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## Creator

[Xinhao Zhong](https://orcid.org/0000-0002-4468-6847), Debdatta Panigrahi, [Ryoma Hayakawa](https://orcid.org/0000-0002-1442-8230), [Yutaka Wakayama](https://orcid.org/0000-0002-0801-8884), [Koji Harano](https://orcid.org/0000-0001-6800-8023), [Masayuki Takeuchi](https://orcid.org/0000-0002-0207-0665), [Junko Aimi](https://orcid.org/0000-0003-1339-0581)

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[Ambipolar charge-trapping in self-assembled nanostructures of supramolecular miktoarm star-shaped copolymer with a zinc phthalocyanine core](https://mdr.nims.go.jp/datasets/93cd15f7-362b-4d79-8f19-22289a418e2b)

## Fulltext

Ambipolar charge-trapping in self-assembled nanostructures of a supramolecular miktoarm star-shaped copolymer with a zinc phthalocyanine coreThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. CCite this: DOI: 10.1039/d4tc01265hAmbipolar charge-trapping in self-assemblednanostructures of a supramolecular miktoarmstar-shaped copolymer with a zincphthalocyanine core†Xinhao Zhong,ab Debdatta Panigrahi, c Ryoma Hayakawa, cYutaka Wakayama, *c Koji Harano, d Masayuki Takeuchi *ab andJunko Aimi *aNonvolatile organic field-effect transistor (OFET) memories have attracted considerable attention owingto their potential applications in flexible and wearable electronic devices. The novel design of a charge-trapping material based on supramolecular miktoarm star copolymers (m-stars) consisting of star-shapedpolystyrene with a zinc phthalocyanine core (ZnPcPS4) and a pyridyl end-functionalized polymer(py-polymer) has been studied to explore the influence of self-assembled morphology on the finaldevice performances. Supramolecular m-stars containing the ZnPc core showed distinctive phase-separated nanostructures in the films that were different from typical polymer blends. The OFETmemory devices embedded with supramolecular m-stars exhibited ambipolar charge-trapping behaviorwith photoresponsive characteristics, resulting in a wide memory window (47 V) with a high on/offcurrent ratio (4107) for a long period of time (4104 s). Furthermore, the charge-trapping properties ofthe polymer memory layer were studied using Kelvin probe force microscopy (KPFM), revealingenhanced charge-trapping capabilities attributed to nanoscale phase separation in the supramolecularm-stars. This study provides the design and concept of charge-trapping materials for next-generationhigh-performance OFET memory devices.IntroductionNonvolatile organic field-effect transistor (OFET) memorydevices, designed to retain data post-power disruption, holdsignificant promise for integration into flexible and wearableorganic electronic devices, such as cost-effective wireless tagsand biosensors.1–3 OFET memory involves an architecturewherein a memory layer is inserted between the charge-transporting layer and the gate electrodes. Various materials,including ferroelectric materials,4–6 polymer electrets,7,8 andnano-floating gates,9–11 have been employed as memory layersin OFET devices. Memory characteristics originate from field-effect modulation by spontaneous polarization in ferroelectricsor charge trapping in dielectrics.12,13 To introduce these mem-ory layers into OFETs while maintaning transistor performance,polymer-based memory materials offer advantages in terms ofease of fabrication and application to flexible devices.The memory performance of the device is assessed based onthe memory window, which denotes the shift in the thresholdvoltage (DVth) caused by trapped charges or polarized dipoleswithin the memory material. A large memory window facilitateseasier differentiation between the ‘‘0’’ and ‘‘1’’ digital states atthe reading voltage. Additionally, controllable memory shiftsare advantageous for achieving multilevel data storage, thusfurther enhancing the memory capacity without enlarging thedevice size.14,15 The magnitude of the memory window isinfluenced by the density of charge-trapping sites and theintensity of the applied electric field in the tunneling layer,potentially affecting the retention ability due to charge leakagefrom insufficient insulation of the adjacent charge-trappingsites.16 Extensive research has demonstrated that morphologi-cal control of the charge-trapping layer is crucial for optimizinga Research Center for Macromolecules and Biomaterials, National Institute forMaterials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.E-mail: AIMI.Junko@nims.go.jp, TAKEUCHI.Masayuki@nims.go.jpb Department of Materials Science and Engineering, Faculty of Pure & AppliedSciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577,Japanc Research Center for Materials Nanoarchitectonics (MANA), NIMS, 1-1 Namiki,Tsukuba, Ibaraki 305-0044, Japan. E-mail: WAKAYAMA.Yutaka@nims.go.jpd Center for Basic Research on Materials, NIMS, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01265hReceived 29th March 2024,Accepted 8th May 2024DOI: 10.1039/d4tc01265hrsc.li/materials-cJournal ofMaterials Chemistry CPAPEROpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journalhttps://orcid.org/0000-0002-0418-8595https://orcid.org/0000-0002-1442-8230https://orcid.org/0000-0002-0801-8884https://orcid.org/0000-0001-6800-8023https://orcid.org/0000-0002-0207-0665https://orcid.org/0000-0003-1339-0581http://crossmark.crossref.org/dialog/?doi=10.1039/d4tc01265h&domain=pdf&date_stamp=2024-06-01https://doi.org/10.1039/d4tc01265hhttps://doi.org/10.1039/d4tc01265hhttps://rsc.li/materials-chttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hhttps://pubs.rsc.org/en/journals/journal/TCJ. Mater. Chem. C This journal is © The Royal Society of Chemistry 2024the memory performance.12,16–18 Notably, microstructuresformed by the self-assembly of block copolymers have beenstudied for preparing well-defined charge-trapping layers.19–22For instance, Leong et al. demonstrated in situ synthesis of Aunanoparticles within self-assembled poly(styrene-b-poly(4-vinylpyridine)) block copolymers to prepare well-defined nano-floating gates. The phase morphology of the block copolymeraffected the loading density of the nanoparticles, thereby con-trolling the memory window.23 Chiu et al. reported OFETmemory utilizing sugar-based block copolymers containingmaltoheptaose (MH), where the orientation of microdomainsinfluenced the memory window.24 Ambipolar charge-trappingwas also achieved by introducing 1-aminopyrene into the poly-mer microdomains via supramolecular interactions. Yang et al.recently introduced a pyrene-functionalized block copolymerfor photoresponsive memory.25 Phototransistor memory, whichoperates via light irradiation rather than voltage application,has been gaining importance because of its low energy con-sumption and rapid data processing.26–30 Phototransistormemory has recently been applied to artificial synaptic memorythat mimics the human brain.31 This uses optical signals tomimic the synapses responsible for information transmissionbetween nerve cells, where memory depends on the intensity orfrequency of external stimuli. Such integration of nonvolatiledata storage and processing functions into a single OFET devicehas potential applications for neuromorphic devices which areattracting attention in the field of artificial intelligence anddeep learning.32,33We recently developed OFET memory devices using a star-shaped polymer with a metallophthalocyanine (MPc) core asa nano-floating gate (Fig. 1(a)).34–36 The MPc core acts as acharge-trapping site, whereas the surrounding polymer armshinder charge leakage to achieve nonvolatile characteristics.Taking advantage of polymer-based nano-floating gates, theMPc-cored star polymer was easily fabricated into logic circuitssuch as inverters, demonstrating multilevel or reconfigur-able logic-in-memory applications.37,38 In this polymer nano-floating gate system, the memory window of the OFET memoryhas been expanded by increasing the density of the MPc core inthe polymer film, which was controlled by the length of thepolymer arms using precision polymer synthesis.35 On theother hand, increasing the core density shortened the memoryretention time and decreased the charge carrier mobility ofthe organic semiconductor. The shorter retention time wasattributed to potential charge leakage arising from insufficientinsulation of the adjacent MPc core charge-trapping sites. Thedecrease in the charge carrier mobility was linked to the crystalgrowth of the organic semiconductor, which was influenced bythe structure of the polymer thin film. The phase morphologyof the memory material appears to be critical to the memoryperformance, particularly the memory window and chargeretention properties.In this study, an OFET memory with ambipolar charge-trapping characteristics is demonstrated. Supramolecular mik-toarm star copolymers (m-stars) composed of star-shaped poly-styrene with a zinc phthalocyanine core (ZnPcPS4) and pyridylend-functionalized polymers (py-polymer) as memory materials(Fig. 1(b)) have been utilized.39 Asymmetric polymers, such asblock copolymers and miktoarm star copolymers, are known toshow unique phase behavior via self-assembly.40–42 By usingmetal–ligand coordination, AB4-type supramolecular m-starswith a functional core were facilely prepared without a tedioussynthetic procedure (Fig. 1(c)). We expect that the morphologyof the asymmetric star-shaped polymers may influence thedevice performance in OFET memory. The thin-film morpho-logy and charge-trapping behavior of supramolecular m-starswere investigated by atomic force microscopy (AFM), transmis-sion electron microscopy (TEM), and Kelvin probe force micro-scopy (KPFM). The polymer films containing supramolecular m-stars showed a unique morphology characteristic of micro-phase separation, which is different from normal blend poly-mers. Polymer films containing the ZnPc core were furtherfabricated for OFET memory, showing ambipolar charge-trapping behavior by electronic and photo-assisted program-ming/erasing operations. This OFET memory possessed longmemory retention capability, which was further enhanced bythe microphase separation of m-stars. The relationship betweenthe phase morphology and charge-trapping behavior wasinvestigated.Experimental sectionMaterialsCommercial chemicals purchased from Aldrich Chemical Co.,Inc., TCI, Wako Chemicals, FUJIFILM Wako Pure ChemicalCorporation, and Kanto Chemicals were used without furtherpurification, unless noted otherwise.MeasurementsAtomic force microscopy (AFM) was performed under ambientconditions using Bruker Dimension Icon and DimensionIconIR. Surface morphology imaging was conducted in theScanAsyst mode using a silicon cantilever (ScanAsyst-Air).Nanoscale infrared (IR) spectroscopy was conducted in theIIR tapping mode using a gold-coated silicon cantilever (PR-UM-TNIR-D-10). X-ray diffraction (XRD) measurements wereFig. 1 Chemical structures of ZnPcPS4 (a) and py-polymers (b). (c) Pre-paration of a supramolecular miktoarm star-shaped copolymer with aZnPc core through coordination interaction.Paper Journal of Materials Chemistry COpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. Cperformed using a Bruker D8 Discover with a Cu Ka X-raysource. Transmission electron microscopy (TEM), high-angleannular dark-field (HAADF) scanning transmission electronmicroscopy (STEM), and energy-dispersive spectroscopy (EDS)analyses were conducted using a Thermo Fisher Scientific TalosF200X G2 equipped with an EDS detector (Super-X G2) at anacceleration voltage of 80 kV. The specimens were prepared byscooping a thin film of polymer in water using a TEM coppergrid with a lacy carbon support (NS-C15, Okenshoji Co., Ltd).The probe current for HAADF-STEM microscopy and EDSmapping was set to 1.6 nA. EDS analysis was xperformed usingVelox software (Thermo Fisher Scientific) with a single three-parameter Bethe-Heitler function as the background correctionparameter. Kelvin probe force microscopy (KPFM) measure-ments were completed using a scanning probe microscope(Shimadzu, SPM-9700HT) under ambient conditions. A con-ductive silicon tip coated with a Pt–Ir alloy was used as thecantilever (Nanoworld, EFM). The voltage applied to the cantileverwas controlled by using a source measurement unit (KeysightTechnology, B2912A).Device fabrication and measurementOFET memory devices with a C8-BTBT organic semiconductorlayer were prepared on a highly doped p+-Si (001) substrate witha 200 nm thick SiO2 layer. First, the substrates were cleanedvia sequential ultrasonication in acetone and ethanol. Subse-quently, a thin layer of the polymer was spin-coated onto theSiO2 surface. The organic semiconductor C8-BTBT was vacuum-deposited onto the polymer film using shadow masks. The top-contact gold electrodes were thermally deposited as the sourceand drain electrodes using another shadow mask to completethe transistor fabrication process. Light-assisted memoryoperations were performed using a xenon lamp (Asahi Spectra,MAX-303) with ultraviolet (UV) light (250–380 nm) and LEDlamps (730 and 365 nm, Asahi Spectra, CL-1501).Electrical measurements were performed using an AgilentB1500A semiconductor parameter analyzer under ambient con-ditions. The charge carrier mobility (m) and threshold voltage(Vth) values were estimated from the slope and intercept of thelinear plot of the square root of the drain-to-source current (I1/2ds )vs. the gate voltage (Vg) in the saturation regime using thefollowing equation:Ids ¼WCtotmVg � Vth� �22L(1)where Ctot is the capacitance per unit area of the total dielectriclayer and L and W are the channel length and width,respectively.The relationships between the capacitances of the device(Ctot), SiO2 wafer (CSiO2), and polymers (Cpoly) and the polymerdielectric constant (e) are defined as follows:1Ctot¼ 1Cpolyþ 1CSiO2(2)Cpoly ¼e0ed(3)where e0 is the vacuum permittivity (8.854 � 10�12 F m�1), d isthe thickness of the dielectric, and CSiO2is 17.7 nF cm�2. Thetotal capacitances (Ctot) were calculated using the estimateddielectric constants of 2.88, 2.35, and 4.66 for ZnPcPS4/pyPMMA, ZnPcPS4, and pyPMMA, respectively.Results and discussionMorphology of a supramolecular miktoarm star copolymerThe supramolecular m-stars were formed by blending ZnPcPS4and pyridine-tethered polymers (py-polymer) in organic sol-vents. In the previous study, we prepared three types of py-polymers, poly(methyl methacrylate) (pyPMMA), poly(vinyl acet-ate) (pyPVAc), and poly(N-vinyl carbazole) (pyPVK), by reversibleaddition–fragmentation chain transfer (RAFT) polymerization(Fig. 1(b)).39 The average molecular weights (Mn) of each py-polymer were 13.9 kg mol�1 for pyPMMA, 16.1 kg mol�1 forpyPVAc, and 13.0 kg mol�1 for pyPVK, respectively. They havesimilar Mn to that of ZnPcPS4 (14.2 kg mol�1) (Table S1, ESI†).The metal–ligand coordination between ZnPcPS4 and threetypes of py-polymers in solution has been confirmed by spectralstudies by means of UV-vis absorption and 1H-NMRmeasurements.39 To investigate the phase morphologies ofpolymer films containing supramolecular m-stars, polymerfilms were prepared by spin-coating each polymer mixture(5 mg mL�1) in a mass ratio of 1 : 1. Notably, the Mn of thepy-tethered polymers was similar to that of ZnPcPS4, which isconsidered to have a molar ratio of approximately 1 : 1, corres-ponding to the 1 : 1 complexation observed between the ZnPccore and the pyridyl end-group. Another set of polymer filmscontaining py-polymers and star-shaped polystyrene without aZnPc core (PS4) was prepared. The Mn of PS4 is 16 kg mol�1,which is similar to that of ZnPcPS4. Binary polymer blends ofPS/PMMA, PS/PVAc, and PS/PVK are known to exhibit variousphase-separated structures due to the strong segregationbetween immiscible polymers.43,44 Therefore, the differencesin film morphology with and without the ZnPc core wereinvestigated to ascertain the influence of the supramolecularinteractions on the polymer blend.The surface morphologies of the polymer blend films wereanalyzed by AFM. Fig. 2(a) shows the AFM height image of afilm spin-coated from a toluene solution of a mixture ofZnPcPS4 and pyPMMA (ZnPcPS4/pyPMMA). The film exhibiteda distinct phase separation with a domain size of approximatelyFig. 2 (a) AFM height image of a blended film of ZnPcPS4/pyPMMA.(b) AFM-IR image of a blended film of ZnPcPS4/pyPMMA by monitoringat 1730 cm�1 (top) and 1492 cm�1 (bottom).Journal of Materials Chemistry C PaperOpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hJ. Mater. Chem. C This journal is © The Royal Society of Chemistry 202450 nm. The phase-separated morphology was analyzed usingAFM combined with nanoscale infrared (IR) spectroscopy(AFM-IR). The AFM image was monitored at selected absorp-tion wavenumbers of 1730 and 1492 cm�1, corresponding tothe carbonyl stretching band of PMMA and the benzene CQCstretching vibrations of PS, respectively. As shown in Fig. 2(b),the relatively higher and brighter parts in the AFM image werecharacterized as the PMMA domain, whereas the otherdomains were dominated by PS. In contrast, the blend polymerfilm containing PS4 and pyPMMA exhibited irregular andmicrometer-sized phase separation in the AFM height image(Fig. S1, ESI†). The root-mean-square (RMS) surface roughness(Rq) of the blended polymer film of PS4 and pyPMMA wasestimated to be 1.36 nm, which was larger than that of theZnPcPS4/pyPMMA film (0.49 nm). The smoother film surfacesand smaller domain sizes of phase separation implied theinfluence of coordination interaction between ZnPc and thepyridyl end-group in the supramolecular m-star.The film morphology of the supramolecular m-star wasfurther investigated by TEM. A 1 wt% toluene solution ofZnPcPS4/pyPMMA was dropped onto the water surface to forma thin film and a portion of the film was scooped onto a TEMgrid with a carbon support layer. A contrast between the phase-separated PS and PMMA domains was observed in the bright-field TEM images of the unstained samples (Fig. S2, ESI†).Fig. 3(a) shows a HAADF-STEM image of the supramolecularm-star film, exhibiting clear phase-separated images with adomain size of 20–50 nm. Elemental mapping of the film usingSTEM-EDS revealed that the brighter regions of the HAADF-STEM image had a higher carbon content than the darkerregions, whereas the darker regions overlapped with oxygen-rich domains (Fig. 3(b)). This result indicates that the brightregion corresponds to the PS domain, whereas the otherdomain is dominated by PMMA. Notably, the elemental map-ping of Zn in ZnPcPS4 overlapped with the carbon-rich domainsand showed a bright contrast in the HAADF-STEM image owingto the presence of heavy atoms (Fig. S3, ESI†). The filmmorphology observed by TEM was similar to that observed inthe aforementioned AFM images, confirming that the ZnPcPS4/pyPMMA films exhibited phase-separated nanodomains. Thecoordination interaction between the ZnPc core in ZnPcPS4 andthe pyridyl end group of pyPMMA might connect the bound-aries between incompatible polymers, reducing the interfacialtension to exhibit microphase separation like that of blockcopolymers rather than macrophase separation, which is oftenobserved in normal polymer blends. In other words, the supra-molecular complexes (m-stars) in the polymer blend act as acompatibilizer, providing nanosized phase-separated morpho-logy and reducing surface roughness.Similarly, the surface morphologies of other supramolecularm-stars (ZnPcPS4/pyPVAc and ZnPcPS4/pyPVK blends) spin-coated from dichloromethane solution were also analyzed byAFM (Fig. 4). A polymer film containing ZnPcPS4 and pyPVAcexhibited distinct phase separation (Fig. 4(a)), whereas the filmfrom a mixture of PS4 and pyPVAc displayed a droplet-likemorphology owing to the strong phase segregation betweenPVAc and PS (Fig. 4(c)). The surface roughness Rq of ZnPcPS4/pyPVAc was 6.28 nm, and it was much smoother than thePS4/pyPVAc blended film (30.4 nm). A similar trend wasobserved for the blended films containing PS and PVK. Thepolymer containing ZnPcPS4/pyPVK showed nanoscale phaseseparation (Fig. 4(b)), whereas stronger phase segregation and arelatively rough surface were observed in the PS4/PVK blendedfilm (Fig. 4(d)). These results support the abovementionedassumption that metal–ligand coordination between the ZnPccore and pyridyl end-groups in the polymers influences thebulk film morphology of the blended polymers. Such nanos-tructures from microphase separation are often observed inblock copolymers or m-stars; however, the preparation of vari-ous asymmetric polymers is normally difficult due to tedioussynthetic procedures. In this study, phase-separated nanostruc-tures of polymer films composed of various polymer blendcombinations were successfully obtained by exploiting supra-molecular interactions.Characteristics of OFET memory using the supramolecularl-starThe unique phase-separated morphology of the supramolecularm-star with the ZnPc core was then utilized in the memory layerof an OFET memory device. To develop the OFET memory, aZnPcPS4/pyPMMA film was chosen that formed a relativelyflat surface, allowing ideal crystal growth of the organicFig. 3 (a) HAADF-STEM image and (b) EDS elemental mapping image ofcarbon (blue) and oxygen (red) for the ZnPcPS4/pyPMMA film.Fig. 4 AFM height images of blended films of (a) ZnPcPS4/pyPVAc, (b)ZnPcPS4/pyPVK, (c) PS4/pyPVAc, and (d) PS4/pyPVK.Paper Journal of Materials Chemistry COpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. Csemiconductor. A 2 wt% toluene solution of ZnPcPS4/pyPMMAin a 1 : 1 mass ratio was spin-coated on a Si wafer with 200 nmSiO2, followed by vacuum deposition of 2,7-dioctyl[1]benzo-thieno[3,2-b][1]benzothiophene (C8-BTBT)45 and gold electro-des. The device architecture with a bottom-gate top-contactconfiguration is shown in Fig. 5(a). The thicknesses of thememory layer and the C8-BTBT layer estimated by X-ray reflec-tivity measurements were 27.0 and 14.9 nm, respectively. OFETmemory devices embedded with ZnPcPS4 or pyPMMA memorylayers were also prepared for comparison.The transfer and output characteristics of the fabricatedOFET devices are shown in Fig. 5(b) and Fig. S4 (ESI†),respectively. A typical p-type accumulation mode with sweepdirection dependence was observed for the fabricated OFETdevices. The estimated charge carrier mobility (m), on/offcurrent ratio (Ion/Ioff), and initial threshold voltage (Vth) arelisted in Table S2 (ESI†). The charge carrier mobility of theOFET with the ZnPcPS4/pyPMMA layer was estimated to be0.43 cm2 V�1 s�1, which is slightly higher than that of theZnPcPS4-embedded device of 0.33 cm2 V�1 s�1. This resultindicated that the phase-separated morphology of the under-lying supramolecular m-star did not disturb the charge trans-port properties in the C8-BTBT semiconductor layer. However,the morphology of the C8-BTBT film on the ZnPcPS4/pyPMMAsurface appeared to be influenced by the underlying polymer,as shown in Fig. 5(c). The surface morphology of C8-BTBT onthe ZnPcPS4 or pyPMMA homopolymers showed islands with asmooth top surface on a continuous layer, which appeared tofollow the Stranski–Krastanov growth modes. The cross-sectional profiles of C8-BTBT on ZnPcPS4 revealed that the stepheight of the islands was approximately 2.9 nm in the top layerand 2.6 nm on the second layer, with an underlying layerapproximately 10 nm below (Fig. 5(c), middle). The long axisof C8-BTBT was approximately 3 nm,46 suggesting that themolecule was aligned in a standing-up orientation normal tothe substrate.47 Meanwhile, the surface morphology of C8-BTBT on ZnPcPS4/pyPMMA showed a similar layer and islands,but a relatively rough surface (Fig. 5(c), left). This indicates thatthe crystal growth of C8-BTBT occurs similarly to the flat PS andPMMA surfaces, but it is in accordance with the initial surfaceroughness of the phase-separated morphology (Fig. 5(d)).Indeed, the out-of-plane X-ray diffraction (XRD) profile of C8-BTBT on the blend or homopolymer showed almost the samepatterns as those of the crystalline structures (Fig. S5, ESI†).The sharp peak at 2y = 3.11, originating from the (001) Braggreflection, was estimated to have a d-spacing of 2.8 nm. Takentogether with the AFM results, the C8-BTBT films formed highlyordered layer-by-layer phases, even on the phase-separatedsurface of ZnPcPS4/pyPMMA, resulting in a comparable chargecarrier mobility in OFET devices.In the transfer curve of the memory devices embedded withZnPc-containing polymers, a clear hysteresis was observedbetween forward and backward sweeps for Vg between +5 and�50 V at a fixed drain voltage (Vd) of �50 V (Fig. 5(b), left andmiddle). This result indicated that the holes accumulated inC8-BTBT were trapped in the memory layer while sweeping to anegative Vg. In contrast, no significant hysteresis was observedin devices with pyPMMA (Fig. 5(b), right). It should also benoted here that the charge carrier mobility values of the OFETdevices with ZnPc-containing polymer layers were lower thanthose with pyPMMA. Since the crystal structure of C8-BTBT onZnPcPS4 or pyPMMA did not show clear differences in the AFMimages, this discrepancy was probably due to charge trappingduring sweeping, which could prevent efficient charge migra-tion in the organic semiconductor.35 The magnitude of thehysteresis was maximal for the ZnPcPS4-embedded memorydevice, which was approximately twice as large as that of theZnPcPS4/pyPMMA-embedded device. This result was reason-able because the concentration of ZnPc, i.e., the density ofFig. 5 (a) Device architecture of C8-BTBT-based OFET memory with apolymer memory layer. (b) Transfer characteristics of OFET memorydevices with memory layers of ZnPcPS4/pyPMMA (left), ZnPcPS4 (middle),and pyPMMA (right). AFM height images and cross-sectional profiles of (c)C8-BTBT and (d) polymer layers.Journal of Materials Chemistry C PaperOpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hJ. Mater. Chem. C This journal is © The Royal Society of Chemistry 2024the charge-trapping site, was reduced by approximately half inthe supramolecular m-star film.To quantitatively evaluate the memory performance, pro-gramming/erasing operations of the OFET memory deviceswere conducted by applying a Vg pulse and light irradiation.When a negative gate bias was applied to the ZnPcPS4/pyPMMA-embedded OFET memory (Vg = �60 V for 1 s), thetransfer curve shifted toward the negative direction (Fig. 6(a),red line). This process is referred to as programming (hole-trapping), in which the holes accumulated in the C8-BTBTsemiconductor layer are transferred to and stored in thememory layer. The average shift of Vth (DVth(+)) was 25.4 V forthe ZnPcPS4/pyPMMA-embedded device and 31.5 V for theZnPcPS4-embedded device (Fig. S6, ESI†). The OFET memorywith the pyPMMA layer showed a small shift of 5.3 V, confirm-ing that significant charge trapping occurred owing to thepresence of the ZnPc core (Fig. S7, ESI†). Interestingly, theerasing process was accomplished by UV light irradiation.When the OFET memory device was exposed to UV light(250–380 nm) with an intensity of 2.5 mW cm�2 for 5 s afterthe programming process, the transfer curve shifted back to theinitial position (Fig. 6(a), purple line). This process is referredto as photo-erasing. By irradiating the device with UV light, asignificant number of excitons (electron–hole pairs) can begenerated on the semiconducting layer because C8-BTBT exhi-bits strong absorption in the UV region of the spectrum (Fig. S8,ESI†). Some electrons might effectively neutralize the trappedholes at the interface between the C8-BTBT layer and theunderlying memory layer, achieving an erasing process in theabsence of an electric field.26,48 Another possible mechanism ofphoto-erasing is the annihilation of the trapped charges byexcitons induced in the memory layer.49,50 However, we foundthat photo-erasing could be completed by LED light irradiationat 365 nm (Fig. S9a, ESI†), while 730 nm LED light irradiation,corresponding to the absorption of ZnPc, cannot complete thisprocess (Fig. S9b, ESI†). Thus, charge annihilation may beprimarily due to the excitons of C8-BTBT, as previously men-tioned. Furthermore, when Vg = +60 V was applied for 5 s underUV irradiation, the transfer curve shifted in the positive direc-tion as +21.9 V (Fig. 6(a), blue line). This process is referred toas photo-assisted programming (electron trapping). In thisprocess, the photogenerated excitons were separated by anexternal electric field and some electrons were trapped in theunderlying polymer layer. This resulted in a positive shift in Vth,eventually giving OFET memory devices a large memory win-dow with ambipolar charge-trapping behavior. This photo-assisted programming process was not achieved by the LEDlight irradiation at 730 nm (Fig. S9b, ESI†). This indicates thatexcitons generated within the semiconductor triggered thememory programming. The proposed mechanisms of holetrapping, photo-erasing, and photo-assisted electron trappingare illustrated in Fig. 6(b). The positive Vth shift of the ZnPcPS4/pyPMMA-embedded OFET memory device was larger than thatof the ZnPcPS4-embedded device. This result suggested thatelectrons were trapped not only in the ZnPc core but also in thepolymer chains and/or interfaces. A similar trend was observedin the OFET memory with a memory layer of phase-separatedblock copolymer, where charge trapping occurred in the inter-faces between the polymer domains.21 Phase separation of thesupramolecular m-star might be favorable for electron trapping,resulting in a comparable memory window of approximately47.0 V for all operations (Fig. S6, ESI†).The conditions of the photo-assisted programming werealso optimized by varying the applied Vg (Fig. S10a, ESI†). Whenthe applied Vg was gradually increased from 0 to +60 V underlight irradiation for 5 s, the memory window saturated atFig. 6 (a) Transfer characteristics of the OFET memory device with a polymer layer of ZnPcPS4/pyPMMA at Vd = �50 V. (b) Schematics and energy-levelalignments during the (i) electrical programming operation, (ii) UV-light-assisted erasing operation, and (iii) UV-light-assisted programming operation.(c) Retention time of the Ids monitored at Vg = 0 V and Vd = �10 V after hole-trapping (triangle) and electron-trapping (circle).Paper Journal of Materials Chemistry COpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. CVg = +40 V, indicating that the applied voltage could be reducedby light irradiation. This result also suggested that the draincurrent can be controlled by applying voltage and/or light,thereby exhibiting potential applications in multilevel datastorage (Fig. S10b, ESI†).The retention time of the OFET memory device after theprogramming process was evaluated to demonstrate its relia-bility. As shown in Fig. 6(c), the drain current was monitored atVg = 0 V under Vd = �10 V after applying Vg = �60 V for 1 s (OFFstate) or Vg = +60 V for 5 s under UV light irradiation (ON state).These correspond to hole-trapped and electron-trapped statesin OFET memory, respectively. The on/off current ratioremained at 105 after 3 h for the ZnPcPS4/pyPMMA-embeddedOFET memory device, indicating the superior charge retentioncapability of the polymer layer. Notably, ZnPcPS4/pyPMMAshowed a longer electron-trapping retention time than theZnPcPS4 and pyPMMA layers (Fig. S11, ESI†). Considering itslarge memory window and long charge-retention capability, thephase-separated morphology of ZnPcPS4/pyPMMA wasassumed to provide efficient charge trapping and suppresscharge leakage. Therefore, the charge-trapping behaviors ofthe polymer films were investigated.Charge-trapping properties in polymer memory layersCharge injection, retention, and diffusion processes in thepolymer films were studied using KPFM. The KPFM techniquehas been widely used to profile the localized electrical proper-ties of films used in electronic devices because it can simulta-neously obtain a high-resolution morphology and surfacepotential.51,52 This technique allows the evaluation of theelectrical potential difference,53,54 charge transport and spatialdistribution in the semiconductor channel,55–57 work functionfor nanostructures,58 and charge-trapping and diffusion prop-erties in the dielectric layer.59–61Three types of polymer films, ZnPcPS4, ZnPcPS4/pyPMMA,and pyPMMA, were prepared by spin-coating toluene solutionsonto a highly doped n-type silicon wafer with a 300 nm SiO2layer. Charges were injected into the polymer film by contactwith a conductive tip on the polymer surface with an appliedvoltage bias (Vtip), while the substrate was ground (Fig. 7(a)). Asshown in Fig. 7(b), after injecting a positive bias (Vtip = +10 V)for 30 s onto the ZnPcPS4/pyPMMA film, a spot with a relativelypositive potential was observed by scanning the surfacepotential using KPFM. This result indicates that the strongelectric field between the tip and polymer surface extractselectrons by tunneling, inducing holes at the specific injectionpoint. In contrast, electron injection was achieved by applying anegative bias of Vtip = �10 V for 30 s by the contact mode inKPFM. Localized charges were visualized by measuring thesurface potential; a relatively negative spot was observed in aspecific area of the injection point. The line profiles of thepotential peaks after injecting holes or electrons into theZnPcPS4/pyPMMA film are plotted in Fig. 7(c), wherethe maximum potential peak decreased very slowly via chargediffusion. Note that the first point of the peak potential wasapproximately 10 min after charge injection owing to the AFMscanning experimental conditions. The peak potential at 10 minafter applying a positive bias was 0.62 V and decreased to 0.32 Vafter 2 h, while the peak potential after a negative bias was�0.51 Vat 10 min and remained at �0.32 V after 2.4 h.To further investigate the charge retention and diffusionbehavior of the polymer film, charge injection experiments wereperformed on other polymer thin films of ZnPcPS4 and pyPMMA(Fig. S12, ESI†). The peak potential after the same injectionexperiments of positive charge was 0.62 V for ZnPcPS4, which isa similar value to that of the ZnPcPS4/pyPMMA film. Meanwhile,the pyPMMA film exhibited the peak potential at 0.42 V aftercharge injection. The higher peak potentials of the ZnPc-containing polymers indicate the efficient charge-trapping cap-ability of the ZnPc cores in the polymer matrix. After applying anegative Vtip, the negative peak potentials were nearly identicalacross polymers, approximately 0.5 V. Subsequently, the peakpotential maps were continuously analyzed every 8 min afterinjecting charges (hole or electron) into each polymer and themaximum peak potential was plotted over time. The time depen-dence of the peak potential of the polymer films is shown inFig. 7(d), representing decay in the number of trapped charges inthe polymer films. The plotted potential peak fits well with theexponential curve according to the following equation:QðtÞ ¼ Q0 exp �tt� �(4)Fig. 7 (a) Schematics of charge-injection experiments using KPFM. (b) Surface potential images of the ZnPcPS4/pyPMMA film after applying Vtip = +10 V(top) and �10 V (bottom). (c) Peak potential change after the charge injection. (d) Decays and exponential curve fittings of the peak potentials with timeafter charge injection into different polymers. (e) Evolution of the full width at half-maximum (FWHM) of potential curves with time for each polymer.Journal of Materials Chemistry C PaperOpen Access Article. Published on 21 May 2024. Downloaded on 6/25/2024 12:26:52 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4tc01265hJ. Mater. Chem. C This journal is © The Royal Society of Chemistry 2024where Q0 is the initial charge, t is the time after charge injection,and t is the characteristic decay time, which is equal to the time toreach approximately 37% of the initial decay. The characteristicdecay time of holes (th) was approximately 1.1 � 104 s for theZnPcPS4 film and 8.5 � 103 s for ZnPcPS4/pyPMMA, relativelylonger than the 5.5 � 103 s for the pyPMMA film. This confirmedthat the ZnPc-containing polymers have superior hole capture andretention capabilities. The dielectric PS arms effectively confinedpositive charges to the ZnPc core, resulting in localized areas ofhigh charge density. More interestingly, the supramolecular m-starfilm also showed an excellent retention capability of electrons.The decay time of electron (te) trapping in the ZnPcPS4/pyPMMAfilm was 2.0 � 104 s. The retention time was much shorter inZnPcPS4 (6.9 � 103 s) and pyPMMA (4.8 � 103 s). This result wasconsistent with the retention characteristics of electron-trappedstates in the OFET memory device (Fig. S11, ESI†). Furthermore,the evolution of the full width at half-maximum (FWHM) of thepotential curve with time was analyzed to investigate the chargediffusion properties in the polymer film (Fig. 7(e)). The potentialspot size derived from the trapped hole was nearly constant for allthe polymer films, indicating that the trapped holes in polymerdielectrics did not show lateral charge diffusion through thepolymer matrix during the charge decay. Meanwhile, a rapidincrease in spot size was monitored after injecting a negativevoltage bias into the pyPMMA film. This electron diffusion featurewas not observed in the case of the ZnPcPS4/pyPMMA film. Thephase separation between the PS and PMMA domains mightinhibit the lateral diffusion of electrons. The ZnPcPS4/pyPMMAfilm originally showed a different surface potential of approxi-mately 0.1 V (Fig. S13, ESI†). This barrier might inhibit uniformpotential diffusion throughout the polymer film. As a result, it wasfound that the ZnPcPS4/pyPMMA film can stably store bothelectrons and holes, making it useful as a memory layer in OFETdevices.ConclusionWe investigated the effect of the film morphology of charge-trapping polymeric materials on the OFET memory devicesusing the supramolecular m-star. The mixtures of ZnPcPS4and pyridine-terminated polymers (pyPMMA, pyPVAc, pyPVK)formed a supramolecular m-star, and their spin-coated filmsexhibited unique microphase separation with smooth filmsurfaces due to the non-covalent interaction between polymers.The OFET memory device was fabricated using a ZnPcPS4/pyPMMA film as the charge-trapping layer and C8-BTBT asthe organic semiconductor. By combining the electric andphoto-assisted programming operations, the memory deviceexhibited ambipolar charge-trapping characteristics. The mem-ory device trapped holes by applying a negative Vg bias, releasedtrapped charges by photoirradiation with UV light, and furthertrapped electrons when UV light and a positive Vg bias wereapplied simultaneously. Consequently, the memory deviceshowed a large memory window (B47 V), a high Ion/Ioff memoryratio (B107), and long-term charge retention (4104 s). KPFMstudies of the polymer films revealed that the ZnPc-containingfilms had efficient hole-trapping and long retention ability,whereas the ZnPcPS4/pyPMMA films also showed superiorretention ability of electrons owing to the surface potentialdifference at the interfaces of nanostructures of supramolecu-lar m-stars. Our study suggests that a design strategy fornanostructured charge-trapping materials with functional aro-matic molecules can improve the performance of photo-transistor memory.Author contributionsJ. A. designed the study. X. Z. synthesized the polymers andinvestigated their morphologies. K. H conducted STEM mea-surements. All device experiments were conducted by X. Z. withsupervision and guidance from D. P., R. H. and Y. W. X. Z. andJ. A. wrote the manuscript with input from all the authors. W. Y.and M. T. supervised the project and finalized the manuscript.Conflicts of interestThe authors declare no competing interests.AcknowledgementsWe deeply appreciate Dr. Masanobu Naito (NIMS) for the use ofthe nano-IR instruments. We are also grateful to Ms. IzumiMatsunaga and Ms. Bo Zhou for their technical support in thepolymer synthesis and AFM measurements. This study wassupported by JSPS KAKENHI (JP21K05220 to J. A., JP21F21052and JP23H00269 to W. Y., and JP23H04874 to K. H.), a Grant-in-Aid from Izumi Science and Technology Foundation to J. A.,and a MEXT ‘‘NIMS Molecule and Material Synthesis Platform’’program.References1 T. Sekitani, T. Yokota, U. Zschieschang, H. Klauk, S. Bauer,K. Takeuchi, M. Takamiya, T. Sakurai and T. Someya,Science, 2009, 326, 1516–1519.2 M. L. Hammock, A. Chortos, B. C. Tee, J. B. Tok and Z. Bao,Adv. Mater., 2013, 25, 5997–6038.3 A. Yamamura, H. Matsui, M. Uno, N. Isahaya, Y. Tanaka,M. Kudo, M. Ito, C. Mitsui, T. Okamoto and J. Takeya,Adv. Electron. 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