High Defect Density Research Articles

Charged defect healing by N, N′–di (naphthalene-1-yl)-N, N′ diphenyl benzidine at the interface of CuInS2 nanoparticle hole transporting materials in carbon-based halide perovskite solar cells

Recently, inorganic nanoparticles have been studied as hole transporting materials (HTM) in perovskite solar cells (PSCs). Inorganic HTMs are believed to provide better stability compared to conventional organic HTMs, such as spiro-OMeTAD. One of the challenges of nanoparticle HTMs is the high density of defects at the interface with the perovskite film, which reduces the device's fill factor. In this study, we use N, N′–di (naphthalene-1-yl)-N, N′ diphenyl benzidine (NPB) small molecules to passivate the surface of perovskite films in carbon-based PSCs with CuInS2 nanoparticles as the HTM. By applying NPB, trap density decreases from 1.35 × 1016 cm−3 to 5.09 × 1015 cm−3. NPB interface passivation results in a power conversion efficiency (PCE) of 16.11% compared to the control device with PCE of 14.92% in carbon-based perovskite solar cells. Moreover, improved contact angle was obtained for NPB surface-treated film (61.9°) compared to the reference film (40.6°), providing better protection against moisture induced degradation. The surface-treated devices maintained 92% of their efficiency after 4000 h of storage at ambient condition, while the control device showed a rapid and notable degradation, losing >58% of the initial efficiency under the same condition.

Designing metal halide perovskite nanoparticle-based resistive random-access memory devices through chemical treatments

Metal-halide perovskite (CsPbX3; X = Cl, Br, and I)-based resistive random-access memory (ReRAM) devices are emerging as promising candidates for neuromorphic systems owing to their low operation voltage and distinct ‘true’ and ‘false’ signals. However, conventional ReRAM devices exploit bulk or a few hundred micrometer-sized perovskites, which are unsuitable for achieving brain-inspired memory devices. This is because nanometer-sized perovskites exhibit a high density of defects on their surfaces, thus resulting in poor or negligible resistive switching characteristics. Herein, defects are engineered on (zero dimensional; 0D) CsPbX3 perovskite nanoparticle (PNP) surfaces, their chemical compositions are tuned by water treatment, and the role of the defects in achieving ReRAM devices is investigated. Theoretical estimations (density functional theory and molecular dynamics) and empirical analyses reveal that H2O molecules reduce the defect density and generate clean surfaces. Through the water- and water-halide treatments on CsPbBr3 PNPs, we successfully demonstrated 0D (18.7±2.4 nm) perovskite-based ReRAM devices and achieve a high ION/IOFF ratio (>105).

Gradient structured high-entropy alloy with high hardness and corrosion resistance after laser shock peening

In the effort to improve the surface properties of FeCoCrNiMn HEA, gradient structure was prepared by laser shock peening (LSP), a versatile and efficient method to achieve distinct surface properties. The underlying mechanism of the LSP strengthening effect was discussed based on microstructure analysis, mechanical properties, and corrosion test. Our results suggested that ultra-fined grains and high density of defects were produced after LSP treatment which accounts for the significant enhancement of hardness, coefficient of friction (CoF), and corrosion resistance. The strengthened mechanical properties owed to the large amount of grain boundaries and high density of defects that significantly attenuate the movement of dislocations. Meanwhile, attributed to the compressive residual stress induced by LSP, the penetration of corrosive solution was effectively impeded and the passivation process was accelerated. Finally, the different mechanisms of LSP strengthening effect have been discussed. This study presented a promising approach to address the challenge of surface modification of FeCoCrNiMn HEA and provided a novel strategy for regulating the microstructure of HEA and achieving outstanding properties.

Synergistic Ion-Anchoring Passivation for Perovskite Solar Cells with Efficiency Exceeding 24% and Ultra-Ambient Stability.

The high-density defect states existing at the grain boundaries and heterojunction interfaces induce nonradiative charge recombination and ion migration processes within perovskite film, which seriously impair the device efficiency and stability. Here, we propose a novel synergistic ion-anchoring passivation (SIP) strategy for high-performance perovskite solar cells, by designing a multifunctional molecule to heal the charged defects via electrostatic interactions. The anion and cation species of the multifunctional molecule are rationally screened via high-throughput DFT simulation and experimental verification, which act as efficient surface passivation agents to heal the lead- and iodine-related defects. As a result, the defect-less perovskite films deliver encouraging device power conversion efficiency >24% with negligible hysteresis. A remarkable open-circuit voltage (Voc) of 1.17 V was obtained with a Voc deficit of 370 mV, featuring the outstanding defect-passivation capability of the SIP strategy. Moreover, the SIP-treated devices show exceptional ambient stability and maintain 70% of the initial efficiency after 150 h of high humidity exposure (relative humidity 70%-80%). Our results highlight the importance of the rational design of passivation agents to realize high-performance perovskite electronics.

Stochastic Formation of Quantum Defects in Carbon Nanotubes.

Small perturbations in the structure of materials significantly affect their properties. One example is single wall carbon nanotubes (SWCNTs), which exhibit chirality-dependent near-infrared (NIR) fluorescence. They can be modified with quantum defects through the reaction with diazonium salts, and the number or distribution of these defects determines their photophysics. However, the presence of multiple chiralities in typical SWCNT samples complicates the identification of defect-related emission features. Here, we show that quantum defects do not affect aqueous two-phase extraction (ATPE) of different SWCNT chiralities into different phases, which suggests low numbers of defects. For bulk samples, the bandgap emission (E11) of monochiral (6,5)-SWCNTs decreases, and the defect-related emission feature (E11*) increases with diazonium salt concentration and represents a proxy for the defect number. The high purity of monochiral samples from ATPE allows us to image NIR fluorescence contributions (E11 = 986 nm and E11* = 1140 nm) on the single SWCNT level. Interestingly, we observe a stochastic (Poisson) distribution of quantum defects. SWCNTs have most likely one to three defects (for low to high (bulk) quantum defect densities). Additionally, we verify this number by following single reaction events that appear as discrete steps in the temporal fluorescence traces. We thereby count single reactions via NIR imaging and demonstrate that stochasticity plays a crucial role in the optical properties of SWCNTs. These results show that there can be a large discrepancy between ensemble and single particle experiments/properties of nanomaterials.

External catalyst-free InGaN photoelectrode for highly efficient energy conversion and H2 generation

GaN is a well-known material whose energy band edges can straddle the redox potentials deep in the visible and infrared wavelengths, thereby promising a drastically improved photon-to-current efficiency under applied bias. However, the material is still limited by the half-reactions of water splitting due to its high defect density, low light absorption, small reaction area, and large energy band bending. Here, our study provides a turn-key solution to all these issues. The synergistic effect of InGaN/GaN quantum pyramids on nanowires (QPs-NWs) directly addresses the performance degradation of the photocathodes (PCs). New InGaN QP structures on non-polar GaN nanowire show a unique tunable energy band (Eg: ∼2 eV to ∼1.36 eV) by quantum-sliding interface recombination effect. Without the use of external catalysts, the photoelectrochemical water splitting (PEC-WS) of QPs-NWs PC demonstrated enhanced performance with a current density of 34.36 mA cm−2 and a photon-to-current efficiency of 13.75 % under the −0.8 to 0 V applied biasing condition, which is much higher than in previous reports. The current density and the H2 production were measured to be ∼61.81 mA cm−2 and 11.5 mmol cm−2 for 10 h. The external catalyst-free electrode and the metal organic chemical vapor deposition (MOCVD) process will open a new platform for the commercialization of III-nitride based water splitting hydrogen technology.

Crystallization mechanism and lasing properties of CsPbBr3 perovskites by chemical vapor deposition

The chemical vapor deposition (CVD) technique is widely used in industry for growing perovskite materials with high crystalline quality and low defect density. However, the underlying crystallization mechanism of all-inorganic lead halide perovskites during the implementation of the CVD process has not been clearly revealed. Under this direction, in this work, hemispheric, rod-shaped, plate-shaped, triangular pyramid-shaped, and irregular-shaped, etc. cesium lead bromide (CsPbBr3) perovskite microcavities were obtained by adjusting the local temperature distribution, substrate types, deposition position, carrier gas flow rate (CGFR), and air pressure within the tube furnace. The crystallization mechanism for the CsPbBr3 microcavity growth with different shapes, densities, and sizes was investigated. It can be seen from the scanning electron microscope (SEM) images that micron hemispheres can be obtained at 5.5 to 6.5 cm (100) Si substrates at standard atmospheric pressure and high deposition temperatures. On the contrary, at low deposition temperatures, microrods, microplates, and microcuboids can be fabricated on the (111) Si, (100) Si, and indium tin oxide (ITO) substrates, respectively, without carrier gas flow and air pressure. We can also modulate the size of the micron hemispheres and microplates by altering the deposition position and CGFR. Moreover, the lasing characteristics of the microplate, microrod, and microcuboid were examined by photoluminescence (PL) and time-resolved PL (TRPL) measurements. Our work provided a feasible method to control the crystal sizes, cavity shapes, and optical properties for the development of various types of lead halide perovskites.

Improved performance and stability in CH3NH3PbI3/Si heterojunction photodetectors realized by ZIF-67 additive assisted Co ion doping

Lead halide perovskites have been regarded as promising materials for fabrication of high-performance photodetectors (PDs). However, spin-coated perovskite thin films generally have suffered from high-density grain boundaries and defect states, which severely deteriorated the performance and stability of resulting PDs. Fortunately, the above issues can be effectively solved via additive assisted metal ions doping. In this work, Co-based zeolite imidazole framework (ZIF-67) additive assisted Co ion doping has been proposed to prepare high-quality perovskite CH3NH3PbI3 (MAPbI3) thin films. Morphological, structural, optical, and electrical studies have shown that Co ion doping decreased grain boundaries as well as suppressed defect states, which greatly enhanced the performance and stability of MAPbI3/Si heterojunction PDs, including reducing dark current, increasing photocurrent, and increasing response speed. Finally, the mechanisms of suppression of defect states benefiting from Co ion doping were discussed based on density functional theory calculations. The results provided in this work will pave the way for development of high-performance and stable MAPbI3 PDs in the future.

Nanosecond laser-induced filamentous modification in silicate glasses

Laser-induced filamentous modification (LIFM) has been extensively studied in transparent materials and is widely used for cutting and drilling with a high aspect ratio. However, the discussion focuses on the domain of ultra-short pulses recently, such as picosecond and femtosecond pulses. In this work, a LIFM by a short pulse in silicate glasses is demonstrated, and the physical mechanism is investigated. In the soda-lime glass, a filamentous modification with a length of >650 μm was introduced by a nanosecond pulse with an energy of 0.86 mJ. The average length of LIFM is positively correlated with the pulse energy but is not affected by the focus depth significantly. Furthermore, LIFM is also observed in borosilicate glass and K9 glass, but not in fused silica glass. Theoretical analysis shows that the contribution of the self-focusing and optical error to LIMB could be ignored in soda-lime glass. An improved Laser-induced moving breakdown (LIMB) model is proposed to describe LIFM by a nanosecond pulse, and the thermal explosion (TE) model is used to explain the micro-structures. High-density defects dominate the LIMB of nanosecond pulses in silicate glasses, which provide seed electrons for laser-induced ionization and induce a thermal explosion through enhanced absorption. The estimation result of the TE model shows that the final temperature of thermal plasma exceeds 1 × 105 K, resulting in micro-explosion and forming micro-voids and micro-crack clusters. Based on the principle of LIFM, stealth cutting was implemented by a high-frequency nanosecond laser in soda-lime glass with a thickness of 2 mm, and the cutting speed reaches 3000 mm/s.

Light-induced hexatic state in a layered quantum material

The tunability of materials properties by light promises a wealth of future applications in energy conversion and information technology. Strongly correlated materials such as transition metal dichalcogenides offer optical control of electronic phases, charge ordering and interlayer correlations by photodoping. Here, we find the emergence of a transient hexatic state during the laser-induced transformation between two charge-density wave phases in a thin-film transition metal dichalcogenide, 1T-type tantalum disulfide (1T-TaS2). Introducing tilt-series ultrafast nanobeam electron diffraction, we reconstruct charge-density wave rocking curves at high momentum resolution. An intermittent suppression of three-dimensional structural correlations promotes a loss of in-plane translational order caused by a high density of unbound topological defects, characteristic of a hexatic intermediate. Our results demonstrate the merit of tomographic ultrafast structural probing in tracing coupled order parameters, heralding universal nanoscale access to laser-induced dimensionality control in functional heterostructures and devices.

Theoretical Insights of Degenerate ZrS2 as a New Buffer for Highly Efficient Emerging Thin‐Film Solar Cells

SnS, Sb2Se3, Cu2SnS3, CuSb(S,Se)2, and Cu2BaSn(S,Se)4 are emerging as promising light absorbers for thin‐film photovoltaics due to their extraordinary optoelectronic properties. However, improper band alignment with the buffer and large open‐circuit voltage (VOC) deficit limits their power conversion efficiencies (PCE). Therefore, finding a suitable buffer that overcomes these obstacles is crucial. Herein, ZrS2 as an alternative buffer for the aforementioned emerging thin‐film solar cells using SCAPS‐1D is proposed. The important ZrS2 parameters are optimized, including bandgap, thickness, carrier concentration, and defect density. Interestingly, ZrS2 behaves as a degenerate semiconductor at carrier concentrations >1E17 cm−3, improving the conductivity of the solar cells; it also demonstrates a high defect tolerance nature when the defect density lies between 1E12 and 1E18 cm−3. After ZrS2 parameters optimization, the built‐in potential of SnS, Sb2Se3, Cu2SnS3, CuSb(S,Se)2, and Cu2BaSn(S,Se)4 solar cells is enhanced by 0.2, 0.58, 0.05, 0.42, and 0.3 V, respectively, reducing recombination rate. Upon optimizing absorbers parameters, a PCE > 35% for SnS, Sb2Se3, and CuSb(S,Se)2 while >32% for Cu2SnS3 and Cu2BaSn(S,Se)4 solar cells is accomplished with low VOC loss (≈0.1 V). The absorbers must have high carrier concentration (1E20 cm−3) and low defect density (1E14 cm−3) to achieve these PCEs.

Defect-rich Cu@CuTCNQ composites for enhanced electrocatalytic nitrate reduction to ammonia

Electrochemical conversion of nitrate (NO3−) pollutants into chemical feedstock and fuel ammonia (NH3) can contribute to sustainable mitigation of the current severe energy and environmental crises. However, the electrocatalytic NO3− reduction to NH3 (NRA) involves a sluggish multielectron and proton transfer process that competes with the hydrogen evolution reaction (HER) in aqueous media, imposing great challenges in developing highly selective catalysts for NRA. In this study, we developed a copper and copper-tetracyanoquinodimethane composite catalyst (Cu@CuTCNQ), which possesses a high density of copper vacancy defects. This catalyst has been proven to be efficient for NRA through an in situ electrochemical reconstruction method. The structural evolution of CuTCNQ during NRA was investigated by in situ Raman spectroscopy, which indicated an accelerated charge transfer from the CuTCNQ substrate to the derived Cu, which facilitated the adsorption activation of NO3−. The obtained Cu@CuTCNQ exhibited an excellent catalytic performance for NRA, with a Faradaic efficiency of 96.4% and productivity of 144.8 μmol h−1 cm−2 at −0.6 V vs. a reversible hydrogen electrode, superior to Cu nanoparticle counterparts and most Cu-based catalysts. Cu vacancy defects and sufficient interfacial charge transfer synergistically optimize the charge distribution of Cu active sites, reduce the energy barrier for NO3− adsorption, and promote deoxidation and hydrogenation processes, thus enhancing NRA and selectivity.

Deformation mechanism of Cu-Al-Ni shape memory alloys fabricated via laser powder bed fusion: Tension-compression asymmetry

Introducing the unique advantage of additive manufacturing technology into copper-based shape memory alloys (SMAs) to fabricate high-performance alloys has garnered great attention in recent years, but the intrinsic relationships between microstructure and mechanical properties need to be further clarified. In this paper, the microstructural evolution of ternary CuAlNi SMAs fabricated by laser powder bed fusion (LPBF) under the tensile-compressive loading was investigated to determine the underlying mechanism of tension-compression asymmetry, that is, excellent compressive but poor tensile properties. Multiscale characterization of the different deformation stages revealed the numerous activated deformation mechanism on the 18R martensite matrix. A twin-related transformation dominated the main plastic deformation process due to lower stacking faults energy and high-density pre-existing planer defects in the CuAlNi SMAs. Deformation twinning nucleated at prior austenite boundaries and developed into parallel and network structures inside the parent grain of different sizes. In addition, the preferred orientation in different stages, the stress-induced γ phase transformation, and the interaction between dislocations and stacking faults are discussed. These results not only provide significant insights to understand the detwinning and deformation twinning process of SMAs but also establish the essential framework of microstructure and mechanical properties of Cu-based SMAs fabricated by LPBF.

Simulation of Native Oxide‐Passivated CsSn0.5Ge0.5I3 Highly Stable Lead‐Free Inorganic Perovskite Solar Cell

The search for a lead‐free, nontoxic, highly stable perovskite phase could end at the native oxide‐passivated composition of CsSn0.5Ge0.5I3. Mainly its remarkable stability is due to a thin native oxide layer (Sn‐doped GeO2) formation over CsSn0.5Ge0.5I3, which suppresses the Sn2+ oxidation to Sn+4. Herein, the performance of this mixed Sn–Ge perovskite and the role of native oxide is analyzed. The experimental device with 7% efficiency is numerically replicated, utilizing reasonable device configuration, material parameters, and defect model. The low efficiency of CsSn0.5Ge0.5I3 is due to high bulk defects. The simulation result shows that Sn oxidation suppressing the native oxide layer imparts stability but does not significantly impact device efficiency. The optimization provides passivated design for absorbers with high bulk defects. Low thickness at high defect density is preferable, and higher thickness at lower defect density is optimal for device design. The final optimized device achieves an efficiency of 21%. The results summarized here can provide a guideline for high‐efficiency, lead‐free, nontoxic stable perovskite solar cells.

Formation of Twin Boundaries in Rapidly Solidified Metals through Deformation Twinning.

The rapid solidification process is relevant to many emerging metallurgical technologies. Compared with conventional solidification processes, high-density microstructure defects and residual thermal stress are commonly seen in rapidly solidified metals. Among the various defects, potentially beneficial twin boundaries have been observed in the rapidly solidified nanocrystalline microstructures of many alloy systems. In this work, a pathway for forming twin boundaries in rapid solidification processes is proposed. A detailed derivation of strain inhomogeneities upon thermal shrinkage and the deformation twinning phase field method is given. By calculating cooling-induced thermal strain inhomogeneity in nanocrystalline metals and growth thresholds for deformation twinning using the phase field method, it is shown that residual thermal strain hotspots in the microstructure can reach the threshold for deformation twinning when the shear elastic property of grain boundaries is significantly different from the bulk.

Remote plasma-enhanced chemical vapor deposition of GeSn on Si: Material and defect characterization

Germanium–tin (GeSn) alloys at sufficiently high Sn concentration, above several atomic percent, are the only group IV semiconductor exhibiting a direct bandgap and have generated much recent interest for optoelectronic applications into the mid-infrared region. Because the large lattice mismatch between GeSn and Si results in considerable strain for thin layers and a high defect density for thicker strain-relaxed layers, most reported GeSn growths incorporate a Ge buffer layer rather than depositing directly on Si substrates. Published reports of GeSn growth directly on Si utilize specialized precursors such as higher order germanes (Ge2H6, Ge3H8, or Ge4H10) or SnD4. In this paper, we report GeSn films with up to 10.6% Sn grown directly on Si substrates by remote plasma-enhanced chemical vapor deposition using GeH4 and SnCl4 precursors. These alloys have been characterized in detail using x-ray diffraction (XRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry with channeling (RBS-C), as well as Raman spectroscopy (RS) and optical microscopy. The films studied are almost fully relaxed, with small residual strain observed, particularly in thinner films, and contain a high interface density of misfit dislocations that increases with Sn concentration. The defect density decreases toward the surface. Good agreement is found between the various characterization methods for the Sn content (XRD and RBS-C), lattice parameter measurement (XRD and TEM), and defect characterization (RBS-C, TEM, and RS). Such characterization of GeSn grown directly on Si substrates is essential to allow growth parameters to be optimized for the realization of the attractive optoelectronic properties of these alloys.

Outstanding specific yield strength of a refractory high-entropy composite at an ultrahigh temperature of 2273 K

We reported on the mechanical properties and microstructural evolution of a W20Ta30Mo20C30 (at.%) refractory high-entropy composite (RHEC). The RHEC exhibited outstanding yield strength at both 2273 and 1873 K. Microstructural investigations revealed that the as-cast RHEC had a triple-phase structure consisting of FCC dendrites, HCP matrix, HCP-BCC eutectic structure, and FCC-BCC eutectoid structure, and exhibited high-density defects owing to the complex phase transformations during solidification. After annealing at 2273 K, the precipitation of the BCC phase from the FCC dendrites and the decomposition of the HCP phase into the FCC-BCC eutectoid structure was observed to significantly refine the grain sizes of all triple phases. After compression at 2273 K, the ceramic phases and solid solution precipitated out from each other, which helps to avoid persistent softening after the yielding of RHEC. Further analyses suggested that the dominant deformation mechanisms of the BCC phase and HCP phase are dislocation glide and transformation-induced plasticity; whereas those of the FCC phase are twinning- and transformation-induced plasticity. The outstanding yield strength of this RHEC at ultrahigh temperatures may originate from the high-content ceramic phases and the structural metastability of the multi-principal composition. These findings provide a novel strategy to design RHECs by alloying high-content nonmetallic elements, which contributes to further breaking through their performance limits at ultrahigh temperatures.

Room-Temperature Laser Planting of High-Loading Single-Atom Catalysts for High-Efficiency Electrocatalytic Hydrogen Evolution.

Despite stunning progress in single-atom catalysis (SAC), it remains a grand challenge to yield a high loading of single atoms (SAs) anchored on substrates. Herein, we report a one-step laser-planting strategy to craft SAs of interest under an atmospheric temperature and pressure on various substrates including carbon, metals, and oxides. Laser pulses render concurrent creation of defects on the substrate and decomposition of precursors into monolithic metal SAs, which are immobilized on the as-produced defects via electronic interactions. Laser planting enables a high defect density, leading to a record-high loading of SAs of 41.8 wt %. Our strategy can also synthesize high-entropy SAs (HESAs) with the coexistence of multiple metal SAs, regardless of their distinct characteristics. An integrated experimental and theoretical study reveals that superior catalytic activity can be achieved when the distribution of metal atom content in HESAs resembles the distribution of their catalytic performance in a volcano plot of electrocatalysis. The noble-metal mass activity for a hydrogen evolution reaction within HESAs is 11-fold over that of commercial Pt/C. The laser-planting strategy is robust, opening up a simple and general route to attaining an array of low-cost, high-density SAs on diverse substrates under ambient conditions for electrochemical energy conversion.

Quasi-2D Lead-Tin Perovskite Memory Devices Fabricated by Blade Coating.

Two terminal passive devices are regarded as one of the promising candidates to solve the processor-memory bottleneck in the Von Neumann computing architectures. Many different materials are used to fabricate memory devices, which have the potential to act as synapses in future neuromorphic electronics. Metal halide perovskites are attractive for memory devices as they display high density of defects with a low migration barrier. However, to become promising for a future neuromorphic technology, attention should be paid on non-toxic materials and scalable deposition processes. Herein, it is reported for the first time the successful fabrication of resistive memory devices using quasi-2D tin-lead perovskite of composition (BA)2 MA4 (Pb0.5 Sn0.5 )5 I16 by blade coating. The devices show typical memory characteristics with excellent endurance (2000 cycles), retention (105 s), and storage stability (3 months). Importantly, the memory devices successfully emulate synaptic behaviors such as spike-timing-dependent plasticity, paired-pulse facilitation, short-term potentiation, and long-term potentiation. A mix of slow (ionic) transport and fast (electronic) transport (charge trapping and de-trapping) is proven to be responsible for the observed resistive switching behavior.

Control of resistive switching type in BaTiO3 thin films grown by high and low laser fluence

A ferroelectric memristor has attracted much attention due to convenient controlling by polarization switching, but the resistive switching has been attributed to the drift or charge trapping of defects. To distinguish the resistive switching mechanism between ferroelectric polarization switching and the normal resistive switching mechanism such as the drift or charge trapping of defects, BaTiO3 (BTO) thin films were grown on a (001) Nb:SrTiO3 single crystal substrate by pulsed laser deposition with high and low laser energy density. Based on a piezoelectric force microscope, ferroelectricity is found in BTO thin films grown at high laser energy density. X-ray photoelectron spectroscopy further confirms the existence of defects in the BTO films grown at low laser energy density. The high energy sample with low density of defects exhibits a resistance hysteresis loop but little current hysteresis loop, while the low energy sample with high density of defects shows a significant resistance and current hysteresis loop simultaneously. These results provide a deep understanding about the resistive switching from ferroelectric polarization switching and the drift or charge trapping of defects.