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Fluorine-Induced Highly-Reproducible Resistive Switching Performance: Facile Morphology Control through the Transition between J and H Aggregation Yang Li, Zhaojun Liu, Hua Li, Qing-Feng Xu, Jing-Hui He, and Jian-Mei Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01128 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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ACS Applied Materials & Interfaces

Fluorine-Induced Highly-Reproducible Resistive Switching Performance: Facile Morphology Control through the Transition between J and H Aggregation Yang Li, †,‡ Zhaojun Liu,† Hua Li,*,† Qingfeng Xu,† Jinghui He,† and Jianmei Lu*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou 215123, P. R. China ‡

School of Materials Science and Engineering, Nanyang Technological University, Singapore

639798, Singapore KEYWORDS: multilevel-cell RS, fluorination, morphological engineering, H-type aggregation, reproducibility

Abstract: Improving the reproducibility and air-endurance of organic resistance switching (RS) device, in particular multilevel-cell RS devices, is critical for the confirmation of its competency to realize big data storage technique. However, such enhancement still remains challenging. In this report, we demonstrated that fluorine (F)-embedding should be an effective way to enhance

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the overall performance of RS devices. Four new azo-cored analogues (IDAZO, FIDAZO, F2IDAZO and F4IDAZO) have been designed and synthesized. These four compounds have similar structures with different number of F substituents. Interestingly, UV–vis measurements reveal that upon F-embedding, the exceptional transition from molecular J-aggregation to Haggregation is achieved. As result, the morphology of RS films becomes more and more uniform, as determined by AFM and XRD. Meanwhile, the hydrophobicity of RS film is promoted, which further improves the device atmospheric stability. The total RS reproducibility increases to 96% (the uppermost value) and the tri-stage RS reproducibility arises to 64%, accompanied by more stable OFF state and lower logic SET voltages. Our study suggests Fembedding would be a promising strategy to achieve highly-reproducible and air-endurable organic multilevel-cell RS devices.

1. INTRODUCTION The exponential increase of digital information1 strongly requires a new information-storage technology with fast speed, high scalability, and low-power operation for central data-driven computation.2-4 Among all data-storage devices, memory electronic devices that hold multi-bitcell nature and reliable reproducibility have been considered to be the forefront.5-9 Although many materials can realize the memory function, organic compounds with resistance-switching (RS) behaviors have received growing interests due to their low cost, high flexibility, and possibility to construct simple device architecture.10-15 In fact, some reported organic RS materials have been demonstrated as promising candidates for big data-storage application through multistage read-out signals (MROS).16-21 Such success would make future information-


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communication technologies (ICTs) realizable through multilevel-cell (MLC) memory electronics.22,23 However, there are still several key concerns regarding these materials besides device integration and architecture issues,11 i.e. insufficient repeatability of the RS behaviors and inconsistent endurable ability, which are subject to material-property-related restrictions.24,25 These material-based problems often cause non-uniform RS performance and suppress the stable output of MROS. In fact, the memory cell-to-cell non-uniformity can play an extremely-negative role in device sustained operation, leading to inferior reproducibility and e-waste. To overcome this obstacle, the basic method is to ameliorate the morphology across the RS layer, which is closely correlated with the strategic molecular internal engineering.26,27 From a material perspective, it is highly desirable for the core RS components to be inert to processing techniques, whilst holding an orderly film morphology that facilitates the efficiency of device. This not only reduces the complexity of device fabrication but also offers superior MROS reproducibility. For instance, manipulating the morphology of donor–acceptor (D–A) polymer has been reported for organic solar cells (OSC) by Jen et al. and realized repeatable high power conversion efficiency in device over 7% with a photocurrent of ~12 mA cm−2.28 This encourages us to investigate how morphological engineering of RS materials improves the reproducibility of MROS performance. From the standpoint of material design principles, heteroatoms (e.g., B, N, O, Si, or P) have been purposely introduced into the conjugated skeletons of organic RS compounds to implement MLC memory devices.16,29-31 However, these heteroatoms sometimes cause the RS layer to be more sensitive to oxidation, moisture, and temperature,33-35 which largely destroys the film morphology and the device endurance. Interestingly, this problem might be solved through


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introducing fluorine (F) into organic system because F-embedded organic materials mostly possess a series of unique features such as great thermal and oxidative robustness,36,37 elevated hydrophobicity,38,39 and enhanced resistance to degradation.39,40 More importantly, Fincorporating can decrease the surface energy of organic films and thus optimize the associated morphology.41 Because of these merits, fluorinated materials have been widely used in OSCs,42,43 organic field-effect transistors (OFET),37,44 and organic light-emitting diodes (OLED).45,46 Unfortunately, to the best of our knowledge, they have been explored scarcely in the RS memory field. Inspired by the charming merits of F, herein, we designed four D–A–A type organic molecules, namely, IDAZO, FIDAZO, F2IDAZO and F4IDAZO by introducing different number of F atoms into the framework of IDAZO (Figure 1). Obviously, F-embedding has great contribution to the morphology of RS layer by shifting from amorphous state to stable crystalline texture. Note that such changes do not require either solvent additives or meticulous annealing post-process, which benefited to address the challenge of e-waste. When F changed from naught to four, the ITO/organic/Al architectural device exhibited highly-increased total RS reproducibility from 74% to 96% (the uppermost value as far as we know), whilst the reproducibility of MROS (tri-stage ROS) triply increased from 22% to 64%. The low-lying OFF state turned out to be more stable and the logic SET voltages showed a decreased trend. Based on the photophysical property and XRD results, one could conclude that the morphological optimization was ascribed to an exceptional structural transition from offset J-aggregation to face-to-face H-aggregation. The F-induced H-aggregation in film state therefore produced more reproducible RS behavior with energy-saving property. This work clearly clarified material-


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property-related RS performance in detail and held a promise for promoting future MLC memorizing data-driven ICTs. 2. RESULTS AND DISCUSSION IDAZO, FIDAZO, F2IDAZO and F4IDAZO were successfully prepared in satisfactory yields through condensation reactions with compound 3 as an intermediate (Figure 1). The detailed synthetic routine and characterizations have been provided in the Supporting Information. These as-prepared four molecules adopted the same azo-cored phthalicimide(PI)-terminated structure, and the only difference is the number of F atoms attached on the core. It is well known that the substitution of H atoms with F would not impose extra steric hindrance between adjacent molecules because H and F have similar size (van der Waals radius F, r = 1.35 Å, only slightly larger than H, r = 1.20 Å).38 Density functional theory (DFT) calculations have been conducted to simulate optimized molecular geometries for IDAZO, FIDAZO, F2IDAZO and F4IDAZO. As shown in Figure S13, these four molecules adopted almost same conformations. The dihedral angles between the PI unit and the phenyl specie were found to be ~45° (44–46°), while those angles between the phenyl and the azo group were calculated to be less than 1°. These calculations suggested that F-embedding did not affect the molecular structures, which can allow us to accurately investigate the effects of F substituents on the film morphology and related RS devices. In addition, these materials not only possessed good thermal stability (Figure S14), but also the thermal stability was largely enhanced through the substitution of F, suggesting that the volatility could be suppressed by F-embedding and the RS device longevity would be enhanced. To explore the F-substituted effects on the RS nature of organic media, we investigated the electrical conductivity of IDAZO, FIDAZO, F2IDAZO and F4IDAZO in their corresponding


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devices. All the devices with film thickness of ~100 nm have been fabricated under the same deposited condition. As shown in Figure 2a, the IDAZO-based device initially exhibited a fluctuant low-conductance (OFF) state when a negative voltage (0 to –5.00 V) was applied. As the voltage proceeded, two abrupt current changes occurred at logic SET voltages (VSETs) of about –2.14 V and –4.54 V (Sweep 1), indicating that the device underwent electrical transitions from OFF state to an intermediate-conductance state (ICS), and further to a high-conductance state (HCS). The three distinct states could be regarded as three different digital storage states: namely, “0”, “1”, and “2”.21,29 Another cell of the device was measured under a voltage bias from 0 to –4.00 V, and the OFF-to-ICS transition was also observed at about –2.06 V (Sweep 2). The cell remained in its ICS during a subsequent repeated scan (Sweep 3), and transited from ICS to HCS when the voltage increased to –5.00 V (Sweep 4). After a following scan from 0 to – 5.00 V, the storage cell retained its HCS (sweep 5), suggestive of non-volatile memory (NVM) nature. The ternary electrical transitions (namely, tri-stage ROS) were also observed for the FIDAZO-, F2IDAZO-, and F4IDAZO-based devices, however, with varied discrepancies in the VSETs and OFF state (Figure 2b–d). When the number of F substituents increased, the fluctuation of OFF state was obviously suppressed, denoting higher RS stability. The HCS/LCS and OFF currents turned to be well-separated by over two order of magnitude, which enabled to identify the tristage ROS (TROS) through the conventional sense amplifier. Simultaneously, the VSETs were detected to show a decreasing trend (–2.14/–4.54 V shift to –1.01/–3.08 V), which indicated that they are promising candidates for energy-saving eco-friendly MLC memory electronics. Furthermore, fifty independent units of each type of devices were tested to evaluate the cellto-cell RS reproducibility upon F-embedding engineering. For non-fluorinated IDAZO, the yield


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of ternary devices, as defined by the frequency of achieving output TROS behavior, was only about 22% (Figure 3a). While incorporating F atoms, it was noted that TROS reproducibility could be significantly improved. The yield of ternary devices remarkably increased to 52% for FIDAZO, 54% for F2IDAZO and 64% for F4IDAZO, which was nearly three times as great as that for IDAZO (Figure 3b–d). Our recent studies19,47 about TROS reproducibility illustrated that the ternary memory yield of some organic-based materials was usually insufficient (~50%). Regarding this flaw, F4IDAZO could be considered as a satisfactory ternary storage media with higher reproducible property. Additionally, the total yield of RS devices greatly aroused from 74% for IDAZO into 96% for F4IDAZO (binary/ternary). To the best of our knowledge, 96% is the uppermost RS statistic result reported so far on the basis of a large sample size. In general, the performance of organic device is a direct reflection of the properties of one material, namely, the performances of above-mentioned RS devices could be further clarified from both the molecular structure and the solid state packing perspectives. On one hand, structural modification is closely related to the electronic energy (HOMO, LUMO etc.),48 which indicates intra-molecular charge transfer process. On the other hand, molecular packing manner in film state plays a key role in controlling the transport of charge carriers throughout the film, which dominates the performance of entire device.49,50 To get insight into the F-induced impact on the molecular electronic properties and RS film states, the UV-vis spectra, emission spectra, cyclic voltammetry (CV) curves, atomic force microscope (AFM), and X-ray diffraction (XRD) patterns were studied as following. The photophysical properties of IDAZO, FIDAZO, F2IDAZO and F4IDAZO in CH2Cl2 solution and in thin films have been tested through UV-vis absorption spectra. We found that the absorption peak of non-fluorinated IDAZO in thin film was red-shifted compared to that in


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solution (Figure 4a, see values in detail in Table S1). This result has been reported in many organic semiconductors51,52 and is well interpreted by J-type aggregation between adjacent molecules, i.e. molecular stacking by a head-to-tail manner (see illustration in Figure 4e, left),53,54 which is likely correlated with the intermolecular D–A polar interactions. Although Jaggregation often occurs in film states, such an offset molecular arrangement could be difficult to provide sufficient π–overlap for the acquisition of higher charge carrier mobility.53 To address this problem, developing side-by-side molecular H-type aggregation is an effective strategy. In contrast to J-aggregation, H-aggregation can produce large area π–stack (face-to-face) between adjacent molecules,55,56 which plays an important role in enhancing the charge mobility of molecular devices (Figure 4e, right).57 However, this aggregation type is very difficult to realize. Interestingly, in our research, we observed that when F atoms were incorporated, the shift from J-aggregation to H-aggregation was induced. The maximum absorption peaks of FIDAZO, F2IDAZO and F4IDAZO in thin films were blue-shifted comparing with those in solution, which could be assigned to H-aggregation of the molecules (Figure 4b–d).54,57 It has been revealed that fluorine atoms could give a great impact on intermolecular assembly through C−F···H, F···F, or C−F···πF interactions.38,58 Therefore, this J- to H-aggregate transition was probably due to the subtle F−H and/or F−F interactions that lead molecules to stack in parallel. Moreover, the emission spectra of FIDAZO, F2IDAZO and F4IDAZO in thin films exhibited quenching behaviors compared with that of IDAZO, which manifested that the strong intermolecular π–π interactions could be triggered by J- to H-aggregate transition (Figure S16). It is noteworthy that F-embedding provided a novel synthetic strategy to realize H-aggregation in the film state. This H-type aggregation was very helpful to obtain more uniform RS layer morphologies and reproducible device performances.


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In order to determine the electronic energy levels of these molecules, CV was employed to measure the electrochemical properties of these four compounds (Figure S17). The HOMO/LUMO energies for IDAZO, FIDAZO, F2IDAZO and F4IDAZO were determined to be –5.12/–2.67 eV, –5.67/–3.29 eV, –5.80/–3.46 eV, and –6.14/–3.81 eV, respectively. It can be concluded that F-embedding was effective in lowering both the HOMO and LUMO levels of the molecules with rare change in the energy band-gaps. Since F atom is a strong electronwithdrawing substituent (most electronegative element with Pauling electronegativity of 4.0),41 the introduction of F atoms into the conjugated skeleton benefits to lower LUMO energy levels of the conjugated molecules. As a result, the Al/LUMO electron-injection energy barriers tended to be smaller than the ITO/HOMO hole-injection energy barriers (see Table S1 and Figure S18 for details), which implied that the electron transfer became easier, suggestive of their potential as ambipolar or n-type semiconductor materials.26 Several studies have revealed that low-lying LUMO levels are necessary for air stable electron transport.59,60 When embedding four F atoms, the LUMO of F4IDAZO decreased to be less than –3.80 eV, which could effectively suppress the possible electron-trapping by atmospheric oxidants.59 This property can lead to a good device air-stability. Note that the electron-injection barrier of F4IDAZO was the smallest one, which would have a great contribution to the superior charge conduction process during RS device operation. The morphologies of RS layers were examined by AFM. As shown in Figure 5, the AFM images clearly indicated that the RS films have a much smoother surface when incorporating F substituents into materials, and the root-mean-square roughness (RRMS) of the active layer improved from 16.5 to 10.2 nm. For non-fluorinated IDAZO, the film exhibited disordered domains with non-uniform orientations (amorphous state). These domains have many uneven


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large sizes (about 100–300 nm in range), which were undesirable in organic films since they could severely reduce charge transportation.61 Upon F-embedding, the amorphous state gradually shifted to an ordered crystalline-granular texture. The domains became much more uniform with smaller grainy size, and preferred homogeneous orientation due to the molecular H-aggregation. This smooth morphology with smaller RMS roughness revealed that more ordered film structures have been formed. The connectivity among neighboring grains was developed, which facilitated to establish efficient pathways for charge transport in devices.49,52 Therefore, the more homogeneous interfaces between the RS layer and top/bottom electrode helped reducing the series contact resistance28,58 and thus lowering the operating VSETs, as observed in the devices. This improvement could also be noted in the leakage of OFF current. The more stable OFF current indicated that the adverse leakage was suppressed by F-embedding, which clearly benefited from the more uniform interface between the RS layer and electrode. The homogeneous morphology warranted the cell-to-cell uniformity of devices, and thus led to more reproducible RS behaviors. Moreover, the introduction of F atoms could effectively decrease the surface energy of the RS layer,38,41 which makes it resistant to atmospheric trapping species such as O2 and H2O, i.e. air stable. The wettability of these RS films was studied in detail by contact angle measurements under ambient condition (Figure S19). It revealed that the hydrophobic property of the film was gradually enhanced with F-embedding. This property is helpful to enhance the non-packaged device endurance during ambient air operation. To further inspect the nanostructural molecular packing, XRD patterns of IDAZO, FIDAZO, F2IDAZO and F4IDAZO films were recorded in order to provide molecular level information about packing (Figure S20). XRD studies manifested the enhancement of molecular ordering after F incorporation. For IDAZO, no discernible XRD signals were detected, indicative of an


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amorphous state,31,62 which was consistent with that observed in AFM image. For FIDAZO, one small diffraction occurred at 2θ = 26.39°, which can be assigned to the π–π stacking peak with the π–π stacking distance of 3.34 Å.52,63 By further F-embedding, both F2IDAZO and F4IDAZO films showed one more diffraction peak at 2θ = 14.52° and 14.64° respectively, different from the π–π stacking peak. These peaks at smaller angle provided notable evidence that the molecular ordering was enhanced to form a well-ordered lamellar structure.64,65 Furthermore, the π–π stacking peak shifted to larger angle (2θ = 29.26° and 29.95°) with smaller π–π stacking distances, accompanied by higher signal intensity. These observations revealed that the intermolecular π–stack interactions became stronger,66 enabling highly-ordered self-assembly behavior throughout the whole RS layer. Generally speaking, the above-mentioned XRD results were closely correlated with the F-induced transition from J-aggregation to H-aggregation. Haggregation could produce sustainable large area π–overlap between adjacent molecules, i.e. strong cofacial π–π interactions.55 The actuated strong intermolecular π–π interactions subsequently guided other molecules to adopt a similar face-to-face alignment, thereby controlling the film to grow into a homogeneous and continuous nanostructure. Thus, the morphological improvement of film state was energetically favored, which accounted for the excellent RS performances of the F-imbedded molecules. To better understand the tri-stage RS switches and electronic properties of these organic molecules, theoretical calculations were performed through DFT method. Computation was operated via generalized gradient approximation (GGA) in DMol3 code, using unrestricted BLYP/DNP level. The calculated orbital energy distributions and electrostatic potential (ESP) isosurfaces were shown in Figure 6. It can be noted that HOMOs for all four molecules were well-delocalized over the electron donors, while LUMOs were mainly distributed on the electron


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accepting sides. These electron distributions generally indicated that the intra-molecular charge transfer (CT) occurred upon the HOMO to LUMO transition.30,35 In addition, the calculated molecular isosurfaces showed continuous positive ESP (in red and white) across the conjugated backbone (Figure 6, top), which generated an open channel for charge carrier transportation. Yet, the negative ESP regions (in blue) aroused from the azo and PI acceptors also appeared. These negative regions could act as “traps” to retard the field-induced motion of charge carriers.67,68 Under a low voltage bias, charge carriers could hardly obtain sufficient energy to surmount the charge injection barriers between the donors and acceptors. Thus, the device remained in the low-conductance (OFF) state. When the voltage bias increased, the charge carriers gradually collected enough activation energy and infused from the donors to acceptors to fill the “traps”.47,69 Accordingly, the conductance of entire device was enhanced to trigger OFF-to-ICS transition. However, the traps resulting from two distinct acceptors could not be filled up at the same time: the trap of azo was totally filled while the trap of PI remained partly unfilled. This process was closely correlated with the electron-withdrawing abilities of two acceptors.20,21,30 Because of the stronger electron-withdrawing ability of PI comparing with azo, the larger depth of PI trap was formed, which required more energy to overcome. As the voltage further proceeded, the charge carriers would eventually accumulate adequate energy and fill up the trap of PI, leading to HCS with higher conductivity. These electrical transitions corresponded to tri-stage ROS, which demonstrated MLC memory behavior. Moreover, the trapped charge carriers could be stabilized by the intra- or intermolecular CT at the field-induced state, forming a charge-segregated state.69,70 These trapped charges could not be easily released after withdrawing the electric field or under reverse voltage bias. Therefore, the resulting HCS could be retained for a long time, indicating that the RS devices possess NVM intrinsic character.


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3. CONCLUSIONS In summary, we have successfully synthesized four new azo-cored molecules (IDAZO, FIDAZO, F2IDAZO and F4IDAZO) that comprised of different number of F atoms. DFT calculations revealed that these four compounds adopted similar molecular geometries, suggesting F-embedding almost has no effect on its geometry. Thus, the impact of F atoms on the morphology, self-assembling behavior, and device performance were accurately explored. Notably, we found that the F-embedding could partially induce H-type aggregation that differed from the non-fluorinated molecule, normally displaying common offset J-type aggregation. Generally, H-aggregation can produce uniform crystalline morphology and superior nanostructural order in the solid state. In addition, F-embedding improved the atmospheric stability of thin films. As a result, the related RS devices gave highly-efficient MLC memory behaviors with excellent reproducibility by far. These key structure–property relations would provide important guidelines to develop highly-reproducible organic RS materials with favorable H-aggregation property and enhanced device performances. 4. EXPERIMENTAL SECTION Fabrication and Characterization of RS Devices. The organic-based RS devices were fabricated on indium-tin-oxide (ITO)-coated glass substrates. The ITO-coated glass substrates were precleaned with deionized (DI) water and ultrasonicated for 20 min each in DI water, acetone, and ethanol sequentially. Afterwards, the organic active layer of IDAZO, FIDAZO, F2IDAZO and F4IDAZO were deposited respectively onto each ITO substrate under a pressure of ~10-6 Torr, with the same deposition rate of 1 Å/s. Then the substrates were transferred into an evaporation chamber and pumped down to a vacuum of about 10-6 Torr. Finally, an aluminum


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layer with thickness of about 100 nm was thermal evaporated through a shadow mask onto the organic films. The cross-sectional scanning electron microscope (SEM) image proves the layerby-layer architecture of the device fabrication (Figure S12). Current–voltage (I–V) measurements of the non-packaged RS devices were performed under ambient conditions using an HP 4145B (DC voltage/current sweep) semiconductor characteristic system equipped with an HP 8110A pulse generator. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, 1H and 13C NMR spectrum, scheme and cross-sectional SEM image of RS device, DFT calculated geometries, thermal properties, emission spectra, optical and electrochemical data, cyclic voltammetry, schematic diagram of HOMO and LUMO energy levels, the film surface wettability tests, and XRD patterns (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.L.). *E-mail: [email protected] (J.L.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully thank the NSF of China (21206102 and 21336005), and National Excellent Doctoral Dissertation funds (201455). Y. Li thanks the China Scholarship Council (201606920044) and the Research Innovation Program for College Graduates of Jiangsu Province (KYZZ16_0086) for the financial support. The authors also sincerely thank Prof. Qichun Zhang from Nanyang Technological University for valuable discussion and revision of this work. REFERENCES (1)

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Figure 1. Synthetic and F-embedding scheme of IDAZO, FIDAZO, F2IDAZO and F4IDAZO.


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Figure 2. Current–voltage (I–V) characteristics of Al/organic/ITO architectural memory cells for IDAZO (a), FIDAZO (b), F2IDAZO (c) and F4IDAZO (d) in the DC sweep mode. The arrows display the sweep process while the numbers denote the sweep sequence. Sweep 1 is conducted on one cell unit, and sweep 2–5 are conducted on another cell.


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Figure 3. The statistical data of reproducibility for IDAZO (a), FIDAZO (b), F2IDAZO (c) and F4IDAZO (d) depicted by a column-chart plot (sweep from 0 to −5 V per storage cell; 50 cells for each kind of device).


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Figure 4. (a–d) UV-vis absorption spectra of IDAZO (a), FIDAZO (b), F2IDAZO (c) and F4IDAZO (d) in CH2Cl2 solution and in a film on a quartz plate. (e) Schematic presentation for self-assembly of molecules (in gray) into head-to-tail slipped J-aggregation (in red, left) and side-by-side cofacial H-aggregation (in blue, right). Strategic inducement of J- to H-aggregate transition produced large area face-to-face π–stack between adjacent molecules and thus enhanced charge carriers mobility.


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Figure 5. AFM height images of IDAZO, FIDAZO, F2IDAZO and F4IDAZO thin films deposited onto ITO substrates, the scan size is 5 µm × 5 µm.

Figure 6. DFT calculated molecular ESP and charge-density isosurfaces (HOMO and LUMO levels) of IDAZO (a), FIDAZO (b), F2IDAZO (c) and F4IDAZO (d) in their optimized geometries (GGA/BLYP/DNP).


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