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We fabricate a ferroelectric field effect transistor (FeFET) based on a semiconducting vanadium dioxide (VO2) nanowire (NW), and we investigate its electron transport characteristics modulated by the ferroelectric effects. The transistor consists of a single VO2 NW as the channel and a ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin film as the dielectric gate. The conductance of the VO2 NW channel is found to be feasibly modulated by the ferroelectric gate with an 85% resistance change under the gate voltage of 18 V (at an applied field of about 0.75 MV cm-1). The electron transport property of the device can be controlled by the remnant polarization of the PZT layer due to the nonvolatile property of the ferroelectric gate, with an off-field change of channel resistance up to 50%. Moreover, multiple resistive states can be achieved by sweeping gate voltage across the device appropriately. These results demonstrate that ferroelectric gate modulation is an efficient tool to regulate the electron transport properties of the VO2 NW, and the VO2-NW-FeFET has potential applications in nonvolatile and low-power consumption devices.

Vanadium dioxide (VO2) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO2 is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO2, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO2 and their corresponding structural features, the typical fabrication methods of VO2 nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO2. In addition, the challenges and opportunities for VO2 in future studies and applications are also discussed.

We propose a doping method by using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to fill the grain boundary interstices of the methylammonium lead iodide (CH3NH3PbI3) perovskite for the elimination of pinholes. A sandwiched PCBM layer is also used between the perovskite and TiO2 layers to improve the interfacial contact. By using these two methods, the fabricated perovskite solar cells show a low hysteresis effect and high current density, which result from the improved compactness at the grain boundaries of the perovskite surface and the interface between the TiO2/perovskite layers. The theoretical and experimental results indicate that PCBM can effectively suppress carrier recombination, regardless of the interfacial layer or dopant. We also found that the dark current reduced during the analysis of dark state current–voltage (I–V) characteristics. The slopes of the I–V curves for the fluorine-doped tin oxide/PCBM-doped perovskite/Au device reduce monotonically with the increase in the PCBM concent...

The possibility of switching the direction of the dipole toroidal moment in ferroelectrics provides exciting opportunities for development of novel nanoscale memory and logic devices. However, a practical control of vortex chirality is rather challenging at present stage, not to mention via mechanical methods. In this paper, we performed the phase-field simulations to show that mechanical switching of vortex chirality can be achieved in ferroelectric nanoplatelet via defect engineering. After introducing a void defect in the nanoplatelet, relative stability of single-vortex state and multi-vortices state is found to be altered. Importantly, during stress-induced vortex multiplication process, the void is a favored nucleation core of new vortex; meanwhile, vortices tend to annihilate away from the void during a vortex annihilation process. As the favored regions of vortex nucleation and annihilation are not the same, a deterministic mechanical switching of vortex chirality can be achieved. The effects of temperature, shape of the nanoplatelet, void size, as well as void position, on the defect-mediated vortex switching behaviors are systematically revealed. Our study demonstrates the feasibility of vortex switching by mechanical loads and provides a route to control and develop electromechanical devices based on ferroic vortices.

Tip-force-induced domain switching in ferroelectrics has recently attracted extensive interest as it provides an alternative switching strategy that might ease the problems brought by electrical switching. From the viewpoint of mechanics, substrate elasticity can largely modify the tip-induced deformation of ferroelectric thin films. However, so far, discussions on the influence of substrate elastic properties on such domain switching still remain exclusive. Here, a phase-field model is employed to study the influence of substrate stiffness on the domain switching in BaTiO3 (BTO) thin films, with the strain and stress distributions in BTO thin films and substrates solved by the finite element method. The results demonstrate that the substrate stiffness and loading modes (i.e., pressing and sliding) have a great influence on the symmetry of strain and stress distributions. The switched domain size is highly dependent on the substrate stiffness and loading modes. The switching is more efficient for thin films on a softer substrate. Moreover, the domain could be switched more effectively by the sliding mode under relatively large forces. Our study thus provides a strategy to increase the mechanical switching efficiency of ferroelectric thin films via tuning the substrate elasticity.

Artificial neural networks have gained intensive attention in recent years because of their potential in effectively reducing energy consumption and improving computation performance. Ferroelectric materials are considered to be promising candidates for artificial synapses because of their multiple and nonvolatile polarization states under external stimuli. Despite artificial ferroelectric synapses with multilevel states, long retention and fast switching speed have been reported, and some key fundamental issues, e.g., the influence of domain wall configuration and evolution on the performance of synapse behaviors, also remain unclear. In this work, we study the performance of artificial synapses based on the motion of 180° ferroelectric domain walls of stripe domain and cylinder domain in ferroelectric thin films via a dynamical phase field model. The results demonstrate that artificial synapses based on the stripe domain exhibit high linearity and symmetry in weight update under a weak electric field, compared with the cylinder domain. Based on such artificial synapses, the accuracy of an artificial neural network for the Modified National Institute of Standards and Technology handwritten digit recognition is over 92%. This work provides a domain-wall-based strategy to improve the weight updating linearity and symmetry of artificial synapse devices and the recognition accuracy of artificial neural networks.

Ferroelectric nanostructures have recently attracted considerable research interests for the discovery of novel topological dipole states. However, currently the preparation of high-quality nanodot arrays remains challenging, precluding the clear characterization of the dipole states in the nanodots. Combining pulsed laser deposition method with circular anodic aluminum oxide template, we reported an unexpected growth of high crystalline quality $\mathrm{PbZ}{\mathrm{r}}_{0.52}\mathrm{T}{\mathrm{i}}_{0.48}{\mathrm{O}}_{3}$ nanodot array in quadrilateral shape with size being \ensuremath{\sim}80 nm. Piezoresponse force microscopy measurement on various regions of the sample shows that the nanodots form rather diverse in-plane domain patterns, with the appearance of abundant topological domain defects including flux-closure vortices, center-divergent vortices, center-convergent vortices, and antivortices. A deterministic control of the domain structure of the nanodots via applying an electric field is further achieved. The nanodots were found to favor a center-divergent vortex pattern under negative tip bias, although the domain patterns of the nanodots at the as-grown state are different. Phase field simulations were performed to reproduce the experimental observation, and our results indicate that the screening condition of the nanodots is far from the ideal ones assumed in previous theoretical modellings. The nanodots are potentially useful in developing multi-state and reconfigurable electronic devices.

Recent experiments have demonstrated the existence of vortex or flux-closure domains in ferroelectric nanostructures, which are attractive to develop high-density data storage and novel configurable electronic devices. However, it remains challenging to stabilize in-plane vortex or flux-closure domains in ferroelectric film for the absence of a lateral geometry confinement. Based on a 3D phase field model, here we show that stabilization of isolated or interacting in-plane vortices in ferroelectric film can be achieved via applying a mechanical tip-force. The formation of such dipole vortices is caused by a conjoint effect of the tip-force-induced depolarization effect and in-plane strain. The effects of factors like film thickness, misfit strain, tip force and temperature on the vortex formation are systematically revealed and summarized as phase diagrams. The interaction between tip-induced vortices is also investigated. It is found that as the two tips get closer than the critical distance, the two initially isolated vortices become coupled, with identical or opposite chirality, depending on the distance between the two tips. A maximum data storage density of isolated in-plane vortices in ferroelectric thin film is estimated to be ~1 Tb in-2. Our work thus demonstrates a mechanical strategy to stabilize dipole vortices, and provides a comprehensive insight into the characteristics of ferroelectric film under a mechanical tip force.

Combining scanning force microscopy characterization and theoretical modeling, in this work, we performed an in-depth study on the electrical/mechanical switching and electroresistance effect in a BaTiO3 thin film. Correlations of the tip load (bias/force and loading time), the switched polarization magnitude, the surface potential, and the tunnel electroresistance are revealed for both electrical and mechanical switching. It is found that electrical switching (with a maximum bias of 4 V) leads to larger saturated switched polarization and sharper switched domain than mechanical switching (with a maximum force of 6600 nN). Meanwhile, mechanical switching exhibits generally smaller surface potential of the switched domain and a more significant tunnel electroresistance effect. However, the load time-dependence of performance is also more serious for mechanical switching. The different characteristics between electrical and mechanical switching are attributed to the charge injection and the switched domain size, which are believed to further affect the surface potential and the tunnel electroresistance of the thin film. At the end, an optimized hybrid switching strategy, which combines tip force and bias, is proposed and shown to be able to achieve complete polarization reversion, low charge injection, high switch speed, and strong tunnel electroresistance effect.

Here we propose a method to detect the degenerated tetragonal vortex states (i.e., the toroidal axis is along the x-, y- or z-axis) in ferroelectric nanodots by applying a controllable surface charge (CSC) condition. Electrodes are placed at two parallel surfaces of the nanodot to form a short circuit. Surface charges with a controllable density are then applied to another two parallel surfaces of the nanodot. Under this CSC condition, a characteristic short-circuit current vs. time (I–t) curve related with the evolution of domain structure in a nanodot can be detected. The evolution paths and the characteristic short-circuit I–t curves of the degenerated vortex states in ferroelectric nanodots have been systematically revealed by our phase field simulations by solving the time-dependent Ginzburg–Landau (TDGL) equations. It is found that the degenerated vortex states exhibit distinct evolution features under the CSC condition. In the stages of placing electrodes and increasing surface charges, one, two, and zero short-circuit I–t peak(s) are observed in the nanodots with 〈100〉, 〈010〉 and 〈001〉 vortex states, respectively. Therefore, the unknown vortex states of a nanodot can be distinguished. We further investigate the effects of temperature and nanodot size on the characteristic I–t curves of the vortex states. The results show that the vortex states can be nondestructively distinguished by applying the CSC condition if the nanodot size is within a moderate range (i.e., 8–12 nm). Our study provides an alternative way of detecting the degenerated tetragonal vortex states in ferroelectric nanodots without the use of a scanning probe microscope, and also sheds light on the application of ferroelectric vortex domain structures in novel devices such as memories, sensors and actuators.

Transition-metal oxides have fascinating characteristics, and have been exploited for various applications, such as Mott transistors, optical switches and strain sensors, etc. Vanadium dioxide is a special and important transition-metal oxide, and exhibits the significant behavior of metal-insulator transition. In this work, single crystalline VO2(A) nanowires have been synthesized by a facile hydrothermal method. Due to the size and surface effects, the nanowires with different widths show great disparities in their hysteresis loops, phase transition temperatures and electrical conductivities. Our results show that the phase transition temperature is linearly dependent on the inverse of the nanowire widths, and a similar relationship between the electrical conductivity and the width of the nanowires has also been found. More interestingly, the first-order phase transition of the nanowire even coverts into high-order continuous phase transition when the width is below a critical size. To explore the intrinsic influence of the size and surface effects, the analysis of the transmission electron microscopy measurements showed that the rough surface structure of the nanowire is very different to the internal structure, and the thickness of this rough surface structure almost remains unchanged as the with of the nanowire decreases. Our results indicated that the surface structure has a remarkable effect on the phase transition characteristics decreasing nanowire width, and the suitable heterogeneous nucleation originating from the rough surface structure should play a crucial role in properties of the VO2(A) nanowires. Size-dependent phase transition features of the VO2(A) nanowires also suggest that the size and surface effects must be taken into consideration when designing VO2 nanodevices.

Dynamical phase field simulation is performed to reveal the dynamic characteristics of the tip-force-induced polarization switching in ferroelectric thin films. We demonstrate nontrivial influences of kinetic coefficient μ related to the second-order time derivative term in the dynamic equation of polarization on the mechanical switching behavior. It is found that such a term causes an oscillation feature of the switching process. Two characteristic switching times, i.e., the time when the inversed polarization begins to appear (denoted as τS1) and the time when the fraction of switched (c−) domain is largest during the loading process (denoted as τS2), can be defined to describe the tip-force-induced switching behavior. Both τS1 and τS2 are found to be affected by factors like misfit strain, temperature, and film thickness. Remarkably, the mechanical switching of polarization can be rather fast, with the switching time comparable to that of electrical switching. Due to the nontrivial dynamical effects, other important phenomena are observed: (a) the size and the pattern of switched domain (i.e., cylinder vs ring) in a single-point switching event strongly depend on the loading time, (b) the critical force of mechanical switching may be largely decreased by choosing a proper loading time, and (c) a large and stable domain pattern can still be written by a sweeping tip despite that the switched domain is not stable in the single-point switching event. Our study should provide new insights into the ultrafast phenomena in ferroelectric polarization switching under mechanical stimuli.

As an important strongly correlated electron material, VO2 undergoes a metal-insulator transition (MIT) accompanied by a huge change of several orders of magnitude in conductance and transmittance. The MIT behavior can be controlled by low-dimensional structures (LDSs) and the interaction between LDSs and substrates. Consequently, fabricating the LDSs and understanding the phase transition behaviors have great significance for the investigation of fundamental properties and applications. Using the pulsed laser deposition technique, we fabricate abundant LDSs (i.e., from zero-dimensional nanodots, one-dimensional nanowires, nanobelts and nanorods to two-dimensional nanoplatelets and ultra-thin films, and zero-/one-/two-dimensional mixed structures), and investigate the controllability of each deposition factor on the growth of the LDSs. TEM results confirm the high crystallinity of the as-synthesized LDSs. AFM results and ab initio calculations demonstrate the great influence of substrates on the growth orientation of the LDSs. More importantly, we systematically investigate the phase transition characteristics of the LDSs by temperature-dependent Raman spectroscopy and XRD. The results clearly reveal the structural dependence of the phase transition features due to the delicate effects of substrates and structures. Our technique provides a rapid, controllable and easy method for fabricating VO2 LDSs, which can lead to a deeper understanding of the electrical, optical, and magnetic properties and potential applications of VO2.

Implementing stretchable memory is the key toward an intelligent device possessing wearability and implantability. In this work, we construct a stretchable ferroelectric field effect transistor (Fe-FET) based on buckled poly(vinylidene fluoride-trifluoroethylene)/poly(3-hexyl thiophene) [P(VDF-TrFE)/P3HT] bilayers. The fabrication procedure avoids complicated etching steps and photolithography process, which significantly reduce the need for equipment and prevent harm to the polymers. Multilevel storage capacity and photomodulated resistance are achieved in the stretchable Fe-FET, in which the conductance of the P3HT layer can be continuously adjusted by the polarization of the P(VDF-TrFE) layer. The stored information remains stable under 20% tensile deformation and is retained even after 2000 stretching/releasing cycles. The good mechanical stability and multilevel storage capacity make this stretchable Fe-FET potential for utilization in smart labels, epidermal systems, and even biointegrated artificial synapses.

Superior thermal stability of resistive switching performance is essential for resistive random access memory (R-RAM) device with high reliability. Thermal stable resistive switching performance can be achieved in ZnO/BiFeO3 bilayer structure by modifying the interface. The bilayer structure with a distinct interface is fabricated with an optimized annealing process in chemical solution deposition method. This bilayer structure shows a better thermal stability compared to the case with an indistinct interface. Attempt has been made to explain such effects based on conductive filament mechanism. We propose that the confinement of the oxygen vacancies migration at distinct interface could be the reason for the thermal stability. Our results indicate that morphology of interface is an important factor to improve the thermal stability of R-RAM.

This work reports a large controllability of domain evolution in BaTiO3 ferroelectric nanodot by isotropic surface charge screening (SCS). The phase field simulations show that the nanodot can exhibit fruitful domain patterns and evolution paths as functions of temperature and screening factor and changing direction of these two variables. Four typical types of experimental processes have been simulated, including the cooling-down and heating-up processes under fixed SCS conditions, and the processes of increasing and decreasing SCS under fixed temperatures. During these processes, the nanodot exhibits up to 13 different kinds of domain patterns, among which some are either polar or toroidal and some are both polar and toroidal. We summarized the phase diagrams as functions of temperature and charge screening factor, and also analyzed the typical domain evolution paths.

Correction for ‘Phase transition characteristics in the conductivity of VO2(A) nanowires: size and surface effects’ by C. Q. Wang et al., Phys. Chem. Chem. Phys., 2016, 18, 10262–10269.

In order to demonstrate the control of BiFeO3 thin film on the resistive switching effect and achieve the high-performance resistive switching device, the single layers and bilayer have been fabricated by chemical solution deposition method, respectively. In comparison with the single films, the composite film exhibits great performance of the resistive switching in endurance and repeatability, high stability and resistance ratio of high resistance state to low resistance state. Resistive switching effect in the BiFeO3 composite structure demonstrates an effective way to improve the endurance and repeatability of the resistive switching characteristics by designing the relative devices.

The high stability to maintain stored information under mechanical deformation is an essential requirement for the practical applications of stretchable electronics. In addition to storage stability, large deformation and easy fabrication are also desirable features for stretchable devices. In this work, we use wavy P(VDF-TrFE) nanoribbons to achieve a mechanical deformation of more than 20%, and the fabricating procedure eliminates the need for complicated etching steps and lithographic masks. The stored information, which is written on the ribbons in the form of ferroelectric domains, is able to remain unchanged after large mechanical deformation. After 10 000 stretching/releasing cycles, the polarization orientation remains the same with very little change of the intensity. These P(VDF-TrFE) nanoribbons with large deformation and high stability demonstrate great potential for the enhanced storage performance of future stretchable electronics.The high stability to maintain stored information under mechanical deformation is an essential requirement for the practical applications of stretchable electronics. In addition to storage stability, large deformation and easy fabrication are also desirable features for stretchable devices. In this work, we use wavy P(VDF-TrFE) nanoribbons to achieve a mechanical deformation of more than 20%, and the fabricating procedure eliminates the need for complicated etching steps and lithographic masks. The stored information, which is written on the ribbons in the form of ferroelectric domains, is able to remain unchanged after large mechanical deformation. After 10 000 stretching/releasing cycles, the polarization orientation remains the same with very little change of the intensity. These P(VDF-TrFE) nanoribbons with large deformation and high stability demonstrate great potential for the enhanced storage performance of future stretchable electronics.

This work aims to give a comprehensive view of magnetic state stability and transformations in PZT-film/FeGa-dot multiferroic composite systems due to the combining effects of size, shape and interfacial coupling strain. It is found that the stable magnetic state of the FeGa nanodots is not only a function of the size and shape of the nanodot but also strongly sensitive to the interfacial coupling strain modified by the polarization state of PZT film. In particular, due to the large magnetostriction of FeGa, the phase boundaries between different magnetic states (i.e., in-plane/out-of-plane polar states, and single-/multi-vortex states) of FeGa nanodots can be effectively tuned by the polarization-mediated strain. Fruitful strain-mediated transformation paths of magnetic states including those between states with different orderings (i.e., one is polar and the other is vortex), as well as those between states with the same ordering (i.e., both are polar or both are vortex) have been revealed in a comprehensive view. Our result sheds light on the potential of utilizing electric field to induce fruitful magnetic state transformation paths in multiferroic film-dot systems towards a development of novel magnetic random access memories.

Phase field simulations are performed to investigate the domain structures of soft ferromagnetic nanodots. It is found that the stability of the domain state is sensitive to its lateral dimensions. As the lateral dimensions increase, the stable domain state gradually changes from polar to vortex, with a transitional region where both the two ordered states are stable. Interestingly, the phase diagram is also a strong function of mechanical loads. By appropriately choosing the lateral dimensions, transformations between polar and vortex states can be induced or controlled by mechanical loads. The study provides instructive information for the applications of ferromagnetic nanostructures.