Fabrication of metal nanowires have received considerable attention and among them template-assisted (ion tracked etched polymers or porous aluminium oxide templates) electrodeposition of metal nanowires assumed significance. Usually, a thin film (< 200 nm) is sputtered over the template and it is subsequently reinforced with a thick (up to 10 μm) metallic layer by plating. However, the difficulty in making electrical connection with the thin and fragile sputtered film as well as in removing the electrodeposited layer to facilitate the release of metal nanowires are the major limitations. Researchers at Centre for Manufacturing and Materials Engineering and Faculty of Health and Life Sciences, Coventry University, UK and Energy Technology Research Group, University of Southampton, UK for the first time have described a new procedure for fabrication of metal nanowires (Cu nanowires) by template-assisted electrodeposition using porous polycarbonate templates.
Polycarbonate templates (pore sizes: 60 nm, 100 nm and 200 nm; thickness: 25 μm) were washed using 1 v./v. % of Neutracon at 40 ºC for 5 min, rinsed, air-dried. They were sputter coated with silver for 3 min on one side of the template (Ar bombardment gas, 15 mA current) followed by electroless plating of Cu using an electroless copper bath at 46 ºC for 10 min to form the electrode layer, rinsed and air dried. Subsequently a layer of Cu was deposited by electrodeposition at -75 mV vs. saturated calomel electrode (SCE) for 120 min to grow the Cu nanowires. A titanium/mixed metal oxide mesh served as a counter electrode. After plating, the coated template was removed from the plating cell, rinsed and air dried. The Cu nanowires were freed from the template by etching away the bottom electrode layer using a 3 v./v.% solution of hydrogen peroxide/sulphuric acid and then by dissolving the polycarbonate template in dichloromethane. The various stages involved in the fabrication of Cu nanowire is schematically illustrated in Fig. 1.
Fig. 1 Schematic illustration of template-assisted deposition of Cu nanowires
The sputtered Ag acts as a seed layer and served as an effective catalyst for electroless deposition of Cu. After 3 min sputtering, a uniform but porous layer of Ag (thickness: ≈ 15 nm) is deposited (Fig. 2(a)). Electroless plating of Cu over the sputtered Ag film for 10 min completely covered and sealed the pores and provides an excellent coverage (Figs. 2(b) and 2(c)). Analysis performed at the reverse side of the electrode layer after dissolving the template indicates that the electroless deposited Cu starts to fill the bottom of the pores and forms the base of the nanowire. The sputtering process directs the Ag atoms into the pores wherein they adhere to the side of the walls and trigger deposition of Cu. Filling-up the bottom and side walls of the porous structure provides an ideal base for subsequent electroplating step to build uniform Cu nanowires (Fig. 2(d)). Plating of Cu into the pores offers an additional advantage of mechanically keying the electrode layer to the smooth surface of the template. A magnified acquired by SEM at the bottom of the Cu nanowires (Fig. 2(e)) clearly indicate the electroless Cu and sputter-coated Ag layers and the interconnection between the electroless Cu and electroplated Cu layer is good.
Fig. 2 SEM images (a) after sputter coating with Ag for 3 min (thickness: ≤ 15 nm); (b, c) after electroless plating with Cu for 10 min (thickness: 300–500 nm); (d) Cu nanowires formed after 120 min of electrodeposition of Cu at -75 V vs. SCE over the sputtered Ag seed layer/electroless Cu; and (e) bottom portion of the Cu nanowire showing a good interconnection between the electroless Cu and electroplated Cu layer
A simple protocol is suggested for the fabrication of template-assisted electrodeposition of metal nanowires. Sputter deposited Ag thin film (≤15 nm) on one side of the polycarbonate template acts as a seed layer and catalyze electroless deposition of a uniform and highly conductive Cu layer (300–500 nm) for subsequent electrolytic deposition of a thick Cu layer. Removal of the electrode layer at the bottom of the template by chemical etching as well as dissolution of the template in dichloromethane yields free standing Cu nanowires.
T.S.N. Sankara Narayanan
For more details, the reader may kindly refer: J.E. Graves et al., A new procedure for the template synthesis of metal nanowires, Electrochemistry Communications (2017) (article in press), doi:10.1016/j.elecom.2017.11.022
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