Thermoresponsive Memory Behavior in Metallosupramolecular

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Thermoresponsive Memory Behavior in MetalloSupramolecular Polymer-based Ternary Memory Devices Peng Wang, Hongliang Wang, Yu Fang, Hua Li, Jing-Hui He, Yujin Ji, Youyong Li, Qing-Feng Xu, Junwei Zheng, and Jian-Mei Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09132 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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ACS Applied Materials & Interfaces

Thermoresponsive Memory Behavior in MetalloSupramolecular Polymer-based Ternary Memory Devices Peng Wang, [a] Hongliang Wang, [a] Yu Fang, [a] Hua Li, [a] Jinghui He, [a] Yujin Ji, [b] Youyong Li, [b] Qingfeng Xu,[a]* Junwei Zheng, and Jianmei Lu[a] * [a]

College of Chemistry Chemical Engineering and Materials Science, Collaborative Innovation Center of

Suzhou Nano Science and Technology, National Center for International Research, Soochow University, Suzhou 215123, China [b]

Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Soochow University, Suzhou 215123, P. R. China

KEYWORDS: (Metallo-Supramolecular Polymers, Ternary Memory, Thermoresponse, Organic Memory Devices, Metal-to-Ligand Charge Transfer)

ABSTRACT: Thermal sensitive materials, such as metallo-supramolecular polymers, have been integrated into devices for a broad range of applications. However, the role of these materials is limited to temperature sensing and lacking of a memory function. Herein, we present novel [PolyCo-L1xL2y-PF6]based organic resistive memories (ORMs) possessing both a thermal response and ternary memory behavior with three electrical resistance states (HRS, IRS and LRS). Furthermore, the thermal behavior can be memorized by the Al/[PolyCoL1xL2y-PF6]/ITO devices. Heating and cooling the devices at a LRS,

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results in a switch from the LRS to a HRS and further to a LRS, indicating the thermal behavior can be efficiently memorized. Following the heating and cooling process, devices at a HRS retain their ternary memory behavior; while an unstable resistance variation behavior is observed at the IRS. We propose a possible mechanism for the thermoresponsive memory behavior and this finding provides a guide for the design of future thermoresponsive ORMs.

Introduction Due to the high sensitivity to thermal stimuli, thermal responsive materials have been extensively explored and integrated into devices for a broad range of applications, such as thermistors, resettable fuses, and temperature sensors1-5. However, in practice, in related devices it is difficult to detect whether temperature-changing events occur under an external environment, largely due to the materials being limited to temperature sensing and lacking a memory function. In other words, a combination of memory function and thermoresponsive behavior has not been reported in one device till now. To achieve the aforementioned properties, organic resistive memories (ORMs) have been selected as potential candidates due to their simple structure and fast electrical signal memory 6-16. By using an active layer consisting of stimuli-responsive materials, resistance switching is possible under the action of both electrical and external stimuli (light, pressure, thermal, etc.)

17-19

. However, thermal stimuli cannot be

efficiently memorized because such ORMs only possess two resistance states. Compared to binary memories, multilevel analogues have more stable resistance states, indicating that they bear a greater range of signal responses under external stimuli and possess the possibility of memorizing such stimuli2023

. To date, thermoresponsive multilevel ORMs have been seldom reported, largely owing to the lagging

of suitable materials bearing both multilevel memory and thermoresponse behavior. Along with various organic memory materials, metallo-supramolecular polymers (MSPs), which incorporate the electronic properties of conventional polymers and gain additional features from the metal centers, have shown potential in conductivity and sensitization24-33. The immobilization of such species


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on conducting surfaces has attracted great interest for the development of ORMs thanks to their metalligand-interactions and inherent stimuli-responsiveness under different conditions25,

34

. For example,

Zhong and coworkers have reported electropolymerized triphenylamine-appended ruthenium complexbased organic resistive memories with binary switching behavior35. However, such ORMs are limited to those based on MSPs with high association constants, while compounds with reversible coordination capability that show sensitization potential under external stimuli have gained less attention. Moreover, the influence of external stimuli on multilevel memory devices based on MSPs remains unclear. In this paper, we try to investigate the relationship between multilevel memory behavior and thermal stimuli. Furthermore, we aim to establish a memory-based sensor prototype for multilevel signal output. Here, we synthesized novel cobalt(II) coordination polymers in which the L1(Bisterpy) and L2 (Triterpy) groups act as ligands to form a series of copolymers, PolyCoL1xL2y-PF6, consisting of different ligand ratios. Subsequently, Al/[PolyCoL1xL2y-PF6]/ITO devices were prepared and exhibited three electrical resistance states ((high resistance state (HRS or ‘OFF’), intermediate resistance state (IRS or ‘ON1’) and low resistance state (LRS or ‘ON2’)) under a bias. More excitingly, a unique thermoresponsive memory behavior was observed under a ‘heating-cooling’ process. At the LRS (‘ON’ state) of the device, heating and cooling is in accord with the switch from a LRS to a HRS and further back to a LRS, indicating that the thermal behavior can be efficiently memorized. When heating and cooling the devices at a HRS, ternary memory behavior is still observed, indicating that the heating process has no influence on the memory cells at a HRS. Conversely, an unstable resistance variation behavior is observed at the IRS, suggesting that the devices have been subjected to thermal stimuli. We have proposed a mechanism for this thermoresponsive memory behavior, and the relationship between thermoresponsiveness and memory behavior is described. This research may provide new ideas for the future design of thermoresponsive ORMs. Experimental Section



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2-acetlpylpyridine,

4-pyridinecarbox

aldehyde,

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α,α’-dibromo-p-xylene,

1,3,5-Tris(bromomethyl)

benzene, polyethyleneglycol (PEG, Mw=300), ammonium acetate (NH4OAc) and ammonium hexafluorophosphate (NH4PF6) were obtained from TCI Chem. Co. Co(OAc)2, sodium hydroxide (NaOH), acetonitrile, methanol (MeOH) and ethanol (EtOH) were obtained from Shanghai Chemical Reagent Co. All chemicals were used as received without further purification. Pyterpy, Bisterpy ([L1] Br) and Triterpy ([L2] Br) was synthesized according to the literature36-37. Different molar ratio of [L1] Br and [L2] Br were used for synthesis of PolyCoL1xL2y-PF6. First, equimolar amounts and [L1] Br and Co(OAc)2 were added to argon-saturated absolute acetic acid and stirred at 70 ℃ for 16 h to synthesized the linear polymer. Then, the calculated amount of L2 which was dissolved in chloroform in advance added into the Co(OAc)2 (the equimolar amount of [L2] Br). Followed the mixture was added into the reaction solution under magnetic stirring for another 24 h. The same anion exchange and purification method used for PolyCoL1-PF6, Synthesis of PolyCoL1xL2y-PF6 was obtained with ＞ 90% yield. Detailed synthetic procedures of PolyCoL1xL2y-PF6 and device fabrication of Al/[PolyCoL1xL2yPF6]/ITO are provided in Supporting Information. Additional information on NMR of ligands, IR, thermogravimetric analysis and cyclic voltammograms (CVs) of PolyCoL1xL2y-PF6, current-voltage characteristics and physical conducive model analysis of the Al/[PolyCoL1xL2y-PF6]/ITO devices. Results and Discussion In this work, the coordination copolymers PolyCoL1xL2y-PF6 with Co (Ⅱ) complex in the main-chain were obtained by copolymerizing the Co (Ⅱ) complex monomer with terpyridine active units from the ligands (L1 and L2) as outlined in Scheme 1. The copolymers PolyCoL1xL2y-PF6 were readily synthesized in high yields by coordination reactions and ion exchange. All copolymers precipitated in methanol, and the ligands were characterized by 1H NMR spectra,

13

C NMR spectra (see detail in

supporting information). The FT-IR spectra of complex PolyCoL1xL2y-PF6 all show an intense peak at 843 cm-1 due to PF6- strething, indicating the conunteranions of copolymer are largely PF6- (Figure S6). The UV-visible diffuse reflectance spectra of pure ligands and complexes PolyCoL1xL2y-PF6 with


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different ligands ratio were recorded in solid state to investigate their photophysical properties. As shown in Figure 1, all of these copolymers exhibit two intense reflection bands in the UV region (250 - 400 nm) and a strong band at λ =500 - 580 nm. The intense reflection band below 400 nm may be associated with the singlet ligand-centered π-π* transitions of the C^N and N^N ligands. The strong reflection at 500 580 nm can be related to the metal-to-ligand charge transfer (MLCT) transition. These results indicate that the Co complex has been successfully introduced into the polymer chain. The band gap energies can be determined by extrapolating the absorption edge onto the energy axis, as shown in the Figure 1d.

Where R’ is the relative reflectance ratio, K is the absorption coefficient, and S is the scattering coefficient. The band gap energy of copolymer PolyCoL1xL2y-PF6 samples are between 1.83 eV and 2.04 eV, indicating the potential application in semiconductor area38. These copolymers exhibited good thermal stability with a decomposition temperature (Td) greater than 500 K and a weight loss of 5% (Figure S7). To obtain uniform and consistent solid films, the copolymer PolyCoL1xL2y-PF6 films have been fabricated on the Indium-Tin Oxide (ITO) glass by spin-coating method. The obtained thin films own low average root-mean-square roughness by atomic microscopy (AFM). Besides, the average root-meansquare roughness of these films slightly increase with the increase of the L2 content in the copolymer, while the surface morphologies exhibit no obvious change (Figure S8). Figure 2 show the anodic cyclic voltammograms (CVs) of the above synthesized cobalt complexes film. In the region between 0 V and 1 V, the anodic scan of the copolymer, PolyCoL1100%-PF6, shows one reversible wave at approximately 0.349 V and another one quasi-reversible wave at around 0.756 V versus SCE, respectively (Figure 2a). And the currents are linearly dependent on the scan rate, which is characteristic of redox processes confined to electrode. Thus, the first redox peaks are chemically reversible due to the stepwise Co(II) processes of the metal segment39-40. Compared with CV curves of pure ligands ([L1] PF6 and [L2] PF6), a well-defined wave at around 0.756 V may come from the N atoms



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in the ligands40-43(Figure S9). Thus the presence of the well-defined 0.756 V redox waves of the copolymer indicates the copolymer has the expected linear structure with Co(II) in Co-terpyridine structure segments. By measuring the CV curves of PolyCoL2100%-PF6 film in the region between 0 V and 1 V, the anodic scan of the polymer shows one reversible wave at approximately 0.349 V and another one quasi-reversible wave at 0.737 V versus SCE, respectively (Figure 2b). When the copolymer mixed the L1 and L2 with different ratio, all of them also show two well-defined redox waves, indicating copolymer owns two oxidation states (Figure 2c-e). Besides, the first redox peaks are due to metal oxidation processes, and the latter wave may be due to the N waves from the ligand. All these results indicate successful introduction of the Co complex into the polymer chain. Five metallo-supramolecular polymer memory devices based on the PolyCoL1xL2y-PF6 framework bearing different ligand ratios were fabricated as Al/[PolyCoL1xL2y-PF6]/ITO devices by sandwiching the polymeric film between the substrate ITO (Indium Tin Oxide) electrode and an Al top electrode (Figure 3a). According to hysteretic current-voltage (I-V) curves, all of these devices can be assigned as possessing nonvolatile ternary memory behavior44-46 (Figure S11). That is, when the bias voltage is added to the memory cell, two abrupt increases in current (Vth1 and Vth2) are observed with increasing voltage. This indicates transition from the HRS to the IRS, and further to the LRS. Figure 3b shows typical I-V curves of the Al/[PolyCoL150%L250%-PF6]/ITO device, in which the IHRS/IIRS/ILRS ratio approaches 1:103:106 with two thresholds at 2.7 V and 3.4 V. Endurance tests were conducted at a reading DC bias voltage of 0.1 V and the SET/RESET distribution plotted over 100 cycles, showing the stability of these devices (Figure S12). Compared to other devices, the resistance gap between the IRS and the LRS is less than 102 Ω in Al/[PolyCoL1100%-PF6]/ITO and Al/[PolyCoL2100%-PF6]/ITO, while the resistance gap between the IRS and the HRS is less than 80 Ω in Al/[PolyCoL120%L280%-PF6]/ITO and Al/[PolyCoL180%L220%-PF6]/ITO. These results suggest that Al/[PolyCoL150%L250%-PF6]/ITO exhibits the most stable and reasonable resistance gap in these devices (Figure 3c).


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Furthermore, these devices possess dual-stimuli (electrical and thermal) responses. As shown in Figure 4a, the Al/[PolyCoL150%L250%-PF6]/ITO device exhibits ternary memory behavior prior to heating. When the device is at a high conductance state (LRS) and heated to 393 K, prompt measurement of the I-V tests shows that the resistance drops back to a HRS irrespective of whether a positive or negative bias is applied. This implies that the circuit breaks at high temperature, similar to a warning function. When the device is completely cooled (around 2400 seconds), the device can be recovered to the LRS when a positive forward voltage is applied. Thus, reworking of the circuit occurs when the environment temperature falls to a safe range, and such heating-cooling process can be memorized simultaneously. This “LRS-HRS-LRS” transition can be also regarded as a “SAFE-WARNING-SAFE” process. In addition, endurance tests of the device were conducted with changing temperature at a reading DC bias voltage of 4 V (Figure 4b). The device remains stable throughout 10 cycles, indicating that the devices exhibit good stability and that they can be used repeatedly. The effects of transient variation on electrical function with regards to temperature were also studied. Heating the device at different temperatures ranging from room temperature (R.T.) to 433 K showed that the resistance of the device provides controllable kinetics of thermal transient behavior (Figure 4c): At the beginning, the device shows low resistance when the temperature is lower than 393 K; rapid transience occurs at around 393K and subsequently the device reaches high resistance. Figure 4d shows the resistance of the device as a function of time when it is cooled from 393 K to R.T. It is clear that the high resistance of the device gradually decreases and finally returns to approximately 200 Ω after 2100 seconds, characteristic of the LRS. Additionally, measurements on all other Al/[PolyCoL1xL2y-PF6]/ITO devices show a similar thermoresponsive behavior (Figure S13). This suggests that the smart memory effect is widespread in PolyCoL1xL2y-PF6-based memory devices. In order to clarify the mechanism of this thermal-related conductance behavior, we attempted to gain an understanding from the viewpoint of both the material and device. Firstly, as the prominent reversible MLCT transition was observed in a solution of PolyCoL1xL2y-PF6, we believe that the thermoresponsive



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memory behavior can be predominantly attributed to the inherent properties of the materials. The photophysical properties of both PolyCoL150%L250%-PF6 solutions and films have been investigated at different treating temperatures. As shown in Figure 5a, when the solution is heated to 393 K, the UV absorption band at about 550 nm arising from a MLCT transition disappears, indicating the dissociation of the metal-ligand coordination. When the heat source is removed and the temperature cooled to R.T., the band at 550 nm reappears, indicating the recovery of the MLCT. Meanwhile, the emission at 380 nm, which can be assigned to that of a mixture of the free ligands themselves, is quenched after coordination and recovered after heating (Figure 5b). All these results suggest the reversible MLCT transition of the copolymer during heating-cooling process47. Similar UV/vis (Figure 5d) and emission (Figure 5e) results are obtained when PolyCoL150%L250%-PF6 films are used. However, since the molecules is fixed in film state compared in solution state, the MLCT change is not significant in film state. Moreover, Figure 5f shows a confocal laser scanning microscopy (CLSM) image of heated film at 393 K. Heated film is shown as a bright green emission and the unheated film exhibits no signal. For comparison, a manganese ion was selected to form a different copolymer PolyMnL150%L250%-PF6. Its fluorescence in DMF solvent and in film was studied using the same heating-cooling process; however, no significant changes were observed (Figure S14). Thus, the aforementioned temperature-dependent reversible MLCT switch may be unique to the PolyCo-L1xL2yPF6 devices. Secondly, the morphology of the PolyCoL150%L250%-PF6 film at different temperatures has also been investigated and is shown in Figure 5g-i. When heated to 393 K, some large particles appeared on the surface of the film (Figure S15) with average roughness increasing from 2.35 nm to 5.41 nm. This can be assigned to partial metal-ligand dissociation in the copolymer films48. When the system is cooled, the morphology and average roughness return to their original state. This behavior may be attributed to the regeneration of the metal–ligand interactions. The above measurements point to a reversible MLCT transition with temperature variation. Anyway, how can this MLCT transition influences the memory behavior at different temperature? We next examined it.


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Firstly, according to conduction process analysis, measured I-V curves at different states of the devices were fitted with appropriate charge transport models. As shown in Figure 6a, the conduction mechanism at the HRS can be considered to be a thermionic emission process, and the space-charge-limited current (SCLC) model can be satisfactorily fitted to the I-V characteristics at the IRS (Figure 6b). These data indicate that the switching is triggered by carrier injection and hopping processes between the copolymer chains49. Combined the results of segmental analysis, the HOMO and LUMO are localized on the metalterpyridine unit and the terpyridine bridging ligand, respectively, indicating that the MLCT process occurs undergoing the HOMO to LUMO transition (Figure 7); The bandgaps of the complex are about 1.61 eV, which are similar with the optical bandgaps that estimate according to Figure 1d. Moreover, the LRS is dominated by the Ohmic model (Figure 6c) and the resistance increases as temperature increases (Figure 6d), thus indicating that the LRS is dictated by conductive filaments. Therefore, the electricstimuli switching mechanism of the Al/[PolyCoL1xL2y-PF6]/ITO devices has been promoted. The schematic diagram in Scheme 2a shows a widely recognized device. During the SET progress, the electrons migrate from bottom electrode to top electrode under the positive DC bias as the case in middle active layer. At a certain applied voltage, according to the DFT calculation result for the basic cobaltterpyridine structure of the copolymer, a high electric transfer through the copolymer chain, resulting in a higher-conducting state (IRS) with a high concentration of charge carriers. When the voltage is sufficiently high, the holes in the top electrode combine with the electrons, so a completely metallic conductive filament path is created, resulting the devices transfer from IRS to LRS50. Joule heating cannot melt the metallic conductive filaments under the subsequent negative voltage51, thus, the device retains at LRS, characteristic of nonvolatile ternary memory behavior. Secondly, when a heating process is applied, the surface morphology of the active layer is altered and such changing may also induce the pre-existing conductive filament paths’ break52. Moreover, due to the metal-ligand dissociation of the copolymer, charge transfer through the copolymer chains becomes difficult, leading to a rapid decline in electrical conductivity, characteristic of the HRS (Scheme 2b).



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When completely cooled, the morphology of the active layer recovers due to the coordination affinity between the Co ion and the terpyridine ligands. The conductive filament paths can be easily formed again when positive voltage is applied, resulting in the device existing in the LRS. This switchable ‘HRS-LRS’ can be assigned to the connection of conductive filaments caused by the MLCT transition. The missing of the IRS may be attributed to the pre-existing conductive filament paths are easier to be connected under bias. We also studied the memory behavior/temperature relationship of the device at the HRS and IRS. Heating a new device at 393 K at the HRS did not alter its resistive state, approximately 108 Ω, under a subsequent electric sweep. When completely cooling back to R.T., the device exhibits ternary performance (Figure S16).

Additionally, heating and cooling the device at the IRS, the electrical

performance is not particularly stable, possibly due to the IRS that is an intermediate state. According to the above mechanism of the thermoresponsive behavior, we believe that this may be related to the absence of conductive filaments in these states (HRS and IRS) when a bias is applied. Conclusions In summary, the novel ternary memory devices based on the Co(II)-copolymers PolyCoL1xL2y-PF6 with unique thermoresponsive memory behavior have been founded and detailed investigated. Specifically, the device at the LRS was found to exhibit reversible switching behavior between the LRS and HRS following heating and cooling. Similar phenomena were not observed when heating and cooling the devices at other states (HRS and IRS). According to photophysical measurements, AFM images, etc., the changes exhibited in organic thin films of the devices were observed with temperature switching. The reversible MLCT transition can be considered to be the key factor leading to the appearance of stable ternary intermediate states and thermoresponsive behavior. Furthermore, this research provides a possible strategy for the development of novel organic thermal sensors, which can be considered as signal storage devices that can be used to detect overheating issues in organic electronic devices. However, to date, related researches have just begun, more in-depth study is very necessary in the future.


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ASSOCIATED CONTENT Supporting Information. Additional information on NMR and cyclic voltammograms of ligands (L1 and L2), IR and TGA data of PolyCoL1xL2y-PF6, photophysical properties and AFM morphology of the PolyCoL1xL2y-PF6 films, current-voltage characteristics and endurance properties of the devices.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors also appreciate the financial support from Chinese Natural Science Foundation (21336005, 21176164 and 21371128), the major research project of Jiangsu Province office of Education (15KJA150008), Qinglan Project and PAPD in Jiangsu Province.

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Scheme 1. Synthesis of the Metallo-Supramolecular Polymers PolyCoL1xL2y-PF6.


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Figure 1. The diffuse reflectance spectras of (a) PolyCoL150%-PF6, PolyCoL1100%-PF6 and [L1] PF6; (b) PolyCoL250%-PF6, PolyCoL2100%-PF6 and [L2] PF6; (c) PolyCoL1xL2y-PF6. And (d) the Kubelka-Munk function of PolyCoL1xL2y-PF6.



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Figure 2. Cyclic voltammograms of (a) PolyCoL1100%-PF6; (b) PolyCoL180%L220%-PF6; (c) PolyCoL150%L250%-PF6; (d) PolyCoL120%L280%-PF6; (e) PolyCoL2100%-PF6 with the same scan rate (50 mV/s) in a 0.1 M Bu4NPF6/CH3CN electrolyte.


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Figure 3. (a) The left shows the structure of PolyCoL1xL2y-PF6, and the schematic diagram of the Al/[PolyCoL1xL2y-PF6]/ITO switching device (right). (b) Typical current-voltage (I-V) characteristics and (c) endurance properties of the of the Al/[PolyCoL150%L250%-PF6]/ITO switching device. (c) The writing pulse are set to 0.5 V/100 ms, 3 V/100 ms, and 4.5 V/100 ms, respectively, and a reading voltage of 0.1 V is applied.



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Figure 4. (a) Typical current-voltage (I-V) characteristics of the used Al/[PolyCoL150%L250%-PF6]/ITO switching device before and after heating at 393 K. (b) Endurance tests of the device after heating-cooling process were conducted at a reading DC bias voltage of 4 V. Measured resistance of Al/[PolyCoL150%L250%-PF6]/ITO devices (c) as a function of temperature; (d) as a function of time after heating at 393 K


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Figure 5. (a)(d) UV/Vis absorption spectra and (b)(e) the emission spectra of (a)(b) PolyCoL150%L250%PF6 (DMF) solution and (d)(e) [PolyCoL150%L250%-PF6]-ITO film under heating process at 393 K and cooling process. (b)(e) The emission spectra were all recorded with excitation at 320 nm. CLSM images of (c) unheated and (f) heated [PolyCoL150%L250%-PF6]-ITO film. (g-i) Morphology characterization of TM-AFM topographic images and corresponding (j-l) cross-section profiles of PolyCoL150%L250%-PF6 on ITO substrates (g) at R.T.; (h) heating to 393 K for 480 seconds; (i) cooling to R.T. The scan size for both images is 5 µm x 5 µm.



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Figure 6. Experiment and fitted I-V characteristics of the Al/[PolyCoL1xL2y-PF6]/ITO device in (a) LRS and (b) IRS and (c) HRS. And (d) temperature is dependent on resistance for the LRS under the read voltage of 0.5 V.


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Figure 7. DFT molecular simulation results of segmental analysis of Co-L1 and Co-L2.



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Scheme 2. Schematic illustration of the Al/[PolyCoL1xL2y-PF6]/ITO device under the electric field at (a) R.T. and (b) 393 K. The white dots represent the Al filament particle.


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Table of Contents Graphic


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Thermoresponsive Memory Behavior in Metallosupramolecular

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