A Novel Electrochemical Synthesis Route for Copper Nanowire Formation

Cindy Schmädicke, Markus Pötschke, Lars David Renner, Gianaurelio Cuniberti

Institute for Materials Science and Max Bergmann Center of Biomaterials TU Dresden

01062 Dresden, Germany g.cuniberti@tu-dresden.de

Gianaurelio Cuniberti Division of IT Convergence Engineering

POSTECH Pohang, Korea

Abstract— We present an alternative electrochemical synthesis process of copper nanowires from aqueous solution. The common widespread methods for the fabrication of copper nanowires suffer a number of drawbacks such as the complexity of procedures steps, the resulting nanowires usually are of short length and have a nonlinear morphology. We address these shortcomings with a one-step method that allows preparing copper nanowires fast and with a high surface-to-volume ratio. The grown nanowires are already connected to the electrodes without further processing. We found that the nanowires diameter can be controlled by the frequency of the applied alternating voltage. Furthermore, we analyze the influence of the electric field profile on the morphology of the nanowires. Using an optimized protocol we are able to grow copper nanowires with a diameter of 100 nm and a length of up to several micrometers that exhibited ohmic behavior. Future uses of the nanowires after their oxidation are sensor applications, particularly gas sensors.

I. INTRODUCTION Semiconducting and metal nanowires are of great interest

for applications in sensor technology and in electronic devices [1,2]. Copper plays a particularly important role in modern technologies, because of its high electrical conductivity and numerous potential applications in the micro- and nanoelectronics industry, particularly interconnects in electronic circuits [3]. In the last years several studies have reported the gas sensing characteristics of CuO [4,5]. Hennemann et al. [6] presented a sensor concept based on CuO nanofibers for the detection of H2S in the low ppm range. These applications require the controlled growth of thin and straight copper nanowires contacted with electrodes.

A potential method is the synthesis of metal nanowires by directed electrochemical nanowire assembly (DENA) [7] followed by oxidation. With this technique, an alternating voltage is applied across a pair of electrodes in metal salt solutions inducing the deposition of metal ions or ion complexes into nanowires.

Figure 1. Schematic representation of a nanowire based resistive sensor for a gas sensing application.

The presented study characterized the experimental parameters to control the nanowire diameter and morphology. The introduced route to synthesize copper nanowires is well suited for a wide variety of applications such as gas sensors, since the wires are already contacted and have a large aspect ratio favorable for detection. The concept of resistive gas sensor based on copper oxide nanowires is shown schematically in Fig. 1.

II. MATERIALS AND METHODS

A. Substrate Fabrication We used cut silicon wafers with a 300 nm oxide layer as

substrates. A photoresist (AR-P 5350, Allresist) was spincoated on top of the silicon wafer. After deposition the photoresist coated substrates were exposed to laser light (at 405 nm) using laser lithography (DWL66fs, Heidelberg Instruments). A 3 nm layer of chromium for improved gold adhesion and then a 15 nm gold layer as electrode material were deposited on the developed structures. After the lift-off process, the resulting substrates were used for the nanowire synthesis process. The electrodes were placed directly on top of the insulating SiO2 layer.

978-1-4673-4642-9/13/$31.00 ©2013 IEEE

Figure 1. Experimental setup for copper nanowire growth from aqueous solution.

B. Nanowire Synthesis The distances between two electrodes ranged from 8 to 40

µm. The gaps were covered with a drop of copper salt solution. The basis for the aqueous solution was copper(II)-nitrate (Cu(NO3)2, Sigma Aldrich), which was adjusted to a concentration of 1mM in deionized water. After applying an AC voltage, nanowires grew in the solution between the electrodes. A function generator (Tektronix AFG320) was used to provide a rectangular voltage signal in the frequency range of 50 kHz - 1 MHz. An oscilloscope (Tektronix TDS3014) was used to monitor the signal and the contacting of the electrodes was carried out with a tip‐probing station (Karl Suss). The schematic setup is illustrated in Fig. 2. We varied the concentration of the metal species in solution (c), the peak-to-peak voltage (Vpp) and the frequency (f) of the applied AC voltage in order to investigate the dependence on the morphology of the copper nanowires formation.

C. Characterization The morphology of the grown nanowires was investigated

by scanning electron microscopy (SEM, Philips XL 30 ESEM-FEG) and a dual channel measurement instrument (Keithley 2602 System SourceMeter) was used to

characterize the electrical conducting properties.

III. RESULTS AND DISCUSSION

A. Growth of Copper Nanowires The nanowires directly grew between the electrodes in an

aqueous solution of Cu(NO3)2. The voltage was manually turned off when the nanowires bridged the gap and short cut the electrodes. The bridging was observed in a sudden increase in the voltage at the oscilloscope. We chose a copper salt solution at a concentration of 1 mM based on previous experiments [8]. While decreasing the salt concentration we observed a higher surface density of the nanowire structures [8]. These results are consistent with previous observations [9]. The peak-to-peak voltage was selected to be slightly above the threshold amplitude at which electrochemical deposition occurs, while incidences of side branching were reduced [10]. These protocol restrictions led to amplitudes from 8-20 V depending on frequency and electrode layout.

Depending on the AC frequency, the diameter of the grown copper nanowires was measured from SEM images. The voltage was set to 8 V peak-to-peak for frequencies from 50 to 500 kHz and 10 V for 750 kHz. The increased voltage at 750 kHz was necessary, because no nanowire growth could be observed for 8 V. The electrode gap was 8 µm and all other process parameters were kept constant. Examples of scanning electron microscopy images of copper nanowires grown at various frequencies of the applied AC voltage and the measured nanowire diameters versus frequency are shown in Fig. 3. The images in Fig. 3(a)-(f) demonstrate that the nanowire diameter decreased with increasing frequency from 50 to 750 kHz. This trend is plotted in Fig. 3(g) where the diameter is shown to decrease from 310-110 nm in the studied frequency range. The mean value and standard deviation are shown as an average of 15 diameter measurements for each

e) 500 kHz

5 µm

f) 750 kHz

5 µm

d) 400 kHz

5 µm

c) 250 kHz

5 µm

b) 150 kHz

5 µm

a) 50 kHz

5 µm

g)

Figure 3. SEM images of copper nanowires grown from 1 mM Cu(NO3)2 solutions at varied frequencies of (a) 50 kHz, (b) 150 kHz, (c) 250 kHz, (d) 400 kHz, (e) 500 kHz (a-e at Vpp = 8 V), (f) 750 kHz (Vpp = 10 V), (g) Wire diameter as a function of the frequency of the alternating voltage. The fitted line is

intended as a guide to the eye.

frequency as determined from image analysis made on scanning electron micrographs. The resulting trend is consistent with results of previous work on the nanowire growth with DENA [11,12].

B. Electric Field Impact on the Nanowire Morphology and Electrical Characterization The growth of the copper nanowires with DENA occurs

based on the application of an electric field. This method can also have undesirable effects if certain parameters are not cautiously selected: (i) if the wire contacts the opposite electrode or another wire, the current through the wire increases suddenly and will damage the wire. This side effect can be avoided by monitoring the current through the electrodes and switching off the frequency generator upon a sudden voltage increase or by using a resistor connected in series with the electrodes to limit the current flowing through completed nanowires [13,14]; (ii) the nanowire morphology can be controlled by the applied electrical field between the electrodes. High electric fields are generated at small curvatures and hence, the straightness of a nanowire can easily be disrupted by slight defects in the wire structure. The nucleation density, which is the number of wire nucleation centers on the electrode, can be controlled by geometrical heterogeneities of the electrode, which can locally lead to high electric fields [15]. We have chosen small curvature geometries of the electrodes causing large electric fields and a homogeneous electric field profile to avoid the above-mentioned shortcomings. An optimized protocol led to the growth of a single straight nanowire from tip to tip (Fig. 4 inset). The distance between the electrodes was 40 µm.

Furthermore, we investigated the electrical properties of the wires. The resistance of the nanowire was 110 Ω and we observed ohmic behavior (Fig. 4). Thus, the resulting conductivity for a wire length of 40 μm and a diameter of 300 nm was 5·106 S/m, as measured at room temperature. This conductance is approximately one order of magnitude lower than the conductivity of bulk copper.

Figure 4. The I-V characteristics show ohmic behavior for the copper nanowire. SEM image of a single copper NW bridging the gap of

40 µm and grown with Vpp = 20 V at f = 750 kHz., inset.

IV. CONCLUSIONS AND OUTLOOK In this work, we demonstrated the electrochemical

synthesis and control of copper nanowires from an aqueous Cu(NO3)2 solution. This approach allows the rapid growth of copper nanowires with certain morphologies at predefined electrode locations. The presented data shows that the frequency of the applied AC voltage can be used to control the diameter of the nanowires. In the investigated frequency range of 50-750 kHz the diameter was reduced from 310 to 110 nm. Adjusting the electric field profile can control the morphology of the nanowires. We showed that straight nanowires with a length of up to 40 µm were fabricated exploiting the specific geometry of the metal electrodes. Further investigations will involve oxidation processes to transform grown copper nanowires into semiconducting CuO (p-type) nanowires for potential gas sensor applications. The nanowires and described synthesis method are a promising approach for a wide variety of further applications in the field of biosensorics.

ACKNOWLEDGMENT This work is funded by the European Union (ERDF) and

the Free State of Saxony via the ESF project 100098212 InnoMedTec. This work is partly supported by the German Research Foundation (DFG) within the Cluster of Excellence "Center for Advancing Electronics Dresden". This research was supported by World Class University program funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea (R31-10100).

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