Co-reporter:Yu Bai;Jian Zhong Cui;Zhan Jie Wang;Yan Na Chen
ACS Applied Materials & Interfaces December 7, 2016 Volume 8(Issue 48) pp:32948-32955
Publication Date(Web):November 15, 2016
DOI:10.1021/acsami.6b10992
In this work, epitaxial Pb(Zr0.4Ti0.6)O3 (PZT) thin films with different thicknesses were deposited on Nb-doped SrTiO3 (NSTO) single-crystal substrates by chemical solution deposition (CSD), and their ferroelectric resistive switching behaviors were investigated. The results showed that the maximum ON/OFF ratio up to 850 could be obtained in the PZT/NSTO heterostructure with the 150 nm thick PZT film. On the basis of the Schottky-Simmons model and the modified semiconductor theory, we also evaluated the interfacial built-in field and the depletion layer at the PZT/NSTO interface, which can be modulated strongly by the ferroelectric polarization, but are independent of the thickness of the PZT thin films. It is clear that the ferroelectric resistive switching is related to the ferroelectric polarization and modulated by the thickness of ferroelectric films. Therefore, there is an optimal thickness of the PZT film for the maximum ON/OFF ratio due to the ferroelectricity and conductivity mutually restricting. It can be expected that by adjusting the ferroelectricity and conductivity of the ferroelectric thin film and its thickness, the maximum switching ratio can be further improved.Keywords: ferroelectric heterostructures; ferroelectric polarization; interfacial built-in field; ON/OFF ratio; resistive switching behaviors;
Co-reporter:Bin He and Zhanjie Wang
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 10) pp:6736
Publication Date(Web):February 25, 2016
DOI:10.1021/acsami.5b12098
In this study, BaTiO3/Pb(Zr0.52Ti0.48)O3 (BTO/PZT) ferroelectric superlattices have been grown on the Nb-doped SrTiO3 (NSTO) single-crystal substrate by pulsed laser deposition, and their electrical properties were investigated in detail. The leakage current was reduced significantly in the BTO/PZT superlattices, and the conduction mechanism could be interpreted as the bulk-limited mechanism. In addition, a more symmetric hysteresis loop was observed in the BTO/PZT superlattices compared with the pure PZT and BTO films. The BTO/PZT superlattices with the modulation thickness of 9.8 nm showed remarkably improved dielectric properties with dielectric constant and loss of 684 and 0.02, respectively, measured at the frequency of 10 kHz. Based on these experimental results, it can be considered that the BTO/PZT interfaces play a very important role for the enhanced electrical properties of the BTO/PZT superlattices.Keywords: dielectric properties; ferroelectric properties; ferroelectric superlattices; leakage current; microstructure;
Co-reporter:Hong Jing Han, Yan Na Chen and Zhan Jie Wang
RSC Advances 2016 vol. 6(Issue 70) pp:66011-66017
Publication Date(Web):05 Jul 2016
DOI:10.1039/C6RA10637D
Electrode materials have a great influence on the performance of ferroelectric film capacitors, and it is still a challenge to find new electrode materials. In this work, highly conductive few-layer graphene (FLG) films were prepared on the surface of the PZT films as top electrodes by dip coating and mechanical exfoliation and their effect on the electric properties of the PZT film capacitors were investigated. The polarization property of the FLG/PZT/Pt capacitor is almost the same as that of the Pt/PZT/Pt capacitor. However, the magnitude of leakage current density for the FLG/PZT/Pt capacitor is largely decreased due to the van der Waals (vdW) gap at the FLG/PZT interface. The FLG films have relatively little oxygen functional groups due to the microwave irradiation treatment, and are more suitable for utilization as the electrode of the PZT capacitors compared with the few-layer-graphene oxide (FLGO) films. Our results demonstrate that the FLG films might be a promising electrode material for application in integrated ferroelectric devices.
Co-reporter:Yingjie Wu, Xingkun Ning, Zhanjie Wang, Qiang Wang, Zhidong Zhang
Journal of Alloys and Compounds 2016 Volume 667() pp:317-322
Publication Date(Web):15 May 2016
DOI:10.1016/j.jallcom.2016.01.193
•The ΔT(TC − TIM) of the LSMO films deposited at different temperatures was studied.•SEM and XPS were used to study the microstructures and the electric structures.•The ΔT was tuned by modulating the Mn3+/Mn4+ ratios and the microstructures.•The ΔT values could strongly affect the LFMR properties of the LSMO films.The polycrystalline La0.7Sr0.3MnO3 films were deposited on SiO2/Si substrates by pulsed laser deposition (PLD), and the temperature gap between Curie temperature (TC) and insulator-metal transition temperature (TIM) was tuned successfully by the deposition temperature and its influence on the LFMR properties was investigated. The results showed that the TC was sensitive to the Mn3+/Mn4+ ratios, whereas the TIM was sensitive to the microstructures in the LSMO films. The LFMR properties of the LSMO films were enhanced with increasing the ΔT values (ΔT = TC − TIM). Our results provide an effective way to modulate the functional properties of LSMO films by modulating the temperature gap between TC and TIM.
Co-reporter:X. K. Ning, Z. J. Wang, Y. N. Chen and Z. D. Zhang
Nanoscale 2015 vol. 7(Issue 48) pp:20635-20641
Publication Date(Web):12 Nov 2015
DOI:10.1039/C5NR06026E
The valence-band offset (VBO) of the La0.67Sr0.33MnO3/NiO (LSMO/NiO), LaMnO3/NiO (LMO/NiO), LSMO/LaNiO3 (LSMO/LNO) and LMO/LaNiO3 (LSMO/LNO) heterostructures has been investigated using X-ray photoemission spectroscopy. The VBO values are calculated to be −0.72, −0.05, +1.43 and +1.51 eV for the LSMO/NiO, LSMO/LNO, LMO/LNO and LMO/NiO heterostructures, respectively. Hence, when compared with NiO and LNO, the valence band of LSMO is shifted to a lower binding energy, whereas that of LMO is shifted to a higher binding energy. In addition, the charge transfer at the interfaces has been depicted as Mn3.3+ + 0.7e → Mn2.6+, Mn3.3+ + 0.1e → Mn3.2+, Mn3.0+ − 0.4e → Mn3.4+ and Mn3.0+ − 0.5e → Mn3.5+ for the LSMO/NiO, LSMO/LNO, LMO/LNO and LMO/NiO heterostructures, respectively. Thus, the charge transfer procedure can be described as electron hopping from NiO and LNO to LSMO in the LSMO/NiO and LSMO/LNO heterostructures, and electron hopping from LMO to NiO and LNO in the LMO/NiO and LSMO/LNO heterostructures. Therefore, the charge transfer is dependent on the VBO, and the charge transfer direction can be determined from the negative or positive values of the VBO.
Co-reporter:Xingkun Ning;Zhidong Zhang
Advanced Materials Interfaces 2015 Volume 2( Issue 15) pp:
Publication Date(Web):
DOI:10.1002/admi.201500302
Two-phase self-assembled nanocomposite films have attracted increasing interest in recent years because of their potential applications in novel technological devices. However, tuning the physical properties by modulating the microstructure of self-assembled nanocomposite films is still a challenge. In this study, epitaxial La0.7Ca0.3MnO3:NiO nanocomposite films are synthesized by pulsed laser deposition. In the composite films with a NiO ratio of 50%, microstructures with nanomultilayer, nanogranular, and nanocolumnar characteristics are successfully obtained by using different growth modes. The metal–insulator transition and magnetic transition can be separately modulated by tuning the microstructures. By precisely modulating the microstructure, a significantly enhanced low-field magnetoresistance (>80% at a magnetic field of 1 T) with an unusual plateau in the temperature interval from 10 to 110 K is realized in these films, which is expected to be applicable in field-sensor devices that can be operated in a wide temperature range.
Co-reporter:Hailing Wang, Yu Bai, Xingkun Ning and Zhanjie Wang
RSC Advances 2015 vol. 5(Issue 126) pp:104203-104209
Publication Date(Web):03 Dec 2015
DOI:10.1039/C5RA22404G
Metal-conductive oxide nanocomposite thin films are regarded as promising electrodes for ferroelectric devices to overcome the intrinsic drawbacks of conventional metal or conductive oxides electrodes. In this work, ferroelectric Pb(Zr,Ti)O3 (PZT) thin films with a thickness of 240 nm were deposited on metal-conductive oxide nanocomposite Au–LaNiO3 (Au–LNO) electrodes by a sol–gel method, and their microstructure and ferroelectric properties were investigated. It was observed that the PZT thin films fabricated on the Au–LNO bottom electrode exhibited enhanced ferroelectricity with the remnant polarization as high as 59.6 μC cm−2, which was much larger than those on the Pt (20 μC cm−2) and the LNO (33 μC cm−2) bottom electrodes. The J–V characteristics fitted by the Schottky emission model revealed the lowest potential barrier height at the interface between the PZT thin film and the Au–LNO electrode. The low potential barrier height contributed to a thin spacing charge layer at the PZT/Au–LNO interface, which was beneficial to the switching of ferroelectric domains. Ultraviolet photoelectron spectroscopy (UPS) results revealed that the decrease of the Schottky potential barrier was understood by the different work functions of the bottom electrodes according to the semiconductor theory. The present work demonstrates that metal-oxide nanocomposite electrodes can effectively improve the electrical properties of ferroelectric films by modulation of the Schottky potential barrier height. In particular, the modulation of the Schottky potential barrier by using nanocomposite electrodes also provides meaningful guidance for designing high performance ferroelectric photovoltaic devices.
Co-reporter:H. J. Han, Y. N. Chen and Z. J. Wang
RSC Advances 2015 vol. 5(Issue 113) pp:92940-92946
Publication Date(Web):23 Oct 2015
DOI:10.1039/C5RA19268D
In this study, the effect of microwave irradiation on reduction of graphene oxide films was investigated. The few-layer graphene (FLG) sheets were prepared by the electrochemical exfoliated method. The reduction process was completed with a microwave power as low as 42 W below a temperature of 250 °C in air. The Raman and XPS spectra demonstrated that a larger amount of oxygen functional groups were removed by microwave irradiation, which is much more effective than the usual method of mild-thermal treatment at 250 °C for 30 min. The FLG films treated by microwave irradiation showed a low value of sheet resistance around 6 × 103 Ω □−1. The results demonstrate that the use of microwave irradiation provides a simple and effective method for eliminating oxygen functional groups to produce highly conductive graphene films.
Co-reporter:H. L. Wang, X. K. Ning and Z. J. Wang
RSC Advances 2015 vol. 5(Issue 94) pp:76783-76787
Publication Date(Web):04 Sep 2015
DOI:10.1039/C5RA15152J
Au–LaNiO3 (Au–LNO) nanocomposite films with 3.84 at% Au were firstly fabricated by one-step chemical solution deposition (CSD), and their electrical properties were investigated. Au nanoparticles with average size of approximately 10 nm were distributed uniformly in the LNO matrix. The nanocomposite films exhibit much lower room temperature resistivity (313 μΩ cm) than the LNO films (1221 μΩ cm). The role of Au nanoparticles in the composite films can be considered from two aspects: firstly, the uniform distribution of Au nanoparticles provides a pathway with less scattering for the conduction of electrons. Secondly, Au nanoparticles can serve as nuclei seeds for the perovskite LNO phase to improve its crystalline quality. Transport property measurement reveals that the Au–LNO nanocomposite films exhibit a lower metal-insulator transition (MIT) temperature and suppression of weak localization due to the lower concentration of oxygen vacancy. The high-quality Au–LNO nanocomposite films with the excellent electrical conductivity are expected to be applicable as electrodes for ferroelectric and multiferroic films.
Co-reporter:Xingkun Ning;Zhidong Zhang
Advanced Functional Materials 2014 Volume 24( Issue 34) pp:5393-5401
Publication Date(Web):
DOI:10.1002/adfm.201400735
Magnetic properties and low-field magnetoresistance (LFMR) in La0.7Sr0.3MnO3 (LSMO):NiO nanocomposite films grown on SrTiO3 (001) substrates, which are shown to be tunable with different microstructures, are investigated. The LSMO:NiO nanocomposite films with NiO volume ratio of 50% have a checkerboard-like structure and show a large LFMR in a temperature range from 200 to 300 K (≈17% at 250 K with a magnetic field of 1 T). As the NiO volume ratio is increased to 70%, a nano-columnar structure formed in the films. Their LFMR is significantly enhanced at a wide temperature range of 10–210 K. The highest value of LFMR with 41% is achieved at 10 K in a magnetic field of 1 T. The enhanced LFMR can be considered to result from the electron scattering at the ferromagnetic LSMO/NiO interfaces and magnetic tunnel junctions (MTJs) of LSMO/NiO/LSMO at the nanometer scale. These results demonstrate that large and tunable LFMR from low temperature to room temperature can be realized by controlling the microstructures in the epitaxial La0.7Sr0.3MnO3:NiO nanocomposite thin films, which will be expected to be applied in the devices using for a wide temperature range.
Co-reporter:Y.N. Chen, Z.J. Wang, T. Yang, Z.D. Zhang
Acta Materialia 2014 Volume 71() pp:1-10
Publication Date(Web):June 2014
DOI:10.1016/j.actamat.2014.03.009
Abstract
We investigated the crystallization process and kinetics of lead zirconate titanate (PZT) thin films fabricated in the magnetic field of microwave irradiation. The PZT thin films were prepared by the sol–gel method and crystallized by microwave irradiation at 2.45 GHz. X-ray diffraction was used to identify the phases and to determine the volume fraction of the perovskite phase transformed during crystallization. Transmission electron microscopy gave information on nucleation, growth and grain structure. We also discussed how the crystallization and phase transformations correlated to the ferroelectric properties of the resultant films. We found that an intermediate phase formed during initial crystallization; it had a perovskite-like crystal structure, but it had a smaller lattice constant than perovskite PZT and contained more Ti. This intermediate phase acted as a nucleation site for the perovskite PZT, which grew with a columnar grain structure into the pyrochlore matrix throughout the film. Using Avrami’s model, we found the effective activation energy for crystallization of the PZT films by microwave irradiation to be ∼214 kJ mol−1, lower than the activation energy for crystallization by conventional thermal processes. These results show that microwave irradiation indeed affected the crystallization of amorphous PZT thin films differently than conventional annealing.
Co-reporter:M.W. Zhu, Z.J. Wang, Y.N. Chen, Z.D. Zhang
Surface and Coatings Technology 2013 Volume 216() pp:139-144
Publication Date(Web):15 February 2013
DOI:10.1016/j.surfcoat.2012.11.041
In the present work, lanthanum nickel oxide (LaNiO3, LNO) films were prepared by the sol–gel method. Microwave processing in separated microwave magnetic field (H field) was introduced during pyrolysis and annealing treatments. The heating behavior of LNO films in microwave H field was explored. The effects of microwave processing on the orientation and microstructure of LNO films were investigated compared to those obtained from conventional furnace processing. The results show that the (100)-oriented LNO films can be prepared through microwave processing at pyrolysis stage and annealing stage, even though the film thickness obtained from a single coating is much larger than the grain size. On the contrary, (110)-orientation predominates in the films prepared through conventional furnace processing. Furthermore, microwave annealing is favorable for improving the crystallinity of LNO films and promoting the transition of the transport property from the insulating-type to the metallic-type. The underlying mechanism of the above-mentioned effects is discussed in terms of the heating behavior of amorphous and crystallized LNO films prepared in microwave field.Highlights► Amorphous and crystallized LaNiO3 films show different microwave heating behaviors. ► Microwave pyrolysis favors the (100)-orientation development in LaNiO3 films. ► Microwave annealing promotes the grain growth of LaNiO3 films.
