The quest for a solid state universal memory with high-storage density, high read/write speed, random access and non-volatility has triggered intense research into new materials and novel device architectures. Though the non-volatile memory market is dominated by flash memory now, it has very low operation speed with ~10 μs programming and ~10 ms erasing time. Furthermore, it can only withstand ~105 rewriting cycles, which prevents it from becoming the universal memory. Here we demonstrate that the significant photovoltaic effect of a ferroelectric material, such as BiFeO3 with a band gap in the visible range, can be used to sense the polarization direction non-destructively in a ferroelectric memory. A prototype 16-cell memory based on the cross-bar architecture has been prepared and tested, demonstrating the feasibility of this technique.

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We report the observation of superconductivity in infinite-layer Ca-doped LaNiO 2 (La 1− x Ca x NiO 2 ) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La 1− x Ca x NiO 2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient R H changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the R H -sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.

Oxygen vacancies induced switchable and nonswitchable photovoltaic effects in Ag/Bi0.9La0.1FeO3 /La0.7Sr0.3MnO3 sandwiched capacitors Appl. Phys. Lett.The switchable photovoltaic effect in BiFeO 3 thin films capacitors has been studied extensively. However, the origin of the photovoltaic response is still under debate. Both bulk depolarization field and interface effects have been used to explain the observations. In this work, we fabricate BiFeO 3 epitaxial films on SrTiO 3 substrate with La 0.7 Sr 0.3 MnO 3 and Pt as electrodes. Much larger switchable photovoltaic response can be observed in the Pt/BiFeO 3 /La 0.7 Sr 0.3 MnO 3 samples, as compared with La 0.7 Sr 0.3 MnO 3 /BiFeO 3 /La 0.7 Sr 0.3 MnO 3 . Moreover, the photovoltaic voltage of the Pt/BiFeO 3 /La 0.7 Sr 0.3 MnO 3 samples is nearly independent of the thickness of the La 0.7 Sr 0.3 MnO 3 bottom electrode. We suggest that the Schottky barrier modulation by ferroelectric polarization at the Pt/BiFeO 3 interface is mainly responsible for the photovoltaic effect, with very small contribution from the bulk depolarization field. V C 2014 AIP Publishing LLC. [http://dx.

For conventional dye or quantum dot sensitized solar cells, which are fabricated using mesoporous films, the inefficient electron transport due to defects such as grain boundaries and surface traps is a major drawback. To simultaneously increase the carrier transport efficiency as well as the surface area, optimal-assembling of hierarchical nanostructures is an attractive approach. Here, a three dimensional (3D) hierarchical heterostructure, consisting of CdS sensitized one dimensional (1D) ZnO nanorods deposited on two dimensional (2D) TiO2 (001) nanosheet, is prepared via a solution-process method. Such heterstructure exhibits significantly enhanced photoelectric and photocatalytic H2 evolution performance compared with CdS sensitized 1D ZnO nanorods/1D TiO2 nanorods photoanode, as a result of the more efficient light harvesting over the entire visible light spectrum and the effective electron transport through a highly connected 3D network.

Wurtzite ZnO nanorod exhibits many unique properties, which make it promising for various optoelectronic applications. To grow well-aligned ZnO nanorod arrays on various substrates, a seed layer is usually required to improve the density and vertical alignment. The reported works about seedless hydrothermal synthesis either require special substrates, or require external electrical field to enhance the ZnO nucleation. Here, we report a general method for the one-pot synthesis of homogenous and well-aligned ZnO nanorods on common conducting substrates without a seed layer. This method, based on the galvanic-cell structure, makes use of the contact potential between different materials as the driving force for ZnO growth. It is applicable to different conducting substrates at low temperature. More importantly, the as-grown ZnO nanorods show enhanced photoelectric response. This unique large scale low-temperature processing method could be of great importance for the application of ZnO nanostructures.

By using piezoelectric force microscopy and scanning Kelvin probe microscopy, we have investigated the domain evolution and space charge distribution in planar BiFeO 3 capacitors with different electrodes. It is observed that charge injection at the film/electrode interface leads to domain pinning and polarization fatigue in BiFeO 3 . Furthermore, the Schottky barrier at the interface is crucial for the charge injection process. Lowering the Schottky barrier by using low work function metals as the electrodes can also improve the fatigue property of the device, similar to what oxide electrodes can achieve. a) Electronic mail: yuanguoliang@mail.njust.edu.cn b) Electronic mail: jlwang@ntu.edu.sg 2 Ferroelectric fatigue refers to the decrease of switchable polarization in a ferroelectric material after repetitive electrical cycling. 1 It is detrimental to ferroelectric based devices and should be minimized. There have been a large number of reports on the mechanism of the polarization fatigue. [2][3][4][5] The models proposed can generally be classified into three categories, namely charge injection, 6-9 defects, i.e. oxygen vacancies, redistribution 10-14 and local phase decomposition. [15][16][17] In our previous report, we have demonstrated that charge injection is likely the cause of fatigue in BiFeO 3 . 18 By using a planar capacitor (inset of Fig. 1 (a)), we have conducted piezoelectric force microscopy (PFM) and scanning Kelvin probe microscopy (SKPM) studies to investigate the domain evolution and space charge activities in BiFeO 3 during fatigue measurement. A clear correlation between injected electrons at the electrode/BiFeO 3 interface and domain pinning is established as shown in Figs. 1(a)-1(c) (For details, please refer to Ref. 18). However, this is not to say that defects are irrelevant to polarization fatigue. On the contrary, these injected electrons must be trapped in gap states which can be associated with existing defects or created by the high energy injected electrons. We emphasize that it is not the redistribution/accumulation of defects, but rather the charging/discharging of defects, leads to polarization fatigue eventually. To clarify the difference between metal and oxide electrodes, we have prepared and investigated BiFeO 3 planar capacitors using (La 0.7 ,Sr 0.3 )MnO 3 as electrodes. All the parameters for BiFeO 3 films deposition are the same as reported in our previous study. 18 Since (La 0.7 ,Sr 0.3 )MnO 3 requires higher deposition temperature than BiFeO 3 , the electrodes are prepared (by standard lithography and etching) first followed by BiFeO 3 (40 nm) deposition. Macroscopic polarizationelectric field measurement reveals no fatigue after 10 10 cycles (data not shown). In the in-plane (IP) PFM images taken after opposite electric field is applied to the film, no domain pinning is 4 observed up to 10 10 cycles (Figs. 1(d) and 1(e)). Furthermore, SKPM image reveals negligible electron injection at the (La 0.7 ,Sr 0.3 )MnO 3 /BiFeO 3 interfaces ( Fig. 1(f)), whereas signi...

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.