Direct Observation of Indium Conductive Filaments in Transparent

Jan 23, 2017 - The use of two-dimensional (2D) materials (such as hexagonal boron nitride (hBN), WO3·H2O, and MoOx) in flexible RRAM devices has the ...
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Direct Observation of Indium Conductive Filaments in Transparent, Flexible, and Transferable Resistive Switching Memory Kai Qian,† Roland Yingjie Tay,‡ Meng-Fang Lin,† Jingwei Chen,† Huakai Li,‡ Jinjun Lin,‡ Jiangxin Wang,† Guofa Cai,† Viet Cuong Nguyen,† Edwin Hang Tong Teo,†,‡ Tupei Chen,‡ and Pooi See Lee*,† †

School of Materials Science and Engineering, and ‡School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore S Supporting Information *

ABSTRACT: Electronics with multifunctionalities such as transparency, portability, and flexibility are anticipated for future circuitry development. Flexible memory is one of the indispensable elements in a hybrid electronic integrated circuit as the information storage device. Herein, we demonstrate a transparent, flexible, and transferable hexagonal boron nitride (hBN)-based resistive switching memory with indium tin oxide (ITO) and graphene electrodes on soft polydimethylsiloxane (PDMS) substrate. The ITO/hBN/graphene/PDMS memory device not only exhibits excellent performance in terms of optical transmittance (∼85% in the visible wavelength), ON/OFF ratio (∼480), retention time (∼5 × 104 s) but also shows robust flexibility under bending conditions and stable operation on arbitrary substrates. More importantly, direct observation of indium filaments in an ITO/hBN/graphene device is found via ex situ transmission electron microscopy, which provides critical insight on the complex resistive switching mechanisms. KEYWORDS: transparent and flexible memory, transferable memory, indium filament, hexagonal boron nitride, graphene, ex situ TEM

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obtain multifunctional RRAM have also aroused great interest.3,19−25 The use of two-dimensional (2D) materials (such as hexagonal boron nitride (hBN), WO3·H2O, and MoOx) in flexible RRAM devices has the potential to outperform conventional bulk metal oxides.17,26−28 Especially, compared with conventional active materials (e.g., bulk and nanoparticles), the cracking and detachment phenomena under repetitive bending or strain conditions, which stemmed from the poor adhesion between switching layer and substrates, can be dramatically avoided in 2D materials.28 Their fascinating properties (e.g., excellent optical and mechanical properties)4,17,29−31 also offer an opportunity for the development of future multifunctional RRAM. On the other hand, graphene, an excellent choice as an electrode for wearable electronics,32−34 can be efficiently transferred to other arbitrary substrates with large area after removal of the supporting

lectronic modules have been going through extensive innovations to meet the ever-increasing technological demands of multiple functionalities, such as transparency, portability, and flexibility.1 Most electronic devices are fabricated on conventional substrates (e.g., flat, rigid, and smooth SiO2/Si and plastics) due to the well-established semiconducting fabrication processes and difficult handling requirements on arbitrary nonconventional substrates (e.g., soft or nonplanar ones).2,3 Recent progress has been made to construct transparent flexible electronic devices in various areas, such as transistors,4,5 charge-trap memory,6 photodetector,7,8 electrochromics,9,10 and triboelectric nanogenerator.11 With the growing interest in the field of flexible or implantable electronics applications on nonconventional substrates, it is an urgent task to establish fabrication strategies for functional electronics on desired arbitrary substrates.1,12,13 Resistive random access memory (RRAM), which can store and process information, is considered one of the most promising candidates for next generation memory due to its excellent storage capability and scalability.14−18 Therefore, efforts to © 2017 American Chemical Society

Received: November 9, 2016 Accepted: January 17, 2017 Published: January 23, 2017 1712

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Figure 1. (a) Schematic of the ITO TE/hBN/FLG memory device fabrication process on any desired substrate. Chemical structures of (b) graphene and (c) hBN for bottom electrode and switching layer, respectively. (d) Schematic of ITO TE/hBN/FLG device on a soft PDMS substrate. (e) Optical transmittance of ITO TE/hBN/FLG/PDMS device. The inset shows the actual memory device (highlighted by the closed region), indicating transparent characteristics.

Figure 2. Electrical properties of the ITO TE/hBN/FLG/PDMS device. (a) I−V characteristics after electroforming process. (b) 500 memory cycles and (c) ∼ 5 × 104 s retention time from the memory device on flat condition, respectively. (d) Retention of the ON and OFF states during the bending test with bending radius (r) of 14 mm. The inset shows the actual transparent flexible ITO TE/hBN/FLG/PDMS device on a PET substrate. The readout voltage is +0.1 V.

metal.3,35 In addition, graphene is also a promising electrode due to its excellent flexibility and strong interfacial adhesive force, i.e., van der Waals interaction (adhesive energy of 0.45 J m−2 on SiO2), rendering efficacy to mount them on desired nonconventional substrates.36,37 Thus, it is rational to suggest that 2D materials would be an attractive solution for multifunctional RRAM applications. In order to obtain excellent memory performances, a thorough understanding of the switching mechanism is necessary to effectively control the switching character-

istics.14,16−18,38 For the transparent RRAM devices with indium tin oxide (ITO) electrodes, the resistive switching behaviors were always associated with the oxygen vacancies migration mechanism or other switching mechanisms, which are not related to the migration of ITO electrodes.39−43 However, the ITO-based device, which can serve as both conducting electrode and switching active material, can also exhibit resistive switching behaviors due to the oxygen migration induced by a relatively high-electric field.44 Therefore, to confirm the role of ITO electrodes in RRAM devices, 1713

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Figure 3. Demonstration of the ITO TE/hBN/FLG/PDMS memory labels tagged on (a) a rough and soft wallet, (b) a centrifuge tube (r ≈ 8 mm), and (c) device attached onto fingers with their corresponding switching characteristics. The memory device is easy to attach onto various curved objects’ surfaces and exhibits excellent flexibility.

(polyethylene terephthalate) (Figure S1e,f, Supporting Information) substrates for the electrical switching behaviors measurement. This as-fabricated ITO TE/hBN/FLG/PDMS memory device shows a good transmittance of ∼85% over the visible wavelength range (Figure 1e). The electrical performances of the ITO TE/hBN/FLG/ PDMS device were measured both on flat and under bending conditions. To avoid damaging the graphene BE when the electrical probe approached during measurement, a contact ITO thin film was predeposited to partially contact the graphene BE on PDMS substrate. Figure 2a showed the current−voltage (I−V) curve from the ITO TE/hBN/FLG/ PDMS device after the initial electroforming process with a relatively high voltage of ∼7.4 V (Figure S2a, Supporting Information). Application of a positive voltage on ITO TE switched the memory device from the OFF state (i.e., high resistance) to the ON state (i.e., low resistance) at ∼0.66 V, which was defined as the SET process. Afterward, the device returned to the OFF state when a reverse bias (∼ −0.9 V) was applied, which was called the RESET process. The ITO TE/ hBN/FLG memory device exhibited a bipolar switching mode and an acceptable ON/OFF ratio above ∼480. On the other hand, compared with the ITO TE/hBN/FLG/PDMS device, the ITO/hBN/ITO/PET memory device (Figure S1f, Supporting Information) also exhibited similar switching behaviors (Figure S3a,b, Supporting Information). Furthermore, additional electrical switching behaviors were also investigated using a reading voltage of 0.1 V, such as write cycles, retention time, and flexibility endurance. The ON and OFF states of the devices were demonstrated with steady operation during the 500 write cycles (Figure 2b), showing no noticeable degradation after 5 × 103 s (Figure 2c). To evaluate the feasibility of the ITO TE/hBN/FLG device for flexible memory applications, the switching behaviors of the memory device were examined under a bending condition (r = 14 mm, Figure S2b,c, Supporting Information). The memory window maintained a ON/OFF ratio of 100 and did not change much during 850× continuous bending cycles (Figure 2d), indicating excellent bending stability and a great potential for nonvolatile memory applications in transparent flexible electronics. In order to broaden the memory device for more realistic applications, ITO TE/hBN/FLG/PDMS device was further characterized as a label on arbitrary surfaces. The correspond-

microscopic experiments that can provide the direct observation of the resulting physical and chemical changes are essential to thoroughly understand the resistive switching mechanism. In order to exclude the influence of oxygen present in the active layer, the active materials without oxygen are necessary, such as the 2D hBN material. Herein, we demonstrate a transparent, flexible, and transferable RRAM device based on an ultrathin hBN (≈ 5 nm) switching layer with ITO top electrode (TE) and a few-layer graphene (FLG) bottom electrode (BE) on a soft polydimethylsiloxane (PDMS) substrate (Figure 1d). The asfabricated ITO/hBN/FLG/PDMS memory device featured a high transmittance of ∼85% in the visible region, long retention time (∼5 × 104 s), stable memory characteristics (ROFF/RON ratio ∼ 480) on different substrates, and reliable flexibility after substrate deformation. More importantly, unlike other transparent metal oxides RRAM with ITO electrodes, the direct observation of indium (In) filaments is reported via ex situ transmission electron microscopy (TEM) to account for the stable resistive switching of the ITO/hBN/FLG/PDMS memory device. This finding undoubtedly contributes to the understanding of the switching mechanism in 2D materials for the rapid development of RRAM.

RESULTS AND DISUSSION Figure 1a illustrates the schematic fabrication processes of the transparent, flexible, and transferable ITO TE/hBN/graphene memory device. In short, a layer of poly(methyl methacrylate) (PMMA) was first coated on the chemical vapor deposition (CVD)-hBN film, which was then transferred to a graphene/ copper (Cu) foil substrate via a conventional wet transfer approach (Figure S1a,b, Supporting Information).17 Subsequently, another layer of PMMA was coated on the hBN/ graphene/Cu foil. Then, the PMMA/hBN/graphene can be easily transferred to any desired substrate after the Cu foil was etched away using ammonia persulfate (APS) solution (Figure S1c,d, Supporting Information). Finally, after removal of the PMMA layer using acetone, the ITO was deposited on the hBN/graphene film as the TE (Figure 1d). To meet the high transparency of the memory device without sacrificing the electrical conductivity of graphene bottom electrode during the transfer process, few-layer graphene was employed to construct ITO TE/hBN/FLG on soft PDMS or flexible ITO/PET 1714

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Figure 4. Observation of conductive filaments in an ITO TE/hBN/FLG/Cu foil memory cell at the ON state. TEM images of (a) a noncomplete and (b) a complete conducting filament in the same memory cell (highlighted by the closed regions), indicating the filament commences growth from ITO TE. (c) EDS elemental mapping result of the memory cell in STEM mode, corresponding to (b).

Figure 5. (a) STEM image of the ITO TE/hBN/FLG device. (b) Line profile of the EDS intensity of Cu, In, and Sn elements along the red line in (a) from Cu foil to ITO TE. (c) EDS spectrum of the filament in region “1” of (a). The intensity of Cu, In, and Sn is shown in the inset table. It indicates that the filament was primarily composed of In. (d−f) Schematic illustration of the In filament growth in the ITO TE/hBN/ FLG memory device.

ing I−V characteristics of these memory devices were measured when it was tagged on different objects (Figure 3). First, Figure 3a,b shows the I−V characteristics of the ITO TE/hBN/FLG/ PDMS memory device labeled on different surfaces: a rough and soft wallet (flat surface, r = ∞, Figure 3a) and a centrifuge tube (r ≈ 8 mm, Figure 3b). As the voltage bias was applied, they exhibited similar switching behaviors. The results indicated that the ITO TE/hBN/MGL/PDMS device can function consistently on both planar and nonplanar surfaces with stable switching properties. As shown in Figure 3c, the ITO TE/ hBN/MGL/PDMS device was further demonstrated as an onskin label conforming to the back of the hand, which was still completely switchable and operated normally. These tests demonstrated that the ITO TE/hBN/FLG/PDMS memory device can conform to various objects with robust operation and reliable switching characteristics on arbitrary substrates, promising an exciting opportunity for epidermal electronics applications. A clear switching mechanism is essential for the design of memory devices in terms of reproducibility and comprehensive modeling.18,45 Understanding the switching behavior is thus of vital importance for device optimization. Therefore, to reveal

the switching mechanism of ITO TE/hBN/FLG memory device, we performed ex situ TEM and EDS analyses for further analysis. The ITO TE/hBN/FLG on Cu foil substrate was fabricated for the investigation of a resistive switching mechanism. First, the specimen for TEM measurements was prepared via focused ion beam (FIB) after being subjected to more than 20 repetitive write cycles to the ON state. As shown in Figure 4a, a noncomplete conductive filament clearly starts growing from the ITO TE, which is in conflict with the conventional electrochemical metallization (ECM) theory.46 In addition, there is a complete cone-shaped filament with a clearly different contrast from the background hBN/FLG film in the memory cell (Figure 4b). The hBN/FLG film has low contrast, while the conducting filament, which has a relatively high contrast, can be seen to clearly connect the ITO TE and graphene BE, leading the memory cell to an ON state. Therefore, it is rationally concluded that the upper region separated by the white dash line was the hBN switching layer where filaments existed, while the lower region was a few-layer graphene BE. The thinnest region and wider base of the filament are located at the hBN/FLG interface and the ITO TE/hBN interface, respectively. To confirm the chemical 1715

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ACS Nano composition of the complete filament, the EDS analysis was conducted in scanning TEM (STEM) mode. Figure 4c presents the filament EDS mapping result of the ITO TE/hBN/FLG/ Cu foil memory cell, corresponding to Figure 4b. It is clear that the filament has an In-rich stoichiometry (Figure 4c), indicating the conducting filaments are primarily composed of an In element in the ITO TE/hBN/FLG memory device. Here in the ITO TE/hBN/FLG memory device, due to the low ion mobility the hBN film offered,17,18 the conductive filaments started growing from the ITO anode (Figure 4a), leading to the inverted cone-shaped In filament with a wider base near the ITO TE/hBN interface (Figure 4b). It is noteworthy that the Cu foil does not contribute to the resistive switching, which was isolated by few-layer graphene BE. During the electroforming process, when a high positive voltage was applied on the graphene electrode while ITO was grounded, the ITO TE/ hBN/FLG/Cu foil device did not show resistive switching (Figure S3c, Supporting Information). To further investigate the chemical composition of the conductive filaments, EDS profiling analysis was carried out along the filament (Figure 5a,b). Despite the semiquantitative nature of EDS analysis, in the filament region (Figure 5b), it is clear that the In intensity was higher than the tin (Sn) intensity. On the other hand, the filament composition was also further studied via EDS point analysis (circle region 1 in Figure 5a). As shown in Figure 5c, there was no Sn element in the filament which may be due to the low Sn content, indicating the filament mainly consists of the In element. Therefore, the ITO electrode, which was similar to other active electrodes (e.g., Ag, Cu) in the RRAM, can also form the In conductive filaments upon an applied high electric field. This finding is critically important for the deep understanding of the complex resistive switching mechanism in the transparent RRAM with ITO electrodes, which will be beneficial to the continued optimization of this important class of transparent memories. It is noteworthy that a higher electric field (∼14.8 MV cm−1) was required for the In evolution and migration in the hBN film during the electroforming process (Figure S2a, Supporting Information). While in the Ag TE/hBN/FLG memory device, a lower electric field (∼7MV cm−1) (Figure S3d, Supporting Information) was required for the Ag movement due to the high electrochemical activity of the Ag electrode.18 Moreover, compared with Ag ions, the diffusion of In ions in solids is suppressed due to its higher charge numbers and sizes, which also lead to a higher electric field required for In ion migration.18 In addition, the Cu element intensity (Figure 5c) is associated with the Cu foil and Cu TEM grid supporting the device. On the basis of the above analysis, the In filament growth process is illustrated in Figure 5d−f and depicted as follows. Upon forward bias applied on the ITO TE, the In−O bond was broken, and the In3+ ions migrated toward the cathode. Then these In3+ ions are reduced to an In atom to form the In nanoclusters and nucleate near the active ITO TE (Figures 5d and 4a) because of the low ion mobility the hBN film offered.17,18 These In clusters then behave as bipolar electrodes during the subsequent filament growth and follow the splitting to the merging processes,18 leading to the clusters migration to the graphene cathode (Figure 5e). In addition, due to the high concentration of In3+ ions near the anode, therefore abundant In clusters will be nucleated near the ITO TE. With the repeated In clusters nucleation, migration inside the hBN film, the conducting filament will gradually grow from the ITO TE

to the graphene bottom electrode (Figure 5f), leading to the formation of a cone-shaped filament with its thinnest region at the hBN/graphene BE interface and switching the memory to ON state (Figure 4b). During the RESET process (Figure S4, Supporting Information), due to the major voltage drop located at the neck of the filament,47 the thinnest part of the conductive filament with high current density at the hBN/graphene interface will be the first to dissolve upon reverse bias application (Figure S4a, Supporting Information), leading to the OFF state.

CONCLUSION We have successfully demonstrated a transparent, flexible, and transferable ITO TE/hBN/graphene resistive memory device on a soft PDMS substrate with a clear switching mechanism: the formation and annihilation of In filaments. Based on the inherent excellent mechanical properties of 2D hBN and graphene materials, the as-fabricated hBN-based memory device can be transferred freely to different substrates with stable switching characteristics, which is beneficial to future multifunctional soft electronic applications. The ITO TE/ hBN/graphene/PDMS memory devices feature high transmittance (∼85%) over the visible region, long retention time (5 × 104 s), large memory window (ON/OFF ratio, ∼ 480), and excellent stability under bending conditions. More importantly, the In filaments were confirmed here via ex situ TEM, which is responsible for the resistive switching. In general, it is of fundamental interest for the community that a material like ITO, which was expected to serve as an electrode, is involved in filament formation/rupture in RRAM. This finding of the existence of In filaments definitely broadens our understanding of the switching mechanism in RRAM with ITO electrodes and allows comprehensive modeling to stimulate comprehensive device optimizations for this important class of devices. METHODS Resistive Switching Memory Device Fabrication. The device structure (Figure 1d) was composed of an ITO TE, chemical vapor deposition (CVD)-grown hBN switching layer,17 and a few-layer graphene film (Graphenea, Spain) bottom electrode. ITO TE with an ∼250 μm diameter was deposited onto the hBN switching layer via a radio frequency (RF) sputtering technique with a hard shadow mask. The ITO TE/hBN/graphene memory device can be transferred to different substrates, such as PDMS (Figure 1d) and ITO/PET (Figure S1f, Supporting Information). The PDMS substrate was fabricated by mixing the PDMS curer and base (Sylgard 184, Dow Corning, USA) at a weight ratio of 1/10, which was then diluted with n-hexane and poured into a glass Petri dish. Finally, the solidified PDMS substrate was peeled off from the Petri dish after curing at 80 °C for 2 h. The PDMS substrate thickness was about ∼220 μm. The hBN films were grown by CVD on Cu substrates, as reported previously.17,48,49 Preparation of Cu foils (25 μm thick, Alfa Asear, product no. 13382) was done by dipping in diluted nitric acid and rinsing with deionized water. Then, the Cu foil was placed in a 1 in. quartz tube under a constant Ar/H2 flow of 200:20 sccm. The furnace was heated up to 1050 °C for 40 min and kept constant for 1 h to anneal the Cu. During growth, ammonia borane (10 mg, Sigma-Aldrich, 97%) was loaded into a ceramic boat and sublimated at 85 °C for 1 h. Finally, the asprepared hBN film on Cu foil was cooled down by opening the lid of the furnace. Device Characterization and Testing. The transmission spectra of the device were measured over the wavelength range from 300 to 900 nm with UV−vis-NIR spectrometer (Cary 5000 and DRA 2500, Agilent Technologies). Electrical characterizations were conducted via Keithley 4200 semiconductor analyzer at a DC voltage sweep rate of 1716

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0.01 V/s. The ON and OFF states resistive memory sample for TEM observation was prepared (Figure S5, Supporting Information) via a dual beam FIB (FEI Helios 450S). JEOL JEM-2100 electron microscope (200 kV) and JEOL JED 2300T EDS were employed for TEM/EDS analyses, respectively.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07577. Additional discussions about the detail of the memory fabrication, the bending test, the electroforming step and I−V characteristics are included (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Kai Qian: 0000-0002-4049-4426 Pooi See Lee: 0000-0003-1383-1623 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the NTU-A*Star Silicon Technologies Centre of Excellence under the program grant no. 112 3510 0003 and National Research Foundation Competitive Research Programme NRF-CRP13-2014-02. K.Q. acknowledges the scholarship awarded by Nanyang Technological University, Singapore. REFERENCES (1) Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; et al. Epidermal Electronics. Science 2011, 333, 838−843. (2) Thanh, Q. N.; Jeong, H.; Kim, J.; Kevek, J. W.; Ahn, Y. H.; Lee, S.; Minot, E. D.; Park, J. Y. Transfer-Printing of As-Fabricated Carbon Nanotube Devices onto Various Substrates. Adv. Mater. 2012, 24, 4499−4504. (3) Lai, Y. C.; Hsu, F. C.; Chen, J. Y.; He, J. H.; Chang, T. C.; Hsieh, Y. P.; Lin, T. Y.; Yang, Y. J.; Chen, Y. F. Transferable and Flexible Label-Like Macromolecular Memory on Arbitrary Substrates with High Performance and a Facile Methodology. Adv. Mater. 2013, 25, 2733−2739. (4) Georgiou, T.; Jalil, R.; Belle, B. D.; Britnell, L.; Gorbachev, R. V.; Morozov, S. V.; Kim, Y. J.; Gholinia, A.; Haigh, S. J.; Makarovsky, O.; Eaves, L.; Ponomarenko, L. A.; Geim, A. K.; Novoselov, K. S.; Mishchenko, A. Vertical Field-Effect Transistor Based on GrapheneWS2 Heterostructures for Flexible and Transparent Electronics. Nat. Nanotechnol. 2013, 8, 100−103. (5) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Room-Temperature Fabrication of Transparent Flexible Thin-Film Transistors Using Amorphous Oxide Semiconductors. Nature 2004, 432, 488−492. (6) Kim, S. M.; Song, E. B.; Lee, S.; Zhu, J. F.; Seo, D. H.; Mecklenburg, M.; Seo, S.; Wang, K. L. Transparent and Flexible Graphene Charge-Trap Memory. ACS Nano 2012, 6, 7879−7884. (7) Zheng, Z.; Gan, L.; Li, H. Q.; Ma, Y.; Bando, Y.; Golberg, D.; Zhai, T. Y. A Fully Transparent and Flexible Ultraviolet-Visible Photodetector Based on Controlled Electrospun ZnO-CdO Heterojunction Nanofiber Arrays. Adv. Funct. Mater. 2015, 25, 5885−5894. (8) Wang, J. X.; Yan, C. Y.; Lin, M. F.; Tsukagoshi, K.; Lee, P. S. Solution-Assembled Nanowires for High Performance Flexible and 1717

DOI: 10.1021/acsnano.6b07577 ACS Nano 2017, 11, 1712−1718

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DOI: 10.1021/acsnano.6b07577 ACS Nano 2017, 11, 1712−1718