Principle Scientist: Chengxiang Xiang
C. Xiang, S.C. Kung, D.K. Taggart, F. Yang, M.A. Thompson, Garcia Güell, Y. Yang, R.M. Penner* , Lithographically Patterned Nanowire Electrodeposition: A Method for Patterning Electrically Continuous Metal Nanowires on Dielectrics, ACS Nano 2 (2008) 1939.pdf.
There are few methods for patterning metal nanowires on surfaces. The most common, electron beam lithography, is a serial writing method capable of a write speed of approximately 10 cm/hour, and a total throughput that is fraction of this - too slow for virtually any industrial process, or for preparing large arrays of nanowires where the total contour length is in the several cm range. We have been working on a new method, called lithographically Patterned Nanowire Electrodeposition (LPNE) in which photolithography is used to position nanowires and electrodeposition is used to actually grow these nanowires. This is accomplished by using photolithography to build a template, consisting of a metal edge that delineates one side of a nanowire, and then growing that nanowire within that template using electrodeposition. Building on prior work in this direction, we describe the details of LPNE for the formation of metal nanowires in this paper.
Figure 1. Process flow for the formation of gold nanowires using the seven-step LPNE Method.
Briefly, the LPNE process involved the thermal evaporation of the nickel or silver film onto a glass, oxidized silicon, or Kapton R surface (step 1). The thickness of this film determined the ultimate thickness of the nanowires produced using this method. Then a (+) photoresist (PR) layer was deposited by spin-coating, it was soft-baked at 90 C for 30 min (step 2) and a contact mask was used to pattern this PR using 365 nm illumination (step 3). After developing this pattern, the exposed nickel was removed by etching in 0.80M nitric acid, and exposed silver was removed using 18% NH4OH and 4% H2O2 (step 4). The duration of etching was adjusted to produce a 300 nm undercut along the perimeter of the photoresist, and it varied some what depending on the thickness, t, of the metal film. Now this surface was immersed in a dilute metal plating solution in which the concentration of metal was between 0.2mM and 6mM and metal was potentiostatically electrodeposited for between 5 s and 500 s depending on the metal and the width of the nanowire that is targeted (step 5). Then the PR layer was removed by washing with acetone (step 6) and the patterned nickel or silver layer
Figure 2. Scanning electron micrographs of linear nanowires: (a) gold, (b) palladium, (c) platinum, (d) bismuth.
The LPNE method produces nanowires with a rectangular cross-section and a flat top. Scanning electron micrographs of linear nanowires of Au, Pd, Pt, and Bi on glass are shown at both low and high magnification in Fig. 2. All of these nanowires were produced in an unfiltered laboratory air ambient and we have verified that the imprecision seen in the nanowire position on the surface reflects the influence of contamination on the photolithography implicit in the LPNE process. In spite of this positioning imprecision, the wire width is narrowly distributed about a mean value that is different for each of the nanowires shown here. This wire width can be systematically varied from a minimum value of 15-30 nm to hundreds of nanometers, depending on the metal and the electrodeposition parameters including the deposition time, solution composition, and the applied potential as shown in Fig 3.
Figure 3. Control of nanowire height and width for wires of Au, Pt, Pd, and Bi: (a) Nanowire height measured by AFM versus the thickness of the nickel or silver sacrificial layer. (b) Nanowire width measured by SEM versus the electrodeposition duration.
Figure 4. a) dR/R0.1V versus Eapp for the oxidation of five gold nanowires with dimensions as indicated. b) Plot of dR/R0.1V evaluated at +1.1 V vs. MSE versus height for the six nanowires in (a) and (b) (red trace). Also plotted are the calculated resistance change caused by the constriction in the wire diameter ((dR/Rred ), green trace) and the change in p from 0.38 to 0.0, blue trace) as well as the sum of these two contributions ((dR/Rred), black trace). c,d) schemtic diagrams depicting two mechanisms for the oxidation of a gold nanowire, the first (top) involving the formation of a superficial oxide layer and the second (bottom) an oxide layer that infiltrates grain boundaries.
Figure 5. Gold nanorods prepared by masking ultra-long gold nanowires with a lithographically patterned photoresist and then etching to remove exposed wire segments. (a) Optical micrograph of gold nanorods with 2 micron spacing. (b) High magnification SEM image of gold nanorods with dimensions of 110 nm x 40 nm x 2 microns.
The nanowire length dimension, as well as its width and height, can be modified by removing nanowire segments after LPNE deposition. This is accomplished by adding a second lithographical patterning step to the baseline 7 step LPNE process: Onto the already fabricated metal nanowires, a photoresist layer is deposited and photopatterned to expose sections of the metal wires that are then removed by a chemical etching step. In the case of gold nanowires, for example, we employed a iodide/triiodide etching solution (16 mM KI3 and 8 mM KI). Gold nanorod arrays (Fig 11) can be produced from linear nanowire arrays using this approach.
In summary, The LPNE method is a new and versatile tool for fabricating metal nanowires in a highly parallel fashion directly on dielectrics ranging from glass to flexible plastic. These nanowires have a rectangular cross-section with height and width dimensions that can be independently specified in the range from 20 nm to 2 microns (w) and 5 - 200 nm (h). Long, 100 micron segments of these wires are electrically continuous and in the case of gold and palladium, these nanowires show a temperature-dependent resistivity that is broadly consistent with the known lateral dimensions and grain diameters of these wires.
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