ARTICLE pubs.acs.org/Langmuir
Mimicking Both Petal and Lotus Effects on a Single Silicon Substrate by Tuning the Wettability of Nanostructured Surfaces M. K. Dawood, H. Zheng, and T. H. Liew Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
K. C. Leong GLOBALFOUNDRIES Singapore Pte. Ltd, Singapore 738406
Y. L. Foo Institute of Materials Research and Engineering (IMRE), Agency of Science Technology and Research (A*STAR), Singapore 117602
R. Rajagopalan and S. A. Khan Department of Chemical and Biomolecular Engineering, National University of Singapore, Chemical and Pharmaceutical Engineering Programme, Singapore-MIT Alliance, Singapore 117576
W. K. Choi* Department of Electrical and Computer Engineering, National University of Singapore, Advanced Materials for Micro- and Nano-Systems Programme, Singapore-MIT Alliance, Singapore 117576
bS Supporting Information ABSTRACT: We describe a new method of fabricating large-area, highly scalable, “hybrid” superhydrophobic surfaces on silicon (Si) substrates with tunable, spatially selective adhesion behavior by controlling the morphologies of Si nanowire arrays. Gold (Au) nanoparticles were deposited on Si by glancingangle deposition, followed by metal-assisted chemical etching of Si to form Si nanowire arrays. These surfaces were chemically modified and rendered hydrophobic by fluorosilane deposition. Au nanoparticles with different size distributions resulted in the synthesis of Si nanowires with very different morphologies (i.e., clumped and straight nanowire surfaces). The difference in nanowire morphology is attributed to capillary force-induced nanocohesion, which is due to the difference in nanowire porosity. The clumped nanowire surface demonstrated the lotus effect, and the straighter nanowires demonstrated the ability to pin water droplets while maintaining large contact angles (i.e., the petal effect). The high contact angles in both cases are explained by invoking the Cassie-Baxter wetting state. The high adhesion behavior of the straight nanowire surface may be explained by a combination of attractive van der Waals forces and capillary adhesion. We demonstrate the spatial patterning of both low- and high-adhesion superhydrophobicity on the same substrate by the simultaneous synthesis of clumped and straight silicon nanowires. The demonstration of hybrid superhydrophobic surfaces with spatially selective, tunable adhesion behavior on single substrates paves the way for future applications in microfluidic channels, substrates for biologically and chemically based analysis and detection where it is necessary to analyze a particular droplet in a defined location on a surface, and as a platform to study in situ chemical mixing and interfacial reactions of liquid pearls.
1. INTRODUCTION Nature presents us with at least two types of superhydrophobic, highly water-repellant surfaces: low-adhesion superhydrophobic surfaces, as observed in lotus leaves (the lotus effect),1,2 and highadhesion superhydrophobic surfaces as observed in the petals of a red rose (rosea Rehd and Rosa hybrid tea, cf. Bairage3-6 (“petal effect”)), where water drops get pinned to the petal surface. Such r 2011 American Chemical Society
fascinating wetting and adhesion properties have sparked much research and have been attributed to a combination of the chemical nature of the surface and hierarchical nano- and microscale surface Received: December 23, 2010 Revised: February 1, 2011 Published: February 28, 2011 4126
dx.doi.org/10.1021/la1050783 | Langmuir 2011, 27, 4126–4133
Langmuir
Figure 1. Diagram illustrating the basic processes utilized in the GLADCE process to fabricate Si nanowires, followed by silanization of the nanowires to achieve superhydrophobicity.
topography.7 Recently, much effort has been directed at artificially replicating such biomimetic surfaces for applications such as selfcleaning and antifogging surfaces, fluid drag reduction, and humidity control for electronic devices.7 Several excellent demonstrations of biomimetic surfaces with tunable wetting and adhesive properties have been achieved using a variety of fabrication techniques.7-12 The ability to address the adhesive properties (i.e., sticking or roll-off behavior) of superhydrophobic surfaces spatially, thereby selectively positioning and pining small spherical drops of liquids, would further enable compelling applications such as in microfluidic lab-on-a-chip devices and as platforms for biological and chemical detection such as the rapid analysis of complex bioactivities. The ability to position and pin small spherical drops of liquids selectively is valuable in developing sensitive methods for biological and chemical analysis and detection.13-16 It is also an excellent platform for the study of interfacial chemical and biological reactions between liquid pearls due to the minimal interaction of the droplets with the underlying substrate.17,18 Thus far, the fabrication of tunable superhydrophobic surfaces is quite challenging in that high-resolution photolithographic steps are required as is the spatial tailoring of wet-chemical experimental conditions over the region of interest, with severely limited scalability and high cost.19-22 The significance of our work lies in the fact that there is no report of a method/process that can simultaneously fabricate both lowand high-adhesion superhydrophobic surfaces on a single substrate. This is primarily due to the different and independent fabrication techniques required to generate petal-like and lotus-like surfaces.19,23 Here we demonstrate (i) a process to fabricate superhydrophobic surfaces on nanostructured silicon (Si) that mimic the lotus- and petal-like wetting behaviors by tuning the morphology of nanowires on the Si surface and (ii) a method to fabricate “hybrid” superhydrophobic surfaces on Si substrates with tunable, spatially selective, adhesive behavior. The basic process steps of our method are schematically illustrated in Figure 1. Briefly, it begins with the deposition of gold (Au) nanoparticles on Si by the glancing angle deposition (GLAD) technique, followed by the metal-assisted chemical etching of Si in a solution of H2O, H2O2, and HF where the Au nanoparticles act as the catalyst that results in the synthesis of Si nanowire arrays. We exploit the tunability of the deposited nanoparticle size distribution that is unique to the GLAD process for obtaining Si nanowire arrays with different morphologies. These nanowire morphologies generated the micro- and nanoscale roughness that enables tuning of the wettability of the nanostructured surfaces. The detailed fabrication process is as follows.
2. EXPERIMENTAL DETAILS N-type (100) Si wafers were first cleaned by standard RCA1 and RCA2 processes. The wafers were subjected to a 1 min etch in 10% HF
ARTICLE
prior to loading into an electron-beam evaporator. The chamber was pumped down to a pressure of 10-6 Torr before commencing the GLAD process. The substrate normal was placed at an angle of 87° with respect to the direction of the incoming flux, and the substrate was rotated at a rate of 0.2 rpm. The samples were then etched for 20 min in a solution of H2O, HF, and H2O2 at room temperature, with the concentrations of HF and H2O2 fixed at 4.6 and 0.44 M, respectively. The Au on the Si surface was then removed using a commercial Au etchant. The samples were next subjected to 10% HF etching for 1 min to remove any native oxide before silanization. The silanization process involved placing the samples in a desiccator for 12 h under house vacuum (mTorr) with 6 μL of tridecafluoro-(1,1,2,2-tetrahydrooctyl)trichlorosilane, [CF3(CF2)5(CH2)2SiCl3], with a surface energy of 13.22 mJ m-2 24 to ensure monolayer coverage. The contact angle measurement of bare Si increased from ∼76.6 to ∼119.1° after the silanization process. Deionized water droplets of 4-6 μL were used for all contact angle measurements. VCA Optima by AST Products Inc. was used for all contact angle measurements. All measurements reported were obtained from an average of five measurements. An FEI NOVA SEM 230 was used for all SEM measurements, and a JEOL 2010F system was used for TEM characterization. This method, which we call GLADCE, is entirely scalable over large areas and does not require complex lithography (such as electron-beam lithography) and etching processes (such as deep reactive ion etching), which are synonymous with the conventional top-down nanofabrication processes.
3. RESULTS AND DISCUSSION The GLAD process involves Au evaporation onto rotating Si substrates placed at large oblique angles to the incoming flux of metal atoms.25,26 Figure 2 shows scanning electron microscope (SEM) images of Au nanoparticles on Si after the GLAD process. Figure 2a corresponds to a shorter-duration GLAD (30 min deposition process), and Figure 2b corresponds to a longerduration GLAD process (90 min deposition process). A longer duration of GLAD evaporation generally leads to larger Au nanoparticles. However, it is important to point out that a longer duration does not imply that all of the nanoparticles would be larger. In fact, a closer and tilted SEM image of Figure 2b, shown as an inset in Figure 2b, reveals many smaller nanoparticles embedded between the larger ones. Figure 2c is a histogram of the Au nanoparticle size distribution between the two abovementioned GLAD durations. The nanoparticle sizes were determined by manually measuring more than 200 nanoparticles from several digital SEM images similar to those in Figure 2a,b. Figure 2 shows that the sample that undergoes a shorterduration GLAD process produces a more uniform distribution in Au nanoparticle size with diameter mostly ranging between 10 and 40 nm whereas a longer-duration GLAD process contains a wide variation in size of Au nanoparticles with diameter ranging from 10 to 100 nm. The nanoparticle packing now resembles Apollonian packing geometries,27 with smaller nanoparticles embedded in the spaces between the larger ones (inset of Figure 2b), which is a consequence of the atomic shadowing process. During the initial stages of the GLAD process, the evaporated Au atoms (adatoms) condense on the Si surface and coalesce to form individual islands (nuclei) that geometrically shadow the substrate because of the oblique arrival angle (87°) and the low surface diffusivity of the adatoms.26 As a result, atomic shadowing takes place and prevents the formation of a continuous thin film. A less oblique arrival angle (
