DOI: 10.1002/adma.200601223 Fabrication and Structural Evaluation of Beaded Inorganic Nanostructures Using Soft Electron-Beam Lithography** By Suresh Donthu, Tao Sun, and Vinayak Dravid* The last decade has witnessed an explosion of research ac- tivity in fabrication and characterization of single-crystalline 1D nanostructures such as nanowires, nanorods, and nano- belts. [1–5] The high surface-area-to-volume ratio of these struc- tures makes them an obvious choice for potential applications involving surface–interface phenomena, such as gas–solid in- teractions in gas sensing and catalysis. [6] The literature on gas sensing is replete with reports [7–9] suggesting a strong correla- tion between the sensitivity and grain size of sensing elements. Higher sensitivity is generally associated with a smaller grain size. However, these previous reports are based on measure- ments conducted on polycrystalline bulk samples without rig- orous control of the grain-size distribution. Notwithstanding, it is clear that structural defects, such as grain boundaries and porosity, are beneficial to sensitivity, and incorporation of these inhomogeneities into monolithic 1D nanostructures might further enhance their sensitivity. Xia et al. [10] recently reported room-temperature gas-sensing performance of poly- crystalline SnO 2 nanowires. Such results can only be observed at elevated temperatures in polycrystalline bulk, thin film, [11–13] or single-crystalline 1D nanostructures [14–16] of SnO 2 . However, the measurements by Xia et al. were per- formed on a film of nanowires and their results are therefore representative of an ensemble average of millions of grains. Remarkably, there are no prior reports that highlight unam- biguously the contribution of individual boundaries of single grains to the overall sensitivity. This requires fabrication of test structures such as lines that are a single grain wide, known as beaded (or bamboolike) structures. [17,18] In addition, it is highly desirable for such a fabrication technique to have site- specific positioning capability so that the fabricated beaded structures are readily amenable to probing and evaluation. We report here one such approach. We have recently demonstrated a high-resolution pattern- ing approach for ceramic materials called soft electron-beam lithography (soft-eBL). [19,20] This technique synergistically combines the advantages of wet chemistry and eBL, enabling fabrication of solid-state nanostructures on almost any sub- strate. In this communication, we demonstrate that soft-eBL is capable of fabricating beaded nanostructures of technologi- cally important ceramic materials such as zinc oxide and bis- muth ferrite (BFO). The soft-eBL process is schematically outlined in Figure 1. After eBL, the substrates were treated with oxygen plasma for about 20 s and spin-coated with a solution precursor. Oxy- COMMUNICATION Adv. Mater. 2007, 19, 125–128 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 125 – [*] Prof. V. Dravid, S. Donthu,T. Sun Department of Materials Science and Engineering and International Institute of Nanotechnology, Northwestern University 2220 Campus Drive, Evanston, IL 60208 (USA) E-mail: v-dravid@northwestern.edu [**] This work was performed in the EPIC/NIFTI/Keck-II facilities of the NUANCE Center at Northwestern University. The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, the Keck Foundation, the State of Illinois, and Northwestern University. This work was sup- ported primarily by the Nanoscale Science and Engineering Initia- tive of the National Science Foundation under NSF Award Number EEC-0647560. Any opinions, findings, and conclusions or recom- mendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation. (a) (b) (c) Figure 1. Schematic illustration of the soft-eBL process: a) eBL of the resist (copolymer/poly(methyl 2-methylpropenoate)) coated substrate; b) spin-casting of the solution precursors after a short oxygen-plasma treatment; and c) the patterned solid structures after lift-off, which was effected by dissolving the resist in acetone. The patterned substrates are subsequently annealed.