1. Introduction

The applications of nanostructures in photonic crystals, [1, 2] biosensors [3, 4] and data storage [5, 6] have stimulated the technical developments in nanolithography and the chemical synthesis of nanostructures. Metallic nanostructures with various shapes such as triangles, [3] shells, [7, 8] rings, [9] rods, [10] disks [11] and cups [12] have been produced. However, many of these techniques are found only suitable for fabrication of metallic nanostructures in a small area due to their costs and productivities. For example, electron beam lithography and focused ion beam (FIB) can beat the diffraction limit of light and make nanometer features, but their throughput is a serious limitation when writing dense patterns over a large area. [13] Therefore, much effort has been focused on developing novel techniques to fabricate metallic nanostructures suitable for large areas and at low costs. Periodic nanobowl structure arrays and pillar arrays are of great interest because of their potential applications in biomedical and nanofluidic devices, [14] nanosphere selection, [15] coercivity enhancement, [14] plasmonic devices for enhancing electromagnetic fields, [16] and electrocatalytic activities. [17] Wang et al. presented an approach that used monolayer self-assembly (MSA) and atomic layer deposition (ALD) to fabricate arrays of TiO₂ nanobowls with a flat bottom. [15] However, this approach is not an economical and simple one since advanced equipments such as ALD equipment and ion milling machine were involved and the milling process must be well controlled to ensure that the top half of the spheres was evenly removed in the fabrication process. Srivastava et al. reported a technique of template-assisted assembly of cobalt nanobowl arrays, but volume shrinkage and cracking of the material were found due to the disadvantages of the infiltration approach. [14] Gold nanopillar arrays have been fabricated by electrochemical deposition of gold into the nanopores of an anodic aluminum oxide (AAO) membrane placed onto the gold thin film electrode surface, [17] but the pillar arrays were not periodic and the pillar size was not uniform due to the variation of the pore diameters in the AAO membrane. [17, 18]

We present a process that combines MSA and electroforming for fabricating a large area of metallic hybrid nanostructures at a low cost. Our approach was based on the following considerations. i) MSA is an effective technique and has been used to inexpensively produce nanoparticle arrays on a relatively large scale with controlled shape, size and interparticle spacing. [4] ii) Electroforming, as a traditional method, has been widely used in the fabrication of metallic MEMS devices. iii) Nickel is a good candidate material for the mold due to its hardness. Unlike the samples reported in [14] and [15], the metallic nanostructures produced by this approach are not amorphous but nanocrytalline material, so the annealing process is not needed. Volume shrinkage and cracking of the material were not observed. This approach is more economical than that in [15] since both MSA and electroforming are cost-effective methods. Each structure unit consists of a hexagonal distributed pillar array upon a nanobowl. Both nanobowl arrays and pillar arrays exhibit uniform sizes. Smooth interior surfaces were observed in the nanobowl arrays. The geometry of the structure can be tuned by controlling the thickness of the conductive layer. The approach presented in this paper can be extended to fabricate ordered hybrid nanostructures of a wide range of metals and alloys with controlled size.

2. Experimental

Figure 1 schematically illustrates the fabrication process. A highly ordered monolayer of PS spheres was self-assembled onto a silicon substrate (Fig. 1(a)). A 50 nm-thick conductive layer (gold film) for electroforming was deposited on the template by thermal evaporation (Fig. 1(b)), and then a 200 μm-thick nickel sheet was electrochemically deposited on the gold coated template (Fig.1(c)). After the nickel sheet was peeled off from the template and ultrasonically etched in ethanol solution for 10 min to completely remove the PS spheres in the metal. The gold film on the metal sheet was removed in a gold etchant. Finally, a large-area of nickel nanostructure arrays was obtained, as shown in Fig. 1(d).

To produce PS sphere template, silicon (100) substrates with an area of 2 cm×2 cm cut from single side polished silicon wafers were used. The dust-free silicon substrates were firstly cleaned in piranha solution (3:1, H₂SO₄:30%H₂O₂) at 80 °C for 1 hour, followed by rinsing repeatedly with ultrapure water (18.2 MΩ, Millipore Simplicity). Next, the substrates were sonicated for 1 hour in RCA (5:1:1, H₂O:NH₄OH:30% H₂O₂), followed again by thorough flushing with ultrapure water. The RCA treatment adds many −OH groups to the surface, which make the substrates hydrophilic. The substrate was dried with nitrogen, and then 2 μl of diluted PS solution with a mean diameter of 500 nm and a size distribution of 3% (Duke Scientific Corporation, USA) was dropped onto the silicon substrate surface using a micropipettor and spread evenly. The single layer of nanospheres was used as the template. A 50 nm-thick gold film was deposited on the PS surface at a rate of ~16 nm min^-1 in a thermal evaporation system at a pressure of 2×10^-5 mbar. A 200 μm-thick nickel was electroformed on the gold coated PS monolayer using a nickel sulfamate solution. The pH value of electrolyte was adjusted to 4.5, in order to avoid chemical damages to the PS monolayer on the treated substrate. A relatively high current density (~40 mA cm^-2) was applied in the electroforming process in order to get smaller grain size of the nickel and shorten the process time. Finally, nickel nanostructures were obtained after the nickel sheet was peeled off from the PS template and washed in ethanol for 10 min to remove the PS spheres on the nickel surface.

 

Fig. 1. Process flow chart of the fabrication of large-area ordered metallic nanostructures arrays. a) The silicon substrate is coated with a monolayer of PS spheres. b) A thin conductive layer is deposited on top of the PS spheres by thermal evaporation. c) A thick metal sheet is electrochemically deposited on top of the conductive layer. d) After removal of PS spheres and the conductive layer, periodic nanostructure arrays of the desired material are obtained.

Download Full Size | PDF

3. Results and discussion

A scanning electron microscope (SEM) was used to examine the result of each step described above. Figure 2(a) shows a monolayer of self-assembled PS spheres on silicon substrate. After being peeled off from the top of the nanospheres and washed in ethanol ultrasonically to remove PS spheres, the periodical nanostructures on the surface of nickel sheet were exposed clearly as shown in Fig. 2(b). The composition of the nanostructure in Fig. 2(b) was confirmed by energy-dispersive X-ray spectroscopy coupled with the SEM (inset in Fig. 2(b)). In addition to nickel, gold and carbon were also shown, which is attributed to the adhesion of gold film and PS residues during the removal process. After this thin layer of gold on the surface was removed in gold etchant, the pure nickel nanostructures are shown in Fig. 2(c). A high magnification image (inset in Fig. 2(c)) indicated that the patterned nanobowl structures have a good uniformity. Figure 2(d) shows the SEM image of side-view of the metallic nanostructures by tilting the sample with an angle of 45°. From this image, hexagonal distributed metallic pillars with an average diameter of 95 nm and an average height of 110 nm were observed on the top of the nanobowls. Volume shrinkage and cracking of the material were not observed. The inset in Fig. 2(d) shows the cross-section of the nickel nanostructures milled by FIB (StrataTM DB 235, FEI) with a beam current of 100 pA, and smooth interior surfaces were observed in the nanobowls.

 

Fig. 2. SEM images of nanostructures during the fabrication process. (a) Monolayer self-assembly of PS spheres on silicon substrates. (b) Nanobowl arrays after PS spheres are removed, and the inset shows the EDS spectra recorded from the nanostructures. (c) Nickel nanobowl array after gold film is removed, and the inset is the high magnification image. (d) Nickel nanostructure arrays with a tilt angle of 45°, hexagonal distributed nickel nanopillars on the top of nanobowls can be clearly seen. The inset is the cross section with a tilt angle of 45°.

Download Full Size | PDF

Drop casting and spin casting are the two effective means of creating ordered arrays of nanospheres. The PS nanospheres were drop coated since this is a simple method of achieving the desired results. The substrate was tilted by 10° to allow gravity to assist in the even distribution of the droplet. The quality of the results strongly depends on the latex/water proportion, properties of the substrate, and the homogeneity of the solvent evaporation. The latex/water proportion was calculated by assuming that a monolayer of spheres should cover the area of substrate evenly. [19] The monolayer assembly process was greatly aided by the negatively-charged carboxylate functionalities. The columbic repulsion between PS spheres causes the spheres to evenly spread out across the surface. The self-assembly process was undertaken in Petri dish under humid conditions to slow down evaporation process. Although the periodic metallic nanostructures at larger scale were separated by line defects into domains (100-500 μm²), the whole surface of metal sheet was still covered with metallic nanostructures. The periodic structures were well preserved and structure distortion was not observed during the entire process.

The morphology of the nanobowls (shown in Fig. 2(d)) is the reverse profile of the top half part of PS nanospheres coated with a thin conductive layer, and the hexagonal distributed pillar arrays are formed by the interstitial spaces between the PS spheres. One remarkable advantage of the presented approach is that the geometry of the structure can be tuned by adjusting the thickness of the conductive layer. The effect of conductive layer thickness on the nanobowl profile was simulated by assuming that the conductive layer should cover evenly on the projection plane of the nanospheres during the deposition process (Fig. 3(a)). As can be seen in the figure, the depth variation of nanobowl is equal to the thickness of conductive layer. Since the geometry of the metallic nanostructures is the reverse profile of top part of nanospheres coated with a conductive layer, there is no flat surface at the bottom of the nanobowls (inset in Fig. 2(d)), which are different from those reported previously. [14] The interstitial spaces between the spheres will be partly filled as a result of the growth of conductive layer (Fig. 3(b)). The yellow area in the inset shows the top view of the void between PS nanospheres. The in-plane pillar size will decrease with the increase of the film thickness, and the pillars will vanish if the interstitial space is completely blocked by a very thick conductive layer.

 

Fig. 3. The tuning of conductive layer thickness on the geometry of the metallic nanostrucutres. (a) The effect of conductive layer thickness on the nanobowl profile. (b) The in-plane metallic pillar size depends on the voids between nanospheres.

Download Full Size | PDF

The approach presented in this paper can be extended to fabricate ordered hybrid nanostructures of a wide range of metals (Au, Cu, Co et al.) and alloys with controlled size. The quality of nanostructures with such geometry is suitable for industrial application and fundamental research. Nickel is a good candidate material for the mold due to its high strength, and such nickel nanostructures can be used as a nanoimprint stamp to massively produce nanobump structures, which have been applied in the hard disk drive industry. [20] The nickel nanostructures were demonstrated to be useful for fabricating ordered Poly (dimethylsiloxane) (PDMS) nanolens array. PDMS precursor (Elastomer Base and Curing Agent mixture 10:1) was cast onto the nickel nanostructures and cured at 90°C for 90 min. Ordered PDMS nanolens arrays were obtained after the PDMS was pulled out of the master. Fig. 4 shows the SEM image of the PDMS nanolens array, which exhibits smooth surface and relatively good uniformity. The shape dependence of magnetic properties has attracted particular interest in the fabrication of magnetic nanostructures of different shapes. Enhanced coercivity was found in cobalt nanobowl arrays due to its shape effects. [13] Nanobowl structures can be used as ultra small containers for holding fluid of nanoscale volume and for nanosphere selection. In metallic systems, the conduction electron charge density and its corresponding electromagnetic field can undergo plasmon oscillations. The charge oscillations in noble metals (e.g. Au, Ag) can propagate along the surface at optical frequencies due to the nature of their optical constants. These surface plasmons can be excited by an incident light in a process that depends on the metal, size, shape and local dielectric environment. Future work will be focused on the optical properties of such hybrid nanostructures of noble metals.

 

Fig. 4. PDMS nanolens array with a tilt angle of 30°.

Download Full Size | PDF

4. Conclusions

In conclusion, a low-cost, high-throughput fabrication process has been developed to produce metallic hybrid nanostructure arrays over large areas without the need of expensive equipments. The process is carried out by a combination of MSA and electroforming techniques. Highly ordered metallic hybrid nanostructure arrays have been fabricated, and both nanobowl arrays and pillar arrays exhibit uniform sizes. Smooth interior surfaces were observed in the nanobowl arrays. The size of the fabricated nanostructures can be adjusted by varying the size of PS nanospheres. The geometry of the structure can be tuned by controlling the thickness of the conductive layer. PDMS nanolens arrays were fabricated by using the nickel hybrid nanostructures as the mold. The choice of materials used in this approach can be extended to a wide range of metals and alloys. Such metallic nanostructures are suitable for industrial application and fundamental research.