupled, composite systems containing ferroelectric and ferri-/ferromagnetic elements have been widely explored, but they have a range of problems, for example, substrate clamping, large leakage, and inability to miniaturize. In this work, through careful material selection, design, and nanoengineering, a high-performance room-temperature magnetoelectric system is demonstrated. The clamping problem is overcome by using a vertically aligned nanocomposite structure in which the strain coupling is independent of the substrate. To overcome the leakage problem, three key novel advances are introduced: a low leakage ferroelectric, Na0.5Bi0.5TiO3; ferroelectric-ferrimagnetic vertical interfaces which are not conducting; and current blockage via a rectifying interface between the film and the Nb-doped SrTiO3 substrate. The new multiferroic nanocomposite (Na0.5Bi0.5TiO3-CoFe2O4) thin-film system enables, for the first time, large-scale in situ electric field control of magnetic anisotropy at room temperature in a system applicable for magnetoelectric random access memory, with a magnetoelectric coefficient of 1.25 × 10-9 s m-1.

Very high commutation quality factor and dielectric tunability in nanocomposite SrTiO3 thin films with Tc enhanced to >300 °C.

We report on nanoengineered SrTiO3-Sm2O3 nanocomposite thin films with the highest reported values of commutation quality factor (CQF or K-factor) of >2800 in SrTiO3 at room temperature. The films also had a large tunability of dielectric constant (49%), low tangent loss (tan δ = 0.01) and a Curie temperature for SrTiO3 > 300 °C, making them very attractive for tunable RF applications. The enhanced properties originate from the unique nanostructure in the films, with <20 nm diameter strain-controlling Sm2O3 nanocolumns embedded in a SrTiO3 matrix. Very large out-of-plane strains (up to 2.6%) and high tetragonality (c/a) (up to 1.013) were induced in the SrTiO3. The K-factor was further enhanced by adding 1 at% Sc3+ (acceptor) dopant in SrTiO3 to a value of 3300 with the tangent loss being ≤0.01 up to 1000 kV cm-1.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI).

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiOx |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Self-assembled oxide films with tailored nanoscale ionic and electronic channels for controlled resistive switching.

Resistive switches are non-volatile memory cells based on nano-ionic redox processes that offer energy efficient device architectures and open pathways to neuromorphics and cognitive computing. However, channel formation typically requires an irreversible, not well controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we report a novel memristive model material system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow the separate tailoring of nanoscale ionic and electronic channels at high density (∼10(12) inch(-2)). We systematically show that these devices allow precise engineering of the resistance states, thus enabling large on-off ratios and high reproducibility. The tunable structure presents an ideal platform to explore ionic and electronic mechanisms and we expect a wide potential impact also on other nascent technologies, ranging from ionic gating to micro-solid oxide fuel cells and neuromorphics.

Ferroelectric Sm-doped BiMnO3 thin films with ferromagnetic transition temperature enhanced to 140 K.

A combined chemical pressure and substrate biaxial pressure crystal engineering approach was demonstrated for producing highly epitaxial Sm-doped BiMnO(3) (BSMO) films on SrTiO(3) single crystal substrates, with enhanced magnetic transition temperatures, TC up to as high as 140 K, 40 K higher than that for standard BiMnO(3) (BMO) films. Strong room temperature ferroelectricity with piezoresponse amplitude, d(33) = 10 pm/V, and long-term retention of polarization were also observed. Furthermore, the BSMO films were much easier to grow than pure BMO films, with excellent phase purity over a wide growth window. The work represents a very effective way to independently control strain in-plane and out-of-plane, which is important not just for BMO but for controlling the properties of many other strongly correlated oxides.

Room Temperature Ferrimagnetism and Ferroelectricity in Strained, Thin Films of BiFe0.5Mn0.5O3.

Highly strained films of BiFe0.5Mn0.5O3 (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (TC ∼ 600K), with a room temperature saturation moment (MS ) of up to 90 emu/cc (∼ 0.58 µB /f.u) on high quality (001) SrTiO3. X-ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe3+ and Mn3+. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.

Extremely high tunability and low loss in nanoscaffold ferroelectric films.

There are numerous radio frequency and microwave device applications which require materials with high electrical tunability and low dielectric loss. For phased array antenna applications there is also a need for materials which can operate above room temperature and which have a low temperature coefficient of capacitance. We have created a nanoscaffold composite ferroelectric material containing Ba(0.6)Sr(0.4)TiO(3) and Sm(2)O(3) which has a very high tunability which scales inversely with loss. This behavior is opposite to what has been demonstrated in any previous report. Furthermore, the materials operate from room temperature to above 150 °C, while maintaining high tunability and low temperature coefficient of tunability. This new paradigm in dielectric property control comes about because of a vertical strain control mechanism which leads to high tetragonality (c/a ratio of 1.0126) in the BSTO. Tunability values of 75% (200 kV/cm field) were achieved at room temperature in micrometer thick films, the value remaining to >50% at 160 °C. Low dielectric loss values of <0.01 were also achieved, significantly lower than reference pure films.