www.advmat.de www.MaterialsViews.com COMMUNICATION © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1871 Adv. Mater. 2011, 23, 1871–1875 Nalae Han, Sung In Kim, Jeong-Do Yang, Kyumin Lee, Hyunchul Sohn, Hye-Mi So, Chi Won Ahn, and Kyung-Hwa Yoo* Phase-Change Memory in Bi 2 Te 3 Nanowires Bismuth telluride (Bi 2 Te 3 ) and its alloys are some of the best available materials for near-room-temperature thermoelec- tric applications. [1] In particular, Bi 2 Te 3 nanowires have been studied extensively [3–5] because low-dimensional thermoelectric materials are expected to have a higher figure of merit due to quantum confinement effects. [2] However, memory switching behavior has never been studied in Bi 2 Te 3 nanowires. Here, we report for the first time reversible memory switching effects in Bi 2 Te 3 nanowires fabricated using anodized aluminum oxide (AAO) membranes. The findings show that Bi 2 Te 3 nanowires display a reversible crystalline–amorphous phase change that is induced by a temperature, laser, or electric field, similar to that reported for chalcogenide materials (Ge–Sb–Te alloys, GST). [6–9] We demonstrate that Bi 2 Te 3 nanowires show considerable promise as building blocks for phase-change random access memory (PRAM). Phase-change materials are used in nonvolatile optical memory (e.g., CDs and DVDs), and are being actively inves- tigated as the media in universal solid-state memory devices that combine rapid read and write speeds, high storage density, and non-volatility. [10] The key feature of PRAM is the revers- ible phase transition of the phase-change material, caused by an electrical pulse, between the crystalline (low resistivity, SET) and amorphous (high resistivity, RESET) states. A major obstacle to achieving high-density PRAM devices is the large writing currents required to generate sufficient thermal energy for a phase change, particularly during the crystal-to-amorphous phase transition, since a high current is required for melting. To reduce the writing currents, GST nanowires have been synthesized and were shown to sat- isfy many of the attributes of universal non-volatile memory devices. [12,13] However, GST nanowires are usually synthesized using vapor transport methods at high temperatures [11–13] and their large-scale assembly is not yet feasible. On the other hand, Bi 2 Te 3 nanowires exhibit memory switching characteris- tics that are comparable to GST nanowires [11,12] and they can be fabricated at room temperature using AAO membranes. Furthermore, the vertical growth of nanowires on a substrate permits a high-density assembly of Bi 2 Te 3 nanowires. Here, we describe in detail the memory switching properties of Bi 2 Te 3 nanowires. We fabricated Bi 2 Te 3 nanowires using electrodeposition within the nanopores of an AAO membrane made by ano- dizing Al plates. The nanowire structures were examined using X-ray diffraction (XRD) and high-resolution transmission elec- tron microscopy (HRTEM). An XRD pattern of the as-grown nanowires ( Figure 1 a) agreed with the rhombohedral crystal structure of Bi 2 Te 3 (JCPDS No. 15-0863) reported by others. [3,4] The spacing between adjacent planes in the HRTEM image (Figure 1b) was 0.202 nm, which corresponded to the (110) lattice planes of the rhombohedral Bi 2 Te 3 crystal structure. In addition, the selected area electron diffraction (SAED) pat- tern was indexed to the rhombohedral crystal structure, both of which are consistent with the XRD data. The chemical com- position of the individual nanowires was characterized using energy dispersive X-ray spectroscopy (EDS) within scanning transmission electron microscopy (STEM). Elemental map- ping images for Bi and Te showed their uniform distribution throughout the nanowire (Figure 1c). The results of EDS point scanning at arbitrary positions on the nanowire revealed that Bi and Te were present in an atomic ratio of approximately 2:3 (Figure 1d). To characterize the electrical properties of Bi 2 Te 3 nanowires, we fabricated the AAO membrane on a SiO 2 /Si substrate and electrodeposited Bi 2 Te 3 inside the nanopores. Then, we deposited a platinum (Pt) electrode on top of the vertically aligned Bi 2 Te 3 nanowires and measured the current–voltage ( I– V) curves using the Pt top electrode and Au bottom electrode ( Figure 2 a). The current increased linearly with the voltage as the voltage was swept from –1 to 1 V. However, at about 0.9 V the current suddenly decreased from 3.3 × 10 -4 to 1.7 × 10 -9 A. Subsequently, when the voltage was swept from 1 to –1 V, a similar abrupt electrical transition to the low resistive state occurred at about –0.7 V. In order to investigate whether BiTe- based nanowires with chemical compositions other than Bi 2 Te 3 exhibited similar hysteric I– V curves, we also fabricated Bi 3 Te, Bi 5 Te 3 , and Bi 4 Te 3 nanowires and measured each of their I– V curves. We did not observe hysteric I– V curves for the Bi 3 Te, Bi 5 Te 3 , and Bi 4 Te 3 nanowires (Figures S1b–d, Supporting Infor- mation), suggesting that only Bi 2 Te 3 nanowires display the described memory switching behavior. Reversible resistive switching phenomena have typically been observed in PRAM, [6] in which thermal processes control a phase change between the amorphous and the crystalline states, and in resistance RAM (ReRAM), [16,19] in which an electrically stimulated change of the resistance in a metal–insulator–metal memory cell. In the case of ReRAM, an initial electroforming DOI: 10.1002/adma.201004746 N. Han, S. I. Kim, J.-D. Yang, Prof. K. H. Yoo Department of Physics, Yonsei Universiy Seoul 120-749, Korea E-mail: khyoo@yonsei.ac.kr K. Lee, Prof. H. C. Sohn Sohn, Department of Material Science and Engineering Yonsei University Seoul 120-749, Korea Dr. H. M. So, Dr. C. W. Ahn National NanoFab Center Daejeon 305-806, Korea