Influence of adsorption small molecules atrazine on nonvolatile resistive switching behavior in Co–Al layered double hydroxide films

Co–Al-layered double hydroxides (LDHs) thin films were prepared by drop-casting process on ITO-coated glass substrates. And then the small molecule atrazine was adsorbed on the Co–Al LDHs film by impregnation method. Current–voltage characteristics revealed nonvolatile resistive switching in Co–Al LDHs adsorbed atrazine films. The Influence of adsorption small molecules atrazine on nonvolatile resistive switching behavior in Co–Al LDHs film has been investigated. By varying the atrazine adsorbed content in Co–Al LDHs thin films, the nonvolatile resistive switching behavior of device could be adjusted in a controlled way. Entirely different nonvolatile resistive switching characteristic, such as write-once read-many-times memory effect and rewritable memory effect are discriminable by the current–voltage curves.


Introduction
Nonvolatile memory is the basic unit in a computer where data is stored to perform logical operations [1][2][3]. With the development of modern electronic technology and further miniaturization of silicon semiconductor devices, new memory technologies have been vigorously developed [4][5][6]. Among these, resistive random access memory (RRAM) is considered the best candidate to replace traditional flash memory technology [7][8][9][10][11].
Two-dimensional (2D) nanocomposite has realized more than one breakthroughs in material and device issues due to their unique structure stability and functionality [3,[12][13][14][15][16][17][18]. In the last few years, the nanocomposite-based resistive switching and photoelectricity switching behaviors have been studied, such as the 2D transition metal dichalcogenidesbased resistive switching memory behavior [19][20][21], enhancement of photoresponse in Cu 2 O/rGO nanocomposites [22], application of AgInZnS-graphene oxide nanocomposite in biomedical optical imaging [23], enhanced X-ray photon response of CsPbBr 3 /rGO nanocomposites [24]. Nevertheless, this material has no high self-selectivity, which prevents large-scale integration without use of additional transistors. The reliability of 2D transition metal dichalcogenides-based memory devices still poses a problem; moreover, the performance of device varies from laboratory to laboratory [25]. While pursuing the robust device performance of 2D materials, we also focus on the robust physical research of 2D materials.
Layered double hydroxides (LDHs), also wellknown as hydrotalcite-like compounds, are inorganic layered materials which have aroused great concern because of their potential applications in adsorption, sensors, electrochemistry and bionanotechnology. The tunable metal ions in a large area without altering material structure and anion exchange properties of LDHs materials render it promising candidate as ion exchanger and absorbent [26]. In recent years, LDHs are increasingly used as biomedical materials. LDHs has a layered structure, a positively charged lamellae structure, exchangeable anions between the layers, and adjustable interlayer space. It not only has the characteristics of biological drug loading, but also has a good electronic channel building function. LDHs is a sandwich compound composed of positively charged laminates and negatively charged interlaminates.
The general formula is ½M 2þ 1Àx M 3þ x ðOHÞ 2 xþ A nÀ x=n Á mH 2 O, where M 2þ refers to the divalent metal cation, M 3þ refers to the trivalent metal cation, A refers to the anion, x represents the ratio of the mass of the M 3þ to the sum of the masses of the M 2þ and M 3þ ; and m refers to the number of moles of crystal water per mole of LDHs. The range of M 2þ and M 3þ in the laminates is wide. Considering that Co is amphoteric and its conductivity is similar to that of semiconductor, it is easy to produce electrical hysteresis effect for resistive switching, and its radius is very close to that of active metal Al, which is conducive to isomeric substitution between lamination plates. Therefore, in this work, Co-Al LDHs is selected as the active layer and the high-speed electron transfer channel formed by its layered structure is used to realize the resistive switching effect. Furthermore, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine (atrazine) is known as a n-type semiconductor, which belongs to the group of symmetric triazine derivatives, due to the introduction of electron-withdrawing group level halogen atoms, they have good charge transfer ability, Moreover, the attraction of the positively charged main plate of Co-Al LDHs to the negatively charged chlorine ions of atrazine, makes it easy to adsorb atrazine on the surface of the Co-Al LDHs.
Recently, Maikap research group fabricated 2D materials MoS 2 -based conducting bridge random access memory devices by engineering Hf/Si interfacial layers. The device shows excellent resistive switching memory and artificial synapse behavior [27]. In addition, this research group made use of dual nanostructure engineering to improve the performance of AlOx-based RRAM devices. The transformation of filamentary switching to nonfilamentary switching due to synergistic effect of dual nanostructure engineering was is implemented. Make allowances for a large number of researches about resistive switching and memory effects [28][29][30][31], the influence of adsorbed level on the resistive switching behavior, which seems that further research is needed. It is reasonable to expect that adsorbed level will have a noticeable impact on charge transport processes, and consequently will have significant influence on device property [32].
In this work, the nonvolatile resistive switching behavior in Co-Al-layered double hydroxide adsorbed atrazine films was reported. The nonvolatile resistive switching behaviors of the Co-Al LDHs adsorbed atrazine films could be adjusted by changing the adsorbed atrazine content in active films. Write-once-read-many-times (WORM) memory and rewritable memory can be realized by adjusting the adsorbed atrazine content on Co-Al LDHs film.

Experimental section 2.1 Materials and solution preparation
Co-Al LDHs were synthesized as our previous work [33,34], atrazine (molecular weight: 217.70) was provided by Shanghai Fusheng Industrial Co., LTD. ITO glasses with a size of 1 cm 9 2 cm were cleaned by ultrasonic in acetone, methanol and deionized water for 35 min successively, and then dried in the vacuum drying phase for standby use. To study the influence of atrazine adsorption content on the resistive switching characteristics of Co-Al LDHs film, the 200, 150, 100, 75, 50, 25 mg of Co-Al LDHs was impregnated in 5 ml (10 mg/ml) atrazine aqueous solution for 24 h (labeled as sample A, sample B, sample C, sample D, sample E, and sample F), respectively.

Device fabrication
2 ml of Co-Al LDHs adsorbed atrazine solution was drop-casted on ITO glass substrate and dried at 50°C for 8 h to form the Co-Al LDHs adsorbed atrazine active layer. Whereafter, the electrode Al matrix was deposited by vacuum evaporation with the help of the mask (100 lm in width, 200 lm in length).

Measurement and characterization
The scanning electron microscope (HITACHI S-4800), transmission electron microscopy (JEOL-2100), atomic force microscopy (Cypher S.), and X-ray photoelectron spectroscopy (Kratos-AXIS ULTRA DLDX) measurements were conducted based on the structure of ITO/Co-Al LDHs. The electrical properties of memory devices were characterized by Keithley 4200 semiconductor parameter testing system. During measurement of current-voltage characteristics of the device, the bottom electrode was always grounded. Figure 1a depicts the chemical structure diagram of Co-Al LDHs. Co-Al LDHs have a typical brucite regular octahedron structure. The synusias are composed of metal-oxygen octahedron. Because the Co in the center of the octahedron is replaced by Al homocrystals with similar radius, there is a large amount of permanent positive charge. The electrical equilibrium is maintained between the layers by exchangeable anions. Figure 1b depicts the chemical structure of atrazine. Co-Al LDHs adsorbed atrazine were used as the resistive switching layer in the devices. Schematic device structure of the Co-Al LDHs adsorbed atrazine-based device was shown in Fig. 1c. The cross section of resistive switching layer before deposition of top electrode Al was characterized by scanning electron microscopy as shown in Fig. 1d, it can be seen the thickness of resistive switching layer about 150 nm. The top-view SEM of resistive switching film was shown in Fig. 1e. And the atomic force microscopy of the resistive switching layer was shown in Fig. 1f. It can be seen that the surface of resistive switching layer was uniform, and the surface roughness was about 8.76 nm, manifesting that resistive switching layer has good flatness.

Results and discussion
X-ray photoelectron spectroscopy (XPS) technology is the most direct characterization method to identify the elements contained in substances and analyze their valence states. Figure 1g shows the Co 2p spectra of Co-Al LDHs. It can be clearly seen from the energy spectrum that Co element in Co-Al LDHs is split into two energy peaks due to the interaction of spin orbits, corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively. Co 2p 3/2 is located at 781.1 eV, and Co 2p 1/2 is located at 797.1 eV, and its spin separation energy is 16.0 eV, which proves that Co ions in the material exist in the form of Co(OH) 2 , while the remaining three satellite peaks indicate that Co ions exist in the form of Co 2? . Figure 1h is the XPS spectrum of O 1s. According to its peak position, the oxygen valence state is O -2 valence. As the O1s spectrum is asymmetric, Gaussian peaks are performed on the O1s to simulate the synthesis of two oxygen peaks, which are located at 531.4 and 532.5 eV, respectively, corresponding to Co-OH and/or Al-OH [35] and chemisorption oxygen [36]. Among them, Co-OH and/or Al-OH accounted for 60.8%, and chemisorption oxygen content accounted for 39.2%. The XPS results of Al element are shown in Fig. 1i. It can be seen that Al 2p energy spectrum is located at 74.1 eV, proving that Al element exists in the material in Al ?3 valence state [37]. Micro-structure of Co-Al LDHs was shown in Fig. 2.
The experimental results show that the resistive switching characteristics change significantly with adsorbed atrazine content. Four of the samples for A, B, and C all show the nonvolatile flash memory characteristic, and the other three samples for D, E, and F shows write-once read-many times (WORM) characteristic. Select one sample for each of the two characteristics to illustrate. Therefore, the two devices are labeled Co-Al LDHs ? Atrazine1 (sample A) and Co-Al LDHs ? Atrazine2 (sample D), respectively. The I-V curves of for device based on sample B and C were shown in Figs. 3 and 4, and the I-V curves of for device based on sample E and F were shown in Figs. 5 and 6. Figure 7a shows that the Co-Al LDHs ? Atrazine1 memory device is initially at a high resistance state (HRS). The DC voltage scanning sequence applied to the device goes from 0 to ? 6 V, from 0 to ? 6 V, from 0 to -6 V, and from 0 to -6 V, the voltage sweep step is 0.01 V. During the first positive voltage sweep from 0 to ? 6 V, a sudden current increase took place at 1.22 V, indicating the conversion of the device from the HRS to low resistance state (LRS), which amounts to a ''writing'' process in digital information storage, the switching threshold voltage of V set is 1.22 V. During the succedent sweep2 from 0 to ? 6 V, this device still remains in LRS and does not relax to HRS even turn off the power. Whereas,  when the negative voltage scans from 0 to -6 V (sweep 3), a sudden current decrease took place at threshold voltage of -4.81 V, manifesting the conversion of the device from LRS to HRS, which equals to the ''erasing'' process in digital information storage. The same voltage sweeping goes, the device holds on HRS during the succedent sweep4 and maintains in the HRS even after turn off the power. In addition, when the switching threshold voltage is reapplied, the device can be rewritten and reerased, manifesting that the device has a storage function, and could be reprogrammable. Figure 7b depicts the cycling endurance of resistive switching characteristics under 176 consecutive DC voltage sweeps. The rewriting ability manifests that Co-Al LDHs ? Atrazine1 memory device exhibits nonvolatile flash memory characteristics [38].
The cumulative probability for resistance in LRS and HRS were also collected for Co-Al LDHs ? Atrazine1 device as depicted in Fig. 7c. And the cumulative probability for threshold voltage of V set and V reset were also analyzed for Co-Al LDHs ?  d Cumulative probability for threshold voltage. e Data retention characteristics of Co-Al LDHs ? Atrazine1 device. f Endurance characteristics of Co-Al LDHs ? Atrazine1 device above 10 4 at -1.5 V. Since high temperature may cause partial memory unit failure, temperature accelerated failure test is used. When the data retention characteristics were tested at 85°C, it was found that the resistance in LRS had little significant change, while the resistance in HRS decreased significantly with time at high temperature, as shown in Fig. 7e. The endurance characteristics of the Co-Al LDHs ? Atrazine1 device was performed as shown in Fig. 7f, inset is the pulse voltage waveform used for endurance testing, where ? 3 V voltage is used to set the device to LRS, -6 V voltage is used to reset the device to HRS, and -1.5 V voltage is used to read the resistance of the device for LRS and HRS. As can be seen the LRS and HRS are also stable up to 10 4 read pulses of -1.5 V (12 ls in period, 6 ls in duration width), as exhibited in Fig. 7f, indicating the distinguished stability of Co-Al LDHs ? Atrazine1 memory device. As shown in Fig. 8a, b, the Co-Al LDHs ? Atrazine2 device shows entirely different resistive switching behaviors. The DC voltage scanning sequence applied to Co-Al LDHs ? Atrazine2 device goes from 0 to ? 6 V, from 0 to ? 6 V, from 0 to -6 V, and from 0 to ? 6 V, the voltage sweep step is 0.01 V. It serves to show, the Co-Al LDHs ? Atrazine2 device is initially in HRS. The current holds on running at a comparatively low level during the initial stage of the first positive voltage sweep until reached to the threshold voltage of 1.37 V. At this voltage site, the current increased abruptly from 9.57 9 10 -6 to 0.024 A, indicating the conversion of the device from HRS to LRS. This conversion is amout to the ''writing'' process in a digital memory cell [39]. After this conversion, the device holds on in LRS, during the succedent positive voltage sweep2. The succedent negative voltage sweep from 0 to -6 V (sweep3) does not switch the device from LRS to HRS, and the device keeps its LRS during the following forward scan (sweep4), which manifests that the HRS-to-LRS conversion is nonreversible. Once the Co-Al LDHs ? Atrazine2 device is converted to LRS, it holds on there and cannot return to the original HRS, indicating its nonvolatile features. The nonreversible and nonvolatile nature of the LRS manifests that the Co-Al LDHs ? Atrazine2 device functions as the WORM memory. The WORM memory device is a write-once, read-many memory device, that is non-rewritable, so that once the data is written, it cannot be changed, erased, or overwritten.
Applications such as archiving, file storage, or permanent record keeping are extremely important for data reliability and security. The WORM device cannot be reset to HRS again, but its ability to writeonce and then read data many times is still working, which is quite different from the breakdown of the device. Compare with the Co-Al LDHs ? Atrazine1 device, the threshold switching voltage of Co-Al LDHs ? Atrazine2 device was high, suggesting that charged defect ions in the active film is increased by absorbed more atrazine micro molecule in Co-Al LDHs ? Atrazine2 device.
The cumulative probability for resistance in LRS and HRS were also collected for Co-Al LDHs ? Atrazine2 device as exhibited in Fig. 8c. And cumulative probability for threshold voltage for positive and negative V set were also analyzed for Co-Al LDHs ? Atrazine2 memory device as exhibited in Fig. 8d. Figure 8e shows the data retention properties of Co-Al LDHs ? Atrazine2 device. Under a constant voltage of 0.5 V, no significant current fluctuations were observed for LRS and HRS, and the ON/OFF resistance ratio can remain as high as about 10 3 at 0.5 V. This good data retention characteristics may be relevant to the stability and low nonrandomness of the the migration of oxygen vacancy between main laminates. Temperature accelerated failure test is performed for Co-Al LDHs ? Atrazine2 device. The data retention characteristics were also tested at 85°C, there is little change in the resistance at LRS, while the resistance in HRS decreased significantly with time at high temperature, demonstrating the typical semiconducting behavior in HRS. This result could be explained with the following considerations. Since the surface of the laminates is alkaline, it can absorb HCl released by the thermal decomposition of Atrazine. At the same time, the laminates can be ion exchanged with chloride ions, giving rise to an increase in the number of charge carriers. At the same time, it is generally accepted that conductive filament can be regarded as the oxygen vacancy chain. Oxygen vacancy can reduce the oxide, favor the reconstitution of the local region, and lead to the insulatorto-metal transition [40]. In our Co-Al LDHs adsorbed atrazine devices, oxygen vacancies chains could also reduce the Co 3? to Co 2? in the vicinity, cause the structure distortion, and give rise to a metallically conductive path [41], resulting in an increase in the HRS current.
Since the WORM type memory device cannot reset back to HRS once it is set to the LRS (when the applied voltage to device does not reach the set voltage: threshold voltage, the device stay in HRS), so the free switch between two states cannot be realized by applying voltage. Hence, the resistances of LRS and HRS are read using continuous pulses to assess the stability of each resistance state. Both of LRS and HRS are also stable up to 10 4 read voltage pulses of 0.5 V (2 ms in period, 1 ms in duration width), as shown in Fig. 8f, indicating the favourable stability of the Co-Al LDHs ? Atrazine2 device.
For a better understanding of the resistive switching behavior, the I-V characteristics of devices were redrawn in log-log scale. Figure 9 depicts its linear fitting for HRS and LRS. For Co-Al LDHs ? Atrazine1 and Co-Al LDHs ? Atrazine2 device, as shown in Fig. 9a, b, the I-V curve has a linear region with a slope of 1.05 and 1.03 in LRS, which is very close to one. This signifies that I-V relationship of the LRS defers to Ohm's law. For HRS of devices, I-V curves show a linear relationship with slope of 1.07 and 1.11, and then I-V relationship converts to Child's law region with slope of 1.87 and 2.09 in the high voltage region, which is very convenient for space charge limited conduction (SCLC) law. This process may occur due to traps created at the interface between the electrode and the active layer. Here Al atoms diffused into the active layer when deposition of the top Al electrode resulting in the formation of impurity band for current conduction. As a result, potential band bending takes place at the electrode-active layer heterojunction by reason of the trapped electrons.
To analyze the composition of the trap, the density of the trap N trap is calculated by: V TFL = eN trap L 2 / (2ee 0 ) [42], where V TFL is trapfilled limit voltage, L is the thickness of resistive switching layer (Refer to SEM image in Fig. 1d), e is the relative dielectric constant of resistive switching materials, and e 0 is the vacuum permittivity. The N trap for Co-Al LDHs ? Atrazine1 device and Co-Al LDHs ? Atrazine2 device is calculated as 3.64 9 10 16 and 2.93 9 10 16 cm -3 , this indicates that the trap centers are coming from metal cation in the Co-Al LDHs main laminate. A conclusion can be drawn from the above analysis, the conductive behaviors of LRS and HRS are totally different. The LRS follows Ohm's law, while the HRS complies with SCLC law.
According to previous reports, the resistance of memory cells has almost nothing to do with area, indicating the resistive switching is a local behavior [43]. The filament conducting can be further confirmed by dependence of the resistance on the area. As exhibited in Fig. 10, there were no significant difference in resistance of LRS and HRS with various areas. As a result, the above results manifested that resistive switching is a filament conducting in Co-Al LDHs adsorbed atrazine device.
In order to analyze the ingredient of the conducting channel, the filament resistivity was calculated. In a general way, the diameter of the filament is 8-10 nm for RRAM [44]. The resistivity of the filament in our Co-Al LDHs adsorbed atrazine devices was about 2.75 9 10 -8 X cm, because the resistance was 3.3577 9 10 5 X (0.477 9 10 -7 A at threshold voltage of 1.22 V) and the supposed diameter and length of the conducting filament is 10 and 152 nm (Refer to SEM image in Fig. 1d), respectively. The resistivity of the filament in our device is close to the resistivity of bulk Al (2.83 9 10 -8 X m at 273 K). The result of the estimate shows that the conducting filament is formed by Al. To further confirm this speculation, we have tested the temperature dependence of the resistance in LRS of the device as shown in Fig. 11. According to the relationship between resistance and temperature, the resistance temperature coefficient is calculated as 4.31 9 10 -3 /K [45], this confirms the existence of the Al conducting filament.
Since AlO x , an oxide layer formed between the functional layer and the Al top electrode, has an effect on the resistive switching characteristics, it has been previously reported that Cu as top electrode can effectively inhibit the formation of oxide layer due to its stable electrical activity, and can effectively improve the performance of the device [46]. So, the I-V curves of Co-Al LDHs ? Atrazine2 device with an Cu act as the top electrode was tested. But the resistive switching behaviors were not observed in device with Cu top electrode when the sweep operating voltage applied is ± 6 V, as depicted in Fig. 12a. In order to investigate the probability of possibility of Cu filaments forming at higher voltages, the operating voltage was increased to ± 10 V as shown in Fig. 12b, c. As can be seen, this device exhibits typical WORM characteristics with a higher resistance ratio (This result is consistent with previous reports [46]) and relatively high threshold switching voltage compared to Al electrode, this may be related to the current overshoot effect. As Co-Al LDHs has a hierarchical structure, the high-speed electron transfer channel formed by the unique layered structure in its laminates can provide a channel for charge transport in the process of resistive switching and guarantee its resistive memory performance. Because Co-Al LDHs is rich in electroactive sites and large specific surface area between laminates, small-molecule atrazine is easily adsorbed on the laminates surface. In addition, the chlorine ions in atrazine has strong reducibility, these characteristics will ensure facilitated charge transfers. Furthermore, Co ions can regulate the crystallinity of materials and the structure of nanosheets. The conversion of the polyvalent states of Co as a transition metal can compensate the charge loss caused by the transfer of Al ions and effectively prevent the collapse of the structure. The long-range disordered structure was formed by the homocrystalline substitution of Al and Co ions. Co(OH) 2 can be converted into CoOOH by reaction with rich hydroxide in the material. And the converted CoOOH has stronger electrical conductivity and is conducive to the rapid transfer of electrons.
Due to strong electron-withdrawing ability of metal cation in the Co-Al LDHs main laminate, which as the center of the trap, plentiful electrons are captured by them. Atrazine adsorbed by the host lamina could offers plentiful electron pathways throughout the adjacent plates. The Al ions introduced during thermal evaporation are easily attracted and reduced to form conductive filaments between the laminate of Co-Al LDHs. Once the threshold voltage is reached, a larger number of electrons are transferred via the Al conductive filaments, leading to a distinct current increase, charge carrier transport along the Al conductive filaments via Co-Al LDHs main laminate becomes easier, resulting in the conversion from HRS to LRS. When a reverse voltage is applied, the electron source supplied by the atrazine is switched off by the reverse voltage, causing the device to switch from LRS to HRS. Hence, Co-Al LDHs ? Atrazine1 devices shows rewritable flash characteristics. With the further increase of adsorbed atrazine and the simultaneous increase in chloride ion, more charge carriers being trapped before switching, and more Al ions were attracted and reduced to form conductive channels, a large number of oxidizing chlorine ions are adsorbed on the Co-Al LDHs main laminate, even if the reverse voltage is applied, the carrier channel still exists, Hence, Co-Al LDHs ? Atrazine2 devices shows nonvolatile WORM characteristics.

Conclusion
The Influence of adsorption small molecules atrazine on nonvolatile resistive switching behavior in Co-Al LDHs Film has been investigated. The nonvolatile resistive switching behavior of device could be adjusted in a controlled way by varying the atrazine adsorbed content in Co-Al LDHs thin films. Distinctly different nonvolatile resistive switching behaviors, WORM memory effect and rewritable memory effect are discernible from the current-voltage characteristics. Both the WORM and rewritable devices are stable under constant voltage and continuous pulse voltage stress. The nonvolatile resistive switching effects of Co-Al LDHs adsorbed atrazine have been analyzed.