Electrical Switching and Phase Transformation in Silver Selenide Nanowires David T. Schoen, Chong Xie, and Yi Cui* Department of Materials Science and Engineering, Stanford UniVersity, Stanford, California 94305 Received November 22, 2006; E-mail: yicui@stanford.edu Resistance switching in solid-state electrolyte materials has been receiving a growing amount of interest for memory device applications. In these memory devices, a polycrystalline or amor- phous Ag + or Cu + ion conductor film is sandwiched between two electrodes. 1-4 Electrical switching is believed to be realized by growing or dissolving a nanoscale metallic Ag or Cu filament when a voltage is applied between the two electrodes, although insufficient work has been done to characterize the nature and formation mechanism of the filament. In addition, the nanoscale conductive pathway is thought to be much smaller than the total device area, and the active device regions are polycrystalline or amorphous, sometimes even having complicated nanostructures. 2 The presence of a large number of random interfaces complicates interpretation of experiments done in these systems. Single-crystal nanowire (NW) devices offer a system with a small number of well-defined interfaces, and their cross section is on a scale similar to that of the proposed conductive pathways. For these reasons, they may offer the potential to study in depth the fundamental processes involved in resistance switching in these devices. Here we report for the first time electrical transport measurements in NW structures of a solid-state electrolyte. By single NW electrical transport and in situ transmission electron microscopy (TEM) studies, we discover two types of electrical behavior linked to Ag 2 Se NW phase transformations. Ag 2 Se is a mixed ionic conductor with a transition from a low- temperature orthorhombic phase () to a high-temperature superi- onic conducting cubic phase (R) at 135 °C. 5 Ag 2 Se NWs were produced by solution-phase synthesis following the procedure published by the Xia group. 6,7 Briefly, crystalline Se NWs are grown from amorphous selenium colloid. These Se NWs are reacted with AgNO 3 (aq) to form Ag 2 Se NWs (Supporting Information). NWs were characterized in a 200 kV Phillips CM20 transmission electron microscope (TEM). Compositional analysis was by energy disper- sive X-ray spectrometry (EDX). Electrical contacts to single NWs were fabricated by electron beam lithography (EBL). The EBL process requires baking the resist. A range of temperatures were used (95-180 °C). The metals used for contact were Au, Ag, and Ni. The Ag 2 Se NWs as synthesized varied in diameter from 40 to 200 nm (Figure 1a). The tetragonal phase observed by Xia for wires smaller than 40 nm was not observed here. 6 The EDX data indicate that NWs consist of Ag and Se with an atomic ratio of 2:1 (Figure S1 in Supporting Information). They were single crystalline orthorhombic Ag 2 Se as determined by TEM and selected area electron diffraction (SAED) (Figure 1b and inset). The growth direction is along [001] direction, different from the [100] direction in Xia’s study. 7 The reason is not yet understood. Ag 2 Se NW electrical devices consist of single NWs contacted by two or more electrodes (Figure 1c and d). The transport measurement was done by applying a voltage (V) across the two metal electrodes and recording the resulting current (I). A resistor (2-3MΩ) is connected in series with the whole circuit to protect the NWs from Joule overheating when they are switched into a highly conducting state. The two-terminal electrical transport shows two different behaviors. The first type of behavior exists in devices fabricated with an electron-beam resist curing temperature below 140 °C. The devices exhibit a linear dependence of I versus V, suggesting ohmic conduction (Figure 1e). Scanning the voltage upward and downward does not change the I-V curve. The resistance of the NW is 4.7 kΩ without a serial resistor (Figure S2). The resistivity is 1.2 × 10 -3 Ω‚cm, consistent with the literature value. 10 Devices baked at 140 °C or above predominately displayed a second type of behavior (Figure 1f). These devices would maintain a very high resistance (off-state) of >GΩ (resistivity >10 3 Ω‚cm), until some threshold voltage, where the resistance would drop precipitously to a value of several 100 Ω (on-state, resistivity ∼10 -4 Ω cm, Figure S3). The resulting on-off ratio approaches a very high value, near 10 7 . The on-state generally persists no more than a few seconds after the voltage is scanned back to a small value or zero, but occasionally on-state persistence times on the order of minutes were observed, so the switching was volatile. Voltage thresholds for switching varied from device to device and from scan to scan on the same device, with values ranging from 0.5 to 3 V. Devices were fabricated both with single Figure 1. (a) SEM image of Se NWs before AgNO3 reaction. Ag2Se NWs are morphologically similar. (b) HRTEM image of Ag2Se NW (SAED inset). (c) Device schematic. (d) SEM of a fabricated device. Contacts are Ag- Ni-Ni-Ag. (e) Electrical behavior of devices baked at 95 °C, in series with a 2.34 MΩ resistor. (f) Electrical behavior of devices baked at 140 °C, in series with a 2.15 MΩ resistor. Published on Web 03/17/2007 4116 9 J. AM. CHEM. SOC. 2007, 129, 4116-4117 10.1021/ja068365s CCC: $37.00 © 2007 American Chemical Society