Fabrication of Copper Nanowires Through Multi-step Anodizing, Electrochemical Barrier Layer Thinning and Electrodeposition

Nanoporous anodic aluminum oxide (AAO) is one of the most commonly employed templates for nanofabrication since the pore diameter, interpore distance, thickness of the oxide, barrier layer and walls can be precisely controlled by a proper choice of anodizing conditions. For the fabrication of metallic nanowires (NW), it is necessary to decrease the barrier layer thickness. In addition, to achieve sufficient electrical contacts at the bottom of the pores, a thin layer of Au has to be deposited either by sputtering or by electrodeposition (ED) using a cyanide bath. Researchers at Military University of Technology, Poland and Delft University of Technology, The Netherlands have proposed a methodology that combines multi-step anodizing, electrochemical barrier layer thinning (BLT) and ED for the fabrication of Cu NW.

Commercial purity aluminum alloy (AA 1050 alloy) was degreased and electropolished (EtOH:HClO₄ 4:1, 0 °C, 20 V, 120 s, Pt grid cathode). A multi-step anodizing protocol was employed to obtain nanoporous anodic aluminium oxide (AAO) templates with a desired nanoporous structure. Mild anodization (MA) in 0.5 M H₂SO₄ with 20 vol.% ethylene glycol (EG) at 0 °C, 20 V and 60 min (Fig. 1, reaction I) was carried out as the first step. The voltage was increased up to 45 V with 0.5 V steps for each 5 s and hard anodizing was performed at 45 V for 1 h (Fig. 1, reaction II). To thin down the bottom of the barrier layer, mild anodizing was carried out in 0.3 M oxalic acid, at 30 °C, 45 V and 30 min (Fig. 1, reaction III). Electrochemical barrier layer thinning (BLT) of multi-step anodized Al alloy was performed in 0.3 M oxalic acid (Fig. 1, reaction IV). A step-wise decrease in voltage and the duration of each voltage step was varied and the suitable conditions for BLT were optimized. For effective opening of the pores at the bottom, the Al alloy at the base was chemically etched using 0.1 M CuCl₂ in HCl. The applicability of the membranes formed using a combination of MA, HA and BLT was ascertained through electrodeposition (ED) of Cu using 0.3 M CuSO₄ and 0.1 M H₃BO₃ at -0.3 V vs. Ag/AgCl for 30 min (Fig. 1, reaction V). The Cu nanowires (NW) were liberated from the AAO template by chemical etching in 5% H₃PO₄ at 30 °C for 45 min (Fig. 1, reaction VI).

Fig. 1 Schematic representation of the various stages involved in the fabrication of Cu nanowires

MA in 0.5 M H₂SO₄ with 20 vol.% EG at 0 °C, 20 V and 60 min enables the formation of a protective oxide layer. The presence of this oxide layer as well as a steady step wise increase in voltage (0.5 V steps for each 5 s) up to 45 V prevents destruction of the Al alloy anode by the high current density avalanche generated during HA at 45 V for 1 h. The MA/HA combination though improved ordering of nanoporous structure in the resultant AAO, the thickness of the barrier layer at the bottom is a critical issue. MA in 0.3 M oxalic acid,    at 30 °C, 45 V and 30 min decreased the thickness of the barrier layer by a reasonable extent, which is suitable for subsequent BLT process. Since the conditions of anodization are mild, the interpore distance and ordering of the pores are maintained. A step-wise decrease in voltage enables BLT of the anodized Al alloy in 0.3 M oxalic acid. Thinning of the barrier layer is effective at selective voltage step and time (Un+1 = 0.75.Un; Δt = 60 s). Under such conditions of BLT, the hexagonal honeycomb-like morphology is maintained at the bottom of the pores (Fig. 2(a)). The applicability of the AAO formed using a combination of MA, HA, MA and BLT is confirmed by ED of Cu NW with a high aspect ratio (Fig. 2(b)). The successful ED of Cu NW confirms efficient BLT and sufficient electrical contact at the electrolyte–aluminum interface that could have facilitated the reduction of Cu²⁺ ions at bottom of the pores.

Fig. 2 FE-SEM micrographs of (a) bottom side of the AAO (after removal of Al alloy using 0.1 M CuCl₂ in HCl) indicating the effectiveness of BLT performed at U_n+1 = 0.75 Un; Δt = 60 s; and (b) ED Cu NW

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: W.J. Stepniowski et al., Journal of Electroanalytical Chemistry, 809 (2018) 59–66.

Template-assisted deposition of metal nanowires

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

3D Printing of Nanotwinned Copper

Nanotwinned (NT)-metals exhibit superior mechanical and electrical properties when compared to their coarse-grained and nano-grained counterparts. NT-metals, either as a film or in bulk, are usually obtained by pulsed electrodeposit­­­­ion (PED), plastic deformation and sputter deposition. However, 3D printing of NT-metals has not been explored. Researchers at Department of Mechanical Engineering and Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, USA have reported a localized pulse electrodeposition (L-PED) process for 3D printing of NT-Cu, which is fully dense, almost free of impurities and low microstructural defects, with no obvious interface between deposited layers, with good mechanical and electrical properties and without the requirement of any post-treatment.

The process is based on L-PED that occurs at the tip of an electrolyte-filled nozzle steered in 3D-XYZ directions, by precisely positioning the stage relative to the substrate for omnidirection (complex 3D, Fig. 1(a)) or layer-by-layer (lateral, Fig. 1(b))) metal printing. The 3D printed Cu structures and patterns are shown in Figs. 1(c) – 1(f). The FIB image of a micropillar printed by vertical deposition at 0.5 V and at an average current density of ≈0.018 A/cm² is shown in Fig. 2(a). The SEM image of a four-layer hollow square shape printed by layer-by-layer process at 0.7 V and at an average current density of 0.045 A/cm² is shown in Fig. 2(b). Both structures exhibit the presence of high density aligned twin boundaries (TBs) within their grains in which most of the TBs are aligned perpendicular to the electric field direction. The TEM image (Fig. 2(c)) confirmed the formation of densely packed nanotwins, mostly aligned in one direction in almost all grains. No noticeable interlayer is observed in the 3D printed Cu by L-PED. With the exception of a few grains, stacking faults, in general, are not frequently observed in 3D-printed Cu.

Fig. 1 (a) Schematic of L-PED process; (b) Schematic side-view of the meniscus between the nozzle tip and the growth front for layer-by-layer deposition of nt-metals; (c-f) SEM images of several 3D-printed Cu structures. (c) A 24-layer structure printed by layer-by-layer DC-ED process, printing time ≈200 min; (d) UTD letter printed, printing time ≈16 min; (e) A micropillar with diameter of ≈10 μm 3D printed by PED, printing time ≈60 min; (f) A helical structure fabricated by pulsed voltage, printing time ≈12 min

Fig. 2 (a) FIB ion channeling contrast image of cross-section of a 3D printed micropillar, printing time ≈60 min; (b) SEM image of a layer-by-layer structure (four-layer) cross-sectioned by FIB, printing time ≈35 min; and (c) FIB ion channel image of the cross-section that shows high-density parallel TBs.

The formation and presence of inter-layers during a step-wise 3D printing of six-layer structure of Cu using a nozzle diameter of 10 μm at 0.5 V pulsed voltage with a duty cycle of 1/100 is studied (Fig. 3(a)). Each layer is printed starting from a point with a shift to the right from the starting point of the former layer. FIB ion channeling contrast images acquired at the cross section (Figs. 3(b) and 3(c)) indicate that the grains in each new layer has continued growing from the grains in the previous layer without the formation of any new grains at each layer. One columnar grain initiated from the first layer grew to the fifth layer (boundary of the grain is shown by a dashed line) with no interlayer could be observed at any step and between any two layers. In L-PED process, the preceding layer functions as the seed layer for the deposition of the next layer, which helps development of an interface-free structure, which is further confirmed by the EDS map of Cu (Fig. 3(d).

Fig. 3 (a) Plan-view FIB image of a six-layer printed Cu structure, printing time ≈35 min; (b, c) Zoomed-in views of the layers that show each layer is deposited on the previous layer, without any noticeable interlayer; (d) EDS map of Cu

The high quality of 3D printed NT-Cu with minimal porosity and structural defects as well as the absence of any interface between the printed layers offer good mechanical property (elastic modulus: 128.2 ± 10.9 GPa; hardness: 2.0 ± 0.19 GPa) and electrical resistance (3.9×10⁻⁷ Ω.m). By modifying the geometry of the nozzle, the geometry of the meniscus between the nozzle tip and the substrate can be varied, which would facilitate fabrication of geometries with more sharp corners and helical pitches. Layer-by-layer deposition seems to be necessary to fabricate entangled structures. The reproducibility of the L-PED process largely depends on the stability of the meniscus, which is influenced by humidity and temperature. In addition, moving speed of the nozzle also influences the reproducibility; high speed could result in breakage of the meniscus while too slow displacement might lead to clogging of the nozzle tip by the deposited metal. The L-PED process can be used for direct 3D printing of layer-by-layer and complex 3D microscale NT-Cu structures for various applications including electronics, micro/nano electromechanical systems (MEMS, and NEMS), metamaterials, plasmonic, and sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Adv. Mater. 2017, 1705107, DOI: 10.1002/adma.201705107