Nanoscale COMMUNICATION Cite this: Nanoscale, 2015, 7, 13373 Received 22nd May 2015, Accepted 7th July 2015 DOI: 10.1039/c5nr03396a www.rsc.org/nanoscale Free-standing NiTi alloy nanowires fabricated by nanoskiving† Huilong Hou and Reginald F. Hamilton* We report on free-standing NiTi alloy nanowires (120 nm × 75 nm) fabricated using a technique referred to as “nanoskiving”, which complements conventional thin film sputter deposition with ultra- microtomy for thin sectioning. To date, the technique has been limited to pure metals without exploring metallic alloys. Lever- aging the technique for the fabrication of shape memory alloy (SMA) nanostructures meets two critical requirements: compo- sitional control (via film deposition) and controlled dimensions (via film deposition and programmable sectioning). Microstructure and composition analysis confirm continuity of the produced nano- wires and Ni and Ti elemental uniformity. Free-standing NiTi nano- wires are robust and remain intact throughout physical manipulation. The fabrication of NiTi alloy nanowires by nano- skiving will advance fundamental characterization of small scale SMA behavior. Nanoskiving combines thin film deposition and ultrathin sec- tioning for the fabrication of free-standing nanowires, without the need to access conventional lithography 4 and clean room protocols, 4–7 and can be leveraged for fabricating shape memory alloy (SMA) nanowires. NiTi SMAs are promising for micro-/nano-scale actuation and sensing functionality, as they offer the highest actuation density among active material families, 8 high strength and ductility, 9 and good corrosion resistance. 9 Conventional fabrication methods for semicon- ductors and pure metals have been used for SMA nanowires such as electrodeposition of Cu-based alloys, 10,11 and of Co–Ni alloys 12 as well as mechanical pressure injection for In–Tl. 13,14 Studies have yet to demonstrate the successful fabrication of NiTi nanowires though the NiTi alloy are the most commer- cially successful and intensively-investigated. 15 An ideal syn- thesis method will have the capability to tune the NiTi alloy composition and control the dimensions of free-standing nanowires. Our recent work on NiTi thin films demonstrated precise NiTi alloy composition control, by sputtering from elemental Ni and Ti targets, as well as controllable film dimen- sion and reduced surface roughness. 16,17 Nanoskiving has pro- duced pure metal (or single material) nanowires from thin films with controllable dimensions 2,5,6,18–26 and its applica- bility for fabricating pure titanium (Ti) and nickel (Ni) metallic films has been assessed. 2 The applicability of nanoskiving to alloys has yet to be ascertained. The NiTi alloy compositional ranges that exhibit SMA be- haviour have a CsCl atomic crystal structure, referred to as body centered cubic (BCC) B2. 27 It is well known that metallic material failure is dependent on the atomic crystal structure and the availability of slip systems; preferred crystallographic planes and crystallographic directions are for dislocation motion that produces plastic deformation in metals. 28 When nanoskiving was undertaken for pure Ni and pure Ti, the Ti nanowires typically fragmented whereas the Ni nanowires did not readily fragment so that intact Ni nanowires were produced that were markedly longer than Ti nanowires. 2,5 The structure of Ti is hexagonal close-packed (HCP) and the structure of Ni is face-centered cubic (FCC). The inherent number of available slip systems in Ti is three and there are twelve for Ni. Due to the stark differential in available slip systems, the mechanical response of pure Ti is expected to be brittle whereas Ni should be ductile and thus the reported fragmentation of Ti is expected. Since BCC structures can have 12 slip systems, we hypothesize that the NiTi alloy films will be amenable to nanoskiving. The nanoskiving approach for the current NiTi alloy nano- wires is illustrated in Fig. 1. Sputtering for film deposition and the ultramicrotome for sectioning can define the nanowire cross-section dimensions. For this work, the NiTi alloy thin film was deposited by magnetron co-sputtering from separate Ti and Ni elemental targets onto an NaCl (100) substrate such that the film was ∼120 nm thick. Details of the thin film depo- sition are reported in our previous work. 16,17 In step A, after film deposition, epoxy (Araldite 502) was cured on the film † Electronic supplementary information (ESI) available: Detailed procedures and associated parameters of sputtering NiTi film, embedding film into epoxy, sec- tioning film into nanowires, transferring nanowire to a V-grid, determining alloy composition, conducting elemental map and high-resolution TEM. See DOI: 10.1039/c5nr03396a Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. E-mail: rfhamilton@psu.edu This journal is © The Royal Society of Chemistry 2015 Nanoscale, 2015, 7, 13373–13378 | 13373 Published on 16 July 2015. Downloaded by Pennsylvania State University on 15/12/2015 14:53:45. View Article Online View Journal | View Issue