Fabrication of Nickel Nanostructure Arrays Via a Modified Nanosphere Lithography
© Wei et al. 2010
Received: 22 July 2010
Accepted: 17 August 2010
Published: 2 September 2010
In this paper, we present a modified nanosphere lithographic scheme that is based on the self-assembly and electroforming techniques. The scheme was demonstrated to fabricate a nickel template of ordered nanobowl arrays together with a nickel nanostructure array-patterned glass substrate. The hemispherical nanobowls exhibit uniform sizes and smooth interior surfaces, and the shallow nanobowls with a flat bottom on the glass substrate are interconnected as a net structure with uniform thickness. A multiphysics model based on the level set method (LSM) was built up to understand this fabricating process by tracking the interface between the growing nickel and the electrolyte. The fabricated nickel nanobowl template can be used as a mold of long lifetime in soft lithography due to the high strength of nickel. The nanostructure–patterned glass substrate can be used in optical and magnetic devices due to their shape effects. This fabrication scheme can also be extended to a wide range of metals and alloys.
Ordered nanostructure arrays have attracted considerable attention due to their unique properties and potential applications in such as data storage , biosensors , catalysis , and photonic crystals [4, 5]. For example, cobalt nanobowl arrays show enhanced coercivity in comparison with spherical particles due to shape effects . However, how to effectively and economically produce such nanostructures in a large area is still a big challenge, even though electron beam lithography, focused ion beam (FIB), and X-ray lithography have been widely used in nanofabrication due to their high resolution. To this end, nanoimprinting lithography (NIL) and nanosphere lithography (NSL), as two low-cost and high-throughput techniques, have been extensively studied in recent time. A high-quality nanoimprint stamp is used in NIL to implement the imprinting, but the stamp fabrication still relies on the aforementioned techniques that are time-consuming and expensive. In the later technique, a monolayer or multilayer of nanospheres is self-assembled and used as the deposition or etch resistive masks. Now, it has been proven to be a promising alternative to the conventional top-down fabrication techniques. For example, through the interstitial spacing between the nanospheres, materials are deposited onto the substrate by using various deposition techniques, such as chemical vapor deposition (CVD) , electroless deposition , atomic layer deposition (ALD) , and even molecular beam epitaxy (MBE) , which is used to deposit single crystals and fabricate quantum dot arrays [11, 12]. Nanostructures of various shapes such as chains , triangles , shells , rings , rods , disks , and cups  have been fabricated on different substrates. Silicon nanopillars were also fabricated using self-assembled nanospheres as a mask in combination with the reactive ion etching (RIE) [20, 21] or deep RIE techniques . Clearly, NSL introduces a controllable way to pattern the substrates but the closely packed nanosphere arrays only work as a shadow mask in most cases and high vacuum chambers are required to perform CVD, e-beam evaporation, sputtering, or ALD, which are expensive and slow processes. In this paper, we present a new scheme to fabricate a metallic master template of ordered nanobowl shape together with a nanobowl structures array–patterned glass substrate. Unlike the reported strategies of using self-assembled polystyrene (PS) nanospheres as mask in literature, a monolayer of PS nanospheres was firstly self-assembled on a gold-coated glass substrate, and then a thin gold film was coated on the top of the PS nanospheres. Followed by nickel electroforming, a nanobowl-shaped template was electroformed on the top PS hemispheres, and shallow nickel nanobowl arrays were grown on the gold surface beneath the bottom PS hemispheres.
The microscopic examination of the produced samples was conducted in our focused ion beam (FIB, StrataTM DB 235, FEI) system. In order to understand the pattern formation around the PS nanospheres, a multiphysics modeling of the process was carried out by using a finite element analysis package (Comsol Multiphysics 3.5a). In this model, the mass transport of chemical species was simulated on the base of the Nernst-Planck application mode, and the moving interface between the electrolyte and metal was analyzed by using the level set application mode.
Results and Discussion
In summary, we proposed and demonstrated a modified NSL method to fabricate metallic nanostructures in a simple and inexpensive way. A nickel template of ordered nanobowls array together with nanostructured modified glass substrate with a gold-coated surface was fabricated by combining self-assembly method with electroforming. The nickel nanobowls exhibit a uniform size and smooth interior surface. No shrinkage of the material occurs in this process when the PS template is removed. The electroforming process can be easily controlled by adjusting operating parameters to produce high-density and fine grain–structured metal deposits. Compared with other metal deposition methods as reported in the literature, electroforming is a facile method to fabricate metallic nanostructures at a low-cost and high-throughput [16, 18].The choice of deposited materials can be also extended to other metals (e.g. Au, Ag) and alloys (e.g. Fe-Pt, Co-Pt).
The authors are grateful to the financial support of European Union project (FP6 RaSP).
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