www.advelectronicmat.de
An All-Inorganic, Transparent, Flexible, and Nonvolatile Resistive Memory
Yuxi Yang, Wenxiu Gao, Zhongshuai Xie, Yaojin Wang, Guoliang Yuan,* and Jun-Ming Liu*
DOI: 10.1002/aelm.201800412
electronic equipment.[7] Among them, the memory chip, as an important com- ponent, should also be transparent and flexible to accommodate the development.
As the promising candidate for the next- generation nonvolatile memory (NVM), the resistive-switching random access memory (ReRAM) offers many techno- logical advantages, such as simple device configuration, fast operation speed, high endurance, excellent scalability, small size, and low-power consumption.[8–11]
Hence, attractions on flexible and trans- parent ReRAM are increasing in recent years.[12–16]
In order to achieve high-performance transparent and flexible characteristics of ReRAM, the functional layers are com- monly organic materials or binary oxides that are prepared near room tempera- ture.[4,17–21] These functional materials are compatible well with the transparent and flexible polymer-based substrates (e.g., polyimide (PI), polyethylene tere- phthalate (PET), polydimethylsiloxane (PDMS), and polyphthalamide (PPA)) and transparent con- ducting films (indium–tin oxides (ITO), Al-doped ZnO (AZO), carbon nanotube (CNT), and some organic con- ducting polymers), which cannot endure safely at >500 °C and oxygen ambience. Previously, Lee and co-workers used polyimide:6-phenyl-C61 butyric acid methyl ester to fabricate the ReRAM, which has the high/low resistance ratio of over 106 and can be maintained over 104 s without notable sub- stantial fluctuation under bending and stretching cycling.[22]
Nagareddy et al. employed hybrid graphene oxide–titanium oxide to fabricate the ReRAM, which has ultrafast program- ming speed (60 ns).[21]
In spite of the extensive studies, it is still a big challenge to directly prepare the transparent, flexible and high-quality memories based on perovskite oxide films (POFs), which gen- erally need to be prepared at oxygen ambience and >500 °C.
Many POFs (e.g., BaTiO3, SrTiO3, and SrZrO3) simultaneously exhibit extraordinary resistive-switching characteristic, favorable thermal stability, outstanding radioresistance, and transpar- ency.[23–25] In order to achieve transparent and flexible ReRAMs, researchers usually grew POFs on rigid substrates (i.e., Si or SrTiO3), peeled off the POFs from substrates, and then trans- ferred them onto flexible organic ones.[3,26] The significant advantage of the “grow-transfer” method is that the transferred layer remains the same in composition and structure.[27,28]
A rapid surge in the research of lightweight, invisible, and flexible
electronics is occurring with the arrival of Internet of Things (IoT). However, multifunctional perovskite oxide electronics are commonly hard and should be synthesized at high temperature and oxygen ambience, where most transparent conductive films will become brittle or highly resistive. Thus, the realization of transparent and flexible nonvolatile perovskite oxide resistive memory remains a big challenge. Here, a transparent, flexible, nonvolatile, and all-inorganic memory with the mica substrate is prepared: the 2.7 wt%
Ag-doped indium–tin oxide (Ag–ITO) film as bottom electrodes, the
BaTi0.95Co0.05O3−δ (BTCO) film as resistive-switching functional layer, and the Ag/ITO films as top electrodes. The Ag–ITO/BTCO/Ag/ITO heterosturcture shows a unipolar resistive-switching behavior, and its high/low resistance ratio is up to 5 × 103 under a low operating voltage (<2.8 V) with fast response speed (≈50 ns). Either high- or low-resistance states remain stable even after the 14 400 cycles’ dynamic bending test with the minimum radius of 3 mm. Additionally, the bipolar resistive-switching characteristic is observed in the ≈85 nm diameter region. As a result, such resistive memory shows potential to be used in many transparent and flexible devices, such as electronic skins and flexible displays.
Y. X. Yang, Dr. W. X. Gao, Z. S. Xie, Prof. Y. J. Wang, Prof. G. L. Yuan School of Materials Science and Engineering
Nanjing University of Science and Technology Nanjing 210094, P. R. China
E-mail: yuanguoliang@njust.edu.cn Prof. J.-M. Liu
National Laboratory of Solid State Microstructures Nanjing University
Nanjing 210093, P. R. China E-mail: liujm@nju.edu.cn Prof. J.-M. Liu
South China Academy of Advanced Optoelectronics South China Normal University
Guangzhou 510006, Guangdong, P. R. China Resistive-Switching Memory
1. Introduction
With the rapid development of the Internet of Things (IoT) and the increasing concerns about bio-electronic devices, embedded and wearable electronics such as radio frequency identification tags, electronic skins, and flexible displays have gained a tremendous interest in recent years.[1–6] Compared with the traditional electronic products, these emergent elec- tronics require to be more conformal, lightweight, and invis- ible for better integration on bodies, boxes, windows, and other
However, such a multistep technique suffers from size limitation and the weak contact strength.[6,27] Consequently, it is extremely essential to develop a one-step and transfer- free method to fabricate the transparent and flexible nonvolatile resistive memory. There- fore, the substrates and bottom electrodes should not only be transparent and flexible but also have high-temperature stability during the POFs growth. First, fluorophlo- gopite mica (AlF2O10Si33Mg) is a silicate crystal with layered structure; it has excellent thermal stability (up to 1200 °C) and high transmittance (over 90%).[29–32] With regard to the bottom electrodes, ITO, AZO, graphene oxide, carbon nanotube, and other organic conducting polymers are well-known trans- parent conducting films (TCFs), but they will become brittle and/or highly resistive after postannealing at >500 °C.[18,33,34] As a result, it is crucial to improve qualities of the original TCF or develop a new TCF that is compatible with the growth condition of POFs.
Here, a transparent, flexible, transfer-free, and nonvolatile resistive memory is achieved through successive deposition of 2.7 wt%
Ag-doped ITO (Ag–ITO), BaTi0.95Co0.05O3−δ (BTCO), and Ag/ITO films on mica substrate.
Such a memory shows the comprehensive resistive-switching characteristics that sus- tain well during the 14 400 bending cycles with 3 mm radius.
2. Results and Discussion
Transparent, flexible, and large-scale Ag–ITO/BTCO/Ag/ITO heterostructural films were deposited on the 2D fluorophlogo- pite mica by pulsed laser deposition (PLD). Fluorophlogopite mica is reduced to 50 µm via mechanical exfoliation, and the fresh surface is nanoscale roughness (Figure S1, Supporting Information). The X-ray diffraction (XRD) patterns of mica, Ag–ITO, and BTCO films on mica substrate are shown in Figure S2 (Supporting Information). Besides the diffraction peaks of mica crystal and ITO film, the result reveals that BTCO thin film exhibits a clear polycrystallized pure perovskite struc- ture with obvious diffraction peaks of (1 0 0) and (1 1 0). An average surface roughness (Ra = 1.03 nm) of the BTCO film has been determined according to the surface morphology observed by atomic force microscopy (AFM), as shown in Figure 1a. The nano meter-scale smooth and uniform surface is favorable for the stability of electrical performance at different positions. The cross-sectional transmission electron microscopy (TEM) image of the ReRAM displays sequentially stacked layers of Ag–ITO (140 nm) and BTCO (45 nm) with clear interface and robust bonding, as shown in Figure 1b. Both Ag–ITO and BTCO layers are of compaction without notable pinholes, probably resulting in good mechanical stability. The content of Ag in Ag–ITO film is about 2.7 wt% according to its energy spectrum analysis
(EDX) in Figure S3 (Supporting Information). Meanwhile, the high-angle annular dark field (HAADF) shows that Ag element is uniformly distributed in the Ag–ITO electrode due to the high diffusion at the growing process (Figure S4, Supporting Information). Besides the homogeneity and compactation of interface, the doping of Ag component makes the bottom elec- trode have better mechanical properties relative to the pure ITO film (Figure S5, Supporting Information).[18,35]
Figure 1c,d shows the structure of mica/Ag–ITO/BTCO/Ag/
ITO and the transparent and flexible ReRAM, respectively. The bandgap of BaTi0.95Co0.05O3−δ is ≈3.2 eV which means most vis- ible lights can go through,[36] without notably attenuation of transparency of the ReRAM. The transmittances of the pure mica (50 µm thick) substrate and the mica/Ag–ITO/BTCO/
Ag/ITO are over 90% and 75%, respectively, in the visible light range, as shown in Figure 1e. This transmittance feature of the proposed ReRAM provides possibility for the storage device to be directly integrated into the flexible displays.
The mica/Ag–ITO/BTCO/Ag/ITO-based ReRAM shows unipolar resistive-switching (URS) characteristics with an operating voltage below 3 V (Figure 2a). The composition of BaTi0.95Co0.05O3−δ was employed via doping 5 at% Co ele- ment (Co2+ or Co3+) into classic BaTiO3 ferroelectric insu- lator, because such modified film is able to form conducting filaments under a relatively low electroforming voltage of 2.3 V (Forming process), that is much smaller than its coercive Figure 1. a) AFM surface morphology and b) the cross-sectional TEM image of mica/Ag–ITO/
BTCO. c) Schematic illustration and d) photograph of mica/Ag–ITO/BTCO/Ag/ITO. e) Optical transmittance of mica substrate and mica/Ag–ITO/BTCO/Ag/ITO.
voltage (Figure S6, Supporting Information).[37] With increasing voltage from 0 to 1 V, the current rapidly increases and then suddenly decreases at 0.5 V; thus, the low-resistance state (LRS or On) shifts to high-resistance state (HRS or Off) (reset proce- dure). After that, during the voltage sweeping from 0 to 2 V, the current reaches the compliance current (Icc= 1 mA) of 1 mA at 1.2 V and then the devices switching from HRS into LRS (set procedure) again. In contrast to other ReRAMs based on POFs, the small operating voltage is mainly attributed to the 5 at% Co dopant, the high electrochemical activity of Ag, and the 45 nm thickness of the BTCO film.[38–40] Furthermore, electric switching endurance between HRS and LRS is exceedingly cru- cial to determine the reproducibility and reliability of ReRAM devices. Figure 2b shows the switching endurance property using the test pulse sequence for the switching endurance measurement (Figure S7, Supporting Information). The resist- ances of HRS and LRS at 0.1 V are defined as RHRS and RLRS, respectively. The typical resistance ratio, RHRS/RLRS (Off/On), is higher than 5 × 103 during the 2500 switching cycles, and is still over 4 × 103 after 15 days retention time (the dashed lines show the extrapolated data in 10 years), which is an important figure of merit for data storage, as shown in Figure 2c. In addi- tion, the resistance of cell can switch normally between HRS and LRS even at a high temperature of 120 °C or after being annealed at 500 °C (Figure S8, Supporting Information).
The mica/Ag–ITO/BTCO/Ag/ITO ReRAM can switch between HRS and LRS with 50 ns pulse voltage via formation/
rupture of Ag conductive filament. For a ReRAM, the pro- gramming speed is also one of the key factors that need to be addressed apart from operating voltage and RHRS/RLRS,
and determines the operating speed of the electronic product to a certain degree.[17] We use a signal generator to emit the voltage pulse for testing programming speed, by applying the first voltage pulse (2.8 V, ≈50 ns) to achieve the set procedure (Figure 2d) and the second pulse (1.2 V, ≈50 ns) to achieve the reset procedure (Figure 2e). The circuit diagrams are shown in Figure S9 (Supporting Information), and the Icc is regulated by changing the fixed resistance of the external circuit. As we can see from Figure 2f, the HRS transformed to the LRS when the 2.8 V pulse applied to the cell. As Ag is a reactive metal in a high potential state, it loses elec- trons and becomes Ag+ in the set process.[41]
Because Ag+ mobility is lower than that of free electrons injected from bottom electrode, Ag+ may capture the free electrons after traveling a short distance in the BTCO layer and be reduced back to Ag atoms before reaching the cathode, as schematically illus- trated in the inset of Figure 2f.[13,42] The LRS parts of the I–V curves show metal character- istics and fit well with I ∝V, which suggests the existence of Ag conductive filaments at LRS (Figure S10, Supporting Information).
After that, the 1.2 V pulse is used to trigger the reset procedure so that the LRS changes to the HRS. In this process, the current greatly increases without the limitation of Icc and finally its Joule heat is large enough to rupture the conducting filaments.[43,44] Both set and reset processes (i.e., write and erase) can be finished within
≈50 ns, so the ReRAM has significant advantages with a faster program speed in contrast to microsecond write/erase speed of flash memory.[45]
The resistive-switching characteristic of the ReRAM at the 2.2 mm bending radius is nearly the same as those of the original flat status. In order to validate memory devices for practical applications, such as wearable devices and flexible display screens, in situ resistive-switching measurements were performed with our flexible memories rolled on a 2.2 mm radius refill (Figure 3a). No obvious change was observed in the I–V curves of the cells under bent state with 2.2 mm radius relative to the flat state (Figure 3b). In principle, the thinner the mica is, the better flexibility the ReRAM will be (Figure S11, Supporting Information).[12] In fact, the bending strain of the Ag–ITO/BTCO/Ag/ITO film is just about 0.5% mainly because its thickness is much smaller than the 10 µm thickness (≈10 µm) of mica substrate. As a result, the identical proper- ties under bent and flat states are probably due to such a slight strain that is too small to change the electrical switching per- formances of mica/Ag–ITO/BTCO/Ag/ITO ReRAM. Figure 3c shows the dependences of RHRS and RLRS on retention time of the 2.2 mm radius bending memory; the RHRS and RLRS show excellent stability with retention time. Figure 3d shows the dependences of RHRS and RLRS on writing/erasing cycles for the identical bending memory, and RHRS/RLRS is still over 103 after the 5000 writing/erasing cycles. Furthermore, such an Figure 2. a) Typical I–V curves of the device showing unipolar resistive-switching behavior.
Dependences of HRS and LRS on b) set–reset cycles and c) retention time. Pulse voltage for d) set and e) reset processes, respectively. f) Transition from HRS to LRS by the set process and from LRS to HRS by the reset process, where the insets show the formation and rupture of conductive filament.
ReRAM also remains stable during the dynamic bending pro- cess with the 3 mm bending radius, and it can switch normally between HRS and LRS after the retentive test. The platform for testing the dynamic resistive-switching performance is shown in Figure S12 (Supporting Information), and such a testing pro- cess is shown in Movie S1 (Supporting Information). During the dynamic bending process, the RHRS and RLRS remain stable, as shown in Figure 3e. In particular, I–V curves are measured again after the cell was bent for 14 400 times and then reat- tached to the 2.2 mm radius refill (Figure 3f). The results show that the resistive-switching performance can maintain well even after repeated bending.
Furthermore, the memories with 3 × 3 cross-bar array configuration are fabricated, and each cell has typical uni- polar resistive-switching behaviors (Figure S13, Supporting Information). Most importantly, the heterostructural memo- ries are also promising to be prepared through stacking these micrometer-scale thick memories one by one, though the sneak
path problem should be considered in this process.[46]
The bipolar resistive-switching (BRS) char- acteristics of the transparent and flexible ReRAM are performed in a 85 nm diameter regions, which is potential for high-density storage. Figure 4a,b shows the morphology image and the corresponding current image of the ≈2 nm thickness BTCO film, which was measured by conducting AFM (C-AFM), respectively. Here the original BTCO film is at HRS, and the current ranges from −2.6 to 2.6 pA at 0.2 V bias. Then I–V curves were measured at the separated 12 points marked with red dots in Figure 4b. When the 85 nm diameter C-AFM tip touches to the BTCO film and the mica/Ag–ITO/BTCO/tip is formed, HRS can switch to LRS at a 0–3 V ramping voltage (set process) and then LRS can switch back to HRS at a 0 to −3 V ramping voltage (reset process). Further- more, the variation from these points was small according to the standard deviation from the error bars in Figure 4c. Meanwhile, the I–V curves of one of the points were measured for 12 times, and the performance is also stable (Figure S14, Supporting Infor- mation). The resistive-switching results in the 85 nm diameter regions show a possi- bility for subsequent high-density storage. In addition, we also observed the BRS charac- teristics in the mica/Ag–ITO/BTCO/Ag/ITO heterostructure with the 100 µm diameter top electrode, as shown in Figure 4d. Both the ion-migration-induced redox reaction and Joule heat were the reason for the reset process of BRS. During the reset process of BRS triggered by an opposite voltage, Ag atoms in conductive filaments lose electrons and become Ag+, and the Ag+ moves back to Ag electrode, leading to the dissolution of conductive filaments, so the resistive memory switches back to HRS. Meanwhile, the current density is huge in conduc- tive filaments region, so Joule heat arises near the conductive filament region that results in accelerated redox reaction. In contrast, Joule heat led directly to the rupture of conductive fila- ment in URS.[43] It is not difficult to switch between BRS and URS, which can realize by adjusting the voltage directions. It is worthy to note that these two states are not contradictory even in the same structure, as previously reported.[47–50]
Table 1 summarizes the properties of the presented ReRAM and other NVMs reported in the past few years.[26,51–57] The PbZr0.48Ti0.52O3 (PZT) ferroelectric NVMs on ultrathin silicon substrate have a monolithic integration advantage over other flexible substrate, but the silicon substrate is not transparent.[51,52] Most flexible NVMs are either fully organic or have organic substrate, which commonly show excellent bending properties but poor temperature stability.[26,53–57]
Here the inorganic mica/Ag–ITO/BTCO/Ag/ITO ReRAM is Figure 3. Performance of the 2.2 mm radius bending mica/Ag–ITO/BTCO/Ag/ITO memories.
a) Photograph of the memories rolled on a 2.2 mm radius refill. b) I–V curves of the first bending memories. Dependences of HRS and LRS on c) retention time and d) set-to-reset cycles of the bending memories. e) Dependences of HRS and LRS on bending cycles/time of the dynamic bending process, where the insets show the bending-to-flattening process. f) I–V curves after the memories undergo the 3 mm radius bending for 14 400 times.
not only transparent and flexible but also shows good tem- perature stability. Most importantly, the ReRAM in this work exhibits outstanding memory performances, such as fast operation speed, long retention time, and small operation voltage. Thus, it is promising to be used in many transparent and flexible electronic devices, such as electronic skins and flexible displays.
3. Conclusions
In summary, we have successfully fabri- cated flexible, transparent, and large-scale mica/Ag–ITO/BTCO/Ag/ITO memory with a high performance of unipolar resistive switching based on a one-step method. The memory device shows a small operation voltage of <3 V and the RHRS/RLRS ratio of over 5000. Both the set and reset processes can be completed within 50 ns. Furthermore, the memory can still work properly when it endured the set-to-reset cycles for 5000 times at a 2.2 mm bending radius or after 14 400 bending cycles with 3 mm minimum radius. The transparent, flexible, and high-quality memory is promising to be applied in flexible screen, electronic skins, and other flexible electronics.
4. Experimental Section
The 50 µm fluorophlogopite mica (Changchun Taiyuan Co., China) substrates were separated by mechanical exfoliation, then Ag–ITO/BTCO/
Ag/ITO films were grown on it by PLD with a KrF excimer laser (248 nm wavelength). Here, the homogeneous Ag–ITO film as the bottom electrode was grown by sputtering the Ag target and ITO (SnO2:In2O3 = 1:9) target alternately with 110 mJ per laser pulse for 375 and 1600 laser pulses for four cycles at 680 °C and 1 Pa oxygen pressure.
The BTCO resistance switching layer was grown at 680 °C and 0.5 Pa oxygen pressure with 80 mJ per laser pulse before it was annealed at 600 °C and 1000 Pa oxygen pressure. For top electrodes with the 100 µm diameter, the ≈5 nm thickness Ag film was grown at 25 °C and 10−4 Pa, and the ≈70 nm thickness ITO film was grown at 300 °C and 1.3 Pa oxygen pressure with a steel mask. Finally, mica substrate was thinned Figure 4. a) Schematic illustration of the C-AFM test and the morphology image of the BTCO
film. b) Current image of mica/Ag–ITO/BTCO. c) I–V curves measured by C-AFM with an 85 nm diameter conducting tip at the red dot positions of panel (b), where the error bars present the standard deviation of point-to-point variations. d) I–V curves of mica/Ag–ITO/
BTCO/Ag/ITO, where the Ag/ITO top electrode has a 100 µm diameter.
Table 1. Eight nonvolatile memories studied extensively.
Reference Ghoneim et al.[51,52] Bakaul et al.[26] Liu et al.[53] Guan et al.[54] Lin et al.[55] Qian et al.[56] Wu et al.[57] This work
Type FRAMa) FRAM ReRAM ReRAM ReRAM ReRAM ReRAM ReRAM
Substrate Ultrathin silicon PETb) PET PET PET PDMSc) PId) Mica
Functional materials PZT PZT CsPbBr3 MAPbBr3e) Fe3O4 hBNf) IGZOg) BTCO
Transferh) No Yes No No Yes Yes No No
Transparent No No No No No Yes No Yes
Voltage [V] 15 4 3 1.5 3 1.5 2 2.8
RHRS/RLRS ratio – – 100 1000 4 × 105 480 10 5000
Programming speed 100 µs 57 ns – – 50 ns – 500 µs 50 ns
Working temperature [°C] >300 – 25 25 25 25 85 120
W/E cycles [N]i) 109 1010 50 1000 300 500 106 5000
Retention [s] 105 105 – 104 104 5 × 104 104 >106
Flexibility Yes Yes Yes Yes Yes Yes Yes Yes
Bending radius [mm] 5 10 ≈10 6 15 14 10 2.2
Bending cycles [N] 1000 100 100 200 10 000 800 1000 14 400
a)Ferroelectric random access memory; b)Polyethylene terephthalate; c)Polydimethylsiloxane; d)Polyimide; e)CH3NH3PbBr3; f)Hexagonal boron nitride; g)Indium–gallium–
zinc oxide; h)Transferring film to flexible substrate; i)Writing/erasing cycles.
to ≈10 µm in thickness by mechanical exfoliation before further bending experiments.
The crystal structures of the Ag–ITO/BTCO films were characterized by XRD (Brucker D8), and its cross-sectional image was observed by TEM (FEI Tecnai G2 F20 S-Twin). The surface morphology and C-AFM results were characterized by atomic force microscopy (Multimode 8, Bruker). The transmittance spectra of mica, mica/ITO, mica/Ag–ITO, and mica/Ag–ITO/BTCO/Ag/ITO were characterized using a UV–vis spectrometer (Shimadzu Co.). The resistive-switching properties of mica/Ag–ITO/BTCO/Ag/ITO were characterized by using a multimeters (Keithley 2635a). The periodic motions of a shaker (SHIAO, JZK-10) were produced by a signal generator (Rigoal, GD1022a) via a power amplifier (GSTYE 5872A).
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Grant Nos. 51790492, 51431006, and 51721001) and the National Key Research Program of China (2016YFA0300101). Besides, G.L.Y. was also supported by the Fundamental Research Funds for the Central Universities (Grant No. 30916011104). Y.X.Y. acknowledges the support from Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0351).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
all-inorganic, flexible, perovskite oxide, resistive-switching memory, transparent
Received: June 28, 2018 Revised: August 6, 2018 Published online:
[1] J. Deng, X. Kuang, R. Liu, W. Ding, A. C. Wang, Y. C. Lai, K. Dong, Z. Wen, Y. Wang, L. Wang, H. J. Qi, T. Zhang, Z. L. Wang, Adv. Mater.
2018, 30, 1705918.
[2] J. Li, Q. Liu, D. Ho, S. Zhao, S. Wu, L. Ling, F. Han, X. Wu, G. Zhang, R. Sun, C. P. Wong, ACS. Appl. Mater. Interfaces 2018, 10, 9727.
[3] S. Kim, J. H. Son, S. H. Lee, B. K. You, K. I. Park, H. K. Lee, M. Byun, K. J. Lee, Adv. Mater. 2014, 26, 7480.
[4] K. L. Kim, W. Lee, S. K. Hwang, S. H. Joo, S. M. Cho, G. Song, S. H. Cho, B. Jeong, I. Hwang, J. H. Ahn, Y. J. Yu, T. J. Shin, S. K. Kwak, S. J. Kang, C. Park, Nano Lett. 2016, 16, 334.
[5] J. Zhong, Y. Zhang, Q. Zhong, Q. Hu, B. Hu, Z. L. Wang, J. Zhou, ACS Nano 2014, 8, 6273.
[6] D. Wang, G. Yuan, G. Hao, Y. Wang, Nano Energy 2018, 43, 351.
[7] P. Barquinha, R. Martins, L. Pereira, E. Fortunato, Transparent Oxide Electronics: From Materials to Devices, John Wiley & Sons, Ltd, Chichester, UK 2012.
[8] T. Tsuruoka, K. Terabe, T. Hasegawa, M. Aono, Nanotechnology 2010, 21, 425205.
[9] J. H. Hur, M. J. Lee, C. B. Lee, Y. B. Kim, C. J. Kim, Phys. Rev. B.
2010, 82, 155321.
[10] D. H. Kwon, K. M. Kim, J. H. Jang, J. M. Jeon, M. H. Lee, G. H. Kim, X. S. Li, G. S. Park, B. Lee, S. Han, M. Kim, C. S. Hwang, Nat. Nano- technol. 2010, 5, 148.
[11] B. Cho, J.-M. Yun, S. Song, Y. Ji, D.-Y. Kim, T. Lee, Adv. Funct. Mater.
2011, 21, 3976.
[12] Y. Yang, G. Yuan, Z. Yan, Y. Wang, X. Lu, J. M. Liu, Adv. Mater. 2017, 29, 1700425.
[13] K. Qian, R. Y. Tay, V. C. Nguyen, J. Wang, G. Cai, T. Chen, E. H. T. Teo, P. S. Lee, Adv. Funct. Mater. 2016, 26, 2176.
[14] H. Tian, H. Y. Chen, T. L. Ren, C. Li, Q. T. Xue, M. A. Mohammad, C. Wu, Y. Yang, H. S. Wong, Nano Lett. 2014, 14, 3214.
[15] K. Qian, R. Y. Tay, M. F. Lin, J. Chen, H. Li, J. Lin, J. Wang, G. Cai, V. C. Nguyen, E. H. Teo, T. Chen, P. S. Lee, ACS Nano 2017, 11, 1712.
[16] M. T. Ghoneim, M. M. Hussain, Electronics 2015, 4, 424.
[17] L. Zhou, J. Mao, Y. Ren, S.-T. Han, V. A. L. Roy, Y. Zhou, Small 2018, 14, 1703126.
[18] J. Shang, W. Xue, Z. Ji, G. Liu, X. Niu, X. Yi, L. Pan, Q. Zhan, X. H. Xu, R. W. Li, Nanoscale 2017, 9, 7037.
[19] R. H. Kim, H. J. Kim, I. Bae, S. K. Hwang, D. B. Velusamy, S. M. Cho, K. Takaishi, T. Muto, D. Hashizume, M. Uchiyama, P. Andre, F. Mathevet, B. Heinrich, T. Aoyama, D. E. Kim, H. Lee, J. C. Ribierre, C. Park, Nat. Commun. 2014, 5, 3583.
[20] X. F. Cheng, X. Hou, J. Zhou, B. J. Gao, J. H. He, H. Li, Q. F. Xu, N. J. Li, D. Y. Chen, J. M. Lu, Small 2018, 14, 1703667.
[21] V. K. Nagareddy, M. D. Barnes, F. Zipoli, K. T. Lai, A. M. Alexeev, M. F. Craciun, C. D. Wright, ACS Nano 2017, 11, 3010.
[22] Y. Ji, S. Lee, B. Cho, S. Song, T. Lee, ACS Nano 2011, 5, 5995.
[23] S. Li, H. Z. Zeng, S. Y. Zhang, X. H. Wei, Appl. Phys. Lett. 2013, 102, 153506.
[24] Z. Miao, L. Chen, F. Zhou, Q. Wang, J. Phys. D: Appl. Phys. 2018, 51, 025107.
[25] M. Janousch, G. I. Meijer, U. Staub, B. Delley, S. F. Karg, B. P. Andreasson, Adv. Mater. 2007, 19, 2232.
[26] S. R. Bakaul, C. R. Serrao, O. Lee, Z. Lu, A. Yadav, C. Carraro, R. Maboudian, R. Ramesh, S. Salahuddin, Adv. Mater. 2017, 29, 1605699.
[27] K.-I. Park, J. H. Son, G.-T. Hwang, C. K. Jeong, J. Ryu, M. Koo, I. Choi, S. H. Lee, M. Byun, Z. L. Wang, K. J. Lee, Adv. Mater. 2014, 26, 2514.
[28] H. Wang, B. Zhu, H. Wang, X. Ma, Y. Hao, X. Chen, Small 2016, 12, 3360.
[29] X. Zhang, Y. He, R. Li, H. Dong, W. Hu, Adv. Mater. 2016, 28, 3755.
[30] Y. Bitla, Y.-H. Chu, FlatChem 2017, 3, 26.
[31] W. Gao, L. You, Y. Wang, G. Yuan, Y.-H. Chu, Z. Liu, J.-M. Liu, Adv. Electron. Mater. 2017, 3, 1600542.
[32] J. Jiang, Y. Bitla, C. W. Huang, T. H. Do, H. J. Liu, Y. H. Hsieh, C. H. Ma, C. Y. Jang, Y. H. Lai, P. W. Chiu, Sci. Adv. 2017, 3, 1700121.
[33] G. Torrisi, I. Crupi, S. Mirabella, A. Terrasi, Sol. Energy Mater. Sol.
Cells 2017, 165, 88.
[34] H. Kang, S. Jung, S. Jeong, G. Kim, K. Lee, Nat. Commun. 2015, 6, 6503.
[35] H.-J. Kim, K.-W. Seo, Y. H. Kim, J. Choi, H.-K. Kim, Appl. Surf. Sci.
2015, 328, 215.
[36] S.-D. Guo, B.-G. Liu, J. Appl. Phys. 2011, 110, 073525.
[37] Z. Yan, Y. Guo, G. Zhang, J. M. Liu, Adv. Mater. 2011, 23, 1351.
[38] Z. C. Wang, J. Miao, P. F. Liu, Y. Ji, F. Shao, K. K. Meng, J. Teng, Y. Wu, X. G. Xu, Y. J. Jiang, J. Mater. Sci.: Mater. Electron. 2017, 28, 12816.
[39] Y. Ahn, J. Y. Son, Mater. Res. Bull. 2017, 85, 255.
[40] Y. Li, S. Long, M. Zhang, Q. Liu, L. Shao, S. Zhang, Y. Wang, Q. Zuo, S. Liu, M. Liu, IEEE Electron Device Lett. 2010, 31, 117.
[41] M. K. Kim, J. S. Lee, ACS Nano 2018, 12, 1680.
[42] E. Yoo, M. Lyu, J.-H. Yun, C. Kang, Y. Choi, L. Wang, J. Mater.
Chem. C. 2016, 4, 7824.
[43] F. Kurnia, C. U. Jung, B. W. Lee, C. Liu, Appl. Phys. Lett. 2015, 107, 073501.
[44] K. M. Kim, T. H. Park, C. S. Hwang, Sci. Rep. 2015, 5, 7844.
[45] A. C. Torrezan, J. P. Strachan, G. Medeiros-Ribeiro, R. S. Williams, Nanotechnology 2011, 22, 485203.
[46] M. A. Zidan, H. A. H. Fahmy, M. M. Hussain, K. N. Salama, Micro- electron. J. 2013, 44, 176.
[47] R. Waser, M. Anon, Nat. Mater. 2007, 6, 833.
[48] S. Z. Rahaman, S. Maikap, W. S. Chen, H. Y. Lee, F. T. Chen, M. J. Kao, M. J. Tsai, Appl. Phys. Lett. 2012, 101, 073106.
[49] L. Liu, S. Zhang, Y. Luo, G. Yuan, J. Liu, J. Yin, Z. J. Liu, J. Appl. Phys.
2012, 111, 104103.
[50] M. Ismail, E. Ahmed, A. M. Rana, I. Talib, M. Y. Nadeem, J. Alloys Compd. 2015, 646, 662.
[51] M. T. Ghoneim, M. M. Hussain, Appl. Phys. Lett. 2015, 107, 052904.
[52] M. T. Ghoneim, M. A. Zidan, M. Y. Alnassar, A. N. Hanna, J. Kosel, K. N. Salama, M. M. Hussain, Adv. Electron. Mater. 2015, 1, 150045.
[53] D. Liu, Q. Lin, Z. Zang, M. Wang, P. Wangyang, X. Tang, M. Zhou, W. Hu, ACS. Appl. Mater. Interfaces 2017, 9, 6171.
[54] X. Guan, W. Hu, M. A. Haque, N. Wei, Z. Liu, A. Chen, T. Wu, Adv. Funct. Mater. 2018, 28, 1704665.
[55] Y. Lin, Y. Xu, Z. Q. Wang, T. Cong, W. Z. Liu, H. L. Ma, Y. C. Liu, Appl. Phys. Lett. 2017, 110, 193503.
[56] K. Qian, R. Y. Tay, M. F. Lin, J. Chen, H. Li, J. Lin, J. Wang, G. Cai, V. C. Nguyen, E. H. T. Teo, T. Chen, P. S. Lee, ACS Nano 2017, 11, 1712.
[57] S. C. Wu, H. T. Feng, M. J. Yu, I. T. Wang, T. H. Hou, IEEE. Electron Device Lett. 2013, 34, 1265.
