Growth of polygonal rings and wires of CuS on structured surfaces
Stoichiometric variants of copper sulfide have attracted interest in fields as diverse as photovoltaics and chemical sensors. By manipulating stoichiometry, crystal structure, and microstructure it is possible to tune copper sulfide's band gap. Using the self-assembly CuS as a test case, Prof. Aizenberg and colleagues demonstrate the feasibility of manipulating morphology via controlled growth of covellite-type "wires" and rings on a structured surface. This paper serves as an example of the power of self-assembly to create structures with tunable, precise order at a small length scale.
Materials and Methods
The structured surfaces were substrates with epoxy micropillars (1 mm diameter, 8 mm height, and 3 mm pitch), functionalized with 1-heptanethiol atop a layer of Pt/Pd for superhydrophobicity. CuS structures were grown by immersing substrates in an aqueous solution with various concentrations of copper sulfate and sodium thiosulfate. Self-assembly of structures was initiated from the tops of the micro pillars only, resulting in the growth of distinctive hexagonal rings and "wires". Morphology/crystallinity of the resulting structures was evaluated via SEM and XRD.
For lower concentrations the growth process was as follows: 1) Due to the selective wetting of the superhydrophobic structured surface, CuS spherical particles nucleated at the tips of the micro pillars, 2) formation of kinked hexagonal and "open" rings with angles of roughly 120 degrees and 110-130 degrees , respectively and a diameter of ~600 nm, 3) formation of larger polygons as the CuS chains exceed the size of the pillars, and 4) formation of longer continuous chains. Figures 1 and 2 shows SEM images of the various stages.
At higher concentrations, the selective wetting process occurs on the tips of the micro pillars, forming wires of 250-350 nm diameter, and 20+um in length. Figure 3 shows the formation of these wires.
Although the material in question is outside the "squishy" material property range, Aizenberg's work is one example of the variety of structures on can achieve by setting certain solution parameters and allowing self-assembly to occur. Through an understanding of system kinetics and thermodynamics, one can achieve greater perfection and precision in a desired structure, and at far smaller length scale than is possible with direct manipulation.