Topic3- Fabrication and Wetting Properties of Metallic Half-Shells with Submicron Diameters
colloids, thin film, self-assembled monolayers, gold half-shells, superhydrophobic
Nanofabrication and nanopatterning have very useful applications in a variety of industries. The semiconductor industry has placed particular emphasis on creating nano-sized features in devices which has lead to the development of many nanofabrication techniques. For the deposition of thin films on the nanometer scale, molecular beam epitaxy (MBE), physical vapor deposition (PVD), and chemical vapor deposition (CVD) have been developed. The high precision of these techniques enable the deposition of thin films on the order of angstroms! Techniques that utilize electron beam and thermal evaporation allow for the formation of thin films consisting of a variety of materials. By combining these techniques, nanostructures can be created for many different applications. Love et. al explored the use of templates to create 100-500 nm diameter hollow metal half spheres. Specifically, they employed sacrificial silica colloids to behave as templates for the deposition of thin metal films. After deposition, the colloids were removed and the metal films released from the template forming hollow half spheres. These nano half spheres have several properties that make them attractive for studies involving surface plasmons or magnetic filtration. They can also be used to create metallic foams, heterogenous catalysts and superhydrophobic surfaces.
The simplicity of the fabrication process for these hollow half shells increases its appeal as a mainstream fabrication technique. Figure 1 provides a brief outline of the process. Aqueous suspensions of silica colloids were initially deposited onto a glass slide as monolayers or multilayers. Titanium or nickel was then deposited on top of the colloids using electron-beam evaporation which formed a 0.5-0.8 nm layer. Following this step, a thin layer of metal (i.e., gold, platinum, palladium) usually around 15 nm was deposited on top of the previous layer. The spherical colloids were released from the glass slide by treating with aqueous HF which acts to etch the silica and titanium/nickel. Once this sacrificial layer is eliminated, the metallic half shells are released from the template.
Variation in Shell Morphology
It was observed that the morphology of the half-shells varied depending on the thickness of the shell. For thin films between 8-15 nm, the gold half-shells had grain sizes between 40-50 nm in diameter with pores lining the boundaries of the grains. On the other hand, palladium and platinum half-shells with thicknesses greater than 8 nm contained no visible pores and grain sizes were 15-20 nm in diameter. Furthermore, the gold shells around 100 nm in diameter were more textured than the platinum shells of similar size. The reason for this inconsistency in morphology was attributed to the ability of the metals to wet the template. Gold is unable to wet the silica template and the titanium/nickel layer as well as palladium or platinum and as a result, pores form within the half-shells. This phenomenon is demonstrated in Figure 2 where the gold half-shells (Figure 2a,d) are clearly more porous than its palladium (Figure 2b) and platinum (Figure 2c) counterparts.
To obtain the most uniform set of hollow shells, the silica template needed to be arranged in a hexagonally close packed monolayer. These shells were found to be very stable even under the conditions required to release the shells from the template or during surface drying before imaging. Fewer than 5% of the shells were found to be deformed or broken after the fabrication process. Furthermore, the shells retained their structure after sonication in an ultrasonication cleaner.
Application: Creation of Superhydrophobic Surface
The metallic hollow half-spheres possess several characteristics that make them attractive options for applications such as the creation of a superhydrophobic surface. The main characteristics of these particles include a.) a high surface area to volume ratio, b.) high density packing of half-shells, c.) high surface roughness, d.) surface property tuning using self assembled monolayers. When a water droplet was placed on the surface of aggregated gold half-shells, the contact angle was 151 degrees. For a surface to be considered superhydrophobic, droplets must form a contact angle of atleast 150 degrees. In this regard, the gold half-shells appear hydrophobic; however, when the surface was tilted to 90 degrees, the <math>5 uL</math> droplet of water remained attached to the surface. In an effort to promote the "rolling" of water droplets off the surface, the surface was modified with hexadecanethiolate. This increased the contact angle to 163 degrees and the droplet easily slid off the surface when tilted above a 1 degree angle.
 Love, J. C., Gates, B.D., Wolfe, D.B., Paul, K.E., and Whitesides, G.M. (2002) Fabrication and Wetting Properties of Metallic Half-Shells with Submicron Diameters. Nano Letters. 2(8):891-894.