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44226-G10
Growth of Nanopillar Arrays for Directed Bio-Molecule Assembly
In this project, we explore how glancing angle deposited (GLAD) nanostructures can be controllably assembled into meso-scale architectures using biological connector molecules that are attached at predetermined locations on the nanostructure surfaces. The processing steps involve (i) the growth of various sets of multi-stack nanorods and nanosprings by GLAD, followed by (ii) selective attachment of biological molecules to specific locations on these nanostructures, and (iii) the directed assembly of the nanostructures into complex meso-scale architectures. The uniqueness of this approach stems from the extreme flexibility of material combinations and nanostructure shapes that can be achieved by the glancing angle deposition process where any element which exhibits a solid phase at room temperature can be stacked on top of any other one, all within 20-500 nm wide nanorods or nanosprings and with a vertical resolution of 1-10 nm. The initial work includes the growth of 25-60 nm wide Cr-Si multi-stack nanosprings and 40-180 nm wide Si-Au multistack nanorods, as well as the directed end-to-end assembly of short and long Si nanorods with Au-caps, using biotin and streptavidin that are selectively attached to the Au section of short and long rods, respectively. The procedure may be ultimately extended to the controlled assembly of nanorods and nanosprings to form interconnected two-dimensional and three-dimensional architectures like, for example, nanoladders and nanohoneycombs, with potential applications in petroleum sensing and processing.
Cr-Si nanosprings and Au-Si nanorods were grown in a load-locked ultra-high vacuum magnetron sputter deposition system. The substrates were continuously rotated about the polar axis (perpendicular to the substrate surface) for the growth of nanorods, and rotated sequentially by 180„a-steps for nanospring growth. A collimating plate covering the substrate was used to prevent any non-directional flux from striking the substrate, and to control the azimuthal deposition angle to be 84„a. Multi-component nanorods were obtained by depositing different materials in discrete deposition steps. The sample heating due to the deposition plasma was monitored by a thermocouple within the sample stage and was below 110 degree C for all depositions, which were carried out at 0.26 Pa (2.0 mTorr) in 99.999% pure Ar that was further purified using a Micro Torr purifier. Sputtering was done at a fixed power of 500 W, yielding approximate rod growth-rates of 2.4, 5.3 and 6.3 nm/min for Cr, Si, and Au respectively. The shape and morphology of the nanorods and their assemblies were analyzed by scanning and transmission electron microscopy (JEOL 6335F Field Emission SEM and Phillips CM12 TEM).
Cr-Si zigzag nanosprings were grown in four sequential steps with alternating deposition of Cr from the right and Si from the left, yielding two-component zigzag nanosprings with four arms consisting of the respective elements. The width of the springs near the substrate ranges from 25 to 30 nm. However, the width increases to 45+/-5 nm at the end of the first (Cr) arm and to 65+/-15 for the fourth (Si) arm, that is, the top of the spring. The broadening as a function of height can be attributed to strong intercolumnar competition, as commonly observed for the growth of narrow („T 100 nm) columns by GLAD. The increase in the width in each consecutive arm of the zigzags is compensated by a fraction of the springs terminating their growth prematurely, leading to a reduction in their number density and resulting in an increase in the average spacing between the springs, from 25+/-5 nm for the first arm to 135+/-10 nm for the fourth arm of the zigzags.
Si-Au multi-stack nanorods were grown with continuous rotation of the substrate about the polar axis using two alternating sequences of Si and Au deposition steps, with a nominal Si:Au atomic ratio of 2:1 The nanorods are 600+/-25-nm tall and their width increases with height from 50+/-15 nm near the substrate to 170+/-10-nm on the top. The increase in width is due to a competitive growth mode and is accompanied by a decrease in the rod number density, similar to the case for the nanosprings discussed above. This leads to rods that terminate their growth prematurely, with a height of 150+/-35 nm and a width of 45+/-15 nm. The larger rods exhibit increasingly rough surfaces with protrusions that elongate along the growth direction. These protrusions can be attributed to the same growth instability that causes the formation of the entire nanorod. That is, small surface irregularities are exacerbated during the growth process since any roughness on the rod surface results in an enhanced capturing rate of the flux, yielding locally increased growth rates and, in turn, surface protrusions.
