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J.C. Fenton Department of Electrical Engineering, London Center for Nanotechnology, University College London, London WC1E 7JE, United Kingdom

Qianqing Jiang Laboratory of Microfabrication, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

P.A. Warburton Department of Electrical Engineering, London Center for Nanotechnology, University College London, London WC1E 7JE, United Kingdom

Tiehan H. Shen Joule Physics Laboratory, University of Salford, Manchester M5 4WT, United Kingdom (Received 30 May 2013; accepted 9 October 2013)

Three-dimensional (3D) nanostructures and nanodevices have attracted tremendous interest in the past few years due to their special mechanical and physical properties. Nanodevices using 3D nanostructures as the building blocks have been demonstrated to exhibit multifunctionality and functions that conventional planar devices cannot achieve. In this article, we report and review focused ion beam techniques for direct site-specific growth of 3D nanostructures and postgrowth shape modification of freestanding nanostructures by ion beam-induced chemical vapor deposition and ion-beam-irradiation-induced plastic bending, respectively. Such techniques have shown nanometer-scale resolution and accuracy in the fabrication of metallic nanoelectrodes, 3D pickup coils, nanogaps, and multibranched structures. Characterization of the resulting nanostructures shows that focused ion beam techniques allow conducting and superconducting freestanding 3D structures to be tailored in size, geometry, and integrated with planar electronic, mechanical, and superconducting nanodevices, potentially enabling lab-on-a-chip experiments.

I. INTRODUCTION

Three-dimensional (3D) structures are important in many applications, including electronic devices,1 nanosensors,2 biological information detectors,3 plasmonics,4 quantum devices,5,6 and micro electro mechanical systems (MEMS). Although nanostructures with some degree of three-dimensionality have been in existence for several decades generated by, e.g., multiple stages of photoand/or e-beam lithography, these structures have to be embedded within films to provide a template for their deposition. For high aspect ratio and complex 3D structure fabrication, in recent years, intensive studies have been performed on the technique of two-photon-absorptionbased direct laser writing to produce 3D structures for a broad range of applications, but the resulting structures Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2013.324 J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013

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can only be formed on polymers, and it is hard to fabricate structures in sub-100-nm scale.7,8 In the past few years, various devices based on freestanding 3D micro/nanostructures have shown excellent functional properties that planar nanod