Comment pubs.acs.org/JPCC
Comment on “Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices” Ilia Valov* and Rainer Waser Peter Gruenberg Institute (Electronic materials), Research Centre Juelich, 52425 Juelich, Germany
J. Phys. Chem. C 2012, 116 (33), 17955−17959. DOI: 10.1021/jp305482c. J. Phys. Chem. C 2013, DOI: 10.1021/jp401828m. way to the filament (Figure 1b) the filament can only grow in the direction of the Pt electrode but cannot dissolve/oxidize. In Figure 1c is shown the situation of a bipolar electrode21 where a Cu particle is placed between the two electrodes. The particle can either initially exist there (as a deposited state) or can appear later on due to segregation or reduction within the electrolyte. At the side of the particle facing the Cu anode a reduction process will take place, and at the side facing the cathode (Pt) oxidation will proceed. Thus, the Cu particle should either grow toward the Cu electrode (if the rate of reduction is much higher than the oxidation rate) or effectively “move” in the field gradient in a direction toward the Cu anode. A verification of the so-called “electrochemical fish” with a solid electrolyte has been experimentally demonstrated with the AgBr system.22 In all cases the oxidation must take place at the anode and reduction at the cathode. Therefore, growth of metallic filament from the anode toward the cathode or dissolution from the cathode is physically not possible. A similar wrong explanation has been recently proposed also by other authors16 believing that different mobilities of cations and electrons can explain such reversed growth.17,18 It is important to note that the mobilities of the ions are not related in any way to their deposition/oxidation (redox) potential. The charge mobility will only influence the rate of the reaction, i.e., the amount of oxidized/reduced ions. The Faradaic current represents the reaction rate (in the sense of heterogeneous reaction rate theory), and therefore, positive current necessarily implies oxidation and negative current implies reduction. Moreover, at the tip of the filament the electric field will be the strongest and respectively the current density the highest. This simple fact is widely used for electrochemical (anodic) polishing, i.e., removing of filaments to ensure high-quality, low roughness surfaces. The conditions described by Gao et al. should not be mixed with the situation provided by the atomic switch.8 The reduction of Ag (or Cu) as discussed in a gap-type atomic switch case occurs at the cathode; i.e., the STM tip and the formed metallic cluster lie at the same (cathodic) potential. The tunnel gap is regarded as an extended Helmholtz layer.8 The conditions described by Gao et al. are completely different; i.e., the filament lies at strongly anodic potential. Under applied anodic voltage the electrochemical potential of electrons or their Fermi energy (μ̃e EF = −eφ) increases
R
edox-based resistive switching memories (ReRAMs) are currently considered as the most promising technology for next generation nonvolatile memories based on an abrupt change of the resistance of the two-electrode electrochemical cell.1−4 Moreover, these cells were found to demonstrate an ability for multilevel switching and scalability down to an atomic level5 and also find application for computing,4 memristive devices,6 and artificial neuromorphic networks.3,7 Despite the huge interest from both academic and industrial communities, at the microscopic level the physicochemical processes responsible for the device operation still remain under debate. The ReRAM cells represent nanoscale electrochemical systems where depending on the particular processes responsible for the resistance state transition the following classification has been introduced: ECM (electrochemical metallization memory), VCM (valence change memories), TCM (thermochemical memeory), etc.1 The ECM type cells used an active electrode, e.g., Ag or Cu, and an inert counter electrode, e.g., Pt. Electrolyte films having a variety of inorganic and organic materials with properties ranging from superionic electrolytes, e.g., RbAg4I5,8 to typical insulators, e.g., Ta2O59 or SiO2,10 have been used, all showing similar switching behavior.11 To explain the switching kinetics and reveal underlying physicochemical processes, several mechanisms have been proposed.8,9,12−18 In a recent publication in the Journal of Physical Chemistry C, Gao et al.19 reported on dynamic processes in Cu/poly (3hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester/ indium−tin oxide (ITO) structured bipolar switching memory cells and suggested a mechanism for filament growth and dissolution. However, the proposed processes and direction for the electrochemical growth and dissolution are violating the laws of physical chemistry and electrochemistry. In Figure 5 of their manuscript the authors suggested that the filament starts growing from the anode (positively charged) despite that Figure 5b clearly shows the mechanical and electrical connection between the Cu electrode and the filament. The dissolution (oxidation) of the filament is stated by the authors to proceed at the cathode (negatively charged). In Figure 1 we show schematically why such a mechanism is physically not possible. As shown in Figure 1a, if the Cu filament (irrespective of its dimensions) is mechanically (and therefore electrically) connected to the Cu anode the electrode reaction at the filament/electrolyte interface must be oxidation; i.e., only dissolution is possible but no deposition/reduction. In the case of applying a negative potential to the Cu electrode and in this © XXXX American Chemical Society
Received: February 8, 2013 Revised: April 5, 2013
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dx.doi.org/10.1021/jp4014306 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Comment
Figure 1. Distribution of the electrical potential in ECM type cells. For anodic polarization (a) the applied potential Δφ drops entirely within the electrolyte film, and the filament has the same potential (polarity) as the Cu electrode. In (b) a cathodic potential difference Δφ is applied to the Cu electrode. In (c)n the situation of the “bipolar electrode” is show. The figure is adapted with permission from ref 20.
from the anode to the cathode (see for a reference, e.g., the φprofile in Figure 1a). If the electrons’ energy is sufficient to reduce Cuz+ ions at an arbitrary distance x from the cathode, then for every distance y < x follows that (EF)x < (EF)y
