The research objective of this award is to perform a systematic study on the nanocarpet behavior of aligned nanorod array surfaces with different structures and morphologies. When high aspect ratio nanorod arrays are treated by liquid and dried, the original morphological structure could suffer a permanent change, which will alter the performance of the nanostructure arrays. By exploring the detailed relationships between the clustering of the nanocarpet effect and the mechanical properties of individual nanorods with different heights, shapes, materials, and the liquid properties, the proposed research seeks answers to the following fundamental questions: (1) How does the mechanical stability of nanostructures affect the nanocarpet effect? (2) What process causes the nanocarpet effect (and the pattern formation), and how to prevent this effect? (3) What will be the novel fluidic properties for nanorod array substrates and how can we use it for different applications?
This transformative research has a potential to obtain fundamental knowledge on liquid-nanostructure interaction and help to design robust nanostructures for applications in liquid environment such as sensors, nanofluidic systems, etc. The success of the project would have a large and immediate impact in the areas of: i) nanostructure fabrication and engineering, ii) fundamental surface science, iii) mechanical properties of nanostructures, iv) sensor design and implement, and v) bioanalytical applications. The lab-based nanotechnology course module will help undergraduate and high school students to obtain hands-on experience with nanofabrication. This project will also establish a rigorous material physics, mechanical engineering, and nanotechnology education and training opportunity for both graduate and undergraduate students.
When high aspect ratio nanorod or nanowire arrays are treated by liquid and dried, the original morphological structure could suffer a permanent change, which will alter the performance of the nanostructure arrays. Such a phenomenon, called nanocarpet effect, could alter the device performance made of nanorod arrays when immersed in liquid, in particularly for biological applications. According to our preliminary studies, such a nanocarpet effect is influenced by the structure and morphology of the nanorod or nanowire arrays, the property of the liquid, and the mechanical properties of the nanostructures. This nanostructure stability issue is critical for applications of the high aspect ratio nanostructures, but so far, no systematical studies have been carried out, and our understanding on this effect is very limited, which prevent us to find an effective way to circumvent this effect. This is mainly due to lack of a nanofabrication technique that could produce nanostructures with arbitrary structures and materials as well as a detailed characterization of the mechanical properties of nanorods with different shapes. We have demonstrated that using aligned Si nanorod arrays fabricated by the glancing angle deposition (GLAD) technique, one can perform systematic studies on the nanocarpet behaviors of nanostructured surfaces. In comparison with existing nanofabrication methods, GLAD offers several strategic advantages, including control of the size, shape, density, alignment, orientation and multilayer composition of the nanorod arrays on a large surface area. For the mechanical property characterization of nanorods, we have developed several novel methodologies including nanoindentation, AFM three point bending test, and tensile and buckling tests inside SEM. The advantages of the GLAD fabrication method and the mechanical testing techniques lay a solid foundation for this study. During the four-year NSF project period, we have focused on four aspects: (1) characterizing the wetting behavior of nanorod arrays; (2) improving the mechanical stability of nanorod arrays; (3) developing new mechanical characterization techniques; and (4) applying wetting phenomenon of nanorod array for biosensing and separation. Our studies show that when the nanorod arrays are deposited onto a patterned microstructured surface, such as a CD surface, the nano-carpet effect can be reduced significantly depending on the size of the pattern. However, if the nanorod arrays are deposited tilted on the patterned surface, the nano-carpet effect is minimized. When the nanorods are bundled together due to nanocarpet effect and treated with hydrophobic coating such as a layer of (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane, the surface becomes super-hydrophobic with a contact angle > 160o and a tilting angle < 2o. In fact, a water droplet cannot even stay on the surface. Such a surface provides a way to study the viscoelastic braking theories developed by Shanahan and de Gennes, and by White. The super-hydrophobic surfaces composed of an array of silicon nanostructures whose Young modulus is 4 orders of magnitude higher than that of surfaces in earlier recorded viscoelastic braking experiments. We find that when the apparent contact angle q < 180°, there is a surface deformation at the three-phase contact line which is associated with a reduction in the hydrophobicity of the surface. For the super-hydrophobic surface, however, there cannot be a three-phase contact line associated with a drop in contact with the surface. To improve the mechanical stability of the nanorods, we have designed a co-deposition process to fabricate composition graded nanorod array. Such a structure was used to improve the adhesion between the nanorods and the substrate. We have successfully made the Cu-Si graded composition nanorods, and the mechanical test shows that they have better stability than Si nanorods. AFM and nanoindentation were employed to probe the mechanical properties of such nanostructures. The elastic modulus of individual Si nanorods on Cu/Ti bilayer film was measure to be 90 ± 6 GPa. For the Cu-Si composition graded nanorods, the elastic modulus was measured to be 135 ± 11 GPa, demonstrating 50.0% increase compared to the Si nanorods. With a fundamental understanding of the wetting properties of tilted nanorod array, we have used the knowledge to combine ultra-thin layer chromatography and surface enhanced Raman scattering technique to generate an on-chip separation and detection system. Recently this system has been demonstrated to be able to directly detect Respiratory syncytial virus (RSV) virus from clinic samples. This project has trained and involved ten graduate students, and three undergraduate students, including an African American female student. Results from the project have been used in the PI’s physics lecture regarding bionanotechnology. It has produced 27 peer reviewed journal publications, 1 conference proceeding, and more than 10 conference talks and seminars.
