This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE ELECTRON DEVICE LETTERS 1 A Flexible Solution-Processed Memristor Nadine Gergel-Hackett, Member, IEEE, Behrang Hamadani, Barbara Dunlap, John Suehle, Senior Member, IEEE, Curt Richter, Senior Member, IEEE, Christina Hacker, and David Gundlach, Member, IEEE Abstract—A rewriteable low-power operation nonvolatile physi- cally flexible memristor device is demonstrated. The active compo- nent of the device is inexpensively fabricated at room temperature by spinning a TiO 2 sol gel on a commercially available polymer sheet. The device exhibits memory behavior consistent with a memristor, demonstrates an on/off ratio greater than 10 000 : 1, is nonvolatile for over 1.2 × 10 6 s, requires less than 10 V, and is still operational after being physically flexed more than 4000 times. Index Terms—Flexible electronics, flexible memory, memristor, sol gel, titanium dioxide. I. I NTRODUCTION W E HAVE fabricated a physically flexible solution- processed device that exhibits electrical behavior con- sistent with that of a memristor, a memory device recently experimentally demonstrated and proposed to be the miss- ing fourth basic circuit element [1], [2]. Although electrical switching behavior has been observed from organic mono- layers and metal oxides from as early 1968 [3]–[11], the unique electrical characteristics associated with the memristor have the potential to revolutionize computing. For example, the functionality of a memristor has been compared to that of a neurological synapse; thus, memristors may eventually enable electronic computation functionally similar that which occurs in the human brain [12]. We have fabricated devices that demonstrate these desirable memristor characteristics as well as: physical flexibility, fabrication by using inexpensive room-temperature solution processing, operation voltages of less than 10 V (relatively low power for flexible electronics), on/off ratios greater than 10 000 : 1, memory potential that is nonvolatile for over 1.2 × 10 6 s, and the ability to be operated after being physically flexed 4000 times. These characteristics Manuscript received March 18, 2009; revised April 6, 2009. This work was supported in part by the NIST Office of Microelectronics Programs, by the NIST Competence on Organic Electronics, and by the DARPA MoleApps Program. The research was performed while the first two authors held National Research Council Research Associate Awards and the third author was a Society of Physics Students Summer Intern. Certain equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental details. Such identification neither implies recommendation by the National Institute of Standards and Technology nor does it imply the materials are necessarily the best available for the purpose. The review of this letter was arranged by Editor C.-P. Chang. N. Gergel-Hackett, B. Hamadani, J. Suehle, C. Richter, C. Hacker, and D. Gundlach are with the National Institute of Standards and Technology, Gaithersburg, MD 20899 USA (e-mail: nadinegh@nist.gov; behrang.hamadani@nist.gov; john.suehle@nist.gov; Curt.Richter@NIST.gov; christina.hacker@nist.gov; David.Gundlach@NIST.gov). B. Dunlap was with the National Institute of Standards and Technology, Gaithersburg, MD 20899 USA. She is now with Alleghany College, Meadville, PA 16335 USA (e-mail: barbara.dunlap@gmail.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2009.2021418 Fig. 1. Flexible polymer sheet patterned with four rewriteable nonvolatile flexible TiO 2 sol gel memory devices with cross-bar aluminum contacts. The inset is a side view cartoon of the flexible TiO 2 device structure. make our flexible memristor device a prime candidate for use in inexpensive flexible lightweight portable electronics, such as disposable sensors [13]–[17]. II. EXPERIMENT Rather than using expensive methods and equipment for the deposition of the active TiO 2 layer of our device, we per- formed a room-temperature deposition through a spin-on sol gel process that required no annealing [18]. This procedure consists of spinning a titanium isopropoxide solution on the flexible plastic substrate (spun on at a rate of approximately 33 r/s for 60 s), and then leaving the precursor in air for at least 1 h to hydrolyze and form a 60-nm-thick amorphous TiO 2 film [18]. To electrically contact the active area, a simple two-terminal crossbar is formed by depositing the bottom contact (80 nm Al) on the substrate (approximately 2.5 cm × 2.5 cm square of HP color laserjet transparency C2934A) prior to spinning the pre- cursor, and depositing the top contact (80 nm Al) after the pre- cursor has hydrolyzed. The devices presented in this letter were fabricated by depositing the top and bottom metal contacts via thermal evaporation through a shadow mask; however, there is the potential to deposit contacts from solution to enable roll-to- roll or inkjet processing [19]. The final Al/TiO 2 /Al stack has an area of approximately 2 mm × 2 mm and is shown in Fig. 1. III. RESULTS AND DISCUSSION The flexible TiO 2 -based devices exhibit electrical switching with memory characteristics that match the electrical behavior reported for memristors [1], [2]. A representative example of the basic switching behavior is shown in Fig. 2. On the first voltage sweep (1), the device starts in the low-conductivity state until the bias is increased to approximately +3 V, when the 0741-3106/$25.00 © 2009 IEEE Authorized licensed use limited to: NIST Researchers. Downloaded on June 3, 2009 at 11:25 from IEEE Xplore. Restrictions apply.