Experimental

3.2.1 Preparation of nanowires

To fabricate the AAO template for the nanowires, we first utilized the two step anodization process of the aluminum foil. However, the anodization process was often time consuming and low yield for production of nanowires. Therefore, we have switched to the commercial AAO membranes purchased form Whatman as the template for the nanowires. To produce nanowires by electrodeposition, alumina oxide membranes with 200 nm pore diameter (Andisc, Whatman) were used as the templates. To perform electrodeposition, one side of membrane was sputter-coated with a 200 nm thick silver film for conducting purpose. This template was placed in contact with a copper plate and restrained with a glass joint and sealed with an o-ring. The counter electrode was a 2 mm diameter platinum wire. The template was rinsed with DI water for 10 minutes and immersed in the desired the electrolytic solution.

Commercial plating solutions were used to produce gold (Technic Gold 25E), Ag (Technic Cy-less Silver 2 RTU,) and Ni (Techni Nickel RTU) nanowires.

3.2.2 Pulse electrodeposition

In this experiment, the pulse electrodeposition was used to produce metallic nanowires.

In a typical electroplating period, a negative current at – 10V was applied 8 ms and followed by a short pulse of positive current at 3 V for 2 ms. Before the next negative current arrived, the system was sustained at 0V for 990 ms [16]. Metallic nanowires with different lengths

could be produced by adjusting the electrodeposition time. For example, the pulse sequence was repeated for 30 minutes to fabricate 3µm long Ag nanowires. However, the deposition time for the 5 µm gold nanowire was 90 minutes. The length of the nanowires can be

controlled by monitoring the total charge passing through the electrochemical cells and the deposition time. The plating solution can be changed during the electroplating process to produce multisegment nanowires.

After the electrodeposition process, the silver film was removed in a 3 M nitric acid and the metal nanowires were released by dissolving alumina membranes in a 3 M NaOH aqueous solution. Template dissolution produces a monodispersed suspension of individual wires, which can then be modified with desired functional group. The nanowires were then washed by DI water and centrifuging at 5000 rpm for 10 minutes several times. Since the length of the nanowires was in the micrometer region, the concentration of the nanowires was determined by counting the number of nanowires using a hemacytometer on an inverted microscope (Olympus, IX 71).

3.3 Results and discussion

When AAO template was used for electroplating, the metal ions can be reduced into the nanochannels where the metal could grow from the bottom of channel to form nanowires.

Shown in figure 1A is the AAO template with silver nanowire. These AAO templates (figure 1B) with 40 to 50 nm pore diameter were fabricated by anodization of the aluminum substrate in 0.3 M oxalic acid aqueous solution. The average pore size was measured to be 43nm. Since the production of such template was time consuming, the commercial AAO membrane was used this experiment for the fabrication of nanowires. Shown in figure 2A is the SEM image of a commercial AAO membrane with irregular pore arrangement where the pore size was about 200nm. When different electroplating solutions were used, it was possible to produce nanowires with multi-components. Shown in figure 2B is the SEM image of the gold-nickel nanowires on the AAO membrane before the removal of the silver backing. To remove the silver backing, the AAO membrane was dissolved in 6 M HNO3 solution for 10 minutes. The alumina membrane was then rinsed with DI water and placed in NaOH solution, and

sonicated for 10 minutes to release the nanowires from the AAO membrane. This process was repeated several times. Figure 2C illustrates the cross-sectional SEM image of 2.5 µm long

nickel nanowires in the nanochannels.

After the AAO templates with high purity nickel nanowires were annealed at 350 ℃for 2 h, the nickel metal nanowires could be released by dissolving alumina membranes. The wires were then rinsed with ethanol solution and spin coated on the ITO substrate. When the

ethanol solution was evaporated, magnetic nickel nanowires bundles could be obtained.

Shown in figure 3 is the back scattered SEM image of the bundles of nickel nanowires. These nanowires were approximately 6 to 7 µm in length. Since AAO templates are stable at high

temperatures, it has been suggested that the annealing process for the magnetic nanowire could produce GMR effect [17].

To fabricate 200 to 300 nm long gold nanorods, the gold plating solution was used and the electrodeposition time was about 20 minutes. After removing the silver backing by nitric acid, the isolated gold nanorods were obtained as shown in figure 4. The same approach can be used to obtain multi-component nanowires composed of silver and nickel segment. Shown

in figure 5 is the back scattered SEM image of Ag-Ni nanowire where the silver portion was about 2 µm long and nickel portion was about 4 µm. Gold –nickel nanowires could also be

obtained by the same process as shown in fig 6 [14]. In the SEM image, the gold portion seems to be brighter than the nickel portion due to the difference in the electron scattering cross section of gold and nickel. It should be noticed that care must be taken to avoid the fracture of the high aspect ratio nanowires.

Figure 7 shows a high density assembly of nanowire composed of silver and gold with 2

~ 3 µm in length. The gold portions of the two-component structure were used to protect

silver domain during the silver backing removal process. In case of the production of the Ag-Au-Ag-Au nanowires, it is very important to control the growth process as shown in

figure 8. Since the extinction coefficient of the silver and gold are different in the UV region and the electron scattering cross section is also different for these two metals, they exhibit different reflectivity in both optical image and electron image as shown in figure 9 and 10 [18].