Principle Scientists: Erik Menke, Mike Thompson, Chengxiang Xiang
E.J. Menke, M.A. Thompson, C. Xiang, L.C. Yang, and R.M. Penner*, Lithographically Patterned Nanowire Electrodeposition, Nature-Materials 5 (2006) 914.
The ESED method has two limitations: 1) Nanowires can not be patterned onto graphite surfaces - they simply nucleate at the step edge defects present on an HOPG surface and the position of these step edges can not be controlled, 2) Single nanowires can not be prepared, because spatially isolated step edges do not exist on these HOPG surface; step edges are always located in pseudo-parallel arrays of hundreds. Nanowires in these arrays are separated from adjacent nanowires by, at most, 500 nm.
We have just developed a new method called Lithographically Patterned Nanowire Electrodeposition (or LPNE) for preparing gold, palladium, or platinum nanowires on glass or SiO2 surfaces. LPNE involves the fabrication of artificial step edges, consisting of linear nickel ultramicroband electrodes, onto the surface of an insulator like SiO2, mica, or glass. These step edges are then used to prepare nanowires by electrodeposition, much like in the ESED method.
Figure 1. Processing steps for preparing gold nanowires using LPNE: 1) Physical Vapor Deposition (PVD) is used to prepare a nickel film that is 10-100 nm in thickness on a glass surface. 2) (+) photoresist is spin-coated onto the PVD nickel, 3) Photoresist is patterned using a conventional mask, UV source, etc., 4) exposed nickel is electrochemically stripped, producing artificial step edges consisting of an exposed nickel band covered with photoresist. IMPORTANT: This nickel band is recessed into the photoresist by about 200-300 nm, 5) gold nanowires are electrodeposited, 6) the photoresist is removed with acetone, 7) nickel is stripped using dilute nitric acid; contact pads are then deposited by through-mask PVD. The resulting gold nanowire has a rectangular cross-section with a height equal to the evaporated nickel film, and a width that is proportional to the electrodeposition time (See Fig. 2).
But there is an important wrinkle: The over-dissolution of nickel undercuts the photoresist (Fig 2, bottom) thereby producing a horizontal trench that is bounded by one nickel edge. This trench confines a nanowire during growth, permitting the height and width of the nano-wire to be independently controlled, as shown by the data presented in Fig 2 below.
Figure 2. a. AFM images of an LPNE nanowire showing rectangular cross section. b. Nanowire height versus evaporated nickel layer thickness for Pd nanowires prepared by LPNE. c. Nanowire width versus electrodeposition duration for LPNE nanowires of Pd, Au, and Pt.
The LPNE method eliminates the necessity for transferring nanowires (e.g. from HOPG to glass). Also eliminated is the chemically reactive cyanoacrylate resin we have em-ployed to carry out this transfer. The nanowires produced by the LPNE method are sup-ported directly on the clean and chemically inert glass, mica, or SiO2 surface on which they are synthesized. SEM images of gold nanowires supported on glass are shown in Fig. 2.
We are unaware of any other method except electron beam lithography (EBL) that is capable of producing metal nanowires in the sub-50 nm diameter range with simultaneous control over nanowire shape and position. The LPNE method literally combines the resolution advantage of EBL with the parallel processing, ease of use, and technological maturity of optical lithography.
Figure 3. SEM images of LPNE nanowires on glass.
The LPNE method eliminates the necessity for transferring nanowires (e.g. from HOPG to glass). Also eliminated is the chemically reactive cyanoacrylate resin we have em-ployed to carry out this transfer. The nanowires produced by the LPNE method are sup-ported directly on the clean and chemically inert glass, mica, or SiO2 surface on which they are synthesized. SEM images of gold nanowires supported on glass are shown in Fig. D12. We are unaware of any other method except electron beam lithography (EBL) that is capable of producing metal nanowires in the sub-50 nm diameter range with simultaneous control over nanowire shape and position. The LPNE method literally combines the resolution advantage of EBL with the parallel processing, ease of use, and technological maturity of optical lithography.
