The current approaches to controlling the geometry of nano-arrays are nanoimprint lithography and unconstrained hydrothermal growth. Nanoimprint lithography is when a polymer imprint is used to form nanoscale patterns that are later cured by UV radiation. This procedure is not suitable for scaling, because it is expensive. Unconstrained hydrothermal growth is another method. Despite many developments in ZnO seed-layer thicknesses, precursor concentrations, and chemical etching treatments, this specific growth does not provide adequate control over the morphology of nanorod arrays.
The approach that
we’ve been using to systematically customize ZnO nanorod arrays is the
self-assembly of block copolymer chains. When two incompatible monomers
are chemically linked, a block copolymer forms. Due to the homopolymers
in the chains and their separation response, the block copolymers
phase-separate into nanoscale microdomains. The following microdomains
covered are spherical (s), cylindrical (c), gyroidal (g), and lamellar
(l). For this particular method, the BCP PS-b-P4VP is used at ratios
that form spherical microdomains. The solution used creates P4VP
micelles that are surrounded by a corona of PS chains. The block
copolymer is dissolved in toluene, which is a strongly selective solvent
for polystyrene (PS).
polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP) is amphiphilic (partially
hydrophobic, partially hydrophilic), allowing for the selective
permeation (penetration through a solid) of aqueous reactants in the
growth solution. As shown in the image, aqueous reactants permeate
through P4VP micellar bodies to react with the substrate and form rods.
The addition of acetone to the hydrothermal growth solution swells PS
chains that surround the P4VP cores enough to allow the aqueous
reactions to reach the depth at which the micelles are located.
procedure that controls nanorod synthesis through block copolymer
self-assembly has already been created for silicon-seeded substrates.
Rather than using silicon, my research involved ZnO rod growth on top of
brass substrates. Brass is an alloy of zinc and copper. This means that
the growth procedure for these substrates would not require Zinc
seeding, which would speed up the process. Brass substrates are also
cheaper, making the procedure more scalable.