1 Introduction

The resistive switching phenomenon basing on the electrical-pulse-regulated resistance in a metal–insulator–metal sandwich structure has recently attracted a great deal of attention due to potential application for nonvolatile random-access memory [1–6]. So far, the resistive switching phenomenon, in which the resistance can be switched between a high resistance state (HRS) and a low resistance state (LRS) by electrical pulse, has been observed in many semiconducting and insulating materials including binary transition metal oxides [7–11], perovskite oxides [12–14], chalcogenides [15, 16], sulfides [17], amorphous silicon [18], organic materials [19, 20], and ferroelectric materials [21, 22]. Accordingly, various models have been suggested to explain the resistive switching phenomenon, including the metal–insulator phase transition [14, 23], the ferroelectric polarization [24–26], the conductive bridge constructed by the migration of localized metal atoms or defects [11, 27], and the formation and elimination of conductive pathways induced by the external electric field [28]. However, the resistive switching mechanisms are still being debated [1, 2].

In past few years, nanoscale transition metal oxides and composites significantly exhibit enhanced physical, chemical, electrical, optical, or magnetic properties, which lead to extensive applications in electronic device, electrochemistry, biomedical device, and other fields [29–33]. Copper–chromium oxide CuCr2O4 often acts as numerous oxidation, hydrogenation, decomposition of alcohols and alkylation reactions, and so on [34]. On the other hand, CuCr2O4, a p-type semiconductor with narrow band gap, is a versatile catalyst due to its stable structure [35]. CuCr2O4 has been reported as an efficient catalyst for various chemical processes such as oxidation, hydrogenation, dehydrogenation, dehydrocyclization, hydrogen production, and decomposition of organic compounds [36, 37]. Therefore, the applications of CuGr2O4 are particularly extensive.

Although there are a large number of reports on various applications of CuCr2O4 in previous works, the resistive switching properties of CuCr2O4 have not been reported so far. Herein, we report resistive switching behavior of Ag/CuCr2O4/FTO device. Specially, the resistive switching characteristics of Ag/CuCr2O4/FTO device can be controlled by white light.

2 Experimental procedures

2.1 Preparation of CuCr2O4 nanoparticles

The CuCr2O4 spinel nanoparticles were prepared by an improved hydrothermal process using cetyltrimethylammonium bromide (CTAB) as the surfactant, which is similar to the methods suggested in previous works [38, 39]. All the chemicals used in this work were of analytical grade and used directly without further purification. The distilled water was used as a solvent throughout the experiments. First, Cu(NO3)2·2.5H2O (3.2 g) and Cr(NO3)3·9H2O (8.0 g) were dissolved in 200 ml distilled water with stirring. Then 200 ml of solution with (CH2)6N4 of 0.01 mol and NH4HCO3 of 0.1 mol was added into above solution. After stirring continuously for 2 h, the precipitate was filtered and washed with distilled water and ethanol for several times until the pH value was 6.5–7.5. Second, the co-precipitate was redispersed in 80 ml of distilled water under vigorous stirring for 30 min. Then 0.5 g cationic surfactant cetyltrimethylammonium bromide (CTAB) was added into above solution under stirring. Then the solution was transferred to a 100-ml sealed Teflon-lined steel autoclave. And the sealed Teflon-lined steel autoclave was heated at 180 °C for 24 h. After the autoclave was cooled to room temperature, the powder obtained was washed with distilled water and ethanol and dried at 60 °C for 12 h. Finally, we annealed the as-prepared CuCr2O4 powder at 900 °C in air for 2 h with a gradual heating rate of 10 °C min−1.

2.2 Preparation of Ag/CuCr2O4/FTO device

First, fluorine-doped tin oxide (FTO) substrates were cleaned by acetone, ethanol, and deionized water and subsequently dried on the spin coater. Second, CuCr2O4 films were prepared on FTO substrate by spin-coating method. The detailed preparation process of CuCr2O4 films is as follows: First, we grinded the as-prepared powder for 2 h. Next, we dissolved the powder in toluene solution to prepare precursor gel. Then the precursor gel was spin-coated on the FTO substrate. The spin-coating process at 5000 rpm for 10 s was used for film deposition. Second, these samples were subsequently dried at 60 °C in vacuum for overnight. The thickness of the films was detected by the step profiler.

2.3 Characterizations

Crystal structure of CuCr2O4 film was characterized at room temperature by X-ray diffraction (XRD) with Cu Kα radiation. The microstructure of the CuCr2O4 film was observed by transmission electron microscopy. In the test of resistive switching characterizations, Ag is top electrode and FTO is bottom electrode. Ag electrodes were prepared by vacuum deposition. And the preparation process of Ag electrodes is as follows: First, we covered surface of CuCr2O4/FTO with a mask. Second, we put it into the vacuum sputtering system to grow Ag electrodes. Finally, we chose the superior electrodes for characterization. Current density–voltage (J–V) and resistance–cycle curves were tested using the electrochemical workstation at room temperature. We used an ordinary filament lamp with various power densities as light source. The wavelength range of light is 400–760 nm.

3 Results and discussion

Figure 1a shows the schematic representation of the device, where the CuCr2O4 film was spin-coated on the FTO substrate, and the electrodes of Ag with the area were deposited onto the CuCr2O4 film. The crystalline structure of the samples was characterized by XRD patterns. Figure 1b displays the XRD of CuCr2O4/FTO structure. The peaks of FTO substrate are obvious (Fig. 1b). In order to make diffraction peaks of CuCr2O4 film more clear, we also present the XRD pattern of the pure FTO substrate without CuCr2O4 film in Fig. 1b. Figure 1b exhibits the XRD pattern of CuCr2O4/FTO. We can see there are only the peaks of CuCr2O4 besides peaks of FTO substrate. The diffraction patterns in Fig. 1b agree with tetragonal CuCr2O4 with spinel structure [38–40]. The XRD demonstrates the characteristic diffraction peaks of CuCr2O4 with spinel structure, which is in good agreement with JCPDS-No 34-0424 [38]. No characteristic diffraction peaks owing to CuO and Cr2O3 are detected. Therefore, the films contain only pure CuCr2O4, and the sharp peaks demonstrate good crystallinity of the CuCr2O4.

Fig. 1
figure 1

a Schematic representation of the device. b The X-ray diffraction (XRD) of CuCr2O4/FTO structure and FTO substrate

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Figure 2a presents the high-resolution transmission electron microscope (HRTEM) image of CuCr2O4 film. The fringes with a spacing of 0.27 nm correspond to (311) planes of CuCr2O4, which indicates that the CuCr2O4 film is single-crystalline structure for individual CuCr2O4 nanoparticle. The composition of CuCr2O4 film is further confirmed by elemental analysis carried out from energy-dispersive X-ray spectra (EDX). The EDX data in Fig. 2b confirm that the compositions of the film are Cu, Cr, and O without any other impurities. And the atomic percentage Cu/Cr/O of CuCr2O4 film is about 1:2:4 from the inset of Fig. 2b.

Fig. 2
figure 2

a High-resolution transmission electron microscope (HRTEM) image of CuCr2O4 film. b The energy-dispersive X-ray (EDX) spectrum of CuCr2O4 film; the inset shows that the atomic percentage of CuCr2O4 film is about 1:2:4

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Figure 3 displays the UV–Vis absorption spectrum of CuCr2O4 nanoparticles without FTO substrate. The onset of the absorption located at about 580 nm indicates that as-prepared CuCr2O4 nanoparticles have good light absorption properties in the visible light region.

Fig. 3
figure 3

PL behavior of CuCr2O4 powders at room temperature

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In order to obtain the resistive switching characteristics of Ag/CuCr2O4/FTO structure under white-light illumination with various power densities, we employed the experimental test circuit shown in inset of Fig. 4a. Figure 4a displays the current density–voltage (J–V) characteristic curves of Ag/CuCr2O4/FTO device in linear scale under illumination with various power densities, which exhibits an asymmetric behavior with significant hysteresis. The arrows in the figure denote the sweeping direction of voltage. We found that the response time is about 0.1–0.2 s.

Fig. 4
figure 4

a Current density–voltage (J–V) characteristic curves in linear scale of Ag/CuCr2O4/FTO structure in the dark and under white-light illumination with various power densities; the inset presents the real test circuit. b The corresponding J–V characteristic curves in logarithmic scale

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Figure 4b shows a corresponding J–V curve of Ag/CuCr2O4/FTO device in logarithmic scale with resistance switching effects. The arrows in Fig. 4b denote the sweeping direction of voltage. We can see that a sudden current increase occurs at about 0.85 V (V Set) in the dark and about 0.9 V (V Set) under white-light illumination with power density of 5 mW/cm2, indicating a resistive switching from the high resistance state (HRS or “OFF”) to the low resistance state (LRS or “ON”), which was called the “Set” process. With further increasing the power density to 10 mW/cm2, the V Set reaches to 0.98 V. When the applied voltage sweeps from zero to negative voltage of about −0.75 V (V Reset) in the dark and −0.98 V under white-light illumination with power density 10 mW/cm2, the device can return to the HRS, which was called the “Reset” process. During the successive “Set” and “Reset” cycles on the same device, the device shows the identical J–V curves. The V Reset and V Set are almost unchanged in subsequent cycles. The threshold voltages in the device are ≤1.0 V, which is an attractive advantage for practical memory applications in an expansive condition [41, 42].

Figure 5a displays the evolutions of V Set and V Reset over 100 successive resistive switching cycles on the device. We find that there is only little fatigue for switching voltages V Set and V Reset. The V Set and V Reset are 0.75 ± 0.06 V and −0.7 ± 0.1 V, respectively, in the dark, and the V Set and V Reset are increased to 0.92 ± 0.08 V and −0.9 ± 0.1 V, respectively, under white-light illumination with power density of 10 mW/cm2, indicating low fatigue for switching voltages of the Ag/CuCr2O4/FTO structure, which reflects excellent switching stability of V Set and V Reset to a certain extent. It is worth noting that the illumination can control the switching voltage V Set and V Reset. The absolute values of V Reset and V Set increase with the increasing power density of illumination. That is to say that the illumination can control the resistive switching, which is consistent with the reported results in previous literature [43–47].

Fig. 5
figure 5

a Evolution of switching voltages including V Set and V Reset during the 100 resistive switching cycles in the dark and under light illumination with power density 10 mW/cm2. b The resistance–cycle curve with a positive bias voltage of 0.1 V in the dark and under light illumination with power density 10 mW/cm2

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To estimate the probably practicability of white-light-controlled resistive switching behaviors of the Ag/CuCr2O4/FTO structure device, the resistance–cycle number curve for the HRS and LRS with a positive bias of 0.1 V is tested and shown in Fig. 5b. It is obvious that the resistances are about 1.1 KΩ at the LRS (ON state) and 2.5 MΩ at the HRS (OFF state) in the dark, and the resistances are about 1.05 KΩ at the LRS (ON state) and 2.48 MΩ at the HRS (OFF state) under illumination with power density of 10 mW/cm2. The OFF/ON-state resistance ratio is up to 103. Both the LRS resistance and the HRS resistance decrease with the increasing power density of illumination. According to the above results, the steady white-light-controlled resistive switching behavior in Ag/CuCr2O4/FTO structure with an OFF/ON-state resistance ratio of about 103 provides the potential for light-controlled nonvolatile memory applications.

The mechanism for resistive switching in a metal/oxides/metal structure has been extensively investigated, but is still controversial [34, 44]. In our works, the asymmetric behavior of J–V curve indicates that a Schottky barrier is formed at the interfaces of Ag/CuCr2O4 and CuCr2O4/FTO. This bipolar resistive switching behavior should be resulted from the trapped and detrapped charge in the Schottky-like depletion layer [48–54]. The white light can modulate the resistive switching behavior by a large number of photogenerated charges [44–47].

4 Conclusions

In brief, the reversible bipolar resistive switching characteristics of Ag/CuCr2O4/FTO device are observed. In particularly, the white light can control the resistance switching behavior. Therefore, the superior resistance switching characteristics of the Ag/CuCr2O4/FTO device hold a great promise for next-generation nonvolatile light-controlled memory applications.