A versatile synthesis method of dendrites-free segmented nanowires with a precise size control
© Sousa et al; licensee Springer. 2012
Received: 29 July 2011
Accepted: 5 March 2012
Published: 5 March 2012
We report an innovative strategy to obtain cylindrical nanowires combining well established and low-cost bottom-up methods such as template-assisted nanowires synthesis and electrodeposition process. This approach allows the growth of single-layer or multi-segmented nanowires with precise control over their length (from few nanometers to several micrometers). The employed techniques give rise to branched pores at the bottom of the templates and consequently dendrites at the end of the nanowires. With our method, these undesired features are easily removed from the nanowires by a selective chemical etching. This is crucial for magnetic characterizations where such non-homogeneous branches may introduce undesired features into the final magnetic response. The obtained structures show extremely narrow distributions in diameter and length, improved robustness and high-yield, making this versatile approach strongly compatible with large scale production at an industrial level. Finally, we show the possibility to tune accurately the size of the nanostructures and consequently provide an easy control over the magnetic properties of these nanostructures.
In this letter, we report an innovative strategy to fabricate dendrite-free nanowires ranging from few nanometers to several micrometers in length. The introduced approach suppresses the main disadvantage of homogeneous pore filling of PAA using PED, i.e., the dendritic structures are selectively etched, leading to nanowires free from end-ramifications. Here, single-layer and segmented nanowires with well controlled dimensions, showing a narrow size distribution (both in diameter and length) were successfully prepared. We support the robustness of the presented method providing the influence of the nanowires aspect ratio on their magnetic properties.
PAA templates were fabricated using a two-step anodization process , in 0.3 M (COOH)2 at 4°C, under 40 V. The nanopores display 35 nm in diameter, 105 nm of interpore distance, and length of 1.5 μ m (pore growth rate of 2.5 μ mh-1) as shown in Figures 1a, b. After the second anodization, the alumina barrier layer at the bottom of the pores shows a thickness of ~ 52 nm, preventing the PED process. Thus, this barrier layer was smoothly thinned by applying an exponentially decreasing voltage from 40 V down to 8 V. This non-steady-state anodization originates tree-like branched pores with various lengths and diameters . The final potential value corresponds to a barrier layer thickness of 10 nm as obtained from , providing the conditions for which the most uniform nanowires length is attained .
Subsequently, the PAA is filled with the desired material by PED method [22, 23]. In this study, we use Ni, Au and Cu: Ni was deposited at 40°C from an aqueous solution containing NiSO4.6H2O (300 g/L), NiCl2.6H2O (300 g/L) and H3BO3 (45 g/L); Au nanosegments were deposited from a 2.5 DWT/Qt. Technic, Inc. electrodeposition solution at 40°C; and Cu was deposited at room temperature from CuSO4.5H2O (1 M) H3BO3 (45 g/L). Segmented nanowires were prepared by first depositing Cu in the branched pores, followed by Au and finally Ni. The three materials were grown using a cycle with three different pulses. Initially, the material is deposited by applying a constant current pulse (70 mA/cm2) for 8 ms. Then, a second pulse with opposite polarity and constant potential of 8 V is used during 2 ms. This pulse is used to discharge the barrier-layer (which acts as a capacitor). It also homogenizes the membrane, helping to repair small cracks that may appear during the first step. Finally, a rest pulse of 0.7 s is applied, introducing a delay time to refresh the metallic ions concentration at the deposition interface and promoting a uniform growth of nanowires. Notice that the dendrites were filled in 55 s, whereas the length of the Au and Ni segments can be varied using the calculated deposition rates of 2.0 nm/s and 1.5 nm/s, respectively. In addition, and when required, the Al substrate was removed with 0.5 M CuCl2 and 10% HCl. The PAA template was dissolved in an aqueous solution of 0.4 M H3PO4 and 0.2 M H2Cr2O7. To selectively etch the Cu dendrites a 1% HNO3 solution was used.
3 Results and discussion
Figures 1a, b show top and cross sectional SEM images of PAA, respectively. Inherent to the process of PAA growth is the presence of a thick continuous oxide-layer at the bottom of each nanopore (Figure 1b) [22, 24]. Therefore, after the second anodization the barrier layer was thinned and the metal electrodeposited. Figure 1c shows single-layer Ni nanowires embedded in a 1 μ m thick PAA template. As seen, the employed barrier layer thinning procedure leads to ~300 nm long non-uniform branched pores at the bottom of the template, which is subsequently replicated by the Ni nanowires. The presence of such structures with several diameters and lengths leads to the loss of the major advantage of this template assisted method, i.e., the nanowires homogeneity in size. Even, after removing the PAA template by chemical etching, the free-standing nanowires remain with the dendrites at the bottom end as shown in the TEM image (Figure 1d). These inhomogeneities confer distinct physical properties and may lead to anomalous behaviors when compared to the main cylindrical nanowires . Such drawback strongly limits practical applications combining PAA and PED to attain extremelly small features.
In summary, we demonstrated an innovative and expedite method to grow cylindrical nanowires in a controlled way ranging from nanometers to micrometers in length. With this method the major limitation of using PAA templates is overcome by selectively removing the dendritic ends, formed due to the inherent barrier layer thinning step required for nanowires growth. Therefore, the presence of undesirable contributions to the magnetic response which may arise from such non-homogenous features are easily removed. Also, several materials can be used to synthesize these nanostructures as long as the dendrites can be chemically etched without affecting the main nanowires dimensions and composition, evidencing the enormous versatility of the presented strategy. We successfully combined well established top-down methods such as PAA template-assisted nanowire synthesis and electrodeposition, which have displayed a large appeal for industrial processing due to their lower-cost and higher-yield. Finally, considering the growing industrial interest in magnetic nanoparticles and high-aspect-ratio segmented nanowires, the developed approach enables the synthesis of nanostructures with high homogeneity in diameter and highly controllable length allowing an effective tune of their magnetic behaviors ultimately enabling reaching the superparamagnetic regime.
The study was supported in part by projects PTDC/FIS/105416/2008 and CERN/FP/123585/2011. Funding from FCT through the Associated Laboratory-IN is acknowledged. CTS and DCL thank FCT for grants SFRH/BPD/82010/2011 and SFRH/BPD/72359/2010, respectively. JV acknowledges financial support through FSE/POPH.
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