Comment pubs.acs.org/JPCC
Reply to “Comment on ‘Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices’” Shuang Gao, Cheng Song,* Chao Chen, Fei Zeng, and Feng Pan* Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
J. Phys. Chem. C 2012, 116 (33), 17955−17959. DOI: 10.1021/jp305482c. J. Phys. Chem. C 2013, 10.1021/jp4014306.
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n our recent publication,1 we attributed the bipolar resistive switching phenomenon in the Cu/poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester/indium−tin oxide (Cu/P3HT:PCBM/ITO in short) structure to the Cu filament, which was proposed to grow from the anode (Cu) to the cathode (ITO) and to rupture initially near the cathode (ITO). The same switching kinetics has also been demonstrated directly by the use of in situ transmission electron microscope (TEM) in Ag/α-Si/Pt,2 Cu(Ag)/ZrO2/Pt,3 and Au/ZnO/Au4 sandwich structures. The switching kinetics in our work is completely opposite to that in memory cells based on traditional solid electrolytes (e.g., Cu-doped GeTe) with extremely high cation mobility.5 This contradictory situation somehow verifies that the filament growth mode is profoundly dependent on cation mobility in the storage media.2−4 Moreover, in our very recent work,6 Ag filaments have been confirmed to nucleate initially at the middle region (neither near the anode nor near the cathode) in the Ag/poly(3,4ethylene-dioxythiophene):poly(styrenesulfonate)/Pt planar device, further supporting the theory that the filament growth mode is dependent on cation mobility. Valov and Waser made a comment, in which they declared that switching kinetics in our work is violating the laws of physical chemistry and electrochemistry.7 In their opinion, if the Cu filament (irrespective of its dimensions) is mechanically (and therefore electrically) connected to the Cu electrode, only dissolution (growth) of the Cu filament is possible in the case that the Cu electrode is positively (negatively) charged. First of all, we strongly agree with Valov and Waser about the basic laws of physical chemistry and electrochemistry described in the comment. Unfortunately, they ignore the core issue in our paper that the organic material adopted has very low cation mobility, which is in sharp contrast to traditional solid electrolytes with extremely high cation mobility. Figure 1 shows a more detailed schematic of the Cu filament growth mode. According to this figure, the Cu ions originate initially from anode dissolution and then move toward the cathode under an external electric field (Figure 1a,b). However, due to very low mobility, the ions can only migrate an extremely short distance before becoming reduced by the oncoming electrons (Figure 1c). The precipitated Cu atoms in the vicinity of the anode share almost the same electrostatic potential as the anode, acting as an extension of the anode. Subsequently, the extended anode tip will be dissolved again and reduced nearby, depicted in Figure 1d,e, thus resulting in the phenomenon that the Cu filament grows from the anode to the cathode. This © XXXX American Chemical Society
Figure 1. Detailed dynamic growth mode of the Cu filament. (a) Cu ions originate initially from anode dissolution. (b) Driven by external electric field, Cu ions migrate toward the cathode. (c) Cu ions are reduced in the vicinity of the anode due to their very low mobility, acting as an extension of the anode. (d,e) The filament tip is ionized again and reduced nearby, leading to the growth of Cu filament from the anode to the cathode.
process means that the growth of metallic filaments is not a simple ″growth″ mode. Instead, it includes growth, dissolution, and the repetition of these two steps. Such a growth mode is reasonable from the viewpoint of kinetics and can not be explained only by the classic laws of physical chemistry and electrochemistry. Our concept is also supported by ref 2 in which the filaments in storage medium are composed of a string of particles, instead of perfect nanowires. We admit that Figure 5b in our recent paper is just a brief illustration of Figure 1 in this response. To directly observe the Cu filament, we fabricated a planar Cu/P3HT:PCBM/Cu device via a two-step ultraviolet lithography process followed by spin-coating. Figure 2a shows the schematic of the planar device and measurement configuration. A scanning electron microscope (SEM) image of the pristine planar device is displayed in Figure 2b. In this figure, one can see that the distance between these two Cu electrodes is about 200 nm. Under positive voltage sweep, a sharp current jump appears at ∼6.3 V (Figure 2c), indicating Received: February 21, 2013 Revised: April 26, 2013
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dx.doi.org/10.1021/jp401828m | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Comment
than the distance between the leftmost Ag dendrite and the cathode because there is clear Ag deposition at the left side of the leftmost Ag dendrite, and therefore oxidation of other Ag dendrites is impossible because their locations are smaller than x. Obviously, this deduction is wrong according to what is shown in that figure. Furthermore, the electrochemical potential of cations decreases from the anode to the cathode, completely opposite to that for electrons. Therefore, it is illogical that they analyze the redox reaction without considering the role of cations involved. In summary, the dynamic growth and rupture processes of the Cu filament have been described with more details in this response. More importantly, the conical Cu filament with its thinnest part near the cathode has been directly observed, according well with the proposed filament growth mode in our recent paper. Therefore, we are more convinced that the Cu filament in our work grows from the anode to the cathode due to very low cation mobility and ruptures initially near the cathode with the help of Joule heat, without violating the laws of physical chemistry and electrochemistry. This concept is not only demonstrated by our experimental results but also greatly supported by in situ TEM observations.2−4
Figure 2. (a) Schematic of the planar Cu/P3HT:PCBM/Cu device and measurement configuration. (b) SEM image of the planar device in a pristine state. (c) I−V curve of the planar device under a positive voltage sweep. The arrows indicate sweep directions. (d) SEM image of the planar device in a LRS. The dashed lines outline the conical conducting filament. The inset shows the Cu element mapping image of the region surrounded by the red rectangle, confirming that the conducting filament is composed of a Cu element.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-10-62781275. E-mail:
[email protected]. edu.cn (C.S). Phone: +86-10-62772907. E-mail: panf@mail. tsinghua.edu.cn (F.P).
that the device is switched into a low resistance state (LRS). The SEM image of the device in LRS is shown in Figure 2d. There exists a conical Cu filament, verified by the Cu elemental mapping image in the inset, with its thinnest part near the cathode, which agrees well with Figure 5c in our recent paper and thus strongly supports the correctness of the proposed filament growth mode. Under negative voltage, the thinnest part of the Cu filament will generate the largest proportion of Joule heat and resultantly exhibit the highest temperature.8 Therefore, this part ruptures first due to thermal-assistant electrochemical dissolution;2,3,8 that is, the Cu filament ruptures initially near the cathode (ITO). Once the thinnest part of the Cu filament has ruptured, the Cu ions generated will migrate toward the bottom of the Cu electrode driven by concentration gradient and external electric field. Hence, the ruptured filament can not grow under negative voltage, which could be attributed to the lack of a Cu ion at the region between the ITO electrode and the tip of the residual filament. Valov and Waser might ignore this fact because there are many Cu ions locating at the region between the Cu filament tip and the Pt electrode (Figure 1b in their comment). Furthermore, with continuous negative voltage, the residual filament will dissolve gradually, which has already been observed directly by in situ TEM in Ag/α-Si/Pt2 and Cu (Ag)/ ZrO2/Pt3 sandwich structures. Besides, Valov and Waser declared in paragraph 6 of their comment that if the electrons’ energy is sufficient to reduce Cu ions at an arbitrary distance x from the cathode, then no oxidation is possible at every distance y < x due to higher electrochemical potential of electrons. Although it sounds plausible, it is untenable and even contradictory to the observed field-driven migration of Ag particles in ref 22 of their comment. Figure 3 in that reference shows a large number of Ag dendrites with clear dissolution at their anodic sides and deposition of silver at their cathodic sides. According to what was declared by Valov and Waser, the distance x must be larger
Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Gao, S.; Song, C.; Chen, C.; Zeng, F.; Pan, F. Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices. J. Phys. Chem. C 2012, 116, 17955−17959. (2) Yang, Y. C.; Gao, P.; Gaba, S.; Chang, T.; Pan, X. Q.; Lu, W. Observation of Conducting Filament Growth in Nanoscale Resistive Memories. Nat. Commun. 2012, 3, 732. (3) Liu, Q.; Sun, J.; Lv, H. B.; Long, S. B.; Yin, K. B.; Wan, N.; Li, Y. T.; Sun, L. T.; Liu, M. Real-Time Observation on Dynamic Growth/ Dissolution of Conductive Filaments in Oxide-Electrolyte-Based ReRAM. Adv. Mater. 2012, 24, 1844−1849. (4) Peng, C. N.; Wang, C. W.; Chan, T. C.; Chang, W. Y.; Wang, Y. C.; Tsai, H. W.; Wu, W. W.; Chen, L. J.; Chueh, Y. L. Resistive Switching of Au/ZnO/Au Resistive Memory: an In Situ Observation of Conductive Bridge Formation. Nanoscale Res. Lett. 2012, 7, 559. (5) Choi, S. J.; Park, G. S.; Kim, K. H.; Cho, S.; Yang, W. Y.; Li, X. S.; Moon, J. H.; Lee, K. J.; Kim, K. In Situ Observation of VoltageInduced Multilevel Resistive Switching in Solid Electrolyte Memory. Adv. Mater. 2011, 23, 3272−3277. (6) Gao, S.; Song, C.; Chen, C.; Zeng, F.; Pan, F. Formation Process of Conducting Filament in Planar Organic Resistive Memory. Appl. Phys. Lett. 2013, 102, 141606. (7) Valov, I.; Waser, R. Comment on “Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices”. J. Phys. Chem. C 2013, DOI: 10.1021/jp4014306. (8) Tsuruoka, T.; Terabe, K.; Hasegawa, T.; Aono, M. Temperature Effects on the Switching Kinetics of a Cu−Ta2O5-Based Atomic Switch. Nanotechnology 2010, 21, 425205.
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dx.doi.org/10.1021/jp401828m | J. Phys. Chem. C XXXX, XXX, XXX−XXX
