Yiping Zhao Research Group

Figure 1 shows some representative shapes of water droplets on the as-deposited Si nanostructured films (left column of Fig. 1) and on the HF treated nanostructured films (right column of Fig. 1). For the as-deposited hydrophilic Si surfaces, the contact angle decreases from 31.1 degrees to 3.2 degrees with increasing normal film thickness from 25 nm to 2000 nm; while for the HF treated hydrophobic Si surfaces, the contact angle increases from 86.8 degrees to 142.7 degrees under the same growth conditions. We observed the following trends: for an originally hydrophilic film, the increasing nanorod height made the film more hydrophilic, while for an originally hydrophobic film, increasing nanorod height made the film more hydrophobic. And the super-hydrophilic and super-hydrophobic properties of the nanostructured films depended strongly on the height of the nanorod. In Fig. 2, we plotted the water contact angles of as-grown Si nanostructures and HF treated Si nanostructures versus the normal film thickness d . The trend observed in Fig. 2 was clearly demonstrated. Surprisingly, the contact angles at both hydrophilic surface and hydrophobic surface experienced a transition at ~ d = 500 nm: for d < 500 nm, the contact angle had a steep increment or decrement, while for d = 500 nm, the contact angle tended to saturate.

When compared to conventional rough surfaces, vertically aligned nanotube or nanorod arrays have their own unique characteristics. The individual nanotubes or nanorods not only have a high aspect ratio (the ratio of feature's height to its diameter), but also have an end anchored to the surface. The separation between the adjacent nanotubes or nanorods is on the order of 50 – 200 nm. In a liquid environment, such an arrangement will result in a large capillary pressure , which could easily cause the nanotubes or nanorods to deform , thus changing the surface of the entire nanostructure assembly permanently. In our study, with the as-grown Si nanorod array, we also noticed a dramatic change in surface structure, which we call it “nanocarpet effect”. Figure 3 shows the water mark left by the nanocapet effect, and Figure 4 illustrates the detailed structural change on vertically aligned Si nanorods. This change in surface structure may play a very important role in affecting the performance of devices developed based on these kinds of nanostructures. The questions remain whether the pattern change follows a statistical rule, and what the physical cause is for such a change. Answering these questions will help us not only to understand the mechanism of droplet spreading on a nanostructured surface, but also to find ways to either utilize or avoid this effect.

Figure 4. An orange watermark formed on 890 nm Si nanorods by a 3 u l water drop.



	Wetting Behavior of Water on Si Nanorod Array Yiping Zhao Department of Physics and Astronomy, University of Georgia , Athens , GA 30602 , USA The wettability of a solid surface is affected by the surface roughness and the surface chemical composition. It has been demonstrated that fractal rough surfaces or patterned microstructured pillar arrays can greatly alter the contact angle depending on the wettability of the original flat surface: for a hydrophilic surface, a super-hydrophilic property can be developed , and for a hydrophobic surface, a super-hydrophobic property can be produced. Very recently, the super-hydrophobic property has also been found in chemically treated, vertically aligned carbon nanotube arrays and Polyacrylonitrile nanofiber arrays. These studies show that a one-dimensional nanowire/rod array has a dramatic effect on the wettability of the surface. Unfortunately the detailed relationship between the wettability and structure parameters has not been investigated. In order to probe how the nanometer features affect the wettability of a surface, the most difficult task is to fabricate and quantify nanostructures that can be systematically varied. Here we would like to perform a systematic investigation on the wettability of vertically aligned Si nanorod arrays with different height s (aspect ratio s ) fabricated by the glancing angle deposition (GLAD) technique.
	Figure 1 shows some representative shapes of water droplets on the as-deposited Si nanostructured films (left column of Fig. 1) and on the HF treated nanostructured films (right column of Fig. 1). For the as-deposited hydrophilic Si surfaces, the contact angle decreases from 31.1 degrees to 3.2 degrees with increasing normal film thickness from 25 nm to 2000 nm; while for the HF treated hydrophobic Si surfaces, the contact angle increases from 86.8 degrees to 142.7 degrees under the same growth conditions. We observed the following trends: for an originally hydrophilic film, the increasing nanorod height made the film more hydrophilic, while for an originally hydrophobic film, increasing nanorod height made the film more hydrophobic. And the super-hydrophilic and super-hydrophobic properties of the nanostructured films depended strongly on the height of the nanorod. In Fig. 2, we plotted the water contact angles of as-grown Si nanostructures and HF treated Si nanostructures versus the normal film thickness d . The trend observed in Fig. 2 was clearly demonstrated. Surprisingly, the contact angles at both hydrophilic surface and hydrophobic surface experienced a transition at ~ d = 500 nm: for d < 500 nm, the contact angle had a steep increment or decrement, while for d = 500 nm, the contact angle tended to saturate. Figure 1. The representative shapes of water droplets on the as-deposited Si nanostructured films (left column) and on the HF treated nanostructured films (right column). Figure 2. The water contact angles versus normal film thickness for the as- deposited and HF-treated nanostructured films. The solid curves are the best fittings according to Wenzel's and Cassie's laws. The dashed curve is the calculation of the contact angles from the SEM data based on Wenzel's law. When compared to conventional rough surfaces, vertically aligned nanotube or nanorod arrays have their own unique characteristics. The individual nanotubes or nanorods not only have a high aspect ratio (the ratio of feature's height to its diameter), but also have an end anchored to the surface. The separation between the adjacent nanotubes or nanorods is on the order of 50 – 200 nm. In a liquid environment, such an arrangement will result in a large capillary pressure , which could easily cause the nanotubes or nanorods to deform , thus changing the surface of the entire nanostructure assembly permanently. In our study, with the as-grown Si nanorod array, we also noticed a dramatic change in surface structure, which we call it “nanocarpet effect”. Figure 3 shows the water mark left by the nanocapet effect, and Figure 4 illustrates the detailed structural change on vertically aligned Si nanorods. This change in surface structure may play a very important role in affecting the performance of devices developed based on these kinds of nanostructures. The questions remain whether the pattern change follows a statistical rule, and what the physical cause is for such a change. Answering these questions will help us not only to understand the mechanism of droplet spreading on a nanostructured surface, but also to find ways to either utilize or avoid this effect. Figure 4. An orange watermark formed on 890 nm Si nanorods by a 3 u l water drop. Figure 5. SEM top-view and cross-section at the center of this watermark showing nanorods bundled together. The scale bars represent 2 um.
	Last updated on July 24, 2006

Wetting Behavior of Water on Si Nanorod Array