Polymer/nanoparticle (NP) composite films have attracted great attention due to their potential applications in electrical bistable devices. The bistable mechanism is usually attributed to electron trapping and detrapping between the NP trap center and the polymer matrix. However, the exact conduction switching mechanism is still in controversy and even in the same polymer/NP system based devices the switch-on mechanism has been explained by two different models: either Fowler-Nordheim (FN) tunneling or trapped charge-limit-current (TCLC). Therefore, the study on the conduction switching mechanism for polymer/NP composite electrical bistable devices is critically necessary. In this work, ZnO NPs embedded in poly (ethylene oxide) (PEO) were first applied in electrical bistable devices using a solution process and the effect of the nanoparticle surface defects on the conduction switching mechanism is studied. The electrical bistability is observed from the device with the structure ITO/PEO : ZnO-NPs/Al and the conduction switch-on process is dependent on the existence of ZnO surface defects. The effect of the nanoparticle surface defects was investigated by current-voltage, Scanning Electron Microscopy (SEM), and photoluminescence measurements. Besides effectively separating nanoparticles, the surface capping can passivate the surface defects and affect the electrical hysteresis. The switch-on mechanism for the devices based on the NPs with surface detects can be modeled by TCLC while the one based on the NPs with the complete surface defect passivation can be explained by FN tunneling. The results demonstrate that the FN tunneling induced conduction switch-on process is more desirable in electrical bistable devices due to the better device performances. rsc.li/rsc-advances 54128 | RSC Adv., 2017, 7, 54128-54135 This journal isFig. 1 (a) XRD pattern of ZnO nanoparticles with standard diffraction lines of monoclinic ZnO; (b) schematic diagram of the ITO/PEO : ZnO/Al bistable devices; (c) PL spectra of PEO capped ZnO nanoparticles on top of glass; (d) I-V curves of the organic bistable devices with the various PEO : ZnO ratios under 5 V sweep voltage.This journal is Fig. 3 Theoretical linear fitting (solid line) of I-V characteristics in positive voltage region: (a) thermionic emission model plot from À0.1 V to À0.5 V in OFF state, (b) space-charge-limited-current (SCLC) theory plot from À0.6 V to À1.9 V in OFF state, (c) TCLC theory plot from À2 V to À3.8 V in OFF state, (d) SCLC model plot from À5 V to À0.1 V in ON state.This journal is Fig. 5 SEM images of PEO : ZnO film with different mass ratio, (a) ZnO only, (b) 1 : 1, (c) 2 : 1, (d) 10 : 1, (e) 20 : 1, (f) PEO only.This journal is

Conductivity switching in a semiconductor nanoparticle/polymer composite electrical bistable device is typically associated with the electron trapping between the semiconductor trap center and the polymer matrix. However, the switch‐on mechanism is still not clear, and even in the same metal oxide (MO) nanoparticle/polymer system, two different models are applied to explain the conductivity switching mechanisms, including Fowler–Nordheim (F–N) tunneling current and trapped charge‐limit‐current (TCLC). Therefore, the analysis of the conductivity‐switching mechanism in MO/polymer composite devices is crucial to facilitate technology development. Herein, poly(ethylene oxide) (PEO) films are deposited on top of the various TiO2 films, and the two different electrical bistabilities are observed. The corresponding conductivity‐switching mechanisms are found to be significantly related to the different TiO2 surface traps. The results demonstrate that the F–N tunneling current dominates in the devices where a large number of hydroxyl groups are present in TiO2, whereas, after removing the hydroxyl groups from the device, the switching‐on current is dominated by the TCLC mechanism.