- Nano Express
- Open Access
Oxygen Effects on Performance of Electrically Bistable Devices Based on Hybrid Silver Sulfide Poly(N-vinylcarbazole) Nanocomposites
© Li et al. 2016
- Received: 28 December 2015
- Accepted: 28 January 2016
- Published: 3 February 2016
An organic/inorganic bistable device is fabricated by using a simple spin-coating technique, in which the hybrid silver sulfide (Ag2S) poly(N-vinylcarbazole) (PVK) nanocomposite film is sandwiched between two electrodes. An obvious electrical hysteresis is observed in the current-voltage (I-V) curve of the device measured in the presence of different oxygen concentrations, and the magnitude of the electrical hysteresis is decreased with a decrease of the oxygen concentrations. The electrical bistability of the device exhibits a strong dependence on the oxygen concentrations, and the current variation of the OFF state is higher than that of the ON state with the gas atmosphere changing from N2 to air. Different theoretical models have been employed to describe the carrier transport mechanisms of the device in the OFF and ON states measured in different gas atmospheres on the basis of the experimental I-V results, and the carrier transport of the device in the ON state measured in air is very different from that measured in N2 and low O2 concentrations due to the participation of oxygen vacancies in the trapping and de-trapping processes of electrons into and out of the Ag2S/PVK heterointerface.
- Electrical bistability
- Oxygen effects
In the past few decades, organic electrically bistable devices have attracted much attention due to their potential applications in the next generation of nonvolatile memory technology [1–3]. To date, various candidates have been exploited to be used in electrically bistable devices, in which hybrid inorganic/organic nanocomposites have currently aroused broad interests due to their simple fabrication and low cost [4–6]. In general, the hybrid organic/inorganic nanocomposites are formed by dissolving colloidal inorganic nanocrystals and polymers into organic solvent to form a homogeneous phase. A typical device structure for the hybrid organic/inorganic bistable device is a single nanocrystal/polymer hybrid layer sandwiched between two electrodes, in which the hybrid layer is formed by using a simple spin-coating method. Substantial research efforts in the exploitation of inorganic nanocrystals have led to the applications of different types of inorganic nanocrystals in the fabrication of hybrid organic/inorganic bistable devices, such as II-VI group semiconductors, noble metals, and graphene and metal oxide nanocrystals [7–9]. Recently, our group reported some electrically bistable devices based on dodecanethiol-capped Cu2S, hybrid silver sulfide (Ag2S), and Ag nanocrystals blending with conjugated polymer, and obvious electrical bistability and negative differential resistance (NDR) effects were clearly observed. Although some different resistive switching mechanisms, such as charge trapping [10, 11], redox reaction , and metallic filament conducting [13, 14], have been brought out to elucidate the charge transport process, the resistive switching mechanism is still under debate. Therefore, there is no end to study the working mechanisms of the electrically bistable devices based on organic/inorganic nanocomposites. In our previous work, an obvious electrical bistability was observed in the hybrid organic/inorganic nanocomposites , but the environmental gas effects on the electrical bistability and working mechanism were not discussed. Based on our previous work, herein, the effects of environmental gas on the electrical bistability of the devices were studied, and the devices were fabricated based on hybrid organic/inorganic Ag2S/poly(N-vinylcarbazole) (PVK) nanocomposites through a simple spin-coating technique. The current-voltage (I-V) characteristics of the hybrid electrically bistable devices exhibit a strong dependence on the oxygen concentrations, in which the current of ON and OFF states changes with the measurement atmosphere variation from N2 to air. On the basis of the experimental results, different organic electronic models were employed to describe the carrier transport of the electrically bistable devices.
High-resolution TEM images were performed on a JEM-2010 with an acceleration voltage of 200 kV. The I-V characteristics of the devices were measured by using a Keithley Source Meter 2410 controlled by a computer. All the measurements were carried out under ambient conditions at room temperature.
Figure 1 depicts a typical high-resolution transmission electron microscopy (HRTEM) image, and it can be seen that the morphology of the as-obtained Ag2S nanocrystals is quasi-spherical with a mean size less than 5 nm.
When a reverse sweeping voltage is applied to the device, the conducting states switch from OFF state to ON state, and the corresponding experimental and fitting I-V results are depicted in Fig. 5c, d. Two different linear fitting regions in a double logarithmic plot are obtained in the sweeping voltage from 15 to 0 V for the ON state. As shown in Fig. 5c, a distinct linear relationship between log I and log V with a slope of 2.4 is observed in the device measured in the air for the sweeping voltage range from 15 to 5 V, indicating the electrical conduction is dominated by the SCLC process in the voltage region. In contrast, the slope of the device measured under three different atmospheres is in the range of 3.2–4, indicating that the TC-SCLC process dominates the carrier transport of the device in the relatively low O2 concentration and N2 in this voltage region. The electrical conduction of the device measured in air changes from TC-SCLC to SCLC, suggesting that the number of the injected carriers is decreased as some trapped carriers are released from the Ag2S nanocrystals due to the breakdown of the oxide dimer, and some other carriers are still trapped by the dodecanethiol-capped Ag2S nanocrystals. Moreover, with the sweeping voltage decreasing from 5 to 0 V, the slope of the linear fitting of the device measured in air is decreased from 2.4 to 1.4 (Fig. 5d), indicating that the carrier transport changes from the TF-SCLC model to Ohmic conduction , and this behavior may be attributed to the formation of conduction filament which acts as a resistor with a low resistivity [28, 29]. However, the slope of the fitting line remains in the range of 2.0–2.3 for the device measured in the relatively low O2 concentration and N2 atmosphere, which suggests that the electrical conduction is governed by the TF-SCLC model in this voltage range. The aforementioned theoretical fitting results indicate that the electrical conduction of the device for the OFF state is mainly governed by the TE model in the low voltage region and TC-SCLC model in the relatively high voltage region when it is measured under different gas atmospheres. After the conduction state switches from OFF state to ON state, the electrical conduction of the device measured in air is very different from that of the device measured in the low O2 concentration and N2. This indicates that the oxygen concentration has an important effect on the carrier transport of the device for the ON state.
In summary, an electrically bistable devices based on hybrid Ag2S/PVK nanocomposites was fabricated by using a simple solution process technique. The I-V characteristics of the device measured in different atmospheres with different oxygen concentrations exhibited electrical bistable behaviors, and the current of ON state and OFF state showed a strong dependence on the oxygen concentration. The conducting and carrier transport mechanisms of the device were studied by using different theoretical models of organic electronics, and the electrical conduction of the device measured in air for the ON state was very different from that of the device measured in low oxygen concentrations. The device may have potential applications both in the industry of gas sensor and memory storage due to its high level of sensitivity to the environmental oxygen and two distinctive resistive conducting states.
This work is partly supported by the Fundamental Research Funds for the Central Universities (2014JBZ010), National Natural Science Foundation of China (61108063, 61377028, 61475014), and National Science Foundation for Distinguished Young Scholars of China (61125505).
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