FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com Although the kinetics of CF formation/ dissolution is still unclear, it is widely accepted that the CF formation/dissolu- tion is strongly related to the electromigra- tion and electrochemical reaction of anion (i.e., oxygen vacancy) [13–16] or cation (i.e., Cu 2+ , Ag + or Ni 2+ ). [17–22] Generally, RS behavior can be classi- fied as two modes: nonvolatile memory switching (MS) and volatile threshold switching (TS). In the MS mode, both LRS and HRS can be maintained after removing the external voltage, while the LRS in the TS mode will be back to the HRS once the applied voltage is smaller than a critical value. [23–25] To avoid confu- sion with MS, the LRS and HRS in TS are renamed as “TS ON-state” and “TS OFF- state” in this article. The MS device can be used for the non-volatile data storage [1–5] while TS device can be as a selector in series with memory cell to suppress crosstalk effect in the crossbar array. [26–30] Recently, some groups reported that TS and MS can coexist and mutually transform in a single device at suitable external excitation. [23–28] Several models have been proposed to explain this phenomenon, including CF thermal instability, [23] strong electron correlation effect, [24] quantum-wire model, [25] interface barrier modulation, [26] and space charge effect. [27] However, the underlying mechanism of the phenom- enon is still unclear, especially lacking of direct evidences to uncover when and how the two RS modes happen and what is the internal relationship between them. Here, we demonstrate that the TS and MS modes can be modulated in the Ag/SiO 2 /Pt structure by controlling the com- pliance current ( I CC ) in electroforming. We systematically inves- tigate the morphologies, chemical components, and dynamic growth of the CF using scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) analysis. The results confirm that the TS and MS modes correspond to the CF consisting of isolated and continuous Ag nanocrystals, respectively. In addition, by Kelvin probe force microscopy (KPFM) studies, the voltage potential distribution of CF in the ON- and OFF-state further indicate that the TS mode is Direct Observation of Conversion Between Threshold Switching and Memory Switching Induced by Conductive Filament Morphology Haitao Sun, Qi Liu,* Congfei Li, Shibing Long, Hangbing Lv, Chong Bi, Zongliang Huo, Ling Li, and Ming Liu* Volatile threshold switching (TS) and non-volatile memory switching (MS) are two typical resistive switching (RS) phenomena in oxides, which could form the basis for memory, analog circuits, and neuromorphic applications. Interestingly, TS and MS can be coexistent and converted in a single device under the suitable external excitation. However, the origin of the transition from TS to MS is still unclear due to the lack of direct experimental evidence. Here, conversion between TS and MS induced by conductive filament (CF) morphology in Ag/SiO 2 /Pt device is directly observed using scanning electron microscopy and high-resolution transmission electron microscopy. The MS mechanism is related to the formation and dissolution of CF consisting of continuous Ag nanocrystals. The TS originates from discontinuous CF with isolated Ag nanocrystals. The results of current–voltage fitting and Kelvin probe force microscopy further indicate that the TS mechanism is related to the modulation of the tunneling barrier between Ag nanocrystals in CF. This work provides clearly experimental evidence to deepen understanding of the mechanism for RS in oxide-electrolyte-based resistive switching memory, contributing to better control of the two RS behaviors to establish high-perfor- mance emerging devices. DOI: 10.1002/adfm.201401304 H. Sun, Dr. Q. Liu, C. Li, Dr. S. Long, Dr. H. Lv, Dr. C. Bi, Prof. Z. Huo, Prof. L. Li, Prof. M. Liu Lab of Nanofabrication and Novel Device Integration Institute of Microelectronics Chinese Academy of Sciences Beijing 100029, China E-mail: liuqi@ime.ac.cn; liuming@ime.ac.cn 1. Introduction Resistive switching (RS) phenomenon induced by redox in oxide electrolyte provides attractive prospects for resistive random access memory (RRAM), analog circuits and other neuromorphic applications. [1–5] Due to simple structure, high speed, low power and excellent scalability, the redox-based RRAM is very suitable for 3D ultrahigh density storage. [6–12] Under external electrical stimulations, the resistance of the RRAM cell can be switched between high resistive state (HRS) and low resistive state (LRS) due to the formation/dissolution of conductive filament (CF) inside the insulator layer. [13–22] Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201401304 www.afm-journal.de www.MaterialsViews.com